Abstract
LaMnO3+δ is a complex oxide, which, depending on the oxygen excess concentration, presents different crystalline structure and interesting magnetic and electric properties such as colossal magnetoresistance, polaron dynamics, multiferroic behavior, and charge-orbital ordering. This complexity requires different characterization techniques to draw a picture as complete as possible allowing a good understanding of these phenomena. Here, we have used the perturbed angular correlation (PAC) technique to measure hyperfine interactions at La and Mn sites of LaMnO3+δ(δ ∼0.15) using 140Ce and 111Cd at La sites as probe nuclei in order to investigate within an atomic scale the magnetic and electric interactions in this compound. The results show that 111Cd nuclei occupy highly symmetric local sites in agreement with a rhombohedral structure. The magnetic hyperfine field (Bhf) measured with 111Cd at La sites is very small (Bhf = 0.40 T) due to the supertransferred magnetic field from Mn neighbors through oxygen orbitals. On the other hand, 140Ce nuclei at La sites present a saturation field of around 3.7 T much higher than that expected for La sites (due to the weak transfer field by superexchange mechanism). In addition, for temperature range above the magnetic ordering (200-300 K) a dynamic hyperfine interaction was observed characterized by the attenuation parameter λ(T) whose temperature dependence allowed to determine the activation energy (Ea) associated to charge transfer. The polarization of the 4f-electron of Ce impurities affects the local magnetic field at impurity sites as well as the Ea.
Highlights
LaMnO3+δ exhibits a complex magnetic behavior such as the colossal magnetoresistance and different magnetic ordering depending on the oxygen concentration given by δ parameter
The stoichiometric compound LaMnO3 with only Mn+3 ion is a semiconductor at room temperature that, at the Néel temperature of TN = 140 K, orders antiferromagnetically2 presenting a cooperative Jahn-Teller distortion of MnO6 octahedra, whereas the replacement of La with alkaline-earth atoms such as Ca, as well as an excess of oxygen in the sample (LaMnO3+δ with δ > 0), induces the coexistence of Mn+3 and Mn+4 which reduces the Jahn-Teller distortion increasing symmetry and favoring the ferromagnetic ordering
LaMnO3+δ has been investigated by measuring hyperfine interactions with different results and the characterization of the magnetic hyperfine field (MHF) at La and Mn sites is not well established yet for the full range of possible δ values
Summary
Besides MHF, studies of hyperfine interactions in LaMnO3+δ have been carried out by measuring the electric field gradient (EFG) using the time differential perturbed angular correlation (TDPAC) spectroscopy.6,8 Because it is a nuclear method, TDPAC has a high sensitivity to the magnetic exchange interactions, local crystallographic variations, and charge distributions around the lattice site where probe nuclei are located and can be used to detect the local magnetism, local symmetry changes as well as dynamic processes due to charge movement or spin fluctuations.. The results are compared with previous findings where Cd probe originated from 111mCd, scitation.org/journal/adv aiming to understand the correlation between the properties of introduced impurity and the behavior of impurity-compound complex Another probe used in this work to study LaMnO3.15 was 140La(140Ce) whose advantage is an almost absolute dominance of magnetic interaction allowing an insight on exchange and dynamic phenomena in LaMnO3+δ
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