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Spin reorientation at the antiferromagnetic NiO(001) surface in response to an adjacent ferromagnet H. Ohldag, A. Scholl, F. Nolting, S. Anders, F.U. Hillebrecht, and J. Stöhr Parallel domain correlation at interface Exchange bias Unidirectional anisotropy in FM adjacent to AFM M AFM Ni XMLD discovered 1956 by Meiklejohn and Bean Co XMCD FM [010] H s E Exchange-bias field (size depends on many parameters) The exchange bias effect is exploited in magnetic data storage devices, though the origin of this effect is still poorly understood. Different models do exist, most of them assume that the magnetic structure at the surface is the same as for the bulk. Interfacial spins Spin-flop Meiklejohn, Bean (1956) Takano et al. (1997) [010] •High Spatial Resolution •Elemental Sensitivity •Ferromagnetic Contrast •Antiferromagnetic Contrast •Surface/Interface Sensitivity AFM grain / domain boundary [100] What causes correlation - exchange coupling at interface ? Nickeloxide interface magnetism Co / NiO(001) • Antiferromagnet, Néel temperature 523 K • Spins lie in ferromagnetic {111} sheets along <112> directions • {111} sheets are coupled antiferromagnetically • Strong rhombohedral compression along <111> axes (10-3) One of 4 "T" domains [001] NiO antiferromagnetic E NiO interface ferromagnetic s s [010] 3 possible "S" domains [100] XMLD XMCD NiO x' [010] NiO Co NiO interface structure is not bulklike! •(±110) walls disappear after Co deposition •AFM axis rotates in-plane along [±110] [100] Bare NiO(001) Co/NiO XMCD Co ~1 monolayer Summary Domain structure of Co/NiO(001) NiO Co ferromagnetic [010] [100] Single crystalline NiO (001) sample preparation : cleaved ex-situ annealed at 380 K in PEEM chamber Co (< 2 nm) deposited in-situ [±110] [100] PEEM Koon (1997) FM step [±110] • Surface magnetic structure can differ from bulk structure => bulk methods are of limited value for the understanding of interface effects • Clear evidence for parallel coupling between AFM and FM in Co/NiO, no indication of spin canting, ruling out spin-flop models • Exchange coupling is related to about 1 ML of uncompensated spins at the AFM surface Co/NiO(001)