Alexander J. Grutter

Interfacial Ferromagnetism in LaNiO3/CaMnO3 Superlattices

We observe interfacial ferromagnetism in superlattices of the paramagnetic metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO3. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of CaMnO3 at the interface. Together these results suggest that ferromagnetism can be attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.

Tailoring exchange couplings in magnetic topological-insulator/antiferromagnet heterostructures

Magnetic topological insulators such as Cr-doped (Bi,Sb)2Te3 provide a platform for the realization of versatile time-reversal symmetry-breaking physics. By constructing heterostructures exhibiting Néel order in an antiferromagnetic CrSb and ferromagnetic order in Cr-doped (Bi,Sb)2Te3, we realize emergent interfacial magnetic phenomena which can be tailored through artificial structural engineering. Through deliberate geometrical design of heterostructures and superlattices, we demonstrate the use of antiferromagnetic exchange coupling in manipulating the magnetic properties of magnetic topological insulators. Proximity effects are shown to induce an interfacial spin texture modulation and establish an effective long-range exchange coupling mediated by antiferromagnetism, which significantly enhances the magnetic ordering temperature in the superlattice. This work provides a new framework on integrating topological insulators with antiferromagnetic materials and unveils new avenues towards dissipationless topological antiferromagnetic spintronics.

Enhanced magnetism in epitaxial SrRuO3 thin films

We observed enhanced magnetization in epitaxial SrRuO3 thin films compared to previously reported bulk and thin film values. The enhancement is strongly dependent on the orientation of the lattice distortions imposed by (001), (110), and (111) oriented SrTiO3 substrates. A larger magnetization enhancement for coherently strained SrRuO3 films on (111) and (110) oriented SrTiO3 compared to films on (001) SrTiO3 confirms the importance of the strain state in determining the magnetic ground state of the Ru ion. Moreover, SrRuO3 films on (111) SrTiO3 exhibit enhanced moments as high as 3.4 μB/Ru ion, suggesting the stabilization of a high-spin Ru4+ state.

Structural and magnetic depth profiles of magneto-ionic heterostructures beyond the interface limit

Electric field control of magnetism provides a promising route towards ultralow power information storage and sensor technologies. The effects of magneto-ionic motion have been prominently featured in the modification of interface characteristics. Here, we demonstrate magnetoelectric coupling moderated by voltage-driven oxygen migration beyond the interface in relatively thick AlOx/GdOx/Co(15 nm) films. Oxygen migration and Co magnetization are quantitatively mapped with polarized neutron reflectometry under electro-thermal conditioning. The depth-resolved profiles uniquely identify interfacial and bulk behaviours and a semi-reversible control of the magnetization. Magnetometry measurements suggest changes in the microstructure which disrupt long-range ferromagnetic ordering, resulting in an additional magnetically soft phase. X-ray spectroscopy confirms changes in the Co oxidation state, but not in the Gd, suggesting that the GdOx transmits oxygen but does not source or sink it. These results together provide crucial insight into controlling magnetism via magneto-ionic motion, both at interfaces and throughout the bulk of the films. Mechanisms allowing electrical manipulation of magnetic material possess potential applications in low power memory and sensor technologies. Here, the authors demonstrate the control of magnetic characteristics via voltage-driven migration of oxygen across a GdOx/Co interface, well into the bulk of the cobalt.Electric-field control of magnetism provides a promising route towards ultralow power information storage and sensor technologies. The effects of magneto-ionic motion have so far been prominently featured in the direct modification of interface chemical and physical characteristics. Here we demonstrate magnetoelectric coupling moderated by voltage-driven oxygen migration beyond the interface limit in relatively thick AlOx/GdOx/Co (15 nm) films. Oxygen migration and its ramifications on the Co magnetization are quantitatively mapped with polarized neutron reflectometry under thermal and electro-thermal conditionings. The depth-resolved profiles uniquely identify interfacial and bulk behaviors and a semi-reversible suppression and recovery of the magnetization. Magnetometry measurements show that the conditioning changes the microstructure so as to disrupt long-range ferromagnetic ordering, resulting in an additional magnetically soft phase. X-ray spectroscopy confirms electric field induced changes in the Co oxidation state but not in the Gd, suggesting that the GdOx transmits oxygen but does not source or sink it. These results together provide crucial insight into controlling magnetic heterostructures via magneto-ionic motion, not only at the interface, but also throughout the bulk of the films.

