Showing papers on "Curie temperature published in 2009"
TL;DR: Using density functional theory, it is shown that when half of the hydrogen in this graphane sheet is removed, the resulting semihydrogenated graphene becomes a ferromagnetic semiconductor with a small indirect gap.
Abstract: Single layer of graphite (graphene) was predicted and later experimentally confirmed to undergo metal-semiconductor transition when fully hydrogenated (graphane). Using density functional theory we show that when half of the hydrogen in this graphane sheet is removed, the resulting semihydrogenated graphene (which we refer to as graphone) becomes a ferromagnetic semiconductor with a small indirect gap. Half-hydrogenation breaks the delocalized pi bonding network of graphene, leaving the electrons in the unhydrogenated carbon atoms localized and unpaired. The magnetic moments at these sites couple ferromagnetically with an estimated Curie temperature between 278 and 417 K, giving rise to an infinite magnetic sheet with structural integrity and magnetic homogeneity. This is very different from the widely studied finite graphene nanostrucures such as one-dimensional nanoribbons and two-dimensional nanoholes, where zigzag edges are necessary for magnetism. From graphene to graphane and to graphone, the system evolves from metallic to semiconducting and from nonmagnetic to magnetic. Hydrogenation provides a novel way to tune the properties with unprecedented potentials for applications.
749 citations
TL;DR: In this article, a modified chemical coprecipitation route was used to synthesize CoFe 2 O 4 ferrite nanoparticles, and structural and magnetic properties were systematically investigated.
290 citations
TL;DR: Sr-doped PbZrO3 antiferroelectric (AFE) thin films have been fabricated on the platinum-buffered silicon substrates via the sol-gel technique.
Abstract: Sr-doped PbZrO3 antiferroelectric (AFE) thin films have been fabricated on the platinum-buffered silicon substrates via the sol–gel technique. The temperature-dependent dielectric properties results indicated that the AFE phase was stabilized for the Sr-modified PbZrO3 thin films with a Curie temperature of 251°C. The recoverable energy density and energy efficiency of the Sr-doped PbZrO3 thin films were enhanced by the doping of strontium. Compared with the pure PbZrO3 AFE thin films, the performance against fatigue of the Sr-doped PbZrO3 thin films were also improved greatly.
222 citations
TL;DR: In this paper, a layer-structured single phase Bi5Fe0.5Co 0.5Ti3O15 ceramics was synthesized following a multicalcination procedure.
Abstract: Layer-structured, single phase Bi5Fe0.5Co0.5Ti3O15 ceramics was synthesized following a multicalcination procedure. Magnetic moment increases more than three times by substituting half Fe sites by Co ions. The material exhibits an Aurivillius phase with a four-layer unit cell structure, and presents a remarkable coexistence of ferroelectricity and ferromagnetism above room temperature. The measured 2Pr and 2Mr are 13 μC/cm2 and 7.8 memu/g, respectively. The material’s magnetic behavior below 275 °C is relaxationlike and its magnetic Curie temperature is ∼345 °C.
215 citations
TL;DR: In this paper, the effect of biaxial strain on thin films of (001) La 0.7Sr0.3MnO3 was studied and it was shown that the dependence of the Curie temperature on BIAXial strain is in very good agreement with the theoretical predictions of Millis et al.
Abstract: We have studied the effect of biaxial strain on thin films of (001) La0.7Sr0.3MnO3. We deposited films by reactive molecular-beam epitaxy on different single crystalline substrates, varying the substrate-induced biaxial strain from −2.3% to +3.2%. Magnetization and electrical transport measurements reveal that the dependence of the Curie temperature on biaxial strain is in very good agreement with the theoretical predictions of Millis et al. [J. Appl. Phys. 83, 1588 (1998)].
183 citations
03 Nov 2009
TL;DR: In this paper, a survey of relaxor-PT-based ferroelectric single crystal systems with respect to their temperature usage range is presented, and general trends of dielectric and piezoelectric properties are discussed in relation to their respective TC/TRT.
