Structure and non-collinear magnetism of iron linear chains

Atomic structures and non-collinear magnetic moments are calculated by the first principle molecular dynamics for Fe₅ and Fe₆ linear chains with several fixed and free chain-lengths. The dimerization appears in the optimized atomic structures of all the chains. For the Fe₅, the magnetic arrangement is parallel for a large chain-length and changes to non-collinear with decreasing the chain-length. For the Fe₆, the magnetic arrangement is antiparallel in a unit of dimer for a small chain-length and changes to non-collinear with increasing the chain-length. These magnetic behaviors are simulated by a simple J₁-J₂ Heisenberg model.     

Magnetic structure of the manganites heavily doped by Fe and Cr ions

Powder neutron diffraction and magnetic studies have been performed for NdMn_0.5Fe_0.5O₃ and NdMn_0.5Cr_0.5O₃ manganites. In NdMn_0.5Cr_0.5O₃, magnetic structure has been revealed consisting of ferromagnetic and G-type antiferromagnetic components as result of a 3d-ions magnetic moments ordering. Magnetic moments of Nd-ions are parallel to the ferromagnetic component. In NdMn_0.5Fe_0.5O₃ only the antiferromagnetic G-type structure has been found whereas Nd-sublattice was not ordered. In the both compounds, magnetic moments of 3d-ions are significantly less than one can expect, what is interpreted in terms of intrinsic chemical inhomogeneity. Magnetic phase diagrams have been constructed for the Nd(Mn_1−xM_x)O₃ (M=Fe, Cr) systems, interpreted assuming superexchange interactions Mn³⁺–O–Cr³⁺ to be positive, Mn³⁺–O–Fe³⁺ negative and taking into account a disordered arrangement of Mn and Cr ions in the crystal structure sublattice as well as interplay between Jahn–Teller effect and superexchange interactions.     

Magnetism of alloyed models of Cr(Mn) V and Mn Cr overlayers on V (001) substrates

The magnetism is calculated for substitutional alloyed of Cr_x(Mn_x)V_1-x monolayers (MLs) and Cr_xMn_1-x on V (001) surface with a variety of concentrations (x = 0.25, 0.50 and 0.75). The substitutional surface alloys were treated by an artificial super cell construction. Parallel magnetic ordering is obtained for all the considered structures. The surface net magnetization increases in terms of Cr (Mn) concentration in Cr_x(Mn_x)V_1-x/V (001) system, while no serious variations occur in Cr_xMn_1-x/V (001). Vanadium atoms at the interfacial layers acquire appreciable magnetic moments antiferromagneticlly (AF) coupled with the surface moments. Received 25 February 2002 / Received in final form 13 May 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: jkalifa@sci.ju.edu.jo     

Magnetic properties of small Yttrium clusters

The magnetic properties of small Y_N clusters are studied by using a tight-binding Hubbard Hamiltonian in the unrestricted Hartree-Fock approximation. Several types of cluster geometries are considered in order to see the effects of the size and symmetry of the structures on the magnetic properties. The average magnetic moments are found to be constant over large domains of variations in the interatomic distance, a fact that can be explained by the existing closed shell electronic configurations at least for one spin direction in all our magnetic solutions. Small energy gains upon the onset of magnetization are obtained, which reveals the low stability of the magnetic solutions. Our results contradict the prediction of a magnetic-nonmagnetic transition at a large cluster size (about 90 atoms) for these kinds of systems. Received: 27 April 1998 / Received in final form: 23 June 1998 / Accepted: 17 July 1998     

Electronic structure and magnetism of disordered bcc Fe alloys

We study the electronic structure and magnetic properties of disordered bcc Co_xFe_1-x, Cr_xFe_1-x and Mn_xFe_1-x alloys in their ferromagnetic phases using the Augmented Space Recursion (ASR) technique coupled with the tight-binding linearized muffin tin orbital (TB-LMTO) method. We calculate the density of states and magnetic moment of these alloys to show the variation upon alloying Fe with the other neighbouring 3d transition metals using arguments based on charge transfer, exchange splitting and hybridization effects. Received 10 April 2001 and Received in final form 15 August 2001     

Magnetism of Fe clusters embedded in Cu,Ag, and Au fcc matrices: density functional calculations

