Showing papers on "Curie temperature published in 1977"

Ferroelectric Liquid Crystals; A Review

Abstract: The development and the current state of knowledge of ferroelectric liquid crystals are reviewed. Symmetry considerations first indicated that a ferroelectric phase should exist. Synthesis of new molecules led to the first experiments on electro-optical properties, indicating a spontaneous polarization. Various studies of bulk samples, including further electro-optical effects, shear induced polarization, pyroelectric effects, and switching experiments are discussed. Studies of the Curie point and effects visible in freely suspended films are summarized. Comparisons with crystalline ferroelectrics are made. Some further work and possible applications are indicated.

Changes in Curie temperature, physical dimensions, and magnetic anisotropy during annealing of amorphous magnetic alloys

Abstract: We have measured the effects of annealing in air on various properties of several amorphous alloys. Reported here are results on the changes in Curie temperature, in the physical dimensions of lengths of amorphous ribbon, and in the magnetic anisotropy. Increases in Curie temperature up to 35°C have been measured. All the alloys examined show a steady increase in T c on annealing at low temperatures, but some compositions show a smaller increase in T c on annealing near the crystallization temperature than on annealing at lower temperatures. There appear to be two competing mechanisms influencing T c . All the alloys examined show a clearly measurable decrease in length on annealing; we interpret this as an increase in bulk density. The kinetics of the annealing are similar to those of the stress relaxation. Finally, annealing experiments on the shape of 60 Hz hysteresis loops show a decrease in the anisotropy associated with non-uniform internal stresses, but in some cases also show the slow development of a fairly strong uniaxial anisotropy with its easy axis perpendicular to the ribbon axis. This uniaxial anisotropy is tentatively ascribed to the development of an oxide layer during annealing, which in turn produces a uniform compressive stress due to differential thermal contraction and therefore a stress-magnetostriction anisotropy. The changes in Curie temperature and in sample dimensions cannot be ascribed to oxidation. All the results described above are for annealing treatments that do not cause crystallization. The time for crystallization at various temperatures has been measured, and activation energies for crystallization derived.

Preparation, Characterization, and Properties of Dy‐Doped Small‐Grained BaTiO3 Ceramics

Abstract: The effects of Dy doping and sintering parameters on the properties of BaTiO3 ceramics were studied. The average grain size decreases with increasing Dy content and is controlled at ∼1.5 μ m by 0.8 at.% Dy. The Curie temperature change, associated with ≤1.2 at.% Dy, is <3°C. The dielectric constant is ∼2600 for specimens doped with 0.8 at.% Dy, calcined at 1200°C, and sintered at 1450°C. The dielectric constant variation with ambient temperature is much less than that of conventional BaTiO3 ceramics. Lattice constant c decreases with increasing Dy concentration whereas a increases slightly. The effects of grain size on dielectric constant, lattice parameters, and linear thermal expansion coefficient are more pronounced than the chemical effects of Dy doping in the ferroelectric state, whereas in the paraelectric state, these characteristics are almost independent of grain size as well as Dy concentration. The decrease in the frequency of occurrence of 90° twins with decreasing grain size implies that internal stress, which develops when BaTiO3 ceramics are cooled below Tc, strongly influences the effects of grain size.

Magnetic Properties of Fe 2 P Single Crystal

Abstract: The magnetic properties of hexagonal Fe 2 P single crystal are studied by the measurements of magnetization, susceptibility, thermal expansion and electrical resistivity along the principal crystallographic axes. It is found that the saturation magnetization per molecule and the Curie temperature are 2.94 µ B and 208.6 K, respectively, and the anisotropy constant K 1 is 2.32×10 7 erg/cm 3 at 4.2 K, which is considerably larger than the values in 3d-transition compounds. The thermal expansion and the electrical resistivity show discontinuous changes at the Curie temperature indicating that the ferromagnetic to paramagnetic transition of Fe 2 P is accompanied by the first-order transition due to magnetoelastic effects. The exchange striction obtained from the measurement of thermal expansion is discussed on the basis of the theory of Bean and Rodbell.

