Pingfan Chen

Electric-field-modulated nonvolatile resistance switching in VO₂/PMN-PT(111) heterostructures.

The electric-field-modulated resistance switching in VO2 thin films grown on piezoelectric (111)-0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-PT) substrates has been investigated. Large relative change in resistance (10.7%) was observed in VO2/PMN-PT(111) hererostructures at room temperature. For a substrate with a given polarization direction, stable resistive states of VO2 films can be realized even when the applied electric fields are removed from the heterostructures. By sweeping electric fields across the heterostructure appropriately, multiple resistive states can be achieved. These stable resistive states result from the different stable remnant strain states of substrate, which is related to the rearrangements of ferroelectric domain structures in PMN-PT(111) substrate. The resistance switching tuned by electric field in our work may have potential applications for novel electronic devices.

Anisotropic-strain-induced antiferromagnetic-insulating state with strong phase instability in epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films

We grow epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films simultaneously on the lattice-closely-matched substrates, cubic (LaAlO3)0.3(Sr2AlTaO6)0.7 [LSAT(001)] and orthorhombic NdGaO3 [NGO(001) and NGO(110)]. While all as-grown films show a ferromagnetic-metallic (FM) ground state as observed for the bulk target, the annealed films show quite different magnetotransport behavior as follows: on NGO(110) they show a robust FM ground state, on LSAT(001) and NGO(001) however, they show surprisingly a coexisted antiferromagnetic insulating state with high phase instability in a wide temperature range. The phase coexistence being easily induced via the control of anisotropic epitaxial strain suggests that the phase separation in manganites could be elastically driven, and thus can be strain-engineered for devices applications.

Pseudomorphic strain induced strong anisotropic magnetoresistance over a wide temperature range in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films

Strong anisotropic magnetoresistance (AMR) was observed in La0.67Ca0.33MnO3 films grown coherently on the orthorhombic NdGaO3(001) substrates. With an increased orthorhombic lattice distortion due to the pseudomorphic strain, the films show not only a ferromagnetic-metal (FM) transition at TC of ∼265 K, but also the phase coexistence of FM and antiferromagnetic-insulator below ∼250 K. The phase competitions are very sensitive to the magnetic field, and more strikingly, to its orientations with respect to the crystal axes resulting in a large AMR in a broad temperature range, in addition to the conventional one peaked near TC. The films also show uniaxial magnetic anisotropy with the easy axis along the elongated b axis, suggesting that it is the strain induced spin-orbit-lattice coupling and the resultant phase competitions that control the AMR in epitaxial manganite films.

Phase evolution and the multiple metal-insulator transitions in epitaxially shear-strained La0.67Ca0.33MnO3/NdGaO3(001) films

The ferromagnetic-metal (FM) to antiferromagnetic-insulator (AFI) transition, near 250 K, has been induced in epitaxially shear-strained La0.67Ca0.33MnO3/NdGaO3(001) films, although they are doped for a FM ground state. We showed that for these films the phase diagram can feature the five regions of paramagnetic-insulator, FM, AFI dominated, FM dominated, and the frozen state, evolving with decreasing temperatures. And the phase instabilities in the temperature/magnetic-field regime can lead to multiple metal-insulator transitions over the various phase boundaries, in addition to a huge low-field magnetoresistance in the wide temperature range. The results underline that with the elastic-driven phase separation and consequently the complex phase evolution, this epitaxial system could be employed for further understanding of the manganites, and also for thin-film devices applications.

All-oxide–based synthetic antiferromagnets exhibiting layer-resolved magnetization reversal

Making an oxide-layered antiferromagnet Antiferromagnetism, a state of matter where ordered neighboring spins point in opposite directions, can be engineered in layered heterostructures, which affords control over their properties. Doing so in oxide heterostructures is tricky because the necessary ferromagnetism of the constituent layers may not survive thinning to nanometer thicknesses. Chen et al. overcame this materials challenge by finding and growing the right combination of substrate, magnetic, and insulating layers to engineer antiferromagnetic coupling. The resulting superlattices, consisting of alternating layers of a ferromagnetic oxide and an insulating material, exhibit layer-by-layer switching of magnetization. Science, this issue p. 191 Superlattices made of layers of ferromagnetic La2/3Ca1/3MnO3 and insulating CaRu1/2Ti1/2O3 show antiferromagnetic coupling. Synthesizing antiferromagnets with correlated oxides has been challenging, owing partly to the markedly degraded ferromagnetism of the magnetic layer at nanoscale thicknesses. Here we report on the engineering of an antiferromagnetic interlayer exchange coupling (AF-IEC) between ultrathin but ferromagnetic La2/3Ca1/3MnO3 layers across an insulating CaRu1/2Ti1/2O3 spacer. The layer-resolved magnetic switching leads to sharp steplike hysteresis loops with magnetization plateaus depending on the repetition number of the stacking bilayers. The magnetization configurations can be switched at moderate fields of hundreds of oersted. Moreover, the AF-IEC can also be realized with an alternative magnetic layer of La2/3Sr1/3MnO3 that possesses a Curie temperature near room temperature. The findings will add functionalities to devices with correlated-oxide interfaces.

