Showing papers on "Curie temperature published in 1995"

Lattice effects on the magnetoresistance in doped lamno3

Abstract: A detailed study of doped LaMn${\mathrm{O}}_{3}$ with fixed carrier concentration reveals a direct relationship between the Curie temperature ${T}_{c}$ and the average ionic radius of the La site $〈{r}_{A}〉$, which is varied by substituting different rare earth ions for La. With decreasing $〈{r}_{A}〉$, magnetic order and significant magnetoresistance occur at lower temperatures with increasing thermal hysteresis, and the magnitude of the magnetoresistance increases dramatically. These results show that the notion of ``double exchange'' must be generalized to include changes in the Mn-Mn electronic hopping parameter as a result of changes in the Mn-O-Mn bond angle.

The development of new ferromagnetic shape memory alloys in Ni-Mn-Ga system

Abstract: The Ferromagnetic Heusler alloy Ni$AnGa is known to undergo a structural phase transformation of marten&c type. Thermoelastic nature, shape memory effect @ME) and superelasticity were found in (1) to be intrinsic to this transformation. Hence the martensitic transformation (MT) in NiwGa appeared to be a subject of further intensive investigations last few years. It tumcd out that the Ni&lnGa alloy was an affluent object to search ftx new phenomena such as magnetoelastic behavior in the vicinity of MT (2) and martensite lattice softening while approaching the intermartensitic transformation temperature during heating (3). In fact all the data published before were obtained using two Ni&lnGa samples of near stoichiometric composition. Despite the small composition difference the two mentioned Ni&lnGa alloys (alloys 3 and 10 in Table 1) studid e.g. in (Z!, 3), possess entirely Werent martensitic temperatures, Iv&. So, the high sensitivity of M. to the deviation ILi-om the stoichiometric composition can be drawn as an important conclusion. In this work we present the results of the investigation of the following problems: how M,, the thermal hysteresis, Curie temperature, transformation heat are a&&d by the composition variation in the Ni-Mn-Ga alloy system in a concentration interval for each component of about 10 at. %. This work was performed to make sure that lhe new family of Ni-Mn-Ga based shape memory alloys @MA) with a wide variety of structural and magnetic properties is actually elaborated.

Thermal and Magnetic Properties of Bulk Fe-Based Glassy Alloys Prepared by Copper Mold Casting

Abstract: Bulk glassy Fe 73 Al 5 Ga 2 P 11 C 5 B 4 alloys in cylindrical form with diameters of 0.5 and 1.0 mm were found to form by a copper mold casting method. The further increase in diameter causes the formation of coexistent glassy, Fe 3 (B, C), Fe 2 B and Fe 3 P phases for the 1.5 mm ? sample and coexistent Fe 3 (B, C), Fe 2 B and Fe 3 P phases for the 2.0 mm ? sample. It is to be noticed that the maximum thickness for glass formation is about 10 times larger than the largest thickness for Fe-based glassy alloys reported up to date. The glass transition temperature (T g ), crystallization temperature (T x ) and heat of crystallization of the 1.0 mm ? glassy alloy are 732 K, 785 K and 3.76 kJ/mol, respectively. No appreciable difference in the thermal stability and magnetic properties is seen between the bulk glassy alloys and the melt-spun ribbon. The 1.0 mm ? glassy alloy has ferromagnetism with a Curie temperature of 606 K and exhibits 1.26 T for saturation magnetization (B s ), 82 A/m for coercivity (H c ) and 0.38 for the ratio of residual magnetization to B s at room temperature. The large ΔT x ( = T x - T g ) and large glass-forming ability can be obtained for the Fe-based alloy containing simultaneously the five solute elements. The effectiveness of the multiplication is presumably due to the combination of the following three effects ; (1) the suppression of crystalline nuclei due to the increase in dense random packing density for the glassy structure containing P, C and B with significantly different atomic sizes, (2) the difficulty of atomic rearrangements for the precipitation of the Fe-metalloid compounds caused by the generation of Al-metalloid pairs with strongly attractive bonding nature, and (3) the decrease in the preferential precipitation tendency of Fe-B and Fe-C compounds by the dissolution of Ga which is immiscible to B and C and soluble to Fe.

