Showing papers on "Curie temperature published in 1981"

The effects of microstructure on the electrocaloric properties of Pb(Zr,Sn,Ti)O3 ceramics

Abstract: When an electric field is applied to a dielectric material under adiabatic conditions there is a resultant change in temperature, a phenomenon known as the electrocaloric effect Compositions in the Pb(Zr,Sn,Ti)O3 system (PZST) have several phases of nearly equal free energy, allowing antiferroelectric (AFE) to ferroelectric (FE) and paraelectric (PE) to ferroelectric (FE) transitions to occur Data are presented for electrocaloric measurements on AFE materials in the vicinity of the Curie point The effect of microstructure is reported, and experimental values are related to dielectric properties through thermodynamic relations Culmination of this work resulted in the largest electrocaloric effect (26°K) measured to date for coarse grain material in the PZST system

The dielectric properties of ice Ih in the range 272–133 K

Abstract: The amplitude of the orientational dielectric dispersion of impure polycrystalline ice Ih has been measured at temperatures down to 133 K in an attempt to find evidence for an ordering transition. The Curie–Weiss temperature is 6.2±1.7 K and so, within the experimental precision, there is no significant evidence that the molecular orientations become more correlated than the ice rules require. From the most recent results on polycrystalline ice, the Curie–Weiss temperature is 15±∼11 K. As this temperature is far below the lowest experimental temperatures, the evidence for an ordering temperature is not firm. The activation energy for dielectric relaxation in the impure ice is 25.5 kJ mol−1 at high temperature and increases at low temperatures. The low activation energy is caused by impurities that generate orientational defects in about the maximum number physically possible, and is mainly the activation energy for diffusion of the defects. At lower temperatures, the impurities produce fewer defects and t...

T‐S diagram for gadolinium near the Curie temperature

Abstract: The magnetocaloric effect in polycrystalline Gd was measured at temperatures from 190 to 370 K in applied fields from 1 to 7 T. The magnetocaloric temperature changes were combined with existing zero‐field specific heat data to construct a T‐S diagram for Gd near the Curie point. Experimental values of ΔT were also compared with values calculated from a simple mean field theory, which predicts rather well both the general shape of the magnetocaloric curve and the relative magnitudes of the temperature changes in various measuring fields. The maximum ΔT with applied field was about 14 K at 7 tesla, and maxima in all applied fiels occurred near 292 K. The relatively large magnetocaloric effect in Gd near room temperature is attractive for potential magnetic refrigeration applications, and the experimental T‐S diagram may now be used to refine estimates of the performance of Gd as a solid magnetic refrigerant.

Magnetic properties of sheet silicates; 2:1:1 layer minerals

Abstract: Magnetization, susceptibility and M6ssbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of the Tzg triplet of 1,120 K. Predominant exchange interactions in the iron-rich samples are ferromagnetic with J= 1.2 K, as for other trioctahedral ferrous minerals. A peak in the sus- ceptibility of thuringite occurs at Tm = 5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the M6ssbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4~ 1). A factor favouring this random ferromagnetic ground state over the planar antiferromag- netic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illus- trate how long range ferromagnetic order may be destroyed by vortices forming around the Fe 3 +--Fe 3 + pairs. tral in chlorites. Substitution of trivalent ions for Mg in the brucite layers is compensated by replacement of Si by A1 in the tetrahedral sheets of the talc layers (Bailey 1980). The general formula for a trioctahedral chlorite is

Magnetism of nickel

Abstract: A previously developed theory of the magnetism of iron, based upon notion of a randomized exchange field, has now been applied to calculate the properties of nickel. We obtained a Curie temperature of 1200 K, about twice the observed value. The magnetization curve, the paramagnetic susceptibility (a Curie-Weiss law), and the distribution of spin lengths are also calculated. Both similarities and differences from the results of the iron calculations are noted.

Magnetovolume Effects in Ferromagnetic Transition Metals

Abstract: Magnetovolume effects in ferromagnetic transition metals and alloys, such as the spontaneous volume magnetostriction, the forced volume magnetostriction, are described by a phenomenological theory based upon a fluctuating local band picture, in which both itinerant electron and local moment characters are taken into account. The magnetovolume effects of Fe, Ni and the Fe-Ni Invar alloy are analyzed on the basis of the proposed theory. It has been shown that the degree of shrinkage of local moments above the Curie temperature can be estimated from this analysis. It is concluded that local moments remain almost unchanged in bcc Fe and, on the contrary, they shrink markedly in the Invar alloy.

