Showing papers on "Curie temperature published in 1969"

Magnetic properties and oxidation of iron‐titanium oxide minerals in Alae and Makaopuhi Lava Lakes, Hawaii

Abstract: Measurements of strong-field magnetization over the temperature range −196° to 700°C have been made on forty-eight drill core samples of tholeiitic basalt from Alae and Makaopuhi lava lakes, Kilauea volcano, Hawaii. These samples were originally obtained at temperatures ranging from 50° to 1020°C. Nearly all samples contain abundant hemoilmenite with Curie temperatures in the range −100° to −160°C. Samples quenched from high temperatures (800° to 1000°C) have second Curie temperatures ranging from 150° to 290°C, due to unoxidized titanomagnetite, and samples obtained at lower temperatures (50° to between 400° and 700°C) have second Curie temperatures ranging from 500° to 580°C. This transition from medium to high Curie temperatures occurs between 850° and 300°C, varying from one drill hole to another, and is accompanied by a marked increase in the strong-field magnetization at room temperature. Oxidation of original titanomagnetite to Ti-poor titanomagnetite containing ilmenite lamellas is the cause of the increase in Curie temperature. Comparison of the compositions of the oxide minerals with the oxygen fugacity data of Sato and Wright and the equilibrium reaction data of Buddington and Lindsley shows that oxygen fugacity was controlled largely by the buffering action of the oxide minerals; hence titanomagnetite was oxidized, whereas the more abundant hemoilmenite was little changed as the lava cooled. This oxidation occurred at temperatures well below equilibrium, the difference being generally of the order of 100°C but as much as 400°C. We conclude that in some basaltic lavas the magnetic minerals may form through subsolidus reactions at temperatures well below their final Curie temperatures. In such lavas the natural remanent magnetization is a mixture of thermoremanent magnetization and high-temperature chemical remanent magnetization.

Temperature Dependence of Ms and K1 of BaFe12O19 and SrFe12O19 Single Crystals

Abstract: Single crystals of BaFe12O19 and SrFe12O19 were grown in a Na2O flux. Static magnetic measurements on the two crystal types were made to reveal Ms and K1 as functions of temperature. It was found that Ms of BaFe12O19 and SrFe12O19 are practically identical over the temperature range 77°K to the Curie point with a room‐temperature value of 380 G. Extrapolations to absolute zero reveal that the Bohr magneton numbers are approximately 19.3 and 19.7 per molecule, respectively, while the Curie points are 740° and 750°K, respectively, for BaFe12O19 and SrFe12O19. K1 for SrFe12O19 is 10% higher than that of BaFe12O19 over the entire temperature range. Room temperature values are 3.25×106 and 3.57×106 erg/cc, respectively, for BaFe12O19 and SrFe12O19. Calculations made on the basis of the Stoner‐Wohlfarth theory indicate that the intrinsic coercive force of SrFe12O19 will be higher than that of BeFe12O19 by about 10%. Theoretical room‐temperature values of coercive force for an isotropic assembly of crystallites ...

Ferroelectric Tungsten Bronze‐Type Crystal Structures. II. Barium Sodium Niobate Ba(4 + x)Na(2 − 2x)Nb10O30

