Showing papers on "Ferromagnetism published in 1985"

Spin Fluctuations in Itinerant Electron Magnetism

Abstract: A general theory of spin fluctuations and thermodynamical properties of itinerant electron magnets is developed, interpolating between the weakly and strongly ferromagnetic limits. A unified expression is given for the Curie temperature and the physical meaning of the Curie-Weiss magnetic susceptibility is discussed. As new phenomena derived from this theory the temperature-induced local magnetic moments as observed in CoS2, CoSe2, etc. and peculiar magnetic and thermal properties of nearly ferromagnetic semiconductors such as FeSi are discussed.

Effect of spin fluctuations on the magnetic equation of state of ferromagnetic or nearly ferromagnetic metals

Abstract: The authors present a quantitative model for the magnetic equation of state of nearly or weakly ferromagnetic metals at low temperatures which includes corrections to conventional Stoner theory arising from enhanced fluctuations in the local magnetisation. The model takes account of both longitudinal and transverse fluctuations in terms of four physically transparent parameters which may be determined independently from the equation of state in the T=0 limit and from inelastic neutron scattering, or calculated directly from a semi-empirical band structure model near the Fermi level fitted for example to experimental Fermi surface areas. For parameters of the same order of magnitude as those recently determined in the weakly spin-polarised metal Ni3Al, the model yields approximately a quadratic temperature dependence of the spontaneous magnetisation over a wide range well below the Curie temperature (Tc), a nearly linear inverse susceptibility well above Tc, and nearly linear magnetic isotherms (Arrott plots) at high magnetic fields. These results are qualitatively consistent with the behaviour observed in many magnetic metals near the ferromagnetic instability at low temperatures. For Ni3Al the model yields good quantitative agreement with experiment for the magnitude of the Curie temperature Tc, for the ratio peff/p0 of the high- to low-temperature effective magnetic moments, and for the coefficient of the quadratic (T2) variation of the magnetisation with temperature well below Tc, without the use of any free adjustable parameters. Finally the authors show that the model also provides a good quantitative description of the paramagnetic susceptibility and transition temperature of the more complex magnetic system MnSi, the only other unsaturated magnetic metal for which all of the microscopic parameters are well known.

Phase diagrams for dilute spin glasses

Abstract: A generalised, dilute, infinite-ranged Ising spin-glass model is introduced and studied as a function of the concentration p and temperature T. The phase diagram is investigated and paramagnetic (P), ferromagnetic (F), spin glass (SG) and mixed (M) phases, meeting at a multicritical point (p*,T*), are identified. The P/F and P/SG phase boundaries are derived, and the F/M and M/SG boundaries are calculated close to (p*,T*). The condition for having a re-entrant spin-glass transition is derived. In non-zero magnetic field a p-dependent A-T instability line is obtained. The authors apply their results to the insulator EuxSr1-xS, it is predicted to exhibit re-entrant behaviour.

Coexistence of superconductivity and magnetism theoretical predictions and experimental results

Abstract: Superconductivity and ferromagnetic ordering are two antagonistic types of ordering, and their mutual influence leads to many interesting phenomena which have been studied recently in ternary compounds. Theoretical analysis of ferromagnetic materials which are type II superconductors near the superconducting transition point T cl shows that they become type I near the magnetic transition point T M. The proposed theory constructed for the case T M « T cl predicts the formation of a transverse domain-like (DS phase) magnetic structure below T M. The electronic spectrum appears to be gapless in the DS phase of clean compounds with a re-entrant transition. The change from type II to type I behaviour as the sample is cooled to T M has been observed in ErRh4B4. Experimental data for HoMo6S8, HoMo6Se8 and ErRh4B4 give evidence for the coexistence of super-conductivity and non-uniform magnetic ordering below T M. Mutual influence of superconducting and magnetic orderings is also studied.

