arXiv subjects

D. C. Johnston

Publications and source records attributed to D. C. Johnston.

At least 55 records · Page 3Linked to original sources

arXiv2017

Antiferromagnetism in semiconducting SrMn2Sb2 and BaMn2Sb2 single crystals

Crystals of SrMn2Sb2 and BaMn2Sb2 were grown using Sn flux and characterized by powder and single-crystal x-ray diffraction, respectively, and by single-crystal electrical resistivity rho, heat capacity Cp, and magnetic susceptibility chi measurements versus temperature T, and magnetization versus field M(H) isotherm measurements. SrMn2Sb2 adopts the trigonal CaAl2Si2-type structure whereas BaMn2Sb2 crystallizes in the tetragonal ThCr2Si2-type structure. The rho(T) data indicate semiconducting behaviors for both compounds with activation energies of 0.35 eV for SrMn2Sb2 and 0.16 eV for BaMn2Sb2. The chi(T) and Cp(T) data reveal antiferromagnetic (AFM) ordering at TN = 110 K for SrMn2Sb2 and 450~K for BaMn2Sb2. The anisotropic chi(T < TN) data also show that the ordered moments in SrMn2Sb2 are aligned in the hexagonal ab plane whereas the ordered moments in BaMn2Sb2 are aligned collinearly along the tetragonal c axis. The ab-plane M(H) data for SrMn2Sb2 exhibit a continuous metamagnetic transition at low fields 0 < H < 1 T, whereas BaMn2Sb2 exhibits no metamagnetic transitions up to 5.5 T. The chi(T) data for both compounds and the Cp(T) data for SrMn2Sb2 and BaMn2Sb2 indicate strong dynamic short-range AFM correlations above their respective TN up to at least 900 K within a local-moment picture, corresponding to quasi-two-dimensional magnetic behavior. The present results and a survey of the literature for Mn pnictides with the CaAl2Si2 and ThCr2Si2 crystal structures show that the TN values for the CaAl2Si2-type compounds are much smaller than those for the ThCr2Si2-type materials.

cond-mat.str-el↗

arXiv2017

Effective One-Dimensional Coupling in the Highly-Frustrated Square-Lattice Itinerant Magnet CaCo$_{\mathrm{2}-y}$As$_{2}$

Inelastic neutron scattering measurements on the itinerant antiferromagnet (AFM) CaCo$_{\mathrm{2}-y}$As$_{2}$ at a temperature of 8 K reveal two orthogonal planes of scattering perpendicular to the Co square lattice in reciprocal space, demonstrating the presence of effective one-dimensional spin interactions. These results are shown to arise from near-perfect bond frustration within the $J_1$-$J_2$ Heisenberg model on a square lattice with ferromagnetic $J_1$, and hence indicate that the extensive previous experimental and theoretical study of the $J_1$-$J_2$ Heisenberg model on local-moment square spin lattices should be expanded to include itinerant spin systems.

cond-mat.supr-con↗

arXiv2017

NMR studies of the incommensurate helical antiferromagnet EuCo2P2 : determination of the antiferromagnetic propagation vector

Recently Ding et al. [Phys. Rev. B 95, 184404 (2017)] reported that their nuclear magnetic resonance (NMR) study on EuCo$_2$As$_2$ successfully characterized the antiferromagnetic (AFM) propagation vector of the incommensurate helix AFM state, showing that NMR is a unique tool for determination of the spin structures in incommensurate helical AFMs. Motivated by this work, we have carried out $^{153}$Eu, $^{31}$P and $^{59}$Co NMR measurements on the helical antiferromagnet EuCo$_2$P$_2$ with an AFM ordering temperature $T_{\rm N}$ = 66.5 K. An incommensurate helical AFM structure was clearly confirmed by $^{153}$Eu and $^{31}$P NMR spectra on single crystalline EuCo$_2$P$_2$ in zero magnetic field at 1.6 K and its external magnetic field dependence. Furthermore, based on $^{59}$Co NMR data in both the paramagnetic and the incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector k = (0, 0, 0.73 $\pm$ 0.09)2$π$/$c$ where $c$ is the $c$-axis lattice parameter. The temperature dependence of k is also discussed.

