SR Signal Research Articles

On the relaxation of? + SR signal and diffusive neutron scattering in AF phase of HTSC

We consider the antiferromagnetic (AF) state of high-T c compounds and assume the existence of the localized magnetic two-level systems (TLS) with the relatively small energyE in the AF copper planes, which is compatible with a series of experimental data implying low-energy scale in these systems. It is shown in our previous paper that these TLS are formed, if one accepts Aharony et al. suggestion that the small doping results in the holes' localization on the oxygen ions in CuO2 planes. Randomly distributed, these TLS cause partial disorder in the average values of AF copper spins. The manifestation of this effect in theμ + SR experiments and the elastic neutron scattering is discussed. Our results are in a qualitative agreement with the temperature dependence and the magnitude of the line shift and the relaxation rate ofμ + SR signal. The elastic diffusive neutron scattering at the small wave-vectors near 2D AF Bragg point (1/2, 1/2,l) and near the point (0, 0,l) is predicted. However, the temperature dependence of this scattering, observed in the experiments with YBa2Cu3O6+x atx=0.38, isn't met by our formulae, obtained for a small doping. It is shown, that μSR data give stronger confidence to the “frustrated bond” rather and “frustrated plaquette” case of the hole localization.

Muon—nuclear quadrupolar level crossing resonance in solid nitrogen. Evidence for N 2μ + ion formation

Both integral and time-differential muon level crossing resonance techniques were used to identify the source of the diamagnetic μ + SR signal in crystalline nitrogen. When the muon Zeeman splitting and the 14N nuclear quadrupole splitting are equal, the longitudinal field μ + spin relaxation rate is increased due to dipolar 14N—μ + coupling. At T = 37 K the resonance occurs at a magnetic field of 172 ± 2 G with a fwhm of 10 ± 2 G. The resonance parameters unambiguously confirm the formation of the N 2μ + ion in solid nitrogen.

Evidence of frustrated magnetism in CeAl3 from muon-spin-rotation spectroscopy.

Zero-, transverse-, and longitudinal-field muon-spin-rotation (${\ensuremath{\mu}}^{+}$SR) spectroscopy was performed on polycrystalline ${\mathrm{CeAl}}_{3}$. The onset of frustrated quasistatic magnetic correlations below about 2 K, becoming partly coherent below 0.7 K, was detected. The occurrence of different components in the ${\ensuremath{\mu}}^{+}$SR signal with relative strengths showing a pronounced temperature dependence leads to the conclusion that these correlations develop in a spatially inhomogeneous, frustrated way. No evidence for a macroscopic phase transition was obtained by specific heat measurements and neutron-diffraction experiments on our sample. This indicates that the coherence length of the correlations must be very short even below 0.7 K. The effective magnetic moment was estimated to be about 0.5${\ensuremath{\mu}}_{B}$ per cerium atom. It is speculated that the observed gradual evolution of magnetism in ${\mathrm{CeAl}}_{3}$ is driven by the temperature dependence of competing magnetic interactions rather than by a divergence in the single-ion susceptibility, similar to what was recently proposed for the heavy-electron compound ${\mathrm{U}}_{2}$${\mathrm{Zn}}_{17}$.

Observation of precipitates in a supersaturated FeCo alloy by muon spin rotation spectroscopy: a step towards understanding embrittlement of reactor pressure vessel steels

AbstractThe technique of μ+SR spectroscopy was applied to detect precipitate formation due to ageing in the binary Fe‐0.8 wt% Cu alloy. As compared to the homogenized specimen, the aged sample gives a μ+SR signal with considerably larger relaxation rate in the temperature interval 180 K < T < 350 K, having a maximum near T = 250 K with Λ = 14 μs−1.Since precipitation of copper plays possibly an important role in the embrittlement of neutron irradiated ferritic steels, the signal from an irradiated steel specimen has also been investigated.

Electron, muon, and nuclear spin dynamics in SmRh4B4 and ErRh4B4.

