Curie temperature

The correlated metal UCoGe is a weak itinerant ferromagnet with a Curie temperature T C = 3 K and a superconductor with a transition temperature T s = 0.6 K. We review its basic thermal, magnetic  on the macro and microscopic scale  and transport properties, as well as the response to high pressure. The data unambiguously show that superconductivity and ferromagnetism coexist below T s = 0.6 K and are carried by the same 5f electrons. We present evidence that UCoGe is a p-wave superconductor and argue that superconductivity is mediated by critical ferromagnetic spin fluctuations.

The mean field solution of the Ising model on a Barabási-Albert scale-free network with ferromagnetic coupling between linked spins is presented. The critical temperature Tc for the ferromagnetic to paramagnetic phase transition ( Curie temperature) is infinite and the effective critical temperature for a finite size system increases as the logarithm of the system size in agreement with recent numerical results of Aleksiejuk, Holyst and Stauffer.

We report the effect of average A-site cation radius on the structural, magnetic and electrical properties of electron doped manganite La 0.7 Ce 0.3 MnO 3 thin films. A site cation radius hr A i is varied systematically by partially replacing La C3 ions by smaller Y C3 ions in the parent compound. The carrier doping, i.e. the fraction of tetravalent Ce atoms at the A-site was kept at 30%. A series of La 0.7Kx Y x Ce 0.3 MnO 3 (xZ0, 0.05, 0.1, 0.15 and 0.25) thin films were prepared under identical conditions by using pulsed laser deposition technique. Metal-insulator transition temperature (T p ) and the ferromagnetic Curie temperature (T c ) are found to be decreasing significantly with increasing yttrium concentration, i.e. decreasing hr A i. The resistivity (r) at T p increases by one order of magnitude when the yttrium concentration is increased from xZ0 to 0.25, while magnetization decreases with decreasing hr A i. Magnetoresistance as measured under field of 1 T is significant near T c . Structural analysis reveals the films are having single phase and c-axis lattice parameter decreasing linearly as hr A i decreases. There is no marked difference in surface morphology of samples with different yttrium concentration. From EDX the Y:Ce:Mn ratio were consistent within 5% of the nominal composition. q

The polycrystalline manganite La 0.75 Sr 0.25 MnO 3 prepared by an alternative carbonate precipitation route reveals the rhombohedral perovskite structure. Magnetization isotherms measured up to 2 T are used to determine Curie temperature of 332 K by means of Arrott plot. Maximum of magnetic entropy change is found at Curie temperature. The relative cooling power equal to 64 J/kg for 1.5 T magnetic field, is superior as compared to the manganite with the same chemical composition from the sol-gel method.

The complexes of N-disubstituted, N'-ethoxy carbonyl thioureas with copper, nickel and cobalt have been synthesized and studied. NMR spectra and magnetic data have shown square-planar NiL2 and facor mer-CoL 3 compounds. The existence of copper(III)/copper(II) and copper(II)/copper(I) redox systems has been chemically and electrochemically proved in solution.

We study the dependence of magnetic interactions and Curie temperature in Ni (1+x) MnSb system on the Ni concentration within the framework of the density-functional theory. The calculation of the exchange parameters is based on the super-cell and frozen-magnon approaches. The Curie temperatures, TC , are calculated within the random-phase approximation. In agreement with experiment we obtain decrease of the Curie temperature with increasing Ni content.

Microcrystalline cellulose was analysed by Curie-point pyrolysis-capillary gas chromatography-mass spectrometry (Py-GC-MS) under electron impact (70 eV) as well as chemical ionisation (isobutane) conditions. Almost all interpretations could be supported by high-resolution mass spectrometric data, obtained by Py-GC-high-resolution MS. The pyrolysis products identified can be classified into several main groups i.e. carbonyl compounds, acids and methyl esters, furans, pyrans, anhydrosugars and hydrocarbons. Altogether 96 compounds have been found, and structural assignments could be given to 94 of them. From these compounds 25 have not been identified earlier in pyrolysates of polysaccharides. Levoglucosan is the major GC amenable pyrolysis product detected. Several formation pathways are proposed to account for the formation of a number of pyrolysis products from levoglucosan and/or cellobiosan. An overview showing the thermal degradation pattern of cellulose through anhydrosaccharides to smaller pyrolysis products is also presented. Carbohydrates; cellulose; gas chromatography; high-resolution mass spectrometry; levoglucosan; mass spectrometry; pyrolysis.

Haag, M., Heller, F., Carracedo, J.C. and Soler, V., 1990. Remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of Nevado del Ruiz (Colombia) reversed due to self-reversal. In: S.N. Williams (Editor), Nevado del Ruiz Volcano, Colombia, I. J. Volcanol. Geotherm. Res., 41: 369-377.

