Longitudinal Field Muon Spin Relaxation Research Articles

Muon spin relaxation and emergence of disorder-induced unconventional dynamic magnetic fluctuations in Dy2Zr2O7

The disordered pyrochlore oxide Dy2Zr2O7 shows the signatures of field-induced spin freezing with remnant zero-point spin-ice entropy at 5 kOe magnetic field. We have performed zero-field and longitudinal field Muon spin relaxation (µSR) studies on Dy2Zr2O7. Our zero field studies reveal the absence of both long-range ordering and spin freezing down to 62 mK. The µSR relaxation rate exhibits a temperature-independent plateau below 4 K, indicating a dynamic ground state of fluctuating spins similar to the well-known spin ice system Dy2Ti2O7. The low-temperature spin fluctuations persist in the longitudinal field of 20 kOe as well and show unusual field dependence of the relaxation rate, which is uncommon for a spin-liquid system. Our results, combined with the previous studies do not show any evidence of spin ice or spin glass ground state, rather point to a disorder-induced dynamic magnetic ground state in the Dy2Zr2O7 material.

Paramagnetic probes in an organic semiconductor: μSR and DFT calculations of the Mu adducts of Alq3 and 8-hydroxyquinoline.

It has been claimed that longitudinal field muon spin relaxation (LF-μSR) experiments on the organic semiconductor (OSC) tris-(8-hydroxyquinoline)aluminum(III) (Alq3) have measured electron hopping rates of ∼1012s-1, while density functional theory (DFT) calculations suggest that electron hopping between a muoniated radical and a neighboring molecule is energetically unfavorable and that the LF-μSR experiments were probing muoniated radicals with localized spin density. We have performed avoided level crossing muon spin resonance (ALC-μSR) and transverse field muon spin rotation (TF-μSR) measurements on Alq3 and 8-hydroxyquinoline (8hq), which is meant to model the muoniated radicals present in Alq3 when they are not in an OSC. These are supplemented by benchmarked DFT calculations. The ALC-μSR and TF-μSR spectra of 8hq and Alq3 are best explained by Mu adding to all six secondary carbons of the quinolate rings with roughly equal yields and localized spin density. There is no evidence in the TF-μSR spectrum of Alq3 for the formation of radicals with muon hyperfine coupling constants of 23 or 91MHz as reported earlier by others. Our measurements support the view that there is localized spin density on the molecule to which Mu is covalently bound and the muon is not a passive probe in organic systems as it can be incorporated into radicals that have different electronic structures to the parent compounds. The muoniated radicals in Alq3 are more short-lived than in 8hq, which could be due to interactions with mobile electrons in the OSC, but with electron spin flip rates on the order of ∼107s-1.

Megahertz dynamics in skyrmion systems probed with muon-spin relaxation

We present longitudinal-field muon-spin relaxation (LF $\mu$SR) measurements on two systems that stabilize a skyrmion lattice (SkL): Cu$_2$OSeO$_3$, and Co$_x$Zn$_y$Mn$_{20-x-y}$ for $(x,y)~=~(10,10)$, $(8,9)$ and $(8,8)$. We find that the SkL phase of Cu$_2$OSeO$_3$ exhibits emergent dynamic behavior at megahertz frequencies, likely due to collective excitations, allowing the SkL to be identified from the $\mu$SR response. From measurements following different cooling protocols and calculations of the muon stopping site, we suggest that the metastable SkL is not the majority phase throughout the bulk of this material at the fields and temperatures where it is often observed. The dynamics of bulk Co$_8$Zn$_9$Mn$_3$ are well described by $\simeq~2$ GHz excitations that reduce in frequency near the critical temperature, while in Co$_8$Zn$_8$Mn$_4$ we observe similar behavior over a wide range of temperatures, implying that dynamics of this kind persist beyond the SkL phase.

