Magnetic properties of FeSiB thin films displaying stripe domains

FeSiB amorphous thin films with thicknesses from 25 to 600 nm have been produced by rf sputtering on Si₃N₄ substrates. A spin reorientation transition has been observed on the as-prepared samples, as a function of thickness and temperature. Spin reorientation transition is shown to depend on the thermal treatments to which the as-prepared samples have been submitted. Static hysteresis loops obtained as a function of temperature, and magnetic force microscopy images taken at room temperature at the remanence and as a function of an applied magnetic field, have been employed to study the magnetic domain configuration of all the samples, and to see how it is affected by sample thickness, measurement temperature and annealing conditions.     

Preparation and magnetoresistance of thin CrO2 films

Highly textured chromium dioxide (CrO₂) films have been deposited on Al₂O₃ single-crystal substrates by atmospheric pressure chemical vapor deposition method (CVD). X-ray diffraction patterns show that the CrO₂ films are (1 0 0)-oriented on Al₂O₃ (0 0 1) substrates, and are (1 0 1)-oriented on Al₂O₃ (0 1 2) substrates. Scanning electron microscopy images indicate that the (1 0 0)-oriented CrO₂ films grown on Al₂O₃ (0 0 1) substrates have smoother surface and better qualities than that grown on Al₂O₃ (0 1 2) substrate. At room temperature, the magnetoresistance of the (1 0 0)- and (1 0 1)-oriented CrO₂ films are nearly same, and both show a linear dependence on applied magnetic field. While at 80 K, the (1 0 1)-oriented CrO₂ films show a much larger magnetoresistance compared with the (1 0 0)-oriented CrO₂ films. The reasons are briefly discussed.     

Enhancement in soft magnetic and ferromagnetic ordering behaviour through nanocrystallisation in Al substituted CoFeSiBNb alloys

The effect of substituting Al for Si in Co₃₆Fe₃₆Si_4−xAl_xB₂₀Nb₄, (X=0, 0.5, 1.0, 1.5, 2.0 at%) alloys prepared in the form of melt-spun ribbons have been investigated. All the alloys were amorphous in their as-cast state. The onset of crystallization as observed using differential scanning calorimetry (DSC) was found to rise at low Al content up to X=1 at% beyond which there was a decreasing trend. The alloys also exhibited glass transition at ‘T_g’. Microstructural studies of optimally annealed samples indicated finer dispersions of nanoparticles in amorphous matrix which were identified as bcc-(FeCo)Si and bcc-(FeCo)SiAl nanophases by X-ray diffraction technique. Alloy with optimum content of Al around X=1 at% exhibited stability in coercivity at elevated temperatures. Though Al addition is known to lower magnetostriction, such consistency in coercivity may also be attributed towards lowering in the nanoparticle size compared to X=0 alloy. In the nanostructured state, the alloy containing optimum Al content (X=1) exhibited further enhancement in ferromagnetic ordering or the Curie temperature by 100 K compared to alloy without Al. Such addition also attributed to better frequency response of coercivity and low core losses.     

Magnetic properties of 3d transition metal monolayers on metal substrates

We report results of systematic calculations for magnetic properties of 3d transition metal monolayers on Pd(001) and Ag(001). We find large similarities to interactions of magnetic 3d impurities in the bulk. Therefore the overlayer results are supplemented with results for 3d dimers in Cu, Ag, and Pd. Differences between the two classes of systems are utilized to reveal the interaction within the overlayers and between overlayers and substrates. In virtually all cases we find both ferromagnetic and antiferromagnetic solutions, showing large magnetic moments and similar densities of states. From the trend of the calculations we conclude that V, Cr, and Mn overlayers favor the antiferromagnetic c(2×2) structure, while Ti, Fe, Co, and Ni prefer the ferromagnetic one.     

Magnetic behavior of MnAs precipitates in Ga1−xMnxAs diluted magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x≈4.7–5.5%) were grown on (1 0 0) GaAs substrates by molecular beam epitaxy. These p-type (Ga,Mn)As films were revealed to have a ferromagnetic structure and ferromagnetism is observed up to a Curie temperature of 318 K, which is ascribed to the presence of MnAs secondary magnetic phases within the film. It is highly likely that the phase segregation occurs due to the high Mn cell temperature around 890–920 °C, as it is well established that GaMnAs is unstable at such a high temperature. The MnAs precipitate in the samples with x≈4.7–5.5% has a Curie temperature T_c≈318 K, which was characterized from field-cooled and zero-field-cooled magnetization curves.     

