Manipulation of magnetization states of ferromagnetic ...

APPLIED PHYSICS LETTERS 98, 242505 共2011兲

Manipulation of magnetization states of ferromagnetic nanorings by an applied azimuthal Oersted field T Yang,2 Nihar R. Pradhan,1 Abby Goldman,1 Abigail S. Licht,1 Yihan Li,1 M. Kemei,1 Mark T. Tuominen,2 and Katherine E. Aidala1,a兲 1

Department of Physics, Mount Holyoke College, South Hadley, Massachusetts 01075, USA Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, USA

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共Received 3 March 2011; accepted 23 May 2011; published online 16 June 2011兲 We manipulate the magnetic states of ferromagnetic nanorings with an azimuthal Oersted field directed along the ring circumference. The circular field is generated by passing current through an atomic force microscope tip positioned at the center of the ring, and can directly control the chirality of the vortex state. We demonstrate switching from an onion state to a vortex state and between two vortex states, using magnetic force microscopy to image the resulting magnetic states. The understanding of the magnetization switching behavior in an azimuthal Oersted field could improve practical magnetic data storage devices. © 2011 American Institute of Physics. 关doi:10.1063/1.3599714兴

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Electronic mail: [email protected].

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zation. The direction and magnitude of the field are determined by passing a current through a solid metal atomic force microscope probe into a metal underlayer 关Fig. 1共a兲兴. We demonstrate control over switching from an onion state to a vortex state 共O-V兲 and the direct switching between two vortex states 共V-V兲, using MFM to image the resulting magnetic states. Domain wall 共DW兲 types 共head-to-head or tailto-tail兲 and vortex chiralities 关clockwise 共CW兲 or counterclockwise 共CCW兲兴 can be predetermined by an in-plane saturation field, and confirmed by MFM contrast. Symmetric and asymmetric nanorings were fabricated using standard electron-beam lithography, using electronbeam evaporation to deposit 25 nm of permalloy on a goldcoated silicon wafer, capped with 4 nm Pt to protect the permalloy from oxidization. MFM images were obtained with an Asylum Research MFP3D atomic force microscope, and circular fields were created by passing a current through a solid Pt tip 共Rocky Mountain Nanotechnology兲. Figure 1共a兲 depicts a schematic of how we generated the circular field. After obtaining a topographic image of the rings, the Pt solid tip was placed at the center of the ring. We ramped a voltage and measured the resulting current across a 100 ⍀ resistor. Figure 1共b兲 is a typical I-V curve of the current. The initial tip diameter is nominally 40 nm, though the high current densities 共3.18⫻ 109 A / cm2兲 will likely deform the tip, creating a blunter tip and, subsequently, lower current densities. We were generally able to use the same tip to locate the rings topographically and pass current through the ring center about ten times. After applying current, we switched back to

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Ferromagnetic nanorings have received increasing attention over the past few years.1–4 The ring shape is of particular interest due to its closed-flux “vortex” state, in which the circular field lines are entirely contained within the structure. Ferromagnetic rings have been proposed as a basis for practical nonvolatile data storage devices.5,8 The vortex chirality can be used to encode binary information, and the lack of stray fields suggests that the bits could be packed densely without coupling between neighbors. Much work has been devoted to the investigation of the orientations of the vortex in rings under the influence of a uniform magnetic field. In symmetric rings, the vortex chirality cannot be controlled by a homogeneous external field,10 while in asymmetric rings, the chirality can be manipulated with a uniform in-plane field in a predictable way.10–13 A metastable onion state was reported as an intermediate remanent state during the switching process of these rings. Direct vortex-to-vortex switching is not achievable with a uniform in-plane magnetic field. It is possible to use the nonuniform field created from the stray fields of a magnetic force microscope 共MFM兲 probe to control the chirality of the vortex in a disk.14 Our work uses a perpendicular current through the center of a ring to generate a local circular magnetic field, and achieves direct switching and control over the magnetic vortex chirality of ferromagnetic nanorings. Creating this field and measuring the resulting vortex chirality presents experimental challenges. Limited experimental efforts15,16 have demonstrated multilayered structures that use spin transfer torque and the accompanying Oersted field to switch two free and reference magnetic rings back and forth between parallel and antiparallel onion states by passing current through the ring structure itself. Recent theoretical work has begun exploring the switching mechanism3,8,17–19 resulting from a local circular field applied to the ring center, to predict magnetic states. We present a method to manipulate the magnetic states of ferromagnetic nanorings using an azimuthal Oersted magnetic field directed along the ring arm circumference. This type of circular field can select the desired vortex magneti-

