Showing papers on "Curie temperature published in 1990"
TL;DR: Pour des couches cubiques a faces centrees inferieures a 3 couches monomoleculaires, les points de Curie sont beaucoup plus bas que ceux des materiaux massifs, and ils decroissent avec l'epaisseur de the couche.
Abstract: We have explored the relationship between epitaxial growth and magnetism in the case of ultrathin fcc-Co films by means of a multitechnique approach. For high-quality fcc-Co films less than 3 monolayers thick the Curie temperatures ${\mathrm{T}}_{\mathrm{C}}$ are dramatically lower than ${\mathrm{T}}_{\mathrm{C}}^{\mathrm{B}}$ of bulk fcc-Co and decease distinctly with film thickness. In contrast to previous claims, ${\mathrm{T}}_{\mathrm{C}}$ of a single monolayer appears to be far below 300 K. Substrate topology is found to strongly influence the structural perfection of the films which in turn determines their magnetic properties (Curie temperature, coercive field, and anisotropies).
368 citations
TL;DR: In this paper, the fracture toughness of a single-phase commercial lead zirconate titanate (PZT) ceramic was measured using the single edge notched beam method above and below the Curie temperature.
Abstract: Fracture toughness, KIC, of a single-phase commercial lead zirconate titanate (PZT) ceramic (Zr/Ti=0.54/0.46) of tetragonal structure (c/a=1.019) was measured using the single edge notched beam method above and below the Curie temperature. Domain switching (poling) under electrical and mechanical loading was examined using X-ray diffraction. Surface grinding, electrical poling, and mechanical poling caused crystallographic texture. Similar texture, indicative of domain switching, was also observed on fracture surfaces of some saples fractured at room temperature. At room temperature, the highest KIC measured was 1.85 MPa·m1/2, while above the Curie temperature it was about 1.0 MPa·m1/2. Cracks emanating from Vickers indents in poled samples were different in the poling and the transverse directions. The difference in crack sizes is explained on the basis of domain switching during crack growth. These results indicate that ferroelastic domain switching (twinning) is a viable toughening mechanism in the PZT materials tested.
276 citations
TL;DR: In this paper, the authors used mechanical alloying and a subsequent two-step heat treatment to produce magnetically isotropic microcrystalline Sm2Fe17Nx samples with room-temperature coercivities up to 24 kA/cm (30 kOe).
Abstract: Using mechanical alloying and a subsequent two‐step heat treatment we produced magnetically isotropic microcrystalline Sm2Fe17Nx samples with room‐temperature coercivities up to 24 kA/cm (30 kOe). The remanence and the energy product are equivalent to similarly prepared Nd‐Fe‐B samples, but the properties at elevated temperatures are superior because of the high Curie temperature of 470 °C and the large anisotropy field of 14 T at room temperature. From differential scanning calorimetry it is concluded that the 2:17 nitride is metastable. It decomposes into Sm nitride and α‐Fe above 600 °C.
237 citations
TL;DR: A series of chargetransfer (CT) salts have been synthesized that utilize the spin S = 1 molecular cation, [Mn(C5(CH3)5)2]+ (decamethylmanganocenium), a bulk molecular ferromagnet with the highest critical temperature and coercive field yet reported for such a material.
Abstract: The study of magnetic phase transitions in insulating molecular solids provides new insights into mechanisms of magnetic coupling in the solid state and into critical phenomena associated with these transitions. Only a few such materials are known to display cooperative magnetic properties. The use of high-spin molecular components would enhance intermolecular spin-spin interactions and thus a series of chargetransfer (CT) salts have been synthesized that utilize the spin S = 1 molecular cation, [Mn(C5(CH3)5)2]+ (decamethylmanganocenium). The structure and cooperative magnetic behavior of [Mn(C5(CH3)5)2]+[TCNQ- (decamethylmanganocenium 7,7,8,8-tetracyano-p-quinodimethanide) are reported. This salt is a bulk molecular ferromagnet with the highest critical (Curie) temperature (Tc = 6.2 K) and coercive field (3.6 x 103 gauss), yet reported for such a material.
163 citations
TL;DR: In this paper, a new theoretical framework is presented for understanding the dielectric response of relaxor ferroelectrics, based on the lead magnesium niobate, a well-known relaxor material, and the existence of a ferroelectric network linking niobium-oxygen coordination octahedra.
146 citations
TL;DR: In this paper, the magnetocrystalline anisotropy of Sm2Fe17N2 was investigated from room temperature up to the Curie temperature, Tc, and the temperature dependence of the anisotropic field was measured up to 700 K by singular point detection (SPD) method and by magnetization measurements on aligned powder samples.