Abstract: Relaxor-PT based ferroelectric single crystals Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZNT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMNT) offer high performance with ultra-high electromechanical coupling factors k33s > 0.9 and piezoelectric coefficients d33s > 1500 pC/N. However, the usage temperature range of these perovskite single crystals is limited by TRT-the rhombohedral to tetragonal phase transition temperature, which occurs at significantly lower temperatures than the Curie temperature TC, a consequence of curved morphotropic phase boundaries (MPBs). Furthermore, these 〈001〉-oriented crystals exhibit low mechanical quality Q and coercive fields, restricting their usage in high-power applications. In this survey, recent developments on binary and ternary perovskite relaxor-PT crystal systems are reviewed with respect to their temperature usage range. General trends of dielectric and piezoelectric properties of relaxor-PT crystal systems are discussed in relation to their respective TC/TRT. In addition, two approaches have been implemented to improve mechanical Q, including acceptor dopants, analogous to hard polycrystalline ceramics, and anisotropic domain engineering, enabling lowloss crystals with high coupling for high-power applications.
158 citations
TL;DR: In this article, a study of the magnetic field-induced martensitic transformation and magnetocaloric effect in Ni45Mn37In13Co5 and Ni46Mn35In14Co5 ribbons prepared by melt-spinning was carried out.
143 citations
TL;DR: In this article, a correlation between increase in hole concentration with decrease in magnetization and Curie temperature in ZnO:Li nanorods was found to explain the observed ferromagnetism.
Abstract: We have observed ferromagnetism in Li-doped ZnO nanorods with Curie temperature up to 554 K. Li forms shallow acceptor states in substitutional zinc sites giving rise to $p$-type conductivity. An explicit correlation emerges between increase in hole concentration with decrease in magnetization and Curie temperature in ZnO:Li. Occurrence of ferromagnetism at room temperature has been established with observed magnetic domain formation in ZnO:Li pellets in magnetic force microscopy and prominent ferromagnetic resonance signal in electron paramagnetic resonance spectrum. Magnetic ZnO:Li nanorods are luminescent, showing strong near UV emission. Substitutional Li atoms can induce local moments on neighboring oxygen atoms, which when considered in a correlated model for oxygen orbitals with random potentials introduced by dopant atom could explain the observed ferromagnetism and high Curie temperature in ZnO:Li nanorods.
142 citations
TL;DR: In this article, it was shown that the Curie point and the temperature where the magnetic entropy change is maximum coincide only in the mean field approximation, but the Heisenberg model implies that Tpeak>TC even for homogeneous materials.
Abstract: Using the Arrott–Noakes equation of state it is shown that the Curie point (TC) and the temperature where the magnetic entropy change is maximum (Tpeak) coincide only in the mean field approximation, but the Heisenberg model implies that Tpeak>TC even for homogeneous materials. The distance between Tpeak and TC increases with applied magnetic field following a power law. In both cases, TC corresponds to a singular point in the temperature dependence of the magnetic entropy change. The field dependence of the magnetic entropy change is exactly the same at the Curie temperature and at the temperature of the peak.
137 citations
TL;DR: In this article, the phase transition temperature of tetragonality to orthorhombic structure (To−t) increases with increasing of Zr content, forming a pinched phase transition.
132 citations
TL;DR: In this paper, an extensive characterization of the spinel ferrimagnetic spinel oxide Zn(x)Fe(3-x)O(4) was performed on MgO(001) substrates with excellent structural properties.
