To determine the origin of ferromagnetism in La0.5Sr0.5-
Ti1−xNixO3 (0.02 ≤ x ≤ 0.05) nanofibers, the mechanism that
has recently been proposed to explain ferromagnetism in
some oxides is the F-center exchange mechanism as shown in
Fig. 12.4,30) Since the XPS results and XANES spectra
confirm the existence of oxygen vacancies, Ni2+ ions, and
some Ni metal in La0.5Sr0.5Ti1−xNixO3 nanofibers, it is
reasonable to propose that Ni2+– –Ni2+ groups are common
in the structure, where denotes an oxygen vacancy.30)
According to Hund’s rule and Pauli’s exclusion principle, the
trapped electrons in a vacancy will come from ↓ and two
neighboring Ni atoms ↑; thus, ferromagnetism is achieved.30)
The schematic diagram for the F-center exchange mechanism
in La0.5Sr0.5Ti1−xNixO3 (0.02 ≤ x ≤ 0.05) nanofibers is illustrated
in Fig. 12. Overlapping Ni2+– –Ni2+ groups are
crucial for the proposed ferromagnetism coupling. On the
other hand, the magnetic properties may be come from Ni
metal in the nanofibers. Owing to oxygen vacancies that can
lead to induce room-temperature ferromagnetism, we further
conclude that F-center exchange is an important exchange
mechanism in both La0.5Sr0.