Pressure effects on the magnetoresistance in doped manganese perovskites

Abstract: We present a detailed study on the effects of applied hydrostatic pressure on the magnetoresistance in doped LaMn${\mathrm{O}}_{3}$ at fixed doping level. In all cases, the application of external pressure monotonically increases the Curie temperature. This is compared with the application of "internal" pressure, which is varied by substituting different rare-earth ions for La. Both effects can be understood in one simple picture that relates the structural modifications to the variation of the Mn-Mn electronic transfer integral. Thus a general phase diagram has been derived with the transfer integral as the implicit microscopic parameter dominating the magnetic and transport properties of doped LaMn${\mathrm{O}}_{3}$.

Fe-Based Ferromagnetic Glassy Alloys with Wide Supercooled Liquid Region

Abstract: A multicomponent Fe 72 Al 5 Ga 2 P 11 C 6 B 4 alloy was found to form a glassy phase with a wide supercooled liquid region before crystallization and of ferromagnetism at room temperature. The glass transition temperature (T g ), crystallization temperature (T x ) and temperature intervals of supercooled liquid, ΔT x (=T x -T g ) for the glassy alloy are 732, 793 and 61 K, respectively. It is to be noticed that the supercooled liquid region exceeds 60 K which is about 3 times larger than the largest ΔT x value for the Fe-based glassy alloys reported up to date. The amorphous alloy also exhibits good bending ductility and rather good soft ferromagnetic properties at room temperature. The soft magnetic properties are improved by annealing at temperatures above the Curie temperature (T c ). The magnetic properties at room temperature for the glassy alloy annealed for 600 s at 723 K are 605 K for T c , 1.07 T for saturation magnetization, 5.1 A/m for coercivity, 9000 for permeability at 1 kHz and 2.0 x 10 -6 for magnetostriction. The finding of the ductile Fe-based glassy alloy exhibiting simultaneously the wide supercooled liquid region and good soft ferromagnetic properties is expected to create a new ferromagnetic bulk material with good deformability.

The positive temperature coefficient of resistivity in barium titanate

Abstract: Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above ∼ 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.

Investigation of tetragonal distortion in the PbTiO 3 –BiFeO 3 system by high-temperature x-ray diffraction

Abstract: Compositions in the (Pb1−xBix (Ti1−xFex)O3 solid solution system for x ⋚ 0.7 show unusually large tetragonal distortion. High-temperature x-ray diffraction was used to study the tetragonal distortion as a function of temperature (25–700 °C) for compositions (x = 0–0.7) using powders prepared by solid-state reaction in the above system. Large changes in the lattice parameters were observed over a narrow temperature range near Curie temperature (TC) for compositions near the morphotropic phase boundary (MPB) (x ≃ 0.7). Compositions near MPB showed a c/a ratio of 1.18 at room temperature. Polar plots of lattice constants at different temperatures indicated strong anisotropic thermal expansion with zero thermal expansion along the [201] direction.

Frequency dispersion and temperature variation of complex permeability of Ni‐Zn ferrite composite materials

Abstract: Permeability spectra in Ni‐Zn ferrite composite materials were studied at the volume loading of ferrite above 30% and at temperatures from 100 to 400 K. The permeability decreased with decreases in the volume loading of ferrite. This decrease was much larger than that expected from the empirical mixing law. This was attributed to the demagnetizing field, generated by the magnetic poles on the surface of the ferrite particles. Simultaneously, the demagnetizing field increased spin resonance frequency. For the sintered ferrite, the primary peak of the permeability was just below the Curie temperature. The peak becomes obscure and disappeared as the volume loading decreased. The temperature dependence of the spin resonance frequency was lower in the ferrite composite material than that in the sintered ferrite. These features were also discussed from the view point of the demagnetizing field.