Magnetic dipolar interaction in ultrathin films: Structural and size effects

A previously introduced formalism for calculating magnetic dipolar anisotropy energy ΔU in atomic layered structures is further developed. Numerical results are presented for ultrathin films with different close-packed (face centered cubic (FCC) [1 1 1]) and non-close-packed (FCC [0 0 1] and body centered cubic (BCC) [0 0 1]) structures. Structural effects become apparent in the magnetocrystalline dipolar anisotropy energy ΔU_L when the ratio between the interlayer separation c and the 2D lattice constant a is changed. Despite the long-range character of the dipolar interaction, it is shown that the number of significantly interacting layers, conventially called coupled layers, is limited and depends on the structural aspect ratio c/a. The slope in the observed linear dependence between ΔU_L and the inverse of the film thickness t is explained by the number of the so-called coupled layers, and not by a surface contribution to volume values. Size effects appearing in ΔU are unambiguously distinguished from structural effects. Effective anisotropy energy ΔU_eff and ΔU are presented for Co [0 0 0 1] and Ni [0 0 1] ultrathin films. It is verified that the dipolar interaction makes an important contribution to ΔU_eff, but the spin reorientation transition is determined by non-dipolar interactions. The former favors the magnetization switching only when the size aspect ratio d/t, with d the characteristic lateral dimension of the film, is sufficiently small. Applications to other layered arrays of magnetic dipoles are straightforward.     

Characterization of individual ferromagnetic nanowires by in-plane magnetic measurements of arrays

We present a phenomenological model for the interaction field in ferromagnetic nanowire arrays and use it to obtain the effective anisotropy field of individual nanowires, from the in-plane saturation field of the array. In contrast to other methods which may be used to estimate this parameter, the proposed strategy requires no knowledge of the saturation magnetization nor of the nanowire radius. Applied to three arrays of different compositions, this approach yields an equivalent anisotropy field of the individual nanowires approximately equal to _Ms/2$M_{\mathrm{s}}/2$, indicating that its origin is the demagnetizing field of the wire.     

MOCVD growth, structure and magnetic properties of Fe films grown on GaAs (001) substrates

Thin iron films have been grown on (001) GaAs substrates by low pressure metal organic chemical vapor deposition (LP-MOCVD) at different temperatures with the pressure of 150 Torr. X-ray diffraction (XRD) analysis showed that all films have only one strong diffraction peak (110). The surface of Fe film became smooth with increasing the growth temperature. Magnetization measurements showed that the Fe films grown at different temperatures were ferromagnetic with easy axis parallel to the film surface and hard axis perpendicular to the substrates. The field dependence of magnetization along two axes showed a remarkable difference, implying that the samples have strong magnetic anisotropy. Furthermore, when the applied magnetic field is perpendicular to the Fe surface, a sharp jump in the hysteresis loop could be observed, followed by a broad shoulder, which is related to the interface effect, the existence of carbon and the formation of 180^°/90^° magnetic domains.     

Calculations of localized modes on surface and impurity layer embedded in a semi-infinite Heisenberg ferromagnet

We investigate the magnetic excitations for the magnetic problem arising from the absence of magnetic translation symmetry in one dimension due to the presence of an impurity layer embedded within a semi-infinite ferromagnet. A Heisenberg model is employed to investigate the possibility that localized modes can occur with an impurity layer implanted within a semi-infinite ferromagnet. No electronic effects are considered. The theoretical approach employs the matching procedure in the mean field approximation and determines the propagating and evanescent spin amplitude fields including the contribution due to an applied field. The results are used to calculate the energies of localized modes associated with the impurity layer and with the surface. Numerical examples of the modes are given and they are found to exhibit various effects due to the interplay between the impurity layer and surface modes. It is shown that more localized modes can occur and the modification of the spin wave spectra can be signaled by the appearance of surface and impurity modes, besides the bulk excitations. Also, the bulk spin fluctuations field, the spin waves localized on the surface as well as on impurity layer depend are shown to depend on the nature of the exchange coupling between spin sites, the values of spin sites and the position of the impurity layer from the surface.     

Effect of Co layer thickness on structural and magnetic properties of C/Co/C films

Granular C/Co/C films have been prepared by magnetron sputtering from C and Co onto glass substrates at room temperature and subsequent in situ annealing. It has been found that the structure and magnetic properties of the C/Co/C films depend strongly on the Co layer thickness. Vibrating sample magnetometer measurements indicate that the in-plane coercivities reach maximum in 20 nm Co thickness of both as-deposited and annealed films. The squareness ratio of annealed films was more than 0.8. X-ray diffraction shows that majority Co nanograins are formed as the hexagonal-close-packed (HCP) structure in 20 nm Co thickness with annealing at 400 °C. Scanning probe microscope was used to scan surface morphology and magnetic domain structures. The values of the surface roughness were lower than 0.6 nm in all annealed samples. The average magnetic cluster size was estimated to be about 10 nm in annealed 20 nm Co thickness films.     

Perpendicular anisotropy of Co3Pt deposited on the NiFeMo/Ru buffer

The multilayer of Ta/NiFeMo/Ru/Co₃Pt was sputter deposited on the Si (1 0 0) wafer. Using the NiFeMo buffer layer greatly enhanced the texture of Co₃Pt layer. The enhanced texture increased the perpendicular magnetic anisotropy of Co₃Pt. According to the VSM and XRD results, only the 5 nm of NiFeMo was good enough to produce the texture and perpendicular anisotropy in Co₃Pt layer. The perpendicular anisotropy was attributed to the existence of short-range-ordered HCP structure of Co₃Pt.