J.A. Hodges

Cu(2) magnetic correlations in YBa2Cu3Ox studied by Mössbauer spectroscopy on substituted 170Yb3+: I. 170Yb3+ hyperfine parameters (x ≅ 7.0) and static properties of the Cu(2) derived molecular field (x ≅ 6.0)

Abstract We describe a method, based on the field dependence of the electro-nuclear levels of 170Yb3+ diluted for yttrium in YBa2Cu3Ox, which probes magnetic fields at the rate earth site. Measurements were made using 170Yb Mossbauer spectroscopy. When x ≅ 6.0, we have evidenced an internal (molecular) field on the Yb3+ attributed to the net exchange interaction with the ordered Cu(2) sublattice. The field has an average size of 1600 G and lies at an average angle of near 45° to the c-axis. Up to a least 60 K it remains static on the frequency scale of the experiment (5×108s−1). Some possible origins of this previously unsuspected field are discussed.

Cu(2) magnetic correlations in YBa2Cu3Ox (6.0⪅x⪅7.0) studied by Mössbauer spectroscopy on substituted 170Yb3+: II. Dynamic properties

Abstract We extend the method used to measure the internal (molecular) field on 170 Yb 3+ diluted for Y 3+ in YBa 2 Cu 3 O x described in the preceding paper (Part 1) to the study of dynamical effects. (The method exploits the field dependence of the 170 Yb 3+ electronuclear levels and the measurements are made by Mossbauer spectroscopy. By studying the fluctuation rates of the Cu(2) derived molecular field acting on the 170 Yb 3+ probes, we follow the fluctuation rates of the short range correlated Cu(2) moments as a function of oxygen content (6.0⩽ x ⩽7.0) and temperature (⪅70 K). When x ∼6.0 the correlated Cu(2) moments are static or fluctuate at frequencies below the lower limit of the accessible window. For x ≅6.35, the average fluctuation rate shows a strong thermal dependence decreasing through the accessible range of 10 12 to 5×10 8 s -1 as the temperature is lowered to 1.4 K and the system shows spin glass-like behaviour. At 1.4 K as x is increased above about 6.5, we observe that a fraction of the probes no long show spin glass-like behaviour and show no evidence of any fields fluctuating in the accessible frequency range. We identify these probes as being situated in the superconducting regions of the sample where at temperatures much lower than the gap there are not expected to be any Cu(2)-based magnetic excitations. From these local probe measurements we find that the amount of the sample that is superconducting depends strongly on x increasing from essentially zero at x ≅6.50 to about 50% at x ≅6.56, to about 75% at x ≅6.66, and reaches 100% as x approaches 7.0. The remaining fraction of the probes which shows a concomitant decrease in importance, continues to show spin glass-like behaviour with a strong thermal dependence of the average local fluctuation rates.

MÖSSBAUER EVIDENCE FOR MAGNETIC ORDERING IN YbP

Abstract 170 Yb Mossbauer experiments confirm that the heavy-electron type compound YbP is magnetically ordered below 0.41 K. The saturated spontaneous moment of Yb is much smaller than that of the Yb 3+ Γ 6 doublet, which is the high-temperature ground-state level.

Magnetic and superconducting properties of ErNi2 − xM2−xB2C (M Co, Pd, Pt)

Abstract We report lattice parameter and superconducting temperature transition ( T c ) measurements on ErNi 2 − x M 2− x B 2 C (M  Co, Pd, Pt), x = 0.20; M  Co, x = 0.05). We discuss the origin of T c lowering which is relatively modest for th substitutions Pd and Pt (d T c /d x ∼ − 12 K) and more pronounced for the non-isoelectronic substitution of Co (d T c /d x ∼ − 120 K). We also report 166 Er Mossbauer measurements for M = Co, x = 0.20 which evidence a main Er 3+ ordering temperature near 5 K and a saturated magnetic moment of 8μ B .

Single ion properties of Er3+ in YBa2Cu3Ox and Er3+ ordering arrangement in ErBa2Cu3Ox (x≅7.0 and 6.0) studied by 166Er Mössbauer spectroscopy

Abstract From Mossbauer measurements on 166 Er 3+ diluted into YBa 2 Cu 3 O x with x ≅7.0 (superconducting) we obtain the hyperfine tensor and the g tensor of the Er 3+ crystal field ground doublet: A ab = 1610, A c = 935 MHz , g ab = 8.0, g c = 4.5. When the Er 3+ is diluted into a sample with x ≅6.0 (non-superconducting) we find that it now experiences a molecular field of approximately 200 G attributed to a net exchange coupling with its magnetically ordered Cu(2) neighbours. The nonzero value for this field shows that the arrangement of the Cu(2) moments does not have inversion symmetry about the Er 3+ ion. From 166 Er 3+ measurements at 0.05 K on grain oriented ErBa 2 Cu 3 O x ( x ≅7.0 and 6.0) we confirm that for x ≅7.0 the ordered Er 3+ magnetic moments are aligned close to directions perpendicular to the c -axis and we show that for x ≅6.0 the additional Cu(2)-derived molecular field does not modify this alignment. We suggest however, that the appearance of this molecular field as the oxygen level is lowered below x ≅6.4 could be a factor contributing to the known x dependence of the breakdown of the 3-dimensional correlations within the Er 3+ sublattice. For the grain oriented samples we describe an X-ray diffraction analysis procedure which gives a comprehensive assessment of the quality of the grain orientation.