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FIG. 1. 共Color兲共a兲 Schematic of the experiment setup. 共b兲 A current-voltage plot generated directly from the experimental setup in 共a兲. 98, 242505-1

© 2011 American Institute of Physics

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a magnetically coated Asylum high coercivity MFM probe to image the resulting magnetic state. Figure 2 shows the magnetic switching of symmetric nanorings, with Figs. 2共a兲 and 2共h兲 showing the topographical AFM images. Before the MFM characterization, an initial in-plane, uniform magnetic field of 5 kOe was applied. In Figs. 2共b兲 and 2共i兲, remanent onion states in all six rings were observed at zero field with MFM, shown schematically in Figs. 2共c兲, 2共d兲, 2共j兲, and 2共l兲. We can identify the white contrast with head-to-head DWs, and dark as tail-to-tail, given the initialization. We passed 40 and 30 mA of current through two identical rings indicated by the double and single red dashed circles. The corresponding Oersted fields in the vicinity of the ring center are around 178 and 133 Oe respectively, as calculated according to Ampere’s Law, assuming an infinite wire approximation. Two different magnetic states, shown by MFM contrast and schematically in Figs. 2共e兲–2共g兲 resulted from the different circular fields. In ring 4, with the smaller applied current, we observed a metastable state, with a 360° DW,6–9,20 resulting from the propagation of two 180° DWs. In ring 2, with the larger applied current, the two DWs moved toward each other and annihilated, entering the CW vortex. We can directly observe the chirality in the 360° DW or so called “twisted onion” state from the light/dark MFM contrast, while for ring 2, we assumed the vortex direction based on the external CW Oersted field. When the circular field is applied, one semicircular domain in the onion state is more energetically favored due to the Zeeman energy, and the two DWs slide toward the minimum energy configuration, leading to the vortex state of desired chirality. A stronger field is required to annihilate the DWs than to move them.6 Figures 2共i兲–2共n兲 show the switching of another symmetric ring set from the initialized onion state 关Figs. 2共i兲 and

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1 FIG. 2. 共Color兲共a兲 Topographic AFM image of two identically designed sets of Ni81Fe19 symmetric rings. Ring 2 and 4 have 300 nm arm width and 1200 nm outer diameter, while 1 and 3 have 300 nm arm width and 800 nm outer diameter. 共b兲 MFM image of the initial remanent states of rings in 共a兲, revealing the onion state as indicated schematically in 共c兲 and 共d兲 for rings 2 and 4. The double red dashed line indicates that we apply a stronger current and CW field 共40 mA and 178 Oe兲 through the ring center, while the single red dashed line indicates a smaller current 共30 mA, 133 Oe兲. 共e兲 MFM image after applying currents, revealing ring 2 is in a vortex state while ring 4 remains in an intermediate state, and rings 1 and 3 are unchanged. 共f兲 Schematic of the state of ring 2, and 共g兲 ring 4. 共h兲 Topographic AFM image of a third set of symmetric rings. 共i兲 MFM image of initial remanent onion states, indicated schematically in 共j兲 for ring 6. Blue circles indicate current direction opposite to red circles. 共k兲 MFM image after applying a ⫺25 mA 共111 Oe兲 CCW current, revealing a 360° DW, shown schematically in 共l兲. 共m兲 MFM image after applying a stronger current 共⫺35 mA and 156 Oe兲, revealing the vortex state, indicated schematically in 共n兲.

