Ferromagnetic SrRuO3 Research Articles

Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular magnetic anisotropy.

Non-coplanar spin textures with scalar spin chirality can generate an effective magnetic field that deflects the motion of charge carriers, resulting in a topological Hall effect (THE)1-3. However, spin chirality fluctuations in two-dimensional ferromagnets with perpendicular magnetic anisotropy have not been considered so far. Here, we report evidence of spin chirality fluctuations by probing the THE above the Curie temperature in two different ferromagnetic ultra-thin films, SrRuO3 and V-doped Sb2Te3. The temperature, magnetic field, thickness and carrier-type dependence of the THE signal, along with Monte Carlo simulations, suggest that spin chirality fluctuations are a common phenomenon in two-dimensional ferromagnets with perpendicular magnetic anisotropy. Our results open a path for exploring spin chirality with topological Hall transport in two-dimensional magnets and beyond4-7.

Magnetization density distribution in the metallic ferromagnet SrRuO3 determined by polarized neutron diffraction

The magnetization-density distribution in the metallic ferromagnet SrRuO$_3$ was studied by means of polarized neutron diffraction. The analyzes by multipole refinements and by the maximum entropy method consistently reveal a strong polarization of all oxygen sites carrying 30\% of the total magnetization. The spin-density distribution on the Ru site exhibits a nearly cubic shape in agreement with an almost equal occupation of $t_{2g}$ orbitals and $pd$ hybridization. The experimental analysis is well reproduced by density functional calculations. There is no qualitative change in the magnetization distribution between 2 and 200 K.

Interfacial oxygen-octahedral-tilting-driven electrically tunable topological Hall effect in ultrathin SrRuO3 films

Topological spin textures as an emerging class of topological matter offer a medium for information storage and processing. The recently discovered topological Hall effect (THE) is considered as a fingerprint for electrically probing the Dzyaloshinskii–Moriya (DM) interaction and corresponding non-trivial spin-textures. In this paper, the THE and its electrical control are observed in ultrathin (⩽8 unit cells. u.c.) 4D ferromagnetic SrRuO3 films grown on SrTiO3(0 0 1) substrates, indicating the existence of gate-bias-tunable DM interaction in the single SrRuO3 layer without contacting 5D oxide SrIrO3 layer. High-resolution lattice structure analysis revealed that the interfacial RuO6 octahedral tilting induced by local orthorhombic-to-tetragonal structural phase transition exists across the SrRuO3/SrTiO3 interface, which naturally breaks the inversion symmetry. Our theoretical calculations demonstrate that the DM interaction arises owing to the broken inversion symmetry and strong spin–orbit interaction of 4D SrRuO3. This interfacial RuO6 octahedral tilting-induced DM interaction can stabilize the Néel-type magnetic skyrmions, which in turn accounts for the observed THE in transport. Besides the fundamental significance, the understanding of THE in oxides and its electrical manipulation presented in this work could advance the low power cost topological electronic and spintronic applications.

Observation of superconducting gap spectra of long-range proximity effect in Au/SrTiO3/SrRuO3/Sr2RuO4 tunnel junctions

We observe an unconventional superconducting minigap induced into a ferromagnet SrRuO$_3$ from a spin-triplet superconductor Sr$_2$RuO$_4$ using a Au/SrTiO$_3$/SrRuO$_3$/Sr$_2$RuO$_4$ tunnel junction. Voltage bias differential conductance of the tunnel junctions exhibits V-shaped gap features around zero bias, corresponding to a decrease in the density-of-states with an opening of a superconducting minigap in SrRuO$_3$. Observation of a minigap at a surface of a 15~nm thick SrRuO$_3$ layers confirms the spin-triplet nature of induced superconductivity. The shape and temperature dependence of the gap features in the differential conductance indicate that the even-frequency $p$-wave correlations dominate, over odd-frequency $s$-wave correlations. Theoretical calculations support this $p$-wave scenario. Our work provides the density-of-states proof for $p$-wave Cooper pair penetration in a ferromagnet and significantly put forward our understanding of the $p$-wave spin-triplet proximity effect between spin-triplet superconductors and ferromagnets.

