Magnetic structure and magnetic excitations in the two-dimensional spin system of the Cu<Subscript>3</Subscript>B<Subscript>2</Subscript>O<Subscript>6</Subscript> compound

This paper reports on the results of measurements of the magnetic susceptibility, heat capacity, neutron scattering, muon spin relaxation, and electron paramagnetic resonance in Cu₃B₂O₆ for the study of the ground state of the spin system of this compound. The results obtained suggest that, at a temperature of 10 K, the spin subsystem of the crystal, which consists of single spins and clusters of pairs and fours of spins interacting with one another, undergoes a transition to a state representing a superposition of the singlet (for clusters) and magnetically ordered (for single spins) states.     

Anomalously strong relaxation of the polarization of muons in the magnetically ordered and paramagnetic states of the TbMnO<Subscript>3</Subscript> multiferroic

An anomalously strong relaxation of the muon polarization in a magnetically ordered state in the TbMnO₃ multiferroic has been revealed by the method below the μSR Néel temperature (42 K). Such a relaxation is due to the muon channel of relaxation of the polarization and the interaction of the magnetic moment of the muon with inhomogeneities of the internal magnetic field of an ordered state in the form of a cycloid. Above the Néel temperature, beginning with temperatures depending on the applied magnetic field, a two-phase state has been revealed where one phase has an anomalously strong relaxation of the muon polarization for a paramagnetic state. These features of the paramagnetic state are due to short-range magnetic order domains that appear in strongly frustrated TbMnO₃. A true paramagnetic state has been observed only at T ≥ 150 K.     

Calculation of the muon polarization function for longitudinal fields

Fields for a muon interacting with a single nucleus is presented. The results demonstrate quantitatively the phenomenon of the level-crossing resonance between the muon Zeeman splitting and the induced quadrupolar coupling of neighboring nuclei. Approximate numerical solutions are given for the cases of a muon interacting with four (tetrahedral site) or six (octahedral site) Cu nuclei.     

Muon diffusion and level crossing in copper

We have studied the quantum diffusion of positive muons in pure copper over the temperature range 12 mK≤T≤150 K using weak longitudinal field μSR. Below 150 K, this technique has proved to be the most sensitive to the muon hop rate. Our final results for the behaviour of the muon hop rate are well explained within the framework of theories for the quantum diffusion of light interstitials in metals of Kondo, Yamada and others. In addition, the use of level-crossing resonance has allowed us to measure the electric quadrupole interaction strength (and sign) of the copper nuclei, ω_Q= −3.314(7) μS⁻¹. These results have enabled us to show that the muon occupies the same octahedral site at all the temperatures studied, ruling out the possibility of metastable muon sites contributing to any significant portion of the muon polarization.     

Photoacoustic detection of ferro and paramagnetic resonance

A commercial ESR spectrometer was modified to detect the photoacoustic signal of modulated magnetic resonance absorption, operating at room temperature and at atmospheric pressure. Both magnetically ordered materials (YIG and FeBO₃) and paramagnetic substances were investigated. The measurements include various phenomena of ferromagnetic resonance: uniform precession, magnetostatic modes and spin-wave instabilities. Paramagnetic resonance was observed in the concentrated system MnF₂ and in the diluted system ZnS : Mn (1%), with well resolved hyperfine splitting.     

Static and dynamic properties of R(Rh1−x Ru x )2Si2 probed by μSR (R=U,Nd,Ce)

Muon spin relaxation experiments have been carried out in the paramagnetic and magnetically ordered states of URh₂Si₂, U(Rh_0.35Ru_0.65)₂Si₂, NdRh₂Si₂ and CeRh₂Si₂. In order to obtain information on the localisation and diffusion properties of the muon, some measurements have been performed also on isostructural diamagnetic compounds. From our measurements. information on the magneto-crystalline anisotropy of the samples has been obtained. Depending on the compound, we found a static field distribution below the Néel temperature. We discuss the implications of this result on the magnetic properties of the various materials.     

Resolved nuclear hyperfine structure of muonium in CuCl by means of muon level-crossing resonance

Datailed muon level-crossing resonance measurements of Mu¹ and Mu¹¹ centres in single crystals of CuCl are presented. The hyperfine and nuclear hyperfine parameters of the closest two shells of nuclei are remarkably similar for the two centres, indicating that both are located at the same tetrahedral interstitial site with four Cu nearest neighbours and six Cl next-nearest neighbours. About 30% of the total unpaired-electron spin density is located on the muon, about 60% on the four nearest neighbours and the rest on the six next-nearest neighbours, with nothing observable for any other shell.     