We present extensive first principles density functional theory (DFT) calculations dedicated to analyze the magnetic properties of small Fe _n clusters (n = 2,3) embedded in Cu fcc, Ag fcc and Au fcc matrices. We consider several dimers and trimers having different interatomic distances. In all cases the Fe atoms are embedded as substitutional impurities in the metallic network. For the case of the Fe dimers we have considered two magnetic configurations: ferromagnetic (antiferromagnetic), when the atomic magnetic moment of the Fe atoms are parallel (antiparallel) each other. For the case of dimers immersed in Cu and Ag matrices, the ground state corresponds to the ferromagnetic Fe dimer whose interatomic distance is a/√2. For Fe dimer immersed in the Au matrix the ground state corresponds to a ferromagnetic coupling when the interatomic distance is a√(3/2). In the case of the Fe trimers we have considered three or four magnetic configurations, depending on the Fe cluster geometry. For the case of Fe trimer immersed in Cu and Ag matrices we have found that the ground state corresponds to the ferromagnetic trimer forming an equilateral triangle with an interatomic distance equal to a/√2. The ground state for the Fe trimer immersed in the Au matrix corresponds to the ferromagnetic Fe trimer forming a right angle triangle.     

Magnetic properties of the Ce2Fe17−xMnx helical magnets up to high magnetic fields

Magnetic properties of the Ce₂Fe_17−xMn_x, x=0–2, alloys in magnetic fields up to 40 T are reported. The compounds with x=0.5–1 are helical antiferromagnets and those with 1<x?2 are helical ferromagnets or helical antiferromagnets at low and high T, respectively. Mn ions in the system carry average magnetic moment of 3.0±0.2 μ_B that couple antiparallelly to the Fe moments. Easy-plane magnetic anisotropy in the Ce₂Fe_17−xMn_x compounds weakens upon substitution of Mn for Fe. The absolute value of the first anisotropy constant in the Ce₂Fe_17−xMn_x helical ferromagnets decreases slower with increasing temperature than that calculated from the third power of the spontaneous magnetization. Noticeable magnetic hysteresis in the Ce₂Fe_17−xMn_x, x=0.5–2, helical magnets over the whole range of magnetic fields reflects mainly irreversible deformation of the helical magnetic structure during the magnetization of the compounds. A contribution from short-range order (SRO) magnetic clusters to the magnetic hysteresis of the helical magnets has been also estimated.     

Effect of sulphur doping on manganese clusters: an ab initio study

We report a structural, electronic and magnetic analysis of minimal Mn_nS clusters, n = 1–13, from ab initio calculations. Total geometry optimizations were performed by considering compact manganese clusters, doped with a single sulphur atom. The doping was added to the cluster by considering substitution, interstitial and adsorbed positions. To further investigate the influence of the sulphur doping on the magnetic properties of manganese clusters, we performed non collinear magnetic calculations within the local spin density approximation (LSDA) for the exchange-correlation. We find that the electronic properties can be better controlled when the cluster is doped with a sulphur atom, and less size dependent. There are no differences in the magnetic properties of doped and non-doped clusters, except for n=7 and 8, in which the total magnetic moment per atom are smaller in doped clusters.     

First-principles study on the electronic structure, magnetic properties and phase stability of alloyed cementite with Cr or Mn

Using the first-principles technique, the electronic structures, magnetic properties and phase stability of alloyed cementite with Cr or Mn were investigated. The calculations show that the chemical and mechanical stability of alloyed cementite can be strengthened by the use of Cr/Mn-doped method. The Magnetic Moments (Ms) of Mn₁Fe₂C, Mn₂Fe₁C, Cr₁Fe₂C and Cr₂Fe₁C are 5.274, 0.941, 1.864 and 0.736 μ_B/f.u, respectively. The Ms of Cr in Fe₂CrC (−1.374 μ_B) and Cr₂FeC (−0.032 μ_B) are different due to replacing different sites Fe atoms. The magnetic behaviors of Mn are different from Cr in alloyed cementite. The Ms of Mn in Fe₂MnC and Mn₂FeC are 2.300 μ_B and −0.147 μ_B, respectively.     

Au和3d过渡金属元素混合团簇结构、电子结构和磁性的研究

采用基于密度泛函理论的第一性原理方法系统研究了Au与3d过渡元素构成的混合小团簇的结构、稳定性、电子结构及磁性,得到了Au与3d过渡元素构成的混合小团簇的稳定结构.计算结果表明,Au与3d元素可形成大量的低能异构体,特别是有些异构体在结构上极相近,这不同于共价或离子键类型的团簇.与纯过渡金属团簇类似,这类团簇也表现出复杂的磁性.过渡金属元素的磁矩相比体材料而言既有增强的、也有减弱的,与轨道的交换劈裂密切相关.对于基态构型,AuCr₂,Au₂Cr_{2关键词： 密度泛函理论 第一性原理方法 团簇 电子结构}     

Fe_nO_m~+(n+m=4)团簇的构型、电子结构特征和磁性

采用二项式方案构建了FenOm+(n+m=4)团簇的大量可能初始结构.运用广义梯度近似(GGA)密度泛函理论中的PW91交换关联泛函对这些初始结构进行优化和频率分析,得到12个稳定的异构体.在此基础上计算和分析了它们的结合能、对称性、键长、磁矩,最高占据轨道与最低未占据轨道的能隙.发现Fe—O键在FenOm+(n+m=4)团簇的稳定中具有重要作用,团簇的总磁矩主要取决于铁原子的磁矩和各个原子磁矩排布情况.     

