Search

This paper reports on results taken to validate the extension to the theory of the magnetomechanical effect reported recently. This theory is based on a “law of approach” but the underlying equations have been generalized to include linear and nonlinear terms which are analogous to those in the well-known Rayleigh law of magnetization. It is shown that the generalized theory can be applied to materials with negative magnetostriction, such as nickel, and that the stress dependent model parameters can be determined from experimental measurements. It has been found that the results show improved agreement with experimental observation compared with the more restricted previous exposition of the model. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper experimental stress-strain and magnetization versus field curves measured for Ni[SUB2]MnGa 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 c[SUBme] = 1.57 × 10[SUP-4] MPa (kA/m)[SUP-2] is found, which is in good agreement with the experimentally determined value of 1.25 × 10[SUP-4] MPa (kA/m)[SUP-2]. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The approach of using the generalized Meiklejohn-Bean model to derive the antiferromagnetic thickness dependence of exchange bias and coercivity in ferromagnetic/antiferromagnetic bilayers is justified. The mathematical procedure to obtain the analytical expressions through finding the critical fields is illuminated. [ABSTRACT FROM AUTHOR]

A magnon parametron is a calculating element that carries binary information by a discretized oscillating phase: 0 and π. Owing to the strong non-linearity of the magnetization dynamics, the oscillation phase flips to another stochastically, which can give a rise to unconventional computing functionalities, including probabilistic computing. Here, we investigated the stochastic dynamics of ferromagnetic-metal magnon parametron, of which the precession phase is discretized into two (0 and π) by parametric pumping of magnons. We found that an AC magnetic field perpendicular to an external field can control the precession phase in the magnon parametron, of which amplitude dependence follows the sigmoid function, a requirement for probabilistic bit operation. We also found that the time scale for flipping between different precession phases grows exponentially as pumping microwave power. Our finding ensures that the magnon parametron can be used as a calculating element for unconventional computing schemes based on bistable systems. [ABSTRACT FROM AUTHOR]

A theoretical analysis is made for the quantized spin waves in single-layered ferromagnetic nanorings with the added effect of interfacial Dzyaloshinski–Moriya interactions (DMI). A microscopic, or Hamiltonian-based, formalism is employed that includes competing terms for the symmetric (bilinear) exchange interactions, the antisymmetric DMI, the magnetic dipole–dipole interactions, and applied magnetic field. It is found that, in our model, the transition field value between vortex and onion states is shifted by the inclusion of DMI effects. Significantly, the spin-wave frequencies are also modified with the effects being largest in the onion state close to the transition field. We present combined analytical and numerical results obtained for the static and dynamical magnetization, including the frequencies and amplitudes (with relative phase) of the spin waves when interfacial DMI is present. [ABSTRACT FROM AUTHOR]

Coherent wave states such as Bose–Einstein condensates (BECs), which spontaneously form in an overpopulated magnon gas even at room temperature, have considerable potential for wave-based computing and information processing at microwave frequencies. The ability to control the transport properties of magnon BECs plays an essential role in their practical use. Here, we demonstrate the spatiotemporal control of the BEC density distribution through the excitation of magnon supercurrents in an inhomogeneously magnetized yttrium iron garnet film. The BEC is created by microwave parametric pumping and probed by Brillouin light scattering spectroscopy. The desired magnetization profile is prepared by heating the film with optical patterns projected onto its surface using a phase-based wavefront modulation technique. Specifically, we observe a pronounced spatially localized magnon accumulation caused by magnon supercurrents flowing toward each other originating in two heated regions. This accumulation effect increases the BEC lifetime due to the constant influx of condensed magnons into the confinement region. The shown approach to manipulate coherent waves provides an opportunity to extend the lifetime of freely evolving magnon BECs, create dynamic magnon textures, and study the interaction of magnon condensates formed in different regions of the sample. [ABSTRACT FROM AUTHOR]

We propose a hybrid magnonic-oscillator system based on the combination of a spin transfer auto-oscillator and a magnonic waveguide to open new perspectives for spin-wave based circuits. The system is composed of a spin transfer oscillator based on a vortex state which is dipolarly coupled to a nanoscale spin-wave waveguide with longitudinal magnetization. In its auto-oscillating regime, the oscillator emits coherent spin waves with tunable and controllable frequencies, directions, and amplitudes into the waveguide. We demonstrate the principle of this method using micromagnetic simulations and show that reconfiguration of the system is possible by changing the chirality and polarity of the magnetic vortex. Spin waves are emitted into the waveguide with high non-reciprocity and the preferred direction depends on the core polarity of the vortex. In contrast, different vortex chiralities lead to different amplitudes of the emitted waves. Our findings open up a novel way to design an agile spintronic device for the coherent and tunable generation of propagating spin waves. [ABSTRACT FROM AUTHOR]

