Muon Site Research Articles

Spin Dynamics in Copper Metaborate CuB2O4 Studied by Muon Spin Relaxation

Copper metaborate CuB$_2$O$_{4}$ was studied by muon spin relaxation measurements in order to clarify its static and dynamic magnetic properties. The time spectra of muon spin depolarization suggest that the local fields at the muon site contain both static and fluctuating components in all ordered phases down to 0.3 K. In the weak ferromagnetic phase (20 K~$>T>$~9.3 K), the static component is dominant. On the other hand, upon cooling the fluctuating component becomes dominant in the incommensurate helix phase (9.3K > T > 1.4K). The dynamical fluctuations of the local fields persist down to 0.3K, where a new incommensurate phase (T < 1.4K) is expected to appear. This result suggests that spins fluctuate even at T \to 0. We propose two possible origins of the remnant dynamical spin fluctuations: frustration of the exchange interactions and the dynamic behavior of the soliton lattice.

Muon-spin relaxation study of the spin-12molecular chain compoundCu(HCO2)2(C4H4N2)

We present the results of a muon-spin relaxation (μ+SR) study of magnetic order in the quasi-one-dimensional spin-1/2 molecular chain compound CU(HCO2)2(C4h4N2). Zero-field μ+SR measurements at low temperatures show oscillations in the measured asymmetry, characteristic of a quasistatic magnetic field at the muon sites. Our measurements provide convincing evidence for long-range magnetic order below a temperature TN=1.97(8) K. This leads to an estimate of the interchain coupling constant of |J'| lkB=1.27 K and to a ratio |J'lJ|=1.6x10-1.

Magnetic order in the quasi-one-dimensional spin-1∕2molecular chain compound copper pyrazine dinitrate

We present evidence of magnetic order in the quasi-one-dimensional spin-1/2 molecular chain compound, copper pyrazine dinitrate $\mathrm{Cu}({\mathrm{C}}_{4}{\mathrm{H}}_{4}{\mathrm{N}}_{2}){(\mathrm{N}{\mathrm{O}}_{3})}_{2}$. Zero field muon-spin relaxation measurements made at dilution refrigerator temperatures show oscillations in the measured asymmetry characteristic of a quasistatic magnetic field at the muon sites. Our measurements provide convincing evidence for long-range magnetic order below a temperature ${T}_{\mathrm{N}}=107(1)\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. This leads to an estimate of the interchain coupling constant of $\ensuremath{\mid}{J}^{\ensuremath{'}}\ensuremath{\mid}∕{k}_{\mathrm{B}}=0.046\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and to a ratio of $\ensuremath{\mid}{J}^{\ensuremath{'}}∕J\ensuremath{\mid}=4.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$.

μSR in two-dimensional hydrogen bonding system squaric acid

Squaric acid, a famous two-dimensional hydrogen-bonded system was studied by μSR. From the temperature dependence of the muon spin relaxation rate as well as from the crystal axis dependences, different muon sites at low temperature (10 K) and at high temperature (300 K) were found. The nuclear dipole field was calculated for possible muon sites. At low temperature the muon attaches C O … HO acceptor oxygen, while at 300 K the muon occupies the regular hydrogen sites.

A [formula omitted]SR study of the unusual magnetism of [formula omitted

We present the results of low longitudinal field μ SR experiments in the intermetallic magnetic system LaCrSb 3 . In the low field phase ( < 250 G ) , we find evidence of at least two magnetically inequivalent muon sites characterized by large static internal fields which are not changed appreciably by the spin-reorientation transition at about 90 K. Other characteristics of the muon spin relaxation function that do reflect the reorientation are discussed.

Magnetic clusters in superconducting lightly doped [formula omitted

We report μ SR experiments on two lightly doped superconducting YBa 2 Cu 3 O 6 + x samples ( 0.39 ⩽ x ⩽ 0.41 ) , performed with the initial muon spin polarization rotated by 47 ∘ from the applied field direction. At low temperature, the muon asymmetry of both samples exhibits a dominant component with very large depolarization rates, due to the presence of frozen magnetic clusters. A two-component model, accounting for muon sites close to the clusters and far from them, respectively, is presented. The model provides a prediction for the time evolution of both the longitudinal and transverse projections of the muon polarization, and yields a good simultaneous fit to the data of each sample at all the applied fields. The best-fit parameters demonstrate that the static internal fields from the magnetic clusters are disordered, in agreement with the absence of long-range magnetic Bragg peaks in neutron diffraction.

