Transport Properties of Co2MnSi Heusler Compounds

Abstract

Co-based Heusler compounds are promising candidates for many emerging spintronic applications regarding the potential full spin polarization and ultra-low magnetic damping, together with their high Curie temperature (900 – 1000 K) and tunable electronic properties [1]. In particular, the existence of a minority spin gap and 100% spin polarization has been demonstrated by spin-resolved photoemission spectroscopy (SR-PES) for Co2MnSi thin films, and an ultra-low magnetic damping coefficient of 4.6*10-4 was obtained for this compound, which is the lowest value reported so far for a conductive material [2]. Tunnel magnetoresistance (TMR) ratios as high as 1995 % and 354 % has been reported for epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions at low and room temperature, respectively [3]. Large magnetoresistance ratios over 30%, at room temperature, and over 60 %, at low temperatures, have been also reported for current perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices, with Co2MnSi as ferromagnetic electrode [15]. However, the electronic properties and the half-metallicity seem to be very sensitive to the structure and composition of the films [1], which can negatively impact the physical properties and the performance of these films when implemented into GMR or TMR magnetoresistive devices.In this work, we report on the transport properties of Co2MnSi Heusler compounds fabricated by molecular beam epitaxy, and patterned into Hall bars by standard UV-photolithography. The formation of the chemically-ordered L21 phase was verified by X-ray diffraction, reflection high-energy electron diffraction (RHEED) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (see Figure 1).A considerable increase in the resistivity of the films was observed with the decrease of thickness as a result of the increase in the inelastic electron scattering at the film surface. Fitting the film’s resistivity versus thickness using Fuchs and Sondheimer model allows us to estimate the electron mean free path for Co2MnSi equal to 13 ± 5 nm, a key parameter to consider when employing these alloys for current-in-plane (CIP) magnetoresistive devices. Furthermore, by fitting the resistivity of the films versus temperature, we access to the distance between the Fermi energy and the minority-spin conduction band minimum (kB * Δ), as shown in Figure 2. These values in combination with spin-resolved photoemission spectroscopy (SR-PES) measurements, performed to a 20-nm Co2MnSi film, permit to estimate the spin gap of Co2MnSi between 0.55 - 0.7 eV, in good agreement with ab initio calculations [6]. A negative anisotropy magnetoresistance (AMR) ratio was measured for all the samples, which is a signature of the Half-metallicity of the Co2MnSi films. Moreover, AMR values ranging from -0.15 to - 0.24 were obtained, which are in good agreement with the highest values reported in the literature for Co2MnSi.The carrier’s concentration and carrier’s mobility of the films and their dependency with temperature was determined from Hall effect measurements. Finally, the evolution of films resistivity with thickness and temperature provided the ideal scenario to study the scaling of Hall resistivity with longitudinal resistivity. The linear trend observed indicates that screw scattering is the dominant temperature-dependent scattering mechanism taking place in Co2MnSi.The electrical parameter obtained here were measured on high-quality Co2MnSi films, with ultra-low magnetic damping and 100% spin polarization, which can be very useful in views of the implementation of Co2MnSi films in magnetoresistive devices. Moreover, we demonstrate the feasibility of the electrical transport measurements as a mean to verify the half-metallicity of Heusler compounds, and to estimate their spin gap, as a complement to SR-PES measurements. **