An attractive avenue for a memory device is offered by thermally induced magnetization switching (TIMS), which is expected to increase the storage density. As an essential step in TIMS, the transient ferromagnetic-like state (TFLS) has attracted much attention. Here, our research analyzes the TFLS from the perspective of magnetization rate based on extensive atomic spin simulations in a parameter space necessary for the formation of TIMS. The results demonstrate that the evolution of the magnetization rate for both Fe and Gd sublattices has two peaks at different timescales, and the two peaks are driven by the Langevin field and the exchange field, respectively. By modulating the pulse fluence, the initial temperature, the pulse duration, the electron–phonon coupling factor, and the damping constant, we find that the first peak of the magnetization rate depends not only on the electron thermal bath and temperature of the phonon but also on the energy transfer rate between the electronic system and the spin system. Moreover, we elucidate the role played by the Langevin field and the exchange field in TFLS formation when modulating the pulse fluence, initial temperature, and pulse duration. The simulation results show that the enhancement of the Langevin field prolongs the duration of TFLS. The enhancement of the exchange field at a long pulse duration before the electron system cools down leads to the shortening of the TFLS duration. However, the time required for Gd demagnetization after the electron system cools down to the Curie temperature changes only slightly when modulating these simulated parameters. [ABSTRACT FROM AUTHOR]

The dynamics of the magnetization of the free layer in a spin-torque nano oscillator (STNO) influenced by a noncollinear alignment between the magnetizations of the free and pinned layers due to an interlayer exchange coupling has been investigated theoretically. The orientations of the magnetization of the free layer with that of the pinned layer have been computed through the macrospin model and they are found to match well with experimental results. The bilinear and biquadratic coupling strengths make the current to switch the magnetization between two states or oscillate steadily. The expressions for the critical currents between which oscillations are possible and the critical bilinear coupling strength below which oscillations are not possible are derived. The frequency of the oscillations is shown to be tuned and increased to or above 300 GHz by the current, which is the largest to date among nanopillar-shaped STNOs. [ABSTRACT FROM AUTHOR]

Part I. Presents a study which derived a theory which described the effects of field cycles on the magnetization in particulate systems. Characteristics of magnetic materials; Equation of the time variation of magnetization; Schematic representation of the reptation technique; Theoretical framework of field cycles on magnetization; Results and discussion.

Presents a study which reported the results of experimental reptation measurements on a chromium dioxide videotape sample. Method used in the magnetic characterization of a chromium dioxide videotape; Description of the irreversible susceptibility calculated from the demagnetization remanence curve and the variation of the time-dependent viscosity parameter; Overview of the time variation of magnetization and in-field reptation for fast and slow sweep rates.

An introduction is presented which discusses articles within the issue on topics including ultrafast spin dynamics in the terahertz frequency region, magnetization dynamics of geometrically frustrated spin systems, and phase-to-property correlation in titanium oxide.

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]

Switching of perpendicular magnetization via spin–orbit torque (SOT) is of particular interest in the development of non-volatile magnetic random access memory (MRAM) devices. We studied current-induced magnetization switching of Ir/GdFeCo/Cu/Pt heterostructures in a Hall cross geometry as a function of the in-plane applied magnetic field. Remarkably, magnetization switching is observed at zero applied field. This is shown to result from the competition between SOT, the Oersted field generated by the charge current, and the material's coercivity. Our results show a means of achieving zero-field switching that can impact the design of future spintronics devices, such as SOT-MRAM. [ABSTRACT FROM AUTHOR]

This article focuses on computational studies evaluating the influence of crystallinity, residual stresses, and out-of-plane (OOP) deterministic switching on Terfenol-D nano/microstructures. The computational models use both coupled and uncoupled Landau–Liftshitz–Gilbert equations with elastodynamics to study strain-induced magnetization reorientation. A Voronoi tessellation approach models the crystal distribution in the microstructures subjected to residual stresses with good agreement to experimental data including large changes in coercivity values, i.e., from 100 to 3000 Oe. Parametric studies show how the coercivity is manipulated with residual stresses, including a magnetoelastically induced perpendicular-magnetic-anisotropy (PMA), important for memory applications. Additional parametric studies focus on epitaxially deposited micro-disks, revealing that residual stresses can create magnetoelastically dominant easy axes along the ⟨ 110 ⟩ directions, which are energetically favorable relative to the intrinsic ⟨ 111 ⟩ magnetocrystalline easy axes. Modification of the global easy axis is used to design a strain-mediated multiferroic composite consisting of a 20 nm epitaxially deposited Terfenol-D memory bit with PMA grown on a PZT substrate. The multiferroic disk achieves OOP deterministic clocking with an applied voltage. [ABSTRACT FROM AUTHOR]

