Positive muon spectroscopy of Nd2Fe14B and Pr2Fe14B

We have performed positive muon spin rotation measurements on polycrystalline samples of Nd₂Fe₁₄B and Pr₂Fe₁₄B in zero applied field. In both samples a single sharp μSR line was observed which was unexpected in this complicated structure. The temperature dependence of the muon frequency for Nd₂Fe₁₄B clearly reflects the spin reorientation below 150 K and can be explained qualitatively by assuming that only the c-axis component of a magnetization is sampled by the muon. A smooth decrease of the muon frequency with increasing temperature is observed for Pr₂Fe₁₄B.     

μ–H Interaction in Hydrogen Bonding Systems

Muon spin rotation (μSR) experiments were performed on single crystal samples of KH₂PO₄(KDP) and KD₂PO₄(dKDP) to study the dynamics of hydrogen in hydrogen bonding systems. At low temperature, the nuclear dipole interaction of muon and proton was confirmed from the angular dependence of precession frequency of the muon spin under zero magnetic field. The muon occupation site was also determined. A clear change in μSR spectra was observed at the antiferroelectric transition temperature (123 K). At 90 K well below the transition temperature, the muon spin starts to relax, possibly due to muon dynamics. This revised version was published online in September 2006 with corrections to the Cover Date.     

Study of magnetic properties of Pd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.95</Subscript>Mn<Subscript>0.05</Subscript> alloy using polarized muons and neutrons

The Pd_1?x Fe _x )_0.95Mn_0.05 alloy with random competing interaction was studied by measuring the muon spin relaxation in an external transverse magnetic field and in a zero magnetic field. Using the measured temperature dependence of the dynamic relaxation rate λ and the characteristics of the distribution of local static fields, the phase states of the sample under study are refined. In particular, it is shown that the ferromagnetic and spin-glass states coexist simultaneously in the sample below 25 K. Combined studies of the sample using the μSR and neutron depolarization methods made it possible to determine the size of magnetic inhomogeneities to be 2–6 μm in the temperature range 5–40 K.     

Study of the spin reorientation phenomenon in Nd2(Fe, Si)14BHy

We have studied the influence of the Si for Fe substitution and of the H insertion on the spin reorientation phenomenon in the Nd₂Fe₁₄B phases. To determine the temperature of spin reorientation and the tilt angle between the c-axis and the easy magnetisation direction, we have measured the angular dependence of the components of the magnetisation vector. Our measurements are based on powder samples that have been previously aligned under an external magnetic field. The Si for Fe substitution induces a decrease of the spin reorientation temperature and of the tilt angle. Meanwhile, we have found that the tilt angles are almost the same for Nd₂Fe₁₃SiB and Nd₂Fe₁₂Si₂B. These features are analysed in terms of changes in the crystal electric field and the unit cell volume induced by the Si for Fe substitution. The insertion of hydrogen in the Si-containing samples leads to an additive decrease of the spin reorientation temperature. This decrease is not linked to a change in the tilt angle at 4 K but to a different thermal behaviour in the H-containing samples. The magnitude of the effects of H insertion on the crystal electric field in comparison with the Si for Fe substitution is discussed together with the role of the lattice expansion.     

MuSR/μ+SR studies in KDP and DKDP

The temperature dependence of muon interactions has been studied in ferroelectric KDP ( H₂KPO₄) and DKDP ( D₂KPO₄) using conventional μSR and muon spin resonance spectroscopy. In longitudinal field measurements, a fast relaxing component and a slow relaxing component were observed. The slow relaxing component is attributed to diamagnetic muons. The muon spin resonance measurements indicate that the fast relaxing component results from some muonium like species: either normal or anomalous. In zero field and weak longitudinal field μSR (0–100 G), a remarkable peak in the fast relaxing component is observed around 220 K in both KDP and DKDP. An additional feature is also seen around 300 K. The amplitude of the resonance measurement has a broad minimum around 200 K which corresponds to the maximum in the relaxation rate in longitudinal field (100 G). The temperature dependence of the muonium relaxation rate in KDP is almost identical to that of DKDP. The diamagnetic fraction also shows almost no difference in relaxation rate or asymmetry for DKDP and KDP. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muon spin relaxation in Li0.6TiO2 anode material

In Li_0.6TiO₂ the longitudinal muon spin relaxation function has been measured for temperatures between 10 and 600 K. The μSR spectra were analyzed with a Markov process for multiple collisions. The time scale found for the Li⁺ diffusion is of the order of the microsecond or shorter. Above T = 100 K the magnetic field distribution at the muon is decreasing with increasing temperature.     

