Magnetic Reorientation Research Articles

Evolution of Fe magnetic order inNdFexGa1 − xO3

The evolution of the crystal structure and magnetic properties with Fe content inNdFexGa1 − xO3 has been studied by magnetization, ac-susceptibility, x-ray and neutron scattering techniques forx ≥ 0.2 in orderto determine the phase diagram of the series. X-ray diffraction shows that the crystallographic structureof NdFexGa1 − xO3 can be described in the space group Pbnm for allx values. Both the magnetic ordering and spin reorientation temperatures of the Femagnetic sublattice decrease with iron concentration due to the presence of magneticvacancies occupied by Ga. The long-range Fe magnetic ordering disappears forx ≤ 0.3, while ac-susceptibility measurements evidence the presence ofshort-range Fe ordered clusters and superspin-glass-like effects forx well below the percolation threshold. The magnetic structure of the compounds,including the spin reorientation temperature range, is determined by high-resolutionneutron diffraction analysis. Although the presence of finite magnetic clusters forx values close to percolation is evidenced, the study of a percolation quantum phase transition in thisseries is hindered by the presence of Nd magnetic moments and a sizeable distribution of compositionΔx around the nominal value.

Large converse magnetoelectric coupling effect at room temperature in CoPd/PMN-PT (001) heterostructure

We report a large converse magnetoelectric (CME) effect at room temperature in a multiferroic heterostructure formed from thin layers of perpendicularly magnetized CoxPd1-x alloys deposited on a piezoelectric single-crystal of lead magnesium niobate-lead titanate PMN-PT(001). The CME results from a strain-induced reorientation of the CoPd magnetization. By varying the composition and thickness of the CoxPd1-x film, a large converse magnetoelectric coupling constant, α=8×10−7 s/m, at room temperature was found for 10 nm Co0.25Pd0.75. This large CME effect results from combining a highly magnetostrictive CoPd alloy with highly piezoelectric PMN-PT.

Magnetization switching in ferromagnetic microwires

Magnetization states of amorphous soft ferromagnetic microwires are studied by experiment and theoretically. It is shown that in low applied axial fields, on increasing the circular magnetic field produced by current, the initial homogeneous circular magnetization distribution evolves in three sequential steps: (i) the appearance of a helical magnetization state, (ii) a jump to the helical state with opposite chirality, and (iii) a gradual rotation of magnetization toward the new circular state. An applied, large-enough axial field stabilizes the axial magnetization states, making these transitions continuous and shifting the magnetic hysteresis loops. The origin of these successive magnetization reorientations and switching is explained in the frameworks of the proposed model.

Spin Polarized Current in Magnetic Nanojunctions

Two channels of the s-d exchange interaction are considered in magnetic junctions. The first channel describes the interaction of transversal spins with the lattice magnetization. The second channel describes the interaction of longitudinal spins with the magnetization. We show that the longitudinal channel leads to a number of significant effects: 1) drastic lowering of the current-instability threshold down to three (or even more) orders of magnitude; 2) creation of sufficiently large distortion of equilibrium due to the current- driven spin injection leading to inversion of populations of the energy spin subbands and laser-like instability in the THz frequency range at room temperature. External magnetic field is likely to tend to additionally lower the instability threshold due to the proximity effect of purely magnetic reorientation phase transition. This effect demonstrates new properties: the giant magnetoresistance (GMR) becomes strongly current-dependent and the exchange switching becomes of very low threshold. We derived some matching condition that should be satisfied to achieve high spin injection level. Some characteristic quantities appeared in the condition, namely, the so called "spin resistances" , where the a number of a layer in the junction. For a three-layer junction ( ), the matching condition is , where the number corresponds to the main functional layer. We investigated also the junctions having variable lateral dimensions of the layers, for example, a ferromagnetic rod contacting with a very thin ferromagnetic film. A large enhancement of the current density can appear near the contact region, leading to the spin injection luminescence.

Structure of [formula omitted] ceramics treated with hot-pressing

BiFeO 3 ceramics synthesized at high temperature and pressure is observed to possess large spontaneous polarization and magnetism. The selected-area electron diffraction patterns indicate the coexistence of polycrystalline and nanocrystalline grains in the hot-pressed samples. Raman spectroscopic studies reveal that the Bi–O bond strain and cations (Fe 3+) away from the oxygen octahedral center induce the polarization. The Fe–O bond strain, oxygen octahedral rotation around the [111] axis, and electromagnons build a long-range interaction between electrical polarization and magnetic moment reorientation to form an incommensurate spiral magnetic structure. This work clearly shows that the hot-pressed treatment can modify the structure and properties of BiFeO 3 ceramics.

