bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetism of ThCo5-based alloys
Authors: Wallace W. E. , Ganapathy E. V. , Craig R. S.
Published: February 2003

Magnetic behavior of ThCo5, Th1−xYxCo5 and Th1−xYxCo4Fe has been determined over the temperature range 4 K to the Curie temperature in applied fields up to 120 kOe. Crystal parameters, saturation magnetization and anisotropy fields are presented for ThCo5, for Th1−xYxCo5 with x=0.1, 0.2, 0.4, 0.6 and 0.8 and for Th1−xYxCo4Fe with x=0.2, 0.4, 0.6 and 0.8. ThCo5 is observed to be exceptionally hard, not achieving saturation at 120 kOe. Co in this compound exhibits an anomalously low moment, ∼1.13 μB (as determined at 120 kOe). The Co moment rapidly increases as Y replaces Th in ThCo5, achieving for x=0.2 a value of 1.5 μB, the same as in YCo5. The anisotropy field (HA) for Th1−xYxCo5 ternaries at 4.2 K rises from 175 to 203 kOe as x increases from 0 to 0.2 and subsequently falls to 175 kOe for Th.2Y.8Co5. HA increases by 41 kOe between ThCo5 and ThCo4Fe. In contrast with the behavior of (Th, Y)Co5 ternaries progressive replacement of Th by Y in ThCo4Fe leads to a monatomic decline in HA. Changes of HA by alloying is ascribed to (a) preferential site occupancy or (b) band modification effects and often these cannot be separated. Present results indicate that these two effects are comparable in magnitude.
The temperature dependence of demagnetization of MMl−xSmxCo5
Authors: Lee R. W.
Published: February 2003

In permanent magnet applications such as electric motors, high reverse magnet fields are experienced by the magnet and a critical design parameter is the onset of the ’’knee’’ of the hysteresis loop. The associated field, H*, marks the onset of irreversible demagnetization. This is particularly important for mischmetal-based magnets which exhibit lower coercivity than SmCo5 magnets. This paper reports measurements of the demagnetization of sintered MMl−xSmxCo5 magnets for 0.02 ?x? 0.43 over the temperature range: 175 K ?T? 425 K. The compositions were formed by alloying ’’natural’’ and Ce-free mischmetal with varying percentages of samarium and then blending with a ’’sintering aid’’. The temperature dependence of H* can be described by H* (T2) = H* (T1) (1 − 0.004 (T2−T1)), where the temperature coefficient −4×10−3K−1 is independent of the composition and coercivity within the limits investigated.
Temperature dependence of the magnetic parameters in the Sm2−xGdxCo17−yMny compounds
Authors: Bergner R. L. , Leupold H. A. , Breslin J. T. , Rothwarf F. , Tauber A.
Published: February 2003

The series of compounds Sm2−xGdxCo17−yMny was studied with regard to its suitability for employment as permanent magnet material in microwave/millimeter-wave traveling-wave tubes (TWT′s). The pertinent properties investigated are saturation magnetization, 4πMs, temperature coefficient of 4πMs, α, and anisotropy field, HA. The composition ranges investigated were from x =0 to 0.6 and y =0 to 2. Values of 4πMs at 25 C ranged from 12.8 kG for x = 0, y = 1 to 9.4 kG for x = 0.6, y = 2. For the compounds studied, maximum HA occurs at y = 1 for x = 0. The compounds, whose formulas contain a single atom of manganese, were found to yield excellent temperature compensation via Gd substitution in the range +20 to +150 C with the best coefficient occurring in Sm1.6Gd0.4Co16Mn for which α = −0.008%/C (4πMs=10.5 kG).
Enhancement of the magnetic properties of the Sm2Cu1.6Zr0.16Fe3.3Co12 compound
Authors: Bergner R. L. , Leupold H. A. , Breslin J. T. , Shappirio J. R. , Tauber A. , Rothwarf F.
Published: February 2003

A recent investigation [1] of the compounds Sm2(Cu,Zr,Fe,Co)17 revealed optimal HA, iHc, and Br as well as an energy product of 30 MGOe in the composition Sm2Cu1.6Zr0.16Fe3.3Co12. However, this compound has a nonlinear demagnetization characteristic. We have further investigated this system by varying heat treatment and/or replacing progressively larger amounts of Co by Mn or Cr. Metallography showed the microstructures of these materials to be very complex and sensitive to heat treatment. Magnetization measurements were carried out to determine the reversible temperature coefficient α as a function of annealing conditions and substitutent content. α was found to vary from 0.022%/C to 0.060%/C over the temperature range 15 C to 150 C. The room temperature saturation magnetization of these materials ranged from 4πMs=8.64 kG to 11.5 kG. Measurements of anisotropy field HA showed an enhancement with the replacement of Co by Mn or Cr by as much as 25% above the 92.0 kOe for the original material. This suggests that a higher coercivity and thus a more linear demagnetization curve may result from the Mn or Cr substituents.
High field magnetization studies in lithium-zinc ferritea)
Authors: White Geoffrey O. , Edmondson Charles A. , Goldfarb Ronald B. , Patton Carl E.
Published: February 2003

The magnetization in Li0.5−z/2ZnzFe2.5−z/2O4 with z=0.0, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 has been measured at 30 K and at applied magnetic fields of up to 100 kOe. The magnetization for samples with z?0.2 saturated at less than 10 kOe. The magnetization for samples with z?0.3 did not saturate. Rather, for applied fields in excess of 30 kOe, these samples exhibited field-independent differential susceptibilities ranging from 1.68×10−5 emu/g-Oe for z=0.3 to 20.66×10−5 emu/g-Oe for z=0.7. The high-field susceptibilities are attributed to localized canting of octahedral site Fe3+ spins with fewer than three tetrahedral site Fe3+ nearest neighbors. Susceptibilities predicted by existing localized canting models are in qualitative agreement with the data, but predicted values for large z are too small.
Stretcher design study for 1.5 μm bubble devices
Authors: Yanase T. , Majima T. , Orihara S.
Published: February 2003