In viscous dynamics, velocity is proportional to the force. An ideal memristor is a device whose resistance changes at a rate proportional to the driving input. We present a proof-of-principle demonstration of the connection between viscous dynamics and memristive functionality by utilizing a thin-film ferromagnet/antiferromagnet bilayer, where viscous magnetization dynamics results from the frustration at the magnetic interface, and driving is provided by an external magnetic field. Thanks to the atomic scale of frustration effects, the presented approach is amenable to downscaling. It can also be adapted for electronic driving by spin torque, making it attractive for applications in neuromorphic circuits. [ABSTRACT FROM AUTHOR]

MgB2 precursor wires were prepared using powder in tube technique by Institute of High Pressure PAS in Warsaw. All samples were annealed under isostatic pressure generated by liquid Argon in the range from 0.3 GPa to 1 GPa. In this paper, we show the effects of different processing routes, namely, cold drawing (CD), cold rolling (CR), hot isostatic pressure (HIP) and doping on critical current density (Jc), pinning force (Fp), irreversible magnetic-field (Birr), critical temperature (Tc), n value, and dominant pinning mechanism in MgB2/Fe wires with ex situ MgB2 barrier. The results show that medium pressures (∼0.35 GPa) lead to high Jc in low and medium magnetic fields (0 T - 9 T). On the other hand, higher pressures (∼1 GPa) lead to enhanced Jc in high magnetic fields (above 9 T). Transport measurements show that CD, CR, and HIP have small effects on Birr and Tc, but CD, CR, HIP, and doping enhance Jc and Fp in in situ MgB2 wires with ex situ MgB2 barrier. Transport measurements on in situ undoped MgB2 wire with ex situ MgB2 barrier yield a Jc of about 100 A/mm2 at 4.2K in 6 T, at 10K in 4 T and at 20K in 2 T. The results also show that cold drawing causes increase of n value. [ABSTRACT FROM AUTHOR]

The microscopic magnetic properties of high-quality GdN thin films have been investigated by electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements. Detailed temperature dependence ESR measurements have shown the existence of two ferromagnetic components at lower temperatures, which was not clear from the previous magnetization measurements. The temperature, where the resonance shift occurs for the major ferromagnetic component, seems to be consistent with the Curie temperature obtained from the previous magnetization measurement. On the other hand, the divergence of line width is observed around 57K for the minor ferromagnetic component. The magnetic anisotropies of GdN thin films have been obtained by the analysis of FMR angular dependence observed at 4.2K. Combining the X-ray diffraction results, the correlation between the magnetic anisotropies and the lattice constants is discussed. [ABSTRACT FROM AUTHOR]

The application of ferromagnetic steel products is pervasive in society, with important applications arising in electrical steel, oil and gas pipelines, transportation infrastructure, naval structures, aircraft landing gear, and automotive components. Magnetic properties of electrical steel materials play a key role in electrical motors and transformers, with a direct impact on energy efficiency. Measurement of response to magnetization has implications for non-destructive inspection methods, such as magnetic flux leakage, magnetic Barkhausen noise, and metal magnetic memory method. Examples include flaw detection, characterization of material properties, and identification of stress state in steel. An understanding of the magnetic response of steel materials can be facilitated by the use of magnetic objects (MOs). MOs are defined as regions of relatively independent magnetic behavior, typically about the size of a grain, to which fundamental magnetic energy considerations may be applied. This Tutorial outlines mechanisms by which MOs may be applied for modeling magnetic response in steel and presents examples of their application. MOs incorporate material physical properties such as microstructure, grain size, crystallographic texture, the presence of dislocations and impurity elements, and the presence of residual stress and stress load on the component. They can also accommodate a description of the evolution of magnetic domain structure under magnetizing conditions. As the MO model incorporates fundamental physics principles, it allows estimates of physical parameters that can be used to provide insights into the connections between magnetic properties and material properties, including hardness, embrittlement, and the presence of applied and residual stress. Practical applications include non-destructive characterization of the stress state of steel and an improved understanding of magnetic processes in electrical steel. Examples where such models may be applied include magnetic Barkhausen noise and magnetic memory method for the characterization of steel materials. This Tutorial summarizes recent advances in the MO model and its applications, providing the foundation for its further development. Magnetic objects have the potential to provide fundamental explanations and could form the basis for magnetic measurements and magnetization processes, including magnetic flux leakage, magnetic Barkhausen noise, and magnetic hysteresis. [ABSTRACT FROM AUTHOR]

Electrical control of magnetization or magnetic control of polarization offers an extra degree of freedom in materials possessing both electric and magnetic dipole moments, viz., magnetoelectric (ME) multiferroics. A microstructure with polycrystalline configurations that enhances the overall polarization/magnetization and that outperforms single crystalline configurations is identified in a 1–3 CoFe 2 O 4 –BaTiO 3 (or CFO–BTO) composite. The characterization of local fields corresponding to the polycrystal configuration underlines a nontrivial role played by randomness in better cross coupling mediated by anisotropic and asymmetric strains. The microscopic field (local field) profile of the composite provides rich information regarding the distribution of key parameters central to the magnetoelectric effect. The differential contractual stress level observed in the local stress profile of CFO–BTO composite upon applying an external magnetic field conforms with the previous experimental magnetostriction observed in CFO. The role played by residual stresses stemming from misalignment of the polarization in the neighboring grains in enhancing the ME coupling is briefly discussed. [ABSTRACT FROM AUTHOR]