Temporal and field evolution of spin excitations in the disorder-free triangular antiferromagnet Na2BaCo(PO4)2

The realization of quantum spin liquids on a triangular lattice magnet remains challenging due to quenched disorders present in real materials. Here, we report local-probe signatures of spin-liquid-like excitations in the structurally perfect triangular antiferromagnet ${\mathrm{Na}}_{2}\mathrm{BaCo}{({\mathrm{PO}}_{4})}_{2}$. Our zero-field and longitudinal-field muon spin relaxation measurements reveal both spatially and temporally correlated spins with no hint of static magnetism down to 80 mK, consistent with a quantum spin liquid. Comprehensive $^{23}\mathrm{Na}$ nuclear magnetic resonance experiments enable us to identify a field-induced crossover of magnetic excitations from a spinonlike to a gapped magnon at the critical field ${\ensuremath{\mu}}_{0}{H}_{C}=1.65$ T. Our findings uncover the temporal, thermal, and magnetic-field characteristics of the magnetic excitations in the triangular-lattice quantum spin liquid candidate ${\mathrm{Na}}_{2}\mathrm{BaCo}{({\mathrm{PO}}_{4})}_{2}$.

Spin Fluctuating Ground State in the Spin-1/2 Kagome Lattice (Rb1−xCsx)2Cu3SnF12 with x = 0.53 Observed by Muon Spin Relaxation

Longitudinal-field muon spin relaxation (LF-μSR) measurements were performed on the spin-1/2 kagome lattice compound (Rb0.47Cs0.53)2Cu3SnF12 down to 26 mK. It is reported that the spin-1/2 kagome l...

Quantum fluctuations in the quasi-one-dimensional non-Fermi liquid system CeCo2Ga8 investigated using μSR

Reduced dimensionality offers a crucial information in deciding the type of the quantum ground state in heavy fermion materials. Here we have examined stoichiometric CeCo$_{2}$Ga$_{8}$ compound, which crystallizes in a quasi-one-dimensional crystal structure with Ga-Ce-Co chains along the $c$-axis. The low-temperature behavior of magnetic susceptibility ($\chi\sim-\ln T$), heat capacity ($C_p/T\sim-\ln T$), and resistivity ($\rho\sim T^{n}$) firmly confirm the non-Fermi liquid ground state of CeCo$_{2}$Ga$_{8}$. We studied the low-energy spin dynamics of CeCo$_{2}$Ga$_{8}$ compound utilizing zero field (ZF-) and longitudinal field (LF-) muon spin relaxation ($\mu$SR) measurements. ZF-$\mu$SR measurement reveals the absence of long-range magnetic ordering down to 70 mK, and interestingly below 1 K, the electronic relaxation rate sharply rises, intimating the appearance of low energy quantum spin fluctuations in CeCo$_{2}$Ga$_{8}$.

Unveiling spin-glass transition and antiferromagnetic order by μSR studies in spin-chain Sm2BaNiO5

We report the zero-field and longitudinal field muon spin relaxation studies in a spin-chain compound Sm2BaNiO5. Two magnetic transitions, that have not been previously detected by the heat capacity and magnetization measurements, are confirmed at 46 and 9 K. The antiferromagnetic order is suggested at 46 K. Analysis of the muon spin polarization unveils the spin-glass transition at 9 K. Time-field scaling relation of the muon spin polarization verifies the spin–spin autocorrelation function following the cut-off power law, which is approximated by the Ogielski form, as employed numerically for characterizing the spin-glasses.

Spin Fluctuations in the Spin-1/2 Kagome Lattice Antiferromagnet (Rb1−xCsx)2Cu3SnF12 around the Quantum Critical Point Detected by Muon Spin Relaxation Technique

Zero- and longitudinal-field muon spin relaxation (ZF-, LF-μSR) measurements were carried out on the spin-1/2 kagome lattice antiferromagnet (Rb1−xCsx)2Cu3SnF12 with x = 0.53 and 0.6, where xc = 0....

Spin correlations in (Mn,Fe)2(P,Si) magnetocaloric compounds above Curie temperature

The longitudinal-field muon-spin relaxation (LF-μSR) technique was employed to study the spin correlations in (Mn,Fe)2(P,Si) compounds above the ferromagnetic transition temperature (TC). The (Mn,Fe)2(P,Si) compound under study is found to show itinerant magnetism. The standard deviation of the magnetic field distribution of electronic origin increases with a decrease in temperature, which is attributed to the development of spin correlations. The anomalously low magnetic fluctuation rate is suggested to be another signature of the spin correlations. The development of pronounced magnetic fluctuations is in agreement with the observed deviation of the paramagnetic susceptibility from Curie–Weiss behavior. Our study sheds light on the magneto-elastic transition and the mixed magnetism in (Mn,Fe)2(P,Si) compounds.

