bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of fields and force densities from finite element approximations of potentials (abstract)
Authors: Kotiuga P. R. , Silvester P.
Published: July 2002

Problems of electrostatics and magnetostatics are usually formulated such that fields and force densities are related by some differential operator to a potential. Because of the necessity of finding potentials numerically, and difficulties inherent in numerical differentiation, fields forces and other expressions involving derivatives of the potential are seldom determined accurately. For the case of a particle situated in a field whose potential is a harmonic function, Bui avoided this difficulty by considering an analytic expression for the potential in a neighborhood of a point, and relating expressions involving the derivatives of the potential at that point to the potential in a neighborhood by an integral operator. In this paper, Bui’s approach is extended to fields whose potentials are harmonic functions or vectors fields whose components in Cartesian coordinates are harmonic functions. In this way, field and force density vectors as well as other expressions involving derivatives of the potential can be estimated from finite element approximations to the potential. The paper contains a derivation of the method from the Green’s function for a sphere, the justification of the particular choice of integral operator among possible variants, and a discussion of the fortran programs used to implement the method.
The anisotropy paradox of Co2+ ions in lithium ferrite
Authors: Dionne Gerald F.
Published: July 2002

Microwave power absorption and hysteresis loops were studied on a series of ferrite materials prepared from a standard LiTi spinel host with Co2+ additions in amounts up to 0.05 ions per formula unit. Experimental results over this concentration range indicate that the rapid relaxation of Co2+ causes X-band spin wave linewidths to increase by more than an order of magnitude. The usual strong effects of Co2+ positive magnetic anisotropy, however, were not found in either ferrimagnetic resonance linewidths or hysteresis loop parameters. To account for the absence of anisotropy effects in the presence of strong relaxation effects, Co2+ is considered to act as a paramagnetic ion through decoupling from the exchange fields of the iron sublattices. This new theory of exchange isolation is based on selective clustering of dilutant ions that results from minimization of magnetic exchange, lattice electrostatic, and crystal field stabilization energies.
Magnetic resonance study of the crystal symmetry of praseodymium bromate nonahydrate
Authors: Taylor D. R. , Sussums A. A. F.
Published: July 2002

Electron spin resonance measurements on Nd ions in Pr(BrO3)39H2O (PrBR) have been carried out at 4.2 K to examine the symmetry of the rare-earth sites. The results are not compatible with recent diffraction studies which identified the crystal as hexagonal with D3h rare-earth site symmetry. The spectra show a large number of inequivalent sites with probably only reflection symmetry, repeating every 60° in the plane perpendicular to the long axis of the crystal. These results appear to be consistent with the twinned pseudohexagonal C122v structure also proposed for this compound.
Magnetic excitations in neodymium
Authors: Stirling W. G. , McEwen K. A. , Stanley H.
Published: July 2002

While magnetic excitations in rare-earth systems in which the full ordered moment develops at each site have been comprehensively investigated, the excitations in sinusoidally modulated magnetic systems have proved to be rather elusive. Moreover, the character of the magnetic excitations in modulated magnets has been a subject of theoretical interest for many years. Neodymium metal exhibits a complicated sequence of modulated antiferromagnetic structures below TN =19.9 K. Following our first observations of magnetic excitations in this element, we have now carried out an extensive study of the spin dynamics of Nd, in both the modulated and paramagnetic phases. The wave vector and temperature dependence of several branches of excitations, whose energies range from 1 to 16 meV, have been measured and will be discussed in detail. Additionally, some results of the field dependence of the excitations will be presented.
Magnetic and quadrupolar excitations in NdSb
Authors: Sablik M. J. , Wang Y. L.
Published: July 2002

We have extended our previous analysis of magnetic and quadrupolar excitations in rare-earth compounds to NdSb, which is a type-I antiferromagnet. The full ten-level manifold of the Nd+++ crystal field is incorporated into the analysis. Also, a study of excitation amplitudes is included as a function of q across the Brillouin zone. Zone edge effects on these amplitudes are discussed. Comparison of our analysis with the experimental data of Furrer et al. reveals that mixing of quadrupolar excitations with magnetic excitations below the ordering temperature can lead to a flattening of the dispersion if there are competing quadrupolar and exchange couplings.
Precision measurements of magnetization in iron by neutron diffraction. I. Problems
Authors: Arrott A. S. , Templeton T. L.
Published: July 2002

A method is proposed for a precision determination of the magnetic contribution to neutron scattering from the (110) Bragg reflection in iron. The method requires that the metallurgical preparation of the sample leads to the behavior expected for an ideally imperfect crystal of a particular thickness. The theory, experimental techniques, and some results are discussed. An anomalous contribution to the scattering in the demagnetized state is observed.
Magnetization of very thin Ni films in epitaxial metal film sandwiches of Cu
Authors: Sill L. R. , Brodsky M. B. , Bowen S. , Hamaker H. C.
Published: July 2002

Single-crystal nickel films varying in thickness from 2-40 Å were prepared by epitaxial growth between thicker layers (500 Å) of single-crystal copper (001). Their magnetization was measured as a function of field up to 10 kOe using a dc SQUID magnetometer. The saturation magnetization of these films was determined as a function of Ni thickness at liquid helium temperatures. A decrease in saturation moment from that of bulk nickel was observed for films of thickness less than 18 Å. The apparent Curie temperature also decreases with decreasing thickness for these films. The magnetization at maximum field as a function of temperature does not follow a Brillouin function.
FMR of cubic cobalt grown by molecular beam epitaxy on GaAs (abstract)
Authors: Prinz G. A. , Vittoria C. , Krebs J. J. , Hathaway K. B.
Published: July 2002

