Temperature evolution of itinerant ferromagnetism in SrRuO3 probed by optical spectroscopy

Electric Control of 2D Van Hove Singularity in Oxide Ultra-Thin Films

Divergent density of states (DOS) can induce extraordinary phenomena such as significant enhancement of superconductivity and unexpected phase transitions. Moreover, van Hove singularities (VHSs) lead to divergent DOS in 2D systems. Despite recent interest in VHSs, only a few controllable cases have been reported to date. In this work, by utilizing an atomically ultra-thin SrRuO₃ film, the electronic structure of a 2D VHS is investigated with angle-resolved photoemission spectroscopy and transport properties are controlled. By applying electric fields with alkali metal deposition and ionic-liquid gating methods, the 2D VHS and the sign of the charge carrier are precisely controlled. Use of a tunable 2D VHS in an atomically flat oxide film could serve as a new strategy to realize infinite DOS near the Fermi level, thereby allowing efficient tuning of electric properties.

Observation of Spin-Dependent Dual Ferromagnetism in Perovskite Ruthenates

We performed in situ angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES (SARPES) experiments to investigate the relationship between electronic band structures and ferromagnetism in SrRuO_{3} (SRO) thin films. Our high quality ARPES and SARPES results show clear spin-lifted band structures. The spin polarization is strongly dependent on momentum around the Fermi level, whereas it becomes less dependent at high-binding energies. This experimental observation matches our dynamical mean-field theory results very well. As temperature increases from low to the Curie temperature, spin-splitting gap decreases and band dispersions become incoherent. Based on the ARPES study and theoretical calculation results, we found that SRO possesses spin-dependent electron correlations in which majority and minority spins are localized and itinerant, respectively. Our finding explains how ferromagnetism and electronic structure are connected, which has been under debate for decades in SRO.

Sign-tunable anomalous Hall effect induced by two-dimensional symmetry-protected nodal structures in ferromagnetic perovskite thin films

Magnetism and spin-orbit coupling are two quintessential ingredients underlying topological transport phenomena in itinerant ferromagnets. When spin-polarized bands support nodal points/lines with band degeneracy that can be lifted by spin-orbit coupling, the nodal structures become a source of Berry curvature, leading to a large anomalous Hall effect. However, two-dimensional systems can possess stable nodal structures only when proper crystalline symmetry exists. Here we show that two-dimensional spin-polarized band structures of perovskite oxides generally support symmetry-protected nodal lines and points that govern both the sign and the magnitude of the anomalous Hall effect. To demonstrate this, we performed angle-resolved photoemission studies of ultrathin films of SrRuO₃, a representative metallic ferromagnet with spin-orbit coupling. We show that the sign-changing anomalous Hall effect upon variation in the film thickness, magnetization and chemical potential can be well explained by theoretical models. Our work may facilitate new switchable devices based on ferromagnetic ultrathin films.

Defect-Induced Exchange Bias in a Single SrRuO3 Layer

Exchange bias stems from the interaction between different magnetic phases, and therefore, it generally occurs in magnetic multilayers. Here, we present a large exchange bias in a single SrRuO₃ layer induced by helium ion irradiation. When the fluence increases, the induced defects not only suppress the magnetization and the Curie temperature but also drive a metal-insulator transition at a low temperature. In particular, a large exchange bias field up to ∼0.36 T can be created by the irradiation. This large exchange bias is related to the coexistence of different magnetic and structural phases that are introduced by embedded defects. Our work demonstrates that spintronic properties in complex oxides can be created and enhanced by applying ion irradiation.

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)[Formula: see text] (GdCrO₃) _x (where [Formula: see text]), which resides between orthorhombic ferromagnetic (FM) metal SrRuO₃ ([Formula: see text] K) and orthorhombic antiferromagnetic (AFM) insulator GdCrO₃ ([Formula: see text] K). Crystal structure analysis reveals that complete solid solution exists only up to [Formula: see text], above which chemical phase separation of two/three phases occurs, and persists up to [Formula: see text]. X-ray photoelectron spectroscopy measurement also corroborates the existence of [Formula: see text] for the intermediate composition [Formula: see text], which reinforces the astonishing scheelite-type GdCrO₄ formation (at ambient pressure) for [Formula: see text] compositions. Electrical resistivity measurements affirm the temperature driven metal to insulator (M-I) transition for [Formula: see text] and [Formula: see text] 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 [Formula: see text] 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 (T _C), and thereby introduces Griffiths phase in phase separated samples. Furthermore, we believe that a sharp rise in magnetization at low temperature for [Formula: see text] samples is due to the formation of AFM GdCrO₄ phase. Prominent thermal hysteresis in temperature dependent magnetization curves for [Formula: see text], and appearance of spin-reorientation transition for [Formula: see text] are the distinct indications for transformation into canted AFM GdCrO₃ oxide at higher x. The effective magnetic moment ([Formula: see text]) continuously increases with the incorporation of higher moment elements (Gd and Cr); while coercive field (H _C) exhibits an abrupt variation as a function of x at the onset of phase separation.

