arXiv subjects

M. A. McGuire

Publications and source records attributed to M. A. McGuire.

At least 37 records · Page 2Linked to original sources

arXiv2017

Slater Insulator in Iridate Perovskites with Strong Spin-Orbit Coupling

The perovskite SrIrO3 is an exotic narrow-band metal owing to a confluence of the strengths of the spin-orbit coupling (SOC) and the electron-electron correlations. It has been proposed that topological and magnetic insulating phases can be achieved by tuning the SOC, Hubbard interactions, and/or lattice symmetry. Here, we report that the substitution of nonmagnetic, isovalent Sn4+ for Ir4+ in the SrIr1-xSnxO3 perovskites synthesized under high pressure leads to a metal-insulator transition to an antiferromagnetic (AF) phase at TN > 225 K. The continuous change of the cell volume as detected by x-ray diffraction and the lamda-shape transition of the specific heat on cooling through TN demonstrate that the metal-insulator transition is of second-order. Neutron powder diffraction results indicate that the Sn substitution enlarges an octahedral-site distortion that reduces the SOC relative to the spin-spin exchange interaction and results in the type-G AF spin ordering below TN. Measurement of high-temperature magnetic susceptibility shows the evolution of magnetic coupling in the paramagnetic phase typical of weak itinerant-electron magnetism in the Sn-substituted samples. A reduced structural symmetry in the magnetically ordered phase leads to an electron gap opening at the Brillouin zone boundary below TN in the same way as proposed by Slater.

cond-mat.str-el↗

arXiv2017

LaCu$_{6-x}$Ag$_{x}$: A promising host of an elastic quantum critical point

Structural properties of LaCu$_{6-x}$Ag$_{x}$ have been investigated using neutron and x-ray diffraction, and resonant ultrasound spectroscopy (RUS) measurements. Diffraction measurements indicate a continuous structural transition from orthorhombic ($Pnma$) to monoclinic ($P2_1/c$) structure. RUS measurements show softening of natural frequencies at the structural transition, consistent with the elastic nature of the structural ground state. The structural transition temperatures in LaCu$_{6-x}$Ag$_{x}$ decrease with Ag composition until the monoclinic phase is completely suppressed at $x_c$ = 0.225. All of the evidence is consistent with the presence of an elastic quantum critical point in LaCu$_{6-x}$Ag$_{x}$.

cond-mat.str-el↗

arXiv2017

Flux growth in a horizontal configuration: an analogue to vapor transport growth

Flux growth of single crystals is normally performed in a vertical configuration with an upright refractory container holding the flux melt. At high temperatures, flux dissolves the charge forming a homogeneous solution before nucleation and growth of crystals take place under proper supersaturation generated by cooling or evaporating the flux. In this work, we report flux growth in a horizontal configuration with a temperature gradient along the horizontal axis: a liquid transport growth analogous to the vapor transport technique. In a typical liquid transport growth, the charge is kept at the hot end of the refractory container and the flux melt dissolves the charge and transfers it to the cold end. Once the concentration of charge is above the solubility limit at the cold end, the thermodynamically stable phase nucleates and grows. Compared to the vertical flux growth, the liquid transport growth can provide a large quantity of crystals in a single growth since the charge/flux ratio is not limited by the solubility limit at the growth temperature. This technique is complementary to the vertical flux growth and can be considered when a large amount of crystals are needed but the yield from the conventional vertical flux growth is limited. We applied this technique to the growth of IrSb$_3$, Mo$_3$Sb$_7$, MnBi from self flux, and the growth of FeSe, CrTe$_3$, NiPSe$_3$, FePSe$_3$, and InCuP$_2$S$_6$ from a halide flux.

cond-mat.mtrl-sci↗

arXiv2016

Candidate Elastic Quantum Critical Point in LaCu$_{6-x}$Au$_x$

The structural properties of LaCu$_{6-x}$Au$_x$ have been studied using neutron diffraction, x-ray diffraction, and heat capacity measurements. The continuous orthorhombic-monoclinic structural phase transition in LaCu$_{6}$ is suppressed linearly with Au substitution until a complete suppression of the structural phase transition occurs at the critical composition, $x_{c}$ = 0.3. Heat capacity measurements at low temperatures indicate residual structural instability at $x_c$ that extends well into the orthorhombic phase. The instability is ferroelastic in nature, with density functional theory (DFT) calculations showing negligible coupling to electronic states near the Fermi level. The data and calculations presented here are consistent with the zero temperature termination of a continuous structural phase transition suggesting that the LaCu$_{6-x}$Au$_x$ series hosts an elastic quantum critical point.

