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High Magnetic Moment of FeCo Nanoparticles Produced in Polyol Medium
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DOI: 10.1109/TMAG.2013.2288411
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Structural, magnetic, and electronic properties of high moment FeCo nanoparticles
K. Zehania,⇑, R. Beza,c, A. Boutaharb, E.K. Hlild, H. Lassrib, J. Moscovicia, N. Mlikic, L. Bessaisa
aCMTR, ICMPE, UMR7182, CNRS – Université Paris Est Créteil, 2-8 rue Henri Dunant, F-94320 Thiais, France
bLPMMAT, Université Hassan II, Faculté des Sciences Ain Chock, B.P.5366 Maârif, Route d’El Jadida, km-8, Casablanca, Morocco
cLMOP, Faculté des Sciences de Tunis, Université de Tunis El Manar, 2092 Tunis, Tunisia
dInstitut Néel, CNRS et Université Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France
a r t i c l e i n f o
Article history:
Received 7 October 2013
Received in revised form 26 November 2013 Accepted 27 November 2013
Available online 16 December 2013
Keywords:
Polyol synthesis
Structural and magnetic properties Random magnetic anisotropy Electronic structure calculations
a b s t r a c t
Soft-magnetic Fe55Co45alloy nanoparticles have been successfully synthesized by the polyol reduction process followed by annealing under argon. The diethylene glycol (DEG) was used as solvent and reduc- ing agent simultaneously in this process. The synthesized samples of nanoparticles were annealed at 873 K for different times. The alloy formation processes, the evolution of the microstructure, the mag- netic properties, and the DOS calculation have been investigated before and after samples annealing.
The X-ray diffraction of the synthesized product before annealing shows that a cobalt ferrite is spinel structure of crystallite size of about 10 nm. X-ray diffraction analysis of the samples annealed at 873 K for different times also shows that of the FeCo alloy has been obtained by reducing the cobalt ferrite.
It has been confirmed the formation of a body-centered-cubic (bcc) single phase structure where the wt.%increases with annealing times leading to a pure phase after annealing during 4 h. These results are confirmed by transmission electron microscopy study. The saturation magnetization of the Fe–Co alloys increases with annealing time, indicating an increasing homogeneity in composition and the single bcc FeCo phase formation. The highest saturation magnetization of 235 emu g1with a low coercivity of 76 Oe was obtained for the Fe55Co45nanoparticles annealed during 4 h. The local random anisotropy con- stantKLhas been extracted. This work presents also detailed information about total, and atom projected density of state functions, as well as the magnetic moment for different atoms in Fe55Co45alloys and cobalt ferrite.
Ó2013 Elsevier B.V. All rights reserved.
1. Introduction
The nanocrystalline ferromagnetic materials exhibit interesting magnetic properties from the point of view of fundamental re- search up to applications. These materials have taken a privileged place in the research of new soft magnetic materials [1,2].
Recently, several researches have been made on the study of the FeCo soft magnetic nanomaterials[3,4].
These materials are interesting for various applications. Because of their unique magnetic properties (high saturation magnetiza- tion, large permeability, low coercivity and ferromagnetic behavior up to 1073 K), they are used in transformer cores, electrical gener- ators, electrical motors, pole pieces and for hyperthermia-based therapy[5–7]. The optimization of the structure and microstruc- ture represents the key of success to develop the magnetic proper- ties of these samples. They may be synthesized by high energy milling or chemical route[8–10].
Poudyal et al. have prepared FeCo nanomaterials by surfactants- assisted ball milling; they have found that saturation magnetiza- tion of FeCo (obtained by ball milling for 1 h) is 209 emu g1with 23 nm of nanoparticles size [8]. Zamanpour et al. were able to synthesize FeCo nanoparticles by polyol using ethylene glycol (EG) as solvent; they obtained a saturation magnetization of 200 emu g1for nanoparticles size of 30 nm[4]. Hiyama et al. were also able to prepare a FeCo alloy in the polyol medium (using EG as solvent), followed by annealing under hydrogen but the particle size and the saturation magnetization were 8lm and 220 emu g1 respectively[10].
In this paper, we present high magnetic moment Fe1xCox
(x= 0.45) nanoparticles synthesized by a novel route – the polyol process – followed with annealing under argon. We have carried out structural investigation by careful powder X-ray diffraction (XRD) analysis with Rietveld refinements[14], transmision electron microscopy coupled with energy dispersive spectroscopy (EDS) analysis, and measurement hysteresis loop. In addition to the exper- iments, the random magnetic anisotropiy (RMA), and a theoretical investigation by density functional theory (DFT) were achieved.
0925-8388/$ - see front matterÓ2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.11.208
⇑Corresponding author. Tel.: +33 149781198.
E-mail address:zehani@icmpe.cnrs.fr(K. Zehani).
Contents lists available atScienceDirect
Journal of Alloys and Compounds
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m
2. Experimental studies
The powders of Fe55Co45nanoparticles were synthesized by reduction polyol method[4,11,12].
High purity analytical grade cobalt acetate anhydrous (Co(Ac)2), iron chloride tertra hydrate (FeCl24H2O), diethylene glycol (DEG), sodium hydroxide (NaOH) and ruthenium chloride (Ru(Cl2)) were used in synthetic reaction without any further treatment.
