Showing papers on "Curie temperature published in 1985"

Dielectric properties of fine‐grained barium titanate ceramics

Abstract: Dielectric properties, lattice‐ and microstructure of ceramic BaTiO3 showing grain sizes of 0.3–100 μm were studied. At grain sizes <10 μm the width of ferroelectric 90° domains decreases proportionally to the square root of the grain diameter. The decreasing width of the domains can be theoretically explained by the equilibrium of elastic field energy and domain wall energy. The smaller the grains, the more the dielectric and the elastic constants are determined by the contribution of 90° domain walls. The permittivity below the Curie point shows a pronounced maximum er ≊5000 at grain sizes 0.8–1 μm. At grain sizes <0.7 μm the permittivity strongly decreases and the lattice gradually changes from tetragonal to pseudocubic.

Spin Fluctuations in Itinerant Electron Magnetism

Abstract: A general theory of spin fluctuations and thermodynamical properties of itinerant electron magnets is developed, interpolating between the weakly and strongly ferromagnetic limits. A unified expression is given for the Curie temperature and the physical meaning of the Curie-Weiss magnetic susceptibility is discussed. As new phenomena derived from this theory the temperature-induced local magnetic moments as observed in CoS2, CoSe2, etc. and peculiar magnetic and thermal properties of nearly ferromagnetic semiconductors such as FeSi are discussed.

A first-principles theory of ferromagnetic phase transitions in metals

Abstract: On the basis of a spin-polarised density functional description of the electrons, the authors develop a 'mean-field' theory of magnetic phase transitions in metals. The one-electron-like finite-temperature Schrodinger equation is solved, formally, for random orientations of local moments and the corresponding grand potential is used in a statistical mechanics of the spin configurations. This latter, in the mean-field approximation, requires the knowledge of the electronic grand potential averaged over various ensembles of such 'spin' configurations. These averages are carried out with the help of the Korringa-Kohn-Rostoker coherent-potential-approximation (KKR CPA) method for dealing with electrons in random potential fields. Then, the whole procedure is made self-consistent on the average. The theory determines the local moment mu , the Curie temperature TC, and the susceptibility chi (q,T) in addition to the electronic structure at finite temperatures without adjustable parameters. The authors illustrate the explicit calculations for iron. The local moment is found to be 1.9 mu B above TC, and the preliminary estimate of Tc is 1250K.

Effect of spin fluctuations on the magnetic equation of state of ferromagnetic or nearly ferromagnetic metals

Abstract: The authors present a quantitative model for the magnetic equation of state of nearly or weakly ferromagnetic metals at low temperatures which includes corrections to conventional Stoner theory arising from enhanced fluctuations in the local magnetisation. The model takes account of both longitudinal and transverse fluctuations in terms of four physically transparent parameters which may be determined independently from the equation of state in the T=0 limit and from inelastic neutron scattering, or calculated directly from a semi-empirical band structure model near the Fermi level fitted for example to experimental Fermi surface areas. For parameters of the same order of magnitude as those recently determined in the weakly spin-polarised metal Ni3Al, the model yields approximately a quadratic temperature dependence of the spontaneous magnetisation over a wide range well below the Curie temperature (Tc), a nearly linear inverse susceptibility well above Tc, and nearly linear magnetic isotherms (Arrott plots) at high magnetic fields. These results are qualitatively consistent with the behaviour observed in many magnetic metals near the ferromagnetic instability at low temperatures. For Ni3Al the model yields good quantitative agreement with experiment for the magnitude of the Curie temperature Tc, for the ratio peff/p0 of the high- to low-temperature effective magnetic moments, and for the coefficient of the quadratic (T2) variation of the magnetisation with temperature well below Tc, without the use of any free adjustable parameters. Finally the authors show that the model also provides a good quantitative description of the paramagnetic susceptibility and transition temperature of the more complex magnetic system MnSi, the only other unsaturated magnetic metal for which all of the microscopic parameters are well known.

Magnetic properties of the Nd2(Fe1−xCox)14B system

Abstract: We have investigated the magnetic properties of the Nd2(Fe1−xCox)14B system to improve the thermal properties of the Nd‐Fe‐B magnets. Nd2(Fe1−xCox)14B exists in the tetragonal form in the entire range of 0≤x≤1. In this system, the replacement of Fe by Co significantly increases the Curie temperature. The room‐temperature magnetization of Nd2(Fe1−xCox)14B has its maximum value at x=0.1. However, because of the decrease in the anisotropy energy and the saturation magnetization by further substitution of Co for Fe, a reasonable substitution range of Co is suggested to be x<0.2 in the sintered Nd‐Fe‐B magnet. In this range of Co, we have succeeded in improving the reversible temperature coefficient of the remanence for the Nd‐Fe‐B magnets.