Single-crystal cobalt films from 50 to 350 Å thick have been grown on GaAs by molecular beam epitaxy techniques used previously to grow single-crystal films of bcc Fe. In situ reflection high-energy electron diffraction during growth indicates that the crystallographic axes of the films are parallel to those of the (110)-face GaAs substrates, and always shows single-crystal diffraction patterns for films less than 50 Å thick. The surface net spacing is very close to what would be expected for a (110) face of bcc Co by extrapolating the lattice constants of Fe-Co alloys to 100% Co. Vibrating sample magnetometer measurements in the plane show 〈001〉 axis easy, 〈111〉 intermediate, and 〈110〉 axis hard in accord with a negative K1. Ferromagnetic resonance (FMR) measurements at fixed frequency (9.5 and 34.6 GHz) and variable frequency (1–18 GHz) were performed on these films. The magnetic field H was in the film plane or perpendicular to the film for the fixed frequency FMR measurements. H was applied along the 〈110〉 axis for the variable frequency measurements. For the thickest single-crystal film grown (357 Å) the following magnetic parameters were determined: 2K1/M=−1.08 kOe, g=2.16, 4πM=15.5 kG, and Hu =1.32 kOe. Hu is a strain-induced uniaxial magnetic anisotropy field whose easy axis is along the 〈001〉 direction. The angular variation of the in-plane FMR fields is consistent with cubic but not hexagonal Co.
Coupled oscillations of domain-domain wall system in garnet films
Authors: Ramesh M. , Jedryka E. , Wigen P. E. , Shone M.
Published: July 2002

Interactions between the domain wall resonance mode (DWR) and the domain ferromagnetic resonance modes ω− and ω+ were observed for the first time in unsaturated Ga- and La-doped thin YIG films. The samples were around 10 μm thick, their 4πM ranged from 1470 to 525 G, uniaxial anisotropy field ranged between 400 and 800 Oe and the in-plane saturation field was between 110 and 690 Oe. A transmission rf spectrometer using a slotted rf structure in the 10–2500 MHz range was used to observe the signals. The zero in-plane field DWR frequencies were an order of magnitude higher than those of typical bubble materials and ω+ and ω− fell in the same frequency-field regime as the DWR. The interactions between the modes resulted in frequency gaps of 70–500 MHz for the DWR-ω− modes and 200–10 MHz for the DWR-ω+ modes. A generalized theory of dynamics of a system of domains and domain walls is proposed. The results of the theory, which predicts a coupling of the form hc  sin 2θ between DWR and ω−, are discussed.
Suppression of magnetostatic backward volume wave end reflections via field gradients
Authors: Wallin S. J. , Stancil D. D.
Published: July 2002

This paper reports the use of longitudinal magnetic field variations to suppress magnetostatic backward volume wave end reflections in an yttrium-iron-garnet (YIG) thin film. The variations were achieved by mounting the YIG film between two mumetal strips. The passband ripple in the insertion loss due to reflections was reduced from an average of about 12 dB peak to peak to 1/2 dB peak to peak with this technique.
Spin-polarized photoemission study of ferromagnetic metallic glasses (abstract)
Authors: Hopster H. , Kurzawa R. , Raue R. , Schmitt W. , Walker K.-H. , Güntherodt G.
Published: July 2002

The spin-resolved density of states (DOS) of the Fe-based ferromagnetic metallic glasses Fe82B12Si6, Fe60Ni20B20, and Fe44Ni37B19 have been determined by spin-polarized photoemission spectroscopy (HeI, hv=21.2 eV, ΔE=100 meV). For the Fe82 sample the maximum of the up-spin DOS is located around 1.1 eV below EF, while the down-spin intensity has its maximum at EF indicating that the maximum of the down-spin DOS is above EF. This shows that the double-peak structure, which has been observed before in conventional photoemission, is due to ferromagnetic exchange splitting. The spin polarization at EF is slightly negative (−5%) and it has a maximum (+45%) at 2-eV binding energy, indicating the bottom of the down-spin Fe 3d states. A minimum in the spin polarization at about 5-eV binding energy corresponds to emission of unpolarized electrons from B 2p and Si 2p states. With increasing Ni concentration the maximum of the DOS shifts to higher binding energy, indicating the filling of the 3d states. The maximum of the polarization decreases and the spin polarization at EF becomes more negative (−8% for Fe60, −15 for Fe44−). No unambiguous ‘‘exchange splitting’’ can be extracted from the spin-resolved energy distribution curves. There is no evidence for strong ferromagnetism, i.e., that the majority states are completely filled and below EF. The data indicate that neither the rigid band model nor the split-band model apply, but that there is a strong hybridization between the Fe 3d and Ni 3d states (common-band regime). Upon oxygen adsorption on the Fe82 sample the intensity of the metallic 3d states decreases (25% for 16 L exposure) and an unpolarized peak due to oxygen 2p grows at 5.5-eV binding energy. The spin polarization of the Fe 3d states is only slightly affected, ruling out the existence of oxygen-induced magnetically ‘‘dead layers.’’
Short-range effects on hyperfine fields in amorphous CoSn alloys (abstract)
Authors: Nabli H. , Durand J. , Piecuch M. , Marchal G.
Published: July 2002

