μSR study of the properties of Fe3O4-based nanostructured magnetic systems

A ferrofluid based on Fe₃O₄ nanoparticles dispersed in heavy water D₂O is studied using the μSR method. The experiment has been carried out at temperatures 26–300 K. It is found that the diamagnetic (muon) fraction is formed in the ferrofluid in about the same amount as in D₂O, but the muon-spin relaxation rate in the ferrofluid is much higher than in D₂O. A significant shift of the muon-spin precession frequency in the ferrofluid is observed. It is shown that the shift of the muon precession frequency as a function of the external magnetic field is described by the Langevin function typical of paramagnetic magnetization. The mean magnetic field in the medium due to magnetic-nanoparticle polarization in an external field is experimentally determined. The nanoparticle sizes are estimated.     

μ+SR study of two-dimensional antiferromagnets,delafossite-type compounds

⁺SR experiments were performed on delafossite-type compounds, CuCrO₂, AgCrO₂, CuFeO₂, which are model compounds of triangular lattice antiferromagnets. The initial asymmetries are much smaller than the expected value, implying muonium formation. The time spectra are composed of slow andfast relaxation components. We attributed the components to signals from ⁺ stopped at the center of O^2– ions andmuonium far from nuclear dipole moments, respectively. The asymmetries decrease belowT _N but no precession spectra were observed. Relaxation rates of slow andfast relaxation components show maxima atT _N.     

Thermal,Raman and dielectric study of 0.5K0.5Bi0.5TiO3–0.5PbTiO3 ceramics

The 0.5K_0.5Bi_0.5TiO₃–0.5PbTiO₃ ceramics were prepared by following a standard solid-state method. The Raman, thermal and dielectric properties of these ceramics were investigated. The X-ray measurements showed that samples have single perovskite-type structure with tetragonal symmetry. Dielectric study revealed that the dielectric behaviour of the investigated ceramics is rather of normal ferroelectrics with large thermal hysteresis. The transition temperature observed by means of differential scanning calorimetry measurements is in good agreement with that obtained from dielectric study.     

μ +SR study of some magnetic superconductors

The relaxation of the ⁺SR signal has been investigated for the three compounds YRh₄B₄, ErRh₄B₄ and SmRh₄B₄. In the non-magnetic superconducting (T _c 11 K) YRh₄B₄, the data display a Kubo-Toyabe (gaussian) shape for zero (transverse) magnetic fields. ErRh₄B₄ (superconducting below 8.7 K and ferromagnetic below 1 K) shows a dominant signal with very slow relaxation. In contrast SmRh₄B₄ (superconducting below 2.7 K and antiferromagnetic-superconducting below 0.87 K) shows a change in relaxation from gaussian above 60 K to exponential between 1 K and 4 K to two exponential signals (fast and slow) belowT _N=0.9 K. In the region 0.9 K <T < 4.5 K, the relaxation time and the asymmetry both increase withT.supported by the U.S. Department of Energy.Supported by NSERC of Canada.supported by the U.S. Department of Energy.We are grateful to Drs. H. Umezawa and H. Matsumoto for interesting discussions regarding the persistent current screening and the results of self-consistent calculations.     

μ+SR in imperfect solids

The use of ⁺SR as a probe of point defects in metals is discussed. The importance of separating the effects due to the presence of the muon from those intrinsic to the defect-host system is stressed. It is suggested that studies of magnetic impurities in non-magnetic hosts may yield data which are relatively probe-independent. A theoretical analysis of the depolarization of muons coupled to magnetic impurities is outlined. It is pointed out that measurements of the field dependence of the depolarization rate can give information about impurity spin correlation times.Work performed under the auspices of the U. S. Department of Energy.     

Maximum-entropy TFμSR study of vortex states in 1/2/3 cuprates

A maximum-entropy (ME) method has been applied to TFSR vortex data of RBa₂Cu₃O₇ (R1237; R=Er, Gd and Eu). The obtained information on the field distribution is of much better quality than that resulting from Fourier transformation and curve-fitting of the same data. ME application to R1237 TFSR data taken below T_c at LAMPF show asymmetrical, nearly triangular, field distributions, which broaden with decreasing temperature. Significant deviations from Abrikosov predictions have been observed for the estimated R1237-vortex field distributions. Below 10 K, for Er1237 and Gd1237, the non-Abrikosov features appear to be influenced by the rare-earth magnetism. The low-field tail in the field distribution and the ME-intensity anomaly near T_c suggest glassy features in the 1/2/3 cuprate vortex states.     

