Showing papers on "Curie temperature published in 2011"
TL;DR: The room-temperature electrical control of the ferromagnetic phase transition in cobalt, one of the most representative transition-metal ferromagnets, is demonstrated at room temperature.
Abstract: Electrical control of magnetic properties is crucial for device applications in the field of spintronics. Although the magnetic coercivity or anisotropy has been successfully controlled electrically in metals as well as in semiconductors, the electrical control of Curie temperature has been realized only in semiconductors at low temperature. Here, we demonstrate the room-temperature electrical control of the ferromagnetic phase transition in cobalt, one of the most representative transition-metal ferromagnets. Solid-state field effect devices consisting of a ultrathin cobalt film covered by a dielectric layer and a gate electrode were fabricated. We prove that the Curie temperature of cobalt can be changed by up to 12 K by applying a gate electric field of about ±2 MV cm(-1). The two-dimensionality of the cobalt film may be relevant to our observations. The demonstrated electric field effect in the ferromagnetic metal at room temperature is a significant step towards realizing future low-power magnetic applications.
405 citations
TL;DR: First-principles calculations based on density functional theory are presented to investigate systematically the electronic and magnetic properties of such novel organometallics (labeled as TMPc, TM = Cr-Zn) as free-standing sheets to provide theoretical insight leading to a better understanding of novel phthalocyanine-based 2D structures beyond graphene and BN sheets.
Abstract: A two-dimensional (2D) periodic Fe phthalocyanine (FePc) single-layer sheet has very recently been synthesized experimentally (Abel, M.; et al. J. Am. Chem. Soc.2011, 133, 1203), providing a novel pathway for achieving 2D atomic sheets with regularly and separately distributed transition-metal atoms for unprecedented applications. Here we present first-principles calculations based on density functional theory to investigate systematically the electronic and magnetic properties of such novel organometallics (labeled as TMPc, TM = Cr-Zn) as free-standing sheets. Among them, we found that only the 2D MnPc framework is ferromagnetic, while 2D CrPc, FePc, CoPc, and CuPc are antiferromagnetic and 2D NiPc and ZnPc are nonmagnetic. The difference in magnetic couplings for the studied systems is related to the different orbital interactions. Only MnPc displays metallic d(xz) and d(yz) orbitals that can hybridize with p electrons of Pc, which mediates the long-range ferromagnetic coupling. Monte Carlo simulations based on the Ising model suggest that the Curie temperature (T(C)) of the 2D MnPc framework is ∼150 K, which is comparable to the highest T(C) achieved experimentally, that of Mn-doped GaAs. The present study provides theoretical insight leading to a better understanding of novel phthalocyanine-based 2D structures beyond graphene and BN sheets.
335 citations
TL;DR: In this paper, the authors systematically document the known magnetic properties of greigite (at high, ambient, and low temperatures and with alternating and direct fields) and illustrate how grain size variations affect magnetic properties.
Abstract: Greigite (Fe3S4) is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction. It can also grow at any time when dissolved iron and sulfide are available during diagenesis. Greigite is important in paleomagnetic, environmental, biological, biogeochemical, tectonic, and industrial processes. Much recent progress has been made in understanding its magnetic properties. Greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites. The crystallographic c axis is the easy axis of magnetization, with magnetic properties dominated by magnetocrystalline anisotropy. Robust empirical estimates of the saturation magnetization, anisotropy constant, and exchange constant for greigite have been obtained recently for the first time, and the first robust estimate of the low-field magnetic susceptibility is reported here. The Curie temperature of greigite remains unknown but must exceed 350°C. Greigite lacks a low-temperature magnetic transition. On the basis of preliminary micromagnetic modeling, the size range for stable single domain behavior is 17–200 nm for cubic crystals and 17–500 nm for octahedral crystals. Gradual variation in magnetic properties is observed through the pseudo-single-domain size range. We systematically document the known magnetic properties of greigite (at high, ambient, and low temperatures and with alternating and direct fields) and illustrate how grain size variations affect magnetic properties. Recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite, which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records, including widespread remagnetizations.