We have studied the 59Co and 119Sn hyperfine fields in amorphous CoSn alloys obtained by coevaporation onto cryogenic substrates. 59Co spin-echo spectra were measured at 1.4 K in zero-applied field on ferromagnetic Cox Sn1−x (0.65≤x≤0.80) alloys. NMR spectra display peaks. These peaks have a relative intensity which varies with composition, but their average location in frequency is concentration independent. From a comparison with 59Co NMR spectra in crystalline Co with dilute Sn impurities, it is inferred that the structures observed in NMR spectra for amorphous CoSn alloys are related to the number of Sn first neighbors of the Co resonant nuclei. Thus, short-range effects are thought to be predominant in determining the Co on-site moments in these alloys, as well as in amorphous Co alloys with other s-p elements. 119Sn Mössbauer spectra were measured at room temperature on paramagnetic Cox Sn1−x (0.40≤x≤0.55) alloys. Comparison is made with the stoichiometric CoSn crystalline compound, where quadrupolar interactions give rise to an admixture of two doublets corresponding to the two inequivalent Sn sites. Mössbauer spectra for the amorphous CoSn alloys display a single doublet identified as arising from Sn atoms in trigonal prisms arrangements. These results are discussed within the frame of current models for the structure of metallic glasses.
Large in-plane anisotropy in amorphous Sm-Co and (Sm+Ti)Fe films
Authors: Cheung T. D. , Wickramasekara L. , Cadieue F. J.
Published: July 2002

SmCo5, Sm2(Co,Fe,Zr)17, and (Sm+Ti)Fe amorphous films were deposited by rf sputtering onto water cooled substrates with an applied field, Hs, of 2.5 or 1.75 kOe in the plane of the substrates during the sputter deposition. All of these systems have been made so as to exhibit in-plane rectangular hysteresis loops when the magnetizing field is applied in plane parallel to the original field applied during synthesis. The in-plane intrinsic coercive forces parellel to Hs are 1, 0.7, and 0.5 kOe for amorphous SmCo5, Sm2(CoFeZr)17, and (Sm+Ti)Fe5 films respectively. These films exhibit a large amount of in-plane anisotropy since inclined hysteresis loops are observed when the magnetizing field is applied in plane but perpendicular to Hs. The in-plane anisotropy constants for magnetizing these different amorphous systems parallel versus perpendicular to Hs range from (1.4–2.6)×106 ergs/cm3. Expected saturation moments were obtained for these systems for magnetizing fields applied in the film plane. Most samples have been made about 2 μm thick. The samples exhibit smooth mirrorlike surfaces.
Connection between surface magnetism and electronic structure of oxygen on Ni(110) (invited)
Authors: Seiler A. , Feigerle C. S. , Peña J. L. , Celotta R. J. , Pierce D. T.
Published: July 2002

The d-band holes which give rise to ferromagnetism in Ni can be directly observed by spin-polarized inverse photoelectron spectroscopy (SPIPES). Only incident electrons polarized in the minority spin direction can fall into unfilled minority spin states and radiate a detected photon. On dissociative chemisorption of O2 we observe a reduction in the number of minority spin d holes. It is this change in electronic structure which gives rise to a decrease in magnetization. A background of minority and majority spin states remains essentially unchanged. Further exposure to oxygen causes formation of NiO; the surface magnetization goes to zero, and a completely different SPIPES spectrum is observed. The relative importance of d electrons and s, p electrons in chemisorptive bonding on Ni has been much discussed. These data suggest that the d states interact strongly with the oxygen and that this interaction has a profound influence on the surface magnetism.
Influence of topology near the MCP of a-(FexNi100−x )0.75P16B6Al3 alloys
Authors: Mazumdar P. , Bhagat S. M. , Manheimer M. A.
Published: July 2002

The onset of ferromagnetism at the multicritical point (MCP) of the series a-(FexNi100−x)0.75P16B6Al3 is examined by studying the low-field reversible magnetization. The peak susceptibility at TSG is found to increase rapidly as one approaches the MCP (xc≊16.5) from below. The plateau magnetization for the reentrant alloys tends to vanish as x→x+c. The sharp change in the magnetic response on crossing xc is interpreted in terms of a percolation transition at xc.
Spin resonance in EuxSr1−xS with x=0.4, 0.5, and 0.54
Authors: Dillon J. F. , Rupp L. W. , Batlogg B. , Maletta H.
Published: July 2002

Electron spin resonance has been studied in single crystal spheres of EuxSr1−xS with samples of nominal composition x=0.4, 0.5, and 0.54. The emphasis has been on the field for resonance of x=0.4 material at 11.65 GHz at temperatures between 4.2 and 1.65 K. Even at the lowest temperatures the lines do not vary with crystal orientation. In all samples there is a depression of the field for resonance beginning at about 10 K, and increasing to the lowest temperatures. This is interpeted as an ‘‘internal’’ field Hi. Measurements at 17 GHz reveal that Hi for all three compositions decreases with increasing frequency. Surprisingly, at 1.65 K, this seems to vary with sphere size for the x=0.4 composition. In addition to the experiments on spheres, a small plate-shaped specimen of x=0.4 was studied. It too showed a large depression of the field for resonance at 1.75 K.
Spin correlations in a spin-glass system studied by light scattering: EuxSr1−xS (abstract)
Authors: Zirngiebl E. , Güntherodt G. , Maletta H.
Published: July 2002

