The exchange bias of epitaxially grown CoO/Fe/Ag(001) was investigated using Xray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD) techniques. A direct XMLD measurement on the CoO layer during the Fe magnetization reversal shows that the CoO compensated spins are rotatable at thinner thickness and frozen at larger thickness. By a quantitative determination of the rotatable and frozen CoO spins as a function of the CoO film thickness, we find the remarkable result that the exchange bias is well established before frozen spins are detectable in the CoO film. We further show that the rotatable and frozen CoO spins are uniformly distributed in the CoO film.

In magnetic thin films, a magnetic vortex is a state in which the magnetization vector curls around the centre of a confined structure 1. In a thin-film disc, vortex states are characterized by the vortex polarity and the winding number 2,3. In ferromagnetic (FM) discs, these two parameters have been shown to govern many fundamental properties of the vortex, such as its gyroscopic rotation 4 , polarity reversal 5-7 , core motion 8 and vortex-pair excitation 9. In antiferromagnetic (AFM) discs 10 , in contrast, there has been only indirect evidence for a vortex state, obtained through the observation of induced FM-ordered spins in the AFM disc 11-14. Here we report the direct observation of an AFM vortex state in the AFM layer of an AFM/FM bilayer system. We have fabricated single-crystalline NiO/Fe/Ag(001) and CoO/Fe/Ag(001) discs, and using X-ray magnetic linear dichroism techniques we observe two types of AFM vortex, one of which has no analogue in FM structures. We also show that a frozen AFM vortex can bias an FM vortex at low temperature. Single-crystalline NiO/Fe(12 nm)/Ag(001) and CoO/Fe(12 nm)/ Ag(001) films were grown by molecular beam epitaxy and patterned into discs using a focused ion beam. The FM Fe and AFM NiO and CoO were measured at the Advanced Light Source of Lawrence Berkeley National Laboratory by X-ray magnetic circular dichroism (XMCD) and X-ray magnetic linear dichroism (XMLD). Although the XMCD measurement is a standard method, the XMLD measurement on NiO and CoO in our experiment was made at the Ni L2 edge and Co L3 edge by changing the X-ray linear polarization angle (φ) relative to the Fe [001] magnetization axis, which is parallel to the NiO or CoO [110] crystalline axis (Fig. 1a; ref. 15). Figure 1 represents a typical CoO XMLD result from CoO (3 nm)/Fe (12 nm)/Ag(001) with the XMLD signal defined by the so-called L3 ratio (R L3),which is the X-ray absorption intensity at the photon energy E = 778.1 eV divided by the absorption intensity at E = 778.9 eV (ref. 15). The L3 ratio follows the expected cos 2 φ dependence for all CoO thicknesses. As the L3 ratio under this condition should reach its maximum value for X-ray polarization parallel to the CoO spin axis 16-18 , the R L3 result in Fig. 1b shows that the CoO spins are coupled collinearly to the Fe spins at smaller CoO thickness (d CoO = 0.6 nm) and perpendicularly to the Fe spins at larger CoO thickness (d CoO = 3.0 nm). This collinear to 90 • coupling transition was also reported in the NiO/Fe(001) system as a function of NiO thickness 19. The underlying mechanism of this coupling transition remains unclear so far and has been a focus of research in this field. Element-specific magnetic domains were imaged for both the FM Fe and the AFM NiO (CoO) of the bilayer discs at

CoO/Ag/Fe/Ag(001) films were grown epitaxially and studied using Magneto-Optic Kerr Effect and X-ray Magnetic Circular Dichroism (XMCD). Instead of exponential decay as reported in previous works, we find that both the exchange bias and the coercivity in the epitaxially grown films exhibit a non-monotonous behavior with the Ag spacer layer thickness. By purposely increasing the film roughness, the non-monotonous interlayer coupling evolves into a monotonic decrease with increasing the Ag thickness. Furthermore, we show that the interlayer coupling peak diminishes or shifts its peak position by inserting a Cr layer between CoO and Ag.

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Co thin films were epitaxially grown on Mo(110) and investigated by spinpolarized low-energy electron microscopy (SPLEEM). We find that the spin asymmetry of the electron reflectivity from the Co film alternates its sign as a function of both the electron energy and the Co film thickness as a result of spin polarized quantum well states in the Co film. By measuring spin dependent quantum well states, we are able to resolve the spin-split energy dispersions of the Co film precisely. The determined spin resolved energy bands are further tested by fitting the quantum well states using the phase accumulation model (PAM), and the result shows an excellent agreement between the fitting and the experimental data.