Showing papers on "Ferromagnetism published in 1975"

Calculation of the M 23 magneto-optical absorption spectrum of ferromagnetic nickel

Abstract: The ${M}_{23}$ magneto-optical absorption spectrum of ferromagnetic nickel is calculated using an approach similar to the component state-density method that has been successfully used in obtaining valence-band emission and absorption x-ray spectra of metals. The ${M}_{23}$ magneto-optical effects result predominantly from spin-orbit splitting of the $3p$ core state in conjunction with the final $d$-state spin polarization. The calculated spectrum exhibits features that are directly related to electronic structure parameters including the $3p$ core spin-orbit splitting, and the unfilled $d$-band spin polarization. Temperature variations in the magneto-optical structure can be used to determine separately the exchange-splitting variation and spin-wave excitation contributions to the decrease in the magnetization. Experimental verification of these predictions should provide insight into the applicability of the Stoner model to ferromagnetic nickel and may be helpful in resolving some of the apparently conflicting results of other experimental probes of the spin polarization near the Fermi level in nickel.

Contribution of Spin Fluctuations to the Electrical and Thermal Resistivities of Weakly and Nearly Ferromagnetic Metals

Abstract: The electrical and thermal resistivities due to the spin fluctuations in weakly and nearly ferromagnetic metals are studied by using the s-d band model and the self-consistently renormalized spin fluctuation theory for the d-band electrons. The resistivities are calculated in the whole temperature range of interest and their temperature dependences are discussed in detail. The resistivities are shown to be suppressed significantly by the renormalization from those calculated by using the RPA results for the spin fluctuations. Particularly the electrical resistivity can have a tendency towards saturation with increasing temperature.

Magnetic behaviour of cobalt, iron and manganese dissolved in palladium

Abstract: This paper is meant to be a report on the experimental work on dilute Pd-based alloys with Co, Fe and Mn. These alloys exhibit the phenomenon of giant moments. The importance of measurements on paramagnetic alloys is emphasized. From these measurements the conclusion can be drawn that Co and Fe dissolved in Pd does not behave like a normal paramagnet, i.e. according to a Brillouin function. This result makes it possible to explain the existing discrepancy in the interpretations of magnetic measurements on one hand and of specific-heat experiments on the other. The main conclusions of this paper are: The giant moment should be accounted for by ‘normal’ values of the magnetic quantum number (3/2 for Co, 2 for Fe and 5/2 for Mn) and a large value of geff. Paramagnetic alloys of Mn in Pd behave according to Brillouin functions, but alloys of Co or Fe in Pd do not. Hence, a number of interpretations of magnetic measurements should be considered as incorrect. The localized model for ferromagnetism can ...

Itinerant-electron ferromagnetism in La 1−x Sr x CoO 3 : a Mössbauer study

Abstract: Mossbauer and other studies establish that in La 1-x Sr x CoO 3 (x>0.125), ferromagnetic Sr 2+ -rich clusters coexist with paramagnetic La 3+ -rich regions in the same crystallographic phase, with the ferromagnetic component increasing with increasing x and decreasing T. The 3d holes created by Sr 2+ substitution are itinerant both above and below T C . All the experimental observations on this system can be explained on the basis of itinerant-electron ferromagnetism.

Spin glass versus ferromagnet

Abstract: An investigation is presented of the conditions for spin glass or ferromagnetic ordering in systems with exchange interactions distributed randomly about a positive mean, using a quantum extension of a recent method of edwards and anderson (see ibid., vol.5, no.5 p.965, (1975)). The results are related to the situation relevant to a metallic magnetic alloy where they lead to a phase diagram and susceptibility cusp in good accord with experiment.

Domain wall kinetics in soft ferromagnetic metallic glasses

Abstract: Domain wall kinetics are studied in glassy iron‐base wire and iron‐nickel–base ribbon. These materials exhibit high wall mobilities. The wire is examined in a single‐wall regime below the nucleation field. Here reversal takes place by propagation of a domain wall of 2.5‐cm axial length along the wire. The mobility normal to the wall is 270 cm/sec Oe. In the case of glassy iron‐nickel–base ribbon the number of walls taking part in reversal is measured to be two. Using this number, the wall mobility is 1700 cm/sec Oe. The viscosity limiting the wall mobility is partitioned into a (calculated) eddy‐current contribution and a spin‐relaxation contribution. The latter constitutes about 40% of the total viscosity in the wire and about 50% in the ribbon samples. Gilbert damping constants for these samples, 0.44 (wire) and 0.19 (ribbon), are small compared with similarly obtained values for comparable polycrystalline alloys.

