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K Zhao,
Z Deng,
X C Wang,
W Han,
J L Zhu,
X Li,
Q Q Liu,
R C Yu,
T Goko,
B Frandsen, [......],
Fanlong Ning, Y J Uemura,
H Dabkowska,
G M Luke,
H Luetkens,
E Morenzoni,
S R Dunsiger,
A Senyshyn,
P Böni,
C Q Jin
[show abstract]
[hide abstract]
ABSTRACT: Diluted magnetic semiconductors have received much attention due to their potential applications for spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since the 1990s. The simultaneous spin and charge doping via hetero-valent (Ga(3+),Mn(2+)) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Here we report the synthesis of a new diluted magnetic semiconductor (Ba(1-x)K(x))(Zn(1-y)Mn(y))(2)As(2), which is isostructural to the 122 iron-based superconductors with the tetragonal ThCr(2)Si(2) (122) structure. Holes are doped via (Ba(2+), K(1+)) replacements, while spins via isovalent (Zn(2+),Mn(2+)) substitutions. Bulk samples with x=0.1-0.3 and y=0.05-0.15 exhibit ferromagnetic order with T(C) up to 180 K, which is comparable to the highest T(C) for (Ga,Mn)As and significantly enhanced from T(C) up to 50 K of the '111'-based Li(Zn,Mn)As. Moreover, ferromagnetic (Ba,K)(Zn,Mn)(2)As(2) shares the same 122 crystal structure with semiconducting BaZn(2)As(2), antiferromagnetic BaMn(2)As(2) and superconducting (Ba,K)Fe(2)As(2), which makes them promising for the development of multilayer functional devices.
Nature Communications 02/2013; 4:1442. · 7.40 Impact Factor
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S. R. Dunsiger,
A. A. Aczel,
C. Arguello,
H. Dabkowska,
A. Dabkowski,
M. -H. Du,
T. Goko,
B. Javanparast,
T. Lin,
F. L. Ning,
H. M. L. Noad,
D. J. Singh,
T. J. Williams, Y. J. Uemura,
M. J. P. Gingras,
G. M. Luke
[show abstract]
[hide abstract]
ABSTRACT: We respond to the comment of Bramwell et al (arXiv:1111.4168v1) to our
original publication (S. R. Dunsiger et al, Phys. Rev. Lett. 107, 207207
(2011)), detailing muon spin rotation measurements of the Spin Ice compound
Dy2Ti2O7.
05/2012;
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J P Carlo,
T Goko,
I M Gat-Malureanu,
P L Russo,
A T Savici,
A A Aczel,
G J MacDougall,
J A Rodriguez,
T J Williams,
G M Luke,
C R Wiebe,
Y Yoshida,
S Nakatsuji,
Y Maeno,
T Taniguchi, Y J Uemura
[show abstract]
[hide abstract]
ABSTRACT: High-T(c) cuprates, iron pnictides, organic BEDT and TMTSF, alkali-doped C(60), and heavy-fermion systems have superconducting states adjacent to competing states exhibiting static antiferromagnetic or spin density wave order. This feature has promoted pictures for their superconducting pairing mediated by spin fluctuations. Sr(2)RuO(4) is another unconventional superconductor which almost certainly has a p-wave pairing. The absence of known signatures of static magnetism in the Sr-rich side of the (Ca, Sr) substitution space, however, has led to a prevailing view that the superconducting state in Sr(2)RuO(4) emerges from a surrounding Fermi-liquid metallic state. Using muon spin relaxation and magnetic susceptibility measurements, we demonstrate here that (Sr,Ca)(2)RuO(4) has a ground state with static magnetic order over nearly the entire range of (Ca, Sr) substitution, with spin-glass behaviour in Sr(1.5)Ca(0.5)RuO(4) and Ca(1.5)Sr(0.5)RuO(4). The resulting new magnetic phase diagram establishes the proximity of superconductivity in Sr(2)RuO(4) to competing static magnetic order.
Nature Material 02/2012; 11(4):323-8. · 32.84 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We have performed detailed $^{57}$Fe M\"ossbauer spectroscopy measurements on
Ba$_{0.78}$K$_{0.22}$Fe$_2$As$_2$ and BaFe$_{2-x}$Ni$_x$As$_2$ single crystal
mosaics showing antiferromagnetic ordering below $T_N$ with superconductivity
below $T_C$. Analysis of the M\"ossbauer spectra shows a decrease in the
magnetic hyperfine (hf) field but no change in the magnetic volume fraction
below $T_C$. This clearly indicates the coexistence of magnetism and
superconductivity in these compounds. The decrease in the magnetic hf field
below $T_C$ depends on the difference between $T_N$ and $T_C$, being the
largest for $T_N$ close to $T_C$. Two different explanations for this
observation are given. We also find that the non-magnetic volume fraction below
$T_N$ correlates with the Ni doping $x$, being large for high $T_C$ and small
for high $T_N$.
