Direct observation of magnetization dynamics generated by nanocontact spin-torque vortex oscillators

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Direct observation of magnetization dynamics generated by nanocontact spin-torque vortex oscillators

P. S. Keatley, S. R. Sani, G. Hrkac, S. M. Mohseni, P. Dürrenfeld, T. H. J. Loughran, J. Åkerman, and R. J. Hicken

Phys. Rev. B 94, 060402(R) – Published 2 August 2016

Abstract

Time-resolved scanning Kerr microscopy has been used to directly image the magnetization dynamics of nanocontact (NC) spin-torque vortex oscillators (STVOs) when phase locked to an injected microwave (rf) current. The Kerr images reveal free-layer magnetization dynamics that extend outside the NC footprint, where they cannot be detected electrically, but which are crucial to phase-lock STVOs that share common magnetic layers. For a single NC, dynamics were observed not only when the STVO frequency was fully locked to that of the rf current, but also for a partially locked state characterized by periodic changes in the core trajectory at the rf frequency. For a pair of NCs, we explore the correlation between the spatial character of injection-locked dynamics and the free-running spectra. Insight gained from these images may improve understanding of the conditions required for mutual phase locking of multiple STVOs, and hence enhanced microwave power emission.

Received 15 January 2016

DOI:https://doi.org/10.1103/PhysRevB.94.060402

Physics Subject Headings (PhySH)

Magnetic vortices Spin torque Spintronics

Magneto-optical Kerr effect

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

P. S. Keatley^1,*, S. R. Sani^2,3,4, G. Hrkac⁵, S. M. Mohseni⁶, P. Dürrenfeld⁷, T. H. J. Loughran¹, J. Åkerman^2,3,7, and R. J. Hicken¹

¹Department of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom
²Materials and Nano Physics, School of ICT, KTH Royal Institute of Technology, Electrum 229, SE-164 60 Kista, Sweden
³NanOsc AB, Electrum 205, SE-164 40 Kista, Sweden
⁴Department of Physics & Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden
⁵College of Engineering, Mathematics and Physical Science, University of Exeter, Exeter EX4 4SB, United Kingdom
⁶Department of Physics, Shahid Beheshti University, G.C., Evin, Tehran 19839, Iran
⁷Physics Department, University of Gothenburg, Fysikgränd 3, SE-412 96 Gothenburg, Sweden

^*p.s.keatley@exeter.ac.uk

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Vol. 94, Iss. 6 — 1 August 2016

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Images

Figure 1
(a) A schematic of the combined apparatus for TRSKM and frequency-domain magnetoelectronic measurements. Inset: An atomic force microscope image of the contact pad geometry allowing for optical access to the extended spin-valve mesa (scale bar 10 µm). The approximate location of the NC (center of the mesa) is indicated by a small black dot. (b) and (c) The free-running signal generated by single and double NC devices subject to dc current only. White corresponds to $0 nV / H z^{1 / 2}$ and black corresponds to 4 and $10 nV / H z^{1 / 2}$ in (b) and (c), respectively. Vertical dashed lines in (c) indicate the values of $I_{dc} (- 19.2$ and $- 17.2 mA)$ at which the spectra of Fig. 3 were extracted. (d) and (e) The signal generated by single and double NC devices subject to an additional rf current with a frequency of 160 MHz and an amplitude of 1.4 mA. White corresponds to $0 nV / H z^{1 / 2}$ and black corresponds to 2 and $6 nV / H z^{1 / 2}$ in (d) and (e), respectively. In (d) and (e) the vertical dashed lines indicate the values of $I_{dc} [- 21.5 mA$ in (d), and $- 17.5$ and $- 15.5 mA$ in (e)] at which Kerr images were acquired. In (b) and (d), $| H_{B} | = 5 mT$ and $θ_{B} = 270^{\circ}$ . In (c) and (e), $| H_{B} | = 3 mT$ and $θ_{B} = 0^{\circ}$ .
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Figure 2
(a)–(c) Time- and vector-resolved Kerr images corresponding to the $Δ M_{x}$ and $Δ M_{y}$ , (in-plane), and $Δ M_{z}$ (out-of-plane) components of the dynamic magnetization (scale bar 5 μm) acquired at $| I_{dc} | = - 21.5 mA$ with $| H_{B} | = 5 mT$ and $θ_{B} = 270^{\circ}$ . The dotted black line depicts the approximate outline of the top contact pad. In (a) and (b) black/white represents a change in magnetization of $+ / - 0.5 M_{s}$ , where $M_{s}$ is the saturation magnetization. A micromagnetic simulation (Sim) shows the contrast expected for phase-modulated imaging (scale bar 200 nm, $| I_{dc} | = 18 mA)$ . (d) Time-resolved traces of $Δ M_{x}$ corresponding to locations 1 (black symbols), 2 (red symbols), and 3 (blue symbols). (e) Amplitude of $Δ M_{x}$ at location 1 (black symbols) and 2 (red symbols), and (f) area of the localized dynamics around location 1 extracted from $Δ M_{x}$ images as a function of $I_{dc}$ . The area was defined by a contour corresponding to $0.25 M_{s}$ in $Δ M_{x}$ images plotted on a contrast scale of $0 - 0.5 M_{s}$ . In (d)–(f) fitted curves are guides to the eye, while gray shading represents an uncertainty of one standard deviation derived from signals extracted from location 3. In (e) and (f) the vertical blue line represents the onset of locking, while the yellow band represents the primary locking range. (g) A simple schematic representation of the partially locked mode, which exhibits a change in the core trajectory at $f_{rf}$ due to the time-dependent amplitude of the Oersted field $(H_{dc + rf})$ , while the core traverses the trajectory at $f_{G}$ . The blue shading of $H_{dc + rf}$ illustrates the “breathing” of the azimuthal Oersted field lines. The center of the NC is represented by the black dot at the center of the diagram (not to scale).
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Figure 3
Spatial character of magnetization dynamics generated by a pair of STVOs. In (a) and (b) time- and vector-resolved Kerr images acquired at a fixed phase of the rf current at $I_{dc} = - 17.5$ and $- 15.5 mA$ show two regions of localized dynamics (indicated by arrows). In (a) and (b), $| H_{B} | = 3 mT$ and $θ_{B} = 0^{\circ}$ . Black/white grayscale represents $+ / - 0.4 M_{s}$ , respectively. (c) The equivalent response for a single NC shows only a single region of localized dynamics (arrow), where black/white represents $+ / - 0.5 M_{s}$ . In (c), $| H_{B} | = 5 mT$ and $θ_{B} = 0^{\circ}$ . (d) and (e) Time-resolved traces extracted from the left (black symbols) and right (red symbols) region of localized dynamics, indicated by the arrows in the $Δ M_{x}$ images, acquired over one rf cycle for $I_{dc}$ values of (d) $- 17.5 mA$ and (e) $- 15.5 mA$ . Fitted curves with a frequency 160 MHz are guides to the eye. (f) Free-running spectra extracted along the vertical dashed lines in Fig. 1 that lie to either side of the mode transition occurring at $I_{dc} = - 18.2 mA$ .
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