The chemical and physical properties of CrO2and tetravalent chromium oxide derivatives

Abstract: The oxide of chromium (IV), Cr02, is a highly interesting compound for materials investigation in that it is ferromagnetic at room temperature and a good electrical conductor. The Curie temperature is 118°C, and the material can readily be demagnetized with little energy input. The electrical conductivity of this black compound is quite high for an oxide; it even rivals some of the metal conductors such as Mn and Gd at roomtemperature. Thls review describes procedures for preparing Cr02, presents its chemical and physical properties including the modifications observed with metal-ion substitutions, and discusses the reactions of Cr02 with other metal oxides together with the physical properties of the ternary products containing tetravalent chromium.

An attempt to define the curie point isotherm in northern and central arizona

Abstract: Magnetic total intensity anomalies in northern and central Arizona have been analyzed to locate the bases of the polarized source bodies. The base of the magnetic crust is interpreted as the position of the Curie point isotherm. Results indicate a zone of shallow Curie depth (∼10 km) in a belt, about 60 km wide, running through the center of the state. This zone, near the northern border of the Basin‐Range province, is flanked on the north and south by areas of greater Curie depth (∼20 km). The results are in agreement with regional variations in Pn velocity in Arizona.

Distance between magnetic ions and fundamental properties in ferrites

Abstract: In the basic spinel NiFe2O4, ions of larger and larger ionic radius, Zn2+ and Cd2+, have been introduced with higher and higher concentrations. The corresponding materials of composition Ni1-xZnxFe2O4 and Ni1-xCdxFe2O4 have been prepared by using a special technologic process. The lattice constant increases linearly with the mean ionic radius in tetrahedral site. The variation of the Curie temperature due to a variation of the distance between magnetic ions is evaluated. The contribution to the Curie temperature due to the magnetic carriers distribution is shown by plotting a single intercomposition hypothetic curve. In the same way the influence upon the magnetization value of the variation of the distance between magnetic ions has been evaluated. Results obtained on spinels have been compared with those deduced from literature data in the case of garnets.

Magnetic moments and curie temperatures of (Fe, Ni) 80 (P, B) 20 amorphous alloys

Abstract: The magnetic moment per transition metal atom at 0°K and the Curie temperature were obtained for a series of (Fe, Ni) 80 (P, B) 20 amorphous quenched alloy ribbons. Fe/Ni and P/B compositions were varied separately. The moment data can be fitted well by assigning 2.1 Bohr magnetons per Fe atom and 0.6 per Ni atom, with the moment being lowered by 0.3 per B atom and 1.0 per P atom. Alternatively, moments varying with composition, as shown by neutron diffraction in crystalline alloys, combined with a lowering of 1.2 per B atom and 2.1 per P atom, also fit well. For a given P/B composition, T c shows a broad maximum at Fe:Ni of about 3:1. For a given transition metal composition, T c increases with increasing B content.

Magnetic properties and structures of some ordered (FeMn)3Si alloys

Abstract: Magnetic and crystallographic properties of the (FeMn)3Si system have been studied by X-ray and neutron diffraction magnetization resistivity and magnetoresistivity measurements. The single-phase alloys have been found to crystallize in the L21A structure and to have ferromagnetic properties at low Mn concentrations. At higher concentrations the alloys exhibit a more complex behaviour, having antiferromagnetic components below a temperature TR which is almost concentration independent. The resistivity showed anomalies at TR as well as at the Curie point. The variation of the magnetic moment on each site has been determined as a function of concentration and is discussed in terms of environmental changes occurring as a result of Mn additions.

Calcium orthovanadate, Ca3(VO4)2-a new high-temperature ferroelectric

Abstract: Ferroelectricity has been discovered in calcium vanadate. Large, transparent crystals have been grown by Czochralski pulling. Electrical 180° domains of millimeter dimensions are observed by etching as-grown crystals. The ferroelectric Curie temperature, Tc = 1383 K, has been determined from dielectric, thermal and pyroelectric studies.

