Cooperative Jahn-Teller effect in TmZn

Specific-heat, resistivity, neutron-spectroscopy, magnetization, and magnetostriction experiments on TmZn are reported. This equiatomic metallic compound crystallizes with the cubic CsCl structure and shows a first-order Jahn-Teller transition to a tetragonal structure at TQ=8.55 K. The cell distortion reaches c/a-1=(-9±1)×10^-3. The magnetic moment orders along a fourfold axis at Tc=8.12 K, its value being largely field dependent. Due to the vicinity of the transitions, they are mixed and strongly raised in temperature by (even low) applied fields as observed by the magnetization and magnetostriction experiments in the paramagnetic state. These results and the elastic constants previously published are analyzed taking into account the quadrupole-quadrupole exchange and the quadrupole-lattice term beside the Heisenberg exchange and the cubic crystal electric field (CEF) terms; the CEF parameters are A₄〈r⁴〉=-38±5 K and A₆〈r₆〉=-19.6±2 K/atom. The ground state is the magnetic triplet Γ₅⁽¹⁾ in the cubic paramagnetic state. The theoretical study of the dependence of Tc and TQ on the bilinear and biquadratic exchange terms reveals a complex phase diagram. In the case of TmZn, the obtained second-order coefficients allow a good description of all the physical properties now investigated.