Abstract: The ferrimagnetic spinel oxide Zn(x)Fe(3-x)O(4) combines high Curie temperature and spin polarization with tunable electrical and magnetic properties, making it a promising functional material for spintronic devices. We have grown epitaxial thin films with 0<=x<=0.9 on MgO(001) substrates with excellent structural properties both in pure Ar atmosphere and an Ar/O2 mixture by laser molecular beam epitaxy. We find that the electrical conductivity and the saturation magnetization can be tuned over a wide range during growth. Our extensive characterization of the films provides a clear picture of the underlying physics of this spinel ferrimagnet with antiparallel Fe moments on the A and B sublattice: (i) Zn substitution removes both Fe3+ moments from the A sublattice and itinerant charge carriers from the B sublattice, (ii) growth in finite oxygen partial pressure generates Fe vacancies on the B sublattice also removing itinerant charge carriers, and (iii) application of both Zn substitution and excess oxygen results in a compensation effect as Zn substitution partially removes the Fe vacancies. A decrease (increase) of charge carrier density results in a weakening (strengthening) of double exchange and thereby a decrease (increase) of conductivity and the saturation magnetization. This scenario is confirmed by the observation that the saturation magnetization scales with the longitudinal conductivity. The combination of tailored films with semiconductor materials such as ZnO in multi-functional heterostructures seems to be particularly appealing.
TL;DR: In this article, the low-temperature magnetotransport behaviors of (Ga,Mn)As films with nominal Mn concentration x larger than 10% were investigated and the ferromagnetic transition temperature T-C can be enhanced to 191 K after postgrowth annealing.
Abstract: We report the low-temperature magnetotransport behaviors of (Ga,Mn)As films with the nominal Mn concentration x larger than 10%. The ferromagnetic transition temperature T-C can be enhanced to 191 K after postgrowth annealing (Ga,Mn)As with x=20%. The temperature T-m, corresponding to the resistivity minimum in the curve of resistivity versus temperature at temperature below T-C, depends on Mn concentration, annealing condition, and magnetic field. Moreover, we find that the variable-range hopping may be the main conductive mechanism when temperature is lower than T-m.
TL;DR: In this article, the properties of Gd and Nd-substituted nickel ferrite were investigated and the existence of the ferroelectricity was confirmed from the Ferroelectric loops and magnetocapacitance of −2% and −3% were observed.
TL;DR: In this article, the authors studied the entropy change and the shift of the martensitic transformation temperatures with magnetic field in samples of a polycrystalline Ni-Co-Mn-In alloy having different degrees of long-range atomic order due to different heat treatments.
Abstract: We studied the entropy change and the shift of the martensitic transformation temperatures with magnetic field in samples of a polycrystalline Ni–Co–Mn–In alloy having different degrees of long-range atomic order due to different heat treatments. We found, for the samples of the same composition, strong variations of the entropy change with the degree of atomic order, mediated by the difference between the Curie and martensitic transformation temperatures. Calculations of the field-induced shift of the transformation using data of entropy variations show good agreement with experimental results.
TL;DR: In this article, the phase transition temperatures and the dielectric, ferroelectric, and piezoelectric properties of bismuth perovskite lead-free BKT-based solid solutions have been reviewed.
Abstract: The phase transition temperatures and the dielectric, ferroelectric, and piezoelectric properties of bismuth perovskite lead-free ferroelectric ceramics such as (Bi1/2Na1/2)TiO3 (BNT)- and (Bi1/2Na1/2)TiO3 (BKT)-based solid solutions have been reviewed According to the results obtained by our group, these ceramics can be considered as superior lead-free piezoelectric materials for reducing environmental damage Perovskite-type ceramics appear to be suitable for actuator and high-power applications that require a large piezoelectric constant d33 and a high Curie temperature TC or a high depolarization temperature Td (> 200degC) In this paper, we summarize the relationship between phase transition temperatures and piezoelectric properties In the case of the BNT-based solid solutions, the highest piezoelectric properties were obtained at the morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases However, d33 and Td were shown to have a tradeoff relationship Considering the high Td and high d33, the tetragonal side of the MPB composition is suitable for piezoelectric actuator application Meanwhile, the Qm values on the rhombohedral side of the MPB composition were better than those on the tetragonal side, and excellent high-power characteristics were obtained for Mn-doped BNT-(Bi1/2Na1/2)TiO3-BKT ternary systems with rhombohedral symmetry BKT ceramics were prepared by the hot-pressing (HP) method, and their ferroelectric and piezoelectric properties were clarified BKT ceramics doped with a small amount of Bi have a relatively high remanent polarization of Pr = 276 muC/cm2 and high piezoelectric properties (k33 = 040 and d33 = 101 pC/N) In addition, it was clarified that BKT ceramics have a high Td of approximately 300degC The solid solution (1-x)BKT-xBaTiO3 (BKT-BT100x) exhibited a high Td of approximately 300degC at x > 04
TL;DR: In this paper, a chemical co-precipitation method was used to synthesize strontium hexaferrite doped with Zr-Zn to obtain a suitable signal to noise ratio for application in the magnetic recording media.
