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 Γ(1)3 ground state, a singlet Γ(1)4 first excited state at 6.1 K, a doublet Γ(1)5 second excited state at 79.5 K, and so on. Temperature variation of the 4f CEF level population, especially that of doublet Γ(1)5, 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 Γ(1)5. An exchange-induced Van Vleck antiferromagnetic transition occurs at 2.8 K, since magnitude of transition probability between the ground Γ(1)3 and excited Γ(1)4 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.
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