Large vertical hysteretic shift and signature of exchange bias in BiFeO3/SrRuO3 heterostructure

Abstract We present the results of magnetic study on G-type antiferromagnetic BiFeO 3 /ferromagnetic SrRuO 3 heterostructure grown on (001) SrTiO 3 substrate using pulsed laser deposition technique. We observe a large positive vertical magnetization shift (M Shift ~35%) in the magnetic hysteresis (M − H) loop in the direction of the cooling fields (±30 kOe). The M Shift is essentially due to the presence of the interfacial pinned Ru 4+ moments and the local defects/vacancies present in the SRO layer. The pinned Ru 4+ moments strongly attached to Fe 3+ moments of BiFeO 3 layer through exchange interaction may result in the exchange bias (EB) effect. The positive M Shift and negative EB effect suggest the ferromagnetic double exchange interaction Fe 3+ -O-Ru 4+ , across the interface.

Interplay of Electronic and Spin Degrees in Ferromagnetic SrRuO3 : Anomalous Softening of the Magnon Gap and Stiffness

The magnon dispersion of ferromagnetic SrRuO_{3} was studied by inelastic neutron scattering experiments on single crystals as a function of temperature. Even at low temperature the magnon modes exhibit substantial broadening pointing to strong interaction with charge carriers. We find an anomalous temperature dependence of both the magnon gap and the magnon stiffness, which soften upon cooling in the ferromagnetic phase. Both effects trace the temperature dependence of the anomalous Hall effect and can be attributed to the impact of Weyl points, which results in the same relative renormalization in the spin stiffness and magnon gap.

Current-Induced Modulation of Coercive Field in the Ferromagnetic Oxide SrRuO3

We investigated the coercive field $H_{\text {c}}$ for domain wall (DW) motion as a function of the current ${I}$ in the ferromagnetic oxide SrRuO3, a model system with narrow DWs for fabricating high-density spintronics devices. The DW is moved by ${I}$ in the direction of the current, and $H_{\text {c}}$ is modulated linearly in ${I}$ . This linear relationship is consistent with an effective magnetic field $H_{\text {eff}}$ driving the DW. The direction of DW motion and the magnitude of $H_{\text {eff}}$ are well described by a model based on the field-like torque arising from the spin relaxation of conduction electrons in the DW.

Magnetic coupling of ferromagnetic SrRuO3 epitaxial layers separated by ultrathin non-magnetic SrZrO3/SrIrO3

Ferromagnetic multilayers with asymmetric interfaces and perpendicular magnetic anisotropy can stabilize non-trivial magnetic structures due to interfacial Dzyaloshinskii-Moriya interactions. Magnetic interlayer coupling between ferromagnetic layers separated by non-magnetic insulating spacers is an additional important ingredient for the properties of multilayers. We addressed the magnetic coupling between ferromagnetic SrRuO3 epitaxial layers separated by ultrathin spacers of SrIrO3/SrZrO3, so that inversion symmetry is broken at the top and bottom interfaces. Major and minor magnetization loops allowed us to assess the type and strength of the magnetic coupling. We inferred that the magnetic coupling of SrRuO3 layers through non-magnetic insulating interlayers (≤1.6 nm thick) is weakly ferromagnetic.

Cooper-Pair Spin Current in a Strontium Ruthenate Heterostructure.

It has been recognized that the condensation of spin-triplet Cooper pairs requires not only broken gauge symmetry but also spin ordering as well. One consequence of this is the possibility of a Cooper-pair spin current analogous to the magnon spin current in magnetic insulators, the analogy also extending to the existence of the Gilbert damping of the collective spin-triplet dynamics. The recently fabricated heterostructure of the thin film of the itinerant ferromagnet SrRuO_{3} on bulk Sr_{2}RuO_{4}, the best-known candidate material for a spin-triplet superconductor, offers a promising platform for generating such spin current. We show how such heterostructure allows us to not only realize the long-range spin valve but also electrically drive the collective spin mode of the spin-triplet order parameter. Our proposal represents both a novel experimental realization of superfluid spin transport and a transport signature of the spin-triplet superconductivity therein.

