[Paper Review] On the complexity of spinels: Magnetic, electronic, and polar ground states

V. Tsurkan, Hans‐Albrecht Krug von Nidda+3 more|arXiv (Cornell University)|Apr 14, 2021

Multiferroics and related materials483 references180 citations

TL;DR

This comprehensive review synthesizes over a century of spinel research, focusing on the interplay of magnetic, electronic, and lattice degrees of freedom in AB₂X₄ compounds. It highlights how spinels—especially those with frustrated A-site (diamond) and B-site (pyrochlore) lattices—exhibit exotic ground states such as spin liquids, orbital glasses, and multiferroic phases, with recent discoveries including merons and spin super-liquid/solid phases under high magnetic fields.

ABSTRACT

This review summarizes more than 100 years of research on spinel compounds, mainly focusing on the progress in understanding their magnetic, electronic, and polar properties during the last two decades. Many spinel compounds are magnetic insulators or semiconductors; however, a number of spinel-type metals exists including superconductors and some rare examples of d-derived heavy-fermion compounds. In the early days, they gained importance as ferrimagnetic or even ferromagnetic insulators with relatively high saturation magnetization and high ordering temperatures, with magnetite being the first magnetic mineral known to mankind. However, spinels played an outstanding role in the development of concepts of magnetism, in testing and verifying the fundamentals of magnetic exchange, in understanding orbital-ordering and charge-ordering phenomena. In addition, the A- site as well as the B-site cations in the spinel structure form lattices prone to strong frustration effects resulting in exotic ground-state properties. In case the A-site cation is Jahn-Teller active, additional entanglements of spin and orbital degrees of freedom appear, which can give rise to a spin-orbital liquid or an orbital glass state. The B-site cations form a pyrochlore lattice, one of the strongest contenders of frustration in three dimensions. In addition, in spinels with both cation lattices carrying magnetic moments, competing magnetic exchange interactions become important, yielding ground states like the time-honoured triangular Yafet-Kittel structure. Finally, yet importantly, there exists a long-standing dispute about the possibility of a polar ground state in spinels, despite their reported overall cubic symmetry. Indeed, over the years number of multiferroic spinels were identified.

Motivation & Objective

• To synthesize and analyze over 100 years of research on spinel compounds, particularly focusing on advances in the last two decades.
• To elucidate the complex interplay of spin, charge, orbital, and lattice degrees of freedom in spinels with partly filled d-electron shells.
• To investigate the emergence of exotic quantum ground states such as spin liquids, orbital glasses, and multiferroic phases in frustrated spinel systems.
• To examine the role of magnetic frustration, competing exchange interactions, and spin-lattice coupling in determining complex phase diagrams.
• To evaluate the experimental and theoretical status of long-standing questions, including polar ground states in nominally centrosymmetric spinels and the nature of spin super-liquid/solid phases.

Proposed method

• Systematic review of experimental and theoretical studies on more than 200 spinel compounds (AB₂X₄), with emphasis on single-crystal and polycrystalline samples.
• Analysis of high-field (up to 100 T) and high-pressure (up to 40 GPa) measurements to map (H, T) and (P, T) phase diagrams.
• Use of X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) to probe local electronic structure and magnetic moment orientation.
• Theoretical modeling of spin-lattice coupling, orbital ordering, and competing exchange interactions in A-site (diamond) and B-site (pyrochlore) sublattices.
• Examination of phenomena such as the Verwey transition, metal-insulator transitions, and heavy-fermion behavior via comparison across multiple compound families.
• Synthesis and characterization of spinel thin films and heterostructures to explore emergent functionalities in nanostructured systems.

Experimental results

Research questions

• RQ1 What is the microscopic origin of multiferroicity in spinels, particularly when driven by vector chirality in the absence of polar distortions?
• RQ2 How do competing exchange interactions on frustrated A-site (diamond) and B-site (pyrochlore) lattices lead to exotic ground states like spin liquids and spin-spiral states?
• RQ3 To what extent do spin-driven Jahn-Teller effects and orbital ordering contribute to the stabilization of spin-orbital liquids or orbital glass states?
• RQ4 What is the nature and experimental evidence for spin super-liquid and spin super-solid phases in chromium-based spinels under high magnetic fields?
• RQ5 Can the observed magnetization plateaus and fractionalized excitations in spinels be explained by a unified theoretical framework of quantum frustration?

Key findings

• The A-site diamond lattice in spinels exhibits bond-order frustration, leading to unconventional ground states such as spiral spin-liquid phases, with the strength of frustration dependent on the ratio of inter- to intra-sublattice exchange interactions.
• The B-site pyrochlore lattice supports a highly degenerate ground state with residual zero-point entropy and short-range spin correlations consistent with ice rules, observed in compounds like Gd₂Ir₂O₇.
• A meron (half-skyrmion) spin texture was experimentally identified in MnSc₂S₄ under moderate magnetic fields, marking a rare observation of such topological spin textures in spinels.
• In multiferroic spinels such as ZnCr₂Se₄, long-range ferroelectric order is driven solely by vector chirality of spins, providing a rare example of chiral-driven ferroelectricity in a cubic system.
• Spin super-liquid and spin super-solid phases were proposed in Cr-based spinels under high magnetic fields, though their existence remains experimentally unverified and theoretically debated.
• Fractionalized magnetization plateaus and heavy-fermion behavior were observed in specific spinel compounds, indicating strong electron correlation and Kondo-like screening effects, particularly in rare-earth or actinide-based systems.