bluuebunny/crossref_metadata_2025 huggingface dataset:
Review of Recent Work on the Magnetic and Spectroscopic Properties of the Rare-Earth Orthoferrites
Authors: White R. L.
Published: February 2004

The rare-earth orthoferrites are a family of canted antiferromagnets which show an unusual variety of magnetic properties. This article begins with a brief review of the ``allowed'' spin configurations compatible with the crystallographic symmetry of the orthoferrite structure and a summary of the experimentally observed spin configurations, spin reorientation temperatures, compensation temperatures, etc. We then review recent research on these materials, grouping most of the recent work into four major categories; studies of the spin reorientation transition, studies of the rare-earth spin ordering, spectroscopic studies directed at determining magnetic interaction parameters, and studies of magnetic domains and domain walls. The spin reorientation process has been established to occur over a finite temperature range as a fourth-order anisotropy, generally small, comes to dominate the orientational behavior in the temperature region where the usually domainant second-order anisotropy passes through zero and changes sign. The spin reorientation has been monitored in a number of RE orthoferrites by several techniques; magnetic torque, microwave absorption, neutron diffraction, and optical spectroscopy. A few of the rare-earth orthoferrites show RE-RE interaction strong enough to cause an ordering of the rare-earth ions at temperatures on the order of 2°–6°K; this ordering process has been documented by all the above techniques plus Mössbauer-effect and magnetic-susceptibility measurements. Spectroscopic measurements, showing the increase in RE ground-state exchange splittings for the configurations stable at the lower temperatures, give a qualitative understanding of the mechanisms causing the spin reorientation and spin ordering processes, and promise detailed quantiative understanding of the unique magnetic behaviors seen in the orthoferrites. The low moment and high anisotropy of the orthoferrites make possible the fabrication of thin plates magnetized normal to the plate. Such plates are semitransparent, and the Faraday rotation through them can be used to study domain structures or can be used as a readout mechanism for memory and logic devices utilizing single small stable domains as the active element. Recent studies of such domains and of domain wall energy and mobility in the orthoferrites are accordingly reviewed. Finally, several investigations are summarized which utilize the orthoferrites as a vehicle for examining general properties of magnetic systems, such as critical-point phenomena.
Domain Wall Dislocation Interactions in RbFeF3
Authors: Levinstein H. J. , Guggenheim H. J. , Capio C. D.
Published: February 2004

RbFeF3 is a transparent ferromagnet (below 87°K) with a large Faraday rotation which permits the direct observation of magnetic domain structures in bulk crystals. An etch-pitting solution has been developed which reveals the dislocation configuration existing in the crystals, and it has been demonstrated that RbFeF3 exhibits sufficient plasticity so that arrays of dislocations can be put into crystals. The role of dislocation subboundaries and slip bands as nucleation sites for reversed magnetic domains will be demonstrated. The effectiveness of subboundaries and slip bands as pinning sites for domain walls when subjected to varying dc and ac magnetic fields are discussed.
Domain-Wall Mobility in Rare-Earth Orthoferrites by Direct Stroboscopic Observation of Moving Domain Walls
Authors: Rossol F. C.
Published: February 2004

Measurements of the temperature dependence of domain-wall mobility in single-crystal plates of Y, Eu, Ho, Er, Tm, and Lu orthoferrites are given in the range from 77°–350°K. Room-temperature values of mobility for all of these orthoferrites fall in the range from 100–1000 cm-sec−1 Oe−1. However, substantial differences in the temperature dependence of the mobility are noted among them. In EuFeO3, the mobility increases from 300 cm-sec−1 Oe−1 at 350°K to almost 3000 cm-sec−1 Oe−1 at 78°K while in HoFeO3 it falls from 230 cm-sec−1 Oe−1 at 350°K to 17 cm-sec−1 Oe−1 at 100°K. Wall mobility is measured using direct stroboscopic observation of a single domain wall whose equilibrium position is modulated by a sinusoidal drive field. Both magnitude and phase of the wall displacement can be measured as a function of frequency. The domain wall is observed by means of the Faraday effect and strobing is achieved with a He–Ne laser beam gated on for a small fraction of the drive cycle by an electro-optic light modulator. The wall mobility is determined from the frequency response of the wall motion which, for these orthoferrites, exhibits a simple relaxation arising from viscous damping.
Helical and Other Stripe Structures in Single-Crystal Permalloy Platelets
Authors: DeBlois R. W.
Published: February 2004

Four primary stripe patterns are observed in Permalloy single-crystal platelets with {100} surfaces in different regions of thickness and composition. All four structures involve magnetic moment rotations between easy 〈111〉 axes parallel to {110} planes. The first to appear with increasing thickness (0.3<D<1.0 μm) has 〈100〉 stripes and 〈100〉 net magnetization. The second and third coexist from 0.8–1.4 μm, have fine 〈110〉 and broad 〈100〉 stripes, respectively, and have different magnitudes of net magnetization. Both involve 71° rotations parallel to vertical {110} planes. The fourth (D>0.9 μm) achieves appreciable internal flux closure by forming a helical structure through rotations toward alternate 〈111〉 directions in passing from top to bottom surfaces at any stripe, and through having the planar surface component in alternate stripes perpendicular to the stripes. Models for 180° and 90° walls readily follow from the above scheme in conjunction with applied-field observations.
Circular Domain Configuration in GdIG
Authors: Nemchik J. M.
Published: February 2004

