μSR Spectra Research Articles

Evolution of magnetism and its interplay with superconductivity in heavy-fermion UPt 3 doped with Pd

The pseudobinary series U(Pt 1− x Pd x ) 3 demonstrates a wealth of magnetic and superconducting properties that are exemplary for heavy-fermion physics. In this paper, I present a survey of recent neutron-diffraction and μSR experiments, conducted to study the evolution of magnetism, and its interplay with superconductivity, in UPt 3 doped with Pd. The small-moment antiferromagnetic order (SMAF) with T N∼6 K reported for pure UPt 3, is robust upon alloying till at least x=0.005. The small ordered moment m( x) grows from 0.018 μ B/U-atom for x=0.00 to 0.048 μ B/U-atom for x=0.005. T N of the SMAF phase does not vary with Pd content. The increase of m( x) correlates with the splitting Δ T c of the superconducting transition and provides evidence for a Ginzburg–Landau scenario for unconventional superconductivity with magnetism as symmetry-breaking field. The absence of a signal of the SMAF phase in zero-field μSR spectra provides strong evidence for a moment fluctuating at a rate >10 MHz. A second large-moment antiferromagnetic phase (LMAF) is found at higher Pd concentrations. For this phase, at optimum doping ( x=0.05) T N,max=5.8 K and m=0.62 μ B/U-atom. The critical Pd concentration for the emergence of the LMAF phase is x c,af∼0.006. At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity ( x c,af= x c,sc≈0.006), yields evidence for odd-parity superconductivity mediated by ferromagnetic spin-fluctuations.

μSR spectroscopy of the Kondo insulators Lu1−xYbxB12

Single crystals of Yb1−xLuxB12(x=0,0.125,0.5,1) were measured between 1.8 and 300K. We found similar spectral shapes in all compounds studied over the whole temperature range scanned. In zero field the spectra alter their appearance around 20, 100 and 150K. In a longitudinal field of 100G, which largely suppresses the contribution from 11B nuclear moments the relaxation rate remained constant up to ∼150K where it suddenly peaks. These findings exclude magnetic correlations as the origin of muon spin relaxational behavior. The dominant features of the μSR spectra at low temperatures arise from lattice dynamics, probably involving the B12 clusters, rather than from spin dynamics. Higher temperature features are most likely muon related.

Magnetic ordering process in diluted Ising spin nets, Rb 2Co cMg 1− cF 4, probed by muon spin relaxation

Muon spin relaxation was measured on Rb 2Co c Mg 1− c F 4 to study magnetic ordering and spin fluctuations in diluted Ising spin nets with concentration near the percolation threshold, c p=0.593. In a c=0.6 compound, the μSR spectrum shows a distinct anomaly due to the long-range ordering of the fractal lattice at 31 K much higher than T N =20 K . Comparing with the results for a c=0.5 compound, the observed spectra for the c=0.6 compound are analysed in terms of the spin fluctuations of the fractal lattice and the isolated spin clusters which are frozen gradually below 15 K.

μ–H Interaction in Hydrogen Bonding Systems

Muon spin rotation (μSR) experiments were performed on single crystal samples of KH2PO4(KDP) and KD2PO4(dKDP) to study the dynamics of hydrogen in hydrogen bonding systems. At low temperature, the nuclear dipole interaction of muon and proton was confirmed from the angular dependence of precession frequency of the muon spin under zero magnetic field. The muon occupation site was also determined. A clear change in μSR spectra was observed at the antiferroelectric transition temperature (123 K). At 90 K well below the transition temperature, the muon spin starts to relax, possibly due to muon dynamics.

