A new model is proposed for ferromagnetism associated with defects in the bulk or at the surface of nanoparticles. The basic idea is that a narrow, structured local density of states Ns(E) is associated with the defects, but the Fermi level (which may lie above or below a mobility edge) will not normally coincide with a peak in Ns(E). However, if there is a local charge reservoir, such as a dopant cation coexisting in two different charge states or a charge-transfer complex at the surface, then it may be possible for electron transfer to raise the Fermi level to a peak in the local density of states, leading to Stoner splitting of Ns(E). Spontaneous Stoner ferromagnetism can arise in percolating defect-rich regions, such as the nanoparticle surface. The charge-transfer ferromagnetism model may be applicable to a wide range of nanoparticles and thin films of dilute magnetic oxides have previously been regarded as dilute magnetic semiconductors.

The authors report room temperature ferromagnetism in La0.5Sr0.5Ti0.985Co0.015O3−δ nanoparticles with particle sizes of ∼12–14nm. Nanoparticles of undoped and Co-doped samples were synthesized by a polymerized complex method and were characterized using x-ray diffraction and transmission electron microscopy. The magnetic properties of the nanoparticles were determined using vibrating sample magnetometry. The undoped samples exhibit a diamagnetic behavior, whereas all the Co-doped samples are ferromagnetic having the specific magnetizations of ∼0.011–0.038emu∕g at 10kOe. The finding of room temperature ferromagnetism in the Co-doped samples would allow an enhanced flexibility for applications.

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We report room-temperature ferromagnetism in ∼11.4–14.6nm nanoparticles of La0.5Sr0.5Ti0.985Fe0.015O3−δ. Nanoparticles of undoped and Fe-doped samples were synthesized by a polymerized complex method and were characterized using x-ray diffraction, transmission electron microscopy, and vibrating-sample magnetometer. The undoped samples exhibit a diamagnetic behavior, whereas the Fe-doped samples are ferromagnetic having the specific magnetizations of 0.041–0.101emu∕g at 10kOe. Our results indicate that the ferromagnetic property of La0.5Sr0.5Ti0.985Fe0.015O3−δ system is intrinsic and is not a result of any secondary magnetic phase or cluster formation. The finding of room-temperature ferromagnetism in this system would stimulate further interest in the area of diluted magnetic oxides.

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