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Register a new userSpin-wave dispersion in half-doped LaSrNiO
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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.
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Magnonics is seen nowadays as a candidate technology for energy-efficient data processing in classical and quantum systems. Pronounced nonlinearity, anisotropy of dispersion relations and phase degree of freedom of spin waves require advanced methodology for probing spin waves at room as well as at mK temperatures. Yet, the use of the established optical techniques like Brillouin light scattering (BLS) or magneto optical Kerr effect (MOKE) at ultra-low temperatures is forbiddingly complicated. By contrast, microwave spectroscopy can be used at all temperatures but is usually lacking spatial and wavenumber resolution. Here, we develop a variable-gap propagating spin-wave spectroscopy (VG-PSWS) method for the deduction of the dispersion relation of spin waves in wide frequency and wavenumber range. The method is based on the phase-resolved analysis of the spin-wave transmission between two antennas with variable spacing, in conjunction with theoretical data treatment. We validate the method for the in-plane magnetized CoFeB and YIG thin films in $kperp B$ and $kparallel B$ geometries by deducing the full set of material and spin-wave parameters, including spin-wave dispersion, hybridization of the fundamental mode with the higher-order perpendicular standing spin-wave modes and surface spin pinning. The compatibility of microwaves with low temperatures makes this approach attractive for cryogenic magnonics at the nanoscale.
We present electronic structure calculations in combination with local and non-local many-body correlation effects for the half-metallic ferromagnet CrO$_2$. Finite-temperature Dynamical Mean Field Theory results show the existence of non-quasiparticle states, which were recently observed as almost currentless minority spin states near the Fermi energy in resonant scattering experients. At zero temperatures, Variational Cluster Approach calculations support the half-metallic nature of CrO$_2$ as seen in superconducting point contact spectroscopy. The combination of these two techniques allowed us to qualitatively describe the spin-polarization in CrO$_2$.
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.
In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped $mathrm{La_{0.5}Ca_{0.5}MnO_3}$(LCMO) synthesized by chemical route, having particle size down to an average diameter of 15nm. It was observed that the size reduction leads to change in crystal structure and the room temperature structure is arrested so that the structure does not evolve on cooling unlike bulk samples. The structural change mainly affects the orthorhombic distortion of the lattice. By making comparison with observed crystal structure data under hydrostatic pressure it is suggested that the change in the crystal structure of the nanocrystals occurs due to an effective hydrostatic pressure created by the surface pressure on size reduction. This not only changes the structure but also causes the room temperature structure to freeze-in. The size reduction also does not allow the long supercell modulation needed for the Charge Ordering, characteristic of this half-doped manganite, to set-in. The magnetic and transport measurements also show that the Charge Ordering (CO) does not occur when the size is reduced below a critical size. Instead, the nanocrystals show ferromagnetic ordering down to the lowest temperatures along with metallic type conductivity. Our investigation establishes a structural basis for the destabilization of CO state observed in half-doped manganite nanocrystals.
We have investigated possible spin and charge ordered states in 3d transition-metal oxides with small or negative charge-transfer energy, which can be regarded as self-doped Mott insulators, using Hartree-Fock calculations on d-p-type lattice models. It was found that an antiferromagnetic state with charge ordering in oxygen 2p orbitals is favored for relatively large charge-transfer energy and may be relevant for PrNiO$_3$ and NdNiO$_3$. On the other hand, an antiferromagnetic state with charge ordering in transition-metal 3$d$ orbitals tends to be stable for highly negative charge-transfer energy and can be stabilized by the breathing-type lattice distortion; this is probably realized in YNiO$_3$.
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