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The molecular triangular lattice system, beta'-EtMe3Sb[Pd(dmit)2]2, is considered as a candidate material for the quantum spin liquid (QSL) state, although ongoing debates arise from recent controversial results. Here, the results of electron spin resonance (ESR) and muon spin relaxation (muSR) measurements on beta'-EtMe3Sb[Pd(dmit)2]2 are presented. Both results indicate characteristic behaviors related to quasi-one-dimensional (q1D) spin dynamics, whereas the direction of anisotropy found in ESR is in contradiction with previous theories. We succeed in interpreting the experiments by combining density-functional theory calculations and analysis of the effective model taking into account the multi-orbital nature of the system. While the QSL-like origin of beta'-EtMe3Sb[Pd(dmit)2]2 was initially attributed to the magnetic frustration of the triangular lattice, it appears that the primary origin is a 1D spin liquid resulting from the dimensional reduction effect., Comment: 5 pages, 4 figures

The layered molecular conductors $\kappa$-(BEDT-TTF)$_2$X are a perfect experimental platform for studying the physics of the Mott transition and related exotic electronic states. In these materials, the subtle balance between various instabilities of the normal metallic state can be efficiently changed by applying a very moderate external pressure or by subtle chemical modifications, e.g. by a replacement of the insulating anion X$^{-}$, frequently referred to as ``chemical pressure''. A crucially important but still unsettled issue is an exact understanding of the influence of physical and chemical pressure on the electronic structure. Here, we use magnetic quantum oscillations to explore in a broad pressure range the behavior of the key parameters governing the Mott physics, the electronic correlation strength ratio $U/t$ and the spin frustration ratio $t'/t$ in two $\kappa$ salts, the ambient-pressure antiferromagnetic insulator with X = Cu[N(CN)$_2$]Cl and the ambient-pressure superconductor with X = Cu(NCS)$_2$. Our analysis shows that pressure effectively changes not only the conduction bandwidth but also the degree of spin frustration, thus weakening both the electronic correlation strength and the magnetic ordering instability. At the same time, we find that the replacement of the anion Cu[N(CN)$_2$]Cl$^-$ by Cu(NCS)$_2^-$ results in a significant increase of the frustration parameter $t'/t$, leaving the correlation strength essentially unchanged., Comment: 13 pages,including 7 Figures. Supplemental Material is available upon request

The triangular-lattice system $\alpha$-RuI$_3$ is isostructural to the widely-studied $\alpha$-RuCl$_3$ compound which was identified as a potential Kitaev system but exhibits, instead of spin liquid behaviour, a magnetically ordered zig-zag ground state which sets in below 14~K. Here we show experimentally that, in contrast, the spins in $\alpha$-RuI$_3$ remain dynamic down to at least 50~mK. We study the spin dynamics using muon-spin relaxation methods and determine the presence of low-frequency fluctuations which are characteristic of a two-dimensional system., Comment: 5 pages, 5 figures

We present the results of a muon-spin relaxation ($\mu^{+}$SR) investigation of the crystalline organic radical compound 4-(2-benzimidazolyl)-1,2,3,5-dithiadiazolyl (HbimDTDA), in which we demonstrate the hysteretic magnetic switching of the system that takes place at $T = 274 \pm 11\,\mathrm{K}$ caused by a structural phase transition. Muon-site analysis using electronic structure calculations suggests a range of candidate muon stopping sites. The sites are numerous and similar in energy but, significantly, differ between the two structural phases of the material. Despite the difference in the sites, the muon remains a faithful probe of the transition, revealing a dynamically-fluctuating magnetically disordered state in the low-temperature structural phase. In contrast, in the high temperature phase the relaxation is caused by static nuclear moments, with rapid electronic dynamics being motionally narrowed from the muon spectra.

