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This paper studies the magnetization mechanism of a hybrid high temperature superconducting (HTS) trapped field magnet. To address the size limitation of traditional HTS bulk materials, hybridization between HTS-stacked ring magnets and HTS bulks is proposed here. A jointless HTS-stacked ring magnet is used to increase the trapped field area for HTS bulks. A hybrid HTS magnet with 90 mm in length and 60 mm in width was tested to provide a trapped field of 7.35 T in a field cooling magnetization. The paper focuses mainly on understanding the novel magnetization mechanism of this hybrid HTS trapped field magnet. A numerical model based on homogenized H formulation was used to compare with experimental results, and a good match was found. Our experimental and numerical study of the electromagnetic interaction between the HTS-stacked ring magnet and the HTS bulks reveals that there are two magnetization stages, and the magnetization speed differs in these two stages by a sing criterion: whether the HTS-stacked ring magnet is fully penetrated or not. This study confirms that hybridization helps to build large HTS trapped field magnets. [ABSTRACT FROM AUTHOR]

Spin–orbit torque (SOT) can provide efficient electrical manipulation of magnetism via applying electrical current to breaking the symmetry of damping-like torque. In the heterojunction of heavy and ferromagnetic metal, Dzyaloshinskii–Moriya interaction (DMI) is one of the key ingredients for stabilizing chiral spin structures, like chiral domain walls. Meanwhile, materials with larger charge-spin conversion rates are also highly expected for the efficient SOT. In this paper, spin–orbit torque magnetic switching is observed in the perpendicularly magnetized Pt/Co/Bi2Se3 and shows relatively high efficiency with low critical switching current density of about 5 × 105 A cm−2. The SOT efficiency and DMI in perpendicularly magnetized Pt/Co/Bi2Se3 were quantitatively investigated by electrical detection of the effective spin Hall field. The DMI constant is about 2.6 mJ m−2, and the effective spin Hall angle of Pt/Co/Bi2Se3 is about 0.14. The work also demonstrates that the Bi2Se3 layer takes the main responsibility for SOT, and the Pt/Co interface is the main source of DMI in Pt/Co/Bi2Se3 structures, which makes it possible to achieve independent optimization of DMI and SOT in the Pt/Co/Bi2Se3 structure at room temperature for the advanced application of spintronic devices. [ABSTRACT FROM AUTHOR]

Spin–orbit torque (SOT) can provide efficient electrical manipulation of magnetism via applying electrical current to breaking the symmetry of damping-like torque. In the heterojunction of heavy and ferromagnetic metal, Dzyaloshinskii–Moriya interaction (DMI) is one of the key ingredients for stabilizing chiral spin structures, like chiral domain walls. Meanwhile, materials with larger charge-spin conversion rates are also highly expected for the efficient SOT. In this paper, spin–orbit torque magnetic switching is observed in the perpendicularly magnetized Pt/Co/Bi2Se3 and shows relatively high efficiency with low critical switching current density of about 5 × 105 A cm−2. The SOT efficiency and DMI in perpendicularly magnetized Pt/Co/Bi2Se3 were quantitatively investigated by electrical detection of the effective spin Hall field. The DMI constant is about 2.6 mJ m−2, and the effective spin Hall angle of Pt/Co/Bi2Se3 is about 0.14. The work also demonstrates that the Bi2Se3 layer takes the main responsibility for SOT, and the Pt/Co interface is the main source of DMI in Pt/Co/Bi2Se3 structures, which makes it possible to achieve independent optimization of DMI and SOT in the Pt/Co/Bi2Se3 structure at room temperature for the advanced application of spintronic devices. [ABSTRACT FROM AUTHOR]

The field-free spin–orbit torque induced 180° reorientation of magnetization is beneficial for the high performance magnetic memory. The antiferromagnetic material (AFM) can provide a higher operation speed than the ferromagnetic counterpart. In this paper, we propose a trilayer AFM/insulator/heavy metal structure as the AFM memory device. We show that the field-free switching of the AFM with a perpendicular Néel vector can be achieved by using two orthogonal currents, which provide a uniform damping-like torque and uniform field-like torque, respectively. The reversible switching can be obtained by reversing either current. A current density of 1.79 × 1011 A/m2 is sufficient to induce the switching. In addition, the two magnetic moments become noncollinear during switching. This enables an ultrafast switching within 40 ps. The device and switching mechanism proposed in this work offer a promising approach to deterministically switch the AFM with the perpendicular Néel vector. It can also stimulate the development of an ultrafast AFM-based MRAM. [ABSTRACT FROM AUTHOR]

In the paper, a magneto-mechanic hysteresis model for giant magnetostrictive materials is suggested by considering the effect of the domain rotation and domain wall motion on the magnetization process under prestress and the applied magnetic field. The coercive force, which is magneto-mechanic dependent, is proposed instead of a pinning constant in the Jiles–Atherton model. The model can well predict the characteristics of a magnetization-applied field curve and magnetostrictive strain-applied field curve shown in the experiment, especially the "overturn phenomenon" under different compressive prestresses. Furthermore, the effect of the microstructure parameter, such as the ratio of the domain wall thickness to the internal stress wavelength, the amplitude of internal stress, the ratio of the domain wall thickness to the inclusion radius, and inclusion consistency, on coercive force under applied prestress can also be described by the model. The comparison between the results predicted by the model and experiment shows that the model is suited for a wide-ranging applied magnetic field. [ABSTRACT FROM AUTHOR]

Based on the micro-domain theory, the macro-thermodynamic relationship, and the effective field theory, an improved nonlinear constitutive model is established for ferromagnetic materials. The constitutive model includes new expressions of some nonlinear functions and high-order nonlinear functions and has simple expressions for some ferromagnetic materials. Compared with the existing models, the nonlinear model established in this paper has a great improvement in the prediction accuracy of the magneto-elastic coupling behavior. In other words, it can accurately predict the complicated nonlinear behaviors of magnetostriction and magnetization under the magnetic field and applied stress. Furthermore, it can correctly describe magnetostriction with high tensile stress and low magnetic fields, the complicated magnetostriction behavior with stress and middle magnetic fields, the complicated magnetization behavior with stress and high magnetic fields, and the maximum point of magnetostriction according to applied stress for ferromagnetic materials. A detailed analysis of the complicated nonlinear behaviors for ferromagnetic materials demonstrates the high effectiveness of the presented model. [ABSTRACT FROM AUTHOR]