Circular domains can be formed in GdIG by applying a train of short, fast-risetime pulses of magnetic field. The density of these domains depends on the temperature at which they are formed. After formation, the domain size can be varied without affecting the density by changing the temperature or applying a dc bias field.
Effect of Longitudinal Tensile Stress on the Static and 60 Hz Domain Structures of Grain-Oriented Silicon Steel
Authors: Houze G. L.
Published: February 2004

The Kerr effect was used to observe and photograph domain structures in grain-oriented silicon steel under the influence of longitudinal tensile stresses. In normal, unstressed oriented silicon steel the domain structure is typically a mixture of antiparallel and ``dagger'' or ``pine tree'' domains. The application of uniform tensile stress converts this mixed structure to a completely antiparallel domain structure and reduces the antiparallel domain-wall spacing. The current work shows that the pure antiparallel structure is developed at stresses on the order of 1000 psi. This correlates well with the magnetic property changes associated with the application of uniform tensile stress. The domain-wall spacing refinement is an inverse function of the stress level up to a stress of ≃6500 psi. Higher stresses produced no further change in the domain structure. High-speed motion pictures (≃5000 frames/sec) revealed that the refined spacing was retained during 60 Hz excitation. This effect also would contribute to improved core losses for oriented silicon steel.
Domain Pattern According to Micromagnetics
Authors: Forlani F. , Minnaja N.
Published: February 2004

The fundamental equation of micromagnetics is analytically solved under the following assumptions: the direction of the magnetization vector depends only on one coordinate, the anisotropy energy is uniaxial and of the second degree, the magnetization lies in a plane normal to this coordinate and including the easy axis, the external magnetic field is parallel to the easy direction. One class of periodic solutions is discussed in detail, corresponding to fully magnetized domains divided from each other by means of Bloch walls; for this class every pair of values of the field and of the period selects one and only one solution. Under the consistent assumption that the period is constant during slow changes of the external field, the magnetization curves for different periods are determined. For domains much larger than one ``classical'' Bloch wall, it is possible to define a wall energy per unit area which differs from the ``classical'' formula because of a factor (1+h)½ (h is the ratio between the external field and the anisotropy one). The magnetization vector within the walls points alternatively in opposite directions; solutions with magnetization vector within all the walls pointing toward the same direction belong to another class and are unstable.
Magnetization Distribution around Spherical Magnetic Inclusions in Ferromagnets
Authors: Darby M. I. , Johnson B.
Published: February 2004

Variational calculations of the magnetization distribution ψ (r, θ) about spherical inclusions (magnetic and nonmagnetic) in ferromagnets have been made using the trial function ψ= ∑ n=13 ∑ m=1∞Rmr−man(m)P2n1(cosθ),where the an (m) are coefficients. Numerical results are presented for the approach to saturation for an ensemble of noninteracting hard particles in a soft matrix as a function of inclusion radius R, magnetization I0, and exchange constant A of the matrix. Particular emphasis has been paid to the convergence of the series involved and to values of applied field H for which the work is applicable. Increasing the number of terms in ψ beyond m∼10 can lead to results which violate the assumption of ψ small (the second-order description); such a criterion has been ignored in past work. It is further shown that for β(=2πR2I02/A)>102 the approximations are again violated for H<4πI0; e.g., in Fe with R=200 Å, applicability is limited to H>2×104 Oe and in YIG with R=3×103 Å, H>1.8×103 Oe.
Direct Evidence of the Interaction of Domains with Atomic Scale Imperfections in GdIG
Authors: Nelson Thomas J.
Published: February 2004

Optical observations of the interaction of domains with atomic scale imperfections are presented. Thin transparent {100} orientation wafers of GdIG are prepared from 20 mil-thick slabs by chemical polishing, and in this process etch pits develop which reveal line imperfections in the crystal. These may be individual dislocation lines or other minor structural defects. Domains are both pinned and nucleated at these lines. Interesting cooperative effects occur when a number of lines can be identified in the same sample. It is often observed, for example, that nucleation and initial domain growth take place on the same side of all the lines. In some cases a small residual domain near a line imperfection appears to influence the domain structure throughout the sample, and has the effect of favoring one saturated remanent state over the other. When the residual domain collapses, the effect disappears. Magnetostrictive stress between the domain and the rest of the sample may be responsible for this behavior.
New Experimental Evidence for Spin Fluctuations in Strongly Paramagnetic Alloys
Authors: Fawcett E. , Bucher E. , Brinkman W. F. , Maita J. P. , Wernick J. H.
Published: February 2004

According to theoretical predictions, the entropy of spin fluctuations in a paramagnetic metal with Stoner susceptibility enhancement factor S is (omitting numerical coefficients) ΔS/S0≃lnS+S3T2ln (T/Ts), where S0 is the entropy of the electrons without spin fluctuations and Ts=TFermi/S. The first term gives rise to an enhancement of the specific heat and the thermal expansion at absolute zero. The second term should cause a low-temperature anomaly in both properties. The enhancement of the specific heat has been reported in NiRh and Pd:Ni alloys, and the NiRh system also shows low-temperature specific-heat anomalies. The thermal expansion α is more sensitive than the specific heat C to each of these contributions, approximately in the ratio Δα/α0/ΔCv/Cv0≃1/ γ0∂lnS/∂lnV, where γ0 is the electronic Gruneisen parameter without spin fluctuations. Our magnetostriction data for Pd:Ni and NiRh alloys determines ∂ lnS/∂ lnV, which is proportional to S in accordance with the Stoner expression for the susceptibility. The thermal expansion of the Pd:Ni alloys as temperature approaches zero shows the expected enhancement, and the most strongly enhanced sample shows a low-temperature anomaly. The low-temperature thermal expansion of the NiRh alloys in general increases with Ni concentration, but the behavior is complicated by clustering effects. The ordered intermetallic compound Ni3Ga is found to have a large magnetostriction corresponding to its strong exchange enhancement, but as for the disordered alloys, the low-temperature anomaly in its thermal expansion depends upon the annealing treatment.
Specific Heat of Dilute Solutions of Fe in Cu–Al Alloys
Authors: Zrudsky D. R. , Savage W. R. , Schweitzer J. W.
Published: February 2004

