Seebeck coefficients of half-metallic ferromagnets

In this article the Co₂ based Heusler compounds are discussed as potential materials for spin voltage generation. The compounds were synthesized by arc melting and consequent annealing. Band structure calculations were performed which revealed the compounds to be half-metallic ferromagnets. Magnetometry was performed on the samples and the Curie temperatures and the magnetic moments were determined. The Seebeck coefficients were measured from low to ambient temperatures for all compounds. For selected compounds high temperature measurements up to 900 K were performed.     

Thermal collapse of spin polarization in half-metallic ferromagnets

We propose two novel approaches to study the temperature dependence of the magnetization and the spin polarization at the Fermi level in magnetic compounds, and apply them to half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab initio calculations. The thermal properties are studied by mapping ab initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method.     

Temperature dependences of resistivity and magnetoresistivity for half-metallic ferromagnets

Peculiarities of transport properties of three- and two-dimensional half-metallic ferromagnets are investigated, which are connected with the absence of spin-flip scattering processes. The temperature and magnetic field dependences of resistivity in various regimes are calculated. The resistivity is proportional to T ^9/2 for T < T ^* and to T ^7/2 for T > T ^*, T^* being the crossover temperature for longitudinal scattering processes. The latter scale plays also an important role in magnetoresistance. The contribution of non-quasiparticle (incoherent) states to the transport properties is discussed. It is shown that they can dominate in the temperature dependence of the impurity-induced resistivity and in the tunnel junction conductivity. Received 16 September 2002 / Received in final form 6 November 2002 Published online 31 December 2002     

Nonreciprocal neutron scattering by helical ferromagnets

Calculations of scattering of unpolarized neutrons by crystals with helical magnetic ordering are presented. It is shown that the position of peaks of low-angle neutron scattering depends of the magnitude and direction of an external magnetic field applied along the axis of the helix.     

Magnetic and electrical properties of the half-metallic ferromagnets Co2CrAl

This paper presents the results of measurements of the magnetic and electrical properties of the ferromagnetic alloy Co₂CrAl in two structural states: (i) after severe plastic deformation and (ii) after shortterm high-temperature annealing of the deformed specimens. The experiments have been performed at temperatures in the range from 2 to 900 K in magnetic fields H ≤ 50 kOe. The ferromagnetic Curie temperature T _C and the paramagnetic Curie temperature Θ have been determined (T _C = 305 K and Θ = 326 K), as well as the spontaneous magnetic moment μ _S and the effective magnetic moment μ_eff per molecule of the alloy (μ _S = 1.62 μ_B and μ _eff ² = 8.2 μ _B ² ). It has been shown that the magnetic crystalline anisotropy energy of the alloy is on the order of ～5 × 10⁵ erg/g. The specific features of the electrical properties are associated with the presence of an energy gap in the electronic spectrum near the Fermi level E _F and with the change in the parameters of the energy gap as a function of the temperature.     

Relativistic multiple scattering theory of electrons by ferromagnets

In view of investigating for 3d- and 4f-ferromagnetic crystals the elastic scattering and the emission of polarized low-energy electrons as well as the bulk electronic structure, a relativistic multiple scattering theory has been developed, in which the exchange interaction with the magnetic ground state electrons is treated in a local density functional approximation. In a layer-KKR-type approach, the Dirac equation, which contains an effective magnetic field term, is first solved for a single crystal atom and subsequently for a monoatomic layer and for a semi-infinite crystal. Spin-orbit coupling and magnetic exchange interaction are thus simultaneously taken into account.     

Non-quasiparticle states in the core level spectra of ferromagnetic semiconductors and half-metallic ferromagnets

The Greens functions that determine X-ray spectra are calculated in the s-d exchange model of a saturated conducting ferromagnet in the presence of the core hole. It is demonstrated that the core level (X-ray absorption, emission and photoelectron) spectroscopy might be an efficient tool to investigate the nonquasiparticle (NQP, spin-polaron) states in electron energy spectrum since the core hole potential can enhance essentially their spectral weight. NQP contributions to resonant X-ray scattering spectra can be also much more pronounced than those to the the density of states.     

