The deuterium-induced changes of the optical transmission in Fe/V (001) and Cr/V (001) superlattices are found experimentally to be dominated by the volume changes of the vanadium layers and thus indirectly linked to concentration. The deuterium-induced expansion is 67% larger in Cr/V 2/14 monolayers (ML) as compared to Fe/V 2/14 ML. This large difference can be explained by a difference in the site of deuterium from tetrahedral in Fe/V to octahedral in Cr/V. First-principles calculations based on this assumption give quantitative agreement with both the measured optical transmission and the deuterium-induced expansion coefficient. Placing hydrogen in the middle of the vanadium layers results in total energies at 0 K that favor tetrahedral occupancy at low concentrations, although the energy difference is of the order of the thermal energy available in the experiments. Hence small changes in strain, defect concentration, and/or vibrational spectrum of the superlattices may tip the balance to octahedral occupancy at low concentrations. Given this link to concentration and the linear scaling, optical transmission can, therefore, be used in a straightforward way to obtain pressure-composition isotherms also in thin metal films that do not undergo metal-insulator transitions upon hydrogenation.

No consensus has been reached so far about the hydrogen anomaly problem in Compton scattering of neutrons, although strongly reduced H cross-sections were first reported almost 20 years ago. Over the years, this phenomenon has been observed in many different hydrogen-containing materials. Here, we use yttrium hydrides as test objects, YH2, YH3, YD2 and YD3, Y(H x D1−x )2 and Y(H x D1−x )3, for which we observe H anomalies increasing with transferred momentum q. We also observe reduced deuteron cross-sections in YD2 and YD3 and have followed those up to scattering angles of 140° corresponding to high momentum transfers. In addition to data taken using the standard Au-197 foils for neutron energy selection, the present work includes experiments with Rh-103 foils and comparisons were also made with data from different detector setups. The H and D anomalies are discussed in terms of the different models proposed for their interpretation. The 'electron loss model' (which assumes energy transfer to excited electrons) is contradicted by the present data, but it is shown here that exchange effects in scattering from two or more protons (or deuterons) in the presence of large zero-point vibrations, can explain quantitatively the reduction of the cross-sections as well as their q-dependence. Decoherence processes also play an essential role. In a scattering time representation, shake-up processes can be followed on the attosecond scale. The theory also shows that large anomalies can appear only when the neutron coherence lengths (determined by energy selection and detector geometry) are about the same size as the distance between the scatterers.

The metallic ruthenium perovskites ACu3Ru4O12 with A = Ca, Pr,Nd were investigated by zero field (ZF) and weak transverse field (TF) muon spin rotation/relaxation (μSR) spectroscopy. The ZF spectra for CaCu3Ru4O12 show pure static Gaussian Kubo-Toyabe relaxation arising from the interaction between the muon spin and the nuclear moments of the Cu ions. This confirms that no atomic magnetic moment exists. A sudden increase of the lattice parameter a when heating above ~150 K had previously been detected by neutron diffraction. The root mean square field at the muon site Brms was found to be 0.15 mT independent of temperature, in particular around 150 K. Also, no change of spectral parameters is seen in the weak TF data in that temperature range. Those findings imply that the structural change around 150 K takes place without noticeably shifting atomic positions. The spectra for PrCu3Ru4O12 and NdCu3Ru4O12 are dominated by the interaction with the dynamic rare earth moments. The analysis requires the use of the electron-nuclear double relaxation formalism. The electronic part shows simple exponential relaxation typical for a paramagnet. It features, with reducing the temperature a steep increase of paramagnetic relaxation rate as is characteristic for an approach to a magnetic spin freezing transition from above. That result suggests that the magnetic ground states of PrCu3Ru4O12 and NdCu3Ru4O12 are spin frozen states, although bulk magnetic data give no direct evidence in that direction.

