Crystalline Electric Field and Magnetic Properties of the TmCu₂Si₂ Intermetallics

Abstract

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 Γ₃⁽¹⁾ ground state, a singlet Γ₄⁽¹⁾ first excited state at 6.1 K, a doublet Γ₅⁽¹⁾ second excited state at 79.5 K, and so on. Temperature variation of the 4f CEF level population, especially that of doublet Γ₅⁽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 Γ₅⁽¹⁾. An exchange-induced Van Vleck antiferromagnetic transition occurs at 2.8 K, since magnitude of transition probability between the ground Γ₃⁽¹⁾ and excited Γ₄⁽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 = (δ, δ, 0) with δ=0.147, where the Tm moments align along the c-axis.