Tuning Quantum Properties in a Novel Magnetic Material by Thin Film Engineering

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 σ_xx, which is the conductivity of the material along the direction of the electric field; and the transverse conductivity σ_xy, which is the conductivity along the perpendicular direction of the electric field. The ratio between σ_xy and σ_xx, defined as the anomalous Hall angle, measures the strength of the intrinsic deflection by the Berry phase: The higher the σ_xy/σ_xx 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^[3] and hybrid complementary metal-oxide-semiconductor (CMOS)/anomalous Hall effect devices^[4].

Recently the ferromagnetic material Co₃Sn₂S₂ has been experimentally proven to be a magnetic Weyl semimetal[5]. 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 (σ_xy/σ_xx =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^[8].

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^[8].

The magneto transport measurement on Co₂MnGa thin films revealed that when the thickness of the Co₂MnGa layer was 50 nm, the anomalous Hall angle σ_xy/σ_xx 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 σ_xy/σ_xx 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 σ_xy/σ_xx in thinner films, the relation between σ_xy and σ_xx was investigated in Fig. 3 (c). In the thicker films, the transverse conductivity σ_xy 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 σ_xx and the transverse conductivity σ_xy on the temperature and the film thickness were summarized in Fig. 3 (d) and (e), respectively. In Fig. 3 (d), we can see that σ_xx increased at low temperature and varied a little for all samples. However, σ_xy 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.

References:

[1] K. Manna, Y. Sun, L. Muechler, J. Kübler and C. Felser, "Heusler, Weyl and Berry", Nat. Rev. Mater. 3, 244 (2018)

[2] J. Hu, S. Y. Xu, N. Ni and Z. Mao, "Transport of Topological Semimetals", Annu. Rev. Mater. Res. 49, 207 (2019)

[3] E. V. Vidal, G. Stryganyuk, H. Schneider, C. Felser and G. Jakob, "Exploring Co₂MnAl Heusler compound for anomalous Hall effect sensors", Appl. Phys. Lett. 99, 132509 (2011)

[4] J. Moritz, B. Rodmacq, S. Auffret and B. Dieny, "Extraordinary Hall effect in thin magnetic films and its potential for sensors, memories and magnetic logic applications", J. Phys. D Appl. Phys. 41, 135001 (2008)

[5] E. Liu, Y. Sun, N. Kumar, L. Muechler, A. Sun L. Jiao, S. Y. Yang, D. Liu, A. Liang, Q. Xu, J. Kroder, V. Süß, H. Borrmann, C. Shekhar, Z. Wang, C. Xi, W. Wang, W. Schnelle, S. Wirth, Y. Chen, S. T. B. Goennenwein and C. Felser, "Giant anomalous Hall effect in a ferromagnetic kagome-lattice semimetal", Nat. Phys. 14, 1125 (2018)

[6] I. Belopolski, K. Manna, D. S. Sanchez, G. Chang, B. Ernst, J. Yin, S. S. Zhang, T. Cochran, N. Shumiya, H. Zheng, B. Singh, G. Bian, D. Multer, M. Litskevich, X. Zhou, S. M. Huang, B. Wang, T. R. Chang, S. Y. Xu, A. Bansil, C. Felser, H. Lin and M. Z. Hasan, "Discovery of topological Weyl fermion lines and drumhead surface states in a room temperature magnet", Science 365, 1278 (2019)

[7] A. Sakai, Y. P. Mizuta, A. A. Nugroho, R. Sihombing, T. Koretsune, M. T. Suzuki, N. Takemori, R. Ishii, D. N. Hamane, R. Arita, P. Goswami and S. Nakatsuji, "Giant anomalous Nernst effect and quantum critical scaling in a ferromagnetic semimetal", Nat. Phys. 14, 1119 (2018)

[8] Y. Zhang, Y. Yin, G. Dubuis, T. Butler, N. V. Medhekar and S. Granville, "Berry curvature origin of the thickness-dependent anomalous Hall effect in a ferromagnetic Weyl semimetal", npj Quantum Mater. 6, 17 (2021)

[9] S. Onoda, N. Sugimoto and N. Nagaosa, "Intrinsic Versus Extrinsic Anomalous Hall Effect in Ferromagnets", Phys. Rev. Lett. 97, 126602 (2006)