Nanocrystalline Soft Magnetic Fe–M–B (M=Zr, Hf, Nb) Alloys Produced by Crystallization of Amorphous Phase (Overview)

Abstract: This paper reviews our results on the development of a new type of soft magnetic material with high saturation magnetic flux density (B s ) combined with excellent soft magnetic properties. A mostly single bcc structure composed of bcc grains with about 10-20 nm in size surrounded by a small amount of intergranular amorphous layers was obtained by crystallization of amorphous phases prepared by melt-spinning and sputtering technique in Fe-rich regions of Fe-M-B (M=Zr, Nb, Hf) ternary systems. The typical nanocrystalline bcc Fe 90 Zr 7 B 3 , Fe 89 Hf 7 B 4 and Fe 84 Nb 7 B 9 alloys subjected to the optimum annealing exhibit high B s above 1.5 T as well as high effective permeability (μ e ) at 1 kHz above 20000. The high B s for the Fe-M-B alloys is resulting from the high Fe concentrations owing to high glass-forming ability of M(Zr, Hf, Nb) and B. The origin of the good soft magnetic properties for the alloys are listed as follows. (1) The apparent anisotropy is decreased by the combined effects of the formation of the nanoscale bcc structure and the achievement of rather strong magnetic coupling between the bcc grains through the intergranular ferromagnetic amorphous phase. (2) The small saturation magnetostriction (λ s ) results from the nonequilibrium bcc phase. The solute-rich inter-glanular amorphous phase with high Curie temperature (T c ) and high thermal stability has an important role in the achievement of the good soft magnetic properties through the formation of the nanoscale bcc structure and the attainment of the rather strong magnetic coupling between the bcc grains. The soft magnetic properties of the nanocrystalline Fe-M-B alloys were improved through the decrease in the bcc grain size and the increase in T c of the intergranular amorphous phase by optimizing heating rate in the crystallization process and adding small amounts of elements. For example, the improved Fe 84 Zr 3.5 Nb 3.5 N 8 Cu 1 alloy shows the high μ e of 100000 combined with the high B s of 1.53 T. This excellent μ e is comparable to those of nanocrystalline Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 and the zero-magnetostrictive Co based amorphous alloys, and the high B s is comparable to those of the Fe based amorphous alloys with good soft magnetic properties. The Fe-M-B based alloys also have very low core losses, the sufficient thermal stability and the low stress-sensibility of the soft magnetic properties. Therefore, the nanocrystalline Fe-M-B alloys are expected as practical magnetic materials for magnetic transformers, inductors, and other devices and parts.

Ferroelectric properties in epitaxially grown BaxSr1−xTiO3 thin films

Abstract: Induced ferroelectricity was observed in epitaxially grown BaxSr1−xTiO3 (BST) thin films with thicknesses of about 220 nm The BST films with various Ba content were deposited at 600 °C on Pt/MgO substrates, by radio frequency magnetron sputtering with double targets, BaTiO3 and SrTiO3 The epitaxial growth of the BST films was confirmed with x‐ray‐diffraction and reflection high‐energy electron‐diffraction analyses The D–E hysteresis curve and the Curie temperature shift confirmed the BST films with Ba content x≥044 had ferroelectricity at room temperature The mechanism of the ferroelectricity appearance was discussed in relation to the elongation of the c axis in the thickness direction, caused by lattice mismatch between Pt and BST

Iron exchange-field penetration into the amorphous interphase of nanocrystalline materials.