Raman measurements in all magnetic phases of Eux Sr1−xS show a new type of q≠0 magnetic scattering at low temperatures. The intensity can be quantitatively described by using in the scattering cross section a modified Ornstein–Zernike ansatz for the spin correlation function. A consistent interpretation of the Raman data in correspondence with the magnetic phase diagram has been achieved. Eux Sr1−xS shows a rather unusual magnetic phase diagram. Ferromagnetism breaks down below x=0.51. Spin-glass behavior is observed at low temperatures for 0.13<x<0.65. Most interest has been attracted recently by the reentrant phase boundary between the (‘‘frustrated’’) ferromagnet and the spin glass for 0.51<x<0.65. We performed Raman scattering on Eux Sr1−xS for x=0.65, 0.54, and 0.40 at temperatures between 1.5 and 300 K and in applied magnetic fields up to 7 T using 5145-Å laser excitation. Besides the well-known spin-disorder induced scattering from the LO(L) phonons near the zone boundary in EuS, one observes in the (frustrated) ferromagnetic phase and in the spin-glass regime additional scattering intensity which is of magnetic origin. A consistent theoretical description of our results is given by the one-phonon one-spin scattering cross section used for EuS. For the spin correlation function, we used a modified Ornstein–Zernike ansatz, with an additional exponent y which describes the spin system from uncorrelated ( y=0) via statistically coupled ( y=2) to ferromagnetically aligned spins ( y>2). Systematic variations of the exponent y as a function of composition and temperature in high magnetic fields characterize the spin correlations and yield a consistent interpretation of the Raman data in correspondence with the magnetic phase diagram. For example, for Eu0.54 Sr0.46S, we observe in the (frustrated) ferromagnetic phase with decreasing temperature an increase of y from y=2.0 to y=2.6 near the reentrant phase boundary and a decrease to y=2.0 in the spin-glass phase. The latter value is also found in the spin glass phase of Eu0.40Sr0.60S. In conclusion, we would like to point out that Raman measurements in Eux Sr1−xS show a new type of q≠0 magnetic scattering at low temperatures and in applied magnetic fields, not observed in the ferromagnetic state of EuS. The deduced exponent y of the spin correlation function agrees well with that deduced from diffuse magnetic neutron scattering. In addition, our measurements in external magnetic fields up to 7 T provide information complementary to the zero-field neutron measurements and yield a microscopic understanding of the magnetic phase diagram.
Investigation of Metglas toroid fabrication techniques for a heavy ion fusion driver
Authors: Faltens Andris , Rosenblum Stephen S. , Smith Carl H.
Published: July 2002

Measurements are reported on the dc and pulsed magnetic properties of three MetglasRa) alloys in the as-cast, box annealed, and rewound conditions. Alloys 2605SC, 2605S-2 and 2605CO were evaluated for possible use in an induction linear accelerator. It is found that the 2605SC and 2605S-2 give similar good performance and the 2605CO slightly less good performance in this application in the microsecond pulse-width region.
Magnetization curves of small isotropic spheres (abstract)
Authors: Aharoni A. , Jakubovics J. P.
Published: July 2002

Magnetization curves of small isotropic spheres have been calculated numerically, and compared with previous Ritz method calculations. Apart from imposing cylindrical symmetry (curling mode), no prior assumptions have been made about the magnetization distribution. The energy was calculated by subdividing the sphere into quasi-toroids within which the magnetization is independent of r and θ, a method that has already been used for the exchange energy. The calculation of the magnetostatic energy was based on another recently developed theory. Figure 1 shows 〈mz〉 plotted against h=3H/2Ms The arrows indicate the theoretical nucleation fields for the four largest radii, and the line shows the results from the Ritz model for 20-nm radius. Magnetization reversal is seen to nucleate more gradually than in the Ritz model. The contribution of the surface charges to the magnetostatic energy was also calculated, and found to be similar to, and in some cases slightly higher than the total magnetostatic energy, showing that the contribution of volume charges is relatively small. This may enable first approximations to magnetization configurations to be obtained more rapidly.
Physical model for static and dynamic scaling in spin glasses
Authors: Malozemoff A. P. , Barbara B.
Published: July 2002

We extend earlier critical fractal cluster models of a spin glass phase transition. At the transition temperature in finite field, a negative excluded-volume term dominates a positive saturated-cluster term, giving the negative H2/δ nonlinear susceptibility observed experimentally. The two competing effects in general have different relaxation times and therefore novel implications for dynamic scaling.
Melting of frustrated spins: Mechanism for reentrant magnetism in disordered spin systems (abstract)
Authors: Saslow W. M. , Parker G.
Published: July 2002

Using ‘‘naive mean-field theory,’’ we first consider a square lattice of XY spins with nearest-neighbor ferromagnetic interactions, except for one ‘‘frustrated’’ (Fr) spin with two antiferromagnetic bonds. At T=0 the Fr spin points perpendicularly to the more distant spins, with its four nearest neighbors tipping slightly towards it (or away from it) according to whether the bonds are ferromagnetic or antiferromagnetic. In this case the mean field on the Fr spin is much less than on the more distant spins, so that it is not surprising that the Fr spin loses its magnetism at a lower temperature than the spins of the ferromagnetic matrix. As the Fr spin ‘‘melts,’’ the rest of the spins become more collinear. Thus the net magnetization is subject to a competition between the length decrease of the ferromagnetic spins, and their increased collinearity, as T is raised. We next consider the case where many such Fr spins are embedded in the ferromagnetic matrix. For a given value of the antiferromagnetic exchange, it is not difficult to find concentrations of Fr spins whose mean-field solutions yield magnetizations which are low at low T, which begin to rise near Tc (leading to a plateaulike shape), and which disappear above Tc. This behavior is in qualitative correspondence with what is observed in the reentrant state.
Study of T
c with the addition of magnetic and nonmagnetic impurities in noble metal spin glasses (abstract)
Authors: Vier D. C. , Schultz S.
Published: July 2002

