J. J. Peng

Magnetoelectric Coupling Induced by Interfacial Orbital Reconstruction

Reversible orbital reconstruction driven by ferroelectric polarization modulates the magnetic performance of model ferroelectric/ferromagnetic heterostructures without onerous limitations. Mn-d(x2-y2) orbital occupancy and related interfacial exotic magnetic states are enhanced and weakened by negative and positive electric fields, respectively, filling the missing member-orbital in the mechanism of magnetoelectric coupling and advancing the application of orbitals to microelectronics.

Interplay between chemical state, electric properties, and ferromagnetism in Fe-doped ZnO films

Valence state of Fe ions plays an important role in the physical properties of Fe doped ZnO films. Here, a series of Zn1−xFexO films with different Fe concentrations (x = 0, 2.3, 5.4, 7.1, and 9.3 at. %) were prepared to investigate their structural, piezoelectric, ferroelectric, bipolar resistive switching properties, and electrical-control of ferromagnetism at room temperature. The structure characterizations indicate that the chemical state of Fe ions substituting Zn2+ site changes from Fe3+ to Fe2+ with the increase of Fe dopant concentration. We found enhanced piezoelectric and ferroelectric properties in Zn0.977Fe0.023O films with more Fe3+ due to the smaller Fe3+ ionic size in comparison with Zn2+ while the increase of Fe2+ concentration by a larger amount of Fe dopant results in the worse ferroelectric and piezoelectric performance. All Pt/Zn1−xFexO/Pt devices show bipolar resistive switching properties. Especially, devices with lower Fe dopant concentration exhibit better endurance properties due...

Tuning the entanglement between orbital reconstruction and charge transfer at a film surface

The interplay between orbital, charge, spin, and lattice degrees of freedom is at the core of correlated oxides. This is extensively studied at the interface of heterostructures constituted of two-layer or multilayer oxide films. Here, we demonstrate the interactions between orbital reconstruction and charge transfer in the surface regime of ultrathin (La,Sr)MnO3, which is a model system of correlated oxides. The interactions are manipulated in a quantitative manner by surface symmetry-breaking and epitaxial strain, both tensile and compressive. The established charge transfer, accompanied by the formation of oxygen vacancies, provides a conceptually novel vision for the long-term problem of manganites—the severe surface/interface magnetization and conductivity deterioration. The oxygen vacancies are then purposefully tuned by cooling oxygen pressure, markedly improving the performances of differently strained films. Our findings offer a broad opportunity to tailor and benefit from the entanglements between orbit, charge, spin, and lattice at the surface of oxide films.

Charge Transfer and Orbital Reconstruction in Strain-Engineered (La,Sr)MnO3/LaNiO3 Heterostructures.

We investigate charge transfer, orbital reconstruction, and the emergence of exchange bias in (La,Sr)MnO3/LaNiO3 heterostructures. We demonstrate that charge transfer from Mn(3+) ions to Ni(3+) ions is accompanied by the formation of hybridized Mn/Ni 3z(2) - r(2) orbits at the interface, instead of strain-stabilized Mn and Ni x(2) - y(2) orbits in the bulk films. In the heterostructures with ultrathin LaNiO3, orbital reconstruction induced by charge transfer results in magnetization frustration of (La,Sr)MnO3 at the interface. But the strain effect exerted by the growth of the LaNiO3 top layer plays a dominant role on orbital reconstruction in the heterostructures with thick LaNiO3, stabilizing 3z(2) - r(2) orbits. In this case, robust spin glass, associated with larger magnetization frustration, accounts for the exchange bias effect. Our work builds a bridge between the microscopic electronic structure and the macroscopic magnetic property, providing the possibility of manipulating the exotic states with the aid of strain engineering in oxide-based electronics.

Interface-modification-enhanced tunnel electroresistance in multiferroic tunnel junctions

We report a large tunnel electroresistance (TER) effect up to ∼104% in La0.67Sr0.33MnO3/BaTiO3/Co (LSMO/BTO/Co) multiferroic tunnel junctions (MFTJs), which couples with well-defined tunnel magnetoresistance. The large TER is related to (LaAlO3)0.3(LaSrTaO6)0.7 substrates which guarantee a high-quality LSMO/BTO interface and robust ferroelectricity in BTO. The insert of 0.5 nm-thick Pt between the Co electrode and BTO barrier further enhances the TER value to 105% and improves the endurance of the MFTJs, ascribed to the shortened screening length and reduced oxidation of BTO/Co interface. Their use would advance the process towards practical MFTJs with four resistance states.

