Strongly-coupled quantum critical point in an all-in-all-out antiferromagnet

Controlling inversion disorder in a stoichiometric spinel magnet

While disorder is an inalienable characteristic of real crystalline materials, the capability of controlling various types of disorder often strongly influences our understanding of science and the advancement of technology. Magnetic spinel represents a class of materials with a pyrochlore-structured sublattice to potentially host three-dimensional spin frustration but is strongly influenced by the inversion disorder of two similarly sized cation species. While it remains challenging to experimentally differentiate these two characteristics, here we mitigate the disorder issue at the crystal growth stage. Our independent control of both stoichiometry and inversion disorder clarifies both magnetism and structure in a spinel oxide of interest for seven decades.

In the study of frustrated quantum magnets, it is essential to be able to control the nature and degree of site disorder during the growth process, as many measurement techniques are incapable of distinguishing between site disorder and frustration-induced spin disorder. Pyrochlore-structured spinel oxides can serve as model systems of geometrically frustrated three-dimensional quantum magnets; however, the nature of the magnetism in one well-studied spinel, ZnFe₂O₄, remains unclear. Here, we demonstrate simultaneous control of both stoichiometry and inversion disorder in the growth of ZnFe₂O₄ single crystals, directly yielding a revised understanding of both the collective spin behavior and lattice symmetry. Crystals grown in the stoichiometric limit with minimal site inversion disorder contravene all the previously suggested exotic spin phases in ZnFe₂O₄. Furthermore, the structure is confirmed on the F4¯3m space group with broken inversion symmetry that induces antiferroelectricity. The effective tuning of magnetic behavior by site disorder in the presence of robust antiferroelectricity makes ZnFe₂O₄ of special interest to multiferroic devices.

Magnetic behavior of Ru substituted skyrmion metal MnSi

We report on the structural and magnetic properties of Ru substituted skyrmion metal MnSi i.e. Mn1-xRu_xSi for the nominal compositions of0⩽x⩽0.5. The composition-temperature (x-T) phase diagram illustrates the substitution-driven changes in the magnetic behavior. It is confirmed that the magnetic ordering temperature (para-to helimagnetic)Ttrand the effective magneticμeffmoment decrease with increasingx. This indicates the suppression of magnetic order by the substitution of Ru in MnSi. However, the magnetic nature is sustained up to a concentration of aboutx= 0.1 above which the system exhibits spin-glass like nature as inferred from the negative Curie-Weiss temperatureθCW, reduced magnetic moment (of the order 10-2 μBf.u.-1) and linearM-H(at 2 K) inx= 0.5. Mn1-xRu_xSi is found to avoid the quantum phase transition and exhibits a composition-driven magnetic to spin-glass like transition.

A continuous metal-insulator transition driven by spin correlations

While Mott insulators induced by Coulomb interactions are a well-recognized class of metal-insulator transitions, insulators purely driven by spin correlations are much less common, as the reduced energy scale often invites competition from other degrees of freedom. Here, we demonstrate a clean example of a spin-correlation-driven metal-insulator transition in the all-in-all-out pyrochlore antiferromagnet Cd₂Os₂O₇, where the lattice symmetry is preserved by the antiferromagnetism. After the antisymmetric linear magnetoresistance from conductive, ferromagnetic domain walls is removed experimentally, the bulk Hall coefficient reveals four Fermi surfaces of both electron and hole types, sequentially departing the Fermi level with decreasing temperature below the Néel temperature, T_N = 227 K. In Cd₂Os₂O₇, the charge gap of a continuous metal-insulator transition opens only at T ~ 10 K << T_N. The insulating mechanism parallels the Slater picture, but without a folded Brillouin zone, and contrasts sharply with Mott insulators and spin density waves, where the electronic gap opens above and at T_N, respectively.

The Mott transition is a celebrated example of a metal-insulator transition driven by electron correlations. Here, using magnetoresistance measurements, the authors demonstrate the presence of a spin-driven metal-insulator transition, where the insulating gap only opens below the Neel temperature.

Optical Raman measurements of low frequency magnons under high pressure

The application of giga-Pascal scale pressures has been widely used as a tool to systematically tune the properties of materials in order to access such general questions as the driving mechanisms underlying phase transitions. While there is a large and growing set of experimental tools successfully applied to high-pressure environments, the compatibility between diamond anvil cells and optical probes offers further potential for examining lattice, magnetic, and electronic states, along with their excitations. Here, we describe the construction of a highly efficient optical Raman spectrometer that enables measurements of magnetic excitations in single crystals down to energies of 9 cm^-1 (1.1 meV or 13 K) at cryogenic temperatures and under pressures of tens of GPa.

X-ray magnetic diffraction under high pressure

Advances in both non-resonant and resonant X-ray magnetic diffraction since the 1980s have provided researchers with a powerful tool for exploring the spin, orbital and ion degrees of freedom in magnetic solids, as well as parsing their interplay. Here, we discuss key issues for performing X-ray magnetic diffraction on single-crystal samples under high pressure (above 40 GPa) and at cryogenic temperatures (4 K). We present case studies of both non-resonant and resonant X-ray magnetic diffraction under pressure for a spin-flip transition in an incommensurate spin-density-wave material and a continuous quantum phase transition of a commensurate all-in-all-out antiferromagnet. Both cases use diamond-anvil-cell technologies at third-generation synchrotron radiation sources. In addition to the exploration of the athermal emergence and evolution of antiferromagnetism discussed here, these techniques can be applied to the study of the pressure evolution of weak charge order such as charge-density waves, antiferro-type orbital order, the charge anisotropic tensor susceptibility and charge superlattices associated with either primary spin order or softened phonons.