Showing papers on "Curie temperature published in 1991"

Organic molecular soft ferromagnetism in a fullerene c60.

Abstract: The properties of an organic molecular ferromagnet [C(60)TDAE(086); TDAE is tetrakis(dimethylamino)ethylene] with a Curie temperature ;T(c) = 161 kelvin are described The ferromagnetic state shows no remanence, and the temperature dependence of the magnetization below ;T(c) does not follow the behavior expected of a conventional ferromagnet These results are interpreted as a reflection of a three-dimensional system leading to a soft ferromagnet

Self-regulating heater utilizing ferrite-type body.

Abstract: A self-regulating heater is provided by placing ferrite-type body member, which is highly lossy when exposed to a high frequency magnetic field and has a predetermined Curie temperature, on or around a central conductor which is connected or is adapted to be connected to a power source which provides high frequency alternating current to the conductor. The current passing through the central conductor produces a magnetic field around the conductor, which causes the ferrite-type body to be heated by internal losses to its Curie temperature. The heater self-regulates at the Curie temperature of the ferrite-type body. The power source is preferably a constant current, impedance matched power source. The ferrite-type body member can be ferromagnetic or ferrimagnetic. The ferrite-type body is preferably ferrimagnetic, such as ferrite beads, rings, and the like, which heat by hysteresis losses.

Texturing of magnetic materials at high temperature by solidification in a magnetic field

Abstract: THE ability to impose a preferred orientation, or 'texture', on a crystalline material is important in many fields of materials science. In general, crystalline materials are more or less anisotropic in their properties, depending on their lattice structure, and texturing allows the most favourable direction (for example, for current flow or magnetic susceptibility) to be used in applications. Ferromagnetic materials can be textured by 'magnetic annealing'—the orientation of powdered material in a magnetic field, usually followed by a sintering step—but this must be done at temperatures below the material's Curie temperature. Here we present a new method of texturing materials that have a residual anisotropy in their magnetic susceptibility at high temperature, by solidification in a magnetic field. This one-step process, which may be called 'paramagnetic annealing', is demonstrated by application to the high-temperature superconductor YBa2Cu3O7.

Giant magnetic moment and other magnetic properties of epitaxially grown Fe16N2 single‐crystal films (invited)

Abstract: Single‐crystal Fe16N2 films have been grown epitaxially on Fe(001)/InGaAs(001) and InGaAs(001) substrates by molecular beam epitaxy (MBE). Saturation flux density Bs of Fe16N2 films has been demonstrated to be 2.8–3.0 T at room temperature, which is very close to the value obtained by Kim and Takahashi using polycrystalline evaporated Fe–N films. Temperature dependence of Bs has been measured. Bs changed with temperature reversibly up to 400 °C, while beyond 400 °C, Bs decreased irreversibly. X‐ray diffraction showed that Fe16N2 crystal is stable up to 400 °C, while beyond 400 °C, Fe16N2 dissolves into Fe and Fe4N, and also some chemical reactions between Fe16N2 and the substrate occurs. This caused the temperature dependence of Bs mentioned above. From the temperature dependence of Bs up to 400 °C, the Curie temperature of Fe16N2 is estimated to be around 540 °C by using the Langevin function. The above mentioned Bs of 2.9 T at room temperature and 3.2 T at −268 °C corresponded to an average magnetic mom...

Structural and hard magnetic properties of rapidly solidified Sm-Fe-N

Abstract: Sm–Fe alloys have been produced by rapid quenching and the resulting phases have been investigated in the as‐quenched state and after nitriding. Besides the well‐known equilibrium phases of the binary Sm–Fe system (Fe, Sm2Fe17, SmFe3, SmFe2 and Sm), a hexagonal TbCu7‐type phase shows up in melt spun ribbons (a=4.88 A, c=4.23 A). Its stoichiometry is about Sm1Fe9 and it is formed only at wheel velocities above 15 m/s. The Curie temperature and the saturation polarization of this new phase is 210 °C and 1.25 T, respectively. At higher Sm concentrations or lower quenching rates the structure changes to the Th2Zn17‐type. The Th2Zn17‐type ribbons are magnetically soft whereas the TbCu7‐type samples show moderate coercivities of up to 1.7 kA/cm. Nitrogenation leads to an expansion of the lattice and to an overall improvement of the hard magnetic properties for both phases. Their Curie temperatures are increased to 470 °C and the saturation polarizations are raised to 1.40 and 1.51 T for the TbCu7‐ and the Th2Zn17‐type phases, respectively. The best hard magnetic properties for isotropic TbCu7‐type material are obtained for quenched, annealed, and nitrided Sm10.6Fe89.4 which shows a coercivity, Hci, of 4.9 kA/cm, a remanence, Jr, of 0.86 T and an energy product, (BH)max, of 69.6 kJ/m3. For similarly treated Sm12Fe88, which crystallizes in the Th2Zn17 structure, a coercivity of 16.7 kA/cm, a remanence of 0.73 T and an energy product of 65.6 kJ/m3 are achieved.