2D Kagomé ordering in the 3D frustrated spinelLi2Mn2O4

Muon spin relaxation (μSR) experiments on the geometrically frustrated spinel oxide,Li2Mn2O4, show the development of spin correlations over a range oflengthscales with decreasing temperature. Increased relaxation below∼150 K isconsistent with the onset of spin correlations. Below 50 K, spin order, on a lengthscale which is long rangefor the μSR probe, appears abruptly in temperature, consistent with prior neutron diffraction results.The oscillations in the zero field asymmetry are analysed using a three frequency model. Bylocating the muon site, this is shown to be consistent with the unexpected 2D structure on the Kagomé planes proposed originally from neutron data. Longitudinal fielddata demonstrate that some spin dynamics persist even at 2 K. Thus, a very complexmagnetic ground state, featuring the coexistence of long lengthscale 2D ordering andsignificant spin dynamics, is proposed. This is unusual considering the 3D topology of theMn3+ spins in this material.

Muons as a probe of magnetism in molecule-based low dimensional magnets

We present the results of muon spin relaxation(μ+SR) studies on low dimensional molecular magnet systems.μ+SR measurements have been carried out on the Cu-based chain compoundsCuX2(pyz) (whereX = Br, Cl,NCS and pyz = pyrazine) as a function of temperature and applied longitudinal magnetic field.Oscillations in the time dependence of the muon polarization, characteristicof magnetic order at two distinct muon sites, are detected in bothCuBr2(pyz) (belowTN = 3.6(1) K)and CuCl2(pyz) (below TN = 3.2(2) K). No evidence of magnetic order is observed inCu(NCS)2(pyz) down to 0.35 K. The results are discussed in terms of the estimatedCu–X–Cu and Cu–(pyz)–Cu exchange constants. The theory ofμ+SR in high spin molecule (HSM) systems, which are effectivelyzero-dimensional magnets, is discussed and results are presented on[Ni12(chp)12(O2CMe)12(H2O)6(THF)6] (S = 12), [Mn9O7(OAc)11(thme)(py)3(H2O)2] (S = 17/2)and [Fe14(bta)6(O)6(OMe)18Cl6] (). Measurements made in applied longitudinal magnetic fields on HSM materials atdilution refrigerator temperatures strongly suggest that dynamic local magnetic fieldfluctuations are responsible for the relaxation of the muon spin ensemble. Trends intemperature and field dependent behaviour in these systems, as probed by the muon, arediscussed.

Observation of the spin-crossover in [Fe(btr) 2(NCS) 2] · H 2O (btr = 4,4′-bis-1,2,4-triazole) with μSR

The electronic spin transition in [Fe(btr) 2(NCS) 2] · H 2O (btr = 4,4′-bis-1,2,4-triazole) is probed by longitudinal field muon spin relaxation measurements. The μSR spectra could be analyzed with two different components, corresponding to two muon sites. For both components, an increase in magnetic fluctuations is observed at the LS → HS transition, i.e. crossing the spin transition temperature coming from low temperature, if a sufficiently high field is applied. Even below the transition temperature, magnetic fluctuations are still found. For one of the two muon sites, muonium formation is clearly found below the spin-crossover temperature.

Antiferromagnetic properties of a water-vapor-insertedYBa2Cu3O6.5compound studied by NMR, NQR, andμSR

We present a detailed NQR, NMR and $\mu $SR study of a magnetic phase obtained during a topotactic chemical reaction of YBa$_{2}$Cu$_{3}$O$_{6.5}$ high- temperature superconductor with low-pressure water vapor. Our studies give straightforward evidence that the ''empty'' Cu(1) chains play the role of an easy water insertion channel. It is shown that the NQR spectrum of the starting material transforms progressively under insertion of water, and completely disappears when one H$_{2}$O molecule is inserted per unit cell. Similarly, a Cu ZFNMR signal characteristic of this water inserted material appears and grows with increasing water content, which indicates that the products of the reaction are non-superconducting antiferromagnetic phases in which the bilayers are ordered. These antiferromagnetic phases are felt by proton NMR which reveals two sites with static internal fields of 150 and about 15 Gauss respectively. Two muon sites are also evidenced with similar local fields which vanish at $T\approx 400$ K. This indicates that the magnetic phases have similar N\'{e}el temperatures as the other bilayer undoped compounds. An analysis of the internal fields on different sites of the structure suggests that they can be all assigned to a single magnetic phase at large water content in which the Cu(1) electron spins order with those of the Cu(2). It appears that even samples packed in Stycast epoxy resin heated moderately at a temperature (200$^{0}$C) undergo a reaction with epoxy decomposition products which yield the formation of the same final compound. It is then quite clear that such effects should be considered quite seriously and avoided in experiments attempting to resolve tiny effects in such materials, as those performed in some recent neutron scattering experiments.

