Spin-wave resonance and magnetic anisotropy in FePt thin films

Ferromagnetic Resonance (FMR) measurements at room temperature and X-band microwave frequency were performed on polycrystalline FePt thin films with face-centered cubic structure. With the external field perpendicular to the film plane, the absorption fields H_n of the odd and even spin-wave resonance modes n detected for the Fe_0.44Pt_0.56(45 nm)/Si(1 0 0) and Fe_0.51Pt_0.49(105 nm)/Pt(55 nm)/MgO(1 0 0) films, were found to obey the well-known H_n×n² ratio, giving for these films the exchange stiffness constant values of 3.9×10⁻⁸ and 4.4×10^–7 erg/cm, respectively. The study of the out-of-plane angular dependence of the absorption field of the uniform FMR mode allowed the measurement of the effective magnetic anisotropy constants of 5.3×10⁶ , 6.4×10⁶ , and 6.7×10⁶ erg/cm³, related, respectively, to the [1 1 1], [1 0 0], and [1 1 0] textures present in the films.     

Magnetic anisotropy and anisotropic ballistic conductance of thin magnetic wires

The magnetocrystalline anisotropy of thin magnetic wires of iron and cobalt is quite different from the bulk phases. The spin moment of monatomic Fe wire may be as high as 3.4 μ_B, while the orbital moment as high as 0.5 μ_B. The magnetocrystalline anisotropy energy (MAE) was calculated for wires up to 0.6 nm in diameter starting from monatomic wire and adding consecutive shells for thicker wires. I observe that Fe wires exhibit the change sign with the stress applied along the wire. It means that easy axis may change from the direction along the wire to perpendicular to the wire. We find that ballistic conductance of the wire depends on the direction of the applied magnetic field, i.e. shows anisotropic ballistic magnetoresistance. This effect occurs due to the symmetry dependence of the splitting of degenerate bands in the applied field which changes the number of bands crossing the Fermi level. We find that the ballistic conductance changes with applied stress. Even for thicker wires the ballistic conductance changes by factor 2 on moderate tensile stain in our 5×4 model wire. Thus, the ballistic conductance of magnetic wires changes in the applied field due to the magnetostriction. This effect can be observed as large anisotropic BMR in the experiment.     

Experimental determination of limit angle of helical anisotropy in amorphous magnetic microwires

The influence of the torsion stress on the surface magnetic structure in Co-rich amorphous glass covered microwires has been investigated. The limit angle of the surface helical anisotropy induced by the torsion stress has been determined in agreement with the model which considers the torsion stress as a interference of two tensile stresses of opposite signs directed at 45° and 135° relative to the longitudinal axis of the wire.     

Theoretical investigation of magnetic property in paramagnetic neodymium gallium garnet under high magnetic field

The magnetic property in neodymium gallium garnet (NdGaG) is studied by the quantum theory. The ground configuration split states are calculated taking into account the spin–orbit interaction and crystal field effect. Taking account of the Nd–Nd exchange interaction, a good agreement between experimental and theoretical values can be obtained for the variation of the magnetic moment with the external magnetic field under “extreme” conditions (low temperature and high magnetic field). Moreover, the temperature dependence of magnetic moment and the magnetic susceptibility χ is also discussed. Above 30 K, the magnetization (M) shows a linear field (H_e) dependence.     

Magnetoresistivity of bismuth films in magnetic fields to 19 Tesla

The longitudinal and transverse magnetoresistance to fields of 19 Tesla and 4.2 K has been measured for bismuth films ranging in thickness from 0.01 to 2 m. We have observed a pronounced maximum in the longitudinal magnetoresistance which is thickness dependent. We compare these results with classical size effect theories for the longitudinal magnetoresistance in terms of magnetic-field-dependent electron scattering at grain and film boundaries. Measurements of the longitudinal magnetoresistance to 38 Tesla show a quenching of the classical size effect in the high field limit. This result strongly supports our analysis.Work supported by NSF grant #DMR 8113456Supported by the National Science Foundation     

Modifications in magnetic anisotropy of M—type strontium hexaferrite crystals by swift heavy ion irradiation.

