Specific heat and modulated magnetic structures in the HoNi0.5Cu0.5 and ErNi0.6Cu0.4 compounds

Specific heat measurements performed on HoNi_0.5Cu_0.5 and ErNi_0.6Cu_0.4 have confirmed the role of the rare earth nature on the thermal stability of modulated magnetic structures. With non Kramers ions (Ho for instance) lying in a very low symmetry site a modulated structure can remain stable down to zero Kelvin; the magnetic moment being induced by the exchange field, no entropy is associated with the modulation. With Kramers ions (Er for instance), for which an intrinsic moment always exists, a modulated structure transforms at low temperature into an antiphase structure.     

Crystal and magnetic structures of the oxyphosphates MFePO 5 (M = Fe, Co, Ni, Cu). Analysis of the magnetic ground state in terms of superexchange interactions

We have studied in detail the crystal and magnetic structures of the oxyphosphates MFePO₅ (M: divalent transition metal) using neutron powder diffraction as a function of temperature. All of them are isomorphic to the mixed valence compound α-Fe₂PO₅ with space-group Pnma. No disorder exists between the two metallic sites. The M²⁺O₆ octahedra share edges between them and faces with Fe³⁺O₆ octahedra building zigzag chains running parallel to the b-axis that are connected by PO₄ tetrahedra. The topology of this structure gives rise to a complex pattern of super-exchange interactions responsible of the observed antiferromagnetic order. The magnetic structures are all collinear with the spin directed along the b-axis except for M = Co. The experimental magnetic moments of Cu⁺² and Ni²⁺ correspond to the expected ionic value, on the contrary the magnetic moment of Fe³⁺ is reduced, probably due to covalence effects, and that of Co²⁺ is greater than the spin-only value indicating a non negligible orbital contribution. Using numerical calculations we have established a magnetic phase diagram adapted for this type of crystal structure and determined the constraints to be satisfied by the values of the exchange interactions in order to obtain the observed magnetic structure as the ground state. Received 15 December 2000 and Received in final form 25 June 2001     

Magnetic Structure of rare earth orthomanganites - 1. YMnO3

In an attempt to determine the magnetic structures of the heavy rare earth manganites of perovskite type, we have studied first the antiferromagnetic order of manganese in YMnO₃. The Néel temperature is about 42 K, the Mn³⁺ ordering is a helix and derives from an A mode. The propagation vector of the helical structure is along the b axis: k=[0 k_y 0] with k_y= 0.0786. The Mn³⁺ ions carry a magnetic moment of only 3.10 ± 0.1 μ_B at 4.2 K. We present a phase diagram of helical and collinear modes in terms of exchange integrals.     

Magnetic properties of R3Rh2 compounds with R=Gd, Tb,Dy, Ho and Er

Bulk magnetic measurements performed on polycrystalline samples of the tetragonal compounds R₃Rh₂ with R=Gd, Tb, Dy, Ho and Er are presented. All the compounds are ferromagnetic at low temperature. However in Tb₃Rh₂ an antiferromagnetic behaviour is observed between 14 and 24 K. In Gd₃Rh₂, where the magnetocrystalline anisotropy must be negligible, it seems that the magnetic structure is not collinear. In the other compounds the observed properties essentially result from indirect exchange interactions and crystal field effects acting on the rare earth ions which lie in low symmetry sites.     

Magnetic properties and structures of PrPt and NdPt

The magnetic properties of the CrB-type orthorhombic phase of PrPt and NdPt are presented. Below their ordering temperature which is 15 and 23 K respectively, these compounds exhibit a ferromagnetic behaviour. The magnetic structures determined by neutron diffraction are collinear. While in PrPt, the magnetic moments are parallel to c, in NdPt, the moments lie in the (a, c) plane and make an angle of 22° with a. These properties and especially the peculiar moment direction in NdPt are discussed in terms of crystalline electric field effects on the rare earth ion.     

Enhancement of the upper critical magnetic field in La1.2−xEuxMo6S8 compounds

Measurements of the upper critical magnetic field (H_c2) of the pseudoternary system La_1.2?xEu_xMo₆S₈ were made at temperatures above 1.5 K. An enhancement of the value of H_c2 was found for 0 < x ? 0.6. The results are attributed to an increase of the orbital critical field with increasing x, compensation of the applied magnetic field by a negative exchange field due to an antiferromagnetic exchange interaction between the conduction electron spins and the Eu magnetic moments (Jaccarino-Peter effect), and exchange scattering of conduction electrons by the rare earth magnetic moments.     