Ferromagnetism in lithium holmium fluoride-LiHoF4. I. Magnetic measurements

Abstract: Magnetic susceptibility measurements on LiHoF4 show that this compound undergoes a transition to a ferromagnetic state below Tc=1.53K, with the Ising-like Ho3+ moments oriented along the crystallographic c-axis. Analysis of the shape-dependent Curie-Weiss O indicates that magnetic dipole interaction dominates the ordering process.

Effect of Spin Fluctuations on the Specific Heat of Weakly and Nearly Ferromagnetic Metals

Abstract: The specific heat due to the effect of spin fluctuations in itinerant ferromagnets is calculated by using the renormalized spin fluctuation theory of Moriya and Kawabata. The calculation continuously covers all the interesting temperature ranges, both below and above the Curie temperature. The specific heat is shown to be generally enhanced in weakly and nearly ferromagnetic metals. This is particularly so at low temperatures and the effect decreases as temperature goes up far more rapidly than the previous results based on the random phase approximation which generally over-estimates the spin fluctuation effect. An anomaly around the Curie temperature turns out to be rather small.

Magnetic properties of lithium rare-earth fluorides: Ferromagnetism in LiErF4 and LiHoF4 and crystal-field parameters at the rare-earth and Li sites

Abstract: and crystal-field parameters at the rare-earth and Li sites DTU Orbit (18/11/2019) Magnetic properties of lithium rare-earth fluorides: Ferromagnetism in LiErF4 and LiHoF4 and crystal-field parameters at the rare-earth and Li sites Single crystals of LiErF4 and LiHoF4 have been grown and their magnetic properties measured from 1.3 K to 300 K. LiHoF4 turned out to be a nearly ideal Ising ferromagnet with TC=1.30±0.05 K and a saturation magnetization along the crystalline c axis of (6.98±0.02)μB. In LiErF4 no ordering was observed, but extrapolation indicates that below 0.5 K it will be ferromagnetic with the magnetic moments in the crytalline ab plane. From the susceptibilities the crystal-field parameters Bnm with (n, m)=(2, 0), (4, 0), (4, 4), (6, 0), (6, 4) have been extracted giving for Er3+ in LiErF4: 430., -985., 1185., -5., 740.+i135. (cm-1) and for Ho3+ in LiHoF4: 470., -825., 1050., -10., 760.+i150 (cm-1). The exchange constants were found to be small compared to the dipole interactions. Furthermore the 7Li NMR spectra have been obtained in these materials as well as in LiTbF4 thereby determining the second-order crystal-field parameters at the Li site: q=0.0363, 0.0354, and 0.0343 Å-3 for the Er, Ho, and Tb compound, respectively. The crystal-field parameters are compared to values calculated from point charges, showing agreement at the Li sites but not at the rare earth sites.

Violation of Dynamical Scaling for Randomly Dilute Ising Ferromagnets near Percolation Threshold

Abstract: Strongly anisotropic magnets at low temperatures with many (quenched) nonmagnetic impurities are shown to deviate from usual dynamical scaling assumptions near the critical concentration; instead of the time, the logarithm of the time is the basic variable. Analysis of previous Monte Carlo data confirms the static droplet picture used here, which fulfills for the first time the homogeneity, analyticity, and symmetry requirements of static scaling.

Effect of Orbital Degeneracy and Intra-Atomic Exchange on the Occurrence of Ferromagnetism

Abstract: The role of orbital degeneracy and intra-atomic exchange for the possible origin of ferromagnetism in transition metals is investigated assuming doubly degenerate bands with intra-atomic Coulomb and exchange interactions. Roth showed in the narrow band limit with one electron per atom that the ground state is ferromagnetic with a superlattice ordering with respect to the orbital states. It is found here that there exist two phase transitions at finite temperatures. At the higher transition temperature only the superlattice ordering of the orbital states appears. The spin susceptibility, which obeys at higher temperatures the antiferromagnetic Curie-Weiss law, is modified by the appearance of this orbital ordering and finally it goes over into a ferromagnetic behavior with the help of intra-atomic exchange. Ferromagnetism thus occurs at the lower transition temperature. This conclusion is suggested in a two-atom system and is confirmed within a molecular field approximation.