Intrinsic versus extrinsic anomalous Hall effect in ferromagnets

A unified theory of the anomalous Hall effect (AHE) is presented for multiband ferromagnetic metals with dilute impurities. In the clean limit, the AHE is mostly due to extrinsic skew scattering. When the Fermi level is located around anticrossing of band dispersions split by spin-orbit interaction, the intrinsic AHE to be calculated ab initio is resonantly enhanced by its nonperturbative nature, revealing the extrinsic-to-intrinsic crossover which occurs when the relaxation rate is comparable to the spin-orbit coupling.     

Magnetic properties of several potential rocksalt half-metallic ferromagnets based on the first-principles calculations

Several rocksalt Sr₄X₃N (X=,O, S, Se, and Te) are predicted to be potential half-metallic ferromagnets free of transition-metal and rare-earth elements by performing the first-principles calculations. Then their magnetic properties, such as the half metallicity and the crystal-cell magnetic moments are investigated. The Sr₄X₃N possibly have higher Curie temperatures and have more stable half metallicity than the Sr₄X₃C. Their crystal-cell magnetic moments are all 1.00 μ_B. The crystal-cell magnetic moments and the half metallicity arise mainly from the N ions. The main mechanism is the strong covalent interaction leading to the sp² hybridized orbitals in the Sr₄X₃N. Then two Sr-5s and three N-2p electrons enter into three sp² hybridized orbitals. Among these five electrons, four electrons are paired and one is unpaired, so there are three spin-up electrons and two spin-down electrons in these sp² hybridized orbitals.     

Crossover behavior of the anomalous Hall effect and anomalous nernst effect in itinerant ferromagnets

The anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) are experimentally investigated in a variety of ferromagnetic metals including pure transition metals, oxides, and chalcogenides, whose resistivities range over 5 orders of magnitude. For these ferromagnets, the transverse conductivity sigma{xy} versus the longitudinal conductivity sigma{xx} shows a crossover behavior with three distinct regimes in accordance qualitatively with a recent unified theory of the intrinsic and extrinsic AHE. We also found that the transverse Peltier coefficient alpha{xy} for the ANE obeys the Mott rule. These results offer a coherent and semiquantitative understanding of the AHE and ANE to an issue of controversy for many decades.     

Peculiarities of the electronic transport in Co2CrAl and Co2CrGa half-metallic ferromagnets

We present results on the transport properties of the half-metallic ferromagnetic Heusler alloys Co₂CrAl and Co₂CrGa in the temperature range from 4 to 900 K. The peculiarities of the resistivity and the absolute differential thermoelectric power are considered within a two-current model of conductivity, taking into account the energy gap at the Fermi level in the electronic spectrum of alloys for electrons with spin opposite to the direction of the magnetization vector.     

Magnetic small-angle scattering of neutrons in amorphous ferromagnets

In amorphous Fe₄₀Ni₄₀C₁₄P₆ alloys the field dependence of the magnetic scattering cross section and the analysis of the scattering profile indicate the presence of stress centres corresponding to quasidislocation dipoles.     

Spin chirality fluctuations and anomalous Hall effect in itinerant ferromagnets

The anomalous Hall effect due to the spin chirality order and fluctuation is studied theoretically in a Kondo lattice model without the relativistic spin-orbit interaction. Even without the correlations of the localized spins, sigma(xy) can emerge depending on the lattice structure and the spin anisotropy. We reveal the condition for this chirality-fluctuation driven mechanism for sigma(xy). Our semiquantitative estimates for a pyrochlore oxide Nd2Mo2O7 give a finite sigma(xy) approximately equal 10 Omega(-1) cm(-1) together with a high resistivity rho(xx) approximately equal 10(-4)-10(-3) Omega cm, in agreement with experiments.     