The first step of the synthesis of metal nanoparticles is to process spinels CoFe2-
O4nanoparticles via forced hydrolysis in a polyol medium[13]. The second step is to reduce them by argon gas at 873 K. CoFe2O4was synthesized, starting from an amount precursor salts of 100 mmol of FeCl24 H2O and 80 mmol of cobalt acetate Co(CH3COO)24H2O. The acetate ratio defined as½OAs½M(Mstands for the total amount of metallic elements), was fixed to 2.2. The total volume of the DEG is 200 ml. We have dissolved also 5 mmol of ruthenium chloride and 1.26 M of sodium hydroxide.
The mixture was then heated at 418 K for 2 h with a rate of 5 K min1. After completion of the reaction the produced powder was filtered, washed with ethanol and acetone, and dried at 353 K.
After polyol synthesis, the nanopowder wrapped in tantalum foils were an- nealed in a sealed silica tube under argon gas (350 Torr) at 873 K during 30 min, 1, 2, and 4 h.
The structural properties of the samples obtained before and after annealing were characterized by the powder X-ray diffraction using the BRUKER diffractome- ter with Cu Katarget (k¼1:5406 Å) to determine the crystallographic structure and to identify phases. The intensities were measured at angles from 2h= 20to 90 with a step size 0.02. The structure refinement for the X-ray pattern was carried out using MAUD computer code based on Rietveld analysis[14]. Transmission elec- tron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were performed using a JEOL 2010 FEG microscope operating at 200 kV. The chemical composition of the grains was determined by energy dispersive spectroscopy. The magnetic measurements of the samples were measured using a Physical Properties Measurement System (PPMS9 Quantum Design) under an applied field up to 3 T.
3. Results and discussion 3.1. Structure analysis
Fig. 1shows typical X-ray diffraction patterns of the FeCo nano- particles as-synthesized and after annealing at 873 K for distinct durations. The annealing of the powders is accompanied by a de- crease of the intensities of Co ferrite peaks and the apparition of FeCo ones. However, the cobalt ferrite phase doesn’t exist in sam- ples annealed for 4 h.
Fig. 2 presents, as an example the Rietveld analysis results of XRD pattern of FeCo sample before and after annealing for 4 h at 873 K. The refinement performed for the as-synthesized sample shows the presence of a main phase of CoFe2O4with spinel struc- ture (space group Fd-3m). The unit cell parameterais equal to 8.4087 Å.
Three characteristic peaks of FeCo phase corresponding to the crystal planes of (1 1 0), (2 0 0) and (2 1 1) were observed for all an- nealed samples. The relative contribution of the two crystalline phases given by the Rietveld analysis varies with annealing time for a given temperature 873 K. With increasing the duration of annealing, the proportion of Fe55Co45 phase increases from 64.28% to 98.89% for 30 min and 4 h respectively. After an anneal- ing during 4 h the structure refinement shows a main phase of Fe55Co45with body centered cubic structure (bcc). The lattice parameter isa= 2.8539 Å. No additionnals peaks such as Co(OH)2, Fe(OH)2, are observed in XRD pattern which indicates the high purity of prepared sample. These results provide that increasing the annealing time is important for co-reduction of metal ions (Co2+ and Fe3+) and favors the formation of alloy phase. The experimental conditions and structural characterization for all samples are summarized inTable 1.
The grain size and the strain of the nanoparticles was calculated using the Williamson–Hall equation[15]:
BcosðhÞ ¼kk
Dþ4sinðhÞ ð1Þ
whichDis the grain size,Bis the full width at half maximum inten- sity of the peak,k is the sheerer’s constant (0.90),kis the X-ray wavelength andhis the Bragg angle. Plots are drawn with 4 sinh along thex-axis andBcoshalong they-axis for all annealed FeCo samples as shown in Fig. 2. From the linear fit of the data, the auto-coherent diffraction domain size was estimated from the y-intercept, and the strain, from the slope of the fit.
Fig. 1.XRD pattern of the Fe55Co45as-synthesized and annealed samples at 873 K during 30 min, 1 h, 2 h and 4 h.
Fig. 2.Rietveld analysis for X-ray diffraction pattern of FeCo as-synthesized (above) and annealed at 873 K for 4 h (below).
The Williamson–Hall plots (Fig. 3) have shown a increase in the size of the FeCo nanoparticles and the strain with increase of annealing time from 30 min to 4 h (Table 1).
3.2. Microstructural analysis
The size, the morphology of as-prepared product as well as those annealed during 30 min and 4 h were further examined by TEM, HREM and SAED. TEM Image shown inFig. 4a indicated that the as-synthesized CoFe2O4are composed of sphere-like nanopar- ticles well dispersed with diameters ranging between 5 and 12 nm (see size distribution in inset). The corresponding HRTEM image (Fig. 4b) shows nanoparticles rather in good crystallinity.Fig. 5 exhibits the typical low and high magnification TEM images of Fe–Co compound annealed for 30 min at 873 K. The mean particles size increases to 30–60 nm range. The increase in the size of the nanoparticles after 30 min annealing, as seen in TEM images, cor- roborates the XRD results. It is interesting to note that the crystal- linity is increased with annealing. The interplanar distance of 0.252 nm indicated in Fig. 5b corresponds to the (3 1 1) plan in agreement with the XRD results.