NMR and $mu;SR investigation of spin dynamics in $lcurlb;Mo72Fe30$rcurlb; molecular clusters

We present μ+SR and proton NMR data on the molecular cluster {Mo72Fe30} in its standard form (std-Fe30) and in its layered form (lay-Fe30) where the quasi-spherical molecules form a grid. μ+SR experiments on std-Fe30 as a function of the magnetic field at T≈30 mK showed that the muon asymmetry has two components, corresponding to different muon sites, near and far from the magnetic ions. At the level crossing we observed an onset of local fields but no anomaly in the relaxation rate. Also a small effect of the spin topology was observed by means of 1H NMR.

NMR and μSR investigation of spin dynamics in {Mo72Fe30} molecular clusters

We present μ + SR and proton NMR data on the molecular cluster {Mo 72 Fe 30 } in its standard form (std-Fe30) and in its layered form (lay-Fe30) where the quasi-spherical molecules form a grid. μ + SR experiments on std-Fe30 as a function of the magnetic field at T ≈30 mK showed that the muon asymmetry has two components, corresponding to different muon sites, near and far from the magnetic ions. At the level crossing we observed an onset of local fields but no anomaly in the relaxation rate. Also a small effect of the spin topology was observed by means of 1 H NMR.

Two hybrid organometallic-inorganic layered magnets from the series [ ZIIICp*2] [ MIIMIII(ox)3] studied with μ+SR

We present zero-field muon spin relaxation (ZF-μ + SR) measurements on two examples of a new series of hybrid organometallic-inorganic layered magnets, namely ferromagnetic [FeCp* 2 ][MnCr(ox) 3 ] and ferrimagnetic [CoCp* 2 ][FeFe(ox) 3 ] (where ox = oxalate and Cp* = pentame- thyl-cyclopentadienyl). Both materials show multi-component muon spin precession signals characteristic of quasistatic magnetic fields at several distinct muon sites. The temperature dependence of the precession frequencies allow critical exponents to be extracted. Possible muon sites are discussed on the basis of dipole field calculations.

Magnetic order and local field distribution in the hybrid magnets [FeCp*2][MnCr(ox)3] and [CoCp*2][FeFe(ox)3]: a muon spin relaxation study

Zero-field muon spin relaxation (μ+SR) measurements on materials from the series [ZIIICp*2][MIIMIII(ox)3] show precession signals at several frequencies, characteristic of quasistatic magnetic fields at up to three distinct muon sites.

A μ+SR study of the rareearth antiferromagnet PrO2

We present zero-field muon spin relaxation(μ+SR) measurements on the rare earth antiferromagnetPrO2. Oscillations in the time dependence of the muon polarization, characteristicof a quasistatic magnetic field at two distinct muon sites, are observed belowTN, along with relaxation dominated by the magnitude of the local field. Candidate muon sites inthe material are suggested on the strength of dipole field calculations. Relaxation aboveTN allows a prediction of the quasielastic neutron scattering linewidth, which is discussed in termsof a simple model of magnetoelastic relaxation involving a crystal field state at an energyΔ above the ground level. The obtained value ofΔ may be taken as an approximate estimate of the splitting of theΓ8 groundstate of Pr4+ in the distorted fluorite structure.