Density function study transition metal chromium-doped alkali clusters: the finding of magnetic superatom

The structures, stabilities and magnetic properties of CrX_n (X = Na, Rb and Cs; n up to 9) clusters are studied using density functional theory to search for the stable magnetic superatoms. The geometrical optimisations indicate the ground-state structures of CrX_n evolve toward a close packed structure with an interior Cr atom surrounded by X atoms as the cluster size increase. Their stabilities are analysed by the relative energy, gain in energy (ΔE(n)) and the highest unoccupied molecular orbital and lowest unoccupied molecular orbital gaps. Furthermore, the magnetic moments of CrX_n clusters show an odd–even oscillation. Here, we mainly focus on the CrX₇ (X = Na, Rb and Cs) clusters due to the same valence count as the known stable magnetic superatoms VNa₈, VCs₈ and TiNa₉. Although these clusters all have a filled electronic configuration 1S²1P⁶ and large magnetic moment 5 μ_B, our studies indicate that only CrNa₇ is highly stable compared to its nearest neighbours, while CrRb₇ and CrCs₇ clusters are less stable. This suggests that Cr-doped Na₇ is most appropriate for filled electronic configuration and CrNa₇ is shown to be a stable magnetic superatom. More interesting, we find CrRb₈ and CrCs₈ with the filled electronic configuration 1S²1P⁶ have higher stability and large magnetic moment 6 μ_B in their respective series.     

Structural,electronic, magnetic and thermoelectric properties of inverse Heusler alloys Ti2CoSi,Mn2CoAl and Cr2ZnSi by employing Ab initio calculations

ABSTRACT

The inverse Heusler alloys such as Ti₂CoSi, Mn₂CoAl and Cr₂ZnSi were studied in the framework of density functional theory using FP-LAPW linearised augmented plane wave method in order to determine the different physical properties such as structural, electronic, magnetic and thermoelectric. The generalised gradient approximation (GGA) was used to treat the exchange–correlation energy and the Beck-Johnson (mBJ) approach was used to calculate the electronic properties. In all studied compounds, the stable type Hg₂CuTi was energetically more favourable than Cu₂MnAl type structure. The results show that two compounds (Ti₂CoSi and Mn₂CoAl) are both ferromagnetic (FM) while Cr₂ZnSi is antiferromagnetic (AFM). The compounds Ti₂CoSi and Mn₂CoAl have a total magnetic moment of 3 and 2?μ_B, respectively, whereas the Cr₂ZnSi alloy has a total magnetic moment equals zero. The Ti₂CoSi, Mn₂CoAl and Cr₂ZnSi compounds exhibit half-metallic (HM) character with 100% spin polarisation at the Fermi level. Finally, the semi-classical Boltzmann theory implicit in the BoltzTraP code was used to calculate the electronic transport coefficients such as thermal and electrical conductivity, the Seebeck coefficient and the factor of merit.     

Magnetism of mass-filtered nanoparticles on ferromagnetic supports

The magnetic properties of 3d-metal clusters significantly differ from bulk behavior and, for small clusters, strongly depend on the number of atoms within each cluster. Such phenomena are caused by a narrowing of electronic states and the high ratio of surface to volume atoms giving rise to enhanced magnetic orbital moments. However, even large Fe nanoparticles (6–12 nm) deposited onto ferromagnetic surfaces show enhanced orbital moments. At a low coverage large iron clusters on a cobalt film exhibit a nearly doubled value for the orbital moments when compared to bulk behaviour. With increasing coverage, the orbital moment is clearly reduced. Additionally, the spin and orbital moments of iron and cobalt in Fe₅₀Co₅₀ alloy clusters with a size of 7.5 nm on a nickel substrate have been investigated. FeCo alloys are known to exhibit very high magnetic moments for soft magnetic materials. PACS 73.22.-f; 75.75.+a; 81.07.-b     

Magnetic structure of the random system near the ferro-antiferromagnetic transition

The magnetic structure of the disordered alloy Fe₆₅Ni₂₈Mn₇ was investigated in the temperature 4.2–300 K by the methods: small angle scattering of neutrons, Mössbauer effect, magnetization, magnetic contribution to the thermal coefficient of the thermal expansion, and resistivity. All measurements show that long-range ferromagnetic order appears below T_c ? 160 K. At the same time for T ? 100 K, a dramatic change of magnetic state takes place which is interpreted as the freezing of “spin glass”. An increase of the magnetic contribution to the resistivity with decreasing temperature was also found. This increase was attributed to the existence of poor-bonded magnetic moments of the Kondo-type. A model of the magnetic ground state is proposed which includes the details of magnetic behavior such as long-range ferromagnetic order, spin glass, finite ferro-and antiferromagnetic clusters, and Kondo-type states. A magnetic phase diagram of the system Fe₆₅(Ni_1?xMn_x)₃₅ is also proposed.     

