Importance of anharmonic terms in the analysis of the specific heat of UNi2Si2 - art. no. 212408

The tetragonal compound UNi2Si2 in zero magnetic field exhibits three diffe rent antiferromagnetic phases below T-N=124 K. They are characterized by th e 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-ord er magnetic phase transitions are observed; namely, at T-1 - 108 K and T-2 = 40 K. The later two transitions are of the first order type. All three ma gnetic phase transitions are reflected by the pronounced anomalies of bulk thermodynamic properties such as thermal expansion, resistivity, susceptibi lity, and specific heat. Here we present the analysis of specific heat data going beyond the harmonic approximation. Realistic values of magnetic spec ific heat and magnetic entropy were obtained by this approach. An additiona l exponential contribution to the specific heat and resistivity below T-2 i s considered. It is attributed to the anisotropy induced gap in the magneti c excitation spectrum.