Showing papers on "Ferromagnetism published in 1978"
TL;DR: In this article, the effect of strong random anisotropy in materials such as amorphous Tb•Fe and Dy•Fe at low temperature and also examine the role which weak random an isotropy might play in even ideally homogeneous soft materials.
Abstract: Random anisotropy is present in all amorphous magnetic materials, and depending on its strength, it can dramatically affect the magnetic behavior. We describe the effect of strong random anisotropy in materials such as amorphous Tb‐Fe and Dy‐Fe at low temperature and also examine the role which weak random anisotropy might play in even ideally homogeneous soft materials. Much of our analysis is based on the simple model proposed by Harris, Plischke and Zuckerman for a ferromagnet with random‐axis uniaxial anisotropy. We describe computer simulation results for this model and then develop scale length arguments which allow us to describe fluctuations in the magnetization direction. For a perfectly isotropic distribution of anisotropy axis we find that the conventional ferromagnetic ground state is unstable. The new ground state has large frozen in fluctuations but probably has a considerable moment and is, therefore, not spin glass‐like. This system does not support domain walls of the conventional type. For a system with both a macroscopic anisotropy axis and random anisotropy, we can have domain walls. We present a theory for the intrinsic coercivity which gives values of about 10−6 Oe for Fe‐metalloids and 0.2 Oe for Gd‐Co‐Mo. This indicates that inhomogeneities of larger than atomic scale are limiting the behavior of present materials. A model illustrating aspects of magnetic resonance behavior is also described.
659 citations
TL;DR: Au-Fe alloys have been studied by magnetic measurements of various types in the composition range where ferromagnetism evolves from spin-glass character, and the percolation limit identified from the results and other data is 15.5 + 0.3 at as mentioned in this paper.
Abstract: Au-Fe alloys have been studied by magnetic measurements of various types in the composition range where ferromagnetism evolves from spin-glass character. The percolation limit identified from the results and other data is 15.5 + 0.3 at.% Fe. Between this composition and ∼28% Fe, finite clusters co-exist in significant concentrations with the infinite cluster, and a fairly well-defined temperature observed in the magnetic properties is ascribed to a freezing of these clusters that parallels that of finite clusters in the pre-percolation alloys. Magnetic resonance is observed in alloys with more than about 13% Fe (i.e. in alloys containing large nearest-neighbour clusters) and the temperature dependence of the linewidth correlates with the interpretation given of the magnetization data.
205 citations
IBM1
TL;DR: Using angle-resolved photoemission and synchrotron radiation, the authors determined the energy-versus-momentum valence-band dispersion relations for a Ni(111) crystal.
Abstract: Using angle-resolved photoemission and synchrotron radiation, we have determined the energy-versus-momentum valence-band dispersion relations for a Ni(111) crystal. The temperature-dependent ferromagnetic exchange splitting has been directly observed. Both the $d$-band width (\ensuremath{\sim}3.4 eV at $L$) and exchange splitting (0.31 eV) are much smaller than theoretical estimates (\ensuremath{\sim}4.5 eV wide at $L$ with \ensuremath{\sim}0.7-eV splitting, respectively, at 293 K).
200 citations
TL;DR: In this article, the Curie temperature and magnetic susceptibility of itinerant electron ferromagnets are discussed from a general point of view by using a simple model with a small number of physical parameters and approximations in the functional integral formalism.
Abstract: Spin fluctuations and the thermodynamical properties of itinerant electron ferromagnets are discussed from a general point of view by using a simple model with a small number of physical parameters and approximations in the functional integral formalism. We give unified expressions for the Curie temperature and the magnetic susceptibility which interpolate between the weakly and strongly ferromagnetic limits. In general, both the transversal and longitudinal components of the local spin fluctuations contribute to the Curie constant and the former (latter) is predominant in the local moment (weakly ferromagnetic) limit. It is pointed out that the amplitude of the local spin fluctuations can be saturated when there is an upper or lower bound of the energy band not too far from the Fermi level, resulting in a Curie-Weiss susceptibility of a local moment type, even without well-defined local moments. A transition between two different kinds of local spin states can also be expected in some cases.
