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Register a new userGiant shifts of crystal-field excitations with temperature as consequence of internal magnetic exchange fields
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Crystal-field excitations, for example in transition-metal oxides where a rare-earth element is used as a spacer between the transition-metal-oxide tetrahedra and octahedra, are assumed to be extremely robust with respect to external perturbations such as temperature. Using inelastic neutron scattering experiments, a giant shift of the energy of the lowest crystal-field excitation of Er3+ (4I15/2) in ErFeO3 from 0.30(2) meV to 0.75(2) meV was measured below the magnetic-ordering temperature of erbium at 4.1 K. Quantum-mechanical point-charge calculations of the crystal-field levels indicate that the shift is caused by the internal magnetic field created by the erbium spins themselves, which causes a Zeeman splitting of the erbium 4f electronic levels, and therefore a change in the energies of crystal-field transitions. To verify this explanation, the effect of an external magnetic field on the crystal-field excitations was measured by inelastic neutron scattering and compared to the field-dependent point-charge calculations. The existence of an internal magnetic exchange interaction will have implications for a deeper understanding of a broader group of phenomena such as multiferroic properties or spin frustration, which are a consequence of various competing electronic and magnetic exchange interactions.
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Crystal-field excitations in transition-metal oxides where -rare-earth elements locate in the space between the transition-metal-oxide tetrahedra and octahedra, are assumed to be robust with respect to external perturbations such as temperature. Using inelastic neutron-scattering experiments, a giant shift of the energy of the lowest crystal-field excitation of Er$^{3+}$ ($^{4}$I$_{15/2}$) in ErFeO$_3$ from 0.35 meV to 0.75 meV was observed on cooling from 10K to 1.5K through the magnetic ordering temperature of Er$^{3+}$ at 4.1 K. A crystal-field model was proposed to explain the observed crystal field excitations in this work. The model indicates the lowest-energy crystal-field excitation in ErFeO$_3$ is the first Kramers doublet above the ground state. Its energy substantially shifts by the internal field induced by the ordered Er$^{3+}$ magnetic moments. Further magnetic-field-dependent measurements provide strong supportive evidence for this scenario. By fitting the external magnetic-field dependency of the crystal-field excitation energy, the internal field generated by Er$^{3+}$ magnetic moments was derived to be ~0.33meV. The result indicates that the internal field of Er$^{3+}$ magnetic moments contribute to the energy shift of the crystal-field excitations. The giant energy shift under fields could be attributed to the anisotropy of the large effective g-factor.
We report a study on the temperature dependence of the charge-neutral crystal field (dd) excitations in cupric oxide, using nonresonant inelastic x-ray scattering (IXS) spectroscopy. Thanks to a very high energy resolution (60 meV), we observe thermal effects on the dd excitation spectrum fine structure between temperatures of 10-320 K. With an increasing temperature, the spectra broaden considerably. We assign the temperature dependence of the dd excitations to the relatively large electron-phonon coupling.
The thermodynamic properties of the pyrochlore Yb2Ti2O7 material are calculated using the numericallinked-cluster (NLC) calculation method for an effective anisotropic-exchange spin-1/2 Hamiltonian with parameters recently determined by fitting the neutron scattering spin wave data obtained at high magnetic field h. Magnetization, M(T,h), as a function of temperature T and for different magnetic fields h applied along the three high symmetry directions [100], [110] and [111], are compared with experimental measurements on the material for temperature T>1.8K. The excellent agreement between experimentally measured and calculated M(T,h) over the entire temperature and magnetic field range considered provides strong quantitative validation of the effective Hamiltonian. It also confirms that fitting the high-field neutron spin wave spectra in the polarized paramagnetic state is an excellent method for determining the microscopic exchange constants of rare-earth insulating magnets that are described by an effective spin-1/2 Hamiltonian. Finally, we present results which demonstrate that a recent analysis of the polarized neutron scattering intensity of Yb2Ti2O7 using a random phase approximation (RPA) method [Chang et al., Nature Communications {3}, 992 (2012)] does not provide a good description of M(T,h) for $Tlesssim 10$ K, that is in the entire temperature regime where correlations become non-negligible.
In contrast to magnetic order formed by electrons dipolar moments, ordering phenomena associated with higher-order multipoles (quadrupoles, octupoles, etc.) are more difficult to characterize because of the limited choice of experimental probes that can distinguish different multipolar moments. The heavy-fermion compound CeB6 and its La-diluted alloys are among the best-studied realizations of the long-range-ordered multipolar phases, often referred to as hidden order. Previously the hidden order in phase II was identified as primary antiferroquadrupolar (AFQ) and field-induced octupolar (AFO) order. Here we present a combined experimental and theoretical investigation of collective excitations in the phase II of CeB6. Inelastic neutron scattering (INS) in fields up to 16.5 T reveals a new high-energy mode above 14 T in addition to the low-energy magnetic excitations. The experimental dependence of their energy on the magnitude and angle of the applied magnetic field is compared to the results of a multipolar interaction model. The magnetic excitation spectrum in rotating field is calculated within a localized approach using the pseudo-spin presentation for the Gamma8 states. We show that the rotating-field technique at fixed momentum can complement conventional INS measurements of the dispersion at constant field and holds great promise for identifying the symmetry of multipolar order parameters and the details of inter-multipolar interactions that stabilize hidden-order phases.
Single crystals of the Kitaev spin-liquid candidate $alpha$-RuCl$_3$ have been studied to determine low-temperature bulk properties, structure and the magnetic ground state. Refinements of x-ray diffraction data show that the low temperature crystal structure is described by space group $C2/m$ with a nearly-perfect honeycomb lattice exhibiting less than 0.2 % in-plane distortion. The as-grown single crystals exhibit only one sharp magnetic transition at $T_{N}$ = 7~K. The magnetic order below this temperature exhibits a propagation vector of $k$ = (0, 1, 1/3), which coincides with a 3-layer stacking of the $C2/m$ unit cells. Magnetic transitions at higher temperatures up to 14~K can be introduced by deformations of the crystal that result in regions in the crystal with a 2-layer stacking sequence. The best fit symmetry allowed magnetic structure of the as-grown crystals shows that the spins lie in the $ac$-plane, with a zigzag configuration in each honeycomb layer. The three layer repeat out-of-plane structure can be refined as a 120$^o$ spiral order or a collinear structure with spin direction 35$^o$ away from the $a$-axis. The collinear spin configuration yields a slightly better fit and also is physically preferred. The average ordered moment in either structure is less than 0.45(5) $mu_B$ per Ru$^{3+}$ ion.
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