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Single-crystalline Bi(2)Fe(4)O(9) nanocrystals (NCs) of various sizes (from 78 to 14 nm), synthesized using the sol gel method, exhibit size-dependent magnetic properties, which can be attributed to the uncompensated spins on the surface of the antiferromagnetic NCs. These tunable magnetic properties have a profound significance and can be widely applied in spintronics. As the crystal size decreases, the energy gap of the Bi(2)Fe(4)O(9) semiconductor NCs increases from 1.91 to 2.04 eV, which results from quantum confinement effect. The Bi(2)Fe(4)O(9) NCs exhibit excellent photocatalytic oxidation of methylene blue (MB) under visible light irradiation with the assistance of a small amount of H(2)O(2). Interestingly, smaller NCs exhibit higher photocatalytic activities. The size-dependence of photocatalytic properties of the Bi(2)Fe(4)O(9) NCs was ascribed to the lower recombination rate of the photogenerated electron/hole pair within smaller NCs and higher specific surface areas, which allow for stronger photon absorption on the surface of smaller crystals.