TABLE OF CONTENTS Adiabatic demagnetization (AD) is one of the cooling methods that have a common principle: the entropy of the working medium (helium, spins, etc.) is controlled by some parameter like pressure or magnetic field. The working medium in AD is a system of magnetic moments (nuclear spins, electronic spins, or orbital moments) and the magnetic field is the control parameter when the magnetocaloric effect is employed. The magnetization, M, of a paramagnet or a ferromagnet increases with an applied magnetic field, H, and simultaneously both the energy of magnetic moments in the field and the internal energy of their exchange interaction decrease. Supposing negligible pressure and volume effects, the entropy can be expressed by the following thermodynamic relation:
The internal energy, U, is a function of the temperature, T, of the sample, and in general it increases with temperature. Consequently, if the magnetization of a paramagnetic sample is reduced adiabatically, the sample temperature should decrease. The cooling cycle of the AD method consist of two steps:
(i) Isothermal magnetization. The sample is in thermal contact with a constant temperature bath kept at T i. The magnetic field is increased from zero to H i and the heat released to the bath compensates for the work done on the sample. It leads to an increase in the order of the magnetic moments and, consequently, reducing their entropy, S,by
This entropy change is determined by the...