Microstructure and electrical properties of Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics of (1?x?y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yLiNbO₃ (BNT–BKT–LN-x/y) have been fabricated by a conventional solid-state reaction method, and their microstructure and electrical properties have been investigated. The results of X-ray diffraction (XRD) measurement show that K⁺, Li⁺ and Nb⁵⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure. The BKT and LN addition has no remarkable effect on the crystal structure. However, a significant change in grain size took place. Simultaneously, with increasing amount of LN, the temperature for a ferroelectric–antiferroelectric phase transition is clearly reduced. The temperature dependence of dielectric properties suggests that the ceramics have diffuse-type phase transition characteristics. The piezoelectric constant d₃₃ and the electromechanical coupling factor k_p of the ceramics attain maximum values of 195 pC/N and 0.336 at x=0.18 and y=0.01.     

Mössbauer study of Fe in 3C-SiC following 57Mn implantation

Our investigations on substitutional and interstitial Fe in the group IV semiconductors, from ⁵⁷Fe Mössbauer measurements following ⁵⁷Mn implantation, have been continued with investigations in 3C-SiC. Mössbauer spectra were collected after implantation and measurement at temperatures from 300 to 905 K. Following comparison with Mössbauer parameters for Fe in Si, diamond and Ge, four Fe species are identified: two due to Fe in tetrahedral interstitial sites surrounded, respectively, by four C atoms (Fe_i.C) or four Si atoms (Fe_i,Si) and two to Fe in (or close to) defect free or implantation damaged substitutional sites. An annealing stage at 300–500 K is evident. Above 600 K the Fe_i,Si fraction decreases markedly, reaching close to zero intensity at 905 K. This is accompanied by a corresponding increase in the Fe_i,C fraction.     

RF-μSR study of muonium charge states and dynamics in Si

The radio frequencySR technique developed at TRIUMF was used to measure the temperature dependence of the diamagnetic muon, Mu, and Mu^* amplitudes in silicon between 10 K and 500 K. Six samples doped with phosphorus (n-type) and boron (p-type) in the concentration range 10¹¹ to 10¹⁵ cm^–3 were studied. In pure Si a very good fit over the whole temperature range is obtained from a model that includes the ionization of Mu^* and Mu to a bond centered ⁺ followed at high temperature by charge exchange involving Mu.     

μSR investigation of cupric oxide

TF and ZFSR-investigations were performed on high purity CuO powder. By TF measurements a phase transition to the ordered state was observed at 227K. A commensurate-incommensurate phase transition was detected at 213K by ZF measurements. In the commensurate phase we observed the Larmor precession. Four signals were detected below 55K, but by increasing temperature above 190K, precession became having only one component. This fact may be explained by muons tunneling between equivalent sites. In the incommensurate phase the Larmor precession was not detected because of too large damping.     

μSR studies of YFeMnO4

Muon spin relaxation (SR) studies were carried out on YFeMnO₄. Two interesting phenomena were found in measurements of longitudinal relaxation time spectra atT>T _cusp for a single crystal: (1) difference of the relaxation rates between the directions parallel with and perpendicular to thec-axis, and (2) suppression of local field fluctuations in a longitudinal external field. The spectra atT >T _cusp were reproduced with the function exp[-(₀ t)]. The results are discussed under a picture of the magnetic spin fluctuations of triangular antiferromagnets.     

A57Fe Mössbauer study of Nd(Fe0.5Co0.5)9Si2

A⁵⁷Fe Mössbauer study of Nd(Fe_0.5Co_0.5)₉Si₂ has been carried out over the temperature range 4.2–295 K. The analysis of the Mössbauer spectra, combined with X-ray diffraction on a magnetically aligned powder sample, shows that the easy-magnetization direction lies in the basal plane of this tetragonal BaCd₁₁ structure at 295 K, but is canted at an angle of 29(5) above the basal plane at 4.2 K. AC susceptibility measurements performed in the temperature range 77–295 K reveal a peak spanning the range 87–106 K, with the maximum occurring at 96 K. From these data, we conclude that there is a spin-reorientation from basal c-plane to a canted magnetic structure in this compound, with an onset at 96 K as the temperature decreases.     

Dielectric and piezoelectric properties of Li-substituted lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 ceramics

Lead-free 0.79(Bi_0.5Na_0.5)TiO₃–0.14[Bi_0.5(K_0.5−xLi_x)]TiO₃–0.07BaTiO₃ (BNBK79 + xLi, x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics were prepared by conventional solid state reaction process. The crystalline structures and surface morphologies are investigated by X-ray diffraction method and scanning electron microscopy. Dielectric and piezoelectric properties were measured. With increasing of lithium substitution, the Curie temperatures of BNBK79 + xLi ceramics increase, but the maximum value of the dielectric constant decreases. And a relatively large remnant polarization of 17.6 μC/cm² and 157 pC/N of d₃₃ has been obtained when x = 0.3.     

