Neutron powder diffraction measurements (NPD) and Mössbauer spectroscopy (MS) have been performed to determine the magnetic phase diagram of the YBa2(Cu1−xFex)3Oy system with 0 ⩽ x ⩽ 0.16 and an oxygen content y close to 6.3. These phases were deoxygenated from the initial oxygen-rich state (y ≈ 7), by a low temperature process. This thermal treatment leaves the iron distribution identical to the one of the original oxygenated compound, whereas the usual high temperature method leads to a modification of the iron distribution. The distribution of iron on the two copper sites, Cu(1) and Cu(2), was carefully checked by both methods. The thermal dependence of the mean magnetic moments on the Cu(1) and Cu(2) sites was determined from Rietveld refinements. For x > 0.04, a collinear phase (AF′2) is observed with a doubling of the cell along the c axis and a ferromagnetic coupling between Cu(1) and Cu(2) sites. At lower iron content, several transitions are observed as the temperature decreases, from a paramagnetic to a collinear structure without any moment on the Cu(1) site, then to a canted structure and at low temperature the AF′2 structure is stabilized.

Mössbauer relaxation measurements have been made on 170Yb3+ in DyAlO3, TbAlO3 and HoAlO3. In DyAlO3 and TbAlO3 between 30 K and TN and in HoAlO3 between 30 and 10K conventional temperature inde pendent spin-spin driven relaxation is observed. In HoAlO3 below 10K, the relaxation rate is tempe rature dependent. This behaviour is interpreted in terms of coupling between Yb3+ and the collective excitations of the two singlet level ground states of the Ho3+ ions.

170Yb Mössbauer measurements in YbBa2Cu3O6 and on Yb3+ ions substituted into TmBa2Cu3O6 show that the ordered Cu(2) sublattice produces a molecular field at the rare earth site. The Yb3+ ion is not at a centre of magnetic inversion symmetry relative to the Cu(2) moments.

The magnetic interactions in superconducting YbBa2Cu3O7 and non-superconducting YbBa2Cu3O6 have been examined using Yb-170(3+) Mossbauer spectroscopy on grain-oriented samples. In YbBa2Cu3O7 where the Yb3+ sublattice orders at 0.35 K and the saturated magnetic moment is 1.75 mu(B), the saturated molecular field is 0.4 T and it is directed perpendicular to the c-axis. In YbBa2Cu3O6, where the Cu(2) sublattice also orders, the average saturated molecular field on Yb3+ is 0.2 T. It is disordered in direction and contains contributions from both the Yb3+ and the Cu(2)(2+) sublattices. The presence of a Cu(2)(2+) derived molecular field shows the Yb3+ ions are not at a centre of magnetic inversion symmetry relative to the ordered Cu(2)(2+) moments.

From measurements of the thermal dependence of 1/T1 the electronic fluctuation rate for the S′ = 12 ground state of 170Yb3+ Mïossbauer probes in YBa2(Cu1−xZnx)3O7, we find that the addition of Zn2+ introduces a density of states at the Fermi level in the superconducting state. Gapless behaviour is reached when x reaches 0.06. We compare the information obtained from the 170Yb3+ probe when Tc is reduced by adding Zn2+ and/or by reducing the oxygen level.

From 170Yb3+ Mössbauer measurements on Yb2Ni17 down to 0.05 K we have obtained the saturated 4f shell magnetic and quadrupole moments at the two Yb3+ sites. The combined influence of the Yb3+ crystal field and the Ni derived exchange interactions leaves an essentially pure fully stretched | Jz = 72〉 state at one site and a mixed state with a 40% reduction of the magnetic moment at the other site. The fully stretched state experiences a saturated molecular field of near 0.5 T. From the magnetisation measurements we obtain the magnetic ordering temperature of the Ni sublattice (145 K) and find that, in contrast to the other R2Ni17, the Yb3+ and Ni sublattice moments do not show a compensation temperature.

We present muon spin relaxation measurements performed on crystals of the heavy fermion superconductor UPt3. In zero applied field, contrary to a previous report, we do not observe an increase of the internal magnetic field in the lower superconducting phase (the B phase). Our result gives an experimental upper bound of the magnetic field that could be associated with the superconducting state.

The magnetic interactions in superconducting YbBa 2 Cu 3 O 7 and non-superconducting YbBa 2 Cu 3 O 6 have been examined using 170 Yb 3+ Mossbauer spectroscopy on grain-oriented samples. In YbBa 2 Cu 3 O 7 where the Yb 3+ sublattice orders at 0.35 K and the saturated magnetic moment is 1.75 μ B , the saturated molecular field is 0.4 T and it is directed perpendicular to the c-axis. In YbBa 2 Cu 3 O 6 , where the Cu(2) sublattice also orders, the average saturated molecular field on Yb 3+ is 0.2 T. It is disordered in direction and contains contributions from both the Yb 3+ and the Cu(2) 2+ sublattices. The presence of a Cu(2) 2+ derived molecular field shows the Yb 3+ ions are not at a centre of magnetic inversion symmetry relative to the ordered Cu(2) 2+ moments

We have recorded at ISIS zero field and longitudinalμSR relaxation spectra on a single crystal of the Van- Vleck paramagnetPrNi5 from room temperature down to 0.09K. Our data clearly indicate that at low temperature the rare earth spin dynamics is quasi-static, i.e. its correlation time is larger than 10−6s. From theμSR methodological point of view, this work provides the first example of aμSR study of the dynamics of an hyperfine-enhanced nuclear dipole system.