New developments in thin-film magnetism are reviewed with an emphasis on the ultrathin regime. The scope includes relatively simple metallic systems in overlayer, sandwich, and superlattice configurations. Sample fabrication, characterization, and magnetic measurement techniques are outlined by highlighting some of the more modern experimental innovations. Current issues and advances that demonstrate the symbiotic relationship between experiment and theory are then examined, including the surface magnetic anisotropy, the two-dimensional critical behavior, the creation of metastable phases via epitaxy, and phenomena associated with coupled magnetic layers. The review ends with a brief account of the impact of the various contemporary developments on the applications area.

Nanostructured Pd-Fe thin films with varied Fe content were prepared by electrodeposition technique from organic electrolytes on Cu and brass substrates. The structure and the magnetic properties of the films were investigated prior to post-deposition annealing. The structure of the Pd 1-

Ultrasonic studies of the temperature behavior of the velocity and damping of sound for the xx and zz longitudinal and yx and zx transverse waves in K 3 Na(CrO 4 ) 2 have been carried out in the temperature interval 185-295 K, which includes the region of the ferroelastic phase transition. The acoustic parameters for both shear and longitudinal waves were found to have anomalies in the region of the phase transition with a Curie temperature of 235.5 K. A theoretical analysis of the softening of the elastic moduli c 44 and c 66 was performed on the basis of the Landau expansion in terms of the strain tensor components ε 4 and ( ε 2 -ε 1 )/2 considered as the linearly coupled primary and secondary order parameter, respectively. The absolute values of the elastic moduli c 11 , c 33 , c 44 , c 66 , c 12 , and c 14 at 295 K were calculated. © 2004 MAIK "Nauka/Interperiodica".

The crystal and magnetic structure of RCrO 4 oxides (R=Nd, Er and Tm) has been studied by owder neutron diffraction. These compounds crystallize with the zircon-type structure, showing tetragonal symmetry, space group I4 1 =amd: In the case of NdCrO 4 , magnetic susceptibility measurements reveal the existence of an antiferromagnetic ordering in which both Cr 5+ and Nd 3+ sublattices are involved. This ordering has been explained on the basis of a propagation vector k ¼ 0 and a collinear structure, described by the symmetry mode A x ; the ordered magnetic moments being 0.62 and 0.66 m B at 2 K for Nd 3+ and Cr 5+ , respectively. Magnetic susceptibility and magnetization measurements reveal that both ErCrO 4 and TmCrO 4 behave as ferromagnetic compounds with a Curie temperature of 15 and 18 K, respectively. Rietveld refinement of the neutron diffraction data for ErCrO 4 yields a collinear magnetic structure described with an F x mode. In the case of the TmCrO 4 oxide, the ferromagnetic sublattices of Tm 3+ and Cr 5+ are aligned antiparallel in the a2b plane, while along the c-axis the magnetic moments point to the same direction. In both compounds, the rather small values obtained for the Er 3+ and Tm 3+ ordered moments compared with the theoretical ones have been attributed to crystal field effects. The differences in the ferromagnetic structure of these compounds have been explained as the result of the higher rare-earth anisotropy of Tm 3+ when compared with Er 3+ , for which no magnetic component is present along the c-direction.

Structural and magnetic transformations in the Heusler-based system Ni 0.50 Mn 0.50−x Sn x are studied by x-ray diffraction, optical microscopy, differential scanning calorimetry, and magnetization. The structural transformations are of austenitic-martensitic character. The austenite state has an L2 1 structure, whereas the structures of the martensite can be 10M, 14M, or L1 0 depending on the Sn composition. For samples that undergo martensitic transformations below and around room temperature, it is observed that the magnetic exchange in both parent and product phases is ferromagnetic, but the ferromagnetic exchange, characteristic of each phase, is found to be of different strength. This gives rise to different Curie temperatures for the austenitic and martensitic states.

X-ray powder diffraction measurements, at room temperature, and magnetic susceptibility χ measurements, in the temperature range from 2 to 300 K, were made on polycrystalline samples of Mn 2 GeTe 4 , Fe 2 GeTe 4 and Fe 2 SnSe 4 compounds, which would be useful for spintronic device production. Magnetization M measurements at various temperatures were carried out on the Fe-compounds. From the analysis of the X-ray diffraction patterns, it was found that the Mn 2 GeTe 4 , Fe 2 GeTe 4 and Fe 2 SnSe 4 have orthorhombic structure, possibly an olivine structure-type (SG: Pnma No. 62, z = 4). It was found that Mn 2 GeTe 4 has a Néel temperature of 30 K, shows mainly antiferromagnetic behavior with a weak superimposed ferromagnetic component which is attributed to spin canting. The resulting susceptibility χ versus T curves for Fe 2 GeTe 4 and Fe 2 SnSe 4 were found to have, in each case, a form which is typical of a ferromagnetic material with Curie temperatures T C of 149.9 and 301 K, respectively. The critical exponent β for the Fe-compounds were found to be very similar and close to the expected value for a ferromagnetic material, in the range 0.33-0.39. The values of the coercive field B C and the remanent magnetization M r were found to vary nonlinearly with the temperature T.

