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Showing papers on "Curie temperature published in 1982"


Patent
11 Mar 1982
TL;DR: In this paper, the electrical heating element of the present invention is consisted of a core 7(25) or a substrate 35 of a non-magnetic material having high thermal and electrical conductivity, clad with a surface layer 9(27, 37) of a ferromagnetic material of relatively low electrical conductivities.
Abstract: There are many situations requiring temperature regulation in heating elements which the prior art feedback control systems are not capable of handling adequately. In order to provide a temperature regulation in a narrow range around the Curie temperature, the electrical heating element of the present invention is consisted of a core 7(25) or a substrate 35 of a non-magnetic material having high thermal and electrical conductivity, clad with a surface layer 9(27, 37) of a ferromagnetic material of relatively low electrical conductivity. When the heating element is energized by an electrical source 3(41) of high frequency alternating current, the skin effect initially confines current flow principally to the surface layer of ferromagnetic material. As temperature rises into the region of the Curie temperature of the ferromagnetic material, however, the decline in magnetic permeability of the ferromagnetic material causes a significant lessening of the skin effect, permitting migration of current into the high conductivity non-magnetic core or the substrate, thereby simultaneously enlarging the cross-sectional area of the current flow path and expanding it into the highly conductive material; the resistance of the heating element becomes less due to both causes. By selecting the proper frequency for energization, by regulating the source to produce constant current, and by selecting dimensions and material parameters for the heating element, temperature regulation in a narrow range around the Curie temperature of the ferromagnetic material can be produced, despite considerable fluctuations in thermal load.

228 citations


Journal ArticleDOI
TL;DR: The electrostrictive properties of relaxor-type ferroelectrics such as PMN have been studied in this paper, where the authors show that the induced elastic strain and electric polarization hold even below the average Curie temperature.
Abstract: Lead magnesium niobate Pb(Mg1/3Nb2/3)O3 (PMN) is a relaxor ferroelectric with a disordered perovskite structure. A statistical inhomogeneity in the distribution of Mg2+ and Nb5+ ions in the B-sites of an ABO3 perovskite lattice leads to a wide Curie temperature range in which the crystal exhibits an intimate mixture of paraelectric and ferroelectric microregions. Relaxor-type ferroelectrics such as PMN show somewhat unusual electrostrictive phenomena. Although the polarization-related electrostrictive coefficients Q (defined as x = QP2 ) are one order of magnitude smaller than those of simple perovskite compounds, large electrostrictive strains can be obtained because of their huge dielectric constants. A quadratic relation between the induced elastic strain and the electric polarization holds even below the average Curie temperature. The electrostrictive properties of PMN and related materials are reviewed in this article. Relaxor ferroelectrics are promising transducer materials for use as acti...

184 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reviewed the current understanding of the mechanisms and their dependence on the composition of the ceramic and on the experimental parameters, and presented models that attribute the losses to the damping of a moving domain wall.

151 citations


Journal ArticleDOI
TL;DR: In this paper, the coercive force of Fe92Zr8 amorphous alloy drastically increases below 30 K, and the high-field susceptibility of the present alloys are extremely large.
Abstract: Several kinds of Fe‐Zr amorphous ribbons have been prepared from the melt in an argon atmosphere, and the spin‐glass‐like behaviors have been investigated. The thermomagnetic history has been observed, and the freezing temperature monotonously decreases, in contrast to the Curie temperature, which increases and then goes through a maximum with increasing Zr content. Asymmetric hysteresis loops and the relaxation of remanence have also been found. The coercive force of Fe92Zr8 amorphous alloy drastically increases below 30 K, and the high‐field susceptibility of the present alloys are extremely large. These behaviors are explained in terms of the coexistence of ferromagnetic and antiferromagnetic states.

