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Two-dimensional van der Waals materials have attracted considerable attention because of their promising applications in spintronic devices. This paper reports on first-principles calculations of the electronic structure and ferromagnetism of Cr 2 Ge 2 Te 6 with surface Ge vacancies. These vacancies are found to remove the bandgap and induce metallicity in Cr 2 Ge 2 Te 6 that persists with decreasing Ge-vacancy concentration. Meanwhile, the Ge vacancies cause an unexpected sharp increase in the magnetic anisotropy energy compared to that of perfect Cr 2 Ge 2 Te 6. More importantly, how thickness affects the magnetic anisotropy energy is studied to show that the latter oscillates upon increasing the number of layers, thereby switching between the out-of-plane and in-plane magnetization directions. The present findings regarding a surface-vacancy-tuned bandgap and magnetism controlled by the layer thickness in a two-dimensional van der Waals magnet could lead to potential applications in next-generation magnetic memory storage, sensors, and spintronics.Two-dimensional van der Waals materials have attracted considerable attention because of their promising applications in spintronic devices. This paper reports on first-principles calculations of the electronic structure and ferromagnetism of Cr 2 Ge 2 Te 6 with surface Ge vacancies. These vacancies are found to remove the bandgap and induce metallicity in Cr 2 Ge 2 Te 6 that persists with decreasing Ge-vacancy concentration. Meanwhile, the Ge vacancies cause an unexpected sharp increase in the magnetic anisotropy energy compared to that of perfect Cr 2 Ge 2 Te 6. More importantly, how thickness affects the magnetic anisotropy energy is studied to show that the latter oscillates upon increasing the number of layers, thereby switching between the out-of-plane and in-plane magnetization directions. The present findings regarding a surface-vacancy-tuned bandgap and magnetism controlled by the layer thickness in a two-dimensional van der Waals magnet could lead to potential application...

A ferrimagnet (FiM) has small domains consisting of oppositely polarized and unequal magnetic moments. Oppositely polarized magnetic moments have strong exchange coupling between them resulting in robust torque in FiM called bulk torque. Due to the strong bulk torque, magnetic moments in FiM do not precess around the switching axis for a long time unlike ferromagnets (FMs) leading to faster switching speed compared to FM. In this paper, we propose and model the switching of FiM (CoTb) with spin current injection from heavy metal with strong spin Hall effect (SHE) such as Pt. We carry out ab initio calculation of the Dzyaloshinskii-Moriya interaction (DMI) at the FiM(CoTb)/heavy metal(Pt) interface using spin polarized relativistic Korringa-Kohn-Rostoker (KKR) Green’s function method. We compute the exchange coupling among the magnetic moments inside the CoTb layer and model how the spin-orbit torque (SOT) along with the bulk torque can efficiently switch the FiM. Then, we consider a ferrimagnet based magnetic tunnel junction (FMTJ) with SHE metal. We perform a comprehensive simulation and performance analysis of FiM(CoTb)/MgO/CoFeB and FiM(CoTb)/MgO/FiM(CoTb) FMTJs with Pt underlayer. Using non-equilibrium Green's function (NEGF) formalism, we calculate the tunneling magnetoresistance (TMR) of the proposed FMTJs at room temperature. Furthermore, the importance of thickness in CoTb magnetization dynamics is shown using our FiM magnetization simulation framework. We analyze and show that the CoTb based FMTJ has large switching energy barrier to ensure thermal stability. Finally, we show the performance comparison (TMR, write performance and power consumption) between our proposed FMTJs and FM based MTJ and our simulation exhibits that for picosecond range switching speed, FMTJ is ∼25 times more energy efficient than FM based MTJ.A ferrimagnet (FiM) has small domains consisting of oppositely polarized and unequal magnetic moments. Oppositely polarized magnetic moments have strong exchange coupling between them resulting in robust torque in FiM called bulk torque. Due to the strong bulk torque, magnetic moments in FiM do not precess around the switching axis for a long time unlike ferromagnets (FMs) leading to faster switching speed compared to FM. In this paper, we propose and model the switching of FiM (CoTb) with spin current injection from heavy metal with strong spin Hall effect (SHE) such as Pt. We carry out ab initio calculation of the Dzyaloshinskii-Moriya interaction (DMI) at the FiM(CoTb)/heavy metal(Pt) interface using spin polarized relativistic Korringa-Kohn-Rostoker (KKR) Green’s function method. We compute the exchange coupling among the magnetic moments inside the CoTb layer and model how the spin-orbit torque (SOT) along with the bulk torque can efficiently switch the FiM. Then, we consider a ferrimagnet based magn...

