Showing papers on "Ferromagnetism published in 1974"
TL;DR: In this article, a semi-polar representation of the spin operators is introduced, which makes possible, in the harmonic approximation, the definition of magnons for any wavelength at low temperature in one-dimensional or two-dimensional magnetic systems without long-range order, provided they are of the planar type.
Abstract: A « semi-polar » representation of the spin operators is introduced, which makes possible, in the harmonic approximation, the definition of magnons for any wavelength at low temperature in one-dimensional (= 1-D) or two-dimensional (= 2-D) magnetic systems without long-range order, provided they are of the « planar » type, i. e. they have an easy magnetization plane. The semi-polar representation is used to calculate the spin pair correlation function at low temperature. Its space-time Fourier transform (directly observable by neutron scattering) consists of a relatively broad peak due to spin fluctuations inside the easy plane, plus a narrower peak due to out-of-plane fluctuations. The intensity, width and lineshape of both peaks are calculated in both 1-D and 2-D cases for all momentum transfers, as well as the frequency shift as a function of temperature.
213 citations
TL;DR: In this paper, the ferromagnetic order is modified near the surface of a magnetic crystal, which is studied most clearly in magnetic thin films consisting of only a few atomic layers.
Abstract: The ferromagnetic order is modified near the surface of a magnetic crystal. This modification is studied most clearly in magnetic thin films consisting of only a few atomic layers. Some essential features of the theoretical description are reported as a basis for the discussion of experiments. The main experimental problem is the preparation of flat single crystal films; the real structure of the best available films is discussed. Their magnetic properties are compared with theoretical predictions. Furthermore, methods and results are reported for probing the magnetic properties of surfaces, by their interaction with low energy electrons.
180 citations
180 citations
TL;DR: The magnetic properties of the distorted perovskite GdCrO3 have been investigated by magnetic moment measurements in the temperature range below 100K and specific heat measurements below 10K as discussed by the authors.
Abstract: The magnetic properties of the distorted perovskite GdCrO3 have been investigated by magnetic moment measurements in the temperature range below 100K and specific heat measurements below 10K. Measurements show that the canting of the Cr3+ ions from a simple antiferromagnetic alignment is due to antisymmetric exchange coupling between Cr ions. The value of the weak ferromagnetic moment Fz is 400 emu mol-1. The interaction between the Cr3+ and Gd3+ ions produces an effective field at the Gd sites of 5.5 kOe, acting in the opposite direction to Fz. At a temperature of approximately 7K the Cr-Gd interaction is sufficient to cause the Cr system to undergo a spontaneous spin reorientation from a GxFz to a GzFx mod. At 2.3K the interaction between the Gd3+ ions is sufficient to bring about a co-operative transition to an antiferromagnetic state.
156 citations
TL;DR: In this paper, a series of ferromagnetic alloys at the composition Co 2 YZ, where Y is a group IVA or VA element and Z is a Group IIIB or IVB element, were studied.
152 citations
TL;DR: In this paper, an exact discussion of the conductivity of disordered resistor networks and spin-wave stiffness in disordered Heisenberg ferromagnets, by considering the special case of the Bethe lattice, is given.
Abstract: An exact discussion is given of the conductivity of disordered resistor networks and of the related problem of spin-wave stiffness in disordered Heisenberg ferromagnets, by considering the special case of the Bethe lattice. For arbitrary substitutional disorder an exact nonlinear integral equation is obtained for a generating function determining the conductivity and spin-wave stiffness in such branching models. An effective medium (coherent potential) approximation is shown to provide an asymptotically exact solution in the limit of high coordination number z. Its extension, to order (1/z)4, is obtained.
114 citations
TL;DR: In this article, the authors compared the order-disorder transition in the face centered cubic (f.c.) lattice and that in the body centered cubic lattice, both calculated based on the tetrahedron approximation of the cluster-variation method.
