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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 Mössbauer 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 Mössbauer 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 °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. [ABSTRACT FROM AUTHOR]

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 Mössbauer 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 Mössbauer 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 °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. [ABSTRACT FROM AUTHOR]