The competition among different phases in perovskite manganites is pronounced since their energies are very close under the interplay of charge, spin, orbital and lattice degrees of freedom. To reveal the roles of underlying interactions, many efforts have been devoted towards directly imaging phase transitions at microscopic scales. Here we show images of the charge-ordered insulator (COI) phase transition from a pure ferromagnetic metal with reducing field or increasing temperature in a strained phase-separated manganite film, using a home-built magnetic force microscope. Compared with the COI melting transition, this reverse transition is sharp, cooperative and martensitic-like with astonishingly unique yet diverse morphologies. The COI domains show variable-dimensional growth at different temperatures and their distribution can illustrate the delicate balance of the underlying interactions in manganites. Our findings also display how phase domain engineering is possible and how the phase competition can be tuned in a controllable manner.

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Evolution and control of the phase competition morphology in a manganite film.

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.

Annealing assisted substrate coherency and high-temperature antiferromagnetic insulating transition in epitaxial La 0.67 Ca 0.33 MnO 3 /NdGaO 3 (001) films

Magneto-resistive anisotropy (AMR) has been studied in 45 nm thick La0.7−xPrxCa0.3MnO3 (LPCMO) manganite films (with Pr doping x between 0 and 0.40) deposited on LaAlO3 (LAO) and SrTiO3 (STO) substrates. The AMR in compressively strained films undergoes a sign change from positive to negative at low temperatures, whereas its sign does not change in films subjected to tensile strain. Temperature dependence of magnetization in a magnetic field applied parallel and perpendicular to the (100)-plane of the films shows that at low temperatures strain-induced rotation of the easy-axis magnetization determines the sign of the AMR. At higher temperatures near the TMI the sign of the AMR is the same in both LPCMO/LAO and LPCMO/STO films, suggesting the dominating influence of percolative transport in the plane of these films at these temperatures.

Mechanism of sign crossover of the anisotropic magneto-resistance in La0.7−xPrxCa0.3MnO3 thin films

In-plane angular dependent magneto-resistance has been studied in La0.3Pr0.4Ca0.3MnO3 (LPCMO) manganite thin films deposited on the (100) oriented NdGaO3, and (001) oriented SrTiO3 and LaAlO3 substrates. At temperatures where the electronic phase separation is the strongest, a metastable irreversible state exists in the films whose resistivity ρ attains a large time dependent value. The ρ decreases sharply with an increasing angle θ between the magnetic field and the current, and does not display an expected oscillatory cos2θ/sin2θ dependence for all films. The regular oscillations are recovered during repetitive sweeping of θ between 0° and 180°. We discuss possible factors that could produce these unusual changes in the resistivity.

Recovery of oscillatory magneto-resistance in phase separated La0.3Pr0.4Ca0.3MnO3 epitaxial thin films

We studied the correlation between the charge carrier localization and the anisotropic magneto-resistance (AMR) of La0.7−xPrxCa0.3MnO3 (0.00≤x≤0.35) epitaxial thin films deposited on SrTiO3 and LaAlO3 substrates. An increase in the carrier localization, as evidenced by the rise of the small polaron activation energy Ea in the paramagnetic state, has been achieved either by increasing the dopant concentration x on the A-site or by decreasing the thickness (increasing the epitaxial lattice strain) in the film. The relationship between the AMRmax (the magnitude of AMR at the maximum near the metal-insulator temperature) and Ea exhibits a trend that is independent of the thickness of the film, the substrate, and the doping concentration x. This implies that knowledge of the value of Ea measured in a film could be sufficient to accurately predict its AMRmax value.

Influence of A-site doping and strain on the relationship between the anisotropic magneto-resistance and charge localization in films of La0.7−xPrxCa0.3MnO3 manganites

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.

G. M. Li

Papers

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