Temperature and Field Dependence of Hyperfine Fields and Magnetization in a Dilute Random Substitutional Ferromagnetic Alloy:Fe2.65Pd97.35

The assumption of proportionality between hyperfine magnetic field and bulk magnetization, which has been shown to obtain in elemental ferromagnetic Fe, is shown to apply as well in the random substitutional cubic dilute ferromagnetic alloy ${\mathrm{Fe}}_{2.65}$ ${\mathrm{Pd}}_{97.35}$. The same sample of this alloy was studied by means of both the Mössbauer effect and bulk magnetization. The Mössbauer data represent the first detailed study of the hyperfine field in a ferromagnet in the presence of large external fields. Proportionality between the bulk magnetization and the hyperfine field is found over an extended range of temperature and applied external magnetic field. Extrapolations of the bulk-magnetization data to yield the Curie point disclose significant discrepancies with the Mössbauer results when standard extrapolation techniques are used. Good agreement is obtained if one assumes that the magnetization varies near the Curie temperature $\theta$ as ${(\theta -T)}^{\beta }$ with $\beta$ between ¼ and $\frac{1}{3}$. This type of behavior suggests that the sample may be describable in the region of the Curie temperature by recent Padé-approximant calculations based upon Ising and Heisenberg models. The temperature variation of the hyperfine field is compared to molecular-field calculations in both zero and nonzero external fields. Although qualitative agreement is found, there are significant quantitative discrepancies, indicating the inapplicability of the molecular-field model. A description is included of the Brookhaven cryogenic Mössbauer apparatus involving a variable-temperature cell mounted inside a high-field superconducting magnet.