334 citations
TL;DR: By magneto-transport measurements that a Curie temperature as high as 200 K can be obtained in nanostructures of (Ga,Mn)As is demonstrated, which may provide useful information on optimal structures for ( Ga,Mm)As-based nanospintronic devices operational at relatively high temperatures.
Abstract: We demonstrate by magneto-transport measurements that a Curie temperature as high as 200 K can be obtained in nanostructures of (Ga,Mn)As. Heavily Mn-doped (Ga,Mn)As films were patterned into nanowires and then subject to low-temperature annealing. Resistance and Hall effect measurements demonstrated a consistent increase of T(C) with decreasing wire width down to about 300 nm. This observation is attributed primarily to the increase of the free surface in the narrower wires, which allows the Mn interstitials to diffuse out at the sidewalls, thus enhancing the efficiency of annealing. These results may provide useful information on optimal structures for (Ga,Mn)As-based nanospintronic devices operational at relatively high temperatures.
266 citations
TL;DR: In this paper, a new form of magnetism has been discovered: the coexistence of strong and weak magnetism in alternate atomic layers, which can be used for refrigeration and energy conversion.
Abstract: The efficient coupling between lattice degrees of freedom and spin degrees of freedom in magnetic materials can be used for refrigeration and energy conversion. This coupling is enhanced in materials exhibiting the giant magnetocaloric effect. First principle electronic structure calculations on hexagonal MnFe(P, Si) reveal a new form of magnetism: the coexistence of strong and weak magnetism in alternate atomic layers. The weak magnetism of Fe layers (disappearance of local magnetic moments at the Curie temperature) is responsible for a strong coupling with the crystal lattice while the strong magnetism in adjacent Mn-layers ensures Curie temperatures high enough to enable operation at and above room temperature. Varying the composition on these magnetic sublattices gives a handle to tune the working temperature and to achieve a strong reduction of the undesired thermal hysteresis. In this way we design novel materials based on abundantly available elements with properties matched to the requirements of an efficient refrigeration or energy-conversion cycle.
226 citations
TL;DR: One-dimensional (1D) undoped and Fe doped ZnO nanorods of average length ∼1μm and diameter ∼50nm have been obtained using a microwave-assisted synthesis as discussed by the authors.
168 citations
TL;DR: A large-size single crystal of nearly stoichiometric SrCoO(3) was prepared with a two-step method combining the floating-zone technique and subsequent high oxygen pressure treatment, indicative of the possible effect of orbital fluctuation in the intermediate spin ferromagnetic metallic state.
Abstract: A large-size single crystal of nearly stoichiometric SrCoO(3) was prepared with a two-step method combining the floating-zone technique and subsequent high oxygen pressure treatment. SrCoO(3) crystallizes in a cubic perovskite structure with space group Pm3m, and displays an itinerant ferromagnetic behavior with the Curie temperature of 305 K. The easy magnetization axis is found to be along the [111] direction, and the saturation moment is 2.5 µ(B)/f.u., in accord with the picture of the intermediate spin state. The resistivity at low temperatures (T) is proportional to T(2), indicative of the possible effect of orbital fluctuation in the intermediate spin ferromagnetic metallic state. Unusual anisotropic magnetoresistance is also observed and its possible origin is discussed.
164 citations
TL;DR: In this article, the magnetic and dielectric properties of the as-prepared Ni-Zn ferrites were investigated using X-ray diffraction data, showing that the saturation magnetization and the coercivity increased with increasing substitution level, x, and the Curie temperature fluctuating between 308 and 320°C.
146 citations
TL;DR: Direct experimental evidence indicates that the spontaneous polarization originating from Pb/Bi-O hybridization is strongly associated with thenegative thermal expansion, which can be used as a guide for the future design of negative thermal expansion of phase-transforming oxides.
