Effect Of Demagnetizing Field Research Articles

Efficient Optical Control of Magnon Dynamics in van der Waals Ferromagnets

Optical control of magnons in two-dimensional (2D) materials promises new functionalities for spintronics and magnonics in atomically thin devices. Here, we report control of magnon dynamics, using laser polarization, in a ferromagnetic van der Waals (vdW) material, Fe 3.6 Co 1.4 GeTe 2 . The magnon amplitude, frequency, and lifetime are controlled and monitored by time-resolved pump-probe spectroscopy. We show substantial (over 25%) and continuous modulation of magnon dynamics as a function of incident laser polarization. Our results suggest that the modification of the effective demagnetization field and magnetic anisotropy by the pump laser pulses with different polarizations is due to anisotropic optical absorption. This implies that pump laser pulses modify the local spin environment, which enables the launch of magnons with tunable dynamics. Our first-principles calculations confirm the anisotropic optical absorption of different crystal orientations. Our findings suggest a new route for the development of opto-spintronic or opto-magnonic devices.

Microstructure and magnetic properties of Sm3Co11−xFexB4 ribbons with superior coercivity

This paper reports the relationship between the microstructure and magnetic properties of a new Sm3Co11B4-type alloy with ultra-high coercivity. Sm3Co11−xFexB4 (x = 0, 0.5 and 1.0) ribbons were prepared by melt-spinning at 30 m/s and then annealing at 800 °C for 30 min. The microstructural analysis demonstrates that the x = 0 ribbons consist of equiaxed Sm3Co11B4, Sm5Co19B6, Sm2Co7B3, and SmCo4B grains. A small amount of Fe addition does not change the phase composition but transforms the microstructure into a strip-like morphology in x = 0.5 ribbons, which increased both the coercivity and magnetization and obtained a quite high coercivity of 4.7 MA/m. When Fe content increases to x = 1.0, a new SmCo12B6-type phase forms, and the microstructure is converted from strip-shaped to equiaxed morphology, which reduces the coercivity due to the non-magnetic characteristic of SmCo12B6 and the increase of effective demagnetization field. The underlying mechanism was revealed by microstructure evolution in connection with the magnetic properties.

Temperature dependence of magnetic properties and ferromagnetic resonance in oriented hcp CoIr soft magnetic films with/without easy-plane magnetocrystalline anisotropy

The static magnetic properties and ferromagnetic resonance of the c-axis oriented hcp Co0.85Ir0.15 and Co0.98Ir0.02 soft magnetic films, with an easy-plane magnetocrystalline anisotropy (MA) and a c-axis MA, respectively, are investigated with temperature from 300 K down to 5 K. As the MA is opposite, opposite behavior in the temperature dependence of the effective demagnetization field Heff is observed: a decrement in Co0.98Ir0.02 film and an increment in Co0.85Ir0.15 film, respectively. Moreover, the resonance field HR(T) obtained under high frequency is different from that under low frequency for Co0.85Ir0.15 film, which is significantly different from the similar behavior in HR(T) under each frequency (6–18 GHz) for Co0.98Ir0.02 film. The fitting in HR(T) indicates that the opposite MA is also the primary reason for the different appearance in HR(T). Our findings provide supports to develop the advantages and applications of the soft magnetic films with easy-plane MA.

Combined effect of demagnetization field and magnetic anisotropy on magnetocaloric behavior and magnetocaloric-magnetoresistance correlation in GdTmErCoAl high-entropy amorphous alloy

The magnetic, magnetocaloric and transport properties were investigated systematically on Gd20Tm20Er20Co20Al20 high-entropy amorphous ribbons with magnetic fields parallel and perpendicular to ribbon surfaces. It is found that the alloy shows greater magnetic entropy change and refrigeration capacity with maximum values of 9.7 J kg−1 K−1 and 551.3 J kg−1 under the perpendicular field change of 5 T, respectively, than those of 8.4 J kg−1 K−1 and 458.1 J kg−1 under the parallel field, respectively. This large difference cannot be simply understood from the demagnetization effect and indicates a significant role of magnetic anisotropy associated with the anisotropy of atomic packing. Besides, magnetoresistance and magnetic entropy change are found to have a power-law correlation with distinct power exponents in different sample orientations with respect to the field. Present results have implications for the understanding and design of amorphous refrigerants with advanced magnetocaloric performance.

