Muon Spin Precession Research Articles

Interaction of Hydrogen with Reduced Graphene Oxide Probed by Muon-Spin Relaxation Technique

Muon-spin relaxation (μSR) spectroscopy has let an understanding of the hydrogen interactions with graphene, providing insights for hydrogen storage technologies based on graphene-based compounds. We report an μSR study on the reduced graphene oxide (rGO, a product of ®Graphenea) at 300 K. Spontaneous muon-spin precession is not observed under the high statistic zero-field measurement. Instead, the spectra show a typical muon diffusion with a small fraction of muon experiencing dipolar interactions with neighboring protons. Measurements under longitudinal field conditions yield the obtained hyperfine field (Bhyp) a way much lower than the required field to recover free muonium, demonstrating a radical formation. Moreover, a noticable ratio of captured muoniums (~35%) in the sample illustrates strong interactions between hydrogen and rGO.

Fully gapped superconductivity with preserved time-reversal symmetry in NiBi3 single crystals

We report a study of ${\mathrm{NiBi}}_{3}$ single crystals by means of electrical-resistivity, magnetization, and muon-spin rotation and relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements. As a single crystal, ${\mathrm{NiBi}}_{3}$ adopts a needlelike shape and exhibits bulk superconductivity with ${T}_{c}\ensuremath{\approx}4.1$ K. By applying magnetic fields parallel and perpendicular to the $b$ axis of ${\mathrm{NiBi}}_{3}$, we establish that its lower and upper critical fields as well as the magnetic penetration depths show slightly different values, suggesting a weakly anisotropic superconductivity. In both cases, the zero-temperature upper critical fields are much smaller than the Pauli-limit value, indicating that the superconducting state is constrained by the orbital pair breaking. The temperature evolution of the superfluid density, obtained from transverse-field $\ensuremath{\mu}\mathrm{SR}$, reveals a fully gapped superconductivity in ${\mathrm{NiBi}}_{3}$, with a shared superconducting gap ${\mathrm{\ensuremath{\Delta}}}_{0}=2.1{k}_{\mathrm{B}}{T}_{c}$ and magnetic penetration depths ${\ensuremath{\lambda}}_{0}$ = 223 and 210 nm for $H\ensuremath{\parallel}b$ and $H\ensuremath{\perp}b$, respectively. The lack of spontaneous fields below ${T}_{c}$ indicates that time-reversal symmetry is preserved in ${\mathrm{NiBi}}_{3}$. The absence of a fast muon-spin relaxation and/or precession in the zero-field $\ensuremath{\mu}\mathrm{SR}$ spectra definitely rules out any type of magnetic ordering in ${\mathrm{NiBi}}_{3}$ single crystals. Overall, our investigation suggests that ${\mathrm{NiBi}}_{3}$ behaves as a conventional $s$-type superconductor.

The internal magnetic field in a ferromagnetic compound Y2Co12P7

The internal magnetic field in a ferromagnetic compound, Y2Co12P7 with T C = 150 K, was studied with µ +SR using a powder sample down to 2 K. The wTF-µ +SR measurements revealed the presence of a sharp magnetic transition at T C = 151 K, and the ZF-µ +SR measurements clarified the formation of static magnetic order below T C. The presence of two muon spin precession signals in the ZF-µ +SR spectrum below T C indicates the existence of the two different muon sites in the lattice. By considering the muon sites and local spin densities at the muon sites predicted with DFT calculations, the ordered magnetic moments of Co were successfully determined.

A development of a compact software package for systematic error studies in muon $$g-2$$/EDM experiment at J-PARC

The J-PARC muon g-2/EDM experiment aims to measure the muon magnetic moment anomaly $a_{\mu} = (g -2 ) / 2 ~ $. The target sensitivity for $a_{\mu}$ is a statistical uncertainty of $450 \times 10^{-9}$, and $a_{\mu}$ can be extracted by measuring the spin precession of muons ($\omega_a$). The $\omega_a$ can be extracted from the distribution of muon decay time, and the time is measured by reconstructing tracks of positrons which are decayed from the muons. To extract $\omega_a$ precisely, the systematic effects on $\omega_a$ should be controlled under the aforementioned sensitivity. To study systematic effects, we develop a compact simulation package, and using our developed package, we study several systematic effects and discuss results. We discuss the package details and studies of systematic effects on $\omega_a$ based on our package.