TL;DR: In this article, the dielectric and electromechanical properties of NBT-LiCe ceramics at elevated temperature were investigated in detail, and the excellent piezoelectric properties, coupled with high Curie temperature (Tc=660 ǫ°C), demonstrated that the nBi0.04Bi4Ti4O15 (NBT)-ceramics are the promising candidates for high temperature applications.
Abstract: The Aurivillius-type bismuth layer-structured (NaBi)0.46(LiCe)0.04Bi4Ti4O15 (NBT-LiCe) piezoelectric ceramics were synthesized using conventional solid-state processing. Phase analysis was performed by x-ray diffraction and microstructural morphology was assessed by scanning electron microscopy. The dielectric, piezoelectric, ferroelectric, and electromechanical properties of NBT-LiCe ceramics were investigated. The piezoelectric activities were found to be significantly enhanced compared to NBT ceramics, which can be attributed to the lattice distortion and the presence of bismuth vacancies. The dielectric and electromechanical properties of NBT-LiCe ceramics at elevated temperature were investigated in detail. The excellent piezoelectric, dielectric, and electromechanical properties, coupled with high Curie temperature (Tc=660 °C), demonstrated that the NBT-LiCe ceramics are the promising candidates for high temperature applications.
TL;DR: In this paper, the authors focus on recent findings in self-assembled nano-structure composite thin films, and various efforts to realize and improve their magnetoelectricity.
Abstract: Two-phase multiferroic nano-composite thin films have been a topic of research interests in the last few years This is because of their expected magnetoelectric coupling, as well as potential applications This review focuses on recent findings in self-assembled nano-structure composite thin films, and various efforts to realize and improve their magnetoelectricity Topics include: (i) nano-pillar and maze structures, and their formation mechanisms, and a nano-belt structure oriented in-plane found by our research group; (ii) the ferroelectric properties of composite thin films, and how they can be enhanced by epitaxial engineering; (iii) a magnetic anisotropy that is induced by constraint stress, and by the nano-structures of the ferromagnetic phase; and (iv) a magnetoelectric coupling that was first observed via a change in magnetization near the Curie temperature of the ferroelectric phase, a magnetization switching assisted by electric field, and recently direct measurements using a magnetic cantilever method yielding values of 18 mV/cm Oe in BiFeO3–CoFe2O4
TL;DR: In this paper, the authors showed that the decrease of the Curie temperature with increasing field for H ∥ b -axis marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped H c2 b curve.
Abstract: Magnetoresistivity measurements with fine tuning of the field direction on high quality single crystals of the ferromagnetic superconductor UCoGe show anomalous anisotropy of the upper critical field H c2 . H c2 for H ∥ b -axis ( H c2 b ) in the orthorhombic crystal structure is strongly enhanced with decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The temperature dependence of H c2 a shows upward curvature with a low temperature value exceeding 30 T, while H c2 c at 0 K is very small (∼0.6 T). Contrary to conventional ferromagnets, the decrease of the Curie temperature with increasing field for H ∥ b -axis marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped H c2 b curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.
