Abstract
In magnetic thin films, a magnetic vortex is a state in which the magnetization vector curls around the centre of a confined structure1. In a thin-film disc, vortex states are characterized by the vortex polarity and the winding number2,3. In ferromagnetic (FM) discs, these two parameters have been shown to govern many fundamental properties of the vortex, such as its gyroscopic rotation4, polarity reversal5,6,7, core motion8 and vortex-pair excitation9. In antiferromagnetic (AFM) discs10, in contrast, there has been only indirect evidence for a vortex state, obtained through the observation of induced FM-ordered spins in the AFM disc11,12,13,14. 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.
Similar content being viewed by others
Main
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 () 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 (RL3),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 cos2 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 axis16,17,18, the RL3 result in Fig. 1b shows that the CoO spins are coupled collinearly to the Fe spins at smaller CoO thickness (dCoO=0.6 nm) and perpendicularly to the Fe spins at larger CoO thickness (dCoO=3.0 nm). This collinear to 90° coupling transition was also reported in the NiO/Fe(001) system as a function of NiO thickness19. 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 low temperature using photoemission electron-beam microscopy (PEEM). Although the FM Fe discs exhibit the expected FM vortices for both circular and square discs, the AFM domain imaging of the NiO and CoO discs reveals unambiguously the existence of the AFM vortex state in the NiO and CoO discs (Fig. 2). It should be emphasized that, in addition to the majority AFM-ordered spins (compensated spins), the AFM layer in an AFM/FM bilayer system sometimes could contain a small number of FM-ordered spins (uncompensated spins) induced by the FM layer. Although the induced FM-ordered spins are located at the AFM/FM interface and have properties that resemble those of the FM layer, the majority AFM-ordered spins represent the magnetic properties of the AFM layer. The AFM vortex state reported in previous works11,12,13 was actually from the FM-ordered uncompensated spins in the AFM layer at the AFM/FM interface rather than from the majority AFM-ordered compensated spins in the AFM layer. As the XMLD signal in our work is from the AFM-ordered compensated spins, the NiO and CoO vortices shown in Fig. 2 represent the first direct observation of the AFM vortex state from the AFM-ordered compensated spins, as opposed to the vortex state from the induced FM-ordered uncompensated spins. The AFM NiO and CoO vortices follow the same pattern as the Fe vortices, showing that the AFM vortex state is imprinted from the FM vortex through the AFM/FM interfacial interaction11,12,13. We found that there exist two types of AFM vortex. In thinner NiO (CoO) films, the AFM spins are coupled collinearly with the Fe spins, so the AFM vortex has the conventional structure with the spins curling around the centre of the disc in the same manner as the FM vortex (Fig. 2b). However, in thicker NiO (CoO) films, the AFM spins are coupled perpendicularly to the Fe spins to result in an AFM vortex structure with a divergent spin configuration (Fig. 2c). The divergent vortex is forbidden both for an FM disc and for the FM-ordered uncompensated spins in an AFM disc because a divergent FM vortex would produce magnetic charges at the disc boundary to increase the magnetostatic energy. We believe that the divergent vortex is allowed only for the AFM-ordered compensated spins in an AFM disc because the opposite magnetic charges from the two magnetic sublattices of the AFM material cancel each other at the disc boundary.
