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ABSTRACT: Measurements of temperature-dependent in-plane resistivity, $\rho(T)$, were
used to determine the upper critical field and its anisotropy in high quality
single crystals of stoichiometric iron arsenide superconductor KFe$_2$As$_2$.
The crystals were characterized by residual resistivity ratio,
$\rho(300K)/\rho(0)$ up to 3000 and resistive transition midpoint temperature,
$T_c$=3.8 K, significantly higher than in previous studies on the same
material. We find increased $H_{c2}(T)$ for both directions of the magnetic
field, which scale with the increased $T_c$. This unusual linear $H_{c2}(T_c)$
scaling is not expected for orbital limiting mechanism of the upper critical
field in clean materials.
04/2013;
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ABSTRACT: We study the relationship between precipitate morphology and
superconductivity in KxFe1.6+ySe2 single crystals grown by self-flux method.
Scanning electron microscopy (SEM) measurements revealed that superconducting
phase forms a network in the samples quenched above iron vacancy order-disorder
transition temperature Ts. For the samples obtained by natural cooling down to
room temperature in the furnace, referred to as furnace cooling,
superconducting phase aggregates into micrometer-sized rectangular bars and
aligns as disconnected chains. Accompanying this change in morphology the
superconducting shielding fraction is strongly reduced in the furnace-cooling
samples. By post-annealing above Ts followed by quenching in room temperature
water, the network recovers with superconducting shielding fraction approaching
80%. A reversible change from network to bar chains was realized by a secondary
heat treatment in annealed samples showing large shielding fraction, i.e.,
heating above Ts followed by slow cooling across Ts. The large shielding
fraction observed in KxFe1.6+ySe2 single crystals actually results from a
uniform and contiguous distribution of superconducting phase. Through the
measurements of temperature dependent x-ray diffraction, it is found that the
reflection corresponding to superconducting phase merges into that from iron
vacancy ordered phase upon warming. It is a solid solution above Ts, where iron
atoms randomly occupy the both Fe1 and Fe2 sites in iron vacancy disordering
status. By cooling across Ts, superconducting phase precipitates while iron
vacancy ordered phase forms together, suggesting that phase separation in
KxFe1.6+ySe2 single crystals is driven by the iron vacancy order-disorder
transition.
08/2012;
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ABSTRACT: Here, we present an electrical transport study in single crystals of
LaFe$_{0.92}$Co$_{0.08}$AsO ($T_c \simeq 9.1$ K) under high magnetic fields. In
contrast to most of the previously reported Fe based superconductors, and
despite its relatively low $T_c$, LaFe$_{1-x}$Co$_x$AsO shows a superconducting
anisotropy which is comparable to those seen for instance in the cuprates or
$\gamma_H = H_{c2}^{ab}/H_{c2}^{c} = m_c/m_{ab} \simeq 9$, where $m_c/m_{ab}$
is the effective mass anisotropy. Although, in the present case and as in all
Fe based superconductors, $\gamma \rightarrow 1$ as $T \rightarrow 0$. Under
the application of an external field, we also observe a remarkable broadening
of the superconducting transition particularly for fields applied along the
inter-planar direction. Both observations indicate that the low dimensionality
of LaFe$_{1-x}$Co$_x$AsO is likely to lead to a more complex vortex
phase-diagram when compared to the other Fe arsenides and consequently, to a
pronounced dissipation associated with the movement of vortices in a possible
vortex liquid phase. When compared to, for instance, F-doped compounds
pertaining to same family, we obtain rather small activation energies for the
motion of vortices. This suggests that the disorder introduced by doping
LaFeAsO with F is more effective in pinning the vortices than alloying it with
Co.
08/2012;
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ABSTRACT: The phonon dispersion was measured at room temperature along (0,0,L) in the
tetragonal phase of LaFeAsO using inelastic x-ray scattering. Spin-polarized
first-principles calculations imposing various types of antiferromagnetic order
are in better agreement with the experimental results than nonmagnetic
calculations, although the measurements were made well above the magnetic
ordering temperature, T_N. Splitting observed between two A_{1g} phonon modes
at 22 and 26 meV is only observed in spin-polarized calculations.
