Magnetic layers are essential in the production of modern data storage devices, e.g. hard disks, MRAM memory etc. Exchange coupling across a non-magnetic layer can lead to an anti-parallel arrangement of the magnetization in adjacent magnetic layers. If the non-magnetic layer has an antiferromagnetic structure, there can be an antiferromagnetic coupling between the ferromagnetic layer and the antiferromagnetic layer, giving rise to the 'exchange bias' phenomenon. This leads to a unidirectional shift of the hysteresis loop after field cooling the system below the Néel temperature of the antiferromagnet. Although in the past there has been significant activity in the field of exchange biased thin films, the phenomenon is still lacking a complete understanding at the microscopic level.
We have studied the epitaxial NiFe/FeMn (ferromagnet/antiferromagnet) exchange bias system. If deposited under suitable circumstances, this system displays exchange bias at room temperature, making it much more attractive for applications than systems which show exchange bias only at lower temperatures. The NiFe/FeMn system is ideally suited to make use of the unique element specificity provided by synchrotron experiments. X-ray absorption experiments have recently led to remarkable advances in the understanding of the microscopic spin structure leading to exchange bias. From such experiments it has become clear that elucidation of the magnetic spin structure at the interface between the ferromagnet and the antiferromagnet is essential for a better understanding of exchange bias. We will present the magnetic properties of the exchange bias system as revealed by neutron reflectivity and element specific x-ray magnetic circular dichroism. The neutron experiments allow us to identify the magnetization reversal mechanism (magnetization rotation and domain wall motion). For the x-ray absorption we have obtained dichroic contributions by measuring the total electron yields at the Fe and Ni edges for samples with FeMn on top of NiFe and also for samples with a reversed layer sequence and with varying thickness of the antiferromagnet. We will show element specific hysteresis loops giving more insight in the magnetic behavior at the interface.
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