Showing papers on "Curie temperature published in 2004"

Multiferroic BaTiO3-CoFe2O4 Nanostructures.

Abstract: We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO3-CoFe2O4 ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO3 (001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe2O4 nanopillars embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases.

Phase transitional behavior and piezoelectric properties of (Na0.5K0.5)NbO3–LiNbO3 ceramics

Abstract: Lead-free piezoelectric ceramics (1−x)(Na0.5K0.5)NbO3–xLiNbO3 {[Lix(Na0.5K0.5)1−x]NbO3} (x=0.04–0.20) have been synthesized by an ordinary sintering technique. The materials with perovskite structure is orthorhombic phase at x⩽0.05 and becomes tetragonal phase at x⩾0.07, a phase K3Li2Nb5O15 with tetragonal tungsten bronze structure begins to appear at x=0.08 and becomes dominant with increasing the content of LiNbO3. A morphotropic phase boundary between orthorhombic and tetragonal phases is found in the composition range 0.05

Ferromagnetism in Fe-doped SnO2 thin films

Abstract: Thin films grown by pulsed-laser deposition from targets of Sn0.95Fe0.05O2 are transparent ferromagnets with Curie temperature and spontaneous magnetization of 610 K and 2.2 A m2 kg−1, respectively. The 57Fe Mossbauer spectra show the iron is all high-spin Fe3+ but the films are magnetically inhomogeneous on an atomic scale, with only 23% of the iron ordering magnetically. The net ferromagnetic moment per ordered iron ion, 1.8 μB, is greater than for any simple iron oxide with superexchange interactions. Ferromagnetic coupling of ferric ions via an electron trapped in a bridging oxygen vacancy (F center) is proposed to explain the high Curie temperature.

Niobium Doping and Dielectric Anomalies in Bismuth Titanate

Abstract: A study of the complex permittivity in bismuth titanate was conducted to reveal the nature of an anomaly in the real part of the permittivity, which occurs below the Curie temperature. This anomaly is frequency dependent and is caused by a combination of two relaxation phenomena that appear in the imaginary part of the permittivity. One of the relaxations showed classic characteristics of an ion-jump process. Niobium doping suppressed this relaxation and eliminated the nonferroelectric anomaly in permittivity. Niobium is proposed to affect the ion-jump relaxation through a decrease in the concentration and possibly the mobility of oxygen vacancies.

Structural and magnetic properties of transition metal substituted ZnO

Abstract: Structural and magnetic properties have been studied for polycrystalline Zn1−xTMxO, where TM (transition metal ions)=Mn, Fe, and Co. No bulk ferromagnetism was observed for single-phase materials, contrary to the existing theories. Single-phase samples demonstrate paramagnetic Curie–Weiss behavior with antiferromagnetic interactions, similar to other diluted magnetic semiconductors. Nonoptimal synthesis conditions lead to formation of second phases that are responsible for spin-glass behavior {ZnMnO3 impurity for Zn1−xMnxO [S. Kolesnik et al., J. Supercond. 15, 251 (2002)]} or high-temperature ferromagnetic ordering [Co metal for Zn1−xCoxO with the Curie temperature TC>800 K or (Zn,Fe)3O4 for Zn1−xFexO with TC=440 K].

First-principles calculation of the intersublattice exchange interactions and Curie temperatures of the full Heusler alloysNi2MnX(X=Ga,In,Sn,Sb)

Abstract: The interatomic exchange interactions and Curie temperatures in $\mathrm{Ni}$-based full Heusler alloys ${\mathrm{Ni}}_{2}\mathrm{Mn}X$ with $X=\mathrm{Ga},\mathrm{In},\mathrm{Sn}$, and $\mathrm{Sb}$ are studied within the framework of the density-functional theory. The calculation of the exchange parameters is based on the frozen-magnon approach. Despite closeness of the experimental Curie temperatures for all four systems, their magnetism appeared to differ strongly. This difference involves both the $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Mn}$ and $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Ni}$ exchange interactions. The Curie temperatures, ${T}_{C}$, are calculated within the mean-field approximation by solving a matrix equation for a multisublattice system. Good agreement with experiment for all four systems is obtained. The role of different exchange interactions in the formation of ${T}_{C}$ of the systems is discussed.