Large modulation of interface magnetization and interface magnetoelectric effect in SrRuO3|KNbO3 oxide heterostructures: Prediction from first-principles study

The interface magnetization and its modulation is explored in oxide heterostructures within the framework of density functional theory. In particular, modulation in interface magnetization is investigated in prototype heterostructure SrRuO3|KNbO3(0 0 1) consisting ferromagnetic SrRuO3 and nonmagnetic KNbO3 as the thickness of KNbO3 is varied. The computed interfacial magnetization is found to alternate between a higher and a lower value with increasing KNbO3 thickness in the heterostructure. The average change in interface magnetization is computed to be ∼−60% as KNbO3 film changes from non-stoichiometric to stoichiometric and ∼+60% as it changes from stoichiometric to non-stoichiometric due to variation in thickness. The magnetic moment per interface cell changes from ∼2.27 μB for non-stoichiometric KNbO3 to ∼1.31 μB for stoichiometric KNbO3 in the heterostructure. The modulation in interface magnetization arises due to spin-dependent screening of effective charge of KNbO3 unit cells by accumulated free carriers at the interface. Large modulation in interface magnetization is also obtained for other heterostructures consisting magnetic substructure and ionized planes at the interface. In addition, the alteration in interface magnetization due to ferroelectric polarization reversal in ferromagnetic|ferroelectric SrRuO3|KNbO3 (0 0 1) superlattice is explored. The resulting interface magnetoelectric effect as well as control of interface magnetization using ionized planes may have interesting implications for technological applications.

Defect‐Induced Anomalous Transverse Resistivity in an Itinerant Ferromagnetic Oxide

Phenomena giving rise to voltages transverse to electric currents, such as anomalous and topological Hall effects, have been at the heart of modern condensed matter physics. Here, we show that epitaxial thin films of the itinerant ferromagnetic oxide SrRuO3 exhibit anomalous transverse resistivity associated with magnetization reversal, which is reminiscent of the emergence of the topological Hall effect. The behavior of the anomalous transverse resistivity of SrRuO3 films is found to depend on Ru vacancies. The anomalous peaking behavior in the transverse resistivity is enhanced when the film is thin enough that Ru vacancies significantly influence the magneto‐transport properties. Furthermore, films with fewer Ru vacancies exhibit no anomaly in transverse resistivity. Our observations imply that the Ru vacancies significantly influence the transverse resistivity in SrRuO3 and that defect‐induced changes in electronic structure are a plausible origin for the emergence of the anomalous peaking behavior in the transverse resistivity.

Atomic Level Engineering of Structural and Functional Properties of Transition Metal Oxides

Metal-oxygen bonds are responsible for a broad spectrum of functional properties of transition-metal oxides, and engineering the bonds is a key for exploring their properties. Artificial oxide heterostructures with chemically abrupt interfaces have provided a platform for engineering bonding geometries that could lead to emergent phenomena not seen in bulk oxides. In this paper, we demonstrate that interfacially engineering oxygen displacement is a good route for exploring structural and functional properties of oxides. Our high-resolution annular bright-field scanning transmission electron microscopy observations revealed that interfacial oxygen displacement determines metal-oxygen bond angles in entire heterostructures and the magnitude of the displacements can be adjusted by controlling propagations of the oxygen octahedral rotations across the heterointerface. We also show that by heterostructuring an itinerant ferromagnet SrRuO3 with Ca0.5Sr0.5TiO3 (0–4 monolayers thick) grown on a GdScO3 substrate, structural phase and magnetic anisotropy of SrRuO3 can be controlled through the interfacial engineering of its oxygen displacement.

Giant Photoresponse in Quantized SrRuO3 Monolayer at Oxide Interfaces

The photoelectric effect in semiconductors is the main mechanism for most modern optoelectronic devices, in which the adequate bandgap plays the key role for acquiring high photoresponse. Among numerous material categories applied in this field, the complex oxides exhibit great possibilities because they present a wide distribution of band gaps for absorbing light with any wavelength. Their physical properties and lattice structures are always strongly coupled and sensitive to light illumination. Moreover, the confinement of dimensionality of the complex oxides in the heterostructures can provide more diversities in designing and modulating the band structures. On the basis of this perspective, we have chosen itinerary ferromagnetic SrRuO3 as the model material, and fabricated it in one-unit-cell thickness in order to open a small band gap for effective utilization of visible light. By inserting this SrRuO3 monolayer at the interface of the well-developed two-dimensional electron gas system (LaAlO3/SrTiO3...

