bluuebunny/crossref_metadata_2025 huggingface dataset:
Antiferromagnetic Resonance in Doped α-Fe2O3
Authors: Foner S. , McNiff E. J. , Remeika J. P.
Published: April 2005

Antiferromagnetic resonance (AFMR) in small single crystals of α-Fe2O3 doped with various impurities has been examined at 120 Gc/sec from 4.2°K to 300°K with pulsed fields. The data are best described by comparison with previous low-temperature AFMR in pure α-Fe2O31 single crystals. These measurements involve the large axial anisotropy rather than the small in-plane anisotropy usually observable at high temperatures in pure α-Fe2O3. Because this anisotropy results from an almost complete cancellation of the dipolar anisotropy by fine structure contributions in α-Fe2O3,2 it is expected that in some cases the AFMR would be quite sensitive to impurities. Crystals prepared with 2% Al2O3 and 2% Gd2O3, nominal additions to the melt, showed only small changes in the AFMR and magnetic transition temperature. The usual low-frequency AFMR, the spin-flop resonance, and the high-frequency canted resonance mode were all observed similar to that of pure α-Fe2O3. Various compositions with up to nominally 1% SiO2 in α-Fe2O3 were also studied as well as α-Fe2O3 doped with small amounts of TiO2 and SnO2. The AFMR results are consistent with previous interpretations.2 Detailed results will be published elsewhere.
Stabilization Prediction for Permanent-Magnet Field Motors and Generators
Authors: Wentworth B. W. , Ellis E. L.
Published: April 2005

A primary concern in the design of permanent magnet field motors is the interaction of the field flux and the flux produced by the armature current. An analytical model has been established to quantitatively predict the degree to which the permanent magnet field is demagnetized by the armature current. As an aid to analysis, the pole face opposite each armature tooth is treated as a separate section. The maximum amount of magnetizing or demagnetizing force is determined for each section from ΔH=−4π(K0−Ka)NI/50.8 lm,where K0 is the number of armature slots opposed to field flux; Ka, the number of armature slots aiding field flux; NI, the maximum ampere turns per slot (generally at stall current); and lm, the radial length of the permanent magnet in inches. The maximum demagnetizing force per section is then used to determine the stabilized flux densities per section, which in turn are used to determine the stabilized flux per pole for the machine. The effects of armature reversal can be included in the stabilization prediction.

The analytical model was compared with a static test model, and it was determined that the predictions were accurate to within a few percent. Employment of this method of field flux analysis with optimization of armature parameters, including tooth width and armature winding, provides the designer with competent design tradeoffs prior to committing expensive tooling operations.
Application of Modern Permanent-Magnet Materials to the Excitation of a Slow-Speed Power Servomotor
Authors: Lord W.
Published: April 2005

The main requirements of low-speed, dc power servomotors used as drive members of director control systems are for fast speed of response coupled with optimization of size and weight for a given torque output. It can be shown that the over-all time constant of such a dc machine is inversely proportional to the gap density squared; thus, where physical size and temperature rise of the field windings are important design limitations, the use of modern, high-energy permanent-magnet materials becomes advantageous.

This paper describes the conversion of a conventional 6-pole, 1-hp, 100-rpm machine to one having permanent magnet excitation employing Columax (Alnico VB, DG) poles. Difficulties associated with machining, and magnetizing and demagnetizing the 6-pole stator in situ are discussed, and a study of the maximum demagnetization forces existing in such a machine, carried out with the aid of Hall plates, is given. A comparison of both the dynamic and the steady-state performance of the conventional machine and the permanent magnet machine, shows the feasibility of using modern permanent magnet materials for such an application.
Improvement in Performance of Bulk Separation Equipment using Permanent Ceramic Magnets
Authors: Staats Gustav W.
Published: April 2005

Theoretical analysis of the performance of flat-plate permanent magnet separation equipment is presented, together with experimental confirmation. By the use of conformal mapping techniques, force on magnetic spheres along the magnet center line is found to be proportional to the square of surface flux density. Addition of a blocking magnet increases surface flux density and force with no increase in magnet size. Test data confirm the predicted increase in flux density and separating force.
Wideband Direct Recording with High-Frequency Bias
Authors: Hendershot William B.
Published: April 2005

Most present day wideband magnetic tape recorders utilize a FM system for encoding and decoding the information applied to the tape. In the past FM has been employed for reasons of linearity and to avoid the amplitude modulation which occurs due to varying head-to-tape contact. Recent developments in heads, tapes, and transports, however, have removed many of the constraints which originally forced the use of a FM system. Consideration of a high-frequency bias system for video and wideband instrumentation recorders, therefore, is again made possible. As a direct recording system holds so much promise in the areas of linearity and distortion properties, it becomes necessary to establish the transfer function for tape when it is acted upon by a high-frequency bias. The purpose of this paper is to develop a method that may be used to establish the feasibility of such a system and to set down its physical and magnetic parameters. A three-dimensional magnetic moment distribution function J(H+, H−) is used to describe the magnetic properties of the tape. Then the actual transfer characteristics of the headtape system are developed by applying the magnetic field about the record head to the three-dimensional distribution function J(H+, H−).
ac Field ``Freezing'' and ``Melting'' of Magnetization in Fine-Particle Assemblies
Authors: Kneller E. , Friedlaender F. J. , Puschert W.
Published: April 2005

Alternating-current-freezing curves Mf(H, H′) and ac-melting curves Mm(H, H′) represent a dynamic recording of the process of storing and erasing, respectively, of magnetic signals under the continuously equilibrating action of an ac-field H′.