Abstract: PbTiO(3)-based compounds are well-known ferroelectrics that exhibit a negative thermal expansion more or less in the tetragonal phase. The mechanism of negative thermal expansion has been studied by high-temperature neutron powder diffraction performed on two representative compounds, 0.7PbTiO(3)-0.3BiFeO(3) and 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3), whose negative thermal expansion is contrarily enhanced and weakened, respectively. With increasing temperature up to the Curie temperature, the spontaneous polarization displacement of Pb/Bi (δz(Pb/Bi)) is weakened in 0.7PbTiO(3)-0.3BiFeO(3) but well-maintained in 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3). There is an apparent correlation between tetragonality (c/a) and spontaneous polarization. Direct experimental evidence indicates that the spontaneous polarization originating from Pb/Bi-O hybridization is strongly associated with the negative thermal expansion. This mechanism can be used as a guide for the future design of negative thermal expansion of phase-transforming oxides.
139 citations
TL;DR: In this article, the exact coordinates of atoms, unit cell dimensions, atom ion occupancy, degree of inversion as well as crystallite size and residual microstrain have been determined.
Abstract: Ferrite samples of Al3+ substituted NiFe2O4 nanoparticles were prepared by wet chemical co-precipitation method. The samples were obtained by annealing at relatively low temperature at 600 °C and characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM), and ac susceptibility. On applying the full pattern fitting of Rietveld method using FullProf program, exact coordinates of atoms, unit cell dimensions, atom ion occupancy, degree of inversion as well as crystallite size and residual microstrain have been determined. The lattice parameter, density, particle size, lattice strain, magnetization, magneton number, and Curie temperature are seen to decrease with increasing A13+ content whereas the specific surface area, porosity, coercive force, shows an increasing trend with A13+ content. Cation distribution is obtained from XRD and Rietveld method and the variation of the cation distribution has been discussed on ...
137 citations
TL;DR: In this paper, a nano ZnFe 2 O 4 with grain size of 30nm has been synthesized via sol-gel auto combustion method and characterized using DSC (differential scanning calorimetry), XRD (X-ray diffraction), SEM (scanning electron microscopy), EDAX (energy dispersive X-ray analysis), FTIR (Fourier transform infrared spectroscopy), VSM (vibrating sample magnetometer) and Ac-electrical conductivity measurement setup.
TL;DR: The anomalous magnetic properties and high superior photodecomposition activity of well shaped Mn₃O₄ nano-octahedra should be related to the special shape of the nanoparticles and the abundantly exposed {101} facets at the external surfaces.
Abstract: Very uniform and well shaped Mn₃O₄ nano-octahedra are synthesized using a simple hydrothermal method under the help of polyethylene glycol (PEG200) as a reductant and shape-directing agent. The nano-octahedra formation mechanism is monitored. The shape and crystal orientation of the nanoparticles is reconstructed by scanning electron microscopy and electron tomography, which reveals that the nano-octahedra only selectively expose {101} facets at the external surfaces. The magnetic testing demonstrates that the Mn₃O₄ nano-octahedra exhibit anomalous magnetic properties: the Mn₃O₄ nano-octahedra around 150 nm show a similar Curie temperature and blocking temperature to Mn₃O₄ nanoparticles with 10 nm size because of the vertical axis of [001] plane and the exposed {101} facets. With these Mn₃O₄ nano-octahedra as a catalyst, the photodecomposition of rhodamine B is evaluated and it is found that the photodecomposition activity of Mn₃O₄ nano-octahedra is much superior to that of commercial Mn₃O₄ powders. The anomalous magnetic properties and high superior photodecomposition activity of well shaped Mn₃O₄ nano-octahedra should be related to the special shape of the nanoparticles and the abundantly exposed {101} facets at the external surfaces. Therefore, the shape preference can largely broaden the application of the Mn₃O₄ nano-octahedra.
