Showing papers on "Curie temperature published in 2003"

High temperature ferromagnetism with a giant magnetic moment in transparent co-doped SnO(2-delta).

Abstract: The occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn(1-x)Co(x)O(2-delta) (x<0.3). Interestingly, films of Sn(0.95)Co(0.05)O(2-delta) grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5+/-0.5 micro(B)/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

Glassy ferromagnetism and magnetic phase separation in La 1 − x Sr x CoO 3

Abstract: We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and magnetotransport properties of the glassy ferromagnet ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}.$ The compositions studied span the range from the end-member ${\mathrm{LaCoO}}_{3}$ $(x=0.0)$ through to $x=0.7.$ These materials have attracted attention recently, primarily due to the spin-state transition phenomena in ${\mathrm{LaCoO}}_{3}$ and the unusual nature of the magnetic ground state for finite x. In this paper we present a consistent picture of the magnetic behavior of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ in terms of short-range ferromagnetic ordering and intrinsic phase separation. At high Sr doping $(xg0.2)$ the system exhibits unconventional ferromagnetism (with a Curie temperature up to 250 K), which is interpreted in terms of the coalescence of short-range-ordered ferromagnetic clusters. Brillouin function fits to the temperature dependence of the magnetization as well as high-temperature Curie-Weiss behavior suggest that the ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions are both in the intermediate spin state. At lower Sr doping $(xl0.18)$ the system enters a mixed phase that displays the characteristics of both a spin glass and a ferromagnet. A cusp in the zero-field-cooled dc magnetization, a frequency-dependent peak in the ac susceptibility and time-dependent effects in both dc and ac magnetic properties all point towards glassy behavior. On the other hand, field cooling results in a relatively large ferromagneticlike moment, with zero-field-cooled and field-cooled magnetizations bifurcating at an irreversibility point. Even in the region above $x=0.2$ the out-of-phase component of the ac susceptibility shows frequency-dependent peaks below the Curie temperature (indicative of glassy behavior) which have previously been interpreted in terms of the freezing of clusters. All of the results are consistent with the existence of a strong tendency towards magnetic phase separation in this material, a conclusion which is further reinforced by consideration of the electronic properties. The metal-insulator transition is observed to be coincident with the onset of ferromagnetic ordering $(x=0.18)$ and has a behavior in the doping dependence of the low-temperature conductivity which is strongly suggestive of percolation. This can be interpreted as a percolation transition within the simple ferromagnetic cluster model. On the metallic side of the transition the system exhibits colossal magnetoresistance-type behavior with a peak in the negative magnetoresistance (\ensuremath{\sim}10% in 90 kOe) in the vicinity of the Curie temperature. As the transition is approached from the metallic side we observe the onset of a negative magnetoresistance that increases in magnitude with decreasing temperature, reaching values as large as 90% in a 90-kOe field. This magnetoresistance is enhanced at the metal-insulator transition, where it persists even to room temperature.

Ferromagnetism in Mn-implanted ZnO:Sn single crystals

Abstract: We have investigated the magnetic properties of Mn-implanted n-type ZnO single crystals that are codoped with Sn. Theory predicts that room-temperature carrier-mediated ferromagnetism should be possible in manganese-doped p-type ZnO, although Mn-doped n-type ZnO should not be ferromagnetic. While previous efforts report only low-temperature ferromagnetism in Mn-doped ZnO that is n type via shallow donors, we find evidence for ferromagnetism with a Curie temperature of ∼250 K in ZnO that is codoped with Mn and Sn. As a 4+ valence cation, Sn should behave as a doubly ionized donor, thus introducing states deep in the gap. Hysteresis is clearly observed in magnetization versus field curves. Differences in zero-field-cooled and field-cooled magnetization persists up to ∼250 K for Sn-doped ZnO crystals implanted with 3 at. % Mn. Increasing the Mn concentration to 5 at. % significantly reduces the magnetic hysteresis. This latter observation is inconsistent with the origin for ferromagnetism being due to segregated secondary phases, and strongly suggests that a near-room-temperature dilute magnetic semiconducting oxide has been realized. Based on these results, ZnO doped with Mn and Sn may prove promising as a ferromagnetic semiconductor for spintronics.