This paper analyzes the magnetic properties of the emerging ferromagnetic chromium tri-iodide (CrI3) monolayer, under compressive and tensile biaxial strains. By combining first-principles density functional theory and Metropolis Monte Carlo methods, the multi-scale simulations are used to quantitatively analyze the strain-dependent magnetocrystalline anisotropy energy, Heisenberg isotropic symmetric exchange effects, anisotropic symmetric exchange effects, magnetic moment, and Curie temperature (Tc). The Villari effect (or the inverse magnetostrictive effect) and the Nagaoka-Honda effect (or the inverse Barret effect) are unraveled. It is shown that a small strain (e.g., smaller than 1%) could change Tc by only less than 1 K. By contrast, a small strain can noticeably influence the hysteresis curve shape and significantly alter the coercive magnetic field (Bc), which offers one of the possible explanations of the large variation of Bc as measured on the strain-prone exfoliated CrI3 monolayers. This also indicates the importance to vanish strain to ensure small device-to-device variation of magnetic properties in the monolayer-based spintronics memory and logic devices. It is revealed that strain can induce changes on a series of key magnetic properties (e.g., the strain-induced magnetization direction flip, the strain-induced ferromagnetic/antiferromagnetic transition, the strain-induced change of magnetic coercivity, etc.), which might be useful to enable monolayer-based sensor applications.This paper analyzes the magnetic properties of the emerging ferromagnetic chromium tri-iodide (CrI3) monolayer, under compressive and tensile biaxial strains. By combining first-principles density functional theory and Metropolis Monte Carlo methods, the multi-scale simulations are used to quantitatively analyze the strain-dependent magnetocrystalline anisotropy energy, Heisenberg isotropic symmetric exchange effects, anisotropic symmetric exchange effects, magnetic moment, and Curie temperature (Tc). The Villari effect (or the inverse magnetostrictive effect) and the Nagaoka-Honda effect (or the inverse Barret effect) are unraveled. It is shown that a small strain (e.g., smaller than 1%) could change Tc by only less than 1 K. By contrast, a small strain can noticeably influence the hysteresis curve shape and significantly alter the coercive magnetic field (Bc), which offers one of the possible explanations of the large variation of Bc as measured on the strain-prone exfoliated CrI3 monolayers. This also ...

This paper is to study ramping turn-to-turn loss and magnetization loss of a no-insulation (NI) hightemperature superconductor (HTS) pancake coil wound with (RE)Ba2Cu3Ox (REBCO) conduc-tors. For insulated (INS) HTS coils, a magnetization loss occurs on superconducting layers during aramping operation. For the NI HTS coil, additional loss is generated by the “bypassing” current onthe turn-to-turn metallic contacts, which is called “turn-to-turn loss” in this study. Therefore, theNI coil’s ramping loss is much different from that of the INS coil, but few studies have beenreported on this aspect. To analyze the ramping losses of NI coils, a numerical method is developedby 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 lattermodel is to calculate the magnetization loss. A test NI pancake coil is wound with REBCO tapesand the reliability of this model is validated by experiments. Then the characteristics of the NIcoil’s ramping losses are studied using this coupling model. Results show that the turn-to-turn lossis much higher than the magnetization loss. The NI coil’s total ramping loss is much higher thanthat of its insulated counterpart, which has to be considered carefully in the design and operation ofNI applications. This paper also discusses the possibility to reduce NI coil’s ramping loss bydecreasing the ramping rate of power supply or increasing the coil’s turn-to-turn resistivity.

This paper presents two-dimensional magnetostriction of electrical steel sheet under vector magnetic characteristic. In conventional measurement method using Single Sheet Tester, the magnetic flux density, the magnetic field strength, and the magnetostriction have been measured in one direction. However, an angle between the magnetic flux density vector and the magnetic field strength vector exists because the magnetic property is vector quantity. An angle between the magnetic flux density vector and the direction of maximum magnetostriction also exists. We developed a new measurement method, which enables measurement of these angles. The vector magnetic characteristic and the two-dimensional magnetostriction have been measured using the new measurement method. The BH and Bλ curves considering the angles are shown in this paper. The analyzed results considering the angles are also made clear.

This paper proposes the method to accurately predict the 180° basic domain width (Dw) in demagnetized states of the grooved Fe3%Si(110) single crystal with the tilt angle of [001] out of the sheet surface (β). The evaluation of Dw enables the estimation of the anomalous eddy current losses. In this paper, Dw is optimized to minimize the magnetic Gibbs free energy represented by vector potentials using the finite element method and the conjugate gradient method. The μ*-method is adopted to approximate the magnetization relaxation. The stray field energy generated by the magnetic charges occurring on both grooves cross section and sheet surfaces is considered in our proposed method. The validity of the proposed method was confirmed by comparison with the observed Dw. As a result, we could reveal the β dependence of Dw against the groove depth. Moreover, the theoretical threshold of the domain refinement due to the grooves has been suggested.