The properties of such curves are discussed in connection with measurements on assemblies of acicular γ−Fe2O3 particles. Indication is made how these curves are calculated from simple particle models. It is especially shown that the extension of the pair model with equal volumes, equal critical fields, and negative interaction field to unequal critical fields accounts surprisingly well for most details found experimentally.
A Brief Review of High-Coercivity Thin Films for Digital Magnetic Recording
Authors: Bate G.
Published: April 2005

Three methods have been used to prepare thin films of the common ferromagnetic elements and their alloys having permanent magnet properties. The methods are (1) electrodeposition, (2) autocatalytic deposition, and (3) vacuum deposition. The effect of the deposition conditions on the magnetic properties is discussed briefly for each method. It is possible to prepare thin (∼1000 Å) films with coercivities 100<Hc<1500 Oe, in which the saturation magnetization is fairly close to that of the bulk material and whose values of remanent/saturation intensity approach 1·0. The magnetic properties of the films prepared by methods (1) and (2) are isotropic and those prepared by method (3) are usually uniaxial in the film plane. Although they are highly (and specularly) reflecting and are electrically continuous, the magnetic structure of the films appears to be basically that of an assembly of strongly interacting single domain particles. The application of these films is in high-density digital magnetic recording; because of their high coercivity and thinness they give output pulses which are narrow and resolvable at densities considerably greater than those possible with conventional oxide recording surfaces.
Morphologies of Acicular γ−Fe2O3 Particles
Authors: Hurt J. , Amendola A. , Smith R. E.
Published: April 2005

Commercial iron-oxide magnetic recording tapes have long been made using acicular particles of γ Fe2O3 dispersed in an organic matrix. X-ray diffraction of coatings of this type showed little or no preferred crystallographic orientation even though these coatings had been oriented by the presence of a strong magnetic field during coating. Low-viscosity binders were used in some experimental coatings while the oxide was subjected to a strong magnetic field, and similar x-ray results were obtained. Electron microscopy revealed aligned particles at or near the surfaces of oriented coatings, and the particle morphology was examined by electron diffraction. The majority of particles examined were found to be polycrystalline, and single-crystal particles were seen to possess a [110] needle axis. X-ray evidence of some preferred orientation in a coating could be seen only when the particles were oriented by stretching the coating. The x-ray results obtained using both magnetically oriented and unoriented coatings are explained by a combination of the polycrystalline nature of the particles and the inefficiency of the magnetic orienting process.
Localized Moments in Transition Metals
Authors: Jaccarino V. , Walker L. R.
Published: April 2005

Two types of nuclear magnetic resonance (NMR) studies are discussed which are pertinent to the magnitude and distribution of the magnetization associated with local moments in transition metals: (1) The unusual temperature dependence of the 55Mn NMR1 of 1.5% Mn in Fe, below Tc, is shown2 to result from the existence of a localized moment on the Mn and the fact that the Fe-Mn exchange interaction is smaller than the Fe-Fe exchange. (2) From studies of the 59Co NMR in dilute Co in Rh-Pd alloys evidence is obtained which suggests that the occurrence of localized moments in a binary alloy system is discontinuous— the apparent continuous behavior observed in magnetization measurements is a consequence of the statistical variation of near neighbors of a given kind to the impurity atom.3
Spin Density Patterns around Impurity Atoms in Iron and Nickel
Authors: Arrott A. , Collins M. F. , Holden T. M. , Low G. G. , Nathans R.
Published: April 2005

The spatial pattern of spin density disturbance around six different impurities in nickel (Al, Si, V, Cr, Mn, and Fe) and around seventeen impurities in iron (Al, Si, Ti, V, Cr, Mn, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, and Pt) has been investigated. Al, Si, V, and Cr impurities in nickel reduce the spin on neighboring nickel atoms very markedly while for iron impurities in nickel the disturbance is essentially confined to the solute atom site. The pattern around many impurities in iron is more complex. V, Cr, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, and Pt produce a small increase in moment on some iron atoms in the neighborhood of the impurity site. Those transition elements that lie to the right of iron in the periodic table, Co, Ni, etc., seem to produce a disturbance in the iron matrix which is always positive while the remaining elements mentioned above give a positive disturbance which is apprently confined to a region about 5 Ȧ distant from the impurity. All the solutes produce a loss of moment relative to pure iron at the impurity sites.
Iron Losses Due to Elliptically Polarized Magnetic Fields
Authors: Young F. J. , Schenk H. L.
Published: April 2005

The iron losses occurring in an 11-mil cube-textured silicon-iron specimen, a 12-mil singly oriented silicon-iron specimen, and a 14-mil nonoriented silicon-iron specimen when subjected to a 60-cps elliptically polarized magnetic field (EPMF) are measured by means of a technique described previously and these EPMF losses are presented and compared. In general, the loss due to the EPMF was found to be equal to or less than the sum of the 60-cps alternating losses at the two induction levels that are the major and minor components of the EPMF. In the singly oriented and nonoriented silicon-iron specimens, a decrease in the losses for increasing flux densities for large elliptical fields is observed. The decrease continues until the eddy current loss becomes sufficiently large compared to the decrease in the rotational hysteresis loss to cause an increase in the total loss. This behavior was not found in the cube-textured specimen. An empirical expression, derived from the known pure alternating losses, is given which predicts the EPMF losses within 10% except at large EPMF's.
Origin of the Constricted B-H Loop in Hard-Rolled Co-10% Fe
Authors: English A. T. , Von Neida A. R. , Chin G. Y.
Published: April 2005