Electric-field control of anomalous and topological Hall effects in oxide bilayer thin films

One of the key goals in spintronics is to tame the spin-orbit coupling (SOC) that links spin and motion of electrons, giving rise to intriguing magneto-transport properties in itinerant magnets. Prominent examples of such SOC-based phenomena are the anomalous and topological Hall effects. However, controlling them with electric fields has remained unachieved since an electric field tends to be screened in itinerant magnets. Here we demonstrate that both anomalous and topological Hall effects can be modulated by electric fields in oxide heterostructures consisting of ferromagnetic SrRuO3 and nonmagnetic SrIrO3. We observe a clear electric field effect only when SrIrO3 is inserted between SrRuO3 and a gate dielectric. Our results establish that strong SOC of nonmagnetic materials such as SrIrO3 is essential in electrical tuning of these Hall effects and possibly other SOC-related phenomena.

Effect of Gd and Cr substitution on the structural, electronic and magnetic phases of SrRuO3: a case study of doping and chemical phase separation

We explore the crystal structure, electrical resistivity and magnetic behavior of the compositional series (SrRuO3) (GdCrO3)x (where ), which resides between orthorhombic ferromagnetic (FM) metal SrRuO3 ( K) and orthorhombic antiferromagnetic (AFM) insulator GdCrO3 ( K). Crystal structure analysis reveals that complete solid solution exists only up to , above which chemical phase separation of two/three phases occurs, and persists up to . X-ray photoelectron spectroscopy measurement also corroborates the existence of for the intermediate composition , which reinforces the astonishing scheelite-type GdCrO4 formation (at ambient pressure) for compositions. Electrical resistivity measurements affirm the temperature driven metal to insulator (M–I) transition for and samples. Low temperature insulating state in these samples is interpreted by electron–electron interaction of weak disordered systems. Precise analysis of temperature dependent resistivity for samples (which have insulating ground state) dictate that the transport phenomenon is mainly associated with Arrhenius-type charge conduction, Mott’s variable range hopping, short-range and long-range Coulomb interaction mediated hopping processes, due to the high degree of randomness. Interruption of magnetic Ru–O–Ru interaction by Ru–O–Cr and Cr–O–Cr interactions lowers the FM transition temperature (TC), and thereby introduces Griffiths phase in phase separated samples. Furthermore, we believe that a sharp rise in magnetization at low temperature for samples is due to the formation of AFM GdCrO4 phase. Prominent thermal hysteresis in temperature dependent magnetization curves for , and appearance of spin-reorientation transition for are the distinct indications for transformation into canted AFM GdCrO3 oxide at higher x. The effective magnetic moment () continuously increases with the incorporation of higher moment elements (Gd and Cr); while coercive field (HC) exhibits an abrupt variation as a function of x at the onset of phase separation.

Finite-size scaling and exchange-bias in SrRuO3/LaNiO3/SrRuO3 trilayers

We report a detailed magnetization study of the coherently strained trilayer heterostructures of metallic ferromagnet SrRuO3 (SRO)/paramagnetic-LaNiO3/SRO grown on a (001) oriented SrTiO3 single crystal substrate using the pulsed laser deposition technique. A tunable positive exchange bias (μoHEB ∼ 0.2 T, at 10 K) across the interface has been observed. Upon varying the SRO layer thickness (2 nm ≤ tSRO ≤ 18 nm), the saturation magnetization (MS ∼ 0.46 μB/Ru) increases significantly owing to electronic-reconstruction across the interfaces and finite size effects. The magnitude of μoHEB increases continuously with increasing tSRO and saturates above 16 nm; however, the loop-asymmetry increases rapidly as the temperature is lowered below the ferromagnetic Curie temperature (TC) of SRO. Notable reductions in both TC (125.6 K ≤ TC ≤ 146.4 K) and blocking temperature TB = T* (90.6 K ≤ T∗ ≤ 119.2 K) are observed due to surface effects that comply with the finite-size scaling law: TC(tSRO) = TC (∞) [1 − (ξo/tSRO)λ] with critical exponent λ = 1.12 ± 0.037 and correlation length ξ0 = 0.59 ± 0.02 nm signifying the fact that the investigated system is an intermediate between the 3D Ising configuration and those associated with mean field approximations.