112 citations
TL;DR: In this paper, the authors examined the low field behavior of Pd-Ni alloys and showed that the susceptibility results for some paramagnetic Pd Ni alloys appear to conform with this value for the critical concentration and in particular the specific heat data, where disagreement has been cited previously, are shown to be in agreement also with the authors value of 2.3+or 0.05 at% Ni.
Abstract: Pd-Ni alloys are of special interest as a system where a critical composition for the paramagnetic to ferromagnetic transition exists. Interpretation of the magnetization as a function of the field, temperature and concentration for this system is not quite straightforward and previous measurements have not examined the low field behaviour in detail. New data on this system have been obtained and careful analysis with the help of Arrott plots yield Curie temperatures T0 which extrapolate to the same value for the critical concentration as that found previously from the resistivity measurements. Existing susceptibility results for some paramagnetic Pd-Ni alloys appear to conform with this value for the critical concentration and in particular the specific heat data, where disagreement has been cited previously, are reanalysed and shown to be in agreement also with the authors value of 2.3+or-0.05 at% Ni.
77 citations
TL;DR: In this article, the impurity spin polarization, static susceptibility, and longitudinal spin relaxation rate were calculated for the s-d model as function of temperature and magnetic field for ferromagnetic and antiferromagnetic exchange coupling.
Abstract: The impurity spin polarization, static susceptibility, and longitudinal impurity spin relaxation rate are calculated for thes-d model as function of temperature and magnetic field for ferromagnetic and antiferromagnetic exchange coupling. The thermodynamic functions and the dynamical susceptibility are obtained from the impurity relaxation spectrum, which is approximated by taking into account the infrared-like singularities. For antiferromagnetic coupling the zero-field susceptibility obeys a Curie-Weiss law1/χ∼4.6(T+θ) for high and intermediate temperatures and it approaches the finite value1/χ∼3.8θ for zero temperature. The zero-field relaxation rate is much larger than the Korringa value; it decreases with temperature and approaches the nonzero value1/T1∼1.2θ for zero temperature. The relaxation rate decreases with increasing field. The results for the spin polarization agree well with the experimental data for the Cu:Fe alloy.
75 citations
TL;DR: The magnetic properties of Ni-Pt alloys containing 42.9, 45.2, 47.6, and 50.2% Ni were measured in magnetic fields up to 60 kOe, at temperatures between 4 and 60 K, and at pressures up to 7 kbar as discussed by the authors.
Abstract: The magnetic properties of ferromagnetic Ni-Pt alloys containing 42.9, 45.2, 47.6, and 50.2 at.% Ni were measured in magnetic fields up to 60 kOe, at temperatures between 4 and 60 K, and at pressures up to 7 kbar. The results are presented in terms of Arrott and Mathon plots whose essential straightness over a broad range of alloying contents and external influences provides evidence for the close applicabilities of weak-ferromagnetic theory based on the intinerant-electron model. This model is used to discuss the results quantitatively and the pressure dependence of the critical concentration is estimated, in good agreement with observation. The effective interaction $U$ is found to be 0.44 eV for these alloys, while the ratio of $U$ to the bare interaction is found to be 0.65. The results are compared quantitatively with those for other similar materials, in particular Zr${\mathrm{Zn}}_{2}$.
70 citations
TL;DR: In this paper, the phase boundary between mixed crystals with MnP-structure and those with NiAsstructure was investigated by neutron diffraction at various temperatures, and the signs and the ratios of the exchange integrals were discussed in a Heisenberg model.
TL;DR: In this paper, a single band description of ferromagnetism in 3D transition metals is given based on a recent formal theory of strong correlations by the author, which is assumed to be important leading to 'Hubbard split bands' and localized moments.
Abstract: A single band description of ferromagnetism in 3d transition metals is given based on a recent formal theory of strong correlations by the author. Correlations are assumed to be important leading to 'Hubbard split bands' and localized moments. Correlations and magnetic effects of the system are calculated selfconsistently. An effective Heisenberg Hamiltonian is obtained. The magnetic excitations of the system are spin waves, their dispersion relation is derived. The spin wave stiffness constant is compared with earlier theories. At the Curie temperature Tc long range magnetic order disappears, localized moments and short range order, however, still exist above Tc. Comparison with experiment is made.
