Static critical properties of disordered ferromagnets studied by superconducting quantum interference device magnetometry and small-angle neutron-scattering techniques

Abstract

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 114Cd 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 TG, the Curie temperature TC, and the critical exponent γ of the susceptibility above TC 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 ξ, its exponent ν above TC, and another independent determination of γ. Whereas the pure compound exhibits exponents (γ=1.37, ν=0.70) compatible with a 3D Heisenberg ferromagnetic model, a pronounced increase of measured exponents with dilution was found, namely γ=1.97, ν=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.