Formation and ordering of local magnetic moments in Fe–Al alloys

The density functional theory is used to study the local magnetic moments in Fe–Al alloys depending on concentration (from 29 to 44 at% Al) and the Fe nearest environment. We have found three different solutions for the system: a spin-spiral wave (SSW) which has a minimum energy and two collinear states, a ferromagnetic one and a state with both positive and negative Fe magnetic moments (the Fe atoms with many neighboring Al atoms around them have negative magnetic moments, while the other Fe atoms—positive). Both the SSW and the negative Fe moments agree with the experiments. Magnetization curves taken from the literature are analyzed. The assumption of percolation character of the size distribution of magnetic clusters describes well the experimental superparamagnetic behavior above 150 K.     

Electronic structures and local magnetic moments in ferromagnetic and antiferromagnetic FexRh1−x alloys

Ground-state magnetic properties of ordered FeRh alloys are discussed by using the realistic canonical d-band model within the Hartree-Fock and coherent potential approximations. Local magnetic moments of Fe and Rh atoms in ferromagnetic and antiferromagnetic Fe_xRh_1-x alloys are computed as a function of x, the Fe concentration. Calculated results are in good agreement with the neutron-diffraction data. The band-energy calculation indicates that in Fe-rich alloys excess Fe atoms substituted on Rh sites having larger magnetic moments than Rh atoms, play an important role in the antiferromagnetic to ferromagnetic transition observed at low temperatures.     

Two successive magnetic transitions in Y2Cu2O5 studied by μSR

Zero field muon spin rotation and magnetic susceptibility experiments on investigation of magnetic properties of cuprateY ₂Cu₂O₅ have been performed in the temperature range 4.2–30 K. Transverse fieldSR-experiments have been also carried out in order to obtain accurate information about transition temperature and to study the influence of the external magnetic field. Our data show that two magnetic phase transitions occur inY ₂Cu₂O₅ with lowering temperature. Upper Neel temperatureT _N=13 K is consistent with previous experimental data. We obtained the temperature dependence of the local magnetic field on the muonB (T) in the antiferromagnetic phase.B (T) reveals a peculiarity (some change of the slope) near the temperatureT _N=7.5 K, which can be interpreted as an additional magnetic phase transition caused by a change in magnetic ordering of the copper subsystem. Applying a small external magnetic field 50 Oe leads to smearing of the peculiarity inB (T) dependence.     

Zero-field μSR and low-temperature μ+ diffusivity in copper

In this paper I review the history of ⁺ diffusion studies in copper, with particular emphasis on the increased low-temperature diffusivity which has been known for several years now. I survey the theory and practice of the zero-fieldSR method, which has come into increasing favor in the study of muon diffusion and trapping in metals, and discuss its application to the low-temperature copper problem.     

Nanocrystalline oxide zinc materials studied by the Mössbauer effect, μSR and neutron diffraction

ZnO and ZnFe₂O₄ were investigated in nanocrystalline form using various hyperfine techniques in conjunction with neutron diffraction and bulk magnetic measurements. For the zinc oxide, the main interest was the influence of the nanocrystalline state on lattice dynamics and on the local symmetry of the Zn atom as reflected in its quadrupolar coupling, employing the high-resolution Mössbauer resonance of ⁶⁷Zn. Measurements were carried out on nanocrystalline materials with particle sizes between 4 and 25 nm. It was found that the asymmetry parameter increases drastically from η=0 in ZnO single crystals to η≈0.5 in nanostructured material, indicating an off-axis displacement of either Zn or O atoms in finite size grains. The dependences of the Lamb-Mössbauer factor on grain size and temperature are well described by a two-component model in which small crystallites are surrounded by a network of grain boundaries. For the zinc ferrite the question of changes in the magnetic properties stood in the foreground. Using the combination of techniques mentioned, the magnetic behavior of nanocrystalline ZnFe₂O₄, which had an average particle size of about 9 nm was compared to differently prepared crystalline samples. It was seen that the role of cation-site occupation, which changes substantially with nanocrystallinity, plays a decisive role for the magnetic properties.     

Multiple Phase Transitions in CuO Studied by μSR

Precise measurements of the local magnetic field at muon sites in CuO were performed. The temperature dependence of the local field including spatial orientation was obtained in the ordered state. Not only zero field μSR measurements were performed, but also external magnetic fields were applied to obtain an unambiguous determination of field direction. Multiple stages of muon signals were observed at 55 and 80 K. An incommensurate phase between 212 K and 227 K was confirmed from the line shapes of the local fields at the muon sites. This revised version was published online in September 2006 with corrections to the Cover Date.     

Study of local magnetic fields in the oxide α-Bi2O3 byNQR andμSR techniques

NQR andSR investigations of the local magnetic field in-Bi₂O₃ were performed. In theNQR experiments on-Bi₂O₃ which is usually considered as diamagnetic, the splitting of the spectral lines revealed a local field on the bismuth nuclei. The internal magnetic field obtained bySR significantly exceeds the dipole field from Bi nuclear magnetic moments. A possible source of the local magnetic fields is partial covalent bonds in-Bi₂O₃.     