Abstract: Ferroelectric Ba(4+x)Na(2 − 2x)Nb10O30, with a Curie temperature of 833°K, has a tungsten bronze‐type structure and crystallizes in the orthorhombic system, with subcell lattice constants a = 17.59182 ± 0.00001, b = 17.62560 ± 0.00005, and c = 3.994915 ± 0.000004 A at 298°K. The space group is Cmm2, and there are two formulas in the subcell. The c axis of the true cell is double that of the subcell. The integrated intensities of 6911 reflections within a reciprocal hemisphere of radius (sinθ)/λ = 1.02 A−1 were measured with PEXRAD, 1873 symmetry‐independent structure factors being significantly above background. The metal atom positions were determined from the three‐dimensional Patterson function and the oxygen atoms from metal‐phased Fourier series. The final agreement index between measured and calculated structure factors is 0.0579. The structure differs only in detail from previously determined tetragonal tungsten bronze structures. In the general formula (A1)2(A2)4C4(B1)2(B2)8O30, the B1 and B2 sites are fully occupied by Nb, the A2 sites by Ba and the A1 site by 87.0% Na and 6.5% Ba. Evidence from chemical analysis, x‐ray density calculations and the present determination suggests that the best approximation to the formula of the crystal studied is Ba4.13Na1.74Nb10O30. The Ba and O atoms at z ≃ 12 are disordered in a manner similar to the O atoms in the Ba layer in barium strontium niobate. The four crystallographically independent Nb atoms, each in octahedral coordination, are linked to O atoms by distances ranging from 1.765 ± 0.021 to 2.270 ± 0.021 A, with a mean value of 1.989 A. Ba is 10 coordinated, with Ba–O distances ranging upward from 2.671 ± 0.013 A. Na is 12 coordinated, with Na–O distances ranging from 2.660 ± 0.014 to 2.990 ± 0.015 A, with a mean of 2.788 A. The Nb‐atom displacements from the mean oxygen planes lie in the range 0.171–0.205 A; the parabolic relation with Curie temperature predicts a displacement of 0.204 A. The measured value of Ps at room temperature is 40 μC cm−2: the linear relation between displacement and polarization predicts a saturation value of 44–53 μC cm−2. All metal atoms are displaced from the oxygen planes in the sense given by the macroscopic positive polarity.

Magnetic Susceptibilities of Rare‐Earth–Indium Compounds: RIn3

Abstract: The magnetic properties and the lattice parameters for 13 compounds of the composition RIn3 (Cu3Au structure) are reported. The magnetic measurements have been performed between 4.2° and 500°K with magnetic fields up to 30 kOe. With the exception of compounds with R=La, Sm, Y, and Yb Curie–Weiss behavior is observed in the high‐temperature region. At low temperatures the magnetic behavior for compounds in which R=Ce, Nd, Sm, Gd, Tb, Dy, Ho, and Er is ascribed to antiferromagnetic ordering. In the case of PrIn3 magnetic ordering is prevented by crystalline fields. The splitting of the J = 4 state of Pr3+ by cubic crystalline fields has been calculated for various combinations of fourth‐ and sixth‐order potentials. Possible energy diagrams are presented.

Theory of Itinerant Ferromagnets Exhibiting Localized-Moment Behavior above the Curie Point

Abstract: By using a functional integral formulation of the theory of itinerant ferromagnets above the Curie point, we show that for strong Coulomb interaction $U$, there are localized moments exhibiting a characteristic Curie-law susceptibility with the correct free spin-\textonehalf{} limiting value of the Curie constant. For weak $U$ the same formulation gives a Pauli-like susceptibility, again with the proper limit, while for intermediate values the theory gives a smooth interpolation between the extreme cases.

Dielectric Properties of Ice I

Abstract: The dielectric properties of ice have been measured from 0° to − 80°C using samples prepared from redistilled water which were further purified by zone refining. The resulting conductivities were low enough that dielectric and electrode space‐charge effects could be resolved in three terminal ac measurements to frequencies extended to 0.1 Hz. The relaxation times of the Debye behavior obey an Arrhenius rate equation only above − 50°C, smaller values being found at lower temperatures, the temperature dependence of static dielectric constant gives a Curie temperature of 38°K, and the dc conductance is 4 × 10−11 mho cm−1 at − 10°C for the best sample with an apparent activation energy which is much larger than for less pure ice but somewhat uncertain. The significance of these data for estimates of defect concentrations and other parameters is discussed.

