Measurement of the energy barrier distribution in the antiferromagnetic layer of exchange-biased materials

Abstract

Abstract-The value of exchange field of two FeMn-NiFeCobased spin valves with varying thickness of the pinned ferromagnetic layer has been determined as a function of temperature. The complexities caused by thermal activation of the antiferromagnet during measurement have been overcome by the development of a measurement protocol. The values of the exchange field obtained provide a measure of the degree of order in the antiferromagnet. Thus it is possible to determine the distribution of energy barriers to reversal for the system. We find that for a 110-A-thick pinned NiFeCo layer a broad distribution exists, whereas for an 80-A layer, the distribution is bimodal and has a component subject to thermal activation at temperatures down to 260 K.Index Terms-Exchange bias, exchange interactions.

Magnetization reversal of the ferromagnetic layer in IrMn/CoFe bilayers

“…The two energy barrier distributions, which have also been seen for the FeMn layer, 39 are believed to result from several factors, such as a distribution of AFM domain size, a distribution of Néel axis orientation in the AFM domains, and random pinning sites and helical domain walls at the FM/AFM interface. The distribution of AFM grain size can be seen from our microstructural results and has also been reported elsewhere, for example, in Ref.…”

Angular dependence of coercivity and exchange bias in IrMn/CoFe bilayers

“…The time spent in reverse bias, i.e. with the F in reversed saturated state, was significantly shorter than the time spent with the F in positive saturated state [2]. As the sweep-rate has been kept constant for all measurements, the maximum magnetic field applied in each measurement was varied to keep the time spent in the reverse bias configuration the same for every measurement.…”

“…The amount by which the hysteresis loop shifts is the exchange field (H E ). It has been shown experimentally that this shift derives from the amount of AF material that exists in a state such that its orientation remains unaltered when the F is reversed [2]. This shift is accompanied by an enhancement of the coercivity (H C ) of the F material, defined as the half-width of the loop, or an anomalous form of the magnetic viscosity in the F layer [3].…”

A new paradigm for exchange bias in polycrystalline thin films

“…In polycrystalline systems each grain has its own blocking temperature and therefore the AF is characterised by a distribution of blocking temperatures. When the conventional measurement procedure is used, the AF is subject to thermal activation during the time of measurement at a logarithmic rate [41]. This leads to changes in the state of order in the AF from measurement to measurement as well as lack of reproducibility in the data.…”