A recent paper by Wenny and Gould reported a pronounced ``Perminvar''-type constricted B-H loop in hard-rolled Co-10% Fe sheet magnetized in the rolling direction. The conditions for obtaining the constricted loop are quite different from those which underlie similar phenomena in the Perminvar alloys. Based on further experiments described in this report, the constricted loop is related to the combined effects of two factors:(1) A very high negative magnetocrystalline anisotropy constant, K1=−7.5×105 erg/cm3; and(2) A crystallographic texture having two distinct {110} 〈112〉 components, present as small crystallites. These factors have been incorporated into a model which quantitatively accounts for the main features of this phenomenon. The role of magnetostatic effects at internal crystal boundaries is emphasized.
Microwave Absorption in Rare-Earth Metals
Authors: Bagguley D. M. S. , Liesegang J.
Published: April 2005

Microwave absorption phenomena have been investigated in single crystals of the rare-earth metals Tb, Dy, Ho, and Er, at frequencies of 9.44 and 35.3 Gc/sec over the temperature range 10° to 290°K. Microwave absorption occurs at the critical field transitions for the antiferromagnetic phase. Additional absorption lines have been observed in the ferromagnetic phases of Tb and Dy.
Microwave Transmission Through Permalloy Films
Authors: Holzer W. C. , Perry B. W. , Portis A. M.
Published: April 2005

A new interference effect has been observed in the transmission of microwave signals through Permalloy films. This effect, which involves interference between the eddy-current microwave transmission and the magnetic field associated with the excited spin waves, permits identification of the symmetry and surface pinning behavior of the standing spin waves. Previous studies of spin waves in Permalloy films have involved a determination of the surface impedance in the vicinity of spin-wave excitation. The present measurements show a decrease in transmitted microwave power for the lowest-order spin-wave mode and for those higher-order modes whose magnetic excitation has even symmetry with respect to the plane of the film. However, corresponding to intermediate modes, which have odd symmetry, we observe a transmission maximum. Thus we observe alternate minima and maxima on transmission. The measured fields correspond exactly with the field positions of the alternate strong and weak maxima in surface impedance, as observed on reflection. This correspondence between transmission and reflection supports the generally accepted interpretation of spin-wave mode symmetry. In addition, the direction and amplitude of this interference as a function of the angle between the magnetic-field direction and the plane of the film agree with the partial-pinning model of Nisenoff and Terhune.
Linewidth Studies in Ordered Lithium Ferrite
Authors: Nilsen W. G. , Comstock R. L. , Remeika J. P.
Published: April 2005

Linewidth measurements have been carried out in ordered lithium ferrite from 4.2°K to room temperature and at several microwave frequencies. Our purpose was to compare the linewidth characteristics of ordered lithium ferrite with those of YIG in the low-temperature range. We find that nominally pure lithium ferrite exhibits a small linewidth peak at about 19°K as compared to roughly 40° to 50°K for YIG. Of particular interest is the fact that in lithium ferrite the linewidth increases with microwave frequency both below and above the linewidth peak and that the peak occurs at the same temperature independent of microwave frequency. For YIG below the peak, the linewidth decreases with increasing microwave frequency.

In an attempt to determine the origin of the linewidth peak in lithium ferrite, several samples were made with small dopings of silicon and manganese. These samples also had a linewidth peak at 19°K, but the height of the peak was roughly the same for undoped and doped samples made at the same time. Chemical analysis and conductivity measurements showed that the pure samples did not contain significant amounts of Fe++ or manganese, that the silicon-doped samples contained Fe++, and that manganese probably enters the lithium ferrite lattice as Mn3+. Apparently, neither Fe++ nor Mn3+ is the primary cause of the linewidth peak in lithium ferrite. Also, calcium-doped YIG, which is known to contain Fe4+, did not show a linewidth peak. It appears that the linewidth peak in lithium ferrite is not due to an impurity ion.
SSWM in Thin Permalloy Films Prepared in Vacuums of 10−5 and 10−9 Torr
Authors: Lykken G. I. , Harman W. L. , Mitchell E. N.
Published: April 2005

Standing spin-wave modes were measured in Permalloy films (82% Ni-18% Fe, thickness range 100–2800 Å, substrate deposition temperature 4.2°K <T<573°K) at room temperature and 12.2 Gc/sec. Two series of films were examined: series I made in an ultrahigh vacuum system (10−10−10−9 Torr) and measured prior to removal from this vacuum and later in air, series II made in an oil diffusion system (10−5 Torr) and subsequently measured in air. The absorption modes of all series I films shifted to higher magnetic fields as they aged in vacuum at room temperature, while aging in air generally shifted them to lesser fields; in both cases the relative intensities of the modes were unchanged. Annealing greatly altered the absorption spectra and shifted them to lesser fields. Most films exhibited a quadratic dependence on mode spacings supporting the surface-spin-pinning model. The series II films could also be fitted to the volume inhomogeneity model by changing the numbering of the modes.