Combined resistance noise and muon-spin relaxation ($\mu$SR) measurements of the ferromagnetic semiconductor HgCr$_2$Se$_4$ suggest a degree of magnetoelectric coupling and provide evidence for the existence of isolated magnetic polarons. These form at elevated temperatures and undergo a percolation transition with a drastic enhancement of the low-frequency 1/$f$-type charge fluctuations at the insulator-to-metal transition at $\sim 95 - 98$ K in the vicinity of the magnetic ordering temperature $T_C \sim 105 - 107$ K. Upon approaching the percolation threshold from above, the strikingly unusual dynamics of a distinct two-level fluctuator superimposed on the $1/f$ noise can be described by a slowing down of the dynamics of a nanoscale magnetic cluster, a magnetic polaron, when taking into account an effective radius of the polaron depending on the spin correlation length. Coinciding temperature scales found in $\mu$SR and noise measurements suggest changes in the magnetic dynamics over a wide range of frequencies and are consistent with the existence of large polarized and domain-wall-like regions at low temperatures, that result from the freezing of spin dynamics at the magnetic polaron percolation transition.

The molecular coordination complex NiI$_2$(3,5-lut)$_4$ [where (3,5-lut) $=$ (3,5-lutidine) $=$ (C$_7$H$_9$N)] has been synthesized and characterized by several techniques including synchrotron X-ray diffraction, ESR, SQUID magnetometry, pulsed-field magnetization, inelastic neutron scattering and muon spin relaxation. Templated by the configuration of 3,5-lut ligands the molecules pack in-registry with the Ni--I$\cdots$I--Ni chains aligned along the $c$--axis. This arrangement leads to through-space I$\cdots$I magnetic coupling which is directly measured for the first time in this work. The net result is a near-ideal realization of the $S = 1$ Haldane chain with $J = 17.5~\rm{K}$ and energy gaps of $\Delta^{\parallel} = 5.3~{\rm K}$ $\Delta^{\perp} =7.7~{\rm K}$, split by the easy-axis single-ion anisotropy $D=-1.2~{\rm K}$. The ratio $D/J = -0.07$ affords one of the most isotropic Haldane systems yet discovered, while the ratio $\Delta_0/J = 0.40(1)$ (where $\Delta_0$ is the average gap size) is close to its ideal theoretical value, suggesting a very high degree of magnetic isolation of the spin chains in this material. The Haldane gap is closed by orientation-dependent critical fields $\mu_0H_{\rm c}^{\parallel} = 5.3~\rm{T}$ and $\mu_0H_{\rm c}^{\perp} = 4.3~\rm{T}$, which are readily accessible experimentally and permit investigations across the entirety of the Haldane phase, with the fully polarized state occurring at $\mu_0 H_{\rm s}^{\parallel}=46.0~\rm{T}$ and $\mu_0 H_{\rm s}^{\perp}=50.7~\rm{T}$. The results are explicable within the so-called fermion model, in contrast to other reported easy-axis Haldane systems. Zero-field magnetic order is absent down to $20~{\rm mK}$ and emergent end-chain effects are observed in the gapped state, as evidenced by detailed low-temperature measurements., Comment: 13 pages, 8 figures plus supporting information

We present the results of a muon-spin spectroscopy investigation of GaV$_4$S$_{8-y}$Se$_{y}$ with $y=0, 2, 4$ and 8. Zero-field measurements suggest that GaV$_{4}$Se$_{8}$ and GaV$_{4}$S$_{8}$ have distinct magnetic ground states, with the latter material showing an anomalous temperature-dependence of the local magnetic field. It is not possible to evolve the magnetic state continuously between these two systems, with the intermediate $y=2$ and $4$ materials showing glassy magnetic behaviour at low temperature. The skyrmion lattice (SkL) phase is evident in the $y=0$ and 8 materials through an enhanced response of the muon-spin relaxation to the emergent dynamics that accompany the SkL. For our polycrystalline samples of GaV$_4$Se$_{8}$, this enhanced dynamic response is confined to a smaller region of the magnetic field-temperature phase diagram than the previous reports of the SkL in single crystals.

