A magnetic, neutron diffraction, and Mȯssbauer spectral study of the Ce2Fe17−xAlx solid solutions (abstract)

Abstract

The magnetic properties of a series of Ce2Fe17−xAlx solid solutions with x equal to 0.0, 0.88, 2.06, 2.80, 3.98, 5.15, 6.08, 7.21, 8.20, 9.08, 9.84, and 10.62 have been studied by magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. Neutron diffraction data indicate that the compounds all crystallize in the rhombohedral Th2Zn17-type structure. The aluminum atoms are excluded from the 9d site and show a distinct preference for the 6c site for x greater than 6. The substitution of aluminum leads to an expansion of the a and c axis by 0.5% and 0.4% per aluminum atom. The unit cell volume increases by approximately 1.4% per aluminum atom. The magnetic moment per formula unit, measured at 295 K, shows very little change for x less than or equal to 4, but decreases rapidly with increasing aluminum content for higher values of x, indicating that aluminum acts as a magnetic hole at the lower concentrations. The Curie temperature increases from 238 K in Ce2Fe17 to a maximum of 384 K in Ce2Fe14Al3. The Ce2Fe17−xAlx solid solutions behave as spin glasses for x greater than 7. The Mössbauer spectra have been fit with a binomial distribution of the near-neighbor environments in terms of a maximum hyperfine field, Hmax, for an iron atom with zero aluminum near neighbors, and a decremental field, ΔH, per aluminum near neighbor. Mössbauer spectral results indicate both that the samples are ferromagnetically ordered in the basal plane for x values between 0.2 and 7 and that a magnetic phase transition occurs from helimagnetic in Ce2Fe17 to ferromagnetic in Ce2Fe16.8Al0.2. However, at 295 K ferromagnetic ordering is observed only for x values between 2 and 5. The average value of Hmax decreases by about 25 kOe per aluminum atom. The isomer shift increases with aluminum content, an increase which can be explained by the screening of the 4s conduction electrons by the 3d band electrons. ΔH decreases by about 1.0 kOe per aluminum atom, a decrease which can be explained by the indirect exchange interaction between iron atoms modified by the presence of non-magnetic aluminum atoms through the 4s conduction band. Note added in proof: A full account of this work has been published in J. Appl. Phys. 79, 3145 (1996).