Abstract: In this work an enhancement of the Curie temperature of the intergranular amorphous region in nanocrystalline alloys with respect to amorphous ribbons of the same composition is shown. The Curie temperature reaches a value of 125 \ifmmode^\circ\else\textdegree\fi{}C. The experimental results are discussed in terms of both an inhomogeneous distribution of Nb atoms in the interphase and the magnetic interactions between the Fe ferromagnetic grains and the matrix. Finally, it is proposed that the existence of a molecular field of about 80 T originated by the penetration of the exchange field of \ensuremath{\alpha}-Fe nanocrystals into the amorphous paramagnetic intergranular region is the main cause of this Curie temperature enhancement. Moreover, the compositional dependence of the Curie temperature in Fe-B-Nb-Cu amorphous ribbons has been reported.

Artificial fabrication of an L10‐type ordered FeAu alloy by alternate monatomic deposition

Abstract: An FeAu ordered alloy has been fabricated artificially by depositing alternately monatomic layers of Fe and Au. Neither intermediate phase nor intermetallic compound is known for the Fe–Au system in the equilibrium state. Nevertheless, the x‐ray diffraction for prepared FeAu films shows definite superlattice lines corresponding to the L10 ordered structure, and it is metastable at room temperature. The obtained FeAu ordered alloy is ferromagnetic, and it has the Fe moment of 2.5±0.3 μB, a considerably high Curie temperature over room temperature, and a large uniaxial magnetic anisotropy perpendicular to Fe and Au atomic planes.

Thickness-dependent electrical characteristics of lead zirconate titanate thin films

Abstract: Ferroelectric lead zirconate titanate thin films of morphotropic phase boundary composition were fabricated through the sol‐gel spin‐on technique to study the thickness dependence of electrical characteristics. At saturation, the films exhibited a dielectric constant of 1300, dissipation factor of 0.03, Curie temperature of 366 °C, remanent polarization of 36 μC/cm2, coercivity of around 30 kV/cm, and dielectric breakdown strength of over 1 MV/cm. The temperature dependence of permittivity showed an anomalous behavior with annealing temperature. The high temperature dielectric behavior of the films were distinguished by smeared and stunted dielectric maximum.

Surface and size effects on ferroelectric films with domain structures

Abstract: The spontaneous polarization and electrostatic potential in ferroelectric films with domain structures have been calculated based on phenomenological theory. The polarization is reduced at the film surface due to a depolarizing field. The domain width, free energy, and Curie temperature as functions of film thickness are obtained.

Orientational and Magnetic Ordering of Buckyballs in TDAE-C60.

Abstract: Spin ordering in the low-temperature magnetic phase is directly linked to the orientational ordering of C(60) molecules in organically doped fullerene derivatives. Electron spin resonance and alternating current susceptometry measurements on tetrakis(dimethylamino)ethylene-C(60) (TDAE-C(60)) (Curie temperature T(c) = 16 kelvin) show a direct coupling between spin and merohedral degrees of freedom. This coupling was experimentally demonstrated by showing that ordering the spins in the magnetic phase imprints a merohedral order on the solid or, conversely, that merohedrally ordering the C(60) molecules influences the spin order at low temperature. The merohedral disorder gives rise to a distribution of pi-lectron exchange interactions between spins on neighboring C(60) molecules, suggesting a microscopic origin for the observed spinglass behavior of the magnetic state.

Evidence of a Surface-Mediated Magnetically Induced Miscibility Gap in Co-Pt Alloy Thin Films

Abstract: (100) and (111) oriented single-crystal CoPt{sub 3} films were deposited over a range of growth temperatures from {minus}50 to 800 {degree}C. The Curie temperature is increased by 200 {degree}C over the value expected for the homogeneous alloy in the as-deposited films (of both orientations) grown near 400 {degree}C. We interpret this as evidence for a previously unobserved, surface-mediated, magnetically driven miscibility gap in vapor-deposited CoPt{sub 3} films. Large perpendicular magnetic anisotropy is also observed in the as-deposited films (of both orientations) grown near 400 {degree}C.

Magnetic Frustration in Ultrathin Fe Films.