Spin dynamics in Na4−xIr3O8 (x = 0.3 and 0.7) investigated by 23Na NMR and μSR

We report 23Na nuclear magnetic resonance (NMR) and zero-field (ZF) and longitudinal-field (LF) muon spin relaxation (μSR) measurements of the depleted hyperkagome compounds Na4−xIr3O8 (x = 0.3 and 0.7), which undergo an insulator-semimetal transition as a function of x. The 23Na spin-lattice relaxation rates, , follow a T2.5 power law behavior at accessible temperatures of T = 120–350 K. A substantial temperature dependence of indicates the presence of gapped excitations at elevated temperatures through the transition to a semimetallic phase. ZF-μSR results reveal that hole-doping leads to a melting of quasi-static order to a dynamically fluctuating state. The very slow muon depolarization rate which varies hardly with temperature indicates that spins are close to an itinerant limit in the largest doping x = 0.7. The dynamic relaxation rates extracted from the LF-μSR spectra show a three-dimensional diffusive transport. Our combined NMR and μSR results suggest the occurrence of intriguing spin and charge excitations across the insulator-semimetal transition.

Molecular Dynamics of Transient Radicals in Solution Probed by Longitudinal-Field Muon Spin Relaxation

Longitudinal field muon spin relaxation (LF-μSR) has been used to measure the magnetic field dependence of the longitudinal field muon spin relaxation rates (λμ = 1/Tμ1) of two muoniated radicals, 2-muoxyprop-2-yl radical (formed by Mu addition to acetone) and the muoniated 1,2-dicarboxyvinyl radical dianion (formed by Mu addition to the 1,2- dicarboxyacetylene dianion), in aqueous solution. The rotational correlations time of the radicals in solution at 298 K were determined from the magnetic field dependence of λμ and were found to be 32±2 ps for the 2-muoxyprop-2-yl radical and 55±3 ps for the muoniated 1,2-dicarboxyvinyl radical dianion.

Probing the magnetic phases in the Ni-V alloy close to the disordered ferromagnetic quantum critical point with μSR

Zero (ZF) and longitudinal field (LF) muon spin relaxation data of the d-metal alloy Ni1-xVx are presented at several vanadium concentrations x below and above the critical xc ≈ 11 % where long-range ferromagnetic (FM) order is suppressed. The clear single precession frequency observed for Ni, as expected for a homogeneous FM, changes to rather damped osciallation with small V substitution at x = 4 %, confirming magnetic inhomogeneities caused by the less magnetic V environments in the magnetic Ni matrix. Furthermore, local fields and spatial field distributions can be estimated to characterize different inhomogeneous regimes developing with x in the FM phase of Ni1-xVx. In the regime of x = 7 – 10 % a Kubo Toyabe function well describes the low temperature ZF and LF asymmetry data supporting a static Gaussian field distribution. Closer to the quantum critical concentration a single scale static Kubo Toyabe function with one field distribution is not sufficient to describe the muon relaxation. These data indicate that further changes in spatial distributions and dynamics are evolving as expected within the critical regime of a disordered quantum critical point.

Disappearance of Gapped Mott Insulating Phase Neighboring Bose Glass Phase in Tl1−xKxCuCl3 Detected by Longitudinal-Field Muon Spin Relaxation

Longitudinal-field muon spin-relaxation (LF-μSR) measurements have been performed in a bond-randomness introduced quantum spin system, Tl1−xKxCuCl3 with x = 0.40, in which the appearance of a Bose glass (BG) phase adjacent to the Bose–Einstein condensation (BEC) phase of excited triplets is observed. In the zero-field limit below 500 G, LF-μSR time spectra shows an exponential-like decay, and the muon spin-relaxation rate does not have a significant temperature dependence down to 25 mK. This indicates that spins fluctuate in time and in space, and that the spin system is not into a spin gapped state. It is suggested that the theoretically predicted gapped Mott insulating phase adjacent to the BG phase disappears in this case. This result obtained by microprobe LF-μSR measurements is consistent with an indication deduced from the results of macroscopic magnetization measurements.

Muon spin spectroscopy of ferrocene: characterization of muoniated ferrocenyl radicals.