Large Piezoelectric Constant and High Curie Temperature of Lead-Free Piezoelectric Ceramic Ternary System Based on Bismuth Sodium Titanate-Bismuth Potassium Titanate-Barium Titanate near the Morphotropic Phase Boundary

Abstract: A lead-free piezoelectric ceramic ternary system based on bismuth sodium titanate, (Bi1/2Na1/2)TiO3 (BNT) - bismuth potassium titanate (Bi1/2K1/2)TiO3 (BKT) - barium titanate BaTiO3 (BT) near the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases has been investigated. In the case of a(Bi1/2Na1/2)TiO3–bBaTiO3–c(Bi1/2K1/2)TiO3 [BNBK(100a/100b/100c)] solid solution ceramics, the highest piezoelectric constant d33=191 pC/N, Curie temperature, Tc=301°C, electromechanical coupling factor, k33=0.56 and dielectric constant, e33T/e0=1141 are observed for the BNBK(85.2/2.8/12) composition which has a tetragonal phase near the MPB. The d33 value is the highest so far reported for all lead-free piezoelectric ceramics with Tc>300°C. The BNT-BKT-BT ternary ceramics system sintered at 1200°C for 2 h in air has a pure perovskite structure and a high density more than 95% of the theoretical density.

Room-temperature ferromagnetism in a II-VI diluted magnetic semiconductor Zn(1-x)Cr(x)Te.

Abstract: The magnetic and magneto-optical properties of a Cr-doped II-VI semiconductor ZnTe were investigated. Magnetic circular dichroism measurements showed a strong interaction between the sp carriers and localized d spins, indicating that Zn(1-x)Cr(x)Te is a diluted magnetic semiconductor. The Curie temperature of the film with x=0.20 was estimated to be 300+/-10 K, which is the highest value ever reported for a diluted magnetic semiconductor in which sp-d interactions were confirmed. In spite of its high Curie temperature, Zn(1-x)Cr(x)Te film shows semiconducting electrical transport properties.

Entangled quantum state of magnetic dipoles

Abstract: Free magnetic moments usually manifest themselves in Curie laws, where weak external magnetic fields produce magnetizations that vary as the reciprocal of the temperature (1/T). For a variety of materials that do not display static magnetism, including doped semiconductors and certain rare-earth intermetallics, the 1/T law is replaced by a power law T^-α with α < 1. Here we show that a much simpler material system—namely, the insulating magnetic salt LiHo_xY_(1-x)F_4—can also display such a power law. Moreover, by comparing the results of numerical simulations of this system with susceptibility and specific-heat data, we show that both energy-level splitting and quantum entanglement are crucial to describing its behaviour. The second of these quantum mechanical effects—entanglement, where the wavefunction of a system with several degrees of freedom cannot be written as a product of wavefunctions for each degree of freedom—becomes visible for remarkably small tunnelling terms, and is activated well before tunnelling has visible effects on the spectrum. This finding is significant because it shows that entanglement, rather than energy-level redistribution, can underlie the magnetic behaviour of a simple insulating quantum spin system.

Curie temperatures of III–V diluted magnetic semiconductors calculated from first principles

Abstract: Curie temperatures of the diluted magnetic semiconductors (Ga, Mn)As, (Ga, Mn)N, (Ga, Cr)As and (Ga, Cr)N are evaluated from first principles. The electronic structure is calculated in the local spin density approximation by using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation to describe the substitutional and spin disorder. From the total energy differences between the ferromagnetic state and the spin-glass state, realistic estimations of Curie temperatures are achieved by using a mapping on the Heisenberg model in the mean-field approximation. Effects of additional carrier doping treatments are also investigated. Very large Curie temperatures are obtained, lying above room temperature for (Ga, Mn)N, (Ga, Cr)As and (Ga, Cr)N. Upon hole doping the Curie temperature of (Ga, Mn)N further increases, while (Ga, Mn)As shows a plateau behavior.