Magnetic properties of the Nd2(Fe1−xCox)14B system

Abstract: We have investigated the magnetic properties of the Nd2(Fe1−xCox)14B system to improve the thermal properties of the Nd‐Fe‐B magnets. Nd2(Fe1−xCox)14B exists in the tetragonal form in the entire range of 0≤x≤1. In this system, the replacement of Fe by Co significantly increases the Curie temperature. The room‐temperature magnetization of Nd2(Fe1−xCox)14B has its maximum value at x=0.1. However, because of the decrease in the anisotropy energy and the saturation magnetization by further substitution of Co for Fe, a reasonable substitution range of Co is suggested to be x<0.2 in the sintered Nd‐Fe‐B magnet. In this range of Co, we have succeeded in improving the reversible temperature coefficient of the remanence for the Nd‐Fe‐B magnets.

Application of permanent magnets in accelerators and electron storage rings (invited)

Abstract: After an explanation of the general circumstances in which the use of permanent magnets in accelerators is desirable, a number of specific magnets will be discussed. That discussion includes magnets needed for the operation of accelerators as well as magnets that are employed for the utilization of charged particle beams, such as the production of synchrotron radiation.

Calculated ground state properties of light actinide metals and their compounds

Abstract: Agreement between calculated and measured atomic volumes, bulk moduli and cohesive energies of the light actinide metals establishes the combination of self-consistent energy band and density functional theory as an appropriate description of these, the last, transition metals in the periodic table. Similar calculations for NaCl-type compounds of the light actinides are able to explain characteristic trends in lattice constant and bulk modulus. Furthermore, the onset of ferromagnetic ordering and the observed relationship between lattice constant and magnetic moment are, at least qualitatively, given correctly. Relativistic, in particular spin-orbit, effects alter the calculated magnetic properties dramatically being responsible for both large magnetic anisotropy and predominant orbital moments in all magnetic light actinide compounds in which the f electrons are itinerant. The calculated magnetic moments of NaCl-type uranium compounds are, however, only in agreement with experiment when the pairing energy between electrons includes an orbital contribution.

Polarized neutron study of the compounds Y2Fe14B and Nd2Fe14B

Abstract: Outstanding permanent magnet properties were recently observed in a Nd‐Fe‐B compound which was shown to crystallize in a new phase, R2Fe14B. Polarized neutron measurements are reported on Y2Fe14B and Nd2Fe14B single crystals. The 3d Fe moment on different sites in Y2Fe14B at 4.2 K is closely related to local environment. Its value is maximum for atoms in σ‐like layers at the center of an Fe antiprism. On the contrary, it is reduced to 1.95 μB by 4d‐3d electron transfer and hybridization for Fe atoms which have the largest coordinance number of Y atoms, 4. The measurements at 250 K reveal a larger thermal decrease of the 3d moment for Fe atoms which exhibit shortest Fe‐Fe interatomic distances. This property reveals a reduction of 3d magnetic interactions for short distances as was previously observed in R2Fe17 compounds. In Nd2Fe14B, the low values obtained at 4.2 K for Nd magnetic moments suggest that the magnetic structure, determined by a competition between 3d‐4f exchange interactions and crystal‐fiel...

