Angular-dependent magnetoresistance modulated by interfacial magnetic state in Pt/LSMO heterostructures.

The interfaces between heavy metals and antiferromagnetic materials have garnered significant attention due to their interesting physical properties. La0.35Sr0.65MnO3 (LSMO), as a typical manganite, exhibits an antiferromagnetic ground state that can be controlled through epitaxial strain and interfacial spin-orbit coupling. In this work, we reported the diverse magnetoresistance, influenced by the interfacial magnetic state, in Pt (3 nm)/LSMO (6-20 nm) heterostructures. The strong spin-orbit coupling of Pt and Dzyaloshinskii-Moriya interaction alter the spin structure and enhance the electron scattering at the Pt/LSMO interface, resulting in positive magnetoresistance. The interfacial angular-dependent magnetoresistance modulated by the interfacial magnetic states was also observed in the Pt/LSMO (20 nm) heterostructures. Our findings contribute to a broader understanding of interfacial properties between heavy metals and antiferromagnetic manganites.

Electrically and optically erasable non-volatile two-dimensional electron gas memory.

The high-mobility two-dimensional electron gas (2DEG) generated at the interface between two wide-band insulators, LaAlO3 (LAO) and SrTiO3 (STO), is an extensively researched topic. In this study, we have successfully realized reversible switching between metallic and insulating states of the 2DEG system via the application of optical illumination and positive pulse voltage induced by the introduction of oxygen vacancies as reservoirs for electrons. The positive pulse voltage irreversibly drives the electron to the defect energy level formed by the oxygen vacancies, which leads to the formation of the insulating state. Subsequently, the metallic state can be achieved via optical illumination, which excites the trapped electron back to the 2DEG potential well. The ON/OFF state is observed to be robust with a ratio exceeding 106; therefore, the interface can be used as an electrically and optically erasable non-volatile 2DEG memory.

Room temperature spontaneous valley polarization in two-dimensional FeClBr monolayer.

The valley degrees of freedom of Bloch electrons provide a proper platform to realize information storage and processing. Using first principles calculations, we propose that the FeClBr monolayer is a ferromagnetic semiconductor with spontaneous valley polarization owing to the combined effect of magnetic exchange interaction and spin-orbit coupling effect. The FeClBr monolayer shows perpendicular magnetic anisotropy, a high Curie temperature of 651 K and a large valley splitting of 188 meV, which are beneficial for the practical applications in valleytronics. Then, the anomalous valley Hall effect can be realized under an in-plane electrical field due to the valley-contrasting berry curvature. According to the optical selectivity rule, the different valleys at K and K- points in momentum space can be excited by the circularly polarized light in honeycomb structures; however, the FeClBr monolayer can also respond to the linear light. Therefore, the valley degree of freedom of the FeClBr monolayer can be modulated by circularly polarized light, linear light and hole doping. Our work enriches the library of valley materials and provides a candidate for the study of spintronics and valleytronics field.

Two-Dimensional Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) Monolayers: Half-Metallic Ferromagnets with Tunable Magnetic Properties under Strain.

Two-dimensional (2D) ferromagnetic materials with high spin polarization are highly desirable for spintronic devices. 2D Janus materials exhibit novel properties due to their broken symmetry. However, the electronic structure and magnetic properties of 2D Janus magnetic materials with high spin polarization are still unclear. Inspired by the successful synthesis of a ferromagnetic FeCl2 monolayer and 2D Janus MoSSe and WSSe, we systematically study the electronic structure and magnetic properties of Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) monolayers. Based on the Goodenough-Kanamori-Anderson theory, the ferromagnetism stems from the superexchange interaction mediated by Fe-X/Y-Fe bonds. The band gaps of spin-up channels are large enough (>4 eV) to prevent spin flipping, which is beneficial for spintronic devices. Additionally, the sizable magnetocrystalline anisotropy energy (MAE) indicates that Janus FeXY monolayers are suitable for information storage. More importantly, the half-metallic character is still kept in Janus FeXY monolayers, and their magnetic properties are enhanced by the biaxial compressive strain. The MAE of FeClI and FeBrI increases by 1 order of magnitude, and the Curie temperature of FeXY monolayers enhances by 100%. These results provide an example of the 2D Janus half-metallic materials and enrich the 2D magnetic material library.