For MRI and NMR magnet design, a highly homogeneous and high magnetic field has to be achieved by active and/or passive shimming. The active and passive shimming commonly homogenize the magnetic field around the magnet center by cancellation of the higher terms of a spherical harmonic series than the 0th term. So far, the spherical harmonic expression in the cylindrical coordinate system is well known for a circular coil, a solenoid coil, and magnetization of ferromagnetic material with/without its volume. In a shimming design, only the z-component of an inhomogeneous magnetic field is compensated because it is dominant to the performance of MRI/NMR. However, various kinds of MRI and NMR systems have recently been developed, and these magnets sometimes have an axially asymmetric configuration or generate a tilted magnetic field. Therefore, the entire x-, y-, and z-components have to be homogenized for such magnets. I derive the spherical harmonic expression of the entire components of magnetic field generated by a circular coil, a dipole coil, and a straight line current. In addition, since the entire magnetization components of the ferromagnetic material having the volume contribute the entire components of the magnetic field around the magnet center, I also derive the equations of the spherical harmonic coefficients of the magnetic field generated by ferromagnetic material. Since these equations need a numerical integration, such as the Gauss quadrature integration, the computation accuracy of the spherical harmonic coefficients is investigated against the number of the evaluation points. We can calculate the highly accurate spherical harmonic coefficients with the small number of the evaluation points. [ABSTRACT FROM AUTHOR]

Close to room temperature, the off-stoichiometric Ni50Mn35In15 Heusler alloy is known to undergo a first-order magnetostructural transition. This paper presents a new model that closely mimics the magnetic behavior of the virgin curve and that of the M-H loops within the temperature range where the alloy undergoes the first-order transition. The virgin curve and the M-H loops relevant to the model were measured at 280 K. Since our data show that 280 K is above the start of the transition, it implies that at this temperature the alloy is in a mixed state. The mixed state refers the presence of two distinct magnetic states. The model and mechanism we propose to explain the complex magnetic behavior of the virgin curve and of the M-H loops pertain to the action of the applied field on the transition between the two magnetic states. Both the model and the proposed mechanism provide new insight about the complex magnetic behavior displayed by the Ni50Mn35In15 alloy within the first-order transition. [ABSTRACT FROM AUTHOR]

This paper presents a semi-physical parameter identification for a recently proposed enhanced iron-loss formula, the IEM-Formula. Measurements are performed on a standardized Epstein frame by the conventional field-metric method under sinusoidal magnetic flux densities up to high magnitudes and frequencies. Quasi-static losses are identified on the one hand by point-by-point dc-measurements using a flux-meter and on the other hand by extrapolating higher frequency measurements to dc magnetization using the statistical loss-separation theory (Jacobs et al., 'Magnetic material optimization for hybrid vehicle PMSM drives,' in Inductica Conference, CD-Rom, Chicago/USA, 2009). Utilizing this material information, possibilities to identify the parameter of the IEM-Formula are analyzed. Along with this, the importance of excess losses in present-day non-grain oriented Fe-Si laminations is investigated. In conclusion, the calculated losses are compared to the measured losses. [ABSTRACT FROM AUTHOR]

This paper presents magnetic properties of highly ordered ultrathin FeRh films deposited on Si/SiO wafers with MgO as a buffer layer. The antiferromagnetic to ferromagnetic (FM) transition is observed with a thickness as low as 3 nm. However, as the thickness decreases, the residual magnetization (Mrs) at low temperature increases and the amplitude of the transition decreases. In addition, the transition becomes much broader for the thinner films. This broadening is related to the grain size reduction in the thinner films. The temperature dependence of the magnetization of a highly ordered B2 FeRh film with a thickness of 10 nm was carefully measured as a function of field. The results show that the transition temperature decreases almost linearly with a rate of 0.93 K/kOe (heating) and 0.97 K/kOe (cooling) close to the value for the bulk samples, while Mrs obtained at 100 K increases rapidly at low field and then linearly at a field larger than 10 kOe, which clearly demonstrates that an applied field would induce FM stabilization in ultrathin FeRh films. [ABSTRACT FROM AUTHOR]

Magnetostriction in various metals has been known since 1842, recently the focus has shifted away from ferrous metals, towards materials with a straightforward or exaggerated stress magnetostriction relationship. However, there is an increasing interest in understanding ferrous metal relationships, especially steels, because of its widespread use in building structures, transportation infrastructure, and pipelines. The aim of this paper is to solve the inverse problem of determining stress from an observed magnetic field which implies a given magnetic structure and to demonstrate that theoretical calculations using a multi-physics modeling technique agree with this experimental observation. [ABSTRACT FROM AUTHOR]

