Experimental evidence of a collinear antiferromagnetic ordering in the frustrated CoAl2O4 spinel

TL;DR

This study provides experimental evidence of collinear antiferromagnetic order in the frustrated CoAl2O4 spinel, revealing details about magnetic moments, fluctuations, and short-range order through multiple measurement techniques.

Contribution

It demonstrates the presence of collinear antiferromagnetic ordering and characterizes magnetic moments and fluctuations in CoAl2O4 using NMR, neutron diffraction, and other methods.

Findings

Antiferromagnetic order below 9.8 K confirmed by multiple techniques.

Ordered magnetic moments are smaller than expected for Co^2+ ions.

Short-range magnetic correlations persist above the Néel temperature.

Abstract

Nuclear magnetic resonance (NMR), neutron diffaction (ND), x-ray diffraction, magnetic susceptibility and specific heat measurements on the frustrated A-site spinel CoAl2O4 compound reveal a collinear antiferromagnetic ordering below Tn = 9.8(2) K. A high quality powder sample characterized by x-ray diffraction that indicates a relatively low Co-Al inversion parameter x = 0.057(20) in (Co1-xAlx)[Al2-xCox]O4, shows a broad maximum around 15 K in magnetic susceptibility and a sharp peak at Tn in heat capacity. The average ordered magnetic moment of Co^2+ (S = 3/2) ions at the A-site is estimated to be 2.4(1) Bohr magneton from NMR and 1.9(5) Bohr magneton from ND which are smaller than the expected value of 3 Bohr magneton for S = 3/2 and g = 2. Antiferromagnetic spin uctuations and correlations in the paramagnetic state are revealed from the magnetic susceptibility, NMR and ND measurements, which are due to spin frustration and site inversion effects in the system. The ND data also show short-range dynamic magnetic ordering that persists to a temperature that is almost twice Tn.