Jan 08,2016|By
Owing to various coupled interactions in the colossal magneto-resistive manganese oxide, the competing thermodynamic phases can coexist. Studying their evolution behavior in real space can help understand the roles these interactions play in the phase separation. Therefore, the real space observation of the phase separation has been the hot topic in the field. Because fairly strong magnetic field is necessary to drive the transitions among the phases and this type of instrument is rather uncommon, the entire phase separation loop from charge order state to its re-appearance has not been observed in manganite. After the intimate collaboration of more than one year between the groups led by LU Qingyou and WU Wenbin, High Magnetic Field Laboratory, Chinese Academy of Sciences and University of Science and Technology of China, this complete process has been successfully observed in a type of anisotropic strain regulated phase separation manganite film using a homebuilt 20 T strong field magnetic force microscope. This work has been published on Nature Communications (6:8980) with the title “Evolution and Control of the Phase Competition Morphology in a Manganite Film”.
A bulk La0.67Ca0.33MnO3is an optimallydoped ferromagnetic metal, but they have found that special phase competition can be induced in the post-annealtreatedLa0.67Ca0.33MnO3 film on NdGaO3 (001) due to the anisotropic strain from the substrate. Below the ferromagnetic phase transition temperature TC, new ferromagnetic-antiferromagnetic transition (TAFI) and antiferromagnetic-ferromagnetic transition at even lower temperature (TC’) appear. From the transport measurement, the resistivity curve as a function of temperature shows large hysterisis below TAFI, suggesting the existence of phase separation state below TAFI. In the phase diagram, this type of phase separation can be further divided into charge order insulator dominant phase separation (COI-PS) and ferromagnetic metal dominant phase separation (FMM-PS), which have different curves of resistivity vs. magnetic field (？-H), as seen in Fig.1b. In the process of field increasing, because the COI gradually transforms into the FMM, resistivity drops at the two temperatures 10 and 150 K. When the field is reduced after the FMM saturates, the COI will re-appear at 150 K, but will not at 10 K.
This type of COI re-appearance in the process of field reduction has also been ovserved in other manganites with COI being the ground state, such as Pr1-xCaxMnO3, Nd1/2Sr1/2MnO3 and LaPrCaMnO3, but the observation was either in zero field or focusing on the process of field increase. Therefore, the details of COI re-appearance in decreasing field have been totally unknown before. This process is nevertheless very important because it can provide us with more information on the mutual transitions between FMM and COI.
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Fig.1. Transport measurements of La0.67Ca0.33MnO3/NdGaO3(001)film. |
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Fig.2. The evolution and control of the morphology of the re-appeared COI in La0.67Ca0.33MnO3/NdGaO3(001) film at different temperatures as a result of decreasing the magnetic field. |
Using a homebuilt high field magnetic force microscope (Ultramicroscopy 147, 133(2014)), they have systematically imaged the sample’s phase separation behaviors, particularly the re-appearance of COI at decreasing magnetic field. It shows that microscopically the re-appearance process of COI is totally different from the COI melting process in increasing magnetic field and astonishingly unique yet diverse morphologies (Fig.2) are observed.
The re-appeared COI domains are in droplets, stripes and puddles at 230, 190 and 130 K, respectively. The domain size of COI increases when the temperature decreases and accordingly, the melting field of the COI phase increases as the temperature reduces, showing that the COI phase becomes stronger. Consequently, the authors propose a physical picture: COI is weak at a high temperature (230 K) and the re-appeared COI domains are in small droplets. It becomes stronger at a lower temperature of 190 K and the corresponding domain size of COI gets larger, where the anisotropic strain drives the COI domains to grow along a certain lattice orientation. Thus, the COI morphology (stripes) reflects the epitaxial strain characteristic.At 130 K, an even lower temperature, the COI becomes so strong that its main cause (Jahn-Teller distortion) will overwhelm the anisotropic strain and it morphology (irregular puddles) reflects the characteristic of the Jahn-Teller distortion. Hence, another importance of this work is that we can learn how the competing aspects interact from the morphology of the competing phases. Eventually, the control of the phase competition morphology is realized.
The first author of this work is ZHOU Haibiao, a Ph.D. student. LU Qingyou and WU Wenbin are the corresponding authors. This work is supported by the National Natural Science Foundation of China and the National Basic Research Program of China.
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