cond-mat.str-el↗

arXiv2017

Itinerant G-type antiferromagnetic order in SrCr$_2$As$_2$

Neutron diffraction and magnetic susceptibility studies of a polycrystalline SrCr$_2$As$_2$ sample reveal that this compound is an itinerant G-type antiferromagnet below the N${\rm \acute{e}}$el temperature $T_{\textrm N}$ = 590(5) K with the Cr magnetic moments aligned along the tetragonal $c$ axis. The system remains tetragonal to the lowest measured temperature ($\sim$12 K). The lattice parameter ratio $c/a$ and the magnetic moment saturate at about the same temperature below $\sim$ 200 K, indicating a possible magnetoelastic coupling. The ordered moment, $μ=1.9(1)~μ_{\rm B}$/Cr, measured at $T = 12$ K, is significantly reduced compared to its localized value ($4~μ_{\rm B}$/Cr) due to the itinerant character brought about by the hybridization between the Cr $3d$ and As $4p$ orbitals.

cond-mat.str-el↗

arXiv2017

NMR Determination of an Incommensurate Helical Antiferromagnetic Structure in EuCo2As2

We report $^{153}$Eu, $^{75}$As and $^{59}$Co nuclear magnetic resonance (NMR) results on EuCo$_2$As$_2$ single crystal. Observations of $^{153}$Eu and $^{75}$As NMR spectra in zero magnetic field at 4.3 K below an antiferromagnetic (AFM) ordering temperature $T_{\rm N}$ = 45 K and its external magnetic field dependence clearly evidence an incommensurate helical AFM structure in EuCo$_2$As$_2$. Furthermore, based on $^{59}$Co NMR data in both the paramagnetic and the incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector ${\bf k}$ = (0, 0, 0.73 $\pm$ 0.07)2$π$/$c$ where $c$ is the $c$ lattice parameter. Thus the incommensurate helical AFM state was characterized by only NMR data with model-independent analyses, showing NMR to be a unique tool for determination of the spin structure in incommensurate helical AFMs.

cond-mat.str-el↗

arXiv2017

Oscillating magnetoresistance due to fragile spin structure in metallic GdPd$_3$

Studies on the phenomenon of magnetoresistance (MR) have produced intriguing and application-oriented outcomes for decades--colossal MR, giant MR and recently discovered extremely large MR of millions of percents in semimetals can be taken as examples. We report here the investigation of oscillating MR in a cubic intermetallic compound GdPd$_3$, which is the only compound that exhibits MR oscillations between positive and negative values. Our study shows that a very strong correlation between magnetic, electrical and magnetotransport properties is present in this compound. The magnetic structure in GdPd$_3$ is highly fragile since applied magnetic fields of moderate strength significantly alter the spin arrangement within the system--a behavior that manifests itself in the oscillating MR. Intriguing magnetotransport characteristics of GdPd$_3$ are appealing for field-sensitive device applications, especially if the MR oscillation could materialize at higher temperature by manipulating the magnetic interaction through perturbations caused by chemical substitutions.

cond-mat.str-el↗

arXiv2017

Suppression of magnetic order in CaCo$_{1.86}$As$_{2}$ with Fe substitution: Magnetization, neutron diffraction, and x-ray diffraction studies of Ca(Co$_{1-x}$Fe$_{x}$)$_{y}$As$_{2}$

Magnetization, neutron diffraction, and high-energy x-ray diffraction results for Sn-flux grown single-crystal samples of Ca(Co$_{1-x}$Fe$_{x}$)$_{y}$As$_{2}$, $0\leq x\leq1$, $1.86\leq y \leq 2$, are presented and reveal that A-type antiferromagnetic order, with ordered moments lying along the $c$ axis, persists for $x\lesssim0.12(1)$. The antiferromagnetic order is smoothly suppressed with increasing $x$, with both the ordered moment and Néel temperature linearly decreasing. Stripe-type antiferromagnetic order does not occur for $x\leq0.25$, nor does ferromagnetic order for $x$ up to at least $x=0.104$, and a smooth crossover from the collapsed-tetragonal (cT) phase of CaCo$_{1.86}$As$_{2}$ to the tetragonal (T) phase of CaFe$_{2}$As$_{2}$ occurs. These results suggest that hole doping CaCo$_{1.86}$As$_{2}$ has a less dramatic effect on the magnetism and structure than steric effects due to substituting Sr for Ca.

cond-mat.str-el↗

arXiv2017

Robust antiferromagnetic spin waves across the metal-insulator transition in hole-doped BaMn$_{2}$As$_{2}$