Magnetization of compositionally modulated CuNi films

Abstract: We report static magnetization measurements on the compositionally modulated ferromagnetic alloy Cu/Ni, which, contrary to earlier ferromagnetic resonance measurements, show the moment per Ni atom is reduced relative to pure Ni. The low‐temperature magnetization is found to vary linearly with modulation amplitude, but, surprisingly, the Curie temperature is found to be almost amplitude independent.

Ferroelectric properties of vinylidene fluoride‐trifluoroethylene copolymers

Abstract: The ferroelectric properties of vinylidene fluoride‐trifluoroethylene copolymers were studied. According to the x‐ray diffraction patterns, the crystalline form of the copolymers with VDF molar content less than 80% was different from that of the copolymer with 82 mole % VDF. The Curie temperature was observed for the copolymers with the molar composition ranging from 37 to 80% in VDF by measuring the dielectric constants and endothermic peaks. The copolymers with low Curie temperature had the crystalline with wide interplanar spacing and small saturation polarization. The copolymer with 55 mole % of VDF showed a typical ferroelectric D‐E hysteresis loop at 25 °C and showed a double hysteresis loop at just below the transition temperature.

Kinetics of low temperature structural relaxation in two (Fe‐Ni)‐based metallic glasses

Abstract: Structural relaxation processes below the glass transition temperature have been investigated calorimetrically via isothermal annealing and the time shift method for (Fe.5Ni.5)75P16B6Al3 and Fe40Ni40P14B6 glasses. The relaxation spectra are described by broad distributions in both activation energies and frequency factors. The low temperature anneals stabilize a portion of the relaxation spectrum and the annealed sample upon heating shows an endothermic reaction above the annealing temperature. The effects of successive annealing are additive provided that the anneals are followed by a lower temperature one. The results may account for many aging behaviors, such as the wide variation in activation energies, ’’reversibility’’ in Curie temperature and the cross‐over phenomenon.

The Curie temperature and magnetization of Fe-base amorphous binary alloys containing transition metal

Abstract: Several kinds of Fe-base amorphous alloys with transition metals were prepared by means of a co-sputtering technique. The reduction of the magnetic moment of Fe-Ta and Fe-Nb amorphous alloys is much more pronounced than that of Fe-Hf and Fe-Zr amorphous alloys. Fe-Mo, Fe-V and Fe-W amorphous alloys are paramagnetic. The concentration dependence of the Curie temperature also exhibits a similar tendency. Such behavior can be explained by a d-band filling model. The magnetic-carrier ratio is larger than unity, indicating itinerant magnetic behavior. The reduced magnetization curves of the present alloys are flatter than those of crystalline Ni and Fe, and Co-base amorphous alloys.

Pressure effect on the Curie point of the Heusler alloys Ni2MnSn and Ni2MnSb

Abstract: The pressure effect on the Curie point of the Heusler alloys Ni2MnSn and Ni2MnSb was measured The pressure coefficients of the Curie point were determined to be (1/Tc) (dTc/dp) = 16×10−3 1/kbar for Ni2MnSn and 9×10−3 1/kbar for Ni2MnSb, respectively The volume exchange striction of Ni2MnSn was also studied The pressure coefficient of the Curie point estimated from the result of exchange striction was found to be 14×10−3 1/kbar, which is in good agreement with that by the direct measurement The results are discussed on the basis of the molecular field theory taking account of the strain dependence of the exchange interaction and the Castellitz’s interaction curve

Thermal stability and magnetic properties of amorphous Zr1−xFex alloys

Abstract: Amorphous Zr 1− x Fe x alloys for concentrations in the range 0.2⩽ x ⩽ 0.9 were prepared by melt spinning. The crystallization behaviour of these alloys was studied using differential scanning calorimetry and X-ray diffraction. It is shown that the overall trend in the crystallization temperature ( T x increases with increasing x ) behaves in accordance with the hole model proposed earlier. It is shown that differences in nucleation and/or growth at different compositions mainly account for the deviations from a continuous increase in T x with increasing x . The crystallization of a number of alloys was studied using a variable heating rate and the results were analysed by Kissinger's method. In several iron-rich alloys we determined the Curie temperature and the saturation magnetization. The results were compared with literature data and with data obtained previously on vapour-quenched Zr-Fe alloys. From these results it is concluded that there are marked differences between the magnetic properties of liquid-quenched alloys and those of vapour-quenched alloys.

