Frustrated magnet for ADR to milli-Kelvin temperatures

Tokiwa, Yoshifumi  ; Bachus, S.*; Kavita, K.*; Jesche, A.*; Tsirlin, A. A.*; Gegenwart, P.*

In adiabatic demagnetization cooling, the entropy due to the fluctuation of the magnetic moment absorbs the ambient heat. However, at low temperatures, the entropy disappeared due to the ordering of the magnetic moment at the temperature corresponding to the energy scale (J) of the magnetic interaction, and the final temperature was limited. We have shown that a substance called KBaYb(BO) in which Yb forms a triangular lattice can be cooled to a temperature below J, which is the original limit. In a cooling experiment in a nearly ideal adiabatic state, sweeping from a magnetic field of 5T to 0T at a temperature of 2 K reached 22 mK, well below the Wyeth temperature of -60 mK estimated from magnetic susceptibility. This is because the entropy maintains a large value even at low temperatures due to the suppression of magnetic order due to frustration. Normally, as the volume density of the magnetic moment decreases, the distance between the moments increases and J decreases. Therefore, the final temperature decreases as the volume density of the moment decreases. Due to the effect of frustration, this substance achieves a lower final temperature despite its higher magnetic moment density than the conventional paramagnetic salt, which is a cooling material. Such a high density moment results in a high amount of heat absorption per volume. In addition, paramagnetic salts contain many water molecules, which causes deterioration such as deliquescent and efflorescence. However, KBaYb(BO) used in this study is stable in air because it does not contain water molecules. be. This substance is an ideal cooling material that has all of the final temperature, stability in air, and high magnetic moment density per volume.