11/2011;
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S R Dunsiger,
A A Aczel,
C Arguello,
H Dabkowska,
A Dabkowski,
M-H Du,
T Goko,
B Javanparast,
T Lin,
F L Ning,
H M L Noad,
D J Singh,
T J Williams, Y J Uemura,
M J P Gingras,
G M Luke
[show abstract]
[hide abstract]
ABSTRACT: Theory predicts the low temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (μSR) experiment [S. T. Bramwell et al., Nature (London) 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation-the so-called second Wien effect. We demonstrate via a new series of μSR experiments in Dy(2)Ti(2)O(7) that such an effect is not observable in a TF μSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.
Physical Review Letters 11/2011; 107(20):207207. · 7.37 Impact Factor
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J. Munevar,
H. Micklitz,
M. Alzamora,
C. Arguello,
T. Goko,
F. L. Ning,
A. A. Aczel,
T. Munsie,
T. J. Williams,
G. F. Chen,
W Yu,
G. M. Luke, Y. J. Uemura,
E. Baggio-Saitovitch
[show abstract]
[hide abstract]
ABSTRACT: We have studied the magnetism in superconducting single crystals of EuFe2
As1.4 P0.6 by using the local probe techniques of zero-field muon spin
rotation/relaxation and 151 Eu/57 Fe M\"ossbauer spec- troscopy. All of these
measurements reveal magnetic hyperfine fields below the magnetic ordering
temperature TM = 18 K of the Eu2+ moments. The analysis of the data shows that
there is a coexistence of ferromagnetism, resulting from Eu2+ moments ordered
along the crystallographic c-axis, and superconductivity below TSC \approx 15
K. We find indications for a change in the dynamics of the small Fe magnetic
moments (\sim 0.07 \mu B) at the onset of superconductivity: below TSC the Fe
magnetic moments seem to be "frozen" within the ab-plane.
05/2011;
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Z Deng,
C Q Jin,
Q Q Liu,
X C Wang,
J L Zhu,
S M Feng,
L C Chen,
R C Yu,
C Arguello,
T Goko, [......],
T J Williams,
G M Luke,
T Kakeshita,
S Uchida,
W Higemoto,
T U Ito,
Bo Gu,
S Maekawa,
G D Morris, Y J Uemura
[show abstract]
[hide abstract]
ABSTRACT: In a prototypical ferromagnet (Ga,Mn)As based on a III-V semiconductor, substitution of divalent Mn atoms into trivalent Ga sites leads to severely limited chemical solubility and metastable specimens available only as thin films. The doping of hole carriers via (Ga,Mn) substitution also prohibits electron doping. To overcome these difficulties, Masek et al. theoretically proposed systems based on a I-II-V semiconductor LiZnAs, where isovalent (Zn,Mn) substitution is decoupled from carrier doping with excess/deficient Li concentrations. Here we show successful synthesis of Li(1+y)(Zn(1-x)Mn(x))As in bulk materials. Ferromagnetism with a critical temperature of up to 50 K is observed in nominally Li-excess (y=0.05-0.2) compounds with Mn concentrations of x=0.02-0.15, which have p-type metallic carriers. This is presumably due to excess Li in substitutional Zn sites. Semiconducting LiZnAs, ferromagnetic Li(Zn,Mn)As, antiferromagnetic LiMnAs, and superconducting LiFeAs systems share square lattice As layers, which may enable development of novel junction devices in the future.