Stress induced shift of the Curie point in epitaxial PbTiO3 thin films

Abstract: A 50 °C shift in Curie temperature has been observed for c‐axis oriented PbTiO3 thin films using x‐ray diffraction. An analysis of the electrostrictive strain based on the Devonshire thermodynamic formalism showed that the shift in the Curie point for these films can be plausibly explained by an effective two‐dimensional compressive stress of ≊400 MPa. The single‐domain, single‐crystal dielectric susceptibility (η33) and piezoelectric coefficient (d33) were calculated and found to be relatively unaffected, at room temperature, by a compressive stress of this magnitude.

Alloys and compounds of 3d elements with main group elements

Abstract: Survey on tables and figures containing data on alloys and compounds between 3d-elements and Cu, Ag and Au, respectively.

Magnetic and crystallographic properties of novel Fe‐rich rare‐earth nitrides of the type RTiFe11N1−δ (invited)

Abstract: We succeed in inserting a number of nitrogen atoms into the RTiFe11 intermetallics. The nitrides retain the ThMn12‐type structure, but with an increase in the unit cell volume. The crystallographic sites located by nitrogen atoms are determined by using neutron diffraction techniques. The nitrogen atoms are found to have an effect of increasing Curie temperature and saturation magnetization. Moreover, an essential change in magnetocrystalline anisotropy is observed upon nitrogenation. By all of these effects, the NdTiFe11N1−δ compounds have excellent intrinsic magnetic properties favorable for permanent magnet applications.

Magnetic properties of a new family of ternary rare‐earth iron nitrides R2Fe17N3−δ (invited)

Abstract: A series of interstitial ternary nitrides R2Fe17N3−δ has been prepared via a gas‐solid reaction for R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu, and Y. These nitrides have structures related to the Th2Zn17 or Th2Ni17 structures of the R2Fe17 parent compounds, but the unit cell volumes are 6%–7% greater, and the Curie temperatures are approximately 400 K higher. The Fe‐Fe exchange interactions are increased by a factor of 2.8 by nitrogenation, whereas the R‐Fe exchange interactions are little changed. All compounds exhibit easy‐plane anisotropy at room temperature, except for Sm2Fe17N3−δ, which shows strong uniaxial anisotropy and may be used to make permanent magnets. The Er and Tm compounds exhibit spin reorientations below room temperature. The anisotropy due to the iron sublattices is easy plane (K1 = −1.3 MJ/m3 at 4.2 K for Y2Fe17N3), but it changes sign to easy axis with cobalt substitution (K1 ≊ 1.0 MJ/m3 at 4.2 K for Y2(Fe1−xCox)17N3−δ when x≥0.2).

Decamethylmanganocenium tetracyanoethenide, [Mn(C5Me5)2]..+[TCNE].- - a molecular ferromagnet with an 8.8-K TC

Abstract: Communication Triplet electron- transfer salts are predicted to exhibit bulk ferromagnetic behavior at higher temperatures than their singlet analogues. To test this expected trend, the title compound, which is isostructural with [FeCp 2 ] ⊙⊕ [TCNE] ⊙⊖ (Cp* = pentamethylcyloopentadienide; TCNE = tetracyanoethylene) which exhibits a Curie temperature, T c , of 4.8 K has been synthesized. The synthesis and magnetic characterization of the new compound, which is found to have a T c of 8.8 K, are reported.

Anisotropy compensation and magnetostriction in TbxDy1−x(Fe1−yTy)1.9 (T=Co,Mn)

Abstract: From magnetization (M) and magnetostriction (λ) measurements as a function of magnetic field and stress, the temperatures of anisotropy compensation, Tm, for technologically important TbxDy1−x(Fe1−yTy)1.9 [T=Co,Mn (0.3≤x≤0.5) (0≤y≤0.3)] were determined. Measurements of M and λ encompassing Tm were made under compressive stresses from 8.8 to 36 MPa and for temperatures from −196 to +130 °C. In agreement with earlier measurements, Tm decreases with increasing Tb. Substitution of Mn for Fe for fixed x also decreases Tm. In contrast with these observations is the increase of the anisotropy compensation temperature with the replacement of Fe by small amounts of Co. In the cases of both (1) increasing Tb content and (2) increasing Co content, the Curie temperature TC increases, yielding, in general, a higher magnetic moment and saturation magnetostriction of these alloys. Thus, compensation at a given temperature may be obtained in an improved class of Laves phase compounds, R(1)xR(2)1−x(Fe1−yCoy)2, where rare ...

