R. Georges

Classical treatment of a heisenberg linear chain with spin alternation; application to the MnNi(EDTA)-6H2O complex

Abstract The thermodynamic behaviour of an exchange-coupled linear system with two alternating spin sublattices is investigated from a classical point of view. A closed formula is derived for the magnetic susceptibility which is easily adapted to real spin systems, using appropriate scaling transformations. The proposed approach is shown to describe successfully the behaviour of a new bimetallic chain complex MnNi(EDTA)-6H 2 O. In particular, a minimum in the xT versus T plot is observed at very low temperature, in agreement with theory.

Spin alternation in one-dimensional exchange-coupled systems: Magnetic behaviour of the (12−1)N chain

Abstract We report on the magnetic behaviour of exchange-coupled closed chains (rings) of increasing length made of two alternating spin sublattices S a = 1 2 , S b = 1 . Taking into account the geometrical and spin space symmetries of such syste the problem is shown to be tractable up to 10 spins for distinct values of the ga/gb ratio between Lande factors; the results are discussed for an antiferromagnetic coupling. The limiting behaviour for the infinite chain is determined by extrapolation of the data obtained for finite rings. At very low temperatures, the analogy with the regular S = 1 2 ferromagnetic chain is underlined.

The magnetic and electrical properties of some rare earth tetraborides

Abstract A study of the magnetic and electrical properties of some rare earth tetraborides (PrB 4 , SmB 4 , GdB 4 , TbB 4 , DyB 4 , HoB 4 , ErB 4 and YbB 4 ) was carried out on powders and single crystals prepared by the flux method. The tetraborides were found to be metallic. The only exceptions are PrB 4 which is ferromagnetic and YbB 4 where no magnetic ordering was found down to 0.34 K; they are antiferromagnetic at low temperatures. PrB 4 and ErB 4 have a strong anisotropy along the c axis from the susceptibility and magnetization measurements. ErB 4 is of special interest because at low temperature it is antiferromagnetic in a weak magnetic field but shows two successive magnetic transitions with increasing field, leading finally to a purely ferromagnetic structure.

Synthese, cristallogenese, proprietes magnetiques et effets magnetostrictifs spontanes de quelques tetraborures de terres rares

Study of the magnetic properties of some rare-earth tetraboride powders shows that NdB/sub 4/, SmB/sub 4/, GdB/sub 4/, TbB/sub 4/, DyB/sub 4/, and HoB/sub 4/ are antiferromagnetic at low temperature, whereas PrB/sub 4/ is ferromagnetic. Preliminary results concerning single crystals are also given. PrB/sub 4/ is strongly anisotropic and its magnetization is observed only along the c ..-->.. axis. At low temperature ErB/sub 4/ is antiferromagnetic in weak fields; as the field strength increases, two magnetic transitions are observed successively, leading finally to a purely ferromagnetic arrangement. No magnetic order has been found for YbB/sub 4/ down to 4.2/sup 0/K, and a Moessbauer study shows that the isomer shift of ytterbium is intermediate between those of Yb/sup 2 +/ and Yb/sup 3 +/ in compounds with metallic behavior; the variation of the crystal parameters vs temperature (4.2--300/sup 0/K) shows that only a is affected near the ordering temperature. 9 figures, 1 table.

Mixed-valence trinuclear cluster (2dn, dn+1); Influence of the electronic coupling on the magnetic properties

Abstract The influence of electronic processes on the magnetic behavior of the mixed-valence trinuclear clusters (2d n , d n +1 ) is discussed. The problem is solved by means of a pseudoparticle formalism by assuming competing effects such as electronic transfer, and localization of the extra-particle due to different transfer pathways or different sites…. The magnetic susceptibility is derived for significant values of the characteristic parameters in the cases n = 1, 3, 5. Finally, the magnetic behavior of Fe 3 O(CH 3 COO) 6 L 3 , with L = H 2 O or pyridine, is discussed on the basis of the proposed model.

Crystal structure and magnetic properties of new rare earth ternary equiatomic silicides RERhSi

Abstract New ternary silicides RERhSi (RE=Y, Gd, Tb, Dy, Ho, Er) have been prepared by arc melting from the elements and annealed for for several days at 800°C. Single crystal studies show these materials to be of orthorhombic symmetry and isostructural with TiNiSi (anti-PbCl 2 structure). Gd, Tb and Dy compounds show a spontaneous magnetization and their ordering consists probably of a non-colinear arrangement of the moments. HoRhSi and ErRhSi order antiferromagnetically at 8K and 7.5K respectively. At 4.2K HoRhSi undergoes a metamagnetic transition above 6kOe. A field transition appears also at 4.2K for ErRhSi above 12kOe.

Thermodynamics of ferrimagnetic ising chains

The exact solutions of the so‐called ferrimagnetic Ising chain made up of two sublattices (S0,S1) are derived from a transfer matrix method. The short‐range ferrimagnetic order occurs when considering different spins and/or different Lande factors on both sublattices. Most of the physical features of interest are shown to be involved in the S0=S1=1 system including an alternation of Lande factors (g0,g1) and local anisotropies (K0,K1). Thus, in the limit K0→∞ stabilizing a Kramers doublet, Sz=±1, on even sites, the system behaves like the ferrimagnetic chain (S0=1/2, S1=1). The susceptibility and magnetization curves are discussed in various situations as a function of the significant parameters.

Electron transfer in tetranuclear mixed-valence iron clusters. Role of the topology on the magnetic properties

Abstract The energy levels and magnetic properties of mixed-valence d 5 d 5 d 5 d 6 clusters are studied from a model that takes into account the electron transfers between orbitally nondegenerate orbitals of the metal ions, and superexchange interactions. The influence of the electron transfer on the spin state energies and magnetic properties is examined as a function of the geometry of the tetranuclear cluster, including the limiting cases with T d and D 4h symmetries. For geometries close to the tetrahedral, effects of antiferromagnetic superexchange interactions are also discussed.

Magnetic properties of ferrimagnetic chains

Abstract The magnetic behaviors of complexes MnM′ (EDTA)⋯6H 2 O (M′ = Ni, Co) are discussed in terms of so-called ferrimagnetic 1-d systems. The data agree very well with theoretical predictions for Heisenberg ([MnNi] complex) or Ising ([MnCo] complex) models, yielding the respective values J = -1,5 K and J = -1,9 K for the exchange constants.

Anisotropy and magnetic phase transitions in the rare earth tetraborides TbB4, HoB4 and ErB4☆

Abstract In order to determine the electronic properties of the rare earth tetraborides TbB 4 , HoB 4 and ErB 4 (UB 4 structure type) large single crystals of 50–100 mg were prepared by the Czochralski and the aluminium flux methods. Magnetic and electrical measurements were performed at temperatures ranging from 2 to 300 K and under magnetic fields up to 80 kOe, applied along various crystallographic directions. All these tetraborides are metallic, and they show long-range antiferromagnetic ordering at low temperatures under a weak field. However, their detailed magnetic structure and field-temperature phase diagrams, confirmed by neutron diffraction experiments, are quite different.