Pressure dependence of ferromagnetic phase boundary in BaVSe3 studied with high-pressure μ+SR

The magnetic nature of a quasi-one-dimensional compound, ${\mathrm{BaVSe}}_{3}$, has been investigated with positive muon spin rotation and relaxation (${\ensuremath{\mu}}^{+}\mathrm{SR}$) measurements at ambient and high pressures. At ambient pressure, the ${\ensuremath{\mu}}^{+}\mathrm{SR}$ spectrum recorded under zero external magnetic field exhibited a clear oscillation below the Curie temperature (${T}_{C}\ensuremath{\sim}41\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) due to the formation of quasistatic ferromagnetic order. The oscillation consisted of two different muon spin precession signals, indicating the presence of two magnetically different muon sites in the lattice. However, the two precession frequencies, which correspond to the internal magnetic fields at the two muon sites, could not be adequately explained with relatively simple ferromagnetic structures using the muon sites predicted by density functional theory calculations. The detailed analysis of the internal magnetic field suggested that the V moments align ferromagnetically along the $c$ axis but slightly canted toward the $a$ axis by ${28}^{\ensuremath{\circ}}$ that is coupled antiferromagnetically. The ordered V moment (${\mathbit{M}}_{\mathrm{V}}$) is estimated as (0.59, 0, 1.11) ${\ensuremath{\mu}}_{\mathrm{B}}$. As pressure increased from ambient pressure, ${T}_{C}$ was found to decrease slightly up to about 1.5 GPa, at which point ${T}_{C}$ started to increase rapidly with the further increase of the pressure. Based on a strong ferromagnetic interaction along the $c$ axis, the high-pressure ${\ensuremath{\mu}}^{+}\mathrm{SR}$ result revealed that there are two magnetic interactions in the $ab$ plane; one is an antiferromagnetic interaction that is enhanced with pressure, mainly at pressures below 1.5 GPa, while the other is a ferromagnetic interaction that becomes predominant at pressures above 1.5 GPa.

Revisiting the A -type antiferromagnet NaNiO2 with muon spin rotation measurements and density functional theory calculations

An $A$-type antiferromagnet, ${\mathrm{NaNiO}}_{2}$, was examined by means of positive muon spin rotation and relaxation (${\ensuremath{\mu}}^{+}\mathrm{SR}$) measurements and first-principles calculations based on a density functional theory (DFT). Below ${T}_{\mathrm{N}}=20$ K, a clear muon spin precession signal was observed in the ${\ensuremath{\mu}}^{+}\mathrm{SR}$ time spectrum recorded under zero field, due to the formation of a static internal magnetic field. The microscopic origin of such an internal field was computed as a sum of dipolar and hyperfine contact fields at the site (0.624, 0, 0.854), where both the muon site and the local spin density at such a site were predicted with DFT calculations. While the computed values were consistent with experimentally obtained ones, in both the antiferromagnetic and the paramagnetic states, the contribution of the hyperfine contact field was shown to be insignificant even below ${T}_{\mathrm{N}}$. Finally, measurements at higher temperatures signified thermally activated Na-ion diffusion with ${E}_{\mathrm{a}}=50(20)$ meV and ${D}_{\mathrm{Na}}(300\phantom{\rule{0.16em}{0ex}}\mathrm{K})=8.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}$/s, commonly observed in layered-type compounds.

