Pressure, together with temperature and magnetic field, is an important thermodynamical parameter in physics. Investigating the response of a compound or of a material to pressure allows to elucidate ground states, investigate their interplay and interactions and determine microscopic parameters. Pressure tuning is used to establish phase diagrams, study phase transitions and identify critical points. Muon spin rotation/relaxation (muSR) is now a standard technique making increasing significant contribution in condensed matter physics, material science research and other fields. In this review, we will discuss specific requirements and challenges to perform muSR experiments under pressure, introduce the high-pressure muon facility at the Paul Scherrer Institute (PSI, Switzerland) and present selected results obtained by combining the sensitivity of the muSR technique with pressure.Comment: Submitted to High Pressure Research. 26 pages, 17 Figure

The behavior of the so-called weak moment antiferromagnetic states, observed in the heavy-fermion superconductors UPt 3 and URu 2 Si 2 , is discussed in view of recent µSR results obtained as function of control parameters like chemical substitution and external pressure. In UPt 3 , the Pd substitution for Pt reveals the dynamical character of the weak moment order. On the other hand, µSR measurements performed on samples in which Th substitutes U suggest that crystallographic disorder on the magnetic sites deeply affects the fluctuation timescale. In URu 2 Si 2 , 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 µSR, it is deduced that the respective order parameters have different symmetries.

The magnetism of LixCoO2 (LCO), which has a similar structure to NaxCoO2 (NCO), has been investigated by muon-spin spectroscopy and susceptibility measurements using samples with x=0.1-1 prepared by an electrochemical reaction. In the x range below 0.75, LCO was found to be Pauli paramagnetic down to 1.8 K, suggesting an intermediate- or weak-coupling regime, although disordered local moments, with volume fractions below approximately 20%, appear at low T for LCO with x > or = 0.5. The phase diagram and interactions of LCO are thus strikingly different from NCO, while the differences cannot be explained simply by structural differences between the two systems.

Transverse-field muon-spin rotation (µSR) experiments were performed on a single crystal sample of the non-centrosymmetric system MnSi. The observed angular dependence of the muon precession frequencies matches perfectly the one of the Mn-dipolar fields acting on the muons stopping at a 4a position of the crystallographic structure. The data provide a precise determination of the magnetic dipolar tensor. In addition, we have calculated the shape of the field distribution expected below the magnetic transition temperature TC at the 4a muon-site when no external magnetic field is applied. We show that this field distribution is consistent with the one reported by zero-field µSR studies. Finally, we present ab initio calculations based on the density-functional theory which confirm the position of the muon stopping site inferred from transverse-field µSR. In view of the presented evidence we conclude that the µSR response of MnSi can be perfectly and fully understood without invoking a hypothetical magnetic polaron state.

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.

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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 <111> 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 <111> axis, thus preserving the overall helical structure.

The antiferromagnetic ͑AF͒ nature of the normal spinel Co 3 O 4 with Néel temperature ͑T N ͒ =30 K was investigated by means of positive muon spin rotation and relaxation ͑ + SR͒ techniques using a polycrystalline sample. Clear muon spin precession signals due to a quasistatic long-range AF order were found in the zero-field + SR spectra below T N. The spectra consist of two oscillating signals with frequencies at T → 0 K of 80 and 60 MHz, respectively, indicating an incommensurate ͑IC͒ AF order in Co 3 O 4. A possible reason for the appearance of the IC-AF order in Co 3 O 4 would be local structural transitions due to a charge and/or a spin state change of Co ions.