126 citations


Journal ArticleDOI
TL;DR: In this paper, a pigment of Fe4N particles for magnetic recording was prepared by nitrogenizing acicular metal iron powder, and the chemical treatment needed to obtain stoichiometric Fe 4N powder was studied, and it was found that Fe 4 N powder was obtained when the Fe powder was heated at about 400°C in a mixture of H2−NH 3 (65-80 vol%).
Abstract: A pigment of Fe4N particles for magnetic recording was prepared by nitrogenizing acicular metal iron powder. The chemical treatment needed to obtain stoichiometric Fe4N powder was studied, and it was found that Fe4N powder was obtained when the Fe powder was heated at about 400°C in a mixture of H2–NH3 (65–80 vol.%). The Curie point of the powder coincided well with that of bulk Fe4N. The coercive force of Fe4N was 640 Oe, which is considerably smaller than that of the Fe powder used as the starting material. The dispersion of magnetic anisotropy was measured by a torque meter, and the decrease in the coercivity of nitrogenized iron powder was attributed to the exchange anisotropy of the surface layer.

96 citations


Journal ArticleDOI
E. M. Gyorgy1, D. B. McWhan1, J. F. Dillon1, L. R. Walker1, Joseph V. Waszczak1 
TL;DR: In this paper, a series of modulated Cu-Ni films with varying wavelength and amplitude of the compositional modulation were presented. But the properties of the films were not analyzed.
Abstract: A series of modulated Cu-Ni films has been prepared with varying wavelength, $\ensuremath{\lambda}$, and amplitude, $A$, of the compositional modulation. The films have coherent interfaces between the copper and nickel regions over at least the range 16 $l\ensuremath{\lambda}l$ 120 \AA{} and several of the films are essentially single crystals containing growth faults. A model for the magnetic behavior based on the assumption that each layer is not significantly different from a random alloy of the same composition agrees with the measurements of the average magnetic moment and the Curie temperature. This suggests the absence of significant two-dimensional effects.

93 citations


20 Dec 1982
TL;DR: In this article, the results of Ozima & Sakamoto as a decomposition process were compared with those of Readman & O'Reilly with some exceptions, which may have been caused by the differences in starting materials.
Abstract: Summary. Low-temperature oxidation of titanomagnetites (x = 0.3, 0.5 and 0.7) was investigated in this study. Starting materials were carefully prepared so that their Curie points and lattice constants were in agreement with reliable values. Changes in Curie points and lattice constants during the process of low-temperature oxidation were then studied in detail. Independent heatings were performed under various conditions and oxidation parameters (z) were determined by chemjcal analyses. Though this method could not provide continuous z values like the weight increase method, reproducibility was very high when samples were heated in the same condition and the various states were consistent with each other. Oxidation states from z = 0.0 to 1.0 were realized in three compositions. When titanomagnetite in basalt was heated under various conditions a change occurred, either oxidation, decomposition or reduction. Natural titanomagnetite minerals closely followed the oxidation-reduction trends established for synthetic samples. The present results are consistent with those of Readman & O’Reilly with some exceptions. The differences may have been caused by the differences in starting materials. The results of Keefer & Shive are similar to those of Ozima & Sakamoto and are completely different from ours. They detected a rhombohedral phase and indicated that the decomposition process proceeds in the same manner reported by Ozima & Sakamoto. We are unable to account for their results. It may be useful to consider the results of Ozima & Sakamoto as a decomposition process. This idea has not been presented previously in the literature.

88 citations


Journal ArticleDOI
TL;DR: In this article, a boundary condition on temperature at depth in the continental crust can in principle be obtained by mapping of the Curie isotherm where it forms the base of the magnetic crust.
Abstract: A boundary condition on temperature at depth in the continental crust can in principle be obtained by mapping of the Curie isotherm where it forms the base of the magnetic crust. An approach to mapping magnetic crustal thickness using satellite magnetic anomaly data is described. Because magnetic crustal thickness is small with respect to satellite altitude, inversion of the anomaly data yields magnetization models expressing regional variations in the vertical integral of magnetization. This parameter is inversely correlated with regional heat flow in the western United States, suggesting that areas of low apparent magnetization are areas of thin magnetic crust resulting from a shallow Curie isotherm. For a simple model of a uniformly magnetized magnetic crust (or intra-crustal layer),Curie depth is linearly related to apparent magnetization. If the relation can be calibrated by two or more Curie depth estimates, regional apparent Curie isotherm configuration can be mapped. In an alternative approach, a family of Curie depth models can be computed using assumed magnetization values. In each case, the Curie depth configuration is used as a boundary condition in a thermal model. That model is selected which gives a heat flow distribution best fitting observations. Conductive heat flow models for a traverse across the Rio Grande Rift using a finite element technique and for a map grid using a one-dimensional thermal model approximation are given as examples.