Nature Communications 01/2011; 2:422. · 7.40 Impact Factor
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J. Munevar,
D. R. Sanchez,
M. Alzamora,
E. Baggio-Saitovitch,
J. P. Carlo,
T. Goko,
A. A. Aczel,
T. J. Williams,
G. M. Luke,
Hai-Hu Wen,
Xiyu Zhu,
Fei Han, Y. J. Uemura
[show abstract]
[hide abstract]
ABSTRACT: Static magnetic order of quasi two-dimensional FeAs compounds Sr4A2O6-xFe2As2, with A = Sc and V, has been detected by 57Fe Moessbauer and muon spin relaxation ({\mu}SR) spectroscopies. The non-superconducting stoichiometric (x = 0) A = Sc system exhibits a static internal/hyperfine magnetic field both at the 57Fe and {\mu}+ sites, indicating antiferromagnetic order of Fe moments below TN = 35 K with ~ 0.1 Bohr magneton per Fe at T = 2 K. The superconducting and oxygen deficient (x = 0.4) A = V system exhibits a static internal field only at the {\mu}+ site below TN ~ 40 K, indicating static magnetic order of V moments co-existing with superconductivity without freezing of Fe moments. These results suggest that the 42622 FeAs systems belong to the same paradigm with the 1111 and 122 FeAs systems with respect to magnetic behavior of Fe moments. Comment: 4 pages 4 figures: for information, contact tomo@lorentz.phys.columbia.edu
11/2010;
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[show abstract]
[hide abstract]
ABSTRACT: LiHo(x)Y(1-x)F4 is an insulator where the magnetic Ho3+ ions have an Ising character and interact mainly through magnetic dipolar fields. We used the muon spin relaxation technique to study the nature of its ground state for samples with x ≤ 0.25. In contrast with some previous works, we did not find canonical spin glass behavior down to ≈ 15 mK. Instead, below ≈300 mK we observed temperature-independent dynamic magnetism characterized by a single correlation time. The 300 mK energy scale corresponds to the Ho3+ hyperfine interaction strength, suggesting that this interaction may be involved in the dynamic behavior of the system.
Physical Review Letters 09/2010; 105(10):107203. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: V2O3 nanocrystals can be synthesized through hydrothermal reduction of
VO(OH)2 using hydrazine as a reducing agent. Addition of different ligands to
the reaction produces nanoparticles, nanorods and nanoplatelets of different
sizes. Small nanoparticles synthesized in this manner show suppression of the
magnetic phase transition to lower temperatures. Using muon spin relaxation
spectroscopy and synchrotron x-ray diffraction, it is determined that the
volume fraction of the high-temperature phase, characterized by a rhombohedral
structure and paramagnetism, gradually declines with decreasing temperature, in
contrast to the sharp transition observed in bulk V2O3.
07/2010;
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T. J. Williams,
A. A. Aczel,
E. Baggio-Saitovitch,
S. L. Bud'ko,
P. C. Canfield,
J. P. Carlo,
T. Goko,
H Kageyama,
A. Kitado,
J. Munevar,
N. Ni,
S. R. Saha,
K. Kirschenbaum,
J. Paglione,
D. R. Sanchez-Candela, Y. J. Uemura,
G. M. Luke
[show abstract]
[hide abstract]
ABSTRACT: We report muon spin rotation ($\mu$SR) measurements of single crystal Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. From measurements of the magnetic field penetration depth $\lambda$ we find that for optimally- and over-doped samples, $1/\lambda(T\to 0)^2$ varies monotonically with the superconducting transition temperature T$_{\rm C}$. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied. Comment: 7 pages, accepted for publication in Phys. Rev. B
05/2010;
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S R Dunsiger,
J P Carlo,
T Goko,
G Nieuwenhuys,
T Prokscha,
A Suter,
E Morenzoni,
D Chiba,
Y Nishitani,
T Tanikawa,
F Matsukura,
H Ohno,
J Ohe,
S Maekawa, Y J Uemura
[show abstract]
[hide abstract]
ABSTRACT: Mn-doped GaAs is a ferromagnetic semiconductor, widely studied because of its possible application for spin-sensitive 'spintronics' devices. The material also attracts great interest in fundamental research regarding its evolution from a paramagnetic insulator to a ferromagnetic metal. The high sensitivity of its physical properties to preparation conditions and heat treatments and the strong doping and temperature dependencies of the magnetic anisotropy have generated a view in the research community that ferromagnetism in (Ga, Mn)As may be associated with unavoidable and intrinsic strong spatial inhomogeneity. Muon spin relaxation (muSR) probes magnetism, yielding unique information about the volume fraction of regions having static magnetic order, as well as the size and distribution of the ordered moments. By combining low-energy muSR, conductivity and a.c. and d.c. magnetization results obtained on high-quality thin-film specimens, we demonstrate here that (Ga, Mn)As shows a sharp onset of ferromagnetic order, developing homogeneously in the full volume fraction, in both insulating and metallic films. Smooth evolution of the ordered moment size across the insulator-metal phase boundary indicates strong ferromagnetic coupling between Mn moments that exists before the emergence of fully itinerant hole carriers.
