EVIDENCE FOR BIQUADRATIC EXCHANGE INTERACTIONS IN GDAG1-XZNX

We have re-examined the magnetic-phase diagram of GdAg1-xZnx, an inter metallic solid solution of a ferromagnet (GdZn) and an antiferromagnet (GdAg). Samples of the intermediate composition range 0.48 < x < 0.59 show new magnetic-ordering phenomena and all exhibit nearly the same phase diagram at the application of a magnetic field. These phase diag rams comprise two ferrimagnetic phases with ground-state magnetization values of m approximate to 1/3 and m approximate to 2/3 and one uncom mon high-temperature phase which is nearly ferromagnetic but which has a finite susceptibility and therefore no spontaneous magnetization. A ll ordered phases seem to coexist at one multicritical point. It is ar gued that the observed variety of magnetic phases is due to biquadrati c exchange interactions which remain as the only active ones in a situ ation where the ferromagnetic and antiferromagnetic bilinear interacti ons compensate each other. The easiest method to identify biquadratic interactions is the Curie-Weiss law of the cubic susceptibility chi(3) . However, in metallic systems the Curie-Weiss temperature regime of c hi(3) is not nearly reached even at five times the ordering temperatur e, thus preventing the evaluation of theta(3) which provides a measure for the strength of the biquadratic interaction in insulating systems . This shows that there are much stronger and strongly temperature-dep endent individual biquadratic interaction processes of either sign tha n one might assume according to the macroscopically observed average i nteraction. Although the existence of the new magnetically ordered sta tes is confirmed with magnetization, dilatometric and specific heat me asurements, no corresponding neutron-diffraction intensities are obser ved in zero-magnetic field which points to strongly fluctuating or non -collinear magnetic structures. Since the outlined observations fall i nto a composition and temperature range in this random magnetic-bond s ystem where magnetic non-equilibrium phenomena become visible, special precautions are necessary to obtain the low-temperature equilibrium m agnetization curves.