Skyrmionics in correlated oxides

While chiral magnets, metal-based magnetic multilayers, or Heusler compounds have been considered as the material workhorses in the field of skyrmionics, oxides are now emerging as promising alternatives, as they host special correlations between the spin–orbital–charge–lattice degrees of freedom and/or coupled ferroic order parameters. These interactions open new possibilities for practically exploiting skyrmionics. In this article, we review the recent advances in the observation and control of topological spin textures in various oxide systems. We start with the discovery of skyrmions and related quasiparticles in bulk and heterostructure ferromagnetic oxides. Next, we emphasize the shortcomings of implementing ferromagnetic textures, which have led to the recent explorations of ferrimagnetic and antiferromagnetic oxide counterparts, with higher Curie temperatures, stray-field immunity, low Gilbert damping, ultrafast magnetic dynamics, and/or absence of skyrmion deflection. Then, we highlight the development of novel pathways to control the stability, motion, and detection of topological textures using electric fields and currents. Finally, we present the outstanding challenges that need to be overcome to achieve all-electrical, nonvolatile, low-power oxide skyrmionic devices. Graphical abstract

Magnetic and transport properties of electronically spin polarised double perovskites and Heusler intermetallics

<p>Half-metallic ferromagnets with 100 % electronic spin polarisation are an interesting class of materials for new spin transport electronics applications. Some of the double perovskites and Heusler alloys are predicted to be half-metallic with Curie temperatures above room temperature. This thesis presents the results from an experimental study of polycrystalline double perovskites Sr₂₋ₓLaₓFeMoO₆ and Ba₂₋ₓLaₓFeMoO₆, and ordered and disordered epitaxial thin films of Co₂MnSi Heusler alloys. A magnetothermopower was observed in Sr₂₋ₓLaₓFeMoO₆ and Ba₂₋ₓLaₓFeMoO₆. This magnetothermopower can be explained in terms of a spin-tunnelling contribution to the thermopower between grains that changes in an applied magnetic field. The results from the high temperature (above 400 K) magnetisation studies on Ba₂₋ₓLaₓFeMoO₆ in the paramagnetic region reveal that a localised electron model with antiferromagnetic coupling to itinerant electrons can account for the carrier concentration dependent effective moments. The correlation between the bare itinerant electron susceptibility and the Curie-Weiss temperature supports the kinetic energy driven model that has been used to account for the electronic spin polarisation and high Curie temperatures. Antisite disorder is evident in the Co₂MnSi thin films that leads to a reduction in the saturation magnetisation. The resistivity of the ordered Co₂MnSi thin film is linear in temperature whereas the resistivity of the disordered film increases at low temperature due to weak localisation. A magnetoresistance is observed in ordered and disordered films. At low fields (below 0.1 T) the magnetoresistance is likely to be due to domain wall scattering. For magnetic fields greater than 0.1 T there is likely to be a contribution from a magnetic-field-induced suppression of the weak localisation resistivity. Similar magnetoresistance behaviour was observed for ordered and disordered films. There is a large anomalous Hall resistivity observed in the ordered and disordered Co₂MnSi thin films. In the case of the ordered film it is found that the anomalous Hall effect is dominated by skew scattering.</p>

Magnetic and transport properties of electronically spin polarised double perovskites and Heusler intermetallics

<p>Half-metallic ferromagnets with 100 % electronic spin polarisation are an interesting class of materials for new spin transport electronics applications. Some of the double perovskites and Heusler alloys are predicted to be half-metallic with Curie temperatures above room temperature. This thesis presents the results from an experimental study of polycrystalline double perovskites Sr₂₋ₓLaₓFeMoO₆ and Ba₂₋ₓLaₓFeMoO₆, and ordered and disordered epitaxial thin films of Co₂MnSi Heusler alloys. A magnetothermopower was observed in Sr₂₋ₓLaₓFeMoO₆ and Ba₂₋ₓLaₓFeMoO₆. This magnetothermopower can be explained in terms of a spin-tunnelling contribution to the thermopower between grains that changes in an applied magnetic field. The results from the high temperature (above 400 K) magnetisation studies on Ba₂₋ₓLaₓFeMoO₆ in the paramagnetic region reveal that a localised electron model with antiferromagnetic coupling to itinerant electrons can account for the carrier concentration dependent effective moments. The correlation between the bare itinerant electron susceptibility and the Curie-Weiss temperature supports the kinetic energy driven model that has been used to account for the electronic spin polarisation and high Curie temperatures. Antisite disorder is evident in the Co₂MnSi thin films that leads to a reduction in the saturation magnetisation. The resistivity of the ordered Co₂MnSi thin film is linear in temperature whereas the resistivity of the disordered film increases at low temperature due to weak localisation. A magnetoresistance is observed in ordered and disordered films. At low fields (below 0.1 T) the magnetoresistance is likely to be due to domain wall scattering. For magnetic fields greater than 0.1 T there is likely to be a contribution from a magnetic-field-induced suppression of the weak localisation resistivity. Similar magnetoresistance behaviour was observed for ordered and disordered films. There is a large anomalous Hall resistivity observed in the ordered and disordered Co₂MnSi thin films. In the case of the ordered film it is found that the anomalous Hall effect is dominated by skew scattering.</p>

