Matter can exist in various phases. These phases are frequently found separated by interfaces, if they can coexist. For example, liquid H2O can not be mixed uniformly with gas H2O, due to their differing densities. Thus, phase separation is a very common phenomena that are encountered every day. Of course, there may also exist systems that exhibit multiple coexisting phases without spatial phase boundaries. One example is multiferroics: BiFeO3 is at the same time an anti-ferromagnet and an ferroelectric, without electrical polarization aliened from magnetic one. They are both homogeneously distributed in the entire sample.
Phase separation can happen on not only large scales, but also on small ones. Examples include multi-domain crystal, in which down magnetization regions are spaced from up magnetization regions by domain walls. Such multi-domain structure is due to long range interactions. Short-range interactions are usually not supposed to raise multi-domain structures. Another example is pnictide superconductors, in which some authors claimed evidences of coexistence of superconductivity and AFM, spatially divorced.
Here[1], the author studied phase separation in thin films of magnese substrated upon STO. Strain caused at the interface is found of important role. The phases are complex and rich.
[1]Science 329, 190 (2010)
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