Showing papers on "Curie temperature 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

Grain‐size effects on dielectric properties in barium titanate ceramics

Abstract: Measurements of the dielectric constant κ in high‐purity BaTiO3 ceramics as a function of temperature show that κ is quite dependent on grain size in the ferroelectric state while it is almost independent of grain size in the paraelectric state. In the ferroelectric orthorhombic and rhombohedral phases, κ decreases rapidly with lowering temperature when grain size is reduced smaller than 3 μm. It is also observed that the Curie temperature Tc is decreased very slightly, but the lower two transition points are shifted to higher temperatures as grain size decreases.

Thermoelectric Power of Metals

Abstract: 1 Introduction- 11 Seebeck, Peltier, and Thomson Effects- 12 Transport Coefficients and Onsager Relations- 2 Survey of the Theory of Electronic Conduction in Metals- 21 Electrons in Metals- 21a Free Electron Gas- 21b Energy Bands- 22 Transport Properties- 23 Relaxation Time Anisotropy for Spherical Fermi Surfaces- 24 Thermopower: Isotropic Relaxation Time Approximation- 25 Thermopower: Real Metals- 25a Alkali and Noble Metals- 25b Polyvalent Metals- 26 Phonon Drag- 27 Thermopower of Alloys- 27a Diffusion Thermopower- 27b Phonon Drag- 3 Techniques in Thermoelectric Measurements- 31 Introduction- 32 Seebeck Effect- 33 Peltier Effect- 34 Thomson Effect: The Absolute Thermopower of Lead- 35 Measurement of Unconventional Thermoelectric Coefficients- 36 Measurement of Temperature and of Small Voltages- 36a Temperature Measurement- 36b Voltage Measurement- 37 Superconducting Devices- 37a Superconducting Modulators- 37b Weak Link and Josephson Junction Devices-SQUIDS and Slugs- 37b1 SQUIDS- 37b2 Slugs- 4 Phonon Drag- 41 Introduction and General Relations- 42 Sg at High Temperatures- 43 Sg at Low Temperatures- 43a Low-Temperature Phonon Drag in the Alkali Metals- 43b Low-Temperature Phonon Drag in Other Metals- 44 Anisotropy of Relaxation Times and Phonon-Drag Thermopower- 45 Sg at Intermediate Temperatures- 46 Sg of Alloys- 47 Phonon Drag or Phony Phonon Drag?- 47a Pure Metals- 47b Dilute Alloys- 47c Evidence for "Phony Phonon Drag"- 47d Effects of Higher-Order Scattering Processes- 5 The Thermoelectric Power of Transition Metals- 51 Special Problems in Transition Metals- 51a s- and d-Conduction- 51b Electron-Electron Collisions- 51c Magnetic Effects: Collective Electrons and Isolated Spins- 51d Magnetic Effects: Magnons and Paramagnons- 51e Magnetic Effects: Spin Mixing- 51f Magnetic Effects: The Curie and Neel Temperatures- 52 The Diffusion Thermopower of Transition Metals- 52a Phonons and Impurities- 52b Electron-Electron Scattering- 52c Magnons and Paramagnons- 52d Two-Current Conduction and Spin Mixing- 52e Curie Point Anomalies- 53 The Phonon-Drag Thermopower of Transition Metals- 54 Magnon Drag- 55 Transition Elements: Summary of Experimental Results- 55a The Magnetic Elements: Cr, Mn, Fe, Co, and Ni- 55b The "Thermocouple" Elements: Fe, Pt, Re, and W- 56 Commercial Thermocouples- 56a Copper vs Constantan (Type T)- 56b Iron vs Constantan (Type J)- 56c Chromel vs Constantan (Type E)- 56d Chromel vs Alumel (Type K)- 56e Tungsten vs Tungsten-Rhenium (Types G* and C*)- 56f Platinum vs Platinum-Rhodium (Types R, S, and B)- 6 Dilute Magnetic Alloys- 61 Introduction- 62 The Virtual Bound State- 62a Electronic Properties for VBS Systems- 62b Survey of Experimental Results for VBS Systems- 63 Kondo Alloys- 63a Theory of the Kondo Effect- 63b Thermoelectric Power of Kondo Alloys- 64 Spin-Fluctuation Models- 65 Closing Comment- 7 Effects of Pressure and Magnetic Field on the Thermoelectric Power- 71 Pressure Dependence- 71a Introduction- 71b Practical Thermocouples- 71c Fundamental Studies- 71c1 Diffusion Thermopower- 71c2 Phonon-Drag Thermopower- 72 Magnetic Field Dependence- 72a Introduction- 72b Practical Thermocouples- 72c Fundamental Studies- 72d Landau Quantization Effects- References- Author Index

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.

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 properties of amorphous Tb‐Fe thin films prepared by rf sputtering

Abstract: The magnetic properties of amorphous TbxFe1−x films prepared by rf cosputtering were studied. For 0.15

Electrical Resistivity of Weak Itinerant Ferromagnet ZrZn2

Abstract: Electrical resistivity of ZrZn 2 has been measured between 1.5 and 300 K. The resistivity shows a T 2 dependence with a coefficient 4.7×10 -8 Ω-cm/ K 2 below about 20 K, a T 2/3 dependence above a Curie point of 21 K up to about 50 K, and a dependence on temperature less rapid than T 1 above about 100 K, in good agreement with the result of a recent theory by Ueda and Moriya. The resistivity shows a slight change of slope at the Curie point.

