32 research outputs found
Combined Transport, Magnetization and Neutron Scattering Study of Correlated Iridates and Iron Pnictide Superconductors:
Thesis advisor: Stephen WilsonThe work performed within this thesis is divided into two parts, each focusing primarily on the study of magnetic phase behavior using neutron scattering techniques. In first part, I present transport, magnetization, and neutron scattering studies of materials within the iridium oxide-based Ruddelsden-Popper series [Srn+1IrnO3n+1] compounds Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). This includes a comprehensive study of the doped bilayer system Sr3(Ir1-xRux )2O7. In second part, I present my studies of the effect of uniaxial pressure on magnetic and structural phase behavior of the iron-based high temperature superconductor Ba(Fe1-xCox)2As2. Iridium-based 5d transition metal oxides host rather unusual electronic/magnetic ground states due to strong interplay between electronic correlation, lattice structure and spin-orbit effects. Out of the many oxides containing iridium, the Ruddelsden-Popper series [Srn+1IrnO3n+1] oxides are some of the most interesting systems to study both from the point of view of physics as well as from potential applications. My work is focused on two members of this series Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). In particular, our combined transport, magnetization and neutron scattering studies of Sr3Ir2O7 (n=2) showed that this system exhibits a complex coupling between charge transport and magnetism. The spin magnetic moments form a G-type antiferromagnetic structure with moments oriented along the c-axis, with an ordered moment of 0.35±0.06 µB/Ir. I also performed experiments doping holes in this bilayer Sr3(Ir1-xRux)2O7 system in order to study the role of electronic correlation in these materials. Our results show that the ruthenium-doped holes remain localized within the Jeff=1/2 Mott insulating background of Sr3Ir2O7, suggestive of `Mott blocking' and the presence of strong electronic correlation in these materials. Antiferromagnetic order however survives deep into the metallic regime with the same ordering q-vector, suggesting an intricate interplay between residual AF correlations in the Jeff=1/2 state and metallic nanoscale hole regions. Our results lead us to propose an electronic/magnetic phase diagram for Sr3(Ir1-xRux)2O7 system showing how the system moves from Jeff=1/2 antiferromagnetic Mott insulator (Sr3Ir2O7) to paramagnetic Fermi liquid metal (Sr3Ru2O7). On the other hand, our neutron scattering measurements on Sr2IrO4 (n=1), a prototypical Jeff=1/2 Mott insulator, showed that the spins arranged antiferromagnetically in ab-plane with an ordered moment comparable to that of Sr3Ir2O7. The second part of my work is comprised of a neutron scattering-based study of the Ba(Fe1-xCox)2As2 system, a bilayer family of iron-based high temperature superconductors. Undoped, this system exhibits either simultaneous or nearly simultaneous magnetic and structural phase transitions from a high temperature paramagnetic tetragonal phase to low temperature orthorhombic antiferromagnetic phase. With the gradual suppression of these two temperatures, the superconducting phase appears with the highest TC obtained just beyond their complete suppression. It has been proposed that these coupled magnetostructural transitions are secondary manifestations which arise as a consequence of electronic nematic ordering that occurs at a temperature higher than either of them. My work is mainly focused on probing the spin behaviors coupling to this electronic nematic phase. I devised a small device to apply uniaxial pressure along an in-plane high symmetry axis and studied the magnetic and structural behavior in series of Ba(Fe1-xCox)2As2 compounds via neutron scattering in presence of uniaxial pressure. There is an upward thermal shift in the onset of structural and magnetic transition temperature caused by this uniaxial pressure which is surprisingly insensitive to cobalt concentration in the absolute scale. Furthermore, on the first order side of the phase diagram (below the tricritical point), the structural and magnetic transitions are decoupled with magnetic transition following structural distortion. This study suggests the importance of both spin-lattice and orbital-lattice interactions in these families of compounds.Thesis (PhD) — Boston College, 2014.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics
Exploring the origins of the Dzyalloshinski-Moria interaction in MnSi
By using magnetization and small-angle neutron scattering (SANS)
measurements, we have investigated the magnetic behavior of Mn_{1-x}Ir_{x}Si
system to explore the effect of increased carrier density and spin-orbit
interaction on the magnetic properties of MnSi. We determine estimates of the
spin wave stiffness and the Dzyalloshinski-Moria, DM, interaction strength and
compare with Mn_{1-x}Co_{x}Si and Mn_{1-x}Fe_{x}Si. Despite the large
differences in atomic mass and size of the substituted elements,
Mn_{1-x}Co_{x}Si and Mn_{1-x}Ir_{x}Si show nearly identical variations in their
magnetic properties with substitution. We find a systematic dependence of the
transition temperature, the ordered moment, the helix period and the DM
interaction strength with electron count for Mn{1-x}Ir{x}Si, Mn_{1-x}Co_{x}Si,
and Mn_{1-x}Fe_{x}Si indicating that the magnetic behavior is primarily
dependent upon the additional carrier density rather than on the mass or size
of the substituting species. This indicates that the variation in magnetic
properties, including the DM interaction strength, are primarily controlled by
the electronic structure as Co and Ir are isovalent. Our work suggests that
although the rigid band model of electronic structure along with Moira's model
of weak itinerant magnetism describe this system surprisingly well,
phenomenological models for the DM interaction strength are not adequate to
describe this system.Comment: 17 pages, 7 Figure
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
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
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