Magnetic mixed valent TmSe

Various thermal, magnetic, and transport properties of single-phase TmₓSe crystals have been measured, partly as a function of temperature (0.04≤T≤300 K), and/or magnetic field (H≤20 kOe), and/or hydrostatic pressure (p≤20 kbar). The Tm to Se ratio of the samples has been varied between 0.87 and 1.05. The valence state of the Tm ions is influenced by the chemical composition and the divalent character increases from {~}0% for Tm0.87Se to {~}28% for Tm1.05Se. The low-temperature magnetic susceptibility and the anomalously high compressibility are consistent with the model that the ground state of each Tm ion contains both divalent and trivalent character. In all crystals with this homogeneously mixed-valent character, the localized $4f$ electrons and the itinerant $5d$ electrons strongly interact below {~}35 K. Magnetic ordering occurs in all compounds investigated, but the type of ordering and the critical parameters of the magnetic phase diagram are again strongly influenced by the chemical composition.