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：     

Spontaneous magnetostriction in R2Fe14B (R=Y,Nd, Gd,Tb, Er)

Thermal expansion anomalies of R₂Fe₁₄B (R=Y, Nd, Gd, Tb, Er) stoichiometric compounds were studied by X-ray diffraction with high-energy synchrotron radiation using a Debye–Scherrer geometry in temperature range of ∼10–1000 K. A large invar effect with a corresponding large temperature dependence of lattice parameters ∼10–15 K above their Curie temperatures (T_c) are observed. The a-axes show a larger invar effect than the c-axes in all compounds. The spontaneous magnetostrain of the lattices and bonds are calculated. The iron sublattice is shown to dominate the volumetric spontaneous magnetostriction of the compounds and the contribution from the rare-earth sublattice is roughly proportional to the spin magnetic moment of the rare earths. The bond-length changes are consistent with the theoretical spin-density calculation. The average bonds magnetostrain around Fe sites is approximately proportional to their magnetic moments.     

Nd2Fe14B的价电子结构分析和磁性计算

应用固体与分子经验电子理论计算了Nd₂Fe₁₄B的价电子结构、磁矩和居里温度,计算结果与实验值相符.计算表明:该合金的磁性与3d磁电子数成正比.从Fe(c)晶位到Fe(k₂)晶位磁矩增加,其机理源于价电子、哑对电子和3d磁电子之间的转化,有78%的哑对电子和18%的3d共价电子转化成了磁电子.居里温度和磁矩与Fe原子配位数成正比,与加权等同键数I^σ成反比,Nd原子 关键词： 2Fe₁₄B')" href="#">Nd₂Fe₁₄B 价电子结构 居里温度     

Atomic origin of magnetocrystalline anisotropy in Nd(2)Fe(14)B

The magnetic moment reversal at each of the two inequivalent Nd sites in a single crystal of ferromagnetic Nd(2)Fe(14)B is probed by dichroic resonant diffraction of circularly polarized x rays. The results, supported by theory, show that the c-axis intrinsic magnetic stability of this superior permanent magnetic material arises predominantly at one of the Nd sites (g). The other site (f) undermines magnetic stability by favoring a magnetic moment orientation in the basal plane.     

非晶态RE—TM(RE=Nd,Pr,TM=Fe,Co,Ni)薄膜的磁性和Pr的变价行为

本文通过对非晶态轻稀土Pr,Nd和过渡族金属Fe,Co,Ni薄膜合金的低温磁性研究,分别得到了(Pr,Nd)_x-(Fe,Co,Ni)_1-x合金中Pr,Nd和Fe,Co,Ni金属磁矩随成份x的变化,并且通过对磁矩的研究得到Pr离子的4f电子可能有退局域化的结论。 关键词：     

Magnetic structure and preferential occupation of Fe in the composite compound Nd2Co6Fe

The crystal and magnetic structures of the composite compound Nd₂Co₆Fe have been investigated by high-resolution neutron powder diffraction and X-ray powder diffraction. The compound crystallizes in the hexagonal Ce₂Ni₇-type structure consisting of Nd(Co,Fe)₂ and Nd(Co,Fe)₅ structural blocks alternately stacked along the c-axis. Multi-pattern Rietveld refinement of neutron diffraction and X-ray diffraction data at room temperature reveal that substitution of Fe for Co occurs exclusively in the Nd(Co,Fe)₅ structural blocks. The preferential occupation of the Fe atoms in the structure is discussed based on the mixing enthalpy between Nd and Fe atoms and on the lattice distortions. In agreement with the reported magnetic phase diagram of the Nd₂Co_7−xFe_x compounds, magnetic structure models with the moments of all atoms in the ab plane at 300 K and along the c-axis at 450 K provide a satisfactory fitting to the experimental neutron diffraction data. The refinement results show that the atomic moments of (Co,Fe) atoms within the Nd(Co,Fe)₅ blocks decrease slightly with temperature, whereas the atomic moments of Nd in the compound and of (Co,Fe) atoms at the interface between the Nd(Co,Fe)₂ and Nd(Co,Fe)₅ blocks are reduced significantly.     