BaMn$_{2}$As$_{2}$ is an antiferromagnetic insulator where a metal-insulator transition occurs with hole doping via the substitution of Ba with K. The metal-insulator transition causes only a small suppression of the Néel temperature ($T_\mathrm{N}$) and the ordered moment, suggesting that doped holes interact weakly with the Mn spin system. Powder inelastic neutron scattering measurements were performed on three different powder samples of Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ with $x=$0, 0.125 and 0.25 to study the effect of hole doping and metallization on the spin dynamics of these compounds. We compare the neutron intensities to a linear spin wave theory approximation to the $J_{1}-J_{2}-J_{c}$ Heisenberg model. Hole doping is found to introduce only minor modifications to the exchange energies and spin gap. The changes observed in the exchange constants are consistent with the small drop of $T_\mathrm{N}$ with doping.

cond-mat.str-el↗

arXiv2016

Metallic behavior induced by potassium doping of the trigonal antiferromagnetic insulator EuMn2As2

We report magnetic susceptibility χ, isothermal magnetization M, heat capacity C_p and electrical resistivity ρmeasurements on undoped EuMn2As2 and K-doped Eu0.96K0.04Mn2As2 and Eu0.93K0.07Mn2As2 single crystals with the trigonal CaAl2Si2-type structure as a function of temperature T and magnetic field H. EuMn2As2 has an insulating ground state with an activation energy of 52 meV and exhibits antiferromagnetic (AFM) ordering of the Eu+2 spins S=7/2 at T_N1 = 15 K from C_p(T) and χ(T) data with a likely spin-reorientation transition at T_N2 = 5.0 K. The Mn+2 3d5 spins-5/2 exhibit AFM ordering at T_N = 142 K from all three types of measurements. The M(H) isotherm and χ(T) data indicate that the Eu AFM structure is both noncollinear and noncoplanar. The AFM structure of the Mn spins is also unclear. A 4% substitution of K for Eu in Eu0.96K0.04Mn2As2 is sufficient to induce a metallic ground state. Evidence is found for a difference in the AFM structure of the Eu moments in the metallic crystals from that of undoped EuMn2As2 versus both T and H. For metallic Eu0.96K0.04Mn2As2 and Eu0.93K0.07Mn2As2, an anomalous S-shape T dependence of ρrelated to the Mn magnetism is found. Upon cooling from 200 K, ρexhibits a strong negative curvature, reaches maximum positive slope at the Mn T_N ~ 150 K, and then continues to decrease but more slowly below T_N. This suggests that dynamic short-range AFM order of the Mn spins above the Mn T_N strongly suppresses the resistivity, contrary to the conventional decrease of ρthat is only observed upon cooling below T_N of an antiferromagnet.

cond-mat.str-el↗

arXiv2016

EuCo2P2: A Model Molecular-Field Helical Heisenberg Antiferromagnet

The metallic compound EuCo2P2 with the body-centered tetragonal ThCr2Si2 structure containing Eu spins 7/2 was previously shown from single-crystal neutron diffraction measurements to exhibit a helical antiferromagnetic (AFM) structure below TN = 66.5 K with the helix axis along the c axis and with the ordered moments aligned within the ab-plane. Here we report crystallography, electrical resistivity, heat capacity, magnetization and magnetic susceptibility measurements on single crystals of this compound. We demonstrate that EuCo2P2 is a model molecular-field helical Heisenberg antiferromagnet from comparisons of the anisotropic magnetic susceptibility chi, high-field magnetization and magnetic heat capacity of EuCo2P2 single crystals at temperature T < TN with the predictions of our recent formulation of molecular field theory. Values of the Heisenberg exchange interactions between the Eu spins are derived from the data. The low-T magnetic heat capacity ~ T^3 arising from spin-wave excitations with no anisotropy gap is calculated and found to be comparable to the lattice heat capacity. The density of states at the Fermi energy of EuCo2P2 and the related compound BaCo2P2 are found from the heat capacity data to be large, 10 and 16 states/eV per formula unit for EuCo2P2 and BaCo2P2, respectively. These values are enhanced by a factor of ~2.5 above those found from DFT electronic structure calculations for the two compounds. The calculations also find ferromagnetic Eu-Eu exchange interactions within the ab-plane and AFM interactions between nearest- and next-nearest planes, in agreement with the MFT analysis of chi{ab}(T < TN).

