Interplay of Electronic and Spin Degrees in Ferromagnetic SrRuO_{3}: Anomalous Softening of the Magnon Gap and Stiffness

Orbital Isotropy of Magnetic Fluctuations in Correlated Electron Materials Induced by Hund's Exchange Coupling

Characterizing nonlocal magnetic fluctuations in materials with strong electronic Coulomb interactions remains one of the major outstanding challenges of modern condensed matter theory. In this Letter, we address the spatial symmetry and orbital structure of magnetic fluctuations in perovskite materials. To this aim, we develop a consistent multiorbital diagrammatic extension of dynamical mean-field theory, which we apply to an anisotropic three-orbital model of cubic t_{2g} symmetry. We find that the form of spatial spin fluctuations is governed by the local Hund's coupling. For small values of the coupling, magnetic fluctuations are anisotropic in orbital space, which reflects the symmetry of the considered t_{2g} model. Large Hund's coupling enhances collective spin excitations, which mixes orbital and spatial degrees of freedom, and magnetic fluctuations become orbitally isotropic. Remarkably, this effect can be seen only in two-particle quantities; single-particle observables remain anisotropic for any value of the Hund's coupling. Importantly, we find that the orbital isotropy can be induced both at half filling and for the case of four electrons per lattice site, where the magnetic instability is associated with different, antiferromagnetic and ferromagnetic, modes, respectively.

High-sensitivity of initial SrO growth on the residual resistivity in epitaxial thin films of SrRuO[Formula: see text] on SrTiO[Formula: see text] (001)

The growth of SrRuO[Formula: see text] (SRO) thin film with high-crystallinity and low residual resistivity (RR) is essential to explore its intrinsic properties. Here, utilizing the adsorption-controlled growth technique, the growth condition of initial SrO layer on TiO[Formula: see text]-terminated SrTiO[Formula: see text] (STO) (001) substrate was found to be crucial for achieving a low RR in the resulting SRO film grown afterward. The optimized initial SrO layer shows a c(2 [Formula: see text] 2) superstructure that was characterized by electron diffraction, and a series of SRO films with different thicknesses (ts) were then grown. The resulting SRO films exhibit excellent crystallinity with orthorhombic-phase down to [Formula: see text] 4.3 nm, which was confirmed by high resolution X-ray measurements. From X-ray azimuthal scan across SRO orthorhombic (02 ± 1) reflections, we uncover four structural domains with a dominant domain of orthorhombic SRO [001] along cubic STO [010] direction. The dominant domain population depends on t, STO miscut angle ([Formula: see text]), and miscut direction ([Formula: see text]), giving a volume fraction of about 92 [Formula: see text] for [Formula: see text] 26.6 nm and [Formula: see text] (0.14[Formula: see text], 5[Formula: see text]). On the other hand, metallic and ferromagnetic properties were well preserved down to t [Formula: see text] 1.2 nm. Residual resistivity ratio (RRR = [Formula: see text]/[Formula: see text]) reduces from 77.1 for t [Formula: see text] 28.5 nm to 2.5 for t [Formula: see text] 1.2 nm, while [Formula: see text] increases from 2.5 [Formula: see text]cm for t [Formula: see text] 28.5 nm to 131.0 [Formula: see text]cm for t [Formula: see text] 1.2 nm. The ferromagnetic onset temperature ([Formula: see text]) of around 151 K remains nearly unchanged down to t [Formula: see text] 9.0 nm and decreases to 90 K for t [Formula: see text] 1.2 nm. Our finding thus provides a practical guideline to achieve high crystallinity and low RR in ultra-thin SRO films by simply adjusting the growth of initial SrO layer.

Quantum transport evidence of Weyl fermions in an epitaxial ferromagnetic oxide

Magnetic Weyl semimetals have novel transport phenomena related to pairs of Weyl nodes in the band structure. Although the existence of Weyl fermions is expected in various oxides, the evidence of Weyl fermions in oxide materials remains elusive. Here we show direct quantum transport evidence of Weyl fermions in an epitaxial 4d ferromagnetic oxide SrRuO₃. We employ machine-learning-assisted molecular beam epitaxy to synthesize SrRuO₃ films whose quality is sufficiently high to probe their intrinsic transport properties. Experimental observation of the five transport signatures of Weyl fermions—the linear positive magnetoresistance, chiral-anomaly-induced negative magnetoresistance, π phase shift in a quantum oscillation, light cyclotron mass, and high quantum mobility of about 10,000 cm²V⁻¹s⁻¹—combined with first-principles electronic structure calculations establishes SrRuO₃ as a magnetic Weyl semimetal. We also clarify the disorder dependence of the transport of the Weyl fermions, which gives a clear guideline for accessing the topologically nontrivial transport phenomena.

Despite various predictions, the evidence of Weyl fermions in oxide materials remains elusive. Here, the authors show evidence of Weyl fermions in quantum transport measurements in an epitaxial ferromagnetic oxide SrRuO₃.

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.