Magnetic and electronic properties of NiAs-type chromium chalcogenides

We have computed spin-dependent energy bands, spin moments and density of states of NiAs-type CrX (X=S, Se and Te) chalcogenides using linear combination of atomic orbitals method within density functional theory as well as full potential augmented plane wave method. In addition, magnetic properties have also been computed using spin polarized relativistic Korringa-Kohn-Rostoker method. We have also obtained the first ever theoretical electron momentum densities of CrX compounds considering linear combination of atomic orbitals and compared the results with the isotropic Compton profiles measured using 20 Ci ¹³⁷Cs Compton spectrometer. The Fermi surface topology and magnetic properties are discussed in terms of majority and minority energy bands and density of states. In addition, to highlight the role of Cr (3d) electrons in such type of chalcogenides, we have also reported the magnetic Compton profile of CrTe using the Korringa-Kohn-Rostoker method.     

Exchange constants in nickel-cobalt alloys from standing spin wave resonance

The exchange parameters and saturation magnetization have been measured for a large number of films of nickel-cobalt alloys. The data are obtained from standing spin wave resonance at room temperature in a conventional 9 GHz spectrometer. The distortions in the spectra are accounted for by a first order perturbation calculation. ¹ The results are analysed using a model proposed by Hatherley et al. ² Within the framework of this model values for the second moment of the exchange integral between NiNi, NiCo and CoCo pairs of atoms are derived.     

Active polarized target: Measurement of proton spin polarizabilities

The static (electric and magnetic) polarizabilities of the proton are known from measurements of low-energy Compton scattering of real photons. However, no experimental data are so far available about its spin polarizabilities, which describe the response of the proton spin to a varying electromagnetic field. These fundamental structure constants can be obtained in the most model-independent way from measurements of double-polarized Compton scattering at energies below the pion threshold using a polarized photon beam and a polarized proton target. An active polarized target based on polystyrene doped with a scintillator allows recoil protons to be detected directly in the target material, background from the coherent Compton effect on heavier nuclei (¹²C etc.) to be eliminated and precise data on spin polarizabilities to be obtained.     

Role of the chemical ordering on the magnetic properties of Fe–Ni cluster alloys

The spin and orbital moments of fcc Fe-Ni cluster alloys are determined within the framework of a d-band Hamiltonian including the spin-orbit coupling non perturbatively. Different sizes (up to 321 atoms), compositions, and chemical configurations (random alloys as well as core-shell arrays of iron and nickel atoms) are considered in order to reveal the crucial role played by local order and stoichiometry on the magnetic moments of the clusters. Interestingly, we have found considerably reduced average magnetizations for Fe-Ni clusters with Fe cores compared to that of the bulk alloy with the same composition. Indeed, in these configurations not only antiparallel arrangements between the local moments of some Fe atoms within the iron core are found, but also the total magnetization of the surface Ni atoms is significantly quenched. On the opposite, the disordered and Ni-core cluster alloys are characterized by high magnetizations resulting from saturated-like contributions from both Ni and Fe atoms, in agreement with recent ab-initio calculations. In general, the local orbital magnetic moments are strongly enhanced with respect to their bulk values. Finally, the variation of the orbital-to-spin moment ratio with the chemical order is discussed.     

自旋转向相变中的条纹磁畴研究

用光激发电子显微镜研究了Fe／Ni铁磁膜和Co／Cu／Fe／Ni磁耦合膜中的条纹磁畴．实验发现：在Fe／Ni体系中，条纹磁畴宽度随着铁层厚度趋近于自旋转向相变点呈指数下降；在Co／Cu／Fe／Ni体系中，Fe／M层中的条纹磁畴会沿着钴层磁矩的方向排列，其磁畴宽度会随着Co-Fe／Ni间的层间耦合强度呈指数下降．理论上推导出条纹磁畴随着磁各向异性能和层间耦合强度变化的统一公式，而实验结果与理论符合得非常好。     

Optical and magnetic characterisation of Co3+ and Ni3+ in LaAlO3: interplay between the spin state and Jahn-Teller effect