Zero-field positive muon spin relaxation (ZF-${\ensuremath{\mu}}^{+}$SR) studies of ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$ and ${\mathrm{ErRh}}_{4}$${\mathrm{B}}_{4}$ have revealed several features of magnetic moment dynamics in these magnetic superconductors. The shape of the \ensuremath{\mu}SR signal is not simple over most of the temperature range studied in either ${\mathrm{SMRh}}_{4}$${\mathrm{B}}_{4}$ or ${\mathrm{ErRh}}_{4}$${\mathrm{B}}_{4}$ because of the complicated nature of both materials. In ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$ above 200 K we observe complete decoupling of the muon spins from the stable, localized samarium ionic moments due to the extremely fast fluctuation of those moments; below 200 K there is a nearly simultaneous onset of coupling of the samarium electronic moments to both the muon spins and the $^{11}\mathrm{B}$ nuclear moments. The muon spin-relaxation rate in ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$ increases smoothly as the temperature is lowered through the superconducting transition toward the antiferromagnetic ordering at 0.87 K. In ${\mathrm{ErRh}}_{4}$${\mathrm{B}}_{4}$ the muon spin-relaxation function is a sum of two exponentials down to 50 K, below which the relaxation becomes too fast to be detected by our instruments, leaving only a signal of much-reduced amplitude at lower temperatures. We attribute the two exponentials to two distinct types of muon site in the crystal structure: a (high rms field) channel structure around the rare earths, and a (low rms field) site at the center of the RhB cluster. The deduced fluctuation rates of the rare-earth dipole fields at the muons are generally consistent with a model of paramagnetic rare-earth fluctuation mechanisms (Ruderman-Kittel-Kasuya-Yosida, Korringa, and spin-lattice interactions) above 4 K in ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$ and above 50 K (i.e., where the signal is fully resolved) in ${\mathrm{ErRh}}_{4}$${\mathrm{B}}_{4}$. Below 4 K in ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$, there is excess relaxation that seems to be associated with the magnetic ordering and that cannot be explained by the model which fits above 4 K.

A mu SR study of paramagnetic fluctuations in rare-earth-aluminium intermetallics

The authors have studied magnetic fluctuations in the paramagnetic phase of rare-earth-aluminium intermetallics of the type (RE)Al2 by the method of transverse-field muon spin rotation ( mu SR). In the case with strong 4f exchange they could observe a strong increase in the damping of the mu SR signal on approaching the Curie point Tc. One important contribution to the increased damping rate is the slowing down of magnetic fluctuations attributed to the formation of paramagnetic spin correlations which are observed to persist far above Tc. The crystallographic site for the mu + is discussed as well as the hyperfine field induced at this site due to interstitial electron polarisation.

Magnetic properties ofMnF2andCoF2determined by implanted positive muons. II. Sublattice magnetization and phase transition

Since it has been established in a companion paper that implanted positive muons occupy well-defined interstitial positions in ${\mathrm{CoF}}_{2}$ and ${\mathrm{MnF}}_{2}$, it has been possible to measure the sublattice magnetization and the critical spin fluctuations near ${T}_{N}$ which are reported in this paper. In ${\mathrm{CoF}}_{2}$, below the N\'eel temperature, the muon-spin-rotation (\ensuremath{\mu} SR) experiments in the absence of external fields reveal a signal whose frequency is proportional to the spontaneous sublattice magnetization. Its temperature dependence clearly reflects the presence of the strong magnetic anisotropy. In particular at the lower temperatures the behavior follows the predictions of spin-wave theory, while on approaching ${T}_{N}$ a simple power-law dependence agrees with the three-dimensional Ising model. The linewidths of the \ensuremath{\mu} SR signal in the neighborhood of ${T}_{N}$ in both crystals display a critical broadening which again can be asymptotically described by power laws. Such data are analyzed and critically compared with existing NMR and neutron scattering experiments. In this respect, our data reveal the anisotropy crossover in ${\mathrm{MnF}}_{2}$ and the existence of a central diffusive peak in the scattering function of ${\mathrm{CoF}}_{2}$, below ${T}_{N}$.

μ +SR study of some magnetic superconductors

The relaxation of theμ +SR signal has been investigated for the three compounds YRh4B4, ErRh4B4 and SmRh4B4. In the non-magnetic superconducting (T c ≈ 11 K) YRh4B4, the data display a Kubo-Toyabe (gaussian) shape for zero (transverse) magnetic fields. ErRh4B4 (superconducting below 8.7 K and ferromagnetic below 1 K) shows a dominant signal with very slow relaxation. In contrast SmRh4B4 (superconducting below 2.7 K and antiferromagnetic-superconducting below 0.87 K) shows a change in relaxation from gaussian above ≈ 60 K to exponential between 1 K and 4 K to two exponential signals (fast and slow) belowT N=0.9 K. In the region 0.9 K <T < 4.5 K, the relaxation time and the asymmetry both increase withT.

Observation ofμ − spin precession in ferromagnetic Ni and giant hyperfine anomaly

Aμ−SR signal in nickel was found representing the first observation ofμ−SR in ferromagnetic materials, and the hyperfine anomaly was determined to be −2.82±0.08% in comparison with the hyperfine field at dilute Co in Ni known from NMR.