We report results of a muon spin rotation and relaxation ($\mu$SR) study of dilute Pd$_{1-x}$Ni$_x$ alloys, with emphasis on Ni concentrations $x =$ 0.0243 and 0.025. These are close to the critical value $x_\mathrm{cr}$ for the onset of ferromagnetic long-range order (LRO), which is a candidate for a quantum critical point. The 2.43 and 2.5 at.% Ni alloys exhibit similar $\mu$SR properties. Both samples are fully magnetic, with average muon local fields $\langle B^\mathrm{loc}\rangle =$ 2.0 and 3.8 mT and Curie temperatures $T_C =$ 1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively, at $T = 0$. The temperature dependence of $\langle B^\mathrm{loc}\rangle$ suggests ordering of Ni spin clusters rather than isolated spins. Just above $T_C$ a two-phase region is found with separate volume fractions of quasistatic short-range order (SRO) and paramagnetism. The SRO fraction decreases to zero with increasing temperature a few kelvin above $T_C$. This mixture of SRO and paramagnetism is consistent with the notion of an inhomogeneous alloy with Ni clustering. The measured values of $T_C$ extrapolate to $x_\mathrm{cr}$ = 0.0236 $\pm$ 0.0027. The dynamic muon spin relaxation in the vicinity of $T_C$ differs for the two samples: a relaxation-rate maximum at $T_C$ is observed for $x$ = 0.0243, reminiscent of critical slowing down, whereas for $x =$ 0.025 no dynamic relaxation is observed within the $\mu$SR time window. The data suggest a mean-field-like transition in this alloy.

Magnetic properties of multiferroic Eu1-xYxMnO3 with x = 0.2, 0.3 were studied by μSR and Mossbauer spectroscopy. Both compounds are known to become antiferromagnetic with Mn3+ being the only magnetic ion. We find TN = 47±0.5K for x = 0.2 and 45±0.5K for x = 0.3. Below TN three different magnetic states (AFM-1, AFM-2, AFM-3) are formed. The μSR parameters of the uppermost magnetic state (AFM-1) are alike in both compounds, and compatible with a commensurate modulated collinear spin structure. The magnetic ground state in x = 0.2 (AFM-3) is shown to be single phase. Its spectral parameters support the proposal of a cone-like spin structure, yet with incommensurate modulation. The ground state for x = 0.3 is found to have an incommensurate spiral spin structure. 151Eu Mössbauer spectroscopy measured the Eu hyperfine field induced by the Mn3+ ions. Its low value (~4T) means that mixing with higher Eu electronic states is small and that the Mn-Eu exchange coupling is weak (~0.5K). The various magnetic transitions appear as small irregularities in the temperature dependence of the hyperfine fields. The present results are discussed in terms of different published magnetic phase diagrams of Eu1-xYxMnO3.

The metallic ruthenium perovskites ACu(3)Ru(4)O(12) with A = Ca, Pr, Nd were investigated by zero field (ZF) and weak transverse field (TF) muon spin rotation/relaxation (mu SR) spectroscopy. The ZF spectra for CaCu3Ru4O12 show pure static Gaussian Kubo-Toyabe relaxation arising from the interaction between the muon spin and the nuclear moments of the Cu ions. This confirms that no atomic magnetic moment exists. A sudden increase of the lattice parameter a when heating above similar to 150K had previously been detected by neutron diffraction. The root mean square field at the muon site B-rms was found to be 0.15 mT independent of temperature, in particular around 150K. Also, no change of spectral parameters is seen in the weak TF data in that temperature range. Those findings imply that the structural change around 150K takes place without noticeably shifting atomic positions. The spectra for PrCu3Ru4O12 and NdCu3Ru4O12 are dominated by the interaction with the dynamic rare earth moments. The analysis requires the use of the electron-nuclear double relaxation formalism. The electronic part shows simple exponential relaxation typical for a paramagnet. It features, with reducing the temperature a steep increase of paramagnetic relaxation rate as is characteristic for an approach to a magnetic spin freezing transition from above. That result suggests that the magnetic ground states of PrCu3Ru4O12 and NdCu3Ru4O12 are spin frozen states, although bulk magnetic data give no direct evidence in that direction.