We have measured the dc magnetization cusp temperature, Tc, and the electrical resistivity ρ as a function of the concentration of various impurities added to noble metal host-transition metal impurity spin-glass systems. For nonmagnetic impurities we identify at least two mechanisms which shift Tc with increasing impurity concentration. One, a reduction of Tc, associated with changes in the conduction electron mean-free path, and another mechanism whose shift is of opposite sign but unknown origin. We have also measured the perpendicular ac susceptibility and find that the change in the anisotropy energy cannot be the unknown mechanism. For nonmagnetic impurities Tc( ρ) is exponentially dependent on the total resistivity, allowing us to determine Tc(0) and Tc(∞). We find that Tc(∞) not only exists, but is surprisingly large, and ≂40% of Tc(0), indicating that there is a strong residual spin interaction which is independent of the mean-free path. For Ag-Mn we find that both Tc(0) and Tc(∞) are linear in the Mn concentration well into the range where the undoped sample Tc exhibits the well-known deviations from linear behavior.
Time, temperature, and field dependence of the remanent magnetization in Cd1−xMnxTe
Authors: Oseroff S. , Gandra F. G.
Published: July 2002

We have measured the time, temperature, and field dependence of thermoremanent magnetization (TRM) and isothermal magnetization (IRM) for Cd1−xMnxTe with x≂0.225, 0.30, and 0.40 and Tg ∼3.7, 6.8, and 11.7 K, respectively. We found that the field and time dependence of TRM and IRM showed a similar behavior that was found before in other spin-glass systems. The temperature dependence of TRM is such that it goes to zero at a temperature very close to Tg. The magnetization curves for T>Tg can be written in terms of odd powers of the magnetic field.
Spin-glass and ferromagnetic behavior of Ni-Mn multilayered films
Authors: Vernon S. P. , Halawith B. N. , Stearns M. B.
Published: July 2002

We report a detailed investigation of the temperature T and field H dependence of the magnetization of equal layer thickness Ni-Mn multilayered films with modulation wavelengths of 50(25–25) and 26(13–13) A, and overall film thickness of approximately 1 μm. The dc magnetization was measured for T between 1.6 and 400 K, for H between ±30 kOe using a SHE VTS-968 SQUID magnetometer. Both samples exhibit spin-glass behavior at low temperatures: Cooling the films in an applied magnetic field displaces the hysteresis loop and the saturation magnetization is reduced at low T. Both films undergo a transition to ferromagnetic (FM) behavior as the temperature is increased above ∼20 K. At higher T, the 13–13 A sample behaves like a ‘‘normal’’ FM, but the 25–25 A film behaves like a ‘‘biased’’ FM; that is, the hysteresis loop continues to be displaced, along the H axis. The displacement gradually decreases with increasing T. The experimental observations are consistent with there being systematic concentration variations in each compositionally modulated film which give rise to adjacent FM and antiferromagnetic regions as well as an interfacial spin-glass-like region.
Cluster-glass behavior of FexNiGe (0.5≤x≤0.9)
Authors: Goto T. , Kanomata T.
Published: July 2002

The compounds FexNiGe have the Ni2In-type crystal structure in the nonstoichiometric composition range of 0.5≲x≲0.9. With decreasing temperature, the magnetization of these compounds shows a broad maximum first and then decreases abruptly at a critical temperature Tf which varies with x from 5 K for x=0.5 to 17 K for x=0.9. Spin-glass-like freezing is considered to occur at Tf. The Mössbauer spectra observed below Tf show a symmetric pattern and exhibit a wide distribution of hyperfine fields. The symmetric pattern of the spectra is attributable to the freezing of iron moments in random directions. The Mössbauer linewidth decreases sharply with increasing temperature near Tf. This fact would mean that the relaxation time of the magnetic moments strongly depends on temperature and exceeds the nucleus Larmor period below Tf. The long relaxation time of the magnetic moments may be due to the formation of magnetic clusters.
Electron spin resonance modes of a reentrant Ni-Mn alloy (abstract)
Authors: Hurdequint H. , Kouvel J. S.
Published: July 2002

We have carried out an electron spin resonance (ESR) study of a disordered Ni–Mn alloy of 26.5 at. % Mn at 9.2 GHz. Investigated in particular was the variation of the resonance spectrum as a function of the orientation of the static magnetic field relative to the plane of the thin-foil sample. The measurements were made at different temperatures and for different magnetothermal histories, thus extending a previous study that had been restricted to the parallel geometry. Magnetization and ac susceptibility measurements on the same sample have revealed that it becomes ferromagnetic at Tc ≊107 K and that below ∼75 K it is in a mixed ferro-spin-glass state. Thus, this reentrant Ni-Mn alloy lies close to and on the ferromagnetic side of the multicritical point in the magnetic phase diagram of temperature versus Mn concentration. In the reentrant temperature regime and also in the ferromagnetic regime, the ESR spectrum is found to consist of two resonance lines, whose splitting is maximum in the perpendicular geometry and decreases smoothly upon turning towards the parallel geometry. Upon field cooling to the reentrant state, the fields for resonance reveal the presence of a macroscopic anisotropy of unidirectional character, superimposed on the dominant effect of the demagnetizing fields. Within the framework of a two-sublattice ferrimagnetic model, the resonance lines observed at the lower and higher fields can be interpreted as corresponding respectively to the ferromagnetic and exchange resonance modes.
ac susceptibility measurements on toroid shaped samples of CdCr2xIn2−2xS4 (0.85<x<1) spinel solid solutions (abstract)
Authors: Saifi A. , Cagan V. , Dormann J. L. , Nogues M.
Published: July 2002