Magnetic moment of rare-earth elements in R2Fe14B estimated with μ+SR

The ferromagnetic (FM) nature of Nd2Fe14B has been investigated with muon spin rotation and relaxation ({\mu}^+SR) measurements on an aligned, sintered plate-shaped sample. A clear muon spin precession frequency (f_{FM}) corresponding to the static internal FM field at the muon site showed an order parameter-like temperature dependence and disappeared above around 582 K (~T_C). This indicated that the implanted muons are static in the Nd2Fe14B lattice even at temperatures above around 600 K. Using the predicted muon site and local spin densities predicted by DFT calculations, the ordered Nd moment (M_{Nd}) was estimated to be 3.31 {\mu}_B at 5 K, when both M_{Fe} and M_{Nd} are parallel to the c-axis and M_{Fe} = 2.1 {\mu}_B. Furthermore, M_R in R2Fe14B with R = Y, Ce, Pr, Sm, Gd, Tb, Dy, Ho, Er, and Tm was estimated from f_{\mu} values reported in earlier {\mu}+SR work, using the FM structure proposed by neutron scattering and the same muon site and local spin density as in Nd2Fe14B. Such estimations yielded M_R values consistent with those obtained by the other methods.

Absence of quantum criticality and bulk 3D magnetism in green dioptase

Green dioptase is a naturally occurring antiferromagnetic mineral that in recent years has been suggested as a candidate to exhibit quantum fluctuations at both above and below TN. Our work uses muon spectroscopy to study the dynamic and static properties of the magnetism in zero applied field. We observe the antiferromagnetic transition through tracking out the evolution of the muon spin precession frequency as a function of temperature. TN is calculated to be 15 K and the critical exponent of the transition matches with that of a 3D Heisenberg system. We also note that no evidence for any quantum magnetic fluctuations is observed either above or below TN.

Spin glass behavior in LaCo1−xRhxO3 ( x=0.4 , 0.5, and 0.6)

We studied the magnetic ground state of Co/Rh-disordered ${\mathrm{LaCo}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}{\mathrm{O}}_{3}$ ($x=0.4$, 0.5, and 0.6) by means of powder neutron diffraction (PND), muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$), ac susceptibility, isothermal magnetic relaxation, and heat capacity measurements. The dc magnetic susceptibility measurements exhibit a cusp around the freezing temperature ${T}_{f}$ of all studied compounds. PND measurements show no indications of long-range magnetic ordering down to 4 K in all studied samples. $\ensuremath{\mu}\mathrm{SR}$ experiments on these compounds show a peak in the muon spin relaxation rate around ${T}_{f}$. However, no spontaneous muon spin precession can be observed evidencing the short-range nature of the magnetic ordering. For the $x=0.5$ sample ac susceptibility measurements show that ${T}_{f}$ shifts to higher temperatures with increasing frequency and becomes suppressed by a dc biasing field. All these observations indicate that the Co/Rh-disordered ${\mathrm{LaCo}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}{\mathrm{O}}_{3}$ system is a spin glass for $x=0.4$, 0.5, and 0.6.

Muon-spin relaxation study of the double perovskite insulators Sr2BOsO6 (B = Fe, Y, ln)

We present the results of zero-field muon-spin relaxation measurements made on the double perovskite insulators Sr2BOsO6 (). Spontaneous muon-spin precession indicative of quasistatic long range magnetic ordering is observed in Sr2FeOsO6 within the AF1 antiferromagnetic phase for temperatures below K. Upon cooling below K the oscillations cease to be resolvable owing to the coexistence of the AF1 and AF2 phases, which leads to a broader range of internal magnetic fields. Using density functional calculations we identify a candidate muon stopping site within the unit cell, which dipole field simulations show to be consistent with the proposed magnetic structure. The possibility of incommensurate magnetic ordering is discussed for temperatures below K and 25 K for Sr2YOsO6 and Sr2InOsO6, respectively.

Muon Site Estimations by DFT Calculations in Metal and Insulating Systems

A way to estimate muon sites in materials is reported. Since the muon has a positive charge, one easiest and conventional way to estimate muon positions is to calculate minimum potential positions. We applied our developed method to estimate those potential minimum positions to some systems which have been studied by mons at the RIKEN-RAL Muon Facility and showed well defined muon-spin precession behavior in magnetically ordered states. Tentative calculation results by using the density functional theory are reported.

