Showing papers on "Ferrimagnetism published in 1994"
TL;DR: In this paper, a tutorial of typical magnetic behavior of molecular materials is presented and three distinct models (intramolecular spin coupling through orthogonal orbitals in the same spatial region within a molecule/ion, intermolecular spins coupling through pairwise configuration interaction between spin-containing moieties, and dipole-dipole, through-space interactions) which enable the design of new molecular-based magnetic materials are discussed.
Abstract: Magnets composed of molecular species or polymers and prepared by relatively low-temperature organic synthetic methodologies are a focus of contemporary materials science research. The anticipated properties of such molecular-species-based magnetic materials, particularly in combination with other properties associated with molecules and polymers, may enable their use in future generations of electronic, magnetic, and/or photonic/photronic devices ranging from information storage and magnetic imaging to static and low-frequency magnetic shielding. A tutorial of typical magnetic behavior of molecular materials is presented. The three distinct models (intramolecular spin coupling through orthogonal orbitals in the same spatial region within a molecule/ion, intermolecular spin coupling through pairwise “configuration interaction” between spin-containing moieties, and dipole—dipole, through-space interactions) which enable the design of new molecular-based magnetic materials are discussed. To achieve the required spin couplings for bulk ferro- or ferrimagnetic behavior it is crucial to prepare materials with the necessary primary, secondary, and tertiary structures akin to proteins. Selected results from the worldwide effort aimed at preparing molecular-based magnetic materials by these mechanisms are described. Some organometallic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) that is, …D•+ A•− D•+ A•−…, exhibit cooperative magnetic phenomena. Bulk ferromagnetic behavior was first observed below the critical (Curie) temperature Tc of 4.8 K for [FeIII(C5Me5)2]•+ [TCNE]•− (Me = methyl; TCNE = tetracyanoethylene). Replacement of FeIII with MnIII leads to a ferromagnet with a Tc of 8.8 K in agreement with mean-field models developed for this class of materials. Replacement with CrIII, however, leads to a ferromagnet with a Tc lowered to 3.65 K which is at variance with this model. Extension to the reaction of a vanadium(o) complex with TCNE leads to the isolation of a magnet with a Tc ≈ 400 K, which exceeds the thermal decomposition temperature of the material. This demonstrates that a magnetic material with a Tc substantially above room temperature is achievable in a molecule/organic/polymeric material. Finally, a new class of one-dimensional ferrimagnetic materials based on metalloporphins is discussed.
1,184 citations
Book•
01 Jan 1994
TL;DR: Soft magnetic alloys amorphous ferromagnetic alloys and ferrofluids ferrimagnetic materials permanent magnetic materials as mentioned in this paper are the most commonly used materials for soft magnetic materials.
Abstract: Soft magnetic alloys amorphous ferromagnetic alloys and ferrofluids ferrimagnetic materials permanent magnetic materials.
326 citations
TL;DR: In this paper, an empirical linear dependence of the specific saturation magnetization σs on the specific surface area Sa of the fine crystallites was obtained in the form of σS(S) = σ s(∞)(1 - ASa), where the slope A is different for γ-Fe2O3 and γCo 0.94O3 particles.
270 citations
TL;DR: In this paper, the authors discuss two methods that quantify the contribution of different groups to the AMS: (1) comparative measurements of the magnetic susceptibility in low fields and high fields and (2) heating curves from 77 K to room temperature (low temperature magnetic susceptibility, LTMS).
148 citations
TL;DR: In this paper, the authors have collected samples from two sites representing the range of magnetic susceptibilities to be found in the Chinese loess record and showed that susceptibility enhancement is associated with the concentration of a ferrimagnetic fine fraction.