Phase diagram of Fe-Cr-Co permanent magnet system

Abstract: The miscibility gap of the α phase in Fe-Cr-Co permanent magnet alloys is constructed using mechanical hardness and Curie temperature measurements. It is found that the miscibility gap of the Fe-Cr binary system develops into Fe-Cr-Co ternary system, and that the addition of cobalt raises the decomposition temperature, and extends the difference in concentrations between the iron rich phase (α 1 ) and the chromium rich phase (α 2 ). For instance, the α phase of an Fe-31%Cr-23%Co alloy exhibits a miscibility gap below 670°C, and decomposes into the (α 1 ) phase of 65%Fe-32%Co-3%Cr and the (α 2 ) phase of 69%Cr-21%Fe-10%Co at 600°C.

Mössbauer spectroscopy of hexagonal iron-nitrogen alloys

Abstract: In Fe−N alloys, the hexagonal-close-packed phase can be completely retained metastably at room temperature by rapid quenching from 700°C, with nitrogen contents ranging from about 17 to 27 at. pct N; (between the latter composition and 33 at. pct N, the hexagonal phase is stable at room temperature). The phase is ferromagnetic; the Curie temperature is a sharp function of nitrogen content, with the maximum Curie point (about 300°C) occurring at 24 at. pct N. The Curie point is below room temperature in the hexagonal phase for nitrogen contents less than about 17.5 at. pct N. For alloys of the Fe3N composition quenched from various temperatures, Mossbauer spectroscopy indicates that the hexagonal phase undergeos ordering of nitrogen atoms on interstitial sites.

Permanent magnet alloy

Abstract: PURPOSE: To increase the Curie point and coercive force (Hc) of a permanent magnet and to improve the thermal stability by substituting Si for part of B in the magnet made of an intermetallic compound consisting of a rare earth metal, Fe, Co and B. CONSTITUTION: An alloy having a composition represented by formula 1 (where R is one or more kinds of rare earth elements, 0≤x≤0.5, 0.02≤y≤0.3, 0.002≤z ≤0.15, and 4≤A≤7.5) is melted in vacuum or a gaseous Ar atmosphere, and the molten alloy is cast into an ingot. This ingot is pulverized in a gaseous N 2 atmosphere, and the resulting powder is press-molded in a magnetic field of 15kOe. The molded body is sintered in a gaseous Ar atmosphere, and the sintered body is rapidly cooled and heat treated at 400W800°C. Thus, an intermetallic compound having a composition contg. Si substituted for part of B is formed, and a permanent magnet made of the compound consisting of rare earth metals, Fe, Co, B and Si is obtd. This permanent magnet has an increased Curie point, increased coercive force (Hc) and superior thermal stability. COPYRIGHT: (C)1985,JPO&Japio

Ferroelectric Phase Transition in (NH 4 ) 3 H(SeO 4 ) 2

Abstract: Dielectric properties of single crystal (NH 4 ) 3 H(SeO 4 ) 2 were measured below room temperature Two phase transitions were found at around +2°C and at -92°C A λ-type peak of the dielectric constant along the c -direction is found at -92°C, and the crystal shows ferroelectricity below the transition The temperature dependence of the spontaneous polarization was measured by a pyroelectric method The Curie temperature linearly increases with increasing hydrostatic pressure up to 85 kbar with a slope of 23 deg kbar -1

Magnetic susceptibility of (rare earth) x Mo 6 S 8

Abstract: We have studied the susceptibility of all (RE) x Mo6S8 compounds between 1.3 and 300 K (RE = rare earth). In the two superconducting compounds Ho 1.2 Mo 6 S 8 and Dy 1.2 Mo6S8 magnetic ordering is expected in the temperature range 0.1–1 K. The magnetic transition in CeMo6S8 at2.25 K is of antiferromagnetic nature. A second anomaly at 1.6 K in this compound may announce a low-temperature ferromagnetic state.

