7,883 research outputs found
Antiferromagnetism in EuCu2As2 and EuCu1.82Sb2 Single Crystals
Single crystals of EuCu2As2 and EuCu2Sb2 were grown from CuAs and CuSb
self-flux, respectively. The crystallographic, magnetic, thermal and electronic
transport properties of the single crystals were investigated by
room-temperature x-ray diffraction (XRD), magnetic susceptibility \chi versus
temperature T, isothermal magnetization M versus magnetic field H, specific
heat Cp(T) and electrical resistivity \rho(T) measurements. EuCu2As2
crystallizes in the body-centered tetragonal ThCr2Si2-type structure (space
group I4/mmm), whereas EuCu2Sb2 crystallizes in the related primitive
tetragonal CaBe2Ge2-type structure (space group P4/nmm). The energy-dispersive
x-ray spectroscopy and XRD data for the EuCu2Sb2 crystals showed the presence
of vacancies on the Cu sites, yielding the actual composition EuCu1.82Sb2. The
\rho(T) and Cp(T) data reveal metallic character for both EuCu2As2 and
EuCu1.82Sb2. Antiferromagnetic (AFM) ordering is indicated from the \chi(T),
Cp(T), and \rho(T) data for both EuCu2As2 (T_N = 17.5 K) and EuCu1.82Sb2 (T_N =
5.1 K). In EuCu1.82Sb2, the ordered-state \chi(T) and M(H) data suggest either
a collinear A-type AFM ordering of Eu+2 spins S=7/2 or a planar noncollinear
AFM structure, with the ordered moments oriented in the tetragonal ab plane in
either case. This ordered-moment orientation for the A-type AFM is consistent
with calculations with magnetic dipole interactions. The anisotropic \chi(T)
and isothermal M(H) data for EuCu2As2, also containing Eu+2 spins S=7/2,
strongly deviate from the predictions of molecular field theory for collinear
AFM ordering and the AFM structure appears to be both noncollinear and
noncoplanar.Comment: 21 pages, 22 figures, 4 Table
Metallic behavior induced by potassium doping of the trigonal antiferromagnetic insulator EuMn2As2
We report magnetic susceptibility \chi, isothermal magnetization M, heat
capacity C_p and electrical resistivity \rho measurements 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 \chi(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 \chi(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 \rho related to the Mn magnetism is found. Upon cooling from 200
K, \rho exhibits 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 \rho that is only observed upon cooling below T_N of an antiferromagnet.Comment: 21 pages, 22 figures, 4 Table
Physical properties of EuPd2As2 single crystals
The physical properties of self-flux grown EuPd2As2 single crystals have been
investigated by magnetization M, magnetic susceptibility chi, specific heat Cp,
and electrical resistivity rho measurements versus temperature T and magnetic
field H. The crystal structure was determined by powder x-ray diffraction
measurements, which confirmed the ThCr2Si2-type body-centered tetragonal
structure (space group I4/mmm) reported previously. The rho(T) data indicate
that state of EuPd2As2 is metallic. Long-range antiferromagnetic (AFM) ordering
is apparent from the chi(T), Cp(T), and rho(T) measurements. For H \parallel c
the chi(T) indicates two transitions at TN1 = 11.0 K and TN2 = 5.5 K, whereas
for H \perp c only one transition is observed at TN1 = 11.0 K. Between TN1 and
TN2 the anisotropic chi(T) data suggest a planar noncollinear AFM structure,
whereas at T < TN2 the chi(T) and M(H,T) data suggest a spin reorientation
transition in which equal numbers of spins cant in opposite directions out of
the ab plane. We estimate the critical field at 2 K at which all Eu moments
become aligned with the field to be about 22 T. The magnetic entropy at 25 K
estimated from the Cp(T) measurements is about smaller than expected,
possibly due to an inaccuracy in the lattice heat capacity contribution. An
upturn in rho at T < TN1 suggests superzone energy gap formation below TN1.
This behavior of rho(T < TN1) is not sensitive to applied magnetic fields up to
H = 12 T.Comment: 11 pages, 10 figures, 2 tables and 52 references; To appear in J.