Electronic structure, dynamics, and metastability of muonium in semiconductors

Muonium centers are light hydrogen-like centers formed when positive muons are stopped in crystalline semiconductors. Detailed information on the hyperfine structure, dynamics and metastability of muonium are obtained using a combination of muon spin rotation or relaxation, muon level-crossing resonance and related methods. The expected close similarity to hydrogen, especially with regard to electronic structure, is important since the equivalent information on isolated hydrogen is either less detailed or completely absent. There are also interesting differences between muonium and hydrogen. In particular muonium dynamics are expected to exhibit enhanced quantum mechanical effects since the muon has only 1/9th the proton mass. In this paper we review the current status of experiments.     

Observation of muon level-crossing resonance in antiferromagnetic MnF2

We report the observation of μ⁺ Level-Crossing Resonances (LCR's) in the ordered phase of an antiferromagnetic material. Two LCR's were observed in MnF₂ as a function of longitudinal magnetic field in the temperature range between 10K and 65K. Both are attributed to a muon in an interstitial octahedral-like site. The low field resonance is attributed to a muon-nuclear spin flip-flop transition involving the two nearest neighbour¹⁹F nuclei. The high field resonance occurs when the applied field cancels the local hyperfine field on the muon. The positions and widths of the LCR's were seen to scale with the sublattice magnetization.     

Magnetic density of states at low energy in geometrically frustrated systems

Using muon-spin-relaxation measurements we show that the pyrochlore compound Gd(2)Ti(2)O(7), in its magnetically ordered phase below approximately 1 K, displays persistent spin dynamics down to temperatures as low as 20 mK. The characteristics of the induced muon relaxation can be accounted for by a scattering process involving two magnetic excitations, with a density of states characterized by an upturn at low energy and a small gap depending linearly on the temperature. We propose that such a density of states is a generic feature of geometrically frustrated magnetic materials.     

Magnetic hyperfine fields on119Sn nuclei in uranium compounds

119Sn Mössbauer spectroscopy studies were performed on 12 uranium intermetallic compounds in order to investigate correlations between the formation of the magnetic moment on the U atom and the magnetic hyperfine field transferred to 119Sn nuclei in magnetically ordered materials. The measured hyperfine fields (H_hf) are related to the values of the ordered U magnetic moments (UB) by U_oH_hf/n=A UU. The parameter A varies between 0.73 (UGa2) and 1.55 (UGe2). It seems to correlate with the extent of the hybridization of the 5f states with the conduction electron states.     

Specific MARY spectrum from radical anion of pentafluorobenzene

The paper presents the first resolved experimental magnetically affected reaction yield (MARY) spectrum for a system with nonequivalent nuclei, radical anion of pentafluorobenzene. This observation dispels the common apprehension that because of a rather involved energy level layout a system with not all nuclei magnetically equivalent cannot produce resolved MARY lines in nonzero fields, and greatly, increases the practical scope of level-crossing techniques for studies of spin-correlated radical pairs. The experimental finding is supported by schemes of energy levels calculated for this system.     

A comparative study of the magnetic heavy-electron materials U2Zn17 and UCu5 by μ+SR

A systematic μ⁺SR study of the magnetic heavy-electron systems U₂Zn₁₇ and UCu₅ in the paramagnetic and in the magnetically ordered state is presented. In both systems the antiferromagnetic nature of the low-temperature phase could be at least partially confirmed, but the muon reveals significant differences with regard to the phase transition itself. UCu₅ behaves like a model-antiferromagnet showing a drastic increase of the relaxation rate both below and above T_N, two spontaneous frequencies in the ordered phase, and a Knight shift above T_N which scales with the bulk susceptibility. In contrast U₂Zn₁₇ shows a loss of μ⁺ asymmetry by 20% below T_N, which is independent of the external field but can be quenched in sufficiently strong longitudinal fields. No scaling of the Knight shift and the susceptibility was observed and no critical increase of the relaxation rate λ. Most astonishing is the strong and nonlinear field dependence of λ above and below T_N in both compounds. The absence of longitudinal relaxation demonstrates the static origin of λ.     

Delayed formation of muon-containing species in organic liquids observed by spin resonance technique

The resonance technique has been applied to observe diamagnetic muons and, for the first time by the resonance, Mu-substituted radicals in organic liquids under strong decoupling magnetic fields. In benzoquinone solutions in benzene the relaxation of Mu-cyclohexadienyl radicals through reaction with quinone was directly observed by the radical resonance technique. The product of this reaction was then observed by the diamagnetic muon resonance as a slow formation. Similar slow formation was observed for diamagnetic muons in neat CS₂ and for Mu-radicals in benzene and styrene. Such slow formation can never be observed by the rotation technique due to dephasing problem, and thus the previous method is expected to provide new source of information on slow reaction dynamics of muon containing species.     