Magnetic penetration depth in V3Si and LiTi2O4 measured by μSR

Muon spin relaxation (SR) studies have been performed in the normal spinel LiTi₂O₄ and the A-15 superconductor V₃Si to measure the magnetic penetration depth . The relaxation rate(T) 1/² in field-cooled measurements shows a sharp increase belowT _c followed by saturation at low temperatures in both systems. This feature implies an isotropic energy gap without anomalous zeros, and most likelys-wave pairing. The low temperature penetration depth (T 0) is determined to be 2100Å for LiTi₂O₄ and 1300Å for V₃Si respectively. Assuming a clean limit relation ^–2 n _s /m ^*, we derive the Fermi temperatureT _F n _s/ ^2/3 m ^* from the relaxation rate and the Sommerfeld constant asT _F ^3/4–1/4. Unlike conventional superconductors, both LiTi₂O₄ and V₃Si have a large ratio ofT _c /T _F 0.01, only slightly smaller than those ratios in more exotic superconductors.We thank C. Ballard and K. Hoyle for technical assistance. Work at Columbia University is supported by NSF Grant No. DMR-89-13784 and Packard Foundation (YJU). Ames Laboratory is operated for the U. S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. Work at Ames was supported by the Director for Energy Research, Office of Basic Energy Sciences.     

High pressure μSR studies: rare earths and related materials

After a short introduction to μSR with respect to the study of magnetic properties, followed by a brief outline of the principle of the high pressure-low temperature μSR spectrometer installed at the Paul Scherrer Institute, we discuss some measurements on rare earth materials employing this instrument. They are concerned with: (1) The pressure dependence of the spin turning process in ferromagnetic Gd. (2) The volume dependence of the internal magnetic field in the heavy rare earth metals Gd, Dy, and Ho in their ordered magnetic states. (3) The response of the (first order) magnetic transition in the frustrated antiferromagnets of type RMn₂ (R = Y,Gd) to pressure. (4) The variation of magnetic parameters with pressure in La₂CuO₄ (powder sample), the antiferromagnetic parent compound of the high T_C superconductors of type La_2−x(Sr, Ba)_xCuO₄. In conclusion a short outlook on further developments is given. This revised version was published online in July 2006 with corrections to the Cover Date.     

μ+SR and Mössbauer studies of low dimensional magnetic ordering in ilvaite

Ilvaite reveals a quasi one dimensional ferromagnetic order along Fe chains between 50–120K. Both Mössbauer spectroscopy and ⁺SR reveal the onset of long range order below 120K with spin dynamics in the MHz range which is interpreted with relatively slowly moving wide domain boundaries. Below 50K the individual chains couple antiferromagnetically, yet a considerable fraction of dynamical spins is detected from ⁺SR down to low temperature.Work supported by the German Federal Minister of Research and Technology (BMFT) under contract Nr. 03-LI3-BRA.     

Interplays ofμSR,susceptibility and neutron studies on dilute-alloy spin glasses

Static spin polarization of Fe moments in a spin glassAuFe, determined by zero-fieldSR, is compared to an ac-susceptibility measurement below the cusp temperatureT _g, and a rather uniform amplitude of the static polarization is pointed out for frozen spins. Completely random orientation of frozen spins is revealed by neutron scattering in a dilute 1%CuMn, and an importance of comparing results of these different methods is demonstrated.     

μ+ diffusion in copper studied by zero field μSR

Diffusion of muons in high purity Cu was studied by the zero-fieldSR method at temperatures from 134 K down to 0.09 K. The experimental data show clear evidence that the muons still diffuse below 15 K; the measured diffusion rate has a minimum at around 15 K and seems to become independent of temperature below about 0.5 K.     

ZF and low-LF μSR in spin-glassy icosahedral Al-Mn-Si quasicrystal

ZF-SR measurements in icosahedral Al₇₃Mn₂₁Si₆, which has a spin glass freezing temperature of 6.4K, show that the spin-freezing process in this quasicrystal is very similar to that observed in crystalline dilute Cu(Mn), but that the temperature range of partial freezing (coexisting frozen and unfrozen regions) is much wider in the quasicrystal. Low-LF spectra confirm that a substantial volume remains unfrozen down to T _g /2 at least. Whether this is more consistent with macroscopic inhomogeneous freezing or any more homogeneous model, such as fractal clusters, depends on resolving and understanding the TT _g static limit.DRN supported by USDOE grant DE-FG05-88ER45353, PM supported at TRIUMF by NSERC Canada.Thanks to Y. Calvayrac of CECM-CNRS Vitry (France) for providing the sample, and to T.L. Duty, S. Johnston, and G.D. Morris for assistance in collecting these data. This work was also partially supported by the NRC of Canada, through its support of TRIUMF.     