Electrical control of magnetism in oxides

This review article aims at illustrating the recent progresses in the electrical control of magnetism in oxides with profound physics and enormous potential applications. In the first part, we provide a comprehensive summary of the electrical control of magnetism in the classic multiferroic heterostructures and clarify their various mechanisms lying behind. The second part focuses on the novel route of electric double layer gating for driving a significantly electronic phase transition in magnetic oxides by a small voltage. The electric field applied on the ordinary dielectric oxide in the third part is used to control the magnetic phenomenon originated from the charge transfer and orbital reconstruction at the interface between dissimilar correlated oxides. At last, we analyze the challenges in electrical control of magnetism in oxides, both on mechanism and practical application, which would inspire more in-depth researches and advance the development in this field.

Tilt engineering of exchange coupling at G-type SrMnO3/(La,Sr)MnO3 interfaces

With the recent realization of hybrid improper ferroelectricity and room-temperature multiferroic by tilt engineering, “functional” octahedral tilting has become a novel concept in multifunctional perovskite oxides, showing great potential for property manipulation and device design. However, the control of magnetism by octahedral tilting has remained a challenging issue. Here a qualitative and quantitative tilt engineering of exchange coupling, one of the magnetic properties, is demonstrated at compensated G-type antiferromagnetic/ferromagnetic (SrMnO3/La2/3Sr1/3MnO3) interfaces. According to interfacial Hamiltonian, exchange bias (EB) in this system originates from an in-plane antiphase rotation (a−) in G-type antiferromagnetic layer. Based on first-principles calculation, tilt patterns in SrMnO3 are artificially designed in experiment with different epitaxial strain and a much stronger EB is attained in the tensile heterostructure than the compressive counterpart. By controlling the magnitude of octahedral tilting, the manipulation of exchange coupling is even performed in a quantitative manner, as expected in the theoretical estimation. This work realized the combination of tilt engineering and exchange coupling, which might be significant for the development of multifunctional materials and antiferromagnetic spintronics.

Manipulation of orbital occupancy by ferroelectric polarization in LaNiO3/BaTiO3−δ heterostructures

We investigate the control of orbital occupancy by ferroelectric polarization in LaNiO3/BaTiO3−δ heterostructures. It is achieved by the ferroelectric displacement of Ti ions polarized downward (upward) to form (break) Ni–O–Ti covalent bonding at the interface, accompanied by charge transfer between them and the modulation of the conductivity of ultrathin LaNiO3. This electrically switchable and bi-direction control of orbital polarization in LaNiO3 is up to 25%, different from the manipulation by epitaxial strain or quantum confinement. Our finding opens a window for electrical control of orbital polarization at an oxide interface.

Designing room-temperature multiferroic materials in a single-phase solid-solution film

The search for multiferroic materials with simultaneous ferroelectric and ferromagnetic properties in a single phase at room temperature continues to be fuelled from the perspective of developing multifunctional devices. Here we design a single-phase multiferroic La0.67Sr0.33MnO3-BaTiO3 film, which possesses epitaxial single-crystal and solid-solution structure, high magnetic Curie temperature (~640 K) as well as switchable ferroelectric polarization. Moreover, a notable strain-mediated magnetoelectric coupling at room temperature in the way of modulating the magnetism with an external applied voltage is also observed. The synthetic solid-solution multiferroic film may open an extraordinary avenue for exploring a series of room-temperature multiferroic materials.

Oscillatory exchange bias effect in La0.67Sr0.33MnO3/G-SrMnO3/La0.67Sr0.33MnO3 sandwiches

We investigate the exchange bias effect in La0.67Sr0.33MnO3/G-SrMnO3/ La0.67Sr0.33MnO3 (LSMO/SMO/LSMO) sandwiches, where the spacer is a G-type antiferromagnet. Because of the cycloid bulk spins in the SMO modulated by the top LSMO layer, the sandwich structure exhibits an oscillatory exchange bias behavior as a function of the SMO thickness. This unprecedented phenomenon vanishes in LSMO/SMO bilayers, where the exchange bias saturates as the SMO thickness increases, giving rise to the antiferromagnetic anisotropy constant of SMO, 1.0 –2.0 × 105 erg cm−3. The results provide a broad opportunity to tailor antiferromagnetic moments by interfacial design for antiferromagnet spintronics.