Using vibrating sample magnetometery (VSM) 50 MeV Li³⁺ ion irradiation effects on magnetic properties of single crystals of SrGa_xIn_yFe_12−(x+y)O₁₉ (where x=0, 5, 7, 9; y=0, 0.8, 1.3, 1.0), are reported. The substitution of Ga and In in strontium hexaferrite crystals decreases the value of magnetization sharply, which is attributed to shifting of collinear magnetic order to a non-collinear one. Reduction of magnetization is also explained to be as a result of the occupation of the crystallographic sites of Fe³⁺ by Ga³⁺ and In³⁺. The Li³⁺ ion irradiation decreases the value of magnetization, irrespective of whether the crystals are Ga–In substituted or unsubstituted crystals of SrFe₁₂O₁₉. The result is interpreted in terms of the occurrence of a paramagnetic doublet in crystals replacing magnetic sextuplet as a result of irradiation. Substitution of Ga–In in Strontium hexaferrite decreases the value of anisotropy constant. Irradiation with Li³⁺ ions increases the values of anisotropy field for both substituted as well as unsubstituted crystals. Substitution with Ga–In also decreases the Curie temperature (T_c) but the irradiation with Li³⁺ ions does not affect the curie temperature of either Ga–In substituted or pure SrFe₁₂O₁₉ crystals.     

Phase segregation and exchange biasing in TbFeCo films with perpendicular anisotropy

A TbFeCo film was deposited by DC magnetron sputtering and studied by transmission electron microscopy, polar and longitudinal magneto-optical Kerr effect, and magnetometry measurements. Transmission electron microscopy has shown the existence of lateral compositional inhomogeneity. Magneto-optical measurements have shown that the initial layer at the bottom consists of only magnetic perpendicular component and the top surface layer has a compositional inhomogeneity and consists of in-plane components and perpendicular one. The perpendicular components in the bottom and the surface layers have identical composition. Two in-plane components have been shown by magnetometry measurements. It is shown that phase segregation exists in the TbFeCo film and possible form of compositional inhomogeneity has been discussed. The two in-plane components are exchange coupled with a magnetization off-alignment of 35°. For the soft in-plane component, the in-plane and out-of-plane angular dependence of the exchange biasing is similar to those of the conventional one. Within temperatures from 100 to 300 K, the exchange field and the coercivity are both linear functions of temperature.     

Magnetic anisotropy engineering in square magnetic elements

Square magnetic elements with side in the 100–500 nm range have been fabricated using the focused ion beam (FIB) milling technique from a 10 nm thick, single-crystal Fe film, epitaxially grown on MgO(0 0 1). Thanks to the good crystal quality of the film, magnetic elements with well-defined magnetocrystalline anisotropy have been prepared, while the fine control of the size and shape of the magnets allows for the effective engineering of the anisotropic behavior of the magnetostatic energy that determines the so-called configurational anisotropy. Micromagnetic calculations and experiments show that the angular dependence of the transverse susceptibility has a strong dependence on the material parameters as well as on the static applied field. This allows the effective engineering of the total anisotropy of the magnets.     

Magnetic anisotropy of Tb1−xGdxMn6Sn6 compounds

Magnetization curves of Tb_1−xGd_xMn₆Sn₆ compounds (0?x?1) have been measured for aligned powder samples in the temperature range 4.2–300 K in pulsed magnetic fields up to 30 T. Temperature and concentration dependences of the magnetocrystalline anisotropy constants K₁ and K₂ and concentration dependence of the temperature of spontaneous spin-reorientation transition have been determined. Using these data, we estimated the contribution of the manganese and terbium atoms to the magnetic anisotropy of Tb_1−xGd_xMn₆Sn₆ and analyzed the origin of the appearance of field-induced first-order magnetic phase transition in these compounds.     