Magnetic phase transition of ultrathin Fe films on Ag(111).

Abstract: High-quality, single-crystalline films of Fe(110)/Ag(111) were grown by molecular-beam epitaxy in the range of \ensuremath{\sim}1\char21{}3 monolayers of Fe, and investigated in situ by means of the surface magneto-optic Kerr effect. The magnetization exhibits a second-order phase transition at the thickness-dependent Curie temperature. The value of the critical exponent \ensuremath{\beta} of 0.137\ifmmode\pm\else\textpm\fi{}0.008 is in good agreement with that of the two-dimensional Ising model.

The magnetic structure of DyMn2

Abstract: The evolution with temperature of the magnetic structure of the C15 Laves-phase compound DyMn2 has been studied using powder neutron diffraction and magnetization techniques. The Dy sublattice assumes a spin-canted ferromagnetic structure with 8.8 mu B per Dy atom. Although all Mn sites within the unit cell are chemically equivalent, only one Mn atom in four is found to possess a magnetic moment (of 1.4 mu B). These magnetic Mn atoms are located at sites with a strongly polarizing magnetic environment resulting from a near-neighbour configuration of ferromagnetically coupled Dy spins. A spin reorientation is observed at 36 K, and is accompanied by a small thermal expansion anomaly. The Curie temperature of DyMn2 is found to be 45 K.

The electronic structure of ordered binary Co-Pt compounds

Abstract: In this article the authors present the results of spin-polarized band-structure calculations on the ordered binary compounds of Co-Pr Comparison is made with experimental values for the magnetic moments The results of these calculations show a nonlocal magnetic moment behaviour in these systems Furthermore the authors find an almost constant number of holes in the Pt 5d band for the different compounds The nature of the magnetic interactions is investigated The coupling between the Co atoms can be described with the Heisenberg Hamiltonian and the exchange coupling parameters are evaluated With these exchange parameters the authors are able to calculate the Curie temperature of ordered and disordered Co-Pt alloys

Effect of nitrogen on the structural and magnetic properties of intermetallic compounds with the ThMn12 structure

Abstract: The effects of nitrogen uptake on the structural and magnetic properties of RFe12−xTx alloys have been studied. Several interstitial nitrides with composition RFe12−xTxNy (R=Nd and Sm and T=Ti and V) were prepared by heat treating fine powders of RFe12−xTx alloys in N2 gas. Preliminary results show the following observations: (1) The ThMn12 structure of the original compounds is retained with the addition of nitrogen but with increased lattice constants (a and c). (2) The Curie temperature is greatly enhanced by about 150–175 K. (3) The character of magnetic anisotropy is changed from ‘‘weak uniaxial’’ type to ‘‘strong uniaxial’’ type for NdFe12−xTxNy and from ‘‘uniaxial’’ to ‘‘planar’’ for SmFe12−xTxNy.

Spin-wave, Stoner single-particle and correlated particle-hole pair contributions to thermal demagnetization in amorphous Fe90+xZr10-x

Abstract: Detailed magnetization (M) measurements have been performed on amorphous (a-)Fe90+xZr10-x alloys with x=0 and 1 at temperatures ranging from 4.2 to 300 K in external magnetic fields (H) up to 15 kOe. Arrott plot isotherms of M2(H,T) against H/M(H,T) are nearly linear at high magnetic fields for temperatures well outside the critical region. Spontaneous magnetization varies with temperature as M(0,T)/M(0,0)=1-BT32/-AT2, (M(0,T)/M(0,0))2=a-bT2 and (M(O,T)/M(0,0))2=a'-b'T43/ in the temperature intervals 0

Sm2(Fe,M)17Nx compounds and magnets

Abstract: The effect of additives has been studied for the Sm2Fe17Nx compounds. Only Co addition increases the Curie temperature. The Ti, V, or Co additions increase the anisotropy field. The additions of these elements are favorable for the magnet fabrication. The magnet fabrication has been carried out by the Zn metal bonding and mechanical alloying. The (BH)max is 8.7 MGOe and the iHc is 14 kOe for the Zn‐bonded magnet. The (BH)max is 10.8 MGOe and the iHc is around 25 kOe for the mechanical alloy magnet.