Acoustic-Phonon Softening in the Invar Alloy Fe 3 Pt

Abstract: Inelastic neutron scattering experiments on an ordered Invar alloy ${\mathrm{Fe}}_{3}$Pt show large softening of the [100]TA and [110]T${\mathrm{A}}_{1}$ acoustic shear modes below the Curie temperature. In addition, a central peak is observed below 100 K, which, however, behaves noncritically. The phonon softening is discussed in terms of the magnetoelastic coupling caused by the electron-phonon interaction.

Magnetization, magnetic anisotropy, and domain patterns of Fe80B20 glass

Abstract: The thermomagnetization behavior, magnetic anisotropy, and magnetic domain patterns of ribbons of the newly synthesized Fe80B20 glass have been examined. This glass exhibits a saturation moment of 1.99 μB/Fe atom, a ferromagnetic Curie temperature of 647 K, and a crystallization temperature of ∼660 K. The direction of magnetic anisotropy lies at ∼60° to the ribbon axis in the plane of the ribbon. Large domains, elongated parallel to the ribbon axis, are observed.

Charge transfer in mixed-valence solids. Part VIII. Contribution of valence delocalisation to the ferromagnetism of Prussian Blue

Abstract: The contribution of mixed-valence electron delocalisation to the ferromagnetic exchange between the iron(III) ions in Prussian Blue {FeIII4[eII(CN)6]3·14H2O} has been estimated theoretically. Agreement between the calculated and observed values of the Curie temperature is quite good.

Advances in Ferromagnetic Metallic Glasses

Abstract: Recent developments in transition metal glasses have resulted in the synthesis of an iron‐rich glass (nominal composition, Fe80B20) in continuous ribbon form. The as‐quenched glass shows a saturation moment of 1.99 μB/Fe atom at 4.2 K, an induction of 16 kG at 300 K, a ferromagnetic Curie temperature of 650 K, a saturation magnetostriction of 30 × 10−6 and a coercivity Hc of about 100 mOe. Large domains, elongated parallel to the ribbon axis, are observed. Both Mossbauer and FMR data indicate that the magnetic anisotropy (K = 3 × 104 erg/cm3) is in the ribbon plane. Evidence is presented that boron donates less electrons to the d‐band of transition metal atoms compared with other glass‐forming metalloids such as phosphorous. Some of the low‐field properties include: the permeability at 20 G, μ(20) = 1,700, μmax = 102,000 and core‐loss, W ≃ 0.3 to 0.4 watts/kg at f=1 kHz and Bmax = 1 kG. Field annealing results in improvements in these properties: A typical field‐annealed toroid shows Hc∼40 mOe, μ(20) = 4,...

Magnetic properties and curie point writing in amorphous metallic films

Abstract: The Curie point writing in the amorphous ferrimagnetic films are described. The bits written on the films with relatively large values of 4\piM_{s} are unsaturated. We could write and erase the saturated bits on the film with low values of 4\piM_{s} with a low external field. The magnetic reversal profile inside the bits can be explained by the "self" demagnetizing field originating within the reversed region itself.

Dielectric Study on the Phase Transitions in Triglycine Selenate-Deuterated Triglycine Selenate System

Abstract: Ferroelectric phase transitions in the solid solution of triglycine selenate (NH 2 CH 2 COOH) 3 ·H 2 SeO 4 (TGSe) and deuterated triglycine selenate (ND 2 CH 2 COOD) 3 ·D 2 SeO 4 (DTGSe) were studied by means of dielectric-constant measurements. The Curie point monotonically increases with increasing deuterium content from 22.3°C for TGSe to 35°C for DTGSe. The deuterium-substitution induces a discontinuous change in dielectric constant at the transition accompanied with a thermal hysteresis of the Curie point. The first order nature of the transition becomes more and more conspicuous as the deuterium-content increases. The critical deuterium content at which the order of the transition changes from second to first seems to be very low.

Narrow Bloch walls in the RCo5‐type rare‐earth–cobalt compounds

Abstract: For compounds of the type YCo5xNi5−5x the saturation moment and Curie temperature were determined. For large Co concentrations the rigid‐band approach applies. The magnetic splitting of the 3d band collapses near x=0.2. Magnetic isotherms were studied at 4.2 K as a function of concentration. The form of the virgin magnetization curve, the presence of large intrinsic coercive forces, and the observation of pronounced thermomagnetic history effects suggest the presence of narrow Bloch walls of only a few interatomic distances in these compounds. It is argued that the presence of narrow Bloch walls is the primary reason for the high coercive forces observed usually in solid pieces of materials of the type RCo5xCu5−5x.

