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Magnetic frustration in ferromagnetic metallic systems is unusual due to the long-range and symmetric nature of the exchange interactions. In this work we prove that it is possible to obtain a highly frustrated ferromagnetic phase in a multi-sublattices cubic structure through a fine tuning of the magnetic interactions. This peculiar state is achieved in Ni-Mn-(In, Sn) Heusler alloys and results in the effective decoupling of their two intertwined ferromagnetic sublattices. One sublattice is ferromagnetic long range ordered below the macroscopic Curie temperature (TC ) whereas the second one remains disordered until a crossover to a polarized state occurs at T << TC . This result points out that a fine engineering of the magnetic interactions in metallic systems can lead to interesting novel and emergent phenomena.

We present a systematic investigation of muon-stopping states in superconductors that reportedly exhibit spontaneous magnetic fields below their transition temperatures due to time-reversal symmetry breaking. These materials include elemental rhenium, several intermetallic systems and Sr$_2$RuO$_4$. We demonstrate that the presence of the muon leads to only a limited and relatively localized perturbation to the local crystal structure, while any small changes to the electronic structure occur several electron volts below the Fermi energy leading to only minimal changes in the charge density on ions close to the muon. Our results imply that the muon-induced perturbation alone is unlikely to lead to the observed spontaneous fields in these materials, whose origin is more likely intrinsic to the time-reversal symmetry broken superconducting state., Comment: Main text: 6 pages, 3 figures. Supplemental Material: 13 pages, 16 figures

The electronic ground state of iron-based materials is unusually sensitive to electronic correlations. Among others, its delicate balance is profoundly affected by the insertion of magnetic impurities in the FeAs layers. Here, we address the effects of Fe-to-Mn substitution in the non-superconducting Sm-1111 pnictide parent compound via a comparative study of SmFe1-xMnxAsO samples with x(Mn) = 0.05 and 0.10. Magnetization, Hall effect, and muon-spin spectroscopy data provide a coherent picture, indicating a weakening of the commensurate Fe spin-density-wave (SDW) order, as shown by the lowering of the SDW transition temperature TSDW with increasing Mn content, and the unexpected appearance of another magnetic order, occurring at T∗≈ 10 and 20 K for x= 0.05 and 0.10, respectively. We attribute the new magnetic transition at T∗, occurring well inside the SDW phase, to a reorganization of the Fermi surface due to Fe-to-Mn substitutions. These give rise to enhanced magnetic fluctuations along the incommensurate wavevector Q2= (π± δ, π± δ) , further increased by the RKKY interactions among Mn impurities., Scientific Reports, 11, ISSN:2045-2322

An accurate experimental characterization of finite antiferromagnetic (AF) spin chains is crucial for controlling and manipulating their magnetic properties and quantum states for potential applications in spintronics or quantum computation. In particular, finite AF chains are expected to show a different magnetic behaviour depending on their length and topology. Molecular AF rings are able to combine the quantum-magnetic behaviour of AF chains with a very remarkable tunability of their topological and geometrical properties. In this work we measure the $^{53}$Cr-NMR spectra of the Cr$_{8}$Cd ring to study the local spin densities on the Cr sites. Cr$_{8}$Cd can in fact be considered a model system of a finite AF open chain with an even number of spins. The NMR resonant frequencies are in good agreement with the theoretical local spin densities, by assuming a core polarization feld AC = -12.7 T/$\mu_B$. Moreover, these NMR results confirm the theoretically predicted non-collinear spin arrangement along the Cr$_{8}$Cd ring, which is typical of an even-open AF spin chain.

The recent discovery of single-ion magnets with magnetic hysteresis above liquid-nitrogen temperatures placed these compounds among the best candidates to realize high-density storage devices. Starting from a prototypical dysprosocenium molecule, showing hysteresis up to 60 K, we derive here a general recipe to design high-blocking-temperature rare-earth single-ion magnets. The complex magnetic relaxation is unraveled by combining magnetization and nuclear magnetic resonance measurements with inelastic neutron scattering experiments and ab initio calculations, thus disentangling the different mechanisms and identifying the key ingredients behind slow relaxation.