cond-mat.str-el↗

arXiv2016

Extended Magnetic Exchange Interactions in the High-Temperature Ferromagnet MnBi

The high-temperature ferromagnet MnBi continues to receive attention as a candidate to replace rare-earth-containing permanent magnets in applications above room temperature. This is due to a high Curie temperature, large magnetic moments, and a coercivity that increases with temperature. The synthesis of MnBi also allows for crystals that are free of interstitial Mn, enabling more direct access to the key interactions underlying the physical properties of binary Mn-based ferromagnets. In this work, we use inelastic neutron scattering to measure the spin waves of MnBi in order to characterize the magnetic exchange at low temperature. Consistent with the spin reorientation that occurs below 140 K, we do not observe a spin gap in this system above our experimental resolution. A Heisenberg model was fit to the spin wave data in order to characterize the long-range nature of the exchange. It was found that interactions up to sixth nearest neighbor are required to fully parameterize the spin waves. Surprisingly, the nearest-neighbor term is antiferromagnetic, and the realization of a ferromagnetic ground state relies on the more numerous ferromagnetic terms beyond nearest neighbor, suggesting that the ferromagnetic ground state arises as a consequence of the long-ranged interactions in the system.

cond-mat.str-el↗

arXiv2016

FeS: Structure and Composition Relations to Superconductivity and Magnetism

Structure and composition of iron chalcogenides have a delicate relationship with magnetism and superconductivity. In this report we investigate the iron sulfide layered tetragonal phase (t-FeS), and compare with three-dimensional hexagonal phase (h-FeS). X-ray diffraction reveals the absence of structural transitions for both t- and h-FeS below room temperature, and gives phase compositions of Fe0.93(1)S and Fe0.84(1)S, respectively, for the samples studied here. The a lattice parameter of bigger than 3.68 A is significant for causing bulk superconductivity in iron sulfide, which is controlled by composition and structural details such as iron stoichiometry and concentration of vacancy. While h-FeS with a = 3.4436(1) A has magnetic ordering well above room temperature, our t-FeS with a =3.6779(8)A shows filamentary superconductivity below Tc = 4 K with less than 15% superconducting volume fraction. Also for t-FeS, the magnetic susceptibility shows an anomaly at ~ 15 K, and neutron diffraction reveals a commensurate antiferromagnetic ordering below TN = 116 K, with wave vector km= (0.25,0.25,0) and 0.46(2)uB/Fe. Although two synthesis routes are used here to stabilize t vs h crystal structures (hydrothermal vs solid-state methods), both FeS compounds order on two length-scales of ~1000 nm sheets or blocks and ~ 20 nm smaller particles, shown by neutron scattering. First principles calculations reveal a high sensitivity to the structure for the electronic and magnetic properties in t-FeS, predicting marginal antiferromagnetic instability for our compound (sulfur height of zS ~0.252) with an ordering energy of ~11 meV/Fe, while h-FeS is magnetically stable.

cond-mat.supr-con↗

arXiv2016

Fragile singlet ground state magnetism in pyrochlore osmates R2Os2O7 (R=Y and Ho)

The singlet ground state magnetism in pyrochlore osmates Y$_2$Os$_2$O$_7$ and Ho$_2$Os$_2$O$_7$ is studied by DC and AC susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected non-magnetic singlet in the strong spin-orbit coupling (SOC) limit for Os$^{4+}$ ($5d^4$), Y$_2$Os$_2$O$_7$ exhibits a spin-glass (SG) ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the non-magnetic state in the strong SOC limit and the magnetic state in the strong superexchange limit. Ho$_2$Os$_2$O$_7$ has the same structural distortion as occurs in Y$_2$Os$_2$O$_7$. However, the Os sublattice in Ho$_2$Os$_2$O$_7$ shows long-range magnetic ordering below 36\,K. The sharp difference of the magnetic ground state between Y$_2$Os$_2$O$_7$ and Ho$_2$Os$_2$O$_7$ signals the singlet ground state magnetism in $R_2$Os$_2$O$_7$ is fragile and can be disturbed by the weak $4f-5d$ interactions.