Domain faceting in an in-plane magnetic reorientation transition

The microscopic structure of the $90\ifmmode^\circ\else\textdegree\fi{}$ in-plane magnetic reorientation transition in Fe(110) films is examined using photoemission x-ray microscopy. At the nanoscale, sharp magnetic boundaries are detected. They are indicative of a first-order transition and are consistent with Fe magnetic anisotropy constants. At the micron scale, the magnetic boundary breaks up into triangular patterns whose characteristic angular dependence is revealed by experiments on conical microwedges. This effect, fully accounted by micromagnetic simulations, opens the possibility to control the sharpness of the transition at the microscopic scale.

Resistive switching via the converse magnetoelectric effect in ferromagnetic multilayers onferroelectric substrates

A voltage-controlled resistive switching is predicted for ferromagnetic multilayers and spinvalves mechanically coupled to a ferroelectric substrate. The switching between low- andhigh-resistance states results from the strain-driven magnetization reorientations by about90°, which are shown to occur in ferromagnetic layers with a high magnetostriction and weakcubic magnetocrystalline anisotropy. Such reorientations, not requiring external magneticfields, can be realized experimentally by applying moderate electric field to athick substrate (bulk or membrane type) made of a relaxor ferroelectric havingultrahigh piezoelectric coefficients. The proposed multiferroic hybrids exhibiting giantmagnetoresistance may be employed as electric-write nonvolatile magnetic memory cellswith nondestructive readout.

Twin-variant reorientation-induced large magnetoresistance effect in Ni50Mn29Ga21 single crystal

We report a significant magnetoresistance (MR) effect arisen from magnetic field-induced reorientation of martensitic twin variants in a ferromagnetic shape memory Ni50Mn29Ga21 single crystal. The measured electrical resistivity shows large anisotropy and the measured MR value is as large as 25% over the wide temperature range of 230-315 K at a moderate magnetic field of 1.2 T. It is found that a proper combination of the initial state of martensitic twin variants and the direction and magnitude of applied magnetic field can give rise to either positive or negative MR value of ∼25%, thus allowing a periodic modulation of the MR effect in response to varying the spatial angle between the directions of applied magnetic field and electric current for every 180°.

Effect of additive elements on exchange coupling and spin reorientation transition of nanocrystalline single-phase Nd-Fe-B alloy

Nd12.3Fe81.7−xZrxB6.0 (x=0, 1.5) and (NdDyTb)12.3(FeZrNbCu)81.7−yCoyB6.0 (y=0, 12) ribbons were prepared by melt spinning at 22 m/s and subsequent annealing. The influences of Zr, Dy/Tb and Co substitutions on magnetic properties and spin reorientation transitions of nanophase Nd2Fe14B have been systematically investigated. Compared with Zr-free sample, the remanence, intrinsic coercivity and maximum energy product for Nd12.3Fe80.2Zr1.5B6.0 ribbon increase by 10.8 %, 17.8 % and 60.2 %, respectively. The significant improvement of magnetic properties originates from the finer grains of the sample by introducing Zr, which leads to the stronger exchange coupling between neighboring grains. The intrinsic coercivity for (DyTb)-substituted ribbon is significantly increased although the remanence is reduced, which could be compensated by the substitution of Co for Fe. The spin reorientation temperature Tsr of nanocrystalline Nd2Fe14B alloys was determined by measuring the ac magnetic susceptibility. It was found to be lower than that of bulk Nd2Fe14B. The substitutions of Zr, Dy/Tb and Co result in reduction of Tsr. The smaller the grain size, the lower the Tsr will be. Influence of spin reorientation on magnetization characteristics of nanophase Nd2Fe14B was discussed.

Triangular hysteresis loops in the spin-rotation region of orthoferrites

A theory is proposed that qualitatively explains the shapes of the hysteresis loops in orthoferrites within the temperature interval of the magnetic reorientation transition. Triangular loops result from the strong temperature dependence of both the magnetic moment and the magnetic domain wall structure.