This Tutorial article focuses on magnetic phenomena and material systems that have gained significant importance since the original development of mumax3, but are challenging to simulate for users who rely solely on the originally provided examples. Alongside the physical background, we provide hands-on examples of advanced magnetic systems, including detailed explanations of complete mumax3 input files (13 in total, often showing different ways to achieve things), and highlighting potential pitfalls where applicable. Specifically, we explore two approaches to incorporate spin–orbit torques in mumax simulations, considering the trade-off between versatility and speed. We also examine complex multilayer material stacks, including synthetic antiferromagnets, demonstrating different implementation methods that again vary in speed, versatility, and realism. A key criterion for selecting the optimal simulation strategy is its suitability for modeling systems where the magnetization varies significantly in the third dimension. The material covered in this Tutorial paper includes content developed for the mumax3 workshop presented during the summer of 2020 within the context of the IEEE online spintronics seminar, along with additional new topics. Throughout the explanations, we ensure broad applicability beyond specific examples. [ABSTRACT FROM AUTHOR]

Power transfer capability of wireless power transfer systems is highly dependent on the magnetic design of the primary and secondary inductors and is measured quantitatively by the coupling factor. The inductors are designed by placing the coil over a ferrite base to increase the coupling factor and reduce magnetic emissions to the surroundings. Effect of adding extra ferrite above the base ferrite at different physical locations on the self-inductance, mutual inductance, and coupling factor is under investigation in this paper. The addition can increase or decrease the mutual inductance depending on the placement of ferrite. Also, the addition of ferrite increases the self-inductance of the coils, and there is a probability for an overall decrease in the coupling factor. Correct placement of ferrite, on the other hand, can increase the coupling factor relatively higher than the base ferrite as it is closer to the other inductor. Ferrite being a heavy compound of iron increases the inductor weight significantly and needs to be added judiciously. Four zones have been identified in the paper, which shows different sensitivity to addition of ferrite in terms of the two inductances and coupling factor. Simulation and measurement results are presented for different air gaps between the coils and at different gap distances between the ferrite base and added ferrite. This paper is beneficial in improving the coupling factor while adding minimum weight to wireless power transfer system. [ABSTRACT FROM AUTHOR]

During the service process of remanufactured components, the stress and crack may occur at the interface between coating and substrate due to their mechanical property mismatch. Based on the spontaneous magnetization phenomenon in ferromagnetic materials, the stress distribution and crack propagation length along the interface can be characterized. In this paper, the magnetomechanical constitutive model for interface is established according to Timoshenko beam theory and Jiles' stress-magnetization model. The distributions of stress and residual magnetization along the interface are analyzed under the effect of typical three-point bending (TPB) load. The results show that the interfacial residual magnetization Mr can reflect the stress distribution very well and its peak value at supporting seat Mr3 is closely related to interfacial crack propagation length. In order to verify the theoretical model, the metal magnetic memory (MMM) technique is used to collect the residual magnetic field along the interface in the TPB testing. The experimental results are consistent with the theoretical predictions. They indicate that the magnetomechanical model for coating/substrate interface established in this paper can provide the theoretical support for magnetic non-destructive testing and the calculation results can be applied to the interfacial crack propagation length characterization in MMM. [ABSTRACT FROM AUTHOR]

Variable inductors have a simple structure that consists of only magnetic core and primary dc and secondary ac windings, but can control effective inductance of the secondary ac winding by the primary dc current due to magnetic saturation effect. Hence, the variable inductors can be applied as a var compensator in electric power systems. In a previous paper, a lap-winding type three-phase variable inductor, which has the primary dc and secondary ac windings concentrically arranged on the same leg, was presented. It has a good controllability and low distortion current. This paper describes size and weight reduction of the lap-winding type three-phase variable inductor by focusing on the leg width. First, an initial design method of the leg width considering the influence of the dc-biased magnetization is presented. Next, a detailed design of the leg width for the size and weight reduction of the proposed variable inductor is described. Finally, characteristics of the size- and weight-reduced variable inductor were compared to those of the previous variable inductor. It is demonstrated that the core diameter is reduced by about 6% and the weight is decreased by about 50% in comparison with the previous variable inductor. [ABSTRACT FROM AUTHOR]

This paper deals with a neural network approach to model magnetic hysteresis at macro-magnetic scale. Such approach to the problem seems promising in order to couple the numerical treatment of magnetic hysteresis to FEM numerical solvers of the Maxwell's equations in time domain, as in case of the nonlinear dynamic analysis of electrical machines, and other similar devices, making possible a full computer simulation in a reasonable time. The neural system proposed consists of four inputs representing the magnetic field and the magnetic inductions components at each time step and it is trained by 2-d measurements performed on the magnetic material to be modeled. The magnetic induction B is assumed as entry point and the output of the neural system returns the predicted value of the field H at the same time step. A suitable partitioning of the neural system, described in the paper, makes the computing process rather fast. Validations with experimental tests and simulations for non-symmetric and minor loops are presented. [ABSTRACT FROM AUTHOR]

The design concept of microwave-assisted magnetic recording (MAMR) using the flux control (FC) effect has been proposed as a technology for hard disk drives (HDDs). In this type of MAMR, the magnetization of an in-gap device (FC device) is reversed against the gap field by spin-transfer torque, enhancing the amplitude and gradient of the recording field. In this paper, we study the magnetization dynamics of an FC device fabricated in the write gap of an HDD write head. The operation of the FC device is analyzed by measuring the temporal resistance change in the sub-nanosecond region. Reversal of the FC device becomes faster as the bias current is increased and can be completed by 0.5 ns after the transition of the write current. The experimental results are reproduced by micromagnetic simulations using a head model, confirming that the simulations correctly describe the magnetization dynamics of the actual device. The simulations show that the recording field gain by the FC device appears with little delay after the rise of the recording field and that the FC device operates effectively even at a fast write rate of approximately 3 Gbit/s. Furthermore, we demonstrate the effectiveness of boosting the bias current, which can realize both fast and reliable operation of the FC device. These results indicate that the FC device operates as designed and that MAMR using the FC effect is promising for extending the recording density of HDDs. [ABSTRACT FROM AUTHOR]

In this paper, we discuss a phenomenological approach for the numerical modeling of static hysteresis of grain oriented (GO) steels at macro-magnetic scale. The modeling is inspired to a generalization in 2-d of the Preisach model (this generalization can be easily extended in 3-d, but it is not necessary here), and the magnetization dependent state of the model parameters is represented by two distinct vectors, named the interaction field and the operative field, suitably defined. In this way, the very complex behavior of the GO steel circularly or linearly polarized is reproduced with accuracy, as shown in the numerical tests reported in the paper. [ABSTRACT FROM AUTHOR]

