Muon Spin Relaxation Data Research Articles

Signatures of pancake bonding in hydrated eumelanin.

Pancake bonding phenomenology is applied for the first time in a bioorganic system, the pigment eumelanin, via a hydration-induced decrease of the interplanar distance down to 3.19 Å. The observation explains the long-term inconsistency between electron paramagnetic resonance and muon spin relaxation data for eumelanin.

Spin-orbit coupling controlled ground states in the double perovskite iridates A2BIrO6 ( A= Ba, Sr; B= Lu, Sc)

Iridates with the $5{d}^{4}$ electronic configuration have attracted recent interest due to reports of magnetically ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a $J=0$ electronic ground state for each ${\mathrm{Ir}}^{5+}$ ion. The major focus of prior research has been on the double perovskite iridates ${\mathrm{Ba}}_{2}{\mathrm{YIrO}}_{6}$ and ${\mathrm{Sr}}_{2}{\mathrm{YIrO}}_{6}$, where the nature of the ground states (i.e., ordered vs nonmagnetic) is still controversial. Here, we present neutron powder diffraction, high-energy-resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), resonant inelastic x-ray scattering (RIXS), magnetic susceptibility, and muon spin relaxation data on the related double perovskite iridates ${\mathrm{Ba}}_{2}{\mathrm{LuIrO}}_{6}, {\mathrm{Sr}}_{2}{\mathrm{LuIrO}}_{6}, {\mathrm{Ba}}_{2}{\mathrm{ScIrO}}_{6}$, and ${\mathrm{Sr}}_{2}{\mathrm{ScIrO}}_{6}$ that enable us to gain a general understanding of the electronic and magnetic properties for this family of materials. Our HERFD-XAS and RIXS measurements establish $J=0$ electronic ground states for the ${\mathrm{Ir}}^{5+}$ ions in all cases, with similar values for Hund's coupling ${J}_{\mathrm{H}}$ and the spin-orbit coupling constant ${\ensuremath{\lambda}}_{\mathrm{SOC}}$. Our bulk susceptibility and muon spin relaxation data find no evidence for long-range magnetic order or spin freezing, but they do exhibit weak magnetic signals that are consistent with extrinsic local moments. Our results indicate that the large ${\ensuremath{\lambda}}_{\mathrm{SOC}}$ is the key driving force behind the electronic and magnetic ground states realized in the $5{d}^{4}$ double perovskite iridates, which agrees well with conventional wisdom.

Experimental evidence for a valence-bond glass in the 5d1 double perovskite Ba2YWO6

We report the magnetic susceptibility, specific heat, and muon spin relaxation results of the $5{d}^{1}$ double perovskite ${\mathrm{Ba}}_{2}{\mathrm{YWO}}_{6}$. The dc magnetic susceptibility shows two distinct Curie-Weiss regimes and a sub-Curie-Weiss increase ${T}^{\ensuremath{-}n}$ with $n=0.75(1)$ for $T=3\text{--}100$ K, alluding to the presence of random magnetism. The ac magnetic susceptibility reveals a spin freezing at ${T}_{f}\ensuremath{\sim}0.3$ K with the activation energy of $\mathrm{\ensuremath{\Delta}}/{k}_{B}=27.6$ K. The specific heat data exhibit the pseudogap behavior at 25 K and the subsequent power-law dependence with decreasing temperature, indicating the gradual spin freezing with the quenching of orbital fluctuations. However, the muon spin relaxation data display only a weak muon spin depolarization with lacking long-range magnetic ordering down to 26 mK. Taken together, our results suggest that ${\mathrm{Ba}}_{2}{\mathrm{YWO}}_{6}$ is the proximate realization of a random spin-orbit dimer state.

Slow magnetic fluctuations and critical slowing down in Sr2Ir1−xRhxO4

Hidden magnetic order of Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$, $x = 0.05$ and 0.1, has been studied using muon spin relaxation spectroscopy. In zero applied field and weak longitudinal fields ($\mu_0H_L \lesssim 2$~mT), muon spin relaxation data can be well described by exponentially-damped static Lorentzian Kubo-Toyabe functions, indicating that static and dynamic local fields coexist at each muon site. For $\mu_0H_L \gtrsim 2$~mT, the static rate is completely decoupled, and the exponential decay is due to dynamic spin fluctuations. In both zero field and $\mu_0H_L = 1$--2~mT, the temperature dependencies of the exponential muon spin relaxation rate exhibit maxima at 215~K for $x = 0.05$ and 175~K for $x = 0.1$, suggesting critical slowing down of electronic spin fluctuations. The field dependencies of the dynamic spin fluctuation rates can be well described by the Redfield relation. The correlation time of this electronic spin fluctuation is in the range of~2--5~ns for Sr$_2$Ir$_{0.9}$Rh$_{0.1}$O$_4$, and shorter than 2~ns for Sr$_2$Ir$_{0.95}$Rh$_{0.05}$O$_4$. The rms fluctuating field is on the order of 1 mT, which is consistent with the polarized neutron diffraction cross-section.