TL;DR: In this article, the L2 1 structure of Fe 3− x Mn x Si alloys has been found and the alloys are ferromagnetic at high Fe concentrations, non-ferromagnetic with high Mn concentrations, and in the range 0.75 x
TL;DR: In this article, the authors studied the high field susceptibility of nickel-platinum alloys in fields up to 150 kOe, at temperatures between 4.2 and 300 K. The results were compared with those obtained for palladium-nickel alloys.
Abstract: The authors have studied the high field susceptibility of nickel-platinum alloys in fields up to 150 kOe, at temperatures between 4.2 and 300K. The high field susceptibility is analysed in the ferromagnetic and paramagnetic range by means of Arrott plots and Landau coefficients. The paramagnetic initial susceptibility follows a Curie-Weiss law, with a Curie constant independent of nickel concentration. The results are compared with those obtained for palladium-nickel alloys. The experimental data are discussed in the framework of recent theories on itinerant ferromagnetism.
TL;DR: In this article, the authors show that the apparent ferromagnetic behaviour and higher ordering temperatures obtained by previous workers in their magnetization measurements have a simple physical explanation based on the intrinsic properties of the alloys.
Abstract: Bulk magnetization measurements on Au-Fe alloys are widely held as indicating the onset of long range ferromagnetism in alloys with Fe concentration above about 12 at.%. The 'ordering' temperatures deduced from these measurements appear to be in reasonable agreement with temperatures of the maxima of d rho /dT in the resistivity of the alloys but are considerably higher than the values obtained from Mossbauer effect measurements for alloys in the concentration range between 12 and 20 at% Fe. Magnetization measurements on a Au-15at% Fe alloys are presented which, contrary to the above, show a behaviour usually associated with spin glass alloys and indicate a spin freezing temperature close to the 'ordering' temperature expected from Mossbauer effect measurements. The results show that the apparent ferromagnetic behaviour and higher ordering temperatures obtained by previous workers in their magnetization measurements have a simple physical explanation based on the intrinsic properties of the alloys.
TL;DR: In this article, a simple physical interpretation for hyperfine fields systematics at the sites of nonmagnetic ions in concentrated ferromagnetic alloys is given and applied to Heusler alloys and numerical results are given.
TL;DR: In this article, the ferromagnetic anisotropy of Ni and Fe metals is calculated based on the differences between the crystal energies with the different directions of the spontaneous magnetization.
Abstract: The ferromagnetic anisotropy energies of Ni and Fe metals are calculated based on the differences between the crystal energies with the different directions of the spontaneous magnetization. These crystal energies are calculated by taking into account the variations of the spin-orbit interaction and the magnitude of the spontaneous magnetization, on the basis of the energy bands of ferromagnetic Ni and Fe metals, where the redistribution of the conduction electrons for each direction of the spontaneous magnetization is also taken into account. The obtained results are in agreement with the experimental results. The doubly degenerate energy levels in the energy bands, intersecting the Fermi level, produce the dominant contributions to the anisotropy energy. In Ni metal the temperature dependence of the anisotropy constants can be explained by the temperature variation of the Fermi level, and for Fe metal it can be explained in consideration of the decrease of the exchange splitting.
TL;DR: In this article, a two-dimensional ferromagnet K 2 CuF 4 was used for a parallel pumping experiment and a magnon relaxation time was measured at a pumping frequency of 8.9 GHz.