cond-mat.str-el↗

arXiv2016

Antiferromagnetism in trigonal SrMn2As2 and CaMn2As2 single crystals

Crystallographic, electronic transport, thermal and magnetic properties are reported for SrMn2As2 and CaMn2As2 single crystals grown using Sn flux. Rietveld refinements of powder x-ray diffraction data show that the two compounds are isostructural and crystallize in the trigonal CaAl2Si2-type structure (space group P-3m1), in agreement with the literature. Electrical resistivity rho versus temperature T measurements demonstrate insulating ground states for both compounds with activation energies of 85 meV for SrMn2As2 and 61 meV for CaMn2As2. In a local-moment picture, the Mn^{+2} 3d^5 ions are expected to have high-spin S = 5/2 with spectroscopic splitting factor g = 2. Magnetic susceptibility chi and heat capacity measurements versus T reveal antiferromagnetic (AFM) transitions at TN = 120(2) K and 62(3) K for SrMn2As2 and CaMn2As2, respectively. The anisotropic chi(T < TN) data indicate that the hexagonal c axis is the hard axis and hence that the ordered Mn moments are aligned in the ab plane. The chi(T) for both compounds and Cp(T) data for SrMn2As2 show strong dynamic short-range AFM correlations from TN up to at least 900 K, likely associated with quasi-two-dimensional connectivity of strong AFM exchange interactions between the Mn spins within the corrugated honeycomb Mn layers parallel to the ab plane.

cond-mat.str-el↗

arXiv2016

Collinear antiferromagnetism in trigonal SrMn$_{2}$As$_{2}$ revealed by single-crystal neutron diffraction

Fe pnictides and related materials have been a topic of intense research for understanding the complex interplay between magnetism and superconductivity. Here we report on the magnetic structure of SrMn$_{2}$As$_{2}$ that crystallizes in a trigonal structure ($P\bar{3}m1$) and undergoes an antiferromagnetic (AFM) transition at $T_{\textrm c}$ $= 118(2)$ K. The magnetic susceptibility remains nearly constant at temperatures $T \le T_{\textrm N}$ with $\textbf{H}\parallel \textbf{c}$ whereas it decreases significantly with $\textbf{H}\parallel \textbf{ab}$. This shows that the ordered Mn moments lie in the $\textbf{ab}$-plane instead of aligning along the $\textbf{c}$-axis as in tetragonal BaMn$_{2}$As$_{2}$. Single-crystal neutron diffraction measurements on SrMn$_{2}$As$_{2}$ demonstrate that the Mn moments are ordered in a collinear Néel AFM phase with $180^\circ$ AFM alignment between a moment and all nearest neighbor moments in the basal plane and also perpendicular to it. Moreover, quasi-two-dimensional AFM order is manifested in SrMn$_{2}$As$_{2}$ as evident from the temperature dependence of the order parameter.

cond-mat.str-el↗

arXiv2016

Volovik Effect and Fermi-Liquid Behavior in the s-Wave Superconductor CaPd2As2: 75As NMR-NQR Measurements

The electronic and magnetic properties of the collapsed-tetragonal CaPd$_2$As$_2$ superconductor (SC) with a transition temperature of 1.27 K have been investigated by $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. The temperature ($T$) dependence of the nuclear spin lattice relaxation rates (1/$T_1$) and the Knight shifts indicate the absence of magnetic correlations in the normal state. In the SC state, 1/$T_1$ measured by $^{75}$As NQR shows a clear Hebel-Slichter (HS) peak just below $T_{\rm c}$ and decreases exponentially at lower $T$, confirming a conventional $s$-wave SC. In addition, the Volovik effect, also known as Doppler shift effect, has been clearly evidenced by the observation of the suppression of the HS peak with applied magnetic field.

cond-mat.supr-con↗

arXiv2015

EuCo2P2: A Model Molecular-Field Helical Heisenberg Antiferromagnet

The Eu spins-7/2 in EuCo2P2 with the tetragonal ThCr2Si2-type structure have the prototypical noncollinear helical antiferromagnetic structure below the Neel temperature TN = 66.5 K as previously determined from single-crystal neutron diffraction measurements. The helix axis is along the c axis with the ordered moments aligned within the ab plane. Our recent formulation of molecular field theory (MFT) is found to quantitatively fit the anisotropic magnetic susceptibility of single-crystal EuCo2P2 at T < TN with a helix turn angle comparable to the neutron diffraction value. The experimental and MFT magnetic heat capacities at T < TN are also in good agreement. Values of the Heisenberg exchange interactions between the Eu spins-7/2 within the J0-Jz1-Jz2 MFT model are derived. The results demonstrate the robust applicability of the MFT to model the thermodynamic properties of noncollinear Heisenberg antiferromagnets.