Spin fluctuations in itinerant electron magnetism

Abstract: 1 Introduction- 11 Local Magnetic Moment and the Weiss Theory of Ferromagnetism- 12 Magnetic Moments of Atoms- 13 Heisenberg Localized Electron Model- 14 Itinerant Electron Model- 15 Localized vs Itinerant Electron Models- 16 Random-Phase Approximation Theory of Spin Fluctuations in Itinerant Electron Magnets- 17 Local Moments in Metals- 18 Self-Consistent Renormalization Theory of Spin Fluctuations and Weakly Ferro- and Antiferromagnetic Metals- 19 Unified Picture of Magnetism- 110 Organization of the Book- 2 Mean-Field Theory of Itinerant Electron Magnetism- 21 Model Hamiltonians- 22 Ferromagnetism- 23 Antiferromagnetism- 24 Spin-Density Waves- 25 Stability of Various Spin Orderings- 3 Dynamical Mean-Field Theory of Spin Fluctuations- 31 Stoner Excitations and Spin Waves in Ferromagnetic Metals- 32 General Spin Fluctuations and Dynamical Susceptibilities- 33 Critical Spin Fluctuations- 34 Antiferromagnets- 35 Limitations of the Hartree-Fock-RPA Theory- 4 Self-Consistent Renormalization (SCR) Theory of Spin Fluctuations- 41 Expressions for the Free Energy of an Interacting Electron System- 42 Paramagnon Theories- 43 SCR Theory of Ferromagnetic Metals- 431 Curie Temperature and Magnetic Susceptibility Above TC- 432 Analytical Explanation of the New CW Law- 433 Magnetization Below TC- 434 Rotationally Invariant Treatment- 44 Phenomenological Mode-Mode Coupling Theory- 45 SCR Theory of Antiferro- and Helimagnetic Metals- 46 Physical Origin of the New Curie-Weiss Susceptibility- 461 Temperature Variation of the Mean-Square Local Amplitude of Spin Fluctuation- 462 Spatial Spin Correlation- 47 Coexistence of and Phase Transitions Between Ferro- and Antiferromagnetism- 48 Quantitative Aspects of the SCR Theory for Weak Itinerant Ferromagnets- 5 Physical Properties of Weakly and Nearly Ferro- and Antiferromagnetic Metals- 51 Properties of Spin Fluctuations- 52 Thermal Expansion- 53 Specific Heat- 54 Nuclear Spin Relaxation- 541 Weakly and Nearly Ferromagnetic Metals- 542 Weakly and Nearly Antiferromagnetic Metals- 55 Electrical and Thermal Resistivities and Magnetoresistance- 551 Weakly and Nearly Ferromagnetic Metals- 552 Antiferromagnets- 56 Coupling Between Spin and Charge Density Fluctuations- 6 Local Magnetic Moments- 61 Local Moments in Insulator Magnets- 62 Metal-Insulator (Mott) Transition- 63 s-d or s-f Exchange Model- 64 Local Moment Formation in Metals- 641 Virtual Bound State- 642 The Anderson Model- 643 The Wolff-Clogston Tight-Binding Model- 65 Physical Properties of a Single Local Moment in Metals- 66 Interaction Between a Pair of Local Moments in Metals- 67 Local Moment Description of Magnetic Transition Metals- 7 A Unified Theory and Its General Consequences- 71 General Considerations Toward a Unified Theory- 72 A Phenomenological Description of the Unified Theory- 73 General Mechanism for the Curie-Weiss Susceptibility- 731 A Mean Mode-Mode Coupling Theory of Magnetic Susceptibility- 732 General Expressions for the Curie Temperature and Susceptibility- 733 Spin Correlations and Magnetic Susceptibility- 74 Possibility for Systematic Analyses of Experimental Results- 75 Temperature Variation of the Local Amplitude of Spin Fluctuation- 751 General Remarks- 752 Temperature-Induced Local Moments- 76 Limitations of the Adiabatic Approximation- 8 Functional Integral Theory- 81 Basic Formalism- 82 Local Moment in Metals- 83 Band Magnetism - A General Formalism- 84 Methods of Calculating the Free Energy Functional- 841 A Closed-Form Expression for ? [?, ?]- 842 Long-Wavelength Approximations- 85 A Unified Description of Magnetism in Narrow-Band Systems- 86 Approximation Methods of Evaluating the Functional Integrals- 861 Unified Theory- 862 Molecular Field (Single-Site) Approximation- 863 Choice of Forms for the Interaction Hamiltonian- 87 Results of Numerical Calculations- 871 Ferromagnetic Transition Metals ?-Fe, Co, Ni- 872 Antiferromagnetic Transition Metals- 873 FeSi, Nearly Ferromagnetic Semiconductor- 874 Temperature-Induced Local Moments in CoS2, CoSe2- 875 Magnetovolume Effects- 88 Supplementary Discussions on Various Approaches and Physical Pictures- 881 The Hartree-Fock Theory for Excited States with Spatially Varying Spin Density- 882 Local Band Theory- 883 Persistence of Exchange Splitting of the Band Above TC- 9 Spin Fluctuations in d-Electron Systems- 91 Spin Fluctuations and Neutron-Scattering Measurements- 92 Local Moment Systems- 921 Heusler Alloys- 922 EuO- 93 Substances Close to the Local Moment Limit- 931 MnPt3, FePd3- 932 FePt3- 94 Substances Close to the Weakly Ferro- and Antiferromagnetic Limits- 941 MnSi- 942 Cr- 95 Substances in the Intermediate Regime: q-Space Description- 951 Fe3Pt- 952 CeFe2- 953 ?-Mn, ?-(FeMn), ?-Fe- 954 ?-Mn- 96 Ferromagnetic Transition Metals Fe, Co, and Ni- 97 Some Systematic Trends in Paramagnetic Scattering- 10 Toward a Unified Theory of Dynamical Spin Fluctuations- 101 General Considerations- 102 Green's Function Theory of Dynamical Susceptibilities- 103 Effect of Electron Lifetime on RPA-SCR Spin Fluctuations- 11 Concluding Remarks- References