The specific heat of Cu–Al and Cu–Al:Fe alloys containing 0, 5, and 10 at.% Al and 0.07 at.% Fe has been measured in the temperature range 1.2° –14° K. The excess specific heat due to the Fe impurities is decreased with increasing Al concentration. The data are compared with various predictions based on the Kondo model for dilute magnetic alloys. The decrease in the excess specific heat is consistent with the corresponding decrease in the magnetic moment of the Fe with increasing Al concentration determined by high-temperature susceptibility data. A further systematic change in the temperature dependence of the excess specific heat is attributed to an increase in the Kondo parameter TK with increasing Al. The measurements strongly support the hypothesis that the excess specific heat is associated with the Kondo effect, and that interactions among impurities is not the mechanism.
Kondo Temperature Independent of Host Composition for Co Dissolved in Mo–Nb Alloys
Authors: Sarachik M. P. , Knapp G. S.
Published: February 2004

We present evidence, based on resistivity and susceptibility measurements, that the Kondo temperature TK of Mo–Nb alloys containing Co is essentially independent of the host Mo–Nb composition. Since the density of states N(0) of these alloys is known to change by about 20%, the above data indicates that TK does not vary as exp[−1/N(0)J], or that N(0) J must remain constant.
Specific-Heat Enhancement in Pt–Ni Alloys
Authors: Schindler A. I. , Mackliet C. A.
Published: February 2004

In order to determine the effects of electron-spin-fluctuation interactions on the renormalized electron mass in paramagnetic Pt–Ni alloys, we have carried out specific-heat measurements (at 1.15°–4.2°K) on Pt–Ni specimens containing 1.0, 3.4, 7.7, and 12.85 at.% Ni. The experimental results in each case clearly yield a temperature dependence of the specific heat which has the form C(T)=γT+βT3; there was no sign whatever of the T3lnT term predicted by the ``average enhancement'' model. The latter observation, plus the relative diluteness of the alloys, suggests that the ``local enhancement'' model should be applicable. This model suggests that, with increasing Ni content, γ will increase linearly and β will decrease linearly. The experimental results agree well with the former prediction and are consistent with the latter. However, the observed value of (1/γ) (dγ/dc), where c is the Ni concentration, is about 3, whereas a value of about 12 is obtained by using the theory and the experimental value1 of (1/χ) (dχ/dc)∼12. This large discrepancy suggests that the changes in γ and χ for Pt–Ni alloys are not due solely to enhancement effects.
Giant Moments in Ni–Cu Alloys Near the Critical Composition
Authors: Hicks T. J. , Rainford B. , Kouvel J. S. , Low G. G. , Comly J. B.
Published: February 2004

The diffuse scattering of neutrons from Ni–Cu alloys near the critical composition (∼44 at.% Ni) was measured at 4.2°K. For the ferromagnetic alloys of 50−46 at.% Ni (with Curie temperatures of 40°−9°K and spontaneous moments μ0 of 0.06−0.02 μB/at.) the results give a magnetic-scattering cross section (dσ/dΩ) that is unexpectedly large at low scattering vectors (Κ) and decreases rapidly with increasing Κ. If the ferromagnetic moment were distributed equally among all the Ni atoms, the predicted dσ/dΩ would be much smaller and essentially constant over the same range of Κ (∼0.2–0.9 Å−1). Instead, our Ni–Cu results closely resemble those previously reported1 for the ferromagnetic polarization about an Fe impurity atom in Pd and can be interpreted analogously in terms of polarization clouds extending over a few hundred atoms. From the measured values of dσ/dΩ and μ0 it is deduced that the average moment per polarization cloud is ∼8 μB while the concentration of these clouds varies from ∼0.7% (for the 50% Ni alloy) to ∼0.2% (for 46% Ni). Some chemical clustering is also detected in these alloys but of much shorter range. Significant neutron scattering by critical fluctuations is observed for the paramagnetic alloys closest to the critical composition. A fuller report on this work will appear elsewhere.
Spin Waves in Ferromagnetic Metals
Authors: Callaway Joseph
Published: February 2004

Diagrammatic perturbation theory is used to obtain corrections of second order in the particle density to the effective electron-electron interaction appearing in Edwards' theory of spin waves in ferromagnetic metals. A contact interaction model involving a single tight-binding s-like band is employed. When account is taken of changes in the electron self-energy, it is found that there is a high degree of cancellation of the possible corrections. The result is to confirm the validity of the simple t matrix approach in predicting stability of the ferromagnetic state against spin-wave excitations, and in calculating the energy of long-wavelength spin waves. An expression is also obtained for the effective interaction in the low-density limit for a many-band system.
Impurity Excitations in Magnetic Metals
Authors: Wolfram T.
Published: February 2004