This paper is to study ramping turn-to-turn loss and magnetization loss of a no-insulation (NI) high temperature superconductor (HTS) pancake coil wound with (RE)Ba2Cu3Ox (REBCO) conductors. For insulated (INS) HTS coils, a magnetization loss occurs on superconducting layers during a ramping operation. For the NI HTS coil, additional loss is generated by the "bypassing" current on the turn-to-turn metallic contacts, which is called "turn-to-turn loss" in this study. Therefore, the NI coil's ramping loss is much different from that of the INS coil, but few studies have been reported on this aspect. To analyze the ramping losses of NI coils, a numerical method is developed by coupling an equivalent circuit network model and a H-formulation finite element method model. The former model is to calculate NI coil's current distribution and turn-to-turn loss, and the latter model is to calculate the magnetization loss. A test NI pancake coil is wound with REBCO tapes and the reliability of this model is validated by experiments. Then the characteristics of the NI coil's ramping losses are studied using this coupling model. Results show that the turn-to-turn loss is much higher than the magnetization loss. The NI coil's total ramping loss is much higher than that of its insulated counterpart, which has to be considered carefully in the design and operation of NI applications. This paper also discusses the possibility to reduce NI coil's ramping loss by decreasing the ramping rate of power supply or increasing the coil's turn-to-turn resistivity. [ABSTRACT FROM AUTHOR]

This paper experimentally demonstrates the operating principles of a near-field multiferroic antenna. The antenna uses a piezoelectric lead-zirconate-titanate stack to apply a time varying strain to a magnetoelastic iron gallium Fe80.77Ga19.23 (FeGa) rod. The voltage induced strain controls the net magnetization of the FeGa rod to generate oscillating magnetic fields in free space surrounding the antenna. Direct experimental measurements of the mechanical force, strain, magnetic field, and magnetic flux inside the FeGa rod were collected and show strong a correlation with the dynamic magnetic field measured in free space. Additionally, an analytical dipole model is used to demonstrate that the free space signal originates from the changing magnetization in the FeGa rod, providing a clear multiferroic antenna proof-of-concept demonstration. Finally, analysis is provided to highlight the importance of device geometry and demagnetization fields when optimizing the antenna response. [ABSTRACT FROM AUTHOR]

This paper presents new experimental data that enable the observed processes in nonlinear ferrite lines to be related with the theoretical positions of the spin waves of the magnetization precession under the conditions of a high-power microwave. Such an approach has not been considered in earlier discussions on the subject and can contribute to the theory of the generation of oscillations in a gyromagnetic ferrite medium. These new aspects were used to design a new type of generator. The specific feature is the presence of regularity in the ferrite lines in the generation mode of microwave oscillations. The repeating regularity enabled the implementation of multistage pulse shape converters capable of operating in two modes and providing extreme parameters of the output pulses. Two variants of multistage converters of nanosecond high-voltage pulse shapes with a duration of ∼4 ns at a half-height and with an amplitude of −500 kV were designed and tested. The assembly of the converters and the driving generator are described in the stationary setup. In the first case, the rise time shortened to ∼45 ps, and the amplitude increased to −850 kV due to the sharpening of a pulse and the formation of a shock wave by the cascade of three nonlinear transmission lines. A record rate for the increase of the leading peak voltage of ∼15.5 MV/ns was reached. In the second case, the new approach for the generation of a sequence of subnanosecond pulses was presented and tested, and each pulse of the previous stage was modulated by the next stage doubling the number of pulses while conserving a deeper modulation. As a result, at the top of the incident pulse, a sequence of subnanosecond peaks with a large modulation depth (∼70%) was formed when the maximum voltage amplitude reached −700 kV. The results of the emission of such pulses are also presented. [ABSTRACT FROM AUTHOR]

The magnetic properties of the low-dimensional trirutile tantalates Co 1 − x Mg x Ta 2 O 6 [ x = 0.0, 0.1, 0.3, 0.5, 0.7, 1.0] are reported in this paper. CoTa 2 O 6 is an antiferromagnet with a transition temperature at T N = 6.6 K, while showing a very broad peak in the magnetic susceptibility at ≈ 12 K owing to the low dimensionality of the magnetic interactions. In our study, where Mg is introduced at the Co position, we see the disappearance of the broad transition for high values of x. For the x = 0.1 composition, three transitions are observed in the magnetization at 28 K, 6 K, and 3.3 K. Our analysis of magnetization data using the anisotropic Heisenberg and the Ising models for spin chains points out that the magnetic interactions in Co 1 − x Mg x Ta 2 O 6 are indeed low dimensional. Field-induced metamagnetic transitions are observed in the case of x = 0 and x = 0.1 compositions of Co 1 − x Mg x Ta 2 O 6 , which are evident in the magnetization isotherms at T = 2 K. Interestingly, we find that with progressive doping up to x = 0.5 and 0.7, we obtain higher magnetization pointing to the possibility of weak-ferromagnetism. This might point toward the formation of single-chain magnets, which opens up a new possibility in low dimensional oxides for spintronics applications. [ABSTRACT FROM AUTHOR]

In this paper, the magnetic properties of Co-rich CoPtP films electrodeposited using an optimized Pulse Reverse (PR) technique are investigated for magnetic MEMS applications. By using a combination of forward and reverse pulses with optimized duty cycles during deposition and suitable bath chemistry, the film stress is reduced significantly, which results in smooth, crack-free films of thickness up to 26 μm. The deposited film of thickness ∼3 μm shows a coercivity of 268 kA/m, a remanence of 0.4 T, and a maximum energy product of 35 kJ/m3 in the out-of-plane direction. The variation in the hard-magnetic properties of the films for changing the film thickness is analyzed in terms of the composition, crystalline structure, and grain size. As the thickness is increased from 0.9 μm to 26 μm, the in-plane coercivity reduces by 17% due to an increase of the grain size and the Co content in the alloy structure. The in-plane squareness factor increases by 1.5 times as the thickness is increased over the above-mentioned range, which results in an enhancement of the in-plane remanence value. The magnetization reversal behavior of the deposited films indicates that the nature of magnetic interaction is significantly influenced by the thickness of the films, where the dipolar interaction for the thinner films changes to exchange coupling at higher thickness due to the increase of the grain size. Finally, an innovative design strategy to integrate CoPtP in magnetic MEMS devices by micro-patterning is proposed and analyzed using the finite element method. The demagnetization fields of the magnetic elements are minimized through optimized micro-patterned structures which improve the viability of PR deposited CoPtP micro-magnets having suitable nano-grains in potential MEMS based applications. [ABSTRACT FROM AUTHOR]

