SPIN-WAVE THEORY OF PRINCIPAL SUSCEPTIBILITIES OF HEISENBERG ANTIFERROMAGNET

The perpendicular susceptibility of MnF2 below T N(67.3°K) measured by Trapp and Stout decreases with increasing temperature at low temperatures and passes a minimum at 45°K. Calculation based on existing spin-wave theories of the susceptibility turns out to decrease too fast above 15°K compared with the data. It is pointed out through a new approach that the theories neglect an important contribution made by excitations of three and more magnons of finite wavenumber; they arise from the nondiagonal part of the spin-wave interaction with the perturbation of external field. The new contribution starts with a 1/S2 term in the 1/S expansion and increases with temperature. A calculation which takes into account whole terms of 1/S2 is in good agreement with the data up to 40°K. A calculation of the parallel susceptibility is also presented, which takes into account a shift of the spin-wave frequency caused by external field through the spin-wave interaction. It agrees with experiment even at 50°K. © 1968 The American Institute of Physics.