Interplay between frustration and spin-orbit coupling in vanadates

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H-T phase diagrams of CdV2O4 and MgV2O4 obtained from M(T, H) and ΔP(H) measurements. Abbreviations PM, PE, and FE are for paramagnetic, paraelectric, and ferroelectric state. The shaded area is the FE state, and the lined area represents a mixed PE-FE state due to the polycrystalline nature of CdV2O4.

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Spin-orbit interactions do not normally play the leading role in determining the magnetic ordering of three-dimensional-based materials. However, in certain magnets the dominant magnetic interactions are frustrated, leaving spin-orbit interactions to provide an important role in selecting the ordering. Magnetic frustration happens when all the magnetic interactions in a system cannot be satisfied simultaneously. Understanding how to model the interplay between spin-orbit interactions, magnetism, and structure is important for an increasing number of frustrated materials because these materials have potential applications as sensors and in memory devices. Los Alamos researchers and collaborators have combined experiment and theory to understand frustration in the archetypal frustrated spinel compounds MV2O4 (M = Cd, Mg). Physical Review Letters published the findings.

Frustrated systems are at a tipping point where small external stimuli can push them from one magnetic configuration to another. This makes them very tunable and sensitive to external fields, and therefore useful for applications. The scientists studied materials that have spin frustration and orbital ordering that occurs on the same energy scale. (Electrons can produce magnetism either through their intrinsic spin, or by the fact that they orbit the nucleus of the atom). The orbital ordering adds an additional degree of complexity and is a challenge for theoretical description. However, it adds functionality because orbital magnetism couples to the structure of the material and to electrical properties. This enables creation of coupling between magnetism and structure, and between magnetism and electric polarization. The potential applications of this general area of research that couples magnetism to structure and to electrical properties include sensors, memories, high-frequency filters, tuners, resonators, energy harvesting, etc.

The team developed a model for the magnetic, orbital, and structural interactions. They conducted magnetization and electric polarization measurements in pulsed magnetic fields up to 65 Tesla at the National High Magnetic Field Laboratory at Los Alamos to chart magnetic field-induced multiferroic transitions and determine the high-field phase diagram.

The model and experiments match beautifully. The high-field results confirm that the spin-orbit interactions create a trigonal distortion of the vanadium ions both in zero and high fields that was previously suggested as a possibility. The researchers concluded that a more complex zero-field magnetic order must occur than previously assumed, which allows relatively small magnetic fields to flip ordered plaquettes of spins.

Figure 12. H-T phase diagrams of CdV2O4 and MgV2O4 obtained from M(T, H) and ΔP(H) measurements. Abbreviations PM, PE, and FE are for paramagnetic, paraelectric, and ferroelectric state. The shaded area is the FE state, and the lined area represents a mixed PE-FE state due to the polycrystalline nature of CdV2O4.

Reference: “Magnetic Field Induced Transition in Vanadium Spinels,” Physical Review Letters 112, 017207 (2014); doi: 10.1103/PhysRevLett.112.017207. Los Alamos contributors include Eun Mun and Vivien Zapf (Condensed Matter and Magnet Science, MPA-CMMS), Gia-Wei Chern and Cristian Batista (Physics of Condensed Matter and Complex Systems, T-4). Coauthors are V. Vardo and F. Rivadulla (University of Santiago de Compostela), R. Sinclair and H. D. Zhou (University of Tennessee). Brian Scott (Materials Synthesis and Integrated Devices, MPA-11) provided single-crystal orientations. Scientists at the University of Santiago de Compostela and the University of Tennessee grew high-quality crystals for the research.

The Laboratory Directed Research and Development (LDRD) program funded the research at Los Alamos. The National Science Foundation, the DOE, and the State of Florida sponsor the National High Magnetic Field Laboratory – Pulsed Field Facility at Los Alamos. The work supports the Lab’s Energy Security and Global Security mission areas, and the Materials for the Future science pillar. Technical contact: Vivien Zapf