Materials in film form for electromechanical transduction have a number of potential applications in ultrasound. They are presently under investigation in flexural transducers for air-coupled ultrasound and underwater sonar operating at frequencies up to a few MHz. At higher frequencies, they have the potential to be integrated with electronics for applications of ultrasound requiring high spatial resolution. However, a number of fabrication difficulties have arisen in studies of such films. These include the high temperatures required in many thick and thin film deposition processes, making them incompatible with other stages in transducer fabrication, and difficulties maintaining film quality when thin film -typically sub-1 µm -processes are extended to higher thicknesses. In this paper, we first outline a process which has allowed us to deposit aluminium nitride (AlN) films capable of electromechanical transduction at thicknesses up to more than 5 µm without substrate heating. As an ultrasonic transduction material, AlN has functional disadvantages, particularly a high acoustic velocity and weak electromechanical transduction. However, it also has a number of advantages relating to practicality of fabrication and functionality. These include the ability to be deposited on a variety of amorphous substrates, a very high Curie temperature, low permittivity, and low electrical and mechanical losses. Here, we present experimental results highlighting the transduction capabilities of AlN deposited on aluminium electrodes on glass and lithium niobate. We compare the results with those from standard simulation processes, highlighting the reasons for discrepancies and discussing the implications for incorporation of AlN into standard ultrasonic transducer design processes.

... doi:10.1016/0304-8853(87)90721-9 | How to Cite or Link Using DOI Copyright © 1987 Published by Elsevier Science BV All rights reserved. Permissions & Reprints. Local spin density functionalapproach to the theory of exchange interactions in ferromagnetic metals and alloys. ...

• Straightforward synthesis for 0-3 Sr 0.5 Ba 0.5 Nb 2 O 6-CoFe 2 O 4 composites. • Monitoring the phase evolution during sintering to ceramics bodies. • Determination of Curie temperature. • Impedance spectroscopy up to 200 • C and 10 MHz. • Investigations of the magnetoelectric coefficient in dependence on magnetic DC field, frequency, and temperature.

A series of CoAl x Cr x Fe 2-2x O 4 systems (x = 0.1 to 0.5 in steps of x = 0.1) spinel ferrites have been synthesized successfully using wet chemical co-precipitation technique. The samples were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR) and magnetization measurements. The powder XRD patterns confirm the single phase spinel structure for the materials synthesized. X -ray diffraction measurements were performed to yield the lattice constant as function of the amount x corresponding to Al-Cr substitution. Lattice parameters, X-ray density, bulk density and particle size decrease whereas porosity increases with the increase in Al-Cr content, 'x'. Infrared studies show two absorption bands at about 400 cm -1 and 600 cm -1 for octahedral and tetrahedral sites, respectively. Saturation magnetization decreases with the increase in Al-Cr content x. AC magnetic susceptibility measurements were carried out as a function of temperature to measure the Curie temperature, which was found to decrease with Al-Cr content x. The decrease of Curie temperature has been explained by A-B interaction.

Dielectric properties of apple, pear, orange, grape and pineapple juices were studied from 20 to 4500 MHz at 15-95°C. The dielectric constants e 0 of fruit juices decreased as frequency increased. The loss factors e 00 decreased with increasing frequency to a minimum between about 1000 and 3000 MHz, depending on temperature, and then increased as frequency increased. The frequency of that turning point increased, whereas the minimum e 00 decreased with increasing temperature. The e 00 values increased with increasing temperature below about 1000 MHz, and decreased with increasing temperature above 3000 MHz. Dielectric constants decreased consistently and linearly with increasing temperature. Linear or polynomial correlations for the dependence of dielectric properties on temperature were developed. The power penetration depth of all fruit juices decreased with increasing frequency. The depth decreased as temperature increased at lower frequencies and increased with temperature above 3000 MHz. The dielectric properties obtained in the study provide important parameters for juice pasteurization by dielectric heating.

High intensity focused ultrasound (HIFU) and ultra sound-guided HIFU therapy require probes capable of both high quality diagnostic imaging and radiating high acoustic power into tissue. Piezoelectric materials with high mechanical quality factor Q, large electromechanical coupling (k ij ) and high dielectric constant K T are desired to meet the demands of advanced high power electromechanical applications. In addition, a high Curie temperature is desired because of wider temperature usage and improved temperature stability. Previous investigations have shown that Q or k ij and K T can be enhanced only at the expense of each other. Also materials with higher Curie temperature were found to possess lower dielectric and piezoelectric performance. Therefore, modified piezoelectric materials (including polycrystalline and single crystals) were reviewed as a function of Curie temperature. Optimization of performance was exemplified with Pb(Sc 1 / 2 Nb 1 / 2 )O 3 -PbTiO 3 (PSNT) compositions close to the morphotropic phase boundary (MPB) and the effect of manganese additions on Q, coupling k 33 and K T , respectively.