Structural and doping effects in the half-metallic double perovskite A 2 CrWO 6 (A=Sr, Ba, and Ca)

Abstract: The structural, transport, magnetic, and optical properties of the double perovskite A2CrWO6 with A = Sr,Ba,Ca have been studied. By varying the alkaline earth ion on the A site, the influence of steric effects on the Curie temperature TC and the saturation magnetization has been determined. A maximum TC = 458 K was found for Sr2CrWO6 having an almost undistorted perovskite structure with a tolerance factor f~1. For Ca2CrWO6 and Ba2CrWO6 structural changes result in a strong reduction of TC. Our study strongly suggests that for the double perovskites in general an optimum TC is achieved only for f~1, that is, for an undistorted perovskite structure. Electron doping in Sr2CrWO6 by a partial substitution of Sr2+ by La3+ was found to reduce both TC and the saturation magnetization Ms. The reduction of Ms could be attributed both to band structure effects and the Cr/W antisites induced by doping. Band structure calculations for Sr2CrWO6 predict an energy gap in the spin-up band, but a finite density of states for the spin-down band. The predictions of the band structure calculation are consistent with our optical measurements. Our experimental results support the presence of a kinetic energy driven mechanism in A2CrWO6, where ferromagnetism is stabilized by a hybridization of states of the nonmagnetic W site positioned in between the high spin Cr sites.

Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers

Abstract: We report Curie temperatures up to 150 K in annealed Ga1−xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of Ga1−xMnxAs epilayers may be important in determining their physical properties.

Shape memory ferromagnets

Abstract: In ferromagnetic alloys with shape memory large reversible strains can be obtained by rearranging the martensitic domain structure by a magnetic field. Magnetization through displacement of domain walls is possible in the presence of high magnetocrystalline anisotropy, when martensitic structure rearrangement is energetically favorable compared to the reorientation of magnetic moments. In ferromagnetic Heusler alloys Ni2+xMn1–xGa the Curie temperature exceeds the martensitic transformation temperature. The fact that these two temperatures are close to room temperature offers the possibility of magnetically controlling the shape and size of ferromagnets in the martensitic state. In Ni2+xMn1–xGa single crystals, a reversible strain of ~6% is obtained in fields of ~1 T.

Martensitic transformation and shape memory effect in ferromagnetic Heusler alloy Ni2FeGa

Abstract: We have synthesized ferromagnetic Heusler alloy Ni2FeGa using the melt-spinning technique. The Ni2FeGa ribbon, having a high chemical ordering L21 structure, exhibits a thermoelastic martensitic transformation from cubic to orthorhombic structure at 142 K and a premartensitic transformation. The alloy has a relatively high Curie temperature of 430 K, a magnetization of 73 Am2/kg, and a low saturated field of 0.6 T. The textured samples with preferentially oriented grains show a completely recoverable two-way shape memory effect with a strain of 0.3% upon the thermoelastic martensitic transformation.

Ferromagnetism in laser deposited anatase Ti 1-x Co x O 2-δ films

Abstract: Pulsed laser deposited films of Co-doped anatase TiO 2 are examined for Co substitutionality, ferromagnetism, transport, magnetotransport, and optical properties. Our results show limited solubility (up to ∼2%) of Co in the as-grown films and formation of Co clusters thereafter. For the Ti 0 . 9 3 Co 0 . 0 7 O 2 - δ sample, which exhibits a Curie temperature (T C ) over 1180 K, we find the presence of 20-50 nm Co clusters as well as a small concentration of Co incorporated into the remaining matrix. After being subjected to the high-temperature anneal during the first magnetization measurement, the very same sample shows a T C ∼650 K and almost full matrix incorporation of Co. This T C is close to that of as-grown Ti 0 . 9 9 Co 0 . 0 1 O 2 - δ sample (∼700 K). The transport, magnetotransport, and optical studies also reveal interesting effects of the matrix incorporation of Co. These results are indicative of an intrinsic Ti 1 - x Co x O 2 - δ diluted magnetic semiconductor with T C of about 650-700 K.

Ferromagnetism and high Curie temperature in semiconductor heterostructures with Mn δ-doped GaAs and p-type selective doping

Abstract: The interaction between the magnetic dopants (Mn) and 2-dimensional hole gas (2DHG) in Mn \ensuremath{\delta}-doped GaAs/Be-doped $p$-type AlGaAs heterostructures, where holes were supplied from the Be-doped AlGaAs to the Mn \ensuremath{\delta}-doped GaAs, realized ferromagnetic ordering. The Curie temperature ${T}_{\mathrm{C}}$ of the heterostructure prepared with suitable growth conditions was 172 K, highest among the ${T}_{\mathrm{C}}$ values reported in III-V (InAs, GaAs) magnetic semiconductors.