Forced magnetostriction in the band model of magnetism

Abstract: The free energy expression of Wohlfarth for a very weak itinerant ferromagnet is amended by the addition of volume-dependent terms given by Belov. Hence the forced magnetostriction and pressure dependence of the Curie temperature are calculated. The effects depend on the volume dependence not only of the effective interaction energy but also of the fine details of the band structure. Magnetostriction measurements on ZrZn2 by Ogawa and Waki are used to estimate that here partial differentialTc/partial differentialP = -2·4 degK kbar-1. A variety of results relating to the effect is deduced from elementary thermodynamics. For a very weak itinerant ferromagnet corrections to the result deduced from the Belov equation are obtained. Reference is made to recent measurements by Fawcett et al. on dilute platinum and palladium alloys.

MAGNETO‐OPTICAL PROPERTIES OF A GREEN ROOM‐TEMPERATURE FERROMAGNET: FeBO3

Abstract: Ferric fluoride (FeF3) is a transparent green uniaxial weak ferromagnet (canted antiferromagnet) with a Curie temperature of 365 °K. FeF3 is far more transparent in the visible and uv than any other known material which has a spontaneous magnetic moment at room temperature. FeF3 at room temperature has 6 absorption bands between 7000 A and the band edge which is at 2440 A. There is a local absorption minimum of α = 4.4 cm−1 in the green at 5225 A. The Faraday rotation is 180 °/cm at 5225 A and increases to 1100 °/cm at 3000 A. The birefringent phase retardation at 5200 A is about 80 000 °/cm. The two indices of refraction are about 1.54 in the visible spectrum.

Theory of Magnetically Dilute Systems

Abstract: The variation of the critical temperature (the Curie or Neel temperature) of a magnetic crystal randomly diluted with non-magnetic atoms is evaluated as a function of concentration p of magnetic atoms by the effective Hamiltonian method based on the Ising and the Heisenberg models. The specific heat of Ising ferromagnet above the Curie temperature is calculated for various concentrations in order to see the existence of Schottky-type maximum. The critical index of susceptibility and the Curie temperature for diluted ferromagnet is also calculated using Pade approximation to check the idea that the lowering of the Curie temperature with the decrease of p is attributed to the decrease of effective dimensionality of ferromagnetic system.

Transition from Ferromagnetism to Paramagnetism in Ni–Cu Alloys

Abstract: The low‐temperature (1.4° to 4.2°K) specific heat, the ferromagnetic Curie temperature, and the magnetic moment of Ni–Cu solid solutions were measured near the composition where ferromagnetism terminates. The anomaly in the specific heat cannot be accounted for by the bT3 ln T term expected from electron‐paramagnon interaction, but it can be interpreted as a magnetic cluster contribution independent of temperature from 1.4° to 4.2°K. Another magnetic contribution is included in the term of the specific heat linear in temperature. Whether the maximum in the linear term at 50–54 at.% Cu should be attributed to this magnetic contribution or to the (enhanced) electronic specific heat is uncertain. For alloys near the critical composition the Curie temperature, as determined by methods based on high field data, is distinctly higher than that measured at low fields.

Piezoelectric and Elastic Properties of LiTaO3: Temperature Characteristics

Abstract: Piezoelectric and elastic constants of lithium tantalate single crystal were measured from room temperature up to the Curie point (630°C). These constants were obtained by measuring resonant and antiresonant frequencies of bars and plates with various orientations. Temperature dependence of piezoelectric constants was discussed on the basis of the electrostrictive effect in the paraelectric phase biased by spontaneous polarization. Anomalous behavior of d33 and d31 was well explained by the dielectric anomaly of e33T.