Ni-Pt Phase Diagram: Experiment and Theory

Abstract: The temperature-composition phase diagram of the ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}$ system was determined by x-ray diffraction and in situ high-temperature measurements of electrical resistivity. The Curie temperatures were also determined for both disordered and ordered alloys. Our results provide direct experimental evidence for the interplay between atomic ordering and magnetism. The experimental measurements are satisfactorily reproduced by a Hamiltonian, solved in the tetrahedron approximation of the cluster-variation method, that contains magnetic interactions dependent on chemical and short-range order.

Magnetic characteristics of R2Fe14B systems prepared with high purity rare earths (R=Ce, Pr, Dy, and Er)

Abstract: Commercial rare earth metals contain several atomic % oxygen, and intermetallics synthesized using these ingredients deviate markedly from the intended compositions. In this work, high purity rare earths (<20 ppm oxygen by weight) were used. Results for R2Fe14B systems with R = Y, Nd, Sm, or Gd have been previously reported. The earlier work has now been extended to include systems containing Ce, Pr, Dy and Er. All were ordered magnetically at room temperature, with Curie temperatures ranging from 437 K (Ce) to 592 K (Dy). Results suggest that Ce in this system exists in the quadripositive state. Saturation magnetizations (77 K) are 29.4, 34.8, 12.1, and 14.7 μB/formula unit, respectively, for R = Ce, Pr, Dy, and Er. These may be contrasted with 30.4 μB/f.u. for Y2Fe14B, which is the Fe magnetization alone. These results imply ferromagnetic coupling for Pr2Fe14B and antiferromagnetic coupling for the Dy and Er compounds. Room temperature anisotropy fields ranged from 37 kOe (R = Ce) to 158 kOe (R = Dy). Er2Fe14B exhibits a spin reorientation near room temperature. Pr2Fe14B, unlike Nd2Fe14B, exhibits no spin reorientation.

Electronic structure of metallic ferromagnets above the Curie temperature

Abstract: The authors describe the electronic structure of Fe and Ni above their Curie temperatures in their disordered local moment (DLM) states as a function of wavevector k and energy epsilon . In particular, they calculate the Bloch spectral function, A(k, epsilon ), averaged over the orientational configurations of the local moments, at selected points in the Brillouin zone and determine the shape and smearing of the 'Fermi surface'. They find that BCC Fe, with a local moment of 1.9 mu B, can show an exchange splitting at some points whilst in other regions of the Brillouin zone no such splitting occurs. For comparison they also study FCC Fe, which also supports a substantial local moment. They find that it has similar features but the smearing of the 'bands' is more pronounced. On the other hand, the electronic structure of Ni is quite different, shows no such local exchange splitting, but is able to support a small local moment of 0.2 mu B. The resulting picture of the electronic structure of Ni is that of a paramagnetic smeared 'Stoner-Wohlfarth' model.

Magnetic and electrical properties of Fe‐B‐N amorphous films (invited)

Abstract: A new type of ferromagnetic amorphous alloy with a two‐phase structure was found in cosputtered Fe‐B‐N films. The microstructure was identified to be composed of two amorphous phases with about 50 A or less in size; one is a ferromagnetic amorphous phase of Fe‐B‐N alloy and the other is a nonmagnetic and high resistive amorphous phase of boron nitride. The homogeneous mixture of these two phases in the films resulted in unique features for magnetic and electrical properties. In comparison to sputtered Fe‐B films, the Fe‐B‐N films exhibit higher saturation magnetizations and the electrical resistivity is about 3–103 times higher. The compositional dependence of Curie and crystallization temperatures also differs from those of the sputtered amorphous Fe‐B films. Furthermore, the corrosion resistance of the films is improved in the presence of nitrogen.

Lattice parameter variation and magnetization studies on titanium‐, zirconium‐, and tin‐substituted nickel‐zinc ferrites

Abstract: The variation of lattice parameter, saturation magnetization, and Curie temperature on the addition of Ti4+, Zr4+, and Sn4+ to some Ni‐Zn ferrite compositions are reported and the results are explained on the basis of the movement of the substituting ions first to the tetrahedral and finally to the octahedral sites of the spinel lattice.