A general nonlinear magneto-elastic coupled constitutive model for soft ferromagnetic materials

In this study, a new general nonlinear magneto-elastic coupled constitutive model was proposed for soft ferromagnetic materials under magnetic field and pre-stress. The model is based on thermodynamic theory of magnetic materials and Jiles theory on magnetomechanical effect, considering demagnetization field. This model can accurately predict the changes of magnetostrictive strain and magnetization under pre-stress; in particular, it can obviously predict the nonlinear change of magnetostrictive strain according to compressive pre-stress. In addition, this model is concise by not including the inverse function of magnetization relationship and the piecewise function of stress in its mathematical form and can be degenerated to the previous general constitutive model for giant magnetostrictive materials. The prediction result by this model is well-consistent with experimental data. In comparison with the existing constitutive models for soft ferromagnetic materials, this model was demonstrated to have obviously-improved prediction accuracy, particularly in the compressive state. In conclusion, the proposed model can fully describe the effect of pre-stress on magnetostrictive strain and magnetization and the effects of magnetic field and pre-stress on Young’s modulus and relative magnetic permeability, reflecting the effect of demagnetization field on magnetomechanical characteristics.

Electromagnetic properties of Fe-Co granular composite materials containing acicular nanoparticles

Electromagnetic properties of acicular (needle-like) Fe76Co24 nanoparticle composite materials have been studied in microwave frequency range up to 20 GHz. The Fe76Co24 particles are commercially available acicular Fe76Co24 nanoparticles with an approximate length and diameter of 100 and 25 nm, respectively. The Fe76Co24 nanocomposites were prepared by embedding the Fe76Co24 nanoparticle in an appropriate resin. Since the metallic Fe76Co24 nanoparticles have an oxidized surface, even high particle content composites at 78 vol.%, which is in the percolated state, does not show metallic conduction; a low frequency plasmonic state with the negative permittivity spectrum was not observed. Meanwhile, the negative permeability spectrum caused by the magnetic resonance in Fe76Co24 alloy was obtained in the high particle content composites. From the measurement of the complex permeability spectra under the external dc magnetic field, it was clarified that the gyromagnetic spin rotation mainly contributes to the permeability spectrum of nanocomposites due to extremely small quantity of domain walls in the acicular nanoparticles. This result suggests that the negative permeability spectrum was caused by the gyromagnetic spin resonance. By the comparison of the complex permeability spectrum between the acicular Fe76Co24 nanocomposite and the spherical Fe50Co50 microcomposite, the gyromagnetic spin resonance frequency of the acicular nanocomposite tends to locate higher than that of the spherical microcomposite owing to the demagnetizing field effect. Therefore, it can be concluded that the negative permeability frequency band of the acicular nanocomposite is higher than that of the spherical microcomposite at the same particle content.

Effects of the Third Harmonic Demagnetization Field on the ac Losses of a Granular FeSeTe Superconductor in ac Field

The first harmonics of the magnetic response, of inter- and intragranular volume fractions of a FeSe0.5Te0.5 granular superconducting sample in an alternate magnetic field, have been numerically extracted from the measured magnetization of the whole system by using a model which considers the effects of the fundamental demagnetization fields generated by the magnetization itself. This allowed us to obtain the temperature and field behaviors of the inter- and intragranular ac losses during an ac cycle. These parameters have been found sensitive to the existence of the third harmonic components of the demagnetization field produced by the corresponding magnetization harmonics of the sample. This confirms that the effects of the magnetic response third harmonic components on the sample magnetization, through the demagnetization field, should be taken into account for a more detailed analysis of the measured ac magnetic curves.

Demagnetizing Field Effect on the Detection Range of a Galfenol-Based Magnetic Field Sensor

This paper investigates the new developments of a class of magnetic field sensors based on the integration of Iron–Gallium magnetostrictive alloys (Galfenol) and fiber Bragg gratings used to detect the magneto-induced mechanical strain. This kind of sensor has the advantage of being able to work also in harsh environments, but on the other hand cannot detect fields beyond 10 kA/m, because of the magnetic softness of the active material. A simple solution consists in the exploitation of the demagnetizing field experienced by the ferromagnetic alloy by effect of its magnetization, generated by the application of the external magnetic field. Since the demagnetizing field effect depends only on geometrical parameters, the use of samples with different aspect ratios allows us to check how the shape of the active material can be used as a control parameter of the sensor detection range.

Transient enhancement of magnetization damping in CoFeB film via pulsed laser excitation

Laser-induced spin dynamics of in-plane magnetized CoFeB films has been studied by using time-resolved magneto-optical Kerr effect measurements. While the effective demagnetization field shows little dependence on the pump laser fluence, the intrinsic damping constant has been found to be increased from 0.008 to 0.076 with the increase in the pump fluence from 2 mJ/cm2 to 20 mJ/cm2. This sharp enhancement has been shown to be transient and ascribed to the heating effect induced by the pump laser excitation, as the damping constant is almost unchanged when the pump-probe measurements are performed at a fixed pump fluence of 5 mJ/cm2 after irradiation by high power pump pulses.

