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Register a new userHelical magnetic structure and the anomalous and topological Hall effects in epitaxial B20 Fe$_{1-y}$Co$_y$Ge films
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Added by
Prof. Christopher Marrows
Publication date
2018
fields
Physics
and research's language is
English
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Epitaxial films of the B20-structure alloy Fe$_{1-y}$Co$_y$Ge were grown by molecular beam epitaxy on Si (111) substrates. The magnetization varied smoothly from the bulk-like values of one Bohr magneton per Fe atom for FeGe to zero for non-magnetic CoGe. The chiral lattice structure leads to a Dzyaloshinskii-Moriya interaction (DMI), and the films helical magnetic ground state was confirmed using polarized neutron reflectometry measurements. The pitch of the spin helix, measured by this method, varies with Co content $y$ and diverges at $y sim 0.45$. This indicates a zero-crossing of the DMI, which we reproduced in calculations using first principle methods. We also measured the longitudinal and Hall resistivity of our films as a function of magnetic field, temperature, and Co content $y$. The Hall resistivity is expected to contain contributions from the ordinary, anomalous, and topological Hall effects. Both the anomalous and topological Hall resistivities show peaks around $y sim 0.5$. Our first principles calculations show a peak in the topological Hall constant at this value of $y$, related to the strong spin-polarisation predicted for intermediate values of $y$. Half-metallicity is predicted for $y = 0.6$, consistent with the experimentally observed linear magnetoresistance at this composition. Whilst it is possible to reconcile theory with experiment for the various Hall effects for FeGe, the large topological Hall resistivities for $y sim 0.5$ are much larger then expected when the very small emergent fields associated with the divergence in the DMI are taken into account.
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Synthesis of crystallographically well-defined thin films of topological materials is important for unraveling their mesoscale quantum properties and for device applications. Mn$_3$Ge, an antiferromagnetic Weyl semimetal with a chiral magnetic structure on a Kagome lattice, is expected to have enhanced Berry curvature around Weyl nodes near the Fermi energy, leading to large anomalous Hall / Nernst effects and a large spin-Hall effect. Using magnetron sputtering, we have grown epitaxial thin films of hexagonal D0$_{19}$ Mn$_3$Ge that are flat and continuous. Large anomalous Nernst and inverse spin-Hall effects are observed in thermoelectric and spin-pumping devices. The anomalous Nernst signal in our Mn$_3$Ge films is estimated to be 0.1 $mu$V / K, and is comparable to that in ferromagnetic Fe, despite Mn$_3$Ge having a weak magnetization of ~3.5 m$mu_B$ at room temperature. The spin mixing conductance is 90.5 nm$^{-2}$ at the Py / Mn$_3$Ge interface, and the spin-Hall angle in Mn$_3$Ge is estimated to be about 8 times of that in Pt.
The purpose of this study was to investigate the magnetotransport properties of the Ge(0.743)Pb(0.183)Mn(0.074)Te mixed crystal. The results of magnetization measurements indicated that the compound is a spin-glass-like diluted magnetic semiconductor with critical temperature TSG = 97.5 K. Nanoclusters in the sample are observed. Both, matrix and clusters are magnetically active. Resistivity as a function of temperature has a minimum at 30 K. Below the minimum a variable-range hopping is observed, while above the minimum a metallic-like behavior occurs. The crystal has high hole concentration, p = 6.6E20 cm-3, temperature-independent. Magnetoresistance amplitude changes from -0.78 to 1.18% with increase of temperature. In the magnetotransport measurements we observed the anomalous Hall effect (AHE) with hysteresis loops. Calculated AHE coefficient, RS = 2.0E6 m3/C, is temperature independent. The analysis indicates the extrinsic skew scattering mechanism to be the main physical mechanism responsible for AHE in Ge(0.743)Pb(0.183)Mn(0.074)Te alloy.
