O. Kalogirou

Existence range, structural and magnetic properties of Nd3Fe27.5Ti1.5−yMoy and Nd3Fe27.5Ti1.5−yMoyNx (0.0 ≤ y ≤ 1.5)

Abstract A series of alloys with the composition Nd 3 Fe 27.5 Ti 1.5− y Mo y (0 ≤ y ≤ 1.5) and the Nd 3 (Fe,Ti) 29 -type structure have been synthesized. The X-ray diffraction pattern of these compounds can be indexed in the monoclinic symmetry with lattice parameters of α = 10.6382(5) A , b = 8.5892(4) A , c = 9.7456(5) A and β = 96.930(2)° and the structure can be described in the A2/m space group ( Z = 2). This is a minimal non-isomorphic supergroup of P2 1/c . With this description the number of Fe sites is eleven. The Curie temperature depends on the Ti, Mo concentration and increases from 400 to 437 K with increasing Ti. The room temperature saturation magnetization for the y = 0 sample is 139.6 A m 2 /kg and the anisotropy field 3.7 T. After nitrogenation a lattice expansion of 5.3% is observed. Four N atoms per formula unit and their probable sites (4i and 4f) are predicted. An empirical model is proposed for the calculation of the number of the N atoms for all phases having a superstructure relation to the 1:5 phase. The Curie temperature increases by 280 K reaching 663 to 712 K depending on the Ti:Mo ratio. The room temperature saturation magnetization of the y = 0 sample becomes 170.6 A m 2 /kg and the anisotropy field 8.0 T after the nitrogenation process. X-ray powder diffraction patterns of magnetically aligned powder samples of the parent and the nitrided compound indicate the presence of an easy-cone-type anisotropy. The calculated room temperature anisotropy constants, K 1 = −1.80 MJ/m 3 , K 2 = 1.86 MJ / m 3 for the parent y = 0 sample and K 1 = −5.17 MJ/m 3 , K 2 = 3.57 MJ/m 3 for the nitrided one, confirm this assumption.

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.

Magnetic Graphene Oxide: Effect of Preparation Route on Reactive Black 5 Adsorption

In this study, the effect of preparation route of magnetic graphene oxide (mGO) on Reactive Black 5 (RB5) adsorption was investigated. The synthesis of mGO was achieved both with (i) impregnation method (mGOi nanoparticles), and (ii) co-precipitation (mGOp nanoparticles). After synthesis, the full characterization with various techniques (SEM, FTIR, XRD, DTA, DTG, VSM) was achieved revealing many possible interactions/forces of dye-composite system. Effects of initial solution pH, effect of temperature, adsorption isotherms and kinetics were investigated in order to conclude about the aforementioned effect of the preparation method on dye adsorption performance of the magnetic nanocomposites. The adsorption evaluation of the magnetic nanoparticles presented higher adsorption capacity of mGOp derivative (188 mg/g) and lower of mGOi (164 mg/g). Equilibrium experiments are also performed studying the effect of contact time (pseudo-first and -second order equations) and temperature (isotherms at 25, 45 and 65 °C fitted to Langmuir and Freundlich model). A full thermodynamic evaluation was carried out, calculating the parameters of enthalpy, free energy and entropy (ΔH0, ΔG0 and ΔS0).

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.

Site occupancy and lattice changes on nitrogenation in Nd3Fe29−xTixNy

The structure of Nd3Fe29−xTixNy has been determined by neutron diffraction and compared with the unnitrided parent. Nitrogen is found in the two large octahedral interstitial sites with four Fe and two rare‐earth neighbors, leading to a concentration of y=4. The lattice expands by 6.4% (ΔV/V) but the expansion is anisotropic in the axes. Although most of the Fe–Fe bond lengths expand, a few show significant contraction, most notably the Fe3–Fe3 dumbbell, which has an extremely short 2.18 A length.

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.