TL;DR: In this paper, Mn/Fe-doped ZnS nanowires and nanobelts doped with Mn and Fe were synthesized by a chemical vapor transport method.
Abstract: ZnS nanowires and nanobelts doped with Mn and Fe were synthesized by a chemical vapor transport method. The Mn/Fe-doped and co-doped ZnS nanostructures were grown on Au-coated Si substrates. The key to this synthetic process of co-doping lies in the use of metal chloride as a metal carrier. Crystal structure and chemical compositional analyses by transmission electron microscopy (TEM) indicate that the nanowires and nanobelts are single-crystalline and uniformly doped with dopants. Strong emission bands were found from photoluminescence (PL) spectra of Mn/Fe-doped and co-doped ZnS nanowires. The magnetic property measurements from the nanostructure ensemble show that the Curie temperature is above room temperature. The synthesized Mn/Fe-doped and co-doped ZnS nanostructures can be employed in the fabrication of nanoscale magnetic and optical devices.
TL;DR: In this paper, the electrical and magnetic properties of the synthesized ferrites have been investigated as a function of temperature, and the drift mobility and activation energy are also reported. But the analysis of X-ray diffraction (XRD) data showed the crystallite size variation within the range 10-13nm.
TL;DR: Using combined resonance and ultrasonic methods, a full set of material constants has been measured for morphotropic phase boundary (MPB) composition xPb(In1/2Nb 1/2)O3-pbTiO3 (PIN-PMN-PT) as mentioned in this paper.
Abstract: Using combined resonance and ultrasonic methods, a full set of material constants has been measured for morphotropic phase boundary (MPB) composition xPb(In1/2Nb1/2)O3–(1−x−y)Pb(Mg1/3Nb2/3)O3–yPbTiO3 (PIN-PMN-PT) single crystals poled along [001]c. Compared with the MPB composition (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN-PT) single crystals, the PIN-PMN-PT single crystals have smaller anisotropy, higher Curie temperature (Tc≈197 °C), and higher rhombohedral to tetragonal phase transition temperature (TR-T≈96 °C). The electromechanical properties obtained here are the best found so far for this ternary system with d33≈2742 pC/N, d31≈−1337 pC/N, k33≈95%, and k31≈65%.
TL;DR: In this article, the authors present results that suggest a breakdown of the recently proposed universal behaviour of the magnetocaloric effect in materials with a second-order magnetic phase transition in some La-containing samples.
TL;DR: In this paper, the structure and magnetic properties of La2NiMnO6 nanoparticles with sizes ranging from about 16 to 66 nm synthesized by a sol-gel method have been investigated.
Abstract: The structure and magnetic properties of La2NiMnO6 nanoparticles with sizes ranging from about 16 to 66 nm synthesized by a sol-gel method have been investigated. The magnetic measurements reveal that the rates of disordered phases increase as particle size decreases, which can be deduced from the changes of Curie temperature and the occurrence of glasslike behaviors, and the similar saturation magnetization was obtained at 4 K. While the particle size becomes smaller, two broad peaks of Raman spectrum at around 535 and 670 cm−1, which are assigned to antisymmetric stretching mode and symmetric stretching mode, move to higher and lower frequencies than the corresponding modes of the bulk compound, respectively. The two peaks also become broader, while the intensity becomes relatively weaker with the decrease in nanoparticle size. All results support that the trend of Mn4++Ni2+→Mn3++Ni3+ bring locally cooperative Jahn–Teller deformation and charge disproportion, and demonstrate that the nanometer effect is a crucial factor to influence the properties of La2NiMnO6.
TL;DR: In this article, the magnetic properties such as magnetization and coercivity were measured using vibrating sample magnetometer, and the existence of a metastable cation distribution together with possible surface effects at the nanoregime.
