Showing papers on "Ferromagnetism published in 1988"
TL;DR: Estimated tunneling rates of the magnetization in a single-domain particle through an energy barrier between easy directions prove to be large enough for observation of the effect with the use of existing experimental techniques.
Abstract: The probability of tunneling of the magnetization in a single-domain particle through an energy barrier between easy directions is calculated for several forms of magnetic anisotropy. Estimated tunneling rates prove to be large enough for observation of the effect with the use of existing experimental techniques.
420 citations
379 citations
TL;DR: In this paper, a new theory involving a random field at the interface which causes the antiferromagnet to break up into domains, whose size is inversely proportional to the exchange field offset is described.
Abstract: Exchange anisotropy refers to a group of phenomena which appear in ferromagnetic‐antiferromagnetic sandwiches, particularly to a field offset in the hysteresis loop. After a brief review of experiment and earlier theory, a new theory is described involving a random field at the interface which causes the antiferromagnet to break up into domains, whose size is inversely proportional to the exchange field offset. The theory is extended by considering the temperature dependence and also the topological properties of the domains, in particular nonzero winding numbers which increase the size of the domains and give them added stability. The metastability of such structures provides an explanation of the magnetic ‘‘training’’ effect observed in multiple cycles of the hysteresis loop.
378 citations
TL;DR: In this article, a cubic discretization procedure of the micromagnetic energy functional is used to carry out numerical studies of the magnetization process in ferromagnetic cubes and their switching behavior is calculated for particle sizes in the range from 100 to 550 A.
Abstract: A cubic discretization procedure of the micromagnetic energy functional is used to carry out numerical studies of the magnetization process in ferromagnetic cubes. Equilibrium magnetization configurations and their switching behavior are calculated for particle sizes in the range from 100 to 550 A. In the model calculations the particles are assumed to have uniaxial crystalline anisotropy with an anisotropy constant of 18 500 erg/cm3, a saturation magnetization of 370 emu/cm3, and an exchange constant of 10−6 erg/cm. For particle sizes smaller than 520 A the remanent state has a flowerlike magnetization configuration. Beyond 520 A this state is replaced by a vortex structure about the easy axis. For particles smaller than 450 A switching occurs by approximately uniform rotation of the flower state. The switching fields are larger than the corresponding Stoner–Wohlfarth value. Beyond 450 A the application of an external field leads to the formation of a vortex configuration. The switching of the vortex con...
307 citations
TL;DR: A new technique to measure conduction electron relaxation times is described, using nonequilibrium magnetization present in a paramagnetic metal can be detected as an open circuit voltage across an interface between the paramagnet and a ferromagnet.
Abstract: Microscopic models are presented to elucidate the concept of interfacial charge-spin coupling. At the interface between a ferromagnet and a paramagnet, the spin subbands are loosely coupled, an interfacial conductance may be defined for each, and a result of their inequivalence is that an electric current flowing from a ferromagnetic metal into a paramagnetic metal will be partially spin polarized, i.e., will have an associated current of magnetization. The inverse is also true; nonequilibrium magnetization present in a paramagnetic metal can be detected as an open circuit voltage across an interface between the paramagnet and a ferromagnet. Using this effect, a new technique to measure conduction electron relaxation times is described.
282 citations
01 Jan 1988
TL;DR: Legvold et al. as discussed by the authors measured the basal plane magnetostrictions of Tb and Dy at low temperatures, which are 100 to 10000 times typical magnetostrains and still remain the largest known (~1%).
Abstract: By the early 1960’s, it was widely recognized that the rare earths possessed many extraordinary magnetic properties. Neutron diffraction measurements, for example, showed that the spin structures were much more complex than those of any of the classical ferromagnets or antiferromagnets. More importantly, in the heavy rare earth metals, the parallel coupling of large orbital and large spin angular momenta yielded huge magnetic moments of 9μ B and 10μ B, dwarfing the conventional values of 0.6 for Ni and 2.2 for Fe. Enormous magnetic anisotropies were also encountered in the heavy rare earth elements. In 1963, a breakthrough in magnetostrictive materials occurred with the measurement of the basal plane magnetostrictions of Tb and Dy at low temperatures (Legvold et al. 1963, Clark et al. 1963, 1965, Rhyne and Legvold 1965). These basal plane strains are 100 to 10000 times typical magnetostrictions and still remain today the largest known (~1%). Over wide temperature ranges, thermal expansions are dominated by the temperature dependences of the magnetostrains. Elastic moduli were found to be strongly influenced by the unprecedented magnetoelastic interactions. However, because of the low ordering temperatures of the rare earths the application of these magnetostrictive properties to devices operating at room temperature could not be achieved with the elements. Only Gd, which is essentially non-magnetostrictive, possesses a Curie point as high as room temperature.
