Mark Laver

Coexistence and Competition of the Short-Range Incommensurate Antiferromagnetic Order with the Superconducting State of BaFe2-xNixAs2

Superconductivity in the iron pnictides develops near antiferromagnetism, and the antiferromagnetic (AF) phase appears to overlap with the superconducting phase in some materials such as BaFe(2-x)T(x)As2 (where T=Co or Ni). Here we use neutron scattering to demonstrate that genuine long-range AF order and superconductivity do not coexist in BaFe(2-x)Ni(x)As2 near optimal superconductivity. In addition, we find a first-order-like AF-to-superconductivity phase transition with no evidence for a magnetic quantum critical point. Instead, the data reveal that incommensurate short-range AF order coexists and competes with superconductivity, where the AF spin correlation length is comparable to the superconducting coherence length.

Avoided Quantum Criticality and Magnetoelastic Coupling in BaFe2-xNixAs2

We study the structural and magnetic orders in electron-doped BaFe(2-x)Ni(x)As2 by high-resolution synchrotron x-ray and neutron scatterings. Upon Ni doping x, the nearly simultaneous tetragonal-to-orthorhombic structural (T(s)) and antiferromagnetic (T(N)) phase transitions in BaFe2As2 are gradually suppressed and separated, resulting in T(s)>T(N) with increasing x, as was previously observed. However, the temperature separation between T(s) and T(N) decreases with increasing x for x≥0.065, tending toward a quantum bicritical point near optimal superconductivity at x≈0.1. The zero-temperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.088≲x≲0.104, resulting in a finite value of T(N)≈T(c) + 10  K above the superconducting dome around x≈0.1. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states.

Origin of electric-field-induced magnetization in multiferroic HoMnO3.

We have performed polarized and unpolarized small angle neutron scattering experiments on single crystals of HoMnO(3) and have found that an increase in magnetic scattering at low momentum transfers begins upon cooling through temperatures close to the spin reorientation transition at T(SR) approximately 40 K. We attribute the increase to an uncompensated magnetization arising within antiferromagnetic domain walls. Polarized neutron scattering experiments performed while applying an electric field show that the field suppresses magnetic scattering below T approximately 50 K, indicating that the electric field affects the magnetization via the antiferromagnetic domain walls rather than through a change to the bulk magnetic order.

Symmetry and disorder of the vitreous vortex lattice in overdoped BaFe2-xCoxAs2: Indication for strong single-vortex pinning

The disordered flux line lattice in single crystals of the slightly overdoped aFe_{2-x}Co_xAs_2 (x = 0.19, Tc = 23 K) superconductor is studied by magnetization measurements, small-angle neutron scattering (SANS), and magnetic force microscopy (MFM). In the whole range of magnetic fields up to 9 T, vortex pinning precludes the formation of an ordered Abrikosov lattice. Instead, a vitreous vortex phase (vortex glass) with a short-range hexagonal order is observed. Statistical processing of MFM datasets lets us directly measure its radial and angular distribution functions and extract the radial correlation length \zeta. In contrast to predictions of the collective pinning model, no increase in the correlated volume with the applied field is observed. Instead, we find that \zeta decreases as 1.3*R1 ~ H^(-1/2) over four decades of the applied magnetic field, where R1 is the radius of the first coordination shell of the vortex lattice. Such universal scaling of \zeta implies that the vortex pinning in iron arsenides remains strong even in the absence of static magnetism. This result is consistent with all the real- and reciprocal-space vortex-lattice measurements in overdoped as-grown aFe_{2-x}Co_xAs_2 published to date and is thus sample-independent. The failure of the collective pinning model suggests that the vortices remain in the single-vortex pinning limit even in high magnetic fields up to 9 T.

Origin of magnetostriction in Fe–Ga

This paper investigates the origin of large magnetostriction in Fe–Ga alloys using small-angle neutron scattering (SANS) and Kerr microscopy. The SANS data for a single-crystal, electron irradiated, and quenched Fe81Ga19 sample under externally applied magnetic and elastic fields revealed the existence of magnetostrictive nanoclusters spaced at ∼15 nm apart that have a different magnetization than the A2 matrix. Combining the SANS results and the magnetization orientation obtained from the magnetic domain images using a Kerr microscope, it appears that the nanoclusters contribute significantly to the macroscopic magnetostriction.

Morphotropic phase boundaries in ferromagnets: Tb1-xDyxFe2 alloys

The structure and properties of the ferromagnet Tb(1-x)Dy(x)Fe(2) are explored through the morphotropic phase boundary (MPB) separating ferroic phases of differing symmetry. Our synchrotron data support a first order structural transition, with a broadening MPB width at higher temperatures. The optimal point for magnetomechanical applications is not centered on the MPB but lies on the rhombohedral side, where the high striction of the rhombohedral majority phase combines with the softened anisotropy of the MPB. We compare our findings with single ion crystal field theory and with ferroelectric MPBs, where the controlling energies are different.

Exploring the fragile antiferromagnetic superconducting phase in CeCoIn5.

CeCoIn5 is a heavy fermion type-II superconductor showing clear signs of Pauli-limited superconductivity. A variety of measurements give evidence for a transition at high magnetic fields inside the superconducting state, when the field is applied either parallel to or perpendicular to the c axis. When the field is perpendicular to the c axis, antiferromagnetic order develops on the high-field side of the transition. This order remains as the field is rotated out of the basal plane, but the associated moment eventually disappears above 17°, indicating that anomalies seen with the field parallel to the c axis are not related to this magnetic order. We discuss the implications of this finding.

Fermi Surface and Order Parameter Driven Vortex Lattice Structure Transitions in Twin-Free YBa2Cu3O7

We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3O7 at 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twin-domain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, marks a crossover from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the order-parameter anisotropy.

Morphology of the Superconducting Vortex Lattice in Ultrapure Niobium

The morphology of the superconducting flux line lattice (FLL) of Nb comprises gradual variations with various lock-in transitions and symmetry breaking rotations. We report a comprehensive small-angle neutron scattering study of the FLL in an ultrapure single crystal of Nb as a function of the orientation of the applied magnetic field. We attribute the general morphology of the FLL and its orientation to three dominant mechanisms; first, nonlocal contributions, second, the transition between open and closed Fermi surface sheets and, third, the intermediate mixed state between the Meissner and the Shubnikov phase.

Correlating material-specific layers and magnetic distributions within onion-like Fe3O4/MnO/γ-Mn2O3 core/shell nanoparticles

The magnetic responses of two nanoparticle systems comprised of Fe3O4/γ-Mn2O3 (soft ferrimagnetic, FM/hard FM) and Fe3O4/MnO/γ-Mn2O3 (soft FM/antiferromagnetic, AFM/hard FM) are compared, where the MnO serves to physically decouple the FM layers. Variation in the temperature and applied field allows for Small Angle Neutron Scattering (SANS) measurements of the magnetic moments both parallel and perpendicular to an applied field. Data for the bilayer particle indicate that the graded ferrimagnetic layers are coupled and respond to the field as a single unit. For the trilayer nanoparticles, magnetometry suggests a Curie temperature (TC) ≈ 40 K for the outer γ-Mn2O3 component, yet SANS reveals an increase in the magnetization associated with outer layer that is perpendicular to the applied field above TC during magnetic reversal. This result suggests that the γ-Mn2O3 magnetically reorients relative to the applied field as the temperature is increased above 40 K.