Field Dependence of the Magnetization for an Amorphous Ferrimagnet Gd₆₅Co₃₅

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We have investigated the longitudinal fluctuation of the local magnetization for an amorphous ferrimagnet Gd₆₅Co₃₅ using the precise measurement of the magnetization curve below 5.3, T. The longitudinal fluctuation varies as ST² with temperature T. The parameter S depends on the magnetic field B as S=S₀(1-S₁B). A linear combination of the longitudinal and transverse fluctuations well accounts for the field and temperature dependence of the magnetization below 120, K except near 4.2, K. The ferrimagnet shows a weak random magnetic anisotropy at low temperatures, which causes the magnetic isotherm at 4.2, K to approach to saturation as B^-1/2.