Abstract: In Fe(110) films prepared at 300 K on W(110), ferromagnetic long range order is suppressed in a range of coverages between 1.20 and 1.48 pseudomorphic monolayers, whereas below and above this gap of long range order, monolayer and double layer ferromagnetism is observed with Curie temperatures of 230 and roughly 450 K, respectively. Apparently, the frustration of long range order in the gap is caused by indirect interactions of electronic origin between double layer islands, mediated by the surrounding monolayer and its W substrate.

Spontaneous Magnetic Ordering in the Fullerene Charge-Transfer Salt (TDAE)C60.

Abstract: The zero-field muon spin relaxation technique has been used in the direct observation of spontaneous magnetic order below a Curie temperature (T(c)) of approximately 16.1 kelvin in the fullerene charge-transfer salt (tetrakisdimethylaminoethylene)C(60) [(TDAE)C(60)]. Coherent ordering of the electronic magnetic moments leads to a local field of 68(1) gauss at the muon site at 3.2 kelvin (parentheses indicate the error in the last digit). Substantial spatially inhomogeneous effects are manifested in the distribution of the local fields, whose width amounts to 48(2) gauss at the same temperature. The temperature evolution of the internal magnetic field below the freezing temperature mirrors that of the saturation magnetization, closely following the behavior expected for collective spin wave (magnon) excitations. The transition to a ferromagnetic state with a T(c) higher than that of any other organic material is now authenticated.

Thermodynamics and kinetics of nanostructure formation in soft-magnetic nanocrystalline alloys (overview)

Abstract: Thermodynamic and kinetic conditions for nanocrystallization of Fe-based amorphous precursors into a duplex structure of α-Fe nanocrystals within a residual amorphous matrix are considered in view of recent literature. The effect of copper addition on nanograin size is explained in terms of the CuFe metastable phase diagram and the nature of the various heteroatomic interactions. Addition of larger atoms such as Nb and Zr that are rejected together with boron atoms at the α-Fe nanocrystallization interface is found to generate diffusion double-layers with sharp concentration gradients. The diffusion double-layers explain measured differences in Curie temperatures T c of the remaining grain boundary amorphous phase and bulk glasses of the same composition. They also successfully predict the occurence of a broad maximum in T c of the remaining amorphous phase during nanocrystallization annealing. The concentration gradients have an additional thermodynamic stabilization effect on the amorphous interlayers by reducing the driving force for the formation of intermetallics.

Dielectric and Piezoelectric Properties of Pb(Yb1/2Nb1/2)O3-PbTiO3 Solid Solution System

Abstract: Solid solutions of (1- x)Pb(Yb1/2Nb1/2)O3-( x)PbTiO3x=0 to 0.8 were investigated from the viewpoint of dielectric and piezoelectric properties. With increasing PbTiO3 content (x), phases at room temperature continuously changed from antiferroelectric (x=0), ferroelectric (x=0.1 to 0.15), relaxor (x=0.2 to 0.49), and again to ferroelectric (x>0.49), where crystal structures of all these phases were perovskite. Sintering temperature was lowered to around 950° C in the relaxor phase (x=0.2 to 0.49). Piezoelectric constants exhibited a maximum at x=0.49 to 0.50 near the morphotropic phase boundary (x=0.49). In the sample with x=0.50, Curie point was 349° C, and remanent polarization and coercive field were 32.8 µ C/cm2 and 2.75 kV/mm, respectively. Also, electromechanical coupling factors for planar (k p) and thickness (k t) modes were 0.527 and 0.436, respectively.

Magnetic Transition Temperature of (La,Sr)MnO3

Abstract: Using the Kondo lattice model with classical spins in infinite dimension, magnetic phase transition in the perovskite-type 3 d transition-metal oxide La 1- x Sr x MnO 3 is theoretically studied. On the Bethe lattice, the self-consistency equations are solved exactly. Curie temperatures in the region of double-exchange ferromagnetism 0.1≤ x ≤0.25 as well as the Neel temperature at x =0 are well reproduced quantitatively. Pressure effect on the Curie temperature is also discussed.