Magneto-structural effects similar to those observed in the AFe_2As_2 materials
are confirmed present in LaFeAsO. The presence of Fe-spin is necessary to find
reasonable agreement of the calculations with the measured spectrum well above
T_N. On-site Fe and As force constants show significant softening compared to
nonmagnetic calculations, however an investigation of the real-space force
constants associates the magnetoelastic coupling with a complex renormalization
instead of softening of a specific pairwise force.
06/2012;
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ABSTRACT: The Ni-Pr phase diagram on the Pr-rich side was revised using differential thermal analysis, differential scanning calorimetry,
and scanning electron microscopy. The existence of four stoichiometric compounds, Ni2Pr, NiPr, Ni3Pr7, and NiPr3, was confirmed; however, the melting temperatures of 766 °C (NiPr), 566 °C (Ni3Pr7), and 562 °C (NiPr3) are considerably different from the previously reported values. The same was true for the four eutectic reactions, which
were determined to be as follows: liquid (L) (45±1 at.% Pr) ↔ Ni2Pr+NiPr at a mean temperature of 732±5 °C; L (65±1 at.% Pr) ↔ NiPr+Ni3Pr7 at a mean temperature of 550±2 °C; L (72±2 at.% Pr) ↔ Ni3Pr7+NiPr3 at a mean temperature of 557±2 °C; and L (77±1 at.% Pr) ↔ NiPr3+Pr.
Journal of Phase Equilibria and Diffusion 04/2012; 26(3):209-214. · 0.62 Impact Factor
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ABSTRACT: This article will review the development of the Fe–Ga (Galfenol) alloy system for magnetostriction applications including
work on substitutional ternary alloying additions for magnetic property enhancement. A majority of the alloying addition research
has focused on substitutional ternary elements in Bridgman grown single crystals with the intent of improving the magnetostrictive
capability of the Galfenol system. Single crystals provide the ideal vehicle to assess the effectiveness of the addition on
the magnetostrictive properties by eliminating grain boundary effects, orientation variations, and grain-to-grain interactions
that occur when polycrystals respond to applied magnetic fields. In almost all cases, ternary additions of transition metal
elements have decreased the magnetostriction values from the binary Fe–Ga alloy. Most of the ternary additions are known to
stabilize the D03 chemical order and could be a primary contribution to the observed reduction in magnetostriction. In contrast, both Sn and
Al are found to substitute chemically for Ga. For Sn additions, whose solubility is limited, no reduction in magnetostriction
strains are observed when compared to the equivalent binary alloy composition. Aluminum additions, whose effect on the magnetoelastic
coupling on Fe is similar to Ga, result in a rule of mixture relationship. The reviewed research suggests that phase stabilization
of the disordered bcc structure is a key component to increase the magnetostriction of Fe–Ga alloys.
Journal of Materials Science 04/2012; 42(23):9582-9594. · 2.02 Impact Factor
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S. Vélez,
J. M. Hernandez,
A. García-Santiago,
J. Tejada,
V. K. Pecharsky,
Gschneidner, Jr., K. A,
D. L. Schlagel, T. A. Lograsso,
P. V. Santos
[show abstract]
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ABSTRACT: Experimental evidence of the anisotropy of the magnetic deflagration associated with the low-temperature first order antiferromagnetic (AFM) --> ferromagnetic (FM) phase-transition in single crystals of Gd5Ge4 is reported. The deflagrations have been induced by controlled pulses of surface acoustic waves (SAW) allowing us to explore both the magnetic field and temperature dependencies on the characteristic times of the phenomenon. The study was done using samples with different geometries and configurations between the SAW pulses and the direction of the applied magnetic field with respect to the three main crystallographic directions of the samples. The effect of temperature is nearly negligible, whereas observed strong magnetic field dependence correlates with the magnetic anisotropy of the sample. Finally, the
role of the SAW pulses in both the ignition and formation of the deflagration front was also studied, and we show that the thermal diffusivity of Gd5Ge4 must be anisotropic, following \kappaa>\kappab>\kappac.