Production of high circular polarization by ion surface scattering of ferromagnets

From a theoretical analysis we predict large fractions of circular polarization of light emitted by ions scattered from surfaces of ferromagnetic single crystals. Maximum enhancement of circular polarization as compared to surface ion scattering from non-magnetic materials is again expected at grazing incidence. A circular polarization of 89% was estimated for the case of Ar⁺ ions scattered from the (110) plane of a Ni single crystal.     

Spin-dependent scattering versus spin-dependent tunneling in heterogeneous ferromagnets

Can spin-dependent scattering mimic spin-dependent tunneling in heterogeneous ferromagnets? The discussion is triggered by the surprising similarity in magnetoresistance properties of granular ferromagnets with metallic, insulating or point-contact spacers.     

Observation of nonconventional spin waves in composite-fermion ferromagnets

We find unexpected low energy excitations of fully spin-polarized composite-fermion ferromagnets in the fractional quantum Hall liquid, resulting from a complex interplay between a topological order manifesting through new energy levels and a magnetic order due to spin polarization. The lowest energy modes, which involve spin reversal, are remarkable in displaying unconventional negative dispersion at small momenta followed by a deep roton minimum at larger momenta. This behavior results from a nontrivial mixing of spin-wave and spin-flip modes creating a spin-flip excitonic state of composite-fermion particle-hole pairs. The striking properties of spin-flip excitons imply highly tunable mode couplings that enable fine control of topological states of itinerant two-dimensional ferromagnets.     

Cr含量对ZB型半金属铁磁体Cr_xZn_(1-x)Se磁电性能的影响

使用基于密度泛函理论的第一性原理方法,优化了闪锌矿结构的Cr_xZn_(1-x)Se(x=0.000,0.125,0.250和0.375)的2×2×2超胞的几何结构,计算了其自旋极化的态密度和能带结构、离子磁矩、电荷分布等磁电性能,详细分析了Cr含量对Cr_xZn_(1-x)Se磁电性能的影响.结果表明,Cr掺杂后ZB型ZnSe具有明显的半金属特性;当x=0.125,0.250和0.375时,Cr_xZn_(1-x)Se均有较宽的半金属带隙,从而可能具有较高的居里温度;当x=0.125时,Cr_xZn_(1-x)Se的半金属性最稳定;x=0.125,0.250和0.375时,Cr_xZn_(1-x)Se的超胞磁矩分别为整数磁矩4.0,8.0和12.0μ_B,而具有整数磁矩是半金属铁磁体非常重要的特征之一.Cr_xZn_(1-x)Se的半金属性和磁性主要来源于Cr离子的自旋极化,Cr离子的电子结构为Cr e_g~2↑e_g~1↓t_(2g)~3↑.     

Cr含量对ZB型半金属铁磁体CrxZn1-xSe磁电性能的影响

使用基于密度泛函理论的第一性原理方法,优化了闪锌矿结构的CrxZn1-xSe (x = 0.000,0.125,0.250和0.375)的2×2×2超胞的几何结构,计算了其自旋极化的态密度和能带结构、离子磁矩、电荷分布等磁电性能,详细分析了Cr含量对CrxZn1-xSe磁电性能的影响。结果表明,Cr掺杂后ZB型ZnSe具有明显的半金属特性;当x=0.125, 0.250和0.375时,CrxZn1-xSe均有较宽的半金属带隙,从而可能具有较高的居里温度;当x=0.125时,CrxZn1-xSe的半金属性最稳定;x=0.125, 0.250和0.375时,CrxZn1-xSe的超胞磁矩分别为整数磁矩4.0, 8.0和12.0 µB,而具有整数磁矩是半金属铁磁体非常重要的特征之一。CrxZn1-xSe的半金属性和磁性主要来源于Cr离子的自旋极化,Cr离子的电子结构为Cr e2g↑e1g↓t2g3↑。