We have determined the superconducting and magnetic properties of four samples of Lu$_x$Zr$_{1-x}$B$_{12}$ ($x=0.04$, $0.07$, $0.17$, and $0.8$) using muon spin rotation ($\mu$SR) and magnetometry measurements. We observed a strong magnetic signal in both the $\mu$SR and magnetometry data in one sample ($x=0.07$), likely caused by the formation of static moments of size $\approx 1\,\mu_{\rm B}$ due to a clustering effect of the Lu$^{3+}$ ions. In all other samples, we find only a small magnetic signal in the $\mu$SR data thought to originate from boron nuclei in the B$_{12}$ cages. The superconductivity is found to evolve with $x$, with a decrease in $x$ resulting in an increase in critical temperature and a decrease of the penetration depth. Most remarkably, we find the formation of nodes in the superconducting gap for $x \leq 0.17$, providing a new example of an $s$-to-$d$-wave crossover in a superconductor., Comment: 5 pages, 2 figures

LiOsO$_3$ is the first example of a new class of material called a ferroelectric metal. We performed zero-field and longitudinal-field $\mu$SR, along with a combination of electronic structure and dipole field calculations, to determine the magnetic ground state of LiOsO$_3$. We find that the sample contains both static Li nuclear moments and dynamic Os electronic moments. Below $\approx 0.7\,$K, the fluctuations of the Os moments slow down, though remain dynamic down to 0.08$\,$K. We expect this could result in a frozen-out, disordered ground state at even lower temperatures., Comment: 6 pages, 1 figure

We study by means of bulk and local probes the d-metal alloy Ni$_{1-x}$V$_x$ close to the quantum critical concentration, $x_c \approx 11.6\%$, where the ferromagnetic transition temperature vanishes. The magnetization-field curve in the ferromagnetic phase takes an anomalous power-law form with a nonuniversal exponent that is strongly $x$-dependent and mirrors the behavior in the paramagnetic phase. Muon spin rotation experiments demonstrate inhomogeneous magnetic order and indicate the presence of dynamic fluctuating magnetic clusters. These results provide strong evidence for a quantum Griffiths phase on the ferromagnetic side of the quantum phase transition., Comment: 6 pages + 3 pages supplemental material, final version as published

We have determined the superconducting and magnetic properties of a hydrothermally synthesized powder sample of tetragonal FeS using muon spin rotation ({\mu}SR). The superconducting properties are entirely consistent with those of a recently published study, showing fully gapped behavior and giving a penetration depth of {\lambda}_{ab} = 204(3) nm. However, our zero-field {\mu}SR data are rather different and indicate the presence of a small, non-superconducting magnetic phase within the sample. These results highlight that sample-to-sample variations in magnetism can arise in hydrothermally prepared phases, but interestingly the superconducting behavior is remarkably insensitive to these variations., Comment: 5 pages, 2 figures

Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional antiferromagnets, namely, [Cu(HF$_2$)(pyz)$_2$]ClO$_4$ [pyz = pyrazine], [Cu$L_2$(pyz)$_2$](ClO$_4$)$_2$ [$L$ = pyO = pyridine-N-oxide and 4-phpyO = 4-phenylpyridine-N-oxide. These materials are shown to possess equivalent two-dimensional [Cu(pyz)$_2$]$^{2+}$ nearly square layers, but exhibit interlayer spacings that vary from 6.5713~\AA ~to 16.777~\AA, as dictated by the axial ligands. We present the structural and magnetic properties of this family as determined via x-ray diffraction, electron-spin resonance, pulsed- and quasistatic-field magnetometry and muon-spin rotation, and compare them to those of the prototypical two-dimensional magnetic polymer Cu(pyz)$_2$(ClO$_4$)$_2$. We find that, within the limits of the experimental error, the two-dimensional, {\it intralayer} exchange coupling in our family of materials remains largely unaffected by the axial ligand substitution, while the observed magnetic ordering temperature decreases slowly with increasing layer separation. Despite the structural motifs common to this family and Cu(pyz)$_2$(ClO$_4$)$_2$, the latter has significantly stronger two-dimensional exchange interactions and hence a higher ordering temperature. We discuss these results, as well as the mechanisms that might drive the long-range order in these materials, in terms of departures from the ideal $S=1/2$ two-dimensional square-lattice Heisenberg antiferromagnet. In particular, we find that both spin exchange anisotropy in the intralayer interaction and interlayer couplings (exchange, dipolar, or both) are needed to account for the observed ordering temperatures, with the intralayer anisotropy becoming more important as the layers are pulled further apart., Comment: 17 pages, 5 figures