Dielectric Properties of Bi 4 Ti 3 O 12 below the Curie Temperature

Abstract: The dielectric permittivity, optical transmission spectrum and temperature dependence of resistance were investigated in single crystals of Bi4Ti3O12 and BaBi4Ti4O15. It is found that the dielectric anomalies previously reported below the Curie temperature are due electron space charges of inhomogeneous distribution in the material and near the non-ohmic electrodes.

Magnetic Properties of (Co1-XMnX)2P Compounds

Abstract: Measurements of the magnetization, magnetic susceptibility and NMR have been made for (Co 1- x Mn x ) 2 P single crystal compounds. The compounds with orthorhombic structure are ferromagnetic for 0.06< x <0.63, ferromagnetic at high temperatures and screw antiferromagnetic at low temperatures for 0.63 ≤ x ≤0.73 and antiferromagnetic for 0.73 ≤ x ≤0.80. The magnetic moments of Mn I and Mn II are expected to have about 1 and 3 µ B , respectively, from the measurement of NMR. The magnetocrystalline anisotropy constants K 1 and K 2 are both negative and their absolute values take maxima at x =0.5. The Curie temperatures and saturation magnetic moments for x ≤0.5 compounds are briefly discussed with the exchange interaction and the local environment of Mn II site.

Electrical conductivity of Manganese-Magnesium ferrites

Abstract: The electrical conductivity of Mn-Mg ferrites of various compositions was studied as a function of composition and of temperature. Based on the Seebeck coefficients observed the mixed ferrites were divided into two groups. The mixed ferrites of group I have a slight excess of cations and show n-type conduction, whilst those of group II have a cation deficiency and show p-type conduction. The mechanism of the electrical conduction is explained in terms of the oxidation of Fe2+ by Mn3+ on octahedral sites. Plots of conductivity (log (σT)) versus temperature ( 10 3 T ) are almost linear; they show a transition near the Curie temperature and a change in slope of the line with further increase in temperature.

Magnetic properties of monodispersed submicromic α‐Fe2O3 particles

Abstract: Magnetic properties are reported of recently prepared spherical hematite (α‐Fe2O3) particles of narrow size distributions with average diameters (d) varying between 20 and 300 nm. Mass magnetization (σ) of these particles has been determined as a function of temperature (T) up to 1000 K and of magnetic field (H) as high as 22 kOe. Specifically, magnetic behavior of α‐Fe2O3 particles of d = 300 nm is described in detail. Their Curie (Neel) temperature has been determined to be 960 K. The Bean‐Livingston plot indicates a change in magnetic anisotropy from higher symmetry at room temperatures to unidirectional behavior at elevated temperatures. This observation is consistent with the change of saturation and remanent magnetization with T. The measurements of σ on different size particles indicate that below 100 nm the particle magnetic susceptibility increases with decreasing size.

The pressure dependence of the Curie temperature of heterogeneous ferromagnetic alloys

Abstract: The empirical evidence for a systematic dependence of the laws regarding the pressure dependence of the Curie temperature of alloys on their structural and magnetic heterogeneity is presented. The materials covered include both crystalline and amorphous alloys. Previous interpretations of the results are given, followed by a discussion on the basis of the Landau-Ginzburg formalism which is concerned with concentration fluctuations and which reduces the magnetovolume problem to a purely magnetic one. A reasonable approach to the problem gives results in semi-quantitative agreement with the empirical evidence.

Experimental studies on quantum ferroelectrics

Abstract: The balance between dipolar forces and temporal fluctuations of the dipoles determines the temperature at which polar order sets in. If this temperature falls into the region of dominant zero-point vibration, the term “quantum ferroelectric” (QFE) is applied to denote the nature of the balance. To obtain this condition of quantum ferroelectricity, it is necessary to shift the Curie point into the desired temperature range. Experiments are reported for KDP-base QFE by application of hydrostatic pressure, and for KTaO3 by admixing isoelectronic impurities (Nb) during crystal growth (KTN = KTa1-x Nbx 03). Dielectric, ultrasonic and pyroelectric measurements show the particularities of QFE's: Their phase diagrams are given by Tc = (x - xc)1/2; x being pressure (KDP) or impurity concentration (KTN). The dielectric susceptibility varies as e-1 ∼T2 for x = xc, the polarization P ∼(x - xc)1/2 at T = 0, and the elastic compliance is given by a step function. At temperatures outside the quantum range, T > ...