Magnetocrystalline anisotropy of light rare-earth cobalt compounds

Abstract: The magnetocrystalline anisotropy constants of Y-, Ce-, Pr-, Nd-, Sm- and CeMMCo5 were determined from magnetization and torque curve measurements on single crystals in the temperature range between 77 K and the ferromagnetic Curie point and in fields up to 16 kOe. The anisotropy of Y-, Ce-, Sm- and CeMMCo5 can be described by the constant K1 alone, whereas K2 and K4 are necessary for NdCo5 and PrCo5, for temperatures below 300 K. A comparison of the temperature dependence of K1 with the classical temperature dependence of magnetocrystalline anisotropy shows that the intrinsic anisotropy itself is a function of temperature. The change in type of anisotropy from NdCo5 and PrCo5 is consistent with crystal field calculations.

Theory of capture of particles in magnetic high-intensity filters

Abstract: This paper considers the capture of paramagnetic particles, of radius R and susceptibility χ from a slurry, of viscosity η, by a ferromagnetic wire, of radius a and saturation magnetization M s , in a uniform magnetic field H o , large enough to saturate the wire, applied perpendicular to the axis of the wire. When the slurry is far away from the wire it's velocity is V o . Capture is considered for various orientations of V o with respect to H o . In the calculations the fluid is treated in the inviscid, incompressible approximation. Calculation shows that when M s /2μ o H o is large, the capture radius R c increases with M s /2μ o H o . For values of M s /2μ o H o m /V o , where V m , is the magnetic velocity, V m = (2χM s H o R2/9ηa). The increased performance observed in magnetic filters when μ o H o m /V o , is consistent with the increase of R c with M s /2μ o H o . Effects observed in filters when various configurations of flow in relation to field are used could not be explained in terms of the differences in the bare wire values of R c . It is argued that these differences are due to different dependences of R c on the volume of material captured in the various configurations.

Magnetooptik und elektronische Struktur der magnetisch ordnenden Europiumchalkogenide

Abstract: The absorption coefficient and the interband Faraday rotation of EuS, EuSe and EuTe thin films have been measured as function of the photon energy (1–6 eV), the temperature (2.7–300 K) and the applied magnetic field (0–11.5 kOe). In addition a magnetic field modulation technique has been developed, with a resolution of 2 ⋅ 10−4 deg. This allows the measurement of the Faraday rotation in fields of only 100 Oe, which is important for metamagnetic samples with low critical fields. A Kramers-Kronig transformation of the Faraday rotation leads to the circular dichroism and from these two quantities and the optical constants the off-diagonal elements of the conductivity tensor have been computed. From a comparison of this experimental result with values obtained from a modified atomic model, we deduce the character of the involved transitions and the spin polarization of the occupied ground states (4f 7,p(anion)). In addition the ratio of exchange splitting to band width of the empty 5d final state can be evaluated. The fine structure of the first main peak is discussed in terms of Kasuya's coupling scheme between the 4f 6 multiplett and the excited 5d electron. In the antiferromagnetic EuTe the temperature dependence of the Faraday rotation does not follow the net magnetization of the sample for all photon energies, but some transitions show a “ferromagnetic” behavior. This is interpreted in Slater's model of the magnetic Brillouin zone.

Studies of the Magnetic Structure of FeSn Using the Mössbauer Effect

Abstract: From the analysis of 57Fe and 119Sn Mossbauer spectra a modified anti-ferromagnetic spin structure is proposed. The ferromagnetic ordering of the iron moments in the basal planes is partly destroyed and the magnetic unit cell is four times as large as the chemical unit cell, being doubled both along the a and the c axis. The temperature variation of the reduced magnetic hyperfine field for Sn(1) strongly deviates from the corresponding quantity found for iron. The Neel temperature is found to be 368 K and the critical exponent β obtained is 0.340.