The formation and magnetic behaviour of nanoparticles of various manganese oxides have been studied. For synthesis, manganese oxalate dihydrate is first produced via sol-gel process and then decomposed in oxygen, air, nitrogen, or argon ambient at different temperatures. The decomposition process leads to emergence of Mn 3 O 4 which undergoes oxidation, oxidation-reduction, or reduction process subsequently depending upon the availability of oxygen to yield Mn 2 O 3 in air/O 2 and MnO in argon/nitrogen ambient. A partial reaction causes retention of some Mn 3 O 4 alone or Mn 3 O 4 and Mn 5 O 8 together as secondary phase(s). The magnetic susceptibility ( ) data reveal (i) antiferro-magnetic nature for Mn 2 O 3 , Mn 5 O 8 and MnO with Néel temperature 75 K, 131 K and 115 K, respectively, (ii) orthorhombic to cubic crystallographic transition in Mn 2 O 3 at 310 K for the first time, and (iii) ferri-magnetic character for Mn 3 O 4 with Curie temperature 46 K. Further, it is demonstrated that a small quantity of Mn 3 O 4 always remains in the products and can be detected by susceptibility data below 50 K only with bifurcation in ZFC and FC curves and blocking temperature in the range 30-40 K. Also, the decomposition of manganese oxalate at 1100 • C for 4 h in air leads to formation of pure Mn 3 O 4 with saturation magnetization of 1.85 B per molecule and coercivity of 4750 Oe at 10 K.

Ba 1Àx Sr x TiO 3 solid solutions were prepared by solid-state reaction from raw materials. Four compositions with x ¼ 0.25, 0.50, 0.75 and 0.90 have been investigated. The perovskite type and polycrystalline structure of the BST samples were revealed by X-ray diffraction data. The morphology, grain size distribution, porous structure and elemental composition of the sintered ceramics were analyzed by using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) microanalysis. The temperature dependence of permittivity and of dielectric loss tangent at low frequency (1 kHz) showed decrease of Curie temperature with increase of Sr content. Microwave measurements (1 GHz) showed substantial decrease of the dielectric constant from about 1600 to 200 and also of the losses from 12% to less than 2% with the Sr concentration increase from x ¼ 0.25 to x ¼ 0.90. Moreover, the addition of MgO and MnO 2 1 wt.% each, improved sintering process and lowered the microwave losses up to 0.2%.

Intermetallic Heusler alloys are amongst the most attractive half-metallic systems due to the high Curie temperatures and the structural similarity to the binary semiconductors. In this review we present an overview of the basic electronic and magnetic properties of the half-metallic full-Heusler alloys like Co2MnGe. Ab-initio results suggest that the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The total spin magnetic moment in the unit cell, Mt, scales linearly with the number of the valence electrons, Zt, such that Mt = Zt − 24 for the full-Heusler alloys opening the way to engineer new half-metallic alloys with the desired magnetic properties. Moreover we present analytical results on the disorder in Co2Cr(Mn)Al(Si) alloys, which is susceptible to destroy the perfect half-metallicity of the bulk compounds and thus degrade the performance of devices. Finally we discuss the appearance of the half-metallic ferrimagnetism due to the creation of Cr(Mn) antisites in these compounds and the Co-doping in Mn2VAl(Si) alloys which leads to the fully-compensated half-metallic ferrimagnetism.

Manganese-substituted nickel–zinc ferrites have been investigated as power core materials for applications in switched-mode-power supplies. High frequency operation of these power supplies requires high performance cores with low power losses. The main contributors to the power loss are eddy current loss, hysteresis loss and residual loss. The ferrites have been synthesized by the citrate precursor technique and their electromagnetic properties such as resistivity, permeability, saturation magnetization and Curie temperature studied. A power loss of 500 mW/cc could be obtained at a frequency of 500 kHz, flux density of 50 mT and temperature 100 °C.

The generalized Heusler compounds Mn2CoZ (Z = Al, Ga, In, Si, Ge, Sn, Sb) with the Hg2CuTi structure are of large interest due to their half-metallic ferrimagnetism. The complex magnetic interactions between the constituents are studied by 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, while the total moment increases when changing Z from one group to another. 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 decreasing magnetic moments. Curie temperatures of more than 800 K are predicted for Mn2CoAl (890 K), Mn2CoGa (886 K), and Mn2CoIn (845 K).

Intermetallic Heusler alloys are amongst the most attractive half-metallic systems due to the high Curie temperatures and the structural similarity to the binary semiconductors. In this review we present an overview of the basic electronic and magnetic properties of both Heusler families: the so-called half-Heusler alloys like NiMnSb and the the full-Heusler alloys like Co 2 MnGe. Ab-initio results suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The total spin magnetic moment M t scales linearly with the number of the valence electrons Z t , such that M t = Z t − 24 for the full-Heusler and M t = Z t − 18 for the half-Heusler alloys, thus opening the way to engineer new half-metallic alloys with the desired magnetic properties.

