Electronic transport properties of antiperovskite Mn4N epitaxial films

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The control of magnetization by electric current plays an important role in the applications of magnetic recording, nonvolatile memories, etc. As a key way to realize the manipulation of magnetization efficiently, the application of spin transfer torque (STT) and spin-orbit torques (SOT) requires materials with high spin polarization in electron transport.1-3 Besides, magnetic materials with low magnetization could reduce the critical switching current effectively due to the switching current is proportional to the saturation magnetization.4 Antiperovskite type Mn4N films have attracted a lot of attention due to the perpendicular magnetic anisotropy (PMA) and low saturation magnetization.5,6 However, the details about electronic transport properties in Mn4N films are not yet clear. Thus, the electronic transport properties of the Mn4N films with different thicknesses on MgO substrates have been investigated systematically. Fig. 1(a) presents the ρxx(T)/ρxx(300 K) of Mn4N films with different thicknesses on MgO(001) substrates. It is obvious that the resistivity continuously increases as the temperature increases in most films, showing a metallic conductance mechanism for the Mn4N films. However, there is an abnormal trend in the 4.7-nm-thick Mn4N films whose resistivity decreases as the temperature increases. In order to evaluate the purity and integrity of the Mn4N films, the residual resistivity ρxx0 at 0 K originated from impurity scattering is extracted from the ρxx-T curves.7 Fig. 1(b) shows the ρxx-T and fitting curve of the 78.8-nm-thick Mn4N film. The ρxx0 is obtained by fitting the ρxx-T at low temperatures using the expression as ρxx(T) = ρxx0+αTβ, where ρxx0 and T are the residual resistivity and temperature, and the coefficient α and β will change with the scattering model. The calculated ρxx0 of the 78.8-nm-thick Mn4N film is 30.56 μΩ cm, which is slightly less than ρxx(2 K) = 30.95 μΩ cm. The calculated β is 2.014 below 85 K, which is closely proportional to 2 for the ferromagnets. And the ρxx0 of other thicknesses were obtained by fitting the ρxx-T curves with the thickness from 7.8 to 47.3 nm in the same way. The thickness-dependent ρxx0(t) curve is shown in the inset of Fig. 1(b). It is obvious that the ρxx0 increases with the decrease of the film thickness, which imply the enhancement of the electron-electron interaction effects and interfacial scattering.8 Anomalous Hall effect of the Mn4N films were measured in the temperature range of 5-300K and in magnetic fields to 7 T. The Hall bars were used for measuring the Hall resistivity ρxy of Mn4N films with the thickness of 4.7-78.8 nm. The AHE is analyzed by the expression as ρxy=ρOH+ρAH=R0H+RsM, where R0 and Rs are ordinary and anomalous Hall coefficients, respectively. Fig. 2(a)-(h) shows the hysteretic property of the Hall resistivity ρxy for the different thickness as a function of applied magnetic field H at the temperatures of 5-300 K. The hysteretic property of ρxy due to the AHE caused by quantum mechanical spin-orbit interaction, and the linear property at the high-field regions due to the ordinary Hall effect (OHE) originated from the Lorentz force.9 The definition of MR is [ρxx(H)-ρxx(0)]/ρxx(0)×100%, where ρxx(0) is the resistivity of the film at 0 T. MR of the Mn4N film displays a negative signal and butterfly shape. The films with the thickness of 47.3 nm and 23.6 nm show a sudden lift as the magnetic field increases from 0 to 0.5 T, and the sign of MR is positive. Then, MR gradually decreases and sign turns negative with the increasing magnetic field above 0.5 T. The MR of 11.0 nm and 7.8 nm thick films shows the same trend except for the turning point at about 1 T. The upturn of MR in the low fields is mainly due to the Lorentz force effect, which causes a decline of mean free paths and a rise of the scattering rates.10 And the downturn of MR at high fields could be attributed to the enhancement of ferrimagnetic order and the reduction of electron scattering by spin-wave excitations and local magnetic anisotropy.11 At a certain temperature, MR of the Mn4N films increases with the increase of film thickness. These results provide experimental basis for application of SOT devices.This work is supported by National Natural Science Foundation of China (51871161 and 52071233). **