When a magnetic field (B in Fig. 1 (a)) is applied to a metal, the electric current (J in Fig. 1 (a)) flowing through the metal will be deflected by the magnetic field so that it is not parallel with the applied electric field (E in Fig. 1 (a)). This phenomenon, known as the Hall effect, is well understood in the classical physics. Later, the research on magnetic materials discovered that in certain magnetic materials, the Berry phase, one of the quantum properties of electrons, can alter the motion of electrons without the presence of an external magnetic field (Fig. 1 (b)). This phenomenon is known as the anomalous Hall effect (AHE). In AHE experiments, two quantities are measured: The longitudinal conductivity σxx, which is the conductivity of the material along the direction of the electric field; and the transverse conductivity σxy, which is the conductivity along the perpendicular direction of the electric field. The ratio between σxy and σxx, defined as the anomalous Hall angle, measures the strength of the intrinsic deflection by the Berry phase: The higher the σxy/σxx is, the more prominent the Berry phase is.
Weyl semimetals are a class of crystals in which the quantum nature of electrons results in exotic properties such as negative magnetoresistance, giant anomalous Hall effect and anomalous Nernst effect[1,2]. Magnetic Weyl semimetals are the materials that integrate magnetic orderings with the electronic properties of Weyl semimetals. As a result, they form a unique material system for realizing unprecedented applications including anomalous Hall effect sensors and hybrid complementary metal-oxide-semiconductor (CMOS)/anomalous Hall effect devices.
Recently the ferromagnetic material Co3Sn2S2 has been experimentally proven to be a magnetic Weyl semimetal. However, Co3Sn2S2 shows a low Curie temperature of 177 K, meaning that above 177 K (-96°C or -141°F), Co3Sn2S2 becomes nonmagnetic. Consequently, the implement of Co3Sn2S2 for room temperature applications is hindered. Many Co-based compounds that have similar crystalline structures as Co3Sn2S2 have been predicted to be magnetic Weyl semimetals. Among them, the ferromagnetic crystal Co2MnGa shows not only high Curie temperature of 700 K (427°C or 800°F) but also large anomalous Hall angle at room temperature (σxy/σxx =12%), making Co2MnGa an ideal candidate for room temperature quantum devices[6,7].
While the properties of bulk single crystal Co2MnGa are well understood, the quantum properties of Co2MnGa in the form of thin films are little known. To elucidate the effect of the low dimensionality on the Berry phase of Co2MnGa thin films, the research group led by Dr. Simon Granville at Victoria University of Wellington, New Zealand, fabricated epitaxial Co2MnGa thin films on single crystal MgO (001) substrates.
Co2MnGa thin films with various thicknesses (from 5 nm to 50 nm) were synthesized by DC magnetron sputtering in a Kurt J Lesker CMS-18 system with the base pressure lower than 5x10-8 Torr (Fig. 2). Before deposition, the single crystal MgO substrates were cleaned by Ar plasma, and then annealed at 400°C for 1 hour in the vacuum. A stoichiometric polycrystalline target was employed, and the growth rate was 0.8 Å/s at 100 W under 6 mTorr of Ar. During the growth, the substrate temperature was 400 °C. All samples were post-annealed in-situ at 550°C for 1 hour. After cooling down to the ambient temperature, a 2 nm protective MgO layer was grown on the top. The crystalline structures of the Co2MnGa films were analyzed by a high-resolution X-ray Diffractometer. The film thickness and smoothness were measured by X-ray reflectivity and atomic force microscopy. Photolithography was exploited to define Hall bar structures for characterization by a Physical Property Measurement System and a Superconducting Quantum Interference Device Magnetometer.
The magneto transport measurement on Co2MnGa thin films revealed that when the thickness of the Co2MnGa layer was 50 nm, the anomalous Hall angle σxy/σxx was 9.7% at room temperature and reached its maximum value of 11.4% at 113 K (Fig. 3 (a)). Although some non-magnetic Weyl semimetals exhibit large anomalous Hall angle in single crystals, to date, only Co2MnGa shows a large anomalous Hall angle in thin films at room temperature. In Fig. 3 (b), the thickness dependence of the anomalous Hall angle was investigated. When the thickness of the film was above 20 nm, the ratio σxy/σxx showed similar values. When the thickness was lower than 20 nm, however, the anomalous Hall angle decreased significantly. To understand the mechanism resulting in lower σxy/σxx in thinner films, the relation between σxy and σxx was investigated in Fig. 3 (c). In the thicker films, the transverse conductivity σxy agreed with the value expected from the intrinsic mechanism resulted from the Berry phase. In comparison, in the thinner films, the transverse conductivity was much smaller than the value predicted by the intrinsic contribution, therefore extrinsic mechanisms such as side-jump model should be considered to explain the results[8,9]. The dependence of both the longitudinal conductivity σxx and the transverse conductivity σxy on the temperature and the film thickness were summarized in Fig. 3 (d) and (e), respectively. In Fig. 3 (d), we can see that σxx increased at low temperature and varied a little for all samples. However, σxy dropped suddenly when the thickness was below 15 nm (Fig. 3 (e)). The characterization results suggest that Co2MnGa is an excellent material to realize room temperature spintronic applications; and the thickness dependence of the Berry phase opens the opportunity to tune the quantum properties by thin film design.
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