The structure of Eurofer 97 has been determined by diffraction and electron microscopy techniques. The magnetic (coercive field, saturation and remanence magnetization and Curie temperature), electrical and thermal properties of Eurofer 97 steel are reported in the temperature range from room temperature up to 900 K. The experimental data are described by empirical equations and are compared with data from similar steel alloys.

The magnetic properties (coercive force, saturation and residual magnetization, and Curie temperature) of an AISI 420 steel were measured for different heat treatment conditions (quenching and tempering). The results show that the material is magnetically softer in the annealed condition and after oil quenching an increase of coercive force ( H ) and residual induction (B ) and a decrease of saturation M c r magnetization (s ) are observed. Tempering in the 300-5008C range promotes a further increase of H and B values. Tempering in the s M c r 500-6508C range promotes an increase of s and a magnetic softening. The magnetization vs. temperature curves of quenched samples s showed a pronounced increase of magnetization before reaching the Curie point. The results are discussed taking the knowledge of the physical metallurgy of the martensitic stainless steels into account.

In this work a simple concept was used for a systematic search for new materials with high spin polarization. It is based on two semi-empirical models. Firstly, the Slater-Pauling rule was used for estimation of the magnetic moment. This model is well supported by electronic structure calculations. The second model was found particularly for Co2 based Heusler compounds when comparing their magnetic properties. It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment.

Determination of Curie temperature by plotting magnetic moment vs. temperature curves requires a small applied field, which influences the measurement and temporarily disturbs the temperature of the sample especially for highly magnetocaloric materials. The Arrott plot technique was therefore used in order to determine the Curie temperature for a magnetocaloric Gd 5 Si 2.7 Ge 1.3 (x ¼ 0.675) single crystal sample. This technique was compared with other methods such as the inflection point technique and the line projection method. The results show how applied magnetic field influences the determination of Curie temperature. Using the Arrott plot the second-order transition Curie temperature for Gd 5 Si 2.7 Ge 1.3 was determined to be 304 K. r

We report on the magnetic and electrical properties of Pr 0.6 Sr 0.4 MnO 3 single crystals. This compound undergoes a continuous paramagnetic-ferromagnetic transition with a Curie temperature T C ∼ 301 K and a first-order structural transition at T S ∼ 64 K. At T S , the magnetic susceptibility exhibits an abrupt jump, and a corresponding small hump is seen in the resistivity. The critical behavior of the static magnetization and the temperature dependence of the resistivity are consistent with the behavior expected for a nearly isotropic ferromagnet with short-range exchange belonging to the Heisenberg universality class. The magnetization (M-H ) curves below T S are anomalous in that the virgin curve lies outside the subsequent M-H loops. The hysteretic structural transition at T S as well as the irreversible magnetization processes below T S can be explained by phase separation between a high-temperature orthorhombic and a low-temperature monoclinic ferromagnetic phase.

Nanocrystalline Al 3+ ions doped Mg 0.2 Mn 0.5 Ni 0.3 Al y Fe 2Ày O 4 compositions, where y = 0.0, 0.05 and 0.10 have been synthesized by citrate precursor method. Crystal structure and magnetic properties have been investigated at 300 K by means of X-ray diffraction, transmission electron microscope (TEM), vibrating sample magnetometer (VSM) and Mössbauer spectra measurements. XRD study reveals that particle size decreases from 102.25 nm to 41.65 nm. A decrease in lattice constant and saturation magnetization was attributed to smaller ionic radius of Al 3+ ions and weakening of super exchange interaction. Experimental and X-ray density decrease with increasing aluminum concentration. Though Mössbauer spectra of y = 0.0 exhibit normal Zeeman split sextets, spectra of samples for y = 0.05 and 0.10 are characterized by simultaneous presence of a central paramagnetic doublet. Dependence of Mössbauer parameters such as isomer shift, quadrupole splitting, linewidth and hyperfine magnetic field on Al 3+ ions concentration have been discussed. Initial permeability 'l i ', saturation magnetization (4 pM S ), retentivity (M R ), Bohr magneton number (n N B ), magneto crystalline anisotropy constant (K 1 ) and magnetic loss decreases while coercivity (H C ) increases with increasing substitution of Al 3+ ions. Magnetic loss has very low value in the range of 10 À3 which is two orders of magnitude less than samples prepared by conventional method.