Column period dependence of an asymmetrical chevron stretcher for 1.5 μm bubble devices has been studied under some film thickness conditions. The asymmetrical chevron stretchers with large column periods improve upper thresholds of bias margins although the 15 μm period degrades both upper and lower thresholds of the bias margins. A newly designed stretcher called Mt.Fuji has been developed, which has excellent stretching margin even at the period of 15 μm, typically 40 Oe bias margin at 55 Oe with a triangular wave drive. Nelson type detector for 1.5 μm bubble has been studied. Sensitivity of 0.25 % has been obtained.
Dynamics of 1μm bubbles in ion-implanted contiguous disk structures
Authors: Sanders Ian L. , Kryder Mark H.
Published: February 2003

The dynamic behaviour of 1μm bubble domains in double-layer garnet films in which the driving mechanism is by charged walls at ion-implanted pattern edges has been investigated using a 7 nsec pulsewidth stroboscopic laser system. Bubble propagation around circular and square-shaped structures, and between adjacent cusps forming part of a propagation loop was studied at frequencies up to 500 kHz. The data show that the tri-directional magnetocrystalline anisotropy of the driving layer has a strong influence on the dynamic behavior of the bubbles and is necessary to explain observed dependences of bubble velocity on propagation direction. The effective mobility μeff, of the charged wall coupled to a bubble was measured and found to be linearly related to the ion-implanted layer thickness t2, in agreement with theory. For a typical garnet composite of EuTmYGaIG/GdTmYGaIG with t2=0.4 μm, μeff∼280 cm/sec⋅Oe in comparison with a bubble domain wall mobility of ∼730 cm/sec⋅Oe. Expansion of domains along straight non-implanted edges, bubble translation and bubble propagation around disks consistently yield saturation velocities of 2000–3000 cm/sec in these composites.
Ion implant profiles in garnet films
Authors: Washburn H. A. , Galli G.
Published: February 2003

Chemical etch rate and magnetization profiles of neon implanted LPE garnet films were obtained by successively etching thin layers and measuring both weight loss and soft bubble collapse fields. Implant energies and doses in the ranges 60 to 160 keV and .5 to 10×1014 cm−2, respectively, were used. Etch rate profiles are similar in shape to typical implanted ion distributions with a maximum for 100 keV at .06μm from the original surface and the bulk film etch rate reached at .2μm. The bubble collapse field increases with dosage up to 2×1014 cm−2 then decreases. For the higher dosages, a profile is observed in which the collapse field is independent of thickness through most of the implanted layer. It appears that the damage, which causes the increase in etch rate, eventually creates a non-magnetic layer. Annealing restores magnetization and reduces crystallographic damage.
Magnetic ordering and anomalous ground state in CeAl2
Authors: Barbara B. , Rossignol M. F. , Boucherle J. X. , Schweizer J. , Buevoz J. L.
Published: February 2003

The intermetallic Laves phase compound CeAl2 presents some unusual properties, which makes the exact nature of its ground state a matter of great interest. A first step towards an explanation would be the knowledge of any magnetic structure below 3.8 K (phase transition temperature). We have carried out very accurate neutron diffraction experiments on a polycrystalline sample, completed by experiments on a single crystal. Several magnetic reflections have been found which are characteristic of the propagation vector k↘=1/2+τ, 1/2−τ, 1/2. The corresponding magnetic structure consists of antiferromagnetic (11̄0) planes in which the magnetic moments, lying along [111], are modulated according to a sine wave propagation along [11̄0]. No variation of k↘ nor any tendency towards antiphase structure appear down to 0.4 K. This result is not coherent with the Kramers’ degeneracy of the Ce3+ ion because it implies a non magnetic level as a ground state. On the other hand previous polarized neutron experiments have shown an antiparallel coupling between the spins of the cerium and the conduction electrons. Both results, as well as other physical properties, agree with the existence of a singlet ground state associated with a strong d-f coupling.
SmTmGa- and YSmTmGa-garnets for small−bubble applications
Authors: Bekebrede W. R. , Kestigian M. , Smith A. B. , Josephs R. M.
Published: February 2003

SmTmGa-garnet has a high growth-induced anisotropy, typical results being Ku = 1.1×105 ergs/cm3 and q = 7 for a sample with ℓ = 0.20 μm. Thus the anisotropy of this material is high enough so that a high-q, 1 μm-diameter bubble material could readily be produced by properly adjusting the Ga content. Rocking experiments indicate that hard bubbles are successfully suppressed by ion implantation even though the implanted layer does not exert the same keepering effect as in lower anisotropy materials. Temperature dependence data show that (ΔHcol/Hcol)/ΔT at 50°C is only −0.24%/°C in the ℓ = 0.20 μm sample mentioned above. The mobility of this sample is 140 cm sec−1Oe−1. The mobility can be increased (at the expense of reducing Ku and q) by adding yttrium to the composition. We have formulated such YSmTmGa-garnets for use as 2 μm bubble materials and have grown films with μ = 660 cm sec−1Oe−1, Ku = 2.3 × 104 erg/cm3 and μ = 340 cm sec −1Oe−1, Ku = 3.8 ×104 erg/cm3, depending on the yttrium content. An 11 μm-period symmetrical half-disc-type circuit fabricated on the latter film demonstrated an ∼20. Oe margin at 330 kHz with 50. Oe-peak triangular drive field.
Coercivity in 1.7 μm bubble garnet films
Authors: Vella-Coleiro G. P. , Hagedorn F. B. , Blank S. L. , Luther L. C.
Published: February 2003