The coordination, the electronic structures and the spin of the ground state of Ni(3+) (3d(7)) and Co(3+) (3d(6)) introduced as impurities in LaAlO(3) are investigated through optical spectroscopy and magnetic measurements. The unusual trivalent valence state in both transition-metal ions was stabilised via a sol-gel process followed by high oxygen pressure treatments. We show that the crystal-field strength at the nearly O(h) transition-metal site in LaAlO(3) locates Ni(3+) and Co(3+) near the spin state crossover, yielding a low-spin ground state in both cases. We analyse how the interplay between the Jahn-Teller (JT) effect and the spin state affects the magnetic moment of the ion and its temperature dependence. The optical spectra reveal a JT effect associated with a low-spin ground state in Ni(3+) and with a thermally populated high-spin low-lying first excited state in Co(3+). The corresponding JT distortions are derived from structural correlations. We conclude that the JT effect is unable to stabilise the intermediate spin state in Co(3+). A low-spin ground state in thermal equilibrium with a high-spin low-lying first excited state is detected in diluted Co(3+)-doped LaAlO(3). These results are compared with those obtained in the parent pure compounds LaNiO(3) and LaCoO(3).     

Au-Ni弱铁磁合金的自发磁矩和居里点

x=0.42—0.55的Au_1-xNi_x晶态合金是从液态直接淬火到室温,测量低温下这些合金的磁化曲线(2—70kOe),得到在温度1.5K下的自发磁矩值,并用Arrott-Noakes图线方法确定居里点。首先用Perrier等人提出的最近邻模型计算平均磁矩,和实验结果进行了比较。并提出最及次近邻模型,即考虑到次近邻的影响,假设一个Ni原子的最、次近邻18个原子有11个以上是Ni原子,它就具有和纯Ni一样的磁矩(0.606μ_B),否则磁矩为零,由此模型计算的平均磁矩值和实验结果比较符合。本文还对合金中自旋集团(spincluster)的存在和自旅集团之间的相互作用作了讨论,给出了表示自旋集团之间铁磁相互作用的平均内场μ_BH/k_B的值。 关键词：     

Ab-initio investigation of electronic properties and magnetism of half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In)

The electronic structures and magnetism of the half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In) have been investigated to search for new candidate half-metallic materials. Here, we predict that NiCrAl, and NiCrGa and NiCrIn are possible half-metals with an energy gap in the minority spin and a completely spin polarization at the Fermi level. The energy gap can be attributed to the covalent hybridization between the d states of the Ni and Cr atoms, which leads to the formation of bonding and antibonding peaks with a gap in between them. Their total magnetic moments are 1μ_B per unit cell; agree with the Slater-Pauling rule. The partial moment of Cr is largest in NiCrZ alloys and moments of Ni and Al are in antiferromagnetic alignment with Cr. Meanwhile, it is also found that FeCrAl is a normal ferromagnetic metal with a magnetic moment of 0.25μ_B per unit cell and CoCrAl is a semi-metal and non-magnetic.     

Magnetic stripe domains in coupled magnetic sandwiches

Magnetic stripe domains in the spin reorientation transition region are investigated in (Fe/Ni)/Cu(001) and Co/Cu/(Fe/Ni)/Cu(001) using photoemission electron microscopy. For (Fe/Ni)/Cu(001), the stripe domain width decreases exponentially as the Fe/Ni film approaches the spin reorientation transition point. For Co/Cu/(Fe/Ni)/Cu(001), the Fe/Ni stripe orientation is aligned with the Co in-plane magnetization, and the stripe domain width decreases exponentially with increasing the interlayer coupling between the Fe/Ni and Co films. By considering magnetic stripes within an in-plane magnetic field, we reveal a universal dependence of the stripe domain width on the magnetic anisotropy and on the interlayer coupling.     

