Inverse magnetocaloric and exchange bias effects in single crystalline La_(0.5)Sr_(0.5)MnO_3 nanowires

摘要

We report the first observation of inverse magnetocaloric effect (IMCE) in hydrothermally synthesized single crystalline La_(0.5)Sr_(0.5)MnO_3 nanowires. The core of the nanowires is phase separated with the development of double exchange driven ferromagnetism (FM) in the antiferromagnetic (AFM) matrix, whereas the surface is found to be composed of disordered magnetic spins. The FM phase scales with the effective magnetic anisotropy, which is directly probed by transverse susceptibility experiments. The surface exhibits a glassy behavior and undergoes spin freezing, which manifests as a positive peak (T_L ~ 42 K) in the magnetic entropy change (-ΔS_M) curves, thereby stabilizing the re-entrance of the conventional magnetocaloric effect. Precisely at T_L, the nanowires develop the exchange bias (EB) effect. Our results conclusively demonstrate that the mere coexistence of FM and AFM phases along with a disordered surface below their Néel temperature (T_N ~ 210 K) does not trigger EB, but this develops only below the surface spin freezing temperature.