Fourth-order exchange interactions in GdxEu1-xS

We report on investigations of fourth-order exchange interactions, i.e. biquadratic, three-spin and four-spin interactions in GdxEu1-xS using measurements of the cubic susceptibility chi(3) and the magnetic specific heat c(p). In the paramagnetic phase of GdS these interactions are ferromagnetic since chi(3) is negative (the magnetization increases stronger than linearly with magnetic field). At the Neel temperature of GdS of 59 K the onset of a strong field dependence of the linear susceptibility chi(1), has early been observed. This feature is attributed here to a quasi-ferromagnetic ordering process induced by the fourth-order exchange interactions. Evidence for this is provided by a divergence of the cubic susceptibility chi(3) at 59 K. Two further magnetic phase transformations were observed in GdS: (a) at 47 It the magnetic susceptibility and specific heat exhibit small but definite anomalies and, (b) below 27 K a second critical field curve reaching only 2.6 T for T --> 0 is observed. Also these phase transitions are attributed to the fourth-order interactions which apparently are not constant but change from ferromagnetic at high temperature to antiferromagnetic at low temperature. The critical magnetic behavior of the magnetic specific heat, c(mag), at T-N = 59 K is as complicated in GdS as in EuTe because two ordering phenomena superpone. A description by power laws with Heisenberg critical exponents is inadequate. On substituting Gd by Eu fourth-order interactions change, as usual, rapidly as function of composition. One sample with composition Gd0.8Eu0.2 was investigated in detail. In this sample the total fourth-order interaction is antiferromagnetic at the Neel temperature of T-N = 55 K. As a consequence, the ferromagnetic component observed in GdS is absent and a second critical field curve reaching only 5.5 T occurs below 55 K. (C) 1999 Elsevier Science B.V. All rights reserved.