Measurements of magnetic bubble coercivity in Ga substituted and in Ca-Ge substituted garnet films capable of supporting 1.7 μm diameter bubbles show that the Ga substituted films have lower coercivity which increases less rapidly with decreasing thickness than in Ca-Ge substituted films. High temperature annealing experiments indicate that the coercivity may be proportional to the material length and the saturation moment.
Magnetic characterization of thin bubble films
Authors: Mier M. G. , Blasingame J. M. , Searle C. W. , Maartense I. , Wigen P. E. , Omaggio J. P. , Globe M. P. , Limaye P. S.
Published: February 2003

Standard procedures for evaluating magnetic bubble materials are found to yield (∼9.25 micrometer) films. Several alternative procedures are described which yield values of magnetic parameters to 1% accuracy for thick (≳3 micrometer) bubble films. Application of these new procedures to thin (<3 micrometer) bubble films decreases the accuracy somewhat, but magnetic parameters are consistent within about 2%. The new procedure is described and results are presented for a number of thin films for comparison to the results achieved by the standard procedure.
Exchange stiffness coefficients of YIG:CaGe and YIG:LuSc from 30 to 300 K
Authors: Krahn D. R. , Wigen P. E. , Blank S. L.
Published: February 2003

The exchange stiffness coefficients A of tetrahedrally substituted YIG:CaGe and octahedrally substituted YIG:LuSc have been determined over the temperature range of 30 to 300 K. The magnetization M of each sample was measured range of 30 to 300 K. The magnetization M of each sample was measured with a vibrating sample magnetometer. The spin wave dispersion coefficient 2A/M was measured using microwave resonance techniques. Values of A were determined as a function of substitution level, the microscopic site of the substitution, and temperature. The results of this study indicate that the Kirkpatrick and Harris percolation model provides a close agreement with the experimental data obtained in this work. The most significant result of this work is represented by a plot of the room temperature values A versus the Curie temperature of each sample. Data for both systems studied here fall on the same curve. It appears that this general curve will yield values of A for a wide range of substituted garnets. Available data on magnetic garnet bubble device materials has verified the predictability of this general curve.
Coherent precession mode (’’2π-mode’’ oscillations) of twisted wall in bubble materials
Authors: Honda Shigeo , Hashimoto Shozo , Kusuda Tetsuzo
Published: February 2003

A model for the wall motion in bubble materials is proposed. When a sufficiently large pulse field is applied (Hz≫Hc=2.36πMα), the coherent precession of magnetization in the wall occurs while maintaining the twisted structure of the static state, so that the wall velocity varies alternately positive and negative, with ’’2π-mode’’ oscillation, after reaching the peak velocity Vp=2.36πMγΔ. This model explains well the empirical relation T=2π/γHz of the oscillatory period for the bubble collapse data, and also is compared with other experimental data.
290KBit, 15μm cell bubble chip
Authors: Gergis I. S. , Tocci L. R. , Williams J. L. , Lee W. P.
Published: February 2003

A block replicate bubble memory chip with a 14×16μm2 bit cell size has been developed using state-of-the art bubble technology. Organized into 282 minor loops and 1025 bits per minor loop, the total capacity of the chip is 289,050 bits. At 25°C and 45 Oe drive the start-stop operating bias margin is 18 Oe up to at least 300 kHz field rate. For operation over the temperature range −25°C to 100°C and up to 300 kHz, all chip drive parameters have at least ±10 percent operating range indicating that no drive parameter is required to be temperature compensated.
Design and functional characteristics of a 3μm, 254 K bit block replicate bubble memory device
Authors: Bullock D. C. , Fontana R. E. , Singh S. K. , Seitchik J. , Closson A.
Published: February 2003

The design and operating characteristics of a quarter mega-bit, block replicate bubble memory device are reported in this paper. Replicate gate and swap gate designs are detailed. Characterization data obtained from packaged devices with computer controlled test systems show operating margins of 20 Oe from 0 to 50°C at 100 KHz rotating field frequency and 60 Oe peak triangular drive amplitude.
Design and operation of a 550 kbit enhanced density 3 μm bubble memory chip
Authors: Dimyan M. Y. , Hubbell W. C.
Published: February 2003

A 550 kbit bubble memory chip that employs standard 3 μm bubbles and an enhanced density concept has been designed, fabricated and characterized at 100 kHz. The chip employs conventional 16 μm period swap-in/replicate-out gates, and 10.5 μm X and Y circuit periods in folded minor loops. Worst case overall write/read and minor loop propagation margins of 13 Oe and 14.5 Oe respectively, are obtained with a 45 Oe drive field. Also, minor loop longevity high and low end margin slopes of −0.25 Oe/decade and 0.2 Oe/decade, respectively are achieved. It is concluded that it is possible to almost double the storage density of 3 μm bubble devices while still maintaining adequate device performance.
Intermediate valency
Authors: Stevens K. W. H.
Published: February 2003

Following a brief outline of the meaning of intermediate valency, by reference to collapsed SmS, a simple three-site model is described. Its main features are a localized orbital at each site, three delocalized orbitals, representing the ’band’ structure, and six electrons arranged so that every site has at least one electron. The interest in this model is that the matrix of the transfer processes between localized and delocalized orbitals can be diagonalized by elementary methods for certain choices of parameters. The ground state is readily seen to be of intermediate valence character. It is shown that, to a very good approximation, the ground state can be written in the form {ΠmXm} ‖vac〉 where Xm is an operator associated with the mth site. A simple physical meaning can be given to Xm, that it creates an S=0 state by placing two electrons in orbitals, one of which is a localized orbital and the other is less localized and built from the occupied band states. It is suggested that a similar description can be given for the ground state of the full lattice model.
Nuclear resonance and relaxation in intermediate-valent YbCuAl
Authors: MacLaughlin D. E. , de Boer F. R. , Bijvoet J. , de Châtel P. F. , Mattens W. C. M.
Published: February 2003

