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ABSTRACT: Single crystals of Ba(Fe1-xMnx)2As2, 0<x<0.148, have been grown and characterized by structural, magnetic, electrical transport, and thermopower measurements. Although growths of single crystals of Ba(Fe1-xMnx)2As2 for the full 0⩽x⩽1 range were made, we find evidence for phase separation (associated with some form of immiscibility) starting for x>0.1–0.2. Our measurements show that whereas the structural/magnetic phase transition found in pure BaFe2As2 at 134 K is initially suppressed by Mn substitution, superconductivity is not observed at any substitution level. Although the effect of hydrostatic pressure up to 20 kbar in the parent BaFe2As2 compound is to suppress the structural/magnetic transition at the approximate rate of 0.9 K/kbar, the effects of pressure and Mn substitution in the x=0.102 compound are not cumulative. Phase diagrams of transition temperature versus substitution concentration x based on electrical transport, magnetization, and thermopower measurements have been constructed and compared to those of the Ba(Fe1-xTMx)2As2 (TM= Co and Cr) series.
Phys. Rev. B. 10/2011; 84(14).
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ABSTRACT: Single crystals of Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$, $x<0.37$, have been grown and characterized by structural, magnetic and transport measurements. These measurements show that the structural/magnetic phase transition found in pure BaFe$_2$As$_2$ at 134 K is suppressed monotonically by Ru doping, but, unlike doping with TM=Co, Ni, Cu, Rh or Pd, the coupled transition seen in the parent compound does not detectably split into two separate ones. Superconductivity is stabilized at low temperatures for $x>0.2$ and continues through the highest doping levels we report. The superconducting region is dome like, with maximum T$_c$ ($\sim16.5$ K) found around $x\sim 0.29$. A phase diagram of temperature versus doping, based on electrical transport and magnetization measurements, has been constructed and compared to those of the Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$ (TM=Co, Ni, Rh, Pd) series as well as to the temperature-pressure phase diagram for pure BaFe$_2$As$_2$. Suppression of the structural/magnetic phase transition as well as the appearance of superconductivity is much more gradual in Ru doping, as compared to Co, Ni, Rh and Pd doping, and appears to have more in common with BaFe$_2$As$_2$ tuned with pressure; by plotting $T_S/T_m$ and $T_c$ as a function of changes in unit cell dimensions, we find that changed in the $c/a$ ratio, rather than changes in $c$, $a$ or V, unify the $T(p)$ and $T(x)$ phase diagrams for BaFe$_2$As$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ respectively. Comment: 16 pages, 10 figures
06/2010;
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ABSTRACT: Microscopic, structural, transport and thermodynamic measurements of single
crystalline Ba(Fe1-xTMx)2As2 (TM = Ni and Cu) series, as well as two mixed TM =
Cu / Co series, are reported. All the transport and thermodynamic measurements
indicate that the structural and magnetic phase transitions at 134 K in pure
BaFe2As2 are monotonically suppressed and increasingly separated in a similar
manner by these dopants. In the Ba(Fe1-xNix)2As2 (x =< 0.072),
superconductivity, with Tc up to 19 K, is stabilized for 0.024 =< x =< 0.072.
In the Ba(Fe1-xCux)2As2 (x =< 0.356) series, although the structural and
magnetic transitions are suppressed, there is only a very limited region of
superconductivity: a sharp drop of the resistivity to zero near 2.1 K is found
only for the x = 0.044 samples. In the Ba(Fe1-x-yCoxCuy)2As2 series,
superconductivity, with Tc values up to 12 K (x ~ 0.022 series) and 20 K (x ~
0.047 series), is stabilized. Quantitative analysis of the detailed
temperature-dopant concentration (T-x) and temperature-extra electrons (T-e)
phase diagrams of these series shows that there exists a limited range of the
number of extra electrons added, inside which the superconductivity can be
stabilized if the structural and magnetic phase transitions are suppressed
enough. Moreover, comparison with pressure-temperature phase diagram data, for
samples spanning the whole doping range, further reenforces the conclusion that
suppression of the structural / magnetic phase transition temperature enhances
Tc on the underdoped side, but for the overdoped side Tcmax is determined by e.