Electrical conductivity of lithium-nickel ferrites

Abstract: The electrical conductivity of Li-Ni ferrites was studied as a function of composition and of temperature. X-ray analysis and Seebeck coefficient studies were also carried out so that the conductivity of these mixed ferrites could be discussed. The dependence of the electrical conductivity of Li-Ni ferrites on composition is explained on the basis of the cation distribution. All the ferrites exhibit a transition near the Curie temperature with a change in slope of the conductivity (log(σT)) versus temperature (103T) curve. It is found that the activation energy in the paramagnetic region is higher than that in the ferrimagnetic region for all the ferrites.

AC susceptibility and resistivity of the ferromagnetic phase of PdMn

Abstract: AC susceptibility and electrical resistivity measurements have been carried out on a series of dilute PdMn alloys containing between 0.5 and 2.5 at.% Mn. The AC susceptibility taken in various collinear biasing fields exhibits a maximum in the vicinity of the Curie temperature as predicted by the scaling law equation of state. Further, the field dependence of these maxima is uniquely governed by the critical index delta , and estimates for this latter index have been made along the index gamma and the Curie temperature Tc. The electrical resistivity data, taken on the same samples, are in good overall agreement with the results obtained from the susceptibility study.

Structure and magnetism of amorphous alloys

Abstract: The effect of structure on the magnetic properties of amorphous alloys is discussed, with emphasis on the effect of fluctuations in the local atomic structure. It is pointed out that the structural fluctuations are best described by the internal stresses of long, medium or atomic range, and some of the magnetic properties are discussed in terms of these stresses. The properties discussed include the Curie temperature, saturation magnetization, magnetostriction, spin wave stiffness, paramagnetic susceptibility, changes in the Curie temperature, induced anisotropy and magnetic after-effect. The role of the structural defects on the last three is also discussed.

Evidence of short range magnetic order at four times Tc in a metallic compound containing Fe: Susceptibility and paramagnetic scattering in CeFe2

Abstract: Below 230 K CeFe2 orders with a saturation magnetic moment of 1.15 μB/Fe at 4.2 K. The paramagnetic effective moment deduced from the Curie‐Weiss law is 3.7 μB. Reported here are the results of neutron paramagnetic scattering measurements obtained between 300 K and 870 K on CeFe2. The technique of polarization analysis has been used to obtain a unique measurement of the magnetic cross‐section. The Fe moment determined in this way is about 1 μB, close to that observed in the ferromagnetic state. However, strong ferromagnetic correlations are found to persist up to the highest temperature used in this study, i.e., 870 K. The results are in complete agreement with recent theories which tend to reconcile 3d band magnetism with the persistence of a moment in the paramagnetic state. In conclusion, effective moments derived from susceptibility measurements do not allow an understanding of the 3d magnetism in the paramagnetic state.

Liquid phase sintered BaTiO3

Abstract: BaTiO3 has been sintered at 1000-1100°C for periods from 1/6 to 2 hours with 10-20 wt% glass of 74mole% Bi2O3 - 26% B2O3 composition. The resulting composites, consisting of BaTiO3 grains surrounded by a glass, show ϵRT=1300-1500 and ϵmax=2500. The Curie temperature is nearly unaffected by the amount of the glass content.

Magnetic heat pumping

Abstract: The method of the invention employs ferromagnetic or ferrimagnetic elements, preferably of rare-earth based material, for example gadolinium, and preferably employs a regenerator. The steps of the method comprise controlling the temperature and applied magnetic field of the element to cause the state of the element as represented on a temperature-magnetic entropy diagram repeatedly to traverse a loop. The loop may have a first portion of concurrent substantially isothermal or constant temperature and increasing applied magnetic field, a second portion of lowering temperature and constant applied magnetic field, a third portion of isothermal and decreasing applied magnetic field, and a fourth portion of increasing temperature and constant applied magnetic field. Other loops may be four-sided, with, for example, two isotherms and two adiabats (constant entropy portions). Preferably, a regenerator may be employed to enhance desired cooling or heating effects, with varied magnetic fields or varying temperatures including three-sided figures traversed by the representative point.

Liquid phase sintered BaTiO 3

Abstract: BaTiO 3 has been sintered at 1000-1100°C for periods from 1/6 to 2 hours with 10-20 wt% glass of 74mole% Bi 2 O 3 -26% B 2 O 3 composition. The resulting composites, consisting of BaTiO 3 grains surrounded by a glass, show e RT =1300-1500 and e max =2500. The Curie temperature is nearly unaffected by the amount of the glass content.