Effects of oxygen vacancy location on the electronic structure and spin density of Co-doped rutile TiO₂ dilute magnetic semiconductors

According to density functional theory (DFT) using the plane wave base and pseudo-potential, we investigate the effects of the specific location of oxygen vacancy (VO) in a (Ti,Co)O 6 distorted octahedron on the spin density and magnetic properties of Co-doped rutile TiO2 dilute magnetic semiconductors. Our calculations suggest that the V O location has a significant influence on the magnetic moment of individual Co cations. In the case where two Co atoms are separated far away from each other, when the V O is located at the equatorial site of a Co-contained octahedron, the ground state of the two Co cations is d6 (t3 2g ↑, t 3 2g ↓) without any magnetic moment. However, if the V O is located at the apical site, these two Co sites have different ground states and magnetic moments. The spin densities are also observed to be modified by the exchange coupling between the Co cations and the location of V O . Some positive spin polarization is induced around the adjacent O ions.     

Magnetism of Fe,Co, and Ni nanowires encapsulated in carbon nanotubes

We have investigated the electronic and magnetic properties of Fe, Co, and Ni nanowires encapsulated in carbon nanotubes (CNTs) using spin polarized ab initio calculation. The incorporated systems with hollow region between the nanowire and the C shell have the enhanced magnetic moments compared to the ferromagnetic nanowires tightly wrapped by CNTs. The Co nanowire encapsulated in CNTs is a strong ferromagnet and has high spin polarization regardless of the distance between the nanowire and the C shell. The results show that the Co-filled CNTs are useful for spin polarized transport nanodevice.     

Spin dependent resistivity in magnetic alloys and multilayers

Material dependence of the spin dependent resistivity and magnetoresistance in magnetic multilayers as well as heterogenous alloys has been studied theoretically in a two-band model, by putting emphasis on random configuration of atoms near the interfaces. When the non-magnetic atoms are on the right (left) of the magnetic atoms, e.g., Fe or Co, in the periodic table, the ratio between the spin dependent resistivity, ϱ_↓ / ϱ_↑, is greater (smaller) than 1. It is also shown that the two-band model unifies the results obtained in the tight-binding model and the Anderson model.     

Metallic magnetism in amorphous TM-Y (TM = Mn,Fe, Co,Ni) alloys

Systematic variations of magnetic properties in amorphous TM-Y (TM = Mn, Fe, Co, Ni) alloys are investigated on the basis of a finite temperature theory of amorphous metallic magnetism which takes into account both thermal spin fluctuations and the fluctuations due to structural and configurations disorder. It is shown that the magnetic phase diagrams calculated in the most random atomic configuration explain qualitatively the spinglass (SG) in Mn-Y, the SG ferromagnetism (F) transition in Fe-Y, and the F-paramagnetism transition in Co-Y and Ni-Y alloys. Magnetization vs concentration curves and susceptibility vs concentration curves as well as the effective Bohr magneton numbers are also shown to be explained qualitatively or semi-quantitatively by the theory. Their miscroscopic mechanisms are elucidated by means of their electronic structures, magnetic couplings, and atomic short range order. It is found that the magnetism in Fe-Y and Mn-Y amorphous alloys is strongly influenced by the atomic short range order. The result explains different magnetic phase diagrams in amorphous Fe-Y alloys and experimental SG transition temperatures in amorphous Mn-Y alloys.     

Co-Ni/FeMn双层膜中磁化强度对交换偏置的影响

固定CoNiFeMn双层膜中反铁磁层的厚度,改变CoNi铁磁层的成分来调节磁化强度,从而研究铁磁层的饱和磁化强度对CoNiFeMn双层膜中交换偏置的影响.研究表明,CoNiFeMn界面的交换耦合能U不是一个常量,而是随(MFM)12的增加而线性增加.其原因是铁磁层磁矩通过界面相互作用在反铁磁层中形成的局域交换磁场,在磁场冷却时影响反铁磁层的自旋结构或磁畴结构及双层膜中的交换偏置 关键词： 交换偏置 磁化强度     

Electronic properties of the magnetocaloric compound GdAl2: A Compton scattering study

We present Compton profiles of the GdAl₂ compound and its constituents using a 20Ci ¹³⁷Cs Compton spectrometer. The experimental Compton data have been analysed using theoretical data obtained from the spin polarised relativistic Korringa–Kohn–Rostoker (SPR-KKR) method and also the charge transfer on the formation of the compound. Both the experimental and the SPR-KKR theoretical Compton data support a charge transfer from Al→Gd in GdAl₂, which is in accordance with the conclusions drawn from the partial, total and integrated density of states of GdAl₂ and its constituents.     