240 citations
TL;DR: This work investigates the magnetic-field- and temperature-dependent equilibrium structure of magnetic superlattices formed from two ferromagnetic materials which couple antiferromagnetically at the interfaces with a macroscopic, Landau-Ginzburg, and microscopic approach.
Abstract: We investigate the magnetic-field- and temperature-dependent equilibrium structure of magnetic superlattices formed from two ferromagnetic materials which couple antiferromagnetically at the interfaces. Both a macroscopic, Landau-Ginzburg, and a microscopic approach are used. Due to competing exchange and Zeeman interactions, a variety of phases exist in the superlattice. There are aligned phases where all the spins are either parallel or antiparallel to the applied field, and there is a twisted phase where the spins in each layer lie at a different angle with respect to the applied field. We show that small changes in the layering structure can lead to dramatic changes in the phase diagram.
225 citations
TL;DR: Combined longitudinal and polar Kerr-effect measurements show that the anisotropy reverts to being in plane for films thicker than approx.
Abstract: The predicted perpendicular surface anisotropy is observed for ferromagnetic fcc Fe/Cu(100) via in situ polar Kerr-effect measurements. Square hysteresis loops are obtained for films 1.5 to 5.7 monolayers thick for 100-K growth. The region of stability of the perpendicularly magnetized state as a function of growth temperature and film thickness is delineated. Combined longitudinal and polar Kerr-effect measurements show that the anisotropy reverts to being in plane for films thicker than \ensuremath{\simeq}6 monolayers.
206 citations
TL;DR: The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems and a model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed.
Abstract: Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Above 16 K the dominant magnetic interactions are along a 1-D chain and, near T(c), 3-D bulk effects as evidenced by the value of the critical exponents dominate the susceptibility. The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems. Assuming the electron-transfer excitation arises from the POMO, ferromagnetic coupling by the McConnell mechanism requires stable radicals (neutral, cations/anions, or ions with small diamagnetic counterions) with a non-half-filled POMO. The lowest excited state formed via virtual charge transfer (retro or forward) must also have the same spin multiplicity and mix with the ground state. These requirements limit the structure of a radical to D(2d) or C>/=(3) symmetry where symmetry breaking distortions do not occur. Intrinsic doubly and triply degenerate orbitals are not necessary and accidental degeneracies suffice. To achieve bulk ferromagnetism, ferromagnetic coupling must be established throughout the solid and a microscopic model has been discussed. These requirements are met by [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Additionally this model suggests that the Ni(III) and Cr(III) analogs should be antiferromagnetic and ferrimagnetic, respectively, as preliminary data suggest. Additional studies are necessary to test and further develop the consequences of these concepts. Some molecular/organic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) with spin state S = 1/2 (...D.(+)A.(-)D.(+)A.(-)...) exhibit cooperative magnetic phenomena, that is, ferro-, antiferro-, ferri-, and metamagnetism. For [Fe(III)(C(5)Me(5))(2)].(+)[TCNE](-). (Me = methyl; TCNE = tetracyanoethylene), bulk ferromagnetic behavior is observed below the Curie temperature of 4.8 K. A model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed to understand the magnetic coupling as a function of electron configuration and direction of charge transfer. This model predicts that ferromagnetic coupling requires stable radicals with a non-half-filled degenerate valence orbital and a charge-transfer excited state with the same spin multiplicity that mixes with the ground state. Ferromagnetic coupling must dominate in all directions to achieve a bulk ferromagnet. Thus, the primary, secondary, and tertiary structures are crucial considerations for the design of molecular/organic ferromagnets.
