Short-range Charge and Spin Superstructures in Doped Layered Co Perovskites

TL;DR

This study reveals doping-dependent short-range charge and spin orderings in layered cobaltite perovskites, with charge order present at room temperature and magnetic order emerging at low temperatures, highlighting complex electronic correlations.

Contribution

It uncovers doping-dependent incommensurate charge and magnetic superstructures in cobaltite perovskites using neutron scattering, a novel observation in these materials.

Findings

Charge order exists at room temperature and is doping-dependent.

Magnetic order appears below 40 K with a doubled periodicity.

Charge incommensurability scales with Co$^{2+}$ concentration.

Abstract

We have investigated cobaltite relatives of the layered perovskite cuprates and nickelates, Pr$_{2-x}$Ca$_x$CoO$_4$ ($0.39 \leq x \leq 0.73$) and La$_{2-x}$Sr$_x$CoO$_4$ ($x = 0.61$), using elastic neutron scattering. We have discovered doping-dependent incommensurate short-range ordering of charges and magnetic moments, which in cobaltites occur in the range of heavy doping, $ 0.5 \lesssim x \lesssim 0.75$. The charge order exists already at room temperature and shows no change on cooling. The incommensurability of its propagation vector, ${\bf Q}_c = (\epsilon_c,0,l)$, roughly scales with the concentration of Co$^{2+}$ ions, $\epsilon_c \sim (1-x)$. Magnetic order is only established at low T$\lesssim 40$ K and has twice larger periodicity, indicating a dominant antiferromagnetic correlation between the nearest Co$^{2+}$ spins.