CRITICAL MAGNETIC-BEHAVIOR OF A FERROMAGNET WITH ANTIFERROMAGNETIC 4TH-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 intera ctions (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 susc eptibility chi(3) Although the Curie temperature can be assumed to be defined mainly by bilinear (Heisenberg) interactions? the critical ind ices for the linear susceptibility chi(1) and the spontaneous magnetiz ation are molecular field like gamma = gamma' = 1, beta = 0.5. The thi rd-order susceptibility chi(3) is discontinuous at T-c(parallel to): f or 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 criti cal temperature T-N(perpendicular to) = 91 K the rise of an ordered an tiferromagnetic component oriented perpendicular to the ferromagnetic one has been reported two decades ago based on neutron scattering inve stigations. Here we will show that the third transition driven by the fourth-order interactions. We must therefore assume that the transvers e components of the ferromagnetically ordered moments order antiferrom agnetically 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 para meter perpendicular to the spontaneous magnetization is generated by t he fourth-order interactions. As a further consequence of the perpendi cular 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 fe rromagnets with fourth-order interactions, such as EuS and CrBr3. (C) 1998 Elsevier Science B.V. All rights reserved.