Nonreciprocal behavior of the spin pumping in ultra-thin film of CoFeB

The processional magnetization induced spin current at the interface between CoFeB and Ta has been studied experimentally using spin pumping and inverse spin Hall effect for different thicknesses of CoFeB film down to 1.6 nm. It is found that upon decreasing the thickness of the CoFeB, the frequency of the peak position of the spin pumping signal reduces and dispersion relation of the ferromagnetic resonance changes from a quadratic to a linear behavior indicating the presence of an interfacial perpendicular anisotropy. Furthermore, a nonreciprocal behavior between the spin pumping signal amplitude at positive and negative fields is observed which could be as large as 100%. Our experimental results suggest reduction of the effective demagnetization field and possibly the spin waves nonreciprocal behavior mediated by the Dzyaloshinskii-Moriya interaction at the Ta/CoFeB interface are responsible for the large nonreciprocity of the spin pumping signal.

Frequency and field dependent dynamic properties of CoFe2−xAlxO4 ferrite nanoparticles

Aluminum doped CoFe2−xAlxO4 (0≤x≤0.9) nanoparticles were synthesized via auto-combustion. Formation of single phase cubic spinel structure was confirmed by X-ray diffraction (XRD) analysis. XRD analysis suggests a linear decrease in lattice cell parameters and grain size (90–55nm) with the increase in Al3+ content. The saturation magnetization of samples decrease with increasing Al3+ content due to magnetic dilution effect. A concomitant linear reduction in coercivity was also observed mainly due to decrease in magnetic anisotropy. Frequency and field dependent dynamic properties of nanoparticles were studied by ferromagnetic resonance (FMR) technique. The resonance frequency increases linearly with magnetic field for all nanoparticles. Magnetic field dependent experimental absorption data (S21 vs. frequency) were compared with effective medium theory considering an effective demagnetization field and was observed to be in good agreement with each other. High Al3+ content reduces the Gilbert damping parameter thus making CoFe2−xAlxO4 as an attractive material for high frequency applications.

Influence of Particle Size and Filling Factor of Galfenol Flakes on Sensing Performance of Magnetostrictive Composite Transducers

Ultrasonic guided wave (GW) techniques have been used widely for structural health monitoring applications in various engineering structures such as rods, tubes, pipes and plates [1]. Magneto-strictive sensors/transducers [2] provide an attractive alternative to piezoelectric transducers for generating and detecting GWs because there are some advantages such as cost-effectiveness, flexibility, durability, no depolarization effect, and non-contact sensing. Results from our previous work validated that a magnetostrictive patch transducer (MPT) composed of a highly textured Fe-Ga alloy (Galfenol) sheet with a preferred orientation was capable of directional sensing of incoming guided Lamb waves (GLWs) in thin aluminum plates [3]. The sensing feature of the proposed MPT resulted from the unique anisotropic magnetostriction nature of the textured Galfenol patch. The Galfenol patch exhibited high magnetostriction values of ∼270 ppm along the orientation, approaching to around 80% of the single crystal magnetostriction performance at the same composition [4]. The use of the magnetostrictive component in the Galfenol-based MPT was recently extended to a polymer-bonded composite with Galfenol flakes for substituting the bulk Galfenol patch [5]. The metal-polymer composite offers useful band width at high frequencies due to high electrical resistivity and a non-conducting binder [6]. The prior work demonstrated the effect of particle shape on sensing performance and directional sensitivity of three different MPTs composed of spherical nickel, granular and flake-type Galfenol powders, respectively [5]. The use of Galfenol flakes in the composite-based MPT showed the best sensing response among those three kinds of powders. It was because the demagnetizing field caused by shape anisotropy was possibly minimized in the flake-type particles along the in-plane direction [7]. In addition to the small demagnetizing field effect, the packing density of composites can be enhanced if the particles are well aligned. Thus, in order to make the composite compact, the processing parameters such as particle size and epoxy content should be optimized for maximizing sensing performance.

