Importance of anharmonic terms in the analysis of the specific heat of UNi₂Si₂

The tetragonal compound UNi₂Si₂ in zero magnetic field exhibits three different antiferromagnetic phases below T_N = 124 K. They are characterized by the U moments parallel to the c axis, forming ferromagnetic basal planes and by a propagation vector q= (0,0, q_z). Consequently, below T_N two order-order magnetic phase transitions are observed; namely, at T₁ = 108 K and T₂= 40 K. The later two transitions are of the first order type. All three magnetic phase transitions are reflected by the pronounced anomalies of bulk thermodynamic properties such as thermal expansion, resistivity, susceptibility, and specific heat. Here we present the analysis of specific heat data going beyond the harmonic approximation. Realistic values of magnetic specific heat and magnetic entropy were obtained by this approach. An additional exponential contribution to the specific heat and resistivity below T₂ is considered. It is attributed to the anisotropy induced gap in the magnetic excitation spectrum.