The effect of flexoelectricity on the dielectric properties of inhomogeneously strained ferroelectric thin films

Abstract: Recent experimental measurements of large flexoelectric coefficients in ferroelectric ceramics suggest that strain gradients can affect the polarization and permittivity behaviour of inhomogeneously strained ferroelectrics. Here we present a phenomenological model of the effect of flexoelectricity on the dielectric constant, polarization, Curie temperature (TC), temperature of maximum dielectric constant (Tm) and temperature of the onset of reversible polarization (Tferro) for ferroelectric thin films subject to substrate-induced epitaxial strains that are allowed to relax with thickness, and the qualitative and quantitative predictions of the model are compared with experimental results for (Ba0.5Sr0.5)TiO3 thin films on SrRuO3 electrodes. It is shown that flexoelectricity can play an important role in decreasing the maximum dielectric constant of ferroelectric thin films under inhomogeneous in-plane strain, regardless of the sign of the strain gradient.

Anomalous Hall effect governed by electron doping in a room-temperature transparent ferromagnetic semiconductor

Abstract: Ferromagnetic semiconductors are believed to be suitable for future spintronics, because both charge and spin degrees of freedom can be manipulated by external stimuli. One of the most important characteristics of ferromagnetic semiconductors is the anomalous Hall effect. This is because the ferromagnetically spin-polarized carrier can be probed and controlled electrically, leading to direct application for electronics. Control of the Curie temperature and magnetization direction by electronic field, and photo-induced ferromagnetism have been performed successfully using the anomalous Hall effect for group III-V ferromagnetic semiconductors. In these cases, the operation temperature was much below room temperature because of the limited Curie temperature of less than 160 K (ref. 6). Here, we report on the anomalous Hall effect governed by electron doping in a room-temperature transparent ferromagnetic semiconductor, rutile Ti(1-x)Co(x)O(2-delta) (of oxygen deficiency delta). This result manifests the intrinsic nature of ferromagnetism in this compound, and represents the possible realization of transparent semiconductor spintronics devices operable at room temperature.

Superhigh strength and good soft-magnetic properties of (Fe,Co)–B–Si–Nb bulk glassy alloys with high glass-forming ability

Abstract: Fe-based bulk ferromagnetic glassy alloys with diameters up to 5mm were formed in [(Fe1−xCox)0.75B0.2Si0.05]96Nb4 system by the copper mold casting method. The substitution of Co for Fe caused an increase of glass-forming ability as well as an improvement of mechanical and magnetic properties. The bulk glassy alloys exhibited superhigh fracture strength of 3900–4250MPa, Young’s modulus of 190–210GPa, elastic strain of 0.02, and plastic strain of 0.0025. The bulk glassy alloys also exhibited good soft magnetic properties, i.e., saturation magnetization of 0.84–1.13T, low coercive force of 1.5–2.7A∕m, high permeability exceeding 1.2×104, and Curie temperature of 600–690K.

Above-room-temperature ferromagnetic Ni2+-doped ZnO thin films prepared from colloidal diluted magnetic semiconductor quantum dots

Abstract: We report the preparation of spin-coated nickel-doped zinc oxide nanocrystalline thin films using high-quality colloidal diluted magnetic semiconductor (DMS) quantum dots as solution precursors. These films show robust ferromagnetism with Curie temperatures above 350K and 300K saturation moments up to 0.1Bohr magnetons per nickel. These results demonstrate a step toward the use of colloidal zero-dimensional DMS nanocrystals as building blocks for the bottom-up construction of more complex ferromagnetic semiconductor nanostructures.

