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 CdC r2S4, a well-known Heisenberg 3D ferromagnet. A study is presented of its static critical properties as a function of concentration, perform ed by low-field superconducting quantum interference device SQUID magn etometry and small-angle neutron-scattering SANS techniques. Samples w ere 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 Cu rie temperature T(C), and the critical exponent gamma of the susceptib ility 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 pro nounced increase of measured exponents with dilution was found, namely gamma = 1.97, nu = 1.03. The results are compared to the ones of simi lar studies in the related system Eu(1-x)Sr(x)S and to the renormaliza tion group treatment of strongly random ferromagnets, and possible mec hanisms to explain how a small dilution can provoke large alterations in the critical behavior and move the system in the strong disorder re gion are discussed.