Exchange Bias Effect and Suppression of Charge Ordering Induced by a Surface Phase Separation in La0.25Ca0.75MnO3 Nanowires

The microstructure and magnetic properties of the polycrystalline La0.25Ca0.75MnO3 nanowires with diameter of about 60 nm prepared by a sol-gel template method were studied with use of X-ray diffraction, transmission electron microscopy, and superconducting quantum interference device magnetometry. Compared with the robust charge ordering (CO) state with CO transition temperature T-CO approximate to 240 K in the bulk counterpart, the CO is suppressed with T-CO approximate to 138 K and CO transition becomes wide, at the same time the ferromagnetic (FM) cluster glass state and the exchange bias effect are detected in La0.25Ca0.75MnO3 nanowires. This exotic behavior is attributed to the uncompensated surface spins which change the spin configuration of nanowires and enhance the e(g) charge density at the surface compared to bulk compound and thereby induce the FM cluster glass state as well as exchange bias effect, and weaken the CO.