Magnetic and structural effects of anelastic deformation of an amorphous alloy

Abstract: We present the first direct evidence of structural anisotropy due to anelastic polarization produced by mechanical creep, in the form of the anisotropic diffraction intensities observed by energy dispersive x‐ray diffraction method. We suggest that this structural anisotropy is the origin of the creep‐induced magnetic anisotropy.

Onset of ferromagnetism in 3d-substituted FeAl alloys. I. Ti, V and Cr substitutions

Abstract: For pt.I see ibid., vol.15, p.681 (1985). Magnetic and structural studies using X-ray, neutron diffraction, magnetisation and small-angle neutron scattering (SANS) techniques are reported for FeAl1-xTx alloys in which partial substitution of Al by Mn or Co has been effected. The results of the neutron diffraction studies are consistent with a B2 model in which the Co atoms exercise a site preference for Fe sites, whereas Mn atoms appear to enter both Fe and Al sites. Consequently, for both systems displacement of Fe atoms into Al sites creates 'antistructure' (AS) Fe atoms. The susceptibilities of both alloy systems were observed to increase with the concentration of the transition metal solute. An apparent onset of spontaneous magnetisation was observed beyond certain 'critical' concentrations: x=0.3 and x=0.2 for Mn and Co substitutions respectively. However, it was not until a Co concentration with x=0.3 was reached that a Curie temperature could be established in the Arrott plots. No TC could be established at all in similar plots for the Mn series having x

Some heat treatment experiments for Nd-Fe-B alloys

Abstract: Since the Curie temperature (Tc) of R-Fe-B permanent magnets is lower than conventional 1/5 and 2/17 magnets, the irreversible loss due to the change of coercivity is critical when considering their application. The simple way to guarantee the thermal properties is to raise the coercivity at room temperature. The effect of heat treatment on coercivity was studied. The typical heat treatment is as follows: (1) The first heating at T 1 , 900°C × 2 hrs is followed by the continuous cooling at the rate of 1.3°C/min to room temperature. (2) The second heating at T 2 , near the eutectic temperature for 1 hr is followed by quenching. When employing this heat treatment, the following magnetic properties were obtained for Nd(Fe 0.92 B 0.08 )6. Br=13800 G, bH c =9150 Oe, iHc=9200 Oe and (BH)max=44.0 MGOe.

Magnetic properties of high-concentration Fe-Ag alloys produced by vapour quenching

Abstract: Alloys of Fe1-xAgx obtained by vapour quenching have been investigated by X-ray diffraction and magnetisation measurements. The single BCC phase extends up to about x=0.14, while the single FCC phase exists above x=0.6. For x=0.14-0.6, there is a mixed BCC and FCC phase. The average magnetic moment decreases with increasing x. It deviates upwards from the simple dilution law in the BCC and FCC alloys with x=0.6-0.7. The Curie temperature, TC, is about 900K for the BCC alloy with x=0.1, while TC is about 550K for the FCC alloy with x=0.6 and decreases with increasing x to zero at about x=0.95.

Effect of magnetic transition on solubility of carbon in bcc Fe and fee Co-Ni alloys

Abstract: The solubility of carbon in bcc Fe and fcc Co-Ni alloys has been examined by the diffusion couple method, and a thermodynamic analysis of the data has been carried out, taking into account the effect of magnetic transition. It has been detected that the solubility slightly deviates from the ordinary linear relation between log [pct C] and 1/T below the Curie point. The deviation has been well explained by the magnetic effect on the Gibbs energy for the bcc Fe-C and fcc Co-Ni-C phases.

Curie isotherm surfaces inferred from high-altitude magnetic anomaly data

Abstract: Two-dimensional Curie depth models along two sections across the western United States are derived using an equivalent layer magnetization model derived from Magsat data, and the results are used to constrain finite element thermal models developed by Mayhew and Majer (1980). Regional heat flow variations predicted by the models compare favorably with those inferred from direct measurements. The methodology is applied to upward-continued aeromagnetic data, and the results are found to be in good agreement with previous Curie depth estimates and average measured heat flow.

Highly heat-resistant Nd-Fe-Co-B system permanent magnets

Abstract: The magnetic properties and crystal structures of sintered Nd-Fe-Co-B magnets have been investigated over a wide range of chemical composition. Magnetic hardening is performed for samples by a post sintering heat treatment at 500\sim1000\deg C. The curie temperature of these samples are significantly improved with the increase of cobalt content. The sample with the composition of Nd 16 Fe 66 Co 11 B 7 shows remarkable magnetic properties, such as the maximum energy product of 42 MGOe and the reversible temperature coefficient of residual magnetization of 0.02 %/°C, Electron probe microanalysis and x-ray data suggests that the highly heat resistant magnet is produced when cobalt distributes only in the tetragonal matrix phase.