Abstract: SUMMARY
Various mechanisms have been proposed for the observed link between palaeoclimate and magnetic susceptibility in the Chinese loess. Many authors have recently pointed to the controlling influence that ultrafine (stable single domain/ superparamagnetic) magnetite/maghemite has on the magnetic susceptibility signal. However, there is still no clear evidence as to the origin of this susceptibility-enhancing magnetic fraction. We have collected samples from two sites representing the range of magnetic susceptibilities to be found in the Chinese loess record. Magnetic measurements indicate that susceptibility enhancement, throughout the loess plateau, is associated with the concentration of a ferrimagnetic fine fraction. This magnetic fraction has a uniform, non-variable grain-size distribution which spans the stable single-domain/superparamagnetic boundary. High-field thermomagnetic analysis reveals a trend in behaviour with increasing susceptibility, towards linear, more reversible curves. We propose that both the particular grain size of the enhanced ferrimagnetic fraction and thermomagnetic reversibility, can be explained by size-induced phase transitions in Fe2O3, which results in maghemite being the energetically favoured phase for certain grain sizes. Thus, the production of Fe2O3, which spans this grain-size range, is the only requirement for magnetic enhancement in the loess.
88 citations
01 Jan 1994
Abstract: Magnetic Clusters: Magnetic Properties of Small Transition Metal Clusters in a Molecular Beam (W.A. de Heer, I.M.L. Billas). Magnetic Properties of Transition Metal and Rareearth Metal Clusters (P.J. Jensen, K.H. Bennemann). One and Twodimensional Systems: The Onedimensional Hubbard Model with Attractive U as a Soluble Model for Exciton Bands and Electronhole Droplets (P. Schlottmann). Superlattices and Multilayers: Magnetization Patterns of Exchange Coupled Metallic Multilayers (D. Altbir, M. Kiwi). Surfaces and Interfaces: Spinpolarized Scanning Tunneling Spectroscopy on Fe and Ni (S.F. Alvarado). Alloys and Amorphous Materials: New Trends in Ferrimagnetism (T. Kaneyoshi). Special Techniques and Materials: Lessconventional Magnetic Domain Investigations (M. Schlenker). General Theoretical Developments: Slave Boson Approach to Local Moment Formation in the Hubbard Model (R.M. RibeiroTeixeira, M. Avignon). Concluding Remarks. 39 additional articles. Index.
82 citations
TL;DR: The electrical conductivity of polycrystalline mixed ferrite system having the chemical formula MnxZn1−xFe2O4 was investigated from room temperature to the neighborhood of the Curie temperature by the two-probe method as mentioned in this paper.
Abstract: The electrical conductivity of polycrystalline mixed ferrite system having the chemical formula MnxZn1−xFe2O4 (where x=0.0, 0.2, 0.4, 0.6, and 0.8) was investigated from room temperature to the neighborhood of the Curie temperature by the two‐probe method. The electrical conduction in these ferrites is explained on the basis of the hopping mechanism. Plots of log (σT) versus 103/T are almost linear and show a transition near the Curie temperature. The activation energy in the ferrimagnetic region is in general less than that in the paramagnetic region.
57 citations
TL;DR: In high anisotropy systems the magnetocrystalline anisotropic energy can be comparable to the exchange energy as mentioned in this paper, and the thickness of domain walls can be as small as the interatomic distance (narrow domain walls).
51 citations
TL;DR: The magnetic susceptibility of ferrimagnetic pyrrhotite is determined empirically from the variation trend in a density-susceptibility diagram of pyrhotite-containing rocks as mentioned in this paper.
48 citations
TL;DR: In this article, the authors performed band structure calculations for alpha and beta-Mn with 58 or 20 atoms, respectively, per unit cell, employing the augmented-spherical-wave (ASW) method.
Abstract: Band structure calculations for alpha - and beta -Mn with 58 or 20 atoms, respectively, per unit cell are performed employing the augmented-spherical-wave (ASW) method. For the alpha -phase a collinear antiferromagnetic spin structure is assumed although some experiments suggest a canting of spins. The magnetic moments depend on the local environment of the respective atoms and are found to be in good agreement with experiments. beta -Mn is found to be a Pauli paramagnet but on the verge of ferrimagnetism. Its electric field gradients as calculated by the ASW method are within the experimental range.