146 citations
TL;DR: In this paper, it was shown that ternary rare earth nitrides of the approximate composition R2Fe17N2.5 can be prepared by the reaction of R 2Fe17 with nitrogen gas at 500°C for all members of this series.
128 citations
TL;DR: In this article, an expansion mismatch between the core and the shell placed the core in compression resulting in a decrease in the lattice parameter and a pseudocubic structure with respect to the unmodified BaTiO3.
Abstract: BaTiO3 modified with grain-boundary resident ZrO2 (≤ 2 wt%) results in substantial chemical modification of the microstructure when sintered at 1320°C for 2 h. These modified regions were seen as core-shell grains in the micro-structure and were accompanied by an increase in the internal stress level. The shells had a variable composition (Ba(Ti1−xZrx)O3) and a tetragonal structure. An expansion mismatch between the core and the shell placed the core in compression resulting in a decrease in the lattice parameter and a pseudocubic structure with respect to the unmodified BaTiO3. The combination of core-shell grains and increased internal stress resulted in an essentially flat permittivity response with temperature, a condition consistent with a distribution of Curie points within the ceramic.
121 citations
TL;DR: In this paper, the authors summarized current thinking about thermally activated magnetic processes, and put them in the context of recent discoveries about magnetic domain structure in crystals containing only one or a few domains.
Abstract: Rock magnetism is concerned with the remanent magnetisation of ferrimagnetic fine particles in weak fields ( approximately=0.1 mT), at temperatures ranging from ambient to the Curie point, and over times of geological length. Since most magnetisations in rocks or their synthetic analogues are thermally activated, the blocking temperature below which the approach to equilibrium is effectively frozen is of particular importance. The author summarises current thinking about thermally activated magnetic processes, and puts them in the context of recent discoveries about magnetic domain structure in crystals containing only one or a few domains. Pseudo-single-domain effects in such crystals, which bridge the gap between classic single-domain and multidomain behaviour, are one of the current frontiers in rock magnetism.
112 citations
TL;DR: In this paper, it was found that copolymers with VDF content ranging from ∼82 to 90 mol% crystallize under ordinary pressure into thick lamellar single crystals composed of a mixture of α, β, and possibly γ phases.
Abstract: Field‐induced phase transformation and thermally induced phase transition in vinylidene fluoride (VDF) and trifluoroethylene copolymers with VDF content higher than 82 mol% and their ferroelectric and piezoelectric properties have been studied by means of x‐ray diffraction, DSC, D‐E hysteresis, and piezoelectric resonance. It is found that copolymers with VDF content ranging from ∼82 to 90 mol% crystallize under ordinary pressure into thick lamellar single crystals composed of a mixture of α, β, and possibly γ phases. Application of a strong ac electric field transforms these mixed phase crystals into β‐phase crystals completely. The β‐phase crystals are ferroelectric and exhibit strong piezoelectric activity stable up to the melting temperature (160–180 °C depending on VDF content); the Curie point coincides with the melting point. Unpoled, mixed phase film has a paraelectric or rotational phase below the melting temperature, in which thick lamellar single crystals are grown extensively, whereas β‐phase...
108 citations
TL;DR: The magnetic correlations, susceptibility, specific heat, and thermal relaxation in the dipolar-coupled Ising system LiHo_xY_(1-x)F_4.4 is measured to be consistent with a single low-degeneracy ground state with a large gap for excitations.
Abstract: We have measured the magnetic correlations, susceptibility, specific heat, and thermal relaxation in the dipolar-coupled Ising system LiHo_xY_(1-x)F_4. The material is ferromagnetic for spin concentrations at least as low as x=0.46, with a Curie temperature obeying mean-field scaling relative to that of pure LiHoF_4. In contrast, an x=0.167 sample behaves as a spin glass above its transition temperature, while an x=0.045 crystal shows very different glassy properties characterized by decreasing barriers to relaxation and nonexponential thermal relaxation as T→0. We find the properties of the x=0.045 system to be consistent with a single low-degeneracy ground state with a large gap for excitations. The x=0.167 sample, however, supports a complex ground state with no appreciable gap, in accordance with prevailing theories of spin glasses. The underlying causes of such disparate behavior are discussed in terms of random clusters as probed by neutron studies of the x=0.167 sample.
TL;DR: In this paper, the authors measured magnetic properties of acicular magnetite crystals of single-domain (SD) size between room temperature and the Curie temperature T C of 645°C.
