Absence of Spontaneous Magnetic Fields due to Time-Reversal Symmetry Breaking in Bulk Superconducting UTe_{2}

We have investigated the low-temperature local magnetic properties in the bulk of molten salt-flux (MSF)-grown single crystals of the candidate odd-parity superconductor UTe_{2} by zero-field muon spin relaxation (μSR). In contrast to previous μSR studies of UTe_{2} single crystals grown by a chemical vapor transport method, we find no evidence of magnetic clusters or electronic moments fluctuating slow enough to cause a discernible relaxation of the zero-field μSR asymmetry spectrum. Consequently, our measurements on MSF-grown single crystals rule out the generation of spontaneous magnetic fields in the bulk that would occur near impurities or lattice defects if the superconducting state of UTe_{2} breaks time-reversal symmetry. This result suggests that UTe_{2} is characterized by a single-component superconducting order parameter.

Intrinsic Low-Temperature Magnetism in SmB_{6}

By means of new muon spin relaxation experiments, we disentangle extrinsic and intrinsic sources of low-temperature bulk magnetism in the candidate topological Kondo insulator (TKI) SmB_{6}. Results on Al-flux-grown SmB_{6} single crystals are compared to those on a large floating-zone-grown ^{154}Sm ^{11}B_{6} single crystal in which a 14 meV bulk spin exciton has been detected by inelastic neutron scattering. Below ∼10  K, we detect the gradual development of quasistatic magnetism due to rare-earth impurities and Sm vacancies. Our measurements also reveal two additional forms of intrinsic magnetism: (1) underlying low-energy (∼100  meV) weak magnetic moment (∼10^{-2}  μ_{B}) fluctuations similar to those detected in the related candidate TKI YbB_{12} that persist down to millikelvin temperatures, and (2) magnetic fluctuations consistent with a 2.6 meV bulk magnetic excitation at zero magnetic field that appears to hinder surface conductivity above ∼4.5  K. We discuss potential origins of the magnetism.

Coexistence of ferromagnetic fluctuations and superconductivity in the actinide superconductor UTe2

We report low-temperature muon spin relaxation/rotation (μSR) measurements on single crystals of the actinide superconductor UTe₂. Below 5 K we observe a continuous slowing down of magnetic fluctuations that persists through the superconducting transition temperature (T _c = 1.6 K), but we find no evidence of long-range or local magnetic order down to 0.025 K. The temperature dependence of the dynamic relaxation rate down to 0.4 K agrees with the self-consistent renormalization theory of spin fluctuations for a three-dimensional weak itinerant ferromagnetic metal. Our μSR measurements also indicate that the superconductivity coexists with the magnetic fluctuations.

Bi-Arrhenius Diffusion and Surface Trapping of ^{8}Li^{+} in Rutile TiO_{2}

We report measurements of the diffusion rate of isolated ion-implanted ^{8}Li^{+} within ∼120  nm of the surface of oriented single-crystal rutile TiO_{2} using a radiotracer technique. The α particles from the ^{8}Li decay provide a sensitive monitor of the distance from the surface and how the depth profile of ^{8}Li evolves with time. The main findings are that the implanted Li^{+} diffuses and traps at the (001) surface. The T dependence of the diffusivity is described by a bi-Arrhenius expression with activation energies of 0.3341(21) eV above 200 K, whereas at lower temperatures it has a much smaller barrier of 0.0313(15) eV. We consider possible origins for the surface trapping, as well the nature of the low-T barrier.

Incommensurate Magnetism Near Quantum Criticality in CeNiAsO

We report the discovery of incommensurate magnetism near quantum criticality in CeNiAsO through neutron scattering and zero field muon spin rotation. For T<T_{N1}=8.7(3)  K, a second order phase transition yields an incommensurate spin density with a wave vector k=(0.44(4),0,0). For T<T_{N2}=7.6(3)  K, we find coplanar commensurate order with a moment of 0.37(5)μ_{B}, reduced to 30% of the saturation moment of the |±1/2⟩ Kramers doublet ground state, which we establish through inelastic neutron scattering. Muon spin rotation in CeNiAs_{1-x}P_{x}O shows the commensurate order only exists for x≤0.1 so we infer the transition at x_{c}=0.4(1) is between an incommensurate longitudinal spin density wave and a paramagnetic Fermi liquid.

μSR study of spin freezing and persistent spin dynamics in NaCaNi2F7

A new pyrochlore compound, NaCaNi₂F₇, was recently synthesized and has a single magnetic site with spin-1 Ni²⁺ . We present zero field and longitudinal field muon spin rotation (μSR) measurements on this pyrochlore. Density functional theory calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-μ-F muon spin polarization function is observed at high temperatures where Ni spin fluctuations are sufficiently rapid. The Ni²⁺ spins undergo spin freezing into a disordered ground state below 4 K, with a characteristic internal field strength of 140 G. Persistent Ni spin dynamics are present to our lowest temperatures (75 mK), a feature characteristic of many geometrically frustrated magnetic systems.