μ+SR Study of Ordering Phenomena in the Heavy-Fermion State of Ce3Pd20Ge6

Magnetic and quadrupolar ordering phenomena in a Ce₃Pd₂₀Ge₆ single crystal have been investigated by muon spin rotation/relaxation (μ⁺SR) spectroscopy. We have observed spontaneous precession of muons in zero-field below T _N =0.7 K in the antiferromagnetic state. The precession frequency follows the power law: ν(T)=ν(0)(1−T/T _N ) ⁿ . The exponent n=0.43(2) is close to the mean-field value of 0.5. The muon longitudinal spin relaxation rate 1/T ₁ is found to be nearly independent of temperature in the range of 0.3 to 2 K, i.e., across either T _N or T _Q =1.2 K, the quadrupolar ordering temperature. Two likely mechanisms for the temperature independent behavior of 1/T ₁ are suggested. This revised version was published online in September 2006 with corrections to the Cover Date.     

Grain-Size Effect on the Dielectric Properties of Bi0.5Na0.5TiO3

We prepared bismuth sodium titanate (Bi0.5Na0.5TiO3)ultrafine powders by the sol-gel method.The dielctric properties of the pressed pellets and fired ceramics with different grain sizes as a function of tempernature at various frequencies were studied.With decreasing grain size,the dielecric anomaly around 200℃ increases,while the dielectric thermal hysteresis decreases,All the samples with grain sizes larger than 100nm show dielectric peaks at temperature of about 350℃.The very little change in Tm observed down to the critical size indicates that Bi0.5Na0.5TiO3 is an order-disorder system above 200℃,In addition,the dielectric peak becomes lower with decreasing grain size and the ferroelectric critical size of Bi0.5Nan0.5TiO3 was eventually determined to be about 100nm according to the disappearance of dielectric peak.     

μ+ Diffusion in copper below 4 K reconfirmed by LF-μ+SR

Muon spin relaxation in longitudinal magnetic field (LF-μ⁺SR) reveals separately the static width of the local dipolar field distribution at the μ⁺ and the μ⁺ hopping rate in the copper lattice at 4.2 and 0.7 K, confirming that muon hopping between octahedral interstitial sites increases at lower temperatures.     

Mössbauer study of 57Fe in GaAs and GaP following 57Mn+ implantation

Ion implantation provides a precise method of incorporating dopant atoms in semiconductors, provided lattice damage due to the implantation process can be annealed and the dopant atoms located on regular lattice sites. We have undertaken ⁵⁷Fe emission Mössbauer spectroscopy measurements on GaAs and GaP single crystals following implantation of radioactive ⁵⁷Mn^?+? ions, to study the lattice sites of the implanted ions, the annealing of implantation induced damage and impurity–vacancy complexes formed. The Mössbauer spectra were analyzed with four spectral components: an asymmetric doublet (D1) attributed to Fe atoms in distorted environments due to implantation damage, two single lines, S1 assigned to Fe on substitutional Ga sites, and S2 to Fe on interstitial sites, and a low intensity symmetric doublet (D2) assigned to impurity–vacancy complexes. The variations in the extracted hyperfine parameters of D1 for both materials at high temperatures (T?> 400 K) suggests changes in the immediate environment of the Fe impurity atoms and different bonding mechanism to the Mössbauer probe atom. The results show that the annealing of the radiation induced damage is more prominent in GaAs compared to GaP.     

μSR Study of Zn-Substituted YBa2Cu3O x

We report zero field and transverse fieldSR. measurements in magnetic YBa₂(Cu_1–y Zn _y )₃O _x . Mild disorder of the antiferromagnetic state (x<6.1), a new disordered magnetic phase abovex=6.4 where superconductivity is destroyed by Zn substitution and, finally, local moments in the paramagnetic state are the most salient features related to Zn substitution described here.     

Influence of magnetic field sweep rate on the hysteresis loops of Ni0.8Fe0.2/Fe0.5Mn0.5 exchange bias bilayer

The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Ni0.8Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer. It was found that the sweep rate of 13.6 kA/4πms is high enough to bring about obvious changes in the hysteresis loops of the exchange bias bilayer. High sweep rate in the magnetization reversal stage enlarges the coercivity of the sample, while high sweep rate in the saturation state reduces the coercivity. The above phenomena were attributed to magnetic viscosity in the ferromagnetic layer enhanced by the interface exchange interaction and domain magnetization reversals assisted by thermal fluctuation in the antiferromagnetic layer respectively.