STATIC CRITICAL PROPERTIES OF DISORDERED FERROMAGNETS STUDIED BY SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE MAGNETOMETRY AND SMALL-ANGLE NEUTRON-SCATTERING TECHNIQUES

CdCr(2-2x)In(2x)S4 is a highly frustrated magnetic system based on CdCr2S4, a well-known Heisenberg 3D ferromagnet. A study is presented of its static critical properties as a function of concentration, performed by low-field superconducting quantum interference device SQUID magnetometry and small-angle neutron-scattering SANS techniques. Samples were prepared from isotopic cadmium Cd-114 with a low absorption cross section for thermal neutrons. Magnetic measurements performed with low dc fields in the temperature range 5-300 K allow one to determine the magnetic state of the samples, the reentrant temperature T(G), the Curie temperature T(C), and the critical exponent gamma of the susceptibility above T(C) as a function of dilution in the domain 0<x<0.15. By directly measuring the critical magnetic fluctuations in zero field in SANS experiments, access is gained to the magnetic correlation length xi, its exponent nu above T(C), and another independent determination of gamma. Whereas the pure compound exhibits exponents (gamma = 1.37, nu = 0.70) compatible with a 3D Heisenberg ferromagnetic model, a pronounced increase of measured exponents with dilution was found, namely gamma = 1.97, nu = 1.03. The results are compared to the ones of similar studies in the related system Eu(1-x)Sr(x)S and to the renormalization group treatment of strongly random ferromagnets, and possible mechanisms to explain how a small dilution can provoke large alterations in the critical behavior and move the system in the strong disorder region are discussed.