Fourth-order exchange interactions in GdxEu1-xS

We report on investigations of fourth-order exchange interactions, i.e. biq uadratic, three-spin and four-spin interactions in GdxEu1-xS using measurem ents of the cubic susceptibility chi(3) and the magnetic specific heat c(p) . In the paramagnetic phase of GdS these interactions are ferromagnetic sin ce chi(3) is negative (the magnetization increases stronger than linearly w ith 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 orderin g process induced by the fourth-order exchange interactions. Evidence for t his 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 definit e 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 cha nge from ferromagnetic at high temperature to antiferromagnetic at low temp erature. The critical magnetic behavior of the magnetic specific heat, c(ma g), 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 e xponents is inadequate. On substituting Gd by Eu fourth-order interactions change, as usual, rapidly as function of composition. One sample with compo sition Gd0.8Eu0.2 was investigated in detail. In this sample the total four th-order interaction is antiferromagnetic at the Neel temperature of T-N = 55 K. As a consequence, the ferromagnetic component observed in GdS is abse nt and a second critical field curve reaching only 5.5 T occurs below 55 K. (C) 1999 Elsevier Science B.V. All rights reserved.