Direct Observation of Anapoles by Neutron Diffraction.

The scope of magnetic neutron scattering has been expanded by the observation of electronic Dirac dipoles (anapoles) that are polar (parity odd) and magnetic (time odd). A zero-magnetization ferromagnet Sm_{0.976}Gd_{0.024}Al_{2} with a diamond-type structure presents Dirac multipoles at basis-forbidden reflections that include the standard (2, 2, 2) reflection. Magnetic amplitudes measured at four such reflections are in full accord with a structure factor calculated from the appropriate magnetic space group.

Single to Multiquasiparticle Excitations in the Itinerant Helical Magnet CeRhIn(5).

CeRhIn(5) is an itinerant magnet where the Ce(3+) spins order in a simple helical phase. We investigate the spin excitations and observe sharp spin waves parameterized by a nearest-neighbor exchange, J(RKKY)=0.88±0.05 meV. At higher energies, the spin fluctuations are heavily damped, where single-quasiparticle excitations are replaced by a momentum- and energy-broadened continuum constrained by kinematics of energy and momentum conservation. The delicate energy balance between localized and itinerant characters results in the breakdown of the single-quasiparticle picture in CeRhIn(5).

Hidden order in URu2Si2 unveiled.

We report on measurements, by polarized neutron elastic scattering, of the magnetization distribution induced in a single crystal of URu2Si2 under a magnetic field applied along the tetragonal c axis. A subtle change in this distribution, revealed by maximum entropy analysis of the data, is found when the temperature is decreased to the range of the hidden order. An analysis in terms of U(4+) ionic states reveals that this change is a fingerprint of a freezing of rank 5 multipoles, i.e., dotriacontapoles.

Single crystal polarized neutron diffraction study of the magnetic structure of HoFeO3.

Polarised neutron diffraction measurements have been made on HoFeO3 single crystals magnetised in both the [0 0 1] and [1 0 0] directions (Pbnm setting). The polarisation dependencies of Bragg reflection intensities were measured both with a high field of [Formula: see text] T parallel to [0 0 1] at [Formula: see text] K and with the lower field [Formula: see text] T parallel to [1 0 0] at [Formula: see text] K. A Fourier projection of magnetization induced parallel to [0 0 1], made using the hk0 reflections measured in 9 T, indicates that almost all of it is due to alignment of Ho moments. Further analysis of the asymmetries of general reflections in these data showed that although, at 70 K, 9 T applied parallel to [0 0 1] hardly perturbs the antiferromagnetic order of the Fe sublattices, it induces significant antiferromagnetic order of the Ho sublattices in the [Formula: see text] plane, with the antiferromagnetic components of moment having the same order of magnitude as the induced ferromagnetic ones. Strong intensity asymmetries measured in the low temperature [Formula: see text] structure with a lower field, 0.5 T [Formula: see text] [1 0 0] allowed the variation of the ordered components of the Ho and Fe moments to be followed. Their absolute orientations, in the [Formula: see text] domain stabilised by the field were determined relative to the distorted perovskite structure. This relationship fixes the sign of the Dzyalshinski-Moriya (D-M) interaction which leads to the weak ferromagnetism. Our results indicate that the combination of strong y-axis anisotropy of the Ho moments and Ho-Fe exchange interactions breaks the centrosymmetry of the structure and could lead to ferroelectric polarization.

Propagation of Nd magnetic phases in Nd/Sm(001) superlattices.

The propagation of Nd long range magnetic order in the hexagonal and cubic sublattices has been investigated in double hexagonal compact Nd/Sm(001) superlattices by resonant x-ray magnetic scattering at the Nd L(2) absorption edge. For a superlattice with 3.7 nm thick Sm layers, the magnetic structure of the hexagonal sublattice propagates coherently through several bilayers, whereas the order in the cubic sublattice remains confined to single Nd blocks. For a superlattice with 1.4 nm thick Sm layers, the magnetic structures of both sublattices appear to propagate coherently through the superlattice. This is the first observation (i) of the long range coherent propagation of Nd order on the cubic sites between Nd blocks and (ii) of a different thickness dependence of the propagation of the Nd magnetic phases associated with the hexagonal and cubic sublattices. The propagation of the Nd magnetic order through Sm is interpreted in terms of generalized susceptibility of the Nd conduction electrons.

Direct observation of finite size effects in chains of antiferromagnetically coupled spins.

Finite spin chains made of few magnetic ions are the ultimate-size structures that can be engineered to perform spin manipulations for quantum information devices. Their spin structure is expected to show finite size effects and its knowledge is of great importance both for fundamental physics and applications. Until now a direct and quantitative measurement of the spatial distribution of the magnetization of such small structures has not been achieved even with the most advanced microscopic techniques. Here we present measurements of the spin density distribution of a finite chain of eight spin-3/2 ions using polarized neutron diffraction. The data reveal edge effects that are a consequence of the finite size and of the parity of the chain and indicate a noncollinear spin arrangement. This is in contrast with the uniform spin distribution observed in the parent closed chain and the collinear arrangement in odd-open chains.

Direct observation of double-k lattice modulation in double-k magnetic structures. The case of CeAl(2).