Abstract: There is a very flat branch in the spin-wave spectrum of two-dimensional ferromagnets. Since the energy of the lowest Brillouin-zone-boundary magnon is very small in this case, those magnons can be excited by a conventional parallel pumping experiment. Instability of the zone-boundary magnons in a two-dimensional ferromagnet K 2 CuF 4 is observed and a magnon relaxation time is measured at a pumping frequency of 8.9 GHz. Another feature of this case is the existence of measurable steady state absorption below the instability threshold (subthreshold absorption), because the magnon density of states of the lowest branch is very large. It is usually difficult to observe this absorption in three-dimensional ferromagnets. From the shape of the subthreshold absorption observed at pumping frequencies of 8.9, 35 and 73 GHz, the interlayer exchange field in K 2 CuF 4 is determined to be 126 Oe (or equivalent to 0.017 K). The ratio of the interlayer to the intralayer exchange fields is estimated to be 0.00042.
TL;DR: In this article, a general dynamical theory of magnetizable, electrically and thermally conducting media is developed for soft ferromagnetic or paramagnetic materials in external electromagnetic fields, where the general equations are linearized by assuming infinitesimal strains, linear constitutive equations and that all field variables may be divided into two parts: a rigid body state and a perturbation state.
Abstract: A general dynamical theory of magnetizable, electrically and thermally conducting media is developed for soft ferromagnetic or paramagnetic materials in external electromagnetic fields. The general equations are linearized by assuming infinitesimal strains, linear constitutive equations and that all field variables may be divided into two parts: a "rigid body state" and a "perturbation state". The former is the same as the one in rigid body electrodynamics, and the latter which accounts for electromagnetic interaction with the deformable continuum is coupled with stress and strain through linearized field equations. The theory is developed for general anisotropy but specialized for materials with uniaxial, or higher, symmetry.
TL;DR: In this paper, a powder sample of the monoclinic, weak ferromagnet CuF 2 was investigated by neutron diffraction using the profile method, the crystal structure was refined and the spin configuration determined.
TL;DR: In this article, single domain particles of Fe alloys (Fe versus Ni, Cu, Si, Cr, Gd and Ho) were prepared by the evaporation method and the saturation magnetization, remanent magnetization and coercive force of these particles were measured by an automatic magnetic balance.
Abstract: Single domain particles of Fe alloys (Fe versus Ni, Cu, Si, Cr, Gd and Ho) were prepared by the evaporation method. The saturation magnetization, remanent magnetization and coercive force of these particles were measured by an automatic magnetic balance. The dispersion of the anisotropy of Fe, Co and Ni particles were determined by a torque meter. The characteristics of these alloy particles were discussed in terms of their applicability to industrial use. The stability for oxydation and the magnetic orientation of particles have already been reported in the previous paper of this series. By choosing a suitable composition of alloys, the coercivity of the prepared particles can be controlled in the range of 200 to 1800 Oe. It was found that the anisotropy dispersion of Fe-Ni alloy was smaller than that of a commercial CrO2 tape.
TL;DR: In this article, a magnetoresistance reading head (MRH) was built into the gap of a ferrite write head; minima and maxima were noted in the wavelength response that can be explained theoretically.
Abstract: The anisotropy in the spontaneous magnetoresistance of ferromagnetic metals can be used to monitor the magnetic fields of cylindrical domains and of magnetic recordings. Depending on which application is chosen, either the magnitude of the anisotropy effect must be optimized by itself, or in a suitable combination with the saturation magnetisation and thickness to account for the demagnetizing field and the thickness dependence of the resistivity. Ni(89)Fe(11) and Ni(70)Co(30) films are shown generally to be acceptable for device materials. Various magnetoresistance reading heads (MRH) have been fabricated and investigated. A MRH was built into the gap of a ferrite write head; minima and maxima were noted in the wavelength response that can be explained theoretically. A new source of noise was found to originate from variations of the heat flow between head and contacting tape. Its influence can be reduced by diminishing the excitation current or by increasing the thermal conductivity near the magneto-resistive material. For analog reading, the head is biased with a field of 50-100 Oe, which has a negligible influence on the recorded information. Track sensing and compensation of cross-talk and external noise are shown to be feasible.
