Easy Direction Research Articles

Magnetostrictive Property and Magnetic Domain Structure of Fe-Ga Alloy Single Crystal under Tensile Strain

The saturation value of the magnetostriction curve in the [100] direction of a Fe-Ga alloy single crystal was decreased from 226 to 55 ppm by applying the tensile strain of 533 ppm to the measured direction. By magnetic domain observation using a magneto-optic Kerr effect microscope, a complex structure composed of various magnetic domains was observed under zero applied strain. On the other hand, a stripe structure composed of magnetic domains with the magnetization direction in two kinds of <100> magnetic easy directions parallel to the tensile direction, which were separated by straight 180° domain walls, was observed under the tensile strain of 533 ppm. The characteristic magnetic domain structure due to the tensile strain was successfully observed as a cause of the significant decrease of the saturation value of the magnetostriction curve.

Significant reduction in Young's modulus of Fe–Ga alloy single crystal by inverse magnetostrictive effect under tensile stress

A characteristic modulation of the magnetic domain structure on the (001) plane of an Fe–12.8 at. % Ga alloy single crystal under tensile stress was observed by magneto-optic Kerr effect microscopy. The magnetic domain structure under zero applied stress was composed of magnetic domains with magnetizations in four types of <100> magnetic easy directions on the (001) plane, which were separated by straight 90° domain walls and stair-like 180° domain walls. When a tensile stress of 11.4 MPa was applied parallel to the [010] direction, these 90° domain walls disappeared to release the tensile stress. A stripe domain structure that was composed of straight 180° domain walls parallel to the tensile stress direction was thus formed by the inverse magnetostrictive effect. Young's modulus of the alloy below the applied 11.4 MPa tensile stress was estimated to be 32 GPa, which is 57% smaller than the corresponding value above that tensile stress value. Consequently, a significant reduction in Young's modulus is caused by the characteristic modulation of the magnetic domain structure that occurs as a result of the inverse magnetostrictive effect.

Influence of crystal orientation on the magnetostriction behavior of Fe films formed on MgO single-crystal substrates

Abstract Fe thin films are prepared on MgO substrates of (001), (110), and (111) orientations by ultra-high vacuum magnetron sputtering. The magnetostriction behaviors under rotating magnetic fields are observed by using a cantilever method. The relationship between in-plane magnetic anisotropy and magnetostriction behavior is investigated. An Fe(001)bcc single-crystal film is obtained on MgO(001) substrate. A four-fold symmetry in in-plane magnetic anisotropy is observed for the Fe(001)bcc film, where the easy magnetization directions are parallel to bcc[100] and bcc[010]. An Fe(211)bcc bi-crystalline film is formed on MgO(110) substrate and the film also shows an almost four-fold symmetric magnetic anisotropy. The reason is interpreted to be due to an influence of overlap of magnetic anisotropies of two bcc(211) variants with two effective easy magnetization directions which are obtained by projecting bcc[010] and bcc[001] on the bcc(211) plane. An Fe(110)bcc film is epitaxially grown on MgO(111) substrate with two types of variant with the crystallographic orientation relationships similar to Nishiyama-Wasserman and Kurdjumov-Sachs. The Fe(110) film shows an isotropic in-plane magnetic property due to an influence of the variant structure. Usual sinusoidal waveforms of magnetostriction are observed for the Fe(110) film. On the other hand, waveforms measured for the Fe(001) and the Fe(211) films are deformed from sinusoidal shape under low in-plane rotating magnetic fields and the magnetostriction behaviors are similar between the two films. Rectangular waveforms are observed for the bending parallel to easy magnetization directions, whereas triangular waveforms are recognized for the bending parallel to hard directions. The phenomenon is related to the direction difference between applied magnetic field and magnetization in magnetically unsaturated films with in-plane anisotropies. The magnetostriction behavior is influenced by the symmetry of in-plane magnetic anisotropy.