化合物SmCo5的电子结构、自旋和轨道磁矩及其交换作用分析

用自旋极化的MS-Xα方法研究了稀土-过渡族化合物SmCo5₅的电子态密度、自 旋能级劈裂及原子磁矩.研究结果显示，由于化合物中Sm-Co间的轨道杂化效应，使Sm原子原来的5d00空轨道上占据了少量5d电子.由于Co(3d)-Sm(5d)电子间的直接交换作用，导致了Sm-Co间的磁性交换耦合，这是化合物中形成Sm-Co铁磁性长程序的一个重要原因.在SmCo5_{5化合物中存在6个能级呈现负交换耦合，导致了SmCo55化 关键词： 电子结构 自旋极化 原子磁矩 交换耦合}     

Neutron diffraction study of the magnetic ordering in the series R 2 BaNiO 5 (R = Rare Earth)

A neutron diffraction study, as a function of temperature, of the title compounds is presented. The whole family (space group Immm, a ≈ 3.8?, b ≈ 5.8?, c ≈ 11.3?) is structurally characterised by the presence of flattened NiO₆ octahedra that form chains along the a-axis, giving rise to a strong Ni-O-Ni antiferromagnetic interaction. Whereas for Y-compound only strong 1D correlations exist above 1.5 K, presenting the Haldane gap characteristic of 1D AF chain with integer spin, 3D AF ordering is established simultaneously for both R and Ni sublattices at temperatures depending on the rare earth size and magnetic moment. The magnetic structures of R₂BaNiO₅ ( R = Nd, Tb, Dy, Ho, Er and Tm) have been determined and refined as a function of temperature. The whole family orders with a magnetic structure characterised by the temperature-independent propagation vector = (1/2, 0, 1/2). At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. Except for Tm and Yb (which does not order above 1.5 K), the magnetic moment of the R³⁺ cations are close to the free-ion value. The magnetic moment of Ni²⁺ is around 1.4 , the strong reduction with respect to the free-ion value is probably due to a combination of low-dimensional quantum effects and covalency. The thermal evolution of the magnetic structures from T _N down to 1.5 K is studied in detail. A smooth re-orientation, governed by the magnetic anisotropy of R³⁺, seems to occur below and very close to T _N in some of these compounds: the Ni moment rotates from nearly parallel to the a-axis toward the c-axis following the R moments. We demonstrate that for setting up the 3D magnetic ordering the R-R exchange interactions cannot be neglected. Received 19 July 2001     

Magnetic ordering in Ni–Cd ferrite

A series of Ni_1−xCd_xFe₂O₄ (0.0≤x≤0.8) were prepared by conventional double sintering ceramic method and sintered at 1200 °C for 6 h. X-ray diffraction results confirmed the single-phase spinel structures of all the samples. The Curie temperature decreases linearly with increasing Cd content, which is explained due to the weakening of the A–B exchange interaction. The sample with x=0.7 shows re-entrant type of spin glass phase transitions. The magnetic moment and saturation magnetization at 20 K are found to increase with Cd content up to x=0.5 and then tends to decrease for x>0.5. The increase in magnetic moment with cadmium is attributed to Neel's two sublattice (A- and B-sublattice) collinear models according to which the magnetic moment is the vector sum of the lattice magnetic moment. The decrease in magnetization for x>0.5 obeys the Yafet–Kittel (Y–K) model. The increase in Y–K angles for x>0.3 indicates the increased tendency for triangular spin arrangements on B-sites. This suggests the existence of a canted spin structure in the ferrite system with higher content of Cd.     

The antiferromagnetic structure of TbB4

The magnetic structure of the rare earth tetraboride TbB₄ (crystallographic space group P4/mbm) has been determined by neutron diffraction on a polycrystalline sample. Below the experimentally determined Néel temperature of T_N = (43±1) K TbB₄ is ordered antiferromagnetically. The data refinement yielded a magnetic moment value of (7.7 ± 0.2) μ_B/Tb ion at 4.2 K which we interpret as Tb⁴⁺. The magnetic structure is antiferromagnetic collinear with the moments perpendicular to the tetragonal axis.     

