We submit that the magnetic space-group Cac (#9.41) is consistent with the established magnetic structure of BaFe2Se3, with magnetic dipole moments in a motif that uses two ladders [Caron J M et al 2011 Phys. Rev. B 84 180409(R)]. The corresponding c
rystal class m1 allows axial and polar dipoles and forbids bulk ferromagnetism. The compound supports magneto-electric multipoles, including a magnetic charge (monopole) and an anapole (magnetic toroidal dipole) visible in the Bragg diffraction of x-rays and neutrons. Our comprehensive simulation of neutron Bragg diffraction by BaFe2Se3 exploits expressions of a general nature that can be of use with other magnetic materials. Electric toroidal moments are also allowed in BaFe2Se3. A discussion of our findings for resonant x-ray Bragg diffraction illustrates the benefit of a common platform for neutron and x-ray diffraction.
Magneto-structural phase transitions in Ba1-xAxFe2As2 (A = K, Na) materials are discussed for both magnetically and orbitally driven mechanisms, using a symmetry analysis formulated within the Landau theory of phase transitions. Both mechanisms predi
ct identical orthorhombic space-group symmetries for the nematic and magnetic phases observed over much of the phase diagram, but they predict different tetragonal space-group symmetries for the newly discovered re-entrant tetragonal phase in Ba1-xNaxFe2As2 (x ~ 0.24-0.28). In a magnetic scenario, magnetic order with moments along the c-axis, as found experimentally, does not allow any type of orbital order, but in an orbital scenario, we have determined two possible orbital patterns, specified by P4/mnc1 and I4221 space groups, which do not require atomic displacements relative to the parent I4/mmm1 symmetry and, in consequence, are indistinguishable in conventional diffraction experiments. We demonstrate that the three possible space groups are however, distinct in resonant X-ray Bragg diffraction patterns created by Templeton & Templeton scattering. This provides an experimental method of distinguishing between magnetic and orbital models.
A theoretical investigation of a plausible construct for electronic structure in iridate perovskites demonstrates the existence of magnetic multipoles hitherto not identified. The strange multipoles, which are parity-even, time-odd and even rank tens
ors, are absent from the so-called jeff = 1/2 model. We prove that the strange multipoles contribute to magnetic neutron diffraction, and we estimate their contribution to intensities of Bragg spots for Sr2IrO4. The construct encompasses the jeff = 1/2 model, and it is consistent with the known magnetic structure, ordered magnetic moment, and published resonant x-ray Bragg diffraction data. Over and above time-odd quadrupoles and hexadecapoles, whose contribution changes neutron Bragg intensities by an order of magnitude, according to our estimates, are relatively small triakontadipoles recently proposed as the primary magnetic order-parameter of Sr2IrO4.
The lightly hole-doped system CeOs1.94Re0.06Al10 has been studied by muon spin relaxation and neutron diffraction measurements. A long-range antiferromagnetic ordering of the Ce-sublattice with substantially reduced value of the magnetic moment 0.18(
1) mu_B has been found below T_N = 21 K. Similar to the undoped parent compound, the magnetic ground state of CeOs1.94Re0.06Al10 preserves the anomalous direction of the ordered moments along the c-axis. The obtained result reveals the crucial difference between electron- and hole-doping effects on the magnetic ordering in CeOs2Al10. The former suppresses the anisotropic c-f hybridization and promotes localized Ce moments. On the contrary, the latter increases the hybridization and shifts the system towards delocalized non-magnetic state.
Magnetic charges, or magnetic monopoles, may form in the electronic structure of magnetic materials where ions are deprived of symmetry with respect to spatial inversion. Predicted in 2009, the strange magnetic, pseudoscalars have recently been found
different from zero in simulations of electronic structures of some magnetically ordered, orthorhombic, lithium orthophosphates (LiMPO4). We prove that magnetic charges in lithium orthophosphates diffract x-rays tuned in energy to an atomic resonance, and to guide future experiments we calculate appropriate unit-cell structure factors for monoclinic LiCoPO4 and orthorhombic LiNiPO4.
The effect of Ir substitution for Os in CeOs2Al10, with an unusually high Neel temperature of T~28.5K, has been studied by high-resolution neutron diffraction and magnetization measurements. A small amount of Ir (~ 8%) results in a pronounced change
of the magnetic structure of the Ce-sublattice. The new magnetic ground state is controlled by the single ion anisotropy and implies antiferromagnetic arrangement of the Ce-moments along the a-axis, as expected from the anisotropy of the paramagnetic susceptibility. The value of the ordered moments, 0.92(1) mu_B, is substantially bigger than in the undoped compound, whereas the transition temperature is reduced down to 21K. A comparison of the observed phenomena with the recently studied CeRu1.9Rh0.1Al10 system, exhibiting similar behaviour [A. Kondo et al., J. Phys. Soc. Jpn. 82, 054709 (2013)], strongly suggests the electron doping as the main origin of the ground state changes. This provides a new way of exploring the anomalous magnetic properties of the Ce(Ru/Os)2Al10 compounds.
The $4f$-electron system YbAl$_3$C$_3$ with a non-magnetic spin-dimer ground state has been studied by neutron diffraction in an applied magnetic field. A long-range magnetic order involving both ferromagnetic and antiferromagnetic components has bee
n revealed above the critical field H$_Csim $ 6T at temperature T=0.05K. The magnetic structure indicates that the geometrical frustration of the prototype hexagonal lattice is not fully relieved in the low-temperature orthorhombic phase. The suppression of magnetic ordering by the remanent frustration is the key factor stabilizing the non-magnetic singlet ground state in zero field. Temperature dependent measurements in the applied field H=12T revealed that the long-range ordering persists up to temperatures significantly higher than the spin gap indicating that this phase is not directly related to the singlet-triplet excitation. Combining our neutron diffraction results with the previously published phase diagram, we support the existence of an intermediate disordered phase as the first excitation from the non-magnetic singlet ground state. Based on our results, we propose YbAl$_3$C$_3$ as a new material for studying the quantum phase transitions of heavy-fermion metals under the influence of geometrical frustration.