Magnetic properties of RCuAl3 (R = Pr and Nd) compounds

Magnetic properties of PrCuAl3 and NdCuAl3 single crystals, adopting the tetragonal BaNiSn3-structure-type, are reported. Our magnetization and specific heat measurements revealed no magnetic ordering in PrCuAl3, while NdCuAl3 orders antiferromagnetically below TN = 2.45 K. The paramagnetic regime was investigated for both analogues revealing (i) the a-axis as the easy direction of magnetization with (ii) a relatively high uniaxial anisotropy energy of 13.2 meV for PrCuAl3 compared to a smaller value of 4.7 meV for NdCuAl3. The measured data are well explained by rare-earth ground state multiplet splitting in crystal electric field. A powder neutron diffraction experiment confirmed PrCuAl3 to remain paramagnetic down to 1.5 K. On the other hand, NdCuAl3 orders antiferromagnetically below TN with an incommensurate magnetic structure described by two propagation vectors k1 = (1/3 - δ1, 0, 0) and k2 = (1/7 - δ2, 1/7 - δ2, 0), where δ1 = 0.035(1) and δ2 = 0.007(1). The magnetic moments (of both magnetic modulations) are arranged within the basal plane with a small z-component moment allowed by the symmetry. Our results are discussed with respect to other RCuAl3 compounds.

Magnetic-field-induced synthesis and growth mechanism of ultra-long nickel microwires from nanospheres

Abstract One-dimensional nickel (Ni) microwires were synthesized by the magnetic-field-induced (MFI) microemulsion-hydrothermal method using Ni nanospheres as seeds. The influence of the chemical conditions and magnetic field on the morphology and magnetic properties of Ni microwires were investigated by X-Ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). The strength of magnetic field greatly affected the morphology and magnetic properties of Ni microwires. The growth and morphology of Ni microwires depended on the competition between Brownian motion and magnetic dipole moments from magnetic field. The larger nanoparticles with the higher magnetic moments preferred to form one-dimensional Ni microwires and their length increased up to several millimeters with the increasing magnetic field strength, while the average diameter of Ni microwires decreased. Furthermore, the easy magnetization direction of Ni microwires was parallel to the direction of magnetic field and the Ni microwires axis.

Magnetic anisotropy and stress-magnetoimpedance (S-MI) in current-annealed Co-rich glass-coated microwires with positive magnetostriction

Abstract Effects of a dc current annealing, in terms of magnitude and inflow time, on magnetic hysteresis properties, magnetoimpedance (MI) and stress-magnetoimpedance (S-MI) in glass-coated amorphous Co71Fe5B11Si10Cr3 microwires with a small positive magnetostriction were investigated in this work. The annealing process combining Joule-heating and circular magnetic field leads to the creation of a specific magnetic anisotropy of a helical or circular type, and consequently, is helpful to control the MI and S-MI behaviours making them more suitable for particular sensing applications. After annealing, the magnetoimpedance plots transformed from a bell-shaped typical of an axial easy anisotropy to the shape with two symmetrical peaks, the magnitude and position of which are controlled with the annealing current parameters. Applying a tensile stress on the annealed wires changes the easy anisotropy direction which leads to the potential for highly sensitive S-MI without use of dc bias fields. A stress sensitivity of more than 260% at σ ex = 250 MPa at zero magnetic field is achieved.

Ab initio and Monte Carlo studies of phase transitions and magnetic properties of YCrO3: Heisenberg model

By using the density functional theory (DFT) and Monte Carlo simulations (MCS) with the Heisenberg model, we have studied magnetic properties of the bulk perovskite YCrO3. The exchange couplings of the Heisenberg model and the magnetic anisotropy are investigated. The 110 direction in the crystalline structure of the compound has shown the minimum energy, it is the easy magnetic direction. Using Monte Carlo simulations, the magnetizations behavior, the effects of system parameters and the critical exponents of the compound YCrO3 are implemented. It is shown that the bulk perovskite YCrO3 belongs to the 3D Heisenberg universality class.

In-situ synthesis and room temperature magnetic properties of cobalt vanadate nanowire array

Abstract Room temperature diluted magnetic semiconductor nanomaterials play an important role in the preparation of novel micro-nano semiconductor devices. Herein, magnetic cobalt vanadate nanowire array is fabricated via a simple one-step cation-exchange method, which is confirmed to be a new kind of one-dimensional room temperature ferromagnetic semiconductors. Anisotropic magnetic with an easy direction of magnetization either perpendicular or parallel to the nanowire axis are found in the samples. Structural reorganization during calcination is beneficial to enhance the magnetic properties of the materials (Ms = 0.081 emu g−1 after calcination at 500 °C). Our investigation indicates that the room temperature anisotropic ferromagnetic of the samples is derived from the uniformly distributed cobalt element in the samples. The cation-exchange method in this work is easily applicable to synthesis similar room temperature ferromagnetic vanadate nanomaterials.