An accurate experimental characterization of finite antiferromagnetic (AF) spin chains is crucial for controlling and manipulating their magnetic properties and quantum states for potential applications in spintronics or quantum computation. In particular, finite AF chains are expected to show a different magnetic behaviour depending on their length and topology. Molecular AF rings are able to combine the quantum-magnetic behaviour of AF chains with a very remarkable tunability of their topological and geometrical properties. In this work we measure the

In the heavy-fermion system $Yb_2Pd_2In_{1-x}Sn_x$, the interplay of crystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida interactions leads to complex chemical-, pressure-, and magnetic-field phase diagrams, still to be explored in full detail. By using a series of techniques, we show that even modest changes of parameters other than temperature are sufficient to induce multiple quantum-critical transitions in this highly susceptible heavy-fermion family. In particular, we show that, above $\sim 10$ kbar, hydrostatic pressure not only induces an antiferromagnetic phase at low temperature, but it likely leads to a reorientation of the Yb magnetic moments and/or the competition among different antiferromagnetic configurations.

We report on a study on a representative set of FeP-based MnFePSi samples by means of Mn NMR in both zero and applied magnetic field. The first-order nature of the magnetic transition is demonstrated by truncated order parameter curves with a large value of the local ordered moment at the Curie point, even at compositions where the transition appears second order from magnetic measurements. No weak ferromagnetic order could be detected at Si-poor compositions showing the kinetic arrest phenomenon, but rather the phase separation of fully ferromagnetic domains from volume fractions where Mn spins are fluctuating. The more pronounced decrease of the ordered moment at the $3f$ sites than at the $3g$ sites on approaching ${T}_{C}$, characteristic of the mixed magnetism of these materials, is shown to be driven by the drop of the $3f$ spin density instead of enhanced spin fluctuations. The temperature-driven $3f$ spin extinction is demonstrated to evolve into a truly nonmagnetic state of the $3f$ Mn ions well above ${T}_{C}$, in agreement with theoretical models and in contrast with previous experiments who detected just a partial moment quenching. Besides ``normal'' $3f$ Mn ions undergoing a magnetic to spinless state transition, NMR in Mn-rich compositions detects the disproportionation at $3f$ sites of a significant minority Mn fraction with negligible hyperfine couplings, which retains its diamagnetic character independent of temperature. Such a diamagnetic fraction qualitatively accounts for the reduced average $3f$ moment previously reported at large Mn concentrations.

We present a muon spin relaxation study of the Mott transition in BaCoS_2 using two independent control parameters: (i) pressure p to tune the electronic bandwidth and (ii) Ni-substitution x on the Co site to tune the band filling. For both tuning parameters, the antiferromagnetic insulating state first transitions to an antiferromagnetic metal and finally to a paramagnetic metal without undergoing any structural phase transition. BaCoS_2 under pressure displays minimal change in the ordered magnetic moment S_ord until it collapses abruptly upon entering the antiferromagnetic metallic state at p_cr ~ 1.3 GPa. In contrast, S_ord in the Ni-doped system Ba(Co_{1-x}Ni_{x})S_{2} steadily decreases with increasing x until the antiferromagnetic metallic region is reached at x_cr ~ 0.22. In both cases, significant phase separation between magnetic and nonmagnetic regions develops when approaching p_cr or x_cr, and the antiferromagnetic metallic state is characterized by weak, random, static magnetism in a small volume fraction. No dynamical critical behavior is observed near the transition for either tuning parameter. These results demonstrate that the quantum evolution of both the bandwidth- and filling-controlled metal-insulator transition at zero temperature proceeds as a first-order transition. This behavior is common to magnetic Mott transitions in RENiO_3 and V_2O_3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase., 8 pages, 6 Figures

We report an experimental study on the effect of Mn impurities in the optimally doped [Formula: see text] compound. The results show that a very tiny amount of Mn, of the order of 0.1%, is enough to destroy superconductivity and to recover at low temperatures both the magnetic ground state and the orthorhombic structure of the pristine LaFeAsO parent compound. The results are discussed within a model where electron correlations enhance the Ruderman-Kittel-Kasuya-Yosida interaction among impurities.