The close-packed AB$_2$ structures called Laves phases constitute the largest group of intermetallic compounds. In this paper we computationally investigated the pseudo-binary Laves phase system Y$_{1-x}$Gd$_x$(Fe$_{1-y}$Co$_y$)$_2$ spanning between the YFe$_2$, YCo$_2$, GdFe$_2$, and GdCo$_2$ vertices. While the vast majority of the Y$_{1-x}$Gd$_x$(Fe$_{1-y}$Co$_y$)$_2$ phase diagram is the ferrimagnetic phase, YCo$_2$ along with a narrow range of concentrations around it is the paramagnetic phase. We presented results obtained by Monte Carlo simulations of the Heisenberg model with parameters derived from first-principles calculations. For calculations, we used the Uppsala atomistic spin dynamics (UppASD) code together with the spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) code. From first principles we calculated the magnetic moments and exchange integrals for the considered pseudo-binary system, together with spin-polarized densities of states for boundary compositions. Furthermore, we showed how the compensation point with the effective zero total moment depends on the concentration in the considered ferrimagnetic phases. However, the main result of our study was the determination of the Curie temperature dependence for the system Y$_{1-x}$Gd$_x$(Fe$_{1-y}$Co$_y$)$_2$. Except for the paramagnetic region around YCo$_2$, the predicted temperatures were in good qualitative and quantitative agreement with experimental results, which confirmed the ability of the method to predict magnetic transition temperatures for systems containing up to three different magnetic elements (Fe, Co, and Gd) simultaneously. For the Y(Fe$_{1-y}$Co$_y$)$_2$ and Gd(Fe$_{1-y}$Co$_y$)$_2$ systems our calculations matched the experimentally-confirmed Slater-Pauling-like behavior of T$_C$ dependence on the Co concentration.
The combination of ferromagnetism and semiconducting behavior offers an avenue for realizing novel spintronics and spin-enhanced thermoelectrics. Here we demonstrate the synthesis of doped and nanocomposite half Heusler Fe$_{1+x}$VSb films by molecular beam epitaxy. For dilute excess Fe ($x < 0.1$), we observe a decrease in the Hall electron concentration and no secondary phases in X-ray diffraction, consistent with Fe doping into FeVSb. Magnetotransport measurements suggest weak ferromagnetism that onsets at a temperature of $T_{c} approx$ 5K. For higher Fe content ($x > 0.1$), ferromagnetic Fe nanostructures precipitate from the semiconducting FeVSb matrix. The Fe/FeVSb interfaces are epitaxial, as observed by transmission electron microscopy and X-ray diffraction. Magnetotransport measurements suggest proximity-induced magnetism in the FeVSb, from the Fe/FeVSb interfaces, at an onset temperature of $T_{c} approx$ 20K.
Noncollinear antiferromagnets with a D0$_{19}$ (space group = 194, P6$_{3}$/mmc) hexagonal structure have garnered much attention for their potential applications in topological spintronics. Here, we report the deposition of continuous epitaxial thin films of such a material, Mn$_{3}$Sn, and characterize their crystal structure using a combination of x-ray diffraction and transmission electron microscopy. Growth of Mn$_{3}$Sn films with both (0001) c-axis orientation and (40$bar{4}$3) texture is achieved. In the latter case, the thin films exhibit a small uncompensated Mn moment in the basal plane, quantified via magnetometry and x-ray magnetic circular dichroism experiments. This cannot account for the large anomalous Hall effect simultaneously observed in these films, even at room temperature, with magnitude $sigma_{mathrm{xy}}$ ($mu_{0}H$ = 0 T) = 21 $mathrm{Omega}^{-1}mathrm{cm}^{-1}$ and coercive field $mu_{0}H_{mathrm{C}}$ = 1.3 T. We attribute the origin of this anomalous Hall effect to momentum-space Berry curvature arising from the symmetry-breaking inverse triangular spin structure of Mn$_{3}$Sn. Upon cooling through the transition to a glassy ferromagnetic state at around 50 K, a peak in the Hall resistivity close to the coercive field indicates the onset of a topological Hall effect contribution, due to the emergence of a scalar spin chirality generating a real-space Berry phase. We demonstrate that the polarity of this topological Hall effect, and hence the chiral-nature of the noncoplanar magnetic structure driving it, can be controlled using different field cooling conditions.
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