bluuebunny/crossref_metadata_2025 huggingface dataset:
55Mn Nuclear Acoustic Resonance in Antiferromagnetic RbMnF3
Authors: Merry J. B. , Bolef D. I.
Published: February 2004

An intense, frequency-dependent absorption of ultrasound in antiferromagnetic RbMnF3 has been observed and attributed to resonant phonon coupling to the 55Mn nuclei. At 4.2 K, the maximum resonant absorption using longitudinal waves at ν0=650 MHz was >40 dB/cm and was accompanied by a dispersion (shift of the acoustic phase velocity) of greater than 1%. Both the field dependent and field independent modes were seen. The angular dependence of nuclear acoustic resonance (NAR) absorption has been studied at 4.2 K for H0> (2HeHa)1/2. For k ∥[100] and H0 in either the (001) or(011̄) plane, the NAR absorption for the field dependent mode vanished for φ=nπ/2 where φ=(k, H0). For φ≅π/4 the absorption was a maximum. Qualitatively, the same angular dependence was observed in a second crystal of RbMnF3, for k ∥[110] and H0 in the (100) plane. NAR absorption for the field independent mode showed a different angular dependence. For k ∥[100] and H0 in the (001) plane, NAR absorption was a maximum for φ=π/2 and vanished for φ=0. The observed nuclear acoustic interaction is well described by a theory due to Fedders.1 In this theory, phonons are coupled to the electronic spins by a magnetoelastic interaction. The electronic susceptibility is calculated by a random phase approximation for the coupled electronic and nuclear spins. The NAR dispersion calculated by Fedders agrees well with experiment.
Magnon-Phonon Interactions in MnF2
Authors: Melcher R. L.
Published: February 2004

The elastic constants c33 and c44 associated with longitudinal and transverse wave propagation along the [001] axis in MnF2 at 4.2°K have been measured as a function of the applied magnetic field H0 ∥ [001]. The results are in good agreement with linear magnetoelastic theory and yield a value for the magnetoelastic coupling constant of (5.4±0.5)×106 ergs/cm3. In the neighborhood of the spin-flop transition the behavior of c33 is discussed in terms of a possible mechanism involving higher-order processes arising from volume magnetostrictive coupling.
Anomalies in the Compressional and Shear Properties of Hematite in the Region of the Morin Transition
Authors: Liebermann Robert C. , Banerjee Subir K.
Published: February 2004

Compressional (vp) and shear (vs) wave velocities have been measured in a hot-pressed (ρ=5.254 g/cc) polycrystalline hematite (α-Fe2O3) as a function of temperature from 200° to 300°K using the pulse superposition method of McSkimin. Both vp and vs exhibit anomalous behavior in the region of the Morin transition (TM≅253°K), but the effect is much more pronounced for vs than for vp; vs at TM is changed by less than 0.02% by performing the experiment in an applied field of 650 Oe. In order to test whether the observed anomalies in vp and vs have their origin in domain wall-stress interaction, vp was measured along the trigonal (c) axis of a high-purity single crystal kindly lent by Y. Shapira. A sharp increase (Δvp=0.2%) was observed as the crystal was cooled through TM. In the absence of an applied field, TM increases by 3.6°K/kbar; this acoustic determination of (∂TM/∂P) is in agreement with that determined from neutron diffraction and NMR studies but differs from that measured by Kawai and Ôno. The vp jump at TM did not vanish when the experiment was repeated with a field of 650 Oe applied in the c plane; this is in marked contrast with the behavior of the Young's modulus (E) anomaly across TM observed by Makkay, Geiger, and Fine. We conclude that the vp anomaly is intrinsic to the saturated crystal.
Measurements of Longitudinal Elastic-Wave/Spin-Wave Conversion Efficiencies in an Axially Magnetized YIG Rod
Authors: Hu H. L. , Rezende S. M. , Morgenthaler F. R.
Published: February 2004

We present direct measurements of the longitudinal elastic/spin-wave conversion efficiencies in a single-crystal axially magnetized YIG rod. Past experiments were carried out on YIG rods axially magnetized along a principal crystallographic direction (for which the longitudinal splitting vanished) and were therefore limited to shear waves. To obtain nearly maximum longitudinal splitting, our experiments were performed on a single-crystal YIG rod with the symmetry axis oriented in a (100) plane, at an angle of 22.5° to a 〈100〉 axis. For this case defocusing effects can be prevented and the longitudinal critical field gradients are at least two orders of magnitude smaller than the shear gradients. Therefore for the first time we were able to measure the conversion efficiencies over a range of gradients that included the region of weak conversion. The room-temperature experiments were performed at L band frequencies using both magnetic- and acoustic-wave injection; the results are in fair agreement to the theoretical predictions of previous authors.
Effect of Magnetic Ordering on the Thermoelectric Power of Concentrated Au(Fe) Alloys
Authors: Cannella V. , Mydosh J. A. , Kawatra M. P.
Published: February 2004

We have measured the temperature dependence of the thermoelectric power S(T) for eight samples of Au(Fe) alloys with concentrations c up to 22 at.% Fe in the temperature range 4°–300°K. S(T) for all samples is always negative and shows a maximum in | S | between 80° and 110°K. For samples with c>12% Fe there are well-defined transitions in the behavior of S(T) at temperatures agreeing with the ordering temperatures as determined by other measurements, whereas no distinct transitions are observed for c<12%. These two concentration groups also differ in the concentration dependence of the magnitude of S(T) and in the temperature dependence of the position of the maximum of | S |. At low temperatures we find that S(T) follows a behavior: S = AT3/2+BT5/2 which suggests a magnon drag contribution to S(T).
Spin Scattering and Magnetic Ordering in EuB6
Authors: Bachmann R. , Lee K. N. , Geballe T. H. , Menth A.
Published: February 2004

