D. Niarchos

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.

Mössbauer study of La0.75Ca0.25Mn0.98Fe0.02O3 compound

A thorough study of the Mossbauer spectra (MS) of a lightly iron doped (2%) polycrystalline La0.75Ca0.25MnO3 compound has been carried out from T=4.2 K up to 300 K. The MS for T>Tc and at T=4.2 K show one component. The corresponding hyperfine parameters are compatible with S=5/2 Fe3+ in octahedral coordination. As temperature increases from 4.2 K the spectra consist of several broad sextets and doublets (especially the Tc). This first reported behavior of MS resembles that of ferromagnetic clusters.A thorough study of the M{umlt o}ssbauer spectra (MS) of a lightly iron doped (2{percent}) polycrystalline La{sub 0.75}Ca{sub 0.25}MnO{sub 3} compound has been carried out from T=4.2 K up to 300 K. The MS for T{gt}T{sub c} and at T=4.2 K show one component. The corresponding hyperfine parameters are compatible with S=5/2 Fe{sup 3+} in octahedral coordination. As temperature increases from 4.2 K the spectra consist of several broad sextets and doublets (especially the T{sub c}). This first reported behavior of MS resembles that of ferromagnetic clusters. {copyright} {ital 1997 American Institute of Physics.}

Preparation and characterization of the NdFe10T2Nx (T = Mo,V) compounds with the ThMn12 tetragonal-type structure

The dramatic changes in the magnetic properties observed in the R2Fe17 alloys upon nitrogen absorption have also been observed in the NdFe10T2Nx (T=Mo,V). The nitrides, prepared at T=773 K and 21/2 atm pressure of N2, showed an increased anisotropy HA ≳ 5 T at RT compared to the HA = 0 for the non‐nitrogenated materials. The Curie temperatures increase about 44% and the observed volume expansion is 3.6% for Mo and 2.8% for the V compound although the structure remains the same. A coercive field of 0.97 and 0.45 T has been observed at 4.2 K for NdFe10Mo2Nx and NdFe10V2Nx compounds, respectively.

Nitrogenation of the RFe10Mo2 (R = rare earth) compounds with ThMn12 type structure

Abstract Interstitial nitrides with the general formula RFe10Mo2Nx have been prepared for R = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu. All compounds have the ThMn12 tetragonal structure with unit cell volumes about 3% larger than the non nitrogenated parent compounds. Curie temperature are enhanced in the nitrides from 30% in Gd up to 80% in Lu compounds. The NdFe10Mo2Nx alloy exhibits an uniaxial anisotropy with BA > 9 Tesla at 4.5 K.

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.

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 510u2009°C. The crystalline state and the magnetic properties of the samples were investigated in relation to the substrate temperature. Films deposited at TS=330–400u2009°C were amorphous. Crystallization started at TS=425u2009°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 315u2009°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...

Exchange biasing in La2/3Ca1/3MnO3/La1/3Ca2/3MnO3 multilayers

A series of [La2/3Ca1/3MnO3/La1/3Ca2/3MnO3]15 multilayers, with bilayer thicknesses between 2 and 32 nm, has been prepared by pulsed laser deposition. The study of their magnetic and magnetotransport properties reveals, for the first time in this category of materials, the presence of an exchange biasing mechanism at low temperatures. Zero-field-cooling and field-cooling magnetic measurements reveal a blocking temperature around 70 K that is independent of the bilayer thickness, whereas the average film magnetization becomes zero at 250 K.

Mössbauer and magnetic studies of the R2Fe17Nx (R = Lu, Sm) intermetallic compounds

Abstract Interstitial ternary nitrides R2Fe17Nx have been prepared for R=Lu and Sm. The compound with Lu has the Th2Ni17 type structure whereas the compound with Sm has the Th2Zn17 type structure. The crystallographic constants were determined by means of Rietvelds profile refinement technique. Magnetic measurements and Mossbauer spectra of the compounds R2Fe17Nx were obtained between 4.2 and 300 K and compared with the non nitrogenated ones. The Curie temperatures of these compounds are 753 K for R=Sm and 683 K for R=Lu.