Synthesis, optical, and magnetic properties of diluted magnetic semiconductor Zn1−xMnxO nanowires via vapor phase growth

Abstract: Diluted magnetic semiconductor Zn1−xMnxO nanowires were synthesized via an in situ doping of manganese in ZnO nanowires using vapor phase growth at 500 °C. The maximum content of the manganese in the ZnO is around 13 at. %, approaching the maximum thermal equilibrium limit of Mn solubility in ZnO at the growth temperature. Structure and composition analysis revealed that the manganese was doped into the lattice structure, forming solid solution instead of precipitation. Magnetic property measurements revealed that the as-doped Zn1−xMnxO nanowires exhibit ferromagnetic behavior with Curie temperature around 37 K.

Effect of low-temperature annealing on (Ga,Mn)As trilayer structures

Abstract: The effect of low-temperature annealing on (Ga,Mn)As/GaAs/(Ga,Mn)As trilayer structures is studied. Low-temperature annealing significantly increases the ferromagnetic transition temperature TC of top (Ga,Mn)As layers, reaching as high as 160 K, whereas no apparent effect is observed on bottom (Ga,Mn)As layers. The annealing effect on Be-doped trilayers is also presented.

Structural phase transition at the percolation threshold in epitaxial (La0.7Ca0.3MnO3)1-x:(MgO)x nanocomposite films.

Abstract: 'Colossal magnetoresistance' in perovskite manganites such as La0.7Ca0.3MnO3 (LCMO), is caused by the interplay of ferro-paramagnetic, metal-insulator and structural phase transitions. Moreover, different electronic phases can coexist on a very fine scale resulting in percolative electron transport. Here we report on (LCMO)1-x:(MgO)x (0 < x < or = 0.8) epitaxial nano-composite films in which the structure and magnetotransport properties of the manganite nanoclusters can be tuned by the tensile stress originating from the MgO second phase. With increasing x, the lattice of LCMO was found to expand, yielding a bulk tensile strain. The largest colossal magnetoresistance of 10(5)% was observed at the percolation threshold in the conductivity at xc 0.3, which is coupled to a structural phase transition from orthorhombic (0 < x < or 0.1) to rhombohedral R3c structure (0.33 < or = x < or = 0.8). An increase of the Curie temperature for the Rc phase was observed. These results may provide a general method for controlling the magnetotransport properties of manganite-based composite films by appropriate choice of the second phase.

High Curie temperature piezocrystals in the BiScO3-PbTiO3 perovskite system

Abstract: Perovskite ferroelectric single crystals in the (1−x)BiScO3-xPbTiO3 (BSPT) system were grown using a flux method. The Curie temperature of rhombohedral BSPT(43/57) crystals was found to be about 404 °C, with a rhombohedral–tetragonal phase transition temperature (Tr−t) around 350 °C. The coercive field (Ec) was found to be 13.7 kV/cm for 〈001〉-oriented crystals, while the coupling factor k33 was determined to be ∼90% at room temperature and was nearly temperature independent until 330 °C. High piezoelectric properties together with a high Curie temperature and coercive field make rhombohedral BSPT crystals promising candidates for the next generation of high-performance, high-temperature actuators and transducers.

Synthesis, characterization, and modeling of high quality ferromagnetic Cr-doped AlN thin films

Abstract: We report a theoretical and experimental investigation of Cr-doped AlN. Density functional calculations predict that the isolated Cr t2 defect level in AlN is 1/3 full, falls approximately at midgap, and broadens into an impurity band for concentrations over 5%. Substitutional Al1−xCrxN random alloys with 0.05⩽x⩽0.15 are predicted to have Curie temperatures over 600 K. Experimentally, we have characterized and optimized the molecular beam epitaxy thin film growth process, and observed room temperature ferromagnetism with a coercive field, Hc, of 120 Oe. The measured magnetic susceptibility indicates that over 33% of the Cr is magnetically active at room temperature and 40% at low temperature.

Epitaxial ferromagnetic Mn5Ge3 on Ge(111)

Abstract: Ferromagnetic Mn5Ge3 thin films were grown on Ge(111) with solid-phase epitaxy. The epitaxial relationship between the alloy film and substrate is Mn5Ge3(001)//Ge(111) with [100]Mn5Ge3//[110]Ge. The alloy films exhibit metallic conductivity and strong ferromagnetism up to the Curie temperature, TC=296 K. These epitaxial alloy films are promising candidates for germanium-based spintronics.