The first-order reversal curve (FORC) diagram method is one of the most successful characterization techniques used to characterize complex hysteretic phenomena not only in magnetism but also in other areas of science like in ferroelectricity, geology, archeology, in spin-transition materials, etc. Because the definition of the FORC diagram involves a second-order derivative, the main problem in their numerical calculation is that the derivative of a function for which only discrete noise-contaminated data values are available magnifies the noise that is inevitably present in measurements. In this paper, we present the doFORC tool for calculating FORC diagrams of noise scattered data. It can provide both a smooth approximation of the measured magnetization and all its partial derivatives. Even if doFORC is mainly dedicated to FORC diagrams' computation, it can process a general set of arbitrarily distributed two-dimensional points. doFORC is a free, portable application working on various operating systems, with an easy to use graphical interface, with four regression methods implemented to obtain a smooth approximation of the data which may then be differentiated to obtain approximations for derivatives. In order to perform the diagnostics and goodness of fit, doFORC computes residuals to characterize the difference between the observed and predicted values, generalized cross-validation to measure the predictive performance, two information criteria to quantify the information that is lost by using an approximate model, and three degrees of freedom to compare different amounts of smoothing being performed by different smoothing methods. Based on these, doFORC can perform automatic smoothing parameter selection. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical tool, which combines a generalized permeability tensor model, a homemade 3D magneto-static solver, and a commercial electromagnetic simulation software Ansys HFSS™ for accurate modeling of ferrite-based devices regardless of their state of magnetization. A magneto-static analysis is carried out to find the internal biasing fields in the ferrite sample. Permeability tensor components are computed using a generalized permeability tensor model. Real and imaginary parts of permeability tensor components are then integrated into the commercial 3D electromagnetic simulation software HFSS retaining the spatial variations of internal biasing fields in the ferrite sample. Frequency domain simulations are done using HFSS. This theoretical modeling approach is validated by comparing the theoretical simulation results with experimental ones in the case of a coaxial line loaded by a magnetized ferrite. Proposed approach is then applied to the design of a microstrip Y-junction circulator. Finally, a demonstration prototype is manufactured in the Low Temperature Co-fired Ceramics technology. [ABSTRACT FROM AUTHOR]

Rare earth chromites (RCrO3) continue to be of considerable interest due to their intriguing physical properties such as spin-reorientation, multiferroicity, and magnetocaloric effect. In this paper, we compare the structural, magnetic, and magnetocaloric properties of bulk ErCrO3 with those of bulk Er0.33Gd0.67CrO3, the latter obtained by Gd substitution at the A-site (Er-site) and report substantial enhancement in the magnetocaloric response by Gd substitution. The samples prepared by the citrate route were structurally characterized at room temperature using x-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy. The XRD measurements refined by Rietveld analysis indicate that both samples crystallized in the orthorhombically distorted perovskite structure with Pbnm space group. Magnetic measurements on both samples were carried out between 5 K and 300 K in magnetic fields up to 7 T and show that T N C r (where Cr3+ orders) for Er0.33Gd0.67CrO3 is enhanced to 155 K vs. 133 K for ErCrO3 with analogous changes in the other magnetic parameters. Isothermal magnetization M vs. H data at different temperatures were used to determine changes in the magnetic entropy ( − Δ S ) and relative cooling power (RCP) for the two samples showing considerable improvement with Gd substitution in bulk ErCrO3. The maximum value of − Δ S for Er0.33Gd0.67CrO3 is 27.6 J kg−1 K−1 at 5 K and 7 T with a RCP of 531.1 J kg−1, in comparison to maximum − Δ S = 10.7 J kg−1 K−1 at 15 K with an RCP of 416.4 J kg−1 for ErCrO3 at 7 T. [ABSTRACT FROM AUTHOR]

This paper presents numerical and experimental data for dipole-dipole coupled Ni nanodots on a piezoelectric [Pb(Mg1/3Nb2/3)O3]0.68[PbTiO3]0.32 substrate. Simulation results show that the dipole coupling produces artificial ferromagnetic (parallel magnetization alignment in the nanodot arrays) behavior that can be modified to artificial antiferromagnetic behavior with an applied voltage. Experimental results show the trends in Mr and Hc predicted by the model, but discrepancies arise due to geometric defects present in the fabricated samples. Geometric defects are introduced into the Ni nanodot models, thus dramatically improving the correlation between experiments and analysis. This work shows, through numerical simulations, that artificial multiferroic nanostructures can be designed to produce switching from parallel (artificial ferromagnetic) to antiparallel (artificial antiferromagnetic) magnetization ordering by leveraging dipole coupling with voltage induced changes in magnetic anisotropy. [ABSTRACT FROM AUTHOR]

This paper follows previous works which investigated the strength of dipolar coupling in twomagnet systems. While those works focused on qualitative analyses, this manuscript elucidates reversal through dipolar coupling culminating in analytical expressions for reversal reliability in identical two-magnet systems. The dipolar field generated by a mono-domain magnetic body can be represented by a tensor containing both longitudinal and perpendicular field components; this field changes orientation and magnitude based on the magnetization of neighboring nanomagnets. While the dipolar field does reduce to its longitudinal component at short time-scales, for slow magnetization reversals, the simple longitudinal field representation greatly underestimates the scope of parameters that ensure reliable coupling. For the first time, analytical models that map the geometric and material parameters required for reliable coupling in two-magnet systems are developed. It is shown that in biaxial nanomagnets, the ̂x and ̂y components of the dipolar field contribute to the coupling, while all three dimensions contribute to the coupling between a pair of uniaxial magnets. Additionally, the ratio of the longitudinal and perpendicular components of the dipolar field is also very important. If the perpendicular components in the dipolar tensor are too large, the nanomagnet pair may come to rest in an undesirable meta-stable state away from the free axis. The analytical models formulated in this manuscript map the minimum and maximum parameters for reliable coupling. Using these models, it is shown that there is a very small range of material parameters which can facilitate reliable coupling between perpendicular-magnetic-anisotropy nanomagnets; hence, in-plane nanomagnets are more suitable for coupled systems. [ABSTRACT FROM AUTHOR]