SnO2 doped with Mn, Fe or Co: Room temperature dilute magnetic semiconductors

Abstract: Room temperature ferromagnetism is found in (Sn1−xMx)O2 (M=Mn, Fe, Co, x=0.05) ceramics where x-ray diffraction confirms the formation of a rutile-structure phase. Room temperature saturation magnetization of 0.2 and 1.8 Am2 kg−1 for (Sn0.95Mn0.05)O2 and (Sn0.95Fe0.05)O2, respectively, corresponds to a moment of 0.11 or 0.95 μB per Mn or Fe atom. The Curie temperatures are 340 and 360 K, respectively. The magnetization cannot be attributed to any identified impurity phase. 57Fe Mossbauer spectra of the Fe-doped SnO2 samples, recorded at room temperature and 16 K, show that about 85% of the iron is in a magnetically ordered high spin Fe3+ state, the remainder being paramagnetic.

Magnetic properties of Mn doped ZnO tetrapod structures

Abstract: ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn diffusion doping was performed at two different temperatures (600 and 800 °C). The samples were characterized by scanning electron microscopy, transmission electron microscopy, x-ray fluorescence, x-ray diffraction (XRD), superconducting quantum interference device magnetometer, and photoluminescence. Diffusion doping resulted in the increase of the size of tetrapods, but no new peaks were found in XRD spectrum. Mn doped ZnO tetrapod structures were found to be ferromagnetic with Curie temperature ∼50 K, and showed large coercive field (∼3500 Oe for 800 °C sample, ∼5500 Oe for 600 °C sample).

Microstructure and dielectric properties of Mg-doped barium strontium titanate ceramics

Abstract: The dielectric properties of Ba0.8Sr0.2TiO3 ceramics doped with Mg from 0.5 to 10 mol % have been investigated systematically. Two effects of Mg doping on the dielectric properties of the Ba0.8Sr0.2TiO3 ceramics have been observed. At low Mg doping concentrations (<2 mol %), Mg mainly acts as an acceptor dopant to replace Ti in the B site of ABO3 perovskite structure, leading to a shift of Curie point (TC) to lower temperatures and a diffused phase transition. The permittivity and dielectric loss are suppressed gradually as the Mg content increases and the grain size decreases drastically. At higher Mg doping concentrations (⩾2 mol %), further suppression of permittivity and losses without further TC shift are observed, which indicates a “composite” mixing effect. The grain size remains almost constant. The solubility limit of Mg in the Ba0.8Sr0.2TiO3 appears to be ∼2 mol %.

Phase Diagram of the Perovskite System (1−x)BiScO3-xPbTiO3

Abstract: The perovskite (1−x)BiScO3-xPbTiO3 (BS-PT) system exhibits a large piezoelectric coefficient (d33>460pC∕N in polycrystalline form) and a high Curie temperature TC=450°C in the vicinity of the morphotropic phase boundary (x=0.64mol fraction PbTiO3), which separates the rhombohedral and tetragonal phases. The present contribution reports on revisions to the BiScO3-PbTiO3 phase diagram specifically, (i) the compositional dependence of the octahedral tilt-transition temperature (x<0.62mol fraction PT), as determined using variable-temperature transmission electron microscopy, (ii) high-temperature curvature of the rhombohedral-tetragonal morphotropic phase boundary determined using dielectric measurements on single crystals, and (iii) Curie temperatures in the tetragonal phase field, which exceed that of the PbTiO3 end member.

Temperature dependent magnetic anisotropy in (Ga, Mn)As layers

Abstract: It is demonstrated by SQUID measurements that (Ga,Mn)As films can exhibit perpendicular easy axis at low temperatures, even under compressive strain, provided that the hole concentration is sufficiently low. In such films, the easy axis assumes a standard in-plane orientation when the temperature is raised towards the Curie temperature or the hole concentration is increased by low temperature annealing. These findings are shown to corroborate quantitatively the predictions of the mean-field Zener model for ferromagnetic semiconductors. The in-plane anisotropy is also examined, and possible mechanisms accounting for its character and magnitude are discussed.

Temperature dependent magnetic anisotropy in metallic magnets from an ab initio electronic structure theory: L1(0)-ordered FePt.