Phys.: Condens. Matte
Recommended from our members
Implant Prophylaxis: The Next Best Practice Toward Asepsis in Spine Surgery.
Study designA literature review.ObjectivesAn evaluation of the contaminants prevalent on implants used for surgery and the aseptic methods being employed against them.MethodsPubMed was searched for articles published between 2000 and 2017 for studies evaluating the contaminants present on spine implants, and associated pre- and intraoperative implant processing and handling methodology suggested to avoid them. Systematic reviews, observational studies, bench-top studies, and expert opinions were included.ResultsEleven studies were identified whose major focus was the asepsis of implants to reduce the incidence of surgical site infection incidences during surgery. These studies measured the colony forming units of bacteria on sterilized implants and/or gloves from the surgeon, scrub nurse, and assistants, as well as reductions of surgical site infection rates in spine surgery due to changes in implant handling techniques. Additionally, the search included assessments of endotoxins and carbohydrates present on reprocessed implants. The suggested changes to surgical practice based on these studies included handling implants with only fresh gloves, keeping implants covered until the immediate time of use, reducing operating room traffic, avoiding reprocessing of implants (ie, providing terminally sterilized implants), and avoiding touching the implants altogether.ConclusionsBoth reprocessing (preoperative) and handling (intraoperative) of implants seem to lead to contamination of sterilized implants. Using a terminally sterilized device may mitigate reprocessing (preoperative implant prophylaxis), whereas the use of fresh gloves for handling each implant and/or a permanent shielding technique (intraoperative implant prophylaxis) could potentially avoid recontamination at the theatre
Observation of long range magnetic ordering in pyrohafnate Nd2Hf2O7: A neutron diffraction study
We have investigated the physical properties of a pyrochlore hafnate Nd2Hf2O7
using ac magnetic susceptibility \chi_ac(T), dc magnetic susceptibility
\chi(T), isothermal magnetization M(H) and heat capacity C_p(T) measurements,
and determined the magnetic ground state by neutron powder diffraction study.
An upturn is observed below 6 K in C_p(T)/T, however both C_p(T) and \chi(T) do
not show any clear anomaly down to 2 K. The \chi_ac(T) shows a well pronounced
anomaly indicating an antiferromagnetic transition at T_N = 0.55 K. The long
range antiferromagnetic ordering is confirmed by neutron diffraction. The
refinement of neutron diffraction pattern reveals an all-in/all-out
antiferromagnetic structure, where for successive tetrahedra, the four Nd3+
magnetic moments point alternatively all-into or all-out-of the tetrahedron,
with an ordering wavevector k = (0, 0, 0) and an ordered state magnetic moment
of m = 0.62(1) \mu_B/Nd at 0.1 K. The ordered moment is strongly reduced
reflecting strong quantum fluctuations in ordered state.Comment: 10 pages, 9 figures and 2 tables; to appear in Phys. Rev.
Large Miscibility Gap in the Ba(Mn_xFe_{1-x})2As2 System
The compounds BaMn2As2 and BaFe2As2 both crystallize in the
body-centered-tetragonal ThCr2Si2-type (122-type) structure at room temperature
but exhibit quite different unit cell volumes and very different magnetic and
electronic transport properties. Evidently reflecting these disparities, we
have discovered a large miscibility gap in the system Ba(Mn_xFe_{1-x})2As2.
Rietveld refinements of powder x-ray diffraction (XRD) measurements on samples
slow-cooled from 1000 C to room temperature (RT) reveal a two-phase mixture of
BaMn2As2 and Ba(Mn_{0.12}Fe_{0.88})2As2 phases together with impurity phases
for x = 0.2, 0.4, 0.5, 0.6 and 0.8. We infer that there exists a miscibility
gap in this system at 300 K with composition limits 0.12 < x < 1. For samples
quenched from 1000 C to 77 K, the refinements of RT XRD data indicate that the
miscibility gap at RT narrows at 1000 C to 0.2 < x < 0.8. Samples with x=0.4,
0.5 and 0.6 quenched from 1100-1400 C to 77 K contain a single 122-type phase
together with significant amounts of Fe_{1-x}Mn_xAs and FeAs2 impurity phases.