Engineering Co Vacancies for Tuning Electrical Properties of p-Type Semiconducting Co3O4 Films.

Spinel oxide Co3O4 has attracted more and more attention for energy- and environment-related applications. In order to tune the electrical properties of Co3O4, p-type semiconducting Co3O4 films were fabricated on the Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT), MgAl2O4 (MAO), and SrTiO3 substrates by reactive magnetron sputtering. The Co3O4 film on the MAO substrate exhibits perfect epitaxial growth. However, the Co3O4 film on the PMN-PT substrate presents dislocation defects between the [011] and [112] orientations. The special ferroelectric domain shape surface and phase transition of the PMN-PT substrate induce the higher concentration of Co vacancies in the Co3O4 film, which further reduce the resistivity by several orders of magnitude. The calculated results indicate that introducing Co vacancies can enhance the electrical properties of Co3O4 by building impurity levels near the Fermi level, which is beneficial to form free-moving holes in the valence band. The free-moving holes can also be accumulated/dissipated by the ferroelectric field effect of PMN-PT substrates, leading to upward/downward bending of conduction, valence bands, and low/high-resistance states. This work helps us to tune and improve the electrical properties of Co3O4.

Berry Phase Engineering in SrRuO3/SrIrO3/SrTiO3 Superlattices Induced by Band Structure Reconstruction.

The Berry phase, which reveals the intimate geometrical structure underlying quantum mechanics, plays a central role in the anomalous Hall effect. In this work, we observed a sign change of Berry curvatures at the interface between the ferromagnet SrRuO3 (SRO) layer and the SrIrO3 (SIO) layer with strong spin-orbit coupling. The negative Berry curvature at the interface, induced by the strongly spin-orbit-coupled Ir 5d bands near the Fermi level, makes the SRO/SIO interface different from the SRO layer that has a positive Berry curvature. These opposite Berry curvatures led to two anomalous Hall effect (AHE) channels with opposite signs at the SRO/SIO interface and in the SRO layer, respectively, resulting in a hump-like feature in the Hall resistivity loop. This observation offers a straightforward explanation of the hump-like feature that is usually associated with the chiral magnetic structure or magnetic skyrmions. Hence, this study provides evidence to oppose the widely accepted claim that magnetic skyrmions induce the hump-like feature.

Electric field induced reversal of spin polarization, magnetic anisotropy and tailored Dzyaloshinskii-Moriya interaction in underoxidized SrRuO3/SrTiO3 heterostructures.

Electric field tailored magnetic properties of the perovskite-type oxide heterostructures are important in spintronic devices with low energy consumption and small size. Here, the electric field modulated magnetic properties of underoxidized SrRuO3 (SRO)/SrTiO3 (STO) heterostructures are investigated using first-principles calculations. The spin polarization of underoxidized SRO/STO heterostructures turns from negative to positive as the electric field changes from -0.2 to 0.2 V nm-1. The underoxidized SRO/STO heterostructure with 7 SRO atomic layers turns from perpendicular magnetic anisotropy to in-plane magnetic anisotropy as the electric field turns from -0.2 to 0.2 V nm-1, which can be attributed to the in-plane dx2-y2 and out-of-plane dxz, dyz orbitals. The Dzyaloshinskii-Moriya interaction of underoxidized SRO/STO heterostructures can also be effectively tailored using an electric field. These results indicate that the use of electric field is an effective method to modulate magnetic properties of perovskite-type oxide heterostructures, which is beneficial for the development of the high-performance spintronic devices.

Ionic Liquid Gating and Phase Transition Induced Semiconducting to Metallic Transition in La0.67Sr0.33MnO3/BaTiO3 Heterostructures.