Magnetic properties of YMn2 and related compounds studied by μSR

Zero, longitudinal and transverse field SR was carried out between 3 and 250 K in YMn₂, Y_0.9Tb_0.1Mn₂, Y_0.98Sc_0.02Mn₂ and TbMn₂. The data had to be analyzed in the frame of electronic-nuclear double relaxation. The dynamics of Mn magnetic moments in the paramagnetic state is typical for an itinerant antiferromagnet approaching magnetic order. This behavior is suppressed abruptly when the first-order transition occurs. In all samples, a part enters a randomly ordered (spin glass like) magnetic state within a certain temperature range aboveT _N as an outcome of frustration. The temperature range of existence and the fraction of the sample taking part in this stale is enhanced by the presence of Tb³⁺ paramagnetic ions and also by the lattice pressure in the Sc doped compound. It is suspected that muon dynamics makes the extraction of information on magnetic relaxation questionable above 150–180 K.Part of this work was funded by the German Federal Minister for Research and Technology (BMFT) under Contract No. 03-KA2-TUM.     

Hyperfine fields and Fe magnetic moments in Fe−Ni invar alloys

Mössbauer spectra of Fe_1?x Ni _x withx=0.40, 0.35 and 0.32 were taken at various temperatures and fitted withB _hf distributions. TheP(B _hf) curves generally show a peak and a low-field tail. The peakB ^* and the average \(\bar B\) were obtained in theT→0 limit. Fe moments were deduced from these data with help of a semiempirical equation. While the average \(\bar \mu _{Fe} \) decreases withx (invar anomaly), a large fraction of Fe atoms retain the strong moment (≈ 2.7 μ_B) typical of Ni-rich alloys. This result is consistent with a two-states model for invar.     

Redox reaction of μ−O in water determined by the chemical shift

The chemical shift of ^–O in water, ice and in aqueous hydrogen peroxide (H₂O₂) was examined using ^–O resonance. Two distinct signals are obtained in water and ice. Using ⁺ resonance signal as a standard, the observed shiftsg are –7.0×10^–4 (signal A) and –12.1×10^–4 (signal B) respectively. The intensity of the two fractions are almost equivalent in water; however, in ice, the intensity of signal B decreases with increase of signal A. After the addition of hydrogen peroxide to the water signal B disappears, A increases, and the total amplitude is increased.     

Hydrogen in semiconductors: The roles of μSR and theory

Hydrogen is an unavoidable impurity in all semiconductors. It interacts with intrinsic defects (from monovacancies to dislocations), with impurities (shallow dopants, deep centers, even electrically inactive impurities), with the crystal, and with otherH interstitials. These interactions profoundly affect the electrical and optical properties of the host. Conventional experimental techniques used to study the properties of hydrogen (EPR, IR or Raman spectroscopy, etc.) have provided information on a number ofH-related defects. Theory has played a major role in these studies, not only by confirming the models proposed on the basis of experimental data, but often by explaining the data altogether or predicting new features. So far,SR has provided fundamental information on isolated hydrogen-like species in many semiconductors. It would be wonderful if thespectroscopic signature of muonium-impurity pairs could be identified or ifquantitative information regarding the stability of the various charge states of muonium could be obtained.     

Mössbauer and μSR spectroscopy: Similarities and difference in applicability

Mössbauer spectroscopy and the muon spin rotation technique are two methods in which information on the atomic surrounding is obtained through the hyperfine interaction. This similarity is contrasted by the differences in observational parameters.While in Mössbauer spectroscopy information originates (in the most general case) from four different interactions, one has in SR only a magnetic interaction to detect. Furthermore, the lattice position probed are also different since the muon is, in general, found in interstitial positions and a further difference is related to the necessary probe concentrations. The differences and similarities are discussed through a number of examples from magnetism, diffusion and superconductivity.     

Photon recycling transport in bulk GaAs probed by μSR with laser irradiation

A newly developed μSR method combined with a tunable, high power and sharply pulsed laser was applied to GaAs. Anomalous transport phenomena of the photo excited effects into bulk crystal were observed, given that the stopping range of implanted muons is about 200 times larger than photo-absorption depth. In addition, the measurement of photon energy dependence of muon spin depolarization under laser irradiation suggests the impurity level associated with Mu _BC ⁰ is located within the band gap of GaAs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spin dynamics of diluted antiferromagnets MnxMg1-xTiO3 by μSR method

The diluted antiferromagnet Mn_xMg_1-xTiO₃ transforms from the antiferromagnet (AF) to the spin-glass (SG) system at x ∼ 0.6. In order to study the microscopic nature of spin dynamics and ordering, we performed muon spin relaxation measurements for the powder samples of x=1.0∼ 0.55 and examined the concentration dependence. From these results, a drastic change in the spin dynamics was found to occur at x ∼ 0.6. This revised version was published online in August 2006 with corrections to the Cover Date.