Muon-spin relaxation and AC magnetometry study of the ferrimagnet LaSr2Cr2SbO9

AC susceptibility and muon spin relaxation data have been collected from a polycrystalline sample of LaSr2Cr2SbO9, a perovskite-like compound wherein the unequal distribution of Cr3+ and Sb5+ cations over two crystallographically-distinct six-coordinate B sites results in the onset of ferrimagnetism at ~150 K. The data are used to elucidate the dynamics of the ferrimagnetic domain walls, thought to lie in Sb-rich regions, and suggest an energy barrier of ~0.13 eV to their motion. The muon data confirm that the ferrimagnetic order is a true bulk phenomenon. The behaviour of this material is briefly compared to that of Ni2+- based relaxor ferromagnets.

Multigap superconductivity in chiral noncentrosymmetric TaRh2B2

We report the first observation of multigap superconductivity in TaRh$_{2}$B$_{2}$. We show TaRh$_{2}$B$_{2}$ is a bulk type-II superconductor with a transition temperature, $T_{\mathrm{c}} = 6.00(5)$ K. We present transverse-field muon spin relaxation data where the superconducting gap can be fit using a two-gap $\left(s+s\right)$-wave model. We also report the zero-field electronic specific heat in the superconducting state that is best described by the same $\left(s+s\right)$ model providing further evidence of multiband behavior in this superconductor. Zero-field muon spin relaxation measurements show time-reversal symmetry is preserved in the superconducting state. We demonstrate that TaRh$_{2}$B$_{2}$ has an upper critical field of $15.2(1)$ T, which is significantly higher than previously reported and exceeds the Pauli limit.

Coupled commensurate charge density wave and lattice distortion inNa2Ti2Pn2O(Pn=As,Sb)determined by x-ray diffraction and angle-resolved photoemission spectroscopy

We report single-crystal x-ray-diffraction measurements on ${\mathrm{Na}}_{2}{\mathrm{Ti}}_{2}P{n}_{2}\mathrm{O}\phantom{\rule{0.28em}{0ex}}(Pn=\mathrm{As},\mathrm{Sb})$ which reveal a charge superstructure that appears below the density wave transitions previously observed in bulk data. From symmetry-constrained structure refinements we establish that the associated distortion mode can be described by two propagation vectors ${\mathbf{q}}_{1}=(1/2,0,l)$ and ${\mathbf{q}}_{2}=(0,1/2,l)$ with $l=0$ (Sb) or $l=1/2$ (As) and primarily involves in-plane displacements of the Ti atoms perpendicular to the Ti-O bonds. We also present angle-resolved photoemission spectroscopy measurements, which show band folding and backbending consistent with a density wave with the same wave-vectors ${\mathbf{q}}_{1}$ and ${\mathbf{q}}_{2}$ associated with Fermi-surface nesting, and muon-spin relaxation data, which show no indication of spin density wave order. The results provide direct evidence for phonon-assisted charge density wave order in ${\mathrm{Na}}_{2}{\mathrm{Ti}}_{2}P{n}_{2}\mathrm{O}$ and fully characterize a proximate ordered phase that could compete with superconductivity in doped ${\mathrm{BaTi}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$.

Magnetization dynamics and frustration in the multiferroic double perovskiteLu2MnCoO6

We investigate the magnetic ordering and the magnetization dynamics (from kHz to THz time scales) of the double perovskite Lu2MnCoO6 using elastic neutron diffraction, muon spin relaxation, and micro-Hall magnetization measurements. This compound is known to be a type II multiferroic with the interesting feature that a ferromagneticlike magnetization hysteresis loop couples to an equally hysteretic electric polarization in the bulk of the material despite a zero-field magnetic ordering of the type ↑↑↓↓ along Co-Mn spin chains. Here we explore the unusual dynamics of this compound and find extremely strong fluctuations, consistent with the axial next-nearest-neighbor Ising (ANNNI) model for frustrated spin chains. We identify three temperature scales in Lu2MnCoO6 corresponding to the onset of highly fluctuating long-range order below TN=50±3 K identified from neutron scattering, the onset of magnetic and electric hysteresis, with change in kHz magnetic and electric dynamics below a 30 K temperature scale, and partial freezing of ∼MHz spin fluctuations in the muon spin relaxation data below 12±3 K. Our results provide a framework for understanding the multiferroic behavior of this compound and its hysteresis and dynamics.

