Binbin Chen

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-relaxation-induced crosshatch morphology in epitaxial SrTiO3/NdGaO3 thin films

We investigate the strain relaxation and surface morphology of epitaxial SrTiO3 (STO) films grown on (001)O and (110)O planes of orthorhombic NdGaO3 (NGO), and (001) plane of cubic (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates. Although the average lattice mismatches are similar, strikingly regular crosshatched surface patterns can be found on STO/NGO(001)O[(110)O] films, contrary to the uniform surface of STO/LSAT(001). Based on the orientation and thickness dependent patterns and high-resolution x-ray diffractions, we ascribe the crosshatch morphology to the anisotropic strain relaxation with possibly the 60° misfit dislocation formation and lateral surface step flow in STO/NGO films, while an isotropic strain relaxation in STO/LSAT. Further, we show that the crosshatched STO/NGO(110)O surface could be utilized as a template to modify the magnetotransport properties of epitaxial La0.6Ca0.4MnO3 films. This study highlights the crucial role of symmetry mismatch in determining the surface morphology of the perovskite oxide films, in addition to their epitaxial strain states, and offers a different route for designing and fabricating functional perovskite-oxide devices.

Controlling the sharpness of metal-insulator transition in epitaxial (La1−xPrx)0.67Ca0.33MnO3 (0 ≤ x ≤ 0.35) films

We report that epitaxial strain and chemical doping can be used cooperatively to tune the sharpness of metal-insulator transition (MIT) in epitaxial (La1−xPrx)0.67Ca0.33MnO3 (LPCMO) films. Compared to multiple MITs in anisotropically strained LPCMO/(LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7(001)C (LSAT) films with a phase-separated ground state, the lattice-matched LPCMO/NdGaO3(110)Or (NGO) films show a sharp MIT near the Curie temperature (TC), with a ferromagnetic-metallic ground state. The sharpness of MIT, as evaluated by the temperature coefficient of resistance (TCR), can be two times larger in LPCMO/NGO films than in LPCMO/LSAT films. Moreover, for LPCMO/NGO films, TCR greatly relies on the Pr doping level x, where a maximum TCR value of 88.17% K−1 can be obtained at x = 0.25, but shows less dependence on the film thicknesses. These results suggest that the combination of epitaxial strain and chemical doping could be employed to control not only the ground state of the manganite films, but the sharpness of MIT...

Antiferromagnetic interlayer exchange coupling in all-perovskite La0.7Sr0.3MnO3/SrRu1-xTixO3 superlattices

An unambiguous antiferromagnetic interlayer exchange coupling (IEC) is realized in all-perovskite oxide La0.7Sr0.3MnO3(LSMO)/SrRu1-xTixO3(SR1-xTxO) (x < 0.3) superlattices above the Curie temperature of the SR1-xTxO spacer layer, which is different from the traditional interfacial antiferromagnetic coupling. For 0.3 ≤ x ≤ 0.5, the superlattices behave as a ferromagnetic coupling. Meanwhile, this antiferromagnetic IEC between the ferromagnetic LSMO layers across the SR1-xTxO spacer can be further modulated by changing the thickness of SR1-xTxO spacer. Combining the high Curie temperature of LSMO, these findings may have potential applications in future spintronic devices.

Structural and electrical properties of epitaxial perovskite CaIr1−xRuxO3 thin films