A Mössbauer study of amorphous iron-phosphorus alloys

Abstract: The Mossbauer spectrum of amorphous iron-phosphorus has been measured for alloys having compositions ranging from 15 to 25.8 at% phosphorus. Measurements were made at ambient temperature and also above the Curie temperature. The Curie temperature was measured by an induction method and increased linearly from 280 to 300°C with decreasing phosphorus content. The results indicated a large number of different iron sites having the same compositional short-range order as the crystal.

Magnetic properties of rare earth-magnesium compounds (RMg3)

Abstract: The RMg3 compounds (R = La, Ce, Pr, Nd, Sm, Gd, Tb) were investigated by X-ray diffraction; the corresponding powder diffractograms were indexed according to the cubic Fe3Al type-structure. The magnetic properties of these compounds were studied in the range 4–300 K, with magnetic field strengths up to 18 kOe. The asymptotic Curie temperature was found to be negative for CeMg3, PrMg3 and NdMg3 but positive for GdMg3 and TbMg3. The compound SmMg3 proved to have a Neel temperature TN = 6.5 K, but no magnetic ordering was detected in CeMg3, PrMg3 and NdMg3. The compounds GdMg3 and TbMg3 apparently order ferromagnetically. The much-too-low saturation moments for these compounds are an indication, however, of a more complex magnetic structure. The observed magnetic properties are in disagreement with an RKKY type description. It is suggested that 5d electrons may have some influence on the coupling between the rare-earth moments.

The effect of structural relaxation on the Curie temperature of Fe based metallic glasses

Abstract: The Curie temperature, T c , of (Fe 100-x M x ) 0.75 P 0.16 B .06 Al .03 glasses with M = Ni, Mo or Cr were measured. T c of the glasses increases monotonically with annealing temperature T a . The rate of increase, dT_{c}/dT_{a} , however, exhibits maxima at T_{a} = 500 and 600 K; temperatures at which the structural relaxation spectra also exhibit peaks. At lower temperatures, T c increases at rates that are greater than those of structural relaxation processes. Since the low and high temperature relaxation peaks may be attributed to local atomic regroupings and cooperative atomic rearrangements, respectively, the present results imply that the magnetic properties of metallic glasses are more sensitive to changes in local atomic arrangement than to longer ranged correlations. The change in T c is larger for the Ni containing glasses than for glasses containing Mo, Cr, or Mn.

Concentration dependence of the magnetic properties in amorphous Fe-P-B alloys

Abstract: The concentration dependence of the magnetic properties of amorphous Fe-P-B alloys obtained by splat-cooling is discussed. For a given Fe concentration, the magnetic moment \bar{\mu} averaged over all atoms of material and the Curie temperature T c both increase with the substitution of B for P. We observed for the variation of \bar{\mu} and T c as a function of the B content the same discontinuity as that occurring between the two phases \varepsilon and \varepsilon_{1} of the crystalline Fe 3 P 1-x B x compounds. We studied also the variation of \bar{\mu} and T c when one metalloid (P or B) substitutes for Fe, the other metalloid remaining constant. \bar{\mu} was found to decrease when the P (or B) content is increased, as expected from a rigid band model. For the same concentrations, T c increases. The different behaviours of \bar{\mu} and T c are tentatively explained by assuming that the amorphous Fe has two magnetic states (ferro and antiferromagnetic) as already suggested for crystalline Fe in a fcc environment.

Piezoelectric study of poling mechanism in lithium niobate crystals at temperature close to the curie point

Abstract: Using a piezoelectric technique, we show that when cooling a lithium niobate crystal through its curie point, three different mechanisms may lead to the orientation of its spontaneous polarization: the first one is the well known effect of an applied electric field. The second is the effect of a thermal gradient and the third one is related to the existence of a composition gradient. These last two effects are reported for the first time. All these mechanisms are shown to be effective only in a very narrow temperature range, around the Curie point (of the order of 3° C). Possible explanations for the last two effects are finally given and discussed.

Magnetic properties and phase relationships of gadolinium-gallium compounds

Abstract: The Gd-Ga system was investigated by X-ray diffraction, thermal analysis, metallography and magnetic measurements. The phase diagram was determined. The compounds observed were Gd 5 Ga 3 , Gd 3 Ga 2 , GdGa and GdGa 2 . Their lattice constants are given. The compounds Gd 5 Ga 3 and Gd 3 Ga 2 are antiferromagnetic, GdGa is ferromagnetic, but no magnetic order was detected for GdGa 2 . The effective moment per Gd atom and the asymptotic Curie temperature of all compounds were determined.

Single crystal magnetization of ErAl2 and interpretation in terms of the crystalline field

Abstract: Measurements are reported of the magnetization of single crystals of ErAl2 between 4.2K and 16K for magnetic fields up to 150 kOe applied along the (111), (110) and (100) directions. The results are interpreted in terms of the crystal field using a two-dimensional molecular-field approach. Within this microscopic description of the magnetization and the related magnetocrystalline anisotropy the authors use only two temperature-independent crystal field parameters and the Curie temperature. A least-square fitting procedure gives B4=1.10*10-4 meV, B6=-1.34*10-6 meV and Tc=14K. With these parameters they obtain good agreement between calculated and measured magnetization curves.