Therefore, by choosing the combination of dopants that are used, we can adjust
the relative positions of the upper phase lines (structural and magnetic phase
transitions) and the superconducting dome to control the occurrence and
disappearance of the superconductivity in transition metal, electron-doped
BaFe2As2.
06/2010;
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ABSTRACT: Single crystals of NdFe1 − xCoxAsO with x = 0, 0.025, 0.05, 0.075, 0.10 and 0.15 were grown via high pressure synthesis. Structural and elemental analysis showed successful substitution of Co for Fe with no detectable change in the oxygen site occupation. Magnetic, electrical transport and tunnel-diode resonator measurements were used to characterize the NdFe1 − xCoxAsO crystals. These measurements indicated that superconductivity was achieved with x ≥ 0.025 and optimal doping was established with x = 0.05 with the maximum superconducting critical temperature of 25 K. Measurement of temperature dependent resistance in applied magnetic fields as high as 14 T showed a broadening of the superconducting transition and an Hc2(T) curve that was consistent with those of other iron pnictide superconductors.
Superconductor Science and Technology 04/2010; 23(5):054008. · 2.66 Impact Factor
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ABSTRACT: Single crystals of NdFe(1-x)Co(x)AsO with x = 0, 0.025, 0.05, 0.075, 0.10 and 0.15 were grown via high pressure synthesis. Structural and elemental analysis showed successful substitution of Co for Fe with no detectable change in the oxygen site occupation. Magnetic, electrical transport and tunnel-diode resonator measurements were used to characterize the NdFe(1-x)Co(x)AsO crystals. These measurements indicated that superconductivity was achieved with x >= 0.025 and optimal doping was established with x = 0.05 with the maximum superconducting critical temperature of 25 K. Measurement of temperature dependent resistance in applied magnetic fields as high as 14 T showed a broadening of the superconducting transition and an H(c2)(T) curve that was consistent with those of other iron pnictide superconductors.
Superc. Sci. Technol. 01/2010; 23(5).
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P. C. Canfield,
M. L. Caudle,
C. -S. Ho,
A. Kreyssig,
S Nandi,
M. G. Kim,
X Lin, A. Kracher,
K. W. Dennis,
R W McCallum,
A. I. Goldman
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ABSTRACT: We report the discovery of a new binary icosahedral phase in a Sc-Zn alloy obtained through solution-growth, producing millimeter-sized, facetted, single grain, quasicrystals that exhibit different growth morphologies, pentagonal dodecahedra and rhombic triacontahedra, under only marginally different growth conditions. These two morphologies manifest different degrees of quasicrystalline order, or phason strain. The discovery of i-Sc$_12$Zn$_88$ suggests that a reexamination of binary phase diagrams at compositions close to crystalline approximant structures may reveal other, new binary quasicrystalline phases. Comment: Incorrect spelling in author list resolved
10/2009;
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J. -Q. Yan,
S Nandi,
J. L. Zarestky,
W. Tian,
A. Kreyssig,
B. Jensen, A. Kracher,
K. W. Dennis,
R. J. McQueeney,
A. I. Goldman,
R W McCallum,
T. A. Lograsso
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ABSTRACT: Millimeter-sized single crystals of LaFeAsO, LaFeAsO1-xFx, and LaFe1-xCoxAsO were grown in NaAs flux at ambient pressure. The detailed growth procedure and crystal characterizations are reported. The as-grown crystals have typical dimensions of 3 * 4 * 0.05-0.3 mm3 with the crystallographic c-axis perpendicular to the plane of the plate-like single crystals. Some crystals manifest linear dimensions as large as 4-5 mm. X-ray and neutron single crystal scattering confirmed that LaFeAsO crystals exhibit a structural phase transition at Ts ~ 154 K and a magnetic phase transition at TSDW ~ 140 K. The transition temperatures agree with those determined by anisotropic magnetization, in-plane electrical resistivity and specific heat measurements and are consistent with previous reports on polycrystalline samples. Co and F were successfully introduced into the lattice leading to superconducting LaFe1-xCoxAsO and LaFeAsO1-xFx single crystals, respectively. This growth protocol has been successfully employed to grow single crystals of NdFeAsO. Thus it is expected to be broadly applicable to grow other RMAsO (R = rare earth, M = transition metal) compounds. These large crystals will facilitate the efforts of unraveling the underlying physics of iron pniticide superconductors.