At low temperature CdCr2xIn2−2xS4 spinel solid solutions show a ferromagnetic-spin-glass transition. In the present work the ac susceptibility measurements (130 Hz<ν<100 kHz) have been carried out on toroid-shaped samples, which allows us to minimize the macroscopic demagnetizing fields, by using a technique based on a direct flux determination. Our results are compared with previous measurements, performed by using standard methods. The shape of the χ′(ν) curves is similar to that obtained from the standard methods: no maximum in the transition temperature region (4 K<Tg<18 K); however, the χ′ values are higher: as an example, for x=0.95, χ′max ≂0.5 emu/cm3≂1/N (N being the demagnetizing factor) with standard methods, while our result is χmax ∼3.5 emu/cm3. Such a result is discussed notably with respect to the influence of porosity; a precise comparison should take into account the actual density of samples. Just below Tc (∼40–85 K), χ′ does not depend on frequency, but we observe, in the transition temperature region, a decrease of χ′ when ν increases. χ″ presents two well-defined peaks around Tg and Tc. The latter maximum does not depend on frequency, while the first one shifts as a function of ν, which indicates dynamical effects (as an example for x=0.95, Tg =10.5 K at 130 Hz and Tg =13.0 K at 100 kHz).
Transverse susceptibility of the Ising model
Authors: Wang Yung-Li , Wentworth Chris
Published: July 2002

A high-temperature series for the transverse susceptibility of the Ising model is obtained to the tenth order in J/kBT for the plane square lattice and the fcc lattice. The temperature behavior of the three-dimensional lattice is studied and compared with the temperature behavior of the two-dimensional lattice.
Exchange mechanisms in CsMnxMg1−xBr3
Authors: Falk U. , Furrer A. , Kjems J. K. , Güdel H. U.
Published: July 2002

The inelastic neutron scattering technique was used to measure the excitation spectra of pairs and triads of magnetic Mn2+ ions in the one-dimensional S-state magnet CsMnxMg1−xBr3 (x=0.14, 0.28, 0.5). It turns out that the Heisenberg exchange interactions in pairs and triads are slightly different from each other. In the cluster regime dipolar interactions are negligibly small.
A brief study of the classical statistical mechanics of spin–Peierls chains
Authors: Marchand M. , Caille A.
Published: July 2002

We present and discuss the statistical mechanics of the static properties associated with a one-dimensional classical spin–Peierls Heisenberg chain. Thermodynamic properties and correlation functions are calculated numerically from the eigenvalues of the transfer matrix. All fluctuations from the magnetic and elastic degrees of freedom are considered. Among other results, it is shown that magnetization and dimerization do not tend to occur simultaneously.
Relaxation of a nuclear spin interacting with a dilute gas of π-solitons in the strong collision limit
Authors: Huber D. L.
Published: July 2002

We calculate the longitudinal relaxation time of a nuclear spin interacting with a dilute gas of π-solitons in a quasi-one-dimensional magnetic chain. The calculation applies to the strong collision limit and thus is appropriate for characterizing the relaxation of the nuclear spins of the magnetic ions. The relaxation rate is proportional to the soliton density for all values of the collision rate. Results are compared with the predictions of the weak collision model, which is applicable to the nuclear spins of the nonmagnetic ions.
First-order line and critical end point of cubic rare-earth pseudobinaries in R
x
R′1−x
T2 in the H, X plane (abstract)
Authors: Cullen J. , Callen E.
Published: July 2002

We study the effects that an applied magnetic field has on the nature of the off-principal axis magnetic phases of rare-earth-transition metal pseudobinaries. The magnetic field is taken to be in the (01̄1) plane of these cubic materials but is otherwise arbitrary. Besides the expected second-order transition which takes place only when the field is in the [100] direction, there occurs a low-field first-order transition which separates two spin-canted phases. This transition is predicted for a range of concentration between the zero-field spin-canted/[111] boundary x0 and a somewhat larger concentration of [111]-easy rare earths x1. The field at the first-order transition is a monotonically increasing function of concentration within this range, starting at zero at x0 and reaching a critical field Hc of the order of D2/JMs at x1, where D is the fourth-order crystal-field strength and  J is the exchange energy density. The point Hc, x1 is a critical end point in the H, x plane. The random fluctuations in sign of the crystalline field will most strongly effect the above results. Estimates of the effects of these fluctuations on the phase diagram will be made.
Numerical simulations on spin glasses (invited)
Authors: Young A. P.
Published: July 2002

We discuss the results of numerical simulations of two spin-glass models, the infinite range Sherrington–Kirkpatrick (SK) model, and a nearest–neighbor model in three dimensions. The probability distribution PN(q) for the spin glass order parameter q has been evaluated for the SK model below Tc with N, the number of spins, up to 192. It shows the same basic features as the Parisi function dx/dq as expected, but with sizeable finite size corrections. Results for the short-range model show evidence for a finite temperature transition but further work is needed to confirm this.
High-field magnetization measurements of the metastability boundary in a d=2 random-field Ising system
Authors: King A. R. , Jaccarino V. , Motokawa M. , Sugiyama K. , Date M.
Published: July 2002

