Abstract
1. Introduction
Heusler compounds have drawn much attention in recent years due to their high Curie temperature (TC)[
Different from Co2MnAl or Mn2CoAl, we have synthesized Co1.65Mn1.35Al (CMA), an off-stoichiometric alloy Co2MnAl, in which perpendicular magnetic anisotropy (PMA) emerges as a result of residual strain. This material provides a material platform for studying the effect of strain on spin-orbit coupling related phenomena such as anomalous Hall effect (AHE), and could also be a promising candidate for potential applications in spintronic devices due to its PMA characteristic[
In this work, we report the temperature T dependence of the longitudinal resistivity ρxx and the negative magneto-resistance (MR) at different temperatures of a 20 nm-thick CMA film. The semiconducting-like transport character in the T range between 5 and 300 K implies the existence of an activation energy Ea. It is found that ρxx can be described by a simple activation model with varying Ea in different T regions. The T dependences of the magnetization M are also different correspondingly. The first transition temperature at around 110 K could be ascribed to a transition from localized to itinerant ferromagnetism. We measure the Hall resistivity at several temperatures and obtain the intrinsic anomalous Hall conductivity of ~55 Ω–1cm–1, which is almost twenty times smaller than that of Co2MnAl[
2. Experiments
The single-crystalline film of CMA was epitaxially grown on GaAs (001) substrate by molecular-beam epitaxy (MBE). After the GaAs de-oxidation and buffer layer deposition, a 20 nm-thick CMA film was grown at 200 °C, capped with 2 nm-thick aluminum for protection from oxidation. The sample was patterned to a standard Hall bar, with the channel along the GaAs [110] direction for transport measurements. The magnetic field H was applied perpendicular to the film plane, i.e., parallel with the GaAs [001] direction. The transport data of the film were collected with T between 5 and 300 K and H up to 50 kOe using a physical property measurement system (Quantum Design, PPMS-9). The magnetic properties of CMA film were measured using a superconducting quantum interference device (SQUID) down to 5 K.
3. Analyses and results
Fig. 1(a) shows the dependence of ρxx on T varying from 5 to 300 K with a zero magnetic field. ρxx increases with decreasing temperature in the whole range of T in our experiment. Different from Co2MnAl[
Figure 1.(Color online) (a)
The relations of MR with H are selectively presented in Fig. 1(b). The MR is defined as (ρxx– ρxx0)/ρxx0 × 100%, where ρxx0 = ρxx(H = 0). The negative MR is observed over the whole T range, which is usually ascribed to the spin-dependent scattering in similar systems[
In order to learn whether a similar transition happens for the ferromagnetism of this sample, we carried out the magnetic measurements using a SQUID magnetometer. The sample was cooled down to 5 K with H = 0 and then saturated by a magnetic field as high as 50 kOe. After H was set at 20 Oe, the T dependence of M along the [001] direction was measured by heating the sample from 5 to 280 K. The results are shown in Fig. 2(a). For our CMA film, a linear dependence of M2 on T2 is observed with T ranging from about 110 to 280 K, which is ordinary in itinerant ferromagnetic materials such as MnSi[
Figure 2.(Color online)
Fig. 3(a) shows the measured Hall resistivity ρxy varying with H and T after the subtraction of the background signals coming from ρxx. ρxy is dominated by the anomalous Hall signal, which reflects the ferromagnetism corresponding to the hysteresis loop shown in the inset. The Hall resistivity is generally described by the formula: ρxy(H) = RoH + RAM(H), where Ro and RA are the ordinary and anomalous Hall coefficient, respectively. When H is large enough to saturate a ferromagnetic material, the slope of ρxy(H) is mainly determined by Ro = 1/(nq) based on the one band model, where n is the carrier concentration and q equals the charge of an electron. However, in the CMA film, we notice that the slope of ρxy(H) gradually becomes larger when H is increased, which seems to be contradicted with the saturation state shown in the hysteresis loop. Considering the different behaviors of MR when H is lower and higher than 30 kOe, M could also change gradually with H which is not evident compared to the remanent magnetization. We then obtain ρAH as the value of ρxy(H = 0) for estimating the anomalous Hall conductivity (AHC). In general, the anomalous Hall resistivity ρAH can be described in terms of ρxx in the form of ρAH = βρxx + γρxx2, where β characterizes the extrinsic skew scattering and γ denotes the intrinsic origin or the extrinsic side-jump scattering[
Figure 3.(Color online) (a)
4. Conclusion
In conclusion, we have systematically investigated the magneto-transport properties of a 20 nm-thick CMA film. The film exhibits semiconducting-like transport behavior below 300 K. Its longitudinal resistivity ρxx(T) is properly described by a simple activation model with different activation energies in three T regions separated by 110 and 35 K. The activation energies are 34, 15 and 1.4 meV in the temperature range of 110–300, 35–110, and 5–35 K, respectively. Similar transitions at nearly the same temperature in M(T) have also been discovered, implying a same origin of the transitions of ρxx(T) and M(T), which might probably be related to the bands’ shift. The Curie temperature is estimated to be ~640 K, which is a typical value among full-Heusler alloys. An intrinsic AHC of about 55 Ω–1cm–1 is obtained from the linear dependence of ρAH versus ρxx2. Further experiments and calculations are needed to understand the relations between the transitions and the band structures of the film.
Acknowledgements
This work was supported by the Ministry of Science and Technology under Grant Nos. 2015CB921500, 2017YFB0405701 and the National Natural Science Foundation of China under Grant Nos. U1632264 and 11704374.
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