NMR evidence for triple-q multipole structure in NpO2

Intrinsic giant magnetoresistance due to exchange-bias-type effects at the surface of single-crystalline NiS2 nanoflakes

The coexistence of different properties in the same material often results in exciting physical effects. At low temperatures, the pyrite transition-metal disulphide NiS₂ hosts both antiferromagnetic and weak ferromagnetic orders, along with surface metallicity dominating its electronic transport. The interplay between such a complex magnetic structure and surface-dominated conduction in NiS₂, however, is still not understood. A possible reason for this limited understanding is that NiS₂ has been available primarily in bulk single-crystal form, which makes it difficult to perform studies combining magnetometry and transport measurements with high spatial resolution. Here, NiS₂ nanoflakes are produced via mechanical cleaving and exfoliation of NiS₂ single crystals and their properties are studied on a local (micron-size) scale. Strongly field-asymmetric magnetotransport features are found at low temperatures, which resemble those of more complex magnetic thin film heterostructures. Using nitrogen vacancy magnetometry, these magnetotransport features are related to exchange-bias-type effects between ferromagnetic and antiferromagnetic regions forming near step edges at the nanoflake surface. Nanoflakes with bigger steps exhibit giant magnetoresistance, which suggests a strong influence of magnetic spin textures at the NiS₂ surface on its electronic transport. These findings pave the way for the application of NiS₂ nanoflakes in van der Waals heterostructures for low-temperature spintronics and superconducting spintronics.

Hidden order and multipolar exchange striction in a correlated f-electron system

Second-order phase transitions in solids occur due to spontaneous symmetry breaking with an order parameter continuously emerging from the disordered high-temperature phase. In some materials, the phase transitions are clearly detected in thermodynamic functions (e.g., specific heat), but the microscopic order parameters remain “hidden” from researchers, in some cases for decades. Here, we show how such hidden-order parameters can be unambiguously identified and the corresponding ordered phase fully described using a first-principles many-body linear response theory. Considering the canonical “hidden-order” system neptunium dioxide, we also identify an unconventional mechanism of spontaneous multipolar exchange striction that induces an anomalous volume contraction of the hidden-order phase in NpO₂.

The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such “hidden orders” (HOs) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f or d shells that may harbor high-rank multipole moments. Coupled by intersite exchange, those moments form a vast space of competing order parameters. Here, we show how the ground-state order and magnetic excitations of a prototypical HO system, neptunium dioxide NpO₂, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force theorem method. Superexchange interactions between the lowest crystal-field quadruplet of Np⁴⁺ ions induce a primary noncollinear order of time-odd rank 5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO₂. Our study also reveals an unconventional multipolar exchange striction mechanism behind the anomalous volume contraction of the NpO₂ HO phase.

Magnetic structure of UO2and NpO2by first-principle methods

The magnetic structure of the actinide dioxides (AnO₂) remains a field of intense research. A noncollinear relativistic computational study of the AnO₂(An = U, Np) magnetic structure has been completed.

Hidden magnetic order in plutonium dioxide nuclear fuel

The magnetic structure of PuO₂ has been investigated by computational methods. A hereto unknown longitudinal 3k AFM ground-state that retains Fm3̄m crystal symmetry has been identified.

NMR evidence for higher-order multipole order parameters in NpO2

We report a microscopic investigation of multipolar order parameters in the ordered state of NpO2 conducted via 17O NMR on a single crystal. From the angular dependence of hyperfine fields at 17O nuclei, we have obtained clear evidence for the appearance of field-induced antiferro-octupolar as well as field-induced antiferro-dipolar moments below T0 = 26 K. We have also observed oscillatory spin-echo decay, which is well understood in terms of small electric field gradients created by antiferro-quadrupolar ordering. This reveals that the quadrupolar order parameter is directly observable by means of NMR. The present NMR studies provide definitive support for a proposed longitudinal triple-q type octupolar-quadrupolar ordering model for NpO2.

Hidden order and low-energy excitations in NpO2

We investigate the nature of the hidden order parameter in the ordered phase of NpO2, which had been identified with a staggered arrangement of Gamma5 magnetic multipoles. By analyzing the existing experimental data, we show that the most likely driving order parameter is not provided by octupoles, as usually assumed, but rather by the rank-5 triakontadipoles. Calculations of the coupled dynamics of spins, Gamma5 quadrupoles, and Gamma5 triakontadipoles in the ordered phase enable us to analyze the resulting structure of low-energy excitations. We show that the powder inelastic neutron scattering cross section should contain, in addition to the already-observed peak at 6.5 meV, a second weaker peak at about 14 meV.