Resistive switching (RS) characteristics of a Pr0.7Ca0.3MnO3 (PCMO) film sandwiched between a Pt bottom electrode and top electrodes (TE) made of various metals are found to belong to two categories. Devices with TE made of Al, Ti, and Ta exhibit a large I-V hysteresis loop and bipolar RS, but those with TE made of Pt, Ag, Au, and Cu do not. Transmission electron microscopy reveals that a thin metal-oxide layer formed at the interface between the former group of TE and PCMO, but not for the latter group of TE. Analysis shows that the categorization depends on the Gibbs free energy of oxidation of the TEs with respect to that of PCMO.
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
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