We investigate the specific influence of structural disorder on the suppression of antiferromagnetic order and on the emergence of cuprate superconductivity. We single out pure disorder, by focusing on a series of Y$_{z}$Eu$_{1-z}$Ba$_2$Cu$_3$O$_{6+y}$ samples at fixed oxygen content $y=0.35$, in the range $0\le z\le 1$. The gradual Y/Eu isovalent substitution smoothly drives the system through the Mott-insulator to superconductor transition from a full antiferromagnet with N\'eel transition $T_N=320$ K at $z=0$ to a bulk superconductor with superconducting critical temperature $T_c=18$ K at $z=1$, YBa$_2$Cu$_3$O$_{6.35}$. The electronic properties are finely tuned by gradual lattice deformations induced by the different cationic radii of the two lanthanides, inducing a continuous change of the basal Cu(1)-O chain length, as well as a controlled amount of disorder in the active Cu(2)O$_2$ bilayers. We check that internal charge transfer from the basal to the active plane is entirely responsible for the doping of the latter and we show that superconductivity emerges with orthorhombicity. By comparing transition temperatures with those of the isoelectronic clean system we deterime the influence of pure structural disorder connected with the Y/Eu alloy., Comment: 10 pages 11 figures, submitted to Journal of Physics: Condensed Matter, Special Issue in memory of Prof. Sandro Massida

55 Mn NMR was used to perform the atomic-scale study of the anisotropic properties of Mn 5 Ge 3 /Ge(111) epitaxial films with thicknesses between 9 and 300 nm. The NMR spectra have been recorded as a function of strong external magnetic field applied in the film plane and perpendicular to it. Two 55 Mn NMR resonances have been observed, corresponding to the two manganese sites 4d and 6g, in the hexagonal D8 8 structure; in zero field their frequency is centered around 207.5 and 428 MHz, respectively. The anisotropy of 55 Mn hyperfine fields between the hexagonal c direction and the c plane at both Mn sites was evidenced and attributed to the anisotropic term due to the unquenched Mn orbital momentum. The anisotropy of the orbital contribution to hyperfine fields was determined as 1.52 T in the 4d site and up to 2.77 T in the 6g site. The 4d site reveals a quadrupolar interaction due to the strong electric field gradient: V zz = 5.3 × 10 19 V/m 2 in this site, which is shown to be oriented along the hexagonal c axis.

We report on an experimental study of the effect of Mn impurities in the optimally doped LaFeAsO$_{0.89}$F$_{0.11}$ compound. The results show that a very tiny amount of Mn, of the order of 0.1\%, is enough to destroy superconductivity and to recover at low temperatures both the magnetic ground state and the orthorhombic structure of the pristine LaFeAsO parent compound. The results are discussed within a model where electron correlations enhance the Ruderman-Kittel-Kasuya-Yosida interaction among impurities.

The interplay between magnetism and superconductivity is tuned as a function of the external pressure in LnFeAsO[Formula: see text]Fx at [Formula: see text] values close to the appearance of superconductivity for lanthanide ion Ln=La[Formula: see text]Y[Formula: see text] and Ce. The results show that in these compounds, both superconductivity and magnetism are reinforced by pressure. Furthermore, muon spin rotation and relaxation measurements reveal qualitative remarkable differences among the samples: Ln=Ce displays nanoscopic coexistence of magnetism and superconductivity in the same volume fraction while Ln=La[Formula: see text]Y[Formula: see text] shows macroscopic phase separation. The Ln-dependent results can be due to a complex combination of the external and chemical pressure, the latter being driven by the different average ionic Ln radius.

We revise the simple and yet powerful approach of the Double Adiabatic Approximation (DAA) to study positive muon embedding sites in condensed matter. This method introduces a tractable and reasona...

