Showing papers on "Magnetization published in 1976"

Magnetic heat pumping near room temperature

Abstract: It is shown that magnetic heat pumping can be made practical at room temperature by using a ferromagnetic material with a Curie point at or near operating temperature and an appropriate regenerative thermodynamic cycle. Measurements are performed which show that gadolinium is a resonable working material and it is found that the application of a 7-T magnetic field to gadolinium at the Curie point (293 K) causes a heat release of 4 kJ/kg under isothermal conditions or a temperature rise of 14 K under adiabatic conditions. A regeneration technique can be used to lift the load of the lattice and electronic heat capacities off the magnetic system in order to span a reasonable temperature difference and to pump as much entropy per cycle as possible

Band model for magnetism of transition metals in the spin-density-functional formalism

Abstract: The Hohenberg-Kohn-Sham spin-density-functional (SDF) formalism is applied. A Stoner-like band mode is derived in the SDF formalism. The solutions of the SDF equations are assumed to have Bloch character and perturbation theory is used to show that the energy splitting between spin up and spin down states is approximately wavevector independent and proportional to an energy-dependent Stoner parameter. This result makes it possible to obtain magnetic properties from the paramagnetic density of states and the Stoner parameter alone. Results in the local-spin-density approximation for the Stoner parameters of V, Fe, Co, Ni, Pd, and Pt are presented. The relative stability of the para and ferromagnetic states is found to be correct for all the elements investigated and the total magnetization compares favourably with experiment. Values for the Curie temperature are systematically too high. Experimental estimates of the Stoner parameter are compared with the calculated values and the deviation is at most a few tenths of an eV.

Mössbauer studies of thermal excitations in magnetically ordered microcrystals

Abstract: A simple model is developed to explain the often-observed diminished values of the magnetic hyperfine fields in microcrystals. It is shown that below the superparamagnetic blocking temperature thermally excited oscillations of the magnetization around an energy minimum reduce the average magnetization and the magnetic hyperfine splitting in the Mossbauer spectrum. In a microcrystal of volumeV these quantities are reduced by a factor of about 1−kT/(2κV), wherekT is the thermal energy and κ is related to the anisotropy constant. Mossbauer spectra of 60 A, 100 A, and 120 A Fe3O4 particles and 120 A α-Fe2O3 particles show excellent agreement with the theory.

Magnetic Structure of an Amorphous Rare-Earth Transition-Metal Alloy

Abstract: /sup 161/Dy Mossbauer spectra show that the dysprosium moments in amorphous DyCo/sub 3.4/ are strongly coupled to the local crystal field axes. The magnetic structure is one in which the cobalt ''sublattice'' is strongly ferromagnetic, and the dysprosium moments are almost randomly distributed in directions between antiparallel and perpendicular to the cobalt. (AIP)

Spin-reorientation transitions in rare-earth magnets

Abstract: This review treats the little-studied magnetic phase transitions of the order-order, spin-reorientation type, in which a rotation of the magnetization vector (or of die antiferromagnetism vector) with respect to the crystallographic axes occurs upon change of temperature, magnetic field, or external pressure. It presents theoretical ideas about magnetic phase transitions of the spin-reorientation type; it considers the effect of domain structure on spin-reorientation phase transitions, aad critical fluctuations during such transitions. Taking as an example rare-earth magnets (orthoferrites, iron-garnets, and rare-earth metals and intermetallic compounds), it considers the experimental data on various types of spin-reorientation transitions and on the anomalies of physical properties at such transitions.

Magnetic Properties of SrRuO 3 Single Crystal

Abstract: Magnetic measurements on single-crystal specimens of SrRuO 3 having perovskite structure were made by a magnetic balance. It was found that the ferromagnetic moment in SrRuO 3 could be technically saturated in a finite magnetic field along an easy axis, . The spontaneous moment was estimated to be 1.10±0.09 µ B at 4.2 K. The thermomagnetic curve could be well reproduced by the one calculated in the molecular-field approximation for S =1/2. The χ -1 vs T curve measured obeys the Curie-Weiss law from 300 K to 700 K. SrRuO 3 has a fairly large amount of cubic magnetocrystalline anisotropy (\(K_{1}{\cong}-1.6{\times}10^{6}\) erg/cm 3 at 4.2 K.). It is concluded that the reduced ferromagnetic moment in this case is smaller than the one expected for the low-spin state of Ru ions at the octahedral sites, and that this can not be attributed to any technical magnetization processes.

