Antiferromagnetic (AF) oxides are of particular interest since adjacent ferromagnetic (F) - AF layers are the key configuration of so called "spin-valve" sensor devices [1]. The AF pins the magnetization of the F layer through either an increase of the coercive field of the F layer or through an exchange field born at the interface which shifts the hysteresis loop of the F layer. This exchange coupling phenomenon itself still raises some controversies. Nevertheless, it is now well accepted that the magnetic exchange coupling is promoted through the uncompensated interfacial spins [2]. Consequently a complete description of the F-AF interface is required in order to correlate magnetic and interfacial properties.
We have studied the Co-hematite (α-Fe2O3) interface, a promising candidate with a high stability due to the large Curie and Néel temperatures of Co (1388 K) and hematite (955 K) respectively. Here we report on the magnetic properties derived from the interfacial characterization.
Cobalt layers were deposited on 20 nm thick films of hematite grown by atomic oxygen plasma assisted molecular beam epitaxy. The layers were investigated by complementary conventional and synchrotron radiation techniques at ELETTRA, BESSY, SLS and ESRF as well as by polarized neutrons. Spectroscopy measurements (photoemission, absorption and magnetic dichroism) have been used to characterize the chemical reactivity of the interface. Our investigations evidence the presence of a reactive interface. The Co layer adjacent to the hematite layer is partially oxidized, whereas we observe the presence of uncompensated Fe moments at the interface. Interestingly, this reaction occurs at - and seems limited to - the very early stages of the growth. We have studied the magnetic properties of these uncompensated Fe-moments by means of element specific dichroism and X-PEEM. Uncompensated Fe-moments are ferromagnetically coupled with the Co layer. Moreover, X-PEEM images directly reveal the onset of an exchange bias since the Co layer exactly reproduces the hematite magnetic domains arrangement.
[1] B. Dieny et al., J. Appl. Phys. 69, (8, pt.2A), 4774 (1991)
[2] H. Ohldag et al. ,Phys. Rev. Lett. 96, 027203 (2006)
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