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.

Magnetic properties of the helimagnetCr1/3NbS2observed byμSR

We have performed muon spin rotation/relaxation (muSR) measurements on single crystals of the chiral helimagnet Cr1/3NbS2 at zero to low magnetic field. The transition from the paramagnetic to helical magnetically ordered phase at zero field is marked by the onset of a coherent oscillation of the zero-field muon spin polarization below a critical temperature Tc. An enhancement of the muon spin precession frequency is observed below T ~ 50K, where anomalous behavior has been observed in bulk transport measurements. The enhanced precession frequency indicates a low-temperature modification of the helical magnetic structure. A Landau free energy analysis suggests that the low-temperature change in the magnetic structure is caused by a structural change, whereas the magnetic order above Tc is the result of an attractive interaction between the ferromagnetic moment induced by the applied field and the magnetic moments of the helical structure. We also suggest a longer periodicity of helicity below T ~ 50K, which can be verified by neutron scattering.

Muonic atom as an acceptor centre in diamond

Polarized negative muons were used to study the behaviour of the boron acceptor centre in diamond produced by the chemical vapour deposition (CVD) method. The temperature dependence of the muon spin relaxation rate and spin precession frequency were measured in the range of 20 – 330 K in a transverse magnetic field of 14 kOe. The muon polarization amplitude P(t = 0) does not depend on the temperature and the muon spin relaxation rate decreases as the temperature increases. For the first time a negative shift of the muon spin precession was observed in diamond. The measurements show that the magnetic susceptibility of the CVD sample is negligible (χ = -4.13(2)·10−7cm3/g at 20 K) and it could not be the reason of the negative shift muon spin precession frequency. The negative shift of the muon spin precession frequency is tentatively attributed to an anisotropic hyperfine interaction in the boron acceptor in diamond.

New insights for the description of magnetic correlations inferred from μSR

Whenever a compound exhibits a spontaneous μSR oscillation, long-range magnetic ordering is usually inferred. Here we show that some caution is required. The coherence length needs not to be large for a spontaneous muon spin precession to be observed. Establishing the incommensurate nature of a magnetic structure, solely based on μSR measurements, may not be reliable. The absence of a spontaneous muon precession at low temperature does not mean that the system under investigation does not display long-range magnetic ordering.The relaxation measured in zero and longitudinal field in the quasi-static limit is usually analyzed in the framework of the strong-collision model, the static polarization function being taken to be the famous Kubo-Toyabe function. This might not be satisfactory if short-range correlation effects are strong. Here we propose a method based on the maximum entropy concept and reverse Monte Carlo technique which gives results consistent with those obtained in 2013 by analytical means for the considered example.

Internal magnetic field in the zigzag-chain family (Na,Ca)Cr2O4

In order to elucidate the magnetic nature for a novel one-dimensional zigzag chain compound, NaCr2O4, we have measured μ+SR spectra using a powder sample in the temperature range between 2 and 200 K. Weak transverse field (wTF-) μ+SR measurements indicated that the whole volume of the sample enters into an antiferromagnetic (AF) phase below TN = 125 K. The zero field (ZF-) μ+SR spectrum obtained below TN exhibits a clear oscillation with a single muon-spin precession frequency (fμ). This suggests that static AF order is formed below TN and that all the implanted muons sense the same internal magnetic field. The temperature dependence of fμ was found to be very similar to that for the intensity of the magnetic Bragg peak in neutron diffraction (ND) measurements. On the other hand, the ZF-μ+SR spectrum for the isostructural compound, β-CaCr2O4, showed a rapidly damped oscillation below TN = 21 K, supporting the formation of incommensurate AF order, as proposed by ND.