It is shown that in most cases, the shifting of the spectra is due to changes in g, 4πM and (A/M) ⊥ and that the SSWM in the series II films are described best by the surface-spin-pinning model. It is concluded that there is strong spin pinning at the substrate-film interface and that this pinning is much greater than at the top surface, except in extreme cases such as highly oxidized films.
Ultrabroad Ferromagnetic Resonance in Dysprosium Metal
Authors: Rossol F. C. , Jones R. V.
Published: April 2005

An extremely broad ferromagnetic resonance has been observed in single-crystal Dy metal at 37 Gc/sec. Although the large planar anisotropy results in a much higher resonance frequency, the resonance was brought down by applying a high field along a hard direction of the in-plane anisotropy. The temperature dependence of the field for resonance, in the temperature range from 78° to 103°K, was found to follow the behavior of the in-plane anisotropy. Above 87°K the zero field ordering is spiral but ferromagnetic ordering is induced by the applied field. The resonance peak cannot be seen above 103°K because the field for resonance is no longer sufficient to induce ferromagnetic ordering. The absorption was found to be extremely broad in field so that appreciable absorption occurs even where the resonant frequency is far above the experimental frequency. This feature may be interpreted on the basis of either or both of two distinct models. The first relies upon the fact that the spin-wave spectrum decreases to a minimum at a nonzero wave vector k0 and might thereby provide a number of k≠0 modes degenerate with the experimental frequency which may be coupled to through various sample inhomogeneities. The second model considers a very large dynamic broadening of the ferromagnetic resonance such as might arise from large magnon-phonon coupling due to the extremely large magnetoelastic interaction in dysprosium. On this model, the shape and width of the absorption curve indicate a relaxation frequency on the order of the resonance frequency.
Magnetic Effects in the Optical Spectrum of MnF2
Authors: Sell D. D. , Greene R. L. , Yen W. M. , Schawlow A. L. , White R. M.
Published: April 2005

Sharp optical transitions between the 6A1g (ground state) and the 4T1g(4G) levels of MnF2 have been studied as a function of temperature (2°–40°K) and magnetic field (0–10 kOe) in both emission and absorption. Properties suggesting that two emission lines are spin-wave sidebands are discussed. Temperature-dependent shifts of the spectra are attributed to the antiferromagnetic nature of the crystal. A simple molecular field model is presented which describes the magnetic field behavior of the absorption spectra.
55Mn Spin-Lattice Relaxation in MnFe2O4
Authors: Blocker T. G. , Heeger A. J.
Published: April 2005

We report here measurements of the 55Mn spin-lattice relaxation time as a function of temperature and external magnetic field in manganese-iron spinels; and a comparison of the experimental results with current relaxation theories. The results offer evidence for nuclear relaxation via coupling of the nuclear spins to low-frequency fluctuations in the electronic spin system resulting from the presence of impurities, as proposed by de Gennes and Hartmann-Boutron. The field dependence of T1 indicates that relaxation due to frequency-pulling effects gives a negligible contribution to the total relaxation rate in comparison with the above mechanism even at the lowest temperature studied. This conclusion is in direct contradiction with the theory based on the classical electron-nuclear equations of motion. A more detailed calculation taking explicit account of the existence of a well-defined nuclear spin-wave spectrum in this temperature range shows that frequency-pulling effects are indeed negligible. The results therefore provide experimental evidence for nuclear spin waves in this system.
19F NMR Spin Echo in Antiferromagnetic MnF2
Authors: Kaplan N. , Pincus P. , Jaccarino V.
Published: April 2005

Spin echoes of 19F NMR have been observed, in zero external field, in various samples of antiferromagnetic MnF2. The decay of the echo envelope was always found to be exponential. This corresponds to a Lorentzian line profile, indicating that the Gaussian line shape previously observed with a cw spectrometer and found to be an order of magnitude broader, arises from inhomogeneous broadening. A previous calculation of the indirect nuclear spin-spin interaction via virtual magnon excitations predicts a 3-μsec T2 with a Gaussian envelope. However, (1) destructive interference between the 19F dipolar and indirect coupling mechanisms and (2) the appreciable contribution via the 19F−55Mn dipolar interaction of the much larger indirect coupling between 55Mn nuclei to the fourth moment of the line will reduce the 19F relaxation rate, the first, by only 20% but the second by approximately a factor of eight, and will change the line shape from Gaussian to Lorentzian. The derived value T2≅30 μsec is in agreement with the relaxation time T2=29±1 measured for a high-purity MnF2 sample. T2 is observed to monotonically decrease to a value of 22 μsec at 20.2°K. The temperature dependence of the nuclear spin lattice relaxation T1 in the same region is more dramatic, varying from 130 sec at 4.2°K to 3 msec at 20.2°K.
Ferrimagnetism of the Rare-Earth-Cobalt Intermetallic Compounds R2Co17
Authors: Strnat K. , Hoffer G. , Ostertag W. , Olson J. C.
Published: April 2005

Saturation moments, Curie temperatures, and supporting crystallographic data are reported for a series of ferrimagnetic intermetallic compounds of the stoichiometry R2Co17, where R is any of the rare-earth elements Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The saturation moment of Y2Co17 is 27.2, that of Lu2Co17 is 27.6 Bohr magnetons per formula unit, indicating a cobalt moment of 1.61 μB in this series. Substitution of Pr and Nd for R raises the saturation to 32.8 μB/F.U. for Pr2Co17; all other lanthanides lower it (minimum: 5.7 μB/F.U. for Ho2Co17). Except for Sm2Co17, the values agree well with the concept that the Co moments couple antiparallel with the rare-earth spin sublattice. For Ce and Pr, a valence>3 and electron transfer from R to Co must be assumed. The Curie temperatures lie between 795° and 940°C. The variation of Tc with the atomic number suggests that two coupling mechanisms are active: direct Co-Co exchange interaction and indirect exchange between R and Co via polarized conduction electrons.
Magnetic Studies of Gadolinium Compounds with the CsCl Structure
Authors: Alfieri G. T. , Banks E. , Kanematsu K.
Published: April 2005

