Magnetic Reorientation Transition Research Articles

Magnetic transitions and butterfly-shaped hysteresis of Sm-Fe-Al-based perovskite-type orthoferrite

Al doped SmFeO3 (SmFe1–xAlxO3; 0.0≤x≤0.15; step 0.05) were prepared by double sintering ceramic technique. The obtained samples were crystallized in single phase structure except the sample with x=0.15. The unit cell volume was found to decrease with increasing Al substitution in orthoferrite. The effective magnetic moment (<ι>μ</ι>eff) and the Curie constant (C) were calculated from the reciprocal of the molar magnetic susceptibility (χ−1) versus absolute temperature plot and found to attain maximum value for the parent sample. The magnetic behavior showed two different magnetic transitions, viz, Néel temperature (TN) and spin reorientation (TSR) transitions. The M-H hysteresis loop of the parent sample took butterfly-shape as a result of different contributions anisotropies. From the magnetic properties measurements, it was obviously found that B-site cation dilution resulted in a drastic decrease in the magnetization. Surprisingly large value of the coercive field was obtained for the undoped sample; Hc=6198.8 Oe. Based on the mentioned results, one can recommend the use of such orthoferrite in magnetic recording media and as pining layer in spin valve for spintronic applications.

Magnetocrystalline anisotropic effect inGdCo1−xFexAsO(x=0,0.05)

From a systematic study of the electrical resistivity $\rho(T,H)$, magnetic susceptibility $\chi(T,H)$, isothermal magnetization $M(H)$ and the specific heat $C(T,H)$, a temperature-magnetic field ($T$-$H$) phase diagram has been established for GdCo$_{1-x}$Fe$_x$AsO ($x = 0$ and $0.05$) polycrystalline compounds. GdCoAsO undergoes two long-range magnetic transitions: ferromagnetic (FM) transition of Co $3d$ electrons ($T_\textup{C}^\textup{Co}$) and antiferromagnetic (AFM) transition of Gd $4f$ electrons ($T_\textup{N}^\textup{Gd}$). For the Fe-doped sample ($x=0.05$), an extra magnetic reorientation transition takes place below $T_\textup{N}^\textup{Gd}$, which is likely associated with Co moments. The two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic (FIM) behavior in the magnetic susceptibility. Upon decreasing the temperature ($T < T_\textup{C}^\textup{Co}$), the magnetocrystalline anisotropy breaks up the FM order of Co by aligning the moments with the local easy axes of the various grains, leading to a spin reorientation transition at $T_\textup{R}^\textup{Co}$. By applying a magnetic field, $T_\textup{R}^\textup{Co}$ monotonically decreases to lower temperatures, while the $T_\textup{N}^\textup{Gd}$ is relatively robust against the external field. On the other hand, the applied magnetic field pulls the magnetization of grains from the local easy direction to the field direction via a first-order reorientation transition, with the transition field ($H_\textup{M}$) increasing upon cooling the temperature.

Strain induced magnetic domain evolution and spin reorientation transition in epitaxial manganite films

The evolution of magnetic domain structure in epitaxial La0.625Ca0.375MnO3 films on (001) NdGaO3 is monitored as a function of temperature and magnetic field using magnetic force microscopy. We see two distinct regions of magnetic orientational order: one in-plane displaying contrast-less image and the other tilted away from the film plane forming a distinct stripe pattern. A strong domain splitting is observed at the boundary of two regions, which is resilient to reorientation with temperature and magnetic field. We propose a model magnetic free energy functional to explain the mechanism of domain splitting seen in manganite films.

Microwave calorimetry using X-rays

An alternative approach for microwave calorimetry is proposed which relies on the synchrotron radiation powder diffraction technique as well as on the Grüneisen formalism for the analysis of thermal expansion. Cobalt was selected as suitable magnetic material for the present evaluation of the method. First results are reported concerning the calorimetric assessment of the HCP (hexagonal close-packed) to FCC (face centered cubic) transition of cobalt from in situ time-resolved X-ray diffraction experiments performed during magnetic (H-field) microwave heating. The X-ray calorimetry method yields specific heat capacity estimations that compare well with results from conventional differential scanning calorimetry measurements. In the presence of the 2.45 GHz microwave H-field, an ‘anomalous’ behaviour of the heat capacity across the structural phase transition is detected, which can be correlated with the magnetic spin reorientation transition of cobalt in the same temperature range.

