arXiv subjects

S. J. Blundell

Publications and source records attributed to S. J. Blundell.

At least 73 records · Page 4Linked to original sources

arXiv2013

Local magnetism and spin correlations in the geometrically frustrated cluster magnet LiZn$_2$Mo$_3$O$_8$

LiZn$_2$Mo$_3$O$_8$ has been proposed to contain $S~=~1/2$ Mo$_3$O$_{13}$ magnetic clusters arranged on a triangular lattice with antiferromagnetic nearest-neighbor interactions. Here, microwave and terahertz electron spin resonance (ESR), $^7$Li nuclear magnetic resonance (NMR), and muon spin rotation ($μ\textrm{SR}$) spectroscopies are used to characterize the local magnetic properties of LiZn$_2$Mo$_3$O$_8$. These results show the magnetism in LiZn$_2$Mo$_3$O$_8$ arises from a single isotropic $S~=~1/2$ electron per cluster and that there is no static long-range magnetic ordering down to $T~=~0.07\,\textrm{K}$. Further, there is evidence of gapless spin excitations with spin fluctuations slowing down as the temperature is lowered. These data indicate strong spin correlations which, together with previous data, suggest a low-temperature resonating valence-bond state in LiZn$_2$Mo$_3$O$_8$.

cond-mat.str-el↗

arXiv2013

Controlling magnetic order and quantum disorder in one- and zero-dimensional molecule-based magnets

We investigate the structural and magnetic properties of two molecule-based magnets synthesized from the same starting components. Their different structural motifs promote contrasting exchange pathways and consequently lead to markedly different magnetic ground states. Through examination of their structural and magnetic properties we show that [Cu(pyz)(H$_{2}$O)(gly)$_{2}$](ClO$_{4}$)$_{2}$ may be considered a quasi-one-dimensional quantum Heisenberg antiferromagnet while the related compound [Cu(pyz)(gly)](ClO$_{4}$), which is formed from dimers of antiferromagnetically interacting Cu$^{2+}$ spins, remains disordered down to at least 0.03 K in zero field, but shows a field-temperature phase diagram reminiscent of that seen in materials showing a Bose-Einstein condensation of magnons.

cond-mat.str-el↗

arXiv2013

Magnetic fluctuations and spin freezing in nonsuperconducting LiFeAs derivatives

We present detailed magnetometry and muon-spin rotation data on polycrystalline samples of overdoped, non-superconducting LiFe$_{1-x}$Ni$_x$As ($x = 0.1,\,0.2$) and Li$_{1-y}$Fe$_{1+y}$As ($0\leq y\leq 0.04$) as well as superconducting LiFeAs. While LiFe$_{1-x}$Ni$_x$As exhibits weak antiferromagnetic fluctuations down to $1.5\,{\rm K}$, Li$_{1-y}$Fe$_{1+y}$As samples, which have a much smaller deviation from the $1:1:1$ stoichiometry, show a crossover from ferromagnetic to antiferromagnetic fluctuations on cooling and a freezing of dynamically fluctuating moments at low temperatures. We do not find any signatures of time-reversal symmetry breaking in stoichiometric LiFeAs that would support recent predictions of triplet pairing.

cond-mat.supr-con↗

arXiv2013

Magnetic transition and spin dynamics in the triangular Heisenberg antiferromagnet $α$-KCrO$_{2}$

We present the results of muon-spin relaxation measurements on the triangular lattice Heisenberg antiferromagnet $α$-KCrO$_{2}$. We observe sharp changes in behaviour at an ordering temperature of $T_{\mathrm{c}}=23$ K, with an additional broad feature in the muon-spin relaxation rate evident at T=13 K, both of which correspond to features in the magnetic contribution to the heat capacity. This behaviour is distinct from both the Li- and Na- containing members of the series. These data may be qualitatively described with the established theoretical predictions for the underlying spin system.

cond-mat.str-el↗

arXiv2013

Playing quantum hide-and-seek with the muon: localizing muon stopping sites

One of the most fundamental limitations of a muon-spin relaxation experiment can be the lack of knowledge of the implantation site of the muon and the uncertainty about the muon's perturbation of its host. Here we review some of the work done on the `muon site problem' in the solid state and highlight some recent applications of electronic structure calculations that have successfully characterized the quantum states of muons in a number of insulating compounds containing fluorine, in a number of pnictide superconductors, and in ZnO.

cond-mat.str-el↗

arXiv2013

Weak magnetic transitions in pyrochlore Bi2Ir2O7

Muon spin relaxation measurements on Bi2Ir2O7 show that it undergoes a bulk magnetic transition at 1.84(3) K. This is accompanied by increases in the muon spin relaxation rate and the amplitude of the non-relaxing part of the signal. The magnetic field experienced by muons is estimated to be 0.7 mT at low-temperature, around two orders of magnitude smaller than that in other pyrochlore iridates. These results suggest that the low-temperature state represents ordering of exceptionally small magnetic moments with persistent weak dynamics. The relaxation rate increases further below 0.23(4) K, coincident with a growth in the specific heat, suggesting the existence of a second low-temperature magnetic transition.

