Adiabatic Magnetization Cooling in Ytterbium and Gadolinium Iron Garnets

Abstract

We have observed large adiabatic magnetization cooling (≈6°K) in ytterbium iron garnet (YbIG) and dilute gadolinium iron garnet in the 3°–30°K temperature range. In samples of YbIG diluted with yttrium, reductions in temperature up to 10°K were observed. In pure YbIG, isentropic curves taken near the compensation point outline a magnetic phase boundary in the H-T plane at which the cooling suddenly ceases. The boundary extends from the compensation point (≈8°K) at H=0 to above 17°K for H=110 kOe. The high-temperature phase is characterized by a collinear sublattice configuration with the rare earth antiparallel to the field and thus possesses a large negative magnetocaloric effect (−0.1°K/kOe). Isentropes and magnetization curves were calculated for YbIG in the molecular field approximation using an iterative method to find the lowest-energy configuration of the iron sublattice and the six rare-earth sublattices. Depending upon the direction of the field, several phase transitions arise from the competing influences of anisotropy, exchange, and Zeeman energies. Some transitions are first order and others are second order.