Magnetic Properties, Spin Glass State, and Exchange Bias Effect in Strained [La_0.8Ca_0.2MnO₃/La_0.5Ca_0.5MnO₃]₂₀ Films

Perovskite manganites have been widely studied due to their rich magnetic and electrical properties, such as the colossal magnetoresistance (CMR) effect, ferromagnetic transition, and exchange bias effect. This work reports the preparation of five [La_0.8Ca_0.2MnO₃/La_0.5Ca_0.5MnO₃]₂₀ superlattice films with different La_0.8Ca_0.2MnO₃ layer thicknesses on (001) LaAlO₃ substrate using pulsed laser deposition technology. The thickness of the La_0.5Ca_0.5MnO₃ sublayers for all five superlattice films during one growth cycle is 48 Å. The thickness of the La_0.8Ca_0.2MnO₃ sublayers in the superlattice films during one growth cycle is 24 Å, 48 Å, 72 Å, 96 Å, and 120 Å, designated as SL-24, SL-48, SL-72, SL-96, and SL-120, respectively. The crystal structure (lattice constants) of samples is obtained using x-ray diffraction technology, and the strain states of the samples are determined based on the lattice constant. Three superlattice samples (SL-48, SL-72, and SL-96) are in a state of strain relaxation. The SL-24 and SL-120 samples are in a strain state, which is the result of the interlayer strain effect due to the asymmetric thickness of the two components between the sublayers. The magnetic properties of samples are analyzed based on the stress state and magnetic properties. All superlattice samples show a ferromagnetism-to-paramagnetism transition, and with the increase in the thickness of the La_0.8Ca_0.2MnO₃ sublayer, the Curie temperature (T_C) shows a gradual decreasing trend. In addition to the ferromagnetic phase transition in SL-96, the presence of the Néel temperature (representing antiferromagnetic phase transition) is also observed, further indicating that the magnetic performance is affected by factors in addition to strain, such as multiple magnetic phases. There is no significant exchange bias in SL-48, SL-72, and SL-96, since the percolation of ferromagnetic clusters at the interface of ferromagnetic La_0.8Ca_0.2MnO₃ and charge-ordered antiferromagnetic La_0.5Ca_0.5MnO₃ in superlattices destroys the exchange coupling between ferromagnetic and antiferromagnetic layers, resulting in a very small exchange bias. On the contrary, for the SL-24 and SL-120 samples with significant strain states, the strain can suppress the percolation behaviors at the interfaces of the ferromagnetic and antiferromagnetic layers, leading to a pronounced exchange bias effect.