Angular dependence of the superexchange interaction Fe 3+ -O 2- -Cr 3+

Abstract: The super-exchange interaction parametersI(Fe, Cr) of Fe3+ and Cr3+ in iron doped rate-earth orthochromitesRCr0.99Fe0.01 O3 (R=La, La0.5Nd0.5, Nd, Sm, Gd, Dy, Y, Er, Yb or Lu) have been obtained from57Fe magnetic hyperfine structure measurementsvia the Mossbauer effect. The dependence of the experimental valuesI(Fe, Cr) on the Fe−O−Cr average superexchange angle θ (depending upon the relative size of the rare-earth (RE) ionR 3+) is well described by the equationI(d5, d N ) = α N + β N cosθ + γ N cos2 θ. Within the accessible range of super-exchange angles 142°≲θ≲156°, the Fe3+−O2−−Cr3+ interaction is negative (antiferromagnetic). However, a theoretical analysis predicts a sign reversal forI(Fe, Cr.) at about θ ≈ 162° and thus ferromagnetic character of the interaction between 162° and 180°. The spin-only super-exchange interaction integrals fore g andt 2g electrons, separately, are also calculated. Their angular dependence is accounted for by the behaviour of the antiferromagnetic kinetic and ferromagnetic potential exchange which are of different character when passing from 180° to 90° super-exchange geometry. The magnitude and the sign of the spin-only super-exchange integral for an arbitrary 3d cation, pair is predicted.

Limited ferromagnetism and other magnetic properties of Pd-Mn alloys

Abstract: Solid solutions of Mn in Pd have been studied by magnetization, paramagnetic resonance and electrical resistivity measurements and have been found to yield ferromagnetism only for solute concentrations below 5% and temperatures below 8K. At higher concentrations a susceptibility maximum and other effects characteristic of spin glass ordering appear, and in the transition region there is evidence for a large amount of freezing of spin entropy of a less well defined character. This behaviour is discussed in terms of the roles of solute-solvent interactions and solute-solute interactions for close and distant neighbours.

Ferromagnetic and antiferromagnetic semiconductors

Abstract: A review is given of the experimental and theoretical data on ferromagnetic and antiferromagnetic semiconductors. Information on the magnetic, electrical, and optical properties is analyzed systematically. The main theoretical ideas on the physics of magnetic semiconductors are given without detailed proof and are used in the interpretation of the experimental results relating to the influence of the magnetic order on the electrical and optical properties of magnetic semiconductors and to the influence of conduction electrons on the magnetic order.

Neutron Diffraction Study of NiS 2 with Pyrite Structure

Abstract: Neutron diffraction measurements have been made on two single crystals of NiS 2 at low temperatures. Two types of antiferromagnetic reflections, i.e. M1 and M2, were observed. M1 corresponds to the first kind of antiferromagnetic ordering in fcc lattice and M2 to the second kind. The Neel temperature for M2 reflections, T N2 , was 30±1 K, the same for two crystals, and that for M1, T N1 , were 37 K and 51 K, different for two crystals. A distinct anomaly in the intensity of (002) M1 reflection, was observed at T N2 . This suggests a strong correlation between M1 and M2 reflections. A spin structure is proposed, which permits the coexistence of both M1 and M2 reflections. This structure explains the observed intensities. The magnetic moment of Ni 2+ obtained is considerably smaller than 2µ B . Discussions are given on the proposed spin structure, and also on the mechanism for two Neel temperatures and weak ferromagnetism.

Magnetic properties of LaCo5xNi5-5x

Abstract: The magnetic properties, the pressure dependence of the Curie temperature (Tc) and the initial compressibilities of the series of compounds LaCo5xNi5-x were determined. The compounds with x>or=0.6 are ferromagnets whilst for the compounds with 0.2

Spin-wave dispersion and sublattice magnetization in NiCl2

Abstract: NiCl2 is a Heisenberg planar antiferromagnet composed of hexagonal ferromagnetic Ni2+ sheets with effective XY symmetry weakly coupled antiferromagnetically to adjacent Ni2+ sheets. The near two-dimensionality dimples a directionally-dependent spin-wave renormalization together with an unusual temperature dependence on the sublattice magnetization, gap energy and specific heat. The authors report an inelastic neutron scattering study of the spin waves both at low temperatures and, for selected q-vectors, for temperatures up to TN=52.3K. The sublattice magnetization has been measured from 1.5K to TN. A renormalized spin-wave theory, which does not contain any assumptions about the temperature dependence of the energy gap, has been developed. The authors deduce JNN=21.70K and JNNN=-4.85K (H=-2 Sigma i>jJijSi.Sj). Using these interaction constants and the renormalized spin-wave theory, the temperature-dependent dispersion relations (together with the sublattice magnetization) and the gap energy up to approximately 0.4 TN are properly predicted.