TL;DR: A synchrotron x-ray study of the equilibrium polarization structure of ultrathin PbTiO(3) films on SrRuO( 3) electrodes epitaxially grown on SrTiO (001) substrates finds that the ferroelectric Curie temperature varies with pO(2) and has a minimum at the intermediate pO (2), where the polarization below T(C) changes sign.
Abstract: We present a synchrotron x-ray study of the equilibrium polarization structure of ultrathin PbTiO(3) films on SrRuO(3) electrodes epitaxially grown on SrTiO(3) (001) substrates, as a function of temperature and the external oxygen partial pressure (pO(2)) controlling their surface charge compensation. We find that the ferroelectric Curie temperature (T(C)) varies with pO(2) and has a minimum at the intermediate pO(2), where the polarization below T(C) changes sign. The experiments are in qualitative agreement with a model based on Landau theory that takes into account the interaction of the phase transition with the electrochemical equilibria for charged surface species. The paraelectric phase is stabilized at intermediate pO(2) when the concentrations of surface species are insufficient to compensate either polar orientation.
TL;DR: It is found that most thermodynamic properties of nanoparticles vary linearly with 1/D as a first approximation, which may be regarded as a scaling law for most of the size dependent thermodynamics properties for different materials.
Abstract: The previous model on surface free energy has been extended to calculate size dependent thermodynamic properties (i.e., melting temperature, melting enthalpy, melting entropy, evaporation temperature, Curie temperature, Debye temperature and specific heat capacity) of nanoparticles. According to the quantitative calculation of size effects on the calculated thermodynamic properties, it is found that most thermodynamic properties of nanoparticles vary linearly with 1/D as a first approximation. In other words, the size dependent thermodynamic properties Pn have the form of Pn = Pb(1 − K/D), in which Pb is the corresponding bulk value and K is the material constant. This may be regarded as a scaling law for most of the size dependent thermodynamic properties for different materials. The present predictions are consistent literature values.
TL;DR: In this article, the authors show that if partially hydrogenated LaFe13-x-6Hz alloys are held at their Curie temperature for several days, they decomposes into a fraction with an increased curie temperature and another one with decreased Curie temperatures compared to the starting material, and that these degrading materials cannot be used in applications which require constant properties over a long period of time.
Abstract: Long-term aging treatments of partially hydrogenated ternary La-Fe-Si alloys show that these materials do not have stable magnetic properties. If partially hydrogenated LaFe13- xSixHz is held at its Curie temperature for several days it decomposes into a fraction with an increased Curie temperature and another one with decreased Curie temperature compared to the starting material. Such degrading material cannot be used in applications which require constant properties over a long period of time. By fully hydrogenating manganese containing LaFe13-x-yMnySixHz, stable alloys with adjustable Curie temperatures around room temperature can be produced. These hydrides do not change their magnetic properties if they are operated close to their Curie temperature.
TL;DR: In this paper, a computational model of crystallographically amorphous ferrimagnetic alloys using a stochastic Landau-Lifshitzitz-Gilbert equation of motion for atomistic spins and an atomistic spin Hamiltonian with Heisenberg exchange is presented.
Abstract: We present a computational model of crystallographically amorphous ferrimagnetic alloys using a stochastic Landau-Lifshitz-Gilbert equation of motion for atomistic spins and an atomistic spin Hamiltonian with Heisenberg exchange. The spontaneous equilibrium magnetization is calculated and a comparison with a mean field model is made. The simulations show excellent agreement with experiments on GdFeCo using x-ray magnetic circular dichroism to determine the individual sublattice magnetizations. The calculated temperature dependence of the magnetization shows a polarization of the Gd sublattice leading to a common Curie temperature, in agreement with the experimental data. The intersublattice exchange is shown to be an important energy transfer channel for ultrafast dynamics.