The plastic deformation of ferromagnetic face-centred cubic Fe-Ni-C alloys

Abstract: When face-centred cubic alloys with the physical properties typical of Invar are strained over a sufficient range of deformation variables, peculiar deformation characteristics become apparent. For example, there is an unusually large temperature-dependence of the flow stress, completely atypical of f.c.c. metals. Factors such as compositional departure from the Invar range, the intentional formation of martensite, arranged prohibition of martensite, and compositional control of Curie temperatures, were manipulated to determine the origin of the deformation behaviour. Only the Invars reacted as a class, and ferromagnetism appeared causal both to the onset of the special deformation and to its nature. It was observed that the increase of flow stress produced by cooling below the Curie temperature can be completely reversed upon heating, that the formation of martensite is irrelevant, and that the nucleation of martensite is inhibited by the spontaneous ferromagnetism. Observations such as these is...

CURIE‐POINT WRITING OF MAGNETIC HOLOGRAMS ON MnBi

Abstract: Reversible phase holograms have been made on thin films of MnBi by Curie‐point writing. Magnetic holograms are formed when the interference pattern, created by conventional holographic techniques, heats the film above its Curie temperature, causing a magnetic domain pattern that corresponds to the interference pattern. The magnetic hologram can be nondestructively read out, without the use of polarizers or analyzers, by means of either the Faraday or Kerr magneto‐optic effect. The hologram can be erased by application of a magnetic field.

Optical Properties of Single-Crystal Films of Cd Cr 2 S 4

Abstract: We report on the preparation and optical properties of single-crystal films of Cd${\mathrm{Cr}}_{2}$${\mathrm{S}}_{4}$. The present data show no large blue shift associated with magnetic ordering near the Curie temperature contrary to that previously observed in bulk Cd${\mathrm{Cr}}_{2}$${\mathrm{S}}_{4}$ data. A large red shift correlated to the magnetic ordering is observed for a peak in the highly absorbing region. The weakness and narrowness of this absorption is inconsistent with that expected for band-to-band transitions resulting in unbound electrons and holes. We suggest an exciton model to explain this absorption.

The optical properties of nickel above and below the Curie temperature

Abstract: The temperature dependence of the optical absorption spectrum of nickel has been measured over the wavelength range of 021-27 μm by means of Drude's method up to 500 °K in ultra-high vacuum. The following characteristics were found: (i) a small bump near 14 ev which nearly disappears above the Curie temperature, whose energy is unchanged with temperature; (ii) a large absorption peak centred at about 5 ev. The peak position decreases linearly with temperature at a rate of 44 × 10-4 ev °K-1. The width of this peak decreases with increasing temperature up to the Curie temperature. The change in width is interpreted in terms of exchange splitting of the bands. We assign the low energy (14 ev) bump to the transition between the bands near the W and K symmetry points and the main peak (similar 5 ev) to the transition from L2prime to L1 upper.

23 Na Nuclear Magnetic Resonance Study of the Phase Transition in Ferroelectric Sodium Nitrite, NaNO 2

Abstract: The temperature dependence of the electric field gradient (EFG) at the sodium site has been determined in the ferroelectric and paraelectric phases of sodium nitrite (NaNO 2 ). Special attentions have been paid to the narrow temperature range near the ferroelectric Curie point T c where the antiferroelectric phase had been discovered. The main results obtained are: (1) the field gradient varies linearly with the square of the long range order parameter determined by the X-ray study in the ferroelectric phase; (2) a small, but abrupt, change at T c in the field gradient fives an evidence for a phase transition of the first kind; (3) the existence of the antiferroelectric phase can not be evidenced from the present study;(4) slight anomalies exist at 168°C and 178°C in the temperature variation of the field gradient; and (5) the field gradient decreases linearly with increasing temperature in the temperature ranges between 168°C and 178°C and also between 180°C and 230°C. It has been shown that these unusua...