cond-mat.str-el↗

arXiv2015

Pressure-induced collapsed-tetragonal phase in SrCo2As2

We present high-energy x-ray diffraction data under applied pressures up to p = 29 GPa, neutron diffraction measurements up to p = 1.1 GPa, and electrical resistance measurements up to p = 5.9 GPa, on SrCo2As2. Our x-ray diffraction data demonstrate that there is a first-order transition between the tetragonal (T) and collapsed-tetragonal (cT) phases, with an onset above approximately 6 GPa at T = 7 K. The pressure for the onset of the cT phase and the range of coexistence between the T and cT phases appears to be nearly temperature independent. The compressibility along the a-axis is the same for the T and cT phases whereas, along the c-axis, the cT phase is significantly stiffer, which may be due to the formation of an As-As bond in the cT phase. Our resistivity measurements found no evidence of superconductivity in SrCo2As2 for p <= 5.9 GPa and T >= 1.8 K. The resistivity data also show signatures consistent with a pressure-induced phase transition for p >= 5.5 GPa. Single-crystal neutron diffraction measurements performed up to 1.1 GPa in the T phase found no evidence of stripe-type or A-type antiferromagnetic ordering down to 10 K. Spin-polarized total-energy calculations demonstrate that the cT phase is the stable phase at high pressure with a c/a ratio of 2.54. Furthermore, these calculations indicate that the cT phase of SrCo2As2 should manifest either A-type antiferromagnetic or ferromagnetic order.

cond-mat.supr-con↗

arXiv2015

Thermodynamics of the Noninteracting Bose Gas in a Two-Dimensional Box

Bose-Einstein condensation (BEC) of a noninteracting Bose gas of N particles in a two-dimensional box with Dirichlet boundary conditions is studied. Confirming previous work, we find that BEC occurs at finite N at low temperatures T without the occurrence of a phase transition. The conventionally-defined transition temperature TE for an infinite 3D system is shown to correspond in a 2D system with finite N to a crossover temperature between a slow and rapid increase in the fractional boson occupation N0/N of the ground state with decreasing T. We further show that TE ~ 1/log(N) at fixed area per boson, so in the thermodynamic limit there is no significant BEC in 2D at finite T. Thus, paradoxically, BEC only occurs in 2D at finite N with no phase transition associated with it. Calculations of thermodynamic properties versus T and area A are presented, including Helmholtz free energy, entropy S , pressure p, ratio of p to the energy density U/A, heat capacity at constant volume (area) CV and at constant pressure Cp, isothermal compressibility kappa_T and thermal expansion coefficient alpha_p, obtained using both the grand canonical ensemble (GCE) and canonical ensemble (CE) formalisms. The GCE formalism gives acceptable predictions for S, p, p/(U/A), kappa_T and alpha_p at large N, T and A, but fails for smaller values of these three parameters for which BEC becomes significant, whereas the CE formalism gives accurate results for all thermodynamic properties of finite systems even at low T and/or A where BEC occurs.

cond-mat.stat-mech↗

arXiv2015

Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR

In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM) wavevectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using $^{75}$As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of $T_{\rm c}$ and the shape of the superconducting dome in these and other iron-pnictide families.

cond-mat.supr-con↗

arXiv2015

Ba{0.4}Rb{0.6}Mn2As2: A Prototype Half-Metallic Ferromagnet

Half-metallic ferromagnetism (FM) in single-crystal Ba{0.39(1)}Rb{0.61(1)}Mn2As2 below its Curie temperature TC = 103(2) K is reported. The magnetization M versus applied magnetic field H isotherm data at 1.8 K show complete polarization of the itinerant doped-hole magnetic moments that are introduced by substituting Rb for Ba. The material exhibits extremely soft FM, with unobservably small remanent magnetization and coercive field. Surprisingly, and contrary to typical itinerant FMs, the M(H) data follow the Arrott-plot paradigm that is based on a mean-field theory of local-moment FMs. The in-plane electrical resistivity data are fitted well by an activated-T^2 expression for T < TC, whereas the data sharply deviate from this model for T > TC. Hence the activated-T^2 resistivity model is an excellent diagnostic for determining the onset of half-metallic FM in this compound, which in turn demonstrates the presence of a strong correlation between the electronic transport and magnetic properties of the material. Together with previous data on 40% hole-doped Ba{0.6}K{0.4}Mn2As2, these measurements establish 61%-doped Ba{0.39}Rb{0.61}Mn2As2 as a prototype for a new class of half-metallic ferromagnets in which all the itinerant carriers in the material are ferromagnetic.

cond-mat.str-el↗