Muon spin rotation evidence for a glassy superconducting state in YBa2Cu3O7-y

We report on muon spin rotation (μSR) measurements of the internal magnetization in polycrystalline samples of the high T_c superconductor YBa₂Cu₃O_7-y. The mean value as well as the mean square deviation of the internal magnetic field seen by the muons undergo large variations with temperature. This effect shows a marked dependence on the magnetic history of the sample, which is what is expected of a glassy superconducting state.     

Spin state transition in Ca-doped Na0.7CoO2 with the nominal Co valence below 3.16

In order to clarify the nature of the transition around 300 K in Ca-doped Na_0.7CoO₂, the magnetism of Na_0.7Ca_yCoO₂ with y=0.035 and 0.07 was investigated in a positive muon spin rotation and relaxation (μ⁺SR) experiment. Transverse field μ⁺SR measurements showed that the spin state of the Co ions in Na_0.7Ca_0.07CoO₂ changes at around 300 K; i.e. the exponential relaxation rate vs. T curve exhibited a large maximum around 300 K with an accompanying small peak in the muonic Knight shift, whereas no changes were found in the asymmetry, similar to [Ca₂CoO₃]_0.62[CoO₂] at around 400 K.Although the spin-density-wave (SDW) state exists for Na_xCoO₂ with x≥0.75 at low temperatures, zero-field μ⁺SR spectra in the Ca-doped samples showed no muon spin precessions down to 1.8 K but only fast relaxations indicating disorder. This is probably because the Ca²⁺ ions in the Na planes alter the charge and/or spin distribution in the CoO₂ planes. As a result, the SDW order is hindered, as the nominal Co valence is decreased below 3.16.     

μSR in antiferromagnetic α-Fe2O3

The first muon spin rotation (μSR) measurement of an internal magnetic field in an antiferromagnet is reported. Experiments on powder and single crystal α-Fe₂O₃ show a discontinuous change in the magnitude and direction of the local field at the Morin transition. The muon is localized at low temperatures, and thermally activated diffusion sets in at about 500 K.     

Mössbauer study of the intermetallic compound Nd2Fe14B. II. Temperature dependence and spin reorientation

The ⁵⁷Fe Mössbauer effects of Nd₂Fe₁₄B were measured in a temperature range of 4.2−300 K. Below the spin reorientation transition temperature T_sc = 148 K, the spectra were satisfactorily analyzed with twelve Zeeman sextuplets due to splitting of six crystallographic Fe-sites into twelve non-equivalent sites. It was shown that the magnetic moments of the Fe and the Nd atoms are non-collinearly coupled in the magnetic structure with canted moments below T_sc. The directions of the moments at 4.2 K are inclined at 27° for Fe and at 58° for Nd from the c-axis to the [110] direction. The average moments are 2.27μ_B for Fe and 3.3μ_B for Nd at 4.2 K. The increase of the average hyperfine field with decreasing temperature is suppressed below T_sc, and its value at 4.2 K is reduced by 1% from the value of 337 kOe which is observed in Y₂Fe₁₄B and also estimated for Nd₂Fe₁₄B by extrapolating the values above T_sc. On the other hand, the Nd moment increases abruptly around T_sc as the temperature decreases. The directions of the principal axes of electric field gradients on the six distinct Fe-sites were also obtained. The anomalous temperature dependence of quadrupole splittings and isomer shifts was observed around T_sc. They were discussed in a framework of the changes in the band structure and the lattice parameters incidental to the spin reorientation transition.     

