Stacks of alternating high-resistance and low-resistance manganite may perform well as field-switchable devices.
The ease with which electrons conduct through most materials is relatively insensitive to whether they are located in a magnetic field. But in materials called perovskite manganites, the resistance can change by many orders of magnitude in a magnetic field, earning these materials the name ‘colossal magnetoresistive’ manganites.
Although this effect has been known since the 1990s, there is some debate about its origin. One of the main ideas is that two phases — a high-resistance state and a low-resistance, magnetic state — compete energetically so that even a small magnetic field can cause a switch between the two phases. Now, Masao Nakamura of RIKEN in Wako, Japan and colleagues at several institutes in Japan1 are taking advantage of thin-film technology to control the competition between these phases in the path toward making practical devices from manganites.
Nakamura and his team prepared a series of multilayer structures in which they alternated films, several atomic layers thick, of two different manganites: La0.5Sr0.5MnO3 and Pr0.5Ca0.5MnO3. The lanthanum-strontium manganite is a ferromagnetic metal, whereas the praseodymium-calcium manganite is an insulator that has an ‘orbital order’ phase in which the manganese sites are surrounded by an ordered pattern of electron clouds. By artificially placing the two phases that contribute to the magnetoresistive effect next to each other at various ratios, the researchers were able to tune the overall properties of the heterostructure.
Depending on the ratio of the two components, the films dominantly exhibited either the ferromagnetic, metallic behavior of pure La0.5Sr0.5MnO3 or the insulating behavior of Pr0.5Ca0.5MnO3 — or a combination of the two. When displaying both types of behavior, the properties of the multilayer structure are unstable — and therefore easily switched — in the presence of a magnetic field, just as in many magnetoresistive manganites.
In pure manganite, the interfaces between domains appear randomly, as they tend to form near imperfections in the crystal. The team's new materials are much ‘cleaner’ and offer greater control over the balance between the two phases. And, because only a small magnetic field is required to alter the magnetic and electronic properties of the multilayer structure, the researchers hope to find applications for their technology in field-switchable devices.
References
Nakamura, M., Okuyama, D., Lee, J.S., Arima, T., Wakabayashi, Y., Kumai, R., Kawasaki, M., & Tokura, Y. Magnetically tunable metal-insulator superlattices. Adv. Mater. Published online: 2 November 2009; doi:10.1002/adma.200902734
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Manganites: Controlling the competition. NPG Asia Mater 2, 43 (2010). https://doi.org/10.1038/asiamat.2010.8
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DOI: https://doi.org/10.1038/asiamat.2010.8