186 citations
IBM1
TL;DR: In this article, the ground-state properties of ferromagnetic fcc Co have been calculated in the local spin-density approximation for exchange and correlation, and large magnetic effects on the lattice constant and bulk modulus are found.
140 citations
TL;DR: Amorphous thin films of NdxCo1−x and NdxFe 1−x alloys were prepared over the compositional range 0.08⩽x ⩽0.71 by e −beam evaporation as discussed by the authors.
Abstract: Amorphous thin films of NdxCo1−x and NdxFe1−x alloys were prepared over the compositional range 0.08⩽x⩽0.71 by e‐beam evaporation. Magnetization and anisotropy of the samples were studied over a wide temperature range with the aid of a force balance magnetometer, Hall effect measurements, and Mossbauer spectroscopy. It was found that the magnetization of the alloys could not be accounted for by a completely collinear alignment (ferromagnetic) of the Nd and transition‐metal subnetworks. Mean field analysis of the magnetization data showed a large reduction of the Nd–transition‐metal exchange coupling as compared to their Gd analogs. A model was developed which requires that Nd be dispersed in a cone whose axis is parallel to that of the transition‐metal subnetwork by strong coupling to randomly oriented local crystal field axes. This dispersion reduces the Nd net moment to 77% of its free‐ion moment in NdxCo1−x alloys and to 25% of its free‐ion moment in NdxFe1−x alloys. There is evidence that some dispers...
135 citations
TL;DR: In this article, the low-field ac susceptibility on ternary alloys of Pd + 035 at% Fe and a Mn concentration of 0 to 8 at% reveal, for three distinct regimes of Mn concentration, a giant-moment ferromagnetic phase followed by a lower-temperature spinglass transition, and a spin-glass phase.
Abstract: Measurements of the low-field ac susceptibility on ternary alloys of Pd + 035 at% Fe and a Mn concentration of 0 to 8 at% reveal, for three distinct regimes of Mn concentration, (a) a giant-moment ferromagnetism, (b) a high-temperature ferromagnetic phase followed by a lower-temperature spin-glass transition, and (c) a spin-glass phase Our results for the susceptibility and the $T\ensuremath{-}c$ phase diagram are satisfactorily explained by the spin-glass theory of Sherrington and Kirkpatrick
128 citations
IBM1
TL;DR: In this article, the effect of a random uniaxial anisotropy axis on the magnetic properties of amorphous ferromagnet is considered, and it is found that in a quenched system random anisotropic with an isotropic angular distribution destroys the ferromagnetic state in fewer than four dimensions.
Abstract: The effect of a random uniaxial anisotropy axis on the magnetic properties of an amorphous ferromagnet is considered. It is found that in a quenched system random anisotropy with an isotropic angular distribution destroys the ferromagnetic state in fewer than four dimensions. The low-temperature phase is, instead, an Edwards-Anderson spin-glass. In more than four dimensions ferromagnetism or spin-glass ordering is found depending on the degree of disorder. Some properties of the ferromagnetic state are outlined.
127 citations
TL;DR: In this article, the effects of local magnetic anisotropy (RMA) on the magnetic properties of amorphous rare earth alloys have been examined with a view to assessing the effect of RMA on their magnetic properties.
123 citations
TL;DR: In this paper, anomalous-Hall effect measurements were used to determine the magnetization and the susceptibility of the Ni films with a thickness smaller than two atomic layers but show an enhanced susceptibility.
Abstract: Ni, Co, and Fe films of a few atomic layers are condensed in UHV at 10 K on metallic substrates. Anomalous-Hall-effect measurements are used to determine the magnetization and the susceptibility of the films. Ni films with a thickness smaller than two atomic layers possess no magnetic moment but show an enhanced susceptibility. Between two and three atomic layers of Ni the susceptibility diverges and thicker films possess a magnetic moment. For Co and Fe, the first monolayers already shows a magnetic moment.