The colossal piezoelectric effect of relaxor-based ferroelectrics has attracted much attention due to the technological importance of the conversion between electric and mechanical energies. The mechanism of the significant field effect on piezoelectric properties in this disordered system has become one of the current topics in pure and applied physics. The important key factor in the understanding of relaxors is polar nanoregions (PNRs) which appear at temperatures far above the dielectric maximum or Curie temperature. PNRs play a dominant role in the formation of unique characteristics of relaxors and are regarded to be caused by spatial and dynamic heterogeneity. However, a detailed microscopic picture of their evolution depending on temperature or electric field is still the subject of controversy. The elastic and dynamical properties of the Pb(B1/3B′2/3)O3-type relaxor-based ferroelectrics and their solid solutions with pure ferroelectrics have recently been studied extensively by the broadband micro-Brillouin scattering spectroscopy (BMBS) to clarify the role of PNRs. This paper reviews the recent progress in the understanding of fast dynamics of PNRs of typical relaxor-based ferroelectrics of (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 and (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 in the gigahertz frequency range achieved by using BMBS.

Using vibrating sample magnetometery (VSM) 50MeV Li3+ ion irradiation effects on magnetic properties of single crystals of SrGaxInyFe12−(x+ y) O19 (where x= 0, 5, 7, 9; y= 0, 0.8, 1.3, 1.0), are reported. The substitution of Ga and In in strontium hexaferrite crystals ...

The projection of integrating optical, magnetic and electronic functionalities into a single material have aggravated passionate attention in mounting wide band gap diluted magnetic semiconductor (DMS) in the midst of room temperature ferromagnetism. We report the evidence of ferromagnetism in Cu-doped ZnSe quantum dots (QDs) below room temperature, grown from a single source precursor by lyothermal method with the sizes of approximately 3.2-5.14 nm. QDs mainly exhibit paramagnetic behavior between 80 and 300 K, with a weak ferromagnetic/anti-ferromagnetic exchange at lower temperature as observed by superconducting quantum interference device (SQUID) magnetometer. From the Curie-Weiss behavior of the susceptibility, Curie temperature (T c ) of Cu-doped ZnSe sample has been evaluated. From EPR, we obtain the Lande-g factor in the Zeeman interaction term as 2.060. Photoluminescence and EPR measurements support and confirm the view that Cu 2? substitutes for Zn 2? in Cu-doped ZnSe quantum dots.

Soft magnets are extensively deployed in rotating electric machines. Accelerated development of new high Curie temperature, mechanically strong, magnetically soft magnets is urgently needed for next generation rotating electrical machines. The ternary Fe-Co-Ni alloy system was investigated. Spark plasma sintering (SPS) was employed to process compositionally graded (Ni-21Fe)-xCo samples. A combinatorial assessment of structural, magnetic, mechanical and electrical properties of sample sections was performed. Co-lean alloys exhibited a good combination of magnetic, mechanical, and electrical properties but low Curie temperatures. On the other hand, Co rich alloys exhibited elevated T c , high M s , and large hardness. An alloy library with a wide range of properties, M s : 100-150 emu/g, H c : 0.8-30 Oe, T c : 571-1108 °C, ρ: 9-12.9 μΩ-cm, nanohardness: 0.96-1.56 GPa was developed. Thus, SPS processed samples can be employed for the accelerated screening of magnetic materials.

The correlated motion of electrons in multi-orbital metallic ferromagnets is investigated in terms of a realistic Hubbard model with {\cal N}-fold orbital degeneracy and arbitrary intra- and inter-orbital Coulomb interactions U and J using a Goldstone-mode-preserving non-perturbative scheme. An effective quantum parameter '\hbar'=\frac{U^2+({\cal N}-1)J^2}{(U+({\cal N}-1)J)^2} is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large {\cal N}, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. This approach is illustrated for the case of ferromagnetic iron and the half metallic Heusler alloy Co_2 Mn Si. For realistic values for iron, the calculated spin stiffness and Curie temperature values obtained are in quantitative agreement with measurements. Significantly, the contribution of long wavelength modes is shown to yield a nearly ~25% reduction in the calculated Curie temperature. Finally, an outline is presented for extending the approach to generic multi-band metallic ferromagnets including realistic band-structure features of non-degenerate orbitals and inter-orbital hopping as obtained from LDA calculations.

The basal depth of the outer layer with internal magnetic sources was calculated from magnetic data available within a roughly 500 km wide and 1200 km long area, running from central Germany to southern Italy. The dataset, deriving from different aeromagnetic surveys, is reduced to the reference altitude of 3000 m a.s.l. and a reference year of 1980.0. The adopted method, which transforms the spatial data into the frequency domain, provides a relationship between the two-dimensional spectrum of the magnetic anomalies and the top and centroid depths of the magnetic sources. The magnetic layer bottom depth (MLBD) thus obtained is 29-33 km deep in the stable areas (central Europe Variscan units, Corsica-Sardinia Variscan block) and corresponds to the Moho, having an average temperature of 560 • C. From the Alps to the Apennines, MLBD ranges between 22 and 28 km and is clearly shallower than the Moho. In these units, the wide variation of MLBD appears to be compatible with the presence of shallow magnetised bodies, consisting of lower crustal rocks (Ivrea-Verbano zone), ophiolitic units (Penninic zone and Voltri Massif) and intrasedimentary basic volcanic bodies (Po Basin). An average value of 25 km can be attributed to MLBD, which corresponds to a temperature of 550 • C. In the peri-Tyrrhenian zone and the Ligurian Sea, MLBD is below the Moho, which ranges from 17 to 20 km depth, and it has a temperature matching approximately to the Curie temperature of magnetite (580 • C).

