Dongxing Zheng

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.

Enhanced exchange bias in fully epitaxial Fe3O4/tetragonal-like BiFeO3 magnetoelectric bilayers

Fully epitaxial Fe3O4/BiFeO3 bilayers were deposited on (001) LaAlO3 and SrTiO3 by magnetron sputtering. BiFeO3 remained tetragonal on highly misfit-strain LaAlO3 substrates. Obvious exchange bias with a reciprocal relation to the thickness of ferromagnetic Fe3O4 was observed in the Fe3O4/tetragonal-like BiFeO3 (Fe3O4/T-BiFeO3) bilayers below which is similar to that of Fe3O4/rhombohedral-like BiFeO3 (Fe3O4/R-BiFeO3) bilayers. An unexpected enhancement of exchange bias from in Fe3O4/R-BiFeO3 to in the Fe3O4/T-BiFeO3 bilayer was observed at 3 K, which could be attributed to the enhanced Fe–O–Fe interfacial superexchange (SE) interaction induced by distorted T-BiFeO3.

Crystal-Orientation-Modulated Exchange Bias in Orthorhombic-YMnO3/La0.6Sr0.4MnO3 Multiferroic Heterostructures.

The magnetic properties of the all-oxide multiferroic heterostructures composed of orthorhombic YMnO3 (YMO) with E-type antiferromagnetic and double-exchange ferromagnetic (FM) La0.6Sr0.4MnO3 (LSMO) were studied. An orientation-modulated exchange bias effect, which is related to the interfacial Mn-O-Mn bond angle, was discovered. Because of the large bond angle in YMO/LSMO(100) heterostructures, a strong exchange coupling at the interface is formed. This strong exchange coupling sustains an FM phase in YMO at the interface region. The FM phase with strong magnetocrystalline anisotropy contributes to the vertical shift and exchange bias effect in (100) orientation heterostructures. When LSMO (110) and (111) were layered with YMO, the Mn-O-Mn bond angle was reduced, leading to a weakened exchange coupling at the interface, and only a relatively small exchange bias at low temperatures was visible.

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.

Oxygen vacancies influenced interfacial coupling effect in epitaxial Fe2.6V0.4O4/BiFeO3 multiferroic heterostructures

Fully epitaxial Fe2.6V0.4O4(FVO)/BiFeO3(BFO) heterostructures were deposited on SrTiO3(001) substrates by magnetron sputtering. The microstructure of the heterostructures was studied using high-resolution transmission electron microscopy (HRTEM). In order to investigate the oxygen-vacancies?influenced interfacial coupling, the FVO layers were deposited on BFO films with various oxygen partial pressures. With the increase of oxygen vacancy contents, the exchange bias effects of the FVO/BFO heterostructures were significantly suppressed, showing weakened interfacial coupling. The possible mechanism is that the oxygen vacancies weaken the interface Fe3+(A)?O2??Fe3+(BFO) bonds by decreasing the bond angle.

Anomalous thickness-dependent exchange bias effect in Fe3O4/YMnO3 multiferroic heterostructures

Full-oxide epitaxial heterostructures consisting of multiferroic hexagonal YMnO3 and ferromagnetic Fe3O4 with variable thicknesses were fabricated by magnetron sputtering. A parabolic-like exchange bias (EB) effect varying with the thickness of Fe3O4 was discovered, while the EB effect in Fe/YMnO3 and Fe3O4/BiFeO3 heterostructures follows a reciprocal relation (, where is the thickness of FM layer). The parabolic-like EB effect is considered to be associated with the joint effect from the special uncompensated moment at the antiferromagnetic domain walls in YMnO3 and the interactions among antiphase domains (APDs) in Fe3O4. By changing , the strength of the interaction among APDs can be modulated, giving rise to the anomalous thickness-dependent EB effect.

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 3z2 − r2 and low energy x2 − y2 orbitals, which makes electrons occupy the localized x2 − y2 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.

Electric field modulated conduction mechanism in Al/BaTiO3/La0.67Sr0.33MnO3 heterostructures

Mediating a metastable state is a promising way to achieve a giant modulation of physical properties in artificial heterostructures. A metastable state La0.67Sr0.33MnO3 (LSMO) layer suffering tensile strain was grown on MgO substrates. Incorporating with the ferroelectric BaTiO3 (BTO) layer, an accumulation or depletion state controlled by electric fields can be formed at the BTO/LSMO interface, which drives a switching of the conduction mechanism between space charge limited conduction and Poole-Frenkel emission, corresponding to the low and high resistance states. Our results lighten an effective way for electric-field modulated resistance states in multiferroic magnetoelectric 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.

Uniaxial strain tuning of the Verwey transition in flexible Fe3O4/muscovite epitaxial heterostructures

Fe3O4 with high Curie temperature and 100% spin polarization is a potential candidate for practical applications in flexible spintronics. In this work, combined with flexible muscovite substrates, the dynamic strain influenced Verwey transition of Fe3O4 has been studied. The Verwey transition temperature increases (decreases) in the inward (outward) bending heterostructures. From the analyses of Fe L2,3 edge X-ray absorption spectroscopy, the reversible modulation originates from the charge reconstruction effect with the valence variations of Fe ions on the tetrahedral site and octahedral site in different bending states. Meanwhile, the charge reconstruction effect enhances the net magnetic moments of the Fe3O4/muscovite heterostructures in bending states.Fe3O4 with high Curie temperature and 100% spin polarization is a potential candidate for practical applications in flexible spintronics. In this work, combined with flexible muscovite substrates, the dynamic strain influenced Verwey transition of Fe3O4 has been studied. The Verwey transition temperature increases (decreases) in the inward (outward) bending heterostructures. From the analyses of Fe L2,3 edge X-ray absorption spectroscopy, the reversible modulation originates from the charge reconstruction effect with the valence variations of Fe ions on the tetrahedral site and octahedral site in different bending states. Meanwhile, the charge reconstruction effect enhances the net magnetic moments of the Fe3O4/muscovite heterostructures in bending states.