Factors Affecting Exchange Bias in Polycrystalline Metallic Thin Films
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1032-I03-03
Factors Affecting Exchange Bias in Polycrystalline Metallic Thin Films Luis Eugenio Fernandez-Outon, Gonzalo Vallejo-Fernandez, and Kevin O'Grady Department of Physics, The University of York, Heslington, York, YO10 5DD, United Kingdom ABSTRACT We describe the factors which control the measured value of exchange bias (HEX) in bilayers consisting of sputtered metallic thin films of an antiferromagnet (AF) in contact with a ferromagnetic (F) layer. Experimental measurements show that the value of HEX is determined by the grain volume distribution which limits the exchange bias via small grains which are thermally unstable, and large grains which cannot be set when the system is field annealed to set the AF at temperatures below TN. All the results are interpreted in terms of a granular model where the energy barrier to reversal within the AF is grain volume dependent. We show how this affects setting in metallic AFs at TTN (TN~700K for IrMn) but such high temperatures would damage the microstructure of the films. Hence, the films are set by a thermal activation process at TSET ) =
ln(τf 0 )k B < TB > Vm
(6)
where Vm is the median volume of the distribution [26]. We have undertaken these calculations using the high resolution particle size distribution measurements shown in figure 6b. Figure 7a shows the values of as a function of the AF layer thickness. Figure 7b shows the value of the anisotropy constant for the same systems. For our materials this gives a value of KAF at room temperature averaging (5.5 ±0.5)x106ergs/cc. This value is typically a factor 2 larger than reports from other measurements which do not take account of the interfacial spin and setting effects [e.g.27]. It should also be noted that the value of KAF is highly sensitive to temperature as indicated by equation 4.
Figure 7. a) Variation of the median blocking temperature with the thickness of the AF layer (left) and b) Variation of KIrMn with the thickness of the AF layer (right). Having obtained a measured value for KAF and knowing the grain volume distribution to high resolution, we are now able to calculate the limits of the grain size distribution which give rise to the exchange bias. In figure 5b we have shown the variation of the exchange bias at room temperature as a function of the thickness of the AF layer (tAF). The solid line in figure 5b is a calculation of the exchange bias from equation 7 using the limits on the integral across the volume distribution determined from the measurement of KAF and the time of measurement. H EX ∝
V2
∫ f (V )dV
V1
(7)
The remarkable agreement between this simple theoretical calculation and entirely measured parameters indicates that a grain volume model clearly explains the behaviour of exchange bias with film thickness and grain size. Previously data such as that shown in figure 5b has been interpreted as arising due to the fact that domain walls parallel to the interface are unable to form in thinner films until a critical thickness is reached whereupon domain walls appear and the
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