Magnetic phase transitions in the Nd(Mn0.9Me0.1)O3 (Me=Al, Fe, Cr, Zn) perovskites

It is shown that perovskite NdMnO₃ is a weak ferromagnet with an anomalous magnetization behavior due to Nd sublattice contribution. Ferromagnetic component drastically increases whereas T_N slightly decreases when a part of manganese ions is replaced with Cr, Al, Fe, Zn. It is suggested that the Mn³⁺–O–Mn³⁺ superexchange interaction changes a sign in the microdomains enriched with Me=Cr, Al, Fe, Zn ions due to removing static Jahn–Teller distortions. All these substituted perovskites show a sharp drop of the magnetization as temperature decreases. A large temperature hysteresis indicates first-order phase transition. Below this transition neodymium magnetic moments orient opposite to a moment of manganese magnetic sublattice. It is supposed that this phase transition results from a change of the ground state of Nd ions.     

Magnetic properties of Fe/Nd artificial superstructure films

Multilayered films with artificial superstructures were prepared by alternately depositing Fe and Nd in ultrahigh vacuum. The magnetic properties are studied from⁵⁷Fe Mössbauer spectroscopy. The hyperfine field in Fe layers and the direction of Fe magnetic moments depend on the Fe and Nd layer thicknesses. For films with certain Fe and Nd layer thicknesses, the direction of Fe magnetic moments is in-plane at 300 K but changes to be perpendicular at low temperatures. The direction of Fe magnetic moments is discussed in relation with the magnetization of interface Nd atoms.     

Origin of the giant magnetic moments of Fe impurities on and in Cs films

We have explored the origin of the observed giant magnetic moments ( approximately 7&mgr;(B)) of Fe impurities on the surface and in the bulk of Cs films, using the relativistic local-spin-density-approximation method. We have found that Fe impurities in Cs behave differently from those in noble metals or in Pd. Whereas the induced spin polarization of Cs atoms is negligible, the Fe ion itself is a source of the giant magnetic moment. The 3d electrons of Fe in Cs are localized as the 4f electrons in rare-earth ions so that the orbital magnetic moment becomes as large as the spin magnetic moment. The calculated total magnetic moment M = 6.43&mgr;(B) is close to the experimentally observed value.     

On the magnetic properties of Y(FexCo1-x)2 compounds

The composition dependence of the mean magnetic moment of cobalt atoms in Y(Fe_xCo_1-x)₂ compounds is analysed in the local environment model. Cobalt has a magnetic moment of 1.56 μ_B if there are at least two Fe atoms as nearest neighbours. The maximum in the composition dependence of the transition metal moments is due to the magnetic contributions of iron atoms only. The thermal variation of reciprocal susceptibility obeys a Curie-Weiss behaviour, in addition to the Pauli paramagnetic term. Finally, the influence of the variable magnetic interactions on the transition metal moments is discussed.     

Site preference and magnetism of Fe3−xCrxAl0.5Si0.5

In order to gain better insight into the origin of the observed differences between Fe_3−xCr_xAl and Fe_3−xCr_xSi, alloys of Fe_3−xCr_xAl_0.5Si_0.5 (x=0, 0.125, 0.250, 0.375 and 0.5) were prepared and studied by means of X-ray and neutron diffraction as well as by magnetization measurements. Electronic structure calculations of these alloys have been performed by means of TB-LMTO-ASA method. It was expected, and experimentally verified, that the presence of silicon and aluminum atoms in 1:1 proportion will result in the independence of the lattice parameter on the iron/chromium concentration. All samples have been proved to be a single phase of the DO₃-type of structure. Theoretical and experimental results indicate that chromium atoms locate preferentially in B sublattice. Cr magnetic moments are oriented antiparallel to Fe magnetic moments. Neutron measurements show a linear dependence of the magnetic moments of Fe(A,C), Fe(B) and Cr(B) as a function of Cr concentration. However the calculated total magnetic moment decreases faster with chromium content than indicated by the experiment.     