40 citations
TL;DR: The magnetic properties of numerous compositions in the ternary CaO•P2O5•Fe2O3 system have been studied in this paper, where a relatively wide glass formation region is located around the P2O 5 corner of the system.
Abstract: The magnetic properties of numerous compositions in the ternary CaO‐P2O5‐Fe2O3 system have been studied. A relatively wide glass formation region is located around the P2O5 corner of the system. The Fe2O3‐containing glasses exhibit paramagnetic behavior. The specimens inside the composition range Fe2O3≥30% consist of both paramagnetic and ferrimagnetic phases.
TL;DR: In this paper, self-consistent band structure calculations were performed for the nitrides PdFe3N and MnFe3Ns, at several lattice parameters, in order to investigate the influence on the magnetic and electronic structure of Fe4N due to the substitution of one Fe atom by another transition metal, as well as to study the behavior of the magnetism of these materials with pressure.
TL;DR: In this paper, fine particles of ferrimagnetic MgFe2O4 were synthesized through a solgel supercritical drying procedure, then cold-pressed and heated at 500 and 800°C.
Abstract: Fine particles of ferrimagnetic MgFe2O4 were synthesized through a sol‐gel supercritical drying procedure, then cold‐pressed and heated at 500 and 800 °C. These materials were characterized by Mossbauer spectrometry in the temperature range 25–298 K, and in the presence of a small applied magnetic field. The Mossbauer measurements show that heat treatment results in the formation of a small percentage of α‐Fe2O3, an increase in the mean particle size and hyperfine magnetic field, and a decrease in the inversion parameter of the magnesioferrite. An average anisotropy constant equivalent to the bulk magnetocrystalline anisotropy is estimated for particles with diameter D=14 nm.
TL;DR: In this paper, the ternary silicide TbMnSi has been investigated by powder X-ray diffraction, susceptibility measurements and neutron diffraction experiments on the tetragonal structure of the CeFeSi-type.
TL;DR: In this paper, a new class of two phased materials, called macroscopic ferrimagnets, displays many properties associated with ferrimagnetism including magnetic compensation points.
Abstract: A new class of two phased materials, called macroscopic ferrimagnets, displays many properties associated with ferrimagnetism including magnetic compensation points. A typical material system which shows this phenomenon is cobalt containing /spl cong/100 /spl Aring/ precipitates of EuS. The negative exchange in these systems is relatively weak so the remanent magneto-optic properties show the ferrimagnetic state, but at fields above 2 to 3 T, alignment of both the Co and EuS with the field can occur. In addition to the usual magnetic compensation point at which the sign of the magneto-optic loop reverses, we observe a second, magneto-optic, compensation point which occurs when the Kerr rotation of the two phases are equal and opposite. >
TL;DR: In this paper, the response of a system of small (∼4 nm) ferrimagnetic particles to a field reversal at low temperature (06 to 12 K) was studied, and the reversal of the particle magnetic moments through the anisotropy barriers was shown to be governed by thermally activated dynamics down to 06 K.
Abstract: We have studied the response of a system of small (∼4 nm) ferrimagnetic particles to a field reversal at low temperature (06 to 12 K) The reversal of the particle magnetic moments through the anisotropy barriers is shown to be governed by thermally activated dynamics down to 06 K, with no evidence for quantum effects We propose a data analysis in terms of the unique variable T-Ln(t/τ 0 ), which accounts for a non-constant distribution of anisotropy barriers
TL;DR: In this article, the transition temperature, compensation temperature, and magnetization curve of a ferrimagnetic multilayer system, consisting periodically of two layers of spin- 1/2 A atoms, two layer of spin 3/2 B atoms, and a disordered interface in between that is characterized by a random arrangement of A and B atoms of ApB1-p type and a negative A-B coupling, are examined.