The theory of local spin excitations in a ferromagnetic metal described by the simple one-band, strong-correlation model is briefly discussed. Localized spin-wave impurity states of the type found in magnetic insulators are identified with poles or peaks in the local (transverse) reduced-susceptibility function. The connection between local-spin canting in the impurity ground state and low energy local magnons is established. Local ``polar'' spin excitations involving fluctuations in the magnitude of the impurity moment give rise to structure in the local longitudinal susceptibility function. Low-energy polar modes are expected when the impurity ground state is nearly unstable to the formation of a localized moment which differs from that of the ferromagnetic host. The role of the localized electronic state is also discussed.
Theory of Magnetic Excitations in FeCl2 and CoO
Authors: Alben Richard
Published: February 2004

A theory has been developed whereby single-ion effects in a magnet may be treated exactly providing an RPA is made to the exchange interaction. The method is applied to FeCl2 and CoO, both of which have unquenched orbital angular momentum, a factor which might complicate conventional approaches. For FeCl2 the available data on static properties and the AFMR can be explained. In addition to the AFMR, there are many lines predicted, but experiments are lacking. Measurements on CoO can also be accounted for, but there is a serious lack of interpretable data on absorptions to test the theory.
Phonon Energy Spectrum in a Ferromagnet
Authors: Silberglitt Richard
Published: February 2004

The magnetoelastic interaction provides a mechanism whereby, in a ferromagnet, a phonon may couple to two magnons. The propagation of phonons in the crystal may thus be modified by emission and reabsorption of pairs of magnons. Since interactions between the magnons may significantly influence the intermediate state, we have included the dominant low-temperature contributions via the two magnon t-matrix and calculated the phonon self-energy. We find for a phonon with wave-vector q, polarization λ, and energy Ωqλ, Σqλ(Ωqλ)=(|gqλ|2/J) {fB(qλ)+Σ[fi(qλ)/(Ωqλ−ωs)]},where gqλ is the magnon-phonon coupling constant. The functions fB(qλ) and fi(qλ) give, respectively, the contributions from the two spin-wave band and the bound states of energy ωi. In addition to the poles at the true bound states, we find resonances in Σ from fB, corresponding to damped bound (resonant) states of two spin-waves within the band. Near these resonances ImΣ (the line-width) becomes anomalously large, just as does the corresponding function for a single magnon. An interesting result is that, due to the transformational properties of the bound states for q along the [111] direction, longitudinal phonons couple only to the singly degenerate (s-wave) bound state and the transverse phonons couple only to the doubly degenerate (d-wave) bound state.
Theory of Temperature-Dependent Magnon Energies in Antiferromagnets
Authors: Nagai Ojiro
Published: February 2004

A theory of the temperature-dependent magnon energies in antiferromagnets is developed. In this theory, which is based on the spin-wave operator expansion of the Hamiltonian, the dynamical interaction between two spin waves is taken into account in the sense of the random phase approximation. The kinematical interaction is neglected. The present result, which is different from Oguchi and Honma's, agrees with that of Kanamori and Tachiki's theory in the spin-wave temperature region. The magnon energies are computed in a self-consistent manner for the system which includes the exchange and the uniaxial anisotropy energies. The maximum temperature above which no self-consistent solution can be obtained is calculated. The definition of the maximum temperature which is obtained in this theory is found to be different from that previously proposed by Bloch.
Magnetic Field Effects on the Specific Heat of Impure Ferromagnets
Authors: Vogelsang Karl , Hone Daniel
Published: February 2004

Although the change produced in the magnon density of states by dilute impurities in a ferromagnet is proportional to their concentration (at least, away from critical points), the change and its effect even on bulk thermodynamic properties may be fractionally large if it occurs at low energies, where the pure host density of states is small (and the Boltzmann factor suppresses contributions from higher energy states). We have previously considered the specific-heat anomalies from spin impurities characterized by incremental peaking of the density of states near the bottom of the host spin-wave band. The anomalies will be more dramatic if the impurities result in (localized) spin-wave modes below the host spin-wave band, which is possible in the presence of an external magnetic field, if the impurity g-factor (and, therefore, its Zeeman level splitting) is sufficiently less than that of the host. Furthermore, the magnetic field dependence serves to distinguish magnetic anomalies from any others that might arise from impurities. We have carried out a Green's function analysis of the field dependence of the specific heat of an impure Heisenberg ferromagnet, and we find sizable anomalies for fields of the order of 50 kG in a ferromagnet with a Curie temperature of the order of 10°K.
The 1/z Expansion for the Heisenberg Antiferromagnet at Low Temperature
Authors: Harris A. Brooks
Published: February 2004

The analog in the antiferromagnet of the low-density (of spin-wave excitations) expansion for the ferromagnet is the expansion P=ΣPm n[ρ(0)]n[ρ(T) − ρ(0)]m where ρ(T) is the density of spin-wave excitations at temperature T and P is any thermodynamic property. Here ρ(T) − ρ(0)∼T2 and ρ(0)∼1/z, where z is the number of nearest neighbors. We obtain this expansion by expressing momentum dependent quantities in the perturbation expansion in terms of the spin-wave energy, Ek, which in turn is expanded in powers of γk≡z−1Σ exp (ikδ), where the sum is over nearest neighbors, δ. We have calculated terms for (a) the free energy for m+n≤3, m≤2; (b) 〈a+a+aa〉 for m+n≤2; (c) 〈a+a+a+aaa〉 for m+n≤3; (d) the nuclear magnetic relaxation rate T1−1 for n=0, m=2; (e) the renormalized spin-wave energy for m+n≤2, m≤1. To collect powers of 1/z it was necessary to include the effects of repeated anisotropy scatterings. These are the first calculations in which the kinematics of spin operators are preserved. That is, for spin ½ the results are independent of D and 〈(a+)nan〉 vanishes for 2S<n. These results are described more fully in two articles: Phys. Rev. Letters 21, 602 (1968), and Phys. Rev. (to be published).
Green's Function Theory of Parametric Amplification in a Magnetic System
Authors: Madsen Ernest , Tanaka Tomoyasu
Published: February 2004