The strongly correlated La0.67Sr0.33MnO3 (LSMO) thin films were deposited on the BaTiO3 (BTO) substrates to study the influence of temperature-dependent phase transition of BTO on the magnetic and electric transport properties of the LSMO films. Because of the large lattice mismatch between the LSMO and BTO substrates, the ultrathin LSMO film exhibits semiconducting transport behavior rather than the normal metal-insulator transition behavior. A Mn2+-O2--Mn3+ dominated double exchange interaction, which leads to a semiconducting to metallic transition behavior, was formed by the joint effect of the ion-liquid gating and phase transition of the BTO substrates. Our work shows a special way to modulate the conduction mechanism, which opens up a new field for further study of the magnetoelectric coupling of the LSMO/BTO heterostructures.

Ferroelectric resistance switching in Pt/Fe/BiFeO3/SrRuO3/SrTiO3 heterostructures.

BiFeO3 (BFO)-based heterostructures have been widely studied to develop high-speed, high-density and low-consumption nonvolatile memory. In this study, the resistive switching (RS) behavior in metal/BFO/SrRuO3 (SRO) heterostructures was investigated. The I-V curves of Pt/Fe/BFO/SRO and Pt/BFO/SRO heterostructures demonstrate that the RS behavior in the Pt/Fe/BFO/SRO heterostructures results from the fact that ferroelectric polarization modulated the depletion layer width around the BFO/SRO interface. According to the fitting results of the I-V curves, the conductivity mechanisms are the interface-limited Fowler-Nordheim tunneling mechanism in the negative bias and the space-charge-limited conduction mechanism in the positive bias. Compared with the memory performance in the Pt/BFO/SRO heterostructures, the memory performance in the Pt/Fe/BFO/SRO heterostructures evidently improved. The Fe layer with a work function similar to that of the BFO layer can decrease the barrier height and reduce the accumulation of the injected charges at the top-electrode/BFO interface, which further improves the ferroelectric performance of the BFO layer.

Electric-field-mediated magnetic properties of all-oxide CoFe2O4/La0.67Sr0.33MnO3/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures.

Electric-field-mediated magnetic properties were investigated in CoFe2O4/La0.67Sr0.33MnO3/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (CFO/LSMO/PMN-PT) heterostructures. The butterfly-like behavior of the magnetization under different electric fields indicates that the strain effect plays a critical role in the electric-field-mediated magnetic properties, leading to an increase in magnetization along the [100] direction but a decrease along the [01-1] direction in the CFO/LSMO/PMN-PT heterostructures. More interestingly, due to the large magnetostriction of the CFO layer, the coercivity of the CFO/LSMO/PMN-PT heterostructures decreases ∼50% along the [01-1] direction under the electric fields. The large modulation of the coercivity makes it possible to achieve electric-field-controlled magnetoresistance in the metal/CFO/LSMO/PMN-PT spin filter magnetic tunneling junctions.

Ferroelectric field manipulated nonvolatile resistance switching in Al:ZnO/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures at room temperature.

Resistance switching was obtained in Al:ZnO/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures at room temperature by applying an external electric field. The modulation of the resistance is more pronounced in the thinner samples, indicating that it is an interfacial effect. In addition, the resistance of Al:ZnO films is significantly reduced by the photoexcited carriers when illumination is applied. The results indicate that the carrier density in the Al:ZnO films is modulated under external electric fields, due to the accumulation and depletion of charge at the interface between Al:ZnO and Pb(Mg1/3Nb2/3)0.7Ti0.3O3. Hence, reversible and nonvolatile resistance states can be achieved by the ferroelectric field effect, and it is expected that multilevel storage will be realized.

Ferroelectric Field Effect Tuned Giant Electroresistance in La0.67Sr0.33MnO3/BaTiO3 Heterostructures.

The mediation of metastable state has been approved to be a promising tool to achieve giant modulations of the physical properties in artificial structures. In this work, the metastable state La0.67Sr0.33MnO3 (LSMO) films with the coexistence of two phases were fabricated on the tensile ferroelectric BaTiO3 (BTO) substrates. Upon application of pulse electric fields to the BTO substrates, the oxygen vacancies and charge redistribute and result in giant and volatile electroresistance (∼230%) and normal and nonvolatile electroresistance (∼5%) in the LSMO films, respectively. The observation of binary resistance states is very interesting and totally unexpected because only a normal electroresistance has been reported in the similar LSMO/piezoelectric structures previously. Here, we attribute the binary state performance to the model of oxygen redistribution and charge accumulation/depletion modulated by the ferroelectric field effect. The oxygen redistribution strongly affects the double exchange interaction in the LSMO layer, which leads to the giant and volatile electroresistance. This work indicates that the mediation of metastable states by electric field is a promising way to enrich the physical properties in artificial structures.

