Abstract
Ilvaite, Ca(Fe2+,Fe3+)Fe2+Si2O8(OH) shows two magnetic phase transitions, which have been studied by Mossbauer spectroscopy within the temperature range 120–4 K. The continued charge localization between Fe2+ and Fe3+ ions in octahedral A-sites causes the Fe2+-Fe3+ interaction to be ferromagnetic, although the overall magnetic order is antiferromagnetic. The thermal evolution of the hyperfine fields at the Fe2+ (A) and Fe3+ (A) sites indicates Bhf: 328 and 523 kOe respectively at 0 K and TN1= 116K. The corresponding values for Fe2+ (B) site are: Bhf 186 kOe and TN2=36K. An additional hyperfine field exists at the Fe2+(B) site within the temperature range 116–36K due to short-range order induced by the spin ordering in A sites. The considerable difference between the two magnetic transition temperatures is due to spin frustration, because the Fe2+ (B) site occurs on a corner common between two triangles with respect to two sets of Fe2+ (A) and Fe3+ (A) sites with opposite spin directions.
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