For materials with a particulate structure, Neel was the first to point out the importance of statistical fluctuations of the magnetic interaction fields. He has shown how they influence the material behavior and how reptation can be generated. These aspects are not taken into account by some classical hysteresis models, e.g., the Preisach model. A recent paper by Della Torre and Kadar [IEEE Trans. Magn. MAG‐23, 2820 (1987)] improved the Preisach model in order to consider the influence of fluctuations. Our paper presents an hysteresis model, based on the superposition of the elementary characteristics of single bistable particles. The interaction field is assumed to have a Gaussian statistic distribution, with a mean value and a standard deviation depending on the macroscopic magnetization. The spatial fluctuations of the field acting on the bistable particles are seen to enhance the mean‐field components. Being that the fluctuations are stronger when the material is macroscopically demagnetized, it is e...

In this paper we present a study of switching characteristics of a series of synthetic antiferromagnet (SAF) structures using reversible susceptibility experiments. Three series of SAF samples were considered in our study with (t1, t2), the thickness of the FeCoB layers of (80nm, 80nm), (50nm, 50nm), and (80nm, 20nm) and with the interlayer of Ru ranging from 0to2nm. A vector vibrating sample magnetometer was used to measure the hysteresis loops along the different directions in the plane of the samples. The reversible susceptibility experiments were performed using a resonant method based on a tunnel diode oscillator. We showed that the switching peaks in the susceptibility versus field plots obtained for different orientations of the applied dc field can be used to construct the switching diagram of the SAF structure. The critical curve constitutes the fingerprint of the switching behavior and provides information about micromagnetic and structural properties of SAF which is an essential component of modern magnetic random access memories.In this paper we present a study of switching characteristics of a series of synthetic antiferromagnet (SAF) structures using reversible susceptibility experiments. Three series of SAF samples were considered in our study with (t1, t2), the thickness of the FeCoB layers of (80nm, 80nm), (50nm, 50nm), and (80nm, 20nm) and with the interlayer of Ru ranging from 0to2nm. A vector vibrating sample magnetometer was used to measure the hysteresis loops along the different directions in the plane of the samples. The reversible susceptibility experiments were performed using a resonant method based on a tunnel diode oscillator. We showed that the switching peaks in the susceptibility versus field plots obtained for different orientations of the applied dc field can be used to construct the switching diagram of the SAF structure. The critical curve constitutes the fingerprint of the switching behavior and provides information about micromagnetic and structural properties of SAF which is an essential component of mo...

This paper discusses some aspects of practical modeling of spin-torque effects in current-perpendicular-to-plane (CPP) giant magnetoresistive magnetic multilayers, with emphasis on considerations of primary importance for sensor and/or read head applications of CPP spin valves (SVs). These include an extension of the well-known Valet-Fert transport model [Phys. Rev. B 48, 7099 (1993)] to the general case of noncollinear magnetization vectors in CPP multilayer of arbitrary layer structure as well as the application of the model to realistic CPP-SV structures. Simple analytical models for the angular dependence of spin-torque critical currents in CPP-SV devices are reviewed and compared with numerical simulations which include thermal fluctuations. Finally, the paper also contains some recent high frequency spectral measurements of spin-torque-induced noise in dual spin-valve sensors.This paper discusses some aspects of practical modeling of spin-torque effects in current-perpendicular-to-plane (CPP) giant magnetoresistive magnetic multilayers, with emphasis on considerations of primary importance for sensor and/or read head applications of CPP spin valves (SVs). These include an extension of the well-known Valet-Fert transport model [Phys. Rev. B 48, 7099 (1993)] to the general case of noncollinear magnetization vectors in CPP multilayer of arbitrary layer structure as well as the application of the model to realistic CPP-SV structures. Simple analytical models for the angular dependence of spin-torque critical currents in CPP-SV devices are reviewed and compared with numerical simulations which include thermal fluctuations. Finally, the paper also contains some recent high frequency spectral measurements of spin-torque-induced noise in dual spin-valve sensors.