Magnetic ordering and spin dynamics in the quasi-one-dimensionalspin−12chains ofCuNb2O6observed by muon spin rotation and relaxation

Muon spin rotation/relaxation (μ + SR) has been measured in the quasi-one-dimensional zig-zag spin chain compounds α-CuNb 2 O 6 and β-CuNb 2 O 6 . Spontaneous precession frequencies f 1 and f 2 observed at two muon sites in zero-field μ + SR demonstrate antiferromagnetic ordering in β-CuNb 2 O 6 with a Neel temperature of 7.22 K. The frequencies give a measure of the temperature dependence of the intrachain field (αf 2 ) and the interchain field (Αf 1 ). The temperature dependence of f 1 suggests a ferromagnetic intrachain coupling J in the β phase. The interchain coupling J' that is responsible for the three-dimensional ordering is found to be 3.1 K. Using the values of J and J', we estimate the magnetic moment of Cu to be 0.31μ B . Muon longitudinal spin relaxation rate λ L F exhibits stretched exponential behavior in α-CuNb 2 O 6 in the temperature range of 150 to 2 K. λ L F increases sharply below 8 K, and the relaxation function becomes nearly root exponential at 4 K. These features show that the spin dynamics is very similar to that observed in a dilute spin-glass near its spin freezing temperature. The glasslike dynamics, which coexists with the spin-singlet state in the α phase, has been attributed to a small fraction of Cu ions, which become bond impurities of spin-½, due to defects in the spin chains.

μSR studies of organic and molecular magnets

Muon-spin rotation and relaxation (μSR) experiments have been performed on a variety of novel organic and molecular magnetic systems. In these experiments, implanted muons are used to study the magnitude, distribution and dynamics of the local field at the muon site. Calculations of the spatial dependence of the dipole-field inside the unit cell are used to interpret the data and determine the muon site in certain cases. We describe and review muon experiments on nitronyl nitroxide organic ferromagnets and antiferromagnets. We discuss a muon study of the spin crossover phenomenon which has been studied in Fe(PM-PEA)2(NCS)2, and which shows Gaussian and root-exponential muon relaxation in the high-spin and low-spin phases, respectively. The effects of high temperature annealing on TDAE-C60 have also been studied with μSR. Experiments on a disc-shaped molecular complex containing Fe19 (with spin 31/2) reveal the effects of fluctuations of magnetization and allow an estimate of the fluctuation rate. These experiments demonstrate the wide range of problems which can be tackled using the μSR technique.

μSR studies of magnetic superconductors based on the BETS molecule

Muon spin rotation and relaxation measurements have been made on the molecular magnetic superconductors κ-BETS2FeCl4 and κ-BETS2FeBr4 and the non-magnetic molecular metals κ-BETS2GaCl4 and λ-BETS2GaCl4. In the magnetic materials, zero field muon spin relaxation signals show the formation of static antiferromagnetically ordered states and multiple precession frequencies are observed, corresponding to muon sites situated both within the anion layers and within the BETS layers. Studies of the relaxation behavior in the FeCl4 salt have previously shown significant changes around its superconducting transition and the FeBr4 salt reveals similar changes in the region of its superconducting transition, whose onset is around 1.5 K. In the non-magnetic GaCl4 salts the formation of a superconducting vortex lattice was observed and the penetration depth λ was derived from the transverse field muon relaxation. When the BETS superconductors are compared with the ET-based superconductors that we have already studied using μSR, a striking correlation was observed between Tc and λ−3.

μSR study on multi-layered HgBa2Ca4Cu5Oy (Hg-1245) superconductor

We have carried out zero-field muon spin relaxation (ZF-μSR) measurements in multi-layered HgBa2Ca4Cu5Oy (Hg-1245) superconductor with Tc of 108 K. The variation of ZF-μSR time spectra from Gaussian-type to exponential-type behavior was observed with decreasing temperature below 60 K and the muon precessions were also observed below 45 K. These results indicate the appearance of a static magnetic order of Cu moments at muon sites below Tc. Compared with the results of NMR measurements, they suggest that this magnetic order is caused in three under-doped inner planes of five CuO2 planes in Hg-1245.

μSR studies of the interaction between magnetism and superconductivity in κ-BETS 2FeCl 4

Zero field (ZF) muon spin relaxation measurements on the organic metal κ-BETS 2FeCl 4 clearly show the formation of an antiferromagnetically ordered state below T N=0.45 K. The magnetic order remains unperturbed on cooling through the superconducting transition T c ∼0.17 K, providing unambiguous evidence for the coexistence of antiferromagnetic order and superconductivity in this system. Detailed studies of the internal field distribution show contrasting behavior for different muon sites and provide experimental evidence for an ordered spin structure in the BETS layers in addition to that taking place in the Fe layers.