Epitaxial growth of iridate pyrochlore Nd2Ir2O7 films

Epitaxial films of the pyrochlore Nd₂Ir₂O₇ have been grown on (111)-oriented yttria-stabilized zirconia (YSZ) substrates by off-axis sputtering followed by post-growth annealing. X-ray diffraction (XRD) results demonstrate phase-pure epitaxial growth of the pyrochlore films on YSZ. Scanning transmission electron microscopy (STEM) investigation of an Nd₂Ir₂O₇ film with a short post-annealing provides insight into the mechanism for crystallization of Nd₂Ir₂O₇ during the post-annealing process. STEM images reveal clear pyrochlore ordering of Nd and Ir in the films. The epitaxial relationship between the YSZ and Nd₂Ir₂O₇ is observed clearly while some interfacial regions show a thin region with polycrystalline Ir nanocrystals.

Vibrating-coil magnetometry of the spin liquid properties of Tb2Ti2O7

We have explored the spin liquid state in Tb(2)Ti(2)O(7) with vibrating-coil magnetometry down to ~0.04 K under magnetic fields up to 5 T. We observe magnetic history dependence below T*~0.2 K reminiscent of the classical spin ice systems Ho(2)Ti(2)O(7) and Dy(2)TiTi(2)O(7). The magnetic phase diagram inferred from the magnetization is essentially isotropic, without evidence of magnetization plateaus as anticipated for so-called quantum spin ice, predicted theoretically for [111] when quantum fluctuations renormalize the interactions. Instead, the magnetization for T≪T* agrees semiquantitatively with the predictions of "all-in-all-out" (AIAO) antiferromagnetism. Taken together, this suggests that the spin liquid state in Tb(2)Ti(2)O(7) is akin to an incipient AIAO antiferromagnet.

First order metamagnetic transition in Ho2Ti2O7 observed by vibrating coil magnetometry at Milli-Kelvin temperatures

We report vibrating coil magnetometry of the spin-ice system Ho(2)Ti(2)O(7) down to ~0.04 K for magnetic fields up to 5 T applied parallel to the [111] axis. History-dependent behavior emerges below T(0)(*) ~ 0.6 K near zero magnetic field, in common with other spin-ice compounds. In large magnetic fields, we observe a magnetization plateau followed by a hysteretic metamagnetic transition. The temperature dependence of the coercive fields as well as the susceptibility calculated from the magnetization identify the metamagnetic transition as a line of first order transitions terminating in a critical end point at T(m)(*) 0.37 ~/= K, B(m) ~/= 1.5 T. The metamagnetic transition in Ho(2)Ti(2)O(7) is strongly reminiscent of that observed in Dy(2)Ti(2)O(7), suggestive of a general feature of the spin ices.

Spin ice: magnetic excitations without monopole signatures using muon spin rotation

Theory predicts the low temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (μSR) experiment [S. T. Bramwell et al., Nature (London) 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation-the so-called second Wien effect. We demonstrate via a new series of μSR experiments in Dy(2)Ti(2)O(7) that such an effect is not observable in a TF μSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.

Study of the ground state properties of LiHo(x)Y(1-x)F4 using muon spin relaxation

LiHo(x)Y(1-x)F4 is an insulator where the magnetic Ho3+ ions have an Ising character and interact mainly through magnetic dipolar fields. We used the muon spin relaxation technique to study the nature of its ground state for samples with x ≤ 0.25. In contrast with some previous works, we did not find canonical spin glass behavior down to ≈ 15 mK. Instead, below ≈300 mK we observed temperature-independent dynamic magnetism characterized by a single correlation time. The 300 mK energy scale corresponds to the Ho3+ hyperfine interaction strength, suggesting that this interaction may be involved in the dynamic behavior of the system.

Spatially homogeneous ferromagnetism of (Ga, Mn)As

Mn-doped GaAs is a ferromagnetic semiconductor, widely studied because of its possible application for spin-sensitive 'spintronics' devices. The material also attracts great interest in fundamental research regarding its evolution from a paramagnetic insulator to a ferromagnetic metal. The high sensitivity of its physical properties to preparation conditions and heat treatments and the strong doping and temperature dependencies of the magnetic anisotropy have generated a view in the research community that ferromagnetism in (Ga, Mn)As may be associated with unavoidable and intrinsic strong spatial inhomogeneity. Muon spin relaxation (muSR) probes magnetism, yielding unique information about the volume fraction of regions having static magnetic order, as well as the size and distribution of the ordered moments. By combining low-energy muSR, conductivity and a.c. and d.c. magnetization results obtained on high-quality thin-film specimens, we demonstrate here that (Ga, Mn)As shows a sharp onset of ferromagnetic order, developing homogeneously in the full volume fraction, in both insulating and metallic films. Smooth evolution of the ordered moment size across the insulator-metal phase boundary indicates strong ferromagnetic coupling between Mn moments that exists before the emergence of fully itinerant hole carriers.