76 citations


Journal ArticleDOI
TL;DR: In this paper, the authors report detailed measurements of the magnetization of Cu/Ni composition modulated foils as a function of temperature, magnetic field, composition wavelength, and composition amplitude.
Abstract: We report detailed measurements of the magnetization of Cu/Ni composition modulated foils as a function of temperature, magnetic field, composition wavelength, and composition amplitude. We find a Curie temperature that initially increases rapidly with wavelength but quickly saturates. We also find that the Curie temperature is independent of composition amplitude which we ascribe to the existence of disk‐like Ni clusters.

72 citations


Patent
20 Jan 1982
TL;DR: In this article, a wheel-type magnetic refrigerator capable of cooling over a large temperature range is described, where Ferromagnetic or paramagnetic porous materials are layered circumferentially according to their Curie temperature.
Abstract: The disclosure is directed to a wheel-type magnetic refrigerator capable of cooling over a large temperature range. Ferromagnetic or paramagnetic porous materials are layered circumferentially according to their Curie temperature. The innermost layer has the lowest Curie temperature and the outermost layer has the highest Curie temperature. The wheel is rotated through a magnetic field perpendicular to the axis of the wheel and parallel to its direction of rotation. A fluid is pumped through portions of the layers using inner and outer manifolds to achieve refrigeration of a thermal load.

69 citations


01 Jan 1982
TL;DR: In this article, an active magnetic regenerator is described along with the method of calculation and temperature profiles for a normal regenerator and a magnetic regenerative refrigerator are shown, along with a comparison between the two types of devices.
Abstract: The adiabatic temperature change with field is limited to about 2 K/Tesla for ferromagnets near their Curie temperatures by the change of magnetization with temperature and the lattice heat capacity. Hence, practical magnetic refrigerators operate on a regenerative cycle such as the Brayton cycle. One way to execute this cycle is through the use of an active magnetic regenerator, i.e. a regenerator composed of magnetic material that is cycled in an out of a magnetic field with appropriate fluid flows. The theory of these devices can be predicted by solving the partial differential equations that describe fluid and the magnetic solid. The active magnetic regenerator is described along with the method of calculation. Temperature profiles for a normal regenerator and a magnetic regenerative refrigerator are shown.

Journal ArticleDOI
TL;DR: In this paper, a qualitative fit was found for M versus x for an exchange ratio JAB/JBB = 3-4, but the experimental and theoretical χinc versus x results were completely different.
Abstract: The magnetization of Li‐Zn ferrite, Li0.5−x/2ZnxFe2.5−x/2O4, with x = 0 to x = 1.0 Zn at./f.u. has been examined over the temperature range 4–950 K and for fields up to 90 kOe. Magnetization M and high‐field incremental susceptibility χinc were compared with a calculation based on a localized canting model. A qualitative fit was found for M versus x for an exchange ratio JAB/JBB = 3–4, but the experimental and theoretical χinc versus x results were completely different. On the other hand, a good correlation was found between χinc and the probability of a B‐site Fe having only one A‐site Fe neighbor. The data also reveal low‐temperature maxima in M in the 30–80 K range for x = 0.4 up to 1.0; the effect is most pronounced for x = 0.7. Weak mictomagnetic behavior is found for x = 0.5 and 0.6. Samples with x = 0.9 and 1.0 show no spontaneous moment down to 4 K, but exhibit initial susceptibility maxima at about 15 K with extrapolated Curie temperatures of 10 K and 4 K, respectively. For x = 0.8, a Curie temperature of 238 K is obtained. These are considerably lower than literature values.