Magnetic and transport measurements in ferromagnetic EuB6 show (a) evidence for a Fisher-Langer-type resistance anomaly near Tc, (b) that EuB6 may not be a simple ferromagnet, and (c) that the magnetic coupling might be influenced by a temperature-dependent conduction electron concentration.
Spin Wave and Critical Fluctuations in Magnetite
Authors: Collins M. F. , Saunderson D. H.
Published: February 2004

The long-wavelength spin dynamics of the ferrimagnet magnetite (Fe3O4) have been investigated at and below the critical temperature Tc. The magnetic fluctuations were observed in zero magnetic field near the (111) Bragg peak by triple axis neutron spectrometry. Well below Tc the dynamics can be described in terms of spin-wave excitations whose energy is Dq2. Over the temperature range 0.6>1 - T/Tc>0.01, D is found to vary as (Tc - T) to the power 0.265±0.03. Within about 1% of Tc the spin-wave lines broaden rapidly and become over-critically damped. At T = Tc there are no signs of spin-wave excitations and the energy width of the neutron scattering function varies as q to the power 1.65±0.20 over the range 0.05≤q≤0.16 Å−1.
Magnetic Scattering of Phonons in the Thermal Conductivity of MnCl2·4H2O
Authors: Rives J. E. , Walton D. , Dixon G. S.
Published: February 2004

The thermal conductivity of the antiferromagnet MnCl2·4H2O has been measured from 3° to 0.25°K with particular attention to the region of the Néel point (TN = 1.62°K). The effects of magnetic scattering are separated from the thermal resistance arising from nonmagnetic processes by comparing the thermal conductivity measured in a magnetic field of 40 kOe with that measured in zero applied field. It is found that in the paramagnetic phase there is no critically enhanced phonon scattering associated with the magnetic ordering transition. In the ordered phase the magnetic contribution to the scattering has a maximum near 0.75°K and diminishes as the transition is approached. Detailed comparison with theoretical predictions is made, and the relationship of this work to recent measurements of thermal expansion and specific heat is discussed.
Critical Scattering from Cobalt Fluoride
Authors: Buyers W. J. L. , Cowley R. A. , Martel P. , Stevenson R. W. H.
Published: February 2004

The scattering of slow neutrons by the critical fluctuations of the z component of the magnetic moments in the antiferromagnet CoF2 has been measured at fourteen temperatures both above and below the Néel temperature TN. The most unexpected feature of the results is that in contrast to similar measurements on MnF2 and RbMnF3 no inelasticity of the scattering was observed at any temperature. This implies long-lived magnetic fluctuations (>1.6 × 10−11 sec) at temperatures up to 2.1 TN, and is probably associated with the occurrence of the fairly well-defined spin waves observed earlier. The results for the intensity of the scattering as a function of wave vector transfer are compared with recent theories of the critical effects in the Ising model.
Production and Observation of a Nuclear Antiferromagnetic State
Authors: Chapellier M. , Goldman M. , Chau Vu Hoang , Abragam A.
Published: February 2004

In most diamagnetic substances the strength of dipolar interactions between nuclear spins is such that temperatures of the order of a microdegree or less must be obtained for cooperative effects to take place. Rather than cooling the whole sample we have cooled directly the nuclear spins of 19F in CaF2, which are thermally isolated for the duration of the experiment by the weakness of the spin-lattice relaxation mechanism. The cooling takes place in two steps. First, the spins are polarized in a high field by the ``solid effect'' to an equivalent temperature of the order of 5 mdeg. Second, they are demagnetized in the rotating frame reaching a temperature below one microdegree. For negative temperatures their transverse susceptibility as a function of initial polarization exhibits a plateau characteristic of antiferromagnetic behavior. Results are in qualitative agreement with a molecular field theory.
Experimental Evidence for Anisotropic Interactions in Some Rare-Earth Intermetallic Compounds
Authors: Bécle Christian , Lemaire Remy , Paccard Dominique
Published: February 2004

We have studied the magnetic properties of the rare-earth alloys RAl, RNi, and R3Ni, with R = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, in high magnetic fields up to 70 kOe. For the three series, the magnetic properties differ according to the alloyed rare earth. The compounds RAl of DyAl type and RNi of FeB type are ferromagnetic or antiferromagnetic. The compounds R3Ni exhibit metamagnetism, except Tm3Ni which has a ferromagnetic behavior. We have studied the magnetic structure of NdAl, TbAl, HoAl, ErAl, TmAl, NdNi, ErNi, HoNi, and Er3Ni by neutron diffraction. They have all a noncolinear magnetic structure except NdNi, in which the magnetic moments are parallel. We have studied the stability of the magnetic structures by means of group theory; they are all stabilized by strongly anisotropic magnetic interactions between rare-earth atoms.
High-Pressure Suppression of the Magnetic State of V2O3: 51V Nuclear Resonance at 4.2°K and 65 kbar
Authors: Gossard A. C. , McWhan D. B. , Remeika J. P.
Published: February 2004