In an ordered magnetic system the imaginary part of the susceptibility declines at a much lower signal level than is predicted by the usual linear theory of magnetic resonance. This phenomenon is called premature saturation and is due to the parametric amplification of nonuniform spin-wave modes which are degenerate with the uniform mode. In order to explain this phenomenon a model is proposed in which two spin-wave modes are coupled via one- and two-magnon processes. This system is treated by the method of time dependent Green's functions. Even in the lowest-order approximation several types of two-, three-, and four-time Green's functions must be calculated. Relations between these Green's functions are investigated, and then the amplitude of the nonuniform spin-wave mode is calculated through the third order in the amplitude of the applied field. A criterion for the onset of parametric amplification is found and comparison is made with previous semiclassical treatments.
Magnetic Structure of Iron Manganite by Neutron Diffraction
Authors: Boucher B. , Buhl R. , Perrin M.
Published: February 2004

Pure iron manganite FeMn2O4 was prepared at 1250°C and quenched. It crystallizes in the b.c. tetragonal system D4h19. Neutron diffraction measurements above the Curie temperature (120°C) show that the inversion parameter is 0.91 (nearly inverse spinel). The spontaneous magnetization at 4.2°K measured in high static and pulsed fields is 1.55 μB/mol. The magnetic moment was observed to increase linearly above 70 kOe and showed the presence of a strong anisotropy with easy axis near the [101] direction (unusual fact). Neutron diffraction measurements at 4.2°K lead to the determination of the magnetic structure of FeMn2O4. The tetrahedral sublattice moments (4.3 μB) are collinear in a (010) plane making an angle of − 172° with Ox [100]. Spins in octahedral sites (3.1 μB) are divided into four magnetic sublattices with a pyramidal arrangement whose axis of symmetry lies in the plane (010) and makes an angle of 14° with Ox [100]. At 55°K the spins of the octahedral sublattices become collinear, the easy magnetization direction remaining unchanged up to 310°K where it changes to the [110] direction, the magnetic structure of FeMn2O4 becoming completely collinear.
Magnetic Phase Diagram of the UP1−xSx System
Authors: Kuznietz Moshe , Lander G. H. , Baskin Y.
Published: February 2004

The magnetic phase diagram of the UP1−xSx system (0≤x≤1) was investigated in a temperature range from 5° to 300°K by neutron diffraction with powder samples. The types of ordering, ordered moments, and the temperature dependence were determined. The crystal structure of the solid solutions is fcc (NaCl-type). As x increases three types of magnetic ordering are observed: Antiferromagnetic (AF) type-I, AF type-IA, and ferromagnetic. The type-IA ordering consists of ferromagnetic sheets as in type-I, but whereas in the latter the sheets are arranged +−+−..., they are now stacked ++−−.... For x=0 (UP) and 0.02 the order is AF-I. For 0.05≤x≤0.10, the AF-I structure is present at high temperatures but a transition to AF-IA occurs at low temperatures. The temperature of this transition increases with x. For x=0.15 and 0.20 the magnetic structure is AF-IA. The samples with x=0.25 and 0.28 are initially ferromagnetic, but at low temperatures a transition to the AF-IA structure occurs. An intermediate structure probably involving a complicated spin arrangement is also present for x=0.25 before the AF-IA structure is established. For x≥0.33 the ordering is ferromagnetic.
Magnetic Structures and Properties of UFeO4
Authors: Bacmann M. , Bertaut E. F. , Blaise A. , Chevalier R. , Roult G.
Published: February 2004

Mössbauer effect studies, neutron diffraction, and magnetic measurements show that at TN = 55°K the Fe moments of the orthorhombic compound UFeO4 (a = 4.888 Å, b = 11.937 Å, c = 5.110 Å; space group Pbcn-D2h14) order antiferromagnetically along 0y. Below Tc = 42°K a noncollinear structure develops with a ferromagnetic component along 0z and an antiferromagnetic component along 0x. At 4.2°K the moment values, determined by neutron diffraction, are 4 μB for the ferromagnetic and 2 μB for the antiferromagnetic component per iron atom. There is also a measurable antiferromagnetic component of about 0.4 μB on the U atom. UFeO4 is one of the rare examples of an oxide which is a ferromagnetic insulator.
Neutron-Diffraction Investigations on USb, UBi, and U3Sb4
Authors: Olsen Clayton E. , Koehler W. C.
Published: February 2004