Radicals formed by the reaction of muonium (Mu), a light isotope of hydrogen, with ferrocene and ferrocene-d10 have been studied with the avoided level crossing muon spin resonance (ALC-μSR) and longitudinal field muon spin relaxation (LF-μSR) techniques between 10 and 100 K. A single type of radical was observed in each compound and the muon hyperfine coupling constants (hfcc) and the muon spin relaxation rates were measured as a function of temperature. A previous report concerning the observation of Mu adducts of ferrocene (U. A. Jayasooriya et al. Chem. - Eur. J., 2007, 13, 2266-2276) appears to be incorrect. DFT calculations were performed to aid in the assignment of the ALC-μSR spectra. A tentative assignment is that the observed radicals were formed by Mu addition to the exterior of the cyclopentadienyl rings and that the structures are distorted due to interactions with neighbouring molecules. The temperature dependence of the muon hfcc can be explained assuming the population of two levels with different muon hfccs separated by 1.4 ± 0.1 kJ mol(-1). The temperature dependence of the width and amplitude of the Δ1 resonance and the muon spin relaxation rate suggests that the electron spin relaxation rate increase with temperature, but the relaxation mechanism is unknown.

Magnetic ground state of theMn+1AXn-phase nitride Cr2GaN

Aiming to find magnetic states in the ${M}_{n+1}A{X}_{n}$ phases, we measured the magnetic susceptibility, the resistivity, and nuclear magnetic resonance of Cr${}_{2}$Ga$X$ ($X=\text{N}$ and C). Zero-field and longitudinal-field muon spin relaxation experiments were also applied to Cr${}_{2}$GaN. A magnetic phase transition, most probably to the spin-density-wave (SDW) state, has been found in Cr${}_{2}$GaN at ${T}_{\mathrm{N}}=170$ K in contrast to the Pauli paramagnetism in the carbide counterpart Cr${}_{2}$GaC. The origin of the SDW transition is interpreted in terms of possible Fermi-surface nesting in the two-dimensional-like electronic structure.

Low temperature ballistic spin transport in the S = 1/2 antiferromagnetic Heisenberg chain compound SrCuO2

We report zero and longitudinal magnetic field muon spin relaxation (μSR) measurements of the spin S = 1/2 antiferromagnetic Heisenberg chain material SrCuO2. We find that in a weak applied magnetic field B0 the spin–lattice relaxation rate λ follows a power law with n = 0.9(3). This result is temperature independent for 5 K ≤ T ≤ 300 K. Within conformal field theory and using the Müller ansatz we conclude ballistic spin transport in SrCuO2.

Muonium--the second radioisotope of hydrogen: a remarkable and unique radiotracer in the chemical, materials, biological and environmental sciences.

Muonium (Mu), may be regarded as a radioactive hydrogen atom with a positive muon as its nucleus, and is formed in a range of media which are irradiated with positive muons. This exotic atom can be considered as a second radioisotope of hydrogen, along with tritium. Addition of this light atom (with a mass 1/9th that of a normal hydrogen, protium, atom) to unsaturated organic molecules forms free radicals, in which the muon serves as a radioactive and magnetic probe of their kinetic and structural properties. Suitable examples are chosen to illustrate the very large functionality of organic radicals which have been measured using muons and various methods of muSR, where mu stands for muon, S for spin and R may refer to rotation, resonance or relaxation. The principal techniques illustrated are transverse-field muon spin rotation (TF-muSR), avoided level crossing muon spin resonance (ALC-muSR) and longitudinal-field muon spin relaxation (LF-muSRx). Structural studies of radicals, the determination of mechanisms for radical formation, the measurement of radical stabilisation energies, the determination of the kinetics of reactions of free muonium atoms and of free radicals have all been accomplished using TF-muSR methods. It is further shown that TF-muSR is most useful in measuring radical reaction rates in non-aqueous media, to provide information of relevance to cell membrane damage and repair Muonium may further be used as a mechanistic probe since it determines a true pattern of H-atom reactivity in molecules, against which results from similar radiolysed materials may be compared. [In many solid materials that are exposed to ionising radiation, apparent H-atom adduct radicals are detected but which originate from charge-neutralisation of positive holes (radical cations) and ejected electrons, without free H-atoms being formed. DNA is the superlative example of this. Free H-atoms normally feature in the province of radiolysed aqueous media]. The applications of ALC-muSR and LF-muSRx in studying the reorientation of reactive radicals on reactive surfaces forms the substantive proportion of the review: considered specifically are radicals sorbed in zeolites, in clays and in porous silica, in porous carbons and on ice-surfaces, in connection with their role as intermediates in catalytic systems, particularly hydrocarbon cracking and oxidation processes, and in atmospheric aerosol chemistry. The formation of muonium and other muon species in cation-exchanged zeolite-X samples are also considered, according to the evidence of longitudinal field repolarisation measurements. Finally, mention is given of the use of muSR techniques for studying radicals in the gas-phase.