09/2009;
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ABSTRACT: We present thermodynamic, structural and transport measurements on Ba(Fe0.973Cr0.027)2As2 single crystals. All measurements reveal sharp anomalies at ~ 112 K. Single crystal x-ray diffraction identifies the structural transition as a first order, from the high-temperature tetragonal I4/mmm to the low-temperature orthorhombic Fmmm structure, in contrast to an earlier report.
06/2009;
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ABSTRACT: Single crystalline Ba(Fe(1-x)TMx)2As2 (TM = Rh, Pd) series have been grown and characterized by structural, thermodynamic and transport measurements. These measurements show that the structural/magnetic phase transitions, found in pure BaFe2As2 at 134 K, are suppressed monotonically by the doping and that superconductivity can be stablized over a dome-like region. Temperature-composition (T-x) phase diagrams based on electrical transport and magnetization measurements are constructed and compared to those of the Ba(Fe(1-x)TMx)2As2 (TM = Co, Ni) series. Despite the generic difference between 3d and 4d shells and the specific, conspicuous differences in the changes to the unit cell parameters, the effects of Rh doping are exceptionally similar to the effects of Co doping and the effects of Pd doping are exceptionally similar to the effects of Ni doping. These data show that whereas the structural / antiferromagnetic phase transition temperatures can be parameterized by x and the superconducting transition temperature can be parameterized by some combination of x and e, the number of extra electrons associated with the TM doping, the transition temperatures of 3d- and 4d- doped BaFe2As2 can not be simply parameterized by the changes in the unit cell dimensions or their ratios.
05/2009;
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ABSTRACT: Study and comparison of over 30 examples of electron doped BaFe2As2 for transition metal (TM) = Co, Ni, Cu, and (Co/Cu mixtures) have lead to an understanding that the suppression of the structural/antiferromagnetic phase transition to low enough temperature in these compounds is a necessary condition for superconductivity, but not a sufficient one. Whereas the structural/antiferromagnetic transitions are suppressed by the number of TM dopant ions (or changes in the c-axis) the superconducting dome exists over a limited range of values of the number of electrons added by doping (or values of the {a/c} ratio). By choosing which combination of dopants are used we can change the relative positions of the upper phase lines and the superconducting dome, even to the extreme limit of suppressing the upper structural and magnetic phase transitions without the stabilization of low temperature superconducting dome.
04/2009;
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ABSTRACT: Single crystalline samples of Ba(Fe1−xCox)2As2 with x<0.12 have been grown and characterized via microscopic, thermodynamic, and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high-temperature tetragonal to low-temperature orthorhombic) and magnetic (high-temperature nonmagnetic to low-temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K/% Co. In addition, for x≥0.038 superconductivity is stabilized, rising to a maximum Tc of approximately 23 K for x≈0.07 and decreasing for higher x values. The T-x phase diagram for Ba(Fe1−xCox)2As2 indicates that either superconductivity can exist in both low-temperature crystallographic phases or that there is a structural phase separation. Anisotropic superconducting upper critical-field data [Hc2(T)] show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.
Phys. Rev. B. 12/2008; 78(21).
