Direct Imaging of Thermally Driven Domain Wall Motion in Magnetic Insulators

Wanjun Jiang, Pramey Upadhyaya, Yabin Fan, Jing Zhao, Minsheng Wang, Li-Te Chang, Murong Lang, Kin L. Wong, Mark Lewis, Yen-Ting Lin, Jianshi Tang, Sergiy Cherepov, Xuezhi Zhou, Yaroslav Tserkovnyak, Robert N. Schwartz, and Kang L. Wang

Phys. Rev. Lett. 110, 177202 – Published 22 April 2013

Abstract

Thermally induced domain wall motion in a magnetic insulator was observed using spatiotemporally resolved polar magneto-optical Kerr effect microscopy. The following results were found: (i) the domain wall moves towards hot regime; (ii) a threshold temperature gradient ( $5 K / mm$ ), i.e., a minimal temperature gradient required to induce domain wall motion; (iii) a finite domain wall velocity outside of the region with a temperature gradient, slowly decreasing as a function of distance, which is interpreted to result from the penetration of a magnonic current into the constant temperature region; and (iv) a linear dependence of the average domain wall velocity on temperature gradient, beyond a threshold thermal bias. Our observations can be qualitatively explained using a magnonic spin transfer torque mechanism, which suggests the utility of magnonic spin transfer torque for controlling magnetization dynamics.

Received 25 July 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.177202

Authors & Affiliations

Wanjun Jiang¹, Pramey Upadhyaya¹, Yabin Fan¹, Jing Zhao¹, Minsheng Wang¹, Li-Te Chang¹, Murong Lang¹, Kin L. Wong¹, Mark Lewis¹, Yen-Ting Lin¹, Jianshi Tang¹, Sergiy Cherepov¹, Xuezhi Zhou², Yaroslav Tserkovnyak³, Robert N. Schwartz¹, and Kang L. Wang^1,*

¹Department of Electrical Engineering, Device Research Laboratory, University of California, Los Angeles, California 90095, USA
²Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
³Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA

^*wang@ee.ucla.edu; http://drl.ee.ucla.edu/

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Vol. 110, Iss. 17 — 26 April 2013

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Images

Figure 1
Experimental demonstration of DW motion driven by a temperature gradient. (a) The schematic illustration of polar MOKE microscope for DW imaging. (b) and (c) $M$ vs $H$ hysteresis loops measured using a MOKE magnetometer for polar and longitudinal orientations, respectively. (d) Snapshots of the position of the DW as a function of time in the cold region, and hot region (e), respectively. It is noted that there is no time correlation between figures (d) and (e). The blue color corresponds to magnetization along $+ z$ direction, and white color associates with the magnetization along $- z$ direction. Each fingerlike pattern thus contains two parallel DWs. These sequential images demonstrate that the direction of motion of the DWs in both cold and hot regions is the same, viz., from the cold region (RT) to the hot region ( $RT + 45 K$ ), with an average DW velocity of $\bar{v} = 200 \pm 5 μ m / s$ .Reuse & Permissions
Figure 2
Dependence of DW motion on the negative direction of the temperature gradient. Applying the same positive bias magnetic field ( $H = + 60 Oe$ ) as used in Fig. 1, with the temperature gradient orientation reversed (viz., $\nabla T = - 22.5 K / mm$ ); again, the direction of motion is preserved, i.e., from the cold end towards the hot end, with an average DW velocity of $\bar{v} = 185 \pm 5 μ m / s$ .Reuse & Permissions
Figure 3
Observation of DW motion outside of the region with a temperature gradient. (a) Schematic demonstration of DW motion due to the decaying magnonic current in region (III) of constant temperature (RT), which is away from region (II) with a temperature gradient. Specifically, in region III, $\sim 0.4 mm$ away from region II with a constant temperature gradient $\nabla T = 22.5 K / mm$ , the DW motion is found with an average velocity of $\bar{v} = 90 \pm 5 μ m / s$ , as shown in Figs. 3b, 3c, 3d, 3e. The reduction in the DW velocity as a function of distance to region II is shown in (f), which can be fitted as $v = v_{0} e^{- L / L_{0}}$ with the characteristic decay length of $L_{0} \approx 1 mm$ .Reuse & Permissions
Figure 4
Evolutionary demonstration of DW motion under varying temperature gradients. In the presence of the progressively decreasing temperature gradient, one clearly sees that the overall DW displacements decrease and the time intervals spent in traveling increase. These images together yield the decreasing average DW velocity. Here the corresponding temperature gradients for each measurement are (a) $\nabla T = 18.5$ , (b) $\nabla T = 15$ , (c) $\nabla T = 11.5$ , (d) $\nabla T = 8.5$ , and (e) $\nabla T = 5 K / mm$ .Reuse & Permissions
Figure 5
Average DW velocity dependence on temperature gradient. The plot of $\bar{v}$ vs $\nabla T$ manifests the existence of a threshold temperature gradient $\nabla T_{th} \approx 5 K / mm$ . Here, $\bar{v}$ is an approximately linear function in the temperature range of $5 K / mm < \nabla T < 22.5 K / mm$ , $- 22.5 K / mm < \nabla T < - 5 K / mm$ , and in the range of $- 5 K / mm < \nabla T < 5 K / mm$ , $\bar{ν}$ is zero.Reuse & Permissions

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