Showing papers on "Curie temperature published in 2010"
TL;DR: It is shown that a model based on electron-phonon-mediated spin-flip scattering explains both timescales on equal footing and concludes that the ultrafast magnetization dynamics can be well described disregarding highly excited electronic states, merely considering the thermalized electron system.
Abstract: Demagnetization in metals occurs on very different timescales depending on the material. It is now shown that electron–phonon-mediated spin scattering describes the process of demagnetization well in every case, and the differences in timescale are mainly determined by the ratio between Curie temperature and the atomic magnetic moment.
773 citations
TL;DR: Ferroelectric tunnel junctions with ferromagnetic electrodes are used to demonstrate local, large, and nonvolatile control of carrier spin polarization by electrically switching ferroelectric polarization and suggest a low-power approach for spin-based information control.
Abstract: A current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics, which relies on "zero-current," gate-controlled operations. Efforts have been made to control the spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. We used ferroelectric tunnel junctions with ferromagnetic electrodes to demonstrate local, large, and nonvolatile control of carrier spin polarization by electrically switching ferroelectric polarization. Our results represent a giant type of interfacial magnetoelectric coupling and suggest a low-power approach for spin-based information control.
639 citations
TL;DR: In this article, the authors studied the polarization-electric-field hysteresis, the dielectric permittivity dispersion, the piezoelectric properties, the electric-field-induced strain, and the interrelations between these properties for bismuth ferrite (BiFeO3) ceramics.
Abstract: We have studied the polarization-electric-field hysteresis, the dielectric permittivity dispersion, the piezoelectric properties, the electric-field-induced strain, and the interrelations between these properties for bismuth ferrite (BiFeO3) ceramics. The results indicate that the domain-wall movement in BiFeO3 is strongly inhibited by charged defects, most probably acceptor-oxygen-vacancy defect pairs. The domain-wall mobility can be considerably increased by preventing the defects from migrating into their stable configuration; this can be achieved by thermal quenching from above the Curie temperature, which freezes the disordered defect state. Similarly, Bi2O3 loss during annealing at high temperatures contributes to depinning of the domain walls and an increase in the remanent polarization. The possible defects causing the pinning effect are analyzed and discussed. A weakening of the contacts between the grains in the ceramics and crack propagation were observed during poling with constant field at 100 kV/cm. This is probably caused by an electrically induced strain associated with ferroelastic domain reversal. A relatively large piezoelectric d33 constant of 44 pC/N was obtained by “cyclic poling,” in which the electric field was released after each applied cycle with the purpose to relax the mechanical stresses and minimize the problem of cracking.
272 citations
TL;DR: In this paper, the Anderson-Mott transition from a ferromagnetic to a paramagnetic state is observed directly as the density of carriers mediating spin-spin coupling is varied.
Abstract: The transition from a ferromagnetic to a paramagnetic state is observed directly as the density of carriers that mediate spin–spin coupling is varied. The measurement was performed on thin films of GaMnAs and was made possible by superconducting quantum interference devices (SQUIDS). The question of whether the Anderson–Mott localization enhances or reduces magnetic correlations is central to the physics of magnetic alloys1. Particularly intriguing is the case of (Ga, Mn)As and related magnetic semiconductors, for which diverging theoretical scenarios have been proposed2,3,4,5,6,7,8,9. Here, by direct magnetization measurements we demonstrate how magnetism evolves when the density of carriers mediating the spin–spin coupling is diminished by the gate electric field in metal–insulator–semiconductor structures of (Ga, Mn)As. Our findings show that the channel depletion results in a monotonic decrease of the Curie temperature, with no evidence for the maximum expected within the impurity-band models3,5,8,9. We find that the transition from the ferromagnetic to the paramagnetic state proceeds by means of the emergence of a superparamagnetic-like spin arrangement. This implies that carrier localization leads to a phase separation into ferromagnetic and non-magnetic regions, which we attribute to critical fluctuations in the local density of states, specific to the Anderson–Mott quantum transition.
203 citations
TL;DR: In this paper, the magnetic properties of nanoparticles of CoFe2O4 were studied in a broad range of temperature varying from room temperature to 870 K. The results were discussed in terms of interparticle interactions induced by the thermal fluctuations, cation distribution, and other imperfections that exert fields on Co2+ ions could increase with temperature.