A magnetic, neutron diffraction, and Mössbauer spectral study of the Ce2Fe17−xAlx solid solutions

Abstract: The magnetic properties of a series of Ce2Fe17−xAlx solid solutions with x equal to 0.00, 0.88, 2.06, 2.81, 3.98, 5.15, 6.08, 7.21, 8.20, 9.08, 9.84, and 10.62 have been studied by magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. The compounds crystallize in the rhombohedral Th2Zn17‐type structure. Magnetization studies indicate that the Curie temperature increases uniformly from 238 K for Ce2Fe17 to 384 K for Ce2Fe14Al3 and then decreases at higher aluminum content. Powder neutron diffraction results, obtained at 295 K, indicate that aluminum avoids the 9d site for all x values and preferentially occupies the 18h site at low aluminum content. Aluminum shows a marked preference for the 6c site for x≳6. The room‐temperature iron magnetic moments increase from x=0 to 2 and then decrease for x≳2. The Mossbauer spectra have been fit with a binomial distribution of the near‐neighbor environments in terms of a maximum hyperfine field, Hmax, for an iron with zero aluminum near neighbors, a...

Curie Temperature of BaTiO3

Abstract: The Curie point (T C ) of BaTiO 3 was earlier reported to be 120°C and has been recently believed to be 130°C. Some experiments have been performed here to reconfirm the T C . Firing conditions used for preparing BaTiO 3 were examined first with the use of pure BaCO 3 or Ba(NO 3 ) 2 and TiO 2 . The x-dependence of T C in (BaO) 1-x (TiO 2 ) 1+x solid solution was measured. Data were scattered and suffered from individual variations. According to probability considerations, T C of BaTiO 3 was evaluated from the intercept at x=0. On the other hand, composition dependence of T C in some related solid solution systems, (Ba 1-y Pb y )TiO 3 . (Ba 1-y Sr y )TiO 3 and (BaTiO 3 ) 1-y (KF) y , was examined and the T C of BaTiO 3 was estimated by extrapolation toward the limit y→0. In conclusion, the Curie point of BaTiO 3 is determined as 123.0±0.6°C.

Evidence for 4d Ferromagnetism in 2D Systems: Ru Monolayers on C(0001) Substrates.

Abstract: The first observation of $4d$ ferromagnetism in two-dimensional systems is reported. Using Auger electron spectroscopy, we find that the initial growth of Ru on a C(0001) substrate is lateral until a homogeneous monolayer film is formed. The magnetic properties of the films are studied using spin polarized secondary electron spectroscopy. For one monolayer Ru on C(0001), below a surface Curie temperature of approximately 250 K, nonzero in-plane spin polarization is observed and found to saturate in an applied field of a few tenths of an Oe. The results are discussed in the light of theoretical predictions.

Magnetoresistance properties of thin films of the metallic oxide ferromagnet SrRuO3.

Abstract: The magnetoresistance of epitaxial thin films (250 to 1000 \AA{} thick) of the metallic oxide ferromagnet ${\mathrm{SrRuO}}_{3}$ has been measured at temperatures ranging from well below to just above the Curie point (\ensuremath{\approxeq}160 K). Measurements using both transverse (nonzero Lorentz force) and longitudinal (zero Lorentz force) geometries cleanly distinguish between an orbital contribution, present only at low temperature, and a spin-flip scattering contribution, present at all temperatures, to the resistivity in magnetic field. The magnetoresistance also shows strongly hysteretic behavior with high coercive and saturation fields. Through the Curie point, the magnetoresistance magnitude shows a maximum, which results from the suppression of the phase transition in magnetic field. The temperature derivative of the zero-field resistivity also shows a discontinuous jump, as predicted by standard theory.