Temperature dependence and temperature limits of magnetic shape memory effect

Abstract: We study the temperature dependence and low and high temperature limits of the magnetic shape memory effect (MSME) in five-layered tetragonal Ni–Mn–Ga martensite. Using a simple model we show that, additionally to the limits posed by transformation to austenite or an intermartensitic transformation, the temperature dependence of the magnetic anisotropy, tetragonality of the lattice, and twinning stress play important role when considering the temperature limits of the MSME. With decreasing temperature, the lattice distortion and magnetic anisotropy increase, but saturate in a low temperature region. The twinning stress does not saturate and its temperature dependence has exponential-like character that increases rapidly in the low temperature region. The model predicts that the low-temperature limit of the MSME is 165 K for Ni49.7Mn29.1Ga21.2 composition. This agrees very well with the value of 173 K determined from direct measurements. The high temperature limit is transformation to austenite at 315 K. T...

Ruderman-Kittel-Kasuya-Yosida-like ferromagnetism in MnxGe1-x.

Abstract: The nature and origin of ferromagnetism in magnetic semiconductors is investigated by means of highly precise electronic and magnetic property calculations on MnxGe1-x as a function of the location of Mn sites in a large supercell. Surprisingly, the coupling is not always ferromagnetic (FM), even for large Mn-Mn distances. The exchange interaction between Mn ions oscillates as a function of the distance between them and obeys the Ruderman-Kittel-Kasuya-Yosida analytic formula. The estimated Curie temperature is in good agreement with recent experiments, and the estimated effective magnetic moment is about 1.7mu(B)/Mn, in excellent agreement with the experimental values, (1.4-1.9)mu(B)/Mn.

Superelasticity in high-temperature Ni–Mn–Ga alloys

Abstract: The stress–strain behavior of two single crystalline Ni–Mn–Ga alloys with martensitic transformation temperatures above the Curie temperature has been studied during uniaxial compression along the [001] and [110] axes. The superelastic effect associated to the reversible stress-induced martensitic transformation has been obtained in these high-temperature shape memory alloys. The experimental values of the critical stress to induce the martensitic transformation, σc, depend linearly on the test temperature. This dependence is described well by the thermodynamics of the stress-induced martensitic transformation through a Clausius–Clapeyron-type equation. However, the experimental values of slopes dσc/dT obtained in the two compression directions can only be interpreted well if it is assumed that different martensitic phases are formed for each compression axis. Good agreement with the thermodynamic model exists if tetragonal martensites with c/a 1 are induced in the [001] and [110] axes, respectively.

Extraordinary Hall effect and ferromagnetism in Fe-doped reduced rutile

Abstract: Room-temperature ferromagnetism is observed in reduced rutile TiO2−δ by Fe doping. The epitaxial films grown by pulsed-laser deposition are carefully examined by x-ray diffraction, transmission electron microscopy, and magnetic and transport measurements. The films exhibit the extraordinary Hall-effect and thin-film magnetic shape anisotropy. The magnetic moments and anticipated Curie temperatures of the films rule out Fe particles, iron oxides, and Ti–Fe oxides as possible sources for the observed magnetic signals. The carriers of the Fe-doped reduced rutile are p-type, with a carrier density of 1×1022/cm3. This room-temperature dilute magnetic semiconductor should find potential applications in spintronics.

Antisites and electron-doping effects on the magnetic transition of Sr 2 FeMoO 6 double perovskite

Abstract: The effect of antisite (AS) defects and electron doping on the ferromagnetic-paramagnetic (FM-PM) transition of Sr 2 FeMoO 6 (SFMO) double perovskite has been studied. From a detailed analysis of the magnetization curves across the FM-PM transition we conclude that AS defects decrease the average strength of the magnetic interactions, although, remarkably enough, some of them are actually enhanced. It follows that the mean-field Curie temperature lowers whereas the onset of magnetization occurs at somewhat higher temperatures. The FM-PM transition has also been analyzed in electron-doped La x Sr 2 - x FeMoO 6 , where the AS defects concentration has been found to increase upon La doping. It turns out that in spite of the presence of AS, the mean-field Curie temperature significantly rises upon La and electron doping. This experimental finding contrasts with some recent predictions and emphasizes the role of itinerant electrons in the ferromagnetic coupling in these oxides. Moreover, our results indicate that disordered (i.e., with AS) double perovskite materials should be described as systems with random magnetic anisotropy.