Abstract: Cobalt ferrite materials have a wide variety of technological applications that requires temperatures higher than room temperature. Thus the magnetic properties such as saturation magnetization, Ms remanent magnetization, Mr, coercivitty, Hc, and Curie temperature, Tc, of nanoparticles of CoFe2O4 were studied in a broad range of temperature varying from room temperature to 870 K. It was observed that, for temperatures 100 K above room temperature, these magnetic properties are still the same as at room temperature. The results were discussed in terms of interparticle interactions induced by the thermal fluctuations, cation distribution, and other imperfections that exert fields on Co2+ ions could increase with temperature.
195 citations
TL;DR: In this article, the polymorphic phase transitions from orthorhombic to tetragonal phase around room temperature were identified in the composition range of 0.06
Abstract: Lead-free (Ba1−xCax)(Ti0.95Zr0.05)O3 (x=0.02–0.20) ceramics were prepared successfully using a solid-state reaction technique. The polymorphic phase transitions from orthorhombic to tetragonal phase around room temperature were identified in the composition range of 0.06
171 citations
TL;DR: In this paper, the phase diagram of (1−−x)(Bi0.5Na 0.5)-SrTiO3 was completed and investigations on polarization and strain in this system were carried out.
Abstract: The phase diagram of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 was completed and investigations on polarization and strain in this system were carried out. (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3-ceramics were prepared by conventional mixed oxide processing. The depolarization temperature (Td), the temperature of the rhombohedral–tetragonal phase transition (Tr–t) and the Curie temperature (Tm) were determined by measuring the temperature dependence of the relative permittivity. All solid solutions of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 show relaxor behavior (A-site relaxor). From XRD-measurements a broad maximum of the lattice parameter can be observed around x = 0.5 but no structural evidence for a morphotropic phase boundary was found. SEM-analysis revealed a decrease of the grain size for increasing SrTiO3-content. At room temperature a maximum of strain of about 0.29% was found at x = 0.25 which coincides with a transition from a ferroelectric to an antiferroelectric phase. The temperature dependence of the displacement indicates an additional contribution from a structural transition (rhombohedral–tetragonal), which would be of certain relevance for the existence of a morphotropic phase boundary.
164 citations
TL;DR: In this article, an analysis of X-ray diffraction patterns reveals the formation of a single phase cubic spinel structure in ferrite samples of a chemical formula Ni 0.5− x Mn x Zn0.5 Fe 2 O 4.
156 citations
TL;DR: It is found that by applying electric fields to a MOS gate structure, the ferromagnetism of the channel layer can be effectively modulated through the change of hole concentration inside the quantum dots.
Abstract: Controlling the magnetic properties of a materials system by electric means can lead to efficient electronic and memory devices. Now, for the first time, the control of ferromagnetism by the application of an electric voltage is demonstrated in germanium quantum dots for temperatures up to 100 K.
154 citations
TL;DR: In this article, the Zn substituted cobalt ferrite nanoparticles having the generic formula Co1−xZnxFe2O4 (x=0.0−0.7) were prepared by wet chemical coprecipitation technique using analytical reagent (AR) grade sulphates.
Abstract: The Zn substituted cobalt ferrite nanoparticles having the generic formula Co1−xZnxFe2O4 (x=0.0–0.7) were prepared by wet chemical coprecipitation technique using analytical reagent (AR) grade sulphates. The prepared samples were heated at 150 °C to remove water molecules and then annealed at 725 °C for 16 h. Investigation of the structural properties were carried out using x-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy techniques. The nanocrystalline nature of the samples is confirmed by TEM data. Substitution of the nonmagnetic Zn2+ ions considerably changes the magnetic properties. Neel’s model fails to explain the observed magnetic behavior above x=0.2. For x≥0.2 the Yafet–Kittel model can be fitted. AC susceptibility measurements confirm the decrease in Curie temperature.
151 citations
TL;DR: In this article, the effects of confinement of polyvinylidene fluoride (PVdF) with trifluoroethylene (TrFE) copolymers with varying pore diameter were investigated.