The metal to insulator transition of V2O3 proceeds from a low-temperature antiferromagnetically ordered insulating state1 to a high-temperature metallic state. Pressures greater than 26 kbar completely suppress the metal-insulator transition,2 leaving V2O3 metallic to the lowest temperatures. This allows study of the metallic-phase magnetic properties at low temperature. Here we report high-pressure 51V nuclear magnetic resonance (NMR) measurements in V2O3 which show that the metallic phase, unlike the insulating phase, does not order magnetically down to 4.2 K. The measurements were made with spin echo and free induction decay techniques at a frequency of 6.9 MHz in a cryogenic press with a nonmagnetic high-pressure die, and are the highest pressure resonance measurements which have been made at cryogenic temperatures. The 51V NMR was observed at pressures greater than 26 kbar with a linewidth of 300 G and a Knight shift of (− 1.0±0.2) % + (0.01±0.005) P% (P in kbar). At lower pressures the resonance intensity decreased rapidly, evidently as antiferromagnetic ordering produced larger frequency shifts and moved the resonance out of the observable range. The linewidth value implies that no static configuration of vanadium antiferromagnetism with moments larger than 10−3 μB exists in the metallic phase, while the Knightshift value shows that no space-average spin magnetization greater than 0.5×10−3 μB per vanadium atom exists under the experimental conditions. It is concluded that there are no static localized moments in metallic V2O3. Any fluctuating moments would have to fluctuate at frequencies ≥1013 sec−1 to account for the 4.2 K relaxation rate of ≤200 sec−1. This is nearly a band frequency, indicating that metallic V2O3 is best described by an exchange-enhanced band picture. The pressure dependence of the Knight shift implies that the d-spin component of the susceptibility is unusually strongly dependent on volume, with dlnχd/dln V = 8±5. This strong volume dependence, together with a correspondingly anomalous volume dependence of the resistivity,2 suggests that metallic V2O3 is very nearly critical with respect to formation of an insulating paramagnetic state. Indeed, alloying of ≳1% of Cr with V2O3 produces a first-order transition at T>170 K to a paramagnetic insulating phase.3 This transition upon Cr alloying involves neither a lattice symmetry change nor magnetic ordering and has all the features of a Mott metal-insulator transition.
Magnetic Moment Distributions for 3d-Transition Metal Impurities in Cobalt
Authors: Cable J. W. , Hicks T. J.
Published: February 2004

Neutron diffuse scattering measurements were made on Co alloys containing 5 at.% V, Cr, Mn, and Ni impurities in order to determine the distribution of magnetic moment around the impurity sites. The measurements were made at room temperature on polycrystalline samples with 1.09-Å neutrons. The observed cross sections are of two distinct types. For the alloy containing Ni impurities, the cross section is independent of the scattering vector and indicates that Ni impurities produce no moment disturbances on surrounding Co atoms. With V, Cr, or Mn impurities the cross section decreases with increasing scattering vector in a manner which indicates that these impurities produce moment disturbances on their Co neighbors out to distances of approximately 5 Å. All three of these metals produce a net moment reduction when added to Co. For V and Cr this is mostly due to the negative disturbances produced on the neighboring Co atoms. For Mn impurities, an oppositely aligned Mn moment makes an equally significant contribution to the observed moment reduction.
Giant Moments in Paramagnetic Ni–Cu Alloys
Authors: Kouvel J. S. , Comly J. B.
Published: February 2004

For Ni–Cu alloys on the paramagnetic side of the critical composition (∼44 at % Ni), the variation of the initial susceptibility with temperature is found to be resolvable into a Curie-Weiss term Ccw/(T−θ) and a temperature-independent term χ′. In conjunction with the high-field magnetizations measured at 4.2°K,1 the values for Ccw reveal the existence of large superparamagnetic units whose number per atom falls rapidly from ∼0.3% (for the 44% Ni alloy) to ∼0.01% (for 32% Ni), but whose moment is essentially constant at ∼10 μB over this composition range. Thus, the giant polarization clouds recently discovered in the weakly ferromagnetic alloys (50%–46% Ni) by neutron diffraction2 are shown to persist in the paramagnetic alloys. These results, including the smooth variation of χ′ through the critical composition, indicate that critical exchange enhancement in Ni–Cu is manifested in the spontaneous nucleation of magnetic polarization clouds at local, highly Ni-rich regions, and not in the bulk ferromagnetism, which arises from the coupling between neighboring clouds.
Antiferromagnetism in Narrow Band Solids
Authors: Sokoloff J. B.
Published: February 2004

The existence of antiferromagnetism is investigated in the single-band Hubbard Hamiltonian in the limit of bandwidth much less than intra-atomic Coulomb interaction of electrons. We make use of the canonical transformation and ``spectral decomposition'' of the electron creation operators proposed by Harris and Lange, to write down a Green's function which describes electrons in the lower of the split bands of Hubbard's solution. The equation of motion is solved using the moment-conserving decoupling approximation of Tahir-Kheli and Jarrett [R. Tahir-Kheli and H. S. Jarrett, Phys. Rev. 180, 544 (1968)]. We find within our approximation that it is impossible to have an antiferromagnetic state for other than one electron per site. To remedy this defect of the single-band model we investigate a simplified two-band model in the limit of intra-atomic Coulomb and exchange interaction much greater than the bandwidth, and find that antiferromagnetism is possible for the two nearly half-filled bands. We also discuss effects of the antiferromagnetic ordering on the conductivity in our simplified model, and discuss applicability of the theory to real transition metals and transition-metal oxides.
Collective Electron Theory of the Metal-Semiconductor Transition in Magnetite
Authors: Cullen James R. , Callen Earl
Published: February 2004