Magnetic structures and magnetic susceptibilities of terbium oxysulfide and oxyselenide

For the two isomorphous compounds Tb₂O₂S and Tb₂O₂Se, the magnetic susceptibility measurements on powder samples show an antiferromagnetic ordering with Néel temperatures of about 7.7 and 7K respectively. Differing in this respect from the other rare earth oxyselenides, the magnetic anisotropy of Tb₂O₂Se at low temperature is weaker than that of Tb₂O₂S.We also determine the magnetic structures of these two compounds by neutron diffraction experiments at 1.5K. The magnetic cell is orthohexagonal and doubled along the c-axis; the magnetic moments make an angle, with the c-axis, of 47 ± 10° for Tb₂O₂S and 30 ± 10° for Tb₂O₂Se and the moment values at 1.5K are 8.14 ± 0.2μ_B and 6.5 ± 0.2μ_B, respectively.It is rather exceptional that in a rare earth uniaxial compound the magnetic moment makes an angle with the c-axis. However we interpret this situation by the fact that several levels exist very near to the ground state. The crystal field calculations are in good agreement with the experimental results.     

Electronic structure and magnetic properties of metastable SmCo<Subscript>7</Subscript> compound

The electronic density of states, spin-splittings and atomic magnetic moments of SmCO₇-compound have been studied using spin-polarized MS-Xα method. The results show that a few of electrons are transferred to Sm(5d⁰) orbital because of orbital hybridization between Sm and Co atoms in the compound. The exchange interactions between 3d and 5d electrons lead to the magnetic coupling between Sm and Co, and therefore, result in the long-range ferromagnetic order inside the SmCo₇ compound. There are negative exchange couplings occurring at some levels, which weakens the strength of average coupling around Co lattice. So, the Curie temperature and Co-moment of SmCo₇-decrease distinctly compared with pure Co. Compared with SmCo₅ compound, the disordered substitution of Co-Co “dumbbell-atom” pairs for Sm changes the local environment of Co lattice, which makes the 2e site bear negative magnetic moment. The strength of hybridization near Fermi level weakens and the free energy of the compound increases obviously. Thus, SmCo₇ is a metastable compound at room temperature. Considering the localization of 4f electrons and a few of 5d electrons arising from the orbital hybridization, the magnetic moment of Sm atom will be 1.61μ_B in SmCo₇ compound, which is in agreement with the experimental values of Sm³⁺ ion-moment and Sm atom-moment in metals.     

Magnetic properties of GdxLa1−x CoSi

The magnetic properties and crystal structure of Gd_xLa_1−x CoSi compounds (x=0, 0.25, 0.5, 0.65, 0.8, 1) are investigated. The anisotropy of the magnetic properties are analyzed for a GdCoSi single crystal. The temperature dependences of the magnetic susceptibility and the tetragonal lattice parameters of GdCoSi are characterized by anomalies in the vicinity of the magnetic phase transition temperatures, 166 K and 300 K. The Néel temperatures, the effective magnetic moments, and the paramagnetic Curie temperatures of these compounds decrease as gadolinium is replaced by lanthanum. The compound LaCoSi is a Pauli paramagnet. The results are discussed on the basis of a model that takes into account the presence of positive and negative exchange interactions and the itinerant magnetism of the cobalt sublattice. Fiz. Tverd. Tela (St. Petersburg) 39, 1270–1274 (July 1997)     

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.     

Magnetic phase transitions in layered intermetallic compounds

Magnetic, magnetoelastic, and magnetotransport properties have been studied for the RMn₂Si₂ and RMn₆Sn₆ (R is a rare earth metal) intermetallic compounds with natural layered structure. The compounds exhibit wide variety of magnetic structures and magnetic phase transitions. Substitution of different R atoms allows us to modify the interatomic distances and interlayer exchange interactions thus providing the transition from antiferromagnetic to ferromagnetic state. Near the boundary of this transition the magnetic structures are very sensitive to the external field, temperature and pressure. The field-induced transitions are accompanied by considerable change in the sample size and resistivity. It has been shown that various magnetic structures and magnetic phase transitions observed in the layered compounds arise as a result of competition of the Mn–Mn and Mn–R exchange interactions.     