Abstract: Nanoscale patterning of piezoelectric and ferroelectric polymers, such as polyvinylidene fluoride (PVdF) and its copolymers with trifluoroethylene (PVdF-TrFE), is increasingly important in organic electronics, memory, and sensing. The nanoscale processing of polymers can lead to materials behavior that is strikingly different from the bulk because of confinement effects. Here we report the effects of confinement of PVdF-TrFE melt-wetted in porous templates of varying pore diameter. PVdF-TrFE is particularly interesting because it possesses a solid-state Curie transition, where both ferro and nonferroelectric phases crystallize into a paraelectric phase. Using modulated differential scanning calorimetry (MDSC), X-ray diffraction (XRD), and broadband dielectric spectroscopy (BDS), we demonstrate that confined PVdF-TrFE crystallizes into an oriented ferroelectric β phase. Both melting and crystallization temperatures decrease with decreasing pore diameter, and the Curie temperature is weakly affected. Result...
TL;DR: The structural and electronic properties of BiCoO(3) under high pressure have been investigated and results show that Co(3+) is present in the low spin state at high pressures, but X-ray emission spectra suggest that the intermediate spin state is present.
Abstract: The structural and electronic properties of BiCoO(3) under high pressure have been investigated. Synchrotron X-ray and neutron powder diffraction studies show that the structure changes from a polar PbTiO(3) type to a centrosymmetric GdFeO(3) type above 3 GPa with a large volume decrease of 13% at room temperature revealing a spin-state change. The first-order transition is accompanied by a drop of electrical resistivity. Structural results show that Co(3+) is present in the low spin state at high pressures, but X-ray emission spectra suggest that the intermediate spin state is present. The pressure-temperature phase diagram of BiCoO(3) has been constructed enabling the transition temperature at ambient pressure to be estimated as 800-900 K.
TL;DR: In this article, a member of the hybrid perovskite family is shown to be an electrically polar and magnetic compound with dielectric anomaly around the Curie point (247 K).
Abstract: We demonstrate that ethylammonium copper chloride, (C2H5NH3)2CuCl4, a member of the hybrid perovskite family is an electrically polar and magnetic compound with dielectric anomaly around the Curie point (247 K). We have found large spontaneous electric polarization below this point accompanied with a color change in the sample. The system is also ferroelectric, with large remnant polarization (37??C/cm2) that is comparable to classical ferroelectric compounds. The results are ascribed to hydrogen-bond ordering of the organic chains. The coexistence of ferroelectricity and dominant ferromagnetic interactions allows to relate the sample to a rare group of magnetic multiferroic compounds. In such hybrid perovskites the underlying hydrogen bonding of easily tunable organic building blocks in combination with the 3d transition-metal layers offers an emerging pathway to engineer multifuctional multiferroics.
TL;DR: In this article, the effects of La and Ti cosubstitutions on ferroelectric and impedance behavior of BiFeO3, (Bi0.90La0.10)(Fe0.95Ti0.05)O3 and (Bi 0.95 Ti0.
Abstract: To study the effects of La and Ti cosubstitutions on ferroelectric and impedance behavior of BiFeO3, (Bi0.90La0.10)FeO3, Bi(Fe0.95Ti0.05)O3, and (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin films were deposited on LaNiO3-buffered Pt/TiO2/SiO2/Si(100) substrates by off-axis radio frequency sputtering. The (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin film exhibits an Ohmic conduction behavior in the range of the electric field investigated, and its leakage current at high electric fields is greatly suppressed. The Curie temperature of (Bi0.90La0.10)(Fe0.95Ti0.05)O3 decreases to ∼690°C due to La and Ti cosubstitutions, and a direct band gap of 2.88 eV is identified for the (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin film, demonstrating the increase of a direct band gap with La and Ti codoping. A high remanent polarization (2Pr∼102.6 μc/cm2 and 2Ec∼538.0 kV/cm) as confirmed by positive up negative down was obtained for the (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin film at room temperature because of the great inhibition of the leakage current density at high electric fields. The La and Ti cosubstitutions also improve the fatigue behavior of BiFeO3 thin film. Impedance analyses at different temperatures and frequencies show that the La- and Ti-codoped BFO thin film exhibits rather different dielectric relaxation and conduction mechanism as compared with those of the undoped and La- or Ti-doped BFO thin films, where oxygen vacancies appear to be involved in the dielectric relaxation and conduction processes of these thin films.