These results indicate that the system is not a pseudo-binary system over the
whole composition range and that the 122-type phase has a significant
homogeneity range at these temperatures. Magnetic susceptibility, electrical
resistivity and heat capacity measurements versus temperature of the
single-phase quenched polycrystalline samples with x = 0.2 and 0.8 and for
lightly doped BaMn2As2 crystals are reported.Comment: 14 pages, 16 figures, 3 tables; published versio
Superconductivity and Physical Properties of CaPd2Ge2 Single Crystals
We present the superconducting and normal state properties of CaPd2Ge2 single
crystal investigated by magnetic susceptibility \chi, isothermal magnetization
M, heat capacity C_p, in-plane electrical resistivity \rho and London
penetration depth \lambda versus temperature T and magnetic field H
measurements. Bulk superconductivity is inferred from the \rho(T) and C_p(T)
data. The \rho(T) data exhibit metallic behavior and undergoes a
superconducting transition with T_c onset = 1.98 K and zero resistivity state
at T_c 0 = 1.67 K. The \chi(T) reveal the onset of superconductivity at 2.0 K.
For T>2.0 K, the \chi(T) and M(H) are weakly anisotropic paramagnetic with
\chi_ab > \chi_c. The C_p(T) confirm the bulk superconductivity below T_c =
1.69(3) K. The superconducting state electronic heat capacity is analyzed
within the framework of a single-band \alpha-model of BCS superconductivity and
various normal and superconducting state parameters are estimated. Within the
\alpha-model, the C_p(T) data and the ab plane \lambda(T) data consistently
indicate a moderately anisotropic s-wave gap with \Delta(0)/k_B T_c ~ 1.6,
somewhat smaller than the BCS value of 1.764. The relationship of the heat
capacity jump at T_c and the penetration depth measurement to the anisotropy in
the s-wave gap is discussed.Comment: 12 pages, 9 figures, 2 Tables; Submitted to PR
Effect of Ni-doping on magnetism and superconductivity in Eu0.5K0.5Fe2As2
The effect of Ni-doping on the magnetism and superconductivity in
Eu0.5K0.5Fe2As2 has been studied through a systematic investigation of magnetic
and superconducting properties of Eu0.5K0.5(Fe1-xNix)2As2 (x = 0, 0.03, 0.05,
0.08 and 0.12) compounds by means of dc and ac magnetic susceptibilities,
electrical resistivity and specific heat measurements. Eu0.5K0.5Fe2As2 is known
to exhibit superconductivity with superconducting transition temperature Tc as
high as 33 K. The Ni-doping leads to a rapid decrease in Tc; Tc is reduced to
23 K with 3% Ni-doping, and 8% Ni-doping suppresses the superconductivity to
below 1.8 K. In 3% Ni-doped sample Eu0.5K0.5(Fe0.97Ni0.03)2As2
superconductivity coexists with short range ordering of Eu2+ magnetic moments
at Tm ~ 6 K. The suppression of superconductivity with Ni-doping is accompanied
with the emergence of a long range antiferromagnetic ordering with TN = 8.5 K
and 7 K for Eu0.5K0.5(Fe0.92Ni0.08)2As2 and Eu0.5K0.5(Fe0.88Ni0.12)2As2,
respectively. The temperature and field dependent magnetic measurements for x =
0.08 and 0.12 samples reflect the possibility of a helical magnetic ordering of
Eu2 moments. We suspect that the helimagnetism of Eu spins could be responsible
for the destruction of superconductivity as has been observed in Co-doped
EuFe2As2. The most striking feature seen in the resistivity data for x = 0.08
is the reappearance of the anomaly presumably due to spin density wave
transition at around 60 K. This could be attributed to the compensation of
holes (K-doping at Eu-site) by the electrons (Ni-doping at Fe site). The
anomaly associated with spin density wave further shifts to 200 K for x = 0.12
for which the electron doping has almost compensated the holes in the system.Comment: 9 pages, 10 figure
- …