Magnetic anomalies and canting effects in nanocrystalline spinel copper ferrites CuxFe3-xO4

摘要

The magnetic behavior of nanocrystalline copper spinel ferrites CuxFe3-xO4 (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 approximate to 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 Fe3+ 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 degrees C, 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. (C) 2005 American Institute of Physics.