Crystal structure and magnetic properties of R5Sn4 alloys, where R is Tb, Dy, Ho, and Er

Crystal structure and magnetic properties of R5Sn4 alloys with R = Tb, Dy, Ho, and Er have been studied. R5Sn4, R11Sn10, and R5Sn3 phases coexist in the annealed alloys and the content of 11:10 and 5:3 phases varies between 9 and 17 wt.%. The R5Sn4 major phase has Sm5Ge4-type orthorhombic structure with space group Pnma. Tb5Sn4 has a complex magnetic structure, spin re-orientation and ferrimagnetic-paramagnetic phase transitions occur at ∼54 and ∼84 K, respectively. For Dy5Sn4, Ho5Sn4, and Er5Sn4, the antiferromagnetic-paramagnetic phase transitions occur at about 22, 15, and 8 K, respectively. The magnetic entropy changes (−ΔSM) of all alloys are negative at low temperature and changes to positive at higher temperatures, which could be attributed to the change of magnetic states.

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.

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.

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.

Competing anisotropies in bcc Fe81Ni19/Co(001) superlattices

A magnetization reorientation transition has been observed in Fe81Ni19/Co(001) superlattices by means of magneto-optical Kerr effect and magnetic force microscopy measurements. The transition is driven by the variation of the interface density. First-principles calculations are combined with a linear stability analysis of the Landau–Lifshitz equation to clarify the mechanism that drives the transition. We are able to identify it as being driven by competing interface in-plane and uniaxial bulk out-of-plane anisotropies. The origin of the bulk anisotropy is attributed to tetragonal distortions experimentally observed in these superlattices.

Temperature dependent dynamic and static magnetic response in magnetic tunnel junctions with Permalloy layers

Ferromagnetic resonance and static magnetic properties of CoFe∕Al2O3∕CoFe∕Py and CoFe∕Al2O3∕CoFeB∕Py magnetic tunnel junctions and of 25nm thick single-layer Permalloy (Py) films have been studied as a function of temperature down to 2K. The temperature dependence of the ferromagnetic resonance excited in the Py layers in magnetic tunnel junctions shows “kneelike” enhancement of the resonance frequency accompanied by an anomaly in the magnetization near 60K. We attribute the anomalous static and dynamic magnetic response at low temperatures to interface stress induced magnetic reorientation transition at the Py interface which could be influenced by dipolar soft-hard layer coupling through the Al2O3 barrier.

Influence of grain boundaries on the magnetization reorientation transition in ultrathin films

A magnetic reorientation transition is studied in situ in model experimental systems of Co grown on Au(111) and Au(788). Results from magneto-optic measurements and scanning tunneling microscopy, obtained on these two different self-ordered systems, demonstrate that grain boundaries formed during the film growth strongly influence the magnetic properties of the films. The nanometer scale of the magnetization reversal at the transition makes the temperature an essential parameter to predict experimental features observed in the magnetic reorientation transition.

Changes in magnetic properties of ultrathin cobalt films as induced by Mo, V, Au overlayers

Magneto-optical (based on polar and longitudinal Kerr effects) and FMR studies were carried out on ultrathin cobalt films deposited on Au(1 1 1) and covered with either a V or Mo cover layer, and finally by an Au protective layer. Co thickness at which the magnetization reorientation transition from an easy axis into an easy plane occurs is influenced by the type of cover layer. Even 0.1 nm thick V-coverage is efficient enough to induce full reorientation. A strong decrease of domain structure size (down to the submicrometer scale) was observed while approaching reorientation phase transition (RPT). The influence of a Mo-overlayer on Co in-plane magnetic anisotropy is shown to be much weaker in comparison to the influence of Mo-buffer.

