Magnetic anomalies and canting effects in nanocrystalline spinel copper ferrites Cu_(x)Fe_(3-x)O_(4)

摘要

The magnetic behavior of nanocrystalline copper spinel ferrites Cu_(x)Fe_(3-x)O_(4) (0.42≤x≤1) exhibiting different degrees of crystallinity, have been investigated by means of in-field Mossbauer and dc magnetic measurements. The samples were obtained by a coprecipitation method and were subsequently high-energy ball milled for different times, which induced a progressive improvement of the ferrite crystallinity and cation redistribution. Their magnetism shows some peculiar features; at 5 K, the virgin magnetization curves remain outside the main hysteresis loop for applied fields of ≈1 T and display an S-shaped form. Below 60 K, the field-cooled hysteresis loops are shifted toward the negative field direction. In addition, in the same temperature range, the field-cooled magnetization of the samples with poorest crystallinity decreases sharply due to the spin freezing at the surface. The in-field Mossbauer spectra at 4.2 K show that the magnetic moments of the Fe~(3+) ions are in a canted spin state for both the tetrahedral and octahedral sites of the spinel structure, and evidence a decrease of the average spin canting with the degree of crystallinity. After calcination at 950℃, the coprecipitated sample no longer displays the magnetic anomalous and canting features. We explain our results as due to the partially disordered surface within the frame of the core-shell model rather than to the effect of cation redistribution.