27Al NMR shifts and spin-lattice relaxation times have been measured in the intermediate-valent compound YbCuAl over the temperature range 4.2 K to 295 K. Above Tmax?25 K, the temperature for which the bulk susceptibility χ exhibits a maximum, a linear relation was observed between K and χ: K=7.5(5) ×10−4−0.19(1) χ (emu/mole). For T<Tmax this relation was no longer obeyed; at 4.2 K the shift was about 20% more positive than predicted from the linear relation and the measured χ. Satellite NMR in the Kondo alloy CuFe exhibits no such nonlinearity. It is tentatively attributed to a temperature-dependent effect of intermediate valence on the transferred hyperfine coupling between Yb moments and 27Al nuclei. Relaxation data yield a 4f spin fluctuation rate which decreases by about a factor of 2 between Tmax and 100 K. A simple ionic model for the intermediate-valent state accounts qualitatively for this observation. Further corroborating experiments are suggested.
Magnetoresistance of rare earth iron garnets
Authors: Gyorgy E. M. , LeCraw R. C. , Blount E. I. , Jones W. R. , Blank S. L. , Grodkiewicz W. H.
Published: February 2003

The magnetoresistance of bulk and epitaxial films of YIG(Si) has been previously reported by us (Ref. 10). These results can be summarized by the cubic expression ΔR/R=kl(α2iβ2i–αiαjβiβj) where kl at 300°K is between −0.002 and −0.02 and essentially independent of the value of the applied field once the sample is saturated. Similar results have now been obtained for Eu3Fe5O12 (Si) and Eu2.9Lu0.1Fe5O12(Si). The behavior of Tb3Fe5O12(Si) is dominated by additional phenomena. The relatively large field independent term observed in YIG(Si) is small here. However there is a large, isotropic field dependent term and ‖ΔR‖ increases linearly with H. The value of (1/R) (dR/dH) is positive at 300°K and increases with decreasing T. At the compensation temperature this quantity abruptly changes sign and then decreases rapidly with decreasing T. The absolute value of (1/R)(dR/dH) is continuous except for a break right at the compensation temperature. (It is interesting to note that the addition of Si in the sample discussed here has raised the compensation temperature by 20°K.) The effect becomes quite large at low temperatures. The data show that at 215°K and a field of 104 Oe, ΔR/R=10%. Possible mechanisms are discussed.
Temperature dependence of the parallel-pump spin-wave instability in porous yttrium iron garnet
Authors: White Geoffrey O. , Patton Carl E. , Edmondson Charles A.
Published: February 2003

The parallel-pump spin-wave instability threshold has been measured at 8.88 GHz in porous polycrystalline YIG at temperatures ranging from 88 K to 368 K. Sample porosities ranged from <0.5% to 16.3%. The shapes of the low-temperature butterfly curves suggest that the previously inferred inverse wavenumber dependence of the spin-wave linewidth ΔHk may not be valid in porous YIG. The temperature dependences of the ΔHk are incongruous with those predicted by the various transit-time models of spin-wave relaxation. They are also markedly different from those reported for the off-resonance effective linewidth ΔHeff, which suggests that different mechanisms determine ΔHeff and ΔHk.
The magnetic aftereffect in 180 degree magnetically bowed Bloch walls
Authors: Heinrich B. , Hanham S. D. , Arrott A. S.
Published: February 2003

A domain wall in an iron whisker is held in a stationary position as carbon atoms redistribute on interstitial sites. The wall is then moved back and forth from one side to the other of the rest position of the wall before the carbon atoms can follow. As the magnetostatic self energy gives rise to a bowed wall it is necessary to determine the dependence of wall displacement on positions along the whisker by considering the demagnetizing fields, the applied field and the stabilization fields from the interaction of the wall with the carbon distribution. Calculations of wall displacement along the whisker with applied field are carried out for various forms for the potential well. Comparison with experiment leads to the conclusion that the potential well is much narrower than would be expected in the usual treatment of magnetic aftereffect.
Domain drag effect in the presence of variable magnetic field or variable transport current
Authors: Berger L.
Published: February 2003

The predicted dragging of ferromagnetic domain walls by hydromagnetic forces generated by a current crossing the walls has recently been observed in amorphous Gd-Co-Mo films. We extend the basic theory to the case where a uniform external field parallel to the easy direction is changing the sample magnetization. Dragging of walls in a specific direction becomes possible at arbitrarily small d.c. current density and in materials with arbitrarily small Hall angle and arbitrarily large coercive field. At external fields large enough to force successive walls to move in opposite directions, the ’’excess’’ resistivity δρ associated with walls decreases. Inversely, the presence of a d.c. current and associated dragging forces changes the macroscopic magnetic properties: for example, the coercive field decreases or even vanishes. We also treat the case of dragging of walls by an a.c. transport current of angular frequency ω. Reversible bowing of pinned walls is induced at arbitrarily small current magnitude. At small current, δρ decreases with increasing frequency according to δρ (ω) =δρ (0) ωo/(iω+ωo), where ωo is a characteristic frequency of order 104 rad./s. in the case of pure metals at low temperature.
Evidence for the interaction between 90° domain walls and dislocation tangles in high-purity iron from magnetomechanical damping experiments
Authors: Degauque J. , Astié B. , Kubin L. P.
Published: February 2003