Electronic structure and magnetism of Mn-doped GaN

Abstract: Mn-doped semiconductors are extremely interesting systems due to their novel magnetic properties suitable for the spintronics applications. It has been shown recently by both theory and experiment that Mn-doped GaN systems have a very high Curie temperature compared to that of Mn-doped GaAs systems. To understand the electronic and magnetic properties, we have studied Mn-doped GaN system in detail by a first-principles plane-wave method. We show here the effect of varying Mn concentration on the electronic and magnetic properties. In agreement with previous studies, d states of Mn form an impurity band completely separated from the valence-band states of the host GaN for dilute Mn concentration. This is in contrast to the Mn-doped GaAs system where Mn d states in the gap lie very close to the valence-band edge and hybridize strongly with the delocalized valence-band states. To study the effects of electron correlation, LSDA+U calculations have been performed. Calculated exchange interaction in (Mn,Ga)N is short ranged contrary to that in (Mn,Ga)As where the strength of the ferromagnetic coupling between Mn spins is not decreased substantially for large Mn-Mn separation. Also, the exchange interactions are anisotropic in different crystallographic directions due to the presence or absence of connectivity between Mn atoms through As bonds.

The effect of Mn substitution in LaFe11.7Si1.3 compound on the magnetic properties and magnetic entropy changes

Abstract: The magnetic properties and magnetic entropy changes of La(Fe1−xMnx)11.7Si1.3 (x = 0–0.03) have been studied. The Curie temperatures TC decrease monotonously with increasing Mn concentration from 188 to 127 K, and the saturation magnetization μS decreases from 23.9 μB/fu to 22.2 μB/fu respectively, as x increases from 0 to 0.03. The maximal magnetic entropy changes |ΔS|, under a magnetic field change of 0–5 T, are 26.0 J kg−1K −1, 25.7 J kg−1K −1, 20.8 J kg−1K −1 and 17.1 J kg−1K −1 for x = 0, 0.01, 0.02 and 0.03, respectively. The appearance of negative slopes in the Arrott plots for all samples confirms the occurrence of a first-order field-induced itinerant-electron metamagnetic (IEM) transition. Furthermore, the full-width at half-maximum (FWHM) of |ΔS| peak, δTFWHM, increases obviously with increasing Mn content, which results in the decrease of the maximum magnetic entropy change.

Curie temperatures of zinc-blende half-metallic ferromagnets

Abstract: Using density-functional theory in the local-density approximation and spin-fluctuation theory, the Curie temperatures of the zinc-blende half-metallic ferromagnets VAs, CrAs, MnAs and the Heusler compound NiMnSb have been estimated, the lower bounds obtained being ab initio. The orders of magnitude are the same as the Curie temperature of the half-metallic ferromagnet NiMnSb with ${T}_{c}=701\mathrm{K}.$ Of the three compounds VAs, CrAs, and MnAs, the highest ${T}_{c}$ is calculated for CrAs, for which the Fermi energy is in the middle of the minority-spin electron gap; its Curie temperature might be as high as 1000 K. The Curie temperature drops sharply when the Fermi energy moves into the minority-electron conduction band as in the case of MnAs.

Ferroelectric and Piezoelectric Properties of KNbO3 Ceramics Containing Small Amounts of LaFeO3

Abstract: Dense KNbO3 ceramics have been successfully synthesized by pressure-less sintering under optimized heat-treatment conditions using a small amount of La2O3 and Fe2O3 additives KNbO3 forms (K1-xLax)(Nb1-xFex)O3 solid solutions and changes in the crystal system, depending on the additive content, from orthorhombic to tetragonal at x of 0020, and from tetragonal to cubic at x of 0200 or higher When only 0002 mol of La2O3 and Fe2O3 (x=0002) was added into KNbO3, the highest value (988%) of the theoretical density was obtained This specimen showed orthorhombic symmetry with a high Curie temperature of 420°C, and demonstrated a well-saturated ferroelectric hysteresis loop with large remanent polarization (Pr) of 18 µC/cm2, which is comparable to the value reported for pure KNbO3 ceramics fabricated by hot pressing Furthermore, the x=0002 specimen showed a planar electromechanical coupling ratio (kp) of 017 and piezoelectric d33 constant of 98 pC/N, regardless of the unsaturated poling state