Zero-field Muon Spin Rotation Research Articles

Ir 5d-band derived superconductivity in LaIr3

The superconducting properties of rhombohedral LaIr3 were examined using susceptibility, resistivity, heat capacity, and zero-field (ZF) and transverse-field (TF) muon spin relaxation and rotation (SR) measurements. The susceptibility and resistivity measurements confirm a superconducting transition below K. Two successive transitions are observed in the heat capacity data, one at K and a second at 1.2 K below . The heat capacity jump is , which is lower than 1.43 expected for Bardeen–Cooper–Schrieffer (BCS) weak-coupling limit. TF-SR measurements reveal a fully gapped s-wave superconductivity with , which is small compared to the BCS value of 3.56, suggesting weak-coupling superconductivity. The magnetic penetration depth, , estimated from TF-SR gives nm, a superconducting carrier density carriers m−3 and a carrier effective-mass enhancement factor . ZF-SR data show no evidence for any spontaneous magnetic fields below , which demonstrates that time-reversal symmetry is preserved in the superconducting state of LaIr3.

Quantum criticality in the coupled two-leg spin ladder Ba2CuTeO6

We report on zero-field muon spin rotation, electron spin resonance and polarized Raman scattering measurements of the coupled quantum spin ladder Ba2CuTeO6. Zero-field muon spin rotation and electron spin resonance probes disclose a successive crossover from a paramagnetic through a spin-liquid-like into a magnetically ordered state with decreasing temperature. More significantly, the two-magnon Raman response obeys a T-linear scaling relation in its peak energy, linewidth and intensity. This critical scaling behavior presents an experimental signature of proximity to a quantum critical point from an ordered side in Ba2CuTeO6.

Determination of the zero-field magnetic structure of the helimagnet MnSi at low temperature

Below a temperature of approximately 29 K the manganese magnetic moments of the cubic binary compound MnSi order to a long-range incommensurate helical magnetic structure. Here we quantitatively analyze a high-statistic zero-field muon spin rotation spectrum recorded in the magnetically ordered phase of MnSi by exploiting the result of representation theory as applied to the determination of magnetic structures. Instead of a gradual rotation of the magnetic moments when moving along a $\ensuremath{\langle}111\ensuremath{\rangle}$ axis, we find that the angle of rotation between the moments of certain subsequent planes is essentially quenched. It is the magnetization of pairs of planes which rotates when moving along a $\ensuremath{\langle}111\ensuremath{\rangle}$ axis, thus preserving the overall helical structure.

Zero-Field Magnetism of a Two-Dimensional Antiferromagnet, κ-(BEDT-TTF)2Cu[N(CN)2]Cl, Determined by Muon Spin Rotation and Magnetization Measurements

The zero-field magnetism of a two-dimensional noncollinear antiferromagnet, κ-(BEDT-TTF)2Cu[N(CN)2]Cl, has been investigated by magnetization and zero-field muon spin rotation (μSR) measurements. Low-field magnetization measurements enabled us to determine the magnetic transition temperature TN as 22.80 ± 0.02 K. Distinct muon spin precession signals appeared below 21.4 K. μSR spectra below 21.4 K were well described by two types of precession components and a relaxation one. The temperature dependence of internal field converted by μSR data was in good agreement with that of macroscopic residual magnetism. These results suggest that the tiny interlayer interaction, which has been suggested to be almost 106 times less than the intralayer exchange interaction, spontaneously causes the three-dimensional long-range order.

Absence of time-reversal symmetry breaking in the noncentrosymmetric superconductorMo3Al2C

Zero-field muon spin rotation and relaxation $(\ensuremath{\mu}\mathrm{SR})$ studies carried out on the strongly coupled, noncentrosymmetric superconductor ${\mathrm{Mo}}_{3}{\mathrm{Al}}_{2}\mathrm{C},{T}_{c}=9$ K, did not reveal hints of time-reversal symmetry breaking as was found for a number of other noncentrosymmetric systems. Transverse field measurements performed above and below the superconducting transition temperature defined the temperature dependent London penetration depth, which in turn served to derive from a microscopic point of view a simple $s$-wave superconducting state in ${\mathrm{Mo}}_{3}{\mathrm{Al}}_{2}\mathrm{C}$. The present investigations also provide fairly solid grounds to conclude that time-reversal symmetry breaking is not an immanent feature of noncentrosymmetric superconductors.