The Magnetism of Frustrated RFe6Al6 Compounds Studied by μSR and Mössbauer Spectroscopy

μSR studies on the RFe6Al6 (R = Tb, Ho, Er) ferrimagnets (TN≈340 K) show that the presence of frustration by competing exchange causes a slow approach of the ordered spins towards their static limit. This explains the non-Brillouin-like behavior of the magnetic Bragg peak intensities seen in neutron diffraction. Furthermore, considerable local spin disorder is found, despite the existence of long-range order. In YFe6Al6, having no moments on the normally strongly-magnetic R sublattice, neutron diffraction was unable to detect any magnetic Bragg peaks. The μSR and Mossbauer spectra however, clearly reveal a magnetic transition near 340 K, but not all of the Fe ions order at this point. The Fe ions on the mixed Fe-Al sublattice (8j) order first, while the pure Fe sublattice (8f) orders only below ∼70 K. The Fe ions on this site must be inherently frustrated which is also reflected in distributions of moment size and orientation, short correlation lengths and strong spin fluctuations.

μSR Study of the Unusual Magnetic Ordering in the Frustrated Antiferromagnet Zn(Cr x Ga 1−x ) 2 O 4

μSR spectra on the spin frustrating spinel antiferromagnet Zn(CrxGa1−x)2O4 (x=0.9,1.0) have been measured. For x=1.0 compound, both the relaxation rate and the initial asymmetry showed distinct anomalies at the Neel temperature. The magnetic susceptibility for the x=0.9 compound was known to have a faint peak at around 12 K, whose origin was not clear so far. Our μSR study revealed that this temperature is the onset temperature of development of the magnetic correlation accompanied by appreciable spin fluctuations.

Magnetic ordering in La 2Cu 2O 5

Muon spin relaxation (μSR) measurements were carried out on La 2Cu 2O 5, which is a candidate for the spin 1 2 4-leg Heisenberg antiferromagnetic ladder system. At room temperature, the μSR spectrum can be fitted with the static Gaussian Kubo–Toyabe function. This spectrum originates from static nuclear dipole fields of copper. At ∼140 K , a sudden decrease of the initial asymmetry was observed. This suggests that a long-range magnetic ordering of the Cu 2+ spins occurs due to nonnegligible interladder interactions.

μSR study of YMn 12

The Mn ions occupy three different sites (8i, 8j, 8f ) in equal population in YMn 12, which has the tetragonal ThMn 12 structure with space group I4/mmm. They possess a different magnetic moment at the 8f site (0.14 μ B) compared to the 8i and 8j sites (0.42 μ B). The compound orders antiferromagnetically with T N ≈100 K . Above T N the muon-spin relaxation is dominated by the nuclear dipole fields of 55Mn. The influence of the electronic moments on Mn is only weakly felt near T N when critical slowing down occurs. Below T N, no spontaneous spin precession is observed and the zero-field spectra can be described by a Lorentzian Kubo–Toyabe pattern. This shows that the muon must occupy an interstitial site where all contributions to the local field cancel for the antiferromagnetic spin structure. Longitudinal field measurements demonstrate that the spectra are essentially static in nature. It is remarkable that the different Mn sites with their different Mn moments do not show in the μSR spectra. The likely muon stopping sites are discussed.

Magnetic ordering and fluctuations in Ising spin nets Rb 2Co 1− xMg xF 4

We measured muon spin relaxation in single crystals of a two-dimensional Ising spin antiferromagnet Rb 2CoF 4 and magnetically diluted compound Rb 2Co 0.6Mg 0.4F 4 just above the percolation limit ( x=0.407). The zero-field μSR spectra show oscillation above their transition temperatures and were interpreted by a three spin model for F −–μ +–F − bond. This model allows us to identify the μ + sites but also reveals a sizable disturbance to the lattice by μ +. The temperature dependence of the μSR spectra in a longitudinal field of 30 mT shows T N at 102.7 K for Rb 2CoF 4 in excellent agreement with the reported T N=102.9 K, whereas at 31 K for Rb 2Co 0.6Mg 0.4F 4, to be compared with the reported T N=20 K. Both the static fields from the ordered percolating net and the fluctuating fields from spin clusters relax the muon polarization in Rb 2Co 0.6Mg 0.4F 4 below T N.