TL;DR: In this article, the phase transitions in ultrathin Ising films were studied within the mean field approximation, and analytical expressions for the thickness dependence of the Curie temperature were given.
TL;DR: Sintering of a KSr2Nb5O15 powder compact at 1350°C resulted in a duplex structure, and the Curie temperature and dielectric constant were dependent on the microstructure as discussed by the authors.
Abstract: Sintering of a KSr2Nb5O15 powder compact at 1350°C resulted in a duplex structure. Prefiring of the compact between 1200° and 1300°C inhibited the abnormal grain growth responsible for the duplex structure. The Curie temperature and dielectric constant were dependent on the microstructure.
TL;DR: The magnetic properties of R2PdSi3 compounds, where R is Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm and Y, have been investigated from 4.2 to 300 K as discussed by the authors.
TL;DR: In this article, a ferroelectric inversion layer can be formed in LiTaO3 by proton exchange in benzoic acid melts followed by heat treatment at temperatures just below the Curie point.
Abstract: We report that a ferroelectric inversion layer can be formed in LiTaO3 by proton exchange in benzoic acid melts followed by heat treatment at temperatures just below the Curie point It is demonstrated that the inversion layer appears at the −c surface for LiTaO3, whereas it appears at the +c surface for LiNbO3 The dependence of the inversion layer thickness on the conditions of proton exchange and heat treatment is also reported
TL;DR: In this paper, the relationship between the CaCu5 and the Th2Ni17 or ThMn12 structure types is discussed in terms of substitution of transition metal dumbbells on Ca sites, and a structural explanation is given for the directions of the iron anisotropy in the 2:17 and 1:12 compounds.
Abstract: A series of YFe11−xTix alloys with x=0.10, 0.25, 0.40, 0.50, 0.70, 1.00, 1.25, 1.50, and 2.00 have been investigated using x‐ray diffraction, thermomagnetic analysis, the singular point detection technique, and 57Fe Mossbauer spectroscopy. A pure ThMn12 ‐type phase was observed when 0.7≤x≤1.25. It has Curie temperature of 524 K, anisotropy field of 2.3 T, and average iron magnetic moment of 1.47 μB at room temperature. The relationship between the CaCu5 and the Th2Ni17 or ThMn12 structure types is discussed in terms of substitution of transition‐metal dumbbells on Ca sites, and a structural explanation is given for the directions of the iron anisotropy in the 2:17 and 1:12 compounds.
17 Apr 1990
TL;DR: In this paper, a hard magnetic phase is interpreted as a Zr/sub 2/Co/sub 11/ intermetallic compound with a Curie temperature of 500 degrees C and uniaxial magnetic anisotropy with an anisotropic field of 34 kOe.
Abstract: In order to identify a hard magnetic phase in rapidly quenched Zr-Co-B alloys and clarify its magnetic properties, Zr-Co-B ribbons, Zr-Co ribbons, and Zr-Co ingots were studied. The hard magnetic phase is interpreted as a Zr/sub 2/Co/sub 11/ intermetallic compound. This compound has a Curie temperature of 500 degrees C and uniaxial magnetic anisotropy with an anisotropy field of 34 kOe. The magnetization of this compound was estimated to be 67 emu/g at 15 kOe. In addition to the hard magnetic phase, the low and high T/sub c/ phases appear in both binary and ternary alloys. The low T/sub c/ phase is FCC Zr/sub 6/Co/sub 23/ with T/sub c/=180 degrees C. The magnetization of Zr/sub 6/Co/sub 23/ was estimated to be 44 emu/g at 15 kOe. The high T/sub c/ phase is cobalt including a small amount of zirconium. In Zr-Co-B alloys, suitable boron addition is shown to enhance the coercive force. On the other hand, the addition increases the magnetization. While the boron addition produces cobalt, it reduces Zr/sub 6/Co/sub 23/ or quenches its ferromagnetism. >
TL;DR: The low-field magnetic susceptibility anisotropy (LMA) of weakly magnetic rocks is dominated by paramagnetic minerals among which micas, and mainly biotite, is important as discussed by the authors.
TL;DR: In this article, the transition character of paraelectric-to-ferroelectric phase transition and its relation to lattice softmode behavior in the solid solutions of (Ba,Sr)TiO3,(Ba,Zr)O3 and Ba(Ti,Sn),O3 were investigated by the measurements of dielectric constants in the temperature range from −30 to 150°C, and x-ray line breadth of 100, 110, and 111 Bragg reflections at temperatures above the transition point.