The Verwey transition in magnetite is described on a band model, with the electrons self-consistently breaking the symmetry by ordering in the self-induced Coulomb potential. The order parameter is the difference in occupation numbers on alternate sites. There is a gap in the single-particle excitation spectrum, and the gap and order parameter undergo a second-order phase transition. The theory is BCS-like, with the Verwey temperature being given in the weak-coupling limit by an exponential in the reciprocal of the density of states at the Fermi level.
Antiferromagnetism in V2O3
Authors: Moon R. M.
Published: February 2004

Neutron diffraction measurements on powder and single-crystal samples of V2O3 have confirmed the existence of long-range magnetic order in the low temperature (monoclinic) phase. Paoletti and Pickart1 discovered an extra diffraction peak at low temperature which they attributed to nuclear scattering. A polarization analysis experiment on a powder sample showed that this peak was magnetic in origin. In addition, several much weaker magnetic peaks were identified. These peaks index in the monoclinic cell given by McWahn,2 with indices satisfying the relation h+k+l=2n+1. The magnetic peaks show little temperature dependence from 77°K up to the crystallographic transition at 170°K, where they vanish abruptly. On cooling, they suddenly reappear at about 154°K. The data agree with a model in which the V moments are ferromagnetically aligned in (010)m layers (perpendicular to hexagonal a axis) with a reversal between adjacent layers. The ordered moment of 1.2 μB per V atom is oriented perpendicular to the hexagonal a axis at an angle of about 71° from the hexagonal c axis.
High-Magnetic-Field Spectroscopy of MnF2 and MnF2:Fe2+ in the Far Infrared
Authors: Blewitt R. , Weber R.
Published: February 2004

Far-infrared transmission measurements have been made on pure and Fe2+-doped MnF2 at 4.2°K using magnetic fields up to 105 kOe. By means of Fourier-transform spectroscopy we have investigated the behavior of the spin-wave modes near the spin-flop transition. Both the AFMR and the two-magnon absorption with the E field perpendicular to the c axis have been measured. In Fe2+-doped MnF2 the spin flop occurs at higher fields depending on the Fe2+-concentration. The localized mode which splits linearly in low fields no longer shows a linear dependence in the spin-flop region.
Temperature Dependence of Magnon-Pair Modes in Antiferromagnets and Paramagnets
Authors: Fleury Paul A.
Published: February 2004

The temperature dependence of magnon-pair excitations has been obtained in the simple antiferromagnets RbMnF3 and FeF2, by the technique of inelastic light scattering. At low temperatures the pair-mode frequency is nearly twice that of a Brillouin-zone boundary magnon, decreased slightly by magnon-magnon interaction effects. As the temperature is increased, the magnon-pair modes broaden and decrease in frequency. The excitation evolves smoothly as T is raised above TN into the paramagnetic phase. In both materials, light scattering from the paramagnetic vestiges of the magnon-pair modes could be observed up to 300°K. Together with earlier results on NiF2, these data indicate the trends in magnon-pair temperature behavior for materials with different spins and different magnon-magnon interaction effects. These trends may be useful in guiding theories for magnon-pair excitations.
Impurity Magnons in MnF2:Ni
Authors: Dietz R. E. , Parisot G. , Meixner A. E. , Guggenheim H. J.
Published: February 2004

Fluorescence from the lowest spin orbital component of the 3T2 manifold of Ni ions in MnF2 is observed to terminate on the ground state 3A2 of the Ni with excitation of 0, 1, and 2 spin deviations centered on the nickel ion and corresponding, respectively, to the crystal ground state (G), and excitation of the s0 and s02 impurity magnon modes. Application of a magnetic field lifts the degeneracy of otherwise equivalent states of ions on different sublattices resulting in splittings of the ground and excited states. g factors and frequencies of these states were obtained by taking to second order the exchange off-diagonal to the Ising-Hamiltonian. gs02 - gs0 and gG - gs0 are predicted and found to have essentially the ionic Ni2+ g factor of 2.33 in agreement with that measured by ESR in ZnF2:Ni; magnetic field broadening of the ir s0f pair mode observed at 168 cm−1 is also as predicted. Frequencies of the three pair modes observed in Raman scattering and far ir absorption together with the s0 and s02 frequencies were computed to an accuracy of 0.5% or better by adjusting the impurity exchange integrals, J1′ = −1.1 cm−1 and J2′ = −3.41 cm−1.
Two-Magnon Raman Scattering in KNiF3
Authors: Chinn S. R. , Zeiger H. J. , O'Connor J. R.
Published: February 2004

Raman scattering of 5145-Å argon laser radiation by two magnons in the perovskite antiferromagnet KNiF3 has been observed. At low temperature, the spectrum of the scattered light is in excellent agreement with a Green's function theory for a perovskite with S = 1 and nearest-neighbor exchange constant J = (71.0±0.8) cm−1. If the effects of anisotropy (obtained from far-infrared antiferromagnetic resonance measurements) are included, the value of J is changed only slightly, decreasing ≈0.5 cm−1. The line shape and position were also observed as functions of temperature. A comparison is made with similar scattering observed in RbMnF3 (S=52) and in related magnetic Ni2+ fluorides.
Electronic Configuration of SmB6
Authors: Cohen R. L. , Eibschütz M. , West K. W. , Buehler E.
Published: February 2004