μ+SR study of LaNi2As2, URh2Si2 and CeRh2Si2

Muon spin relaxation experiments have been carried out in the paramagnetic and magnetically ordered states of URh₂Si₂ and CeRh₂Si₂. As the magnetic structure of these compounds is well known, these measurements can help to characterise their magnetic properties probed by μSR and to understand the μSR results of the heavy fermion compounds of the same crystallographic family. Our measurements show that the muons occupy two different crystallographic sites. The spectra of URh₂Si₂ and CeRh₂Si₂ in the magnetically ordered states are very different, probably reflecting their different magnetic structures. The spectra obtained on CeRh₂Si₂ are similar to the published spectra of the heavy fermion compound CeCu_2.1 Si₂. Muon spin rotation measurements on LaNi₂As₂ indicate that the muon is diffusing at 150 K.     

Nuclear and muon hyperfine techniques in the study of heavy-fermion superconductors

Nuclear magnetic resonance and relaxation, and the related techniques of muon spin rotation and relaxation, have been used to study local spin polarization and quasiparticle excitations in the heavy-fermion superconductors CeCu₂Si₂, pure and thoriated UBe₁₃, and UPt₃. Measurements of nuclear and positive muon Knight shifts, linewidths, and spinlattice relaxation rates give some hints as to the nature of Cooper pairing in these exotic materials.     

Effect of external pressure on the magnetic properties of RCoAsO (R=La,Pr, Sm): a μSR study

We report on a detailed investigation of the itinerant ferromagnets LaCoAsO, PrCoAsO and SmCoAsO performed by means of muon spin spectroscopy upon the application of external hydrostatic pressures p up to 2.4 GPa. These materials are shown to be magnetically hard in view of the weak dependence of both critical temperatures T_C and internal fields at the muon site on p. In the cases R=La and Sm, the behaviour of the internal field is substantially unaltered up to p=2.4 GPa. A much richer phenomenology is detected in PrCoAsO instead, possibly associated with a strong p dependence of the statistical population of the two different crystallographic sites for the muon. Surprisingly, results are notably different from what is observed in the case of the isostructural compounds RCoPO, where the full As/P substitution is leading to smaller values of unit cell volume already at ambient pressure but p is still very effective in further affecting the magnetic properties.     

NMR investigation of monodomain formation in a weak ferromagnet FeBO3

Process of monodomain formation in FeBO₃ antiferromagnet in the static magnetic field applied in the magnetically easy plane is studied by NMR technique at the temperatures 4.2 and 77 K. It is found that the⁵⁷Fe NMR signal splits into three absorption lines. Dependence of the intensities and the resonance frequency changes of the “central” and of the “side” signals on the static field amplitude is investigated. It is shown that the observed effects are determined both by layered character of domain structure and weak ferromagnetism of iron borate. Possible utilizations of method (based on the NMR frequency differences of nuclei in different domains in the static magnetic field) for investigation of the domain structure character and the remagnetization dynamics of magnetically ordered materials are discussed.     

Magnetic properties of crystalline and amorphous DyAg from μSR measurements

Muon spin relaxation in crystalline and amorphous DyAg has been studied in the paramagnetic and magnetically ordered regimes. In cr-DyAg the critical slowing down of the Dy³⁺ spin fluctuation is seen on approaching T_N = 60 K. In the ordered state we observe a Lorentzian field distribution. The muon spin relaxation in am-DyAg approaches at lower temperatures a root exponential law usually encountered in spin glasses.     

Influence of doping admixture and anisotropic tension on behaviour of muon spin polarization in insulators

Zero‐field μSR measurements have been carried out in the paramagnetic and magnetically ordered states of the heavy fermion alloys: Ce_1-xLa_xRu₂Si₂ (x=0.05,0.13,0.25). In Ce_1-xLa_xRu₂Si₂ system, T_K drastically decreases with x from 24 K for x=0 and becomes very close to the magnetic ordering temperature T_N. As functions of T_K and T_N, the magnetic instabilities were studied in this system. For x=0.05,\ 0.13\ \mboxand\ 0.25 samples, longitudinal muon spin relaxation was measured to study non‐magnetic to magnetic transition and the change from itinerant to localized electron state with increasing La impurities. We discuss the dynamics of muon spins in the paramagnetic and antiferromagnetic state. This revised version was published online in July 2006 with corrections to the Cover Date.