This paper describes a theoretical approach to calculate the permeability tensor of polycrystalline ferrites in various magnetization states, from the completely demagnetized state to saturation. Demagnetizing dynamic fields related to the magnetic domain and grain shapes are introduced in the initial equations to describe the interactions between domains in a grain and between grains. In this paper, we focus on the calculation of the initial permeability of polycrystalline ferrites. Results obtained from the model are compared in a completely demagnetized state to those given by the well-known Schloemann model and to experiments. In all the models given in the literature, an unusually high value of the damping factor must be introduced into the calculations to correctly describe the low-field loss region. Values of the damping factor comparable to those calculated from the measured resonance linewidth of ferrites permit the prediction of the spreading of the magnetic losses as a function of frequency exp...

In a previous paper, we made a complete analysis of the interaction between the probe tip of a magnetic force scanning tunneling microscope (MFSTM) and the magnetic fields emanating from a typical recorded pattern. In this paper we show how the magnetization distribution in the recorded media can be determined from the measurements by obtaining expressions for the magnetic fields from a Fourier series expansion for the recorded magnetizations. We have used these techniques to find the magnetic fields from many different distributions, including all those we could find in the literature. The probe tip displacement, which is the quantity measured using the MFSTM, can be calculated using these magnetic fields. The results can then be compared to the experimental data. For one set of experiments on high density recording we have found that the best fit is with a magnetization that has a modified arctan transition. The modification eliminates the discontinuity in the slope of the transitions as they are joined together, giving a more realistic representation of the magnetic distribution. The transition width can then be used as an adjustable parameter to find the best fit to the data. The MFSTM can, therefore, be used as a quantitative tool to find the magnetic recording transition widths. These theoretical techniques are not necessarily restricted to the use of a MFSTM, but can be applied to other problems in magnetic recording. For instance, we show how the probe tip displacement corresponds to the flux picked up by a conventional read head. The response of the head as a function of different magnetization patterns can then be studied and compared to experimental results. The measurable quantities are expressed in Fourier series but we show how these series can be easily evaluated with a PC and the appropriate software.

The results of in situ studies of the magnetostructural transition occurring in single crystals of an Ni2.19Mn0.81Ga Heusler alloy are presented in this paper. The formation of martensitic twins on exposure to high magnetic fields up to 10 T in the magnetostructural transition regime was observed using an indigenously developed optical microscope. Experiments on magnetization were performed on single crystals of Ni2.19Mn0.81Ga in high magnetic fields, a phase diagram between magnetic field and temperature was constructed, and the isothermal entropy change was estimated. Based on the experimental data and the results from optical microscopic examination, the influence of magnetic field and thermal cycling on martensitic twins and, in turn, on the magnetocaloric effect, is discussed.

This paper reports a current-induced magnetization switching with a nanosecond-scale pulse current in a nanomagnet using a perpendicularly magnetized synthetic antiferromagnetic coupling (p-AFC) structure. The results indicate that the magnetization switching current in the p-AFC structure is less than that in the single-nanomagnet structure with perpendicular anisotropy when the differences in thickness and saturation magnetization between the upper and lower layers of the p-AFC structure are small and the Gilbert damping constant is also small. The results also show that the p-AFC structure can reduce the switching current when the pulse duration is short and its structure is effective for a high-speed switching. The results of this study shall be useful in the design of spin-transfer torque random access memory.

γMn-based alloys mainly possess an antiferromagnetic martensitic structure at room temperature, in which the type of crystal structure changes with the magnetic structure. Therefore, it is commonly believed that the cubic–tetragonal structural transformation is caused by the magnetic transition. Due to small transformation distortion and high magnetic field being required to present strong magnetism, the microstructure evolution of γMn-based alloys under an external magnetic field has rarely been experimentally revealed. In this paper, a phase-field model based on the related antiferromagnetic energies is proposed to investigate the microstructure evolution during magnetization. The simulated results show that the microstructure after martensitic transformation exhibits a self-accommodated twinning pattern. Upon magnetic loading, two transition stages, i.e., the reorientation stage of antiferromagnetic domains and the antiferromagnetic–ferromagnetic transition stage, would successively appear. External stress directly affects the relative stability among martensitic variants, thereby indirectly influencing the magnetization behavior, and the effect is related to the stress direction. Although the microstructure changes with the external condition, the interfacial migration of martensitic variant domain (or the martensitic twin boundary) and magnetic domain could keep in accordance with each other due to the strong magnetoelastic coupling.

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 a strong 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.

The magnetic properties of the low-dimensional trirutile tantalates Co 1 − xMg xTa 2O 6 [ x = 0.0, 0.1, 0.3, 0.5, 0.7, 1.0] are reported in this paper. CoTa 2O 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 − xMg xTa 2O 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 − xMg xTa 2O 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.