Muon-spin rotation data collected at ambient pressure ($p$) and at $p=2.42$ GPa in MnP were analyzed to check their consistency with various low- and high-pressure magnetic structures reported in the literature. Our analysis confirms that in MnP the low-temperature and low-pressure helimagnetic phase is characterised by an increased value of the average magnetic moment compared to the high-temperature ferromagnetic phase. An elliptical double-helical structure with a propagation vector ${\bf Q}=(0,0,0.117)$, an $a-$axis moment elongated by approximately 18% and an additional tilt of the rotation plane towards $c-$direction by $\simeq 4-8^{\rm o}$ leads to a good agreement between the theory and the experiment. The analysis of the high-pressure $\mu$SR data reveals that the new magnetic order appearing for pressures exceeding $1.5$ GPa can not be described by keeping the propagation vector ${\bf Q} \parallel c$. Even the extreme case -- decoupling the double-helical structure into four individual helices -- remains inconsistent with the experiment. It is shown that the high-pressure magnetic phase which is a precursor of superconductivity is an incommensurate helical state with ${\bf Q} \parallel b$., Comment: 9 pages, 9 figures. arXiv admin note: substantial text overlap with arXiv:1603.03367

In this paper we report a detailed $\mu^{+}$SR and {}$^{19}$F-NMR study of the La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$ class of materials. Here, the diamagnetic La$_{1-y}$Y$_{y}$ substitution increases chemical pressure and, accordingly, sizeably enhances the optimal superconducting transition temperature. We investigate the magnetic-superconducting phase transition by keeping the Y content constant ($y = 0.3$) and by varying the F content in the range $0.025 \leq x \leq 0.15$. Our results show how magnetism and superconductivity coexist for $x = 0.065$. Such coexistence is due to segregation of the two phases in macroscopic regions, resembling what was observed in LaFeAsO$_{1-x}$F$_{x}$ materials under applied hydrostatic pressure. This scenario is qualitatively different from the nanoscopic coexistence of the two order parameters observed when La is fully substituted by magnetic rare-earth ions like Sm or Ce.

Muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) and $^{19}\mathrm{F}$ nuclear magnetic resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in ${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors. While for $y=0$ a very low critical concentration of Mn ($x=0.2%$) is needed to quench superconductivity, as $y$ increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for $y=20%$ it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for $y=0$ and for $y=20%$. Low-energy spin fluctuations give rise to a peak in $^{19}\mathrm{F}$ NMR $1/{T}_{1}$ with an onset well above the superconducting transition temperature and whose magnitude increases with $x$. Also the static magnetic correlations probed by $^{19}\mathrm{F}$ NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground state are discussed in the light of the proximity of ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ to a quantum critical point.

This work aims at understanding the large reduction of superconducting critical temperature Tc observed in YSr 2 Cu 3 0 6+x as compared to its YBa 2 Cu 3 O 6+x counterpart (ΔTc = -30 K ). We report on a combined study of structural and electronic properties of RESr 2 Cu 3 O 6+x(RE = Y, Yb) polycrystalline samples. Neutron diffraction data and Cu NQR spectra show that, contrary to REBa 2 Cu 3 O 6+x RESr 2 Cu 3 O 6+x is locally tetragonal and no CuO chains are formed. This arises from the random occupancy of oxygen along the a- or b- direction in the basal planes. Ab-initio calculations of the electronic structure using the full-potential linearized-augmented-plane-wave method (FLAPW) in the local density approximation (LDA) suggest that the CuO chains are not formed because of the large elastic strain associated with the orthorhombic distortion produced by the chain formation. In addition, by using a [Formula: see text] supercell simulating the absence of chains, we find that oxygen disorder greatly alters the band structure near the Fermi level. Our analysis indicates that this alteration leads to a reduction of hole transfer from the CuO chains to the CuO 2 planes, which accounts for the reduction of Tc experimentally observed.

63 Cu(1) NQR signals and their relaxation have been measured in YBa 2 Cu 3 O 6.1 and ErBa 2 Cu 3 O 6+ x at temperatures between 10 K and 300 K. In YBa 2 Cu 3 O 6.1 a peak is observed in the spin-lattice relaxation, T 1 -1 , around 140 K. An explanation in terms of doped-hole motion is presented and alternative O(4)-O(5) diffusion mechanisms are critically discussed. In ErBa 2 Cu 3 O 6+ x the low-temparature T 1 -1 is dominated by magnetic fluctuations of Er; quantitative agreement is found when the correct ground state and magnetic dynamic properties of the rare earth are taken into account.