Physical Review B 02/2012; 85:054432. · 3.69 Impact Factor
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ABSTRACT: Millimeter-sized LaFeAsO single crystals have been successfully grown out of NaAs flux starting with (LaAs+1/2Fe2O3):20NaAs. The factors which allow the growth of these crystals out of NaAs but not out of many other fluxes, such as FeAs, have been investigated. X-ray powder diffraction found that NaAs synthesized for the growth of RFeAsO superconductors has monoclinic LiAs structure. Thermal analysis confirmed that NaAs melts congruently at about 600 °C. The ability to extract RFeAsO crystals from this NaAs flux suggests that NaAs has a significant oxygen solubility, possibly due to the formation of the ternary compound NaAsO2, and enough oxygen diffusivity to transport solute to the growth front. Oxygen solubility and diffusivity are two important factors in searching for an alternative environmentally benign flux for the growth of RFeAsO superconductors.
Applied Physics Letters 02/2011; 98(7):072504-072504-3. · 3.84 Impact Factor
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ABSTRACT: The growth of Bi on a pseudomorphic Bi monolayer on the fivefold surface of the icosahedral Al-Pd-Mn quasicrystal has been investigated using low energy electron diffraction and scanning tunnelling microscopy. Initially randomly oriented pseudocubic islands are formed with a preference for an even number of layers. Subsequently a morphological transformation takes place to hexagonal Bi islands, which align along high symmetry directions of the substrate. The Bi flux is found to have a strong effect on which island structure is preferred. When C(60) is adsorbed on the three different allotropes of Bi present in this system, hexagonal C(60) islands are formed in each case. On the pseudocubic and hexagonal islands, the C(60) islands are aligned with the substrate. We discuss the energetic, kinetic and geometrical factors which influence the morphological transformation referred to above.
Journal of Physics Condensed Matter 09/2010; 22(34):345002. · 2.55 Impact Factor
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Alessandro Ricci,
Nicola Poccia,
B. Joseph,
Luisa Barba,
Gianmichele Arrighetti,
Gabriele Ciasca,
J. -Q. Yan,
R W McCallum, T. A. Lograsso,
N. D. Zhigadlo,
J. Karpinski,
Antonio Bianconi
[show abstract]
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ABSTRACT: The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO (La-1111) and SmFeAsO (Sm-1111) single crystals measured by high resolution x-ray diffraction is found to be sharp while the RFeAsO (R=La, Nd, Pr, Sm) polycrystalline samples show a broad continuous SPT. Comparing the polycrystalline and the single crystal 1111 samples, the critical exponents of the SPT are found to be the same while the correlation length critical exponents are found to be very different. These results imply that the lattice fluctuations in 1111 systems change in samples with different surface to volume ratio that is assigned to the relieve of the temperature dependent superlattice misfit strain between active iron layers and the spacer layers in 1111 systems. This phenomenon that is missing in the AFe2As2 (A=Ca, Sr, Ba) "122" systems, with the same electronic structure but different for the thickness and the elastic constant of the spacer layers, is related with the different maximum superconducting transition temperature in the 1111 (55 K) versus 122 (35 K) systems and implies the surface reconstruction in 1111 single crystals. Comment: Accepted for publication in Phys Rev B
07/2010;
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ABSTRACT: Elastic neutron scattering studies of an optimized LaFeAsO single crystal reveal that upon cooling, an onset of the tetragonal (T)-to-orthorhombic (O) structural transition occurs at $T_\texttt{S} \approx 156$ K, and it exhibits a sharp transition at $T_\texttt{P} \approx 148$ K. We argue that in the temperature range $T_\texttt{S}$ to $T_\texttt{P}$, T and O structures may dynamically coexist possibly due to nematic spin correlations recently proposed for the iron pnictides, and we attribute $T_\texttt{P}$ to the formation of long-range O domains from the finite local precursors. The antiferromagnetic structure emerges at $T_\texttt{N} \approx 140$ K, with the iron moment direction along the O \emph{a} axis. We extract the iron magnetic form factor and use the tabulated $\langle j_0\rangle$ of Fe, Fe$^{2+}$ and Fe$^{3+}$ to obtain a magnetic moment size of $\sim$0.8 $\mu_\texttt{B}$ at 9.5 K. Comment: 7 pages, 6 figures, 3 tables
