M. V. Tovar Costa

Local magnetization and hyperfine field systematics of s−p and noble impurities in Gd and Ni hosts

Magnetic hyperfine data of s−p and noble impurities diluted in ferromagnetic Gd and Ni hosts are described within a simple model which is an extension on that one of Daniel and Friedel. We also include in the present model the effect of next-neighbor perturbation, due to the translational invariance break introduced by the impurity. Performing a self-consistent calculation of the local magnetic moments at the impurity site, one obtains the conduction electron polarization (CEP) hyperfine field. It is found that the model can explain the different hyperfine field trends observed in Gd and Ni hosts, e.g., a remaining negative value along the s−p series in the Gd case and a change of sign behavior in the Ni case. Period effects observed in noble impurities are also discussed and the theoretical calculations are in good agreement with available experimental data.

Magnetocaloric effect in the Laves phase pseudobinary Er1−cYcCo2

In this article we study the magnetocaloric effect in the pseudobinary Er1−cYcCo2 using a theoretical model, in which the localized 4f spins are immersed in an effective subsystem of itinerant electrons. The interaction between the localized 4f spins is treated in the molecular field approximation and the disorder entering in the Er site is considered in the virtual crystal approximation. Our theoretical results for the adiabatic temperature change and for the isothermal magnetic entropy change with magnetic field are in good agreement with experimental data.

Systematics of magnetic hyperfine fields at diluted impurities in ferromagnetic rare-earth compounds GdX (X = Zn and Cd): A theoretical study

Abstract The hyperfine fields at 3d transition impurities and at non-magnetic (s-p and noble) impurities in ferromagnetic GdZn and GdCd are theoretically discussed in the framework of an ‘effective’ Gd host. The observed general trend of the measured hyperfine fields is well reproduced by our model calculations. The calculated local moments at Sc and Ni impurities in these intermetallics are very small, whereas in the case of Mn and Fe impurities one gets a finite moment of about 1.1 μ B and 0.8 μ B , respectively, which couples antiferromagnetically with the host Gd moments in a quite good agreement with experimental data.

On the local magnetic moments at X sites in XFe2 compounds: Pressure effects (X=Y, Lu, Zr, Hf)

Adopting a simple model to describe the intermetallic cubic Laves phase compounds XFe2 (X=Y, Lu, Zr, Hf) we discuss the effect of an external applied pressure on the local magnetic moments at both sublattices. We assume that pressure acts on the system, modifying the widths of the sublattice bands and the spin-dependent hybridization between d states of the two sublattices. Using realistic densities of states we obtain, in a self-consistent way, the local d-magnetic moments at X and Fe sites which turn out to be antiparallel. As pressure increases one shows that the magnitude of the local magnetic moment at the X site increases until saturating, whereas the local magnetic moment at the Fe site decreases until saturating. Similarly to the experimental data, which show that the hyperfine field at the X site increases with applied pressure, our theoretical model predicts that the magnetic moment at the X site also increases with pressure. The calculated hyperfine and magnetic moments at the Fe site are also ...

Change of universality class of metal–insulator transition due to magnetic ordering

Using a two-band model we report a theory to describe the metal–insulator (MI) transition as a function of an external applied magnetic field in Kondo insulators. To deal with electronic correlations we use a functional integral approach in the static approximation. We show the existence of a critical value of the Coulomb correlation Uc, such that for U Uc, the transition is to a ferromagnetic metal and it is described by different critical exponents.

Study of magnetic hyperfine data on rare-earth impurities in Fe and Ni: Non-orbital contribution

Hyperfine data on rare-earth impurities in the ferromagnetic transition host Fe and Ni are theoretically discussed, adopting a simple model, where one considers the local-charge perturbation and the next-neighbor perturbation due to the impurity. A self-consistent procedure enables us to calculate the hyperfine fields at the rare-earth sites in LuM, LaM and GdM (M = Fe, Ni), where no orbital contribution exists, and one gets a good agreement with experimental data. We also discuss the non-orbital contribution and the sign of the total magnetic hyperfine field in cases where the orbital contribution is dominant as, e.g. EuM, TbM and YbM alloys.

Theoretical study of hyperfine fields at impurity nuclei in GdX (X=Zn,Cd) compounds: A two-center model

We theoretically discuss the local moment formation and the hyperfine field behavior of nonmagnetic (s−p and noble) impurities diluted in ferromagnetic rare-earth compounds GdX (X=Zn, Cd). It is experimentally observed in these systems that all impurities enter substitutionally at the “nonmagnetic” X lattice site, creating a strong local charge perturbation. In our simplified model, one calculates the local magnetic moment in the perturbed conduction band at the origin (a charge perturbed X site). The polarization of the conduction band arises from the Gd 4f moments and the conduction electron polarization produced by the host magnetic moment at the origin is treated within the Born approximation. The general trend of the experimental hyperfine field data is well reproduced by our self-consistent calculations.

Contribution of the Spin Current to the Damping of the Magnetic Moment Precession of Fe Impurities in Pd

We investigate the spin dynamics of a magnetic adatom on a non-magnetic surface with strong Stoner enhancement. We find a strong damping of the adatoms magnetization precession and a large shift of the resonance frequency from its bare value. Stoner enhancement in the substrate reduces the damping. We explore the damping dependence on features of the electronic structure.

Local stiffness and stability of the ferromagnetic state in metals

The concept of local stiffness associated with transverse local spin fluctuations is introduced and used to investigate the coupling between the magnetization of atomic planes in the ferromagnetic state of a metallic material. It is shown that the coupling strength in the surface region may differ significantly from that in the bulk, and the possibility of an antiferromagnetic alignment between surface and bulk magnetizations is pointed out. A comparison between calculated values of local and spin-wave stiffnesses is presented, and the stability of the ferromagnetic state against both short- and long-wavelength spin fluctuations is discussed.

Theoretical study of hyperfine fields at diluted s–p, noble, and nd impurities in ferromagnetic compounds GdX (X=Zn, Cd)

The hyperfine fields at nd (n=3,4,5) impurities and at noble impurities diluted in ferromagnetic compounds GdX (X=Zn, Cd) are discussed in the framework of a two center model. In these systems it is observed that all impurities enter substitutionally at the “nonmagnetic” X lattice site, thus creating a strong local charge perturbation. In our model, one calculates the local magnetic moment in the charge perturbed conduction band at the origin. The magnetization of the s–p conduction band arises from the Gd 4f moments. Then one obtains the conduction electron polarization contribution to the hyperfine field. The transition (and noble) impurities also create an Anderson–Moriya d resonance in the strongly perturbed Slater–Koster local density of states at the origin, thus originating an additional local moment and thereby a core polarization contribution to the magnetic hyperfine field. The self-consistently calculated hyperfine fields are in quite good agreement with available experimental data.