Magneto-transport properties in Mn4-xNixN films with large current induced domain wall mobility and investigation in their large anomalous Hall effect

Abstract

Nowadays, our lives are greatly enriched by information society. Meanwhile, energy resource issue is becoming a serious problem, leading to the demand for devices with lower power consumption. When we focus on the memory, the spread of non-volatile memory, represented by the racetrack memory [1] based on current induced domain wall motion (CIDWM), has been longed for.To realize this, our groups have been investigating anti-perovskite Mn4N films as candidate materials for the racetrack memory. They show a small saturation magnetization (MS) of 80 kA m-1 and clear perpendicular magnetic anisotropy (PMA) [2], contributing to high velocity and low threshold current density, respectively. We experimentally demonstrated the domain wall velocity (vDW) of 900 ms-1 at 1.3×1012 Am-2 only by spin-transfer torque (STT) [2]. Note that we achieved this record by rare-earth free material unlikely other reports. Our recent work also pays attention to Mn4-xNixN films because they have compensation point at x ~ 0.15 where vDW takes its maximum value [3]. In Mn3.85Ni0.15N, we recorded vDW of 2000 m/s at 1.16×1012 Am-2, the fastest purely STT-driven CIDWM [4].Despite of these amazing performances, large parts of magneto-transport properties in Mn4-xNixN films are still unknown although they are deeply linked to the relationship between conduction electrons and localized ones. Interestingly, we confirmed large anomalous Hall effect (AHE) with a relatively large anomalous Hall angle (θAHE = ρAHE/ρxx) of 2% in Mn4N films although they don’t contain any heavy metal with large spin-orbit coupling [2]. There were attempts to discover the origin of such large AHE, however, it has not been proved by now. In this work, we performed AHE and conductivity measurements at temperatures in the range of 5-300 K, and we plotted σAHE against σxx to find out the origin of AHE [6]. Also, we performed anisotropic magnetoresistance (AMR) measurements for the better understanding of spin-dependent scattering in these materials.Mn4-xNixN films (30 nm) were fabricated onto SrTiO3 (STO)(001) substrates by molecular beam epitaxy. Epitaxial growth was confirmed by X-ray diffraction, and the thickness of each layer was examined by X-ray reflectivity. AHE and AMR measurements were performed by physical properties measurement system. Before measurements, film samples were processed into Hall bars with a width of 200 mm by Ar ion milling. During AMR measurements, DC current was set to flow in the [100] azimuth and a magnetic field of 9 T was applied parallel to the plane.Figure 1 shows the σxx dependence of σAHE in Mn4-xNixN/STO in the temperature range of 5-300 K. Note that σxx became larger at lower temperatures, a typical behavior in metals. The region of σxx < 104 Ω-1cm-1 is called “Bad metal regime”, where σAHE is proportional to σxxα (α~1.8) in many 3d transition metals [5]. Mn4-xNixN films (x > 0) also followed this trend, while Mn4N showed rather constant σAHE relative to σxx. On the other hand, the region of σxx > 104 Ω-1cm-1 is called “Good metal regime”, where σAHE did not change so much against σxx. In Mn4-xNixN at small x (x = 0 and 0.05), however, σAHE decreased with increasing σxx, thereby, with decreasing temperature. It’s also remarkable that σAHE remained almost unchanged when x = 0.15 and 0.2. This discrepancy can be explained in the following.Figure 2 shows the Fourier coefficients C2 (cos2θ) and C4 (cos4θ) of AMR curves of Mn4-xNixN/STO. θ is the angle made by the current and magnetization. C2 and C4 components derive from cubic and tetragonal crystalline fields, respectively [6]. Thus, large C4 components at temperatures lower than 100 K at x = 0 and 0.05 originate from tetragonal one although they have cubic structures. Note that C4 component was very small at x = 0.15, and AMR was barely observed at x > 0.2.In 3d transition metals, intrinsic AHE comes from hopping of electrons between electronic states dyz and dzx [7]. However, these states rise under the tetragonal crystalline field due to the resolution of degeneracy, leading to smaller number of electrons related to the origin of AHE. Therefore, reflecting our experimental results, unexpected decrease in σAHE at low temperatures in Mn4-xNixN (x= 0, 0.05) can be explained by the emergence of tetragonal field. On the basis of these discussions, we conclude that large AHE in Mn4-xNixN originates from intrinsic AHE at low temperatures. Although we have not fully revealed the case at room temperature due to a limited number of reports on AHE in “Bad metal regime”, we suggest that it originates from intrinsic AHE and extrinsic AHE such as phonon-scattering. We believe this work opens the interest in the band structure of Mn4-xNixN films. **