Spin dynamics in the concentrated spin-glass system Y(Al 1− xFe x) 2

The zero-field μSR spectra obtained from Y(Al 1− x Fe x ) 2 alloys at concentrations below x<0.75 are stretched exponential in form and characteristic of those of a concentrated spin glass in which an increasingly broad distribution of spin relaxation rates evolves as the glass temperature is approached. These μSR results are discussed within the context of recent high-field Mössbauer effect measurements on Y(Al 1− x Fe x ) 2 alloys from which estimates of the lifetime of magnetic correlations have been obtained.

Formula omitted] study of a spin-ladder system [formula omitted]-diazacycloheptane [formula omitted

We measured the muon spin relaxation ( μSR ) for Zn doped spin-ladder compounds ( Cu 1−x Zn x) 2(1,4 -diazacycloheptane ) 2 Cl 4 with x=0 and 0.1 in the temperature region from 2 to 150 K . In both of x=0 and 0.1 samples, only a slight enhancement of the relaxation is observed at low temperatures. This result indicates that both samples exhibit no magnetic order. Besides, we have not observed significant difference of the μSR spectra in both samples. This fact indicates that the Cu 2+ spins induced by the impurity fluctuate fast.

Chemical pressure effects in the Yb(Cu 1− xNi x) 2Si 2 system

μSR results on the Yb(Cu 1− x Ni x ) 2Si 2 system are reported. The chemical pressure obtained by gradual substitution of Cu by Ni changes the nonmagnetic ground state of YbCu 2Si 2 into the magnetic one of YbNi 2Si 2 ( T N=2.1 K). For Ni concentrations x lower than 0.125, no magnetic transition could be resolved in the investigated temperature range of 0.02 –300 K. For 0.125⩽ x⩽1 magnetic correlations were observed at temperatures T⩽ T corr with 0.7 K⩽ T corr( x)⩽2.1 K via a fast-relaxing Gaussian signal reflecting a broad internal field distribution at the muon site. The magnetic correlations develop in an x dependent fraction of the sample. This behaviour is discussed in the frame of the Jaccarino–Walker model assuming that the Yb ions are magnetic if they have at least two Ni ions as nearest neighbours. The fits of the zero field μSR spectra for x>0 show a two-channel relaxation, one due to a nuclear static field distribution and the other due to the fluctuations of the Yb moments.

Location of the superconducting hole condensate in Sr 2YRu 1− uCu uO 6 by μ +SR

The magnetic and superconducting characteristics of sintered Sr 2YRu 1− u Cu u O 6 (for u=0.05, 0.10, 0.15) were probed using transverse- and zero-field muon-spin rotation (μ +SR). Since positive muons are attracted to oxygen ions in the high- T C oxides, Sr 2YRu 1− u Cu u O 6 should (and does) present two types of μ + sites, one in the YRuO 4 layers and the other in the SrO layers. The transverse- and zero-field μSR spectra for all three stoichiometries exhibit magnetic ordering at and below T N ≈ 30 K, with a static local field of ∼3 kG. This transition is marked by a dramatic increase in the μ + spin relaxation rate as the temperature decreases below T N, corresponding to an increased static disordering of the magnetic moments. Above T N no static fields are observed. Instead, the data exhibit a slow dynamic depolarization, presumably due to rapid fluctuation of magnetic moments. Both transverse- and zero-field data also indicate a smaller second component (∼10%) that we associate with the SrO layer, exhibiting superconducting behavior in transverse field with an observed bulk T C≈ T N∼30 K.

Investigation of the effect of electric current on the dynamics of flux-line lattice

We investigate the microscopic and dynamic properties of the flux-line lattice (FLL) in the single crystals of 2H–NbSe 2 and Nb under a pulsed applied electric current, by pulsed μSR technique. Recent theory predicts a dynamic phase transition of the FLL under an applied current from “moving flux crystal” to “incoherent fluidlike flux motion” in a mixed state of a type-II superconductor. We have not observed any significant effect of flux-line lattice flow for μSR spectra under the pulsed electric current. We discuss the motion of flux-line lattice under an external current.