Mn x Zn 1−x Fe 2 O 4-based magnetic fluids with x = 0.1-0.9 are synthesized by coprecipitation. The samples are heated in a radio frequency (rf) magnetic field using an rf generator at different powers, and the temperature is measured as function of time using an optical thermometer. The heating effect of the dispersed magnetic nanoparticles is proportional to the imaginary part of the dynamic magnetic susceptibility of the ferrofluid, a quantity that depends on the temperature through the magnetization of the ferrite nanoparticles and the Néel or Brownian relaxation times, respectively. We propose an extrapolation method to actuate the Curie temperatures of the dispersed magnetic nanoparticles. By means of appropriate fitting functions for (dT /dt) versus T for both the heating and the cooling process, we deduce the Curie temperature of the samples under investigation. For Mn x Zn 1−x Fe 2 O 4-based magnetic nanoparticles the Curie temperatures decrease with increasing Zn content. They turn out to be lower than the literature values for bulk Mn x Zn 1−x Fe 2 O 4 , a phenomenon which is generally observed for phase transitions of nanocrystalline materials.

The emergence of the field of spintronics brought half-metallic ferromagnets to the center of scientific research. A lot of interest was focused on newly created transition-metal pnictides (such as CrAs) and chalcogenides (such as CrTe) in the metastable zinc-blende lattice structure. These compounds were found to present the advantage of high Curie temperature values in addition to their structural similarity to semiconductors. Significant theoretical activity has been devoted to the study of the electronic and magnetic properties of these compounds in an effort to achieve a better control of their experimental behaviour in realistic applications. This review is devoted to an overview of the studies of these compounds, with emphasis on theoretical results, covering their bulk properties (electronic structure, magnetism, stability of the zincblende phase, stability of ferromagnetism) as well as low-dimensional structures (surfaces, interfaces, nanodots and transition-metal delta-doped semiconductors) and phenomena that can possibly destroy the half-metallic property, like structural distortions or defects.

Nanoparticles of Co1−xZnxFe2−xCrxO4 (x = 0.0–0.5) ferrites were prepared by chemical co-precipitation technique using metal sulphates. The structural and magnetic properties were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and AC susceptibility measurements. X-ray diffraction patterns indicate that the samples possess single phase cubic spinel structure. The lattice constant initially increases for x ≤ 0.3 and thereafter for x > 0.3 it decreases with increasing x. The saturation magnetization (Ms), magneton number (nB) and coercivity (Hc) decreases with increasing Cr–Zn content x. Curie temperature deduced from AC susceptibility data decreases with increasing x.► Preparation of nanostructured ferrite particles. ► Co-substitution of Zn2+ and Cr3+ in place of Co2+ and Fe3+ ions. ► Structural properties markedly vary with increasing Cr–Zn ions. ► Magnetic properties decrease with increasing Cr–Zn ions.

We report the results of magnetization, zero-field resistivity, and magnetoresistance measurements in the ferromagnetic Heusler alloy Co 2 TiSn. There is a striking change in the character of electron transport as the system undergoes the paramagnetic to ferromagnetic transition. In the paramagnetic state, the nature of the electron transport is like that of a semiconductor. This changes abruptly to metallic behavior at the onset of ferromagnetic ordering. Application of external magnetic field tends to suppress this semiconducting-like transport leading to a negative magnetoresistance that reaches a peak in the vicinity of the Curie temperature. Comparison is made with the similar unusual behavior observed in other systems, including UNiSn and manganites.

The main phase in as-cast SmCo 7Àx Ti x (x ¼ 0:3; 0.4, 0.5, 0.6) magnets is found to have the TbCu 7 -type structure. The c=a ratio decreases with increasing Ti concentration x; but the unit cell volume increase with x: A minor amount of a rhombohedral Th 2 Zn 17 -type phase was detected at x ¼ 0:3; 0.4, and an another Ce 2 Ni 7 -type phase with low Curie temperature appeared when x exceeds 0.5. No more than 0.1 T of intrinsic coercivity was obtained in the as-cast samples. After annealing, the TbCu 7 -type SmCo 7Àx Ti x magnets decomposed into 1 : 5 and 2 : 17 phases, with a slight increase in coercivity compared with the corresponding as-cast samples up to a maximum of 0.24 T. The as-milled amorphous SmCo 7Àx Ti x magnets recrystallised with TbCu 7 -type structure after annealing at 7501C for 2 h. High intrinsic coercivities of more than 2.0 T were obtained for almost all compositions, and maximum coercivity of 2.5 T was found for the ball-milled SmCo 6.6 Ti 0.4 magnet. The high coercivity of the ball-milled SmCo 7Àx Ti x magnets is attributed to the strong pinning of the walls of the 'interaction domains' by the network of grain boundaries between the nanocrystalline grains. r