Abstract: Using a first-principles, relativistic electronic structure theory of finite temperature metallic magnetism, we investigate the variation of magnetic anisotropy $K$ with magnetization $M$ in metallic ferromagnets. We apply the theory to the high uniaxial $K$ material, $L{1}_{0}$-ordered FePt, and find its magnetic easy axis perpendicular to the $\mathrm{F}\mathrm{e}/\mathrm{P}\mathrm{t}$ layers for all $M$ and $K$ to be proportional to ${M}^{2}$ for a broad range of values of $M$. For small $M$, near the Curie temperature, the calculations pick out the easy axis for the onset of magnetic order. Our ab initio results for this important magnetic material agree well with recent experimental measurements, whereas the single-ion anisotropy model fails to give the correct qualitative behavior.

Pressure-induced superconductivity in ferromagnetic UIr without inversion symmetry

Abstract: We report the discovery of pressure-induced superconductivity in ferromagnetic UIr, which lacks inversion symmetry in the crystal structure. The Curie temperature TC1 = 46 K at ambient pressure decreases with increasing pressure, reaching a value of 11 K at 1.5 GPa. It presumably decreases further up to about Pc1 = 1.7 GPa. The ferromagnetic region named 'F1' exists up to Pc1. A second magnetic phase named 'F2' with a low ferromagnetic moment appears in the pressure range from 1.9 to 2.4 GPa. In the 'F2' phase, the magnetic transition temperature TC2 decreases with pressure, from 18 K at 1.9 GPa to approximately zero at PC2 = 2.6–2.7 GPa. In this critical pressure region, superconductivity appears below Tsc = 0.14 K.

Room-temperature ferromagnetism in well-aligned Zn1−xCoxO nanorods

Abstract: Diluted magnetic semiconductor Zn1−xCoxO nanorods with a Curie temperature higher than 350K have been synthesized by in situ doping of Co in ZnO nanorods using a simple thermal chemical vapor deposition method. Structural analyses indicated that the nanorod possesses the single-crystalline wurtzite structure and there is no segregated cluster of impurity phase appearing throughout the nanorod. The transparence of the Zn1−xCoxO nanorods in the visible region has been examined by UV-visible absorption. The fundamental absorptions of the Zn1−xCoxO nanorods estimated from the absorption spectra do not reveal pronounced difference from that of pure ZnO nanorods.

Large magnetocaloric effect in a La0.7Ca0.3MnO3 single crystal

Abstract: We report the results of a thorough study of the magnetocaloric effect (MCE) in a La0.7Ca0.3MnO3 single crystal, which undergoes a first-order magnetic phase transition at ∼227 K. The magnetic entropy change ΔSM and the adiabatic temperature change ΔTad reach, respectively, ∼6.42 J/kg K and 4.76 K for ΔB=5 T and even if both are smaller than those exhibited by gadolinium (∼9.8 J/kg K and ∼11.7 K), the ΔSM distribution here is much more uniform than that of gadolinium and polycrystalline manganites. This is desirable for an Ericson-cycle magnetic refrigerator. The MCE is larger in the single-crystalline manganite compared to the polycrystalline one. The manganite single crystal has large ΔSM induced by low magnetic field change, which is beneficial for the household application of active magnetic refrigerant (AMR) materials. All these make the lanthanum manganite single crystal an attractive candidate as a working substance for AMR. The molecular field model provides a fairly good description of the magnet...

Field-Induced Martensitic Transformation in New Ferromagnetic Shape Memory Compound Mn1.07Co0.92Ge

Abstract: The structural transformation of a ferromagnetic Mn1.07Co0.92Ge compound was studied by magnetization measurement in magnetic fields up to 13 T and powder X-ray diffraction measurement in fields up to 5 T. The Curie temperature is determined to be 275 K. At room temperature, the compound has a hexagonal Ni2In-type structure, and it transforms diffusionlessly into an orthorhombic TiNiSi-type structure at MS=210 K with hysteresis. The cell volume expands by 5.3% and the magnetic moment increases by 24% at MS, accompanied with this martensitic structural transformation. In addition, we confirmed for the first time that magnetic field induces the structural transformation in Mn1.07Co0.92Ge at the vicinity of MS.