MAGNETIC ORDERING AND PARAMAGNETIC RELAXATION OF YB3+ IN YBNI2SI2

Abstract The magnetically ordered phase and the Yb 3+ paramagnetic relaxation in the compound YbNi 2 Si 2 have been investigated by 170 Yb Mossbauer spectroscopy between 0.05 and 80 K. The magnetic ordering transition is found to occur at T N = 2.1 K, the Yb 3+ magnetic moments being perpendicular to the fourfold symmetry axis ( c -axis). Above T N and upto 30 K, the thermal variation of the Yb 3+ paramagnetic relaxation rate is a superposition of an interionic exchange term (spin-spin relaxation) and of a T -linear conduction electron-4f spin exchange term (Korringa relaxation).

Crystal electric field in YbNi2B2C and YbNiBC observed by 172Yb perturbed angular correlations

We have measured the thermal variation of the quadrupole hyperfine interaction in the non-superconducting tetragonal borocarbides YbNi 2 B 2 C and YbNiBC over the temperature range from 20 to 1000 K using the 172 Yb perturbed angular correlations technique. The observed variations are due to the changes in the 4f shell contribution to the electric field gradient at the 172 Yb nucleus which are related to the thermal dependent populations of the crystal electric field levels of the Yb 3+2 F 7/2 ground state. In YbNiBC, we determine the complete Yb 3+ crystal field level scheme and we confirm that any hybridisation between the 4f and band electrons is weak. In YbNi 2 B 2 C, the electric field gradient does show a thermal dependence, thus evidencing the presence of crystal-field levels, but the total variation is relatively small. We consider this result in terms of the combined influence of the crystal field and 4f-conduction band hybridisation.

Magnetic properties of YB2(Cu0.94Zn0.06)3Ox (x = 6.0 to 7.0) studied by Mössbauer spectroscopy on substituted 170Yb3+

Abstract We have used 170Yb Mossbauer spectroscopy on Yb3+ diluted at the Y3+ sites to study YBa2(Cu0.94Zn0.06)3Ox. In the fully oxygenated sample, the presence of Zn2+ introduces a Korringa dependence of the Yb3+ paramagnetic relaxation rate, indicating an increase in the density of states at the Fermi level, which is visible both above and below Tc (32 K). A molecular field coming from the Cu (2) is observed on all the Yb3+ probes in non-superconducting samples with x = 6.0 and 6.5 showing that at these oxygen levels, all the Cu (2)s are locally magnetically correlated. For x = 6.88 molecular fields suggesting there is phase separation between local magnetic and non-magnetic superconducting regions. For x = 7.0, none of the Cu(2)s are magnetically correlated. The general features of the oxygen-level dependence of the local magnetic properties are similar in the zinc-substituted and in the zinc-free samples, but magnetic ordering persists to higher oxygen levels when Zn2+ is present.

Giant exchange fields and reduced moments in Yb Kondo lattices

Abstract We report 170 Yb Mossbauer spectroscopy and magnetisation measurements in the magnetically ordered Kondo lattices YbP, YbN, YbAuCu 4 and YbNiSn, which reveal hybridisation-induced reductions of the Yb spontaneous moment and of the magnetic ordering temperature. Using the self-consistent NCA method with a RKKY derived molecular field, we calculate these reductions and compare them with experiment.

57Fe Mössbauer absorption and emission spectroscopy and X-ray measurements in oriented crystallites of La2−xSrxCuO4: Directional properties of the 57Fe hyperfine parameters

Abstract We have performed Mossbauer measurements on oriented powders of 57Fe-doped La2−xSrxCuO4 (x=0 and 0.2 ) and 57Co-doped La2CuO4. We find that the main axis OZ of the 57Fe electric field gradient in the series lies at an angle β=26°±5° with respect to the crystal c axis, which is much less than recently published values. Our data were corrected for a fraction of unoriented crystallites, which we find to be systematically present, but which was unsuspected in previous experiments. We describe a convenient X-ray examination procedure for detecting and evaluating the unoriented fraction. We discuss possible origins of the angle β and we examine the relative disposition of the direction of c, OZ and of the hyperfine field on 57Fe in La2CuO 4 below TN. We also consider the influence of the oxygen stoichiometry on the symmetry at the Fe site.