In-gap spin excitations and finite triplet lifetimes in the dilute singlet ground state system SrCu(2-x)Mgx(BO3)2

High resolution neutron scattering measurements on a single crystal of SrCu(2-x)Mgx(BO3)2 with x approximately 0.05 reveal the presence of new spin excitations within the gap of this quasi-two-dimensional, singlet ground state system. The application of a magnetic field induces Zeeman-split states associated with S=1/2 unpaired spins which are antiferromagnetically correlated with the bulk singlet. Substantial broadening of both the one- and two-triplet excitations in the doped single crystal is observed, as compared with pure SrCu2(BO3)2. Theoretical calculations using a variational algorithm and a single quenched magnetic vacancy on an infinite lattice are shown to qualitatively account for these effects.

Field-induced order and spin waves in the pyrochlore antiferromagnet Tb2Ti207

High resolution time-of-flight neutron scattering measurements on Tb(2)Ti(2)0(7) reveal a rich low temperature phase diagram in the presence of a magnetic field applied along [110]. In zero field at T = 0.4 K, Tb(2)Ti(2)0(7) is a highly correlated cooperative paramagnet with disordered spins residing on a pyrochlore lattice of corner-sharing tetrahedra. Application of a small field condenses much of the magnetic diffuse scattering, characteristic of the disordered spins, into a new Bragg peak characteristic of a polarized paramagnet. At higher fields, a magnetically ordered phase is induced, which supports spin wave excitations indicative of continuous, rather than Ising-like, spin degrees of freedom.

Local magnetic susceptibility of the positive muon in the quasi-one-dimensional S=1/2 antiferromagnet dichlorobis (pyridine) copper (II)

We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi-one-dimensional spin 1/2 antiferromagnet dichlorobis (pyridine) copper (II). Signals from three distinct sites are resolved and have a temperature dependent frequency shift which is significantly different than the magnetic susceptibility. This difference is attributed to a muon induced perturbation of the spin 1/2 chain. The obtained frequency shifts are compared with transfer matrix density-matrix renormalization-group numerical simulations.

Temperature dependence of the magnetic penetration depth in the vortex state of the pyrochlore superconductor, Cd2Re2O7

We report transverse-field and zero-field muon spin rotation and relaxation studies of the superconducting rhenium oxide pyrochlore, Cd2Re2O7. Transverse-field measurements (H=0.007 T) show line broadening below T(c), which is characteristic of a vortex state, demonstrating conclusively the type-II nature of this superconductor. The penetration depth is seen to level off below about 400 mK (T/T(c) approximately 0.4), with a rather large value of lambda(T=0) approximately 7500 A. The temperature independent behavior below approximately 400 mK is consistent with a nodeless superconducting energy gap. Zero-field measurements indicate no static magnetic fields developing below the transition temperature.

Anomalous weak magnetism in superconducting YBa2Cu3O6+x

For some time now, there has been considerable experimental and theoretical effort to understand the role of the normal-state "pseudogap" phase in underdoped high-temperature cuprate superconductors. Recent debate has centered on the question of whether the pseudogap is independent of superconductivity. We provide evidence from zero-field muon spin relaxation measurements in YBa2Cu3O6+x for the presence of small spontaneous static magnetic fields of electronic origin intimately related to the pseudogap transition. Our most significant finding is that, for optimal doping, these weak static magnetic fields appear well below the superconducting transition temperature. The two compositions measured suggest the existence of a quantum critical point somewhat above optimal doping.

Low temperature spin dynamics of the geometrically frustrated antiferromagnetic garnet Gd3Ga5O12

The low temperature spin dynamics of the geometrically frustrated antiferromagnet Gd 3Ga 5O (12) (GGG) have been investigated using muon spin relaxation. No evidence for static order is seen down to a temperature of 25 mK or a few percent of the Curie-Weiss temperature. Instead there is a linear decrease in the Gd spin fluctuation rate below 1 K which extrapolates to a small but finite value of 2 GHz at zero temperature. In terms of the spin fluctuations the system appears essentially to remain dynamic at low temperatures (T>0.02 K) and magnetic fields up to 1.8 T.