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Fig.1 Schematic diagram of nano checkerboard structure and nanocolumn structure
Figure 2 TEM and HRTEM cross-sectional images of LSMO-NiO nanocomposite films with different microstructures and FFT spots: (a) nano-checkerboard structure, (b) nano-columnar structure.
Fig. 3 (a) Magnetoresistance versus magnetic field of LSMO-NiO nanocomposite films with different compositions, (b) Magnetoresistance versus magnetic field of LSMO:NiO films with 70% NiO nanocolumn structure.
Recently, the Research Group of Magnetic Materials and Magnetics of the Shenyang Institute of Materials Science and Technology, Chinese Academy of Sciences, Wang Zhanjie's research group used pulsed laser deposition to prepare a variety of composite structures of manganese oxide nanoparticles through self-assembled growth mode. Composite film; By controlling the microstructure of the manganese oxide nanocomposite film, a large low-field magnetoresistance effect with adjustable temperature region is achieved.
Among them, composite thin films with a checkerboard nanostructure exhibit a large low-field magnetoresistance effect near room temperature, and thus have a wide application prospect in room temperature magnetoresistive microelectronic components. This research result will play an important role in the research and application of giant magnetoresistance manganese oxide materials.
Magnetoresistance (MR) refers to the phenomenon that the resistance of a material changes with a magnetic field. In recent years, the giant magnetoresistance effect has been widely applied to microelectronic components such as data read heads, magnetic random memories, and magnetic sensors. In the early 90s of last century, people found more than the giant magnetoresistance effect MR value in the doped manganese oxide film, so it is called Colossal magnetoresistance (CMR) effect.
Therefore, manganese oxide materials have attracted wide attention from researchers. A large number of studies have shown that although the intrinsic magnetoresistance of manganese oxide is very large, there are problems such as narrow application temperature range, high applied magnetic field (~3 Tesla), and it has not yet been practically used. In 1996, Hwang et al. found that a polycrystalline perovskite manganese oxide film has a significant magnetoresistance effect at a low temperature far below the Curie temperature under a small applied magnetic field, which is called a low-field magnetoresistance effect (Low -field magnetoresistance, LFMR).
Various methods have been attempted to increase the low field magnetoresistance of manganese oxides, including the artificial formation of grain boundaries, the introduction of defects, and the second equal of high resistance. However, the temperature range of the low-field magnetoresistance effect is mostly at a low temperature of 10-150 K and cannot be applied near room temperature. Therefore, while increasing the low-field magnetoresistance of the manganese oxide film, how to increase its temperature is a key issue that needs to be solved.
In response to this problem, the research group researchers introduced the second phase of NiO in La0.7Sr0.3MnO3 (LSMO), and prepared a nano checkerboard structure through a self-assembled growth mode using pulsed laser deposition (PLD). Composite films with nano-columnar structures (Figures 1 and 2).
Figure 1 is a schematic of two ideal microstructures, where L represents LSMO and N represents NiO. Between the LSMO and NiO phases, the nano-checkerboard structure and the nano-columnar structure are formed. Among them, the size of the NiO phase should be controlled at 1 to 2 nm to form a nanoscale LSMO/NiO/LSMO magnetic tunnel junction.
The magnetic resistance of the composite film is improved by the tunneling resistance of the LSMO/NiO/LSMO and the scattering at the LSMO/NiO interface; by controlling the strain of the LSMO parent phase and the changes of the Curie temperature and the metal-insulator transition temperature caused by the strain, Increasing the low field magnetoresistance while increasing the temperature of the low field magnetoresistance effect.
The reason why anti-ferromagnetic and semi-conductor NiO are selected as the second phase is to consider the following factors: (1) NiO and LSMO have good lattice matching. The researchers of the research group studied the exchange bias phenomenon of LSMO:NiO particle composite film and layered composite film and found that NiO and LSMO have a good crystallographic epitaxial relationship at the NiO/LSMO interface (Journal of Applied Physics, 113 ( (2013), 223903., IEEE Transactions on Magnetics, 50 (2014), 1000304).
(2) Since the ion radius of Ni2+ (0.69Ã…) is much larger than that of Mn3+ and Mn4+ (0.58 Ã… and 0.53 Ã…, respectively), Ni2+ does not replace the Mn3+ and Mn4+ ions and enter the LSMO lattice. In this way, changes in LSMO magnetism, Curie temperature, and metal-insulator transition temperature, etc. due to compositional changes are avoided.
(3) Due to the semiconducting properties of NiO, a relatively high barrier difference can be formed at the LSMO/NiO interface to satisfy the requirement for a high-impedance second phase in the magnetic tunnel junction.
(4) If the size and distribution of NiO can be controlled, it is possible to form a LSMO/NiO/LSMO tunnel junction composed of NiO and LSMO. FIG. 2 is a TEM and HRTEM cross-sectional photograph of the prepared LSMO-NiO composite film having a nano-checkerboard structure and a nano-columnar structure.
The magnetoresistance test results show that the 50% NiO volume ratio of the checkerboard structure of the LSMO-NiO composite film shows a large low field magnetoresistance in the temperature range of 200~300 K (LFMR = ~ at 250 K and 1 T). 17%); 70% NiO LSMO-NiO nanocomposite films with nano-columnar structures show a large low field magnetoresistance in the 10 to 210 K temperature range (LFMR = ~41% at 10 K and 1 T) 3).
By controlling the microstructure of LSMO:NiO nanocomposite film, a large LFMR with adjustable temperature region was realized. The effect of microstructure on the magnetoresistance of the composite film can be explained by an effective circuit model. Its mechanism of action is due to the presence of electron spin scattering at the LSMO/NiO interface and nanoscale LSMO/NiO/LSMO magnetic tunnel junctions in the composite film. Related research results have been published in Advanced Functional Materials, 24 (2014) 5393–5401.
This research work was supported by the Chinese Academy of Sciences' 100-person plan, the Ministry of Science and Technology's "973", the National Natural Science Foundation of China, and the Shenyang Institute of Materials Science (joint) laboratory frontier innovations.
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