Nature Material 03/2010; 9(4):299-303. · 32.84 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We have searched for time-reversal symmetry breaking fields in the non-centrosymmetric superconductor Mg$_{10}$Ir$_{19}$B$_{16}$ via muon spin relaxation in zero applied field, and we measured the temperature dependence of the superfluid density by muon spin rotation in transverse field to investigate the superconducting pairing symmetry in two polycrystalline samples of signficantly different purities. In the high purity sample, we detected no time-reversal symmetry breaking fields greater than 0.05 G. The superfluid density was also found to be exponentially-flat as T$\to $0, and so can be fit to a single-gap BCS model. In contrast, the lower purity sample showed an increase in the zero-field $\mu$SR relaxation rate below T$_c$ corresponding to a characteristic field strength of 0.6 G. While the temperature-dependence of the superfluid density was also found to be consistent with a single-gap BCS model, the magnitude as T$\to $0 was found to be much lower for a given applied field than in the case of the high purity sample. These findings suggest that the dominant pairing symmetry in high quality Mg$_{10}$Ir$_{19}$B$_{16}$ samples corresponds to the spin-singlet channel, while sample quality drastically affects the superconducting properties of this system. Comment: 6 pages, 5 figures, revised version resubmitted to PRB
01/2010;
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G. J. MacDougall,
A. T. Savici,
A. A. Aczel,
R. J. Birgeneau,
H. Kim,
S.-J. Kim,
T. Ito,
J. A. Rodriguez,
P. L. Russo, Y. J. Uemura,
S. Wakimoto,
C. R. Wiebe,
G. M. Luke
[show abstract]
[hide abstract]
ABSTRACT: Transverse-field muon spin rotation measurements of overdoped La2−xSrxCuO4 reveal a large broadening of the local magnetic field distribution in response to applied field, persisting to high temperatures. The field response is approximately Curie-Weiss-like in temperature and is largest for the highest doping investigated. Such behavior is contrary to the canonical Fermi-liquid picture commonly associated with the overdoped cuprates and implies extensive heterogeneity in this region of the phase diagram. A possible explanation for the result lies in regions of staggered magnetization about dopant cations, analogous to what is argued to exist in underdoped systems.
Phys. Rev. B. 01/2010; 81(1).
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[show abstract]
[hide abstract]
ABSTRACT: $LiHo_xY_{1-x}F_4$ is an insulating system where the magnetic Ho$^{3+}$ ions have an Ising character, and interact mainly through magnetic dipolar fields. We used the muon spin relaxation technique to study the nature of the ground state for samples with x=0.25, 0.12, 0.08, 0.045 and 0.018. In contrast with some previous works, we have not found any signature of canonical spin glass behavior down to $\approx$15mK. Instead, below $\approx$300mK we observed dynamic magnetism characterized by a single correlation time with a temperature independent fluctuation rate. We observed that this low temperature fluctuation rate increases with x up to 0.08, above which it levels off. The 300mK energy scale corresponds to the Ho3+ hyperfine interaction strength, suggesting that the hyperfine interaction may be intimately involved with the spin dynamics in this system.
10/2009;
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T. J. Williams,
A. A. Aczel,
E. Baggio-Saitovitch,
S. L. Budko,
P. C. Canfield,
J. P. Carlo,
T. Goko,
J. Munevar,
N. Ni, Y. J. Uemura,
W. Yu,
G. M. Luke
[show abstract]
[hide abstract]
ABSTRACT: We have performed transverse field muon spin rotation measurements of single
crystals of Ba(Fe$_{0.93}$Co$_{0.07})_2$As$_2$ with the applied magnetic field
along the $\hat{c}$ direction. Fourier transforms of the measured spectra
reveal an anisotropic lineshape characteristic of an Abrikosov vortex lattice.
We have fit the $\mu$SRSR spectra to a microscopic model in terms of the
penetration depth $\lambda$ and the Ginzburg-Landau parameter $\kappa$.
We find that as a function of temperature, the penetration depth varies more
rapidly than in standard weak coupled BCS theory. For this reason we first fit
the temperature dependence to a power law where the power varies from 1.6 to
2.2 as the field changes from 200G to 1000G. Due to the surprisingly strong
field dependence of the power and the superfluid density we proceeded to fit
the temperature dependence to a two gap model, where the size of the two gaps
is field independent. From this model, we obtained gaps of
$2\Delta_1=3.7k_BT_c$ and $2\Delta_2=1.6k_BT_c$, corresponding to roughly 6 meV
and 3 meV respectively.
Physical review. B, Condensed matter 05/2009; 80(9).
