Magnetic Phase Diagram of CuO

TL;DR

This study maps the magnetic phase diagram of CuO using ultrasonic velocity measurements, revealing a new intermediate phase and transitions, and highlights the importance of an incommensurate collinear phase in its magnetic behavior.

Contribution

It introduces a detailed magnetic phase diagram of CuO, identifying a new intermediate phase and the necessity of an incommensurate collinear phase, using ultrasonic measurements and Landau theory.

Findings

Discovery of a new transition at 230 K indicating an intermediate phase.

Observation of a first order transition at 213 K to a collinear phase.

Detection of a spin-flop transition between 11 T and 13 T.

Abstract

High resolution ultrasonic velocity measurements have been used to determine the temperature -- magnetic-field phase diagram of the monoclinic multiferroic CuO. A new transition at TN3 = 230 K, corresponding to an intermediate state between the antiferromagnetic non-collinear spiral phase observed below TN2 = 229.3 K and the paramagnetic phase, is revealed. Anomalies associated with a first order transition to the commensurate collinear phase are also observed at TN1 = 213 K. For fields with B along the b axis, a spin-flop transition is detected between 11 T - 13 T at lower temperatures. Moreover, our analysis using a Landau-type free energy clearly reveals the necessity for an incommensurate collinear phase between the spiral and the paramagnetic phase. This model is also relevant to the phase diagrams of other monoclinic multiferroic systems.