arXiv subjects

F. L. Pratt

Publications and source records attributed to F. L. Pratt.

At least 55 records · Page 3Linked to original sources

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

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↗

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

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

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↗

arXiv2010

The phase transition in the localized ferromagnet EuO probed by muSR

We report results of muon spin rotation measurements performed on the ferromagnetic semiconductor EuO, which is one of the best approximations to a localized ferromagnet. We argue that implanted muons are sensitive to the internal field primarily through a combination of hyperfine and Lorentz fields. The temperature dependences of the internal field and the relaxation rate have been measured and are compared with previous theoretical predictions.

cond-mat.mtrl-sci↗

arXiv2010

Muon spin relaxation investigation of magnetic ordering in the hybrid organic-inorganic perovskites [(CH3)2NH2]M(HCOO)3, M = Ni,Co,Mn,Cu

Muon spin relaxation measurements are reported on samples of dimethylammonium metal formates containing magnetic divalent nickel, cobalt, manganese, and copper ions. These hybrid organic-inorganic perovskites exhibit weak ferromagnetism and are, apart from the copper system, multiferroics with well separated magnetic and antiferroelectric transitions. We use muons to follow the sublattice magnetization, observing the effect of the spin reorientation transitions below TN and the criticality approaching TN. The multiferroic samples have three-dimensional antiferromagnetic interactions, but the copper sample shows quasi-one-dimensional behavior due to its Jahn-Teller distorted structure, with a ratio of its inter- and intrachain exchange constants j/J=0.037.

cond-mat.str-el↗

arXiv2010

Superconductivity and fluctuating magnetism in quasi two-dimensional kappa-(BEDT-TTF)2Cu[N(CN)2]Br probed with implanted muons

A muon-spin relaxation (muSR) investigation is presented for the molecular superconductor kappa-(BEDT-TTF)2Cu[N(CN)2Br]. Evidence is found for low-temperature phase-separation, with only a fraction of the sample showing a superconducting signal, even for slow cooling. Rapid cooling reduces the superconducting fraction still further. For the superconducting phase, the in-plane penetration depth is measured to be lambda_{parallel} = 0.47(1) mu m and evidence is seen for a vortex decoupling transition in applied fields above 40 mT. The magnetic fluctuations in the normal state produce Korringa behavior of the muon spin relaxation rate below 100 K, a precipitous drop in relaxation rate is seen at higher temperatures and an enhanced local spin susceptibility occurs just above T_c.

cond-mat.supr-con↗

arXiv2010

The importance of intra-molecular electron spin relaxation in small molecular semiconductors

Electron spin relaxation rate (eSR) is investigated on several organic semiconductors of different morphologies and molecular structures, using avoided level crossing muon spectroscopy as a local spin probe. We find that two functionalized acenes (polycrystalline tri(isopropyl)silyl-pentacene and amorphous 5,6,11,12-tetraphenyltetracene) exhibit eSRs with an Arrhenius-like temperature dependence, each with two characteristic energy scales similar to those expected from vibrations. Polycrystalline tris(8-hydroxyquinolate)gallium shows a similar behavior. The observed eSR for these molecules is no greater than 0.85 MHz at 300 K. The variety of crystal structures and transport regimes that these molecules possess, as well as the local nature of the probe, strongly suggest an intra-molecular phenomenon general to many organic semiconductors, contrasting the commonly assumed spin relaxation models based on inter-molecular charge carrier transport.

cond-mat.mtrl-sci↗

arXiv2010

Muon-spin relaxation and heat capacity measurements on the magnetoelectric and multiferroic pyroxenes LiFeSi2O6 and NaFeSi2O6

The results of muon-spin relaxation and heat capacity measurements on two pyroxene compounds LiFeSi2O6 and NaFeSi2O6 demonstrate that despite their underlying structural similarity the magnetic ordering is considerably different. In LiFeSi2O6 a single muon precession frequency is observed below TN, consistent with a single peak at TN in the heat capacity and a commensurate magnetic structure. In applied magnetic fields the heat capacity peak splits in two. In contrast, for natural NaFeSi2O6, where multiferroicity has been observed in zero-magnetic-field, a rapid Gaussian depolarization is observed showing that the magnetic structure is more complex. Synthetic NaFeSi2O6 shows a single muon precession frequency but with a far larger damping rate than in the lithium compound. Heat capacity measurements reproduce the phase diagrams previously derived from other techniques and demonstrate that the magnetic entropy is mostly associated with the build up of correlations in the quasi-one-dimensional Fe3+ chains.

cond-mat.str-el↗

arXiv2009

What relaxes muon spins in molecular nanomagnets?

We address the cause of the unusual muon spin relaxation (muSR) results on molecular nanomagnets (MNMs). Through measurements on protonated and deuterated samples of the MNMs Cr7Mn (S=1) and Cr8 (S=0), we show that the muon spin for $S \neq 0$ MNMs is relaxed via dynamic fluctuations of the electronic spins. A freezing out of dynamic processes occurs on cooling and at low temperatures the muon spins are relaxed by the electronic spins which themselves are dephased by incoherent nuclear field fluctuations.We observe a transition to a state of static magnetic order of the MNM electronic spins in Cr7Mn below 2 K.

cond-mat.str-el↗

arXiv2009

Local magnetism and magnetoelectric effect in HoMnO3 studied with muon-spin relaxation

We present the results of muon-spin relaxation (muSR) measurements on the hexagonal manganite HoMnO3. Features in the temperature-dependent relaxation rate, lambda, correlate with the magnetic transitions at 76 K, 38 K and 34 K. The highest temperature transition, associated with the ordering of Mn3+ moments has the largest effect on lambda. The application of a static electric field of E=10^4 Vm^-1 below T=50 K causes a small reduction in lambda which is suggestive of coupling between ferroelectric and magnetic domain walls in the ordered state of the material.

cond-mat.str-el↗