μSR Investigation of magnetic phases in R1−xSrxCoO3 oxides (R=Pr, Nd)

We report results of our muon spin relaxation measurements in the series of polycrystalline compounds Pr_1−xSr_xCoO₃ (x=0.3, 0.4 and 0.5) and Nd_1−xSr_xCoO₃ (x=0.3 and 0.5). For the Pr-based samples our data clearly indicate the existence of two magnetic transitions, as also inferred from macroscopic measurements. While the high temperature transition is typical for cobaltites (∼200 K), the low temperature one is unusual. In our experiments it occurs below about 120 K and it manifests itself as a change in the slope of the temperature variation of the muon spin depolarization rate λ(T). For the Nd-based samples we found an increase of the muon spin depolarization rate below 45 K, temperature at which the sample is ferrimagnetic, when the Nd sublattice coupled antiparallel to the Co lattice. No phase separation could be evidenced in our samples by μSR experiments.     

Magnetocrystalline anisotropy of Er2Fe14B

A rotation-magnetic-alignment method was used to align fine-powdered (< 20 μm) Er₂Fe₁₄B at room temperature while the easy magnetization direction of Er₂Fe₁₄B lies in the basal plane. X-ray diffraction was used to check the magnetic alignment. For the first time, the temperature dependence of the anisotropy field of Er₂Fe₁₄B was measured in a wide temperature range from about 170 to 530 K. The anisotropy field was determined using the SPD technique in a pulsed-field magnetometer from 170 to 320 K (T_SI = 323 K) on a magnetically aligned sample and from 330 to 530 K (T_C = 550 K) on a bulk polycrystal.     

μ + Knight shift and spin relaxation in indium single crystalKnight shift and spin relaxation in indium single crystal

μ ⁺ SR measurements have been performed in a single crystal indium sample between 12 K and 300 K with a stroboscopic μSR spectrometer. The muonic Knight shiftK _μ and the muonic depolarization rate σ were obtained for various angles θ between the tetragonal crystallinec-axis and the direction of the external field. The isotropic part ofK _μ is only weakly temperature dependent and is consistent with the estimated Pauli spin susceptibility value. At a temperature of 12 K the angular dependence ofM ₂ (the second moment of the field distribution at the muon, obtained from the measured σ(θ) values) allows a clear determination of the muon location — the symmetric tetrahedral site. The observed anisotropicK _μ cannot be explained by the dipoles at the In atoms responsible for the bulk magnetic susceptibility but probably originates from an anisotropic Pauli spin susceptibility.     

Spin dynamics in CuGeO3 studied by muon spin rotation

We report a muon spin rotation (SR) study of the magnetic properties of the Cu²⁺ quasi-one-dimensional CuGeO₃ system and its lightly-doped derivative Cu_0.97Zn_0.03GeO₃. Susceptibility measurements on CuGeO₃ show a sudden change in the vicinity of 14 K that has been interpreted before as a magnetic transition to a spin-Peierls state. SR shows no evidence of spin freezing below 14 K, implying that the transition is to a magnetic state with no static (random or ordered) electronic moments. A modest slowing down of the electronic spin dynamics is also identified at this temperature. Similarly, no evidence of a transition to a static magnetic state is found for Cu_0.97Zn_0.03GeO₃ whose susceptibility shows hysteretic behaviour between zero-field and field cooled measurements at 4 K, previously ascribed to spinglass-like behaviour. Given the nature of the muon spin as a local magnetic probe, the present results necessitate a re-interpretation of the origin of the susceptibility anomaly observed in the doped system.     

Muon-spin-rotation study of muon polarization losses in plastic scintillators and quartz

Muon polarization losses in plastic scintillators of two types and in fused quartz have been studied by the μSR method. The muon and muonium spin precession spectra have been measured on the μSR setup placed at the output of the muon channel of the Gatchina synchrocyclotron. It has been shown that a significant fraction of stopped muons participate in the formation of the muonium. As a result, these muons lose their polarization completely. The magnitude of muon depolarization depends considerably on the type of plastic. It has been found that the muon spin precession frequency in fused quartz in an external magnetic field (F _{Q, μ} = 0.116 ± 0.002 MHz) is shifted with respect to that in plastic scintillators (F _{1, μ} = 0.101 ± 0.005 MHz and F _{2, μ} = 0.101 ± 0.002 MHz).     

