Complex magnetic phases and thermodynamics of CuB_2O_4

TL;DR

This study comprehensively maps the complex magnetic phase diagram of CuB_2O_4 across a range of temperatures and magnetic fields, revealing new magnetic phases and the underlying mechanisms through experimental and theoretical methods.

Contribution

It provides the first detailed B-T phase diagram of CuB_2O_4, identifying new magnetic phases and elucidating their origins via combined experimental and first-principles approaches.

Findings

Identification of multiple magnetic phases including soliton states

Discovery of a magnetically frustrated ground state with low entropy release

Elucidation of the competition between ferromagnetic and antiferromagnetic interactions

Abstract

The copper metaborate CuB_2O_4(CBO) has been studied within the parametric space of magnetic field (B) and temperature (T), showing a series of distinct phases, including commensurate and incommensurate magnetic orders, as well as magnetic soliton phases. However, no single study has simultaneously demonstrated all these phases. moreover, the existence of additional phases below 2 K or above 2 T for B perpenducular to c has been scarcely explored. This work presents the first comprehensive construction of the complex B-T phase diagram for T = 0.1 to 25 K and B = 0 to 9 T (B perpenducular to c), utilizing thermodynamic probes, magnetic measurements, and neutron scattering on a single batch of CBO crystals. Our findings elucidate new magnetic phases and regimes. First-principles calculations help to gain a deeper understanding of this complex phase diagram. The emergence of various phases within such a narrow B and T regime is attributed to two primary mechanisms: the delicate competition between ferromagnetic and antiferromagnetic interactions, and the weak exchange and superexchange interactions within and between the Cu(A) and Cu(B) sublattices. This competition also leads to a magnetically frustrated ground state at absolute zero where the total magnetic entropy release from high to low temperatures is only 0.76 Rln2, lower than Rln2 expected for copper spin 1/2. Using CBO as a model system, the present work elucidates the intricate mechanisms underlying the formation of complex magnetic phases.