Critical magnetic behaviour of a ferromagnet with antiferromagnetic fourth-order exchange interactions: GdMg

We report on investigations of the critical magnetic behaviour of GdMg. In this intermetallic ferromagnet the sum of all fourth-order interactions (i.e. biquadratic, three-spin and four-spin interactions) is 50 K and antiferromagnetic at the Curie temperature of T(c)(parallel to) = 110 K, This has been shown with measurements of the third-order susceptibility chi(3) Although the Curie temperature can be assumed to be defined mainly by bilinear (Heisenberg) interactions? the critical indices for the linear susceptibility chi(1) and the spontaneous magnetization are molecular field like gamma = gamma' = 1, beta = 0.5. The third-order susceptibility chi(3) is discontinuous at T(c)(parallel to): for T > T(c)(parallel to), chi(3) is finite but for T < T(c)(parallel to) it diverges with a critical exponent of one. The ferromagnetic order-disorder transition is therefore weakly first order. At a second critical temperature T(N)(perpendicular to) = 91 K the rise of an ordered antiferromagnetic component oriented perpendicular to the ferromagnetic one has been reported two decades ago based on neutron scattering investigations. Here we will show that the third transition driven by the fourth-order interactions. We must therefore assume that the transverse components of the ferromagnetically ordered moments order antiferromagnetically at T(N)(perpendicular to). This view is in accordance with measurements of the susceptibility chi(perpendicular to) perpendicular to an applied static magnetic field. As a result, a second-order parameter perpendicular to the spontaneous magnetization is generated by the fourth-order interactions. As a further consequence of the perpendicular moment configuration, the spontaneous longitudinal magnetization reaches only 0.75 of the theoretical saturation value for T --> 0 and decreases like T(2) with temperature as has been observed for other ferromagnets with fourth-order interactions, such as EuS and CrBr(3). (C) 1998 Elsevier Science B.V. All rights reserved.