Nucleation and annihilation of skyrmions in Mn2CoAl observed through the topological Hall effect

Magnetic skyrmions are topologically protected spin textures with great technological potential. These topologically non-trivial non-coplanar spin textures give rise to a topological Hall effect, enabling the purely electronic detection of magnetic skyrmions. We report a clear topological Hall effect in thin films of the the Heusler alloy Mn2CoAl, a ferromagnetic spin-gapless semiconductor, capped by a thin layer of Pd. We exploit the strong thickness- and temperature-dependence of the anomalous Hall effect in this system, tuning it to zero to enable the unambiguous measurement of the topological Hall effect, which is observed for temperatures between 3 K and 280 K. The topological Hall effect is evidence of skyrmions, and we demonstrate the simultaneous coexistence of opposite polarity skyrmions using a novel method involving minor field loops of the Hall effect.

For transport measurements, we used a Quantum Design Physical Property Measurement 20 System (PPMS) with the resistivity option. Samples were mounted using a sample holder from 21 Wimbush Science and Technology, with spring-loaded contacts. Typical bias current used on the 22 device was 100 µA. All samples were measured in a similar way, with complete magnetic hystere-23 sis loops measured between 3 T and -1 T, at angles of -90°and 90°relative to the sample plane at 24 a set of temperatures (375 K, 250 K, 150 K, 75 K and 3 K). The temperature ramp rate was kept at 25 a constant 3 K/min. During cool down (warm up), the sample was kept at 3 T (7 T) and measured 26 to obtain a temperature dependent measurement of the AHE between 375 K and 3 K. Finally each 27 sample was measured in detailed magnetic hysteresis loops between +3000 and −3000 Oe every 28 25 K between 375 K and 75 K, and between +4000 and −4000 Oe at 25 K and 50 K. Additional While we cannot rule out some interdiffusion at the interfaces, no evidence for secondary 45 phases was detected in the TEM image of Fig. 1(a) or from X-ray diffraction (not shown).

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Magnetization and resistivity properties of Mn 2 CoAl film and trilayers

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The longitudinal resistivity of a 33 nm thick Mn 2 CoAl film with MgO capping layer increases with 48 decreasing temperature as shown in Fig. 1(d). This behavior is consistent with previous reports in This same sample measured with the field in-plane ( Fig. 1(b)) has a saturation magnetization 52 of about 250 emu/cm 3 . This corresponds to ∼1.3 µ B /f.u., which is lower than the expected ∼2 µ B 53 per formula unit in bulk Mn 2 CoAl 1 , but reasonable compared to the ∼1 µ B per formula unit of 54 thin films reported previously in the literature. 3, 4 55 The magnetization of a trilayer sample measured at 200 K with field out-of-plane is shown 56 in Fig. 1(c), normalised to its saturation magnetization. This sample is the same as that of  netic field while the ordinate shows the Hall resistance with an arbitrary offset for better legibility.
All samples were measured at 250 K, 150 K, 75 K and 3 K. The hysteresis loops clearly show the AHE in all samples. AHE is positive (resp. negative) for the thin (resp. thick) samples at 250K. It is remarkable that the sign of the AHE changes for samples 2 and 3 (1.7 nm and 2.0 nm) as they are cooled down. The THE can be seen is sample 2 and 3 in form of a peak near the edge of the hysteresis loop. This is most visible at the temperature at which the AHE is canceled out.

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We present data for three samples that show a sign change in the AHE with temperature in 78 Figs. 3(a)-(c). The 1.5 nm thick sample in Fig. 3(a) is not patterned into a Hall bar like the others, 79 and the Hall effect measurements were done in a Van der Pauw geometry. This helps to establish 80 that there are no artifacts arising from the geometry of the Hall bar or from the sample processing.