W. D. Wu

Static magnetic order in La1.875Ba0.125CuO4

Zero field muon spin relaxation measurements of high quality, single phase La1.875Ba0.125CuO4 indicate that this system exhibits static magnetic order below 38 K. At this x = 18 concentration, superconductivity is suppressed. The ordered spins most likely reside on the copper sites.

Searching for spontaneous magnetic order in an organic ferromagnet. μSR studies of β-phase p-NPNN

Abstract Zero-field muon spin rotation (ZF-μSR) studies in the recently synthesized organic ferromagnet β-phase p-NPNN are reported. Our measurements, using a microscopic magnetic probe, provide the first direct observation of spontaneous magnetic order in p-NPNN. The behaviour of the system is close to that of an isotropic three-dimensional (3D) Heisenberg spin system.

Muon spin relaxation investigation of the spin dynamics of geometrically frustrated antiferromagnets Y2Mo2O7 and Tb2Mo2O7.

The spin dynamics of geometrically frustrated pyrochlore antiferromagnets Y2Mo2O7 and Tb2Mo2O7 have been investigated using muon spin relaxation. A dramatic slowing down of the moment fluctuations occurs as one approaches the spin freezing temperatures (TF=22 K and 25 K respectively) from above. Below TF there is a disordered magnetic state similar to that found in a spin glass but with a residual muon spin relaxation rate at low temperatures. These results show that there is a large density of states for magnetic excitations in these systems near zero energy.

Muon spin relaxation in heavy fermion systems

Heavy fermion systems have received a great deal of study by a wide variety of techniques, includingμSR. In a number of systems, coexisting superconducting and magnetic states have been reported, leading to speculation of an intimate connection between magnetism and superconductivity in these compounds. We observe a spontaneous magnetic field in the superconducting phase of UPt3. In addition, the broadening of the transverse field muon precession signal only onsets approximately 60 mK below the superconductingTc. Our results provide evidence that the “lower superconducting phase” in theH-T phase diagram of UPt3 is characterised by broken time-reversal symmetry. Measurements of URu2Si2 and CeCu2.2Si2 indicate that the magnetically ordered volume fraction is temperature dependent in both systems.

Zero-field muon spin rotation studies in (TMTSF)2PF6

We report zero-field muon spin rotation studies of the organic compound (TMTSF)2PF6 at ambient pressure. An extremely sharp onset of the spin density wave state (SDW) is observed below the transition temperature TM = 12.2 K, which may be first order in nature. The saturation of the magnetic order at low temperatures can be ascribed to the existence of the SDW gap. The broad distribution of local magnetic fields deduced from μSR time spectra is consistent with that expected from an incommensurate SDW.

Muon spin relaxation studies in frustrated and/or low-dimensional spin systems

An overview is given on muon spin relaxation (μSR) measurements in frustrated and/or low dimensional spin systems. In the frustrated Kagomé lattice system SrCr8Ga4O19, we observed dynamic spin fluctuations of ∼ 30 GHz, without any static frozen component even atT=0.1 K, much below the susceptibility-cusp temperatureTg=3.5 K. This is in clear contrast with the case in dilute-alloy spin glassesCuMn andAuFe, where static order develops belowTg. We also present the dimensionality dependence of the sub-lattice magnetization curves in 2-d Heisenberg systems, the remarkable suppression of the ordering temperature in a 1-d system Sr2CuO3, the observation of activation type spin dynamics in a 1-d Ising ferromagnet (DMeFc)(TCNE) aboveTc, slow spin fluctuations (∼ 60 MHz) in Haldane-gap systems at low temperatures, and some results from organic 1-d and 2-d magnetic systems.

Muon spin relaxation in the heavy fermion system UPt3

Abstract We report muon spin rotation/relaxation (μSR) measurements of the heavy fermion superconductor UPt3. We observe the appearance of a spontaneous internal magnetic field at T = 475 mK, which is about 80 mK below Tc. This transition likely corresponds to the lower superconducting transition observed in specific-heat and ultrasound measurements. Our results provide microscopic evidence that the low-temperature/low-field phase in the superconducting phase diagram of UPt3 may break time-reversal symmetry.

μ+SR measurements on pure and copper doped samples of the 1D antiferromagnet Ni(C2H8N2)2NO2(ClO4) (NENP)

Abstract Relaxation reflecting the freezing/slowing down of spins is seen in pure and Cu-doped NENP below ≈ 30 K. This relaxation increases sharply for both materials near 2.5 K and saturates below ≈ 1 K. The applied field dependence of the μ + SR spectra at 20 mK indicates that the internal fields corresponding to these spins are still fluctuating and are probably too large to be attributed to loose chain ends or impurities.

Muon-spin-rotation measurements in the ‘infinite-chain’ Ca2CuO3

We performed zero-field muon-spin-rotation measurements of the «infinite-chain» compound Ca 2 CuO 3 . A spontaneous magnetic field is observed below 13 K. The exchange interaction inferred from susceptibility measurements and two-magnon Raman scattering is approximately 10 3 K, which implies a remarkable suppression of the ordering temperature (k B T N /J 1d ∼ 0.01). We discuss the relevance of these measurements to the problem of one-dimensional spin 1/2 antiferromagnets

Muon spin relaxation measurements in kagomé lattice system SrCr8Ga4O19

Dynamical spin fluctuations in SrCr8−xGa4+xO19 a frustrated spin system on a kagomé lattice, is examined by the longitudinal field muon spin relaxation technique. This system shows a spin-glass (SG)-like cusp in the susceptibility atTg=3.5(2) K. The slowing down of Cr spin fluctuations is found to occur over a very wide temperature rangeTg<T<30Tg. AsT/Tg → 0 these fluctuations remain without static polarization (order parameter). Such strong fluctuations belowTg have not been observed before in a conventional SG system.