The measurement of MOKE (Magneto-Optical Kerr Effect) magnetization loops in a wide-field Kerr microscope offers the advantage that the relevant domain images along the loop can be readily recorded. As the microscope’s objective lens is exposed to the magnetic field, the loops are usually strongly distorted by non-linear Faraday rotations of the polarized light that occur in the objective lens and that are superimposed to the MOKE signal. In this paper, an experimental method, based on a motorized analyzer, is introduced which allows to compensate the Faraday contributions, thus leading to pure MOKE loops. A wide field Kerr microscope, equipped with this technology, works well as a laser-based MOKE magnetometer, additionally offering domain images and thus providing the basis for loop interpretation. [ABSTRACT FROM AUTHOR]

In micro- and nano- electronic device fabrication, and particularly 3D designs, the diffusion of a metal into sublayers during annealing needs to be minimized as it is usually detrimental to device performance. Diffusion also causes the formation and growth of nanoprecipitates in solid matrices. In this paper, the diffusion behavior of low energy, low fluence, ion implanted iron into a thermally grown silicon oxide layer on silicon is investigated. Different ion beam analysis and imaging techniques were used. Magnetization measurements were also undertaken to provide evidence of nanocrystalline ordering. While standard vacuum furnace annealing and electron beam annealing lead to fast diffusion of the implanted species towards the Si/SiO2 interface, we show that furnace annealing in an oxygen rich atmosphere prevents the diffusion of iron that, in turn, limits the growth of the nanoparticles. The diffusion and particle growth is also greatly reduced when oxygen atoms are implanted in the SiO2 prior to Fe implantation, effectively acting as a diffusion barrier. The excess oxygen is hypothesized to trap Fe atoms and reduce their mean free path during the diffusion. Monte-Carlo simulations of the diffusion process which consider the random walk of Fe, Fick’s diffusion of O atoms, Fe precipitation, and desorption of the SiO2 layer under the electron beam annealing were performed. Simulation results for the three preparation conditions are found in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

This paper reports the effect of the film thickness on the magnetostrictive behavior of (Fe,Co) rich Tb-(Fe,Co) films grown on Si <100> by electron beam evaporation. Magnetostriction was found to decrease with an increase in film thicknesses. To understand the variation of magnetostriction with the film thickness, detailed structural, microstructural, magnetization, and magnetic microscopy studies were carried out. X-ray diffraction studies indicated the presence of two phases, viz., Tb2 (Fe, Co)17 and Fe-Co phases, for all the films. With the increase in the film thickness, the peak intensity of the Tb2 (Fe, Co)17 phase decreased and that of the Fe-Co phase increased. Magnetization studies showed the presence of strong in-plane anisotropy for all the films. In addition to this, the presence of the out-of-plane component of magnetization was also observed for the films grown with higher thicknesses. This anisotropic behavior was also validated through magnetic microscopy studies carried out along the in-plane and out-of-plane directions employing magneto-optic Kerr microscopy and magnetic force microscopy, respectively. The decrease in magnetostriction was explained on the basis of dual phase formation and complex interplay between the in-plane and out-of-plane magnetic anisotropies present in the film. [ABSTRACT FROM AUTHOR]

This paper presents a study on the ramping losses of a high temperature superconductor (HTS) magnet consisting of multiple no-insulation (NI) (RE)Ba2Cu3Ox coils. The (RE)Ba2Cu3O2 (REBCO) conductor is the second generation HTS thin tape, where RE stands for rare-earth. During a ramping operation of the NI HTS magnet, losses are generated both across turn-to-turn resistances and inside superconducting layers. The former comes with radial current, which is called “turn-to-turn loss;” the latter one is induced by flux creep and jump, called “magnetization loss.” The modeling and experimental studies on the ramping losses have been reported on single NI pancake coils in the previous part. In a HTS magnet consisting of multiple NI coils, the electromagnetic coupling between coils has a considerable influence on the distribution of ramping losses. Here, the experimentally validated model is used to investigate a HTS magnet consisting of 14 single pancake REBCO coils. The results show that both the turn-to-turn loss and the magnetization loss present a significant non-uniform distribution among the coils. The highest turn-to-turn loss occurs on the middle coils of the magnet, while the highest magnetization loss happens on the end coils. The nonuniform distribution of ramping losses can result in a considerable temperature difference among coils in the NI HTS magnet. It leads to an additional quench risk on the magnet and requires more attention in design. The distribution of the turn-to-to-turn loss can be optimized by adjusting the turn-to-turn resistivity. A much more uniform turn-to-turn loss distribution among coils is achieved by applying a graded turn-to-turn resistivity on the multiple coils. [ABSTRACT FROM AUTHOR]

This paper reports a simple and robust route to stabilize metastable ɛ-Fe2O3 as an epitaxial thin film using a GaFeO3 (GFO) buffer in pulsed laser deposition. The use of an isostructural GFO buffer widens the range of film growth, allowing for stabilization on a range of substrates even with different surface symmetries, such as Al2O3 (0001), SrTiO3 (111), and Y:ZrO2 (100). All films grown on these substrates were c-axis oriented with the characteristic in-plane domain structures. Magnetization of these buffered thin films showed considerable magnetic anisotropy at 350K. In particular, in-plane magnetization was found to be relatively larger in the films grown on the Y:ZrO2 (100) compared to the other cases. Heterostructuring ɛ-Fe2O3 on GFO is promising in the applications of ɛ-Fe2O3 for magnetic and multiferroic devices in that it provides a larger growth window and an effective means to optimize the film properties beyond the simple substitution of Fe with Ga. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of dc magnetic field strength on the resolution of a magnetic nanoparticle (MNP) thermometer, which employs the fundamental ƒ0 and 2ƒ0 harmonics of the MNP magnetization induced by ac and superimposed dc magnetic fields. In ac and parallel dc magnetic fields, the strength of dc magnetic field modulates the harmonics of the MNP magnetization, which affects their temperature sensitivities and measurement signal-to-noise ratios (SNRs). A temperature-adjustable fluxgate-based magnetic particle spectrometer was used to measure the spectra of the MNP magnetization at different temperatures. To determine the temperature, the amplitudes of the measured ƒ0 and 2ƒ0 harmonics were modeled based on the static Langevin function. AC susceptibility measurements on a MNP sample demonstrate the applicability of the static Langevin function for the description of the MNP magnetization spectra at a low frequency ac magnetic field without taking into account the MNP dynamics. Our simulations and experiments show that with increasing dc magnetic field from 0.2mT to 2.0 mT, both the amplitude of the 2ƒ0 harmonic and the temperature sensitivity of the amplitude ratio of the 2ƒ0 to ƒ0 harmonics increase by a factor of about 10 in an ac magnetic field with a frequency of 70 Hz and an amplitude of 1mT. Concomitantly, the SNR of the 2ƒ0 harmonic significantly increases by about 20 dB. Consequently, the temperature resolution of the MNP thermometer is improved from 1.97K to 0.26 K. [ABSTRACT FROM AUTHOR]