Abstract: The transition character of paraelectric‐to‐ferroelectric phase transition and its relation to lattice soft‐mode behavior in the solid solutions of (Ba,Sr)TiO3, (Ba,Ca)TiO3, Ba(Ti,Zr)O3, and Ba(Ti,Sn)O3 were investigated by the measurements of dielectric constants in the temperature range from −30 to 150 °C, and x‐ray line breadth of 100, 110, and 111 Bragg reflections at temperatures above the transition point. From the result of dielectric measurement, it was found that the specimens containing Ti‐site substituents have shown a diffuse phase transition character, whereas those having Ba‐site substituents have preserved a sharp phase transition. It was also noted from the change of line breath of Bragg reflections that for the solutions of Ba(Ti,Zr)O3 and Ba(Ti,Sn)O3, the high dynamic strain at the 〈1 0 0〉 axis, induced from the soft‐mode of lattice vibration, become repressed when a large amount of solutes is contained, but for the solutions of (Ba,Sr)TiO3 and (Ba,Ca)TiO3, it is recovered at temperatures near the transition point. Moreover, the dynamic strain at the 〈1 1 1〉 axis can be rapidly repressed by the addition of Ti‐site substituents. The different effects on the dynamic strains are attributed to the disparate distortions of oxygen octahedra in the perovskite lattice, induced by the solutes of different site substitution. It is also believed that the influence on the soft‐mode of lattice vibration has caused the different phase transition characters of the four BaTiO3 solid solutions.
TL;DR: The 20:70:10 phase has a Curie temperature of 380°C and an estimated saturation magnetization of 6-7 kG as discussed by the authors. But it is not a phase with a large amount of the Fe2 Sm phase.
Abstract: Using mechanical alloying and an additional annealing, we prepared bulk material of a new Sm20 Fe70 Ti10 phase observed before only in sputtered films deposited in the amorphous state and then crystallized. Crystallization of rapidly quenched and partially amorphous ribbons leads to a two‐phase material with a considerable amount of this phase but with the Fe2 Sm phase as a majority phase. This new phase (named 20:70:10 phase) has a Curie temperature of 380 °C and an estimated saturation magnetization of 6–7 kG. The magnetically isotropic, mechanically alloyed samples show room‐temperature coercivities of up to 50.3 kOe.
TL;DR: In this paper, the magnetic properties of peridotites from ODP Hole 670A have been studied and the degree of serpentinization was determined by a new method which combines saturation magnetization and whole-rock iron content.
TL;DR: In this paper, large coercive fields have been obtained in melt-spun Co-Zr(Hf)•B based alloys and the highest coercivities have been observed in Co76 Zr18 B3 Si3 and Co78 Hf16 B3Si3 samples with values 67 and 65 kOe, respectively.
Abstract: Large coercive fields have been obtained in melt‐spun Co‐Zr(Hf)‐B based alloys The highest coercivities have been observed in Co76 Zr18 B3 Si3 and Co78 Hf16 B3 Si3 samples with values 67 and 65 kOe, respectively The saturation magnetization for both samples is estimated to be around 60 emu/g Thermomagnetic data showed the presence of two phases with the major phase having a Curie temperature at around 450 °C TEM and SEM studies showed a microstructure consisting of a mixture of two phases with the grain size in the range of 1000–5000 A The origin of the high coercivity is believed to be due to the fine grain microstructure leading to a single‐domain particle‐type behavior
TL;DR: The Curie temperature of Y2Fe17Cx increases linearly with carbon content in the hexagonal phase, 0 ⩽ × 1.1, where the phase becomes rhombohedral as mentioned in this paper.
TL;DR: In this paper, the electrical conductivity and thermoelectric power of polycrystalline lithium-cadmium ferrites having the chemical formula Li0.5-x/2CdxFe2.
Abstract: The electrical conductivity (σ) and thermoelectric power (Q) of polycrystalline lithiumcadmium ferrites having the chemical formula Li0.5–x/2CdxFe2.5–x/2O4 where (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) have been investigated as a function of temperature. Lithium-cadmium ferrite with x = 0.4 is found to possess minimum electrical conductivity and lowest Seebeck coefficient. Plots of log (σT) versus 103T are almost linear and have shown a transition near the Curie temperature except in the case of cadmium ferrite. The Seebeck coefficient is negative for all the compositions showing that these ferrites behave as n-type semiconductors. The values of charge carrier concentration and mobility have also been computed. The properties of cadmium-substituted lithium ferrites have been correlated with those of zinc-substituted lithium ferrites, cadmium and zinc being two non-magnetic divalent ions occupying essentially tetrahedral A sites when substituted in ferrites.