We have studied by muon spin resonance ({\mu}SR) the helical ground state and fluctuating chiral phase recently observed in the MnGe chiral magnet. At low temperature, the muon polarization shows double period oscillations at short time scales. Their analysis, akin to that recently developed for MnSi [A. Amato et al., Phys. Rev. B 89, 184425 (2014)], provides an estimation of the field distribution induced by the Mn helical order at the muon site. The refined muon position agrees nicely with ab initio calculations. With increasing temperature, an inhomogeneous fluctuating chiral phase sets in, characterized by two well separated frequency ranges which coexist in the sample. Rapid and slow fluctuations, respectively associated with short range and long range ordered helices, coexist in a large temperature range below T$_{N}$ = 170 K. We discuss the results with respect to MnSi, taking the short helical period, metastable quenched state and peculiar band structure of MnGe into account., Comment: 13 pages, 11 figures

We report the results of a muon-spin relaxation (μSR) investigation of La2−xSrxCoO4, an antiferromagnetic insulating series which has been shown to support charge ordered and magnetic stripe phases and an hourglass magnetic excitation spectrum. We present a revised magnetic phase diagram, which shows that the suppression of the magnetic ordering temperature is highly sensitive to small concentrations of holes. Distinct behavior within an intermediate x range (0.2≤x≲0.6) suggests that the putative stripe ordered phase extends to lower x than previously thought. Further charge doping (0.67≤x≤0.9) prevents magnetic ordering for T≳1.5K.

We report a detailed $\mu$SR study of the pressure evolution of the magnetic order in the manganese based pnictide MnP, which has been recently found to undergo a superconducting transition under pressure once the magnetic ground state is suppressed. Using the muon as a volume sensitive local magnetic probe, we identify a ferromagnetic state as well as two incommensurate helical states (with propagation vectors ${\bf Q}$ aligned along the crystallographic $c-$ and $b-$directions, respectively) which transform into each other through first order phase transitions as a function of pressure and temperature. Our data appear to support that the magnetic state from which superconductivity develops at higher pressures is an incommensurate helical phase., Comment: 11 pages, 9 figures

We investigate the magnetic properties and the phonon-induced relaxation dynamics of the first regular ${\mathrm{Cr}}_{9}$ antiferromagnetic (AF) ring, which represents a prototype frustrated AF ring. Geometrical frustration in ${\mathrm{Cr}}_{9}$ yields an energy spectrum with twofold degenerate low-lying levels and a low-spin ground state. The electronic relaxation dynamics is probed by ${}^{1}\mathrm{H}$-NMR through the temperature dependence of the spin-lattice relaxation rate $1/{T}_{1}$. We develop a microscopic model that reproduces $1/{T}_{1}(T)$ curves, taking also into account the wipeout effect. By interpreting these measurements we determine the spin-phonon coupling strength and we investigate the decay of the cluster magnetization due to the spin-phonon interaction. We find that at very low temperatures, the relaxation is characterized by a single dominating Arrhenius-type relaxation process, whereas several relevant processes emerge at higher temperatures. In addition, we calculate the temperature and magnetic field dependence of level lifetimes.

The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimally doped ${\mathrm{SmFeAsO}}_{0.88}{\mathrm{F}}_{0.12}$ iron-based superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide range of manganese contents ($x$) by means of muon-spin spectroscopy ($\ensuremath{\mu}\mathrm{SR}$), both in zero and in transverse fields, to probe the magnetic and the superconducting order, respectively. The pure superconducting phase (at $xl0.03$) is replaced by a crossover region at intermediate Mn values ($0.03\ensuremath{\le}xl0.08$), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for $x=0.08$, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of ${T}_{c}$ and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors (M. N. Gastiasoro et al., Phys. Rev. Lett. 117, 257002 (2016)), which includes correlation-enhanced RKKY couplings between the impurity moments.