Abstract: A calculation is presented via the standard mean-field theory for the magnetic properties (transition temperature, compensation temperature, and magnetization curve) of a ferrimagnetic multilayer system, consisting periodically of two layers of spin- 1/2 A atoms, two layers of spin-3/2 B atoms, and a disordered interface in between that is characterized by a random arrangement of A and B atoms of ApB1-p type and a negative A-B coupling. The consequences of the disordered interfaces and different anisotropies between the bulk and interface for the magnetic properties are examined. A number of characteristic behaviours, such as the possibility of two compensation points, are found for the magnetic properties.
TL;DR: The magnetic structure of Nd6Fe13Si has been studied by magnetization measurement, the singular point detection technique and neutron powder diffraction as mentioned in this paper, and it was found that the Nd 6Fe13 Si is ferrimagnetic with a compensation temperature of 357 K and a Curie temperature of 441 K.
Abstract: The magnetic structure of Nd6Fe13 Si has been studied by magnetization measurement, the singular point detection technique and neutron powder diffraction. It was found that the Nd6Fe13Si is ferrimagnetic with a compensation temperature of 357 K and a Curie temperature of 441 K. The spontaneous magnetization values are 12.9 J T-1 kg-1 at 5 K and 7.4 J T-1 kg-1 at 300 K. There are two values B0cr1 and B0cr2 of the magnetic phase transition field below 225 K: B0cr1=6.2 T and B0cr2=8.2 T at 77 K and B0cr=5.3 T at 300 K. The atomic magnetic moments are ordered in an antiparallel manner, and the two magnetosublattices are MI up arrow (8f(Nd), 16l(Nd), 4d(Fe), 16l2(Fe)) and Mn down arrow (16k(Fe), 161(Fe)).
TL;DR: In this paper, a new nitronyl-nitroxide radical, 2-(p-dimethylaminophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, hereafter denoted NIT(p-NMe2)Ph, has been synthesized.
Abstract: A new nitronyl–nitroxide radical, 2-(p-dimethylaminophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, hereafter denoted NIT(p-NMe2)Ph, has been synthesized. It crystallizes in the P21/c, space group, with a= 15.742(2)A, b= 10.875(4)A, c= 19.011(5)A and β= 113.26(2)°, the asymmetric unit containing two independent radical molecules. Its reaction with manganese hexafluoroacetylacetonate, Mn(hfac)2, in the presence of chloroform leads to the solvent-containing compound Mn(hfac)2[NIT(p-NMe2)Ph]·0.25HCCl3, which crystallizes in the C2/c space group, with a= 28.183(4)A, b= 13.899(2)A, c= 23.425(5)A, β= 125.28(2)°. Its structure presents chains of 21 symmetry running along the b, axis, where the manganeses are bridged by the nitronyl-nitroxide radicals. The magnetic properties in the 5–300 K range are typically those of a ferrimagnetic chain where S= 1/2 and S= 5/2 spins alternate regularly. Below 5 K, a three-dimensional (3D) ferrimagnetic ordering occurs. Rationalization of the ferrimagnetic character of Mn(hfac)2[NIT(p-Nme2)Ph]·0.25HCCl3 and of some other Mn(hfac)2NITR chain compounds on the basis of magnetic dipolar energy calculations has been attempted. Results show that, as far as only collinear magnetic structures and localized moments are considered, the dipolar interaction alone cannot explain the type of magnetic order adopted by these compounds. This questions the possible role of other physical parameters, such as single-ion anisotropy, in the 3D ordering.
TL;DR: In this article, the long range order of ternary RFe 5 Al 7 (R = Tb, Dy, Ho, Er, Tm) intermetallics of ThMn 12 -type structure is analyzed by neutron powder diffraction.
