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Register a new userMuon spin relaxation and inelastic neutron scattering investigations of all-in/all-out antiferromagnet Nd2Hf2O7
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Nd2Hf2O7, belonging to the family of geometrically frustrated cubic rare earth pyrochlore oxides, was recently identified to order antiferromagnetically below T_N = 0.55 K with an all-in/all-out arrangement of Nd3+ moments, however with a much reduced ordered state moment. Herein we investigate the spin dynamics and crystal field states of Nd2Hf2O7 using muon spin relaxation (muSR) and inelastic neutron scattering (INS) measurements. Our muSR study confirms the long range magnetic ordering and shows evidence for coexisting persistent dynamic spin fluctuations deep inside the ordered state down to 42 mK. The INS data show the crytal electric field (CEF) excitations due to the transitions both within the ground state multiplet and to the first excited state multiplet. The INS data are analyzed by a model based on CEF and crystal field states are determined. Strong Ising-type anisotropy is inferred from the ground state wavefunction. The CEF parameters indicate the CEF-split Kramers doublet ground state of Nd3+ to be consistent with the dipolar-octupolar character.
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The magnetic states of the non-centrosymmetric, pressure induced superconductor CeCoGe3 have been studied with magnetic susceptibility, muon spin relaxation(muSR), single crystal neutron diffraction and inelastic neutron scattering (INS). CeCoGe3 exhibits three magnetic phase transitions at T_N1 = 21 K, T_N2 = 12 K and T_N3 = 8 K. The presence of long range magnetic order below T_N1 is revealed by the observation of oscillations of the asymmetry in the muSR spectra between 13 K and 20 K and a sharp increase in the muon depolarization rate. Single crystal neutron diffraction measurements reveal magnetic Bragg peaks consistent with propagation vectors of k = 2/3 between T_N1 and T_N2, k = 5/8between T_N2 and T_N3 and k = 1/2 below T_N3. An increase in intensity of the (1 1 0) reflection between T_N1 and T_N3 also indicates a ferromagnetic component in these phases. These measurements are consistent with an equal moment, two-up, two-down magnetic structure below T_N3, with a magnetic moment of 0.405(5) mu_B/Ce. Above T_N2, the results are consistent with an equal moment, two-up, one-down structure with a moment of 0.360(6) mu_B/Ce. INS studies reveal two crystal-field (CEF) excitations at 19 and 27 meV. From an analysis with a CEF model, the wave-functions of the J = 5/2 multiplet are evaluated along with a prediction for the magnitude and direction of the ground state magnetic moment. Our model correctly predicts that the moments order along the c axis but the observed magnetic moment of 0.405(5) mu_B is reduced compared to the predicted moment of 1.01 mu_B. This is ascribed to hybridization between the localized Ce^3+ f-electrons and the conduction band. This suggests that CeCoGe3 has a degree of hybridization between that of CeRhGe3 and the non-centrosymmetric superconductor CeRhSi3.
Dimensionality and symmetry play deterministic roles in the laws of Nature. They are important tools to characterize and understand quantum phase transitions, especially in the limit of strong correlations between spin, orbit, charge, and structural degrees of freedom. Using newly-developed, high-pressure resonant x-ray magnetic and charge diffraction techniques, we have discovered a quantum critical point in Cd2Os2O7 as the all-in-all-out (AIAO) antiferromagnetic order is continuously suppressed to zero temperature and, concomitantly, the cubic lattice structure continuously changes from space group Fd-3m to F-43m. Surrounded by three phases of different time reversal and spatial inversion symmetries, the quantum critical region anchors two phase lines of opposite curvature, with striking departures from a mean-field form at high pressure. As spin fluctuations, lattice breathing modes, and quasiparticle excitations interact in the quantum critical region, we argue that they present the necessary components for strongly-coupled quantum criticality in this three-dimensional compound.
We report on a novel spin-charge fluctuation in the all-in-all-out pyrochlore magnet Cd$_2$Os$_2$O$_7$, where the spin fluctuation is driven by the conduction of thermally excited electrons/holes and associated fluctuation of Os valence. The fluctuation exhibits an activation energy significantly greater than the spin-charge excitation gap and a peculiar frequency range of $10^{6}$--$10^{10}$ s$^{-1}$. These features are attributed to the hopping motion of carriers as small polarons in the insulating phase, where the polaron state is presumably induced by the magnetoelastic coupling via the strong spin-orbit interaction. Such a coupled spin-charge-phonon fluctuation manifests as a part of the metal-insulator transition that is extended over a wide temperature range due to the modest electron correlation comparable with other interactions characteristic for 5$d$-subshell systems.
Continuous quantum phase transitions involving all-in-all-out (AIAO) antiferromagnetic order in strongly spin-orbit-coupled 5d compounds could give rise to various exotic electronic phases and strongly-coupled quantum critical phenomena. Here we experimentally trace the AIAO spin order in Sm2Ir2O7 using direct resonant x-ray magnetic diffraction techniques under high pressure. The magnetic order is suppressed at a critical pressure Pc=6.30 GPa, while the lattice symmetry remains in the cubic Fd-3m space group across the quantum critical point. Comparing pressure tuning and the chemical series R2Ir2O7 reveals that the suppression of the AIAO order and the approach to the spin-disordered state is characterized by contrasting evolutions of both the pyrochlore lattice constant a and the trigonal distortion x. The former affects the 5d bandwidth, the latter the Ising anisotropy, and as such we posit that the opposite effects of pressure and chemical tuning lead to spin fluctuations with different Ising and Heisenberg character in the quantum critical region. Finally, the observed low-pressure scale of the AIAO quantum phase transition in Sm2Ir2O7 identifies a circumscribed region of P-T space for investigating the putative magnetic Weyl-semimetal state.
The magnetic and quadrupolar ordered states of polycrystalline YbRu2Ge2 have been investigated using zero-field muon spin relaxation ({mu}SR) and neutron diffraction measurements. Specific heat measurements show three successive phase transitions, with decreasing temperature from a paramagnetic to a quadrupolar state at T0 ~ 10 K, from the quadrupolar to a magnetic state at T1 ~ 6.5 K and a possible change in the magnetic ground state at T2 ~ 5.5 K. Clear evidence for the magnetic transition below 7 K (spectrum at 8 K reveals paramagnetic state) and a likely change in the magnetic structure near 5.8 K is observed in the zero-field {mu}SR measurements. The {mu}SR data, however, do not reveal any signature of magnetic order in the temperature range 8 - 45 K. This result is further supported by neutron diffraction measurements, where clear magnetic Bragg peaks have been observed below 8 K, but not above it. Below 8 K, the magnetic Bragg peaks can be characterized by an incommensurate antiferromagnetic ordering with the propagation vector q = [0.352, 0, 0] and the magnetic moment 2.9(3) {mu}B of Yb along the b-axis. These results are discussed in terms of quadrupolar ordered and magnetically ordered states.
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