The unusual magnetic and electrical properties of the semiconductor SmB6 have been closely investigated recently. Primarily from magnetic susceptibility measurements, it has been hypothesized that the Sm valence changes from 3 + (4f5, 6H5/2) at high temperatures to 2 + (4f6, 7F0) at low temperatures. We have used the Mössbauer effect in 149Sm to make a direct determination of the valence state by means of the ``isomer shift,'' which measures the s-electron density at the Sm nucleus. This density changes with the 4f configuration because of coulombic shielding effects. Unfortunately, the change between 4f5 and 4f6 is too small to resolve resonance lines from the two charge states. The observed isomer shift of −0.4 mm/sec lies approximately halfway between the values (∼−0.9 and ∼0. mm/sec−1 respectively) obtained for well characterized ionic di- and trivalent Sm compounds, and is not observed to vary between room temperature and 1.1°K. A new model is presented to explain both the magnetic susceptibility data and the Mössbauer results.
Indirect Exchange in the Rare-Earth Intermetallics
Authors: Levy Peter M.
Published: February 2004

There is no satisfactory fit to the ferromagnetic Curie points of the rare-earth intermetallic compounds having a Laves-phase structure using the Ruderman-Kittel-Kasuya-Yosida exchange interaction. By including a pair interaction of the form LA·SB we are able to improve the fit to the Curie points. This term enters when we include the effects of spin-orbit coupling of the conduction electrons on the indirect exchange interaction.
Numerical Solution of the Behavior of Helical Spin Structures in Applied Magnetic Fields
Authors: Erdös P. , Robinson J. M.
Published: February 2004

The behavior of helical spin structures and the phase transitions they exhibit in applied magnetic fields are studied theoretically.1 Approximate solutions have been developed in the cases of high and low fields.2 We have obtained a numerical solution valid for the case of the planar helix with an arbitrary number of interplanar exchange interactions, an arbitrary form of in-plane anisotropy, and an external magnetic field applied in the plane. We calculated the phase diagram for the case of two interplanar exchange constants and no anisotropy in order to compare with the previous theory and found different behavior in the intermediate field region. As a new result in the case of an in-plane anisotropy the angle dependence of the transition fields has been obtained.
Local Stability, Dielectric Constant, and Spin Susceptibility of Ferromagnetic Itinerant Electrons
Authors: Shimizu M. , Takahashi I.
Published: February 2004

By the perturbation calculation within the Hartree-Fock approximation, the free energy of electrons in metals is expanded as a power series of deviations of electron and spin densities from these in the ferromagnetic state up to the fourth powers. From the expansion, the wavevector-dependent dielectric constant and spin susceptibilities are calculated. It is shown that in ferromagnetic itinerant electrons the external electric (magnetic) field induces not only an electron (spin) density but also a spin (electron) density. The discontinuity of the longitudinal spin susceptibility at zero wavevector in the case of a δ-function type interaction has been explained by the nature of interaction. Coefficients in the expansion are calculated at 0°K in the cases of sinusoidal distributions of electron and spin densities and of a helical distribution of spin density in a paramagnetic gas, where the effective mass approximation and the Hubbard approximation for the exchange terms are used. It is found that the coefficients of the fourth powers of these density waves diverge at a wavenumber 2kf, where kf is the Fermi momentum divided by ℏ.
Giant Moments of Dilute Fe in Binary and Ternary Pd Alloys
Authors: Guertin R. P. , Foner S.
Published: February 2004

Low-temperature measurements have been made of the saturated local moment psat and the ordering temperature Tc of very dilute Fe in various Pd alloys. The alloy systems include Pd1−xAx, where A is Rh, Ag, Pt, and Ni and Pd1−2xRhxAgx (pseudo Pd). Both psat and the high-field matrix susceptibility, χHF, were measured from 1.2° to 4.2°K and with fields to 60 kG. The alloys were fabricated with very dilute (∼1000 ppm) Fe in order to minimize impurity interactions and band splitting effects. The data indicate that the magnitude of psat is not simply proportional to χHF. These results are compared to previous higher temperature measurements of Pd alloys (with 1% Fe). The effects of impurity scattering on psat and Tc are discussed.
New Resistance Minimum Phenomenon in Ferromagnetic Alloys
Authors: Gillespie D. J. , Schindler A. I.
Published: February 2004

The electrical resistivity of Pt–Ni alloys containing approximately 30, 35, 37, 40, 45, and 50 at.% Ni has been measured between 1.4° and 4.2°K. A clear resistance minimum is found for the two ferromagnetic samples, 45 and 50 at.% Ni, at 3.6° and 2.7°K, respectively. Although no minimum was found for the paramagnetic alloys, there was evidence of a turnup at low temperatures. All of the data could be fitted by the equation ρ = ρ0+AT2−B In T. Both coefficients, A and B, increase rapidly as the critical concentration for ferromagnetism is approached from both the paramagnetic and ferromagnetic sides. The application of a 20-kG field to the 50 at.% Ni sample produced no detectable change in the temperature dependence of the resistivity. Ordering of this sample (by annealing at 600°C) yielded a paramagnetic state for which the coefficient A increased by a factor of three and the In T term was no longer observed. Although the T2 term originates from electron-electron scattering, the origin of the In T term is not clear at the present.
Electron-Electron Scattering Effects in the Low-Temperature Lorenz Numbers of Dilute Pd(Ni) Alloys
Authors: Schriempf J. T. , Schindler A. I. , Mills D. L.
Published: February 2004