Shape and strain-induced magnetization reorientation and magnetic anisotropy in thin film Ti/CoCrPt/Ti lines and rings

The contributions to the magnetic anisotropy of thin-film rings and lines of width 50 nm and above made from $\text{Ti}(5\text{ }\text{nm})/{\text{Co}}_{0.66}{\text{Cr}}_{0.22}{\text{Pt}}_{0.12}$ (10 and 20 nm)/Ti (3 nm) with a perpendicular magnetocrystalline anisotropy are investigated, using magnetic force microscopy to image the ac-demagnetized state. Four regimes of behavior were observed in both lines and rings. Samples with the largest widths $(>500\text{ }\text{nm})$ showed an out-of-plane maze domain structure typical of unpatterned films with domain widths of $\ensuremath{\sim}200\text{ }\text{nm}$. As the linewidth decreased, a ''bamboo'' domain structure forms in which the domain walls lie approximately perpendicular to the linewidth. Further linewidth decreases result in a reorientation to a net in-plane anisotropy perpendicular to the linewidth, and for the narrowest lines, $<200\text{-nm}$ wide, the anisotropy reorients in plane parallel to the line. The evolution of anisotropy is modeled in terms of contributions from magnetocrystalline, shape, and first- and second-order magnetoelastic terms, and good agreement with experiment is obtained, considering both bulk and surface anisotropy contributions.

Hard Magnets after Freezing of Spin Dynamics of Soft Magnets in Cobalt(II)–Radical Chain Compounds

Abstract Several metal–radical alternating chains ([Co(hfac)2(CnPNN)]) were prepared, and their crystal structures and magnetic properties were systematically studied, where CnPNN stands for a phenylnitronyl nitroxide ligand having a linear Cn alkoxy group at the ortho or para position. X-ray crystal structure analysis was successfully performed for [Co(hfac)2(p-C5PNN)] (p-5). The chain structure is similar to those of the known p-butoxy and o-ethoxy derivatives. Compound p-5 showed a very large coercive field of 51 kOe (4.1 MA m−1) and a saturation magnetization of 9.7 × 103 erg Oe−1 mol−1 at 6 K. o-Propoxy and o-pentoxy derivatives also exhibited large coercive fields of 50–54 kOe at 5–6 K. The activation energies of magnetization reorientation for the present compounds were estimated to be as large as 290–360 K from Arrhenius analysis using ac susceptibility data. In a temperature range of ca. 10–40 K, all of the present compounds behaved as very soft magnets, as indicated by no hysteresis in the ferromagnetic M–H curves. We propose a mechanism for the drastic soft–hard switch. The hard character grows immediately after freezing of the spin dynamics owing to the strong magnetic anisotropy.

Flux growth and spin reorientation in trigonal Nd 1 −xDy xFe 3(BO 3) 4 single crystals

Data on the stability area of trigonal Nd 1− x Dy x Fe 3(BO 3) 4 phase and the dynamics of labile phases in fluxes based on bismuth trimolibdate are presented. The procedure for crystal nucleation in a thin layer of flux cooled on a platinum rod is reported. Conditions for the growth of Nd 1− x Dy x Fe 3(BO 3) 4 single crystals on seeds are described. Temperature and field dependences of magnetization and specific heat for single crystals with х=0.1 and 0.25 were measured. It is shown that these single crystals, unlike easy-plane NdFe 3(BO 3) 4 and easy-axis DyFe 3(BO 3) 4 antiferromagnets, undergo spontaneous spin-flop phase transitions. Both spontaneous and magnetic field-induced spin reorientations were found to occur in one or several jumps depending on the Nd:Dy ratio in the crystals.

Magnetization reorientation inGaxMn1−xAsfilms: Planar Hall effect measurements

The process of magnetization reorientation in a ferromagnetic semiconductor GaMnAs films was investigated using planar Hall effect measurements. In addition to the well-known two-step switching behavior that occurs during this process, we have observed two additional distinct features in field scan data of the planar Hall resistance (PHR). First, the region of the external field required to begin and complete the reorientation of magnetization from one easy axis to another strongly depends on the direction of the applied field. And second, the maximum amplitude of PHR is significantly reduced during magnetization reversal when the applied field is oriented near one of the easy axes of the GaMnAs film. We provide an explanation of these phenomena using the magnetic field dependence of the free-energy density and assuming the coexistence of multiple domains with three different directions of magnetization in the sample.

Asymmetry in the reorientation process of magnetization for crossing the [11¯] and the [110] directions in Ga1−xMnxAs epilayers

The magnetization reversal processes of ferromagnetic Ga1−xMnxAs films with different Mn concentrations have been investigated using the planar Hall effect. The field scan of the planar Hall resistance (PHR) showed an asymmetric behavior for the reorientation of magnetization crossing the [11¯0] and the [110] directions. The magnetic anisotropy fields and the domain pinning field distributions of the films are obtained from the angle dependence of the PHR measurements. The magnetic anisotropy and pinning field distribution in the samples provided explanation for the observed asymmetric behavior in the magnetization reorientation process of Ga1−xMnxAs film.