The internal stress field, σi, which mobile 90° magnetic domain walls have to overcome can be determined by magnetomechanical damping experiments. It is shown on iron specimens strained up to 3% at room temperature, that σi is proportional to the square root of the dislocation density i.e. to the density of tangled dislocations. During further straining at 77 K, it is observed that the mobility of the 90° domain walls increases while the dislocation density inside the tangles decreases. A satisfactory interpretation of the low temperature results is obtained by calculating the dependance of σi on the density of tangled dislocations for a specimen prestrained 3% at room temperature.
Domain structure and the approach to saturation in [111] oriented iron whiskers
Authors: Hanham S. D. , Heinrich B. , Arrott A. S.
Published: February 2003

The approach to saturation in the [111] direction for an iron whisker with its long axis in that direction is studied by ac susceptibility measurements. The data is analyzed to give the magnetization, intrinsic susceptibility, and demagnetizing field at each of 15 cross sections along the length for applied fields from 10 to ∼1000 Oe. The results are interpreted using postulated domain structures for hexagonal prism whiskers. It is concluded that mean field anisotropy theory does not account for the results. The approach to saturation in the [111] direction may represent the behavior of the 3-component 3-dimensional Potts model.
Domain mode ferromagnetic resonance in materials with K1 and Kua
Authors: Artman J. O. , Charap S. H.
Published: February 2003

We have performed multidomain FMR analyses applicable to thin (111) -plane disks including both cubic, K1, and uniaxial, Ku, magnetocrystalline anisotropy and with the applied field H normal to as well as in the plane of the platelets. The solutions apply to both stripe and bubble arrays. The interplay of K1 and Ku makes nontrivial the determination of the static orientation conditions and the dynamic resonance relations. In some cases the determination of the magnetization orientation angles is sufficiently laborious to make computer solutions expedient. However the resonance relations still can be expressed concisely in the Smit and Beljers manner. In particular, we have computed the multidomain FMR expressions for H along [1̄10] and [111] respectively and, for fully magnetized specimens, the FMR expressions and magnetization tilts obtaining for H at various orientations within the (111) -plane.
Light scattering study of phonons parametrically excited in the weak ferromagnet FeBO3
Authors: Wettling W. , Jantz W. , Patton C. E.
Published: February 2003

We report on a new type of ferromagnetic resonance (FMR) instability which is due to the parametric excitation of phonons in FeBO3, and its observation by inelastic light scattering. The new and most remarkable feature of the process is its low power threshold, since it appears even prior to the competing perpendicular pumping Suhl I magnon instability in the coincidence region. The measurements were performed at room temperature and X-band ⊥ pumping configuration. The FMR absorption saturates above a certain power threshold, simultaneously a strong generation of phonons at half the microwave frequency is observed by light scattering. Using a high contrast Fabry-Perot interferometer, the onset of the phonon generation could be measured starting from the thermal level. At higher microwave power the scattered light is so strong that it can be observed visually. It is thereby revealed that the phonons propagate in sharply defined directions as determined by the energy and momentum conservation conditions of the parametric process.
Light scattering in a dilute antiferromagnet : Fe1−xZnxF2
Authors: Montarroyos E. , de Araujo Cid B. , Rezende S. M.
Published: February 2003

Inelastic scattering of light by one-magnon and magnon-pair excitations has been studied in the dilute antiferromagnet Fe1−xZnxF2 as a function of the concentration x and of the temperature T. As observed in other disordered systems, the two-magnon peak moves to lower frequencies and broadens with increasing T at fixed x, or with fixed temperature and increasing zinc concentration. Due to the role of short range order the two-magnon peak does not disappear above the percolation concentration (xp=0.76) or above the Néel temperature for each particular dilution. The one-magnon peak shows a behavior with temperature similar to that of the k=0 mode in pure FeF2, i.e., it broadens and falls in frequency with increasing T. However, in the high dilution samples the frequency does not tend to zero at the Néel temperature, indicating that the spin excitations in paramagnetically coupled finite clusters can also produce single particle light scattering. At low temperatures the frequency of the one-magnon peak decreases with increasing Zn concentration from the value 52.7 cm−1 at x=0 to 33.54 cm−1 at x=0.75, which is essentially the percolation limit.
Abstract: Magnetic excitons in a type II antiferromagnet with bilinear and biquadratic coupling in a cubic crystal field
Authors: Sablik M. J. , Wang Y. L.
Published: February 2003

We extend the diagrammatic Green’s function technique to a type II antiferromagnet in order to be able to treat a particular system, DySb, in which biquadratic effects are well-established.(AIP)
Theory of spin wave spectra in Heusler alloys
Authors: Reitz John R. , Stearns Mary Beth
Published: February 2003

The spin wave spectra of three Heusler alloys (X2MnZ) measured by Ishikawa et al have been analysed by means of a model containing the three magnetic interactions which we believe are present in these materials: Coulomb exchange between localized dl and itinerant di electrons, s-dl exchange, and superexchange via the Z atoms. Reasonable fits to the data were obtained in which the main variation between alloys was a differing di-dl interaction representing the change in di electron density as X is varied. This di-dl interaction dominates the first three near-neighbor exchange parameters and appears to be responsible for ferromagnetism in these alloys.
Preparation, characterization, mechanical and heat treatment effects on the Heusler alloy Pd2MnSb
Authors: Blizzard M. , Boolchand P. , Langouche G.
Published: February 2003