Direct evidence for the all-in/all-out magnetic structure in the pyrochlore iridates from muon spin relaxation

In the pyrochlore iridates R${}_{2}$Ir${}_{2}$O${}_{7}$ (R = lanthanide, Y) determination of the magnetic structure of the iridium moments remains an outstanding problem despite the role this is expected to play in the formation of novel band structures and topologies in these materials. In this work, an analysis of the experimentally measured spontaneous muon spin precession frequency is presented which incorporates both probabilistic and ab initio modeling techniques to determine the ground state magnetic structure. It is shown that the experimentally observed results are consistent only with a magnetically ordered Ir${}^{4+}$ sublattice with the so-called ``all-in/all-out'' magnetic structure, and that the electronic state of the Ir${}^{4+}$ is best described by the ${J}_{\mathrm{eff}}=\frac{1}{2}$ model in several member compounds. Through this approach it is also demonstrated that such a simple structure is not likely to be present on the rare-earth sublattice which contains much larger localized moments.

Possible Existence of the Stripe Correlations in Electron-Doped Superconducting Cuprates Eu<sub>1.85</sub>Ce<sub>0.15</sub>Cu<sub>1-y</sub>Ni<sub>y</sub>O<sub>4+α-δ</sub> Studied by Muon-Spin-Relaxation

Partially Ni-substituted electron-doped superconducting cuprates of Eu1.85Ce0.15Cu1-yNiyO4+α-δ (ECCNO) with y = 0, 0.01, 0.02 and 0.05 have been studied by muon-spin-relaxation (mSR) measurements, in order to elucidate whether or not the dynamical stripe correlations of spins and charges exist in electron-doped cuprates. It has been found that the development of the Cu-spin correlation is induced at low temperatures through the Ni substitution and that, for y = 0.02, a muon-spin precession due to a long-range magnetic order has been observed at 10 K. These results suggest a possibility that the stripe model can globally explain the high-Tc superconductivity in both hole- and electron-doped cuprates.

Partially disordered spin structure in Ag2CrO2studied withμ+SR

The magnetism of a metallic two-dimensional triangular antiferromagnetic (AF) compound, Ag${}_{2}$CrO${}_{2}$, has been investigated by muon-spin rotation and relaxation (${\ensuremath{\mu}}^{+}$SR) using a powder sample in the temperature range between 1.8 and 40 K. Below ${T}_{\mathrm{N}}=24$ K, a muon-spin precession signal was clearly observed in the zero-field spectrum, indicating the formation of static AF order. It was also found that the internal field is temperature independent except for in the vicinity of ${T}_{\mathrm{N}}$, as in the case for the susceptibility versus temperature curve. This suggests that the AF transition is induced by a first-order structural phase transition at ${T}_{\mathrm{N}}$. Combining the ${\ensuremath{\mu}}^{+}$SR result with the prediction for muon sites in the lattice by first-principles calculations, a partially disordered AF state was found to be the most reasonable spin structure for Ag${}_{2}$CrO${}_{2}$.

Magnetic instability induced by Rh doping in the Kondo semiconductor CeRu2Al10

The magnetic ground state of the Rh-doped Kondo semiconductor CeRu${}_{2}$Al${}_{10}$ [Ce(Ru${}_{1\ensuremath{-}x}$Rh${}_{x}$)${}_{2}$Al${}_{10}$] is investigated with the muon spin relaxation method. Muon spin precession with two frequencies is observed in the $x=0$ sample, while only one frequency is present in the $x=0.05$ and 0.10 samples, which is attributed to the broad static field distribution at the muon site. The internal field at the muon site is enhanced from about 180 G in the $x=0$ sample to about 800 G in the Rh-doped samples, supporting the spin-flop transition as suggested by the magnetization measurement, and the boundary of different magnetic ground states is identified around $x=0.03$. The drastic change of magnetic ground state by a small amount of Rh doping (3%) indicates that the magnetic structure in CeRu${}_{2}$Al${}_{10}$ is not robust and can be easily tuned by external perturbations such as electron doping. The anomalous temperature dependence of the internal field in CeRu${}_{2}$Al${}_{10}$ is suggested to be attributed to the hyperfine interaction between muons and conduction electrons.