Magnetostriction and magnetic anisotropy of (Sm,Ce)Fe2 compounds

The structural, magnetic and magnetostrictive properties of Sm_1−xCe_xFe₂ (0≤x≤1) Laves compounds have been investigated. The magnetostriction coefficient λ₁₁₁ and the anisotropy of the Sm_1−xCe_xFe₂ compounds decrease with increasing Ce content. The decreasing of anisotropy results in an enhancement of the anisotropic magnetostriction at low magnetic fields for the compounds with a small amount of Ce substituted for Sm.     

Determination of anisotropy constants of protein encapsulated iron oxide nanoparticles by electron magnetic resonance

Angle-dependent electron magnetic resonance was performed on 4.9, 8.0, and 19 nm iron oxide nanoparticles encapsulated within protein capsids and suspended in water. Measurements were taken at liquid nitrogen temperature after cooling in a 1 T field to partially align the particles. The angle dependence of the shifts in the resonance field for the iron oxide nanoparticles (synthesized within Listeria-Dps, horse spleen ferritin, and cowpea chlorotic mottle virus) all show evidence of a uniaxial anisotropy. Using a Boltzmann distribution for the particles’ easy-axis direction, we are able to use the resonance field shifts to extract a value for the anisotropy energy, showing that the anisotropy energy density increases with decreasing particle size. This suggests that surface anisotropy plays a significant role in magnetic nanoparticles of this size.     

Magnetic anisotropy of textured CrO2 thin films investigated by X-ray magnetic circular dichroism

Highly a-axis-textured CrO₂ films have been deposited on Al₂O₃ (0001) substrates by chemical vapor deposition. CrO₂ has been found to have highly a-axis (010)-oriented columnar growth on a Cr₂O₃ (0001) initial layer. The six-fold surface symmetry of the Cr₂O₃ initial layer leads to three equivalent in-plane orientations of the a-axis-oriented CrO₂ unit cell. We report Cr L_2,3 X-ray magnetic circular dichroism data along the surface normal and at 60° off-normal sample orientation. For a 60° sample alignment, a strong increase of the projected orbital moment could be observed for unoccupied majority t_2g states using moment analysis. Therefore, the c axis is identified as the intrinsic magnetic easy axis of CrO₂. In addition, a small spin moment and a very strong magnetic dipole term T_z have been found. Received: 8 January 2002 / Accepted: 8 January 2002     

Magnetic anisotropy in colossal magnetoresistive FeCr2S4 single crystals

The magnetic properties of spinel FeCr₂S₄ single crystals were investigated by ferromagnetic resonance (FMR). The FMR spectrum displays a single absorption line in the whole temperature range measured for both H∥(111) and H⊥(111). With decreasing temperature, the line with H∥(111) shifts to lower fields, while that with H⊥(111) shifts to higher fields. By superposing all the FMR spectra measured in different directions at 110 K, a double-peak is obtained, which clarifies the origin of the FMR double-peak in polycrystalline sample. By taking account of magnetic anisotropy and demagnetizing effect, the orientation dependence of resonance field is well fitted. It is found that the magnetic anisotropy strengthens with decreasing temperature; however, it has no evident influence on transport and colossal magnetoresistance behavior.     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

The large anisotropy of the magnetic and transport properties in the Ba5Co5ClO13 single crystal

In this Letter, the single crystals of Ba₅Co₅ClO₁₃ were grown by the flux method successfully. Their structure, magnetic and transport properties were studied. A large anisotropy of the magnetic and transport properties has been detected in this compound. Below the TN∼108 K, the magnetic susceptibility exhibits an antiferromagnetic peak in χc and an upturn transition in χab. We suggest that this behavior is consistent with the competition of the ferromagnetic (FM) intra-blocks coupling and antiferromagnetic (AFM) inter-blocks coupling in this compound. The temperature dependence of the resistivity displays a hump in ρab and a kink in ρc around TN, suggesting the strong coupling between the transport and magnetic properties. Above and below the transition, the transport properties in ab plane follow the three-dimensional (3D) variable range hopping (VRH) mechanism.     