The influence of composition and aging on the Curie temperature of metallic glasses

Abstract: The influence of composition, particularly glass formers P,B,C, and Si and transition metals Ni and Co, on the Curie temperature T c and the change in T c upon isochronal annealing of Fe based glasses is reported. Magnetic order-disorder transition in a "liquid" state is manifested for some (Fe,Co) glasses. The T c vs. y curves of some Fe 100-y G y glasses are seen to exhibit a maximum. Here G represents glass formers. T c increases by 2 ∼ 4 K with annealing just above T c for all glasses observed. With further increase in annealing temperature, T c continues to increase for B and Si containing glasses, while for glasses containing P and C, T c tends to decrease. The influence of metalloid addition and alloying among Fe, Co and Ni on T c of metallic glasses will be interpreted in the framework of the Bethe curve.

Bulk Magnetic properties of amorphous Tb52Ag48. Evidence of a strong local anisotropy

Abstract: The amorphous alloy Tb52Ag48 prepared by sputtering shows “ferromagnetic” ordering below a Curie temperature of 64 K. The results of magnetic susceptibility and magnetization measurements at low temperatures indicate the presence of a strong local anisotropy. The influence of ferromagnetic exchange, local anisotropy, and applied field on the magnetic behaviour are discussed. L'alliage amorphe Tb52Ag48 a ete prepare par pulverisation cathodique. On met en evidence un ordre “ferromagnetique” au dessous de 64 K. Les mesures de susceptibilite et d'aimantation montrent l'existence d'une forte anisotropie locale. L'influence de l'echange ferromagnetique, de l'anisotropie locale et du champ applique sur le comportement magnetique de l'alliage est discutee.

Magnetic anisotropy of substituted R 2 Co 17 compounds (R = Nd, Sm, Er and Yb)

Abstract: Saturation magnetization (M s ), Curie temperature (T c ) and easy direction of magnetization have been measured on Yb 2 Co 17-x Fe x , Er 2 Co 17-x M x (M = Fe, Mn, Ni), Nd 2 Co 17-x Al x and Sm 2 Co 17-x Al x compounds. The T c and M s vary in a similar manner for all the compounds investigated. Anisotropy field variation also shows a similar trend. It is proposed that the observed variations can be used as a guide for future experiments on the substituted R 2 Co 17 compounds.

Magnetic properties to TM80P20 glasses

Abstract: Saturation magnetic moments (4.2 K), room‐temperature magnetizations, and some Curie temperatures are reported for several TM80P20 metallic glasses (TM=Fe, Co, and Ni). These data complement previously published results for TM80B20 glasses and confirm the model in which approximately 2.4 and 1.6 electrons/metalloid atom are transferred by P and B, respectively, to TM d states.

Investigations on electronically conducting oxide systems (X)

Abstract: This paper deals with the preparation and characterization of single-domain crystals of BaNbxVTixIIITi1-2xIVO3 (x = 3 × 10-4 to 3.1 × 10-2). x = 0.01 yields the Curie temperature of 55°C. The characteristic platelet-like appearance of BaTiO3a single crystals grown from a basic BaCl2 flux is maintained up to x = 5 × 10-3. The anisotropy of the specific conductivity is about half an order of magnitude at room temperature and decreases at rising temperatures. The largely analogous dependence of the lattice constants on temperature makes the conclusion possible that the mobility of the polarons is essentially determined by the changes in distance between the Ti-atoms. The absorption spectrum was taken between 400 and 26,000 cm-1. Besides the absorption at 4000 cm-1 characteristic of polaron states it has a band of weak intensity at 16,000 cm-1 which is due to d-d electron transitions in Ti(III)-cations. Altogether, the consideration of the crystal field results in the establishment of a term scheme which perm...

Domain Structure of Titanomagnetites and Its Variation with Temperature

Abstract: In a Tertiary lava flow with optically apparently homogeneous titanomagnetites (Curie temperatures less than 100°C) a range of Curie temperatures from less than —40°C in the central to more than 70°C in the marginal zones of the crystals was found by observing the variation of the domain structure with temperature. The effect is explained by the different degree of maghemitization. TRM production in fields of 35 Oe revealed that the direction of the external field had little influence on the magnetic domain pattern.