The dominant interaction in rare-earth compounds with the CsCl structure is the Ruderman-Kittel-Yosida exchange due to polarization of the conduction electrons. In the rare-earth systems RA (where A=Ag, Cu, Zn, Cd, In), it was found that RAg and RCu are antiferromagnetic; RZn and RCd are ferromagnetic; RIn is antiferromagnetic. This change in magnetic properties can be attributed to the variation of conduction-electron concentration. We have prepared the ternary solid solutions: Gd(ZnxAg1−x), Gd(ZnxIn1−x), Gd(ZnxCu1−x), and (Gd1.2Cd)x(GdAg)1−x [0≦x≦1] to study the effect of changes in electron concentration on the magnetic properties of these materials. The Curie temperature of GdZn and Gd1.2Cd are about 280°K; the Néel temperatures of GdAg, GdCu, and GdIn are reported to be 145°, 41°, and 28°K. The change of the paramagnetic Curie temperatures in the Gd(ZnxAg1−x) and Gd(ZnxIn1−x) systems is in qualitative agreement with the Gd(AgxIn1−x) system as reported by Sekizawa et al. and is consistent with theoretical expectations, assuming that Ag, Zn, and In give one, two, and three conduction electrons. In the composition region 0.4≦x≦1, both systems are ferromagnetic and the Curie temperature decreases as x decreases. At x=0.2 the compounds are probably antiferromagnetic. The total magnetic moment also diminishes as x decreases, in a field of 4 kOe. We expect the total moment to remain constant at 7 μB/Gd for the ferromagnetic compounds at higher fields. The Gd(ZnxCu1−x) and (Gd1.2Cd)x(GdAg)1−x systems show the same trend.
Magnetic Properties of the Monosilicides of Some 3d Transition Elements
Authors: Williams H. J. , Wernick J. H. , Sherwood R. C. , Wertheim G. K.
Published: April 2005

Motivated by the recent observation1 of the unusual electronic properties of FeSi, as manifested in the 57Fe Mössbauer effect and 29Si NMR, the magnetic susceptibilities of the monosilicides of Cr, Mn, Fe, Co, and Ni were determined as a function of temperature from 1.4° to 300°K in fields up to 15 kOe. The silicides of Cr, Mn, Fe, and Co have the cubic B-20 structure and NiSi has the orthorhombic B-31 structure. NiSi and CoSi are diamagnetic and show essentially a temperature-independent susceptibility. CrSi exhibits temperature-independent paramagnetism. FeSi is paramagnetic with an unusual temperature dependence of the susceptibility.1 The susceptibility increases with increasing temperature with χg=0.77×10−6 at T=90°K and χg=8.3×10−6 at T=300°K. Although FeSi is paramagnetic and CoSi is diamagnetic some solid solutions of the two compounds are ferromagnetic at low temperatures. MnSi orders magnetically at 30°K. At 1.4°K the magnetization curve is linear up to a field of 6200 Oe where it saturates abruptly. The saturation moment per Mn atom at 1.4°K is 0.4 μB, which is smaller than the value 1.4 μB one obtains in the paramagnetic region. Dilute solutions of Ru, Rh, Cr, Mn, Fe, and Ni in CoSi become paramagnetic with Curie-Weiss behavior.
Electronic Band Structure, Fermi Surface, and Magnetic Properties of Palladium Metal
Authors: Freeman A. J. , Furdyna A. M. , Dimmock J. O.
Published: April 2005

The electronic energy bands of Pd metal have been determined using the nonrelativistic A.P.W. method. The Fermi surface, which consists of two hole surfaces and a compensating electron surface, agrees very well with the de Haas-van Alphen data of Vuillemin and Priestly. The electron surface is centered at Γ and contains approximately 0.3 electrons per Pd atom. One hole surface consists of a small pocket centered at X. The other hole surface is open along the [100] directions in agreement with galvanomagnetic measurements. The Fermi energy lies slightly above a maximum in the calculated density of states N (E) as expected from studies of dilute alloys. Using the computed N (EF) we obtain an electronic contribution to the specific heat which is only about half the observed value, indicating an electron-phonon enhancement typical for d-band metals. The computed paramagnetic susceptibility χ(T) is much smaller than the experimental value, emphasizing the importance of exchange in markedly enhancing χ(T), particularly at low temperatures.

A full account of this work will be submitted to the Physical Review.
Unusual Nature of the Abrupt Magnetic Transition in FeRh and Its Pseudobinary Variants
Authors: Kouvel J. S.
Published: April 2005

Previous measurements of the critical temperature (Tcrit) for the first-order antiferromagnetic-ferromagnetic transition in FeRh as a function of magnetic field have established that the total entropy change at the transition (ΔS) is much larger than the estimated change in lattice entropy (ΔS)lat. This study has now been extended to pseudobinary variants of FeRh where the Rh is partially replaced by Pd, Pt, or Ir, resulting in a large decrease or increase of Tcrit from its value (about 330°K) for FeRh. In each case, a value for ΔS was deduced from the measured field dependence of Tcrit. The difference between each ΔS and an estimated (ΔS)lat value, when plotted vs Tcrit, defines a smooth curve with a maximum at about 500°K, which is just below the Curie points of these alloys. It is therefore concluded that ΔS−(ΔS)lat represents an entropy change of magnetic origin. This anomalous change in magnetic entropy is attributed to thermal excitation of the Rh moments which in the ferromagnetic state are induced by the net exchange field from neighboring Fe moments; in the antiferromagnetic state this net exchange field vanishes, and the induced Rh moments and their contribution to the entropy go essentially to zero.
Changes in Apparent Symmetry with Magnetic Field in Chromium
Authors: Werner S. A. , Arrott A. , Kendrick H.
Published: April 2005