In the preceding paper [J. Appl. Phys. 57, 2110 (1985)], it was shown that the behavior of eddy current losses in ferromagnetic materials can be fully characterized, in general, in terms of two parameters, the effective number n0 of active magnetic objects present at low magnetizing frequency fm, and the field V0, determining the ability of the external field to increase the number of active objects with increasing fm. In this paper, the connection of n0, V0 with several microstructural and magnetic properties of iron‐based ferromagnetic alloys is investigated. It is shown that, in agreement with theoretical predictions, n0 is, in general, weakly dependent on other physical properties, attaining values ∼1 in most cases. Only the application of a tensile stress leads to a drastic increase of n0 above this limit. V0, on the contrary, turns out to be very sensitive to the type of material. It continuously decreases with decreasing degree of magnetic orientation, changing by about two orders of magnitude ...

Spin-ordered materials may exhibit a magnetic polarization which is proportional to an applied electric field and an electric polarization which is proportional to an applied magnetic field. In this paper a comprehensive discussion is given of the present knowledge of these magnetoelectric (ME) effects. The specific topics covered include the thermodynamic and magnetic symmetry considerations which are relevant to the ME effects and also to the piezomagnetic and piezomagnetoelectric effects. The major part of the paper is a review of the experimental and theoretical work on ME effects in Cr2O3 carried out by the present authors. This includes measurements of the anisotropy of the ME effects, theory of the temperature dependence and atomic mechanism of the ME effects, observation of the magnetically as well as of the electrically induced ME effect, experiments on magnetic annealing and other structure sensitive aspects of ME effects, and the role of antiferromagnetic domains in the interpretation of the results.

The general behavior of the domain structure in Nd2 Fe14 B as a function of temperature has been reported in an earlier paper [Y. Luo, N. Zhang, and X. Su, Acta Metall. Sin. 23, A136 (1987)]. In order to clarify some details of the temperature variation of domain structure in the basal plane of this material, we have now made observations on a single‐phase sample of Nd2 Fe14 B. We find that the optical contrast between antiparallel domains, using the Kerr effect for observation, is approximately proportional to the saturation magnetization; both decrease with increasing temperature. The main basal‐plane domains are unchanged by heating, but the supplementary closure domains (cone‐shaped reversal domains) change in size and shape, and may even disappear, when the sample is heated above 240 °C. Since the domain nucleation process is to some extent random, the domain pattern is different after each thermal demagnetization cycle above the Curie temperature.The general behavior of the domain structure in Nd2 Fe14 B as a function of temperature has been reported in an earlier paper [Y. Luo, N. Zhang, and X. Su, Acta Metall. Sin. 23, A136 (1987)]. In order to clarify some details of the temperature variation of domain structure in the basal plane of this material, we have now made observations on a single‐phase sample of Nd2 Fe14 B. We find that the optical contrast between antiparallel domains, using the Kerr effect for observation, is approximately proportional to the saturation magnetization; both decrease with increasing temperature. The main basal‐plane domains are unchanged by heating, but the supplementary closure domains (cone‐shaped reversal domains) change in size and shape, and may even disappear, when the sample is heated above 240 °C. Since the domain nucleation process is to some extent random, the domain pattern is different after each thermal demagnetization cycle above the Curie temperature.

This paper follows previous works which investigated the strength of dipolar coupling in two-magnet 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 an...

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.

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.

This paper considers the response of the magnetization of a small ferrimagnetic ellipsoid to spatially uniform rf magnetic fields of one or more frequencies. Only a few pertient cases can be considered in this short paper. The analysis is applicable to other excitations and a more complete report will be published elsewhere. Some of the work presented here duplicates that of Pippin [Proc. Inst. Radio Engrs. 44, 1054 (1956)], Jepsen, and others, but the approach to the problem is quite different and allows one to extend previous results, as well as to derive new ones.The effects of symmetry planes and anisotropy will be considered as well as the permeability tensor for the general ellipsoid when modulation terms of the magnetization are neglected.

While magnetization reversal in a large film sheet under uniform‐field excitation is determined by the macroscopic anisotropy property and microscopic magnetization ripple structure, the magnetization reversal in miniaturized multilayer devices, in addition, depends strongly on the geometry of the device structures. Earlier papers reported the study of coupled‐film strips using a nanosecond microscopic Kerr apparatus and simultaneous inductive and magneto‐optic‐measurement techniques. The present paper describes the reversal behavior in two coupled‐film structures, in which the easy axis of each Permalloy layer may be aligned parallel to the strip length (closed hard axis or CHA) or perpendicular to the strip length (closed easy axis or CEA). The Permalloy layers may also be mismatched in thickness. When a Permalloy backing is added to the external drive line as a keeper, the reversal phenomena in both the storage layers and the keeper have been observed.