High quality soft heating method utilizing temperature dependence of permeability and core loss of low Curie temperature Ferrite

Abstract: High quality Soft Heating method utilizing low Curie temperature ferrite is developed. The heater generates sufficient heat at room temperature and stops heating abruptly an the Curie temperature. The heat characteristics of the heater and the suitable exciting conditions are clarified.

Variational approach to finite-temperature magnetism.

Abstract: A theory is presented for magnetism at finite temperatures which includes local electron correlations. It goes beyond the static approximation to the functional-integral method. The theory is based on variational methods. At T=0 it reduces to a correlated ground state of the form proposed by Gutzwiller [Phys. Rev. 134, A293 (1964); 137, A1726 (1965)]. In the high-temperature limit the static approximation is recovered. A single-site approximation is made in order to make numerical calculations possible. The theory is applied to Fe and Ni. A large reduction of the Curie temperature of Fe is found due to correlations. The amplitude of the local moment is increased by the electron correlations. It hardly changes with temperature in contrast to the results of the static approximation. We also discuss the magnetization-versus-temperature curves, the paramagnetic susceptibility, and the charge fluctuations.

Temperature and stress dependence of Young’s modulus in semiconducting barium titanate ceramics

Abstract: In order to study the temperature and stress dependence of Young’s modulus in semiconducting barium titanate ceramics, the longitudinal sound velocity was measured at a frequency of 5 MHz in the temperature range of 20–180 °C. It is striking that at temperatures above the Curie point (Tc=120 °C) the sound velocity reaches values that are 10–20% higher than those at room temperature. Furthermore, a shift of the Curie point of about 2×10−8 K/Pa is observed if uniaxial stress is applied perpendicular to the sound propagation. Both effects can be understood by means of the phenomenological theory of Devonshire.

Permanent magnet properties of sintered Nd-Fe-B between -40 and +200 °C

Abstract: Easy and hard‐axis magnetization curves, and second quadrant demagnetization lines were measured on two early commercial Nd‐Fe‐B based pilot products at temperatures between −40 and +200 °C. Open‐circuit flux losses during short‐term heating were determined. The high‐Hc grade 30‐H shows generally better temperature stability of magnetic properties than the lower‐coercivity grade 35. The (negative) temperature coefficients of Br, BHc, and (BH)max at 100 °C of Nd‐Fe‐B are approximately twice those of SmCo5, that of MHc is three times larger. After thermal demagnetization by cycling to the Curie temperature all losses are fully recoverable by remagnetizing. The required recharging field strength of previously field‐demagnetized magnets can be reduced by the simultaneous application of field and heat.

Diffusion of chromium in α‐iron

Abstract: In the temperature range from 915 to 1124 K interdiffusion coefficients in b.c.c. iron-chromium alloys are determined for concentrations up to 28 at% Cr. Above the Curie temperature of pure iron the concentration dependence of the interdiffusion coefficients shows a maximum at 13 at% Cr correlated with the minimum of the solidus line. For lower temperatures the magnetic ordering shifts the maximum to higher concentrations. The transition to magnetic long range order is marked by a discontinuity in the concentration gradient. The temperature minimum in the γ loop is found between 1124 and 1129 K.

Magnetic and structural properties of Nd2Fe14B, Th2Fe14B, Nd2Co14B and related materials

Abstract: Several alloys close in composition to Nd 2 Fe 14 B and Nd 2 Co 14 B were investigated by standard metallography. It was found that the tetragonal Nd 2 Fe 14 B-type structure applies also to Nd 2 Co 14 B. This structure does not tolerate significant deviations from stoichiometric composition. Annealed off-stoichiometric alloys are composed of impurity phases, the presence of which can have an unfavourable effect on the magnetic properties. The Curie temperature of Nd 2 (Fe 1− x Co x ) 14 B was studied as a function of composition in the range 0 ⩽ x ⩽ 0.6. The compound Th 2 Fe 14 B also crystallizes in the tetragonal Nd 2 Fe 14 B-type structure. Its Curie temperature is 480 K and the room temperature anisotropy field is of the order of 3 T. From a study of the 57 Fe Mossbauer spectra of Nd 2 Fe 14 B and Th 2 Fe 14 B it was deduced that different crystallographic iron sites give rise to different sizes of the iron moments, the average iron moment in Th 2 Fe 14 B being only slightly lower than that in Nd 2 Fe 14 B.