-
[show abstract]
[hide abstract]
ABSTRACT: Copper-divanadate Cu 2Ày Zn y V 2 O 7 comprises trans-edge sharing chains of CuO 5 polyhedra where Cu spins S ¼ 1 2 are antiferromagnetically coupled. Zn substitution for Cu induces a structural phase transition which affects strongly the magnetic properties. In particular, a substantial magnetic anisotropy found in one of the structural phases is strongly suppressed in the other one, owing to the changes of the local bonding geometry.
04/2013; 272276:933-934.
-
[show abstract]
[hide abstract]
ABSTRACT: The spin valve effect for the superconducting current based on the
superconductor/ferromagnet proximity effect has been studied for a
CoO_x/Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect $\Delta T_c$ =
T_c^{AP} - T_c^{P}, where T_c^{P} and T_c^{AP} are the superconducting
transition temperatures for the parallel (P) and antiparallel (AP) orientation
of magnetizations, respectively, has been measured for different thicknesses of
the Fe1 layer d_{Fe1}. The obtained dependence of the effect on d_{Fe1} reveals
that $\Delta T_c$ can be increased in comparison with the case of a
half-infinite Fe1 layer considered by the previous theory. A maximum of the
spin valve effect occurs at d_{Fe1} ~ d_{Fe2}. At the optimal value of d_{Fe1},
almost full switching from the normal to the superconducting state when
changing the mutual orientation of magnetizations of the iron layers Fe1 and
Fe2 from P to AP is demonstrated.
04/2013;
-
-
Yu. Talanov,
N Beisengulov,
G Kornilov,
T Shaposhnikova,
E Vavilova,
C Nacke,
S Wurmehl,
N Panarina,
C Hess, V Kataev,
B Buechner
Supercond. Sci. Technol. 01/2013; 26:045015.
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the measurement of the magnetic susceptibility and of ESR
transitions in the garnet substance Tb$_3$Ga$_5$O$_{12}$ (TGG). The results are
compared with a calculation in the framework of crystal field theory for the
orthorhombic surroundings of the six inequivalent Tb ions of TGG. We also
present a calculation of the magnetization for the three main crystal
directions.
10/2012;
-
Yu. Talanov,
N. Beisengulov,
G. Kornilov,
T. Shaposhnikova,
E. Vavilova,
C. Nacke,
S. Wurmehl,
N. Panarina,
C. Hess, V. Kataev,
B. Büchner
[show abstract]
[hide abstract]
ABSTRACT: Magnetic field dependent modulated microwave absorption (MMWA) measurements
have been carried out to investigate vortex pinning effects in single crystals
of the iron-based high-Tc superconductor Ba(Fe_{1-x}Co_x)_2As_2 with three
different cobalt doping levels of x = 0.07, 0.09, and 0.11. The dependence of
the MMWA hysteresis loops on temperature, magnetic field, and Co-concentration
have been measured and analyzed using a theoretical model of microwave
absorption in superconductors. The analysis reveals that in an underdoped
crystal (x = 0.07) the so called \dtc-pinning due to magnetically ordered
regions defines the temperature dependence of the critical current density,
while in the optimally doped (x = 0.09) and overdoped (x = 0.11) samples the
pinning is governed by structural imperfections due to inhomogeneous
distribution of cobalt dopant and has the so called \delta-l character.
10/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: We have studied the dependence of the superconducting (SC) transition
temperature on the mutual orientation of magnetizations of Fe1 and Fe2 layers
in the spin valve system CoO_x/Fe1/Cu/Fe2/Pb. We find that this dependence is
nonmonotonic when passing from the parallel to the antiparallel case and
reveals a distinct minimum near the orthogonal configuration. The analysis of
the data in the framework of the SC triplet spin valve theory gives direct
evidence for the long-range triplet superconductivity arising due to
noncollinearity of the two magnetizations.
07/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: In this work we present the results of the bulk magnetization measurements in
a superconducting state of single crystals of
Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$. The isothermal magnetic field ($H||c$ axis)
dependent magnetization ($M$) loops exhibit a second peak (SP) or `fishtail
effect', as well as remarkable flux jumps at low temperatures. The critical
current density $J_c$ obtained from the $M(H)$ loops is rather high, of the
order of 10$^6$ A/cm$^2$. The analysis of the temperature and field dependent
$J_c$ implies that high $J_c$ is mainly due to collective (weak) pinning of
vortices by dense microscopic point defects with some contribution from a
strong pinning mechanism. Pronounced magnetic instabilities in terms of flux
jumps depend strongly on temperature as well on the field sweep rate. The field
for first flux jump as calculated from an adiabatic model, however, is much
lower than the experimentally observed values, and this enhanced stability is
attributed to a flux creep phenomenon. The analysis of field dependent magnetic
relaxation data additionally supports a collective pinning model. The data
further suggests that SP in M(H) is likely related to crossover in creep
dynamics from elastic to plastic mechanism. We have constructed the vortex
phase diagram on field-temperature plane.