The thermal and electrical resistivities of Pd(Ni) alloys at concentrations ranging from 0 to 1.0 at.% Ni have been measured at temperatures between 2 and 20 K. The Lorenz numbers Le appropriate to electron-electron scattering have been extracted from the data and are found to be relatively insensitive to Ni concentration. A theoretical calculation of Le has been carried out under the assumption that the entire current is carried by s electrons which are scattered from spin density fluctuations of the electrons in the d band. A consistent interpretation of the data is obtained by employing the local enhancement model, while the uniform enhancement model is found to yield values of Le which decrease too rapidly with increasing Ni concentration to reasonably represent the experimental results. The calculated value of Le is significantly smaller than the experimental value for pure Pd; it is felt this may be due to the neglect of detailed band-structure effects in this work. Lorenz numbers for Baber scattering have also been calculated by using Coulomb interaction screened by the d electrons. It is found that the presence of Baber scattering cannot remove the discrepancy between theory and experiment for pure Pd. The Baber scattering model is also used to calculate the Le for rhenium and gives very good agreement with experiment.
Comments on the Magnetization and Neutron Scattering in Local Moment-Ferromagnetic Alloys
Authors: Silverstein S. D. , Rice M. J.
Published: February 2004

The results of calculations of the magnetization and elastic neutron scattering in local moment-ferromagnetic alloys are discussed and compared critically to other treatments in the literature.
Study of the Ordered Magnetic State of Copper Formate Tetrahydrate by AFMR
Authors: Seehra Mohindar S. , Castner T. G.
Published: February 2004

A brief account of the antiferromagnetic resonance (AFMR) measurements in single crystals of monoclinic Cu(HCOO)2·4H2O has been reported1 and it was shown that a model of weak ferromagnetism of the easy plane type is not consistent with the data. This investigation shows that most of the AFMR and magnetization data can be explained if one assumes that Cu(HCOO)2·4H2O is an antiferromagnet in zero magnetic field. AFMR measurements have been made between 1.4° and 15°K. Resonance transitions have been observed at 9 GHz, from 32 to 36 GHz, from 50 to 80 GHz, and from 107 to 124 GHz in magnetic fields up to 26 kOe. From the angular dependence of the field positions of the resonance transitions a set of ``principal axes'' a″bc″, of the magnetic crystal have been determined. a″, the antiferromagnetic axis, is shown to be 8.5° away from the a-axis toward the c-axis. The AFMR modes and the magnetization have been calculated on a two sublattice model, based on a bilinear exchange interaction, assuming a″ as the antiferromagnetic axis, b as the intermediate axis, and c″ as the hard axis. This calculation is an extension of one by Cinader2 by including the hard-plane anisotropy and an anisotropic, antisymmetric g-tensor. It is found that a magnetic field (perpendicular to a″) larger than a critical field induces a weak ferromagnetic state. The calculation also shows that both antisymmetric exchange and the antisymmetric Zeeman interaction contribute to the effective canting fields. For the c″-axis case the calculations are in good agreement (except for a constant gap in the low-frequency mode above 5.3 kOe) with the data and an effective canting field H′DM=84 kOe. is obtained. For the b-axis the experiment indicates an instability in the antiferromagnetic axis at 5.3 kOe. The calculations also suggest an instability in the antiferromagnetic axis if a second effective canting field, h′DM, is negative, however a quantitative fit to the AFMR modes has not been possible. A simplified calculation for the a″-axis is in fair agreement with some of the a″-axis data. The temperature dependence of the low-frequency zero-field mode is found to deviate substantially from the prediction of the molecular field approximation. A temperature-dependent contribution to the AFMR linewidth of T3.3±0.2 is found. A detailed paper on the experimental results and the calculations mentioned above will be published elsewhere.3
Antiferromagnetic Echoes in CsMnF3
Authors: Hill R. M. , Kaplan D. E. , Herrmann G. F. , Ichiki S. K.
Published: February 2004

We observe 2- and 3-pulse echoes in CsMnF3 at 2°K, by pulsing the antiferromagnetic resonance. The 2-pulse echoes have a short decay time <0.2 μsec while the 3-pulse echo decay is quite long, >1 sec. The 3-pulse echo is thought to result from indirect excitation of 55Mn nuclear spins which are strongly coupled to the AFR mode. The AFR echo shows double resonance effects at all four NMR modes.
Direct Excitation of Two Nuclear Spin Waves by Parallel Pumping in CsMnF3
Authors: Adams Bill T. , Hinderks Larry W. , Richards Peter M.
Published: February 2004

Excitation of two nuclear spin waves, as predicted by Hinderks and Richards, has been accomplished by parallel pumping in CsMnF3 at 988 MHz. This is the first time to our knowledge nuclear spin waves of nonzero wavevector have been excited directly by electromagnetic fields without simultaneous excitation of electronic spin waves or other modes. Wavevectors k from near zero to 105 cm−1 are excited at 4.2° and 1.1°K. Relaxation times are sufficiently long that they can be measured directly from the transient decay, without having to rely on absolute measurement of critical rf fields. Order of magnitude or better agreement is obtained with the theory of Richards on the relaxation rate ηk although the k dependence does not agree at 4.2°K. At 1.1°K, the k dependence is very close to that predicted by theory. The values of ηk at 4.2°K are very close to those inferred by Seavey from more indirect measurements. A second threshold has also been detected at higher powers. This may be the result of nuclear spin and lattice heating which causes first an increase and then a decrease in the nuclear spin-wave frequency.
Covalency, Spin Densities, and Neutron Magnetic Scattering
Authors: Ellis D. E. , Freeman A. J.
Published: February 2004

Effects of covalent spin densities on the neutron magnetic scattering are described using results of UHF-MO-SCF calculations for the (MnF6)4− cluster and for a model three-atom system (Sc2+–F−–Sc2+) in a linear configuration. Full variational freedom was given to all the electrons and all multicenter integrals were included. From the computed spin densities magnetic form factors were calculated including nonspherical terms and core polarization effects. Both ferromagnetic and antiferromagnetic states of the three-atom system were studied in order to investigate possible differences in covalent behavior and the contribution of covalency to measurements of zero-point spin deviation.
Crystal and Magnetic Structure of MnAs0.92P0.08
Authors: Hall E. L. , Schwartz L. H. , Felcher G. P. , Ridgley D. H.
Published: February 2004