On the origin of the giant magnetic moments of Fe and Co in Cs Films

To unravel the mystery of the recently observed giant magnetic moments of Fe and Co in Cs films, orbital-polarization corrected relativistic spin density functional calculations have been performed. Unlike other transition–metal systems where the orbital magnetic moments are quenched, Fe and Co in Cs as well as in other alkali metals are found to possess a giant orbital moment of 2–3 μ_B along with a large spin moment. Also, these free atom-like spin and orbital magnetic moments in Cs would not be squashed under large lattice contractions up to 23% around the impurity atoms. The induced moments on the host atoms are small. The results offer an explanation for the origin of the giant magnetic moments of Fe and Co in Cs films.     

Mössbauer study of the intermetallic compound Nd2Fe14B. II. Temperature dependence and spin reorientation

The ⁵⁷Fe Mössbauer effects of Nd₂Fe₁₄B were measured in a temperature range of 4.2−300 K. Below the spin reorientation transition temperature T_sc = 148 K, the spectra were satisfactorily analyzed with twelve Zeeman sextuplets due to splitting of six crystallographic Fe-sites into twelve non-equivalent sites. It was shown that the magnetic moments of the Fe and the Nd atoms are non-collinearly coupled in the magnetic structure with canted moments below T_sc. The directions of the moments at 4.2 K are inclined at 27° for Fe and at 58° for Nd from the c-axis to the [110] direction. The average moments are 2.27μ_B for Fe and 3.3μ_B for Nd at 4.2 K. The increase of the average hyperfine field with decreasing temperature is suppressed below T_sc, and its value at 4.2 K is reduced by 1% from the value of 337 kOe which is observed in Y₂Fe₁₄B and also estimated for Nd₂Fe₁₄B by extrapolating the values above T_sc. On the other hand, the Nd moment increases abruptly around T_sc as the temperature decreases. The directions of the principal axes of electric field gradients on the six distinct Fe-sites were also obtained. The anomalous temperature dependence of quadrupole splittings and isomer shifts was observed around T_sc. They were discussed in a framework of the changes in the band structure and the lattice parameters incidental to the spin reorientation transition.     

Electronic structure and magnetism of R(Fe,Si)12 (R = Y, Nd)

The newly developed full-potential linearized augmented plane wave (LAPW) and local orbitals (lo) based on standard APW methods are briefly introduced, and the structure and magnetic properties of R(Fe, Si)12 compounds (R = Y, Nd) are calculated using the method. The distribution of Si at different sites is analyzed based on total energy of one crystal unit with structure having been optimized. The characters of magnetic moments, total density of states (TDOS) and partial density of states (PDOS) for different crystal sites Si occupies are obtained and analyzed. The results show that the total magnetic moments of RFe10Si2 (R = Y, Nd) are larger than those of RFe10M2 (M = Ti, V, Cr, Mn, Mo and W) and the hybridization mechanism is seen as follows. Si(8j) reduce the magnetic moments of Fe at three sites, however, Si(8f) mainly reduce the magnetic moments of Fe(8i) and Fe(8j) atoms. The Curie temperature is markedly enhanced by the introduction of Si atoms according to spin fluctuation of DOS at Fermi level.     

Magnetic sublattices in nickel ferrite

A definition of the concept of “magnetic sublattice” is proposed by the following three steps, i.e.,i) the “site sublattice”, ii) the “cationic sublattice” and iii) the “magnetic sublattice”. The most important feature of this concept is the condition of long range order in the occupancy of the cation sites, including the direction of the magnetic moment. These ideas are applied to the case of nickel ferrite. Under the assumption that an ordered configuration on B sites (alternate rows of Ni and Fe cations) is obtained by slow cooling of samples, an estimation of the Curie temperature of the disordered (quenched) configuration is given. The calculated value (826 K) is in good agreement with the experimental value (838 K).     