The d=2 random-field Ising system Rb2Co0.85 Mg0.15F4 has been studied using pulsed, high-field magnetization M measurements. A relatively sharp metastability boundary TF(H) has been discovered which separates a frozen region below from an equilibrium one above TF(H). TF(H) scales as does the (destroyed) phase transition ‘‘Tc(H)’’ as TN −TF(H)  ∝H2/φ, with the measured crossover exponent φ=1.8±0.1 in good agreement with the d=2 susceptibility exponent γ=7/4. As measured in the pulsed field experiment, TF(H) lies well below ‘‘Tc(H)’’ but above the value of TF(H) obtained from recent neutron scattering studies. The difference in the two values of TF(H) is attributed to the log t time dependence of the approach to equilibrium and to magnetocaloric effects.
Suppression of long-range order by random fields in TbxFe1−x alloys
Authors: Spano M. L. , Rhyne J. J.
Published: July 2002

The effect of random anisotropy and exchange fields on the ‘‘phase transitions’’ in amorphous magnets has been examined in Tb2Fe98 and Tb75Fe25 using small wave-vector (Q) neutron scattering. These two alloys represent opposite extremes in relative magnitude of random anisotropy fields. The neutron results for the spin correlation lengths show that neither alloy exhibits long-range magnetic order well below its spin freezing temperature of 210 K for Tb75Fe25 and approximately 245 K for Tb2Fe98. At Tf the correlation range, ξ, in Tb75Fe25 does reach a resolution limited maximum value before falling sharply to a plateau of ≊50 Å extending to low temperatures. Tb2Fe98 shows only a rounded peak of 75 Å near the magnetization-determined Tf, and, below Tf, ξ remains near its maximum value. Both alloys show marked departures from the Lorentzian cross section below Tf, and are fitted to a Lorentzian (L) plus Lorentzian squared (L2) form appropriate for random field systems.
Simple cubic fully frustrated Ising crystal by Monte Carlo simulations
Authors: Diep Hung T. , Lallemand P. , Nagai O.
Published: July 2002

Using the Monte Carlo technique, we study a simple cubic fully frustrated Ising crystal. We find a sharp second-order phase transition, contrary to what is predicted by various theories. By finite size scaling we find the critical temperature kBTc/J =1.355 for an infinite lattice. Various physical properties are studied in detail. The behavior of the system at low temperatures is particularly interesting. There are two ordered phases: the high-temperature ordered phase where local disorders move from one sublattice to another and the low-temperature ordered phase where the disorder is frozen on one pair of sublattices. This causes a shoulder observed in the specific heat far below Tc.
Phase transitions in (1+1)-dimensional transverse Ising models for spin S≥1/2
Authors: Penson K. A. , Kolb M.
Published: July 2002

The phase transitions in the ground state of (1+1)-dimensional Ising models in a transverse field are studied for spin S=1/2, 1, 3/2, and 2 using the finite-size scaling method. The critical fields, thermal and correlation exponents are calculated as a function of S. The critical fields are monotonic functions of S but the exponents are S-independent and take their values for S=1/2. A novel calculational method based on universality of ratios of finite-size correlation length amplitudes is proposed and tested. It results in a much improved accuracy of extrapolation procedures. Such an universality appears to be a consequence of recently postulated conformal invariance of correlation functions of a class of two-dimensional classical models.
Wave-vector-dependent spectra of quantum systems with competing interactions
Authors: Kolb M. , Penson K. A.
Published: July 2002

The analysis of phase diagrams of one-dimensional quantum versions of axial next-nearest-neighbor Ising (ANNNI) and competing two- and four-spin interaction models in two dimensions was made. The zero-temperature wave-vector-dependent spectra for finite chains with up to L≤16 spins indicate a qualitative difference between the two models: while the existence of an incommensurate phase is confirmed for the ANNNI case, our results do not suggest that such a phase exists in the two- and four-spin model.
Kondo effect on crystal field splitting
Authors: Maekawa S. , Takahashi S. , Kashiba S. , Tachiki M.
Published: July 2002

A theory is developed for the temperature dependence of crystal field energy levels of Ce ions in metals caused by the Kondo effect. Abrikosov’s pseudofermion technique is applied to the Coqblin–Schrieffer exchange model, and the self energy is derived in the self-consistent ladder approximation. The neutron quasi-elastic and inelastic scattering spectra are calculated in the system with cubic crystal field.
Intermediate valence and the hybridization model: A study on SmB6, ‘‘gold’’ SmS, and YbB12
Authors: Travaglini G. , Wachter P.
Published: July 2002

The optical reflectivity of SmB6, ‘‘golden’’ SmS, and YbB12 has been measured between 1 meV and 12 eV at various temperatures down to 4.2 K. The results are analyzed with a Kramers–Kronig transformation. All three compounds reveal a hybridization gap of 4, 7, and 6 meV, respectively, with EF in the gap. The interband density of states could be estimated, and with simplifying assumptions it is 11 000 and 1200 states per Rydberg and cell for SmB6 and gold SmS, respectively, factors of 200 and 25 larger than the density of d states.
Crystal-field ‘‘dressing’’ and enhanced magnetoelastic effects
Authors: Banerjea Amitava , Cooper Bernard R.
Published: July 2002