A sample of isocubanite (CuFe2S3) collected in a suboceanic hydrothermal deposit was found to have transformed formed after a few months into a very intimate mixture of similar amounts of chalcopyrite (CuFeS2) and a new phase of composition close to CuFe3S4. This new phase is not a disordered solid solution like isocubanite, but has a tetragonal superstructure. Its magnetization, electric resistivity, 57Fe Mössbauer spectra, X-ray and neutron diffraction spectra, have been studied at various temperatures. The Fe occupies two different crystallographic sites with, at room temperature, a common mixed valency (nominally 2.33+). Three transitions are observed on cooling: at TN ≅ 285 K where one of the Fe sites becomes antiferromagnetic; then at Tt ≅ 190 K, where the second Fe site becomes ferromagnetic with medium-range ordering; finally at Tv ≅ 105 K, where a first-order Verwey transition results in the appearance of integral valencies Fe2+ on one site and Fe3+ on the other, and in a rearrangement of the magnetic ordering to a ferrimagnetic structure.

Mössbauer emission measurements show that the isolated 57Fe3+ probe pins a Cu spin-glass cluster in superconducting La2−xSrxCuO4 for x xt the probe is essentially coupled with an induced localized Cu magnetic moment.

When the Zn2+ level is increased in YBa2(Cu1−xZnx)3O7, we observe that the thermal variation of 1/T1 for the S′=12 ground state of Yb3+ probes progressively crosses over from a dependence showing gap-like features (x = 0) to one showing gapless features (x = 0.06 and above).

We present some results obtained by Mossbauer spectroscopy on Yb3+ when substituted into YBa2Cu3Ox-like compounds. For the fully substituted compound YbBa2Cu3O7, we describe the properties of the rare earth sublattice magnetic ordering which coexists with superconductivity. From measurements at the dilute substitution level, we study the Yb3+ crystal field properties and the thermal dependence of the Yb3+ paramagnetic relaxation rate. By making use of the molecular field produced on the Yb3+ probe by the magnetically correlated Cu(2), we examine the evolution of the Cu(2)-based magnetism as a function of carrier density and study the phase separation which occurs in the superconducting samples with intermediate oxygen levels. We also examine how the properties of the Yb3+ probe are influenced by the substitution of Pr3+ or Zn2+ (these substitutions are known to considerably influence the superconducting properties).

We report a new Mössbauer “relaxation window” which allows the fluctuation rate of a molecular field acting on an impurity in a strongly correlated spin system to be obtained. For a molecular field of 1 T, the fluctuation rate can be measured up to 10 THz, which is three orders of magnitude higher than for the classical relaxation window. The calculated line shapes show that Ising-like fluctuations of the molecular field can be distinguished from other types of fluctuations (in-plane or in space). An application and some other possible uses of this new “relaxation window” are discussed.

The magnetic properties of the rhombohedral intermetallic compound Yb3Pd4 have been investigated using Yb-170 Mossbauer spectroscopy and neutron diffraction. Magnetic ordering of the Yb3+ moments sets in below T(N) = 3.2 K, the saturated moment per Yb3+ being mus congruent-to 0.6 mu(B). The magnetic structure is antiferromagnetic and we discuss the possible moment orientations. In the paramagnetic phase, the thermal variation of the Yb3+ 4f shell quadrupolar moment has been measured.

We present some results obtained by Mössbauer spectroscopy on Yb3+ when substituted into YBa2Cu3O x -like compounds. For the fully substituted compound YbBa2Cu3O7, we describe the properties of the rare earth sublattice magnetic ordering which coexists with superconductivity. From measurements at the dilute substitution level, we study the Yb3+ crystal field properties and the thermal dependence of the Yb3+ paramagnetic relaxation rate. By making use of the molecular field produced on the Yb3+ probe by the magnetically correlated Cu(2), we examine the evolution of the Cu(2)-based magnetism as a function of carrier density and study the phase separation which occurs in the superconducting samples with intermediate oxygen levels. We also examine how the properties of the Yb3+ probe are influenced by the substitution of Pr3+ or Zn2+ (these substitutions are known to considerably influence the superconducting properties).

Mossbauer emission measurements on quasi-isolated Fe-57 impurities substituted for Cu in La2-xSrxCuO4 show a rapid change in the local magnetic behaviour when the Sr content 2 exceeds an effective value x(t), which is roughly at the centre of the superconducting x range in the phase diagram. For x < x(t), the Fe3+ impurity systematically pins a Cu spin-glass cluster where the spins are oriented near the basal plane, with a freezing temperature of 8 +/- 1 K with respect to the characteristic time scale of the Mossbauer probe. For x > x(t), the probe no longer detects local antiferromagnetic ordering, but it reveals the presence of an induced localized magnetization in the Cu sublattice, which is preferentially oriented near the (c) over bar axis for the largest x values. The results are discussed and compared with other available measurements, including our previous Fe-57 Mossbauer absorption study in the same series.

Specific heat and 170Yb Mössbauer spectroscopy measurements in cubic YbPdSb show that the Yb3+ CEF ground state is the Γ8 quartet with a Kondo temperature TK = 7 K. In our sample, a first-order-like magnetic transition is observed at T = 1 K, with a small specific heat jump ΔC ≅2 J/K/mol at the transition. The saturated spontaneous Yb3+ moment is 1.3μB, showing a Kondo reduction with respect to the Γ8 saturated moment (≅2μB). The Mössbauer spectra with applied magnetic field and the magnetisation measurements at T = 0.1 K are consistent with an anisotropic RKKY exchange, of magnitude TRKKY ∼ 6 K. YbPdSb is thus close to the magnetic/nonmagnetic border in Doniach's phase diagram, and the presence of long range ordering seems to be very sensitive to sample preparation.