Intrinsic magnetic properties of R2Fe17CyNx compounds: (R=Y, Sm, Er, and Tm)

Abstract: Samples of R2Fe17C(y)N(x) (R = Y, Sm, Er, Tm) were prepared by arc melting appropriate amount of R, Fe, and C, vacuum annealing at 1373 K and finally annealing at 740 K in nitrogen for 10 h. The magnetic properties of these compounds were investigated by means of ac initial susceptibility, magnetization measurements, and x-ray diffraction. The thermal stability of the nitride phase was studied by differential scanning calorimetry. It was found that, when heated above 600 K, R2Fe17C(y)N(x) irreversibly decomposes N which is irrespective of the carbon concentration and rare-earth element. The Curie temperatures of R2Fe17C(y)N(x) are independent of the carbon concentration and are approximately 400 K higher than those of the corresponding pure R2Fe17 compounds. However, the Curie temperatures cannot be correlated to the composition x of the initial R2Fe17C(y)N(x) compounds at room temperature because some N was lost during the heating to T(c). In the Er and Tm compounds spin reorientation transitions were found, marking the change of the easy magnetization direction from the c axis to the basal plane with increasing temperature. The Tm compounds show an additional magnetic transition at low temperatures (below 40 K). A coexistence of the hexagonal and the rhombohedral structural modifications was found in Er2Fe17C(y)N(x) when y < 1.5, characterized by two different spin reorientation temperatures. The anisotropy fields of Sm2Fe17C(y)N(x) are higher than that of Sm2Fe17N(x). Indications of a magnetic phase transition were found also in Sm2Fe17C0.7N(x) and Sm2Fe17C0.9N(x).

The anisotropy of r2fe17cx (r = sm and tm)

Abstract: The temperature dependence of the anisotropy field of Sm2Fe17C(x) and Tm2Fe17C(x) has been determined over the entire uniaxial ferro- or ferrimagnetic temperature range using the singularity point detection technique. The temperature dependence of the ac initial susceptibility was measured on these two systems from 4.2 to 300 K. The Curie temperature, lattice constants, spin-reorientation temperature (of Tm2Fe17C(x)) and the magnetic anisotropy field are increasing with increasing carbon concentration. A "magnetic-phase-like transition" is observed in Sm2Fe17C(x) for x < 0.7 at low temperatures, which is proved not to be a spin reorientation. Additionally, the transition temperature increases systematically with increasing carbon concentration. However, the nature of this transition is not yet well understood.

Effect of composition on the magnetic properties of (Ce1−xNdx)13.5 (Fe1−y−zCoySiz)80B6.5 sintered magnets

Abstract: The influences of varying cerium, neodymium, cobalt, and silicon concentrations on the magnetic properties of Ce‐Fe‐B type magnets were investigated. The results indicated that magnets containing 13.5 at.% of cerium exhibits superior magnetic properties to those exhibited by magnets with higher rare‐earth element compositions. High‐temperature annealing of the cast alloy containing 13.5 at.% cerium at 1010 °C for 120 min results in the elimination of the α‐Fe phase. Increasing the cobalt concentration from 0 to 17 at.% raises the Curie point from 174 °C to 324 °C. The addition of 1 at.% of silicon to the alloy improves intrinsic coercivity by up to 100%. The substitution of up to 5.4 at.% of cerium by neodymium increases magnetic properties of sintered magnets with a 13.5 at.% total rare‐earth content. Neodymium contents exceeding 5.4 at.% decrease coercivity. The highest maximum energy product of 27.2 MGOe was exhibited by magnets with a composition of Ce8.1Nd5.4Fe62Co17SiB6.5.

Modification of the magnetic properties and corrosion resistance of Nd-Fe-B permanent magnets with addition of cobalt

Abstract: The effect of partial substitution of iron by cobalt on the Curie temperature, demagnetization curves and corrosion behaviour under various conditions of sintered Nd 16 Fe 76−x Co x B 8 ( O ⩽ x ⩽ 30) permanent magnets was investigated. The microstructure and chemical composition of phase structures were also determined. A new intergranular Nd(Fe, Co) 2 B 2 phase was found for x ⩾ 20.

Novel Permanent Magnetic Materials

Abstract: Materials for high-performance permanent magnets must have a high Curie temperature, a large spontaneous magnetization and strong uniaxial magnetocrystalline anisotropy. Binary rare earth iron intermetallics are deficient in one or more of these respects. New series of rare-earth intermetallic compounds may be generated from existing series by a process known as gas-phase interstitial modification, whereby interstitial atoms such as nitrogen or carbon are introduced into the structure by reaction of a powder of the intermetallic with an appropriate gas. The interstitials occupy sites adjacent to the rare earth, thereby creating a crystal field that reflects the new local symmetry. They also expand the lattice, which greatly enhances the Fe-Fe exchange interactions, and may increase slightly the iron moment. These effects are discussed in detail for four systems, R2Fe17N3-δ, R2Fe17C3-δ R(Fe11 Ti)N1-δ and R(Fe11 Ti)C1-δ (R = rare earth), all of which include compounds with potential for making new parmanent magnets.