Daniel Andreica

Muon spin rotation and relaxation in the superconducting ferromagnet UCoGe

We report zero-field muon-spin rotation and relaxation measurements on the superconducting ferromagnet UCoGe. Weak itinerant ferromagnetic order is detected by a spontaneous muon-spin precession frequency below the Curie temperature TC=3 K. The micro+ precession frequency persists below the bulk superconducting transition temperature Tsc=0.5 K, where it measures a local magnetic field Bloc=0.015 T. The amplitude of the microSR signal provides unambiguous proof for ferromagnetism present in the whole sample volume. We conclude ferromagnetism coexists with superconductivity on the microscopic scale.

Inhomogeneous magnetism in URu2Si2 studied by muon spin relaxation under high pressure

Abstract We have studied the low-temperature phase below 17.5 K in the heavy-fermion superconductor URu 2 Si 2 , using the zero-field μSR technique under hydrostatic pressures P up to 0.85 GPa . The measurements qualitatively confirm the previous results of NMR and neutron scattering that pressure induces unusual evolution of an inhomogeneous antiferromagnetic (AF) phase which spatially competes with some nonmagnetic ‘hidden order’. The AF volume fraction, however, develops abruptly at around a pressure of 0.6 GPa , in contrast to the P -linear variations in the NMR results. Moreover, the onset temperature of the AF-volume evolution is clearly lower than 17.5 K , whereas the nominal Neel temperature seems to be about 20 K . These experimental results suggest that the AF phase exists as a metastable state.

Spin fluctuations in YNi5 and CeNi5

Abstract The temperature dependences of the magnetic susceptibilities of YNi 5 and CeNi 5 exchange-enhanced Pauli paramagnets show broad maxima at T max ≈ 250 and 100 K, respectively, and at high temperatures fit a Curie-Weiss law. The experimental data may be described in terms of the self-consistent renormalization theory of spin fluctuations.

Weak magnetism phenomena in heavy-fermion superconductors: selected µSR studies

The behaviour of the so-called weak-moment antiferromagnetic states, observed in the heavy-fermion superconductors UPt3 and URu2Si2, is discussed in view of recent ?SR results obtained as functions of control parameters such as chemical substitution and external pressure. In UPt3, the Pd substitution for Pt reveals the dynamical character of the weak-moment order. On the other hand, ?SR measurements performed on samples in which Th substitutes U suggest that crystallographic disorder on the magnetic sites deeply affects the fluctuation timescale. In URu2Si2, a phase separation between the so-called hidden order state, present at ambient pressure, and an antiferromagnetic state, occurring under pressure, is observed. In view of the pressure?temperature phase diagram obtained by ?SR, it is deduced that the respective order parameters have different symmetries.

Magnetic quantum critical point and superconductivity in UPt3 doped with Pd.

Transverse-field muon spin relaxation measurements have been carried out on the heavy-fermion superconductor UPt (3) doped with small amounts of Pd. We find that the critical Pd concentration for the emergence of the large-moment antiferromagnetic phase is approximately 0.6 at. %Pd. At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity, provides evidence for ferromagnetic spin-fluctuation mediated odd-parity superconductivity, which competes with antiferromagnetic order.

Static magnetic order in metallic K0.49CoO2.

By means of muon-spin spectroscopy, we have found that K0.49CoO2 crystals undergo successive magnetic transitions from a high-T paramagnetic state to a magnetic ordered state below 60 K and then to a second ordered state below 16 K, even though K0.49CoO2 is metallic at least down to 4 K. An isotropic magnetic behavior and wide internal-field distributions suggest the formation of a commensurate helical spin density wave (SDW) state below 16 K, while a linear SDW state is likely to exist above 16 K. It was also found that exhibits a further transition at 150 K presumably due to a change in the spin state of the Co ions. Since the dependence of the internal-field below 60 K was similar to that for Na0.5CoO2, this suggests that magnetic order is more strongly affected by the Co valence than by the interlayer distance or interaction and/or the charge ordering.

Muon-induced break up of spin-singlet pairs in the double-chain compound KCuCl3

Abstract Monoclinic KCuCl3 is a spin- 1 2 ladder compound with a dimerized nonmagnetic spin-singlet ground state at low temperatures. Yet a μSR study by Higemoto et al. [1] showed a pronounced μ+-spin relaxation below 20 K, signaling the onset of a broad static field distribution, reminiscent of a frustrated spin system. With the aim to gain further insight into this phenomenon we have performed ZF-, LF- and TF-measurements on high-quality single crystals. The TF-measurements with B ext || b -axis, || [2 0 1] and || [1 0 2 ] (B ext =0.6 T ) yielded a split signal consisting of up to seven resolvable components. The frequency shifts follow a Curie-behavior from 6 up to at least 40 K and do not scale at all with the bulk susceptibility. This result implies the presence of free electron spins down to 6 K. Below 6 K the lines broaden severely and it is difficult to follow the shifts to lower temperatures. However ZF- and LF-measurements display a slowing down of the spin dynamics and the transition into a spin-glass-like state below 5 K in qualitative agreement with Ref. [1] . We conclude that the presence of the μ+ breaks the near-neighbor dimerized spin pairs and also that those few liberated spins develop a quasi-static behavior below 5 K in part of the volume (35%). Relative slow fluctuations persist in the remaining volume.

Transport properties and μSR spectroscopy of Yb(NixCu1−x)2Si2

We report on transport measurements and μSR data for the solid solution Yb(NixCu1−x)2Si2 (0<x<1). By increasing x, a crossover from an intermediate valence regime to a magnetic Kondo lattice regime is observed. The magnetic order develops above x=0.125.

Determination of the zero-field magnetic structure of the helimagnet MnSi at low temperature

Below a temperature of approximately 29 K the manganese magnetic moments of the cubic binary compound MnSi order to a long-range incommensurate helical magnetic structure. Here, we quantitatively analyze a high-statistic zero-field muon spin rotation spectrum recorded in the magnetically ordered phase of MnSi by exploiting the result of representation theory as applied to the determination of magnetic structures. Instead of a gradual rotation of the magnetic moments when moving along a axis, we find that the angle of rotation between the moments of certain subsequent planes is essentially quenched. It is the magnetization of pairs of planes which rotates when moving along a axis, thus preserving the overall helical structure.

Understanding the μSR spectra of MnSi without magnetic polarons

Transverse-field muon-spin rotation (