Modified mean‐field model for rare‐earth–iron amorphous alloys

Abstract: A mean‐field model of the magnetic properties of amorphous rare‐earth–iron alloys has been developed which incorporates an Fe spin whose dependence on both the concentration and the species of rare earth is based on Mossbauer spectra. Using this model with a single fixed set of exchange constants, we have been able to calculate Curie temperatures which are in reasonable agreement with the data for amorphous rare‐earth (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu–iron alloys containing between 50 and 100 at.% iron. Better agreement with the measured Curie temperatures is achieved if the magnitudes of the Fe‐Fe and rare‐earth–Fe exchange constants are allowed to increase linearly with rare‐earth concentration. The latter case has the additional advantage of reproducing the observed shape of the magnetization‐vs‐temperature curves for wide range of composition and rare‐earth species.

Magnetic viscosity in ferromagnets

Abstract: The thermal activation theory of magnetic viscosity is reviewed Equations are derived relating the maximum slope of the magnetization decrement in (time) curve to absolute temperature, irreversible susceptibility, and the rate of change of activation energy with field at constant temperature, (∂∊/∂H)T The activation energy ∊ is, in general, a function of the applied field H and an intrinsic function of the temperature T If, however, (∂∊/∂T)H=0 then (∂∊/∂H)T≃28·5k/(dH/dT) where H is the field at which the viscosity curve slope is a maximum Deviations from this relationship indicate an intrinsic temperature variation of the activation energy ∊ and thus enable thermal activation and intrinsic temperature effects to be separated Experimental data on magnetic viscosity are re-examined from this point of view

Constraints Imposed by the Shape of Marine Magnetic Anomalies on the Magnetic Source

Abstract: A two-layer source model for marine magnetic anomalies can accommodate several observations made on the shapes of anomalies in the Pacific and southeast Indian oceans. The layers are defined on the basis of cooling history and magnetic properties. The upper layer consists of rapidly cooled basalts, which acquire a strong magnetization near the ridge axis. This layer, with narrow transition zones, can account for the observed short polarity events. The lower layer consists of moderately magnetized, slowly cooled intrusive rocks in the lower oceanic crust. The transition zones in this layer are broad, sloping boundaries reflecting the delayed acquisition of magnetization with depth as, for example, along a sloping Curie point isotherm. The lower layer can account for a skewness discrepancy of 10°–15° in the observed skewness of some anomalies. It is shown that the upper layer has to contribute about three quarters of the total amplitude of magnetic anomalies in order for this model to simulate the observed shape of the anomalies. The model predicts that a deep drill hole located just to the older side of a reversal boundary in the upper part of the oceanic crust should encounter a magnetization polarity reversal within the lower oceanic crust.

Magnetic hysteresis properties of synthetic titanomagnetites

Abstract: Synthetic titanomagnetites ((1−x)Fe3O4, - xFe2TiO4, 0 ≤ × ≤ 0.6) were prepared by the ceramic oxide method and the equilibrium atmosphere method, and two size fractions were separated out. The first contained particles of size >150 μm, while the second, which was ground in a water slurry, had a mean particle size of about 0.1μm. The magnetic hysteresis properties, i.e., coercive force HC, remanent coercive force ERC, saturation remanence JRS, JRS/JS, and initial susceptibility x0, were measured at room temperature. The coarse grain samples were found to be magnetically soft (HC ∼ 30 Oe) and the hysteretic properties independent of composition. In contrast, the fine grain samples were extremely hard (HC ∼ 400–2000 Oe) and their magnetic characteristics strongly dependent on composition. When compared with the Stoner-Wohlfarth model for single-domain magnetization, the agreement was good, indicating that these latter samples contain mainly single-domain grains. Samples containing both hard and soft fractions exhibited hysteretic properties that depended on the relative proportions of each size fraction.

Magnetization and magnetic anisotropy in evaporated GdCo amorphous films

Abstract: Saturation magnetizations and magnetic anisotropy constants were determined for a series of amorphous GdCo films prepared by thermal evaporation. The films covered the composition range from Gd0.05Co0.95 to Gd0.40Co0.60 and were studied by means of ferromagnetic resonance and a force balance magnetometer. The films, in contrast to those prepared by sputtering, had a hard perpendicular direction of magnetization when Co was less than 91 at.%, and easy and moderate directions within the plane of the film. The anisotropy constant can be expressed in terms of sublattice magnetizations by Ku=0.660MCo2 +2.218MCoMGd+0.097MGd2. The results are explained based on a pair‐ordering mechanism. The composition and temperature dependence of saturation magnetization is essentially the same in evaporated and sputtered films.