TL;DR: In this article, the influence of epitaxial strain on the multiferroic phase transitions of BiFeO3 films was investigated and it was shown that strain can drive both transitions close together to yield increased magnetoelectric responses.
Abstract: We report the influence of epitaxial strain on the multiferroic phase transitions of BiFeO3 films. Using advanced characterization techniques and calculations we show that while the magnetic Ne'el temperature hardly varies, the ferroelectric Curie temperature TC decreases dramatically with strain. This is in contrast with the behavior of standard ferroelectrics where strain enhances the polar cation shifts and thus TC. We argue that this is caused by an interplay of polar and oxygen tilting instabilities and that strain can drive both transitions close together to yield increased magnetoelectric responses.
TL;DR: In this article, the average crystallite size was calculated using the Scherrer formula and found within 17-27nm range, and the optical absorption spectra of all the samples showed two clear electronic transitions, first is around 0.75 eV while the second at about 0.84 eV.
TL;DR: Diluted magnetic semiconductors with a Curie temperature exceeding 300 K are promising candidates for spintronic devices and spin-based electronic technologies.
Abstract: Diluted magnetic semiconductors with a Curie temperature exceeding 300 K are promising candidates for spintronic devices and spin-based electronic technologies We review recent achievements in the field of one of them: Co-doped ZnO at the nanoparticulate scale
TL;DR: In this article, the polymorphic phase transitions from orthorhombic to tetragonal phase around room temperature were identified in the composition range of 0.1 to 0.03.
TL;DR: In this paper, the impact of thermal treatment and thickness on the polarization and leakage current of polyvinylidene fluoride-trifluoroethylene copolymer thin film capacitors has been studied.
TL;DR: These measurements provide the first estimation of the location of the quantum critical point in the p-H plane and will inspire similar studies of the other weak itinerant ferromagnets.
Abstract: Precise resistivity measurements on the ferromagnetic superconductor UGe2 under pressure p and magnetic field H reveal a previously unobserved change of the anomaly at the Curie temperature. Therefore, the tricritical point (TCP) where the paramagnetic-to-ferromagnetic transition changes from a second order to a first order transition is located in the p-T phase diagram. Moreover, the evolution of the TCP can be followed under the magnetic field in the same way. It is the first report of the boundary of the first order plane which appears in the p-T-H phase diagram of weak itinerant ferromagnets. This line of critical points starts from the TCP and will terminate at a quantum critical point. These measurements provide the first estimation of the location of the quantum critical point in the p-H plane and will inspire similar studies of the other weak itinerant ferromagnets.
TL;DR: In this paper, a single phase by doping Fe-site of BiFeO3 (BFO) using tetravalent Zr4+ ions can be achieved by introducing cation (Bi3+) vacancies.
Abstract: In this paper, we show that a pure single phase by doping Fe-site of BiFeO3 (BFO) using tetravalent Zr4+ ions can be achieved by introducing cation (Bi3+) vacancies. The structural analysis reveals that the ferroelectric nature of BFO should be weakly affected by 10% of Zr4+ doping as the c/a ratio and the Curie temperature TC remain roughly unchanged compared to that of pure BFO. In contrast, the magnetic properties are affected as a weak ferromagnetism and a change of Neel temperature TN are observed. Beyond the double-exchange interactions arising from the creation of Fe2+, we propose another simple model inducing a local ferromagnetic coupling rather than an antiferromagnetic which considers the replacement of the magnetically active Fe3+, time to time, by a nonactive Zr4+.
TL;DR: In this paper, the field dependence of the magnetic entropy change has been studied for a series of doped Gd 5 Si 2 Ge 2 alloys, which possess a magnetic phase transition that is either entirely second order or a combination of primarily second-order mixed to a very minor degree with a first-order transition arising from a magneto-structural phase change.
TL;DR: In this paper, the magnetocaloric effect in a series of Fe88−2xCoxNixZr7B4Cu1 alloys was studied and the results showed that partial substitution of Fe by Co and Ni leads to a monotonic increase in the Curie temperature (TC) of the alloys from 287 K for x = 0 to 626 K for X = 11.25.