Hybridization of moderately delocalized f electrons with band electrons gives rise to a highly anisotropic two-ion interaction. If a bare, point-charge-like crystal field is included, it is ‘‘dressed’’ by the hybridization both through the two-ion interaction and through the modification of the single-ion interactions. We have investigated the relative effects of the anisotropic two-ion interaction on cubic and tetragonal crystal-field terms in the distorted paramagnetic phase. We have found a difference in the degree and the temperature dependence of the dressing of the two terms. This provides a possible explanation for enhanced magnetoelastic effects in materials such as CeAl2.
Photoemission theory of mixed valent semiconductors
Authors: Parlebas J. C. , Kotani A.
Published: July 2002

By using the Anderson model with a filled valence band (VB) we first give a formal description of the core hole photoemission, the direct f-electron photoemission, and the resonant photoemission. Then we apply our formalism to a CeO2-type semiconductor and we calculate the core photoemission spectra: taking account for a strong final state interaction we discuss our results in terms of (i) the relative positions of the final f states with respect to the VB states, (ii) the hybridization between those two types of states, and (iii) the Coulombic Uff interaction energy.
Electronic structure of amorphous Mo-Ru-Si alloys
Authors: Tenhover M. , Lukco D. , Surman D. , Johnson W. L.
Published: July 2002

The results of x-ray photoemission spectroscopy (XPS) and electrical resistivity measurements on amorphous metal alloys of the form (Mo1−xRux)80Si20 are reported. The XPS valence band spectra are dominated by the Mo-4d subbands and the electronic structure of the amorphous alloys can best be described in the virtual crystal limit of the coherent potential approximation (CPA) formalism. The density of electronic states at the Fermi level sharply decreases in the alloys as the Ru/Mo ratio is increased. Associated with this drop is an increase in electrical resistivity and decrease in the superconducting transition temperatures of the alloys. These relationships are discussed in the context of current theories of the electrical resistivity and superconductivity in amorphous alloys.
Magnetoresistance of Cd1−xMnxSe near the Mott transition
Authors: Shapira Y. , Ridgley D. H. , Dwight K. , Wold A. , Martin K. P. , Brooks J. S.
Published: July 2002

At 0.5≤T≤4.2 K and for fields of order 10 kOe, a large positive magnetoresistance is observed in Cd0.95Mn0.05Se and Cd0.99Mn0.01Se samples which are near the Mott transition. The effect is attributed to an increase in the screening radius, caused by the splitting of the conduction band. At higher fields, but below ∼80 kOe, samples with 5% Mn show negative magnetoresistance which is attributed to a decrease in the separation between the Fermi level and the mobility edge.
Anomalous transport properties in n-type HgCr2Se4
Authors: Selmi A. , Mauger A. , Héritier M.
Published: July 2002

The authors report transport properties of n-type HgCr2Se4 and analyze the temperature dependence of the electron concentration, n. Above 200 K, n decreases with T. For donor concentrations Nd>nc with nc ≊2×1018 cm−3 the Mott concentration, n saturates below 200 K, but a decrease of n(T) is observed at T≊120 K when Nd<nc. These features are associated with an increase of the energy gap and the effective mass of the electrons vs T. For all samples, the electron mobility decreases monotonically vs T. It is argued that these properties are associated with exchange interactions between highly correlated 3d electrons.
Effects of spin dispersion on the higher-order anisotropy constants of mixed Rx  1R21−x Fe2 crystals
Authors: Koon N. C. , Williams C. M.
Published: July 2002

Mean-field theory has been extremely successful in interpreting the complex spin reorientations which take place as a function of temperature and composition in mixed rare-earth–iron Laves-phase crystals, but no effort has been made to interpret the higher-order (κ8) Cubic harmonic term which has been observed experimentally. We generalize the usual mean-field treatment of these alloys to take into account deviations of individual spin directions away from the mean-field direction, and then calculate the free energy as a function of the net magnetization direction, which corresponds directly to what is measured experimentally. At low temperatures the net magnetization direction can differ by as much as 15° to 20° from the assumed mean-field direction, which has a very large effect on the measured anisotropy constants.
Magnetic ordering in YGd alloys
Authors: Pureur P. , Fert A. , Wendler R. , Baberschke K.
Published: July 2002

We present magnetization and resistivity measurements on single crystals of yttrium doped with small concentrations of gadolinium (c=1, 2, and 3 at. %). The low-field susceptibility exhibits a spin-glass-like sharp cusp for the magnetic field along the c axis. However, there is no irreversible behavior below the temperature of the cusp, which rules out the existence of a spin-glass state. Moreover, our magnetization measurements show up a spin-flop transition for H in the basal plane. The existence of a long-range ordering is confirmed by our resistivity measurements. The resistivity parallel to the c axis exhibits a maximum just below the ordering temperature, which is reminiscent of the superzone effects observed in many rare-earth metals. The antiferro- magneticlike properties of the YGd alloys contrast with the spin-glass behavior observed in other Y–rare-earth alloys at similar concentrations.
Induced Fe and Co moments in rare earths and uranium compounds
Authors: Burzo E.
Published: July 2002

The results of magnetic measurements performed on (GdxY1−x)2Fe14B compounds, in the temperature range 4.2 K and 1000 K are firstly reported. Then, the molecular field coefficients characterizing the magnetic interactions inside and between magnetic sublattices, as well as the exchange fields acting on the Fe atoms are determined. The variations of themean iron momrnts, as compared to x=0 value, are linear dependent on those of the exchange fields. Similar behavior is observed for TM=Fe or Co in (GdxY1−x)TMm, Gd(TMyAl1−y)m and U(FexTM1−x’)2 compounds with TM’=Co, Mn, Ni, or Al and m=2, 3, and 3.5. The composition dependence of TM moments in the above systems is mainly attributed to the variation of the exchange interactions.