197 citations
TL;DR: In this article, the demagnetizing field and magnetostatic energy of a thin film with surface roughness has been calculated, and it has been shown that the roughness gives rise to an effective perpendicular anisotropy.
Abstract: The demagnetizing field and magnetostatic energy of a thin film with surface roughness has been calculated. It is shown that the surface roughness gives rise to an effective perpendicular anisotropy whose order of magnitude is evaluated as a function of the parameters characterizing the roughness. The results are discussed in connection with experimental situations.
153 citations
TL;DR: In this paper, a full-potential linearized augmented plane wave method for the whole transition-metal series (V,Cr,Mn,Fe,Co,Ni) as overlayers on the Pd(001) surface is presented.
Abstract: We report systematic calculations based on the full-potential linearized augmented-plane-wave method for the whole transition-metal series (V,Cr,Mn,Fe,Co,Ni) as overlayers on the Pd(001) surface An energy analysis shows that Fe, Co, and Ni overlayers favor the ferromagnetic p(1 x 1) configuration, but V, Cr, and Mn, the antiferromagnetic c(2 x 2) superstructure We conjecture that this result is a general trend which should also be found on the (001) surfaces of Pt and the noble metals
TL;DR: It is shown that a phase transition to ferromagnetism occurs always for arbitrarily small anisotropy, and renormalization-group scaling relations for the transition temperature and the temperature variation of the correlation length are obtained.
Abstract: We discuss the nature of ferromagnetism in ultrathin films of magnetic ions, here regarded as two-dimensional Heisenberg ferromagnets subject to uniaxial anisotropy with the easy axis normal to the film We show that a phase transition to ferromagnetism occurs always for arbitrarily small anisotropy Renormalization-group scaling relations for the transition temperature and the temperature variation of the correlation length are obtained Implications of these results are discussed
TL;DR: Theoretic generalization of ferromagnetic ordering in low and three-dimensional organic substances has been achieved as discussed by the authors, and a number of 3D carbon structures have been forwarded.
TL;DR: In this paper, the constitutive law relating the time rate of change of the magnetic field H to that of the flux density B, via a differential equation, yields a faithful and yet computationally tractable representation of magnetic hysteresis.
Abstract: The constitutive law relating the time rate of change of the magnetic field H to that of the flux density B, via a differential equation, yields a faithful and yet computationally tractable representation of magnetic hysteresis. The equation is used to develop a theory of rate-independent and rate-dependent hysteresis in ferrites, ferromagnetic materials, magnetic thin films, and permanent magnetic materials. The theory provides mathematical expressions for the initial magnetization curve, the anhysteretic curve, the major loop, the symmetric and asymmetric minor loops, and the energy loss associated with their traversal. Functional forms for two material functions that appear in the equation can be scaled to measured values of the closure point, the remanence, the coercivity and, for rate-dependent applications, the nonlinear changes in the loop area and energy loss that accompany increases in dB/dt and dH/dt. Variations in loop shape and coercive point with angle observed in uniaxially anisotropic materials are described. Sample calculations are presented. >
TL;DR: Nucleation of the superconducting phase in superconductor-ferromagnet multilayers is studied theoretically and the theoretical results are in good agreement with experimental data for V/Fe multilayer.
Abstract: Nucleation of the superconducting phase in superconductor-ferromagnet multilayers is studied theoretically. When the superconducting layers are thin and decoupled by pair breaking in the ferromagnetic layers, the parallel critical field exhibits a nonlinear temperature dependence and nonmonotonic thickness dependence. The perpendicular critical field, corresponding to the nucleation of strongly modulated vortices, is also calculated. The theoretical results are in good agreement with experimental data for V/Fe multilayers.
TL;DR: In this article, it was shown that the demagnetization factor for an ultrathin ferromagnetic film is thickness dependent and that the anisotropy resulting from the dipole-dipole interaction can be interpreted as a surface and a volume anisotropic which depend on the crystalline structure and orientation of the film.