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ABSTRACT: Single crystalline samples of
Ba(Fe1-xCox)2As2 with
x<0.12 have been grown and characterized via microscopic,
thermodynamic, and transport measurements. With increasing Co
substitution, the thermodynamic and transport signatures of the
structural (high-temperature tetragonal to low-temperature orthorhombic)
and magnetic (high-temperature nonmagnetic to low-temperature
antiferromagnetic) transitions are suppressed at a rate of roughly 15
K/% Co. In addition, for x≥0.038 superconductivity is stabilized,
rising to a maximum Tc of approximately 23 K for x≈0.07
and decreasing for higher x values. The T-x phase diagram for
Ba(Fe1-xCox)2As2
indicates that either superconductivity can exist in both
low-temperature crystallographic phases or that there is a structural
phase separation. Anisotropic superconducting upper critical-field data
[Hc2(T)] show a significant and clear change in anisotropy
between samples that have higher temperature structural phase
transitions and those that do not. These data show that the
superconductivity is sensitive to the suppression of the higher
temperature phase transition.
Physical review. B, Condensed matter 11/2008; 78(21):214515.
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ABSTRACT: Single crystalline samples of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x < 0.12$ have been grown and characterized via microscopic, thermodynamic and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high temperature tetragonal to low temperature orthorhombic) and magnetic (high temperature non magnetic to low temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K per percent Co. In addition, for $x \ge 0.038$ superconductivity is stabilized, rising to a maximum $T_c$ of approximately 23 K for $x \approx 0.07$ and decreasing for higher $x$ values. The $T - x$ phase diagram for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ indicates that either superconductivity can exist in both low temperature crystallographic phases or that there is a structural phase separation. Anisotropic, superconducting, upper critical field data ($H_{c2}(T)$) show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.
11/2008;
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J.-Q. Yan,
A. Kreyssig,
S. Nandi,
N. Ni,
S. L. Bud’ko, A. Kracher,
R. J. McQueeney,
R. W. McCallum,
T. A. Lograsso,
A. I. Goldman,
P. C. Canfield
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ABSTRACT: Platelike single crystals of SrFe2As2 as large as 3×3×0.5 mm3 have been grown out of Sn flux. The SrFe2As2 single crystals show a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase at To=198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 to 160 K, single-crystal x-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.
Phys. Rev. B. 07/2008; 78(2).
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J. -Q. Yan,
A. Kreyssig,
S Nandi,
N. Ni,
S. L. Bud'ko, A. Kracher,
R. J. McQueeney,
R W McCallum,
T. A. Lograsso,
A. I. Goldman,
P. C. Canfield
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ABSTRACT: Plate-like single crystals of SrFe2As2 as large as 3x3x0.5 mm3 have been grown out of Sn flux. The SrFe2As2 single crystals show a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase at To = 198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 K to 160 K, single crystal X-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.
07/2008;
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ABSTRACT: Single crystals of BaFe$_2$As$_2$ and (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ have been grown out of excess Sn with 1% or less incorporation of solvent. The crystals are exceptionally micaceous, are easily exfoliated and can have dimensions as large as 3 x 3 x 0.2 mm$^3$. The BaFe$_2$As$_2$ single crystals manifest a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase near 85 K and do not show any sign of superconductivity down to 1.8 K. This transition can be detected in the electrical resistivity, Hall resistivity, specific heat and the anisotropic magnetic susceptibility. In the (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single crystals this transition is suppressed and instead superconductivity occurs with a transition temperature near 30 K. Whereas the superconducting transition is easily detected in resistivity and magnetization measurements, the change in specific heat near $T_c$ is small, but resolvable, giving $\Delta C_p/\gamma T_c \approx 1$. The application of a 140 kOe magnetic field suppresses $T_c$ by only $\sim 4$ K when applied along the c-axis and by $\sim 2$ K when applied perpendicular to the c-axis. The ratio of the anisotropic upper critical fields, $\gamma = H_{c2}^{\perp c} / H_{c2}^{\| c}$, varies between 2.5 and 3.5 for temperatures down to $\sim 2$ K below $T_c$.