Controllable Positive Exchange Bias via Redox-Driven Oxygen Migration

Ionic transport in metal/oxide heterostructures offers a highly effective means to tailor material properties via modification of the interfacial characteristics. However, direct observation of ionic motion under buried interfaces and demonstration of its correlation with physical properties has been challenging. Using the strong oxygen affinity of gadolinium, we design a model system of GdxFe1−x/NiCoO bilayer films, where the oxygen migration is observed and manifested in a controlled positive exchange bias over a relatively small cooling field range. The exchange bias characteristics are shown to be the result of an interfacial layer of elemental nickel and cobalt, a few nanometres in thickness, whose moments are larger than expected from uncompensated NiCoO moments. This interface layer is attributed to a redox-driven oxygen migration from NiCoO to the gadolinium, during growth or soon after. These results demonstrate an effective path to tailoring the interfacial characteristics and interlayer exchange coupling in metal/oxide heterostructures.

Enhanced magnetization in epitaxial SrRuO3 thin films via substrate-induced strain

Epitaxial SrRuO3 thin films were grown on SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and LaAlO3 substrates inducing different compressive strains. Coherently strained SrRuO3 films exhibit enhanced magnetization compared to previously reported bulk and thin film values of 1.1–1.6μB per formula unit. A comparison of (001) SrRuO3 films on each substrate indicates that strained films have consistently higher saturated moments than corresponding relaxed films, which exhibit bulk moments. These observations indicate the importance of lattice distortions in controlling the magnetic ground state in this transitional metal oxide.Epitaxial SrRuO3 thin films were grown on SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and LaAlO3 substrates inducing different compressive strains. Coherently strained SrRuO3 films exhibit enhanced magnetization compared to previously reported bulk and thin film values of 1.1–1.6μB per formula unit. A comparison of (001) SrRuO3 films on each substrate indicates that strained films have consistently higher saturated moments than corresponding relaxed films, which exhibit bulk moments. These observations indicate the importance of lattice distortions in controlling the magnetic ground state in this transitional metal oxide.

Interfacial Symmetry Control of Emergent Ferromagnetism at the Nanoscale

The emergence of complex new ground states at interfaces has been identified as one of the most promising routes to highly tunable nanoscale materials. Despite recent progress, isolating and controlling the underlying mechanisms behind these emergent properties remains among the most challenging materials physics problems to date. In particular, generating ferromagnetism localized at the interface of two nonferromagnetic materials is of fundamental and technological interest. Moreover, the ability to turn the ferromagnetism on and off would shed light on the origin of such emergent phenomena and is promising for spintronic applications. We demonstrate that ferromagnetism confined within one unit cell at the interface of CaRuO3 and CaMnO3 can be switched on and off by changing the symmetry of the oxygen octahedra connectivity at the boundary. Interfaces that are symmetry-matched across the boundary exhibit interfacial CaMnO3 ferromagnetism while the ferromagnetism at symmetry-mismatched interfaces is suppressed. We attribute the suppression of ferromagnetic order to a reduction in charge transfer at symmetry-mismatched interfaces, where frustrated bonding weakens the orbital overlap. Thus, interfacial symmetry is a new route to control emergent ferromagnetism in materials such as CaMnO3 that exhibit antiferromagnetism in bulk form.

Quasi-two-dimensional electron gas behavior in doped LaAlO3 thin films on SrTiO3 substrates

We have demonstrated the growth of Tm and Lu doped LaAlO3 epitaxial thin films on single crystal (001) SrTiO3 substrates. These rare-earth dopants potentially act as sources of localized moment and spin-orbit scattering centers at the interface. Through structural and chemical characterization, we confirm the incorporation of Tm and Lu dopants into highly crystalline LaAlO3 films. The rare earth doping of the La site does not significantly modify the sheet carrier concentration or mobility compared to undoped samples despite the evolution of sheet carrier concentration, mobility, and sheet resistance with LaAlO3 thickness in undoped LaAlO3 films on SrTiO3.

Reversible control of magnetism in La0.67Sr0.33MnO3 through chemically-induced oxygen migration

We demonstrate reversible control of magnetization and anisotropy in La0.67Sr0.33MnO3 films through interfacial oxygen migration. Gd metal capping layers deposited onto La0.67Sr0.33MnO3 leach oxygen from the film through a solid-state redox reaction to form porous Gd2O3. X-ray absorption and polarized neutron reflectometry measurements show Mn valence alterations consistent with high oxygen vacancy concentrations, resulting in suppressed magnetization and increased coercive fields. Effects of the oxygen migration are observed both at the interface and also throughout the majority of a 40 nm thick film, suggesting extensive diffusion of oxygen vacancies. After Gd-capped La0.67Sr0.33MnO3 is exposed to atmospheric oxygen for a prolonged period of time, oxygen diffuses through the Gd2O3 layer and the magnetization of the La0.67Sr0.33MnO3 returns to the uncapped value. These findings showcase perovskite heterostructures as ideal candidates for developing functional interfaces through chemically-induced oxygen ...

Measurement and modeling of polarized specular neutron reflectivity in large magnetic fields

A procedure is described for polarized neutron reflectometry when the Zeeman corrections are significant, which occurs when both the magnetic anisotropy and the applied magnetic field are significant. Calculations and a recommended procedure for an example system are provided.