The magnetic spin-reorientation transitions in the RGa (R=rare earth) intermetallic compounds studied by measurements of the hyperfine interactions of the 119Sn probe atoms

The magnetic and electric hyperfine interactions of the probe nucleus 119Sn on the Ga site of the ferromagnetic rare-earth (R) gallium compounds RGa (R=Pr–Er) have been investigated by Mössbauer spectroscopy technique. For all of the compounds, the directions of the magnetic moments of the R 3+ ions have been determined as a function of temperature in the range from 5 K to T C. For NdGa, SmGa, HoGa, and ErGa compounds, the magnetic reorientation transitions due to the competition between the exchange interaction and the interaction with crystal field have been investigated. At high temperatures, when the electric interaction dominates, the orientation of the magnetic moments is unambiguously determined by the sign of the quadrupole moment of 4f shell of the R 3+ ion. With decreasing temperature, the magnetic moments rotate gradually from the bc-plane toward the crystallographic a-axis. In the temperature range 5 K⩽ T<100 K, the ferromagnetic structure of the GdGa compound is noncollinear. At 5 K the magnetic moments of the Gd 3+ ions point in two distinct directions with respect to the crystallographic a-axis ( θ 1 ≈ 30 ° and θ 2 ≈ 60 ° ).

The magnetic reorientation transition in thin ferromagnetic films treated by many-body Green's function theory

This contribution describes the reorientation of the magnetization of thin ferromagnetic Heisenberg films as function of the temperature and/or an external field. Working in a rotating frame allows an exact treatment of the single-ion anisotropy when going to higher order Green's functions. Terms due to the exchange interaction are treated by a generalized Tyablicov (RPA) decoupling.

Magnetic properties of (Ga,Mn)As

A summary of experimental findings and theoretical modelling of micromagnetic properties of zinc-blende ferromagnetic semiconductor (Ga,Mn)As is presented. It is shown that the Zener p–d model explains quantitatively observed Curie temperatures in compensation free samples and that major strain-related effects are correctly accounted for, including the presence of the magnetization reorientation transition, observed as a function of hole concentration and temperature. It is evidenced that a presence of a small trigonal distortion could account for both the presence and properties of uniaxial in-plane magnetic anisotropy.

Magnetic reorientation transitions along the crossover from one-dimensional to two-dimensional transition-metal nanostructures

The crossover from one-dimensional (1D) to two-dimensional (2D) magnetic anisotropy of transition-metal nanostructures is investigated theoretically. The in-plane and off-plane magnetic anisotropy energies (MAEs) of stripes are determined systematically as a function of stripe width $m$ thereby bridging the gap between the 1D chain $(m=1)$ and the 2D monolayer $(m=\ensuremath{\infty})$. The MAEs are in general enhanced upon reduction of dimensionality. They oscillate as a function of $m$, especially in the ultranarrow limit $(m\ensuremath{\leqslant}10)$ where the easy and hard axis depend strongly on $m$ and on the considered transition metal. The convergence to the 2D limit with increasing $m$ is found to be relatively slow, particularly concerning the in-plane anisotropy. Comparison between square and triangular lattices demonstrates the importance of the transversal structure of the stripe. The local contributions to the MAE and the correlation between MAE and average orbital moments are discussed. The magnetization-reorientation transitions observed as a function of $m$ reveal the remarkable richness and complexity of the crossover from 1D to 2D magneto-anisotropic behavior.

Magnetic Properties of (Ga,Mn)As

AbstractFor Abstract see ChemInform Abstract in Full Text.

Magnetization reversal of CrO2 nanomagnet arrays

We report on fabrication techniques and the study of magnetic behavior of submicron-sized ferromagnetic CrO2 arrays. Periodic rectangular arrays composed of circular dots with diameters ranging from 100nmto2μm were fabricated by electron-beam lithography and reactive ion etching of CrO2 epitaxial thin films. The magnetization reversal of the nanomagnet arrays was measured by the magneto-optical Kerr effect, with hysteresis characteristics found to be sensitively dependent on array details, film thickness, and its built-in strain. A rich variety of behaviors such as single domain, probable multidomain, as well as magnetization reorientation transition were observed, with magnetostatic interaction between the neighboring dots affecting the collective behavior of the nanomagnet arrays.

Magnetic Properties of (Ga,Mn)As

Abstract A summary of experimental findings and theoretical modelling of micromagnetic properties of zinc-blende ferromagnetic semiconductor (Ga,Mn)As is presented. It is shown that the Zener p–d model explains quantitatively observed Curie temperatures in compensation free samples and that major strain-related effects are correctly accounted for, including the presence of the magnetization reorientation transition, observed as a function of hole concentration and temperature. It is evidenced that a presence of a small trigonal distortion could account for both the presence and properties of uniaxial in-plane magnetic anisotropy.