Soft magnetic composite containing magnetic flakes with in-plane uniaxial magnetic anisotropy

A novel soft magnetic composite (SMC) containing magnetic flakes with in-plane uniaxial magnetic anisotropy was successfully fabricated. The magnetic flakes of amorphous FeCoBSi phase were compacted in an insulating matrix to fabricate the SMC. The fabricated SMC was annealed under a magnetic field, and in-plane uniaxial magnetic anisotropy was observed in the flattened planes of the magnetic flakes after annealing. The coercivity of the SMC in the easy axis direction was approximately half of that in the hard axis direction after annealing. The minimum coercivity in the easy axis direction was 0.1 Oe, and much smaller than those of conventional SMCs, SMC developed in the other work, and electrical steel sheet. An extremely small core loss of 16 kW/m3 can be expected at 1 T and 400 Hz, reflecting the small coercivity value (0.1 Oe).

Surface anisotropy and spin wave resonance in submicron amorphous wires

Abstract The influence of surface magnetic anisotropy on ferromagnetic resonance (FMR) in submicron wires is investigated both theoretically and experimentally. An analytical formula for resonance fields of radial spin waves in a long axially magnetized circular cylinder is obtained. Three types of surface anisotropy with the easy direction along one of the cylindrical coordinates are considered. For sufficiently strong surface anisotropy with hard direction parallel to the cylinder axis the surface spin wave mode is observed. The theoretical results are verified by FMR measurements at six microwave frequencies from 9 to 69 GHz on glass-covered amorphous FeCrSiB wires with diameters from 541 to 1032 nm. From the bulk spin wave resonances the exchange stiffness constant about 6.5 10−7 erg/cm is obtained. The frequency dependence of the surface mode resonance field indicates that a perpendicular surface anisotropy with anisotropy constant Ks = 6.2 erg/cm2 is present at the metal/glass interface.

Crystal structure, magnetic and microwave absorption properties of Ce2−xSmxFe17N3−δ/paraffin composites

Crystal structure, magnetic and microwave absorption properties of Ce2−xSmxFe17N3−δ (x = 0.0, 0.1, 0.2, 0.3, 0.4) have been investigated. In these compounds, the substitution of Ce by Sm atoms, effectively modulating the c-axial and ab-planar components of the magneto-crystalline anisotropy field, exerts a vital effect on the precession of magnetic moments. Thereby, the permeability and microwave absorption at high frequency are tuned. The easy magnetization directions (EMDs) of all the Ce2−xSmxFe17 (0 ≤ x ≤ 0.4) samples are in the ab-plane, while for Ce2−xSmxFe17N3−δ samples, they are in ab-plane with x = 0.0, 0.1, in cone with x = 0.2, and finally parallel to c-axis with x = 0.3, 0.4, respectively. Compared with other compounds, the one with cone-like anisotropy shows superior microwave absorption properties due to better impedance match. The Ce1.8Sm0.2Fe17N3−δ/paraffin composite exhibits a maximum reflection loss (RL) (−39 dB) at 3.3 GHz with a thickness of 3.0 mm and is expected to be an excellent high frequency electromagnetic absorbing material with strong absorption and relatively small thickness.

Effect of laser pulse repetition rate on morphology and magnetic properties of cobalt ferrite films grown by pulsed laser deposition

In this paper, we report a switch of magnetic anisotropy in CoFe2O4 (CFO) films induced by change in laser pulse repetition rate. CFO films were deposited on MgO (0 0 1) substrates using pulsed laser deposition method with 2, 3, 5, 7 and 10 Hz laser pulse repetition rates. Atomic force microscopy images exhibited the film surface aggregation tends to compact shape with large size islands on the surface of the samples deposited by 2 and 3 Hz while the size of islands decreased on the surface of the 5, 7 and 10 Hz CFO films. The hysteresis curve of films indicated magnetic anisotropy and switching of the easy axis direction by changing laser repetition rates. The films grown at 2 and 3 Hz exhibit an in-plane easy axis, while by increasing the laser repetition rate, the easy axis switches to out-of-plane. The Raman spectra showed the rearrangement of Fe3+ and Co2+ cations on octahedral and tetrahedral sites in the structure by changing the laser pulse repetition rate. Change in the strain, morphology evolution and cation distribution are responsible for the change in easy axis direction and existence of shrinking at M–H loops.