Ising spin model is considered as an efficient computing method to solve combinatorial optimization problems based on its natural tendency of convergence towards low energy state. The underlying basic functions facilitating the Ising model can be categorized into two parts, “Annealing and Majority vote.” In this paper, we propose an Ising cell based on Spin Hall Effect (SHE) induced magnetization switching in a Magnetic Tunnel Junction (MTJ). The stochasticity of our proposed Ising cell based on SHE induced MTJ switching can implement the natural annealing process by preventing the system from being stuck in solutions with local minima. Further, by controlling the current through the Heavy-Metal (HM) underlying the MTJ, we can mimic the majority vote function which determines the next state of the individual spins. By solving coupled Landau-Lifshitz-Gilbert equations, we demonstrate that our Ising cell can be replicated to map certain combinatorial problems. We present results for two representative probl...

In magnetic random access memory, free layers are used to store data bits as “0” or “1.” For free layers with slotted nanoring structures, the magnetization configuration of counterclockwise vortex state can be defined as “0” state, while clockwise vortex state as “1” state. It is important to have a controllable and stable state transition process to make sure that “0” state can switch to “1” state and vice versa. Up to now, it has seldom been reported that the transition process is heavily affected by the static anisotropy field Hk-stat of free layers. A sufficient Hk-stat will substantially reduce the probability of successful state transition. To increase the accuracy of writing data, the free layers must be prepared with a low anisotropy field. In this paper, we present a rotational sputtering method, which can finish the desired isotropic film preparation, and thus realize a stable state transition.

This paper presents an investigation of the mechanical response of a finite-thickness superconducting strip containing an elliptical cavity in oblique magnetic fields. After the Bean critical state model and the minimum magnetic energy variation procedure are employed, the dependency of the magnetic and mechanical properties on the aspect ratio of the strip and the tilt angles of the applied field and elliptical cavity is discussed. The results show that for a strip in an oblique magnetic field, the current front penetrates non-monotonically from the surface inwards in the initial stage. The magnetization of the strip and the applied field are not collinear, and the angle between them becomes smaller with increasing field. Simultaneously, the strip suffers from a torque produced by the electromagnetic force and then has a tendency to rotate. Compared with the defect-free case, the appearance of the elliptical cavity affects the magnetic property of the strip and further causes significant stress concentra...

This paper investigates the influence of dc magnetic field strength on the resolution of a magnetic nanoparticle (MNP) thermometer, which employs the fundamental f0 and 2f0 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 f0 and 2f0 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 simulat...

The magnetic, Raman, ferroelectric, and in-field 57Fe Mossbauer 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 130 K and change in magnetic crystalline anisotropy from conical to planar structure at 250 K. 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 Mossbauer spectroscopy, spin up and spin down site occupations of Fe ions are calculated in the unit cell.

It has been recently shown that broad-band, thermally induced magnetization fluctuations in submicron magnetoresistive (MR) read sensors (used in all present and foreseeable magnetic hard-disk-drives), will serve as a fundamental limit to their achievable signal-to-noise ratio, independent of how large the intrinsic sensitivity parametrics, e.g., ΔR/R, may be. This type of magnetization noise may also be of consequence and/or interest for a broader class of future nanoscale magnetic devices. The foundation for understanding and quantitatively modeling this phenomenon on a practical device-physics level is the fluctuation-dissipation theorem (FDT). This paper develops a theoretical methodology for application of the FDT to the problem of micromagnetic modeling of thermal magnetization fluctuations in small, magnetically soft thin-film magnetic devices, using a generalized Gilbert formulation of magnetization dynamics with phenomenological damping.

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.

The magnetocaloric effect (MCE) is an intrinsic property of magnetic materials that enable magnetic refrigeration devices without using the traditional vapour-compression. Temperature sensitive and anisotropic magnetic solids might give rise to a large rotating MCE for building compact and efficient magnetic cooling systems by simply rotating the sample. Here, we report an unprecedented maximal refrigeration capacity of 497.36 J/kg (at 70 kOe) in perovskite TbFeO3 single crystal, resulting from its giant anisotropic magnetic entropy change along a axis. Our paper reveals that interaction between Fe-3d and Tb-4f electrons drives extremely interesting spin reorientation transition, which is highly sensitive to magnetic field and temperature. These findings highlight potential applications of an emerging material for high efficient low temperature magnetic refrigeration, which is compact and quiet, and does not use ozone-depleting coolant gases.

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.