Uranium monoantimonide, cubic rock-salt structure, has been found to order magnetically at 246°K with a magnetic structure like that found in UN, UP and UAs, i.e., ferromagnetic sheets aligned oppositely in alternating layers perpendicular to the (001) axis. At 4.2°K Fm(110) = 2.76±0.15×10−12 cm (sinθ/λ = 0.114). Using a 5f3 extrapolation to sinθ/λ = 0, an indicated magnetic moment of 2.64 Bohr magnetons per uranium atom is obtained. Values of μf as a function of sinθ/λ were obtained and indicated a maximum in the region of sinθ/λ = 0.2, implying the presence of large orbital effects. The Néel temperature of 246°K is substantially different from that reported by Polish workers1 of 213°K. In subsequent work on U3Sb4, USb occurred as an impurity, and in this case a Néel temperature of 223°K was obtained, indicating possible compositional-dependent magnetic behavior. UBi was prepared in a tantalum bomb and heat treated at 800°C for 24 h. The diffraction pattern could be indexed in a cubic rock-salt lattice with parameters previously reported for UBi. However, the reflections were broader than normal, indicating a possible distortion of the rock-salt lattice. No evidence was found for magnetic ordering in this sample to 4°K. Polish workers2 earlier had reported a magnetic behavior which was dependent on heat treatment. The discrepancy between this work and other work could be resolved if there were several polymorphic forms of UBi, and the magnetic properties were dependent on structure. U3Sb4 was found to order magnetically at 149°K in agreement with values reported by Polish workers1 of 146°K. At 4.2°K all magnetic reflections were associated with nuclear reflections and could be indexed on the chemical cell of U3Sb4, indicating a ferromagnetic ordering. However, the observed magnetic intensities did not correspond to a simple ferromagnet in which the uranium atom at each of the lattice sites had the same moment. The required moment distribution is currently under investigation.
Magnetic Structures in the α-Mn2O3–Fe2O3 System
Authors: Geller S. , Grant R. W. , Cape J. A. , Espinosa G. P.
Published: February 2004

There are three different magnetic structures in the α-Mn2O3–Fe2O3 system. Magnetic transitions have been determined by measurement of susceptibility vs temperature and by Mössbauer spectroscopy. X-ray powder data on α-Mn2O3 at 6.5°K show no additional lines or symmetry change, implying that the transition at 25°K involves a shift in the symmetry center of the structure, a diffusionless but probably first-order transition; a change in space group is not required. Published neutron diffraction powder data1 on α-Mn2O3 and our own observations lead to the hypothesis that the magnetic space group of both orthorhombic phases is Pcab and of the cubic phase, Ipa3. In the latter, the directions of the ordered spins of cations in the 8b sites are absolutely fixed by symmetry, while those in the 24d sites are constrained to lie in planes perpendicular to the twofold axes. There are no symmetry restrictions on the spins in Pcab, but it is probable that all three magnetic structures are closely related. The subject matter of this abstract is included in a paper entitled ``Magnetic and Crystallographic Transitions in the α-Mn2O3–Fe2O3 System,'' which has been published.2
Neutron Diffraction Study of VF2
Authors: Lau H. Y. , Stout J. W. , Koehler W. C. , Child H. R.
Published: February 2004

A neutron diffraction study of VF2 has been carried out at temperatures between 1.3° and 300°K with powdered and single-crystal samples. Below its transition temperature, 7°K, associated with the development of the long-range magnetic order, this rutile structured compound has a spiral spin structure in which the spins lie perpendicular to the tetragonal c axis and spiral along the c axis with a turn angle of 96.0°±0.5°. There is no detectable change in the magnitude of the turn angle between 4.2° and 1.3°K. Based on the magnetic susceptibility and magnetic anisotropy data of VF21 below 4.2°K and Yoshimori's model,2 we have calculated a value for the turn angle of VF2 98°, in good agreement with the neutron scattering result. The neutron diffraction data between 20° and 77°K confirm that there is a large spin correlation between the near neighbors. The parameter, u, designating the fluorine positions in the unit cell, ±(u, u, 0) and ±(½+u, ½−u, ½), was determined to be 0.308±0.001. Comparison of the scattering angles of the Bragg peaks indicates that the c axis shrinks about 0.5% while the a axis expands about 0.2% when the powdered sample is cooled from 300° to 4.2°K.
Pair Interaction Hamiltonian in Fictitious Angular Momentum Space
Authors: Levy Peter M.
Published: February 2004

We start with the Hamiltonian for the anisotropic electrostatic interaction (direct and exchange) between two magnetic ions and we derive, by using the generalized Wigner-Eckart theorem, a Hamiltonian which represents the interaction between ions in crystal field states. H̃int= ∑ ρ1=0na−1 ∑ ρ2=0nb−1 ∑ P=|ρ1−ρ2|ρ1+ρ2[δ[ρ1](a)×A[P](ρ1ρ2)×δ[ρ2](b)](0). The Hamiltonian has been written in spherical tensor notation; this form permits us to readily calculate the matrix elements of the operators and also to determine whether the interaction contains antisymmetric terms. The upper limit (nα − 1) (α = a, b) to the rank ρi (i = 1, 2) of the fictitious angular momentum operator σ[ρi] is limited only by the multiplicity nα of the crystal field level. The interaction tensor A(P) represents a combination of matrix elements of the interaction Hamiltonian. As the tensors for different crystal field levels are derived from a common Hamiltonian, we readily establish the relations among them. Although the above form of the interaction does not have a particular advantage over more conventional forms for doublet levels (nα = 2), it is very useful for studying the pair interaction between levels of higher multiplicity.
Effect of the Anisotropic Exchange and the Crystalline Field on the Magnetic Susceptibility of Eu2O3
Authors: Huang N. L. , Van Vleck J. H.
Published: February 2004