The chemical and magnetic structure of MnAs0.92P0.08 has been studied from 10° to 427°K using x-ray and neutron diffraction. Below 440°K MnAs0.92P0.08 is orthorhombic (Pnma; c>a>b), and a refinement of the x-ray data at lower temperatures showed that there was a continuous change of the Mn and As(P) atomic coordinates. The nuclear scattering, calculated using the x-ray parameters, was subtracted from the measured neutron intensities. The resultant magnetic scattering indicated there must be both ferromagnetic and antiferromagnetic spin components. The orthorhombic magnetic space group, Pn′m′a, is consistent with both types of spin components and the observation of the 001 antiferromagnetic peak. A least-squares fit at 200°K gave a ferromagnetic component of 2.0 μB along the c axis and an antiferromagnetic component of 1.6 μB along the a axis. Below 150°K further weak magnetic lines appear, indicating the onset of long period modulations. The Mn moment is consistent with a high spin rather than a low spin, in disagreement with published bulk magnetization data.
Effect of Atomic Ordering on the Magnetic Structure of the MnPd3 Phase
Authors: Krén E. , Kádár G. , Pál L.
Published: February 2004

Neutron diffraction and magnetic susceptibility measurements were performed on the MnPd3 phase in both completely ordered and partially disordered states. The antiferromagnetic configuration of MnPd3 was confirmed in the range of 23–30 at.% Mn. The angle φc of the magnetic moments to the tetragonal axis, which is 0° below 25% and 90° at higher Mn concentrations, was observed to become atomic order-dependent at 25%, increasing from 0° towards 90° with decreasing order. A decrease in the Néel temperature with decreasing order was also observed. The 90° turn in the direction of the magnetic moments is thought to be due to the effect of the nearest-neighbor Mn pairs which are formed either because of the partial disorder of the stoichiometric alloy or owing to the excess Mn atoms in the ordered nonstoichiometric alloys.
Neutron Diffraction Measurement of the Effective Magnetic Moment of Ni2+ in KNiF3
Authors: Hutchings M. T. , Guggenheim H. J.
Published: February 2004

Two main effects reduce the apparent moment associated with a cation in an ordered antiferromagnet. Covalency tends to spread the moment onto the adjacent anions resulting in a loss due to cancellation at the ligand,1 and the nature of the antiferromagnetic ground state gives rise to zero point deviations from fully aligned spins.2 The latter effect has proved very difficult to measure experimentally with accuracy. In principle the moment associated with the cation may be measured using neutron diffraction by extrapolation of the magnetic Bragg peak intensities to forward scattering. The present measurements were carried out on an ideal antiferromagnet, KNiF3, in order to test the current theories of covalency and spin deviations. The magnitude of both effects may be predicted in this compound1,2 as the covalency parameters are known from NMR work.3 From powder diffraction at 4.2°K we find a value of 1.495 (±0.020) for [g〈Sz〉0 f(τ111)/F200] giving a value of 0.851 (±0.050) for the effective value of 〈Sz〉0. The error in 〈Sz〉0 here includes both the random error of the experiment (0.011), and possible systematic errors (0.039) due to the uncertainties in the current values of the Ni2+ g factor, the form factor, and the nuclear scattering lengths. This value for 〈Sz〉0 may be compared with 0.803 calculated using the molecular orbital theory of covalency and spin wave theory, and 0.832 calculated using covalency parameters determined from a configuration interaction approach4 and perturbation calculations of the spin deviation.5 The experiments thus confirm the order-of-magnitude of the theoretical prediction, but the current systematic errors preclude an exact test of the theoretical approach at the present time. The measurements illustrate the difficulties in obtaining precise values of either covalency parameters or the spin deviation using this technique.
Specific Heat of EuO
Authors: Henderson A. J. , Brown G. R. , Reed T. B. , Meyer H.
Published: February 2004

We report measurements of the specific heat C of two single crystals of EuO over the range of temperatures from 0.37° to 4.4°K and in the presence of external magnetic fields up to 13 kOe. The results for these two samples, which were grown from different batches, are consistent within a few percent. The primary contribution to C above 0.7°K is described in terms of spin-wave theory. From an analysis of the data we find J1/kB=(0.76±0.02) °K and J2/J1=−0.11±0.02 in excellent agreement with another determination. Below 0.7°K, the dominant contribution to C arises from the hyperfine splitting of the 151Eu and 153Eu nuclear levels. The magnitude of this contribution corresponds to an effective hyperfine field HN=300±5 kOe in agreement with the value deduced from NMR and Mössbauer experiments.
Low-Temperature Expansion for Impure Heisenberg Ferromagnets
Authors: Hone Daniel , Scalapino D. J. , Silberglitt Richard
Published: February 2004

The thermodynamics of impure Heisenberg magnets have been studied previously within the random phase approximation (RPA) which neglects some important correlations between exchange-coupled spins. To test the accuracy of the RPA and to allow more detailed comparison with experiment we have extended the rigorous Dyson low-temperature boson formalism to impure systems. Spin-wave interactions are treated in Hartree-Fock approximation (equivalent to Dyson's first Born results for the pure system). We find only very small differences from the RPA impurity magnetization results, but there is substantial variation from RPA predictions of the temperature dependence of local mode energies.
Theory of the Magnetoelectric Effect in Polycrystalline Powders
Authors: Hornreich R. M.
Published: February 2004