Magnetic characteristics of RPd2Si2 (R = Rare earth)

Magnetic characteristics of HoPd2Si2 and ErPd2Si2 with the ThCr2Si2-type crystal structure have been studied from measurements of magnetic susceptibility χ(T) and magnetization M(B) on the single crystals. The compound HoPd2Si2 orders antiferromagnetically below TN = 5.1 K with the magnetic easy direction of [110]. The [110] magnetization process is a three-step metamagnetic one; metamagnetic transitions at B1 = 0.9 T, B2 = 1.5 T and B3 = 3.4 T. The [100] process is a two-step one. Between [110] and [100], strong magnetic anisotropy is seen for high magnetic fields. ErPd2Si2 shows an antiferromagnetic behavior with the magnetic easy direction of [001] below TN=3.9 K. The χ(T) shows four anomalies, which may come from magnetic transitions, at Tmax = 5.1 K, TN = T1 = 3.9 K, T2 = 3.1 K and T3 = 2.2 K. The [001] magnetization process at 2 K is a three-step metamagnetic process; metamagnetic transitions at B1 = 0.5 T, B2 = 1.0 T and B3 = 1.3 T. In the basal plane magnetization processes, a very small metamagnetic transition is at 1.9 T. Crystalline electric field (CEF) analysis was performed, and anisotropic magnetic behaviors are explained. The magnetic data of the RPd2Si2 compound series are summarized.

Anomalous specific heat and magnetic properties of TmxDy1-xAl2 (0 ≤ x ≤ 1)

We study crystal structure, phase transitions and magnetism of pseudo-binary TmxDy1-xAl2 (0 ≤ x ≤ 1) compounds using temperature dependent X-ray powder diffraction, specific heat and magnetization measurements, first principles, and model calculations. In low external magnetic fields, Dy-rich compounds undergo continuous, second-order phase transitions at the respective Curie temperatures, TC. In contrast, the Tm-rich compounds exhibit discontinuous, first-order anomalies in the magnetically ordered states. These sharp transitions correlate with a substantial energy difference between the room temperature cubic and ground state rhombohedral structures of TmAl2. A clear anomaly in the lattice parameter is observed at ∼30 K for x = 0.5, which nearly coincides with TC = 31.2 K. The effective quadrupolar moment of the lanthanides changes sign around x = 0.5, which leads to a nearly zero anisotropy constant and approximately spherical effective 4f charge densities, providing an explanation for the lack of structural distortions below TC for x = 0.5. The calculations confirm [001] as the easy magnetization axis in the ground state tetragonal structure of DyAl2, and reveal collapse of the orbital magnetic moment when the easy magnetization direction changes to [111]. Within the rhombohedral ground state of TmAl2 [111] is the easy magnetization direction.

Characteristics of signal propagation in multiferroic majority logic gates subjected to thermal noise

The error rates of multiferroic majority logic gate (MLG) in the presence of thermal noise fluctuation are evaluated via solving the Landau–Lifshitz–Gilbert equations. It is found that in the pre-stress condition, magnetisation orientations of nanomagnets in the MLG denote an initial 4°–10° departure from the easy axis direction due to random thermal noise field. This phenomenon may be the main origin of switching errors of multiferroic logic gates at high temperature. Specifically, in the post-stress condition, it has been seen that larger spacing between nanomagnets leads to a high error rate, up to 50%, which arises from weak dipolar coupling interactions and strong thermal fluctuations. However, opposed to their expected, small spacing between nanomagnets leads to a higher error rate, up to 90%, which mainly arises from spontaneous magnetisation reversal induced by weak stress anisotropy energy. Therefore, moderate spacing is very important to MLG switching at room temperature, for example the optimal spacing is 250–310 nm for MLG constructed with nanomagnets having shape anisotropy energy of 623 kT. These findings can provide guides on multiferroic logic circuit design.

The Imry–Ma Phase in a Nanocrystalline Ferromagnet

A possibility of appearing a disordered Imry–Ma state induced by fluctuations of the anisotropy easy axis direction in a nanocrystalline sample in the case of a weak exchange interaction between crystallites has been studied. A phase diagram of the system has been built in variables “the characteristic crystallite size–the exchange integral of the intercrystallite interaction.” The characteristic value of statistic fluctuations of the crystallographic anisotropy has been estimated, and the dependences of the coercive field on the crystallite size have been found for systems of various space dimensions.

