Valence and magnetic ordering in the mixed valent compound TmSe

Abstract

A new AC microcalorimetry set-up, which facilitates scanning the pressure (P) at low temperatures (T) permits the precise determination of the (P, T) phase diagram of the intermediate valence compound TmSe in a wide pressure range (0–14 GPa). In the low-pressure area, the occurrence of a phase transition at 3 GPa is confirmed. Contrary to previous measurements which indicated a possible new transition at 6 GPa, our results show a continuous behaviour with a Néel temperature increasing linearly with P. Above 10 GPa, the splitting of the magnetic anomaly might indicate a new phase transition. The same kind of experiment was performed for TmS, and after a P shift of 7 GPa, a scaling can be realized between the two compounds.

Introduction

In typical heavy fermion systems, the valence is usually only very slightly changed under pressure. In order to study a compound in which the valence changes in a wider range under pressure, we investigated the mixed valence compound TmSe to see the influence of valence change on magnetism. The intermediate valence behaviour can be represented by the following equilibrium between the divalent $4{\mathrm{f}}^{13}$ and the trivalent $4{\mathrm{f}}^{12}$5d configurations: ${\mathrm{Tm}}^{2+}\iff {\mathrm{Tm}}^{3+}+{\mathrm{e}}^{-}$. The two configurations are magnetic but the angular momentum are different: the divalent ${\mathrm{Tm}}^{2+}$ is a Kramer's configuration with $J_{{\mathrm{Tm}}^{2+}}=\frac{7}{2}$, whereas the trivalent ${\mathrm{Tm}}^{3+}$ is a non-Kramer's ion with $J_{{\mathrm{Tm}}^{3+}}=6$. In a schematic view of electron density, the 4f level is situated around the Fermi level and the 5d band is above. With increasing pressure, the broadening of the band implies a change in the relative position of the Fermi level. The 5d band becomes populated and the equilibrium moves toward the trivalent configuration. Therefore, a clear change occurs under pressure around 3 GPa [1]. The transition observed is an insulator to metal transition associated with a change in the magnetic structure.

In order to see the behaviour of TmSe in a wide pressure range and with a different technique than resistivity measurements, we proceeded with AC microcalorimetry. At very high pressures, TmSe becomes almost trivalent and can be compared to TmS which is already quasi-trivalent at ambient pressure. Thus, the same kind of experiment was undertaken with TmS for comparison.