cond-mat.str-el↗

arXiv2015

Structural and Magnetic Phase Transitions in CeCu$_{6-x}T_x$ ($T$ = Ag, Pd)

The structural and the magnetic properties of CeCu$_{6-x}$Ag$_x$ (0 $\leq$ $x$ $\leq$ 0.85) and CeCu$_{6-x}$Pd$_x$ (0 $\leq$ $x$ $\leq$ 0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), heat capacity, x-ray diffraction measurements and first principles calculations. The structural and magnetic phase diagrams of CeCu$_{6-x}$Ag$_x$ and CeCu$_{6-x}$Pd$_x$ as a function of Ag/Pd composition are reported. The end member, CeCu$_6$, undergoes a structural phase transition from an orthorhombic ($Pnma$) to a monoclinic ($P2_1/c$) phase at 240 K. In CeCu$_{6-x}$Ag$_x$, the structural phase transition temperature (${T_{s}}$) decreases linearly with Ag concentration and extrapolates to zero at $x_{S}$ $\approx$ 0.1. The structural transition in CeCu$_{6-x}$Pd$_x$ remains unperturbed with Pd substitution within the range of our study. The lattice constant $b$ slightly decreases with Ag/Pd doping, whereas, $a$ and $c$ increase with an overall increase in the unit cell volume. Both systems, CeCu$_{6-x}$Ag$_x$ and CeCu$_{6-x}$Pd$_x$, exhibit a magnetic quantum critical point (QCP), at $x$ $\approx$ 0.2 and $x$ $\approx$ 0.05 respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector ($δ_1$ 0 $δ_2$) where $δ_1 \sim 0.62$, $δ_2 \sim 0.25$, $x$ = 0.125 for CeCu$_{6-x}$Pd$_x$ and $δ_1 \sim 0.64$, $δ_2 \sim 0.3$, $x$ = 0.3 for CeCu$_{6-x}$Ag$_x$. The magnetic structure consists of an amplitude modulation of the Ce-moments which are aligned along the $c$-axis of the orthorhombic unit cell.

cond-mat.str-el↗

arXiv2015

Anomalous magneto-elastic and charge doping effects in thallium-doped BaFe2As2

Within the BaFe2As2 crystal lattice, we partially substitute thallium for barium and report the effects of interlayer coupling in Ba1-xTlxFe2As2 crystals. We demonstrate the unusual effects of magneto-elastic coupling and charge doping in this iron-arsenide material, whereby Neel temperature rises with small x, and then falls with additional x. Specifically, we find that Neel and structural transitions in BaFe2As2 (TN =Ts= 133 K) increase for x=0.05 (TN = 138 K, Ts = 140 K) from magnetization, heat capacity, resistivity, and neutron diffraction measurements. Evidence from single crystal X-ray diffraction and first principles calculations attributes the stronger magnetism in x=0.05 to magneto-elastic coupling related to the shorter intraplanar Fe-Fe bond distance. With further thallium substitution, the transition temperatures decrease for x = 0.09 (TN = Ts = 131 K), and this is due to charge doping. We illustrate that small changes related to 3d transition-metal state can have profound effects on magnetism.

cond-mat.supr-con↗

arXiv2015

Growth and Characterization of Ce- Substituted Nd2Fe14B Single Crystals

Single crystals of (Nd1-xCex)2Fe14B are grown out of Fe-(Nd,Ce) flux. Chemical and structural analysis of the crystals indicates that (Nd1-xCex)2Fe14B forms a solid solution until at least x = 0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over the 4f site. Magnetization measurements show that for x = 0.38 the saturation magnetization at 400 K, a temperature important to applications, falls from 29.8 for the parent Nd2Fe14B to 27.6 (mu)B/f.u., the anisotropy field decreases from 5.5 T to 4.7 T, and the Curie temperature decreases from 586 to 543 K. First principles calculations carried out within density functional theory are used to explain the decrease in magnetic properties due to Ce substitution. Though the presence of the lower-cost and more abundant Ce slightly affects these important magnetic characteristics, this decrease is not large enough to affect a multitude of applications. Ce-substituted Nd2Fe14B is therefore a potential high-performance permanent magnet material with substantially reduced Nd content.