Electromagnetic properties of Permendur granular composite materials containing flaky particles

Electromagnetic properties of Permendur (Fe50Co50 alloy) granular composite materials containing flaky particle have been studied from the RF to microwave frequency range. Properties of the flaky particle composites were compared with the spherical particle ones. The electrical conductivity of the flaky particle composite was higher than that of the spherical particle composite at the same particle content. An insulator to metal transition was observed at the percolation threshold φc in both composites. The φc of the flaky particle composite was lower than that of the spherical one. The relative complex permittivity indicates that the insulating state has dielectric properties. For the spherical particle composite, the permittivity enhancement caused by particle cluster formation can be described by the effective cluster model (ECM). The enhancement of the dielectric constant in the flaky particle composite is larger than the ECM prediction. A negative permittivity spectrum indicating a low frequency plasmonic state was observed in the metallic 70 vol. % flaky particle composite. The relative complex permeability spectra of the flaky particle composite are different from those of the spherical one. The flaky particle composite shows a larger permeability value and lower permeability dispersion frequency than the spherical particle composite. Negative permeability spectra were observed in the both composite materials. The negative permeability frequency band of the flaky particle composite is lower than that of the spherical particle composite owing to the demagnetizing field effect.

Magnetization states in epitaxial thin films subjected to misfit strains and demagnetization field

The effect of non-equally biaxial in-plane misfit strains and demagnetization field on the magnetization states of ferromagnetic thin films is analytically investigated by using a nonlinear thermodynamic theory. The “misfit strain–misfit strain” phase diagrams of the magnetization states at room temperature for α-Fe and CoFe2O4 thin films epitaxially grown on tetragonal substrates are developed by minimizing the total free energy. For a cubic ferromagnetic thin film, an out-of-plane magnetization state exists in the region of tensile misfit strains if the magnetostrictive coefficient of λ100 is negative and the demagnetization field is small, whereas the out-of-plane magnetization vanishes in the whole region of misfit strains when the demagnetization field is large.

전기 도금법으로 제작한 Ni 박막의 강자성 공명 선폭 분석

전기 도금법으로 제작한 Ni 박막(240 nm)의 자기장 각도에 따른 강자성 공명 신호를 측정하여 공명 자기장(<TEX>$H_{res}$</TEX>) 및 선폭(<TEX>${\Delta}H_{PP}$</TEX>)을 도출하였다. 자기장 각도에 따른 <TEX>$H_{res}$</TEX>는 이론적인 분석 결과와 일치하였으며 이들 결과로부터 제조된 Ni 박막의 g-factor는 2.18임을 확인하였다. 자기장 각도에 따른 <TEX>${\Delta}H_{PP}$</TEX>는 박막의 수평 방향에서 매우 큰 값을 나타냈으며, 이러한 특성은 Gilbert 감쇠에 기인하는 균일한 선폭 특성과 약 1 nm의 표면에서 나타나는 자구들의 각도 변화 및 자화량 변화에 기인하는 비균일한 선폭 특성으로는 설명되지 않았다. 따라서 본 연구에서는 박막의 두께가 10 GHz에서 임계 두께(약 50 nm) 이상으로 증가하면 나타나는 two magnon scattering 이론을 적용하여 비균일한 선폭 특성을 분석하였다. 이러한 분석 결과로부터 전기 도금법으로 제작한 240 nm 두께를 갖는 Ni 박막에서 각도에 따른 비균일한 선폭 변화의 주요한 원인은 재료 내부 결함들에 의한 스핀파 산란이었음을 알 수 있었다. We obtained resonance field (<TEX>$H_{res}$</TEX>) and linewidth (<TEX>${\Delta}H_{PP}$</TEX>) from measured ferromagnetic resonance signal in the functions of polar angle (<TEX>${\Theta}_H$</TEX>) in Ni thin film of 240 nm thickness fabricated by electrodeposition method. The angular dependence of <TEX>$H_{res}$</TEX> was well fitted with the calculated ones. We confirmed that the g-factor and effective demagnetization field were 2.18 and 445 emu/cc by the theoretical analysis of the resonance field, respectively. The angular dependence of <TEX>${\Delta}H_{PP}$</TEX> showed very large values at in-plane direction (<TEX>${\Theta}_H=90^{\circ}$</TEX>), which could not explained by the homogenous linewidth due to the Gilbert damping and inhomogeneous linewidth due to the angular variations and magnetization variations by the surface layer. Therefore, we considered the spin wave scattering (two magnon scattering) process in order to analyze the measured inhomogeneous linewidth, which was appeared in thicker film than the critical thickness of 50 nm. The defect medicated spin wave scattering played a key role in the electrodoposited Ni thin film of 240 nm thickness.

