1. Andreas Scholl, 1 Marco Liberati, 2 Hendrik Ohldag, 3 Frithjof Nolting, 4 Joachim Stöhr 3 1 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 2 Department of Physics, Montana State University, Bozeman, MT 59717, USA 3 Stanford Synchrotron Radiation Laboratory, Stanford, CA 94309, USA 4 Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland The role of planar and vertical domain walls and uncompensated interface spins in exchange bias. X -RAY S PECTROSCOPY & I MAGING OF A NTIFERROMAGNETS V ERTICAL D OMAIN W ALLS P LANAR W ALLS U NCOMPENSATED I NTERFACE S PINS Malozemoff, PRB 35, 1987 Random field problem: Competition between domain wall and interface energy: Meiklejohn & Bean, Phys. Rev. 102 1956 Ideal interface: Experiment: AFM FM I NTRODUCTION Observations about exchange bias: Exchange bias is an interface effect. Exchange bias is a result of symmetry breaking. Exchange bias appears in a variety of materials. Basic physics understood – Exchange interaction X-ray microscopy and spectroscopy bring forth a microscopic understanding. Co domain switching LaFeO 3 Local loops -223 Oe92 Oe 103 Oe223 Oe XMCD XMLD [100] [010] 10 mm 1 2 +30 Oe -30 Oe LaFeO 3 F. Nolting et al., Nature 2000 Co/LaFeO 3 Co A. Scholl et al., APL 2004 Domain area distribution Local exchange bias Bias field vs. domain area Local, remanent hysteresis loops of Co/LaFeO 3 show a dependence of the variance of the local bias field with the domain size. Large domains show a small variance. For small domains a large width of the domain size distribution was observed. The sample was measured as-grown and did not possess a macroscopic bias, explaining that both directions of the bias occurred with equal probability. An approximately linear dependence of the width of the bias distribution with the inverse domain diameter is in accordance with predictions of the model proposed by Malozemoff. C ONCLUSIONS Co ferromagnetic NiO interface ferromagnetic NiO antiferromagnetic XMCD XMLD NiOCo [010] [100] ~1 monolayer E  Ohldag et al. PRL 2001 0.20.3 0.1 0.2 3nmCo/NiO 2nmCo/IrMn 2nmCoFe/PtMn[a] 1nmCoFe/PtMn[a] 3nmCoFe/PtMn[b] S pinned (m Bohr ) Interfacial Energy (mJ/m 2 ) Ohldag et al. PRL 2003 Redox reaction at Co/NiO interface AFM Interface loop compared with FM loop Coupling energy scales with density of pinned spins NiO(001) NiO/Si Stöhr et al., PRL 1999 Scholl et al., Science 2000 Ohldag et al., PRL 2001 s XMCDCo L 3 /L 2 Co LaFeO 3 E B/A XMLD Ferromagnet Antiferromagnet [100] [110] X h NiO XMLD Co XMCD 5  m [011] H A B AB H H [110] [100] X h Y Z PEEM imaging shows that the exchange coupling of Co to a NiO(001) substrate results in a uniaxial anistropy. Two classes of Co domains possess easy axes along in- plane directions. A magnetic field applied along one direction leads to switching of one class of domains at low field (A) and rotation of the other class of domains at high field (B). Linear dichroism spectroscopy (XMLD) measures the rotation of the NiO anti- ferromagnetic axis in response to the rotation of the Co magnetization in an external field. Spectra show no spin-flop of NiO without Co cap layer but evidence of a rotation of the surface magnetic moments in the presence of a Co layer. The layer acts like a lever that twists the magnetic structure of the AFM. A planar domain wall parallel to the interface is wound up. The data was fitted applying a model developed by Mauri et al. Fitting parameters are the interface exchange stiffness A 12 and the antiferromagnetic domain wall energy E AFM. For Co/NiO(001) we find E AFM = 0.66 mJ/m 2 and A 12 = 1.52·10 -13 J/m. AFM domain wall energy Interface energy FM anisotropy Zeeman energy Uncompensated spins at the surface of the NiO single crystal are magnetically coupled to a Co layer, indicated by the identical FM domain images of Ni and Co. The uncompensated spins are the result of a chemical reaction at the Co/NiO interface leading to a reduction of the NiO surface and partial oxidization of the Co layer. Uncompensated interface spins were also observed for other combinations of ferromagnets and ferromagnets, for example CoFe/PtMn and Co/IrMn. Element-specific hysteresis loops of the uncompensated moments at the surface of the antiferromagnet showed a pinned fraction, which was aligned with the bias direction of the material. Rotation of the bias direction led to a clear vertical loop shift. The amount of pinned uncompensated spins in several materials was found to be proportional to the exchange bias interfacial energy, suggesting that pinned moments are responsible for the bias. NiO/MgO(001) LaFeO 3 PEEM-2 at BL 7.3.1.1Octupole magnet at BL 4.0.2 PEEM images of Antiferromagnets X-ray absorption spectroscopy (XAS) is an element-specific technique that measures the chemical state and the electronic structure of materials. The magnetization of a ferromagnet relatively to the x-ray propagation direction can be determined using circularly polarized x-rays (X-ray Magnetic Circular Dichroism). Sum rules quantitatively determine the spin and orbital moment. The magnetic axis of antiferromagnets can be sensed using linearly polarized x-rays (X-ray Magnetic Linear Dichroism). In combination with microscopy techniques like Photoemission Electron Microscopy (PEEM), x-ray techniques are uniquely capable of visualizing domain structures of ferromagnets and antiferromagnets at high spatial resolution. The domain wall rotation of different NiO materials in contact with 2.5 nm Co is compared. A strong rotation is found in a NiO single crystal. An epitaxial NiO film on Ag(001) shows an intermediate rotation while a polycrystalline NiO film on Si shows a very small rotation. Only the polycrystalline film possesses a significant exchange bias. The decreasing Mauri-Model: structural quality going from a single crystal over an epitaxial film to a polycrystalline film results in a higher defect density and better pinning of domain walls in the soft, low-anisotropy antiferromagnet NiO. On one hand this leads to a greatly reduced planar wall rotation, on the other hand to much better biasing properties. We learn that good exchange bias materials are characterized by a strong anisotropy or a high defect density to prevent erasure of the bias state by the generation of a planar wall. A planar wall will likely play no role in high-anisotropy antiferromagnets but needs to be taken into consideration in soft antiferromagnets, like NiO. Exchange bias is mediated by uncompensated spins at the surface of the antiferromagnet. Uncompensated spins are randomly distributed and lead to an enhanced bias in small, lateral domains in accordance with Malozemoff’s model. Planar walls appear in soft, structurally perfect antiferromagnets but likely play no role in hard, polycrystalline antiferromagnets with good exchange bias properties. Scholl et al., PRL 2004 The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. To become an ALS user: http://www-als.lbl.gov/als/quickguide/