Spin separation in a metal overlayer.

Abstract: The electronic properties of a ferromagnetic Fe monolayer epitaxially adsorbed on a Ag{100} substrate have been computed fully self-consistently. The minority $d$-band spin states of the Fe overlayer were found to be separated energetically from the majority-spin band. The magnetic moment per overlayer atom is 3.0${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$, 36% larger than that of bulk iron.

Magnetic properties of R2Fe14B single crystals

Abstract: A whole new class of high‐performance permanent magnet materials is based on the ternary tetragonal structure R2Fe14B, where R is one of the rare‐earth elements. We have successfully grown single crystals of this structure with R=Y, Nd, and Tb. Y is a nonmagnetic rare‐earth substitute, while Nd and Tb couple ferro‐ and ferrimagnetically, respectively, relative to the iron moment. All three of the compounds have [001] easy axes at room temperature, although the Nd compound exhibits a spin reorientation away from the [001] below about 150 K. Nd2Fe14B has a saturation induction at room temperature of 16.2 kG, which places an upper limit of approximately 65.6 MGOe on the energy product obtainable by magnets based on that material. While Tb2Fe14B exhibits a smaller magnetization because of ferrimagnetic coupling of the rare earths and the iron, it also has an extremely large magnetic anisotropy which is nearly temperature independent between 4.2 and 300 K.

X-ray resonance magnetic scattering

Abstract: A resonance-type X-ray magnetic scattering at the K -absorption edge was first observed in 220 Bragg reflection of ferromagnetic Ni single crystal. Further a theoretical discussion on the resonance magnetic scattering curve observed was carried out; A difference of the oscillator-strength density for each direction of the spins was also derived from this curve. The oscillator-strength density for minority spin excesses that for the majority spin between the Fermi level and 25 eV above the level. The difference of oscillator-strength density amounts to about 1×10 -4 /Ry at the maximum.

Magnetic properties of R 2 Fe 1 4 B single crystals

Abstract: A whole new class of high‐performance permanent magnet materials is based on the ternary tetragonal structure R2Fe14B, where R is one of the rare‐earth elements. We have successfully grown single crystals of this structure with R=Y, Nd, and Tb. Y is a nonmagnetic rare‐earth substitute, while Nd and Tb couple ferro‐ and ferrimagnetically, respectively, relative to the iron moment. All three of the compounds have [001] easy axes at room temperature, although the Nd compound exhibits a spin reorientation away from the [001] below about 150 K. Nd2Fe14B has a saturation induction at room temperature of 16.2 kG, which places an upper limit of approximately 65.6 MGOe on the energy product obtainable by magnets based on that material. While Tb2Fe14B exhibits a smaller magnetization because of ferrimagnetic coupling of the rare earths and the iron, it also has an extremely large magnetic anisotropy which is nearly temperature independent between 4.2 and 300 K.