Nonmajor cubic symmetry axes of easy magnetization in rare-earth-iron-laves compounds

Abstract: Unusual [$uuw$]- and [$uv0$]-type axes of easy magnetization have been observed in some cubic rare-earth-iron Laves compounds. The presence of these directions of spontaneous magnetization can be accounted for, within the phenomenological treatment of the magnetic anisotropy, by including eighth-power direction-cosine terms in the power expansion of the magnetic anisotropy energy. It will also be shown that the single-ion model predicts the existence of these directions. The conditions imposed on the bulk magnetic anisotropy constants are derived. Typical values of these constants in rare-earth-iron Laves phases are calculated using the single-ion model.

Ferromagnetism in iron-chromium alloys. I. Bulk magnetization measurements

Abstract: Bulk magnetization measurements were made between 5 and 300 K in fields up to 14 kOe on a series of iron-chromium alloys containing from 2- to 70-at.% Cr. The temperature dependence of the magnetization was analyzed to yield spin-wave stiffness coefficients that are systematically different from values obtained directly by inelastic-neutron-scattering measurements. The concentration dependence of the mean moment is approximately linear and is close to the Slater-Pauling curve. However, subtle deviations from linearity reflect the concentration dependence of the individual iron and chromium moments, which is presented in part II. The concentration dependence of the moments can be approximately reproduced by existing coherent-potential-approximation calculations.

Magnetization distribution in an amorphous ferromagnet

Abstract: Experimental results of magnetization and M\"ossbauer measurements on an amorphous alloy ${\mathrm{Fe}}_{75}$${\mathrm{P}}_{15}$${\mathrm{C}}_{10}$ are presented to show that there is a sizable fluctuation in exchange interaction in amorphous ferromagnets. Like several other amorphous ferromagnetic alloys, the spontaneous magnetization as a function of the reduced temperature for the amorphous alloy falls characteristically below those of crystalline ferromagnets such as ${\mathrm{Fe}}_{3}$P and Fe. A comparison of the magnetization data with Hadrich's theory for amorphous ferromagnets suggests that in the amorphous alloy the average variation in the exchange constant ($J$) can be as large as $J$ itself. The M\"ossbauer spectrum is analyzed in terms of a distribution of hyperfine fields which suggests a spatial distribution of magnetization that is compatible with the exchange fluctuation as deduced from the temperature dependence of magnetization. The origin of magnetization fluctuation in amorphous alloys is discussed. It is pointed out that a broad distribution of magnetic ordering is consistent with the experimental observation of the coexistence of Kondo-type resistance minimum and ferromagnetism in amorphous alloys.

Magnetization of the oceanic crust

Abstract: Summary. The magnetization of the oceanic crust can be studied both directly, by looking at the magnetization of rocks collected from the oceans, and also indirectly, by looking at the magnetic anomalies. These methods are discussed. Magnetic measurements on basement samples collected by the Deep Sea Drilling Project (DSDP) show that these samples have lower magnetizations than dredged samples. The DSDP results suggest that the layer causing the magnetic anomalies has to be much thicker than the normally assumed thickness of 0.5 km. A theoretical power spectrum for lineated magnetic anomalies is developed. It was assumed that the anomalies were caused by normally- and reversely-magnetized bands of magnetic material formed during periods of normal and reversed magnetic field, the lengths of the times of constant field polarity being assumed to be exponentially distributed. Comparison of this theoretical power spectrum with published power spectra also suggests that the magnetic anomaly sources are more deeply located within the oceanic crust. The rock types which may contribute to the magnetic anomalies are discussed.

Electronic Structure of Ni-Mn and Ni-Fe Alloys

Abstract: The electronic structures of ferromagnetic Ni-Mn and Ni-Fe alloys are investigated theoretically. The Hartree-Fock calculation combined with the coherent potential approximation is shown to give two magnetic states for Mn and Fe atoms in Ni-Mn and Ni-Fe alloys, respectively. By introducing the ternary alloy picture in which Mn(Fe) atoms having antiparallel moments to the bulk magnetization are distinguished from Mn(Fe) atoms having parallel moments for a given Ni-Mn(Ni-Fe) alloy, the possibility of the coexistence of the two magnetic states is shown by an energy consideration. On the basis of the results obtained by this calculation, various magnetic properties are discussed.