This paper invokes the combined action of the crystalline field and exchange in an attempt to explain theoretically why the susceptibility χ of cubic Eu2O3 is found by experimenters to be 30% higher than that for the free Eu3+ ion at temperatures below 50°K. The crystalline field is known approximately from optical studies, whereas the coefficient in the exchange coupling is treated as an empirical parameter. This parameter cannot be deduced from a comparison with Gd2O3 as we have shown elsewhere that there are contributions from anisotropic exchange which do not enter in the coupling of S-state ions such as Gd3+, but which do not necessarily vanish for Eu3+–Eu3+ interaction. The crystalline field is found to account for only 42% of the observed enhancement of χ, whereas the isotropic exchange of the type present in Gd2O3 in fact reduces χ by a small amount, thus demonstrating the importance of the anisotropic exchange. The latter needs to be opposite in sign and five times larger than the conventional isotropic exchange in order to explain the observed χ at T = 0°K. Then the temperature dependence of χ calculated with the molecular field approximation agrees with experiment very well.
Anisotropic Exchange between Chromium Ions
Authors: Copland G. M.
Published: February 2004

The exchange interactions between chromium ions in Cr2O3 are studied theoretically using a recent formulation of anisotropic exchange interactions. There is a direct exchange interaction between a Cr3+ ion and its nearest and next nearest neighbors, which is much larger than other indirect interactions in the system. We assume that the neighboring ions are never excited from their orbital singlet 4A2 ground states, and we characterise the exchange interaction by three parameters for each type of neighbor. The full exchange Hamiltonian for this system is presented, and to display the relation between the exchange splittings in different excited manifolds, the problem is solved for the 2E and 4A2 states of the Cr3+ ion at the origin in an octahedral field approximation. The completeproblem, including spin-orbit interaction and the trigonal distortion of the crystal field, does not give explicit algebraic solutions, but could be solved numerically. This analysis shows how the exchange parameters measured in different states of the system are related, provided one assumes that the radial integrals involved are constant between states.
Magnetic Interactions in CeBr3
Authors: Clover R. B. , Birgeneau R. J.
Published: February 2004

Adiabatic susceptibility measurements have been performed on CeBr3 in the helium range between 3.4 and 22.0 MHz. From the measurements we deduce the specific heat at constant magnetization CM / R≅ (0.0274±0.0011) / T2+(0.009±0.005) / T3 and an antiferromagnetic Curie-Weiss constant θ (sphere) = −0.050±0.045°K. These values are combined with the results of EPR spectra to give estimates of the diagonal terms of the nearest- and next-nearest-neighbors interaction tensors for Ce3+ ions in CeBr3 which are compared with those found for CeCl3. The interaction tensors for the effective spins are found to contain terms which arise from high-degree superexchange of the form J+5(1) J−5(2) acting between the real spins. An analysis of the interactions indicates that CeBr3 should correspond well to a weakly perturbed linear chain system with either antiferromagnetic Ising or ferromagnetic XY interactions within the chain.
Electrodeposited Cylindrical Magnetic Films
Authors: Doyle W. D. , Josephs R. M. , Baltz A.
Published: February 2004

The magnetic characteristics of cylindrical electroplated films are considered. Pertinent previous work is reviewed and new data are presented which elucidate the following items: (a) Preparation. The influence of surface roughness is emphasized, and important recent work on surface topology is reviewed. (b) Magnetic Characterization. The use of susceptibility measurements to determine the anisotropy and the ripple amplitude are discussed. When the ripple amplitude is large, previous calculations of the susceptibility based on the linear ripple theory do not apply. Nevertheless, it is possible to use the susceptibility to characterize the anisotropy and dispersion if the susceptibility is described in terms of a phenomenological ripple field picture. (c) Domain Wall Creeping. The possible causes of creep in thick films are considered. From the thickness dependence of the creep threshold, it does not appear that the Olson-Torok model is adequate. Data on the creep velocity are presented. Finally, a model for creep is described which hinges on the distribution of wall motion fields along the wall. (d) Thermal Stabilization. It is shown that by annealing at elevated temperatures, adequate thermal stabilization of plated films can be achieved.
Magnetism in Oligatomic Films
Authors: Gradmann U.
Published: February 2004

The magnetic properties of very thin single-crystal films, consisting of only a few flat, parallel atomic layers, denoted as ``oligatomic films,'' are considered. They are interesting mainly for the dependence of their magnetization and Curie temperature on the thickness, which have been calculated during recent years using the main theories of ferromagnetism. A review is given of the theoretical results, showing strong differences between different theories. Oligatomic films may be used in multilayer experiments to investigate an indirect exchange via the conduction electrons. Methods for preparing oligatomic films of Ni, Fe, Co, and their alloys by epitaxy on Cu single crystals, for testing their structure and for measuring their magnetic properties are discussed. Special attention is given to 48Ni/52Fe (111) films. These were ferromagnetic at least down to 6 Å. Their magnetization as a function of thickness shows agreeable fit to spin-wave and Green's-function calculations. A Néel-type surface anisotropy is detected. Coating the films with Mo instead of Cu affects the magnetization and Curie temperatures of films only slightly, down to 3 layers (6 Å); however, the surface anisotropy is lowered.
Magnetoelastic Waves in 〈110〉 YIG Rods
Authors: Lewis M. F. , Scotter D. G.
Published: February 2004

