The characteristic relaxation frequency of a granular antiferromagnet f0 in the Néel–Arrhenius expression has been calculated from a high resolution measurement of the time dependence of the median blocking temperature of a CoFe/IrMn exchange biased system. Calculations based on a granular model give f0=(2.1±0.4)×1012 s−1, three orders of magnitude greater than values commonly found in the literature.
We report on a study of the effect of Ir content on the loop shift (HEX) and anisotropy constant (KAF) in the CoFe/IrMn system. The sample structure investigated was Si/NiCr(5 nm)/Ru(5 nm)/IrxMn1-x/CoFe(2 nm)/Ta(3 nm). All samples were produced by sputtering and the Ir and Mn levels were varied using a specially made composite target and deposited at ∼120 °C. The composition of the samples was analyzed using energy dispersive x-ray analysis. KAF was calculated from thermal activation measurements using the York Protocols. A plateau in HEX was found for Ir levels between 16– 20.5 at. %. HEX was found to decrease by 50% on either side of this window. This result is consistent with previous studies where the enhancement of HEX was attributed to an increase in the atomic ordering of the IrMn alloy. However, KAF decreases linearly with increasing Ir concentration and does not appear to correlate with the change in HEX.
The time and temperature dependences of the exchange field achieved by control of the setting conditions in CoFe/IrMn exchange couples have been investigated. This has been achieved by varying the temperature at which the antiferromagnet was set (TSET) and by measuring the time dependence of the exchange field (Hex) at each temperature. The exchange field was observed to vary logarithmically with setting time, indicating that a wide distribution of energy barriers is present in the antiferromagnetic (AF) layer. The results are consistent with an independent AF grain model of exchange bias where the energy barrier distribution to reversal is governed by the grain volume distribution and the temperature dependence of the magnetocrystalline anisotropy. Calculations of the time dependence of the setting process are in good agreement with the grain volume model.
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