102 citations
TL;DR: In this paper, the emission process from W-EuS junction field emitters was studied by combination of several experimental methods: measurement of emission current, spin polarization, energy distribution, and energy-selective spin polarization as function of emitter temperature for different annealing conditions.
Abstract: The emission process from W-EuS junction field emitters was studied by combination of several experimental methods: measurement of emission current, spin polarization, energy distribution, and energy-selective spin polarization as function of emitter temperature for different annealing conditions. The results show that with critical annealing a stoichiometric crystalline EuS layer can be obtained; the interface of such a layer on tungsten acts as a spin filter below the EuS Curie temperature. With emitters annealed at higher temperatures, evidence for electron trapping and the formation of spin clusters has been obtained.
TL;DR: In this paper, the microstructures of permanent magnet alloys after isothermal aging, thermomagnetic treatment, and step aging have been characterized by transmission electron microscopy and by measurement of the Curie temperature.
Abstract: The microstructures of an Fe-31wt%Cr-23wt%Co ductile permanent magnet alloy after isothermal aging, thermomagnetic treatment, and step aging have been characterized by transmission electron microscopy and by measurement of the Curie temperature. Isothermal aging itself produces the undesirable microstructure. Aging at 600°C develops the magnetic chromium-rich phase. Aging at 600°C produces a nonmagnetic chromium-rich phase dispersed within the iron-rich phase. The effect of thermomagnetic treatment on the microstructure of the alloy is discussed in comparison with that of Alnico alloys. Step aging produces the deskable microstructure, viz., the elongated ferromagnetic phase imbedded in the paramagnetic phase.
TL;DR: In this paper, the nature of the ferromagnetic interactions in cubic-perovskite type ARu 4+ O 3 (A=nonmagnetic cation) was investigated by using a magnetic balance technique.
Abstract: To clarify the nature of the ferromagnetic interactions in SrRuO 3 , the Ru 4+ – –Ru 4+ exchange couplings in cubic-perovskite type ARu 4+ O 3 (A=nonmagnetic cation) were investigated Magnetic measurements were done on these sample oxides (including single crystals of CaRuO 3 ) by using a magnetic balance technique In contrast to the ferromagnetic behaviors in (Ca 1- x Sr x )RuO 3 ( x ≧04), both the (La 05 Na 05 )RuO 3 metallic and (Ca 1- x Mg x )RuO 3 ( x =01, 03 and 05) semiconductive systems were found to be Curie-Weiss paramagnets having negative asymptotic Curie points On the (Ca 08 Sr 08 )RuO 3 sample, an anomalous hump of χ vs T curve was observed, which could be interpreted as being due to ferromagnetic clusters of Sr 2+ It was suggested that, besides the 180° superexchange coupling, another indirect-exchange interaction via Sr 2+ would be indispensable for the ferromagnetism in SrRuO 3
IBM1
TL;DR: The magnetic properties of rare-earth orthochromites have been intensively studied by neutron diffraction and inelastic scattering, bulk magnetization, optical absorption spectroscopy, specific heat, acoustic velocity, and resonance measurements as mentioned in this paper.
Abstract: Rare‐earth transition metal compounds are known to be able to (reversibly) absorb large quantities of hydrogen gas at moderate pressures. This absorption of H2 gas leads to substantial changes of the magnetic properties. These include hydrogen induced transitions from Pauli paramagnetism to ferromagnetism. Examples also exist of the reverse effect, i.e., where in ferromagnetic compounds the 3d moment disappears upon hydrogen absorption. The changes in magnetic properties, together with experimental results of X‐ray diffraction and Mossbauer‐effect spectroscopy are discussed in terms of charge transfer and changes in interatomic distances. The metastable character of the ternary hydrides often leads to a loss of the long range atomic order upon hydrogen absorption. This affects not only the absorption capacity (after repeated cycling) but also influences the magnetic behavior. Examples are shown where the atomic disorder has led to pronounced thermomagnetic history effects.
