Dynamic magnetic properties of ferromagnetic and antiferromagnetic Heusler alloys

Inelastic neutron scattering measurements on a ferromagnetic Heusler alloy, Pd₂MnIn_1?xSn_x at the composition x = 0.75, have established the spin wave dispersion in the three principal symmetry directions. The results have been interpreted using a simple Heisenberg model in which the exchange constants are of long range, extending beyond 12 Å. The Curie temperature, spin wave stiffness constant and the thermal variation of the magnetisation, calculated using the derived exchange parameters are in close agreement with observation.Anomalies in the spin wave dispersion, which are also present to a lesser degree in Pd₂MnSn, have been interpreted as precursor effects associated with the onset of antiferromagnetic ordering, type AF3A, which is the magnetic structure observed in the range 0.2 ? x ? 0.6.     

Electronic structure and magnetic properties of Co- and Mn-based Heusler alloys and thin films

The full-potential linearized augmented-plane-wave method is used to investigate the electronic structure of several Co- and Mn-based ferromagnetic Heusler alloys. It is shown that calculated lattice constants and spin magnetic moments are in good agreement with experimental values. Electronic structure of Ni₂MnGa(001) surface as well as Ni₂MnGa thin film on GaAs(001) substrate is also investigated. The changes of electronic structure and magnetic properties at surface are analyzed.     

Atomic disorder and the magnetic, electrical, and optical properties of a Co2CrAl Heusler alloy

Two Co₂CrAl alloy samples subjected to different heat treatment regimes are studied. An exact distribution of atoms over the sublattices in the samples is determined by X-ray diffraction and neutron diffraction methods. These data are used to perform ab initio density of states calculations and to calculate the magnetic moments of the samples in a coherent potential approximation. The calculated magnetic moments are compared to the experimental values. The effect of atomic ordering on the electronic structure near the Fermi level is analyzed using optical methods. The possible causes of the detected temperature dependence of the electrical resistivity, unusual for metallic alloys, are discussed.     

Electrical properties of ferromagnetic Ni2MnGa and Co2CrGa Heusler alloys

The electrical properties of ferromagnetic Ni₂MnGa and Co₂CrGa Heusler alloys are measured in the temperature range 4–900 K. The effect of the energy gap near the Fermi level in the electronic spectrum on the behavior of electrical resistivity and absolute differential thermopower is discussed.     

Amorphous Heusler alloys: The effect of annealing on structure and magnetic and transport properties

Alloys with the Heusler compositions Cu₂MnIn, Cu₂MnAl and Cu₂MnSn have been prepared as amorphous films for the first time. The structural disorder results in as-deposited films which are not ferromagnetic and, in fact, exhibit spin-glass properties due to the distribution of MnMn distances and co-existence of positive and negative exchange interactions. Annealing of the In and Al alloys can restore single phase ordered Heusler structures which are ferromagnets with Curie temperatures typical of bulk alloys. The calculated Mn moments remain lower than the bulk values of 4 μ_B probably due to disorder at grain boundaries. Films annealed under non-optimum conditions crystallized into several non-magnetic structures as well as the magnetic L2₁ Heusler phase. Lattice constants of all phases were determined. The temperature dependence of resistivity through the amorphous-crystalline transformation was found to confirm the results of thermal annealing on structure and magnetization.     

Structural, magnetic, magnetocaloric and magnetotransport properties in Ge doped Ni-Mn-Sb Heusler alloys

The effect of Ge substitution on the magnetic, magnetocaloric and transport properties of Ni₄₅Co₅Mn₃₈Sb_12−xGe_x (x=0-3) has been investigated. The decrease in the exchange interaction brought by Ge substitution can be seen from the reduction in the magnetization of austenite phase and the increase in the martensitic transition temperature. The magnetocaloric effect and the magnetoresistance values are found to be quite sensitive to small changes in Sb/Ge ratio. Taking into account various properties, the present series seems to be a promising multifunctional system.     

