Mn valence instability in La2/3Ca1/3MnO3 thin films

TL;DR

This study investigates Mn valence instability in La2/3Ca1/3MnO3 thin films, revealing how air exposure and annealing affect magnetic properties and Mn oxidation states, with implications for device fabrication.

Contribution

It demonstrates that Mn valence in thin films can change with environmental exposure and processing, highlighting the importance of annealing in stabilizing magnetic properties.

Findings

Mn2+ signature develops with air exposure

High-temperature annealing stabilizes Mn valence

Valence anomalies extend throughout the film thickness

Abstract

A Mn valence instability on La2/3Ca1/3MnO3 thin films, grown on LaAlO3 (001)substrates is observed by x-ray absorption spectroscopy at the Mn L-edge and O K-edge. As-grown samples, in situ annealed at 800 C in oxygen, exhibit a Curie temperature well below that of the bulk material. Upon air exposure a reduction of the saturation magnetization, MS, of the films is detected. Simultaneously a Mn2+ spectral signature develops, in addition to the expected Mn3+ and Mn4+ contributions, which increases with time. The similarity of the spectral results obtained by total electron yield and fluorescence yield spectroscopy indicates that the location of the Mn valence anomalies is not confined to a narrow surface region of the film, but can extend throughout the whole thickness of the sample. High temperature annealing at 1000 C in air, immediately after growth, improves the magnetic and transport properties of such films towards the bulk values and the Mn2+ signature in the spectra does not appear. The Mn valence is then stable even to prolonged air exposure. We propose a mechanism for the Mn2+ ions formation and discuss the importance of these observations with respect to previous findings and production of thin films devices.