Spin Waves and Revised Crystal Structure of Honeycomb IridateNa2IrO3

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below T(N)=15.3 K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90° Ir-O-Ir bonds required for dominant Kitaev exchange.

Size-dependent magnetic ordering and spin dynamics in DyPO4and GdPO4nanoparticles

Low-temperature magnetic susceptibility and heat-capacity measurements on nanoparticles (d? 2.6 nm) of the antiferromagnetic compounds DyPO4 (TN=3.4 K) and GdPO4 (TN=0.77 K) provide clear demonstrations of finite-size effects, which limit the divergence of the magnetic correlation lengths, thereby suppressing the bulk long-range magnetic ordering transitions. Instead, the incomplete antiferromagnetic order inside the particles leads to the formation of net magnetic moments on the particles. For the nanoparticles of Ising-type DyPO4 superparamagnetic blocking is found in the ac susceptibility at ?1 K, those of the XY-type GdPO4 analog show a dipolar spin-glass transition at ?0.2 K. Monte Carlo simulations for the magnetic heat capacities of both bulk and nanoparticle samples are in agreement with the experimental data. Strong size effects are also apparent in the Dy3+ and Gd3+ spin dynamics, which were studied by zero-field muon spin rotation (?SR) and high-field 31P-nuclear magnetic resonance (31P-NMR) nuclear relaxation measurements. The freezing transitions observed in the ac susceptibility of the nanoparticles also appear as peaks in the temperature dependence of the zero-field ?SR rates, but at slightly higher temperatures, as to be expected from the higher frequency of the muon probe. For both bulk and nanoparticles of GdPO4, the muon and 31P-NMR rates are for T?5 K dominated by exchange-narrowed hyperfine broadening arising from the electron spin-spin interactions inside the particles. The dipolar hyperfine interactions acting on the muons and the 31P are, however, much reduced in the nanoparticles. For the DyPO4 analogs the high-temperature rates appear to be fully determined by electron spin-lattice relaxation processes.

μSR investigation of magnetism and magnetoelectric coupling in Cu2OSeO3

A detailed zero and transverse field (ZF&TF) muon spin rotation (muSR) investigation of magnetism and the magneto-electric coupling in Cu2OSeO3 is reported. An internal magnetic field B_int(T=0) = 85.37(25) mT was found, in agreement with a ferrimagnetic state below Tc = 57.0(1) K. The temperature dependence of the magnetic order parameter is well described by the relation B_int = B(0)(1-(T/Tc)^2)^b with an effective exponent b = 0.39(1) which is close to the critical exponent B ~ 1/3 for a three dimensional (3D) magnetic system. Just above Tc the muon relaxation rate follows the power low \lambda (T)\propto (T/Tc - 1)^\omega with \omega = 1.06(9), which is characteristic for 3D ferromagnets. Measurements of B_int(T) with and without an applied electrostatic field E = 1.66 x 10^5 V/m suggest a possible electric field effect of magnitude \Delta Bv = Bv(0 V)-Bv(500 V) = - 0.4(4) mT.

Evidence for impurity-induced frustration in La2CuO4

Zero-field muon spin rotation and magnetization measurements were performed in La2Cu{1-x}MxO4, for 0<x< 0.12, where Cu2+ is replaced either by M=Zn2+ or by M=Mg2+ spinless impurity. It is shown that while the doping dependence of the sublattice magnetization (M(x)) is nearly the same for both compounds, the N\'eel temperature (T_N(x)) decreases unambiguously more rapidly in the Zn-doped compound. This difference, not taken into account within a simple dilution model, is associated with the frustration induced by the Zn2+ impurity onto the Cu2+ antiferromagnetic lattice. In fact, from T_N(x) and M(x) the spin stiffness is derived and found to be reduced by Zn doping more significantly than expected within a dilution model. The effect of the structural modifications induced by doping on the exchange coupling is also discussed.