The magnetic states of chromium are described in terms of fundamental wave vectors Q1, Q2, and Q3 and the polarizations associated with these Q vectors. If the magnetic reflections in neutron-diffraction experiments show Q1, Q2, and Q3 all present and contributing equally to the single-crystal diffraction pattern, we speak of the ``apparently cubic'' state. If only one Q vector is present, we speak of a ``single Q'' state. Cooling through the first-order paramagnetic-antiferromagnetic transformation at 38.5°C produces an ``apparently cubic'' state if no field is applied during cooling while the application of a field of 40 kG can produce a ``single Q'' state in some crystals. In other crystals 40 kG is not sufficient to produce an entirely ``single Q'' state. The preferred Q is parallel to the applied field; the polarization is then perpendicular to the field. If the field applied during cooling is less than necessary to completely suppress Q1 and Q3 while enhancing the contributions from Q2, the polarizations of Q1 and Q3 prefer the field direction. In the low-temperature phase of chromium, where the polarizations are parallel to the Q's, the application of 160 kG is sufficient to change an ``apparently cubic'' crystal into a state with one Q missing. The ``missing Q'' is parallel to the direction of the field.

The critical scattering at 40°C shows that all three wave vectors Q1, Q2, and Q3 are equally represented in the wave-vector-dependent susceptibility and that the longitudinally polarized fluctuations are comparable to the transversely polarized fluctuations. The tetragonality of the Fermi surface of the ``single-Q'' state does not cause a large enough lattice distortion to be detected with an x-ray technique sensitive to 1 part in 2×104.
De Haas-van Alphen Effect in Antiferromagnetic Chromium
Authors: Graebner J. , Marcus J. A.
Published: April 2005

Cooling chromium through the Néel temperature in a magnetic field Hc alters the magnetic symmetry. The dH-vA amplitudes, but not the frequencies, change with increasing cooling field so as to reduce the symmetry from fourfold to twofold, the changes reaching saturation at cooling field Hs. For several crystals Hs falls in the range 15–25 kG, with an inverse correlation between purity and Hs. For the saturated case, the dH-vA oscillations exhibit the full tetragonal symmetry observed by Watts; furthermore, in several crystals cooled in zero field we have observed all the oscillations of the saturated case but with approximately cubic symmetry and no others. This is consistent with Hc aligning tetragonal domains each containing a single, constant amplitude magnetization wave along a 〈100〉 direction but inconsistent with Hc changing the relative amplitudes of three coexisting magnetization waves. Other crystals, including higher-purity samples, seem to exhibit a large residual tetragonality for Hc=0, suggesting that a perfect crystal of chromium might show full tetragonality even when cooled in zero field.
Initial Susceptibility of Iron and Iron Alloys Just Above Their Curie Temperatures
Authors: Noakes John E. , Tornberg Neal E. , Arrott Anthony
Published: April 2005

The volume susceptibility of Fe and dilute alloys of Fe with various solutes have been measured from within 0.05°K of the Curie temperature Tc to 15°K above Tc. The results have been analyzed using the equation χv−1 = [(T−Tc)/T1]γ. The results yield a value of γ = 1.333±0.015 for Fe and its dilute alloys. For Fe T1 = 1.27°±0.01°K, T1 appears to vary rapidly with solute concentration for some solutes. The measurements do not show any temperature dependence of γ within 15°K of Tc.
Magnetic Behavior of Thin Films of Nickel and Gadolinium Near the Curie Point
Authors: Livesay B. R. , Jordan L. K. , Scheibner E. J.
Published: April 2005

Magnetization measurements have been made on thin films of nickel and gadolinium near the Curie point using a torque technique. The films were prepared by evaporation in pressures of 10−7 and 10−8 Torr. Most of the measurements were made with a torsion magnetometer and the others with an automatic torquemeter system. The relative magnetization σ near but below Tc is described by σ = D(1−τ)f, where τ = T/Tc. The value of f is obtained by using the procedure of Callen and Callen. Plots of σ(1−f)/fdσ/dτ for nickel films were flat for f = ½ for temperatures 0.90<τ<0.95 and peaks for f = ⅓ are observed at lower temperatures. Gd films gave f = 0.6 for temperatures 0.8<τ<0.9 and f = ½ for τ>0.9. The ½-power behavior is in agreement with molecular-field theory and the Landau theory for second-order transitions.
Structure of Domain Walls in Multiple Films
Authors: Slonczewski J. C.
Published: April 2005

We derive an approximate solution of the micromagnetic equations for a Néel-type wall in multiple films. A small normal component of magnetization M permits most of the pole distribution to concentrate on the film surfaces. Thus, the magnetic-field energy resides mostly in the nonmagnetic gaps between magnetic films. The wall structure is found to consist of two ``wings'' having little exchange energy which flank a ``kernel'' in which only exchange and anisotropy energy terms are significantly involved. The kernel makes secondary contributions to the total width and energy of the wall. In first order, the wing shape is given by Mx = M0 exp{−[(K/πDb)½ |x|/M0]}, where K = anisotropy, b = thickness of nonmagnetic layer, x = distance normal to the wall plane, and D = one magnetic layer thickness for a double film or one-half of a thickness for an infinite number of layers. The first-order energy per unit area is 2M0(πKDb)½. This energy is lower than previous results and extends the region of stability for Néel walls (as opposed to Bloch walls) to greater D, especially for small b.
Composite Films as a Domain-Wall Barrier
Authors: Hunt R. P. , Jaecklin A. A.
Published: April 2005

The use of multilayer magnetic films to constrain domain walls to localized regions is discussed. In particular, films which consist of a composite of hard and soft films are investigated; the soft film is continuous, whereas a window is provided in the hard film.