TL;DR: The magnetic field triggers the changes of shape caused by either inducing the structural transition or rearranging the martensite variants as discussed by the authors, which is the best known functionalities of magnetic shape memory alloys.
Abstract: Publisher Summary The discovery of the ferromagnetic (FM) Heusler alloy Cu 2 MnAl, in the beginning of the twentieth century, made considerable impact in the field of magnetism. Although Cu 2 MnAl contained no FM element, it had a very high Curie temperature in excess of 600 K. Features related to the magnetostructural interplay in Heusler alloys are observed in the phonon and magnetization properties. This chapter discusses Heusler-based magnetic shape memory alloys. The complex behavior displayed by these materials is mainly a consequence of the strong coupling between magnetism and structure, which is driven by the martensitic transition. Magnetic shape memory properties are the best known functionalities shown by this class of materials. They refer to the ability of these alloys to show strong response in shape, strain, and dimensions to applied magnetic fields. The magnetic field triggers the changes of shape caused by either inducing the structural transition or rearranging the martensite variants. The first observation of the magnetic-field-controlled shape memory effect was made in Ni 2 MnGa. It is suggested that the mechanism giving rise to the large magnetostriction in Ni 2 MnGa consisted of a twin-related variant reorientation through field-induced twin-boundary motion.
TL;DR: In this paper, the half-metallic gap is wide and the Fermi level falls at the middle of the gap and thus, it presents high degree of spin-polarization for a wide range of lattice constants.
Abstract: Extensive ab-initio electronic structure calculations on Heusler alloys suggest that Cr2CoGa is the alloy of choice to achieve the half-metallic fully-compensated ferrimagnetism since (1) it has been already grown experimentally [T. Graf et al., Z. Anorg. Allg. Chem. 635, 976 (2009)], (2) half-metallic XA structure is favored energetically over all the studied lattice constant range with respect to the L21 which is not half-metallic, (3) the half-metallic gap is wide and the Fermi level falls at the middle of the gap and thus, it presents high degree of spin-polarization for a wide range of lattice constants, and (4) the Curie temperature is extremely high reaching the 1520 K.
TL;DR: In this paper, the effect of Ce4+ substitution in NiFe2O4, with a chemical formula Ni 1-2xCexFe 2O4 (0 ≤ x ≤ 0.25), prepared by a solid-state reaction is presented.
Abstract: The effect of Ce4+ substitution in NiFe2O4, with a chemical formula Ni1-2xCexFe2O4 (0 ≤ x ≤ 0.25), ferrite prepared by a solid-state reaction is presented in this paper. Ce4+ ions enter the NiFe2O4 lattice by replacing Ni2+ and swell the lattice. This enlarges the lattice constant, which results in a moderate distortion of the lattice. The r.m.s. strain increases from 0.411 × 10−3 to 0.471 × 10−3 with increasing Ce4+ content. SEM images revealed that Ce4+ promotes grain growth in NiFe2O4. It was also revealed that x-ray density and porosity decreases, whereas a significant increase in the bulk density is observed with the Ce4+ content. Substitution of Ce4+ for Ni2+ caused a decrease in the saturation magnetization from 41.3 to 25.12 emu/g and a decrease in the Curie temperature of the nickel ferrite from 830 to 594 K, whereas the coercivity increased from 59.48 to 458.25 Oe.
TL;DR: In this article, powder X-ray diffraction was used to characterize polycrystalline Ni 0.65− x Cd x Zn 0.35 Fe 2 O 4 ferrites with x varying from 0.00 to 0.20 in steps of 0.04 have been prepared by conventional ceramic route.
TL;DR: In this paper, the effect of Eu doping on phase transformation and energy storage performance of PZ thin films have been investigated in detail, and it has been seen that on extent of eu dopant the Curie temperature and electric field induced phase transformation can be altered.