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FIG. 3. 共Color兲共a兲 Topographic AFM image of a set of three Ni81Fe19 asymmetric rings. The offset from the center of the ring is 50 nm and outer diameters of three rings are 770 nm, 870 nm, and 1050 nm, respectively. 共b兲 MFM images of initial onion states of the rings at zero field, indicated schematically for rings 2 and 3 in 共c兲 and 共d兲. Blue and red dashed lines indicate out-of-plane and into-plane applied currents, respectively. 共e兲 MFM image after applying ⫺18 mA 共97 Oe, CCW兲 to ring 2 and 19 mA 共90 Oe CW兲 to ring 3, showing different light/dark contrast indicating opposite vortex chiralities, as shown schematically in 共f兲 and 共g兲. 共h兲 MFM image of ring 1 and 2 with initial CW vortex states at zero field, shown schematically in 共i兲. 共j兲 MFM images of rings in 共h兲 after ⫺30 mA 共191 Oe, CCW兲 is applied to ring 1, revealing switched vortex chirality, as indicated schematically in 共k兲. 共l兲 MFM images of ring 1 and 2 with initial CCW vortex states at zero field, shown schematically in 共m兲. 共n兲 MFM image after applying 30 mA 共162 Oe, CW兲 to ring 2, revealing switched chirality, as shown schematically in 共o兲. 共p兲 MFM image of ring 3 with initial CW vortex states at zero field, shown schematically in 共q兲. 共r兲 MFM image of ring 3 after ⫺18 mA 共85 Oe CCW兲 is applied, revealing an intermediate state with a 360° DW, indicated schematically in 共s兲. 共t兲 Simulated MFM image of a permalloy asymmetric ring in CW vortex state. 共u兲 Simulated MFM image of a permalloy asymmetric ring in CCW vortex state, revealing reversed light/ dark contrast.

2共j兲兴 to the opposite, CCW vortex state, achieved by passing a current in the opposite direction at the center of ring 6. A small negative current of ⫺25 mA generated an intermediate state 关Figs. 2共k兲 and 2共l兲兴, with a 360° DW. The light/dark contrast of this DW is opposite to the one created with the positive current in ring 4 关Fig. 2共e兲兴, revealing the underlying CCW circulation. Passing a larger current of ⫺35 mA annihilated the 360° DW, forming the CCW vortex state 关Figs. 2共m兲 and 2共n兲兴, as seen by the lack of MFM contrast. Minor changes in magnetic contrast for nominally identical states 共the small rings with no applied Oersted fields兲 can be expected due to the wear on the tip from the substantial topographic imaging required to find the identical ring after applying the circular field with the solid metal tip. To directly measure the direction of vortex chirality, we introduced asymmetry into the ferromagnetic permalloy nanoring design. Since asymmetric ferromagnetic nanorings have incomplete vortex magnetization circulation, there is stray magnetic field around the edge of the decentered hole. Therefore, MFM images will show contrast that distinguishes the two directions of vortex, as shown in the MFM simulations of Figs. 3共t兲 and 3共u兲, in which the light/dark contrast is reversed for the opposite chiralities. Figures 3共a兲–3共g兲 show the evolution of magnetic states in three rings, initialized in onion states 关Figs. 3共b兲–3共d兲兴. We


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applied-18 mA 共blue circle, ring 2兲 and 19 mA 共red circle, ring 3兲 to generate the CCW and CW vortex states in Fig. 3共e兲, schematically shown in Figs. 3共f兲 and 3共g兲. Compared to the strong MFM signal of the DWs in onion states in Fig. 3共b兲, the MFM contrast of the vortex shows soft light and dark patterns 关Fig. 3共e兲兴 that indicate stray field distribution of the incomplete vortex loops due to the decentered ring geometry 关Figs. 3共t兲 and 3共u兲兴. In our MFM images, topbright-bottom-dark represents CCW vortex and top-darkbottom-bright represents CW vortex. We are able to switch directly between the stable vortex states, as shown in Figs. 3共h兲–3共o兲. Figure 3共h兲 shows two rings, both initialized in the CW vortex states, indicated schematically in Fig. 3共i兲, shown by dark-top/light-bottom contrast. After passing ⫺30 mA of current through blue circled ring 1, we see that the contrast has reversed, indicating the vortex state has switched to CCW, while ring 2 remains unchanged. In Fig. 3共l兲, two rings are initialized in the CCW state 关Fig. 3共m兲兴, and +30 mA is applied to red circled ring 2, which switches into the CW vortex state in Figs. 3共n兲 and 3共o兲, while ring 1 remains unchanged. After multiple trials, we observed that the required current for V-V switching was greater than the current required for O-V switching for identical asymmetric rings. V-V switching includes DW nucleation, DW motion and annihilation, while O-V switching only includes the last two parts, hence requiring less energy.3 We were able to observe an intermediate state showing DW nucleation in the process of the V-V switching. In Figs. 3共p兲 and 3共r兲, a 360° DW was nucleated after passing ⫺18 mA of current through ring 3 initialized in CW vortex, shown schematically in Figs. 3共q兲 and 3共s兲. In summary, we demonstrate a method to manipulate the magnetic configurations in ferromagnetic nanorings through an azimuthal field. Controlled O-V and V-V switching are achieved by our technique and confirmed by MFM measurements. This experimental study could improve the explora-

