M. Gjoka

High coercivity cobalt carbide nanoparticles processed via polyol reaction: a new permanent magnet material

Cobalt carbide nanoparticles were processed using polyol reduction chemistry that offers high product yields in a cost effective single-step process. Particles are shown to be acicular in morphology and typically assembled as clusters with room temperature coercivities greater than 3.4 kOe and maximum energy products greater than 20 kJ m−3. Consisting of Co3C and Co2C phases, the ratio of phase volume, particle size and particle morphology all play important roles in determining permanent magnet properties. Further, the acicular particle shape provides an enhancement to the coercivity via dipolar anisotropy energy as well as offering potential for particle alignment in nanocomposite cores. While Curie temperatures are near 510 K at temperatures approaching 700 K the carbide powders experience an irreversible dissociation to metallic cobalt and carbon thus limiting operational temperatures to near room temperature. These findings warrant more extensive investigation of this and other magnetic carbide systems in which particle size, chemistry and morphology are optimized.

Structure and magnetic properties of RCo7-xMnx alloys (R = Sm, Gd; x = 0.1-1.4)

Abstract Alloys with starting stoichiometry RCo 7− x Mn x (R=Sm, Gd; x =0.1–1.4) were prepared by arc melting. The effect of Mn doping on the formation of the TbCu 7 -type structure and magnetic properties are studied. The SmCo 7− x Mn x as cast alloys with x c -axis as determined by X-ray diffraction on magnetically oriented samples.

Structural and intrinsic magnetic material parameters of Pr3(Fe,Ti)29 and Pr3(Fe,Ti)29Nx

We report the study of the structural and the intrinsic magnetic properties of the Pr member of the newly discovered class of R3(Fe,Ti)29 compounds and its nitride. The X-ray powder diffraction pattern of the alloy is indexed in monoclinic symmetry with lattice parameters a = 10.647(1) A, b = 8.6014(7) A, c = 9.755(1) Aand β = 96.92(1)° and the structure is described in the A2/m space group. Atomic positions and bond lengths are given. Nitrogenation results in a lattice expansion of 6.6% corresponding to ∼ 4N atoms per formula unit. The Curie temperature is 392(5) K, and the saturation magnetization, the anisotropy field and the average hyperfine field at room temperature are 135.4 A m2/kg, 3.9 and 20.3 T, respectively. A magnetic phase transition is observed at ∼ 160 K. After nitrogenation the Curie temperature increases to 721(5) K, and the saturation magnetization to 174.8 A m2/kg, the anisotropy field 7.2 T and the average hyperfine field 30.1 T at room temperature. Mossbauer spectroscopy, X-ray powder diffraction and magnetization measurements on magnetically oriented powder samples provide evidence of the presence of an easy-cone-type magnetocrystalline anisotropy for both the parent and nitrided compounds in the temperature range 85–300 K. The cone angles calculated from the fitted Mossbauer spectra are 34° for the parent compound and 36° for the nitrided compound.

Structural and magnetic properties of Nd3(Fe1−xCox)27.7Ti1.3 (0<x≤0.4) alloys

Abstract Structural and magnetic properties of a novel series of intermetallic compounds, with nominal stoichiometry Nd 3 (Fe 1− x Co x ) 27.7 Ti 1.3 (0 x ≤0.4) are presented. The samples crystallise in the Nd 3 (Fe,Ti) 29 -type structure with monoclinic symmetry (space group A2/m ). The unit cell volume is decreasing as the Co content increases; the cell parameters show anisotropic decrease with the Co content. The Curie temperature increases monotonically with x from 437 to 878 K and the room temperature saturation magnetisation increases from 143.3 for x =0 to 172.5 Am 2 /kg for x =0.3 and remains practically the same for x =0.4. For x =0 and 0.1 a tilted magnetic structure is observed. For x ≥0.2 the compounds present an easy-magnetisation direction along the [4 0 −2] direction. Ac susceptibility curves in the whole range of the Co content ( x =0–0.4) reveal a broad transition at about 160 K, whereas for x =0–0.2 a sharp one with the corresponding transition temperature decreasing with increasing Co content. The observed changes of the critical temperatures observed in the ac susceptibility curves and the obtained anisotropy field values are related to the change of the magnetic anisotropy at x =0.2. The average hyperfine field values depend on the Co content in a way similar to the dependence of the saturation magnetisation.

Ab initio crystal structure solution of the novel intermetallic compound Nd3(Fe,Ti)29

X-ray powder diffraction data have been used for the ab initio crystal structure solution of the novel intermetallic compound Nd3(Fe,Ti)29. The structure has been determined by a combined use of the Patterson and Fourier synthesis methods and the Rietveld decomposition formula. The compound crystallizes in the monoclinic A2/m space group: a = 10.6463(7) A, b = 8.5958(6) A, c = 9.7534(7) A, β = 96.94(1)°. The 6 Nd atoms per cell occupy two crystallographically different sites (2a, 4i) and the 58 (Fe,Ti) atoms are distributed in 11 crystalographically different sites. The structure is a mixture of the tetragonal ThMn12 and the rhombohedral Th2Zn17 types of structures.