07/2010;
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W. Tian,
W. Ratcliff II,
M. G. Kim,
J. -Q. Yan,
P. A. Kienzle,
Q Huang,
B. Jensen,
K. W. Dennis,
R W McCallum, T. A. Lograsso,
R. J. McQueeney,
A. I. Goldman,
J. W. Lynn,
A. Kreyssig
[show abstract]
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ABSTRACT: The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142 K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe moments order AFM in the well-known stripe-like structure in the (ab) plane, but change from AFM to ferromagnetic (FM) arrangement along the c direction below T* = 15 K accompanied by the onset of Nd AFM order below T_Nd = 6 K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO. Comment: revised; 4 pages, 3 figures; accepted for publication in Phys. Rev. B
06/2010;
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ABSTRACT: We measured the electronic structure of an iron arsenic parent compound LaFeAsO using angle resolved photoemission spectroscopy (ARPES). By comparing with a full-potential Linear Augmented PlaneWave calculation we show that the extra large Gamma hole pocket measured via ARPES comes from electronic structure at the sample surface. Based on this we discuss the strong polarization dependence of the band structure and a temperature-dependent hole-like band around the M point. The two phenomena give additional evidences for the existence of the surface-driven electronic structure. Comment: 6 pages, 6 figures
06/2010;
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ABSTRACT: The tetragonal magnetostriction constant, (3/2)λ<sub>100</sub> , of Fe–Si single crystals was measured and was found to be structure dependent. Similar to that of Fe–Ge single crystals, (3/2)λ<sub>100</sub> is positive in the single phase A2 regime, becomes negative in the single phase D 0<sub>3</sub> regime, and changes from positive to negative between the two regimes. Short-range order in the A2 regime decreases the magnetostriction prior to the onset of long range order. In the single phase regions of both A2 and D 0<sub>3</sub> , thermal history does not show any obvious effect on the magnetostriction, contrary to that found for Fe–Ga alloys. However, in the regions of phase mixture involving A2, B2, and D 0<sub>3</sub> phases, quenching pushes the change in magnetostriction from positive to negative to higher Si contents.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: In this paper we investigate the magnetostrictive [ (3/2)λ<sub>100</sub> and (3/2)λ<sub>111</sub> ] and elastic ( c <sup>′</sup> and c <sub>44</sub> ) behavior of single crystalline alloys Fe <sub>100- x </sub> Mo <sub> x </sub> for 2< x <12 ; the magnetoelastic coupling constants ( - b <sub>1</sub> and - b <sub>2</sub> ) are computed from the measurements. Similar to other Fe–X ( X = Al , Ga, and Ge) alloys, the tetragonal magnetostriction (3/2)λ<sub>100</sub> increases monotonically from ∼70×10<sup>-6</sup> at ∼2.5 at . % Mo to a maximum of either ∼100×10<sup>-6</sup> at ∼8 at . % Mo for the slow cooled crystals or ∼125×10<sup>-6</sup> at ∼11 at . % Mo for quenched crystals. A sharp decrease after the peak is observed for the slow cooled crystals due to the formation of a second phase. The rhombohedral magnetostriction (3/2)λ<sub>111</sub> of the Fe–Mo alloys is found to be insensitive to the Mo content. This behavior is distinctly different from other Fe–X ( X = Al , Ga, and Ge) alloys where a slight decrease in magnitude and a sign reversal upon chemical ordering was observed for (3/2)λ<sub>111</sub> . Both shear elastic constants ( c <sup>-
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032;</sup> and c <sub>44</sub> ) for Fe–Mo are remarkably insensitive to the Mo content, which is also distinct from the other Fe-based alloys used in the comparison. The two magnetoelastic coupling constants - b <sub>1</sub>=3λ<sub>100</sub> c <sup>′</sup> (with values from 7.15 to 9.77 MJ / m <sup>3</sup> ) and - b <sub>2</sub>=3λ<sub>111</sub> c <sub>44</sub> (with values from -4.96 to -5.81 MJ / m <sup>3</sup> ) were calculated and compared with those of other Fe–X ( X = Al , Ga, and Ge) alloys.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: Copper deposited on the five-fold surface of icosahedral Al-Pd-Mn forms domains of a structure whose surface has a one-dimensional aperiodic modulation. It is shown that C60deposited on this aperiodic film has highly reduced mobility as compared to C60 deposited on periodic Cu surfaces. This finding is explained in terms of the recently proposed structural model of this system.