cond-mat.str-el↗

arXiv2013

Another dimension: investigations of molecular magnetism using muon-spin relaxation

We review examples of muon-spin relaxation measurements on molecule-based magnetic coordination polymers, classified by their magnetic dimensionality. These include the one-dimensional s=1/2 spin chain Cu(pyz)(NO3)2 and the two-dimensional s=1/2 layered material [Cu(HF2)(pyz)2]BF4. We also describe some of the more exotic ground states that may become accessible in the future given the ability to tune the interaction strengths of our materials through crystal engineering.

cond-mat.str-el↗

arXiv2013

Evolution of magnetic interactions in a pressure-induced Jahn-Teller driven magnetic dimensionality switch

We present the results of high-field magnetization and muon-spin relaxation measurements on the coordination polymer CuF_2(H_2O)_2(pyrazine) in pressures up to 22.5 kbar. We observe a transition from a quasi-two-dimensional to a quasi-one-dimensional antiferromagnetic phase at 9.1 kbar, driven by a rotation of the Jahn-Teller axis. Long-range antiferromagnetic ordering is seen in both regimes, as well as a phase separation in the critical pressure region. The magnetic dimensionality switching as pressure is increased is accompanied by a halving of the primary magnetic exchange energy J and a fivefold decrease in the ordering temperature T_N. J decreases gradually with pressure in the two-dimensional phase, and then increases in the one-dimensional regime. We relate both effects to the changes in the crystal structure with applied pressure.

cond-mat.str-el↗

arXiv2013

Quantum states of muons in fluorides

Muon-spin relaxation (μSR) is a sensitive probe of magnetism, but its utility can be severely limited by the lack of knowledge of the muon implantation site and the extent to which the muon perturbs its host. We demonstrate systematically that these problems can be addressed accurately using electronic-structure calculations. We show that diamagnetic muons introduce significant short-ranged distortions in ionic insulators that would lead to systematic errors on magnetic moments determined by μSR, and quantify these. The F--μ--F complex formed by muons in many fluorides can be understood as an exotic molecule-in-a-crystal defect with a zero-point energy larger than that of any naturally-occurring triatomic molecule.

cond-mat.str-el↗

arXiv2012

Odd-frequency pairing in (TMTSF)2ClO4

The low field phase of the organic superconductor (TMTSF)2ClO4 is studied by muon spin rotation. The zero temperature limit of the magnetic penetration depth within the TMTSF layers is obtained to be lambda_{ab}(0) = 0.86(2) um. Temperature dependence of the muon spin relaxation shows no indication of gap nodes on the Fermi surface nor of any spontaneous fields due to time-reversal-symmetry breaking. The weight of evidence suggests that the symmetry of this low field phase is odd-frequency p-wave singlet; a novel example of odd-frequency pairing in a bulk superconductor.

cond-mat.supr-con↗

arXiv2012

Monopoles, magnetricity and the stray field from spin ice

An analysis is presented of the behaviour of muons in the low-temperature state in spin ice. It is shown in detail how the behavior observed in some previous muon experiments on spin ice in a weak transverse field may result from the macroscopic stray field of magnetized spin ice. A model is presented which allows these macroscopic field effects to be simulated and the results agree with experiment. The persistent spin dynamics at low temperature originate from the sample and could be a muon-induced implantation effect that is operative in out-of-equilibrium systems with long relaxation times.

cond-mat.str-el↗

arXiv2012

Persistent dynamics in the S=1/2 quasi-one-dimensional chain compound Rb4Cu(MoO4)3 probed with muon-spin relaxation

We report the results of muon-spin relaxation measurements on the low-dimensional antiferromagnet Rb4Cu(MoO4)3. No long-range magnetic order is observed down to 50 mK implying a ratio T_N/J<0.005 (where J is the principal exchange strength along the spin chains) and an effective ratio of interchain to intrachain exchange of |J_perp/J|<2 x 10^-3, making the material an excellent realization of a one-dimensional quantum Heisenberg antiferromagnet. We probe the persistent spin excitations at low temperatures and find that ballistic spin transport dominates the excitations detected below 0.3 K.

cond-mat.str-el↗

arXiv2011

The gradual destruction of magnetism in the superconducting family NaFe(1-x)CoxAs

The interplay and coexistence of superconducting, magnetic and structural order parameters in NaFe{1-x}Co{x}As has been studied using SQUID magnetometry, muon-spin rotation and synchrotron x-ray powder diffraction. Substituting Fe by Co weakens the ordered magnetic state through both a suppression of T_N and a reduction in the size of the ordered moment. Upon further substitution of Fe by Co the high sensitivity of the muon as a local magnetic probe reveals a magnetically disordered phase, in which the size of the moment continues to decrease and falls to zero around the same point at which the magnetically-driven structural distortion is no longer resolvable. Both magnetism and the structural distortion are weakened as the robust superconducting state is established.