Evidence for magnetic clusters in Ni1-xVx close to the quantum critical concentration

The d-metal alloy Ni1-xVx undergoes a quantum phase transition from a ferromagnetic ground state to a paramagnetic ground state as the vanadium concentration x is increased. We present magnetization, ac-susceptibility and muon-spin relaxation data at several vanadium concentrations near the critical concentration xc ~ 11.6 % at which the onset of ferromagnetic order is suppressed to zero temperature. Below xc, the muon data reveal a broad magnetic field distribution indicative of a long-range ordered ferromagnetic state with spatial disorder. We show evidence of magnetic clusters in the ferromagnetic phase and close to the phase boundary in this disordered itinerant system as an important generic ingredient of a disordered quantum phase transition. In contrast, the temperature dependence of the magnetic susceptibility above xc is best described in terms of a magnetic quantum Griffiths phase with a power-law distribution of fluctuation rates of dynamic magnetic clusters. At the lowest temperatures, the onset of a short-range ordered cluster-glass phase is recognized by an increase in the muon depolarization in transverse fields and maxima in ac-susceptibility.

Probing the magnetic phases in the Ni-V alloy close to the disordered ferromagnetic quantum critical point with μSR

Zero (ZF) and longitudinal field (LF) muon spin relaxation data of the d-metal alloy Ni1-xVx are presented at several vanadium concentrations x below and above the critical xc ≈ 11 % where long-range ferromagnetic (FM) order is suppressed. The clear single precession frequency observed for Ni, as expected for a homogeneous FM, changes to rather damped osciallation with small V substitution at x = 4 %, confirming magnetic inhomogeneities caused by the less magnetic V environments in the magnetic Ni matrix. Furthermore, local fields and spatial field distributions can be estimated to characterize different inhomogeneous regimes developing with x in the FM phase of Ni1-xVx. In the regime of x = 7 – 10 % a Kubo Toyabe function well describes the low temperature ZF and LF asymmetry data supporting a static Gaussian field distribution. Closer to the quantum critical concentration a single scale static Kubo Toyabe function with one field distribution is not sufficient to describe the muon relaxation. These data indicate that further changes in spatial distributions and dynamics are evolving as expected within the critical regime of a disordered quantum critical point.

Mn(dca)2(o-phen) {dca = dicyanamide; o-phen = 1,10-phenanthroline}: Long-range magnetic order in a low-dimensional Mn-dca polymer

The crystal structure, phonon density-of-states, and magnetic properties of Mn(dca)2(o-phen) (dca=dicyanamide, N(CN)2−; o-phen=1,10-phenanthroline) have been studied using several methods including magnetization, muon-spin relaxation, and neutron and X-ray scattering. From X-ray crystallography the structure was shown to consist of octahedral MnN6 sites that are connected via four μ1,5-bridging dca ligands while the two remaining cis-positions are occupied by N-donors of the o-phen ligand. Two Mn2+ ions are bridged together by two dca anions to afford [Mn2(dca)2(o-phen)2]2+ “dimers.” These dimers are linked in 2D via μ1,5-dca ligands to afford a polymeric structure whereas the o-phen ligands act as spacers. Bulk magnetic susceptibility data show a characteristic broad maximum at 2.65K while neutron scattering and muon-spin relaxation data establish TN=1.85K. For T≪TN, the magnetization, as obtained in pulsed-fields, saturates at 5.75T and reaches a moment typical of an isotropic S=5/2 ion. The magnetic structure was determined and found to consist of antiferromagnetically ordered Mn2+ moments oriented in the ac-plane with no spin-canting being evident. The magnetic space group symmetry for the Mn2+ moments is P21′/c with each Mn2+ ion carrying a magnetic moment of 4.7(1)μB, in good agreement with the value of 5.0μB expected for an isotropic, high-spin S=5/2 ion. The fact that LRO is observed in Mn(dca)2(o-phen) is unusual among low-dimensional dicyanamide complexes especially since only μ1,5-dca modes are involved.

