Effect of dipolar coupling on the magnetism of disordered solids

Abstract

A study is made of the stability of ferromagnetic long-range order under fluctuations due to random easy magnetic axes, including the dipole–dipole coupling. This coupling produces two effects. There is a random dipole field that acts on the spins in the same way as does the random uniaxial anisotropy. Secondly, there is an average dipole field that acts to prevent the appearance of spin-density gradients. The net result is very dependent on the number of spin components. For the planar model in three dimensions, there is a logarithmic divergence of the magnetization reversal and power-law spatial dependence of the correlation function. Thus the lower critical dimension is reduced from four to three by the presence of dipole interactions in the planar (x-y) version of the random easy axis model. For the Heisenberg model the random magnetic anisotropy destroys the long-range ferromagnetic order. There is a correlation length that depends on the direction of the local magnetization. Results of a calculation of the spin–spin correlation function are used to predict qualitatively different ordered regions (domains) for small random anisotropy as compared to large random anisotropy.