Abstract: Undoped and Eu-doped (1, 3 and 5 mol%) PbZrO3 (PZ) antiferroelectric (AFE) thin films have been deposited on Pt (111)/Ti/SiO2/Si substrates by a sol–gel method. The effect of Eu doping on phase transformation and energy storage performance of PZ thin films have been investigated in detail. It has been seen that on extent of Eu dopant the Curie temperature and electric field-induced phase transformation can be altered. The energy storage properties have been found to be strongly dependent on Eu doping content. With the increase of Eu contents, recoverable energy storage density has been enhanced followed by their subsequent reduction. A maximum energy density (~18.8 J/cm3 at ~900 kV/cm) and minimum energy loss (~7.3 J/cm3) have been achieved on 3 mol% Eu-doped PZ thin films.
TL;DR: In this paper, the effect of Fe substitution on magnetic and magnetocaloric properties in La 07 Sr 03 Mn 1− x Fe x O 3 (x =005, 007, 010, 015, and 020) over a wide temperature range was studied.
TL;DR: The La0.8Ca0.2−x□xMnO3 compounds were prepared by the solid-state reaction and X-ray diffraction (XRD) and magnetic measurements were used to investigate the calcium-vacancy effect on the physical properties as mentioned in this paper.
TL;DR: The properties of PIN-PMN-PT crystals were studied as function of phase and orientation as discussed by the authors, including the Curie temperature TC, ferroelectric-ferroelectric phase transition temperature TR/O-T, coercive field, and piezoelectric/dielectric responses.
Abstract: The Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN-PMN-PT) crystals were studied as function of phase and orientation The properties, including the Curie temperature TC, ferroelectric-ferroelectric phase transition temperature TR/O-T, coercive field, and piezoelectric/dielectric responses, were systematically investigated with respect to the composition of PIN-PMN-PT crystals The Curie temperature TC was found to increase from 160 to 220 °C with ferroelectric-ferroelectric phase transition temperature TR-T and TO-T being in the range of 120–105 °C and 105–50 °C, respectively The piezoelectric activity of PIN-PMN-PT crystals was analyzed by Rayleigh approach The ultrahigh piezoelectric response for domain engineered [001] (1600–2200 pC/N) and [011] (830–1550 pC/N) crystals was believed to be mainly from the intrinsic contribution, whereas the enhanced level of piezoelectric and dielectric losses at the compositions around morphotropic phase boundaries (MPBs) was attributed to the phase boundaries motion
TL;DR: It was found that Zn substitution has a big effect in decreasing the Curie temperature (T(c), which is due to facilitation of demagnetisation by substitution of the non-magnetic Zn ions and by production of very small nanoparticles.
Abstract: Zn substituted cobalt ferrite spinels with the general formula ZnxCo1−xFe2O4 (with x varying from 0 to 0.5) were synthesized by a co-precipitation method and calcined at 500 °C and 800 °C. It was found that Zn substitution has a big effect in decreasing the Curie temperature (Tc), from around 440 °C for the undoped sample to ∼180 °C with x = 0.5. However, these values were also strongly affected by the pre-calcination temperature of the samples, thus TC shifts from ∼275 °C for the x = 0.3 sample to ∼296 °C after calcination at 500 °C and 800 °C respectively. These effects are due to facilitation of demagnetisation by substitution of the non-magnetic Zn ions and by production of very small nanoparticles. The latter are removed by higher temperature calcinations and so TC increases.
TL;DR: In this article, a mixture of copper and nickel powders has been used for the preparation of copper-nickel alloy particles in the nanometer range, which were designed to be used for controlled magnetic hyperthermia applications.
TL;DR: In this article, an unaggregated La0.82Sr0.18MnO3 + δ perovskite nanoparticles with a mean crystallite size of 22 nm were successfully synthesized through an aqueous combustion process (Glycine Nitrate Process, GNP) which takes advantage of exothermic, fast and self-sustaining chemical reactions between metal nitrates and glycine as a suitable organic reducing agent.
