SECOND ORDER PHASE TRANSITION IN THE FERRIMAGNETIC THIOSPINEL FeCr₂S₄

Résumé
FeCr₂S₄, de structure ferrimagnétique spinelle, reste cubique jusqu'à 4,2 K. Il présente une transition de phase du second ordre vers 13 K qui affecte les spectres Mössbauer : les inter- prétations antérieures /2-5/ sont basées pour la plupart sur un modèle de champ cristallin à un ion supposant les électrons d du Fe²⁺ A localisés . Une décomposition préalable de nos spectres entre 1,5 K et 300 K en singulets montre que les solutions à 1 site antérieurement suggérées ne sont pas uniques. De plus, les intensités expérimentales des raies diffèrent des intensités théoriques et les largeurs des raies présentent des variations incohérentes avec T. Une reproduction cohérente nécessite deux sites Mössbauer : à T > 13 K, les deux sites d'intensité égale sont caractérisés par V_zz > 0, η = 0, θ = ψ = 0 et par V_zz < 0, η = 1, θ = ψ = &Pi/2 A T< 13 K, le site le plus intense (0.75) est tel que V_zz < 0, η ~ 0.25, θ = &Pi/2, ψ = 0 et l'autre tel que V_zz < 0, η ~ 0.25, θ = ψ = &Pi/2. Les électrons 3d du Fe IIA sont mieux décrits dans un modèle de bande étroite /6/ doublement dégénérée : à T > 13 K, le 6ème électron du fer relaxe entre les deux états de bande, l'un de symétrie (x² - y²), l'autre de symétrie (3z² - r² ) hybridé avec l'état excité du Cr^III en B. A T < 13 K, seul ce dernier état est occupé. Ceci traduitle passage d'un paramagnétisme orbital (T > 13 K) à un ferromagnétisme orbital (T < 13 K) par analogie avec /7/.

Abstract
The spinel ferrimagnet FeCr₂S₄ exhibits a second order phase transition around 13 K anomaly, in the specific heat /1/. It remains cubic down to 4.2 K. Previous interpretations of Mössbauer spectra have suggested : a static to dynamic Jahn Teller transition /2/, an antiferrodistortive transition /3/, an induced E.F.G. by the magnetic order /4/ or by a lattice distorsion due to magnetostrictive effects/5/. Most of these interpretations were based upon a single ion crystal field approach, the 3d electrons of Fe²⁺ A were supposed to be localized. Mössbauer studies on stoichiometric polycrystalline FeCr₂S₄ were carried out from 1.5 to 300 K. An initial decomposition of the spectra into singlets shows that the previous single site solutions /2-5/ are not unique; moreover the experimental and calculated line intensities do not agree and the different linewidths show incoherent variations with T. A more reliable two sites solution is suggested. At T > 13 K, the two sites have the same intensity and are caracterized by V_zz > 0, η = 0, θ = ψ = 0 and V_zz < 0, η = 1, θ = ψ = &Pi/2. At T < 13 K, the two sites are caracterized by V_zz > 0, η ~ 0.25, θ = &Pi/2 but ψ = 0 and ψ = &Pi/2 are respectively related with the higher (0.75) and the lower (0.25) site interisities . Moreover, the experimental mean linewidth, the hyperfine magnetic fields and the isomer shifts show discontinuities at 13 K. Finally, low temperature X rays patterns show no dilatation anomaly and no discontinuity of the ∂a/∂T values (a : unit cell parameter). These features are explained assuming a narrow doubly degenerate d band of Fe^IIA, owing to the low mobility of 3d electrons /6/. The sixth 3d electron of Fe^IIA is assumed to occupy orbital band states with |ε> or (|θ> + |ψCr^II B>) symmetry, owing to the hybridization of |θ> with the B site energetically close Cr^II state. At T > 13 K, the sixth 3d electron relaxes between the |ε> and the hybridized |θ> orbital band states , leading to two equal intensity sites. At T < 13 K, only the hybridized |θ> orbital band state is occupied. This corresponds to an orbital paramagnetism for T > 13 K and to a ferromagnetic orbital ordering for T < 13 K. This is related with the fact that an Hubbard Hamiltonian in the atomic limit leads not only to spin ordering but also to orbital ordering /7/. Moreover, the transition shows at T < 13 K a slow relaxation of the E.F.G. The coupling of the band with the phonons at T < 13 K is different from alinear and weak dynamic Jahn Teller effect as for example for the Asite dilute Fe²⁺.