Out-of-plane exchange bias and magnetic anisotropy in MnPd/Co multilayers

Magnetic and structural properties in [MnPd/Co]₁₀ multilayers deposited onto Si(1 1 1) substrates have been investigated. The dependences of anisotropy and exchange bias on the thicknesses of both MnPd and Co layers have been studied. In most of the samples, the out-of-plane magnetic anisotropy and both large out-of-plane and in-plane exchange biases have been observed at cryogenic temperature below the blocking temperature T_B≈240 K. With appropriate MnPd and Co thicknesses, we have obtained samples with a large out-of-plane exchange bias along with a large out-of-plane magnetic anisotropy. The origin of the out-of-plane magnetic anisotropy in the samples has been suggested to be due to the formation of CoPd interfacial alloys which have tensile in-plane strains, while the spin structure of the antiferromagnetic layer at the interface which is believed to be responsible for exchange bias may be the same as that of the bulk material. Also, the present study shows that the interplay between the out-of-plane magnetic anisotropy and exchange bias is evident in our multilayers and plays an important role in the out-of-plane exchange-bias mechanism.     

Investigation of temperature dependence of surface anisotropy field and constant in the system of BaFe12O19 nanocrystals

We show a possibility to determine the parameters characterizing surface magnetic anisotropy of platelet BaFe₁₂O₁₉ nanocrystals with thickness comparable to the magnetoperturbed surface layer of high-anisotropic hexagonal crystal. Taking into account the above-mentioned specific character of the particles, we introduce the constant K_S as the surface anisotropy energy per unit volume of near-surface layer. Temperature dependence of the surface anisotropy field as one of the components of effective anisotropy was corrected taking into account the thermal fluctuations. Discovered anomaly initiated calculation of the surface anisotropy constant with different approaches for two different temperature ranges. Analyzing the K_S sign changes with temperature, we conclude about the transformation of surface anisotropy from plane to axis type.     

Magnetic anisotropy in icosahedral cobalt clusters

Magnetic anisotropy has been measured in multiply twinned, icosahedral cobalt clusters. It is found that the low-temperature magnetization of deposited cluster layers is well defined with the Stoner–Wohlfarth model by averaging over clusters with a range of anisotropy energy. Anisotropy energy calculation based on Néel's pair model shows that the icosahedral structure and the layer-by-layer growth of the clusters induce oscillations of the magnetic anisotropy as a function of the filling of the outer surface of the particle. The magnetization measurement at room temperature indicates a weakly correlated cluster glass, as deduced from the approach to saturation that is well described with 2D random anisotropy model.     

Ground-state phase diagrams of Ising metamagnet in a mixed longitudinal and transverse magnetic field

The ground-state magnetic properties of a two-sublattice Ising metamagnet in a mixed longitudinal and transverse magnetic field are studied within the effective-field theory. A parameter j₂=J₂/J₁ is introduced, which reflects the strength ratio of spin coupling between adjacent planes and in each plane. In addition to the second-order transition lines, the first-order transition lines are also presented, since the ground-state energy can be calculated numerically. The ground-state phase diagrams in h_x–h_z are presented. The results show that when j₂<0 the phase transition of the system is always first-order for h_x<2.751, and when −1000?j₂<0 it is always second-order for h_x>4.36. For the given h_x (0<h_x<14.71), the longitudinal critical magnetic field increases as j₂ decreases. The reentrant phenomenon occurs in the range of j₂<−11.89, h_x>14.71. There is no fourth-order critical point in the phase diagrams given by using EFT as found by using mean field theory (MFT).     

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.