Fe-B团簇的结构与磁性的第一性原理研究

采用局域密度近似下的密度泛函理论和原子轨道的线性组合方法,通过离散变分法自洽求解Kohn-Sham方程,详细地研究了Fe_nB(n≤6)团簇的结构和磁性,所得主要结论如下:第一,B原子更倾向于在团簇的表面而不是在团簇的内部,通常计算非晶态的四面体结构和三棱柱结构,对于孤立团簇而言是不稳定的,这说明环境对团簇的结构稳定性有重要影响;第二,当Fe_n+1团簇中的一个Fe原子被B原子取代形成Fe_n团簇时,其结合能增大而Fe原子的磁矩减小;第三,Fe<     

Effect of Gd and Fe doping on magnetic properties of Al87Y5Ni8 amorphous alloy

In this paper we present and discuss magnetic properties of the Al₈₇Y₅Ni₈, Al₈₇Y₄Gd₁Ni₈, Al₈₇Gd₅Ni₈, Al₈₇Y₄Gd₁Ni₄Fe₄ and Al₈₇Gd₅Ni₄Fe₄ amorphous alloys. The examinations have been concentrated on a possible magnetic ordering at low temperatures and its modification by amorphous surroundings as well as different magnetic moment of alloying additions. It was shown that magnetic properties of the Al₈₇Y₅Ni₈ amorphous base alloy correspond to a superparamagnetic body with Ni magnetic clusters. Magnetic moment of Ni atom in amorphous aluminum matrix is found to be 0.3 μ_B that corresponds to less than 50 Ni atoms per one cluster. Gd doping of the base alloy leads to a decrease of the resultant magnetic moment of Ni clusters that can be explained by some antiferromagnetic coupling Ni-Gd and Ni-Ni within magnetic clusters.     

A giant enhancement of magnetic moment in a ternary three-shell icosahedral cluster: Fe@Mn12@Au20

ABSTRACT

The average magnetic moment per atom of Mn₁₃ cluster is expected to be enhanced by doping or coating with a shell. Several ternary core–shell icosahedral clusters TM@Mn₁₂@Au₂₀ were constructed by combining substituting the central Mn with VIII elements (Fe, Co, Ni, Ru, Rh, Pd and Pt) and coating with a icosahedral Au₂₀ shell, and systematically studied by using the first-principles density functional method. Compared to Mn₁₃, Fe@Mn₁₂@Au₂₀ cluster shows a giant enhancement on total magnetic moment (52?µ_B) which can be greatly attributed to the ferromagnetic coupling between spin moments of atoms. Coating with Au₂₀ shell enlarged the average distances of TM-Mn and Mn-Mn and is a useful way to change the magnetic coupling style. By analysis of density of states and electron localisation functional, we can conclude that the weak hybridisation between Fe and Mn in Fe@Mn₁₂@Au₂₀ is propitious to maintain their original direction of spin moments of atoms and then form ferromagnetic coupling.     

Classical simulations of magnetic structures for chromium clusters: Size effects

Classical (Heisenberg) simulations show that the total magnetization of the lowest-energy states of clusters made of antiferromagnetically coupled chromium atoms is planar, rather than collinear, depending on the arrangement of the atoms. Although the model Hamiltonian is not restrictive, many cluster configurations of various numbers of atoms do not use all three directions for the spins. This result confirms the conclusion drawn from the local-spin DFT calculation by Kohl and Bertsch that clusters of N≤13 have non-collinear magnetic moments. The present simulations show non-collinear spin ordering also for bigger clusters, designed to be as spherical as possible following the bcc arrangement, when atoms interact both with the nearest and next-nearest neighbours. Depending on the signs of the coupling constants frustration appears. The advantage of the discrete model, despite the simplicity, is that very large clusters and magnetization at finite temperatures can be studied. This model predicts that clusters with specific numbers of atoms interacting only with the nearest neighbours have collinear spins as in the bulk. We also apply the model to simulate the destruction of the anti-ferromagnetic ordering by thermal fluctuations. This model shows no unique magnetization of mixed Fe _0.33 Cr _0.67, which is consistent with experimental observations.