05/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: In the present work we have studied polycrystalline samples of the GdO1−x
F
x
FeAs superconductor by means of high field electron spin resonance (HF-ESR) spectroscopy. Aset of the samples with different
levels of fluorine doping was measured in the frequency range from 10GHz to 400GHz in magnetic fields up to 15T. Surprising
results have been obtained on the GdO0.85F0.15FeAs sample with the superconducting transition temperature T
c
=20K. Gd ESR gives clear indications of the enhancement of (quasi)-static magnetic correlations which set in below ∼80K
and continue to develop even in the superconducting state. This suggests an occurrence of an intimate interplay between magnetism
and superconductivity in the FeAs planes which evolves in GdO1−x
F
x
FeAs compound upon the fluorine doping.
KeywordsGdOFeAs-Pnictides-Unconventional superconductivity
Journal of Low Temperature Physics 04/2012; 159(1):172-175. · 1.19 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: An experimental study of magnetic and superconducting properties of a trilayer Ni/V/Ni thin film system grown on single-crystalline
MgO(001) substrate is reported. The field dependence of the superconducting transition temperature T
c
for samples comprising Ni layers with similar values of the coercive field H
c
reveals no anomalies. However, in samples with different thicknesses of the nickel layers the difference in H
c
amounts up to ΔH
c
∼ 1.8 kOe, thus enabling to manipulate the relative orientations of the layers’ magnetization by an external magnetic field.
Surprisingly, for these samples the T
c
for the parallel orientation of the magnetizations of the Ni layers is higher, in a certain magnetic field range, than for
the antiparallel one, at odds with theoretical predictions. Possible reasons of this contradiction are discussed.
JETP Letters 04/2012; 90(1):59-63. · 1.35 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on structural, magnetic, and superconducting properties of the spin-valve multilayer system CoO x /Fe1/Cu/Fe2/In. For most of the thicknesses of the second iron layer d Fe2 up to 2 nm, we have observed a full spin-valve effect for the superconducting current, i.e., a complete transition from the normal to the superconducting state by changing the mutual orientation of the magnetizations of the Fe1 and Fe2 layers. For d Fe2 < 1 nm, the superconducting transition temperature T P c for the parallel orientation of magnetizations of the Fe1 and Fe2 layers is smaller than that for the antiparallel orientation T AP c , which corresponds to the direct spin-valve effect. For d Fe2 1 nm, we have found the inverse spin-valve effect with T c = T AP c − T P c < 0. Further, in samples with a fixed thickness of the In layer, we have observed an oscillating dependence of its superconducting transition temperature T c on d Fe2 . The analysis of the T c (d Fe2) dependence using the theory of the superconducting-ferromagnetic proximity effect has enabled determination of all microscopic parameters of the studied system. With these parameters, a satisfactory description of the sign-changing oscillating behavior of the spin-valve effect T c (d Fe2) has been obtained using a recent theory by Fominov et al.
Physical Review B 01/2012; 91(91-85):24502-308. · 3.69 Impact Factor
-
A. U. B. Wolter,
F. Lipps,
M. Schäpers,
S. -L. Drechsler,
S. Nishimoto,
R. Vogel, V. Kataev,
B. Büchner,
H. Rosner,
M Schmitt,
M. Uhlarz,
Y. Skourski,
J. Wosnitza,
S. Süllow,
K. C. Rule
[show abstract]
[hide abstract]
ABSTRACT: We present a detailed study in the paramagnetic regime of the frustrated $s$
= 1/2 spin-compound linarite, PbCuSO$_4$(OH)$_2$, with competing ferromagnetic
nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange
interactions. Our data reveal highly anisotropic values for the saturation
field along the crystallographic main directions, with $\sim$ 7.6, $\sim$ 10.5
and $\sim$ 8.5\,T for the $a$, $b$, and $c$ axes, respectively. In the
paramagnetic regime, this behavior is explained mainly by the anisotropy of the
\textit{g}-factor but leaving room for an easy-axis exchange anisotropy. Within
the isotropic $J_1$-$J_2$ spin model our experimental data are described by
various theoretical approaches yielding values for the exchange interactions
$J_1$ $\sim$ -100\,K and $J_2$ $\sim$ 36\,K. These main intrachain exchange
integrals are significantly larger as compared to the values derived in two
previous studies in the literature and shift the frustration ratio $\alpha =
J_2/|J_1|$ $\approx$ 0.36 of linarite closer to the 1D critical point at 0.25.