La 0.7 Sr 0.3 MnO 3 polycrystalline manganite thin films were grown on silicon (Si) substrates covered by SiO x amorphous native oxide. Curie temperatures of about 325 K were achieved for 70-nm-thick films. Strong room temperature XMCD signal was detected indicating high spin polarization at the surface. Cross-sectional TEM images show sharp interface between SiO x and manganite without signature of chemical reaction at the interface. Unusual sharp splitting of the manganite film was observed: on the top of a transition layer characterized by low crystalline order, a magnetically robust layer is formed. r

To understand the nature of grain boundaries in polycrystalline materials, magneto-transport and ferromagnetic resonance measurement have been performed in polycrystalline La 0.6 Pb 0.4 MnO 3 (LPMO) thin films prepared by pulsed laser deposition. Films are found to undergo a semiconductor to metal transition at 230 K and re-enter into the semiconducting state below 130 K. Microwave absorption measurements carried out as function of applied field show two components of resonant absorption signal. First component is in accordance with ferromagnetic transition of grains at Curie temperature and the second component shows antiferromagnetic transition of grain boundaries at 160 K. An additional non-resonant absorption signal centered at zero field has also been observed that supports transition from conducting to insulating grain boundaries at w160 K. Further, temperature dependence of resistance in semiconducting state at low temperatures is in accordance with coulomb blockade model indicating insulating nature of AFM grain boundaries.

The collective behavior of spins in a dilute magnetic semiconductor is determined by their mutual interactions and influenced by the underlying crystal structure. Hence, we begin with the atomic quantum-mechanical description of this system using the proposed variational-perturbation calculus, and then turn to the emerging macroscopic picture employing phenomenological constants. Within this framework we study spin waves and exchange stiffness in the p-d Zener model of (Ga,Mn)As, its thin layers and bulk crystals described by the spds * tight-binding approximation. Analyzing the anisotropic part of exchange, we find that the Dzyaloshinskii-Moriya interaction may lead to a cycloidal spin arrangement and uniaxial in-plane anisotropy of diagonal directions in thin layers, resulting in a surface-like anisotropy in thicker films. We also derive and discuss the spin-wave contribution to magnetization and Curie temperature. Our theory reconstructs the values of stiffness determined from the temperature dependence of magnetization, but reproduces only partly those obtained from analyzing precession modes in (Ga,Mn)As thin films.

Europium-doped cubic barium titanate (BT) nanocrystals with % [Eu/Ti] mol ratio varying from 0.05 to 0.25 were prepared through hydrothermal route. The nano nature of these powders was confirmed by XRD and TEM studies. Pellets were prepared after calcining the powders at 1000 °C for 2 h. These pellets were annealed at 200, 500, 700 and 1000 °C for 2 h at each temperature and used for dielectric measurements. Raman spectra of two typical pellets with %[Eu/Ti] Eu/Ti mol ratios of 0.15 and 0.25 showed all the peaks characteristic of tetragonal BaTiO3. Pure BT showed a low dielectric constant (DC) with a value of 398. Doping with small amounts of Eu resulted in many fold increase of DC values. A maximum value of 10576 at 1 KHz frequency was observed for the sample with % [Eu/Ti] mol ratio of 0.15. Lowering of Curie temperature Tc (95 to 110 °C) was observed for pure as well as Eu-doped barium titanate.

If magnetic semiconductors are ever to find wide application in real spintronic devices, their magnetic and electronic properties will require tailoring in much the same way that bandgaps are engineered in conventional semiconductors. Unfortunately, no systematic understanding yet exists of how, or even whether, properties such as Curie temperatures and bandgaps are related in magnetic semiconductors. Here we explore theoretically these and other relationships within 64 members of a single materials class, the Mn-doped II-IV-V(2) chalcopyrites (where II, IV and V represent elements from groups II, IV and V, respectively); three of these compounds are already known experimentally to be ferromagnetic semiconductors. Our first-principles results reveal a variation of magnetic properties across different materials that cannot be explained by either of the two dominant models of ferromagnetism in semiconductors. On the basis of our results for structural, electronic and magnetic properti...

Intermetallic Heusler alloys are amongst the most attractive half-metallic systems due to the high Curie temperatures and the structural similarity to the binary semiconductors. In this review we present an overview of the basic electronic and magnetic properties of the half-metallic full-Heusler alloys like Co$_2$MnGe. Ab-initio results suggest that the electronic and magnetic properties in these compounds are intrinsically related