The Laves phase intermetallics REAl2 were studied by μSR over several decades. Results had mainly been obtained for the temperature and field dependencies of relaxation rates in the paramagnetic state. However, it turned out that spontaneous precession signals in the ordered magnetic state were very difficult to observe. We report here the observation a weak precession signal in a single crystalline sample of DyAl2. The precession signal seen has low asymmetry and is heavily damped. This is understood with the notion that the magnetic structure of DyAl2 splits the internal field at the interstitial muon stopping site into several components, and that of those only one of the field components leads to the visible precession signal. The observed temperature dependence of the precession frequency is compatible with the change of easy axis of magnetization at T = 40K.

Several R2TIn3 samples (Nd2AgIn3, Ho2AgIn3, Er2AgIn3, Ho2CuIn3 and Er2CuIn3 were studied by muon spin rotation/relaxation (mu SR). The R2TIn3 intermetallics are magnetically frustrated, mainly by competing ferro- and antiferromagnetic exchange. Susceptibility data suggested antiferromagnetic order with T-N in the 10K range for the materials under consideration. Later neutron diffraction studies were, however, unable to detect any magnetic Bragg peaks. The mu SR spectra of all compounds consist of two signals. One fast relaxing, reflecting strong interaction between the muon and the rare earth magnetic moments, the other slowly relaxing, arising largely from the interaction with the nuclear moments of In, its coupling to rare earth magnetism being extremely weak. Suggested origin are two different muon stopping sites in the binary layered crystal structure of the R2TIn3 series. Results on the magnetic behavior of the compounds studied are drawn from the fast relaxing signal. They show that below the susceptibility peak temperatures the systems enter into a spin-glass-type (Nd) or a spin-liquid-type (Ho,Er) short-range ordered magnetic structures, long-range order being definitely absent. Spin fluctuations persist in all cases in the limit T -> 0, a well established mu SR signature for frustrated magnetic materials.

Powder samples of Fe1-xCuxCr2S4 with x = 0,0.2,0.5,0.8 were studied, between 5 and 300 K. The results reveal that for x < 1, the magnetic order in the series is more varied than the simple collinear ferrimagnetic structure traditionally assumed to exist everywhere from the Curie point to T → 0. In FeCr2S4 several ordered magnetic phases are present, with the ground state likely to have an incommensurate cone-like helical structure. Fe0.8Cu0.2Cr2S4 is the compound for which simple collinear ferrimagnetism is best developed. In Fe0.5Cu0.5Cr2S4 the ferrimagnetic spin structure is not stable, causing spin reorientation around 90 K. In Fe0.2Cu0.8Cr2S4 the ferrimagnetic structure is at low temperatures considerably distorted locally, but with rising temperature this disorder shows a rapid reduction, coupled to increased spin fluctuation rates. In summary, the present data show that the changes induced by the replacement of Fe by Cu have more profound influences on the magnetic properties of the Fe1-xCuxCr2S4 compounds than merely a shift of Curie temperature, saturation magnetization and internal field magnitude.

Tb2CuIn3 shows exceptional magnetic behavior within the series of R2CuIn3 intermetallics (R denoting one of the rare earth elements), where long-range magnetic order is absent in favor of spin liquid magnetic ground states as a consequence of frustration by competing exchange. In contrast, neutron studies suggested for the magnetic ground state of Tb2CuIn3 the coexistence of long-range antiferromagnetic and short-range order. The present mu SR (muon spin rotation/relaxation) data show that at similar to 45K all Tb magnetic moments freeze into a spin-glass state. At T-N = 33K partial long-range order sets in, its volume fraction being similar to 30%. On cooling to 10 K, the antiferromagnetic fraction reaches the saturation value of 70% and hence remains coexistent with the spin-glass state for T -> 0. The antiferromagnetic state is characterized by considerable short-range spin disorder. At low temperatures the Tb moments in the two coexisting states approach the static limit. This absence of persistent moment fluctuations together with partial formation of long-range order indicates considerably lower frustration in Tb2CuIn3 compared to the rest of the R2CuIn3 compounds.