Symmetry analysis is combined with x-ray scattering experiments to investigate the lattice modulation associated with the incommensurate magnetic structure in the case of a double- k structure. The expansion of the free energy shows that the components of the magnetic structure with propagation vectors k(1) and k(2) can couple via components of lattice modulations. It is shown that the classical diffraction peaks reflecting a 2k propagation vector, associated with magneto-elastic effects in single- k structures, will coexist with diffraction peaks with propagation vectors k(1)-k(2) or k(1)+k(2). The existence of these latter peaks can be considered as a signature of a double- k magnetic structure. In the case of the double- k modulated structure of CeAl(2), group theory is applied directly to the study of the charge modulation. An x-ray scattering study of the 2k satellites shows that the lattice displacements of the two Ce sites of the structure are antiparallel to each other, and perpendicular to the direction of the magnetic modulation. We also confirm experimentally the existence of k(1)+k(2) satellites.

Electronic state of PuCoGa5 and NpCoGa5 as probed by polarized neutrons.

By using single crystals and polarized neutrons, we have measured the orbital and spin components of the microscopic magnetization in the paramagnetic state of NpCoGa(5) and PuCoGa(5). The microscopic magnetization of NpCoGa(5) agrees with that observed in bulk susceptibility measurements and the magnetic moment has spin and orbital contributions as expected for intermediate coupling. In contrast, for PuCoGa(5), which is a superconductor with a high transition temperature, the microscopic magnetization in the paramagnetic state is small, temperature-independent, and significantly below the value found with bulk techniques at low temperatures. The orbital moment dominates the magnetization.

Magnetic-field-induced spin excitations and renormalized spin gap of the underdoped La1.895Sr0.105CuO4 superconductor.

High-resolution neutron inelastic scattering experiments in applied magnetic fields have been performed on La1.895Sr0.105CuO4 (LSCO). In zero field, the temperature dependence of the low-energy peak intensity at the incommensurate momentum transfer QIC=(0.5,0.5+/-delta,0),(0.5+/-delta,0.5,0) exhibits an anomaly at the superconducting Tc which broadens and shifts to lower temperature upon the application of a magnetic field along the c axis. A field-induced enhancement of the spectral weight is observed, but only at finite energy transfers and in an intermediate temperature range. These observations establish the opening of a strongly downward renormalized spin gap in the underdoped regime of LSCO. This behavior contrasts with the observed doping dependence of most electronic energy features.

Spin-liquid correlations in the Nd-langasite anisotropic kagomé antiferromagnet.

Dynamical magnetic correlations in the geometrically frustrated Nd(3)Ga(5)SiO(14) compound were probed by inelastic neutron scattering on a single crystal. A scattering signal with a ring shape distribution in reciprocal space and unprecedented dispersive features was discovered. Comparison with calculated static magnetic scattering from models of correlated spins suggests that the observed phase is a spin liquid inherent to an antiferromagnetic kagomé-like lattice of anisotropic Nd moments.

Spin excitations in an anisotropic bond-alternating quantum s = 1 chain in a magnetic field: contrast to haldane spin chains.

Inelastic neutron scattering experiments on the S = 1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9D24N4)(NO2)ClO4 have been performed under magnetic fields below and above a critical field Hc at which the energy gap closes. Normal field dependence of Zeeman splitting of the excited triplet modes below Hc has been observed, but the highest mode is unusually small and smears out with increasing field. This can be explained by an interaction with a low-lying two magnon continuum at q(parallel) = pi that is present in dimerized chains but absent in uniform ones. Above Hc, we find only one excited mode, in stark contrast with three massive excitations previously observed in the structurally similar Haldane-gap material NDMAP [A. Zheludev, Phys. Rev. B 68, 134438 (2003)].

Polarization analysis in the 3-25 keV range at the ESRF magnetic scattering beamline.

The ESRF magnetic scattering beamline has been optimized for easy tunability of the polarization and energy in the 3-40 keV range. The linear horizontal polarization from the undulator reaches 99.9%, with a flux of approximately 10(12) photons s(-1) at the sample. The diffractometer can operate in horizontal and vertical geometries, with an energy or polarization analyser. The capabilities of this beamline in terms of flux, energy tunability and polarization, permitted polarization analysis of resonant magnetic scattering from antiferromagnetic UPd(2)Si(2) at both the L(2)- and M(4)-edges of uranium, to separate the contributions of the 5f and 6d electrons to the magnetism.

Resonant enhancements at nonmagnetic ions: new possibilities for magnetic X-ray scattering.

Synchrotron experiments with uranium antiferromagnetic compounds have discovered large ( >1000) enhancements of the magnetic scattering intensities at the K edges of nominally nonmagnetic anions, e.g., Ga and As. The width in energy, the position with respect to the white line, and the azimuthal and polarization dependencies permit one to associate the signal with transitions of E1 dipole symmetry from 1s to 4p states. In momentum space, the signal exhibits long-range order at the antiferromagnetic wave vector. We discuss possible channels capable of generating the observed enhancements.

The XMaS beamline at ESRF: instrumental developments and high-resolution diffraction studies.

The beamline, which is situated on a bending magnet at ESRF, comprises a unique combination of instrumentation for high-resolution and magnetic single-crystal diffraction. White-beam operation is possible, as well as focused and unfocused monochromatic modes. In addition to an eleven-axis Huber diffractometer, which facilitates simple operation in both vertical and horizontal scattering geometries, there is an in-vacuum polarization analyser and slit system, mirrors for harmonic rejection, sub 4.2 K and 1 Tesla magnetic field sample environment, plus a diamond phase plate for polarization conditioning. The instrumentation developed specifically for this beamline is described, and its use illustrated by recent scientific results.