Structural, magnetic and electrical properties of SrRuO3 films and SrRuO3/SrTiO3 superlattices

Growth and characterization of the magnetic topological insulator candidate Mn2Sb2Te5

We report a new member of topological insulator (TI) family i.e. Mn2Sb2Te5, which belongs to MnSb2Te4family and is a sister compound of Mn2Bi2Te5. An antiferromagnetic layer of (MnTe)2has been inserted between quintuple layers of Sb2Te3. The crystal structure and chemical composition of as grown Mn2Sb2Te5crystal is experimentally visualized by single crystal x-ray diffractometer and field emission scanning electron microscopy. The valence states of individual constituents i.e., Mn, Sb and Te are ascertained through x-ray photo electron spectroscopy. Different vibrational modes of Mn2Sb2Te5are elucidated through Raman spectroscopy. Temperature-dependent resistivityρ(T) of Mn2Sb2Te5resulted in metallic behavior of the same with an up-turn at below around 20 K. Further, the magneto-transportρ(T) vsHof the same exhibited negative magneto-resistance (MR) at low temperatures below 20 K and small positive at higher temperatures. The low Temperature -ve MR starts decreasing at higher fields. The magnetic moment as a function of temperature at 100 Oe and 1 kOe showed anti-ferromagnetism (AFM) like down turn cusps at around 20 K and 10 K. The isothermal magnetization showed AFM like loops with some embedded ferromagnetic/paramagnetic (PM) domains at 5 K and purely PM like at 100 K. The studied Mn2Sb2Te5clearly exhibited the characteristics of a magnetic TI.

Analysis of the temperature- and fluence-dependent magnetic stress in laser-excited SrRuO3

We use ultrafast x-ray diffraction to investigate the effect of expansive phononic and contractive magnetic stress driving the picosecond strain response of a metallic perovskite SrRuO3 thin film upon femtosecond laser excitation. We exemplify how the anisotropic bulk equilibrium thermal expansion can be used to predict the response of the thin film to ultrafast deposition of energy. It is key to consider that the laterally homogeneous laser excitation changes the strain response compared to the near-equilibrium thermal expansion because the balanced in-plane stresses suppress the Poisson stress on the picosecond timescale. We find a very large negative Grüneisen constant describing the large contractive stress imposed by a small amount of energy in the spin system. The temperature and fluence dependence of the strain response for a double-pulse excitation scheme demonstrates the saturation of the magnetic stress in the high-fluence regime.

Phase Instability amid Dimensional Crossover in Artificial Oxide Crystal

Artificial crystals synthesized by atomic-scale epitaxy provide the ability to control the dimensions of the quantum phases and associated phase transitions via precise thickness modulation. In particular, the reduction in dimensionality via quantized control of atomic layers is a powerful approach to revealing hidden electronic and magnetic phases. Here, we demonstrate a dimensionality-controlled and induced metal-insulator transition (MIT) in atomically designed superlattices by synthesizing a genuine two-dimensional (2D) SrRuO_{3} crystal with highly suppressed charge transfer. The tendency to ferromagnetically align the spins in an SrRuO_{3} layer diminishes in 2D as the interlayer exchange interaction vanishes, accompanying the 2D localization of electrons. Furthermore, electronic and magnetic instabilities in the two SrRuO_{3} unit cell layers induce a thermally driven MIT along with a metamagnetic transition.

Magnon profile on SrRuO3 films studied by inelastic neutron scattering

In this study, the inelastic neutron scattering probe of SIKA in ANSTO is employed to investigate the magnon dispersion curve in ferromagnetic SrRuO3 single crystal epitaxial films and to better understand the underlying mechanisms. This report presents the successful measurement of a magnon peak from the SrRuO3 films which contained an amount of material of only 0.9 mg. We reveal one significant magnon dispersion curve along [002] following the quadraticE ∝ Q 2 ) relation, which shows a magnon gap of 0.32 meV. We have discussed several possible mechanisms, such as the higher symmetry structure and the impurity levels, which may contribute to this smaller gap.

Interface-induced magnetic polar metal phase in complex oxides

Polar metals are commonly defined as metals with polar structural distortions. Strict symmetry restrictions make them an extremely rare breed as the structural constraints favor insulating over metallic phase. Moreover, no polar metals are known to be magnetic. Here we report on the realization of a magnetic polar metal phase in a BaTiO₃/SrRuO₃/BaTiO₃ heterostructure. Electron microscopy reveals polar lattice distortions in three-unit-cells thick SrRuO₃ between BaTiO₃ layers. Electrical transport and magnetization measurements reveal that this heterostructure possesses a metallic phase with high conductivity and ferromagnetic ordering with high saturation moment. The high conductivity in the SrRuO₃ layer can be attributed to the effect of electrostatic carrier accumulation induced by the BaTiO₃ layers. Density-functional-theory calculations provide insights into the origin of the observed properties of the thin SrRuO₃ film. The present results pave a way to design materials with desired functionalities at oxide interfaces.