Mössbauer Spectroscopy and X-rays are utilized to elucidate the presence of heat and mechanical treatment effects in the Heusler alloy Pd2MnSb. Induction melted samples of Pd2MnSb exhibit, in general, the presence of at least three Sb sites, the relative populations of which are sensitive to the thermal and mechanical treatment undergone by the sample. The three sites respectively are: a dominant high field site of 652(3) kOe, an intermediate field site of 300(25) kOe and a zero field site. The high and zero field sites are identified respectively due to the Pd2MnSb Heusler phase and a contaminant PdSb2 phase, while the poorly defined intermediate field site is shown to be a defect site associated with the Pd2MnSb phase.
The United States national small-angle neutron scattering facility
Authors: Koehler W. C. , Hendricks R. W.
Published: February 2003

A status report is presented for the new U. S. national, user-oriented, small-angle neutron scattering facility under construction at Oak Ridge National Laboratory.
Magnetization and neutron scattering measurements on amorphous NdFe2
Authors: Alperin H. A. , Gillmor W. R. , Pickart S. J. , Rhyne J. J.
Published: February 2003

A bulk sputtered sample of NdFe2 was investigated by magnetization and neutron scattering measurements and compared to similar previous measurements for heavy rare-earth alloys. The amorphous structure was found here to be more open but in all other respects the results were found to agree qualitatively with the results for TbFe2 and HoFe2. Magnetic ordering from the neutron measurements was observed below Tc=305 K with a net moment of 1.3 μB/atom at T=0. The coercive force increased to 11.4 kOe at 5 K.
Polarized neutron measurement of ’’forbidden magnon scattering’’ in nickel
Authors: Sokoloff J. B. , Perry C. H. , Lowde R. D. , Soffge F. , Wagner V. , Wiltshire M. C. K.
Published: February 2003

We have performed inelastic neutron scattering on a Ni60 single crystal using neutrons polarized along and against the scattering vector and magnetic field direction. For temperatures above 0.9 TC we observe magnon stokes (antistokes) scattering for neutrons polarized along (opposite) the magnetization direction. The occurrence of such scattering, which is forbidden at zero temperature, is in disagreement with the conventional Stoner theory of magnetism in metals based on the RPA, but is in qualitative agreement with theories put forward by Korenman, Murray and Prange, Sokoloff, and others.
Comparative study of the magnetic form factors of transition metal ions
Authors: Khan D. C. , Kirtane S. M.
Published: February 2003

Spherical and aspherical parts of the magnetic form factors of transition metal difluorides are calculated for spin as well as orbital moment interactions with neutrons using the Trammell method. For CoF2 and NiF2 the aspherical terms are very small. The spherical form factor shows an expansion of 10 percent, a contraction of 3 percent and an expansion of 15 percent with respect to the free iron curve for CoF2, FeF2 and NiF2 respectively. A comparative study of the form factors of X (X=Fe++,Co++,Ni++) in XF2, XF3 and XO compounds is undertaken. It is found that the orbital effect, in general, contracts the magnetization density distribution with respect to the ’spin only’ distribution.
Abstract: Separation of the spin and orbital magnetic densities in nickel
Authors: Felcher G. P. , Maglic R. , Cable J. W.
Published: February 2003

Diffraction measurements were performed on a crystal of Ni60 at room temperature and in a field of 8KOe; the magnetic moment was found to be independent from the spin direction. The null result seems to indicate that an atomic approach to the problem of separating the spin and orbital contributions to the magnetic moment is invalid. (AIP)
Two-dimensionally modulated magnetic structure of neodymium, commensurate-commensurate transitions in CeSb, and the devil’s staircase
Authors: Bak Per
Published: February 2003

The magnetic structure of the rare-earth metal neodymium has remained a mystery for more than a decade. Recently, a magnetic structure which fits the experimental results has been reported [1]. Here it will be shown how the model was derived by combining neutron diffraction data with the results of Landau symmetry arguments and renormalization group theory. The spins form a fascinating two-dimensional pattern with hexagonal symmetry, the ’’triple q’’ structure. The magnetic order is accompanied by a lattice distortion with a similar symmetry. Also, the results of a numerical study of simple model of a one-dimensionally modulated system are reported [2]. The phase diagram includes multiple phase transitions between commensurate phases similar to those observed in CeSb. This model, and CeSb, are possible candidates for ’’the devil’s staircase’’ behavior where the periodicity jumps between an infinity of commensurate values.
Polarized neutron study of HoFe2
Authors: Fuess H. , Givord D. , Gregory A. R. , Schweizer J.
Published: February 2003

Results of a polarized neutron experiment at 4.2 K on a single crystal of HoFe2 are reported. The magnetic moment of the iron atoms is 1.88(10) μB. In comparison with the LuFe2 results, 0.25 μB of the Fe moment is induced by interactions with holmium. Extrapolating the Ho magnetic amplitudes to sinϑ/λ=0 one finds a magnetic moment of 9.15(10) μB. This value is inconsistent with that deduced from exchange and crystal field parameters already known in this compound.
Effect of hydrogen on sublattice magnetization of Laves-phase rare earth iron compounds
Authors: Fish G. E. , Rhyne J. J. , Sankar S. G. , Wallace W. E.
Published: February 2003

We have used elastic neutron scattering to examine the effect of hydriding on the sublattice magnetization of the Laves-phase compounds RFe2 for R=Ho and Er. On addition of hydrogen or deuterium a stable phase of composition about RFe2D3.5 is formed. The pure compounds RFe2 are ferrimagnetic, with the R moment equal to the free ion value and Fe moment of ∼1.6 μB. The bulk Curie temperatures Tc of the hydrides and deuterides are considerably lower than the corresponding RFe2 values (e.g., 440 K for ErFe2D3.5 versus 574 K for ErFe2). The 10 K R moment is also significantly reduced in the deuterides (4.4 and 5.2 μB versus 9 and 10 μB for Er and Ho in RFe2) and drops steadily with increased temperature, reaching zero below Tc in the case of Er. The Fe moment remains nearly constant with temperature until just below Tc where it drops rapidly to zero; in ErFe2D3.5 and HoFe2D3.5 its magnitude is 1.6 μB, and 1.9 μB, respectively. These results indicate that the exchange interactions involving the R atoms are markedly weakened in the hydrides. The distortion of the local anisotropy field due to random site occupancy of H and the weak exchange coupling of R produces a ’’fanning’’ of the R moment which reduces the spatially averaged moment measured in the neutron experiment.
Magnetic structure of TbFe3
Authors: James W. J. , Hardman K. , Yelon W. , Keem J. , Croat J.
Published: February 2003