The electronic, half-metallic, elastic, and magnetic properties of new PtWZ (Z= In, Tl, Sn, and Pb) half-Heusler alloys via GGA and GGA+mBJ methods

The first-principle calculations of PtWZ (Z= In, Tl, Sn, and Pb) half-Heusler alloys were calculated by WIEN2k for GGA and GGA+mBJ methods. First, the ferromagnetic (FM) phases were obtained more energetically stable than non-magnetic (NM) and antiferromagnetic (AFM) phases in each alloy. The Curie temperatures of PtWIn, PtWTl, PtWSn, and PtWPb alloys were obtained as 286.98 K, 467.14 K, 721.98 K, and 1114.31 K, respectively, by utilizing the energy differences of the AFM and FM phases. In each method and alloy used, spin-up electrons showed metallic character. In the GGA method, PtW(In, Tl) alloys have direct band gaps of 0.72044 eV and 0.91488 eV in spin-down electrons, while PtW(Sn, Pb) alloys have indirect band gaps of 1.2558 eV and 1.11892 eV, respectively. In the GGA+mBJ method, the bandgap directions in all compounds remained the same. Here, band gaps in PtW(In, Tl, Sn, and Pb) alloys were obtained as 0.99918 eV, 1.15385 eV, 1.42676 eV, and 1.17497 eV, respectively. While the total magnetic moment values of PtW(In, Tl) half-Heusler alloys were obtained as 1.00 μB/f.u., the total magnetic moments of PtW(Sn, Pb) alloys were obtained as 2.00 μB/f.u. These results are in full agreement with the Slater-Pauling rule. According to elastic calculations, PtWIn, PtWTl, PtWSn, and PtWPb half-Heusler alloys are elastically stable and ductile.

IrCrMnZ (Z=Al, Ga, Si, Ge) Heusler alloys as electrode materials for MgO-based magnetic tunneling junctions:A first-principles study

We study IrCrMnZ (Z=Al, Ga, Si, Ge) systems using first-principles calculations from the perspective of their application as the electrode materials of MgO-based MTJs. These materials have highly spin-polarized conduction electrons with partially occupied Δ1 band, which is important for coherent tunneling in parallel magnetization configuration. The Curie temperatures of IrCrMnAl and IrCrMnGa are very high (above 1300 K) as predicted from mean-field-approximation. The stability of ordered phase against various antisite disorders has been investigated. We discuss here the effect of ``spin-orbit-coupling'' on the electronic structure around Fermi level. Further, we investigate the electronic structure of IrCrMnZ/MgO heterojunction along (001) direction. IrCrMnAl/MgO and IrCrMnGa/MgO maintain half-metallicity even at the MgO interface, with no interfacial states at/around Fermi level in the minority-spin channel. Large majority-spin conductance of IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa is reported from the calculation of ballistic spin-transport property for parallel magnetization configuration. We propose IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa as promising MTJs with a weaker temperature dependence of tunneling magnetoresistance ratio, owing to their very high Curie temperatures.

Increased room temperature ferromagnetism in Co-doped tetrahedral perovskite niobates

Dilute magnetic semiconductors (DMSs), such as (In, Mn)As and (Ga, Mn)As prototypes, are limited to III–V semiconductors with Curie temperatures ( T c ) far from room temperature, thereby hindering their wide application. Here, one kind of DMS based on perovskite niobates is reported. BaM x Nb (1− x ) O 3− δ ( M = Fe, Co) powders are prepared by the composite-hydroxide-mediated method. The addition of M elements endows BaM x Nb (1− x ) O 3− δ with local ferromagnetism. The tetragonal BaCo x Nb (1− x ) O 3− δ nanocrystals can be obtained by Co doping, which shows strong saturation magnetization ( M sat ) of 2.22 emu g −1 , a remnant magnetization ( M r ) of 0.084 emu g −1 and a small coercive field ( H c ) of 167.02 Oe at room temperature. The ab initio calculations indicate that Co doping could lead to a 64% local spin polarization at the Fermi level ( E F ) with net spin DOS of 0.89 electrons eV −1 , this result shows the possibility of maintaining strong ferromagnetism at room temperature. In addition, the trade-off effect between the defect band absorption and ferromagnetic properties of BaM x Nb (1− x ) O 3− δ is verified experimentally and theoretically.

Magnetic and Magnetostrictive Properties of Ni50Mn20Ga27Cu3 Rapidly Quenched Ribbons

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K—which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples.