Successive multipole orderings in TmTe, probed by 125Te-NMR

We report nuclear magnetic resonance (NMR) studies on evolutions of the spin fluctuation character in a divalent semiconductor TmTe with the applications of external magnetic field and pressure. Upon cooling below ∼20K 125Te-resonance shift -ΔH/H in a high-field ∼7T begins to deviate from the Curie-Weiss (CW) type increase at high temperatures, and exhibits a saturation behavior below ∼3-4K. The nuclear spin-lattice relaxation rate T1-1 changes the temperature dependence from the increasing behavior at high temperatures to a monotonic decrease below ∼35K and to a rapid drop below ∼3-4K, indicating successive freezing of different spin fluctuation modes. These NMR features lead to an interpretation that two distinct transitions take place in high-fields: from the paramagnetic state (state I) to an intermediate state (state II) below 20-35K, and to a low temperature state (state III) below ∼3-4K. The mechanism of the two successive transitions is discussed in terms of a possible octupolar order and an order of field-induced dipole or octupole moments of originally quadrupole moments of Tm2+ ions, respectively, with the low-energy splitting of the crystal electric field levels. In a low-field ∼1T, the negative ΔH/H continues the CW type increase down to ∼4K. On the other hand, T1-1 exhibits a pronounced increase below ∼15K. These results indicate a development of the generalized susceptibility χ(q) with the wave vector q≠0 in low-fields, which could largely disturb the ordering in the state II. The application of a finite pressure of 0.9GPa hardly changes the spin fluctuation character in low-fields, leading to a conjecture that the ferromagnetic order in the pressure-driven metallic state (TC=14K for ∼2.7GPa) shares the basic mechanism with the ordering in the state II of the semiconducting state. (author)