Magnetic properties of amorphous metals

Abstract: Neutron scattering has provided unique information about the nature of magnetism in amorphous alloys. This paper reviews some of the results obtained principally on two ribbon-form metallic glass systems - (Fe x Ni 1− x ) 75 P 16 B 5 Al 3 and Fe x B 1− x . The former exhibits three states depending on composition: pure ferromagnetism, re-entrant spin glass, and ordinary spin glass. The neutron results provide evidence for the coexistence of spin glass and ferromagnetic correlations in the intermediate regime. The alloys of Fe and B show Invar phenomena which result in anomalously small values of the effective spin-wave stiffness calculated from low-temperature magnetization and Mossbauer data. The long-wavelength spin waves as measured by the neutrons do not reflect the rapid demagnetization found in the bulk results, implying the presence of excitation processes other than long-wave-length spin waves.

Electronic structure of metallic ferromagnets above the Curie temperature

Abstract: The authors describe the electronic structure of Fe and Ni above their Curie temperatures in their disordered local moment (DLM) states as a function of wavevector k and energy epsilon . In particular, they calculate the Bloch spectral function, A(k, epsilon ), averaged over the orientational configurations of the local moments, at selected points in the Brillouin zone and determine the shape and smearing of the 'Fermi surface'. They find that BCC Fe, with a local moment of 1.9 mu B, can show an exchange splitting at some points whilst in other regions of the Brillouin zone no such splitting occurs. For comparison they also study FCC Fe, which also supports a substantial local moment. They find that it has similar features but the smearing of the 'bands' is more pronounced. On the other hand, the electronic structure of Ni is quite different, shows no such local exchange splitting, but is able to support a small local moment of 0.2 mu B. The resulting picture of the electronic structure of Ni is that of a paramagnetic smeared 'Stoner-Wohlfarth' model.

Ferrimagnetic to spin glass transition in the mixed spinel Mg1+tFe2-2tTitO4: a Mossbauer and DC susceptibility study

Abstract: Mossbauer and DC susceptibility studies are reported on the mixed spinel Mg1+tFe2-2tTitO4 at various temperatures and in an external magnetic field. Results are obtained indicating that on increasing dilution t, the collinear ferrimagnetic phase breaks down before reaching the ferrimagnetic percolation limit as a result of the presence of competing exchange interactions. In this dilution region there is a second transition, at Tf, to a spin-glass-like (SGL) state, well below the ferrimagnetic ordering temperature TN. The lower limit of re-entrant ferrimagnetism is determined by observing the spontaneous spin canting which occurs in the presence of the external magnetic field on going into the SGL state. At higher dilution a transition to a pure spin-glass state occurs. The results are discussed in terms of the model proposed by Villain (1979) based on the competition between dilution and frustration in the spinel lattice. The re-entrant ferrimagnetism found in this insulating system with near-neighbour interactions is similar to the re-entrant ferromagnetism found in some metallic alloys, and the authors propose a canting order parameter for the low-temperature state as in the metallic case.

Crystal and magnetic structure of Pr2Fe14B and Dy2Fe14B

Abstract: Rietveld analyses of neutron powder diffraction data on Pr2Fe14B and Dy2Fe14B are reported. Both phases form the Nd2Fe14B tetragonal crystal structure (P42/mnm). At 77 and 293 K our analyses indicate that all magnetic moments in each compound are collinear with the c axis. The rare‐earth moments in Pr2Fe14B are parallel to the Fe moments (ferromagnetic) and are antiparallel in Dy2Fe14B (ferrimagnetic).