Exchange interactions in diluted magnetic semiconductors

Abstract: On the basis of ab initio calculations for Mn-doped GaN, GaP, GaAs and GaSb, we discuss the origin of ferromagnetism in diluted magnetic semiconductors. The calculations use the Korringa–Kohn–Rostoker method in connection with the coherent potential approximation to describe the substitutional and moment disorder. By mapping the total energy results onto a Heisenberg model, the Curie temperature TC is estimated in the mean field approximation (MFA). If impurity bands are formed in the gap, as is the case for (Ga, Mn)N, double exchange dominates leading to a characteristic dependence of TCMFA as a function of the Mn concentration c. On the other hand, if the d states are localized, as in (Ga, Mn)Sb, Zener's p–d exchange prevails, resulting in a linear c dependence of TCMFA. (Ga, Mn)As is an intermediate case, showing a -like behaviour in the local density approximation (LDA), but a nearly linear c dependence, if the more accurate LDA +U method with U = 4 eV is used.

Effect of Joule heating in current-driven domain wall motion

Abstract: It was found that high current density needed for the current-driven domain wall motion results in the Joule heating of the sample. The sample temperature, when the current-driven domain wall motion occurred, was estimated by measuring the sample resistance during the application of a pulsed-current. The sample temperature was 750 K for the threshold current density of 6.7 x 10^11 A/m2 in a 10 nm-thick Ni81Fe19 wire with a width of 240 nm. The temperature was raised to 830 K for the current density of 7.5 x 10^11 A/m2, which is very close to the Curie temperature of bulk Ni81Fe19. When the current density exceeded 7.5 x 10^11 A/m2, an appearance of a multi-domain structure in the wire was observed by magnetic force microscopy, suggesting that the sample temperature exceeded the Curie temperature.

Optimized Synthesis of the Elusive ε-Fe2O3 Phase via Sol−Gel Chemistry

Abstract: e-Fe2O3 nanoparticles embedded in a SiO2 matrix have been synthesized by sol−gel chemistry and high temperature heat treatments. Virtually pure e-Fe2O3 (in excess of 93%) is obtained, although a two-phase mixture, e-Fe2O3 + α-Fe2O3, is observed for Fe2O3/SiO2 ratios greater than 37 wt %. The e-Fe2O3 nanoparticles are stable up to ∼1600 K. Optimized e-Fe2O3 nanoparticles are ferrimagnetic, with a Curie temperature TC ≈ 510 K, and remarkably high values of room-temperature coercivity, HC = 20 kOe.

Effect of SrTiO3 concentration and sintering temperature on microstructure and dielectric constant of Ba1−xSrxTiO3

Abstract: The Ba 1− x Sr x TiO 3 materials have received increased attention as one of the most important materials for electroceramic components, such as high dielectric ceramic capacitors, tunable phase shifters and PTCR. In this paper, the effect of SrTiO 3 concentration and sintering temperature on the microstructure and dielectric constant of Ba 1− x Sr x TiO 3 materials at the Curie temperature have been investigated. When Ba 1− x Sr x TiO 3 materials were sintered at 1350 °C, the peak value of the dielectric constant, ϵ max , monotonically decreased with increasing SrTiO 3 concentration. At the sintering temperature of 1400 °C the dielectric constant maximum at the T C increased with an increase in the x value, reaching the highest value at around x =0.4 and then decreased. As sintering temperature increased to 1450 °C, ϵ max increased with increasing SrTiO 3 concentration up to x =0.6. The dielectric properties of Ba 1− x Sr x TiO 3 materials were discussed in terms of SrTiO 3 concentration and microstructure.

High Curie temperatures in ferromagnetic Cr-doped AlN thin films

Abstract: Al1−xCrxN thin films with 0.02⩽x⩽0.1 were deposited by reactive co-sputtering onto c-plane (001) sapphire. Room-temperature ferromagnetism with a coercive field of 85 Oe was observed in samples with chromium contents as low as x=0.027 (2.7%). With increasing Cr content the mean magnetic moment is strongly suppressed, with a maximum saturation moment of 0.62 and 0.71 μB per Cr atom at 300 and 50 K, respectively. We show that the Curie temperature of Al1−xCrxN for x=0.027 is greater than 900 K.