$^{75}$As Nuclear Magnetic (NMR) and Quadrupolar (NQR) Resonance were used, together with M\"{o}ssbauer spectroscopy, to investigate the magnetic state induced by Mn for Fe substitutions in F-doped LaFe$_{1-x}$Mn$_{x}$AsO superconductors. The results show that $0.5$% of Mn doping is enough to suppress the superconducting transition temperature $T_c$ from 27 K to zero and to recover the magnetic structure observed in the parent undoped LaFeAsO. Also the tetragonal to orthorhombic transition of the parent compound is recovered by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The NQR spectra also show that a charge localization process is at play in the system. Theoretical calculations using a realistic five-band model show that correlation-enhanced RKKY exchange interactions between nearby Mn ions stabilize the observed magnetic order, dominated by $Q_1=(\pi,0)$ and $Q_2=(0,\pi)$ ordering vectors. These results give compelling evidence that F-doped LaFeAsO is a strongly correlated electron system at the verge of an electronic instability., Comment: 5 pages, 5 figures and 4 pages of supplemental material

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 $R$CoPO, where the full As/P substitution is already inducing a strong chemical pressure within the lattice but $p$ is still very effective in further affecting the magnetic properties., Comment: 8 pages, 9 figures

We present transverse field μ SR measurements of the flux lattice field distribution in dense pellets of MgB 2 and of the alloy Mg 1 - x Al x B 2 . In the present work we focus on the analysis of the data, comparing the second moment of the calculated and experimental field distributions. Starting from the standard model, we modified the calculation to include the presence of two length scales, the diffusion lengths of the σ and π pairs. We further introduce a partially anisotropic single gap model, adapted to the electronic structure of MgB 2 , and a partially anisotropic two-band model. We confirm that the two distinct gaps can better describe the field dependence of the data.

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.

We performed a preliminary zero-field μ SR investigation on a Fe 3 O 4 single crystal in a temperature range (5–275 K) including the Verwey transition, T V = 120 K . Despite the relatively small statistics employed, we identify a larger number of distinct precession frequencies where previous experiments detected only one. This indicates the very good quality of our specimen and allows us to monitor the abrupt changes in the local muon environment at T V . The discussion of these data is relevant to the on-going dispute on the detailed nature of the Verwey transition.

We present magnetization and muon spin resonance ( μ SR ) data on the single layered manganite LaSrMnO4, nominally containing no holes. Magnetization data corroborate antiferromagnetic (AF) spin order but it also suggest additional static moments with a magnetic anisotropy parallel to the AF easy axis, at low temperatures. Our μ SR data confirm the existence of these statics moments and imply that they are in a metastable, frozen state. Upon heating, the metastability disappears and for the additional moments a very slow dynamic is detected.

The new experimental challenges of modern μSR require a critical review of the existing detection systems, whose limitations could be overcome by the use of position-sensitive detectors. The constraints posed by muon spectroscopy suggest silicon detectors and/or scintillating fibers as the most promising solutions. Realistic simulations using state-of-the-art Monte Carlo programs proved essential in detector optimization and in guiding prototype building.

Zero-field μ SR experiments were performed on a series of polycrystalline samples of AA 3 ′ Mn 4 O 12 double perovskites ( A = Na + , Ca 2 + , La 3 + ; A ′ = Cu 2 + , Mn 3 + ) synthesized under high pressure. Spontaneous precession frequencies, of comparable magnitude to those previously reported in LaMnO 3 , were observed in all the family members except CaMn 7 O 12 . For all the samples, μ SR yielded determinations of the magnetic transition temperatures and of the order parameter in agreement with other techniques, but also revealed a peculiar phase separation in CaMn 7 O 12 , absent in the other compounds.

Here we present a reviewed phase diagram of the high-T c superconducting YBa2Cu3O6+ x compound, finely mapped in the strongly underdoped region (0

We present the results of an extensive μSR reinvestigation of the doping dependence of the AF order in polycrystalline YBa 2 Cu 3 O 6+ x samples (0 x x >0.2, we detected a spin freezing correlated to the charge freezing of the holes in the CuO 2 planes. The data may be fitted to a model of size-dependent reduction of the staggered magnetization implied in a stripe scenario. The hole concentration dependence of these freezing phenomena is mapped onto the phase diagram of YBa 2 Cu 3 O 6+ x .