Isotope shift of the ferromagnetic transition temperature in itinerant ferromagnets

We present a theory of the isotope effect of the Curie temperature Tc in itinerant ferromagnets. The isotope effect in ferromagnets occurs via the electron–phonon vertex correction and the effective attractive interaction mediated by the electron–phonon interaction. The decrease of the Debye frequency increases the relative strength of the Coulomb interaction, which results in a positive isotope shift of Tc when the mass M of an atom increases. Following this picture, we evaluate the isotope effect of Tc by using the Stoner theory and a spin-fluctuation theory. When Tc is large enough as large as or more than 100 K, the isotope effect on Tc can be measurable. Recently, precise measurements on the oxygen isotope effect on Tc have been performed for itinerant ferromagnet SrRuO3 with Tc∼160 K. A clear isotope effect has been observed with the positive shift of Tc∼1 K by isotope substitution (O16→O18). This experimental result is consistent with our theory.

Local conductance spectra of itinerant ferromagnetic SrRuO3 through break junction

We have measured the local differential conductance spectra (dI/dV–V) of an itinerant ferromagnet composed of polycrystalline SrRuO3 using the mechanically controllable break junction technique. Below the material’s Curie temperature (TC = 160 K), characteristic peak or dip conductance spectra are observed. The characteristic energy scale is comparable to the exchange spin splitting energy that is based on ferromagnetic band calculations. Both the peak and dip spectral shapes are explained based on the itinerant ferromagnetic characteristics of SrRuO3 in terms of spin-dependent transmission, which is similar to the giant magnetoresistance mechanism.

Weyl fermions and spin dynamics of metallic ferromagnet SrRuO3.

Weyl fermions that emerge at band crossings in momentum space caused by the spin–orbit interaction act as magnetic monopoles of the Berry curvature and contribute to a variety of novel transport phenomena such as anomalous Hall effect and magnetoresistance. However, their roles in other physical properties remain mostly unexplored. Here, we provide evidence by neutron Brillouin scattering that the spin dynamics of the metallic ferromagnet SrRuO3 in the very low energy range of milli-electron volts is closely relevant to Weyl fermions near Fermi energy. Although the observed spin wave dispersion is well described by the quadratic momentum dependence, the temperature dependence of the spin wave gap shows a nonmonotonous behaviour, which can be related to that of the anomalous Hall conductivity. This shows that the spin dynamics directly reflects the crucial role of Weyl fermions in the metallic ferromagnet.

Terahertz conductivity ofSr1−xCaxRuO3

We investigate the optical conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$ across the ferromagnetic to paramagnetic transition that occurs at $x=0.8$. The thin films were grown by metalorganic aerosol deposition with $0 \leq x \leq 1$ onto NdGaO$_3$ substrates. We performed THz frequency domain spectroscopy in a frequency range from 3~cm$^{-1}$ to 40~cm$^{-1}$ (100~GHz to 1.4~THz) and at temperatures ranging from 5~K to 300~K, measuring transmittivity and phase shift through the films. From this we obtained real and imaginary parts of the optical conductivity. The end-members, ferromagnetic SrRuO$_3$ and paramagnetic CaRuO$_3$, show a strongly frequency-dependent metallic response at temperatures below 20~K. Due to the high quality of these samples we can access pronounced intrinsic electronic contributions to the optical scattering rate, which at 1.4~THz exceeds the residual scattering rate by more than a factor of three. Deviations from a Drude response start at about 0.7~THz for both end-members in a remarkably similar way. For the intermediate members a higher residual scattering originating in the compositional disorder leads to a featureless optical response, instead. The relevance of low-lying interband transitions is addressed by a calculation of the optical conductivity within density functional theory in the local density approximation (LDA).