Magnetization, Thermal Expansion and Low Temperature Specific Heat of Fe 72 Pt 28 Invar Alloy

Abstract: In order to clarify the effect of concentration fluctuation on the Invar effect, the temperature dependences of magnetization and thermal expansion and the low temperature specific heat of the Fe 72 Pt 28 alloy were measured in both the ordered and disordered states. The thermal expansion anomaly, which is the most essential Invar effect, was observed in both states of the alloy. This anomaly is more remarkable in the disordered state than in the ordered state, particularly around T C . This different behavior can be attributable to the difference in the magnetization versus temperature curve; the magnetization of the disordered alloy decreases more rapidly around T C . The spontaneous volume magnetostriction at 0 K were estimated for the ordered and disordered states as 1.4×10 -2 and 1.6×10 -2 , respectively. These phenomena can not be explained by the proposed models for the Invar effect based on the inhomogeneity or the fluctuation of composition.

Random Mixture of the Ising Magnets in a Magnetic Field: Quenched Site and Bond Problems

Abstract: (1974), 120] to obtain the specific heat and the susceptibility of the random mixture of magnets, is applied for the low·field expansion of the free energy and the magnetization. The quartic terms of the free energies of the linear chain and of the infinite Bethe lattice for the site and the bond problems are obtained. The exact solution of the infinite Bethe lattice is equivalent to the Bethe approximation of the ordinary lattices. A divergence of the quartic term of the free energy of the bond problem is discussed in connection with a phase transition relating to the glass-like phase. Transparent formal similarity (which serves as a check and an outlook) between the site and the bond problems is found, and a relation (which serves as an approximation) between the quenched and the annealed systems is discussed. § 1. Introduction and conclusion In a previous paperv a method using projection operators to obtain thermo­ dynamic and magnetic properties of the random mixture of the magnets (the site and the bond problems in the quenched Ising spin systems) was presented. The method was applied to the linear chain and to the infinite Bethe lattice (of which the exact solution is equivalent to the Bethe approximation) giving the free energy and the susceptibility at zero field. A remarkable distinction in the phase diagrams between the site and the bond problems was clarified. The method was also applied to the quenched classical Heisenberg model.") For the free energy and the magnetization at a finite magnetic field, a concentration expansion was carried out and anomalous behavior in the magnetization process of the dilute linear chain at low temperatures was explained.') In this paper the method of Ref. 1) is applied to the low-field expansion of the free energy and the magnetization of the site and the bond problems for the linear chain and for the infinite Bethe lattice, and the quartic terms with respect to the magnetic field is obtained. The divergence of the second derivative of the susceptibility (a 2x/(JH2 ) of the bond problem characterizes the appearance of the glass-like phase introduced by Matsubara and Sakata.4l Transparent formal simi­ larity between the site and the bond problems is found and it serves as a good check. An approximation for the bond model and a relation to the annealed system

Correlation between Spin Polarization of Tunnel Currents from 3 d Ferromagnets and Their Magnetic Moments

Abstract: We report measurements of spin-polarized tunnel currents from a wide compositional range of alloys of Ni with Fe, Mn, Cr, or Ti. The interpretation of the data indicates that all the alloys have an electron spin polarization $P$ which is positive and whose magnitude follows closely the saturation magnetic moment of the alloy. This simple result greatly constrains possible explanations of spin-polarized tunneling from $3d$ ferromagnetic metals.

Magnetic properties of amorphous Gd-Fe films prepared by evaporation

Abstract: Amorphous GdxFe1−x films with 0.18

Correlation of the internal fields, magnetic moments, and site preferences in Fe 3-x Mn x Si alloys

Abstract: A detailed study of changes in the internal fields at Mn, Fe, and Si nuclei in the solid solutions of ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Si}$, $0\ensuremath{\le}x\ensuremath{\le}1.6$, as a function of composition has been made by spin-echo NMR at 1.28 K and in zero external magnetic field. Resonances due to Mn in three crystallographically different sites are observed. For $xl0.75$, Mn enters predominantly the site with eight Fe first near neighbors and has an internal field of 259 kG shifted about 5.4 kG to lower values for each added Mn third near neighbor. For $xg0.75$, Mn atoms also enter the other transition-metal site which has four Fe and Mn, and four Si first near neighbors and has an internal field of about 28 kG at $x=0.75$. This field shifts to lower values as $x$ increases. Mn enters the Si sites due to a small residual disorder of about 3% between Si and the Fe site with eight Fe first near neighbors. The internal field of Mn in these sites is about 127 kG at $x=0.08$ and shifts slowly to higher values. The Fe and Si fields are compared with those in ${\mathrm{Fe}}_{3}$Si alloys. The values of the internal fields and their variation with concentration at all sites are consistent with the magnetic moment and magnetization data of Yoon et al. Contributions to the internal fields from various neighbors are analyzed.