The sudden application of magnetic fields either simultaneous or delayed with respect to muon bunches (in pulsed facilities) or to single muons (when available ‘on demand’) offers new/alternative approaches to studying relaxation phenomena in dielectric liquids and solids. We give here an example of a simple experiment on de-ionized water in which the intrinsic muonium lifetime can be determined regardless of the field inhomogeneity. A 20 μs long pulse of magnetic field (up to 10 mT ) is generated by a single flat-loop coil, which is part of a cell containing the liquid. The delay time between the muon bunch and the field pulse is adjusted in order to observe the residual muon or muonium precession amplitude after an interval of time elapsed in zero external field during which only irreversible/chemical loss of polarization can take place. The muonium and diamagnetic muon precessions are measured after different pulse delays. The initial muonium amplitude is found to decrease as a function of delay time, with a decay rate of 0.17(2) μs −1 , which is the intrinsic muonium lifetime and is independent of the inhomogeneity of the applied field.

We investigated the system Mg 1− x Al x B 2 , in which the heterovalent substitution of Al for Mg changes both the band filling (different valence) and the bandwidth (lattice contraction due to different ionic radii). The result is the disappearance of superconductivity at about x =0.5. Contrary to earlier findings, our carefully annealed powder samples show no evidence of phase separation. We determined the μSR lineshape in a flux line lattice. We obtain a nearly linear dependence of the second moment of the flux lattice field distribution upon doping, indicating a strong reduction of the super carrier density with increasing x . Muon diffusion is detected by zero field measurements. It persists down to the lowest temperatures also at x =0.4. The doping dependent activation energies are reported.

Zero-field muon–spin-rotation investigations have been performed on single crystals of La 1− x Sr 1+ x MnO 4 with 0⩽ x ⩽0.5. In the undoped sample spontaneous precession frequencies ω μ have been observed signaling a long-range-ordered magnetic ground state. In contrast at intermediate doping x =0.25 and 0.4 exhibit only short-range order. For the half-doped ( x =0.5) sample long-range magnetic order reappears.

We investigate the substituted spinel Mg1−xMn2+xO4, in five powder compositions 0⩽x⩽1, where divalent Mg is expected to occupy mainly the B spinel site. A zero-field precession is detected for all samples below an x-dependent ordering temperature, which is different from that detected by macroscopic magnetization. The precessing amplitude in the ordered phase displays temperature-dependent deviations from the canonical value of 2 3 , expected for our powder samples, indicating the presence of a radical state. The difference between the SQUID and the μSR-order temperatures is attributed to a collinear intermediate state stabilized by Mg.

We report μ SR data from pseudo-cubic and layered copper fluorides, KCuF 3 , K 2 CuF 4 , and its substitutions K 1.9 Sr 0.1 CuF 4 , K 2 CuF 3.9 O 0.1 , K 2 Cu 1.8 Zn 0.2 F 4 . In KCuF 3 , the extinction of the F–Mu–F fraction and its progressive replacement by a magnetic fraction between 40 and 35 K demonstrate a first-order magnetic transition at T N , probably driven by orbital degrees of freedom. The layered compounds exhibit on the contrary conventional second-order transitions at T c =6.25 K , independent of electron or hole doping up to at least x =0.1. However, 20% substitution of Cu with non-magnetic Zn completely suppresses long-range magnetic order above 1.3 K .

We measured the 27Al and 11B NMR spin lattice relaxation rates and the isotropic Knight shifts in powder samples of Mg1−xAlxB2, as a function of the Al concentration, x. The temperature independence of the Knight shifts and the linear temperature dependence of the relaxation are verified throughout the compositions explored. The variation with x of the measured quantities is discussed in terms of the projected densities of states at the Fermi energy, finding good qualitative as well as quantitative agreement with recent band structure calculations.

The charge and spin freezing phenomena in high T c YBa 2 Cu 3 O 6+x superconductor is investigated in the low doping region (00.2, the observed spin freezing is correlated to the charge freezing of the holes in the Cu(2)O2 planes. The data may be fitted to a model of size-dependent reduction of the staggered magnetization implied in a stripe scenario. The hole concentration dependence of these freezing phenomena is mapped onto the phase diagram of YBa 2 Cu 3 O 6+x.