The effect of pressure on the spin density wave transition in the layered cobaltites [Ca2CoO3]0.62[CoO2] and [Ca2Co4/3Cu2/3O4]0.62[CoO2]

In order to investigate the pressure effect on the magnetism in the layered cobaltites, positive muon spin rotation and relaxation μ⁺SR experiments have been carried out up to 1.3 GPa using c-aligned polycrystalline samples of [Ca₂CoO₃]_0.62[CoO₂] and [Ca₂Co_4/3Cu_2/3O₄]_0.62[CoO₂]. A transverse field μ⁺SR experiment indicates that the transition temperature to an incommensurate spin density wave IC-SDW state is independent of hydrostatic pressure up to 1.3 GPa for the both compounds. Furthermore, there are no changes in the spontanious muon precession frequency in zero field at 5 K even under 1.3 GPa. These results strongly suggest that the IC-SDW exists not in the rocksalt-type block ([Ca₂CoO₃] and/or [Ca₂Co_4/3Cu_2/3O₄]) but in the CoO₂ plane.     

Magnetovolume effects in Fe80-xNixCr20 alloys

Temperature and magnetic field dependences of the thermal expansion between 4 and 300 K and in fields up to 6 T were made on Fe_80-xNi_xCr₂₀ for 14⩽x⩽49 at%. This concentration range covered the regions in which the samples were antiferromagnetic, paramagnetic and ferromagnetic as well as spin glass and reentrant spin glass at low temperatures. We develop a method of determining the lattice contribution to the thermal expansion for such systems showing mixed magnetic behavior and analyze the present data accordingly. We find in ferromagnetic samples large magnetic contributions to the thermal expansion even at temperatures much higher than the Curie temperature. The field dependence of the lenght change shows behavior which is characteristic of the magnetic state of the system.     

Magnetic hyperfine field changes at different Fe-sites below spin reorientation temperature in polycrystalline untextured Nd2Fe14B

In order to estimate the magnetic hyperfine fields at different sites, an analysis of the Mössbauer spectra of polycrystalline untextured Nd₂Fe₁₄B, recorded at temperatures above and below the spin reorientation temperature T_s(≈150 K), has been made. Here the signs of the electric field gradients (efg) and the principal axes of efg components have been constrained according to the available crystal structure information below and above T_s. It has been observed that the magnetic hyperfine field changes are similar to the recently reported magnetic moment changes at different sites in an isostructural alloy Tm₂Fe₁₄B.     

Muon spin relaxation measurements of CsC60, RbC60, KC60

Muon spin relaxation (μ⁺SR) measurements of CsC₆₀ and RbC₆₀ have revealed magnetic ordering at 20 K and 15 K respectively, with static internal fields at the muon site of \sim7 G for both samples. No magnetic ordering is seen in KC₆₀ down to 2.5 K. An apparent small increase in muon spin relaxation which had been seen at 70 K in RbC₆₀ is ascribed to an extrinsic effect, originating in muons which stopped in the aluminum sample holder. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of hydrogenation on the magnetic and magnetoelastic properties of R 2Fe14B compounds (R = Nd, Gd, Er, Lu)

The temperature dependences of the magnetization σ(T), magnetostriction λ(T), and linear thermal expansion coefficient α(T) of R ₂Fe₁₄B intermetallic compounds (R = Nd, Gd, Er, Lu) and of their hydrides R ₂Fe₁₄BH_2.5 are studied. The magnetization was measured with a pendulum magnetometer within the temperature interval 77–700 K in a magnetic field H = 500 Oe. Magnetostriction and thermal expansion were measured using the tensometric technique in the temperature interval 77–420 K. It was established that Gd₂Fe₁₄BH_2.5 undergoes a spin-reorientational (SR) transition at T _SR = 235 K. In compounds with Nd and Er, anomalies associated with the SR transition were found in the σ(T), λ(T), and α(T) curves. The SR transition temperatures were refined and magnetic phase diagrams were constructed for the compounds studied. The α(T) curves of the R ₂Fe₁₄BH_2.5 hydrides (R = Nd, Er) revealed anomalies of a nonmagnetic origin associated with hydrogen ordering in the crystal lattice of these compounds.