Magnetism in Geometrically FrustratedYMnO3under Hydrostatic Pressure Studied with Muon Spin Relaxation

The ferroelectromagnet YMnO3 consists of weakly coupled triangular layers of S=2 spins. Below T(N) approximately equal to 70 K muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no purely relaxing signals resolvable (which would require different coexisting spin distributions). The transition temperature T(N) increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers.

Characterization of the Crystal and Magnetic Structures of the Mixed-Anion Coordination Polymer Cu(HCO2)(NO3)(pyz) {pyz = Pyrazine} by X-ray Diffraction, ac Magnetic Susceptibility, dc Magnetization, Muon-Spin Relaxation, and Spin Dimer Analysis

The mixed-anion coordination polymer Cu(HCO2)(NO3)(pyz) was synthesized, its crystal structure was determined by X-ray diffraction, and its magnetic structure was characterized by ac susceptibility, dc magnetization, muon-spin relaxation, and spin dimer analysis. The crystal structure consists of five-coordinate Cu2+ ions that are connected through syn-anti bridging mu-HCO2- and mu-pyz ligands to form a highly corrugated two-dimensional layered network. Bulk magnetic measurements show a broad maximum in chi(T) at 6.6 K. The HCO2- and pyz ligands mediate ferromagnetic and antiferromagnetic spin exchange interactions between adjacent Cu2+ ions with the spin exchange parameters J/kB = 8.17 and -5.4 K, respectively (H = -JSigmaSi x Sj). The muon-spin relaxation data show a transition to a long-range magnetic ordering below TN = 3.66(3) K. For T < TN, the M(H) and chi'ac measurements provide evidence for a field-induced spin-flop transition at 15.2 kOe. That Cu(HCO2)(NO3)(pyz) undergoes a long-range magnetic ordering is an unexpected result because the one-dimensional Cu(NO3)2(pyz) and three-dimensional Cu(HCO2)2(pyz) compounds display linear chain antiferromagnetism with no long-range magnetic ordering down to 2 K.

A New Basic Motif in Cyanometallate Coordination Polymers: Structure and Magnetic Behavior of M(μ‐OH2)2[Au(CN)2]2(M=Cu, Ni)

The structures of two cyanoaurate-based coordination polymers, M(mu-OH(2))(2)[Au(CN)(2)](2) (M=Cu, Ni), were determined by using a combination of powder and single-crystal X-ray diffraction techniques. The basic structural motif for both polymers contains rarely observed M(mu-OH(2))(2)M double aqua-bridges, which generate an infinite chain; two trans [Au(CN)(2)](-) units also dangle from each metal center. The chains form ribbons that interact three dimensionally through CNH hydrogen bonding. The magnetic properties of both compounds and of the dehydrated analogue Cu[Au(CN)(2)](2) were investigated by direct current (dc) and alternating current (ac) magnetometry; muon spin-relaxation data was also obtained to probe their magnetic properties in zero-field. In M(mu-OH(2))(2)[Au(CN)(2)](2), ferromagnetic chains of M(mu-OH(2))(2)M are present below 20 K. Interchain magnetic interactions mediated through hydrogen bonding, involving water and cyanoaurate units, yield a long-range magnetically ordered system in Cu(mu-OH(2))(2)[Au(CN)(2)](2) below 0.20 K, as indicated by precession in the muon spin polarization decay. Ni(mu-OH(2))(2)[Au(CN)(2)](2) undergoes a transition to a spin-glass state in zero-field at 3.6 K, as indicated by a combination of muon spin-relaxation and ac-susceptibility data. This transition is probably due to competing interactions that lead to spin frustration. A phase transition to a paramagnetic state is possible for Ni(mu-OH(2))(2)[Au(CN)(2)](2) upon application of an external field; the critical field was determined to be 700 Oe at 1.8 K. The dehydrated compound Cu[Au(CN)(2)](2) shows weak antiferromagnetic interactions at low temperatures.

Frustrated magnetism and the effects of titanium substitution in the mixed-valence oxideBaV10−xTixO15

