Specific Heat of the Dilute, Dipolar-Coupled, Ising Magnet LiHo_<em>x</em>Y_1-<em>x</em>F₄

Abstract

The system LiHo_<em>x</em>Y_1-<em>x</em>F₄ is a nearly perfect example of a dilute, dipolar-coupled Ising magnet and, as such, it is an ideal testing ground for many theories in statistical mechanics. At low holmium concentration (<em>x</em> = 0. 045) an unusual spin liquid or "anti-glass" state was discovered in previous work [1]. This state does not exhibit a spin glass freezing transition as is expected for a long-range interaction. Instead, it shows dynamics which are consistent with a collection of low-frequency oscillators [2]. It was also seen to have sharp features in its specific heat [3]. <br /><br /> We present heat capacity measurements on three samples at and around the concentration of the spin liquid state in zero magnetic field and in a temperature range from around 50 mK to 1 K. In contrast to previous measurements, we find no sharp features in the specific heat. The specific heat is a broad feature which is qualitatively consistent with that of a spin glass. The residual entropy as a function of <em>x</em>, obtained through a numerical integral of the data, however, is consistent with numerical simulations which predict a disappearance of spin glass ordering below a critical concentration of dipoles [4]. <br /><br /> Also presented here, is ac susceptibility data on an <em>x</em> = 0. 45 sample which exhibits a paramagnetic to ferromagnetic transition and is found to be consistent with previous work.