μSR study of anomalous phase transition in UCu 2Sn

The phase transition in the ternary uranium compound UCu 2Sn is studied by using muon spin relaxation method (μSR). Previous measurements of magnetic susceptibility and specific heat showed a pronounced anomaly at 16 K, suggesting an antiferromagnetic order. However, no significant change in the μSR spectra has been observed which indicates the absence of the antiferromagnetic phase transition at 16 K. We also found the fast μSR below 10 K, probably due to a magnetic ordering.

Modelling hydrogen transport in molybdenum disulfide catalysts with muon spin relaxation spectroscopy

Muon spin relaxation (µSR) spectra of a molybdenum disulfide and cobalt-promoted molybdenum disulfide catalyst were measured. The aim was to use the positive muon (µ) to model the binding and transport of hydrogen-like species in the catalyst. Muon implantation bypasses the first function of the catalyst, to dissociate molecular hydrogen. We observed classical diffusion at temperatures above 150 K which is easily described by the Arrhenius law. At lower temperatures (T < 100 K) we observed multi-phonon assisted quantum diffusion. The addition of cobalt to molybdenum disulfide decreased the activation energy for the diffusion of hydrogen-like particles, an effect which may account, at least in part, for the promotional role of cobalt in MoS2-based hydrodesulfurization catalysts. Copyright © 2000 John Wiley & Sons, Ltd.

Muon diffusion and trapping in proton conducting oxides

Trapping as a main feature of proton diffusion in proton conducting oxides was investigated by means of muon spin relaxation measurements, with the positive muon as tracer. In Sc-doped SrZrO 3, a system with representative physico-chemical properties and favourable nuclear magnetic moments, the muons are trapped at single Sc ions at a distance of 2.49 Å with a binding energy of 0.2 eV, in accordance with previous proton results. The muons form muoxide ions which are directed along the bisector of two oxygen–oxygen connection lines. The μSR spectra could quantitatively be described in terms of a two-state model for the muon diffusion comprising random sequences of trapping events and free diffusion periods.

μSR studies of Kondo disorder in UCu 5− xPd x and CeCu 5.9Au 0.1

Muon spin rotation (μSR) spectra in the non-Fermi-liquid (NFL) heavy-fermion alloys UCu 5− x Pd x ( x = 1.0 and 1.5) and CeCu 5.9Au 0.1 yield estimates of the rms width ( δ χ ) rms of the inhomogeneous susceptibility distribution in these systems. In UCu 5− x Pd x , the temperature dependence of ( δ χ ) rms indicates a wide distribution of Kondo temperatures T K, resulting in a significant number of uncompensated U spins with low T K which cause NFL behavior. In contrast, μSR lines in CeCu 5.9Au 0.1 are too narrow for such “Kondo disorder” to account for NFL properties unless, as seems unlikely, the correlation length for the susceptibility disorder is long in this alloy.

μSR magnetic response of CeNiSn on impurity content

Recent results of thermodynamic and transport measurements on CeNiSn at low temperatures show a strong dependence on sample purity. A lower impurity content leads to a more metallic behavior of CeNiSn. We present muon spin relaxation (μSR) spectra at T < 1 K for two samples of CeNiSn which show quite different behavior in low temperature specific heat. No significant difference could be detected in decisive μSR parameters for the two specimen. This strengthens the view that μSR probes the intrinsic magnetic properties of the heavy fermion ground state of CeNiSn.

A muon spin relaxation study of the magnetic ordering of (NH4)2FeCl5·H2O

We report muon spin relaxation (μSR) spectra from the antiferromagnetic salt (NH4)2FeCl5· H2O at various temperatures in the region of magnetic ordering (≈ 7 K). The μSR response includes an oscillatory time dependence of the forward-backward asymmetry over a small (≈ 0.4 K) but well-defined temperature interval. Analysis shows this to be consistent with the existence of two ordering temperatures. This confirms that a two-staged ordering is responsible for the two close-lying heat capacity cusps for this compound. It is further suggested that the two Fe spin systems associated with each of the ordering temperatures are closely intermingled rather than well separated in, for example, different domains.