cond-mat.supr-con↗

arXiv2011

A Bayesian approach to magnetic moment determination using muSR

A significant challenge in zero-field muSR experiments arises from the uncertainty in the muon site. It is possible to calculate the dipole field (and hence precession frequency nu) at any particular site given the magnetic moment mu and magnetic structure. One can also evaluate f(nu), the probability distribution function of nu assuming that the muon site can be anywhere within the unit cell with equal probability, excluding physically forbidden sites. Since nu is obtained from experiment, what we would like to know is g(mu|nu), the probability density function of mu given the observed nu. This can be obtained from our calculated f(nu/mu) using Bayes' theorem. We describe an approach to this problem which we have used to extract information about real systems including a low-moment osmate compound, a family of molecular magnets, and an iron-arsenide compound.

cond-mat.other↗

arXiv2011

Probing magnetic order in LiMPO4, M = Ni, Co, Fe and lithium diffusion in LixFePO4

Muon spin relaxation measurements are reported on three members of the LixMPO4 series. The magnetic properties of stoichiometric samples with M = Ni, Co, Fe, were investigated at low-temperature. In LiNiPO4 we observe different forms of the muon decay asymmetry in the commensurate and incommensurate antiferromagnetic phases, accompanied by a change in the temperature dependence of the muon oscillation frequency. In LiCoPO4 the form of the muon decay asymmetry indicates that the correlation between layers decreases as the Neel temperature is approached from below. LiFePO4 shows more conventional behaviour, typical for an antiferromagnet. Measurements on LixFePO4 with x = 0.8, 0.9, and 1 show evidence for lithium diffusion below ~250 K and muon diffusion dominating the form of the relaxation at higher temperature. The thermally activated form of the observed hopping rate suggests an activation barrier for lithium diffusion of ~ 100 meV and a diffusion constant of D_Li ~ 10^-10 - 10^-9 cm2/s at room temperature.

cond-mat.str-el↗

arXiv2011

Proximal magnetometry of monolayers of magnetic moments

We present a method to measure the magnetic properties of monolayers and ultra-thin films of magnetic material. The method is based on low energy muon spin rotation and $β$-detected nuclear magnetic resonance measurements. A spin probe is used as a "proximal" magnetometer by implanting it in the substrate, just below the magnetic material. We calculate the expected magnetic field distribution sensed by the probe and discuss its temperature and implantation depth dependencies. This method is highly suitable for measuring the magnetic properties of monolayers of single molecule magnets, but can also be extended to ultra-thin magnetic films.

cond-mat.mes-hall↗

arXiv2011

Observation of a level crossing in a molecular nanomagnet using implanted muons

We have observed an electronic energy level crossing in a molecular nanomagnet (MNM) using muon-spin relaxation. This effect, not observed previously despite several muon studies of MNM systems, provides further evidence that the spin relaxation of the implanted muon is sensistive to the dynamics of the electronic spin. Our measurements on a broken ring MNM [H_{2}N^{t}Bu^{is}Pr][Cr_{8}CdF_{9}(O_{2}CC(CH_{3})_{3})_{18}] (hereafter Cr_{8}Cd), which contains eight Cr ions, show clear evidence for the S=0 to S=1 transition that takes place at B_{c}=2.3 T. The crossing is observed as a resonance-like dip in the average positron asymmetry and also in the muon-spin relaxation rate, which shows a sharp increase in magnitude at the transition and a peak centred within the S=1 regime.

cond-mat.str-el↗

arXiv2011

Measurement of the internal magnetic field in the correlated iridates Ca$_4$IrO$_6$, Ca$_5$Ir$_3$O$_{12}$, Sr$_3$Ir$_2$O$_7$ and Sr$_2$IrO$_4$

Oxides containing iridium ions display a range of magnetic and conducting properties that depend on the delicate balance between interactions and are controlled, at least in part, by the details of the crystal architecture. We have used muon-spin rotation ($μ$SR) to study the local field in four iridium oxides, Ca$_4$IrO$_6$, Ca$_5$Ir$_3$O$_{12}$, Sr$_3$Ir$_2$O$_7$ and Sr$_2$IrO$_4$, which show contrasting behavior. Our $μ$SR data on Ca$_4$IrO$_6$ and Ca$_5$Ir$_3$O$_{12}$ are consistent with conventional antiferromagnetism where quasistatic magnetic order develops below $T_{\rm N}=13.85(6)$ K and 7.84(7) K respectively. A lower internal field is observed for Ca$_5$Ir$_3$O$_{12}$, as compared to Ca$_4$IrO$_6$ reflecting the presence of both Ir$^{4+}$ and Ir$^{5+}$ ions, resulting in a more magnetically dilute structure. Muon precession is only observed over a restricted range of temperature in Sr$_3$Ir$_2$O$_7$, while the Mott insulator Sr$_2$IrO$_4$ displays more complex behavior, with the $μ$SR signal containing a single, well-resolved precession signal below $T_{\rm N}=230$\,K, which splits into two precession signals at low temperature following a reorientation of the spins in the ordered state.

cond-mat.str-el↗