In recent years, efforts have been made to explore the superconductivity of clathrates containing crystalline frameworks of group-IV elements. The superconducting silicon clathrate is unusual in that the structure is dominated by strong sp3 covalent bonds between silicon atoms, rather than the metallic bonding that is more typical of traditional superconductors. This paper reports on critical magnetic fields of superconducting Al-substituted silicon clathrates, which were investigated by transport, ac susceptibility, and dc magnetization measurements in magnetic fields up to 90 kOe. For the sample Ba8Si42Al4, the critical magnetic fields were measured to be HC1 = 40.2 Oe and HC2 = 66.4 kOe. The London penetration depth of 4360 A and the coherence length 70 A were obtained, whereas the estimated Ginzburg–Landau parameter of κ = 62 revealed that Ba8Si42Al4 is a strong type-II superconductor.

In this paper, we deal with antiferromagnetic (AFM) films using the many-body Green’s function theory under random phase approximation (RPA). This is the first time to be able to calculate all three components of the magnetization of AFM systems simultaneously under RPA. The magnetization of AFM films is not so easily affected by external fields as FM films and the magnetic behavior is rather different from FM films. When an external field Bz is applied along the z direction, the parallel susceptibility χ∥ is zero at 0 K, and then increases with temperature until Neel point TN. If a field along the x-direction Bx is applied, the perpendicular susceptibility can be approximately expressed as χ⊥=1/2γ|J|, which is a temperature-independent constant, where γ is the nearest neighbor and J is the exchange interaction. In this case the z-component magnetization is nonzero above TN. This is because the x-component magnetizations of both sublattices are not zero and the AFM exchange should play a role, which resul...

In this paper one presents a method for statistical characterization of the magnetization processes in two-dimensional networks of single-domain ferromagnetic particles. These systems are similar with the structured materials that are seen as future high-density recording media. The method allows very interesting developments in the characterization of the stability of the magnetic states which is an important factor for a recording medium.

The purpose of the paper is to explain the linear birefringence and dichroism observed in magnetic fluids when subjected to a magnetic field. Linear aggregates of nanosize near spherical particles, which are considered to be responsible for these phenomena, have been analyzed in terms of the classical oscillating dipole model where each particle is treated as a simple dipole under the action of the electric field of a light beam. The analysis is produced in terms of the number of particles in the aggregate for both spherical and slightly nonspherical particles. Optical anisotropy factors and expressions for the magnetic-field dependence of the birefringence and dichroism due to preexisting aggregates are given. Numerical calculations have been carried out for Fe3O4 particles. Finally, the formulae are applied to experimental data where a close fit is obtained.

The ideal magnetic Ericsson refrigeration cycle should have a constant induced magnetic entropy change as a function of temperature over the whole refrigeration range. To realize this condition using composite materials, a numerical method has been developed to determine the optimum proportions of the component materials in such refrigerants. This paper investigates the effects of increasing the number (n) of the components on the constancy of the magnetic entropy change of the composite (ΔScom), and suggests some new composite refrigerants. For this purpose, the Gd1−x–Dyx (with x=0, 0.12, 0.28, 0.49, and 0.70) alloys, have been used. The values of ΔS have been calculated both from mean-field theory as well as from experimental magnetization curves of these alloys, in the 0.1–7 T magnetic field range and the 200–300 K temperature range. Two sets of composite materials have then been proposed as refrigerants, operating, respectively, over the temperature range 240–290 K and 210–290 K. The ΔS data of the in...

The paper proposes the use of a Lorentzian function to describe the irreversible component of the magnetization of soft materials with hysteresis using the Everett’s integral. We will derive an analytical formula to compute the irreversible magnetization, and compute the reversible component by the measurements of the major loop. We shall compare the numerical results with experiments on not-oriented-grain magnetic irons.

The First Order Reversal Curve (FORC) diagrams of interacting single-domain ferromagnetic particle systems have been found experimentally to contain negative regions. In this paper, we use micromagnetic and phenomenological (Preisach-type) models to help explain the occurrence of these negative regions. In Preisach-type modeling, the position of the negative region is correlated with the sign of the mean-field interactions. In micromagnetic modeling, the position of the negative region is correlated with the spatial arrangement of the particles in the model.

In this paper experimental stress-strain and magnetization versus field curves measured for Ni2MnGa alloys are compared with modeling results obtained using a statistical model. The model is based on the assumption of equivalence between the magnetic field and mechanical stress through magnetoelastic interactions. The approach is extended to describe the complex irreversible behavior of strain and magnetization using a statistical distribution of the energy barriers that separate the metastable states of the martensite twins. A theoretical value of the energy conversion coefficient cme=1.57×10−4 MPa (kA/m)−2 is found, which is in good agreement with the experimentally determined value of 1.25×10−4 MPa (kA/m)−2.