Journal of Physics Conference Series 05/2010; 226(1):012006.
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ABSTRACT: The origin of the induced magnetic anisotropy in stress-annealed single crystalline Fe<sub>80.5</sub>Ga<sub>19.5</sub> was investigated by high energy X-ray diffuse scattering. A compressive stress was applied along [010] during annealing. Superlattice reflections associated with D0<sub>3</sub> chemical order were analyzed to clarify the origin of the stress-annealing-induced magnetic anisotropy. Results showed that the D0<sub>3</sub> precipitates in the microstructure exhibited long range ordering of Ga. These clusters were found not to have a crystallographic preference relative to the applied stress axis and no structural anisotropy could be detected. The results indicate that the induced anisotropy following stress annealing likely comes from the anisotropy of the magnetic domain alignment and is not related to the underlying microstructural features of the alloy.
IEEE Transactions on Magnetics 11/2009; · 1.36 Impact Factor
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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
[show abstract]
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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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W. Tian,
A. Kreyssig,
J. L. Zarestky,
L. Tan,
S Nandi,
A. I. Goldman, T. A. Lograsso,
D. L. Schlagel,
K A Gschneidner,
V K Pecharsky,
R. J. McQueeney
[show abstract]
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ABSTRACT: We present a single crystal neutron diffraction study of the magnetic short-range correlations in Tb$_5$Ge$_4$ which orders antiferromagnetically below the Neel temperature $T_N$ $\approx$ 92 K. Strong diffuse scattering arising from magnetic short-range correlations was observed in wide temperature ranges both below and above $T_N$. The antiferromagnetic ordering in Tb$_5$Ge$_4$ can be described as strongly coupled ferromagnetic block layers in the $ac$-plane that stack along the b-axis with weak antiferromagnetic inter-layer coupling. Diffuse scattering was observed along both $a^*$ and $b^*$ directions indicating three-dimensional short-range correlations. Moreover, the $q$-dependence of the diffuse scattering is Squared-Lorentzian in form suggesting a strongly clustered magnetic state that may be related to the proposed Griffiths-like phase in Gd$_5$Ge$_4$. Comment: 6 pages, 5 figures
08/2009;
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R. W. McCallum,
J.-Q. Yan,
G. E. Rustan,
E. D. Mun,
Yogesh Singh,
S. Das,
R. Nath,
S. L. Budko,
K. W. Dennis,
D. C. Johnston,
P. C. Canfield,
M. J. Kramer,
A. Kreyssig, T. A. Lograsso,
A. I. Goldman
[show abstract]
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ABSTRACT: The reaction path for the synthesis of LaFeAsO and La Fe As O <sub>1-x</sub> F <sub>y</sub> by solid state reaction was studied by in situ high temperature x-ray diffraction technique and differential thermal analysis in the temperature interval 100 ° C ≤T≤1150 ° C . Starting with LaAs, Fe <sub>2</sub> O <sub>3</sub> , Fe, and La F <sub>3</sub> as precursors, the results show that the synthesis is characterized by three temperature intervals: (1) Below 500 ° C the sequential reduction of Fe <sub>2</sub> O <sub>3</sub> and Fe <sub>3</sub> O <sub>4</sub> takes place through the oxidization of LaAs. Below 400 ° C , Fe <sub>2</sub> O <sub>3</sub> is reduced to Fe <sub>3</sub> O <sub>4</sub> by LaAs and then at 400 ° C ≪T≪500 ° C Fe <sub>3</sub> O <sub>4</sub> is further reduced to Fe. (2) In the temperature interval 500 ° C ≪T≪800 ° C , multiple intermediate reactions take place resulting in the formation of FeAs and La <sub>2</sub> O <sub>3</sub> . (3) The formation of LaFeAsO based phase could be unambiguously resolved above 800 ° C . For both LaFeAsO and La Fe As O-
<sub>1-x</sub> F <sub>y</sub> , FeAs is a primary impurity at high temperatures that melts at ∼1040 ° C . Possible reaction pathways and the difference between F-free and F-doped samples are discussed.
Journal of Applied Physics 07/2009; · 2.17 Impact Factor
