Crystalline electric field and magnetic properties of the TmCu2Si2 intermetallics

The magnetic properties of TmCu2Si2 are studied by magnetometric measurement, inelastic neutron scattering (INS) experiment, elastic constant measurement using ultrasonic velocity and neutron diffraction experiment on the powdered and single crystalline samples. Crystalline electric field (CEF) parameters which are determined by the magnetic susceptibility and the INS spectrum give 4f CEF levels consisting of a singlet Gamma(3)((1)) ground state, a singlet Gamma(4)((1)) first excited state at 6.1 K, a doublet Gamma(5)((1)) second excited state at 79.5 K, and so on. Temperature variation of the 4f CEF level population, especially that of doublet Gamma(5)((1)), explains well anomalous behavior of the magnetic susceptibility along the a-axis exhibiting the minimum around 15 K and the maximum around 45 K. Weak lattice softening below 120 K is also attributable mainly to strain susceptibility of Gamma(5)((1)). An exchange-induced Van Vleck antiferromagnetic transition occurs at 2.8 K, since magnitude of transition probability between the ground Gamma(3)((1)) and excited Gamma(4)((1)) singlets is so large to induce the magnetic transition. It is confirmed that the antiferromagnetic structure is a sinusoidally moment-modulated one described by a propagation vector of q = (delta, delta, 0) with delta = 0.147, where the Tm moments align along the c-axis.