Abstract: At the boundary of a ferromagnetic material, the local change in the surroundings of the atomic magnetic moments induces an additional magnetic anisotropy. The dipole‐dipole interaction, responsible for the form‐dependent demagnetizing field inside the ferromagnet, differs for magnetic moments at the boundary and magnetic moments inside the bulk material. By calculation it is shown that the demagnetization factor for an ultrathin ferromagnetic film is thickness dependent. However, the anisotropy resulting from the dipole‐dipole interaction can be interpreted as a surface and a volume anisotropy which depend on the crystalline structure and orientation of the film, but are independent of the thickness of the film.
TL;DR: In this article, a serial radio-frequency sputter-microsectioning method with radioactive tracers 55Fe and 59Fe in the temperature range 766-1148 K was used to study the self-diffusion and isotope effect in α-iron.
TL;DR: In this article, the magnetic properties of equiatomic uranium transition metal ternary compounds of general formula UT(Si, Ge) have been investigated in the orthorhombic CeCu 2 -type structure.
TL;DR: The magnetic properties of the compounds AT2P2 (A = Sr, La, Eu; T = Fe, Co) and EuNi2p2 were studied by 57Fe and 151Eu Mossbauer spectroscopy, and magnetic susceptibility measurements.
TL;DR: In this article, the electrical and magnetic properties of several oxide systems of K2NiF4 structure have been compared to those of the corresponding perovskites, and generalizations of the properties of two-and three-dimensional oxide systems have emerged from these experimental observations.
IBM1
TL;DR: Afin de faire une exploration complementaire de la phenomenologie de l'anisotropie d'echange, et dans le but d'obtenir des observations experimentales additionnelles qui pourraient aider a elucider la nature microscopique of l'interaction d'interface, on realise une etude de R.M.F.
Abstract: Ferromagnetic resonance (FMR) spectra of Permalloy thin films exchange-coupled to iron-manganese films are analyzed. Studies were made on bilayer, ferromagnetic-antiferromagnetic (FA) and trilayer (AFA) structures, as a function of both F and A layer thicknesses in the range 20--800 A\r{}. Data are presented at a frequency of 9.3 GHz for both in-plane and perpendicular directions of the applied field, and at 34.1 GHz, in-plane. Analysis of these data enables extraction of the magnetization, gyromagnetic ratio, and an exchange shift due to spin-wave stiffness and perpendicular-surface anisotropy, as a function of layer thickness. The azimuthal dependence of the in-plane resonance is used to determine the magnitude of the exchange anisotropy (bias field). The magnetization and gyromagnetic ratio show little dependence on the thickness of either the F or A layer down to 50 A\r{}, implying that the interfaces are sharp on a scale of a few lattice constants. Within this interfacial region the magnetization is reduced as a result of interaction with the antiferromagnet. We suggest that the perpendicular-surface anisotropy is created by exchange coupling to the antiferromagnet whose easy axes are not in the plane of the interface. Finally, we suggest a model for exchange anisotropy in which the antiferromagnetic domain pattern is not totally locked, but adjusts in response to the ferromagnetization. Such a model qualitatively explains the bias field exerted by the antiferromagnetic layer deposited before the ferromagnet, the field-training effect, the FMR linewidth, and the magnitude of the bias field.
TL;DR: In this paper, the magnetic and crystallographic properties of the RFe/sub 10/T/sub 2/ compounds are discussed together with the properties of 3d-rich ternary compounds of different composition and structure.
Abstract: The crystal structure and the magnetic properties are determined for several novel ternary intermetallic compounds of the type RFe/sub 10/T/sub 2/, where R represents Ti, V, Cr, Mo, W, or Si. All these compounds show a strong ferromagnetic behavior, with Curie temperatures in the range 350 K to 600 K. The magnetic and crystallographic properties of the RFe/sub 10/T/sub 2/ compounds are discussed together with the properties of 3d-rich ternary compounds of different composition and structure. Special attention is paid to the magnetocrystalline anisotropy of these materials, consisting of contributions of the R sublattice and the Fe sublattice. Detailed information on the crystal-field-induced contribution of the former sublattice is derived from /sup 155/Gd Mossbauer spectroscopy performed on the Gd compounds of these series, since the /sup 155/Gd nucleus can be regarded as a sensitive probe of the electric field gradient at the nuclear site produced by the electric charges of the surrounding ions. It can be shown that this electric field gradient is proportional to the second order crystal field parameter A/sup 0//sub 2/, which in turn determines the magnetocrystalline anisotropy of the R sublattice. >
TL;DR: Recherche de termes topologiques dans un Lagrangien de champ moyen pour les ferromagnetiques et les antiferromagnetsiques inspiree par le modele de Hubbard.
