Exchange Bias Effect and Magnetic Properties in ${\hbox {La}}_{0.7}{\hbox {Sr}}_{0.3}{\hbox {MnO}}_{3}\hbox {-}{\hbox {NiO}}$ Nanocomposite Films

The magnetic exchange coupling effect has been investigated in La_0.7Sr_0.3MnO₃ (LSMO)-NiO nanocomposite films prepared by pulsed laser deposition on SrTiO₃ (001) and (110) single crystal substrates. The films show an enhanced coercivity field H_C = 560 Oe compared with the same thickness single pure LSMO thin film. The composite films exhibit a large exchange bias field H_E = 140 Oe after applying a 4000 Oe field cooling. The values of the exchange bias field decrease with increasing temperature and become zero at about 45 K, which corresponds to the conventional exchange bias blocking temperature ( T_B), regardless of epitaxial growth directions on (001) or (110) SrTiO₃ substrates. We speculate that there may be magnetic interaction at the NiO and LSMO boundaries or charge transfer between the Ni and Mn ions, resulting in the formation of ferromagnetic interaction at the interface. The Curie temperature T_C decreased from 340 K of the LSMO single film to 200 K of the nanocomposite film. The results demonstrate that the magnetic state of the composite film is directly related to the Mn-O-Mn and Mn-O-Ni spin interaction energy. In addition to this, the Mn-O-Ni at the boundary may also form the magnetic region that pins the ferromagnetic LSMO phase.