Journal ArticleDOI
TL;DR: In this article, the authors set up a one-electron-like Hartree-Fock-like model with a Stoner-like intra-atomic exchange interaction for calculating the electronic structure of such a system with arbitrary directions for the atomic moments.
Abstract: Considerable evidence indicates that in Fe above the Curie temperature the electron system forms local magnetic moments which are relatively stable in magnitude and rotate to destroy long-range order. The local moments arise from exchange polarisation of the electron gas in a similar way to the conventional picture of the ferromagnetic ground state of the metal. The authors set up a one-electron-like Hartree-Fock-like model with a Stoner-like intra-atomic exchange interaction for calculating the electronic structure of such a system with arbitrary directions for the atomic moments. The directions for a cluster of 500-1000 atoms are assumed and fed into the calculation, from which the magnitudes of the moments are computed self-consistently using Haydock's recursion method (1980). The total energies are also obtainable. Results are presented for two simple types of magnetic configuration: spin spirals represent a smooth variation of the direction of magnetisation whereas alternating tilts form a rough variation back and forth from atom to atom.

Journal ArticleDOI
TL;DR: In this paper, the effect of addition of Ti4+ under oxidizing atmosphere on the structural and magnetic properties of Ni0.3Zn0.7Fe2O4 is studied.
Abstract: The effect of addition of Ti4+ under oxidizing atmosphere on the structural and magnetic properties of Ni0.3Zn0.7Fe2O4 is studied. The results of x‐ray diffraction, magnetization, and Arrott plot measurements are reported. The data are analyzed on the basis of a three sublattice Yafet‐Kittel model.

Journal ArticleDOI
TL;DR: In this paper, the shape of the initial magnetization curve and magnetic domain observations indicate that there is more than one magnetic hardening process involved in this magnet, consistent with microstructure studies which show the existence of nonmagnetic Cu-rich precipitates and 1:5 phase lamellae distributed in a 2:17 rhombohedral matrix.
Abstract: Magnetic hardening has been examined in the commercial Sm(Co, Fe, Cu, Zr)7.22 permanent magnet by correlating the magnetic properties with the microstructure of the sample. The shape of the initial magnetization curve and magnetic domain observations indicate that there is more than one magnetic hardening process involved in this magnet. This is consistent with microstructure studies which show the existence of nonmagnetic Cu‐rich precipitates and 1:5 phase lamellae distributed in a 2:17 rhombohedral matrix. The presence of the lower Tc 1:5 ferromagnetic phase in the higher Tc 2:17 phase is responsible for the peak observed in the M(T) curve around 870 K and in the change of slope in the Hc(T) curve around 800 K.

Journal ArticleDOI
TL;DR: The magneto-optical properties of rf sputtered amorphous TbxFe1−x alloys were investigated using reflectivity and polar Kerr measurements from a surface overcoated in situ with SiO2 as mentioned in this paper.
Abstract: The magneto‐optical properties of rf sputtered amorphous TbxFe1−x alloys (with 0.17≤x≤0.32) were investigated using reflectivity and polar Kerr measurements from a surface overcoated in situ with SiO2. The concentration dependence of the Curie temperature, compensation temperature, and low‐temperature polar Kerr amplitude were all derived and the results compared with a mean‐field theory. The temperature behavior of the coercive force was also determined. While a considerable degree of agreement is obtained with earlier work, it appears that the low‐temperature polar Kerr amplitude can only be determined by the iron sub‐network alone, if the iron moment increases with terbium concentration. The extent to which this view is justified is examined in light of current ideas.The magneto‐optical properties of rf sputtered amorphous TbxFe1−x alloys (with 0.17≤x≤0.32) were investigated using reflectivity and polar Kerr measurements from a surface overcoated in situ with SiO2. The concentration dependence of the Curie temperature, compensation temperature, and low‐temperature polar Kerr amplitude were all derived and the results compared with a mean‐field theory. The temperature behavior of the coercive force was also determined. While a considerable degree of agreement is obtained with earlier work, it appears that the low‐temperature polar Kerr amplitude can only be determined by the iron sub‐network alone, if the iron moment increases with terbium concentration. The extent to which this view is justified is examined in light of current ideas.