Abstract: We look for possible topological terms in a mean-field Lagrangian for ferromagnets and antiferromagnets inspired by the Hubbard model and study their physical effects. A topological term corresponding to Berry's phase causes the order parameter to behave like a spin moment. We find that the Hopf term is not induced and thus there is no evidence for a neutral fermion in this theory.
TL;DR: In this article, the pyrochlores of the type R2Mn2O7 have been characterized where R is Dy, Ho, Er, Tm, Yb, Lu, or Y. All are semiconductors with activation energies ranging from 0.37 to 0.51 eV, which correlate well with the electronegativity of R.
TL;DR: Etude de la dependance en temperature de l'aimantation de surface dans le regime onde de spin dans la regime ond de spin.
Abstract: Etude de la dependance en temperature de l'aimantation de surface dans le regime onde de spin
TL;DR: In this article, the magnetic order at surfaces of the 4f rare earth metal terbium was investigated using electron capture spectroscopy (ECS), which probes the electron spin polarization (ESP) of the topmost surface layer.
Abstract: The magnetic order at surfaces of the 4f rare‐earth metal terbium is investigated using electron capture spectroscopy (ECS), which probes the electron spin polarization (ESP) of the topmost surface layer. In ECS, the capture of spin‐polarized electrons during grazing‐angle ion‐surface interaction is used to determine the ESP due to long‐ and short‐ranged surface magnetic order. It is found that long‐ranged ferromagnetic order exists far above the bulk Curie temperature, measured to be TCb =220 K. At 140 K, the long‐ranged ESP amounts to 24%. With increasing temperature, the ESP first decreases montonically up to the bulk Neel temperature TNb=228 K, then exhibits a pronounced maximum at T=238 K, and ultimately vanishes at the surface Curie temperature TCs =248 K. These striking results on enhanced magnetic order at Tb surfaces suggest the presence of very large surface anisotropies.
TL;DR: In this article, it was shown that starting materials for permanent magnets can be prepared from amorphous alloys after controlled crystallization, and that specific microstructures may have hard magnetic properties far superior to those attainable by normal casting and annealing procedures.
Abstract: It is shown that novel starting materials for permanent magnets can be prepared from amorphous alloys after controlled crystallization. The magnetic properties of several novel intermetallic rare earth compounds of a metastable nature that are absent in the corresponding phase diagram are described. We also discuss the importance of the possibility of generating specific microstructures of multiphase alloys by starting from the amorphous state. Such specific microstructures may have hard magnetic properties far superior to those attainable by normal casting and annealing procedures. Data are presented for the controlled crystallization of amorphous rare earth-iron-boron alloys close in composition to Fe 3 B.
TL;DR: Detailed measurements and analysis suggest that the phase transition can be described by the mean-field theory of Thio et al. only close to the N\'eel temperature.
Abstract: Neutron-scattering studies provide direct evidence for a field-driven magnetic transition which originates from the canting of spins out of the Cu${\mathrm{O}}_{2}$ planes: At the transition between antiferroand ferromagnetic ordering of the canted component of the spins in the layers, the (100) Bragg peak vanishes while the (201) peak appears. Detailed measurements and analysis suggest that the phase transition can be described by the mean-field theory of Thio et al. only close to the N\'eel temperature.
TL;DR: The electronic structure of 3d impurities in ferromagnetic metals is calculated by the linear-muffin-tin-orbital Green's-function method and the relative stability of different configurations is discussed.
Abstract: The electronic structure of 3d impurities (from Ti to Ni) in ferromagnetic metals (bcc and fcc Fe, hcp and fcc Co, fcc Ni) is calculated by the linear-muffin-tin-orbital Green's-function method. The values of local magnetic moments and effective exchange parameters are listed. For Mn impurities in bcc Fe and hcp Co, two possible impurity configurations with different spin direction are found. Based on the analysis of effective exchange parameters, the relative stability of different configurations is discussed.