TL;DR: In this article, the saturation magnetization and magnetocrystalline anisotropy of fine equiaxed Fe2O3 and Co0.94O3 particles were determined from x-ray line broadening measurements.
Abstract: Fine equiaxed γ‐CoxFe2−xO3 (x=0, 0.06) particles with a diameter ranging from 200 to 1000 A were prepared by chemical precipitation. The average crystallite sizes were determined from x‐ray line broadening measurements. The saturation magnetization and magnetocrystalline anisotropy of the particles were determined by using the approach to saturation. An empirical linear dependence of the specific saturation magnetization σs on the specific surface area Sa of the fine crystallites was obtained in the form of σs(S)=σs(∞)(1−ASa). The slope A which reflects the surface spin canting anomaly is different for γ‐Fe2O3 and γ‐Co0.06Fe1.94O3 particles. Under the supposition of the fine crystallite consisting of two parts, i.e., the surface layer, whose magnetic moment cannot be turned entirely along the direction of the applied field, but makes an average canting angle with the field, and the inner part, whose magnetic moment can be aligned along the direction of the applied field, the above formula can be interpret...
TL;DR: The spinel normal structure of all the compounds under study showed that the lattice parameters decrease with an increase in the Cu concentration as discussed by the authors, and the saturation magnetic moment in comparison with the theoretical one was found to be correlated with the high degree of defects in the crystal structure.
TL;DR: In this paper, the magnetic properties of oxides in a Zn-Fe-O system prepared by the twin-roller quenching method have been examined in order to elucidate the magnetization properties of so-called ferromagnetic amorphous oxides.
TL;DR: In this article, it is shown that when a magnetic coupling of sufficient strength is switched on between the paramagnetic subsystems, the whole system becomes unstable towards a magnetically ordered state.
TL;DR: In this paper, the hyperfine parameters of the tetrahedral and octahedral sites are determined as functions of x and so the cation distributions and spontaneous magnetization are deduced.
Abstract: The ferrite system Co0.6Zn0.4CuxFe2−xO4, 0 ≤ x ≤ 1, is studied by Mossbauer spectroscopy at room temperature, infrared spectra, and mobility measurements. The Mossbauer studies show that the samples are ferrimagnetic for all values of x; the hyperfine parameters of the tetrahedral and octahedral sites are determined as functions of x and so the cation distributions and spontaneous magnetization are deduced. Four bands are observed in the IR spectra. The high-frequency band v1 is assigned to the tetrahedral cations and the low-frequency band v2 to the octahedral complexes. The small band v3 is assigned to the divalent octahedral metal ion. The lowest band v4 is attributed to the lattice vibrations of the system. The drift mobility at different temperatures is determined for all compositions of our system and the activation energies are deduced.
TL;DR: In this article, the magnetic and magneto-optical properties of [Tb/Fe] n multilayers have been investigated and the dependence on Tb and Fe thicknesses and on the stacking parameter n have been studied.
TL;DR: In this article, the magnetization curve of a two-phase two-sublattice model is calculated analytically in one phase a ferromagnetic coupling is assumed which contributes to the total magnetization whereas in the other phase an interaction takes place within the framework of a molecular field theory.
TL;DR: In this article, a tutorial of typical magnetic behavior of molecular materials is presented and three distinct models (intramolecular spin coupling through orthogonal orbitals in the same spatial region within a molecule/ion, intermolecular spins coupling through pairwise configuration interaction between spin-containing moieties, and dipole-dipole, through-space interactions) which enable the design of new molecular-based magnetic materials are discussed.
