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We report polarized- and unpolarized- neutron inelastic scattering measurements of the magnetic excitation spectrum in the spin-charge ordered phase of La3/2Sr1/2NiO4. Up to energies of ~30 meV we observe broad magnetic modes characteristic of a near checkerboard ordering. A linear spin-wave model for an ideal checkerboard ordering with a single antiferromagnetic exchange interaction J = 5.8 +/- 0.5 meV between next-nearest-neighbour spins on Ni2+ sites, together with a small XY-like single-ion anisotropy, provides a reasonable description of the measured dispersion. Above 30 meV the excitations are not fully consistent with the linear spin-wave model, with modes near the two-dimensional reciprocal space wavevector (0.5,0.5) having an anomalously large intensity. Furthermore, two additional dispersive modes not predicted by spin wave theory were observed, both of which are probably magnetic. One disperses away from (0.5,0.5) in the energy range between 50-56 meV, and the other appears around (h,k) type positions (h,k = integer) in the energy range 31-39 meV. We propose a model in which these anomalous features are explained by the existence of discommensurations in the checkerboard ordering. At low energies there is additional diffuse scattering centred on the magnetic ordering wavevector. We associate this diffuse scattering with dynamic antiferromagnetic correlations between spins attached to the doped holes.
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Recent neutron scattering measurements reveal spin and charge ordering in the half-doped nickelate, La$_{3/2}$ Sr$_{1/2}$ NiO$_4$. Many of the features of the magnetic excitations have been explained in terms of the spin waves of diagonal stripes with weak single-ion anisotropy. However, an optical mode dispersing away from the (pi,pi) point was not captured by this theory. We show here that this apparent optical mode is a natural consequence of stripe twinning in a diagonal stripe pattern with a magnetic coupling structure which is two-fold symmetric, i.e. one possessing the same spatial rotational symmetry as the ground state.
Recently, based on the refined crystal structure of Pr0.6Ca0.4MnO3 from neutron diffraction, Daoud-Aladine et al.[PRL89,97205(2002)] have proposed a new ground state structure for the half-doped manganites R0.5Ca0.5MnO3, where R is a trivalent ion like Bi,La,Pr,Sm or Y. Their proposal describes the CE magnetic structure attributed to these materials as an arrangement of dimers along the ferromagnetic Mn zig-zag chains that form it. However, the dimers proposal is in conflict with the Goodenough-Kanamori-Anderson rules, which give a coherent description of many transition metal insulating compounds and predict the coexistence of Mn3+ and Mn4+ ions in equal parts in the half-doped manganites. On the other hand, Rivadulla et al.[PRB 66, 174432 (2002)] have studied several single crystal samples of half-doped manganites and propose a phase diagram in terms of the tolerance factor which contains both types of structures. In the present work we have calculated the magnon dispersion relations for the CE magnetic structure, arising for each type of proposal: the charge ordered and the dimer phases, respectively. We consider a three-dimensional unit cell containing 16 spins, and compare the magnetic excitations along different paths in the first Brillouin zone. We conclude that measurement of the magnon dispersion relations should allow a clear distinction between the two proposals, predicting qualitative differences arising along specific directions of propagation in the first Brillouin zone.
As a simple cubic system with only one f electron per cerium ion, CeB6 is of model character to investigate the interplay of orbital phenomena with magnetism. It is also a textbook example of a compound that exhibits magnetically hidden order -- a low-temperature magnetic phase with ordered quadrupolar moments. It is difficult to identify the symmetry of such hidden-order states in common x-ray or neutron scattering experiments, as there is no signal in zero field, however alternative techniques like neutron diffraction in external field, resonant x-ray scattering, or ultrasonic investigations can be applied. Another possible method for characterizing hidden order is to look at the magnetic excitation spectrum, which carries the imprint of the multipolar interactions and the hidden order parameter in its dispersion relations. Using a specific candidate model, the dispersion is calculated and then compared to that measured with inelastic neutron scattering. Until recently, only a limited amount of data which show the presence of dispersing excitations measured along a few high-symmetry directions in an applied magnetic field were available. Early attempts to compare such calculations with experiments showed that only strongest modes at high-symmetry points could be identified. The present review of the most recent neutron-scattering results is intended to satisfy the need of more accurate inelastic neutron-scattering experiments as a function of field and temperature, giving us the opportunity to identify existing excitation branches in CeB6 and conclusively compare them with the theoretically predicted multipolar excitations.
A few years ago we predicted theoretically that in systems with nesting of the Fermi surface the spin-valley half-metal has lower energy than the spin density wave state. In this paper we suggest a possible way to distinguish these phases experimentally. We calculate dynamical spin susceptibility tensor for both states in the framework of the Kubo formalism. Discussed phases have different numbers of the bands: four bands in the spin-valley half-metal and only two bands in the spin density wave. Therefore, their susceptibilities, as functions of frequency, have different number of peaks. Besides, the spin-valley half-metal does not have rotational symmetry, thus, in general the off-diagonal components of susceptibility tensor are non-zero. The spin density wave obeys robust rotational symmetry and off-diagonal components of the susceptibility tensor are zero. These characteristic features can be observed in experiments with inelastic neutron scattering.
We observe how the charge-ordering (CO) temperature of Nd1/2Sr1/2MnO3 decreases with the external pressure p from 160 K at p = 0 down to 30 K at p ~ 4.5 GPa, by measuring the values p, T where the far-infrared spectral weight of the metallic phase is fully recovered. We thus determine the (p, T) phase diagram of CO in that manganite. We also find that the parameter d(lnTCO)/dp which describes this metallization from the CO phase is equal and opposite to the quantity d(lnTc)/dp which governs the metallization of the paramagnetic state at comparable Curie temperatures Tc, in similar manganites at half doping.
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