TL;DR: In this article, the intermetallic compound Fe3Sn2 has been studied by Mossbauer spectroscopy and magnetisation measurements and the transferred fields at the 119Sn nucleus agree with the model deduced from 57Fe MMS.
Abstract: The intermetallic compound Fe3Sn2 has been studied by Mossbauer spectroscopy and magnetisation measurements. This compound is ferromagnetic from approximately 220K to Tc=657K. In this temperature range, the spin directions are close to or parallel to the c axis while at low temperature they are perpendicular to it. The mean magnetic moment is 2.1 mu B per iron atom at 4K. The spread in the 57Fe hyperfine fields is due to anisotropic contributions while the spread in the spin directions at low temperature is related to a weak basal plane anisotropy energy. The transferred fields at the 119Sn nucleus agree with the model deduced from 57Fe Mossbauer spectroscopy.
TL;DR: In this article, the influence of spacially random dipolar, magnetocrystalline, magnetostrictive and exchange fluctuations on the law of approach to ferromagnetic saturation of amorphous materials was calculated.
TL;DR: In this article, it was shown that random off-diagonal exchange interaction coefficients destroy ferromagnetic long range order in isotropic systems with less than four spatial dimensions, and the proof breaks down when (cubic or hexagonal) symmetry breaking terms are introduced.
TL;DR: In this paper, a weak itinerant ferromagnetism model for YNi1−x amorphous alloys is proposed and the model of Edwards and Wohlfarth allows a satisfactory understanding of their major properties.
Abstract: YxNi1−x amorphous alloys are shown to behave as weak itinerant ferromagnets. The model of Edwards and Wohlfarth allows for a satisfactory understanding of their major properties. However, discrepancies with this model lead us to analyse our results in the light of recent theories of itinerant ferromagnetism. Satisfying agreement is found.
TL;DR: In this paper, the magnetic and crystallographic properties of rare earth compounds of the type RMg2 were studied and the lattice constants were determined for all the compounds in which R represents La, Ce, Pr, Nd, Sm and Gd were found to be of the cubic Laves phase structure.
Abstract: The magnetic and crystallographic properties of rare earth compounds of the type RMg2 were studied. The compounds in which R represents La, Ce, Pr, Nd, Sm and Gd were found to be of the cubic Laves phase structure. The remainder of compounds of this series crystallize in the hexagonal Laves phase structure. For all compounds the lattice constants were determined. Most of these compounds order ferromagnetically at temperatures well below 100 K. Exceptions are LaMg2, YbMg2 and YMg2 which exhibit Pauli paramagnetism, whereas SmMg2 and EuMg2 order antiferromagnetically. From the magnetic properties and from the lattice constants it is concluded that Eu and Yb are both divalent in these compounds. A peculiar temperature dependence of the magnetization was observed in SmMg2. A sharp peak‐like rise in the magnetization occurs within a temperature range of 10 K only, the magnetization being nearly temperature‐independent otherwise. Details regarding the nature of the crystal fields in the cubic RMg2 compounds were derived from specific heat measurement performed on LaMg2, CeMg2 in the temperature range 1.5–40 K.
TL;DR: In this paper, a preferential location of Fe 2+ and Fe 3+ ions on each antiferromagnetic sublattice is proposed, which is the first example of an annealed random spin system.
Abstract: Magnetic properties and electrical conductivity of the YFe 2 O 4 single crystal were investigated. It was disclosed that YFe 2 O 4 is an antiferromagnet whose Neel point is 205 K, and the spin lies parallel to the hexagonal c axis. Below T N , the magnetization parallel to the c axis was not proportional to the magnetic field. A large rotational hysteresis was observed at temperatures between 170 and 120 K. Below 140 K, a weak but very hard ferromagnetic moment parallel to the c axis resulted from field cooling. On the basis of these facts, a model of parasitic ferromagnetism, i.e., preferential location of Fe 2+ and Fe 3+ ions on each antiferromagnetic sublattice, is proposed. If such a preferential location is caused by the difference of the exchange interaction constant, YFe 2 O 4 is the first example of an annealed random spin system.