The magnetic, Raman, ferroelectric, and in-field 57Fe Mössbauer studies of polycrystalline multiferroic Sr3Co2Fe24O41 are reported in this paper. From the magnetization studies, it is observed that the sample is soft magnetic in nature with low temperature magnetic spin transitions like longitudinal to transverse conical structure around 130K and change in magnetic crystalline anisotropy from conical to planar structure at 250K. Ferroelectric studies of the sample exhibit the spontaneous polarization at low temperature. Strong spin phonon and spin lattice coupling is observed through low temperature Raman spectroscopy. From the in-field 57Fe Mössbauer spectroscopy, spin up and spin down site occupations of Fe ions are calculated in the unit cell. [ABSTRACT FROM AUTHOR]

FePt-based exchange-coupled composites consisting of a magnetically hard L10-FePt phase exchange-coupled with a soft ferromagnetic material are promising candidates for future ultra-high density (>1 Tbit/in²) perpendicular magnetic recording media, also being of interest for other applications including spin torque oscillators and micro-electro-mechanical systems, among others. In this paper, the effect of the thickness of a soft Co layer (3 < thCo < 20 nm) on the magnetic behavior of ledge-type fcc(100)-Co/L10(001)-FePt composites deposited on an MgO (100) substrate is systematically studied by combining morpho-structural analyses and angular magnetization measurements. Starting from a film consisting of isolated L10(001)-FePt islands, the ledge-type structure was obtained by depositing a Co layer that either covered the FePt islands or filled-up the inter-island region, gradually forming a continuous layer with increasing Co thickness. A perpendicular anisotropy was maintained up to thCo ∼ 9.5 nm and a significant reduction in the coercivity (about 50% for thCo ∼ 3nm) with the increase in thCo was observed, indicating that, by coupling hard FePt and soft Co phases in a ledge-type configuration, the writability can be greatly improved. Recoil loops' measurements confirmed the exchange-coupled behavior, reinforcing a potential interest in these systems for future magnetic recording media. [ABSTRACT FROM AUTHOR]

Various proposals for future magnetic memories, data processing devices, and sensors rely on a precise control of the magnetization ground state and magnetization reversal process in periodically patterned media. In finite dot arrays, such control is hampered by the magnetostatic interactions between the nanomagnets, leading to the non-uniform magnetization state distributions throughout the sample while reversing. In this paper, we evidence how during reversal typical geometric arrangements of dots in an identical magnetization state appear that originate in the dominance of either Global Configurational Anisotropy or Nearest-Neighbor Magnetostatic interactions, which depends on the fields at which the magnetization reversal sets in. Based on our findings, we propose design rules to obtain the uniform magnetization state distributions throughout the array, and also suggest future research directions to achieve non-uniform state distributions of interest, e.g., when aiming at guiding spin wave edge-modes through dot arrays. Our insights are based on the Magneto-Optical Kerr Effect and Magnetic Force Microscopy measurements as well as the extensive micromagnetic simulations. [ABSTRACT FROM AUTHOR]

An ingenious approach to acquire the absolute magnetization fields produced by polarized atoms has been presented in this paper. The method was based on detection of spin precession signal of the hyperpolarized helium-3 with ultra-sensitive atomic magnetometer of potassium by referring to time-domain analysis. At first, dynamic responses of the mixed spin ensembles in the presence of variant external magnetic fields have been analyzed by referring to the Bloch equation. Subsequently, the relevant equipment was established to achieve the functions of hyperpolarizing helium-3 and detecting the precession of spin-polarized noble gas. By analyzing the transient response of the magnetometer in time domain, we obtained the relevant damping ratio and natural frequency. When the value of damping ratio reached the maximum value of 0.0917, the combined atomic magnetometer was in equilibrium. We draw a conclusion from the steady response: the magnetization fields of the polarized electrons and the hyperpolarized nuclei were corresponding 16.12 nT and 90.74 nT. Under this situation, the nuclear magnetization field could offset disturbing magnetic fields perpendicular to the orientation of the electronic polarization, and it preserved the electronic spin staying in a stable axis. Therefore, the combined magnetometer was particularly attractive for inertial measurements. [ABSTRACT FROM AUTHOR]

Magnetic Particle Imaging (MPI) is an emerging imaging technique that uses magnetic nanoparticles as tracers. In order to analyze the quality of nanoparticles developed for MPI, a Magnetic Particle Spectrometer (MPS) is often employed. In this paper, we describe results for predictions of the nanoparticle harmonic spectra obtained in a MPS using three models: the first uses the Langevin function, which does not take into account finite magnetic relaxation; the second model uses the magnetization equation by Shliomis (Sh), which takes into account finite magnetic relaxation using a constant characteristic time scale; and the third model uses the magnetization equation derived by Martsenyuk, Raikher, and Shliomis (MRSh), which takes into account the effect of magnetic field magnitude on the magnetic relaxation time. We make comparisons between these models and with experiments in order to illustrate the effects of field-dependent relaxation in the MPS. The models results suggest that finite relaxation results in a significant drop in signal intensity (magnitude of individual harmonics) and in faster spectral decay. Interestingly, when field dependence of the magnetic relaxation time was taken into account, through the MRSh model, the simulations predict a significant improvement in the performance of the nanoparticles, as compared to the performance predicted by the Sh equation. The comparison between the predictions from models and experimental measurements showed excellent qualitative as well as quantitative agreement up to the 19th harmonic using the Sh and MRSh equations, highlighting the potential of ferrohydrodynamic modeling in MPI. [ABSTRACT FROM AUTHOR]