Abstract: We have studied the magnetocaloric effect in a series of Fe88−2xCoxNixZr7B4Cu1 alloys. The partial substitution of Fe by Co and Ni leads to a monotonic increase in the Curie temperature (TC) of the alloys from 287 K for x=0 to 626 K for x=11. The maximum magnetic entropy change (ΔSMpk) at an applied field of 1.5 T, shows a value of 1.98 J K−1 kg−1 for x=8.25. The refrigerant capacity (RC) has maximum values near 166 J kg−1 (for x=0 and 2.75). These values place the present series of alloys among the best magnetic refrigerant materials, with an RC ∼40% larger than Gd5Si2Ge1.9Fe0.1 and ∼15% larger than Fe-based amorphous alloys.
TL;DR: The structural and magnetic properties of barium hexaferrite nanoparticles (BaFe12−xGaxO19) prepared by ball milling were investigated using XRD, TEM, and VSM as mentioned in this paper.
TL;DR: In this article, the magnetocaloric properties of three samples of LaFe 13− x − y Co x Si y have been measured and compared to measurements of commercial grade Gd.
TL;DR: In this article, the lattice constant and distribution of cation in the tetrahedral and octahedral sites have been deduced through X-ray diffraction (XRD) data analysis.
TL;DR: In this article, experimental results of magnetocaloric effect for several polycrystalline and nanocrystalline manganites La1−xMnO3 (M=Ca and Sr) are analyzed.
Abstract: Experimental results of magnetocaloric effect for several polycrystalline and nanocrystalline manganites La1−xM0.xMnO3 (M=Ca and Sr) are analyzed. Influence of magnetic field is accounted for by the exponent N. The relatively deep N(T) minimum located close to the Curie temperature is found in the polycrystalline manganites. Temperature dependence of N(T) exponent is comparable with those of the soft magnetic and rare earth containing alloys. The slightly higher sensitivity of magnetocaloric effect in nanocrystalline manganites to magnetic fields is revealed by the N exponent.
TL;DR: In this article, the magnetic properties of the particles were discussed in relation with their chemical composition and their cation distribution in the opportunity of using them as heating mediators for hyperthermia application in cancer therapy.
Abstract: Stoichiometric Mn0.2Zn0.8Fe2O4 monodisperse nanoparticles were prepared by the so-called polyol method. The variation of magnetization as a function of magnetic field H (up to ±50 kOe) and temperature (5−320 K) were investigated, for zero-field-cooled (ZFC) and field-cooled (FC) conditions on freshly produced powder. The T variation of the low-field (H = 200 Oe) magnetic susceptibility is characteristic of superparamagnets with a blocking temperature below room temperature. The H variation of the low temperature (T = 5 K) magnetization exhibits a hysteresis loop. The coercivity is weak, about 0.2−0.3 kOe, which is typical of soft-ferrimagnetic materials. The 0 K saturation magnetization and the Curie temperature are found to be 98 emu.g−1 and 360 K, respectively. The magnetic properties of the particles are discussed in relation with their chemical composition and their cation distribution in the opportunity of using them as heating mediators for hyperthermia application in cancer therapy. In this aim, th...
TL;DR: In this article, the powder x-ray diffraction data confirm the single phase nature of the sample and the temperature dependence of the resistivity data shows semiconductor-like behavior in the temperature range of 5-350 K and follows variable range hopping conduction mechanism in temperature range 215 −350 K.
Abstract: Polycrystalline sample of La2CoMnO6 has been synthesized by sol-gel technique. The powder x-ray diffraction data confirm the single phase nature of the sample. This compound has monoclinic crystal structure (space group P21/n) at room temperature. The temperature dependence of magnetization in low field shows considerable variation between zero-field-cooled and field-cooled magnetization curve below ∼210 K (TC) and it follows Curie–Weiss law in the paramagnetic region. The hysteresis loop at 5 K indicates a coercive field of ∼6 kOe and remnant magnetization of ∼2.32 μB/f.u. The temperature dependence of the resistivity data shows semiconductorlike behavior in the temperature range of 5–350 K and follows variable range hopping conduction mechanism in the temperature range 215–350 K. A colossal magnetoresistance of ∼80% is observed at 5 K in an applied field of 80 kOe and MR has a negative sign.
TL;DR: In this paper, the structure, magnetic properties, and magnetocaloric effect of La0.7Ca0.3MnO3 ceramics with different particle sizes have been investigated.