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J P Carlo, Y J Uemura,
T Goko,
G J Macdougall,
J A Rodriguez,
W Yu,
G M Luke,
Pengcheng Dai,
N Shannon,
S Miyasaka,
S Suzuki,
S Tajima,
G F Chen,
W Z Hu,
J L Luo,
N L Wang
[show abstract]
[hide abstract]
ABSTRACT: Muon spin relaxation measurements in iron-oxypnictide systems have revealed: (1) commensurate long-range order in undoped LaFeAsO; (2) a Bessel function line shape in LaFeAs(O0.97F0.03) which indicates possible incommensurate or stripe magnetism; (3) anomalous weak magnetism existing in superconducting LaFePO, CeFeAs(O0.084F0.16), and NdFeAs(O0.88F0.12) but absent in superconducting LaFeAs(O0.92F0.08); and (4) scaling of the superfluid density with T_{c} in the Ce-, La-, and Nd-FeAs superconductors following a nearly linear relationship found in cuprates.
Physical Review Letters 03/2009; 102(8):087001. · 7.37 Impact Factor
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A. A. Aczel,
E. Baggio-Saitovitch,
S. L. Budko,
P. C. Canfield,
J. P. Carlo,
G. F. Chen,
Pengcheng Dai,
T. Goko,
W. Z. Hu,
G. M. Luke,
J. L. Luo,
N. Ni,
D. R. Sanchez-Candela,
F. F. Tafti,
N. L. Wang,
T. J. Williams,
W. Yu, Y. J. Uemura
[show abstract]
[hide abstract]
ABSTRACT: Zero-field (ZF) muon-spin-relaxation (μSR) measurements have revealed static commensurate magnetic order of Fe moments in NdFeAsO below TN∼135 K, with the ordered moment size nearly equal to that in LaFeAsO, and confirmed similar behavior in BaFe2As2. In single crystals of superconducting Ba0.55K0.45Fe2As2, μSR spectra indicate static magnetism with incommensurate or short-ranged spin structure in ∼70% of volume below TN∼80 K, coexisting with the remaining volume which exhibits a superfluid response below Tc∼30 K.
Phys. Rev. B. 12/2008; 78(21).
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T. Goko,
A. A. Aczel,
E. Baggio-Saitovitch,
S. L. Bud'ko,
P. C. Canfield,
J. P. Carlo,
G. F. Chen,
Pengcheng Dai,
A. C. Hamann,
W. Z. Hu, [......],
D. Reznik,
D. R. Sanchez-Candela,
A. T. Savici,
K. J. Sikes,
N. L. Wang,
C. R. Wiebe,
T. J. Williams,
T Yamamoto,
W Yu, Y. J. Uemura
[show abstract]
[hide abstract]
ABSTRACT: The recent discovery and subsequent developments of FeAs-based superconductors have presented novel challenges and opportunities in the quest for superconducting mechanisms in correlated-electron systems. Central issues of ongoing studies include interplay between superconductivity and magnetism as well as the nature of the pairing symmetry reflected in the superconducting energy gap. In the cuprate and RE(O,F)FeAs (RE = rare earth) systems, the superconducting phase appears without being accompanied by static magnetic order, except for narrow phase-separated regions at the border of phase boundaries. By muon spin relaxation measurements on single crystal specimens, here we show that superconductivity in the AFe$_{2}$As$_{2}$ (A = Ca,Ba,Sr) systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of (Ba$_{0.5}$K$_{0.5}$)Fe$_{2}$As$_{2}$ exhibits a nearly linear variation in T at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multi-gap effects. Comment: 14 pages 7 figures (4 for main text and 3 for on-line supplementary documents)
08/2008;
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A. A. Aczel,
E. Baggio-Saitovitch,
S. L. Budko,
P. C. Canfield,
J. P. Carlo,
G. F. Chen,
Pengcheng Dai,
T. Goko,
W. Z. Hu,
G. M. Luke,
J. L. Luo,
N. Ni,
D. R. Sanchez-Candela,
F. F. Tafti,
N. L. Wang,
T. J. Williams,
W Yu, Y. J. Uemura
[show abstract]
[hide abstract]
ABSTRACT: Zero-field (ZF) muon spin relaxation ($\mu$SR) measurements have revealed static commensurate magnetic order of Fe moments in NdOFeAs below $T_{N} \sim 135$ K, with the ordered moment size nearly equal to that in LaOFeAs, and confirmed similar behavior in BaFe$_{2}$As$_{2}$. In single crystals of superconducting (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$, $\mu$SR spectra indicate static magnetism with incommensurate or short-ranged spin structure in $\sim$ 70 % of volume below $T_{N} \sim$ 80 K, coexisting with remaining volume which exhibits superfluid-response consistent with nodeless gap below $T_{c}\sim 30$ K.
08/2008;
