Ferromagnetic Mixed-Valence and Kondo-Lattice State in TmTe at High Pressure

Abstract

Neutron diffraction experiments on TmTe at pressures up to 7 GPa are reported. The semiconductor-to-metal transition occurring at 2 GPa in this compound is accompanied by the appearance of strong ferromagnetic exchange interactions producing a Curie temperature as high as 14 K. This behavior is characteristic for the incipient mixed-valence regime just above the transition pressure and traced back to the release of a small concentration of free charge carriers in the material. The steep decrease of both the Curie temperature and ordered magnetic moment occurring at higher pressures emphasizes the role of Kondo fluctuations, and raises the interesting possibility that a quantum critical transition to a non-magnetic ground state of mixed-valence Tm might take place around 6 GPa.In magnetic materials, the concentration of mobile charge carriers is known to strongly influence the nature and strength of exchange interactions between neighboring atoms. This is evidenced in a particularly striking manner by systems undergoing metal-insulator transitions upon doping such as Eu chalcogenides [1] and Eu hexaboride [2], or by La manganites where the existence of hole-mediated "doubleexchange" interactions is assumed to be responsible for the orders-of-magnitude response of the electrical resistivity to an applied magnetic field [3]. In the above examples, the extra carriers are usually created by substituting the rare-earth element by an ion with a different valence. This clearly produces a local charge singularity, which can have nontrivial consequences such as e.g. the formation of impurity states, bound magnetic polarons, etc. [4]. A more uniform situation may be achieved in the case where the charge carriers result from a homogeneous mixed-valence state of all magnetic atoms. This phenomenon can be viewed as the quantum admixture of two electronic configurations corresponding to different occupations n and n-1 of the rareearth 4f-shell, one electron being able to fluctuate between a localized 4f-orbital and extended 5d-6s states. Numerous examples of this behavior have been found experimentally, mostly in Ce, Sm, Eu, Tm and Yb compounds [5]. The mixed valence can occur spontaneously, or be the result of an external perturbation, typically chemical substitution or applied pressure. However, in the overwhelming majority of these systems, the valence instability is accompanied by large spin fluctuations which cause a complete suppression of the magnetic state and obscure the variation of magnetic exchange interactions. The only notable exception to date has been reported for the cubic (NaCl structure) thulium monochalcogenide family TmX (X = S, Se, Te) [6]. Among these materials, stoichiometric TmSe exhibits antiferromagnetic (AF) order below T N = 3.4 K with a valence estimated to be comprised, depending on the technique used, between 2.55 (lattice constant) and 2.72 (effective moment). This unique behavior has been claimed to result from the fact that both valence states Tm 2+ and Tm 3+ have degenerate low-lying multipl...

Moissanite: A Window for High-Pressure Experiments

“…The sided Nd:yttrium-aluminum-garnet laser large sample size could allow the use of heating to 3700 K at 3 GPa of a graphite additional types of analysis, such as neusample sandwiched between two layers of tron diffraction (28), neutron scattering NaCl platelets. We also heated moissanite (29), inelastic x-ray spectroscopy, x-ray anvils in air to 1400 K for 36 hours without Compton scattering. nuclear magnetic resdamage, showing that the moissanite anvil onance (30), and ultrasonic interferometry is useful for high P-T experiments with (31.…”

Electronic structure and spectroscopic properties of thulium monochalcogenides

“…In addition a crystallographic phase transition has been reported at ϳ8 GPa, from the NaCl structure to a tetragonal phase. [8][9][10] The optical properties of TmSe 1−x Te x alloys have also been measured, 11 in particular the Kerr rotation spectrum.…”

XMCD under pressure at the FeKedge on the energy-dispersive beamline of the ESRF

“…Pressures up to 15 GPa (Cornelius et al, 1996) and temperatures of 30mK have been reported (Webb et al, 1976). Neutron diffraction is very sensitive to the magnetic order (Goncharenko & Mirebeau, 1998), but the determination of the magnetic moment variations remains limited in precision (Link et al, 1998). Although most published work is reported for pressures below 20 GPa, recent studies up to 50 GPa in a DAC at liquid He temperature have been performed (Goncharenko & Mirebeau, 1997).…”