The normal A-site spinels MnAl2O4, FeAl2O4, CoAl2O4, as well as related mixed (Mn0.5Fe0.5Al2O4) and partially inverted (Fe1.4Al1.6O4) spinels have been studied by μSR. The magnetic ions are subject to magnetic frustration by competing interactions. In all materials and at all temperatures the μSR spectra consist of two signals suggesting a bimodal distribution of the fluctuation rates of magnetic moments. A characteristic temperature T M is found in each compound, representing either a magnetic phase transition into a long-range ordered state (MnAl2O4, Fe1.4Al1.6O4) or the formation of a spin liquid phase (FeAl2O4, CoAl2O4, Mn0.5Fe0.5Al2O4). The magnetic ground state of MnAl2O4 shows coexistence of antiferromagnetic and spin liquid phases. In FeAl2O4 and CoAl2O4 long-range order is suppressed altogether, the ground state can be characterized as a fast relaxing spin liquid coexisting with a small fraction of paramagnetic spins. The partial replacement of Mn by Fe in Mn0.5Fe0.5Al2O4 prevents long-range order and leads to a spin liquid state in the low temperature limit. The partial occupancy of B-sites by magnetic ions in Fe1.4Al1.6O4 strengthens the exchange coupling, allowing the formation of long-range magnetic order at a rather high temperature (~100 K). Magnetic phase diagrams are presented demonstrating that for the studied compounds the magnetic properties are determined by the degree of frustration.

FeCr2S4 orders magnetically at TN≈170 K. According to neutron diffraction, the ordered state down to 4.2 K is a simple collinear ferrimagnet maintaining the cubic spinel structure. Later studies, however, claimed trigonal distortions below ∼60 K coupled to the formation of a spin glass type ground state. To obtain further insight, muon spin rotation/relaxation (μSR) spectroscopy was carried out between 5 and 200 K together with new 57Fe Mössbauer measurements. Below ∼50 K, our data point to the formation of an incommensurately modulated noncollinear spin arrangement like a helical spin structure. Above 50 K, the spectra are compatible with collinear ferrimagnetism, albeit with a substantial spin disorder on the scale of a few lattice constants. These spin lattice distortions become very large at 150 K and the magnetic state is now better characterized as consisting of rapidly fluctuating short-range ordered spins. The Néel transition is of second order, but ill defined, extending over a range of ∼10 K. The Mössbauer data around 10 K confirm the onset of orbital freezing and are also compatible with the noncollinear order of iron. The absence of a major change in the quadrupole interaction around 50 K renders the distortion of crystal symmetry to be small.

In the A-site spinels MAl2O4 (M=Mn, Fe, Co) the A-site sublattice is subjected to strong frustration, putting all three compounds on the verge between long-range and short-range magnetic order. The μSR spectra of the three compounds show two signals at all temperatures examined and a dramatic change in spectral shape at characteristic temperatures. Above this temperature, one signal (slowly relaxing) is typical for free paramagnetic spins, the other (fast relaxing) for highly correlated paramagnetic spins. For M=Mn the ratio of their intensities is 1:3 independent of temperature and the spectral change occurs around 40 K (TN from neutron data) where the strongly correlated fraction enters into long-range order and the free paramagnetic fraction into a glassy spin state. For M=Co or Fe, the free paramagnetic fraction increases from ∼20% to 85% with rising temperature. At the cusp temperatures observed in magnetization (5 or 12 K) the dominating, highly correlated paramagnetic fraction turns into a dynamic short-range ordered magnetic ground state. In all three compounds substantial local spin disorder and persistent spin fluctuations characterize their magnetic ground states, typical features of highly frustrated magnets.