Artificial magnetic spin-ice nanostructures provide an ideal platform for the observation of magnetic monopoles. The formation of a magnetic monopole is governed by the motion of a magnetic charge carrier via the propagation of domain walls (DWs) in a lattice. To date, most experiments have been on the static visualization of DW propagation in the lattice. In this paper, we report on the low field dynamics of DW in a unit spin-ice structure measured by magnetoresistance changes. Our results show that reversible DW propagation can be initiated within the spin-ice basis. The initial magnetization configuration of the unit structure strongly influences the direction of DW motion in the branches. Single or multiple domain wall nucleation can be induced in the respective branches of the unit spin ice by the direction of the applied field. [ABSTRACT FROM AUTHOR]

In this paper, we establish the negative spontaneous magnetization in Mn and Zn substituted cobalt ferrite Co0.6Zn0.4Fe1.7Mn0.3O4 (CZFMO). It is suggested that the origin of negative spontaneous magnetization is due to the substitution of small sized Mn+4 ions (compared to Fe+3 ions) at the octahedral B site in compound Co0.6Zn0.4 Fe2O4. The low value of Poisson's ratio ~0.202 for this compound possibly contributes towards the easy distortion in the bond length and bond angle, causing increase in Fe-O bond distance/decrease in Fe-O-Fe bond angle with Mn substitution, leading to considerably weak Fe-O-Fe superexchange interaction at the octahedral B site. The neutron diffraction data clearly illustrated the significant reduction in ordered magnetic moment at the B site, with the resultant negative spontaneous magnetization (M=MB-MA) in this mixed spinel system. The spin disorder also gives rise to an interesting semi-spin glass behavior in CZFMO. [ABSTRACT FROM AUTHOR]

Three-dimensional magnetic memory design based on circular-cross section nanowires with modulated diameter is the emerging field of spintronics. The consequences of the mutual interaction between electron spins and local magnetic moments in such non-trivial geometries are still open to debate. This paper describes the theoretical study of domain wall dynamics within such wires subjected to spin polarized current. We used our home-made finite element software to characterize the variety of domain wall dynamical regimes observed for different constriction to wire diameter ratios d/D. Also, we studied how sizeable geometry irregularities modify the internal micromagnetic configuration and the electron spin spatial distribution in the system, the geometrical reasons underlying the additional contribution to the system's nonadiabaticity, and the specific domain wall width oscillations inherent to fully three-dimensional systems. [ABSTRACT FROM AUTHOR]

The Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation is widely used to describe phase transformation kinetics. This description, however, is not valid in finite size domains, in particular, thin films. A new computational model incorporating the level-set method is employed to study phase evolution in thin film systems. For both homogeneous (bulk) and heterogeneous (surface) nucleation, nucleation density and film thickness were systematically adjusted to study finitethickness effects on the Avrami exponent during the transformation process. Only site-saturated nucleation with isotropic interface-kinetics controlled growth is considered in this paper. We show that the observed Avrami exponent is not constant throughout the phase transformation process in thin films with a value that is not consistent with the dimensionality of the transformation. Finitethickness effects are shown to result in reduced time-dependent Avrami exponents when bulk nucleation is present, but not necessarily when surface nucleation is present. [ABSTRACT FROM AUTHOR]

In this paper, phase locking dynamics of vortices induced by spin transfer torque in bridge-coupled nanodisks are studied by micromagnetic simulations. In the presence of the bridge coupling, the required time for the phase locking is dramatically reduced, and the phase difference between the two vortices keeps at a nonzero value after the phase locking. Moreover, the phase difference is affected significantly by bridge coupling, Oersted field distribution, nanodisk size, as well as inplane bias magnetic field. In addition, the coupled gyrotropic frequency of vortices depends linearly on the perpendicular magnetic field. This systematic study of phase locking parameters, especially the phase difference, is important for the applications of vortex-based spin-torque nano-oscillators. [ABSTRACT FROM AUTHOR]

Smart control is an attracting and important function for modern electromagnetic wave absorber. This paper presents the design, fabrication, and measurement of a frequency and bandwidth tunable metamaterial absorber (MA) in X-band. The unit cell of the MA consists of a microstrip resonator loaded with the varactors. Simulation and measurement results show that by tuning the bias voltage on the varactors, the peak absorption frequency can be tuned by 0.44 GHz with the peak absorption greater than 95%. Field and circuit model analysis is conducted to reveal the working mode and predict the absorbing frequency. After that, by specially designing the bias circuit so as to adjust the bias voltage on neighboring unit cells separately, dual resonance and absorption peaks occur, and the overall absorption bandwidth can thus be tuned conveniently by controlling the difference of the two resonance frequencies. The center absorbing frequency can also be tuned. Simulation and experiment results show that the 75% absorption (-6 dB reflection) bandwidth can be tuned from 0.40 GHz to 0.74 GHz, which is a two-fold tuning range. This work is believed to improve the state-of-the-art smart metamaterial absorber. [ABSTRACT FROM AUTHOR]

Magnetostriction (MS)-caused strain in single-phase three-legged cores with different core cutting forms, which suffer from induced magnetic loss and noise, was studied. It is found that adopting each different core form types induces magnetostriction ε variation in a transformer core operating with a high-frequency AC signal. The results are compared with finite element analysis simulations. It is also indicated that magnetostriction ε variations are significant in the rolling direction and along limbs and yokes. In this paper, it is proposed that core corner sides and T-joint parts without cutting structure, the core exhibits lower core loss and lower heat dissipation due to the fact that the magnetic flux that passes through corner sides shows lower magnetostriction variation. The magnetic properties resulting from magnetostriction variation in core loss and heat dissipation phenomena are significantly different from other core forms because of stronger contributions from magnetostatic forces. The main contribution for reducing core loss and noise, making them much less in corner numbers and cutting-fabricated forms, can be expected to come from lower magnetic flux and magnetostriction variation. [ABSTRACT FROM AUTHOR]

This paper micromagnetically studies the magnetization dynamics driven by the spin-Hall effect in a Platinum/Permalloy bi-layer. For a certain field and current range, the excitation of a uniform mode, characterized by a power with a spatial distribution in the whole ferromagnetic cross section, is observed. We suggest to use the ferromagnet of the bi-layer as basis for the realization of an array of spin-torque oscillators (STOs): the Permalloy ferromagnet will act as shared free layer, whereas the spacers and the polarizers are built on top of it. Following this strategy, the frequency of the uniform mode will be the same for the whole device, creating an intrinsic synchronization. The synchronization of an array of parallely connected STOs will allow to increase the output power, as necessary for technological applications. [ABSTRACT FROM AUTHOR]