Abstract: Unaggregated La0.82Sr0.18MnO3 + δ perovskite nanoparticles with a mean crystallite size of 22 nm were successfully synthesized through an aqueous combustion process (Glycine Nitrate Process, GNP) which takes advantage of exothermic, fast and self-sustaining chemical reactions between metal nitrates and glycine as a suitable organic reducing agent. The influence of G/N molar ratio on the phase purity, crystallite size and manganese valency was screened. Fuel-rich conditions were selected to improve chelation of the cations in acidic pH and ensure an accurate control of the cationic composition. Fast calcination was optimized to enhance crystallinity of the nanoparticles and subsequent milling step was performed to favour their desaggregation. The manganite nanoparticles were thoroughly characterized by X-ray diffraction (XRD), elemental chemical analysis, Mohr salt titration and transmission electron microscopy (TEM). According to a process derived from the Stober's method, they were uniformly coated with a 5 nm thick silica shell, as evidenced by TEM, infrared spectroscopy, ζ potential measurements and dynamic light scattering experiments. Preliminary heating experiments in a ac magnetic field showed these core@shell nanoparticles fulfill the requirements for self-controlled magnetic fluid hyperthermia, considering their size (20–70 nm) and their maximum heating temperature (43 °C) which is controlled by the Curie temperature of the magnetic cores.
TL;DR: In this article, a Raman spectroscopic study of Na1/2Bi 1/2TiO3-x%BaTiO 3 (NBT-x %BT) single crystals with x = 0 and 5.6 has been performed as a function of temperature from 25 to 600°C.
Abstract: A Raman spectroscopic study of Na1/2Bi1/2TiO3-x%BaTiO3 (NBT-x%BT) single crystals with x = 0 and 5.6 has been performed as a function of temperature from 25 to 600 °C. The general features of the Raman spectra for the various compositions were similar over the region of the phase diagram investigated, with only subtle changes between rhombohedral (R), tetragonal (T) and cubic phases. The peaks were broad, with no significant narrowing on cooling through a phase transition. We find evidence of an oxygen octahedral rotational mode in the paraelectric state. On cooling near and below the ferroelectric Curie temperature, a gradual change in intensity of the A-O and B-O peaks occurred with decreasing temperature. Evidence of a ferroelectric → antiferroelectric transition was found near 200–250 °C, consistent with the onset of dispersion in the dielectric constant. The phase transition mechanism was discussed. The findings indicate the presence of a broad distribution of quasistatic local structural distortions...
TL;DR: In this article, the generalized Heusler compounds Mn2CoZ (Z = Al, Ga, In, Si, Ge, Sn, Sb) with the Hg2CuTi structure are studied by means of first principles calculations of the Heisenberg exchange coupling parameters, and Curie temperatures are calculated from those.
Abstract: The generalized Heusler compounds Mn2CoZ (Z = Al, Ga, In, Si, Ge, Sn, Sb) with the Hg2CuTi structure are of great interest due to their half-metallic ferrimagnetism. The complex magnetic interactions between the constituents are studied by means of first principles calculations of the Heisenberg exchange coupling parameters, and Curie temperatures are calculated from those. Due to the direct Mn–Mn exchange interaction in Mn2CoZ, the Curie temperature decreases, although the total moment increases when the valence electron number Z is increased. The exchange interactions are dominated by a strong direct exchange between Co and its nearest neighbor Mn on the B site, which is nearly constant. The coupling between the nearest neighbor Mn atoms scales with the magnetic moment of the Mn atom on the C site. Calculations with different lattice parameters suggest a negative pressure dependence of the Curie temperature, which follows from the decreasing magnetic moments. Curie temperatures of more than 800 K are predicted for Mn2CoAl (890 K), Mn2CoGa (886 K), and Mn2CoIn (845 K).
TL;DR: In this paper, the authors reported that oxygen vacancy and cobalt aggregates are a key factor for inducing superparamagnetism in the vacuum annealed sample of Co doped TiO 2 nanoparticles.