Anomalous magnetic and spin glass behavior in Nb substituted LaCo$_{1-x}$Nb$_x$O$_3$

TL;DR

This study investigates how Nb substitution in LaCoO$_3$ induces structural phase transitions, enhances magnetic anomalies including spin-glass behavior, and alters electronic properties, revealing complex interplay between structure, spin states, and conductivity.

Contribution

It provides new insights into the effects of Nb$^{5+}$ substitution on structural, magnetic, and electronic properties of LaCoO$_3$, including the observation of spin-glass behavior and spin-state stabilization.

Findings

Structural phase transitions from rhombohedral to monoclinic with Nb doping.

Sharp increase in magnetization below 10 K at low Nb concentrations.

Detection of spin-glass behavior in 0.1 Nb-doped sample.

Abstract

We report the structural, magnetic, transport and electronic properties of Nb substituted LaCo$_{1-x}$Nb$_x$O$_3$ ($x =$ 0--0.2). The Rietveld analysis of x-ray diffraction data demonstrate structural phase transitions from rhombohedral to orthorhombic and further to monoclinic with increasing the Nb concentration up to $x \ge$ 0.2. Interestingly, we observed dramatic changes in the magnetization (M) with increasing the Nb concentration, as the M sharply increases below 10~K even at 2.5\% substitution. Furthermore, ac susceptibility data show the spin-glass behavior in $x=$ 0.1 sample. We find that the density of states near the Fermi level decreases and the activation energy increases, which results in the decreasing conductivity with higher Nb concentration. A significant shift in the peak position of A$_{2g}$ phonon mode has been observed using Raman spectroscopy, which indicates the change in the coupling due to the structural distortion with Nb substitution. The core-level photoemission study confirms that the Nb is present in 5+ valence state. Our study reveals that the nonmagnetic Nb$^{5+}$ ($d^0$) substitution converts Co$^{3+}$ ions to Co$^{2+}$ and stabilize both in the high-spin state. Our results suggest that structural and spin-state transitions as well as the difference in the ionic radii between Nb$^{5+}$ and Co$^{3+}$ are playing an important role in tuning the physical properties.