34 research outputs found
Circularly polarized resonant soft x-ray diffraction study of helical magnetism in hexaferrite
Magnetic spiral structures can exhibit ferroelectric moments as recently
demonstrated in various multiferroic materials. In such cases the helicity of
the magnetic spiral is directly correlated with the direction of the
ferroelectric moment and measurement of the helicity of magnetic structures is
of current interest. Soft x-ray resonant diffraction is particularly
advantageous because it combines element selectivity with a large magnetic
cross-section. We calculate the polarization dependence of the resonant
magnetic x-ray cross-section (electric dipole transition) for the basal plane
magnetic spiral in hexaferrite Ba0.8Sr1.2Zn2Fe12O22 and deduce its domain
population using circular polarized incident radiation. We demonstrate there is
a direct correlation between the diffracted radiation and the helicity of the
magnetic spiral.Comment: 4 pages, 4 figure
Commensurate lattice distortion in the layered titanium oxypnictides NaTiO ( As, Sb) determined by X-ray diffraction
We report single crystal X-ray diffraction measurements on
NaTiO ( = As, Sb) which reveal a charge superstructure that
appears below the density wave transitions previously observed in bulk data.
From symmetry-constrained structure refinements we establish that the
associated distortion mode can be described by two propagation vectors, and , with (Sb) or (As), and primarily involves in-plane displacements of the Ti atoms
perpendicular to the Ti--O bonds. The results provide direct evidence for
phonon-assisted charge density wave order in NaTiO and identify
a proximate ordered phase that could compete with superconductivity in doped
BaTiSbO
The Final Chapter In The Saga Of YIG
The magnetic insulator Yttrium Iron Garnet can be grown with exceptional
quality, has a ferrimagnetic transition temperature of nearly 600 K, and is
used in microwave and spintronic devices that can operate at room temperature.
The most accurate prior measurements of the magnon spectrum date back nearly 40
years, but cover only 3 of the lowest energy modes out of 20 distinct magnon
branches. Here we have used time-of-flight inelastic neutron scattering to
measure the full magnon spectrum throughout the Brillouin zone. We find that
the existing model of the excitation spectrum, well known from an earlier work
titled "The Saga of YIG", fails to describe the optical magnon modes. Using a
very general spin Hamiltonian, we show that the magnetic interactions are both
longer-ranged and more complex than was previously understood. The results
provide the basis for accurate microscopic models of the finite temperature
magnetic properties of Yttrium Iron Garnet, necessary for next-generation
electronic devices.Comment: 10 pages, 3 figures, 4 supplementary figures, 1 table, 1
supplementary tabl
Coupling of magnetic order to planar Bi electrons in the anisotropic Dirac metals AMnBi2 (A = Sr, Ca)
We report powder and single crystal neutron diffraction measurements of the
magnetic order in AMnBi2 (A = Sr and Ca), two layered manganese pnictides with
anisotropic Dirac fermions on a Bi square net. Both materials are found to
order at TN approx 300 K in k = 0 antiferromagnetic structures, with ordered Mn
moments at T = 10 K of approximately 3.8 muB aligned along the c axis. The
magnetic structures are Neel-type within the Mn--Bi layers but the inter-layer
ordering is different, being antiferromagnetic in SrMnBi2 and ferromagnetic in
CaMnBi2. This allows a mean-field coupling of the magnetic order to Bi
electrons in CaMnBi2 but not in SrMnBi2. We find clear evidence that magnetic
order influences electrical transport. First principles calculations explain
the experimental observations and suggest that the mechanism for different
inter-layer ordering in the two compounds is the competition between the
anteiferromagnetic superexchange and ferromagnetic double exchange carried by
itinerant Bi electrons.Comment: Accepted for publication in Physical Review B. Version 2 includes
additional sample characterisation and bulk measurements, and ab initio
electronic structure calculation
Spin dynamics and exchange interactions in CuO measured by neutron scattering
The magnetic properties of CuO encompass several contemporary themes in
condensed matter physics, including quantum magnetism, magnetic frustration,
magnetically-induced ferroelectricity and orbital currents. Here we report
polarized and unpolarized neutron inelastic scattering measurements which
provide a comprehensive map of the cooperative spin dynamics in the low
temperature antiferromagnetic (AFM) phase of CuO throughout much of the
Brillouin zone. At high energies (\,meV) the spectrum displays
continuum features consistent with the des Cloizeax--Pearson dispersion for an
ideal Heisenberg AFM chain. At lower energies the spectrum
becomes more three-dimensional, and we find that a linear spin-wave model for a
Heisenberg AFM provides a very good description of the data, allowing for an
accurate determination of the relevant exchange constants in an effective spin
Hamiltonian for CuO. In the high temperature helicoidal phase, there are
features in the measured low-energy spectrum that we could not reproduce with a
spin-only model. We discuss how these might be associated with the
magnetically-induced multiferroic behavior observed in this phase
Magnetically induced metal-insulator transition in Pb2CaOsO6
We report on the structural, magnetic, and electronic properties of two new
double-perovskites synthesized under high pressure; Pb2CaOsO6 and Pb2ZnOsO6.
Upon cooling below 80 K, Pb2CaOsO6 simultaneously undergoes a metal--insulator
transition and develops antiferromagnetic order. Pb2ZnOsO6, on the other hand,
remains a paramagnetic metal down to 2 K. The key difference between the two
compounds lies in their crystal structure. The Os atoms in Pb2ZnOsO6 are
arranged on an approximately face-centred cubic lattice with strong
antiferromagnetic nearest-neighbor exchange couplings. The geometrical
frustration inherent to this lattice prevents magnetic order from forming down
to the lowest temperatures. In contrast, the unit cell of Pb2CaOsO6 is heavily
distorted up to at least 500 K, including antiferroelectric-like displacements
of the Pb and O atoms despite metallic conductivity above 80 K. This distortion
relieves the magnetic frustration, facilitating magnetic order which in turn
drives the metal--insulator transition. Our results suggest that the phase
transition in Pb2CaOsO6 is spin-driven, and could be a rare example of a Slater
transition.Comment: 14 pages, 9 figures. Accepted as a regular article in Phys. Rev.
Spin dynamics in the antiferromagnetic phases of the Dirac metals AMnBi2 (A = Sr, Ca)
The square Bi layers in AMnBi2 (A= Sr, Ca) host Dirac fermions which coexist with antiferromagnetic order on the Mn sublattice below TN=290 K (Sr) and 265 K (Ca). We have measured the spin-wave dispersion in these materials by triple-axis neutron spectroscopy. The spectra show pronounced spin gaps of 10.2(2)meV (Sr) and 8.3(8)meV (Ca) and extend to a maximum energy transfer of 61-63 meV. The observed spectra can be accurately reproduced by linear spin-wave theory from a Heisenberg effective spin Hamiltonian. Detailed global fits of the full magnon dispersion are used to determine the in-plane and interlayer exchange parameters as well as well as the magnetocrystalline anisotropy constant. To within experimental error we find no evidence that the magnetic dynamics are influenced by the Dirac fermions