Electron spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements
further prove that the static susceptibility is dominated by the intrinsic spin
susceptibility. The Knight shift as well as the broadening of the linewidth in
ESR and NMR at elevated temperatures indicate a highly frustrated system with
the onset of magnetic correlations far above the magnetic ordering temperature
$T_\mathrm{N}$ = 2.75(5)\,K, in agreement with the calculated exchange
constants.
10/2011;
-
Y. C. Arango,
E. Vavilova,
M. Abdel-Hafiez,
O. Janson,
A. A. Tsirlin,
H. Rosner,
S.-L. Drechsler,
M. Weil,
G. Nénert,
R. Klingeler,
O. Volkova,
A. Vasiliev, V. Kataev,
B. Büchner
[show abstract]
[hide abstract]
ABSTRACT: In this work, we study the interplay between the crystal structure and magnetism of the pyroarsenate α-Cu2As2O7 by means of magnetization, heat capacity, electron spin resonance, and nuclear magnetic resonance measurements as well as density functional theory (DFT) calculations and quantum Monte Carlo (QMC) simulations. The data reveal that the magnetic Cu-O chains in the crystal structure represent a realization of a quasi-one-dimensional (1D) coupled alternating spin-1/2 Heisenberg chain model with relevant pathways through nonmagnetic AsO4 tetrahedra. Owing to residual 3D interactions, antiferromagnetic long range ordering at TN≃10 K takes place. Application of the external magnetic field B along the magnetically easy axis induces the transition to a spin-flop phase at BSF∼1.7 T (2 K). The experimental data suggest that substantial quantum spin fluctuations take place at low magnetic fields in the ordered state. DFT calculations confirm the quasi-one-dimensional nature of the spin lattice, with the leading coupling J1 within the structural dimers. QMC fits to the magnetic susceptibility evaluate J1=164 K, the weaker intrachain coupling J1′/J1=0.55, and the effective interchain coupling Jic1/J1=0.20.
Phys. Rev. B. 10/2011; 84(13).
-
Y. C. Arango,
E. Vavilova,
M. Abdel-Hafiez,
O. Janson,
A. A. Tsirlin,
H. Rosner,
S. -L. Drechsler,
M. Weil,
G. Nénert,
R. Klingeler,
O. Volkova,
A. Vasiliev, V. Kataev,
B. Büchner
[show abstract]
[hide abstract]
ABSTRACT: In this work we study the interplay between the crystal structure and
magnetism of the pyroarsenate \alpha-Cu_2As_2O_7 by means of magnetization,
heat capacity, electron spin resonance and nuclear magnetic resonance
measurements as well as density functional theory (DFT) calculations and
quantum Monte Carlo (QMC) simulations. The data reveal that the magnetic Cu-O
chains in the crystal structure represent a realization of a quasi-one
dimensional (1D) coupled alternating spin-1/2 Heisenberg chain model with
relevant pathways through non-magnetic AsO_4 tetrahedra. Owing to residual 3D
interactions antiferromagnetic long range ordering at T_N\simeq10K takes place.
Application of external magnetic field B along the magnetically easy axis
induces the transition to a spin-flop phase at B_{SF}~1.7T (2K). The
experimental data suggest that substantial quantum spin fluctuations take place
at low magnetic fields in the ordered state. DFT calculations confirm the
quasi-one-dimensional nature of the spin lattice, with the leading coupling J_1
within the structural dimers. QMC fits to the magnetic susceptibility evaluate
J_1=164K, the weaker intrachain coupling J'_1/J_1 = 0.55, and the effective
interchain coupling J_{ic1}/J_1 = 0.20.
10/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We report a comparative study of (63)Cu nuclear magnetic resonance spin lattice relaxation rates T(1)(-1) on undoped SrCuO(2) and Ca-doped Sr(0.9)Ca(0.1)CuO(2) spin chain compounds. A temperature independent T(1)(-1) is observed for SrCuO(2) as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T(1)(-1) for T<90 K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J(1)-J(2) Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J(2)-exchange coupling as a possible cause of the spin gap.
Physical Review Letters 07/2011; 107(1):017203. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We investigate the electronic properties and the superconducting gap
characteristics of a single crystal of hole-doped 122 Fe-pnictide
Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ by means of specific heat measurements. The
specific heat exhibits a pronounced anomaly around the superconducting
transition temperature $T_c$ = 29.4 K, and a small residual part at low
temperature. In a magnetic field of 90 kOe, the transition is broadened and
$T_c$ is lowered insignificantly by an amount $\sim$ 1.5 K. We estimate a high
electronic coefficient in the normal state with a value 57.5 mJ mol$^{-1}$
K$^2$, being consistent with hole-doped 122 compounds. The
temperature-dependent superconducting electronic specific heat cannot be
described with single-gap BCS theory under weak coupling approach. Instead, our
analysis implies a presence of two s-wave like gaps with magnitudes
$\Delta_1(0)/k_BT_c$ = 1.06 and $\Delta_2(0)/k_BT_c$ = 2.08 with their
respective weights of 48% and 52%. While our results have qualitative
similarities with other hole-doped 122 materials, the gap's magnitude and their
ratio are quite different.