Evidence for time-reversal symmetry breaking in superconductingPrPt4Ge12

Zero and longitudinal field muon spin rotation (muSR) experiments were performed on the superconductors PrPt4Ge12 and LaPt4Ge12. In PrPt4Ge12 below Tc a spontaneous magnetization with a temperature variation resembling that of the superfluid density appears. This observation implies time-reversal symmetry (TRS) breaking in PrPt4Ge12 below Tc = 7.9 K. This remarkably high Tc for an anomalous superconductor and the weak and gradual change of Tc and of the related specific heat anomaly upon La substitution in La_(1-x)Pr_xPt_4Ge_(12) suggests that the TRS breaking is due to orbital degrees of freedom of the Cooper pairs.

Muon Spin Rotation and Relaxation in the Superconducting Ferromagnet UCoGe

We report zero-field muon-spin rotation and relaxation measurements on the superconducting ferromagnet UCoGe. Weak itinerant ferromagnetic order is detected by a spontaneous muon-spin precession frequency below the Curie temperature TC=3 K. The micro+ precession frequency persists below the bulk superconducting transition temperature Tsc=0.5 K, where it measures a local magnetic field Bloc=0.015 T. The amplitude of the microSR signal provides unambiguous proof for ferromagnetism present in the whole sample volume. We conclude ferromagnetism coexists with superconductivity on the microscopic scale.

Magnetism and superconductivity in heavy fermion superconductor

Abstract Zero field (ZF) and transverse field (TF) muon spin relaxation and rotation ( μ SR ) experiments have been carried out in the Cd-doped heavy fermion superconductor CeCoIn 5 to investigate its superconducting state. The ZF- μ SR results in CeCo ( In 0.97 Cd 0.03 ) 5 revealed that no spontaneous magnetic field was induced below its superconducting transition temperature ( T c ), indicating no evidence for time reversal symmetry breaking. The muon Knight shifts obtained from TF- μ SR measurements decrease significantly below T c , consistent with a spin-singlet state as in the parent compound CeCoIn 5 , which is a d-wave superconductor.

Weak ferromagnetic ordering in the anomalous field-insensitive heavy-fermion state in

Abstract Zero field (ZF) and transverse field (TF) muon spin relaxation and rotation ( μ SR ) study has been carried out in filled-skutterudite SmOs 4 Sb 12 in order to investigate the magnetically robust heavy-fermion (HF) state and the weak ferromagnetic anomaly appearing below ∼ 2.5 K . A large-amplitude oscillating signal appears in the ZF- μ SR spectra at low temperatures, confirming that the weak ferromagnetic anomaly is an intrinsic bulk property. Two components with the fraction ratio of ∼ 2 : 1 exist both in the ZF- μ SR spectra of the ferromagnetically ordered state and the TF- μ SR FFT spectra in applied fields along the 〈 0 0 1 〉 direction. This observation can be explained consistently based on the most probable muon stopping site indicated from the value of the static nuclear dipolar width in Kubo–Toyabe function in the high-temperature ZF- μ SR spectra. Analysis reveals that the spontaneous magnetic moment M s lies along the 〈 0 0 1 〉 direction and the size of M s is largely suppressed, indicating that the weak ferromagnetic moment is carried by itinerant heavy quasiparticles.

Magnetism and orbitally driven spin-singlet states in Ru oxides: A muon-spin rotation study

We have used zero-field muon-spin rotation to study the local magnetic properties of $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}$, ${\mathrm{Y}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, and ${\mathrm{La}}_{2}\mathrm{Ru}{\mathrm{O}}_{5}$. While $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}$ and ${\mathrm{Y}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ give rise to coherent precession signals, in ${\mathrm{La}}_{2}\mathrm{Ru}{\mathrm{O}}_{5}$ we find no signature of long range magnetic order below the structural transition, but observe relaxation consistent with that expected due to a dimerization into spin-singlet states. In ${\mathrm{Y}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ we observe two precession signals with different temperature dependences, consistent with the nature of the noncollinear ordering being temperature dependent.

Oxygen isotope effect on the AFM–FM phase transition of the layered cobaltite [formula omitted

Zero-field muon spin rotation ( ZF – μ SR ) and DC magnetization investigations have been performed on the antiferromagnetic-ferromagnetic (AFM–FM) and the ferromagnetic-paramagnetic (FM–PM) transitions of two oxygen isotope ( 16 O or 18 O) substituted HoBaCo 2 O 5.47 powders. While the FM–PM phase transition shows nearly no isotope effect, a small but clear shift of 1.5 K on the transition temperature is observed for the AFM–FM transition. Together with the recent observation of an isotope effect on the metal–insulator transition on the same sample [E. Pomjakushina, et al., Phys. Rev. B 73 (2006) 113105.], our observations clearly show the coupling of electronic, magnetic and structural degrees of freedom in this system.

