Magnetic behaviour of a disordered Ising ferrimagnet in a high magnetic field

Abstract

The magnetic behaviour of a disordered ferrimagnetic system where both A and B represent magnetic atoms with respective spins and in the presence of a high magnetic field is treated theoretically. Assuming the magnetic interaction can be described through the Ising Hamiltonian the approximate free energy is obtained using the cluster-variational method. The field dependence of the magnetization is then obtained for different concentrations p and exchange parameters (J _AA , J _BB and J _AB ). For p = 0.5, the magnetization M in the ferrimagnetic state and in the absence of a compensation temperature T _cm vanishes at T _c. Field-induced reversal of M is found at switching temperature T _S () which is a decreasing function of the field H. A maximum in M is found above T _S and the maximum value of M increases with the field. In the ferrimagnetic state M increases almost linearly in the high H region. For a system with large ferromagnetic J _AA , the compensation temperature T _cm is an increasing function of J _BB and J _AB . The decrease in compensation temperature is linear at small fields and tends to saturate at higher fields. The sharpness of the magnetization reversal is increased with H. For a fully compensated state of the system with p = 2/3, the magnetization in the presence of H also exhibits switching behaviour at T _S. For p = 0.2 the field-induced reversal of magnetization occurs more sharply. The orientational switching of the sublattice magnetization M_A and M_B with field increases the Zeeman energy and is the origin of magnetization reversal at T _S.

Keywords:

• mixed spin Ising model
• disordered ferrimagnetic alloy
• rare-earth-transition metal alloy
• cluster-variational method

Acknowledgements

The author gratefully acknowledges assistance from the authority of R.K.M. Vivekananda University. It is also my great pleasure to thank Prof. D. Sherrington for his help and encouragement during this work on disordered spin systems.