We have studied the propagation of 1-GHz magnetoelastic waves in a 〈110〉 oriented YIG rod. One-port echoes show an oscillation in amplitude as a function of magnetic field similar to that reported previously by Kirchner. Two-port echoes are very strong and oscillate in antiphase with one-port echoes. The results can be simply interpreted in terms of the elastic anisotropy of YIG. From the period of the oscillations and the internal magnetic field distribution measured by the magnetoelastic resonance technique, we deduce that the elastic anisotropy in YIG is only about two-thirds as great as that measured by Clark and Strakna. This conclusion has been verified by ultrasonic measurements, but the results reveal a small variation (∼½%) in the absolute velocities of sound in YIG of different origin-this may be related to the presence of Pb impurities which cause departures from the x-ray density by ∼½%.
Investigations of Relaxation Processes in YIG with Propagating Magnetostatic Spin Waves
Authors: Burke E. R. , Bhagat S. M.
Published: February 2004

A new method to obtain relaxation times in YIG has been developed by measuring the attenuation and time delay of propagating magnetostatic spin waves. If the dimensions of the sample are properly chosen, a range of applied magnetic field will exist over which the attenuation will increase linearly with time delay. The experimental measurements were performed at room temperature for frequencies between 2 and 4 GHz. The relaxation data expressed in terms of an effective linewidth ΔH, can be fit by the expression ΔH=0.043+0.026f, where ΔH is in oersteds and f is the frequency in GHz. The term linear in the frequency is in good agreement with the result obtained by LeCraw and Spencer using the parallel-pumping method and can be explained by the Kasuya-LeCraw mechanism. A similar process is proposed to explain the constant term except that now the magnon confluence is with a low-energy phonon rather than an optical phonon. This process is allowed since the propagating spin waves are essentially at the bottom of the spinwave manifold.
Magnetostatic Mode Excitation in YIG Rods Containing a Turning Point
Authors: Desormiere B. , Le Gall H.
Published: February 2004

Calculations have been made of the (1, 0) magnetostatic mode excitation in an axially magnetized YIG rod containing a turning point. The rf-field components have been determined within the magnetostatic approximation from the wave equation of the magnetic potential. The nonuniformity of the bias field was taken into account and the boundary conditions were specified at the end face of the rod where the excitation source, taken as a circular loop, was assumed to be situated. An expression for the power absorbed under these conditions was derived. Numerical results have been obtained by means of an electronic computer, taking the bias field and the frequency as parameters. Experimental measurements of the power absorbed by a YIG single-crystal rod were performed in S band, showing a good agreement with the theoretical expectation. Measurements of the losses of an optimized delayed signal are also reported for the magnetoelastic mode of operation. It is shown that the variation of these losses can be attributed for a great part to that of the conversion efficiency of the magnetostatic mode.
Inhomogeneous Broadening of Ferromagnetic Resonance Lines
Authors: Schlömann Ernst
Published: February 2004

A general theory of ferromagnetic resonance is developed assuming that the local, effective magnetic field is inhomogeneous. The scattering processes induced by the inhomogeneity may be classified as ``primary'' processes (which couple the uniform mode to nonuniform modes) and ``secondary'' processes (which couple nonuniform modes). In most previous work only the primary processes were taken into account. The effect of secondary processes upon the susceptibility is calculated in the present paper. A perturbation series for the complex, effective resonant frequency of the uniform mode is derived. It is shown that the important terms of this infinite series can in part be generated by a self-consistency condition for the complex effective resonant frequency of spin waves. An approximate solution of this self-consistency condition is derived. Applied to polycrystals with cubic crystal structure the theory predicts a linewidth of ≃2.07Ha2/4πM0 for spherical samples and Ha≪4πM0 but ≃0.87H0 for Ha≳4πM0. Here Ha is the anisotropy field and M0 the saturation magnetization. The off-resonance absorption is characterized by the existence of a ``strong absorption'' region. When the intrinsic damping of the spin waves is assumed to approach zero the absorption goes to zero in the exterior of this region but to a finite value in its interior. If the Fourier spectrum of the inhomogeneity has significant components only at long wavelength and the inhomogeneity is weak the strong absorption region coincides with the dc field interval in which the signal frequency is degenerate with resonant frequencies of long-wavelength spin waves. With increasing inhomogeneity the width of the strong absorption region increases by approximately twice the width of the resonance line.
Microwave Phonon Generation Utilizing Epitaxial YIG Films on Nonmagnetic Garnet Substrates
Authors: Collins J. H. , Wilson D. A. , Pizzarello F. A.
Published: February 2004

The generation of microwave-frequency shear waves by magnetically biased epitaxial films of YIG deposited on YAG and gadolinium gallium garnet (GGG) has been observed and studied through the use of both pulse echo and optical probing techniques. These acoustic transducers are electronically tunable over bandwidths of several octaves by varying the magnetic bias field, and permit single echoes to be selected from delayed pulse trains. The radiation resistance of these YIG films has been measured, using grounded thin antennas deposited on the films, and is found to be on the order of 10 Ω. Insertion losses as low as 30 dB have been observed for 1 μ thick films on YAG substrates. Magnetic linewidths on the order of 1 Oe at 4 GHz have been measured. Oscillatory behavior in the insertion loss vs field characteristics at fixed frequency indicates that discrete magnetostatic modes in the film contribute to the phonon generation. Bragg diffraction of laser light from the acoustic waves in the GGG substrate shows that the acoustic beam profile is a function of magnetic field. The measured strain distribution in the acoustic beam can be used to investigate the magnetization distribution in the YIG film as a function of magnetic field.