Charge-assisted non-volatile magnetoelectric effects in NiFe2O4/PMN-PT heterostructures.

Electric field tuning of magnetization is a viable path to realize magnetoelectric (ME) coupling effect while developing novel multifunctional devices. The combined charge and strain effects can produce multilevel manipulation in multiferroic heterostructures. Both charge and strain co-mediated ME effects were investigated in NiFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (NFO/PMN-PT) and NFO/Pt/PMN-PT heterostructures. The charge effect was detected by the existence of magnetization imparities between the positive and negative fields. The imparities could be enlarged by inserting a Pt buffer layer between the NFO film and PMN-PT substrate. The suppressed depolarization field effect enhanced ferroelectric polarization of PMN-PT, which strengthened the polarization-dependent charge effect in the NFO/Pt/PMN-PT heterostructures. The enhancement of the charge effect helped achieve a stronger ferromagnetic-ferroelectric coupling effect.

High-Performance Photovoltaic Readable Ferroelectric Nonvolatile Memory Based on La-Doped BiFeO3 Films.

Epitaxial La0.1Bi0.9FeO3 (LBFO) films with SrRuO3 (SRO) bottom electrodes were fabricated on SrTiO3(001) substrates by magnetron sputtering. The LBFO thin films exhibit strong ferroelectric properties. Nonvolatile reversible resistance switchings and switchable photovoltaic effects controlled by electric field have been observed in Pt/LBFO/SRO heterostructures. With the optimized LBFO film thickness, the observed room temperature pulsed-read resistance switching ratio can reach 105% magnitude by applying ±2.7 V pulse voltages. Besides, the observed ferroelectric switchable photovoltaic effect in the visible wavelength range shows a large tunable open-circuit photovoltage from -75 to -330 mV. The switching mechanisms in resistance and photovoltaic effects are demonstrated to be directly related to the ferroelectric reversal, which can be attributed to the polarization-modulated interfacial barriers and deep trap states.

Orbital Redistribution Enhanced Perpendicular Magnetic Anisotropy of CoFe3N Nitrides by Adsorbing Organic Molecules.

Organic/ferromagnetic spinterface plays a significant role in organic spintronics and the manipulation of spinterface will help to optimize the performance of molecular devices. Here, we systematically investigate how the magnetic anisotropy can be tailed by adsorbing different organic molecules on CoFe3N surface. It is found that the adsorption of C6H6, C6F6, and SC4H4 molecules on the FeCo-hollow site enhances the perpendicular magnetic anisotropy (PMA) of CoFe3N. The redistribution of Fe/Co d-orbitals near the Fermi level has an important effect on the modulation of PMA. Asymmetric SC4H4 adsorbed system has a larger PMA than symmetric C6H6 and its halide due to the hybridization between S p z and Fe d z2 orbitals instead of C atom. Our results indicate that appropriate organic molecule adsorption can improve the magnetic properties of ferromagnets, which benefits organic spintronic devices.

Self-Poling-Induced Magnetoelectric Effect in Highly Strained Epitaxial BiFeO3/La0.67Sr0.33MnO3-δ Multiferroic Heterostructures.

Highly strained epitaxial BiFeO3/La0.67Sr0.33MnO3-δ (BFO/LSMO) heterostructures were fabricated on LaAlO3 substrates by magnetron sputtering. The as-grown downward self-polarization of BFO capping layers was confirmed by piezoelectric force microscopy. Using the electrostatic field-induced charge screening, a hole depletion state was induced in ultrathin (8 nm) LSMO films. As a result of the interfacial charge coupling, appreciable saturated magnetization (MS) increase of about 500 and 100% can be observed in LSMO with BFO capping at 5 and 300 K, respectively. Besides, LSMO phase translations can be revealed by the BFO thickness-related exchange bias field (HE) and MS of the BFO/LSMO heterostructures. The results established a new approach in achieving interfacial magnetoelectric couplings with thin self-polarized multiferroic layers.