PEEM measurements are not normally compatible with externally applied fields owing to interactions with the photoemitted electrons. However, when the isolation of the bilayer discs from the surrounding Fe thin film is not complete, the discs experience a local magnetic field produced by the surrounding Fe film. This field is mainly localized within the film and thus has limited effect on the photoemitted electrons outside the sample. Therefore, we can apply the PEEM technique to study the vortex state within the magnetic field of the surrounding Fe film. The strength of the magnetic field from the surrounding Fe film can be estimated as , where dFe is the Fe film thickness, r is the radius of the cavity, θ is the azimuth angle and σ(θ) is the local magnetic surface charge density. When the surrounding Fe film has a uniform magnetization (Fig. 3a), the magnetic surface charge density is given by σ(θ)=−MFecosθ so the magnetic field strength can be easily estimated as . For MFe=1,700 e.m.u. cm−3, dFe=12 nm and r=2 μm (Fig. 3a), this estimation gives H≈32 Oe. The vortex-core position should shift in the transverse direction relative to the magnetic-field direction20. Figure 3 shows the Fe PEEM images of the CoO (3 nm)/Fe (12 nm) disc at room temperature, which is above the Néel temperature of the CoO (TN=291 K). Under this condition, the CoO layer is in a paramagnetic state, so it will not bias the Fe disc magnetization in response to the external magnetic field. As expected, the vortex core of the Fe disc is shifted off-centre (Fig. 3a) by ∼0.5 μm, which is about half of the disc radius (R=1 μm). This amount of shift agrees with the experimental observation for a vortex state within a ∼30 Oe magnetic field21. The magnetic field produced by the surrounding Fe film is reduced by increasing the size of the ring that isolates the disc from the rest of the Fe film. When this field is reduced, the distance by which the vortex core shifts relative to the centre of the disc is reduced as expected (Fig. 3b).
The CoO/Fe disc was then cooled to 150 K to establish the CoO AFM order. Because this cooling took place in the presence of the local magnetic field provided by the surrounding Fe film, the AFM CoO layer is expected to produce an exchange bias22,23 on the FM Fe layer. If the magnetic field from the Fe magnetization outside the disc could be removed, the exchange bias should have the effect of retaining the Fe vortex in its current off-centre state12. As we cannot apply an external magnetic field during the PEEM operation, we can only change the magnetic field to the CoO/Fe disc by changing the domain state of the surrounding Fe film to alter the magnetic surface-charge distribution at the boundary of the cavity. To change the surrounding Fe-film magnetic domains, we sputtered away the CoO overlayer of the surrounding Fe film using focused ion-beam sputtering with a careful control of the sputtering time (Fig. 4a). Note that a CoO (3 nm)/Fe (2.2 nm) film produces an exchange-bias field of 480 Oe in the CoO/Fe bilayer with, a coercivity of 2,000–3,000 Oe (ref. 15). The Fe film without the CoO overlayer has a coercivity of only ∼50 Oe.Thus we can apply a 50 Oe magnetic-field pulse to change the Fe surrounding-film domain state without affecting the CoO/Fe bilayer magnetization. A PEEM image confirms that applying a 50-Oe magnetic-field pulse in the opposite direction to the Fe magnetization changes the single-domain state of the Fe film outside the disc to multidomain (Fig. 4b,c). The 50-Oe external magnetic field was turned off during the PEEM measurement so that only the local magnetic field produced by the surrounding Fe film applies to the CoO/Fe disc during the PEEM measurement. Therefore, the local magnetic field produced by the Fe film surrounding the CoO/Fe disc is reduced from H∼32 Oe in Fig. 4b to H∼0 in Fig. 4c. Therefore, the position of the Fe vortex core would move back towards the centre of the disc if there were no exchange bias; however, as shown in Fig. 4c, the vortex-core position of the CoO/Fe disc remains virtually unchanged after switching the outside Fe magnetization. This demonstrates that the exchange-bias effect of the CoO vortex on the Fe vortex is retained even after the magnetic field produced by the surrounding Fe-film magnetization has changed from H∼32 Oe in Fig. 4b to H∼0 in Fig. 4c. This is expected because the 32-Oe field is much weaker than the exchange-bias field of the CoO (3 nm)/Fe (12 nm) film so the Fe vortex-core position should remain unchanged after switching off the 32-Oe local magnetic field. To further verify this effect, we warmed the sample back to room temperature. After the sample temperature is raised above the CoO Néel temperature, as demonstrated by the disappearance of the CoO vortex domain (Fig. 4d), we observe that the Fe vortex-core position moves towards the centre of the disc (Fig. 4d), showing the diminishing of the exchange-bias effect of the CoO layer above its Néel temperature.