The magnetic structure of TbFe3 was determined at room temperature and 77 K by powder neutron diffraction. The magnetic structure is essentially that of HoFe3, a colinear ferrimagnetic structure with the Tb moments coupled antiparallel to those of Fe in the basal plane. The three non-equivalent crystallographic Fe sites exhibit different magnetic moments. On the basis of the neutron diffraction data collected above the ordering temperature, it is concluded that no substitution of Tb by Fe occurs.
Magnetic properties of CsNiCl3 and RbNiCl3
Authors: Johnson P. B. , Rayne J. A. , Friedberg S. A.
Published: February 2003

The magnetic moment and susceptibility of the linear-chain antiferromagnets CsNiCl3 and RbNiCl3 have been studied up to fields of 60 kG from 1.1 K to beyond their respective Néel temperatures. In both compounds clear evidence for a spin-flop transition near 20 kG has been obtained with the applied field parallel to the c-axis. Details of the magnetic phase diagram for CsNiCl3 have been clarified, particularly with respect to the spin reorientation which occurs near 4.4 K at low fields. No evidence has been obtained for the existence of a corresponding reorientation in RbNiCl3.
Resistive behavior and intermediate valence effects in La1−xCexPd3 and CePd3−xRhx
Authors: Scoboria P. , Crow J. E. , Mihalisin T.
Published: February 2003

The electrical resistivity of CePd3, La1−xCexPd3, CeRhxPd3−x and Ce Pd3+x systems has been measured from helium temperature to 300 K. The resistivity of pure CePd3 rises from 125 μ Ω cm to 150 μ Ω cm as one cools from 300 K to 100 K. Below 100 K the resisitivity drops dramatically to 10 μ Ω cm at low temperatures. The same qualitative behavior, though less pronounced, is observed in other systems such as CeSn3 which like CePd3 are believed to be of intermediate valence. For x<0.3 the system La1−xCexPd3 is believed to be trivalent. We observe Kondo-like behavior in this regime. For x near unity we observe that extremely small amounts of La eliminate the rapid fall in resistivity observed below 100 K for pure CePd3. These results suggest that this fall off is due to some type of cooperative Ce ion hehavior. The CeRhxPd3−x system allows one to monitor the effect on resistivity of varying the Ce valence from 3.45 in CePd3 to 4 in CeRh.64Pd2.36. We find that the magnitude of the resistivity rise (upon cooling from 300 K to low temperature) decreases as the Ce valence increases towards four.
Magnetoresistance of Pd3Fe and PdFe
Authors: Hsu Y. , Jen S. , Berger L.
Published: February 2003

The ferromagnetic anisotropy of resistivity Δρ/ρ0= (ρ∥-ρ⊥)/ρ0 of the alloy Pd70Fe30 has been measured below 295 K, for each of five different thermal treatments. At 4.2 K, Δρ/ρ0 reaches 9.4% in the atomically ordered state (of Pd3Fe type), but is only 5.4% in the disordered state. In Pd75Fe25, the effect of ordering on Δρ/ρ0 is much smaller than in Pd70Fe30, and has the opposite sign. A similar sign reversal with varying iron concentration has been observed earlier in the Ni-Fe series. Most features of our Δρ/ρ0 data, including the sign reversal, can be explained on the basis of the well-known linear relation between Δρ/ρ0 and the ratio ρ↓/ρ↑ of the resistivities for spin-down and spin-up 4s electrons. In Pd50Fe50, ordering (of PdFe type) has little effect on Δρ/ρ0. At 295 K, where phonon scattering dominates over impurity scattering, Δρ/ρ0 is always smaller than 1% in all samples.
Dynamical conductivity of EuSe in high magnetic fields
Authors: Faymonville R. , Grosse P. , Mützenich G. , Heleskivi J. , Mäenpää M.
Published: February 2003

The reflectivity of EuSe has been measured in the submillimeter range at temperatures down to 4.2 K and in magnetic fields up to 15 T. The spectral shapes depend on temperature and magnetic field. Besides a strong reststrahlenband a free carrier contribution is observed at 300 K. This contribution disappears at low temperatures, but appears again in an external magnetic field. A similar behavior is observed in dc-conductivity and Hall-measurements. According to the Hall-measurements in fields between 3 and 10 T, the large negative magnetoresistance at low temperatures can be explained by changes in the free carrier concentration. The dielectric function was calculated by performing a Kramers-Kronig-analysis on the reflectivity data. From the imaginary part the dynamical conductivity was separated. The high frequency conductivity turns out to be 20 times larger than the dc value. The results may be explained by means of a mixed conduction model where conduction at high temperatures or high magnetic fields takes place in a normal conduction band whereas at temperatures near the magnetic ordering temperature and low magnetic fields the conduction changes to a hopping mechanism between localized states.
Magnetic and thermal properties of the mixed valence compounds Sm2S3, Sm3S4, and Sm3Se4
Authors: Coey J. M. D. , Cornut B. , Holtzberg F. , von Molnar S.
Published: February 2003