Multiple scattering approach to Co K-edge XMCD and XANES spectra in Gd–Co alloys

We have measured and analyzed Co K-edge X-ray absorption near edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) near edge spectra in crystalline and amorphous GdCo₂ alloys. We have used a semi-relativistic full multiple scattering approach to the analyses of the XMCD spectra. A general formula is obtained which is applicable to randomly oriented systems in space. Useful information is obtained on both the electronic and geometric structure around a Co atom. The difference in XANES and XMCD spectra between crystalline and amorphous GdCo₂ is well explained by models referring to the anomalous X-ray scattering result, where three Gd atoms in the second shell are removed away. The calculated XANES are not so sensitive to the electronic structure, whereas the calculated XMCD spectra are rather sensitive to the spin polarization on Co atoms. The result shows that the spin polarization on Co atoms in GdCo₂ alloys is smaller than that in Co metals.     

Evolution with composition of the d-band density of states at the Fermi level in highly spin polarized Co1-xFexS2

Highly spin polarized (SP) and half-metallic ferromagnetic systems are of considerable current interest and of potential importance for spintronic applications. Recent work has demonstrated that Co1-xFexS2 is a highly polarized ferromagnet (FM) where the spin polarization can be tuned by alloy composition. Using 59Co FM-NMR as a probe, we have measured the low-temperature spin relaxation in this system in magnetic fields from 0 to 1.0 T for 0相似文献   

Magnetization reversal and domains structures in (Co/Ni/Co/Pt) multilayers

Multilayers of [Co/Ni(t_Ni)/Co/Pt]_×4 are investigated for different Ni insertion layer thicknesses. The resulting magnetic properties and magnetic domain structures are compared with [Co/Ni]_×8 multilayers. As determined by magneto-optical Kerr effect microscopy and a vibrating sample magnetometer measurements, all multilayers exhibited a perpendicular magnetic anisotropy. It is found that the nucleation field and magnetic coercivity of [Co/Ni(t)/Co/Pt]_×4 multilayers are lower than (Co/Ni)_×8 and decreased with Ni thickness. Magnetization decay measurements reveal that these multilayers did not show an exponential decay behavior as was observed in rare earth transition metal alloys. Very narrow wires will remain stables for several hours even with an applied magnetic field closer to the coercivity. Insertion of very thin Ni in (Co/Pt) multilayers offers a good way to optimize the magnetic properties of the material and adjust the domain size for nanowire-based devices.     

第一性原理研究Co2MnSi和Co2MnGe半金属与磁性的稳定性

基于广义梯度近似密度泛函和全势能线性缀加平面波方法,对Co2MnSi和Co2MnGe在晶格常数发生变化的情况下进行电子结构和磁矩的自旋极化计算,得到了它们的自旋态密度分布以及总磁矩和各原子磁矩。计算结果的分析表明：（1）Co2MnSi 和Co2MnGe具有半金属性质;（2）晶格常数的改变分别为-5%~ 4%和-6%~1%时,Co2MnSi 和 Co2MnGe仍保持稳定的半金属质性;（3）Co2MnSi 和Co2MnGe的总磁矩为5.00µB/formula。总磁矩主要来源于Mn和Co的原子磁矩,Si和Ge的原子磁矩对总磁矩的贡献极小而且为负值。（4）Co2MnSi 和 Co2MnGe的晶格常数变化分别为-6% ~ 6%和-7%~ 4%时,虽然各原子磁矩都发生了变化,但是它们总磁矩稳定于5.00µB/formula.