Recently, using spark plasma sintering technique, a family of very fine grained, fully dense NiCuZn ferrites have been produced, which show constant permeability up to several 10 MHz. These ferrites can be used for filtering purposes in high frequency applications where a wide frequency band is required. In this paper, we study the magnetization processes taking place in these nano grained materials, in the frequency interval of 100 kHz to 5 MHz. Using a fluxmetric hysteresis graph, permeability, loss, and BH loops are measured at different temperatures, from -5 °C to 110 °C. Results are compared to the behavior of micrometric grain size ferrites, which are commonly used for power electronic and high frequency applications. [ABSTRACT FROM AUTHOR]

The concept of the memory motor is based on the fact that the magnetization level of the AlNiCo permanent magnet in the motor can be regulated by a temporary current pulse and memorized automatically. In this paper, a new type of memory motor is proposed, namely a flux mnemonic double salient motor drive, which is particularly attractive for electric vehicles. To accurately analyze the motor, an improved hysteresis model is employed in the time-stepping finite element method. Both simulation and experimental results are given to verify the validity of the new method. [ABSTRACT FROM AUTHOR]

A model is introduced in this paper to describe the transition boundaries in perpendicular magnetic recording at extremely high density. In contrary to the previous signal generation models, effects of magnetization transition curvature and the track edge fluctuations are adequately included in the new model that is designed to capture the actual transition boundaries, i.e., the variations of magnetization distribution in both down-track and across-track directions. The model is used to predict the readback waveforms based on the detailed information obtained from micromagnetic simulations taking into account the head and media parameters. The model is therefore suitable for magnetic recording at extremely high densities when the impact of the transition curvature and the track edge effect on the recording performance becomes more significant. [ABSTRACT FROM AUTHOR]

In the paper, a jump-noise process is introduced in magnetization dynamics equations in order to account for random thermal effects. It is demonstrated that in the case of small noise, Landau-Lifshitz and Gilbert damping terms emerge as average effects caused by the jump-noise process. This approach leads to simple formulas for the damping constant in terms of the scattering rate of the jump-noise process. These formulas also reveal the dependence of the damping constant on magnetization. The analysis of random switching of magnetization caused by the jump-noise process is presented. It is shown that the switching rate at very low temperatures may appreciably deviate from the predictions of thermal activation theory, which is consistent with experimental observations of low temperature switchings and is usually attributed to the phenomenon of 'macroscopic tunneling' of magnetization. [ABSTRACT FROM AUTHOR]

In this paper, stochastic magnetization dynamics driven by a jump-noise process is discussed and, under the condition of 'detailed balance,' generalized H-theorems for this dynamics are established. These H-theorems reveal the existence of a class of functionals that monotonically decrease in time during stochastic magnetization dynamics. [ABSTRACT FROM AUTHOR]

The paper deals with the mechanical vibrations damping effect induced by a magnetostrictive energy harvesting device properly working. The proposed experimental setup allows us to investigate, through time-domain measurements, the dependence of the mechanical damping from parameters like magnetic bias and external load, affecting the global harvested power. This result would underline the promising possibility to reduce mechanical vibrations and obtain power harvesting at the same time. [ABSTRACT FROM AUTHOR]

In this paper, we use neutron scattering and electrical transport to investigate the paramagnetic to antiferromagnetic phase transition in tetragonal CuMnAs films on GaP(001). X-ray diffraction and cross-sectional transmission electron microscopy measurements show that the films are chemically ordered with high structural quality. The temperature dependence of the structurally forbidden (100) neutron scattering peak is used to determine the Néel temperature, TN. We then demonstrate the presence of a clear peak in the temperature derivative of the resistivity around TN. The effect of disorder-induced broadening on the shape of the peak is discussed. [ABSTRACT FROM AUTHOR]

This article reports on the influence of the magnetization damping on dynamic hysteresis loops in single-domain particles with uniaxial anisotropy. The approach is based on the Néel-Brown theory and the hierarchy of differential recurrence relations, which follow from averaging over the realizations of the stochastic Landau-Lifshitz equation. A new method of solution is proposed, where the resulting system of differential equations is solved directly using optimized algorithms to explore its sparsity. All parameters involved in uniaxial systems are treated in detail, with particular attention given to the frequency dependence. It is shown that in the ferromagnetic resonance region, novel phenomena are observed for even moderately low values of the damping. The hysteresis loops assume remarkably unusual shapes, which are also followed by a pronounced reduction of their heights. Also demonstrated is that these features remain for randomly oriented ensembles and, moreover, are approximately independent of temperature and particle size. [ABSTRACT FROM AUTHOR]

