The role of planar and vertical domain walls and uncompensated interface spins in exchange bias.

1. Co L3/L2 XMCD Co domain switching -223 Oe XMCD 92 Oe [011] Co Ferromagnet [011] E [100] [010] s s s 103 Oe 223 Oe LaFeO3 B/A XMLD 1 2 Antiferromagnet H Z +30 Oe LaFeO3 XMLD E [100] [110] -30 Oe Y X H 10 mm Local loops LaFeO3 h Co/LaFeO3 Co Random field problem: LaFeO3 Competition between domain wall and interface energy: F. Nolting et al., Nature 2000 Malozemoff, PRB 35, 1987 FM AFM Ideal interface: Experiment: INTRODUCTION Meiklejohn & Bean, Phys. Rev. 102 1956 • Observations about exchange bias: • Exchange bias is an interface effect. • Exchange bias is a result ofsymmetry breaking. • Exchange bias appears in a variety of materials. • Basic physics understood – Exchange interaction • X-ray microscopy and spectroscopy bring forth a microscopic understanding. NiO XMLD [110] B [100] A H X 5 mm h Co XMCD A B 15 Co 10 5 3nmCoFe/PtMn[b] 0.2 0 2nmCoFe/PtMn[a] -5 2nmCo/IrMn -10 Interfacial Energy (mJ/m2) XMCD Asymmetry (%) -15 1nmCoFe/PtMn[a] 0.3 Mn 0.1 0.2 0.1 AFM domain wall energy Interface energy 0.0 3nmCo/NiO -0.1 Zeeman energy FM anisotropy -0.2 0.2 0.3 -0.3 Spinned (mBohr) -3k -2k -1k 0 1k 2k 3k Applied Field (Oe) The role of planar and vertical domain walls and uncompensated interface spins in exchange bias. Andreas Scholl,1 Marco Liberati,2 Hendrik Ohldag,3 Frithjof Nolting,4 Joachim Stöhr31Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 2Department of Physics, Montana State University, Bozeman, MT 59717, USA 3Stanford Synchrotron Radiation Laboratory, Stanford, CA 94309, USA 4Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland X-RAY SPECTROSCOPY & IMAGING OF ANTIFERROMAGNETS VERTICAL DOMAIN WALLS PEEM-2 at BL 7.3.1.1 Octupole magnet at BL 4.0.2 NiO/Si NiO(001) NiO/MgO(001) LaFeO3 PEEM images of Antiferromagnets Local exchange bias Domain area distribution Stöhr et al., PRL 1999 Scholl et al., Science 2000 Ohldag et al., PRL 2001 To become an ALS user: http://www-als.lbl.gov/als/quickguide/ Bias field vs. domain area Local, remanent hysteresis loops of Co/LaFeO3 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. 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. A. Scholl et al., APL 2004 PLANAR WALLS UNCOMPENSATED INTERFACE SPINS NiO antiferromagnetic NiO interface ferromagnetic Co ferromagnetic [010] 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 <011> directions. A magnetic field applied along one <011> 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). Redox reaction atCo/NiO interface [100] XMLD XMCD XMCD NiO Co ~1 monolayer Ohldag et al. PRL 2001 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 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 AFM Interface loop compared with FM loop Coupling energy scales with density of pinned spins 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. 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 A12 and the antiferromagnetic domain wall energy EAFM. For Co/NiO(001) we find EAFM = 0.66 mJ/m2 and A12 = 1.52·10-13 J/m. Scholl et al., PRL 2004 Mauri-Model: Ohldag et al. PRL 2003 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 CONCLUSIONS 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. 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.