Electronic structure,magnetic exchange,and electrical transport properties of the magnetic compounds EuS,GdS and GdP

Optical reflectivity measurements between 0.03 eV and 12 eV have been performed on the isomorphic compounds EuS, GdP and GdS. A Kramers-Kronig analysis of these data permits the evaluation of intra- and interband transitions and the derivation of the electronic structure. The conducting behavior changes from semiconducting for undoped EuS to metallic for Eu_0.99Gd_0.01S and GdS; also GdP exhibits metal-like conductivity. In GdP and GdS the stoichiometry can be varied while keeping the phase purity. In these compounds the concentration of free carriers depends on the stoichiometry just as does the mobility, which in addition depends also on temperature. Large scattering anomalies are observed near the magnetic ordering temperatures for all compounds. In spite of the same spectroscopic ground state ${}^8\text{S}\text{7}\text{2}$ for EuS, GdP and GdS only EuS is ferromagnetic, the others being antiferromagnetic. In the alloy system GdPGdS the free carrier concentration could be varied over nearly two orders of magnitude and the superexchange and free carrier contribution to the total exchange interaction could be separated. Clearly the free carriers contribute a RKKY interaction.An attempt is made to correlate and explain the different physical properties of the related compounds EuS, GdP and GdS by their specific electronic structure.     

The electron spin-density space distribution in ferromagnetic Heusler alloys

The hyperfine magnetic fields on Mn⁵⁵, Co⁵⁹, Al²⁷ and Si²⁹ were measured by the spinecho technique in several systems of Heusler alloys. The concentration dependence of local magnetic moments of Mn and Co ions in different lattice positions was determined and the space distribution of delocalized electron spin-density was analyzed. The hyperfine fields on spelements were calculated using the main theoretical models.     

The radial dependence of the magnetic hyperfine field for Sn atoms in the ferromagnetic Heusler alloys

The magnetic hyperfine fields,B _hf, for impurity¹¹⁹Sn atoms in Z sites of ferromagnetic Heusler alloys Co₂MnZ (Z=Si, Ge) are measured by the Mössbauer effect. At 77 KB _hf=–1.43±0.04 T in Co₂MnSi andB _hf=+1.05±0.05 T in Co₂MnGe. From the comparison between the values ofB _hf for Sn atoms in Co₂MnZ (Z=Si, Ge, Sn), it follows that the negative contribution toB _hf drops as the interatomic distance begins to increase. This radial dependence also manifests itself in the anomalies of the temperature dependences of the hyperfine fields. The temperature anomaly is positive for Sn in Co₂MnGe and negative for Sn in Co₂MnSi.     

Electronic structures,magnetic properties and half-metallicity in Heusler alloys Zr2IrZ (Z = Al,Ga, In)

The electronic structures, magnetic properties and half-metallicity in Zr₂IrZ (Z = Al, Ga, In) alloys with the Hg₂CuTi-type structure were systematically investigated by using the first-principle calculations. Zr₂IrZ (Z = Al, Ga, In) alloys are predicted to be half-metallic ferrimagnets which are quite robust against hydrostatic strain and tetragonal deformation. The total magnetic moment of Zr₂IrZ (Z = Al, Ga, In) alloys mainly originates from the 4d electrons of Zr atoms and follows the conventional Slater-Pauling rule: M_t = Z_t−18. (M_t is the total magnetic moment per unit cell and Z_t is the valence concentration). The origin of the band gap for Zr₂IrZ (Z = Al, Ga, In) alloys is also well studied. Unconventionally, Zr₂Ir-based alloys contain element with 5d valence electrons, which implies a wider field to search for new half-metallic materials.     

The electrical, optical and magnetic properties of Si-doped ZnO films

In this paper, the influences of Si-doping on electrical, optical and magnetic properties of ZnO films have been systematically investigated. It is found that the resistivity of the films decreases from 3.0 × 10³ to 6.2 × 10^-2 Ωcm with Si-doping due to the increase of carrier concentration. The bandgap of ZnO films increases from 3.28 to 3.52 eV with increasing of Si concentration, which is found to be due to the collective effects of bandgap narrowing and Burstein-Moss effect induced by high carrier concentration. With increase of Si concentration, the near band edge (NBE) emission decreases due to the deterioration of crystal quality, while the yellow emission enhances due to the increase of extrinsic impurity or defects. The additional Si-doping has a profound influence on the enhancement of magnetic property and the maximum magnetic moment of 2.6 μ_B/Si is obtained. The ferromagnetic ordering is seen to be correlated with carrier concentration and structural defects.     