tion of magnetic states and practical designs of magnetic data storage devices. This work was supported by NSF Grant No. DMR0906832, the Research Corporation for Science Advancement Grant No. 7889, and the NSF Center for Hierarchical Manufacturing 共Grant No. CMMI-0531171兲. 1

C. A. F. Vaz, T. J. Hayward, J. Llandro, F. Schackert, D. Morecroft, J. A. C. Bland, M. Klaui, M. Laufenberg, D. Backes, U. Rudiger, F. J. Castaño, C. A. Ross, L. J. Heyderman, F. Nolting, A. Locatelli, G. Faini, S. Cherifi, and W. Wernsdorfer, J. Phys.: Condens. Matter 19, 255207 共2007兲. 2 D. K. Singh, R. Krotkov, and M. T. Tuominen, Phys. Rev. B 79, 184409 共2009兲. 3 G. D. Chaves-O’Flynn, A. D. Kent, and D. L. Stein, Phys. Rev. B 79, 184421 共2009兲. 4 W. Zhang and S. Haas, arXiv:0902.3024 . 5 J. G. Zhu, Y. F. Zheng, and G. A. Prinz, J. Appl. Phys. 87, 6668 共2000兲. 6 G. D. Chaves-O’Flynn, D. Bedau, E. Vanden-Eijnden, A. D. Kent, and D. L. Stein, IEEE Trans. Magn. 46, 2272 共2010兲. 7 F. J. Castaño, C. A. Ross, and A. Eilez, J. Phys. D: Appl. Phys. 36, 2031 共2003兲. 8 C. B. Muratov and V. V. Osipov, IEEE Trans. Magn. 45, 3207 共2009兲. 9 O. Tchernyshyov and G.-W. Chern, Phys. Rev. Lett. 95, 197204 共2005兲. 10 F. Q. Zhu, G. W. Chern, O. Tchernyshyov, X. C. Zhu, and C. L. Chien, Phys. Rev. Lett. 96, 027205 共2006兲. 11 E. Saitoh, M. Kawabata, K. Harii, and H. Miyajima, J. Appl. Phys. 95, 1986 共2004兲. 12 D. K. Singh, T. Yang, and M. T. Tuominen, Physica B 405, 4377 共2010兲. 13 F. Giesen, J. Podbielski, B. Botters, and D. Grundler, Phys. Rev. B 75, 184428 共2007兲. 14 V. L. Mironov, B. A. Gribkov, A. A. Fraerman, S. A. Gusev, S. N. Vdovichev, I. R. Karetnikova, I. M. Nefedov, and I. A. Shereshevsky, J. Magn. Magn. Mater. 312, 153 共2007兲. 15 T. Yang, M. Hara, A. Hirohata, T. Kimura, and Y. Otani, Appl. Phys. Lett. 90, 022504 共2007兲. 16 H. X. Wei, F. Q. Zhu, X. F. Han, Z. C. Wen, and C. L. Chien, Phys. Rev. B 77, 224432 共2008兲. 17 M. Maicas, Physica B 343, 247 共2004兲. 18 Y.-S. Choi, M.-W. Yoo, K.-S. Lee, Y.-S. Yu, H. Jung, and S.-K. Kim, Appl. Phys. Lett. 96, 072507 共2010兲. 19 M. Carpentieri, G. Finocchio, B. Azzerboni, L. Torres, L. Lopez-Diaz, and E. Martinez, J. Appl. Phys. 97, 10C713 共2005兲. 20 F. J. Castaño and C. A. Ross, Phys. Rev. B 67, 184425 共2003兲.
