Paramagnetic relaxation in iron ammonium alum and diluted iron ammonium alum at very low temperatures

Abstract

Relaxation measurements have been performed on three samples of iron ammonium alum and a number of diluted iron aluminium alums at liquid helium and liquid hydrogen temperatures and in alternating magnetic fields at frequencies below 1300 Hz. As the relaxation parameters of the diluted alums are very long, a new amplifier device has been constructed which makes it possible to do accurate measurements in alternating magnetic fields clown to 3 hertz. In the liquid hydrogen temperature region the relaxation parameters of only the 1 : 61 diluted alum could be measured and that is why the discussion in this paper is limited to the results obtained at liquid helium temperatures. The experimental dispersion and absorption curves are compared with those which follow from the thermodynamical theory of Casimir and Du Pré. Considerable deviations have been found with the salts which have a poor heat contact with the surrounding liquid helium and to a lesser extent with the salts which contain physical impurities. The relaxation parameters depend on the concentration of the magnetic ions and also on the above mentioned heat contact. Qualitative agreement is obtained if it is supposed that the heat developed in the spin system is transported to the bath by means of the lattice oscillators which are excited by transitions in the spin system.