The 4d and 5d transition metal oxides have exhibited a wide spectrum of attracting phenomena. Here, we demonstrate modulations on crystal structure, electrical properties, and spin-orbit couplings (SOCs) in perovskite CaIr1−xRuxO3 (CIRO, 0 ≤ x ≤ 1) thin films. First, all our coherently-grown CIRO films exhibit the orthorhombic perovskite structure, of which the orthorhombicity is tuned by the doping level x. This is different from the post-perovskite structure that is usually observed in the bulk CaIrO3. Second, the metal-semiconductor transition in the perovskite CIRO layers is triggered by changing either the doping level or the layer thickness. Our data suggest the important roles of SOC and disorders in determining the electrical properties in perovskite CIRO layers. Third, the sign reversal of Hall coefficient in CIRO films reveals the complex evolution of electronic structure depending on x and temperature, suggesting the perovskite CIRO films as a new accessible platform for investigating rich physics in 4d and 5d transition metal oxides.The 4d and 5d transition metal oxides have exhibited a wide spectrum of attracting phenomena. Here, we demonstrate modulations on crystal structure, electrical properties, and spin-orbit couplings (SOCs) in perovskite CaIr1−xRuxO3 (CIRO, 0 ≤ x ≤ 1) thin films. First, all our coherently-grown CIRO films exhibit the orthorhombic perovskite structure, of which the orthorhombicity is tuned by the doping level x. This is different from the post-perovskite structure that is usually observed in the bulk CaIrO3. Second, the metal-semiconductor transition in the perovskite CIRO layers is triggered by changing either the doping level or the layer thickness. Our data suggest the important roles of SOC and disorders in determining the electrical properties in perovskite CIRO layers. Third, the sign reversal of Hall coefficient in CIRO films reveals the complex evolution of electronic structure depending on x and temperature, suggesting the perovskite CIRO films as a new accessible platform for investigating rich phys...

Multilevel control of the metastable states in a manganite film

For high density memory applications, the dynamic switching between multilevel resistance states per cell is highly desirable, and for oxide-based memory devices, the multistate operation has been actively explored. We have previously shown that for La2/3Ca1/3MnO3 films, the antiferromagnetic charge-ordered-insulator (COI) phase can be induced via the anisotropic epitaxial strain, and it competes with the doping-determined ferromagnetic-metal (FMM) ground state in a wide temperature range. Here, we show that for the phase competitions, in various magnetic fields and/or thermal cycling, the reappearance of the COI phase and thus the resistance and magnetization can be manipulated and quantified in a multilevel manner at lower temperatures. Furthermore, by using a high-field magnetic force microscope, we image the COI/FMM domain structures in accordance with the transport measurements, and find that the evolving domains or the phase fraction ratios do underline the metastability of the reappeared COI droplets, possibly protected by the energy barriers due to accommodation strain. These results may add new insights into the design and fabrication of future multilevel memory cells.For high density memory applications, the dynamic switching between multilevel resistance states per cell is highly desirable, and for oxide-based memory devices, the multistate operation has been actively explored. We have previously shown that for La2/3Ca1/3MnO3 films, the antiferromagnetic charge-ordered-insulator (COI) phase can be induced via the anisotropic epitaxial strain, and it competes with the doping-determined ferromagnetic-metal (FMM) ground state in a wide temperature range. Here, we show that for the phase competitions, in various magnetic fields and/or thermal cycling, the reappearance of the COI phase and thus the resistance and magnetization can be manipulated and quantified in a multilevel manner at lower temperatures. Furthermore, by using a high-field magnetic force microscope, we image the COI/FMM domain structures in accordance with the transport measurements, and find that the evolving domains or the phase fraction ratios do underline the metastability of the reappeared COI drople...

Enhanced conductivity and metal–insulator transition of ultrathin CaRuO3 in superlattices

Transport characteristics of CaRuO3(CRO)/SmFeO3(SFO) superlattices are studied as a function of the thickness of CRO (0.8 nm ≤ t CRO ≤ 3.2 nm). An abrupt enhancement of the conductivity is observed on superlattices, although ultrathin CRO film show a very high resistance and SFO single layer is insulating. The superlattices with t CRO between 2.0 and 3.2 nm retain a metallic state. As t CRO decreases to 1.6 nm or even thinner in superlattices, the metallic state turns to insulating state. The metal–insulator transition could be attributed to the comparable scale for the disorder length and the electron travel distance at small t CRO value, which causes a change from weak localization to strong localization.