06/2011;
-
A. Alfonsov,
F. Murányi, V. Kataev,
G. Lang,
N. Leps,
L. Wang,
R. Klingeler,
A. Kondrat,
C. Hess,
S. Wurmehl,
A. Köhler,
G. Behr,
S. Hampel,
M. Deutschmann,
S. Katrych,
N. D. Zhigadlo,
Z. Bukowski,
J. Karpinski,
B. Büchner
[show abstract]
[hide abstract]
ABSTRACT: We report a detailed investigation of GdO1-xFxFeAs (x=0, 0.07, and 0.14) samples by means of high-field and high-frequency electron spin resonance (HF-ESR) together with measurements of thermodynamic and transport properties. The parent GdOFeAs compound exhibits Fe long-range magnetic order below 128 K, whereas both doped samples do not show such order and are superconducting with Tc=20 K (x=0.07) and Tc=45 K (x=0.14). The Gd3+ HF-ESR reveals an appreciable exchange coupling between Gd and Fe moments, through which the static magnetic order is clearly seen in the parent compound. Owing to this coupling, HF-ESR can probe sensitively the evolution of the magnetism in the FeAs planes upon F doping. It is found that in both superconducting samples, where the Fe long-range order is absent, there are short-range, static on the ESR time scale magnetic correlations between Fe spins. Their occurrence on a large doping scale may be indicative of the ground states’ coexistence.
Phys. Rev. B. 03/2011; 83(9).
-
[show abstract]
[hide abstract]
ABSTRACT: We report a comparative study of 63Cu Nuclear Magnetic Resonance spin lattice
relaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca doped
Sr_{0.9}Ca_{0.1}CuO_2 spin chain compounds. A temperature independent T_1^{-1}
is observed for SrCuO_2 as expected for an S=1/2 Heisenberg chain.
Surprisingly, we observe an exponential decrease of T_1^{-1} for T < 90,K in
the Ca-doped sample evidencing the opening of a spin gap. The data analysis
within the J_1-J_2 Heisenberg model employing density-matrix renormalization
group calculations suggests an impurity driven small alternation of the
J_2-exchange coupling as a possible cause of the spin gap.
03/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Superconductor-ferromagnet (S/F) spin valve effect theories based on the S/F proximity phenomenon assume that the superconducting transition temperature Tc of F1/F2/S or F1/S/F2 trilayers for parallel magnetizations of the F1 and F2 layers (T(c)(P)) are smaller than for the antiparallel orientations (T(c)(AP)). Here, we report for CoOx/Fe1/Cu/Fe2/In multilayers with varying Fe2-layer thickness the sign-changing oscillating behavior of the spin valve effect ΔT(c) = T(c)(AP) - T(c)(P). We observe the full direct effect with T(c)(AP) > T(c)(P) for Fe2-layer thickness d(Fe2) < 1 nm and the full inverse (T(c)(AP) < T(c((P)) effect for d(Fe2) ≥ 1 nm. Interference of Cooper pair wave functions reflected from both surfaces of the Fe2 layer appear as the most probable reason for the observed behavior of ΔT(c).
Physical Review Letters 02/2011; 106(6):067005. · 7.37 Impact Factor
-
A. Alfonsov,
F. Murányi, V. Kataev,
G. Lang,
N. Leps,
L. Wang,
R. Klingeler,
A. Kondrat,
C. Hess,
S. Wurmehl,
A. Köhler,
G. Behr,
S Hampel,
M. Deutschmann,
S. Katrych,
N. D. Zhigadlo,
Z. Bukowski,
J. Karpinski,
B. Büchner
[show abstract]
[hide abstract]
ABSTRACT: We report a detailed investigation of GdO$_{1-x}$F$_{x}$FeAs (x = 0, 0.07 and
0.14) samples by means of high-field/high-frequency electron spin resonance
(HF-ESR) together with measurements of thermodynamic and transport properties.
The parent GdOFeAs compound exhibits Fe long-range magnetic order below 128 K,
whereas both doped samples do not show such order and are superconducting with
T$_c$ = 20 K (x = 0.07) and T$_c$ = 45 K (x = 0.14). The Gd$^{3+}$ HF-ESR
reveals an appreciable exchange coupling between Gd and Fe moments, through
which the static magnetic order is clearly seen in the parent compound. Owing
to this coupling, HF-ESR can probe sensitively the evolution of the magnetism
in the FeAs planes upon F doping. It is found that in both superconducting
samples, where the Fe long-range order is absent, there are short-range, static
on the ESR time scale magnetic correlations between Fe spins. Their occurrence
on a large doping scale may be indicative of the ground states' coexistence.
10/2010;