In this paper, it is theoretically demonstrated that the electromagnetically induced transparency (EIT) and absorption (EIA) can be achieved in a magnetic metamaterial. Unit cell of metamaterial consists of a split ring resonator and an "I" shaped cut-wire pair, which serves as a bright resonator and a dark resonator, respectively. It is found that the EIT effect in metamaterial results from the magnetic interaction between bright and dark resonators. A classical model is also introduced to describe the EIT behavior in magnetic metamaterial, and its analytical results are in good agreement with numerical results. Significantly, by controlling the space separation between resonators, we can obtain the destructive and constructive interferences, and thus observe the transition between EIT and EIA. These results may achieve potential applications on enhancing the nonlinear interaction. [ABSTRACT FROM AUTHOR]

Heat-Assisted Magnetic Recording (HAMR) is a likely successor of Perpendicular Magnetic Recording (PMR) in the Hard disk drive industry. In PMR, recording performance is strongly affected by the following distributions in magnetic granular media: magnetic anisotropy field (HK), volume/ grain size, and interaction field from neighboring grains. Since HAMR writing occurs in a narrow temperature region below Curie point (Tc), additional grain-to-grain Tc variation would strongly affect HAMR recording performance. Thus, Tc distribution should be examined for successful HAMR media development. In this paper, we demonstrate a new approach of extracting HK and Tc distributions (σHK and σTc) from thermo-remanence measurements. During the measurement process, a thin film is magnetically saturated, laser heated to specific peak temperature (for a time typically of 5 μs), then cooled to room temperature and magnetic thermo-remanence is measured. Analytical fit to the experimental curves enables independent evaluation of both σTc (±0.5% absolute) and σHK (±2% absolute). Parameters of the analytical statistical model include: temperature dependencies Ms(T), HK(T); mean field effective demagnetization factor N; grain size, HK; and Tc distributions. Thermal fluctuations are taken into account using Arrhenius-Neel formalism. Here, we report experimental σTc values as a function of grain volume. Increase of σTc with grain size reduction might be a limiting factor for HAMR extendibility. [ABSTRACT FROM AUTHOR]

In order to understand the magnetocaloric response of materials, it is important to analyze the interactions between the different phases present in them. Recent models have analyzed the influence of these interactions on the magnetocaloric response of composites, providing an estimate value of the interaction field that is consistent with experimental results. This paper analyzes to which extent magnetization first-order reversal curve (FORC) method can be used to calculate these interactions. It is shown that the different field ranges that are explored using these techniques (inside the hysteretic region for FORC; close to magnetic saturation for magnetocaloric effect) produce interaction field values that differ in order of magnitude, with FORC being sensitive to the lower values of the interaction field and magnetocaloric analysis accounting for the larger interactions. [ABSTRACT FROM AUTHOR]

Memory machines with both high-power-density and wide-speed-range are becoming very attractive most recently. The purpose of this paper is to propose a novel type of memory machine, namely, claw pole memory machine. It engages an axially magnetized AlNiCo PM ring on the claw pole rotor to build the main magnetic flux in air-gap which is responsible for the electromechanical energy conversion. A magnetizing coil is equipped to online regulate the magnetization level of the permanent magnet ring, so as to achieve wide-speed-range operation. The operating principle is analyzed. The Preisach hysteresis model is combined with 3D finite element method to conduct performance assessment of the proposed claw pole memory machine. Calculation results demonstrate that the air-gap flux density can be readily adjusted by injecting DC pulse into the magnetizing coil, and the speed-range of the proposed machine can be extended as wide as six times of its base speed. [ABSTRACT FROM AUTHOR]

This paper deals with the effect of a biocompatible surface coating layer on the magnetic properties of ultrasmall iron oxide nanoparticles. Particles were synthesized by laser pyrolysis and fully oxidized to maghemite by acid treatment. The surface of the magnetic nanoparticles was systematically coated with either phosphonate (phosphonoacetic acid or pamidronic acid) or carboxylate-based (carboxymethyl dextran) molecules and the binding to the nanoparticle surface was analyzed. Magnetic properties at low temperature show a decrease in coercivity and an increase in magnetization after the coating process. Hysteresis loop displacement after field cooling is significantly reduced by the coating, in particular, for particles coated with pamidronic acid, which show a 10% reduction of the displacement of the loop. We conclude that the chemical coordination of carboxylates and phosphonates reduces the surface disorder and enhances the magnetic properties of ultrasmall maghemite nanoparticles. [ABSTRACT FROM AUTHOR]

Recently, interest in alnico magnetic alloys has been rekindled due to their potential to substitute for rare-earth based permanent magnets provided modest improvements in their coercivity can be achieved without loss of saturation magnetization. Recent experimental studies have indicated that atomic and magnetic structure of the two phases (one AlNi-based, the other FeCo-based) that comprise these spinodally decomposed alloy is not as simple as previously thought. A key issue that arises is the distribution of Fe, Co, and Ti within the AlNi-based matrix phase. In this paper, we report the results of first-principles calculations of the site preference of ternary alloying additions in DO3 Fe3Al, Co3Al, and Ni3Al alloys, as models for the aluminide phase. For compound compositions that are Al rich, which correspond to experimental situation, Ti and Fe are found to occupy the a sites, while Co and Ni prefer the c sites of the DO3 lattice. An important finding is that the magnetic moments of transition metals in Fe3Al and Co3Al are ordered ferromagnetically, whereas the Ni3Al were found to be nonmagnetic unless the Fe or Co is added as a ternary element. [ABSTRACT FROM AUTHOR]

In this paper, the physical properties of half-doped manganite La0.5Ca0.5MnO3 with crystallite sizes ranging from 15 to 40 nm are investigated. As expected, ferromagnetic order strengthens at expense of antiferromagnetic one as crystallite size is reduced to 15 nm. However, contrary to previously reported works, an enhancement of saturation magnetization is observed as crystallite size increases from 15 to 22 nm. This unexpected behavior is accompanied by an unusual cell volume variation that seems to induce ferromagnetic-like behavior at expense of antiferromagnetic one. Besides, field cooled hysteresis loops show exchange bias field and coercivity enhancement for increasing cooling fields, which suggest a kind of core-shell structure with AFM-FM coupling for crystallite sizes as small as 15 nm. It is expected that inner core orders antiferromagnetically, whereas uncompensated surface spins behave as spin glass with ferromagnetic-like ordering. [ABSTRACT FROM AUTHOR]