Journal ArticleDOI
TL;DR: In this paper, the effect of the electron-phonon interaction makes the temperature dependence of the spin susceptibility Curie-Weiss-type, with the Curie temperature much lower than in the Stoner theory.
Abstract: Starting from a consideration of how the phonon frequency would change with the spin splitting of the conduction-electron energy bands in an itinerant-electron ferromagnet, we find that in the paramagnetic region the effect of the electron-phonon interaction makes the temperature dependence of the spin susceptibility Curie-Weiss-type, with the Curie temperature ${T}_{C}$ much lower than in the Stoner theory. Further, for $Tl{T}_{C}$ we find that the phonon effect on magnetization can be large enough to account for the anomalous temperature dependence of spontaneous magnetization often observed in itinerant-electron ferromagnets, especially in Invar alloys. To confirm such a conclusion, with the same model and approximation we discuss also the very closely related problem of the magnetic field effect on the sound velocity in an itinerant-electron ferromagnet, and our theory is found to explain satisfactorily the varied observed results for both $Tg{T}_{C}$ and $Tl{T}_{C}$.

Journal ArticleDOI
TL;DR: In this paper, it was found that previously claimed evidence for a crossover was in error, and that the dynamical scaling hypothesis unexpectedly breaks down at small wave numbers due to spin-nonconserving forces, which are tentatively identified with the thermodynamic random fields.
Abstract: Modern, high-resolution neutron-scattering techniques, in particular neutron spin echo, were applied to reinvestigate the critical dynamics of iron at temperatures above the Curie point. It was found that previously claimed evidence for a crossover was in error, and that the dynamical scaling hypothesis unexpectedly breaks down at small wave numbers. This breakdown is shown to be due to spin-nonconserving forces, which are tentatively identified with the thermodynamic random fields.

Journal ArticleDOI
TL;DR: In this paper, a new method of heating, "Soft Heating", has been proposed for accurate temperature control in the neighborhood of room temperature, using temperature-sensitive magnetic materials, and the heater generates heat by induction and has an intrinsic reference temperature of the Curie point.
Abstract: A new method of heating, "Soft Heating", has been proposed for accurate temperature control in the neighborhood of room temperature, using temperature-sensitive magnetic materials. The heater generates heat by induction and has an intrinsic reference temperature of the Curie point. No additional temperature control circuits are needed. Some typical applications have been discussed.

Journal ArticleDOI
TL;DR: The term magnetic liquid (or ferrofluid) as used today does not refer to an intrinsic ferromagnetic liquid but to a stable colloidal suspension of small particles of a magnetagnetic material as discussed by the authors.

Journal ArticleDOI
TL;DR: In this paper, the effect of absorbed hydrogen on the magnetic behavior of Laves phase compounds HoCo2-xFex and ErCo2−Fex has been investigated.

Journal ArticleDOI
TL;DR: In this article, the variation in the magnetic hysteretic properties of rf−sputtered amorphous Tb−Fe thin films as a function of the nominal film thickness was investigated, using Kerr magneto−optic and Hall effect measurements.
Abstract: The variation in the magnetic hysteretic properties of rf‐sputtered amorphous Tb–Fe thin films as a function of the nominal film thickness was investigated, using Kerr magneto‐optic and Hall effect measurements. The results on the thickness dependence of coercivity, polarity of the hysteresis loop, and Curie temperature of films prepared at the same sputtering condition indicate that there is a change in the ‘‘effective’’ film composition. This composition change is believed to be due to microstructure‐induced variations in the short‐range order during the film growth.