By solving Brown’s Fokker-Planck equation numerically with the spherical harmonics, the magnetization reversal of a single-domain particle in a field at an oblique angle up to 45° to the easy axis is investigated. Different from the usual definition, the switching time is defined as the time when the averaged z component of the magnetization reaches 90% of its final value. The switching times of the particle under various conditions are calculated. When the oblique angle of the field is 30°-45° and its magnitude is larger than the Stoner-Wohlfarth limit (Hsw), the switching time is dependent slightly on the oblique angle and magnitude of the field, and the temperature. For the oblique angle of 5°, the switching time depends largely on the magnitude of the field and the temperature. When the magnitude of the field is less than Hsw, the switching time is dependent largely on the oblique angle and the temperature. Effects of the damping constant are also studied. [ABSTRACT FROM AUTHOR]

Magnetic force microscopy (MFM) imaging is a useful technique to locally study the magnetic state of nanostructures. In this paper, we have used the MFM to characterize an ordered array of Ni nanowires embedded in porous membrane. Due to the large aspect ratio of the wires (30 nm diameter and 1000 nm length) they present an axial easy axis. Considering the nanowires as nearly single-domain structures and calculating the amount of wires pointing to each direction, we can obtain the average magnetization. An alternative method to analyze the MFM data is here introduced considering the distribution functions of magnetic contrast. By using this method, the magnetization process of the nanowire array is studied and the results are compared with major and minor hysteresis loops measured by superconducting quantum interference device magnetometer. [ABSTRACT FROM AUTHOR]

The substitution of Mn for Fe in Ce2Fe17 suppresses its ferromagnetic ground state completely for Mn contents x>=0.5 Mn/f.u. Ce2Fe16Mn has only an antiferromagnetic phase (incommensurate helix along c axis) below TN=198 K. In this paper, we present and discuss the effects of deuterium insertion (that can be considered as an application of negative chemical pressure) on the magnetostructural properties of Ce2Fe16Mn. Application of (positive) high pressures up to 10 kbar on Ce2Fe16MnDy deuterides (y=1,2.3) allowed us to estimate the role of lattice volume and to divide it from the role of modified electronic band structure that both determines the magnetic states of the deuterated compounds. The results show that a ferromagnetic phase is stabilized by the insertion of D into the antiferromagnetic Ce2Fe16Mn. The Curie temperature TC of Ce2Fe16MnDy deuterides increases with increasing D content reaching TC=258 K for y=2.3 and remarkably decreases with pressure dTC/dP=-5.4 K/kbar and dTC/dP=-3.6 K/kbar for Ce2Fe16MnD1 and Ce2Fe16MnD2.3, respectively. Significant difference between the magnetization of Ce2Fe16MnD1 under pressure and the one of the parent compound at ambient pressure indicates that changes of the volume alone cannot determine the magnetic states upon the initial deuteration. However, the volume expansion becomes dominant when increasing the deuterium content up to 2.3 D/f.u. [ABSTRACT FROM AUTHOR]

This paper is devoted to numerical simulations of the magnetization dynamics driven by a spin-polarized current in extended ferromagnetic multilayers when a point-contact setup is used. We present (i) detailed analysis of methodological problems arising by such simulations and (ii) physical results obtained on a system similar to that studied in Rippard et al. [Phys. Rev. Lett. 92, 027201 (2004)]. We demonstrate that the usage of a standard Slonczewski formalism for the phenomenological treatment of a spin-induced torque leads to a qualitative disagreement between simulation results and experimental observations, and discuss possible reasons for this discrepancy. [ABSTRACT FROM AUTHOR]

The resilience of the exchange bias (Hex) in ferromagnet/antiferromagnet bilayers is generally studied in terms of repeated hysteresis loop cycling or by protracted annealing under reversed field (training and long-term relaxation, respectively). In this paper we report measurements of training and relaxation in NiFe films coupled with polycrystalline FeMn and epitaxial α-Fe2O3. We show that Hex suppressed both by training and relaxation was partially recovered as soon as a field cycling for consecutive hysteresis loop measurement was stopped or the magnetization of the ferromagnet was switched back to the biased direction. In both cases we can model the observed logarithmic time relaxation behavior, and its film thickness and temperature dependence, in terms of a thermally activated reversal of the antiferromagnetic domain configuration to reduce the total magnetic energy. [ABSTRACT FROM AUTHOR]

Magnetization response to in-plane applied fields has been studied, using Stoner-Wohlfarth model, on the bilayer systems of synthetic antiferromagnet having an anisotropy configuration in which the induced intrinsic uniaxial anisotropy is set either parallel or orthogonal to the shape anisotropy. It has been found that the toggle-mode magnetoresistance random access memory operation is possible even when the shape anisotropy exceeds the intrinsic anisotropy to some extent due to the anisotropic effect of the magnetostatic coupling and that the minimum operating field and maximum relative operating field margin are attained when the two anisotropy compensate each other. The optimal operating fields increase with the increase of the absolute value of the total anisotropy constant. However, the tolerance of total anisotropy constant in the positive direction is greater than in the negative direction if zero total anisotropy is chosen to be optimal. [ABSTRACT FROM AUTHOR]