Polar metals—metals with polar structural distortions—are known not to be magnetic. Here, the authors demonstrate a magnetic polar metal phase in a BaTiO₃/SrRuO₃/BaTiO₃ heterostructure displaying high conductivity and ferromagnetic ordering with high saturation moment.

Nanoscale heat transport through the hetero-interface of SrRuO3 thin films

A SrRuO₃ thin film has been widely used as a metal electrode in electronic devices based on transition metal oxides, and hence it is important to understand its thermal transport properties to minimize a thermal degradation problem during the device operation. Using the time-domain thermoreflectance measurement technique, we investigate the cross-plane thermal conductivity of the SrRuO₃ thin films with a thickness variation from 1 μm to 8 nm. We find that the thermal conductivity is reduced from about 6 W m^-1 K^-1 for the 1 μm thick film to about 1.2 W m^-1 K^-1 for the 8 nm thick film, and attribute this behavior to the boundary scattering of thermal carriers which originally have the mean free path of about 20 nm in a bulk state. Also, we observe a clear dip behavior of the thermal conductivity in the intermediate thickness around 30 nm which suggests an existence of a strong scattering source other than the film boundary. We explain this result by considering an additional interfacial scattering at the tetragonal-orthorhombic phase boundary which is formed during the strain relaxation with an increase of the film thickness.

Theoretical investigation of Hall sign change in SrRuO3

The appearance of Hall sign change in perovskite SrRuO₃ thin films at Curie temperature was confirmed from our fabricated samples and the result was simulated by our proposed theoretical model. In particular, our simulation results are consistent with experimental results mainly due to the introduction of an impurity band in a two-band model. We found the other important factors in our theory responsible for observed consistency Hall measurements are the itinerant carrier density and its intrinsic carrier type. Eventually the theory possibly interprets the mechanism of Hall sign change.

Magnetic Properties of Epitaxially Grown SrRuO3 Nanodots

We present the fabrication and exploration of arrays of nanodots of SrRuO₃ with dot sizes between 500 and 15 nm. Down to the smallest dot size explored, the samples were found to be magnetic with a maximum Curie temperature T_C achieved by dots of 30 nm diameter. This peak in T_C is associated with a dot-size-induced relief of the epitaxial strain, as evidenced by scanning transmission electron microscopy.

Spin-Glass-Like Behavior and Topological Hall Effect in SrRuO3/SrIrO3 Superlattices for Oxide Spintronics Applications

The heterostructure interface provides a powerful platform for exploring rich emergent phenomena, such as interfacial superconductivity and nontrivial topological surface states. Here, SrRuO₃/SrIrO₃ superlattices were epitaxially synthesized. The magnetic and magneto-transport properties of these superlattices were characterized. A broad cusp-type splitting in the zero-field-cooling/field-cooling temperature-dependent magnetization and magnetization relaxation, which follows the modified stretched function model, accompanied by double hysteresis magnetization loops were demonstrated. These physical effects were modulated by the SrIrO₃ layer thickness, which confirms the coexistence of interfacial spin glass and ferromagnetic ordering in the superlattices. In addition, the topological Hall effect was observed at low temperatures, and it is weakened with the increase of the SrIrO₃ layer thickness. These results suggest that a noncoplanar spin texture is generated at the SrRuO₃/SrIrO₃ interfaces because of the interfacial Dzyaloshinskii-Moriya interaction. This work demonstrates that SrIrO₃ can effectively induce interfacial Dzyaloshinskii-Moriya interactions in superlattices, which would serve as a mechanism to develop spintronic devices with perovskite oxides.

Interface control of electronic transport across the magnetic phase transition in SrRuO3/SrTiO3 heterointerface

The emerging material class of complex-oxides, where manipulation of physical properties lead to new functionalities at their heterointerfaces, is expected to open new frontiers in Spintronics. For example, SrRuO3 is a promising material where external stimuli like strain, temperature and structural distortions control the stability of electronic and magnetic states, across its magnetic phase transition, useful for Spintronics. Despite this, not much has been studied to understand such correlations in SrRuO3. Here we explore the influence of electron-lattice correlation to electron-transport, at interfaces between SrRuO3 and Nb:SrTiO3 across its ferromagnetic transition, using a nanoscale transport probe and first-principles calculations. We find that the geometrical reconstructions at the interface and hence modifications in electronic structures dominate the transmission across its ferromagnetic transition, eventually flipping the charge-transport length-scale in SrRuO3. This approach can be easily extended to other devices where competing ground states can lead to different functional properties across their heterointerfaces.