The behaviour of monatomic hydrogen defect centres is modelled using implanted positive muons in both allotropes of elemental tin. A new search for paramagnetic muonium is made in α-Sn, the semiconducting allotrope with the diamond structure. No deep state is found, nor any enhanced Korringa relaxation that would suggest localized moment formation on interstitial protons in this intriguing material. In relation to muonium in the other Group-IV semiconductors, tantalizing indications of shallow state formation below 30K suggest that a deep-to-shallow transition may be responsible. The diamagnetic state remains static to 200K and may be tentatively identified as the bond-centred positive ion, by comparison with recent data for silicon. This behaviour contrasts with the abrupt onset of mobility at 50K for muons in β-Sn, the normal metallic allotrope with tetragonal structure, for which a more conventional interstitial site has lately been determined. Mobility studies are admittedly difficult in the presence of such weak nuclear magnetism but a discrepancy arises between reported migration barriers in white tin.

Samples of CoxCu1-x (x = 0.05, 0.02 and 0.01) produced by melt spinning and subsequent annealing were studied by muon spin rotation/relaxation (mu SR) spectroscopy with the aim to gain information on the dependence of the local magnetic properties like the magnitude, the distribution and the temporal behavior of the magnetic field at the muon site as function of Co concentration. In addition, we investigated for Co0.02Cu0.98 the changes in mu SR response as function of annealing times and temperatures. In all alloys, a spin-glass-like magnetic cluster state was seen well below the susceptibility cusp temperature. Above this temperature, cluster spin dynamics sets in but with low fluctuation rates even at high temperatures reflecting the presence of strong cluster - cluster couplings. A free paramagnetic cluster state is not formed. Spin dynamic properties are different for the x = 0(05 and 0.02 compounds and the x = 0(01 alloy. In the former compounds, the coexistence of a slower and a faster fluctuating fraction is found, arising from a non-uniform cluster size distribution or a non-random spatial distribution of clusters. In the latter compound, the spin dynamical state is homogeneous, but shows an unusual mu SR response requiring the presence of simultaneous fluctuations in spin orientation and moment magnitude. It further points to the presence of a partial loss of magnetic clusters. The variation of annealing times and temperatures for Co0.02Cu0.98 influences details of mu SR spectral parameters but leaves the overall response the same.

Zero field mu SR spectroscopy was applied to a polycrystalline sample of the ferrimagnetic sulfur spinel Fe(0.5)Cu(0.5)Cr(2)S(4) between 5 and 315 K. The temperature dependence of the interstitial magnetic field B(mu) as well as the transverse and longitudinal relaxation rates were deduced. At around 100 K, the temperature dependence of the interstitial field exhibits a strong deviation from the expected Brillouin-like behavior together with a maximum of the transverse relaxation rate. These features are characteristic for a spin reorientation transition. This instability of the ferrimagnetic spin-lattice has not been reported previously. (57)Fe Mossbauer data from the same sample show no irregularity in the temperature dependence of the Fe hyperfine field which could indicate that the spin reorientation involves primarily the Cr sublattice. Above and below the spin reorientation regime, disorder in the spin-lattice is sizable, but not excessive. At low temperatures, the spins are essentially static, spin dynamics sets in above the reorientation range. The mu SR data are also complemented by new susceptibility and magnetization data taken on similar material.

Compton scattering experiments with neutrons usually employ Au- or U-foils for energy selection of the scattered neutrons. A series of experiments on various H-containing materials have shown a large deficit in the scattering intensity of protons using Au-foils and it has been claimed that the anomalies arise from a faulty analysis of the data by neglecting effects of the tails of the Au- resonance lines. In the present experiments a Rh-103 resonance foil is used. It has considerably different resonance characteristics, but the H/metal ratio derived shows nearly the same anomalous value as with Au- foils. The present result therefore supports the existence of the mentioned anomalies.