06/2008;
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ABSTRACT: Rare-earth-based permanent-magnet materials rich in iron have relatively low ferromagnetic ordering temperatures. This is believed to be due to the presence of antiferromagnetic exchange interactions, besides the ferromagnetic interactions responsible for the magnetic order. The magnetic properties of Ce2Fe17 are anomalous. Instead of ferromagnetic, it is antiferromagnetic, and instead of one ordering temperature, it shows two, at the Neel temperature TN ~ 208 K and at TT ~ 124 K. Ce2Fe17, doped by 0.5% Ta, also shows two ordering temperatures, one to an antiferromagnetic phase, at TN ~ 214 K, and one to a ferromagnetic phase, at T0 ~ 75 K. In order to clarify this behavior, single-crystalline samples were prepared by solution growth, and characterized by electron microscopy, single crystal x-ray diffraction, temperature-dependent specific heat, and magnetic field and temperature-dependent electrical resistivity and magnetization. From these measurements, magnetic H-T phase diagrams were determined for both Ta-doped Ce2Fe17 and undoped Ce2Fe17. These phase diagrams can be very well described in terms of a theory that gives magnetic phase diagrams of systems with competing antiferro- and ferromagnetism. Comment: 18 pages, 16 figures, submitted to PRB
12/2006;
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ABSTRACT: Single crystals of flux-grown tetragonal GdFe4Al8 were characterized by thermodynamic, transport, and x-ray resonant magnetic scattering measurements. In addition to antiferromagnetic order at TN ~ 155 K, two low-temperature transitions at T1 ~ 21 K and T2 ~ 27 K were identified. The Fe moments order at TN with an incommensurate propagation vector (tau,tau,0) with tau varying between 0.06 and 0.14 as a function of temperature, and maintain this order over the entire T<TN range. The Gd 4f moments order below T2 with a ferromagnetic component mainly out of plane. Below T1, the ferromagnetic components are confined to the crystallographic plane. Remarkably, at low temperatures the Fe moments maintain the same modulation as at high temperatures, but the Gd 4f moments apparently do not follow this modulation. The magnetic phase diagrams for fields applied in [110] and [001] direction are presented and possible magnetic structures are discussed.
07/2005;
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ABSTRACT: There has been much interest in the past ten years in the effects of applying a transverse magnetic field on the freezing temperature of Ising spin glasses. The focus of this study is to search for and characterize a metallic Ising spin-glass system. This is accomplished by site diluting yttrium for terbium in the crystalline material TbNi2Ge2. Pure TbNi2Ge2 is an Ising antiferromagnet with several distinct magnetic states below 17 K. As the terbium is diluted with yttrium, magnetic coupling and ordering temperatures are suppressed in a monotonic way, as is seen in measurements of the transition temperatures and analysis of the high-temperature Curie-Weiss behavior. At low concentrations of terbium, below x∼0.35, long-range order is no longer detected and a spin-glass-like state emerges. This state is studied through a variety of measurements: dc and ac susceptibility, resistivity, and specific heat. These data are then compared to that of other well characterized spin-glass systems. It is concluded that there is a region of concentrations for which an Ising spin-glass state is formed with the best spin glasses existing for x<~0.30.
Phys. Rev. B. 11/2000; 62(22).
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ABSTRACT: Single crystals of BaFe2As2 and Ba0.55K0.45Fe2As2 have been grown out of excess Sn with 1% or less incorporation of solvent. The crystals are exceptionally micaceous, are easily exfoliated, and can have dimensions as large as 3×3×0.2 mm3. The BaFe2As2 single crystals manifest a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase near 85 K and do not show any sign of superconductivity down to 1.8 K. This transition can be detected in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In the Ba0.55K0.45Fe2As2 single crystals this transition is suppressed and instead superconductivity occurs with a transition temperature near 30 K. Whereas the superconducting transition is easily detected in resistivity and magnetization measurements, the change in specific heat near Tc is small, but resolvable, giving ΔCp/γTc≈1. The application of a 140 kOe magnetic field suppresses Tc by only ∼4 K when applied along the c axis and by ∼2 K when applied perpendicular to the c axis. The ratio of the anisotropic upper critical fields, γ=Hc2⊥c/Hc2∥c, varies between 2.5 and 3.5 for temperatures down to ∼2 K below Tc.
Phys. Rev. B. 78(1).