Electronic structure and half-metallicity of the new Heusler alloys PtZrTiAl,PdZrTiAl and Pt0.5Pd0.5ZrTiAl

The electronic structure and magnetic properties of the PtZrTiAl, PdZrTiAl and Pt_0.5Pd_0.5ZrTiAl Heusler alloys were investigated using the full-potential linearized augmented plane wave (FPLAPW) within the generalized gradient approximation (GGA). For the PtZrTiAl, and PdZrTiAl alloys, the results showed that these Heusler alloys were stable in the Type I structure. The (Pt, Pd)ZrTiAl Heusler alloys are found to exhibit half-metallic ferromagnetism for both the Type I and Type II structure. The total magnetic moments of the PtZrTiAl and PdZrTiAl alloys were obtained to be 3 μ_B per formula unit, which are in agreement with the Slater-Pauling rule m_tot = (Nv ? 18). The half-metalliciy characteristic exists in the relatively wide ranges of 6.06–6.78 Å, and 6.13–6.73 Å for the PtZrTiAl and PdZrTiAl alloys, respectively. To complete the fundamental characteristics of these alloys, Pt_0.5Pd_0.5ZrTiAl is predicted to be a half-metallic ferromagnet with an energy gap of 0.90 eV in the minority spin and a complete spin polarization at the Fermi level. These new Heusler alloys may become ideal candidate material for future spintronic applications.     

Electronic structure and magnetic properties of YbCuAl

The electronic band structure of YbCuAl has been calculated using the self-consistent full potential nonorthogonal local orbital minimum basis scheme based on the density functional theory. We investigated the electronic structure with the spin–orbit interaction and on-site Coulomb potential for the Yb-derived 4f orbitals to obtain the correct ground state of YbCuAl. The exchange interaction between local f electrons and conduction electrons play an important role in the heavy fermion characters of them. The fully relativistic band structure scheme shows that spin–orbit coupling splits the 4f states into two manifolds, the 4f_7/2 and the 4f_5/2 multiplet.     

Laser-plasma deposited nanosized layers of ferromagnetic semiconductors and silicon- and germanium-based Heusler alloys

The electric, magnetic resonance, and magneto-optical properties of thin laser-plasma deposited 50–100-nm layers of diluted magnetic semiconductors Ge:(Mn, Al)/GaAs, Ge:(Mn, Al)/Si, and Heusler alloys Co₂MnSi/Si, Co₂MnSi/GaAs, and Fe₂CrSi/GaAs with T _c > 293 K were studied. Anomalous ferromagnetic resonance in Ge:(Mn, Al) layers, ferromagnetism in CoSi/Si characterized by strong hysteresis in the magneto-optic Kerr effect, and the anomalous Hall effect at 293 K were observed.     

Morphology, magnetic, magnetoresistance and optical properties of Co-Ni-Mo alloys thin films

We present in this paper several results concerning the preparation by means of electrolysis and characterization of Co-Ni-Mo thin films. Co-Ni-Mo thin films with different molybdenum content in the range 0-25 at% Mo were prepared from a complex solution containing ions of Co, Ni and Mo, using galvanostatic control, on aluminum substrates. The effects of applied current density on the morphology, magnetic, magnetoresistance, and optical properties of the electrodeposited Co-Ni-Mo films were investigated. The applied current density significantly influenced the film composition and their magnetic properties. The increase of molybdenum content in Co-Ni films (up to 25 at% Mo) enhances the resistivity, but it reduces the magnetoresistance effect. We report the first observation of magnetoresistance as high as 8% in Co-Ni-Mo thin films.