Magnetic surface anisotropy of transition metal ultrathin films

In order to study the magnetic anisotropy of transition metal ultrathin films, we have performed tight-binding calculations including spin-orbit coupling. Beside the anisotropy energy these calculations also yield the orbital moment, which turns out to be much more anisotropic than in bulk materials. The effects of interfacial mismatch and roughness are discussed within phenomenological models. We also briefly review experimental results on the magnetic surface anisotropy (MSA) in transition metal ultrathin films. In some cases such as Au/Co/Au(111) sandwiches the MSA wins the competition with the shape anisotropy arising from the magnetostatic energy: below a critical thickness this leads to aperpendicular spontaneous magnetization. We show the effects of this crossover on the hysteresis loops and on the magnetoresistance, and the effects of interface roughness on the critical thickness.     

Two- and single-ion magnetic anisotropy in the rare-earth metals

The strong magnetic anisotropy of the rare-earth metals (basically due to the large oribtal contributions to the total ionic moments) has normally been attributed to the crystalline field acting on the ions. Within the last few years a number of experiments have revealed that anisotropic two-ion couplings are of comparable importance. A review of the experimental evidence for the presence of anisotropic coupling between the rare-earth ions is given. Such indications are found primarily in the excitation spectra of the metals, but are also implicit in the crystal-field parameters determined in dilute systems.     

Compressive surface stress in magnetic transition metals

Because of the increased electron density within the surface layer, metal surfaces are generally expected to have tensile surface stress. Here, using first-principles density functional calculations, we demonstrate that in magnetic 3d metals surface magnetism can alter this commonly accepted picture. We find that the thermodynamically stable surfaces of chromium and manganese possess compressive surface stress. The revealed negative surface stress is shown to be ascribed to the enhanced magnetic moments within the surface layer relative to the bulk values.     

Shape instability in out of equilibrium magnetic domains observed in ultrathin magnetic films with perpendicular anisotropy

The magnetic configurations induced by the growth process in a thin film with perpendicular magnetisation have been observed by magnetic force microscopy (MFM). The FePd thin film has been grown by molecular beam epitaxy. A high uniaxial chemical ordering of the alloy into the tetragonal L1₀ structure induces the development of a large perpendicular anisotropy. As the growth process is performed below the Curie temperature of the FePd alloy, domain nucleation occurs during the growth process. The magnetic configuration has been imaged in the as grown state. As the equilibrium size of the magnetic domains decreases when the thickness of the layer increases, the domains obtained from spontaneous nucleation at the beginning of the growth of the thin film are submitted to very large strains as the layer thickness increases. At low thicknesses (low strains), the domain wall instability gives rise to an undulation of the domain walls. Thereafter, it leads to the formation of well-defined magnetic fingers, thus giving birth to the coexistence of two length scale in the domain structure. A quantitative estimation of the strain leading to the fingering instability is obtained. Last, the implications of these observations on the kinetic of domain wall distortion in ultrathin layers are discussed.     

Annealing effect on magnetic anisotropy in ultrathin（Ga,Mn）As

We investigated the effect of low temperature annealing on magnetic anisotropy in 7-nm ultrathin Ga0.94Mn0.06As devices by measuring the angle-dependent planar Hall resistance(PHR).Obvious hysteresis loops were observed during the magnetization reversal through the clockwise and counterclockwise rotations under low magnetic fields(below 1000 Gs,1 Gs = 10-4 T),which can be explained by competition between Zeeman energy and magnetic anisotropic energy.It is found that the uniaxial anisotropy is dominant in the whole measured ferromagnetic range for both the as-grown ultrathin Ga0.94Mn0.06As and the annealed one.The cubic anisotropy changes more than the uniaxial anisotropy in the measured temperature ranges after annealing.This gives a useful way to tune the magnetic anisotropy of ultrathin(Ga,Mn)As devices.     

Local moments and magnetic ordering in transition metals

Stoner theory predicts a complete collapse of the local magnetic moment at the Curie temperature. In this paper, the effect of local moment formation in the non-magnetic state is discussed. The energy between the magnetic and non-magnetic state is found to be considerably reduced. This observation is consistent with the low observed magnetic ordering temperatures of the 3d metals when compared to the values derived from band theory.     

Hyperfine magnetic fields at rare-earth elements in ferromagnetic transition metals

Hyperfine magnetic fields for such probes as:¹⁵²SmFe,¹⁵²SmCo,¹⁵²SmNi,¹⁵⁵GdFe,¹⁷²YbFe,¹⁷²YbCo and¹⁷²YbNi have been measured using PAC method. The quantitative analysis of components of these fields has been also performed.     

Anomalous antiferromagnetic structures in metals

The many-sublattice antiferromagnetic NNSS and NNNSSS structures may be stable in metals with isotropic indirect exchange provided it is strongly non-Heisenbergian (e.g. metals with a quasi-one-dimensional electron energy spectrum). This makes it possible to explain other anomalies of metals of the CeSb type by the non-Heisenberg exchange. Under certain conditions it may cause the canted two-sublattice structure to be most energetically favoured in isotropic metals of the GdMg type.     

Strange anisotropy in amorphous metals

The spin orientation in ferromagnetic amorphous Fe₈₃P₅C₁₂ in an external magnetic field, H_ext, applied perpendicular to the ribbon sample has been determined by magnetoresistance measurements and by the angular dependence of the hyperfine interaction. In small fields, H_ext ? 4 kOe, the spins tend to orient themselves perpendicular to H_ext due to the influence of the demagnetizing field and in large fields, H_ext ≈ 50 kOe, a complete alignment was not obtained.     

Anomalous c-axis transport in layered metals

Transport in metals with a strongly anisotropic single-particle spectrum is studied. Coherent band transport in all directions, described by the standard Boltzmann equation, is shown to withstand both elastic and inelastic scattering as long as EFtau > 1. A model of phonon-assisted tunneling via resonant states located in between the layers is suggested to explain a nonmonotonic temperature dependence of the c-axis resistivity observed in experiments.