It is demonstrated that the edge of a hard film (Hchard≃20 Oe) provides an effective barrier for domains propagating in a window of soft Permalloy film (Hcsoft≃2 Oe). This is an alternative technique which offers a number of advantages over tapered edges, alloy compositional variations substrate roughening, and masked evaporation. Specifically, electrodeposition combined with photographic masking facilitates fabrication.

An expression for the effective ``punch-through'' field required to drive a domain wall into the composite film is derived. It is concluded that the ``punch-through'' field Hpt is approximately given by Hpt≃(Hncomp+2Hcsoft)/3,where Hncomp is the nucleation field of the composite film and Hcsoft is the coercive force of the soft film. Measurements on the ``punch-through'' fields from magneto-optic and Bitter pattern observations are presented.
Domain-Wall Structures in Magnetic Double Films
Authors: Middelhoek S.
Published: April 2005

Double films consisting of two Ni-Fe layers separated by a nonmagnetic SiO layer show many properties which differ from those known from single films. In the first part of this paper, the energy and the width of walls in double films are calculated. In the second part, the theory is compared with experimental Bitter observations of the walls. Further, domain structures and the reduction of the wall motion coercive force are discussed.
Dependence of Angular Dispersion on Grain Size in Evaporated Permalloy Films
Authors: West F. G. , Simmons C. L.
Published: April 2005

Permalloy films of nearly zero magnetostriction composition were evaporated in groups corresponding to two different thickness ranges. The mean grain size D was made to vary from 150 to 550 Å corresponding to different substrate temperatures. Angular dispersion α50 was found to increase with increasing grain size; but the product of dispersion and anisotropy field, α50 Hk, was found to be nearly independent of grain size from about 200 to 550 Å. No dependence on film thickness t was observed. From ripple theory Hoffmann has predicted that α50 should be proportional to σ2D3/tHk, where σ is an average of the randomly varying torque resulting from the anisotropy energy in each grain. While the experimental results agree with the predicted dependence on Hk, the lack of dependence of α50 Hk on D and t requires modifications to the ripple theory. Possible modifications are discussed.
Variations in Anisotropy Magnitude and Magnetization Direction in Planar Films under Equilibrium Conditions
Authors: Ehresman V. A. , Olson C. D.
Published: April 2005

NiFe films were studied using an rf probe technique to map remanent magnetization direction and local Hk values over the surface. Two probe sizes were used with tip diameters of 0.8 and 0.25 mm. The film area coupled is smaller than the tip size in each case so that the small probe could resolve parameter differences less than 0.1 mm apart. The inhomogeneity in a 1×1-mm square remote from any film edges, (determined by means of 100 measurements uniformly taken on a rectangular grid) was compared with that measured for the entire film. Results indicate that for the 27 films studied values of Hk with a range and frequency consistent with that implied by a Δ90 measurement are very nearly randomly distributed over the entire surface. Deviations from a random distribution occur near edges or other film features which contribute shape anisotropy. Remanent magnetization direction, in contrast, is almost entirely determined by the demagnetizing fields of film features so that directions in the 1×1-mm area do not represent a random sample from a parent distribution taken from the entire film. These data do not support the conclusion that easy axis angle variations in such local areas (remote from edges, etc.) may be greater than that implied by an α90 measurement as suggested by Dove and Long.
Anisotropic Resistivity in Thin Permalloy Films
Authors: Soohoo R. F.
Published: April 2005

The resistivity of normal-incidence Permalloy films of various composition and thickness has been observed to be dependent upon the direction in the plane of the film, relative to the easy axis, in which the resistance is measured. In these measurements, a saturating magnetic field is applied in the direction in which the resistance is measured, thereby eliminating the magnetoresistance effect. The angular variation of the resistance so measured is strongly dependent upon thickness and, for thinner films, may be an order of magnitude larger than the magnetoresistance. Furthermore, it has been found that the resistance anisotropy exhibits the same symmetry as the uniaxial magnetic anisotropy of these films deposited in the presence of a magnetic field. The interrelationship between the magnetic and resistance anisotropy is therefore examined. An automatic apparatus which records the resistivity as a continuous function of angle in the plane of the film is also described.
Theory of Large-Angle Ripple in Magnetic Films
Authors: Harte K. J.
Published: April 2005