The relationships between the magnetoelectric (ME) susceptibilities in polycrystalline powders and single crystals are derived for antiferromagnetic magnetic crystal classes possessing orthorhombic or higher symmetry. The powder susceptibilities induced by both parallel and perpendicular ME annealing are calculated in terms of the single-crystal ME susceptibilities. The results indicate that: (a) Measurements of the ME effect in a powder specimen, together with a knowledge of its crystallographic point group in the ordered state, are sufficient to determine its magnetic crystal class. (b) In many cases, the elements of the single-crystal ME susceptibility tensor can be determined, at least in principle, from the powder measurements. The application of these results to potentially ME materials is discussed.
Reduction of Ferromagnetic Exchange with Lattice Dilation in EuO
Authors: Lee Kenneth , Suits J. C.
Published: February 2004

The reduction of the ferromagnetic exchange with lattice dilation has been studied in EuO films. Films deposited at high substrate temperatures (400°–500°C) show bulk-like properties and films deposited at lower substrate temperatures (below 350°C) show as much as a 1% room-temperature lattice dilation with a corresponding 14% blue shift (2.03–2.31 eV) in the room-temperature optical absorption band and 28% reduction (69°–50°K) in the Curie temperature. This lattice dilation causes a reduction in the cubic crystal field splitting of the 5d levels, and consequently a shift to higher energy of the 4f7→4f65t2g transition. The Eu2+–Eu2+ interatomic ferromagnetic exchange is reduced via the Goodenough model which involves overlap of a half-filled 4f level on one cation with an empty 5d level on a neighboring cation.
Magnetic Monopoles: Where Are They and Where Aren't They?
Authors: Fleischer R. L. , Hart H. R. , Jacobs I. S. , Price P. B. , Schwarz W. M. , Woods R. T.
Published: February 2004

The existence of isolated magnetic charges (monopoles) in nature would require profound rethinking not only in elementary particle physics but also in high-energy cosmic-ray physics and astrophysics. Monopoles, whose possible existence was first suggested by Dirac in 1931 and whose properties were later recalculated by Schwinger, would be intensely ionizing particles similar in their effects in passing through matter to relativistic rare-earth ions (or still heavier nuclei). As Parker has noted, only a very minute abundance of monopoles in the galaxy would be necessary to have removed or at least grossly altered the galactic magnetic field on an astrophysically interesting time scale. Solid-state nuclear track detectors have led to a new series of intensive monopole searches: One of these takes ferromanganese deposits from the deep oceans (as a material which would have magnetically trapped monopoles) and exposes them to high magnetic fields sufficient to loosen monopoles and accelerate them into a detection system; the other experiment is designed to utilize natural detectors (mica and obsidian) to record over geological times the paths of massive, penetrating monopoles. The negative results obtained set new and highly restrictive limits on the abundance of monopoles. These limits are such that the mysterious, high-energy cosmic rays (1017−1019 eV) cannot be monopoles as has been suggested, nor can the galactic magnetic fields be seriously altered by the maximum permitted abundance—even over the galactic age. Finally, the flux of monopoles onto the earth over the earth's entire age is <½ monopole/cm2. Qualifications on these claims are indicated in the article.
Rockmagnetism Today
Authors: Banerjee Subir K.
Published: February 2004

Rockmagnetism provides an insight into the origin and stability of Natural Remanent Magnetization (NRM) in rocks, which enables one to decipher the geomagnetic field history, continental drift, and seafloor spreading. NRM in some rocks is due to cooling through the Curie point (TRM) and for others, either due to volume growth from superparamagnetic to single domain size (CRM) or alignment along the field vector in a wet sediment (DRM). However, CRM has sometimes been mistaken for TRM; it is important to distinguish between the two so as to assign the correct age for NRM. NRM may also acquire an antiparallel direction to the ambient geomagnetic field (self-reversal); thus providing an ancient direction 180° in error. Theoretical models for self-reversal appeal to exchange anisotropy, compensation point, and chemical phase changes. Self-reversal in nature, however, has been proved only for a limited number of cases. Although some minerals have large magnetostriction, a direct effect on NRM is questioned. Interestingly, stable NRM has recently been found in nominally diamagnetic minerals with 1% iron content and in antiferromagnets.
Magnetic Properties of Liquid Crystals
Authors: Pincus P.
Published: February 2004

Liquid crystals are characterized by an orientational order of the rod-like (or possible disk-like) molecules of which they are composed. The anisotropic part of the molecular diamagnetic susceptibility gives rise to a coupling between external magnetic fields and the orientational structure. In terms of the Frank elasticity theory we discuss several such effects of a static field, e.g. unwinding of a cholesteric helix, Freedericksz transition. The second part of this review is devoted to a consideration of magnetic resonance as a tool to study both the order parameter and the orientational fluctuation modes in a liquid crystal.
Magnetic Resonance in Metals in the Far Infrared
Authors: Sievers A. J.
Published: February 2004

To measure the surface impedance of metals from 2 to 0.2 mm wavelength, we have utilized a lamellar interferometer and a bolometer detector which operates at 0.4°K. A simple transmission-line sample geometry is used to make the metal samples compatible with our Fourier-transform spectroscopic technique. The applicability of this method to magnetic resonance studies has been demonstrated by the observation of ferromagnetic resonance in nickel in large magnetic fields. An investigation of ferromagnetic Dy and Tb metals has proved somewhat more interesting because in both metals the magnetoelastic interaction energy is extremely large. From the temperature dependence of the ferromagnetic resonance in zero field we have found that the lattice does not follow the precessing magnetization during resonance.