The magnetization and low temperature specific heat of small (20 mg) single crystals of Sm2S3, Sm3S4 and Sm3Se4 have been measured to test the crystal field model for Sm3+ in the Th3P4 structure suggested earlier for the compounds EuSm2S4 and SrSm2S4. In this model, the Γ8 quartet is lowest but split (by ∼2.4 K) by a small noncubic crystal field. A pronounced upturn in the specific heat below 7 K is predicted. The present measurements down to 1.5 K confirm this prediction for every compound measured, the same feature occurring at almost precisely the same temperature. In addition, a comparison of the experimental susceptibilities of Sm2S3 and Sm3S4 indicates that simple addition of the contributions of the different ionic species (Sm2+ and Sm3+) is sufficient to explain the magnetic behavior of these mixed valent Sm compounds.
Magnetic behavior of layered tetravalent manganese oxides
Authors: White T. R. , Glaunsinger W. S. , Horowitz H. S. , Longo J. M.
Published: February 2003

Ca2Mn3O8 and Cd2Mn3O8, which contain Mn4+ monolayers, have been prepared and characterized. Their magnetic susceptibility (χ) and electron paramagnetic resonance (EPR) behavior have been examined in detail. The Mn4+ moments in both Ca2Mn3O8 and Cd2Mn3O8 order antiferromagnetically near 60 and 10 K, respectively. Although the Néel temperature in Ca2Mn3O8 is in reasonable agreement with molecular-field theory, that in Cd2Mn3O8 is well below its expected value. Intralayer exchange integrals have been estimated from the magnetic data and increase in the following manner: Ca2MnO4≫CaMnO3≳Ca2Mn3O8≫Cd2Mn3O8. It is proposed that these results, as well as those in the calcium manganite series CaMnO3→Ca2MnO4, may reflect the chemical influence of the divalent cation in modifying the Mn-O covalent mixing.
Crossover behavior of the magnetic phase boundaries of isotropic antiferromagnets
Authors: Oliveira N. F. , Shapira Y.
Published: February 2003

Phase boundaries, Tc vs H, of isotropic antiferromagnets are discussed. The Fisher-Nelson-Kosterlitz predictions for the effect of a spin-dimensionally crossover on Tc(H) are summarized. The stringent experimental requirements for testing these predictions are reviewed. Results for Tc(H) in the very-low-anisotropy cubic antiferromagnets RbMnF3 and KNiF3 confirm the theoretical predictions for the case of a Heisenberg-to-XY crossover. The RbMnF3 data give a crossover exponent φ which agrees with theory. Results for Tc(H) in the easy-plane antiferromagnet CsMnF3 are presented. The data when H↘ is in the easy plane are consistent with an XY-to-Ising crossover, and give φ=1.185±0.03. The results when H↘ is perpendicular to the easy plane are interpreted in terms of a virtual bicritical point at some negative value of H2.
Two-dimensional ferroelectric liquid crystals
Authors: Pelcovits Robert A. , Halperin B. I.
Published: February 2003

The possibility of experimentally realizing two-dimensional XY critical behavior in thin ferroelectric smectic films is explored. We show that an ideal two-dimensional smectic C layer will exhibit XY behavior and then consider the effect of dipolar interactions and external electric field. The dipolar forces stabilize the ferromagnetic order and are relevant along the entire XY fixed line. They may, however, have little effect on the critical behavior of the present experimental systems due to their small dipole moment. We also offer some comments on the critical behavior controlled by the infinite dipolar fixed point.
Study of cubic anisotropy in three dimensions by the scaling-field method
Authors: Domany Eytan , Riedel Eberhard K.
Published: February 2003

Scaling-field equations for the hypercubic N-vector model are derived from Wilson’s exact renormalization-group equation and applied to the determination of NC*, the instability point of the isotropic fixed point as a function of N, and to the estimation of the critical point exponents associated with the isotropic, cubic, and decoupled Ising fixed points in hypercubic systems of three spatial dimensions.
Renormalization group and other calculations for the one-dimensional spin-1/2 dimerized Heisenberg antiferromagnet
Authors: Fields J. N. , Blöte H. W. J. , Bonner J. C.
Published: February 2003

A zero-temperature renormalization group (RG) approach is applied to the one-dimensional, spin-1/2 antiferromagnetic Heisenberg dimerized (alternating) chain. Specifically, the ground state energy and lowest-lying spectral excitations are examined. The calculation indicates the existence of a gap in the spectrum of the dimerized chain which vanishes only in the limit of a uniform spin chain, in contrast to a recent Green’s function approach. The RG results are in reasonable agreement with numerical extrapolations on the exact eigenvalue spectrum of finite chains of up to 12 spins. Both methods are compared with several other approximate treatments of the Heisenberg system, and tested by comparison with exact results for the spin-1/2 XY dimerized chain.
Exact solutions for infinite-range spin Hamiltonians
Authors: Kventsel G. F. , Katriel J.
Published: February 2003

The closed form magnetization equation, s=σ⋅Bσ[−β⋅σ∂H/∂s], is shown to be valid for any infinite range spin Hamiltonian H=N⋅H(s), where N is the number of particles, each with elementary spin σ, s=S/N, S being the total spin, and Bσ is Brillouin’s function. This equation is used to investigate the following problems: (i) determination of the class of Hamiltonians with a spinodal point at 0°K; (ii) spin-phonon coupling, quadratic in the lattice displacements. Magnetostriction of this form may lead to shift of the critical temperature, change of the order of the phase transition, decrease of the paramagnetic spinodal temperature up to 0°K and renormalization of the phonon spectrum; (iii) singlet-ground-state ferromagnetism, exhibiting various types of phase-transitions, including heat magnetization.