TL;DR: In this paper, polycrystalline ferrites with general formula NiFe 2− x Cr x O 4 (0,≤ x ǫ ≥ 1) were prepared through oxalate impregnation method and characterized using DTA-TG, XRD, FT-IR, AC conductivity, VSM and magnetic susceptibility measurements.
TL;DR: In this article, single phase Bi0.9−xLa0.1NdxFeO3 (BLNFOx) (x=0.05, 0.07, and 0.1) multiferroic ceramics were prepared to study the effect of combine substitution of La and Nd in BiFeO 3 (BFO) and X-ray diffraction studies revealed phase transition from rhombohedral (R3c) to triclinic (P1) on substitution of 05 and 07 mol % of Nd and subsequent
Abstract: Single phase Bi0.9−xLa0.1NdxFeO3 (BLNFOx) (x=0.05, 0.07, and 0.1) multiferroic ceramics were prepared to study the effect of combine substitution of La and Nd in BiFeO3 (BFO). X-ray diffraction studies revealed phase transition from rhombohedral (R3c) to triclinic (P1) on substitution of 05 and 07 mol % of Nd and subsequent transition to monoclinic C2/c with 10 mol % of Nd along with 10 mol % of La. These structural phase transitions and weakening of long range ferroelectric order with increasing x are also confirmed from Raman spectra. The existence of ferroelectricity and the corresponding Curie temperature for all noncentrosymmetric composition were determined using differential thermal analysis. Small remnant magnetization of 0.067 emu/g is observed in (BLNFO)x=0.07 as a result of collapse of space modulated spin structure. For pure BFO dielectric anomaly was observed at 355 °C corresponding to Neel temperature. Due to coexistence of long range ferroelectric order and canted antiferromagnetic orders i...
TL;DR: The structure and properties of materials in the BiFeO3-rich side of the pseudo-binary phase diagram are reported in this article, and the thermodynamics of the ambient pressure solid solubility is discussed and a phase diagram for the system BiFe1−xMnxO3+δ is presented, establishing the stability regions of three, two, one, and zero simultaneous ferroic properties.
Abstract: The structure and properties of materials in the BiFeO3-rich side of the pseudo-binary phase diagram BiFeO3−BiMnO3 are reported. Manganese substitution (x) and oxygen hyperstoichiometry (δ) are demonstrated to strongly affect the crystallographic properties, electrical conductivity, and phase-transition temperatures of BiFe1−xMnxO3+δ. Increasing the manganese content and oxygen hyperstoichiometry of the materials depresses the Neel temperature, the ferroelectric Curie temperature, and the transition temperature from the paraelectric, ferroelastic structure to the paraelastic, cubic perovskite structure. The thermodynamics of the ambient pressure solid solubility is discussed and a phase diagram for the system BiFe1−xMnxO3+δ is presented, establishing the stability regions of three, two, one, and zero simultaneous ferroic properties. Finally, technological implications of the influence of manganese and excess oxygen are discussed.
TL;DR: In this paper, a polymorphic phase transition (PPT) is often engineered into lead-free materials to generate high piezoelectric activity at room temperature, limiting their temperature stability.
Abstract: A polymorphic phase transition (PPT) is often engineered into lead-free materials to generate high piezoelectric activity at room temperature, limiting their temperature stability We report [Na05K05NbO3]093–[LiTaO3]007 tetragonal ceramics with favorable properties over a broad temperature range due to a high Curie temperature at 447 °C and PPT at −15 °C involving a transition to a monoclinic rather than low temperature orthorhombic phase Piezoelectric k31 and d31 coefficients varied from 019 to 014 and −53 to −33 pC/N, respectively, over the range of −15 to 300 °C Strain-electric field loops provided strains of ∼02% and a high-field d33 of 205 pm/V
TL;DR: In this paper, the thermal decomposition process of the nitrate-egg white precursors was investigated by thermogravimetric (TG) technique, and the saturation magnetization (Ms) and coercivity (Hc) as a function of Mg content were investigated using vibrating sample magnetometer (VSM).