High-TC ferromagnetic order in CaRuO3/La2/3Ca1/3MnO3 superlattices

Ferromagnetic-metallic ground state with high Curie temperature (TC) of 200–258 K has been observed in CaRuO3/La2/3Ca1/3MnO3 (CRO/LCMO) superlattices with the ultrathin LCMO layer of 0.8–3.2 nm thick. This contradicts the antiferromagnetic or low-TC insulating ground state observed in single-layer LCMO thin-films. TC and the saturated magnetization of the superlattices are determined dominantly by the LCMO layer thicknesses, indicating no direct magnetic contribution from the CRO layers or the interfaces. Also, they are less sensitive to the growth oxygen pressure as compared to the pure LCMO films. We ascribe the stabilized, bulklike ferromagnetism in the ultrathin LCMO layer to charge transfer from CRO at the interfaces, which could enhance the double-exchange and meanwhile suppress the phase separation, contrary to the case for LCMO thin-films. This interface engineering that can greatly depress the notorious “dead layer” in manganites might be significant in designing the correlated spintronic devices.

Annealing assisted substrate coherency and high-temperature antiferromagnetic insulating transition in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films

Bulk La0.67Ca0.33MnO3 (LCMO) and NdGaO3 (NGO) have the same Pbnm symmetry but different orthorhombic lattice distortions, yielding an anisotropic strain state in the LCMO epitaxial film grown on the NGO(001) substrate. The films are optimally doped in a ferromagnetic-metal ground state, after being ex-situ annealed in oxygen atmosphere, however, they show strikingly an antiferromagnetic-insulating (AFI) transition near 250 K, leading to a phase separation state with tunable phase instability at the temperatures below. To explain this drastic strain effect, the films with various thicknesses were ex-situ annealed under various annealing parameters. We demonstrate that the ex-situ annealing can surprisingly improve the epitaxial quality, resulting in the films with true substrate coherency and the AFI ground state. And the close linkage between the film morphology and electronic phase evolution implies that the strain-mediated octahedral deformation and rotation could be assisted by ex-situ annealing, and moreover, play a key role in controlling the properties of oxide heterostructures.

Contrasting size-scaling behavior of ferromagnetism in La0.67Ca0.33MnO3 films and La0.67Ca0.33MnO3/CaRuO3 multilayers

Using La0.67Ca0.33MnO3 (LCMO) and CaRuO3 (CRO) as components, the single-layer films, bilayers, trilayers, and superlattices were fabricated on NdGaO3 (110) substrates. These epitaxial structures show quite different Curie temperature (TC) depending on the LCMO layer thickness (x), especially in the low x region. For LCMO films, TC dramatically decreases with x and disappears below 3.2 nm, as previously reported. For LCMO/CRO (CRO/LCMO) bilayers, however, a smooth decline of TC was observed, retaining a TC near 50 K at 1.6 nm. More strikingly, for the multilayers with LCMO sandwiched between CRO, TC is stabilized at ∼250 K even at x of 1.6 nm, before decreasing to 200 K at 0.8 nm. We ascribed these distinct behaviors to the LCMO/CRO interfaces, and a possible charge transfer from CRO to LCMO was suggested to play a vital role in stabilizing the ferromagnetism in ultrathin LCMO. This finding would shed some lights on the dead layer formation in ultrathin manganites and be significant in improving the perfo...

Anisotropic-strain-controlled metal-insulator transition in epitaxial NdNiO3 films grown on orthorhombic NdGaO3 substrates

NdNiO3 (NNO) films were grown by pulsed laser deposition on orthorhombic (110)-, (001)-, and (100)-oriented NdGaO3 substrates. It is found that all the films are tensile-strained but show dramatically different metal-insulator transition (MIT) temperatures (TMI) (160–280 K), as compared with the NNO bulk (∼200 K). A high resemblance in the sharpness of MIT and lattice variation across the MIT was observed. The TMI is highly dependent on the magnitude of the orthorhombic distortion induced by the different substrate surface plane and tends to recover the bulk value after annealing. Our results suggest that the anisotropic epitaxial strain can effectively tune the MIT of NNO films, and the NiO6 octahedra rotation and deformation involved in accommodating the tensile strain might cause the different TMI.

Anisotropic resistivities in anisotropic-strain-controlled phase-separated La0.67Ca0.33MnO3/NdGaO3(100) films

The anisotropic resistivities (AR) in La0.67Ca0.33MnO3 films grown on orthorhombic NdGaO3(100) substrates were investigated. In this epitaxial system, the large anisotropic misfit strain was demonstrated to induce an antiferromagnetic insulator (AFI) phase transition near ∼250 K, leading to phase separation (PS) with the coexistence of AFI and ferromagnetic-metal (FM) phases at the temperature below. In the PS regime, the resistivity measured along the highly strained b-axis is greater than that along c-axis, giving rise to a huge AR. It can reach ∼12 500% and shows strong dependence on the amplitude and orientation of the magnetic field. We ascribed this unusual AR to the anisotropic-strain-controlled MnO6 octahedral deformations which can organize the competing AFI and FM phases into orientation-preferred PS pattern, thus resulting in the anisotropic percolative transport.