Morphology influence on nanoscale magnetism of Co nanoparticles: Experimental and theoretical aspects of exchange bias

Co-based nanostructures ranging from core-shell to hollow nanoparticles were produced by varying the reaction time and the chemical environment during the thermal decomposition of Co2(CO)8. Both structural characterization and kinetic model simulation illustrate that the diffusivities of Co and oxygen determine the growth ratio and the final morphology of the nanoparticles. Exchange coupling between Co and Co-oxide in core/shell nanoparticles induced a shift of field-cooled hysteresis loops that is proportional to the shell thickness, as verified by numerical studies. The increased nanocomplexity when going from core/shell to hollow particles, also leads to the appearance of hysteresis above 300 K due to an enhancement of the surface anisotropy resulting from the additional spin-disordered surfaces.

Magnetic phase transitions and magnetocrystalline anisotropy in Nd3(Fe,Ti)29 and Nd3(Fe,Ti)29N4

An investigation of the nature of the magnetocrystalline anisotropy of Nd3(Fe,Ti)29 and Nd3(Fe,Ti)29N4 and the magnetic transitions observed at 150 and 230 K in Nd3(Fe,Ti)29 is presented. AC susceptibility measurements on a magnetically aligned powder sample of single phase Nd3(Fe,Ti)29 with the ac field parallel and perpendicular to the alignment direction indicate that these transitions are first order magnetization processes and not spin reorientation phenomena. 57Fe-Mossbauer spectroscopy on magnetically aligned samples of Nd3(Fe,Ti)29 and Nd3(Fe,Ti)29N4 gives evidence for the presence of an easy-cone-type magnetocrystalline anisotropy in both compounds in the temperature range 85 – 293 K. The calculated cone-angle remains constant at 85 K and at 293 K and has the values of θc≈34° and 29° for the parent and nitrided compound, respectively.

Magnetostrictive properties of amorphous and partially crystalline TbDyFe thin films

A series of amorphous and partially crystalline giant magnetostrictive thin films of the composition (Tb0.3Dy0.7)39Fe61 has been prepared by dc magnetron sputtering. The sputtering conditions were the same for all samples apart from the substrate temperature TS, which varied between 330 and 510 °C. The crystalline state and the magnetic properties of the samples were investigated in relation to the substrate temperature. Films deposited at TS=330–400 °C were amorphous. Crystallization started at TS=425 °C. The magnetostrictive coefficient λ, at 4 kOe and at room temperature, increased with increasing TS from 185 to 750 ppm at Hmax. This was related also to an increase of the Curie temperature TC from 35 to 315 °C. The hysteresis loops of the amorphous samples showed coercivities lower than 10 Oe. These samples presented λ≈300 ppm in a field of 1000 Oe. The direction of the magnetic moments in the amorphous films changed gradually from perpendicular to parallel to the film plane with increasing TS. The sam...

In vitro application of Mn-ferrite nanoparticles as novel magnetic hyperthermia agents

Manganese ferrite nanoparticles were synthesized by a facile, low-cost, environmentally friendly and high yield methodology based on the aqueous co-precipitation of proper salts. Firstly, structural, morphological and magnetic characterization schemes were performed to determine crucial factors for optimizing their heating potential, such as size, polydispersity, saturation magnetization and coercivity. In an effort to simulate the in vivo environment of animal tissue phantoms and study the thermal heating effects resulting from Brownian motion and hysteresis losses, nanoparticles at various concentrations were embedded in aqueous media of varying agar concentration. During the in vitro application healthy cells (primary bone marrow-derived osteoblasts and 3T3-L1 fibroblast-like preadipocytes) and human osteosarcoma Saos-2 cells were incubated with manganese ferrite nanoparticles. The heating profile of the particles was studied at different concentrations and in correlation with their potential cytotoxic effect. Our results revealed concentration dependent cytotoxicity profile and uptake efficiency together with variable specific loss power values yet with fast thermal response, opening novel pathways in material selection as hyperthermia agents.