A theory of planar fluctuations of the direction of magnetization in a thin film which takes into consideration the nonlinear longitudinal magnetostatic force is outlined, and the resultant ripple is compared with that predicted by linear theories. The longitudinal magnetostatic force arises from the change along the direction of mean magnetization m0 of the component of magnetization M(r) parallel to m0; it leads to a torque that varies as the cube of the ripple amplitude. As a result of this torque, the linear theory breaks down for dispersion δ≳δ1, where δ is the rms angular deviation of M from m0 and δ1 is typically only 1° or 2° for a 1000 Å Permalloy film in zero field and varies as L−½, where 2L is the film thickness. With the inclusion of this force the range of validity of the theory has been extended to δ≲20°. A number of interesting effects have been found, including (1) an effective field in the direction of m0 (and proportional to δ4 for δ>δ1) acting on ripple components with wavelengths greater than 4πL; (2) an effective field 4πm0δ2, acting on shorter wavelength components; (3) as a result of (1), a decrease in the longitudinal and transverse ripple coherence lengths rl and rt (or, depending on viewpoint, a decrease in the longitudinal and transverse cutoff wavelengths); (4) a dispersion δ∝ (Krc)2/5, where K=local anisotropy energy and rc=crystallite size (in the linear theory δ∝ Krc); (5) almost no change in the mean ripple wall spacing as observed by Lorentz microscopy, ∼πrc, provided rc<rl; and (6) a spin-wave reaction torque coefficient in high-speed switching R∝δ10/3 (in the linear theory R∝δ2).
Influence of Jahn-Teller Ions on the Acoustic and Magnetic Properties of YIG
Authors: Gyorgy E. M. , Le Craw R. C. , Sturge M. D.
Published: April 2005

The presence of Mn3+ ions in YIG leads to a large low-temperature acoustic loss and to a reduction in sound velocity. In the range of 1 to 15 Mc/sec, these effects increase rapidly with decreasing temperature. In addition, the presence of Mn3+ also gives rise to a large microwave loss which peaks at 37°K for 15 kMc/sec and at 58°K for 56 kMc/sec. In the range of 0 to 1 wt % Mn additions, both acoustic and microwave losses increase monotonically with Mn content. The observed effects are attributed to relaxation between the three possible directions of the Jahn-Teller distortion. At high temperatures the relaxation time becomes characteristic of thermal activation over a potential barrier. At low temperatures the relaxation time becomes independent of temperature which is characteristic of quantum-mechanical tunneling. A simple model gives a reasonable description of the observed results in YIG, and a comparison with Mn3+ in YAG or Ni3+ in corundum indicates that the remaining discrepancies may be attributed to the exchange field and magnetostrictive effects. The latter are substantially enhanced by the presence of Mn.
Iron-Iron Exchange Resonance in Ga-Substituted EuIG
Authors: Richards P. L. , Remeika J. P.
Published: April 2005

The iron-iron exchange resonance in the garnet system Eu3Fe5−xGaxO12 has been studied as a function of x at 4.2°K. In zero field, v=200 cm−1 at x=0 and decreases to zero near the angular momentum compensation point x=1.2. It then rises to a maximum of 9 cm−1 at x=1.35 and falls for larger x. Measurements in a magnetic field indicate a spin flop at the magnetic moment compensation point x=0.75, and an anomalous dispersion in the g factor near x=1.2. These results are in good agreement with a two sublattice model of EuIG for x<1. The tetrahedral-octahedral iron exchange molecular field constant λ=500 kOe/μB.
Magnetization of Holmium Iron Garnet at 4.2°K
Authors: Allain Y. , Bichara M. , Herpin A.
Published: April 2005

Magnetic measurements have been made on a spherical single crystal of holmium iron garnet at 4.2°K, in fields of up to 44 kOe. The sample was mounted in such a way that, by rotation about a 〈110〉 direction, any direction lying in the plane perpendicular to the axis of rotation could be brought parallel to the applied field and held there. An extraction method was used for these measurements.

In any direction, domain movement is observable only below 6 kOe; above this the magnetization varies linearly: M (H) = M (0)+χH. M (0) has a flat maximum about the direction of easy magnetization 〈111〉. The directions 〈110〉 and 〈100〉 are directions of difficult magnetization where M (0) exhibits very sharp minima. We find that M (0) varies proportionally with the cosine of the angle between the 〈111〉 axis and the direction of the field. This is in conformity with the results of neutron diffraction experiments. The maximum value of M (0) gives a value for the resultant magnetic moment of 13.6 μB per formula unit Fe5Ho3O12. The differential susceptibility in this direction is equal to 0.02 μB/kOe=1.27×10−4/g, and is larger in the other directions. The canted arrangement of the moments of the holmium ions is thus sensitive to the field in this temperature range.
NMR Study of Iron Sublattice Magnetization in YIG and GdIG
Authors: Litster J. D. , Benedek G. B.
Published: April 2005

The 57Fe NMR frequencies in the two iron sublattices of YIG and GdIG have been measured accurately at 5° to 15°K intervals from 4.2° to 500°K. We have also measured the pressure dependence of the 57Fe NMR frequencies in YIG to 10 kbar at 196°, 273°, 299°, and 346°K. In YIG, we find the pressure dependence of the critical temperature to be ∂Tc/∂P = 0.69°±0.02°K/kbar. The results of the pressure measurements have been used to correct the NMR frequencies for thermal expansion. From these data the temperature dependence at constant volume of each sublattice magnetization has been determined to about 0.01% precision. A spin-wave analysis of the low-temperature data gives the dispersion constant for the acoustic spin-wave mode as D=31.8±1 cm−1. Our NMR measurements of the iron sublattice magnetizations in GdIG have been combined with total magnetization data to obtain the Gd sublattice magnetization up to 0.9 Tc. The Gd magnetization can be fit quite well by a molecular field calculation and yields estimates of the exchange constants to be Jcd=−3.46 cm−1, Jca=−0.22 cm−1, and Jcc=−0.13 cm−1.