Strain and Ferroelectric-Field Effects Co-mediated Magnetism in (011)-CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Multiferroic Heterostructures.

Electric-field mediated magnetism was investigated in CoFe2O4 (CFO, deposited by reactive cosputtering under different oxygen flow rates) films fabricated on (011)-Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) substrates. Ascribed to the volatile strain effect of PMN-PT, the magnetization of the CFO films decreases along the [01-1] direction whereas it increases along the [100] direction under the electric field, which is attributed to the octahedron distortion in the spinel ferrite. Moreover, a nonvolatile mediation was obtained in the CFO film with low oxygen flow rate (4 sccm), deriving from the ferroelectric-field effect, in which the magnetization is different after removing the positive and negative fields. The cooperation of the two effects produces four different magnetization states in the CFO film with low oxygen flow rate (4 sccm), compared to the only two different states in the CFO film with high oxygen flow rate (10 sccm). It is suggested that the ferroelectric-field effect is related to the oxygen vacancies in CFO films.

Orbital Reconstruction Enhanced Exchange Bias in La0.6Sr0.4MnO3/Orthorhombic YMnO3 Heterostructures.

The exchange bias in ferromagnetic/multiferroic heterostructures is usually considered to originate from interfacial coupling. In this work, an orbital reconstruction enhanced exchange bias was discovered. As La0.6Sr0.4MnO3 (LSMO) grown on YMnO3 (YMO) suffers a tensile strain (a > c), the doubly degenerate eg orbital splits into high energy 3z(2) - r(2) and low energy x(2) - y(2) orbitals, which makes electrons occupy the localized x(2) - y(2) orbital and leads to the formation of antiferromagnetic phase in LSMO. The orbital reconstruction induced antiferromagnetic phase enhances the exchange bias in the LSMO/YMO heterostructures, lightening an effective way for electric-field modulated magnetic moments in multiferroic magnetoelectric devices.

Magnetization and Resistance Switchings Induced by Electric Field in Epitaxial Mn:ZnO/BiFeO3 Multiferroic Heterostructures at Room Temperature.

Electric field induced reversible switchings of the magnetization and resistance were achieved at room temperature in epitaxial Mn:ZnO(110)/BiFeO3(001) heterostructures. The observed modulation of magnetic moment is ∼500% accompanying with a coercive field varying from 43 to 300 Oe and a resistive switching ratio up to ∼10(4)% with the applied voltages of ±4 V. The switching mechanisms in magnetization and resistance are attributed to the ferroelectric polarization reversal of the BiFeO3 layer under applied electric fields, combined with the reversible change of oxygen vacancy concentration at the Mn:ZnO/BiFeO3 interface.

Sign change of magnetoresistance in Gd-doped amorphous carbon granular films.

Gd/C granular films with 11% Gd were fabricated by facing-target magnetron sputtering at room temperature and then annealed at 300-650 °C for 1.5 h. A magnetoresistance of -82% was obtained in the Gd/C films annealed at 650 °C at 3 K under a magnetic field of 50 kOe. A sign change of the magnetoresistance from negative to positive and then back to negative was observed in all samples as the temperature decreases. Grain boundary scattering effects, wave-function-shrinkage, cotunneling and Gd-Gd interactions account for the mechanisms of the magnetoresistance effects in different temperature regions. The sign of the magnetoresistance also varies as the magnetic field increases. At the transition temperature of 25 K, the wave-function-shrinkage effect competes with cotunneling and Gd-Gd interactions at different magnetic fields. The competition between the wave-function-shrinkage effect and the grain boundary scattering effect is approximately at the transition temperature of 100 K. The temperature range of positive magnetoresistance expands and transition temperatures are changed as the annealing temperature increases. It is related to the expansion of the temperature region for the wave-function-shrinkage effect which occurs in the Mott variable range hopping conduction mechanism.