We investigated the electric field gradient tensor at the Cu nuclei by Zeeman perturbed nuclear quadrupole resonance in orbitally ordered KCuF 3 . Our data show the presence of two inequivalent crystallographic sites with distinct orientations of the copper wave function, compatibly with the proposed orbital order for this compound. An unusual behaviour of the paramagnetic NQR signal at the Neel transition is reported.

We report on an 55 Mn NMR investigation of calcium-doped manganite samples carried out at 4.2 K. In the insulating members of the family, the localization of hole-rich ferromagnetic clusters is revealed by the peculiar two-peaked spectra, and by distinct shifts of the resonance lines in an external field. Results are discussed in view of inelastic neutron scattering findings and combined 139 La NMR and μSR data.

We present a systematic characterization of Mg1−xLixB2 polycrystalline samples with nominal Li content x up to 0.30. We explore the effectiveness of the substitution and investigate its influence on superconducting and normal state properties. The structural analyses by neutron and x-ray diffraction indicate that, despite the lattice parameters remaining unchanged within the experimental accuracy, around 30% of the nominal Li content actually enters the structure. Also the transition temperature is only weakly affected by Li substitution, but its relationship with the residual resistivity and magnetoresistivity data is compatible with a picture where π bands are filled with holes and/or affected by disorder, as predicted by theory. We also measured the magnetic penetration depth λ by an inductance technique. Data fits based on the standard Bardeen–Cooper–Schrieffer two-band model yield the zero-temperature limit of both λ and superconducting gaps: we find that λ(0) increases weakly and quickly saturates with increasing x, whereas Δπ(0) and Δσ(0) decrease with x. This analysis suggests that lithium substitution induces disorder mainly in the π band. Point contact spectroscopy results on samples obtained from the same batch are in full agreement with magnetic penetration depth data on the undoped sample, but give superconducting gaps almost insensitive to Li substitution, showing at most a small increase in Δπ(0) that may be related to an inhomogeneous distribution of Li content.

We report on a systematic study of the structural, magnetic, and transport properties of high-purity 1T-VS2 powder samples prepared under high pressure. The results differ notably from those previously obtained by deintercalating Li from LiVS2. First, no charge-density wave (CDW) is found by transmission electron microscopy down to 94 K, though ab initio phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector qCDW≈(0.21,0.21,0) previously reported in deintercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature. Second, infrared optical absorption and electrical resistivity measurements give evidence of nonmetallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with ab initio band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and nonmetallic domains in the nm scale. We conclude that 1T-VS2 is at the verge of a CDW transition and suggest that residual electronic doping in Li deintercalated samples stabilizes a uniform CDW phase with metallic properties.

Transverse-field muon-spin rotation ($\mu$SR) experiments were performed on a single crystal sample of the non-centrosymmetric system MnSi. The observed angular dependence of the muon precession frequencies matches perfectly the one of the Mn-dipolar fields acting on the muons stopping at a 4a position of the crystallographic structure. The data provide a precise determination of the magnetic dipolar tensor. In addition, we have calculated the shape of the field distribution expected below the magnetic transition temperature $T_C$ at the 4a muon-site when no external magnetic field is applied. We show that this field distribution is consistent with the one reported by zero-field $\mu$SR studies. Finally, we present ab initio calculations based on the density-functional theory which confirm the position of the muon stopping site inferred from transverse-field $\mu$SR. In view of the presented evidence we conclude that the $\mu$SR response of MnSi can be perfectly and fully understood without invoking a hypothetical magnetic polaron state., Comment: 10 pages, 12 figures

We have measured the nuclear spin-lattice relaxation of 59Co in the geometrically frustrated Ising-type spin-chain system Ca3Co2O6 as a function of temperature and applied magnetic field. In the nearly-saturated ferrimagnetic and ferromagnetic regimes considered here, the field-dependent energy scales governing the spin-lattice relaxation rates reflect the energy cost of a spin flip in the Co ions adjacent to the probed nuclei. This results in a thermally activated form for the nuclear relaxation rates, which can be exploited as a local probe of the exchange interactions. In particular, the measurements enable the values of intra- and interchain exchange constants J1, J2+J3 to be directly extracted from the data. By using a quasi-one-dimensional model, we then determine the value of J2 consistent with the spin-density-wave order observed in zero field.