Effects of biquadratic exchange in ferromagnets

The effects of the biquadratic-exchange interaction on the temperature dependence of the magnetic specific heat and the magnetization have been calculated for ferromagnets with the use of spin-wave theory and compared with those for antiferromagnets. The ferromagnetic spin structure changes abruptly to a canted one at a certain ratio $P_c$ of the biquadratic exchange to the Heisenberg interaction. This change of spin structure is shown to have significant effects on the magnetic properties. The results of calculations for one-, two-, and three-dimensional systems are compared with one another. It is also shown that the qualitative changes of the magnetic properties of ferromagnets due to the biquadratic-exchange interaction are different from those due to the Dzyaloshinski-Moriya interaction.