Our experimental result not only shows the existence of two types of vortex state in the AFM layer of a AFM/FM disc, it also raises many interesting questions for future study. For example, the exchange-bias effect in AFM/FM systems has been mostly studied in continuous films. The exchange-bias field from an AFM vortex state has not been studied deeply, especially the divergent AFM vortex state. To explore the vortex effect on exchange bias, we would need to measure the FM vortex-core position systematically as a function of applied field. Obviously, this kind of study requires different magnetic-domain structures of the surrounding Fe film. Applying an a.c. demagnetization field or a magnetic pulse of different strength could help to realize this. Another new direction could also be developed by imprinting other types of vortex (for example, antivortex and vortex lattice) from the FM layer into the AFM layer.
Methods
NiO/Fe/Ag(001) and CoO/Fe/Ag(001) films were grown epitaxially by molecular beam epitaxy in an ultrahigh-vacuum system with a base pressure of 2×10−10 torr. The Fe film was grown by evaporating Fe from a thermal crucible. The NiO and CoO films were grown by evaporating the Ni and Co in an oxygen environment of 10−6 torr. The typical evaporation rate is ∼1 Å min−1. Low-energy electron diffraction results show that single-crystalline NiO/Fe and CoO/Fe films were formed with the NiO [110] and CoO [110] axes parallel to the Fe [100] and Ag [110] axes (Fig. 2). The formation of single-crystalline NiO and CoO ensures a non-zero XMLD signal from the NiO and CoO films. After covering the film with a 2-nm-thick Ag protection layer, the films were taken out of the vacuum system and patterned using focused ion-beam patterning (Fig. 2). The sample was measured at both Beamline 4 and the PEEM-3 station of the Advanced Light Source at the Lawrence Berkeley National Laboratory. In the PEEM-3 station, an electromagnet near the sample holder enables the application of an external magnetic-field pulse to the sample to change the magnetic-domain state of the sample. However, the external magnetic field must be turned off during the PEEM measurement because the Lorentz force exerted on the photoemitted electrons by the external magnetic field prohibits PEEM image acquisition.
For the FM Fe discs, XMCD was used to image the Fe domain at the Fe L3 edge absorption peak. For AFM NiO and CoO discs, XMLD was used to image the NiO and CoO domains at the Ni L2 and Co L3 edge absorption peaks. XMCD measurements on the Ni and Co L edges were also made. The absence of Ni and Co XMCD signal confirms that the XMLD signal is from the antiferromagnetically ordered compensated spins. For the XMLD spectrum measurement (Fig. 1), the Fe magnetization was aligned to its easy magnetization [100] axis, which is parallel to the NiO or CoO [110] axis. Under this condition, the XMLD signal should reach its maximum value when the X-ray polarization axis coincides with the NiO and CoO spin axes. A detailed description can be found in ref. 15.
References
Shinjo, T., Okuno, T., Hassdorf, R., Shigeto, K. & Ono, T. Magnetic vortex core observation in circular dots of permalloy. Science 289, 930–932 (2000).
Tretiakov, O. A. & Tchernyshyov, O. Vortices in thin ferromagnetic films and the skyrmion number. Phys. Rev. B 75, 012408 (2007).
Yu, X. Z. et al. Real-space observation of a two-dimensional skyrmion crystal. Nature 465, 901–904 (2010).
Choe, S-B. et al. Vortex core-driven magnetization dynamics. Science 304, 420–422 (2004).
Van Waeyenberge, B. et al. Magnetic vortex core reversal by excitation with short bursts of an alternating field. Nature 444, 461–464 (2006).
Hertel, R., Gliga, S., Fähnle, M. & Schneider, C. M. Ultrafast nanomagnetic toggle switching of vortex cores. Phys. Rev. Lett. 98, 117201 (2007).
Yamada, K. et al. Electrical switching of the vortex core in a magnetic disk. Nature Mater. 6, 270–273 (2007).
Vansteenkiste, A. et al. X-ray imaging of the dynamic magnetic vortex core deformation. Nature Phys. 5, 332–334 (2009).
Buchanan, K. S. et al. Soliton-pair dynamics in patterned ferromagnetic ellipses. Nature Phys. 1, 172–176 (2005).
Senthil, T., Vishwanath, A., Balents, L., Sachdev, S. & Fisher, M. P. A. Deconfined quantum critical points. Science 303, 1490–1494 (2004).