TL;DR: In this article, the authors used a collective electron model to examine magnetic superexchange interactions in the distorted perovskite solid-solutions CaxSr1−xRuO3.
TL;DR: In this article, a virtual double exchange mechanism was proposed for the short range ferromagnetic coupling in Heusler alloys. Satisfactory agreement with experiments was obtained with experiments.
TL;DR: In this paper, it was shown theoretically and experimentally that the current density is not uniform in the neighborhood of a 180° domain wall, in a pure ferromagnet traversed by an electric current at low temperature.
Abstract: It is shown theoretically and experimentally that the current density is not uniform in the neighborhood of a 180 ° domain wall, in a pure ferromagnet traversed by an electric current at low temperature. The effect is caused by the abrupt reversal of the internal field B=Ms across the wall, and by the corresponding gradual reversal of the Hall electric field. In a noncompensated ferromagnetic metal (Co, Ni) in the high field limit ωcτ≫1, domain walls would act as if they were electrically insulating; the current could cross the wall only through a small ``hole'' located at one end of the wall, creating an infinite current density there. A cobalt single crystal of limited purity (R300/R4.2 = 65) was cut in the shape of a thin slab normal to the c axis. One‐half of the slab is magnetized to saturation in the c direction and the other half is magnetized in the opposite direction, thus simulating one domain wall. When a dc current crosses the wall, the current density is found to be larger at one end of the w...
TL;DR: In this article, the role of spin waves in the coupling interaction in a perfect ferromagnetic superconductor is considered, and the time correlations in the spin system are taken into account.
Abstract: This review begins with the theory of the paramagnetic effect in a superconductor. The coexistence of superconductivity and ferromagnetism in metals with paramagnetic impurities is considered with the Gorkov--Rusinov theory being developed in another way. The time correlations in the spin system are taken into account, and the coexistence criteria are defined more precisely. The role of spin waves in the coupling interaction in a perfect ferromagnetic superconductor is considered. (47 references) (DLC)
TL;DR: In this paper, a simple model for a nondilute alloy containing giant spin clusters is used to calculate the spin contribution to the resistivity, the neutron cross section, and the specific heat, which is expected to be applicable to Ni-Cu alloys near the critical concentration for ferromagnetism.
Abstract: A simple model for a nondilute alloy containing giant spin clusters is used to calculate the spin contribution to the resistivity, the neutron cross section, and the specific heat ${C}_{V}$. The model is expected to be applicable to Ni-Cu alloys near the critical concentration for ferromagnetism. Intracluster interactions are treated exactly using a near-neighbor Heisenberg exchange interaction. Intercluster interactions are treated within the molecular-field approximation. It is assumed that in paramagnets there is a weak local field, which derives from the magnetic-anisotropy energy. Reasonable semiquantitative agreement with resistivity, elastic neutron scattering, and low-temperature specific-heat measurements on Ni-Cu for a range of concentrations and temperatures is obtained if the average cluster contains 50 Ni spins and if a Ni atom has a spin when eight or more of its near neighbors are also Ni. It is found that the anomalous temperature dependence of the resistivity, which behavior is common to a variety of alloy systems, can be accounted for using the present theory if the Fermi wave vector times the lattice spacing is less than \ensuremath{\cong}2. The previously unexplained behavior of the elastic neutron cross section in paramagnetic alloys can also be understood within this theoretical framework. In contrast to earlier discussions, it is shown that the spin contribution to ${C}_{V}$ is not temperature independent; in ferromagnetic alloys this contribution is found to increase with increasing temperature and in paramagnetic alloys it decreases with temperature. It is believed that this temperature dependence has, in the past, been incorrectly attributed to the electronic contribution to the specific heat. The validity of previous suggestions that the specific-heat and neutron-scattering measurements on these alloys probe different spin clusters is also questioned.
TL;DR: In this paper, the structures of MxTiS2 phases were reported (M = Fe, Co, Ni; x =.25,.33,.40,.50,.75).