TL;DR: In this article, the authors measured the AC susceptibility with applied magnetic fields between 0 and 1500 Oe for two AuFe alloys above the ferromagnetic percolation concentration.
Abstract: The authors have measured the AC susceptibility with applied magnetic fields between 0 and 1500 Oe for two AuFe alloys above the ferromagnetic percolation concentration. The results indicate that spin glass ordering occurs at low temperatures and an extended T-c phase diagram is proposed.
TL;DR: In this article, the relationship between the microstructure and magnetic properties of heat treated Fe −23 wt%Cr −15 wt%, Co −5 wt %V has been studied by transmission electron microscopy and Lorentz microscopy.
Abstract: The relationship between the microstructure and magnetic properties of heat treated Fe–23 wt%Cr–15 wt% Co–5 wt%V has been studied by transmission electron microscopy and Lorentz microscopy. Three different heat treatments were adopted for the present investigations, viz., (1) isothermal aging, (2) thermomagnetic treatment (TMT) + step‐aging, (3) continuous cooling. It has been found that the magnetic properties of the alloy are very sensitive to the temperature of the TMT. Step‐aging gave the best magnetic properties, producing an elongated ferromagnetic phase, 300 A in diameter and 1200 A in length. Lorentz microscopy revealed domain walls and these lie within the Cr‐rich phase and pinned by the Fe‐rich phase in the isothermally aged alloy at 650 °C. Magnetic domains of optimally step‐aged alloys, 0.5 μm in width, are elongated along the direction of the applied magnetic field. The results suggest that the magnetic anistropy is introduced parallel to the direction of the applied magnetic field during TMT...
TL;DR: The theory of disordered magnetic systems was introduced in this article for disordered ferromagnets, in which the magnetic atoms are randomly located on sites of a crystal lattice.
Abstract: This review concerns disordered ferrotnagnets in which the magnetic atoms are randomly located on sites of a crystal lattice. Attention is given principally to magnets with strong spatial fluctuations of the exchange interaction. Naturally the magnetism of such systems differs substantially from the properties of ordinary ferromagnets; and for them, as for strongly disordered nonmagnetic systems, the methods of perturbation theory are inapplicable. The general concepts of the theory of disordered magnetic systems are presented with application to two models: disordered ferromagnets with interaction between nearest neighbors in the lattice, and ferromagnets in which the exchange interaction depends exponentially on the distance between the magnetic atoms. An example of ferromagnets of the second type is provided by dilute alloys of palladium with iron, cobalt, and manganese. The principal experimental results for these alloys are presented, and it is shown that the theory of disordered magnetic systems describes their properties well.
TL;DR: In this paper, the Hartree-Fock approximation of the periodic Anderson model for a lattice of magnetic ions is investigated in Hartree Fock approximation, and the effect of hybridization leading to reduced magnetic moments strongly depends on the position of the localized f levels relative to the conduction band.
Abstract: The periodic Anderson model for a lattice of magnetic ions is investigated in Hartree-Fock approximation. Attention is paid to different solutions of the self-consistency equations corresponding to ferromagnetic or antiferromagnetic ordering of the local magnetic moments. The effect of hybridization leading to reduced magnetic moments strongly depends on the position of the localizedf levels relative to the conduction band. For paramagnetic solutions with a non-integer value for thef level occupation number comparison is made with properties of intermediate valence rare earth compounds. The mean-field results for the susceptibility and specific heat agree with essential features found for these substances.
TL;DR: The magnetisation and Mossbauer effect have been measured for the Invar type FCC Fe-Pt alloys, and the atomic moment of the alloys varies linearly with concentration, following the Slater-Pauling curve, which indicates that the iron atom possesses a full moment of 2.7 mu B as mentioned in this paper.