The ground state of the quantum spin system kappa-(BEDT-TTF)2Cu2(CN)3 in which antiferromagnetically-interacting S=1/2 spins are located on a nearly equilateral triangular lattice attracts considerable interest both from experimental and theoretical aspects, because a simple antiferromagnetic order may be inhibited because of the geometrical frustration and hence an exotic ground state is expected. Furthermore, recent two reports on the ground state of this system have made it further intriguing by showing completely controversial results; one indicates the gapless state and the other gapped. By utilizing microscopic probe of muSR, we have investigated its spin dynamics below 0.1 K, unveiling its microscopically phase separated ground state at zero field.

Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy. Here we demonstrate a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an $S=1/2$ antiferromagnet along one crystal direction, switching the system from being quasi-two- to quasi-one-dimensional while effectively maintaining the nearest-neighbour coupling strength., Comment: 8 pages, 6 figures

We present the results of a muon-spin relaxation (muSR) investigation into magnetic ordering in several families of layered quasi-two-dimensional molecular antiferromagnets based on transition metal ions such as S=1/2 Cu2+ bridged with organic ligands such as pyrazine. In many of these materials magnetic ordering is difficult to detect with conventional magnetic probes. In contrast, muSR allows us to identify ordering temperatures and study the critical behavior close to T_N . Combining this with measurements of in-plane magnetic exchange J and predictions from quantum Monte Carlo simulations we may assess the degree of isolation of the 2D layers through estimates of the effective inter-layer exchange coupling and in-layer correlation lengths at T_N . We also identify the likely metal-ion moment sizes and muon stopping sites in these materials, based on probabilistic analysis of the magnetic structures and of muon-fluorine dipole-dipole coupling in fluorinated materials., Comment: Text and figures as published in Physical Review B

The magnetic ground states of the isostructural double perovskites Ba$_2$NaOsO$_6$ and Ba$_2$LiOsO$_6$ are investigated with muon-spin rotation. In Ba$_2$NaOsO$_6$ long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below $T_{\mathrm{c}}=7.2\pm 0.2\;\mathrm{K}$, while in Ba$_2$LiOsO$_6$ a static but spatially-disordered internal field is found below 8 K. A novel probabilistic argument is used to show from the observed precession frequencies that the magnetic ground state in Ba$_2$NaOsO$_6$ is most likely to be low-moment ($\approx 0.2$\,$\mu_{\rm B}$) ferromagnetism and not canted antiferromagnetism. Ba$_2$LiOsO$_6$ is antiferromagnetic and we find a spin-flop transition at 5.5\,T. A reduced osmium moment is common to both compounds, probably arising from a combination of spin-orbit coupling and frustration., Comment: 4 pages, 4 figures

Muon spin spectroscopy and in particular the avoided level crossing technique is introduced, with the aim of showing it as a very sensitive local probe for electron spin relaxation in organic semiconductors. Avoided level crossing data on TMS-pentacene at different temperatures are presented, and they are analysed to extract the electron spin relaxation rate, that is shown to increase on increasing the temperature from 0.02 MHz to 0.33 MHz at 3 K and 300 K respectively., Comment: International Conference TSN2010 "Trends in spintronics and nanomagnetism"

Using a combination of neutron, muon and synchrotron techniques we show how the magnetic state in NaFeAs can be tuned into superconductivity by replacing Fe by either Co or Ni. Electron count is the dominant factor, since Ni-doping has double the effect of Co-doping for the same doping level. We follow the structural, magnetic and superconducting properties as a function of doping to show how the superconducting state evolves, concluding that the addition of 0.1 electrons per Fe atom is sufficient to traverse the superconducting domain, and that magnetic order coexists with superconductivity at doping levels less than 0.025 electrons per Fe atom., Comment: 4 pages, 6 figures