While most recent studies of the spin-transfer torque effect in nanoscale magnetic structures mainly concern with spin-wave excitation and the magnetization hysteretic switching induced by spin-polarized current, we theoretically investigate the large angle magnetization precessions triggered and sustained by the spin current in great detail using the Landau-Lifshitz-Gilbert equation. The study is conducted particularly on the nanostructures comprising square magnets with an easy-plane anisotropy. A simple nanoscale magnetoelectronic device is proposed based on the structure without mechanical components to function as a motor to convert a dc current into an ac voltage of microwave frequency. The device is unique because the output amplitude and frequency can be continuously tuned by the electrical current in a wide range. Quantitative analysis of the device structure, function, and realization is provided. [ABSTRACT FROM AUTHOR]

We have studied in a systematic way the magnetization reversal processes of single castellated Ni[sub 80]Fe[sub 20] nanowires fabricated by advanced electron beam lithography. The structures consist of two 5 μm wide Ni[sub 80]Fe[sub 20] support wires bridged with a castellated nanowire of width in the range from 80 to 150 nm. We have used the anisotropic magnetoresistance (AMR) effect as a probe of the spin orientations and hence magnetization reversal processes in these complex magnetic structures. When the applied fields are along the horizontal legs of the castellated nanowire, we observe a unique MR response due to the creation of magnetic local easy and hard axes resulting in a number of discontinuous jumps in the magnetoresistance. The magnetoresistance behavior is strongly influenced by spin states at corners of the nanowire for field applied parallel or perpendicular to horizontal legs. These results are in agreement with simple micromagnetic simulations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles (MNPs) have been widely studied for use in biomedical application with the magnetic anisotropy constant K playing an important role in determining the performance. We estimated K near room temperature from the coercive field Hc of an AC magnetization (M–H) curve. First, we performed numerical simulation of the AC M–H curve of immobilized MNPs and clarified the dependencies of Hc on the MNP parameters and excitation conditions. Based on the simulation result, we obtained an analytical expression for Hc that was more general and included the previously obtained expression; and in addition, it could be applied to an MNP sample with a core-size distribution. Next, we measured the AC M–H curves of two commercial MNP samples and determined the dependencies of Hc on the amplitude and frequency of the excitation field. The dependencies agreed reasonably well with the analytical results. The K value was evaluated to obtain the best fit between the measured and analytical Hc, and the obtained K values were consistent with those estimated using other methods. The temperature dependence of K near room temperature was also determined. The present method will provide a useful tool to estimate the K value of MNPs. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles (MNPs) have been widely studied for bio-sensing applications, where suspended and immobilized MNPs can be magnetically distinguished using their different magnetic properties. We study magnetic properties of suspended and immobilized MNPs when the Néel relaxation time is much shorter than the Brownian. We show in both numerical simulation and experiment that they have different magnetic properties such as AC magnetization curves and harmonic spectra even though the dynamic behavior of both MNP types is primarily dominated by Néel relaxation. This difference is caused by the partial alignment of the easy axes in suspended MNPs when an AC magnetic field is applied. We introduce a distribution function for the angle of easy axis alignment. We also show a method to evaluate the distribution function from the measured AC magnetization curve and clarify the relationship between easy axis alignment and the AC field strength. Using the distribution function, we can quantitatively discuss the effect of easy axis alignment on the magnetic properties of suspended MNPs. The obtained results provide a basis for using MNPs in bio-sensing. [ABSTRACT FROM AUTHOR]

This paper presents a general theoretical model of constitutive relationships for the soft ferromagnetic and/or the giant magnetostrictive rods under a prestress and a bias magnetic field on the basis of Gibbs’ free energy function. After Gibbs’ free energy function is expanded by Taylor’s series expansion with the stress and the magnetization in the materials as the independent variables, a transcendental function is appropriately selected to replace the first terms with higher order up to six on the basis of the meaning of mechanics and physical theory of magnetic domains, from which a set of closed and analytical expressions of the constitutive relationships for the soft ferromagnetic material rods are obtained, and the parameters appearing in the relationships are determined by those measurable experiments in mechanics and physics. It is found that the predictions from this model are in good accordance with the experimental data given by Kuruzar and Cullity [Int. J. Magn. 1, 323 (1971)] and Jiles and Atherton [J. Phys. D 17, 1265 (1995)] and that this model can be degenerated into the relevant constitutive relationships for the giant magnetostrictive materials obtained by Zheng and Liu [J. Appl. Phys. 97, 053901 (2005)]. Comparing this model with other existing models in this field, the quantitative results exhibit that the relationships obtained here are more effective to describe the effect of the prestress on the magnetostrictive strain and the magnetization, and the effect of the stress and the magnetic field on Young’s modulus. [ABSTRACT FROM AUTHOR]