TL;DR: In this paper, the authors obtained a coercivity of 11.7 kOe for Sm-Fe-V with the ThMn12 crystal structure, the best value reported so far for 1:12 magnets.
TL;DR: In this paper, a phase with the hexagonal structure of Co5R is stabilized by the addition of Ti and has a rare-earth content just above 12 at. %, but well below 16.7 at.
Abstract: In Fe‐Nd‐Ti and Fe‐Sm‐Ti the phase with the hexagonal structure of Co5R is stabilized by the addition of Ti and has a rare‐earth content just above 12 at. %, but well below 16.7 at. %, so that it must be classified as a Cu7Tb type. It and Fe17R2 dissolve approximately 3 at. % Ti, accompanied by an increase in Curie temperature in the latter. In Fe‐Nd‐Ti the phase with the tetragonal Mn12Th structure is found near Fe11TiNd, and its low‐anisotropy field is verified. It forms in the cast alloys and is stable only above 1000 °C. At 1000 °C and below it decomposes according to Fe11TiNd→Fe17Nd2 +Fe2Ti+Fe. In Fe‐Ti‐Sm the high‐anisotropy phase Fe11TiSm does not undergo this decomposition down to 700 °C, but it is restricted by the coexistence of Fe17Sm2 and Fe2Ti. As a result, Fe11TiSm is found accompanied by either free Fe or easy‐plane Fe17Sm2. A new phase Fe9.5Ti1.5Sm has the structure of tetragonal Ce(Mn,Ni)11 with a=0.8253 and c=0.4825 nm; it has no permanent magnetic moment at room temperature. The high‐anisotropy phase near Fe70Ti10Sm20 is not observed between the melting temperature and 700 °C.
TL;DR: In this paper, the dielectric properties of N(CH 3 ) 4 CdBr 3 single crystal have been studied and the spontaneous polarization obtained by 50 Hz hysteresis loop measurement is about 1.2×10 -3 C/m 2 at 125 K.
Abstract: Dielectric properties of N(CH 3 ) 4 CdBr 3 single crystal have been studied. The dielectric constant along the c -direction shows a slight λ-type maximum at 156 K while that along the a -direction shows a discontinuity. The compound exhibits ferroelectric activity along the c -direction below the Curie temperature of 156 K. The spontaneous polarization obtained by 50 Hz hysteresis loop measurement is about 1.2×10 -3 C/m 2 at 125 K.
TL;DR: In this paper, four polycomponent systems based on LaCo13 have been studied: La(Co1−xFex)13, La (Co 1−xAlx) 13, La(Al 0.4Fe 0.3)13 and La 0.7Nd0.3
Abstract: Four polycomponent systems based on LaCo13 have been studied: La(Co1−xFex)13, La(Co1−xAlx)13, La(Co1−x−yFexAly)13, and La0.7Nd0.3(Co0.7Fe0.3)13. These 1:13 systems were studied because of their potential for permanent magnet fabrication. LaCo13 has a high 3d metal content, the highest for any known rare‐earth intermetallic, a 13 kG saturation induction, and a high Tc (1318 K). Unfortunately, it is cubic and lacks anisotropy. The substituted systems were examined as a portion of a program to find LaCo13‐based systems of applications significance. Replacement of Co in LaCo13 by Fe and/or Al leads to a rapid decline in Tc for all systems studied. Replacement of Co by Al results in a decline in moment, whereas replacement by Fe leads to a rise in moment to 2.39μB/3d atom for La(Co0.4Fe0.6)13, as compared to 2.46μB/3d atom for Fe0.7Co0.3. Analysis of the magnetic data shows that vacancies occur in both half‐bands for LaCo13 (4.8 spin up and 3.24 spin down) but in only one half‐band in La(Co1−xAlx)13 for x≥0.2....
TL;DR: In this paper, the Curie temperatures Tc and spin reorientation temperatures TSR were found to increase strongly when aluminium, gallium or silicon was substituted for iron in Tm2Fe17.
Abstract: The Curie temperatures Tc and spin reorientation temperatures TSR were found to increase strongly when aluminium, gallium or silicon was substituted for iron in Tm2Fe17. By means of 57Fe Mossbauer spectroscopy we were able to show that there is no preferential substitution of these elements into the 4f or dumbbell site. The 169Tm Mossbauer effect was studied in Tm2Fe15Si2. We found that silicon substitution has led to a marked shift of the second-order crystal field parameter A02 to more negative values, the corresponding increase in K1 explaining the shift of TSR towards higher temperatures.