Field‐induced magnetic reorientation in ultrathin ferromagnetic films: Phase diagrams

AbstractThe magnetic reorientation transition can be induced by applying an external transverse magnetic field. The magnetic field at which the magnetization reaches the field direction is called the reorientation magnetic field. The reorientation magnetic field in ultrathin ferromagnetic films is studied within the framework of the many‐body Green function theory. The model Hamiltonian includes a Heisenberg term with different surface exchange couplings with respect to the bulk one, a second‐order single‐ion anisotropy, and a transverse external magnetic field in the x‐direction. The single‐ion anisotropy term is treated by the generalized Anderson–Callen decoupling scheme. We investigate the reorientation magnetic field $h_{0R}^x$ as a function of the surface single‐ion anisotropic parameter $K_{2,S} (T \to 0)$ for different: film thicknesses L, surface exchange coupling enhancements ΔS, temperatures, T and as a function of the temperature for different film thicknesses L.

In-plane reorientation of magnetization in epitaxial exchange biased Fe/MnPd bilayers

We investigated the in-plane reorientation of magnetization in epitaxial Fe/MnPd bilayers. The samples with various thicknesses of MnPd and Fe layers present a conventional square and an unusual two-step exchange biased hysteresis loops at different temperature. The shape of the loops is reproduced using the coherent rotation model and including the relative orientation of the uniaxial anisotropy with respect to the exchange bias. The parallel and perpendicular uniaxial anisotropies in the ferromagnetic layer are linked to the aligned and the reoriented states, respectively. The magnetic reorientation between the aligned and the reoriented states, which is determined by the competition between the interface exchange coupling and the intrinsic uniaxial energies, is shown to be driven by the temperature, as well as the thickness of MnPd and Fe layers.

Spectroscopic study of the magnetic ordering in SmFe 3(BO 3) 4

SmFe 3(BO 3) 4 belongs to the family of rare-earth RFe 3(BO 3) 4 multiferroics. We present the first measurements of the optical spectra of SmFe 3(BO 3) 4 (on single crystals) and report the data on the crystal-field levels and their symmetries for the ground multiplet 6H 5/2 [0 ( Γ 4 ), 135 ( Γ 56 ), and 220 ( Γ 4 ) cm −1] and on the ground-state exchange splitting (13.2 cm −1 at 1.7 K), important to interpret the magnetic, magnetoelectric, and thermodynamic properties of this compound. The observed exchange splitting of the Sm 3+ Kramers doublets manifests the magnetic ordering in SmFe 3(BO 3) 4 at 32 ± 1 K . From a comparison of the temperature-dependent spectra of the Sm 3+ ion in SmFe 3(BO 3) 4 and in Nd–Gd mixed iron borate where the magnetic spin reorientation has been found earlier, we suggest the easy-plane arrangement of the Fe spins in SmFe 3(BO 3) 4. The samarium iron borate preserves the R32 ( D 3 7 ) crystal structure at all the investigated temperatures (down to 1.7 K).

FEM simulation of a Ni–Mn–Ga film bridge actuator

This paper explores the use of the magnetic shape memory (MSM) effect for actuation of a free-standing film bridge by coupled finite element simulations in the framework of a Gibbs free energy model. The material of interest is a Ni–Mn–Ga film showing a 10M tetragonal martensite structure. Two actuation mechanisms (I and II) are considered depending on the magnetic field configuration and the method of stress biasing. In mechanism I, tensile stress is generated by a mechanical point load in out-ofplane direction applied in the film bridge center. In this case, magnetic field-induced reorientation (MIR) requires a magnetic field along the long axis of the film bridge. We show that only a partial MIR effect is possible for mechanism I as the magnetic field has to counteract the applied mechanical force. For mechanism II, variant reorientation and stress biasing are accomplished by different magnetic fields applied along the short axis of the film bridge allowing complete MIR.

Magnetization reorientation in antiferromagnetically coupled Co films and (Co/Pd) multilayers

Magnetization reversal of antiferromagnetically coupled (AFC) Co thin film and (Co/Pd) multilayers has been investigated. For 5 Å thick Co layer, a high exchange coupling field (Hex) of 7.8 kOe at room temperature and 8.5 kOe at 5 K was measured from the shift in the hysteresis loop. This high value of Hex was accompanied by a magnetization reorientation from in-plane to out-of-plane of the thin Co. When Co thickness increases, the magnetization reorientation was not possible. This unusual high Hex can be used to stabilize AFC structures when the patterning to nanoscale size is needed.