In this paper we describe the results of a parameter survey of a 16 segmented Halbach cylinder in three dimensions in which the parameters internal radius, rin, external radius, rex, and length, L, have been varied. Optimal values of rex and L were found for a Halbach cylinder with the least possible volume of magnets with a given mean flux density in the cylinder bore. The volume of the cylinder bore could also be significantly increased by only slightly increasing the volume of the magnets, for a fixed mean flux density. Placing additional blocks of magnets on the end faces of the Halbach cylinder also improved the mean flux density in the cylinder bore, especially so for short Halbach cylinders with large rex. Moreover, magnetic cooling as an application for Halbach cylinders was considered. A magnetic cooling quality parameter, Λcool, was introduced and results showed that this parameter was optimal for long Halbach cylinders with small rex. Using the previously mentioned additional blocks of magnets can improve the parameter by as much as 15% as well as improve the homogeneity of the field in the cylinder bore. [ABSTRACT FROM AUTHOR]

To overcome some deficiencies in previous constitutive models of giant magnetostrictive materials, a nonlinear and coupled model is suggested to describe the constitutive relations for a Terfenol-D rod subjected to an axial pre-stress and then located in an axial magnetic field. The numerical simulation by the model proposed in this paper shows predicted magnetostrictive strain curves for various compressive pre-stresses in good agreement with the experimental data not only in the region of low and moderate magnetic field but also in the region of high field. In comparison with the previous models, the proposed model can more effectively describe the effect of the pre-stress on the maximum magnetostrictive strain. Moreover, the effect of the stress and the magnetic field on the Young’s modulus of the materials, i.e., the ΔE effect, can also be predicted. In the proposed model, there are only five material parameters. They are the saturation magnetization Ms, the saturation magnetostrictive strain λs, the intrinsic (or saturation) Young’s modulus Es and the initial Young’s modulus E0 as well as the linear magnetic susceptibility χm. Since these parameters are easily measured in experiments, the proposed model is convenient to be used in engineering applications. [ABSTRACT FROM AUTHOR]

We excite the gyrotropic mode of a magnetic vortex and observe the resulting effect on the spin state of a nearby nitrogen-vacancy (NV) defect in diamond. Thin permalloy disks fabricated on a diamond sample are magnetized in a vortex state in which the magnetization curls around a central core. The magnetization dynamics of this configuration are described by a discrete spectrum of confined magnon modes as well as a low-frequency gyrotropic mode in which the vortex core precesses about its equilibrium position. Despite the spin transition frequencies being far-detuned from the modes of the ferromagnet, we observe enhanced relaxation of the NV spin when driving the gyrotropic mode. Moreover, we map the spatial dependence of the interaction between the vortex and the spin by translating the vortex core within the disk with an applied magnetic field, resulting in steplike motion as the vortex is pinned and de-pinned. Strong spin relaxation is observed when the vortex core is within approximately 250 nm of the NV center defect. We attribute this effect to the higher frequencies in the spectrum of the magnetic fringe field arising from the soliton-like nature of the gyrotropic mode when driven with sufficiently large amplitude. [ABSTRACT FROM AUTHOR]

Commercial high- T c superconducting (HTS) coated conductors exhibit asymmetric I c (B , θ) characteristics, where B presents a DC magnetic field and θ is defined as the angle between an applied magnetic field and the normal component of the superconductor plane. The asymmetric I c (B , θ) characteristics have a non-trivial influence on the dominant loss component, magnetization loss, of various HTS applications where HTS conductors are exposed to an AC magnetic field. Here, we present measurements of I c (B , θ) and magnetization loss in a 12 mm-wide (Rare Earth)Ba 2 Cu 3 O 7 − d (REBCO) commercial coated conductor at 77, 70, and 65 K. In the I c (B , θ) measurement, θ was varied around a full 360 ° revolution and B was varied up to 0.2 T. In terms of the magnetization loss measurement, the applied AC magnetic field amplitude is up to 110 mT and the field angle varied from 0 ° to 180 °. At the three given temperatures, we observed magnetization loss variations among the field-angle range, in particular, for θ and 180 ° - θ , which are in mirror symmetry relative to the superconducting plane. Furthermore, this asymmetric field-angle dependence of the magnetization loss becomes more apparent at higher applied field amplitudes and lower operating temperatures. A finite element method simulation using H-formulation was carried out by directly interpolating the measured I c (B , θ) data, and the simulation results reproduce the trend of the experimental results. We also found that the magnetization loss is not equivalent within the positive and negative half field cycles due to the asymmetric I c (B , θ) characteristics of the conductor. Numerical simulations revealed a clear correlation between the magnetization loss and the asymmetric I c (B , θ) data for the whole 360 ° field-angle range in the REBCO conductor. The asymmetry in the I c (B , θ) data about the ab-peak causes differences in magnetization loss values for the mirror-symmetric field angles. The asymmetry in the I c (B , θ) data upon field reversal also results in differences between magnetization loss values for the positive and negative cycles. [ABSTRACT FROM AUTHOR]

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. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]