Journal ArticleDOI
TL;DR: The effect of Co substitution is very significant in high temperature range because of the high Curie temperature as discussed by the authors, and the temperature dependence of θk can be fitted to magnetization of 3-d transition metal sublattice using the mean field model.
Abstract: Amorphous Gdx(Fe1−yCoy)1−x (0.25

Journal ArticleDOI
TL;DR: In this paper, the structure and magnetic properties of TlCrX2 (X= S, Se, Te), Tl3CrS3, TlCl5S8 and AgCrO2 have been investigated.

Journal ArticleDOI
TL;DR: Ternary Sm2Co17−xTx systems in which Co is replaced by V, Ti, Zr, and Hf have been studied to ascertain the efficacy of these elements in improving the permanent magnet characteristics of Sm 2Co17-based magnets as discussed by the authors.
Abstract: Ternary Sm2Co17−xTx systems in which Co is replaced by V, Ti, Zr, and Hf have been studied to ascertain the efficacy of these elements in improving the permanent magnet characteristics of Sm2Co17‐based magnets Single phase materials were obtained for 0?x?10 X‐ray diffraction measurements indicate that these compounds exhibit either the hexagonal‐Th2Ni17 or the rhombohedral‐Th2Zn17 structure The lattice parameters are found to increase with x and in the order Hf

Journal ArticleDOI
TL;DR: In this paper, the authors discuss some other possible mechanisms to account for the magnetic properties of such titanomagnetites observable even when domain-wall-related structures do form in them.

Journal ArticleDOI
TL;DR: In this article, the main effect of the hydrogen is to shift a broad, mostly ferromagnetic exchange distribution with some antiferromagnetic interactions to an overwhelmingly ferrous one, attributed to dilation of the iron-iron nearest neighbor distances.
Abstract: Iron‐rich Y1−xFex amorphous alloys have been charged electrolytically with hydrogen to an H:Y ratio of approximately 3. The alloys are originally asperomagnetic, with spin freezing temperatures below 110 K, but they become soft ferromagnets on hydrogenation with Curie temperatures in excess of 400 K. The iron moment in a–Y12Fe88 changes from 1.96 to 2.24 μB in the hydride and the isomer shift increases by 0.15 mm/s. However, it appears that the main effect of the hydrogen is to shift a broad, mostly ferromagnetic exchange distribution with some antiferromagnetic interactions to an overwhelmingly ferromagnetic one. This is attributed to dilation of the iron–iron nearest‐neighbour distances. By contrast, no significant effects on magnetic moment or exchange were detected on hydrogenating amorphous Fe40Ni38Mo4B18 (Metglas 2826 MB), but the in‐plane anisotropy was modified and the soft magnetic properties degraded.

Journal ArticleDOI
A.L. Greer1
TL;DR: In this article, the ferromagnetic Curie temperature, Tc, has been used to monitor the structural relaxation of the metallic glass Fe815B145Si4, which was produced by melt-spinning to various ribbon thicknesses at different quench rates.
Abstract: Measurements of the ferromagnetic Curie temperature, Tc, have been used to monitor the structural relaxation of the metallic glass Fe815B145Si4 The glass was produced by melt-spinning to various ribbon thicknesses at consequently different quench rates It is shown that the faster quenched (ie thinner) ribbons have lower initial Tc values, corresponding to less relaxed structures All the materials, however, tend to the same equilibrium value on annealing An effect which is particularly marked at low annealing temperatures is that the faster quenched ribbons relax more quickly The results are interpreted in terms of two relaxation mechanisms

Journal ArticleDOI
TL;DR: In this paper, the authors report results on magnetization studies of amorphous Fe40−x/2Ni40 −x/Ni40−ex/2CrxMo2B8Si10 ribbons with 0
Abstract: We report here our results on magnetization (σ) studies of amorphous Fe40−x/2Ni40−x/2CrxMo2B8Si10 ribbons with 0

Journal ArticleDOI
TL;DR: In this paper, a general relation between the pressure dependence of the Curie temperature and spontaneous magnetisation is derived in a simple band model, as calculated from the observed values of the pressure dependences of the spontaneous magnetization.