570 research outputs found
Effect of uniaxial strain on the structural and magnetic phase transitions in BaFeAs
We report neutron scattering experiments probing the influence of uniaxial
strain on both the magnetic and structural order parameters in the parent iron
pnictide compound, BaFeAs. Our data show that modest strain fields
along the in-plane orthorhombic b-axis can affect significant changes in phase
behavior simultaneous to the removal of structural twinning effects. As a
result, we demonstrate in BaFeAs samples detwinned via uniaxial strain
that the in-plane C symmetry is broken by \textit{both} the structural
lattice distortion \textit{and} long-range spin ordering at temperatures far
above the nominal (strain-free), phase transition temperatures. Surprising
changes in the magnetic order parameter of this system under relatively small
strain fields also suggest the inherent presence of magnetic domains
fluctuating above the strain-free ordering temperature in this material.Comment: 4 pages, 3 figure
The evolution of antiferromagnetic susceptibility to uniaxial pressure in Ba(Fe{1-x}Co{x})2As2
Neutron diffraction measurements are presented measuring the responses of
both magnetic and structural order parameters of parent and lightly Co-doped
Ba(Fe{1-x}Co{x})2As2 under the application of uniaxial pressure. We find that
the uniaxial pressure induces a thermal shift in the onset of antiferromagnetic
order that grows as a percentage of T_N as Co-doping is increased and the
superconducting phase is approached. Additionally, as uniaxial pressure is
increased within parent and lightly-doped Ba(Fe{1-x}Co{x})2As2 on the first
order side of the tricritical point, we observe a decoupling between the onsets
of the orthorhombic structural distortion and antiferromagnetism. Our findings
place needed constraints on models exploring the nematic susceptibility of the
bilayer pnictides in the tetragonal, paramagnetic regime.Comment: 10 pages, 7 figure
Neutron scattering study of magnetic phase separation in nanocrystalline LaCaMnO
We demonstrate that magnetic phase separation and competing spin order in the
colossal magnetoresistive (CMR) manganites can be directly explored via tuning
strain in bulk samples of nanocrystalline LaCaMnO. Our results
show that strain can be reversibly frozen into the lattice in order to
stabilize coexisting antiferromagnetic domains within the nominally
ferromagnetic metallic state of LaCaMnO. The measurement of
tunable phase separation via magnetic neutron powder diffraction presents a
direct route of exploring the correlated spin properties of phase separated
charge/magnetic order in highly strained CMR materials and opens a potential
avenue for realizing intergrain spin tunnel junction networks with enhanced CMR
behavior in a chemically homogeneous material.Comment: 6 pages, 4 figures. New figure and text added to manuscrip
Unpinning the skyrmion lattice in MnSi: Effect of substitutional disorder
By employing magnetization and small angle neutron scattering measurements, we have investigated the behavior of the skyrmion lattice (SKL) and the helical order in MnS i 0 . 992 G a 0 . 008 Our results indicate that the order of the SKL is sensitive to the orientation of an applied magnetic field with respect to the crystal lattice and to variations in the sequence of small temperature and applied magnetic field changes. The disorder caused by the substitution of the heavier element Ga for Si is sufficient to reduce the pinning of the SKL to the underlying crystalline lattice, reducing the propensity for the SKL to be aligned with the crystal lattice. This tendency is most evident when the applied field is not well oriented with respect to the high symmetry axes of the crystal resulting in disorder in the long range SKL while maintaining sharp short range (radial) order. We have also investigated the effect of substituting heavier elements into MnSi on the reorientation process of the helical domains with field cycling in MnS i 0 . 992 G a 0 . 008 and M n 0 . 985 I r 0 . 015 Si A comparison of the reorientation process in these materials with field reduction indicates that the substitution of heavier elements on either Mn or Si sites creates a higher energy barrier for the reorientation of the helical order and for the formation of domains
Magnetic order and the electronic ground state in the pyrochlore iridate Nd2Ir2O7
We report a combined muon spin relaxation/rotation, bulk magnetization,
neutron scattering, and transport study of the electronic properties of the
pyrochlore iridate Nd2Ir2O7. We observe the onset of strongly hysteretic
behavior in the temperature dependent magnetization below 120 K, and an abrupt
increase in the temperature dependent resistivity below 8 K. Zero field muon
spin relaxation measurements show that the hysteretic magnetization is driven
by a transition to a magnetically disordered state, and that below 8 K a
complex magnetically ordered ground state sets in, as evidenced by the onset of
heavily damped spontaneous muon precession. Our measurements point toward the
absence of a true metal-to-insulator phase transition in this material and
suggest that Nd2Ir2O7 lies either within or on the metallic side of the
boundary of the Dirac semimetal regime within its topological phase diagram.Comment: 21 pages, 7 figure
Zn-induced spin dynamics in overdoped LaSrCuZnO
Spin fluctuations and the local spin susceptibility in isovalently
Zn-substituted LaSrCuZnO (,
) are measured via inelastic neutron scattering techniques. As
Zn is substituted onto the Cu-sites, an anomalous enhancement of
the local spin susceptibility appears due to the
emergence of a commensurate antiferromagnetic excitation centered at wave
vector \textbf{Q} that coexists with the known incommensurate
SDW excitations at \textbf{Q}.
Our results support a picture of Zn-induced antiferromagnetic (AF) fluctuations
appearing through a local staggered polarization of Cu-spins, and the
simultaneous suppression of T as AF fluctuations are slowed in proximity to
Zn-impurities suggests the continued importance of high energy AF fluctuations
at the far overdoped edge of superconductivity in the cuprates.Comment: 10 pages, 8 figure
A Gyrochronology and Microvariability Survey of the Milky Way's Older Stars Using Kepler's Two-Wheels Program
Even with the diminished precision possible with only two reaction wheels,
the Kepler spacecraft can obtain mmag level, time-resolved photometry of tens
of thousands of sources. The presence of such a rich, large data set could be
transformative for stellar astronomy. In this white paper, we discuss how
rotation periods for a large ensemble of single and binary main- sequence
dwarfs can yield a quantitative understanding of the evolution of stellar
spin-down over time. This will allow us to calibrate rotation-based ages beyond
~1 Gyr, which is the oldest benchmark that exists today apart from the Sun.
Measurement of rotation periods of M dwarfs past the fully-convective boundary
will enable extension of gyrochronology to the end of the stellar
main-sequence, yielding precise ages ({\sigma} ~10%) for the vast majority of
nearby stars. It will also help set constraints on the angular momentum
evolution and magnetic field generation in these stars. Our Kepler-based study
would be supported by a suite of ongoing and future ground-based observations.
Finally, we briefly discuss two ancillary science cases, detection of
long-period low-mass eclipsing binaries and microvariability in white dwarfs
and hot subdwarf B stars that the Kepler Two-Wheels Program would facilitate.Comment: Kepler white pape
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