Magnetic behaviour of TbMnFe

Neutron diffraction and M?ssbauer measurements have been carried out on the cubic Laves phase intermetallic TbMnFe. The magnetic moment on the transition metal atom is found to be low, 0.2μ _B, at room temperature. This moment is temperature independent down to 10 K. Magnetic moment on the rare earth atom varies from 2.5μ _B at 296 K to 7.27μ _B at 10 K. M?ssbauer spectra recorded at 298 K and 78 K have magnetic character but there is a large distribution of hyperfine field values. Both these features arise due to magnetic frustration created in the sample due to the competing ferro and antiferromagnetic interactions between the transition metal atoms.     

Strong correlations and magnetic frustration in the high Tc iron pnictides

We consider the iron pnictides in terms of a proximity to a Mott insulator. The superexchange interactions contain competing nearest-neighbor and next-nearest-neighbor components. In the undoped parent compound, these frustrated interactions lead to a two-sublattice collinear antiferromagnet (each sublattice forming a Néel ordering), with a reduced magnitude for the ordered moment. Electron or hole doping, together with the frustration effect, suppresses the magnetic ordering and allows a superconducting state. The exchange interactions favor a d-wave superconducting order parameter; in the notation appropriate for the Fe square lattice, its orbital symmetry is dxy. A number of existing and future experiments are discussed in light of the theoretical considerations.     

Unique magnetic behaviour of TmFeAl

The magnetic properties of the compound TmFeAl have been studied using⁵⁷Fe and¹⁶⁹Tm Mössbauer spectroscopy, magnetic measurements and neutron depolarization (ND). The compound exhibits a complex magnetic behaviour. At 4 K it contains a Tm moment of 7 _B and a distribution of Fe moments with an average of 0.5 _B. Between 10 and 60 K a distribution in magnetic ordering temperatures is observed. ND displays the growth of small magnetic domains when the temperature decreases towards 4 K. The structural disorder of the Fe atoms in the lattice combined with the competing exchange interactions between the different types of atoms and the high anisotropy of the Tm moment gives rise to the observed magnetic behaviour. Results of high field magnetization and⁵⁷Fe Mössbauer spectroscopy are presented also for GdFeAl and YFeAl.     

Magnetic and magnetoelastic properties of the TbMnSi and Tb<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>MnSi compounds

X-ray diffraction studies showed substitution of nonmagnetic lanthanum for terbium in the TbMnSi polycrystalline compound to initiate a structural transition from a TiNiSi-type orthorhombic structure (for TbMnSi) to a CeFeSi-type tetragonal phase (for Tb_0.5La_0.5MnSi). Magnetic measurements (of the magnetization, magnetostriction, thermal expansion) performed on Tb_0.5La_0.5MnSi revealed a change in the character of magnetic ordering, the appearance of a ferromagnetic component in the Mn magnetic moment, a strong increase in magnetization as compared to TbMnSi, and the appearance of a spontaneous magnetic moment. Insertion of the lanthanum ion onto the rare-earth sublattice of TbMnSi brings about a change in the unit cell size and, hence, in the Mn-Mn, Mn-Si, and R-Mn interatomic distances, which causes, in turn, a change in the character of exchange interactions in Tb_0.5La_0.5MnSi and the formation of a complex magnetic structure.     

Microstructure and thermal properties of quasi-equal rare earth substitution Y0.5Gd0.5Ba2Cu3O6.94 superconductor

Sintering effects in YBa₂Cu₃O_z samples with quasi-equal rare earth substitution have been investigated. It has been shown that the Y-123 type compound can be formed when gadolinium is partially substituted (in this case 50% atomic substitution) for yttrium. The superconducting compound was obtained by the optimized ceramic method, including solid-state reaction, melting and sintering, controlled by X-ray diffraction. The microstructure was investigated by SEM and energy dispersive X-ray analyses. An AC susceptibility measurement has shown that T _c =93 K. Temperature-dependece of the thermal conductivity (4.5–300 K) of the polycrystalline Y_0.5Gd_0.5Ba₂Cu₃O_6.94 sample was also measured.