Magnetic ground state of SrRuO3 thin film and applicability of standard first-principles approximations to metallic magnetism

A systematic first-principles study has been performed to understand the magnetism of thin film SrRuO₃ which lots of research efforts have been devoted to but no clear consensus has been reached about its ground state properties. The relative t_2g level difference, lattice distortion as well as the layer thickness play together in determining the spin order. In particular, it is important to understand the difference between two standard approximations, namely LDA and GGA, in describing this metallic magnetism. Landau free energy analysis and the magnetization-energy-ratio plot clearly show the different tendency of favoring the magnetic moment formation, and it is magnified when applied to the thin film limit where the experimental information is severely limited. As a result, LDA gives a qualitatively different prediction from GGA in the experimentally relevant region of strain whereas both approximations give reasonable results for the bulk phase. We discuss the origin of this difference and the applicability of standard methods to the correlated oxide and the metallic magnetic systems.

Site dilution in SrRuO3: effects on structural and magnetic properties

We have investigated the effect of site dilution with substitution of nonmagnetic element in SrRu_1-x Ti _x O₃ (x  ⩽  0.7). The nature of ferromagnetic state in SrRuO₃ is believed to be of itinerant type with transition temperature [Formula: see text] K. Crystallographically, SrRuO₃ has a distorted orthorhombic structure. Substitution of [Formula: see text] (3d ⁰) for Ru⁺⁴ (4d ⁴), however, does not introduce significant structural modification due to their matching ionic radii. This substitution, on the other hand, is expected to tune the electronic correlation effect and the d electron density in the system. With Ti substitution, we find that magnetic moment and Curie temperature decreases but T _c remains unchanged which has been attributed to opposite tuning of electron correlation effect and density of states within the framework of itinerant ferromagnetism. The estimated critical exponent (β) related to magnetization implies a mean-field type of magnetic nature in SrRuO₃. The value of β further increases with x which is understood from the dilution effect of magnetic lattice. The system evolves to exhibit Griffiths phase like behavior above T _c which is usually realized in diluted ferromagnet following local moment model of magnetism. Our detail analysis of magnetization data indicates that magnetic state in SrRuO₃ has contribution from both itinerant and local moment model of magnetism.

Correlation among disorder, electronic and magnetic phases of SrRuO3

Electric and magnetic properties of Sr1-xBaxRu1-xTixO3 (0 ⩽ x ⩽ 0.8) have been investigated to find the interrelationship between metallicity and ferromagnetism in SrRuO3 (SRO). The simultaneous doping of Sr and Ru with Ba and Ti results in single phase SRO at x = 0.1 and mixed phase of SRO and hexagonal BaTiO3 (h-BTO) at x ⩾ 0.2. Co-doping at Sr and Ru sites gives rise to oxygen vacancy and mixed valency of Ru (Ru(3+) and Ru(4+)). Room temperature resistivity increases due to modification of p(O)-d(Ru) hybridization and phase segregation. Temperature dependent resistivity reveals metal-insulator transition around 232 K at x = 0.1 and insulator down to 2 K at x ⩾ 0.2. The insulating state (x = 0.1) at low temperature is well described by weak localization and electron-electron interaction. Temperature dependence of resistivity (x ⩾ 0.2) follows Mott's three dimensional variable range hopping model. Localization length and average hopping distance decrease with the increase of x, indicating the presence of more disorder. Ferromagnetic transition temperature decreases to 149 K at x = 0.1 and remains constant up to x = 0.5. The Curie-Wiess (CW) temperature (ΘCW) decreases monotonically and becomes negative at x = 0.5. The effective magnetic moment estimated from CW law is smaller than that of pure SRO due to the formation of Ru(3+) ions. The saturation magnetization diminishes, suggesting the demagnetization factor owing to diamagnetic h-BTO. The coercivity increases from 6700 Oe (x = 0) to 12 500 Oe (x = 0.4) and then decreases to 3700 Oe (x = 0.5). Ferromagnetic cluster comprising of doped SRO gives rise to the formation of a Griffith-like phase. The co-occurrence of high jump in resistivity ratio and disappearance of ferromagnetism suggests an interplay between transport process and magnetism at low temperature.