In this paper we study the magnetic behavior of amorphous ferromagnetic ribbons having helical anisotropy with the helix axis perpendicular to the sample surface. It is shown that the magnetization processes under the action of a longitudinal magnetic field can be controlled by the nonuniform field produced by an alternating current, of the same frequency than the exciting field, flowing through the sample. Hysteresis loops with very different susceptibilities and coercive fields have been obtained by varying the amplitude of the current intensity and the phase differences between the exciting magnetic field and the current through the sample.

Being able to control the anisotropy of a magnetic core plays an important role in the development of a fluxgate sensor. Our aim is to induce anisotropy orthogonal to the direction of excitation because it generates a stable, low-noise fluxgate, as cited in the literature. In this paper, we present an original method for electroplating a ring core for a fluxgate with built-in radial anisotropy by performing the electroplating in a radial field produced by a novel yoke. The results show that the resulting anisotropy is homogeneously radial and makes the magnetization rotate, avoiding domain wall movement for low excitation fields.

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 e 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 e 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.

In this paper, the Ho20Er20Co20Al20RE20 (RE = Gd, Dy, and Tm) high-entropy bulk metallic glasses (HE-BMGs) with good magnetocaloric properties are fabricated successfully. The HE-BMGs exhibit a second-order magnetic phase transition. The peak of magnetic entropy change ( ΔSMpk) and refrigerant capacity (RC) reaches 15.0 J kg−1 K−1 and 627 J kg−1 at 5 T, respectively, which is larger than most rare earth based BMGs. The heterogeneous nature of glasses also contributes to the large ΔSMpk and RC. In addition, the magnetic ordering temperature, ΔSMpk and RC can be widely tuned by alloying different rare earth elements. These results suggest that the HE-BMGs are promising magnetic refrigerant at low temperatures.

In this paper, the electromagnetic response and shielding behaviour of superconductor-ferromagnetic bilayer structure are studied. The magnetomechanical coupling in ferromagnetic materials is also considered. Based on the linear piezomagnetic coupling model and anti-plane shear deformation, the current density and magnetic field in superconducting strip are obtained firstly. The effect of shear stress on the magnetization of strip is discussed. Then, we consider the magnetic cloak for superconductor-ferromagnetic bilayer structure. The magnetic permeability of ferromagnetic material is obtained for perfect cloaking in uniform magnetic field with magnetomechanical coupling in ferromagnet. The simulation results show that the electromagnetic response in superconductors will change by applying the stress only to the ferromagnetic material. In addition, the performance of invisibility of structure for non-uniform field will be affected by mechanical stress. It may provide a method to achieve tunability of superconducting properties with mechanical loadings.

A short review is made of the key magnetic properties of dilute cubic RxY1−xAl2 (R=Tb, Dy) intermetallics, in order to show their main magnetic features. Dilution by Y introduces a weak random magnetic anisotropy (RMA). The rich magnetic phase diagram is described, including paramagnetic (P), spin glass (SG), correlated spin glass (CSG), random ferromagnetic (RFM), and ferromagnetic (F) phases, with a triple point and two multicritical ones. The paper deals with the induced macroscopic magnetic anisotropy cooling in a magnetic field below TSG or TCSG transition temperatures, which can be either unidirectional or uniaxial or both. High‐field (3 T) magnetostriction in Tb series shows, for x=0.48, 0.59, 0.87, a decrease of the Callen α exponent (λt∼mα, m=reduced magnetization) below 3. The Sompolinsky irreversibility parameter Δ has been determined for the Tb series in the SG regime, and a replica model is presented to explain the Δ(T) dependence. The character of the P→SG or P→CSG transitions is addressed, ...

The structure and magnetic properties of orthorhombic DyMnO3 nanoparticles with different particle sizes are investigated in this paper. With decreasing particle size, all the lattice parameters a, b, and c gradually decrease, whereas the orthorhombic distortion increases. Magnetic measurements reveal that the antiferromagnetic interaction of Mn ions is weakened due to the decrease in Mn-O-Mn bond angle. Above a critical field H*, DyMnO3 undergoes a field-induced metamagnetic transition at 4 K, which is related to the spin reversal of Dy moments. The critical field H* increases monotonically with size reduction, indicating an enhancement of the antiferromagnetic interaction of Dy ions due to the decreased distance between rare earth ions. The magnetization at 4 K and 5 T, i.e., M(4 K, 5 T) shows a non-monotonic variation with particle size d, i.e., M(4 K, 5 T) initially increases with size reduction but decreases again for d