Magnetic Structure of CrAs and Mn‐Substituted CrAs

Abstract: A neutron diffraction study has been made of powdered samples of CrAs and Mn‐substituted CrAs, namely, Cr0.7Mn0.3As, Cr0.4Mn0.6As, and Cr0.3Mn0.7As. Although CrAs shows a kink in susceptibility at 796°K, the anomaly was found to be due to a crystallographic transformation, from MnP‐type structure (low‐temperature side) to NiAs type. The liquid‐nitrogen temperature diffraction pattern contains a number of magnetic peaks. An analysis of the data gives a double‐layered helical structure, with the propagation vector along the c axis (the space group Pnma), like that found in MnP. The Neel temperature as determined from the temperature dependence of (101)− intensity is about 280°K. The magnetic structure of Cr0.7Mn0.3As is much the same as that of CrAs, but with the Neel temperature at 263°K. Cr0.4Mn0.6As is ferromagnetic with a Curie temperature at 160°K, and has a magnetic transition in the paramagnetic range, which separates the low‐temperature low‐spin state from the high‐temperature intermediate‐spin state. Cr0.3Mn0.7As is metamagnetic at low temperatures.

Pressure Dependence of Ferromagnetic Phase Transitions of Chromium Chalcogenide Spinels

Abstract: The shift in the Curie temperature Tc has been measured as a function of hydrostatic pressure up to 10 kbars for the chromium chalcogenide spinels CdCr2Se4, HgCr2Se4, CdCr2S4, and HgCr2S4, by measuring the mutual inductance of an astatic coil assembly containing the sample. Also the compressibility of these compounds has been measured for the same pressure range at three temperatures. The measured values of Tc and dTc/dP for CdCr2Se4, HgCr2Se4, CdCr2S4, and HgCr2S4 are 128°, 105.5°, 84°, and 36.1°K and −0.82, −0.95, −0.58, and +0.14°K/Kbar, respectively. Combining the compressibility measurements with dTc/dP the values of dTc/da=+66, +99, +46, and −10°K/A are obtained for CdCr2Se4, HgCr2Se4, CdCr2S4, HgCr2S4, respectively. These values of dTc/da for CdCr2Se4 and CdCr2S4 are in qualitative agreement with the dilution studies for these compounds. However, the value of dTc/da for HgCr2S4 is of opposite sign from that predicted from the solid‐solution studies for this compound. High‐pressure measurements seem...

Exchangestriction in CdCr2S4 and CdCr2Se4

Abstract: The ferromagnetic semiconductors CdCr2S4 and CdCr2Se4 both show large (∼0.2 eV) shifts in their absorption edges as the temperature decreases below their Curie temperatures. In CdCr2S4, the absorption edge increases with decreasing temperature; while in CdCrSe4, it decreases with decreasing temperature. It has been proposed that these shifts arise from a lattice expansion or contraction which is, in turn, produced by exchangestriction. Using x‐ray diffraction, we have measured the lattice parameters of these two materials from room‐temperature down to 4.2°K. CdCr2S4 is found to expand with decreasing temperature below its Curie point. The total fractional change is of the order of 10−3. CdCrSe4 contracts, but its fractional change is even smaller than that of the sulfide. Therefore, if this small expansivity is to explain the absorption‐edge shift, it requires a deformation potential of the order of 100 eV, which is at least an order of magnitude larger than what one would reasonably expect. Therefore, it appears that exchangestriction is not responsible for the large absorption‐edge shifts in these materials.

FERROMAGNETIC RESONANCE IN FeBO3, A GREEN ROOM‐TEMPERATURE FERROMAGNET

Abstract: Iron borate (FeBO3) is a transparent, green, room‐temperature weak ferromagnet (a canted anti‐ferromagnet) with a Curie temperature of 348°K. Ferromagnetic resonance measurements, i.e., linewidth, anisotropy fields, and g factor vs temperature and frequency are reported for the first time on this material. A low‐temperature linewidth maximum of unknown origin is also observed. Its narrow linewidth, large magnetic anisotropy, large optical Faraday rotation, and transparency in the green (the latter two reported elsewhere) made FeBO3 of interest for devices such as electronically tunable cavities and filters up to ∼100 GHz and high‐speed magneto‐optic modulators.