The low temperature magnetic phase transitions in $\mathrm{Ba}{\mathrm{V}}_{10}{\mathrm{O}}_{15}$ at $\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are investigated in detail. The topology of the V sublattice, which consists of linked ${\mathrm{V}}_{10}$ clusters formed by four tetrahedra sharing edges and corners, leads to geometric magnetic frustration and a depression of the ordering temperature characterized by the frustration index, $f=\ensuremath{\mid}\ensuremath{\theta}\ensuremath{\mid}∕{T}_{c}\ensuremath{\sim}30$. Specific heat data show the coexistence of both long range and short range magnetic order at low temperature. The position of a $\ensuremath{\lambda}$ peak, identified with ${T}_{c}$ for long range antiferromagnetic order, depends on the ${\mathrm{V}}^{2+}∕{\mathrm{V}}^{3+}$ ratio and varies from $43.0\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}41.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for samples with the highest ${\mathrm{V}}^{2+}$ content but is suppressed to zero for highly oxidized samples. A broad maximum in the specific heat occurs at $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and is present in all samples regardless of the level of oxidation. For samples with a well defined ${T}_{c}$, estimates of the entropy removal over the range $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}55\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are only 11% of that expected for $8\phantom{\rule{0.3em}{0ex}}{\mathrm{V}}^{3+}(S=1)$ and $2\phantom{\rule{0.3em}{0ex}}{\mathrm{V}}^{2+}(S=3∕2)$ ions per formula unit, suggesting the presence of considerable short-range order. Neutron diffraction data support the presence of long range antiferromagnetic (AF) order with ${T}_{c}\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The magnetic structure below ${T}_{c}$ is complex: there are five crystallographically independent vanadium ions in the unit cell, leading to 40 magnetic ions in the crystal unit cell and 80 spins per magnetic cell. Furthermore, the magnetic structure is of the multi-$k$ type involving the two unrelated wave vectors $\mathbf{k}=(0\phantom{\rule{0.3em}{0ex}}0\phantom{\rule{0.3em}{0ex}}0)$ and $\mathbf{k}=(1∕2\phantom{\rule{0.3em}{0ex}}0\phantom{\rule{0.3em}{0ex}}0)$. Zero field muon spin relaxation data show spin freezing below $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. ac susceptibility results are anomalous. Two peaks, at $\ensuremath{\sim}0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are seen in ${\ensuremath{\chi}}^{\ensuremath{'}}$ but the maxima are frequency independent as is the prominent maximum in ${\ensuremath{\chi}}^{\ensuremath{''}}$ near $25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, behavior atypical of spin glasses. Ti-doped samples, $\mathrm{Ba}{\mathrm{V}}_{10\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{15}$, show remarkable results as only 5% Ti doping is sufficient to destroy the AF long range order and induce conventional spin glass behavior in the dc and ac susceptibility, with freezing temperatures ranging from $15\phantom{\rule{0.3em}{0ex}}\mathrm{K}--5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for doping levels from 5% to 50%. The frustration indices reach enormous values to $f=130$ for $x=3$. The spin glass state is quenched for 70% Ti doping and only paramagnetic behavior is seen.

Muonium dynamics in silicon at high temperature

The reconciliation of the high temperature longitudinal field muon spin relaxation data with the low temperature radio-frequency muon spin resonance (RF) derived rate parameters is to date an outstanding issue in the high temperature Mu charge state dynamics in silicon. In this paper we suggest an alternative point of view from which to model the critically important activated–capture transition responsible for the disappearance of the diamagnetic RF amplitude around 400 K . The model assumes that the transition in question proceeds via an excited intermediate state. Such a picture leads to relaxation rate expressions which can naturally resolve the discrepancy in question.

Electronic phase transitions and magnetoresistance in a new bilayer manganate,Ca2.5Sr0.5GaMn2O8

The crystal structure of the anion-deficient perovskite Ca2.5Sr0.5GaMn2O8 hasbeen studied at 290 and 5 K by neutron diffraction (290 K; space groupPcm21,a = 5.4294(1), b = 11.3722(3), c = 5.2983(1)Å).The vacant oxide sites order to create a structure in which perovskitebilayers consisting of MnO6 octahedra are isolated from each otheralong [010] by a single layer of GaO4 tetrahedra. At 5 K the materialis antiferromagnetic with an ordered magnetic moment of 3.09(1) μBper Mn cation. Magnetic susceptibility measurements suggest that short-rangemagnetic ordering within the bilayers occurs above 200 K, and muon spinrelaxation data show that the transition to long-range magnetic order occursbetween 150 and 125 K. The resistivity of Ca2.5Sr0.5GaMn2O8 decreases by anorder of magnitude at 125 K, and ∼50% magnetoresistance is seen in a field of 80kOe at 110 K.

Spin dynamics in a frustrated magnet.

180 degrees spin flips have been identified as the dominant fluctuation mechanism at the transverse spin freezing transition in partially frustrated a-Fe92Zr8. The form of the selective excitation double Mössbauer spectra, coupled with the perfect agreement with zero-field muon spin relaxation data, eliminates other relaxation forms.