cond-mat.str-el↗

arXiv2015

Fragile structural transition in Mo3Sb7

Mo$_3$Sb$_7$ single crystals lightly doped with Cr, Ru, or Te are studied in order to explore the interplay between superconductivity, magnetism, and the cubic-tetragonal structural transition. The structural transition at 53\,K is extremely sensitive to Ru or Te substitution which introduce additional electrons, but robust against Cr substitution. No sign of a structure transition was observed in superconducting Mo$_{2.91}$Ru$_{0.09}$Sb$_7$ and Mo$_3$Sb$_{6.975}$Te$_{0.025}$. In contrast, 3at\% Cr doping only slightly suppresses the structural transition to 48\,K while leaving no trace of superconductivity above 1.8\,K. Analysis of magnetic properties suggests that the interdimer interaction in Mo$_3$Sb$_7$ is near a critical value and essential for the structural transition. All dopants suppress the superconductivity of Mo$_3$Sb$_7$. The tetragonal structure is not necessary for superconductivity.

cond-mat.supr-con↗

arXiv2015

High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe$_{12}$O$_{19}$

High quality single crystals of BaFe$_{12}$O$_{19}$ were grown using the floating zone technique in flowing oxygen pressurized to 100 atm. Single crystal neutron diffraction was used to determine the nuclear and magnetic structure of BaFe$_{12}$O$_{19}$ at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe$^{3+}$ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90-300 K. The inverse dielectric permittivity, $1/\varepsilon$, along the c-axis shows a $T^2$ temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in $1/\varepsilon$. These features resemble those of classic quantum paraelectrics such as SrTiO$_3$. The presence of the upturn in $1/\varepsilon$ indicates that BaFe$_{12}$O$_{19}$ is a critical quantum paraelectric system with Fe$^{3+}$ ions involved in both magnetic and electric dipole formation.

cond-mat.mtrl-sci↗

arXiv2014

Glass-like phonon scattering from a spontaneous nanostructure in AgSbTe2

Materials with very low thermal conductivity are of high interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising to suppress thermal conductivity by scattering phonons, but challenges remain in producing bulk samples. We show that in crystalline AgSbTe$_2$, a spontaneously-forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean-free-paths provides a novel bottom-up microscopic account of thermal conductivity, and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe$_2$ leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and points to a new avenue in nano-scale engineering of materials, to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

cond-mat.mtrl-sci↗

arXiv2014

Complex magnetic behavior of the sawtooth Fe chains in Rb$_{2}$Fe$_{2}$O(AsO$_{4}$)$_{2}$

Results of magnetic field and temperature dependent neutron diffraction and magnetization measurements on oxy-arsenate Rb$_{2}$Fe$_{2}$O(AsO$_{4}$)$_{2}$ are reported. The crystal structure of this compound contains pseudo-one-dimensional [Fe$_{2}$O$_{6}$]$^\infty$ sawtooth-like chains, formed by corner sharing isosceles triangles of $Fe^{3+}$ ions occupying two nonequivalent crystallographic sites. The chains extend infinitely along the crystallographic $b$-axis and are structurally confined from one another via diamagnetic (AsO$_{4}$)$^{3-}$ units along the $a$-axis, and Rb$^+$ cations along the $c$-axis direction. Neutron diffraction measurements indicate the onset of a long range antiferromagnetic order below approximately 25 K. The magnetic structure consists of ferrimagnetic chains which are antiferromagnetically coupled with each other. Within each chain, one of the two Fe sites carries a moment which lies along the \emph{b}-axis, while the second site bears a canted moment in the opposite direction. Externally applied magnetic field induces a transition to a ferrimagnetic state, in which the coupling between the sawtooth chains becomes ferromagnetic. Magnetization measurements performed on optically-aligned single crystals reveal evidence for an uncompensated magnetization at low magnetic fields that could emerge from to a phase-segregated state with ferrimagnetic inclusions or from antiferromagnetic domain walls. The observed magnetic states and the competition between them is expected to arise from strongly frustrated interactions within the sawtooth chains and relatively weak coupling between them.