A study of the magnetic moments and Fe site hyperfine fields in iron-chromium alloys

The magnetization of Fe-Cr alloys ranging from 1 to 15 atomic % of Cr has been measured at room temperature in order to study the relationship between the Fe site hyperfine fields and the magnetic moment. The average moment decreases linearly, at a rate of -2.36 μ_B per Cr atom, up to 10% Cr concentration. The Fe site hyperfine fields were measured in a previous study¹ using the same samples. It is found that the hyperfine fields measured are not proportional to the corresponding magnetic moments. The results are interpreted using a model previously developed for other binary alloys of iron².     

Theory of localized magnetic moments in metals I finite cluster approach

The origin of localized magnetic moments formation in metals is investigated theoretically using a self-consistent local spin density molecular cluster approach. Clusters with up to 55 atoms are employed to describe isolated impurity local moment behavior in the cases of Fe$\text{Ag}$ and Fe$\text{Pd}$. Densities of states and spin magnetic moments were determined and compared with results of spectroscopic (notably photoemission) and magnetization measurements, respectively. In the case of a noble metal host, the spin magnetization density is found to be highly localized around the Fe site; the iron moment is ≈ 3.9μ_B and the polarization of the host Ag atoms is small. In the case of a transition metal host, the iron moment is ≈ 3.2 μ_B but here the strong hybridization of the Fe-3d and Pd-4d states results in a large induced magnetic moment in the host PD metal — in essential agreement with experiment for this giant moment system.     

Magnetization process of Fe layers in Fe/Nd multilayered films investigated by57Fe Mössbauer spectroscopy

The magnetization process of Fe and Nd layers at 5K in Fe/Nd multilayered films with strong perpendicular magnetic anisotropy is elucidated from a comparison of⁵⁷Fe Mössbauer spectra in the presence of the external field applied parallel to the film plane with total magnetization. At zero external field, the film has a magnetic multi-domain structure. The Nd layer moment is perpendicular to the film plane and the Fe layer moment points in the out-of-plane direction. The Fe layer moment monotonically rotates to the in-plane direction with increasing external field parallel to the film plane, while the Nd layer moment is oriented to the film normal direction up to the external field of 10kOe, above which the Nd layer moment gradually turns to the direction of the external field.     

Direct determination of the magnetic ground state in the square lattice S = 1/2 antiferromagnet Li2VOSiO4

Powder neutron diffraction and resonant x-ray scattering measurements from a single crystal have been performed to study the low-temperature state of the 2D frustrated, quantum-Heisenberg system Li2VOSiO4. Both techniques indicate a collinear antiferromagnetic ground state, with propagation vector k=(1 / 2 1 / 2 0), and magnetic moments in the a-b plane. Contrary to previous reports, the ordered moment at 1.44 K, m=0.63(3)micro(B), is very close to the value expected for the square lattice Heisenberg model ( approximately 0.6micro(B)). The magnetic order is three dimensional, with antiferromagnetic a-b layers stacked ferromagnetically along the c axis. Neither x-ray nor neutron diffraction shows evidence for a structural distortion between 1.6 and 10 K.     

First-principles study of structural, electronic and magnetic properties in Cd1−xFexS diluted magnetic semiconductors

In this paper, we report theoretical investigations of structural, electronic and magnetic properties of ordered dilute ferromagnetic semiconductors Cd_1−xFe_xS with x=0.25, 0.5 and 0.75 in zinc blende (B3) phase using all-electron full-potential linear muffin tin orbital (FP-LMTO) calculations within the density functional theory and the generalized gradient approximation. The analysis of band structures, density of states, total energy, exchange interactions and magnetic moments reveals that both the alloys may exhibit a half-metallic ferromagnetism character. The value of calculated magnetic moment per Fe impurity atom is found to be 4 μ_B. Moreover, we found that p-d hybridization reduces the local magnetic moment of Fe from its free space charge value of 4 μ_B and produces small local magnetic moments on Cd and S sites.