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- ItemInhomogeneous ferromagnetism mimics signatures of the topological Hall effect in SrRuO3 films(College Park, MD : APS, 2020) Kim, Gideok; Son, K.; Suyolcu, Y.E.; Miao, L.; Schreiber, N.J.; Nair, H.P.; Putzky, D.; Minola, M.; Christiani, G.; van Aken, P.A.; Shen, K.M.; Schlom, D.G.; Logvenov, G.; Keimer, B.Topological transport phenomena in magnetic materials are a major topic of current condensed matter research. One of the most widely studied phenomena is the topological Hall effect (THE), which is generated via spin-orbit interactions between conduction electrons and topological spin textures such as skyrmions. We report a comprehensive set of Hall effect and magnetization measurements on epitaxial films of the prototypical ferromagnetic metal SrRuO3 the magnetic and transport properties of which were systematically modulated by varying the concentration of Ru vacancies. We observe Hall effect anomalies that closely resemble signatures of the THE, but a quantitative analysis demonstrates that they result from inhomogeneities in the ferromagnetic magnetization caused by a nonrandom distribution of Ru vacancies. As such inhomogeneities are difficult to avoid and are rarely characterized independently, our results call into question the identification of topological spin textures in numerous prior transport studies of quantum materials, heterostructures, and devices. Firm conclusions regarding the presence of such textures must meet stringent conditions such as probes that couple directly to the noncollinear magnetization on the atomic scale.
- ItemStrain-stabilized superconductivity([London] : Nature Publishing Group UK, 2021) Ruf, J.P.; Paik, H.; Schreiber, N.J.; Nair, H.P.; Miao, L.; Kawasaki, J.K.; Nelson, J.N.; Faeth, B.D.; Lee, Y.; Goodge, B.H.; Pamuk, B.; Fennie, C.J.; Kourkoutis, L.F.; Schlom, D.G.; Shen, K.M.Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO2 thin films on (110)-oriented TiO2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals.
- ItemEngineering new limits to magnetostriction through metastability in iron-gallium alloys([London] : Nature Publishing Group UK, 2021) Meisenheimer, P.B.; Steinhardt, R.A.; Sung, S.H.; Williams, L.D.; Zhuang, S.; Nowakowski, M.E.; Novakov, S.; Torunbalci, M.M.; Prasad, B.; Zollner, C. J.; Wang, Z.; Dawley, N.M.; Schubert, J.; Hunter, A.H.; Manipatruni, S.; Nikonov, D.E.; Young, I.A.; Chen, L.Q.; Bokor, J.; Bhave, S.A.; Ramesh, R.; Hu, J.-M.; Kioupakis, E.; Hovden, R.; Schlom, D.G.; Heron, J.T.Magnetostrictive materials transduce magnetic and mechanical energies and when combined with piezoelectric elements, evoke magnetoelectric transduction for high-sensitivity magnetic field sensors and energy-efficient beyond-CMOS technologies. The dearth of ductile, rare-earth-free materials with high magnetostrictive coefficients motivates the discovery of superior materials. Fe1−xGax alloys are amongst the highest performing rare-earth-free magnetostrictive materials; however, magnetostriction becomes sharply suppressed beyond x = 19% due to the formation of a parasitic ordered intermetallic phase. Here, we harness epitaxy to extend the stability of the BCC Fe1−xGax alloy to gallium compositions as high as x = 30% and in so doing dramatically boost the magnetostriction by as much as 10x relative to the bulk and 2x larger than canonical rare-earth based magnetostrictors. A Fe1−xGax − [Pb(Mg1/3Nb2/3)O3]0.7−[PbTiO3]0.3 (PMN-PT) composite magnetoelectric shows robust 90° electrical switching of magnetic anisotropy and a converse magnetoelectric coefficient of 2.0 × 10−5 s m−1. When optimally scaled, this high coefficient implies stable switching at ~80 aJ per bit.
- ItemExploring the intrinsic limit of the charge-carrier-induced increase of the Curie temperature of Lu- and La-doped EuO thin films(College Park, MD : APS, 2020) Held, R.; Mairoser, T.; Melville, A.; Mundy, J.A.; Holtz, M.E.; Hodash, D.; Wang, Z.; Heron, J.T.; Dacek, S.T.; Holländer, B.; Muller, D.A.; Schlom, D.G.Raising the Curie temperature TC of the highly spin-polarized semiconductor EuO by doping it with rare-earth elements is a strategy to make EuO more technologically relevant to spintronics. The increase of TC with free carrier density n and the surprisingly low dopant activation p, found in Gd-doped EuO thin films [Mairoser et al., Phys. Rev. Lett. 105, 257206 (2010)], raised the important question of whether TC could be considerably enhanced by increasing p. Using a low-temperature growth method for depositing high-quality Lu-doped EuO films we attain high dopant activation (p) values of up to 67%, effectively more than doubling p as compared to adsorption-controlled growth of Lu- and Gd-doped EuO. Relating n, p, and lattice compression of La- and Lu-doped EuO films grown at different temperatures to the TC of these samples allows us to identify several different mechanisms influencing TC and causing an experimental maximum in TC. In addition, scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements on La-doped EuO indicate that extensive dopant clustering is one, but not the sole reason for dopant deactivation in rare-earth doped EuO films.