cond-mat.str-el↗

arXiv2013

Magneto-Transport Properties of Single Crystalline LaFeAsO

Measurements of magnetization, specific heat, electrical resistivity, Hall effect, and magnetoresistance on single crystalline samples of LaFeAsO grown in a NaAs flux are reported. While this material is known to be a semimetal, the temperature dependence of the electrical resistivity data presented herein is reminiscent of semiconducting behavior and exhibits distinct features associated with a structural transition and spin density wave (SDW) order. Magnetoresistance and Hall coefficient measurements were performed in magnetic fields up to 9 T applied perpendicular to the basal plane using a van der Pauw configuration. The charge carrier density and mobility indicate that electrons are the majority charge carriers and exhibit features indicative of the structural transition and SDW formation. Low temperature X-ray diffraction measurements have confirmed that the structural transition in these samples occurs near 140 K, compared to a transition temperature of 156 K observed in polycrystalline samples. Isotherms of magnetoresistivity measured as a function of magnetic field can be scaled onto a single curve in which the scaling field is a linear function of temperature between 2.2 K and 180 K.

cond-mat.supr-con↗

arXiv2013

Doping dependence of the spin excitations in Fe-based superconductors Fe1+yTe1-xSex

The Fe1+yTe1-xSex series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials we present systematic inelastic neutron scattering measurements of the low energy spin excitations for x=0.27, 0.36, 0.40, 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2 1/2 0) wave vector with doping. Concentrations (x=0.40 and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2 1/2 0) and also exhibit a strong spin resonance in the spin excitation spectrum below Tc. The resonance signal appears to be closer to (1/2 1/2 0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2 1/2 0) wave vector and the role that interstitial Fe may play.

cond-mat.supr-con↗

arXiv2013

Dy-V magnetic interaction and local structure bias on the complex spin and orbital ordering in Dy$_{1-x}$Tb$_{x}$VO$_3$ (x\,=\,0 and 0.2)

The spin and orbital ordering in Dy$_{1-x}$Tb$_{x}$VO$_3$ (x\,=\,0 and 0.2) was studied by measuring x-ray powder diffraction, magnetization, specific heat, and neutron single crystal diffraction. The results show that G-OO/C-AF and C-OO/G-AF phases coexist in Dy$_{0.8}$Tb$_{0.20}$VO$_3$ in the temperature range 2\,K$\sim$60\,K and the volume fraction of each phase is temperature and field dependent. The ordering of Dy moments at T*\,=\,12\,K induces a transition from G-OO/C-AF to a C-OO/G-AF phase. Magnetic fields suppress the long range order of Dy moments and thus the C-OO/G-AF phase below T*. The polarized moments induced at the Dy sublattice by external magnetic fields couple to the V 3d moments and this coupling favors the G-OO/C-AF state. Also discussed is the effect of the Dy-V magnetic interaction and local structure distortion on the spin and orbital ordering in Dy$_{1-x}$Tb$_{x}$VO$_3$.

cond-mat.str-el↗

arXiv2013

Flux growth and physical properties of Mo3Sb7 single crystals

Millimeter sized single crystals of Mo3Sb7 are grown using the self-flux technique and a thorough characterization of their structural, magnetic, thermal and transport properties is reported. The structure parameters for the high-temperature cubic phase and the low-temperature tetragonal phase were, for the first time, determined with neutron single crystal diffraction. Both X-ray powder diffraction and neutron single crystal diffraction at room temperature confirmed that Mo3Sb7 crystallizes in Ir3Ge7-type cubic structure with space group Im-3m. The cubic-tetragonal structure transition at 53K is verified by the peak splitting of (4 0 0) reflection observed by X-ray single crystal diffraction and the dramatic intensity change of (12 0 0) peak observed by neutron single crystal diffraction. The structural transition is accompanied by a sharp drop in magnetic susceptibility, electrical resistivity, and thermopower while cooling. A weak lambda anomaly was also observed around 53K in the temperature dependence of specific heat and the entropy change across the transition is estimated to be 1.80J/molMoK. The temperature dependence of magnetic susceptibility was measured up to 750K and it follows a Curie-Weiss behavior above room temperature. Analysis of the low-temperature magnetic susceptibility suggests a spin gap of 110K around 53K. A typical phonon thermal conductivity was observed in the low temperature tetragonal phase. A glassy phonon thermal conductivity above 53K suggests a structural instability in a wide temperature range. Superconductivity was observed at 2.35K in the as-grown crystals and the dimensionless specific heat jump was determined to be 1.49, which is slightly larger than the BCS value of 1.43 for the weak-coupling limit.

cond-mat.supr-con↗