Abstract: The magnetisation and Mossbauer effect have been measured for the Invar type FCC Fe-Pt alloys. The atomic moment of the alloys varies linearly with concentration, following the Slater-Pauling curve, which indicates that the iron atom possesses a full moment of 2.7 mu B even in the Invar region. The hyperfine field acting on the Fe nucleus in these alloys is independent of concentration. These results imply that the Fe-Pt Invar alloys are strong ferromagnets, i.e., weak itinerant ferromagnetism is not responsible for the Invar properties of Fe-Pt alloys, in contrast to the Fe-Ni Invar alloys.
TL;DR: In this article, the boundaries between the normal paramagnetic, superconducting and normal magnetically ordered phases in the tetragonal pseudoternary rare earth system (Lu 1− x Ho x )Rh 4 B 4 have been established by means of a.c. magnetic susceptibility measurements to temperatures as low as 0.07 K.
Abstract: The boundaries between the normal paramagnetic, superconducting and normal magnetically ordered phases in the tetragonal pseudoternary rare earth system (Lu 1− x Ho x )Rh 4 B 4 have been established by means of a.c. magnetic susceptibility measurements to temperatures as low as 0.07 K. The heat capacities of the ternary compounds LuRh 4 B 4 and HoRh 4 B 4 and the pseudoternary compound (Lu 0.5 Ho 0.5 Rh 4 B 4 have been measured from 0.5 K to 36 K. The static magnetic susceptibility and a.c. electrical resistance have been determined between about 1 K and room temperature for the ferromagnetic ternary compound HoRh 4 B 4 .
TL;DR: In this paper, the authors investigated the magnetic properties of Cr-Fe alloys and found that small-angle scattering peaked in the forward direction at low temperatures, corresponding to the growth of magnetic correlations over distances of the order of 400'A.
Abstract: Although the magnetic properties of Cr–Fe alloys have been widely studied, there are different interpretations of existing data in the literature, especially for alloys in the concentration range 10 to 25% Fe. The onset of ferromagnetism occurs somewhere in this concentration range, but there is little direct evidence of the precise critical concentration. Detailed small-angle scattering measurements have been carried out on alloys containing 16.7, 19.9, 20.9, 21.7, 24 and 25% Fe over a wide range of temperatures and magnetic fields. Small-angle scattering peaked in the forward direction develops at low temperatures, corresponding to the growth of magnetic correlations over distances of the order of 400 A. The response of the scattering to the application of a magnetic field suggests that the scattering at low temperatures arises from superparamagnetic clusters. Curie temperatures were deduced from the critical scattering, and the variation of Tc with composition suggests that the critical concentration is close to 19% Fe.
TL;DR: Amorphous films of GdxAl1−x and GdyCu1−y where x and y vary from ∼0.2 to 0.8 have been prepared by sputtering as discussed by the authors.
Abstract: Amorphous films of GdxAl1−x and GdyCu1−y where x and y vary from ∼0.2 to 0.8 have been prepared by sputtering. Magnetization measurements in fields from 0.1 to 18 000 Oe over the temperature range 4.2 to 300 °K allowed us to determine the magnetic phase diagram for these systems. Spin glass and ferromagnetic states as well as intermediate regions have been identified. For all compositions the paramagnetic Curie temperature is positive having values from 18 to 150 °K. When x ?0.40, GdxAl1−x displays a susceptibility cusp and relaxation effect characteristic of a spin glass. For compositions x=0.56 and 0.81 a peak in the low field susceptibility is found which is 30 °K below the ferromagnetic Curie temperature as estimated by an Arrott plot. Since magnetic saturation at 4.2 °K is not reached for these two alloys, we assign them to an intermediate state. The GdyCu1−y system also has spin glass like behavior at y=0.21 and y=0.31 but at higher concentrations only easily saturated ferromagnetism is found. Since...