Pressure-dependent spin fluctuations and magnetic structure in the topologically frustrated spin glass alloy Y(Mn0.95Al0.05)2

Longitudinal field (LF = 110 G) muon spin relaxation (mu SR) has been used to investigate the pressure dependence (P < 4.5 kbar) of paramagnetic spin fluctuations in the spin glass alloy Y(Mn0.95Al0.05)(2) via observation of the mu(+) spin depolarization. External mechanical force is seen to counteract the Al-induced chemical pressure, fully delocalizing the Mn moment and altering the nature of the spin fluctuation spectrum sensed by the muon. A qualitative change in the functional form of the mu(+) spin depolarization is observed. Complementary ambient and high-pressure neutron diffraction measurements suggest not only pressure-dependent structural transitions but also the instability of the localized manganese moment. The ambient and high-pressure mu(+) spin depolarization results from Y(Mn0.95Al0.05)(2) are likened to P = 0 results reported for other Y(Mn1-xAlx)(2) alloys. Finally, the possibility of using mu(+) spin depolarization rates to predict experimental inelastic neutron scattering (INS) line widths is considered; the muon having the potential to provide information equivalent to that obtained via INS but with greatly reduced data collection times.

Longitudinal Field Muon Spin Rotation Study of Magnetic Freezing in Fe Rich FeSe0.25Te0.75

Abstract We study the freezing of magnetic fluctuations in an Fe rich sample of the “11” iron-based superconductor using longitudinal field muon spin relaxation. The magnetic relaxation rate peaks at 15 K indicating spin glass freezing that nearly coincides with the superconducting transition of the corresponding Fe stoichiometric phase. At this temperature, the magnetic field dependence of the relaxation indicates slow magnetic fluctuations on the nanosecond timescale.

Radiolabelling Measurements of Free Radicals Derived from Aromatic Volatile Organic Compounds Adsorbed in Zeolite Nanomaterials to and above Saturation Loadings

Benzene, para-xylene and mesitylene were adsorbed into silicalite, Na - morde- nite, Na - ZSM5 and Li - X zeolites, and studied using the longitudinal field muon spin relaxation (LF-μSRx) technique. The zeolites ZSM5/silicalite and mordenite contain microporous channels while zeolite X has only supercages. For cyclohex- adienyl radicals/benzene adsorbed in Na-ZSM5, silicalite and Na - mordenite, a fraction was detected with a common reorientational activation energy in the region of ca 5kJ mol−1; however, in all cases there appeared a second fraction with an activation energy of ca 12kJmol’. [In Li-X only a single fraction was observed with Ea = 8.1kJ mol−1, from molecules adsorbed in supercages]. Given that high loadings of benzene, beyond the saturation capacities of the zeolites were employed, we believe the two distinct motional distributions represent the channel intersection and channel/external-surface niche locations in ZSM5/silicalite, the latter having the higher activation energy. An extraneous fraction may also be present (possibly as a thin film coating the zeolite grains), which probably also contributes to the detected ca 5 kJ mol−1 component in which, as at the channel-intersections, the motion tends toward bulk behaviour. Mordenite has only a single accessible channel structure and is hence devoid of the relatively unrestricting channel intersecting pores that are present in ZSM5/silicalite. However, the channels are wider (ca 7 A diameter) and we may ascribe therefore, the ca 5 kJ mol−1 fraction to the formation of benzene clusters within these channels that exhibit bulk-type behaviour, while we assign, in analogy with the results for ZSM5/silicalite, the ca 12kJ mol−1 fraction to molecules adsorbed in niches on the external surface. It is thought that the essential difference between the two activation energies is that the ca 5 kJ mol -1 processes involve molecular motion within clusters of benzene molecules with properties similar to the bulk phase (6.6 kJ mol -1 was measured in pure benzene), while those characterised at ca 12kJ mol−1 reflect single benzene molecules or small, ordered molecular clusters interacting with a zeolite surface. The results for dimethylcyclohexadienyl radicals (derived from p-xylene) are rather similar, but indicate slightly reduced activation energies. The larger mesitylene molecules can penetrate zeolite X and (more slowly) mordenite, but are excluded from the internal pores of ZSM5/silicalite.