Two-dimensional electron-gas-like charge transport at the interface between a magnetic Heusler alloy and SrTiO3

Abstract

We report remarkably low residual resistivity, giant residual resistivity ratio, free-electron-like Hall resistivity, and high mobility (≈10 4 cm 2 V −1 s −1 ) charge transport in epitaxial films of Co 2 MnSi and Co 2 FeSi grown on (001) SrTiO 3 . This unusual behavior is not observed in films deposited on other cubic oxide substrates of comparable lattice parameters. The scaling of the resistivity with thickness of the films allows the extraction of interface conductance, which can be attributed to a layer of oxygen vacancies confined within 1.9 nm of the interface as revealed by atomically resolved electron microscopy and spectroscopy. The high mobility transport observed here at the interface of a fully spin polarized metal is potentially important for spintronics applications. The seemingly extraordinary electronic transport observed in epitaxial films of LaAlO 3 (LAO), LaTiO 3 (LTO), and related perovskites grown on TiO 2 terminated (001) SrTiO 3 (STO) has taken central stage in condensed matter physics research in recent years.1-4 The origin of two-dimensional electron gas (2DEG), whose mobility and carrier density depend strongly on growth temperature and oxygen partial pressure, 1-3 and which can be modified further by ultraviolet light 2 and electric field, 5 has been attributed to interfacial factors such as atomic relaxation, electronic reconstruction, cation intermixing, and/or creation of oxygen vacancies. 6 The electronic properties of such interfaces have been studied extensively owing to unusual charge transport, 1,7 magnetism, [8][9][10] two-dimensional superconductivity, 11-13 and quantum oscillation in the conductivity.14,15 While several types of oxide overlayers show unusually large interfacial conductivity, 16,17 the common denominator in all these cases is STO, which even without any overlayer, but subjected to subtle surface treatments, can show fascinating 2D electronic behavior. 18,19 Departing from the commonly used approach of growing oxide overlayers, here we show a similar electronic transport realized at the interface of a half-metallic Heusler alloy and STO. The Heusler compounds have generated considerable interest in recent years due to a myriad of properties encompassing half-metallicity, shape memory effect, thermoelectricity, superconductivity, and topologically inhibited conducting states. 20 While our discovery of a highly conducting interface between Heusler alloys and STO can have potential technological applications, the fundamental mechanism for the origin of such a state brings into question the several interpretations given for 2DEG previously.The thin films of Co-based full-Heusler alloys such as Co 2 FeSi (CFS) and Co 2 MnSi (CMS) have been grown on a variety of semiconductors and oxide dielectrics. [21][22][23][24][25] The substrates used in present study were (001) LAO, MgO, NdGaO 3 (NGO), and STO, whose face diagonal matches quite well with the lattice parameter (≈0.565 nm) of CM(F)S. We have deposited a large number (over 70) of highly ordered single phase thin ...

Magnetoelastic coupling induced magnetic anisotropy in Co2(Fe/Mn)Si thin films

“…The full width at half maximum of these films are less than 1.9 , which corresponds to a crystallite size of 5 nm. 15 Moreover, the u scans confirm the epitaxial growth of the films with the relation [100] CF(M)S k [110] STO or MgO. The presence of (111) superlattice, which governs the ordering of the Fe (or Mn) and Si sublattices, and (022) fundamental diffraction line, which confirms the presence of L2 1 ordering in the films, are two important indicators of the structural ordering in the films.…”

“…13) as well as on SrTiO 3 (STO). [14][15][16] While the substrate lattice parameter, growth, thermal annealing condition, and film thickness in these cases vary significantly, the effect of such condition on magnetic anisotropy of the films is seldom addressed. In Heusler alloys films, one expects a four-fold anisotropy due to the cubic symmetry of the unit cell, while in-plane uniaxial anisotropy has also been observed for the case of Co 2 FeSi grown on GaAs.…”

“…However, for our thinnest film on STO, we observe a significantly higher 26 We have used the experimental values of M S , which are in reasonable agreement with the theory. 15 The lower panel shows the H C as a function of t along with the fits (solid lines) according to the relation: H C / t Àn . out-of-plane magnetization, which suggests the possibility of tiltedM with respect to the film plane.…”

“…The details of thin film preparation are described in our earlier reports. 6,7,14,15 The structural characterization of the films has been performed by X-ray diffraction (PANalytical X'Pert PRO X-ray diffractometer) in h-2h, x, u, and grazing incidence X-ray diffraction modes. The magnetic measurements were performed in a vibrating sample magnetometer (EV7 VSM) at room temperature.…”

Resistance minimum and electrical conduction mechanism in polycrystalline CoFeB thin films

“…The existence of such dead layers has been reported in thin films of manganite [30] and Heusler alloy [31]. This dead layer can form on the surface and at film-substrate interface due to surface oxidation, roughness effect, and boron segregation at the interface [9,12].…”