Electric field controlled reversible magnetic anisotropy switching studied by spin rectification

In this letter, spin rectification was used to study the electric field controlled dynamic magnetic properties of the multiferroic composite which is a Co stripe with induced in-plane anisotropy deposited onto a Pb(Mg1∕3Nb2∕3)O3-PbTiO3 substrate. Due to the coupling between piezoelectric and magnetoelastic effects, a reversible in-plane anisotropy switching has been realized by varying the history of the applied electric field. This merit results from the electric hysteresis of the polarization in the nonlinear piezoelectric regime, which has been proved by a butterfly type electric field dependence of the in-plane anisotropy field. Moreover, the electric field dependent effective demagnetization field and linewidth have been observed at the same time.

High Q factor over 3000 due to out-of-plane precession in nano-contact spin-torque oscillator based on magnetic tunnel junctions

We investigated microwave oscillation in a sombrero-shaped nano-contact spin-torque oscillator under an external magnetic field with an out-of-plane component. By increasing the out-of-plane field, we observed an abrupt change from in-plane precession to out-of-plane precession around the effective demagnetization field of the free layer. The out-of-plane precession under optimized conditions yielded not only a fundamental oscillation peak with a very large Q factor (3200) but also higher-order harmonic peaks with frequencies up to at least 45 GHz. A high Q factor and high-frequency oscillations are of great importance for device applications such as telecommunications, radar sensors, and high-speed magnetic field sensors.

Fast deterministic switching in orthogonal spin torque devices via the control of the relative spin polarizations

We model 100 ps pulse switching dynamics of orthogonal spin transfer (OST) devices that employ an out-of-plane polarizer and an in-plane polarizer. Simulation results indicate that increasing the spin polarization ratio, CP = PIPP/POPP, results in deterministic switching of the free layer without over-rotation (360° rotation). By using spin torque asymmetry to realize an enhanced effective PIPP, we experimentally demonstrate this behavior in OST devices in parallel to anti-parallel switching. Modeling predicts that decreasing the effective demagnetization field can substantially reduce the minimum CP required to attain deterministic switching, while retaining low critical switching current, Ip ∼ 500 μA.

Multi-domain resonance in textured Z-type hexagonal ferrite

The resonance frequency of a non-saturated Z-type hexagonal ferrite under a demagnetizing field effect was formulated. The permeabilities along the c-axis and the c-plane were measured by a specimen cut from the c-plane-textured Z-type ferrite, and the resonance frequency was measured as a μ″ peak. The resonance frequency measured from the permeability in the c-plane increases dramatically as the specimen’s thickness increases. On the other hand, the resonance frequency measured from the permeability along the c-axis remains fairly constant. We tried to analyze the reason for the large difference in the resonance frequency depending on the crystal orientation using multi-domain resonance theory. As a result, the resonance frequency of a textured Z-type ferrite has been expressed when the high frequency magnetic field is parallel and normal to the c-plane. Therefore, it is confirmed that a demagnetizing field increases the resonance frequency, especially when the high frequency magnetic field is parallel to the c-plane. This result would contribute to the development a high performance inductance element.

Reduction of intrinsic critical current density under a magnetic field along the hard axis of a free layer in a magnetic tunnel junction

We investigated the effect of a magnetic field along a hard in-plane axis ${H}_{\mathrm{hard}}$ on a current-induced magnetization switching (CIMS) in magnetic tunnel junctions (MTJs). Since ${H}_{\mathrm{hard}}$ causes the effective field to tilt away from the easy axis, we evaluated the ${H}_{\mathrm{hard}}$ dependence of two contributing factors in CIMS [the intrinsic critical current density (${J}_{c0}$) and the thermal stability factor ($E/{k}_{B}T$)] as functions of the tilting angle (${\ensuremath{\theta}}_{H}$). Both measurements and numerical simulations showed that the presence of ${H}_{\mathrm{hard}}$ can reduce ${J}_{c0}$ by more than the amount estimated by Slonczewski's polarization function $g$($\ensuremath{\theta}$) by an order of magnitude and that $E/{k}_{B}T$ is independent of ${\ensuremath{\theta}}_{H}$. These findings suggest that the effect of ${H}_{\mathrm{hard}}$ mainly appears in the dynamic properties due to the nonconservative force of the spin-transfer torque based on the Slonczewski's model. A simple stability analysis demonstrated that the tilt of the magnetization direction away from the easy axis caused by the presence of ${H}_{\mathrm{hard}}$ induces an imbalance between the spin-transfer and damping torques and that applying a current achieves the further tilted stable state. Achievement of this stable state can be interpreted as the suppression of the effect of the effective demagnetization field (${H}_{d}^{*}$). Therefore the major reduction in ${J}_{c0}$ is due to the suppression of ${H}_{d}^{*}$ caused by the presence of ${H}_{\mathrm{hard}}$.