Interface Magnetism in La0.7Ca0.3MnO3/PrBa2Cu3O7 Epitaxial Heterostructures

All oxide magnetic tunnel junctions based on epitaxial La0.7Ca0.3MnO3 (LCMO)(8 nm)/PrBa2Cu3O7 (PBCO) (3–8 nm)/LCMO(25–50 nm) heterostructures grown on (100) SrTiO3 are examined. Manganite electrodes show large (bulk‐like) magnetic moments and exhibit different magnetic anisotropies with different easy axes directions. A form of low dimensional magnetism is induced at the interfaces by the superexchange interaction across the reconstructed bonds. It acts as an exchange spring, driving ferromagnetic coupling between the electrodes. Resistance versus magnetic field loops demonstrate that the interfacial coupling strength depends on electric field through its effect on electronic reconstruction and orbital hierarchy at the interface. The electrically controlled magnetic coupling between the magnetic moments of the electrodes signals a new path toward low dissipation spintronics.

Magnetic anisotropy in permalloy: Hidden quantum mechanical features

By means of relativistic, first principles calculations, we investigate the microscopic origin of the vanishingly low magnetic anisotropy of Permalloy, here proposed to be intrinsically related to the local symmetries of the alloy. It is shown that the local magnetic anisotropy of individual atoms in Permalloy can be several orders of magnitude larger than that of the bulk sample, and 5-10 times larger than that of elemental Fe or Ni. We, furthermore, show that locally there are several easy axis directions that are favored, depending on local composition. The results are discussed in the context of perturbation theory, applying the relation between magnetic anisotropy and orbital moment. Permalloy keeps its strong ferromagnetic nature due to the exchange energy to be larger than the magnetocrystalline anisotropy. Our results shine light on the magnetic anisotropy of permalloy and of magnetic materials in general, and in addition enhance the understanding of pump-probe measurements and ultrafast magnetization dynamics.

Pressure-induced spin reorientation transition in layered ferromagnetic insulator Cr2Ge2Te6

Anisotropic magnetoresistance (AMR) of Cr2Ge2Te6 (CGT), a layered ferromagnetic insulator, is investigated under an applied hydrostatic pressure up to 2 GPa. The easy axis direction of the magnetization is inferred from the AMR saturation feature in the presence and absence of the applied pressure. At zero applied pressure, the easy axis is along the c-direction or perpendicular to the layer. Upon application of a hydrostatic pressure>1 GPa, the uniaxial anisotropy switches to easy-plane anisotropy which drives the equilibrium magnetization from the c-axis to the ab-plane at zero magnetic field, which amounts to a giant magnetic anisotropy energy change (>100%). As the temperature is increased across the Curie temperature, the characteristic AMR effect gradually decreases and disappears. Our first-principles calculations confirm the giant magnetic anisotropy energy change with moderate pressure and assign its origin to the increased off-site spin-orbit interaction of Te atoms due to a shorter Cr-Te distance. Such a pressure-induced spin reorientation transition is very rare in three-dimensional ferromagnets, but it may be common to other layered ferromagnets with similar crystal structures to CGT, and therefore offers a unique way to control magnetic anisotropy.

Clogging and transport of driven particles in asymmetric funnel arrays

We numerically examine the flow and clogging of particles driven through asymmetric funnel arrays when the commensurability ratio of the number of particles per plaquette is varied. The particle–particle interactions are modeled with a soft repulsive potential that could represent vortex flow in type-II superconductors or driven charged colloids. The velocity-force curves for driving in the easy flow direction of the funnels exhibit a single depinning threshold; however, for driving in the hard flow direction, we find that there can be both negative mobility where the velocity decreases with increasing driving force as well as a reentrant pinning effect in which the particles flow at low drives but become pinned at intermediate drives. This reentrant pinning is associated with a transition from smooth 1D flow at low drives to a clogged state at higher drives that occurs when the particles cluster in a small number of plaquettes and block the flow. When the drive is further increased, particle rearrangements occur that cause the clog to break apart. We map out the regimes in which the pinned, flowing, and clogged states appear as a function of plaquette filling and drive. The clogged states remain robust at finite temperatures but develop intermittent bursts of flow in which a clog temporarily breaks apart but quickly reforms.