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Register a new userFe3O4 thin films: controlling and manipulating an elusive quantum material
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Fe3O4 (magnetite) is one of the most elusive quantum materials and at the same time one of the most studied transition metal oxide materials for thin film applications. The theoretically expected half-metallic behavior generates high expectations that it can be used in spintronic devices. Yet, despite the tremendous amount of work devoted to preparing thin films, the enigmatic first order metal-insulator transition and the hall mark of magnetite known as the Verwey transition, is in thin films extremely broad and occurs at substantially lower temperatures as compared to that in high quality bulk single crystals. Here we have succeeded in finding and making a particular class of substrates that allows the growth of magnetite thin films with the Verwey transition as sharp as in the bulk. Moreover, we are now able to tune the transition temperature and, using tensile strain, increase it to substantially higher values than in the bulk.
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Polar catastrophe at the interface of oxide materials with strongly correlated electrons has triggered a flurry of new research activities. The expectations are that the design of such advanced interfaces will become a powerful route to engineer devices with novel functionalities. Here we investigate the initial stages of growth and the electronic structure of the spintronic Fe3O4/MgO (001) interface. Using soft x-ray absorption spectroscopy we have discovered that the so-called A-sites are completely missing in the first Fe3O4 monolayer. This allows us to develop an unexpected but elegant growth principle in which during deposition the Fe atoms are constantly on the move to solve the divergent electrostatic potential problem, thereby ensuring epitaxy and stoichiometry at the same time. This growth principle provides a new perspective for the design of interfaces.
Magnetite (Fe3O4) thin films on GaAs have been studied with HArd X-ray PhotoElectron Spectroscopy (HAXPES) and low-energy electron diffraction. Films prepared under different growth conditions are compared with respect to stoichiometry, oxidation, and chemical nature. Employing the considerably enhanced probing depth of HAXPES as compared to conventional x-ray photoelectron spectroscopy (XPS) allows us to investigate the chemical state of the film-substrate interfaces. The degree of oxidation and intermixing at the interface are dependent on the applied growth conditions; in particular, we found that metallic Fe, As2O3, and Ga2O3 exist at the interface. These interface phases might be detrimental for spin injection from magnetite into GaAs.
Magnetite thin fims have been grown epitaxially on ZnO and MgO substrates using molecular beam epitaxy. The film quality was found to be strongly dependent on the oxygen partial pressure during growth. Structural, electronic, and magnetic properties were analyzed utilizing Low Energy Electron Diffraction (LEED), HArd X-ray PhotoElectron Spectroscopy (HAXPES), Magneto Optical Kerr Effect (MOKE), and X-ray Magnetic Circular Dichroism (XMCD). Diffraction patterns show clear indication for growth in the (111) direction on ZnO. Vertical structure analysis by HAXPES depth profiling revealed uniform magnetite thin films on both type of substrates. Both, MOKE and XMCD measurements show in-plane easy magnetization with a reduced magnetic moment in case of the films on ZnO.
Berry curvature plays a crucial role in exotic electronic states of quantum materials, such as intrinsic anomalous Hall effect. As Berry curvature is highly sensitive to subtle changes of electronic band structures, it can be finely tuned via external stimulus. Here, we demonstrate in SrRuO3 thin films that both the magnitude and sign of anomalous Hall resistivity can be effectively controlled with epitaxial strain. Our first-principles calculations reveal that epitaxial strain induces an additional crystal field splitting and changes the order of Ru d orbital energies, which alters the Berry curvature and leads to the sign and magnitude change of anomalous Hall conductivity. Furthermore, we show that the rotation of Ru magnetic moment in real space of tensile strained sample can result in an exotic nonmonotonic change of anomalous Hall resistivity with the sweeping of magnetic field, resembling the topological Hall effect observed in non-coplanar spin systems. These findings not only deepen our understanding of anomalous Hall effect in SrRuO3 systems, but also provide an effective tuning knob to manipulate Berry curvature and related physical properties in a wide range of quantum materials.
We have grown epitaxial thin films of multiferroic BiMnO$_3$ using pulsed laser deposition. The films were grown on SrTiO$_3$ (001) substrates by ablating a Bi-rich target. Using x-ray diffraction we confirmed that the films were epitaxial and the stoichiometry of the films was confirmed using Auger electron spectroscopy. The films have a ferromagnetic Curie temperature ($T_C$) of 85$pm$5 K and a saturation magnetization of 1 $mu_B$/Mn. The electric polarization as a function of electric field ($P-E$) was measured using an interdigital capacitance geometry. The $P-E$ plot shows a clear hysteresis that confirms the multiferroic nature of the thin films.
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