Synthesis and magnetic properties of R3(Fe,Ti)29 and R3(Fe,Ti)29Nx (R=Ce,Pr,Gd)

Abstract R 3 (Fe,Ti) 29 and R 3 (Fe,Ti) 29 N x nitrides (R = Ce,Pr,Gd) have been synthesized. The structure is described in the A 2/m space group. The unit cell dimensions change as a function of rare-earth reflecting the lanthanide contraction, except for Ce, which has anomalously low lattice parameters. The unit cell volumes of the nitrides are 5–7% greater than those of the parent alloys. The Curie temperature increases from Ce to Gd with values of 330, 392 and 524 K for Ce, Pr and Gd respectively, and is enhanced by nitrogenation to 670 K for Ce, 721 K for Pr, and 745 K for Gd. The room temperature magnetization at 5 T is 92.0, 130.75 and 86.58 Am 2 /kg and increases to 151.88, 164.47 and 118.0 Am 2 /kg after nitrogenation for Ce, Pr and Gd respectively.

Synthesis and magnetic properties of rare earth–iron–chromium phases and their nitrides

Synthesis, structural, and magnetic properties of R3(Fe,Cr)29 intermetallic compounds (R=Ce,Nd) and their nitrides are reported. The Ce3(Fe,Cr)29 sample was single phase while the R=Nd ingot contained significant contamination by secondary phases. X‐ray analysis reveals the monoclinic Nd3(Fe,Ti)29‐type structure with space group A2/m. The unit cell dimensions for the parent R=Ce compound are a=10.513(2) A, b=8.478(1) A, c=9.657(1) A, β=96.75(1)°, and V=854.70 A3. In the case of Ce a relative unit cell expansion of 6.5% is observed upon nitrogenation. The Curie temperature is 326 K for Ce and 432 K for Nd and becomes 612 and 633 K for Ce–nitride and Nd–nitride, respectively. The room temperature magnetization at 5 T is 58.2 A m2/kg for Ce and 104.1 A m2/kg for Nd and increases after nitrogenation to 109.8 and 127.2 A m2/kg for Ce and Nd, respectively. The average hyperfine field of the R=Ce compound at 85 K is 19.2 T. X‐ray diffraction measurements on a magnetically aligned powder of Ce3(Fe,Cr)29 indicate ...

Phase stability, structure and magnetic properties of R3(Fe, TM)29, (R=Gd, Dy, Er, Y and Tm=V, Ti) compounds with disordered structures

Abstract The R 3 (Fe,V) 29 compounds, with R=Dy, Er, Y, and the Gd 3 (Fe, TM) 29 , with TM=Ti and V, after heat treatment between 1123 and 1233 K form disordered structures that belong in the CaCu 5 type family. The compounds with R=Y or Dy and TM=V crystallize in a disorder CaCu 5 structure type (space group (S.G.): P6/mmm with cell dimensions a =4.8769(1) A, c =4.1729(3) A and a =4.8692(2) A, c =4.1750(3) A, respectively), with main structural characteristic the substitutional disorder observed at the R crystallographic site. The same structure is formed from the Gd 3 (Fe,Ti) 29 when it is annealed at 1123 K ( a =4.9044 A, c =4.1920 A). The compounds with R=Gd (TM=V) crystallizes in a disordered Th 2 Zn 17 structure type (S.G.: R-3m , a =8.5197(3) A and c =12.481(1) A) and with R=Er in a disordered Th 2 Ni 17 structure type (S.G.: P6 3 /mmc , a =8.4142(6) A and c =8.3319(7) A). A structural study of the Y 3 (Fe,V) 29 compound based on time of flight, high resolution neutron powder diffraction data is presented. The magnetocrystalline anisotropy is of an easy plane type at room temperature for all the examined compounds.

Synthesis and magnetic properties of (R,R′)3(Fe,Ti)29 (R=Pr, Nd and R′=Sm, Er) intermetallic compounds

Abstract The ternary Pr 2 Er(Fe,Ti) 29 and SmNd 2 (Fe,Ti) 29 intermetallic compounds were successfully synthesised and crystallographically and magnetically characterised. The anisotropy constants, the anisotropy field and the easy magnetization directions were determined at 5 and 300 K. The results support the idea of preferential substitution of Pr and Nd at the 4 i -sites in Pr 2 Er(Fe,Ti) 29 and SmNd 2 (Fe,Ti) 29 respectively, and thus the observed changes in anisotropy arise due to the difference in the Stevens factors between the substituting and substituted ions.

57Fe Mössbauer study of novel series of intermetallic compounds R3(Fe1−xCox)29−yTy (R=Nd, Tb, Dy; T=Ti, V)

R3(Fe1−xCox)29−yTy (R=Nd, Tb, Dy; T=Ti, V) intermetallic compounds have been studied by 57Fe Mossbauer spectroscopy at 85 and 293 K. The hyperfine interaction parameters depend on the amount of Co atoms. It is shown that Co atoms are equally distributed in all Fe sites but strongly avoid the Fe2, Fe3 and Fe6 sites which are the dumb-bell sites.