Some ways to modify the spin‐wave mode spectra of magnetic multilayers (invited)

Abstract: Spin‐wave modes in magnetic multilayers are of interest both for basic research and for application—for example, in microwave devices. Here I would like to discuss some ways to manipulate the mode spectra of such multilayers. One way, for example, would be to have reversed magnetization direction in adjacent layers which can be achieved by tailoring the coercive forces of the films in a proper way. The mode spectrum of such a system has been derived only recently and I would like to report a few important results. Also I would like to discuss the case where the films are coupled by exchange as well as by the dipolar interaction which is always there. Here the amount of exchange can be tailored by choosing the proper materials or thickness for the nonmagnetic intermediate layers between the magnetic films. We have studied these effects for a couple of Fe sandwiches with intermediate layers of various materials and find a short coupling length (<5 A) for V and Cr, and a long one (≲20 A) for Cu, Ag, Au, and ...

Direct and exchange contributions in inelastic scattering of spin-polarized electrons from iron.

Abstract: Premiere caracterisation complete experimentale des processus de diffusion elastique et inelastique a deux electrons dans un ferromagnetique

Iron-based rare-earth magnets (invited)

Abstract: Rare‐earth iron alloys offer promise for the realization of the highest‐energy product attainable. We have investigated rare‐earth iron alloys by melt spinning and identified compositions that are beneficial for magnet fabrication. Compositions of the type R3Fe16B and R3Fe20B provide high values of coercive force. Processing of the compositions into magnets resulted in energy product of up to 45 MG Oe with a potential for 47.5 MG Oe. The B‐H loop characteristics indicate that these magnets can be used up to 150 °C.

Spin-1/2 One-Dimensional Heisenberg Ferromagnet at Low-Temperature

Abstract: Using Bethe ansatz integral equations, we calculate the free energy and susceptibility of spin-\(\frac{1}{2}\) one-dimensional Heisenberg ferromagnet \(\mathcal{H}{=}J\Sigma(\mbi{S}_{\text{i}}\mbi{S}_{\text{i}+1}-\frac{1}{4})\), ( J <0) at T ≥0.004| J |. We find that the free energy and susceptibility are expanded by \(\sqrt{T/|J|}\) at low temperature: \(f&=|J|\left\{-1.042\left(\frac{T}{|J|}\right)^{3/2}+1.00\left(\frac{T}{|J|}\right)^{2}-0.9\left(\frac{T}{|J|}\right)^{5/2}+O\left(\frac{T}{|J|}\right)^{3}\right\}\\ \chi&=|J|^{-1}\left\{0.1667\left(\frac{|J|}{T}\right)^{2}+0.581\left(\frac{|J|}{T}\right)^{3/2}+0.68\left(\frac{|J|}{T}\right)^{1}+O\left(\frac{|J|}{T}\right)^{1/2}\right\}.\) The first term of free energy coincides with the spin wave calculation. The first term of susceptibility coincides with Fisher's solution of classical Heisenberg Ferromagnet. We conclude α=-0.5 and γ=2.

Physical interpretation of eddy current losses in ferromagnetic materials. II. Analysis of experimental results

Abstract: In the preceding paper [J. Appl. Phys. 57, 2110 (1985)], it was shown that the behavior of eddy current losses in ferromagnetic materials can be fully characterized, in general, in terms of two parameters, the effective number n0 of active magnetic objects present at low magnetizing frequency fm, and the field V0, determining the ability of the external field to increase the number of active objects with increasing fm. In this paper, the connection of n0, V0 with several microstructural and magnetic properties of iron‐based ferromagnetic alloys is investigated. It is shown that, in agreement with theoretical predictions, n0 is, in general, weakly dependent on other physical properties, attaining values ∼1 in most cases. Only the application of a tensile stress leads to a drastic increase of n0 above this limit. V0, on the contrary, turns out to be very sensitive to the type of material. It continuously decreases with decreasing degree of magnetic orientation, changing by about two orders of magnitude ...