Evidence for an antiferromagnetic component in the magnetic structure of ZrZn 2

Zero-field muon spin rotation experiments provide evidence for an antiferromagnetic component in the magnetic structure of the intermetallic ZrZn 2.

Multiple magnetic phase transitions inPrCoAl4Observed by Muon Spin Rotation and Relaxation Measurements

Zero field muon spin rotation and relaxation $(\ensuremath{\mu}\mathrm{SR})$ measurements on a single crystal of orthorhombic $\mathrm{Pr}\mathrm{Co}{\mathrm{Al}}_{4}$ revealed the presence of spontaneous static fields, extending above the well-known incommensurately modulated antiferromagnetic structure $({T}_{N}=17\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ up to at least $110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Two further transitions at $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $84\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ were clearly visible. The spontaneous fields above ${T}_{N}$ are confined to the $(b,c)$ plane and are tentatively ascribed to a spin density wave involving the Co $3d$-band electrons.

Coexistence of magnetism and superconductivity inEu1.4Ce0.6RuSr2Cu2O10:A muon spin rotation and magnetization study

We performed zero-field muon spin rotation (ZF-μSR) and dc magnetization experiments in the ruthenate-cuprate Eu 1 . 4 Ce 0 . 6 RuSr 2 Cu 2 O 1 0 . It was found that this compound becomes weakly ferromagnetic at T m = 77.6 K and superconducting below T c =40 K. ZF-μSR experiments indicate that the magnetic order accounts for most of the sample volume and magnetism is not significantly affected in the superconducting state. The present investigation provides evidence for the coexistence of magnetism and superconductivity in Eu 1 . 4 Ce 0 . 6 RuSr 2 Cu 2 O 1 0 .

Evidence for ordered moments on the Rh sites inCeRhIn5

The magnetic structure of the tetragonal heavy fermion compound ${\mathrm{CeRhIn}}_{5}$ ${(T}_{N}=3.8\mathrm{K})$ has been studied by zero-field muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ in the range 1.8--4.3 K in a single crystal. In addition, the anisotropic muon $({\ensuremath{\mu}}^{+})$ Knight shift has been measured in the paramagnetic regime with the aim to determine the ${\ensuremath{\mu}}^{+}$ site or sites. It is found that about 70% of the implanted ${\ensuremath{\mu}}^{+}$ are located at the a site $(\frac{1}{2},\frac{1}{2},\frac{1}{2}).$ The site of the other 30% is not unambigeously determined but may be the f sites $(0,\frac{1}{2},0)$ and $(\frac{1}{2},0,0).$ The data also imply that a small negative moment is induced on the Rh sites. Below ${T}_{N}$ we find five components in the $\ensuremath{\mu}\mathrm{SR}$ signal reflecting spontaneous, relatively sharp internal fields of zero, 13, 37, 69, and 98 G at $T=0\mathrm{K},$ which are confined to the surface of cones oriented along the c axis with apertures ranging from $14\ifmmode^\circ\else\textdegree\fi{}$ to $90\ifmmode^\circ\else\textdegree\fi{}.$ These results are not reproducible by the known incommensurate helical magnetic structure of the Ce sublattice but can be explained if, in addition, a small moment order of unusual type on the Rh sublattice is assumed.

Temperature dependence of the magnetic penetration depth in the vortex state of the pyrochlore superconductor, Cd2Re2O7.

We report transverse-field and zero-field muon spin rotation and relaxation studies of the superconducting rhenium oxide pyrochlore, Cd2Re2O7. Transverse-field measurements (H=0.007 T) show line broadening below T(c), which is characteristic of a vortex state, demonstrating conclusively the type-II nature of this superconductor. The penetration depth is seen to level off below about 400 mK (T/T(c) approximately 0.4), with a rather large value of lambda(T=0) approximately 7500 A. The temperature independent behavior below approximately 400 mK is consistent with a nodeless superconducting energy gap. Zero-field measurements indicate no static magnetic fields developing below the transition temperature.