Abstract: Magnets composed of molecular species or polymers and prepared by relatively low-temperature organic synthetic methodologies are a focus of contemporary materials science research. The anticipated properties of such molecular-species-based magnetic materials, particularly in combination with other properties associated with molecules and polymers, may enable their use in future generations of electronic, magnetic, and/or photonic/photronic devices ranging from information storage and magnetic imaging to static and low-frequency magnetic shielding. A tutorial of typical magnetic behavior of molecular materials is presented. The three distinct models (intramolecular spin coupling through orthogonal orbitals in the same spatial region within a molecule/ion, intermolecular spin coupling through pairwise “configuration interaction” between spin-containing moieties, and dipole—dipole, through-space interactions) which enable the design of new molecular-based magnetic materials are discussed. To achieve the required spin couplings for bulk ferro- or ferrimagnetic behavior it is crucial to prepare materials with the necessary primary, secondary, and tertiary structures akin to proteins. Selected results from the worldwide effort aimed at preparing molecular-based magnetic materials by these mechanisms are described. Some organometallic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) that is, …D•+ A•− D•+ A•−…, exhibit cooperative magnetic phenomena. Bulk ferromagnetic behavior was first observed below the critical (Curie) temperature Tc of 4.8 K for [FeIII(C5Me5)2]•+ [TCNE]•− (Me = methyl; TCNE = tetracyanoethylene). Replacement of FeIII with MnIII leads to a ferromagnet with a Tc of 8.8 K in agreement with mean-field models developed for this class of materials. Replacement with CrIII, however, leads to a ferromagnet with a Tc lowered to 3.65 K which is at variance with this model. Extension to the reaction of a vanadium(o) complex with TCNE leads to the isolation of a magnet with a Tc ≈ 400 K, which exceeds the thermal decomposition temperature of the material. This demonstrates that a magnetic material with a Tc substantially above room temperature is achievable in a molecule/organic/polymeric material. Finally, a new class of one-dimensional ferrimagnetic materials based on metalloporphins is discussed.
TL;DR: In this paper, a model for the diffraction data, based upon a Hendricks-Teller description of the interfacial disorder, demonstrates that the observed broadening of selected reflections originates directly from these stacking faults.
Abstract: Neutron diffraction measurements reveal the nature of the magnetic structure in Fe3O4/NiO superlattices grown by molecular beam epitaxy. Taking advantage of differences between the Fe3O4 and NiO crystalline symmetries, we have determined independently the magnetic order parameters of the bilayer components. The NiO antiferromagnetic order propagates coherently through several superlattice bilayers, while the magnetic coherence of the ferrimagnetic Fe3O4 is restricted to a single interlayer due to the random stacking of the spinel unit cells at the interfaces. A model for the diffraction data, based upon a Hendricks–Teller description of the interfacial disorder, demonstrates that the observed broadening of selected reflections originates directly from these stacking faults.
TL;DR: In this article, the magnetic properties of holmium trifluoride (HoF3) were analyzed within the mean-field/random-phase approximation (RPA) framework.
Abstract: Well defined magnetic excitations in holmium trifluoride are observed in inelastic neutron-scattering experiments, both in the paramagnetic phase at 1.6 K and in the ferrimagnetic phase at 90 mK. The dispersion relations at the two temperatures have been determined along the high-symmetry directions, and the field dependence of the excitations has been studied at 1.6 K. These measurements and the previous studies of the magnetic properties of HoF3 are analysed within the mean-field/random-phase approximation (RPA). The two lowest electronic states of Ho3+ ions are singlets, which are well separated from the remaining levels. The dominating coupling between the ions is the classical dipole interaction, which forces the system to order at Tc=0.53 K. The two-ion coupling is below the threshold value for inducing the ordering of the electronic system; it only occurs because the magnetic susceptibility is enhanced by the hyperfine interaction between the electronic and nuclear moments on the Ho ions. The classical dipole coupling is calculated directly from first principles, whereby the response function is nearly fixed by the macroscopic properties of the system. The calculated response is found to agree accurately with the observations in the paramagnetic phase, whereas some discrepancies occur in the ordered phase. These may indicate that correlation effects beyond the RPA are important or that two-ion (magnetoelastic) quadrupole couplings are present.