Sort, J. et al. Imprinting vortices into antiferromagnets. Phys. Rev. Lett. 97, 067201 (2006).
Hoffmann, A., Sort, J., Buchanan, K. S. & Nogués, J. Exchange-biased magnetic vortices. IEEE Trans. Mag. 44, 1968–1973 (2008).
Salazar-Alvarez, G. et al. Direct evidence of imprinted vortex states in the antiferromagnet of exchange biased microdisks. Appl. Phys. Lett. 95, 012510 (2009).
Tanase, M. et al. Magnetization reversal in circularly exchange-biased ferromagnetic disks. Phys. Rev. B 79, 014436 (2009).
Wu, J. et al. Direct measurement of rotatable and frozen CoO spins in exchange bias system of CoO/Fe/Ag(001). Phys. Rev. Lett. 104, 217204 (2010).
Arenholz, E., van der Laan, G., Chopdekar, R. V. & Suzuki, Y. Angle-dependent Ni2+ X-ray magnetic linear dichroism: Interfacial coupling revisited. Phys. Rev. Lett. 98, 197201 (2007).
van der Laan, G., Arenholz, E., Chopdekar, R. V. & Suzuki, Y. Influence of crystal field on anisotropic X-ray magnetic linear dichroism at the Co2+L2,3 edges. Phys. Rev. B 77, 064407 (2008).
Nevidomskyy, A. H., Scheiber, C., Sénéchal, D. & Tremblay, A-M. S. Magnetism and d-wave superconductivity on the half-filled square lattice with frustration. Phys. Rev. B 77, 064427 (2008).
Finazzi, M. et al. Interface coupling transition in a thin epitaxial antiferromagnetic film interacting with a ferromagnetic substrate. Phys. Rev. Lett. 97, 097202 (2006).
Guslienko, K. et al. Field evolution of magnetic vortex state in ferromagnetic disks. Appl. Phys. Lett. 78, 3848–3850 (2001).
Uhlig, T. et al. Shifting and pinning of a magnetic vortex core in a permalloy dot by a magnetic field. Phys. Phys. Lett. 95, 237205 (2005).
Meiklejohn, W. H. & Bean, C. P. New magnetic anisotropy. Phys. Rev. 102, 1413–1414 (1956).
Nogues, J. & Schuller, I. K. Exchange bias. J. Magn. Magn. Mater. 192, 203–232 (1999).
Acknowledgements
This work was supported by National Science Foundation Grant DMR-0803305, US Department of Energy Grant DE-AC02-05CH11231, the Korea Foundation for International Cooperation of Science and Technology through the Global Research Laboratory project, the Chinese Education Department and the Western Institute of Nanoelectronics.
Author information
Authors and Affiliations
Contributions
J.W. and Z.Q.Q. designed and carried out experiments, analysed data and wrote the paper; D.C. and J.B. patterned the discs and wrote the paper; J.S.P., Y.M., E.A., A.D., A.T.Y., A.S., C.H. and H.W.Z. made the X-ray measurements and discussed results.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing financial interests.
Rights and permissions
About this article
Cite this article
Wu, J., Carlton, D., Park, J. et al. Direct observation of imprinted antiferromagnetic vortex states in CoO/Fe/Ag(001) discs. Nature Phys 7, 303–306 (2011). https://doi.org/10.1038/nphys1891
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/nphys1891
This article is cited by
-
Transfer of magnetic anisotropy in epitaxial Co/NiO/Fe trilayers
Scientific Reports (2024)
-
Antiferromagnetic half-skyrmions electrically generated and controlled at room temperature
Nature Nanotechnology (2023)
-
Tunable interplay between exchange coupling and uniaxial magnetic anisotropy in epitaxial CoO/Au/Fe trilayers
Scientific Reports (2023)
-
Creating zero-field skyrmions in exchange-biased multilayers through X-ray illumination
Nature Communications (2020)
-
Electrical manipulation of the magnetic order in antiferromagnetic PtMn pillars
Nature Electronics (2020)