InFig. 6a, we show the morphology of FeCo alloy, annealed for 4 h, at low magnification. It can be seen that the particle size is clearly increased, and the crystallintiy as shown in Fig. 6b is improved. Microdiffraction patterns inFig. 6c and dare obtained from the nano-crystalline phase of the FeCo sample annealed for 4 h at 873 K. They could be respectively ascribed to [1 1 1] and [1 0 0] directions of the cubic crystal lattice. All patterns can be completely indexed with a body-centered cubic lattice and a unit cell parameter as determined from X-ray powder diffraction.
4. Magnetic properties 4.1. Magnetic measurements
The magnetic behavior of the CoFe2O4ferrite particles will vary with different Co2+occupations since the Co2+ion is highly aniso- tropic. To investigate the magnetic properties of the synthesized CoFe2O4ferrite particles in polyol medium, the applied magnetic field dependence of magnetization M(H) are measured at room temperature, RT (300 K, Fig. 7) and low temperature, LT (10 K, Fig. 7).
The hysteresis loops at 300 K shows a typical soft materials, with coercivityHCof 99 Oe, and saturation magnetization,Ms, of 55 emu g1.Mswere estimated from a fit over the high-field data using the approach to saturation magnetization law (see Eq.(2)).
These values of the coercivity and the saturation magnetization are similar to the ones measured by Zhang et al.[16], for cobalt ferrite synthesized by hydrothermal process where they found
HC= 300 Oe and Ms= 52 emu g1. The remanance magnetization value ofMris equal to 4.94 emu g1is obtained for cobalt ferrite (seeFig. 7inset).
At 10 K a hysteresis loops is obtained, ferromagnetic behavior, with coercivity fieldHcof 6.15 kOe, remananceMrof 40 emu g1, and saturation magnetization, Ms, of 65 emu g1 as shown. The remanance ratio,Mr/Ms, is 0.62. This result can be discussed by considering an assembly of single domain particles with randomly oriented easy axis whereHcis expected to be proportional to theK toMsratio, andMr/Ms= 0.5 for axial anisotropy orMr/Ms= 0.85 for the cubic anisotropy one.
The magnetic properties of the powders synthesized are highly influenced by annealing temperature. Hence, as can be seen in Fig. 8andTable 2, the structural and magnetic data exhibit a clear correlation.Fig. 8a depicts the RT hysteresis loop of Fe55Co45before and after annealing under argon. The saturation magnetization at RT for the annealing time during 4 h to be 235 emu g1which is slightly more than the bulk value and polyol process (with ethylen glycol as solvent) to be 209 and 200 emu g1respectively [8,4].
This confirms the results obtained by X-ray diffraction, i.e. the for- mation of bcc FeCo phase. In fact, we have been showed that Ms increasing with wt.% of FeCo up to 235 emu g1for sample anneal- ing during 4 h (seeFig. 8b andTable 2). We can notice thatMralso increases with annealing time and the wt.% of FeCo, it is 4.16 and 8.19 emu g1for annealing of 30 min and 4 h respectively.
To better understand the effect of annealing on the magnetic properties, we were interested in the last section of the article to the ‘Random magnetic anisotropy’, and the ‘Details of electronic structure calculations’.
Table 1
Structural results of FeCo samples,ais the unit cell parameter,Dis the grain size,is the strain and wt.% is the phase abundance. The sample correspondent respectively to as synthesized FeCo and annealed at 873 K for an annealing duration of 30 min, 1 h, 2 h and 4 h.
Sample Phase wt.% a(Å) D(nm) (104)
FeCo as synthesized Fe55Co45 0.28 – – –
CoFe2O4 99.72 8.4087(4) 10
FeCo (30 min) Fe55Co45 64.28 2.8460(3) 14 9.4
CoFe2O4 35.72 8.3919(6) – –
FeCo (1 h) Fe55Co45 77.22 2.8489(5) 16 11.0
CoFe2O4 23.78 8.3948(2) – –
FeCo (2 h) Fe55Co45 81.12 2.8512(6) 23 16.8
CoFe2O4 18.88 8.3952(2) – –
FeCo (4 h) Fe55Co45 98.89 2.8539(6) 66 23.6
CoFe2O4 1.11 – –
Fig. 3.Plot ofBcos vs 4 sin of FeCo samples annealed at 873 K during 30 min, 1 h, 2 h and 4 h.
4.2. Random magnetic anisotropy constant
Random magnetic anisotropy (RMA) was first proposed by Harris and Plischke[17]to explain the anisotropy found in some amorphous alloys and particularly those containing rare earth metals. Based on their Hamiltonian, Chudnovsky[18–21]proposed a model to analyze the approach to saturation. This model was applied successfully to explain the results by several authors. We
had used this model to analyse our results on several rare earth based amorphous alloys and obtained various fundamental param- eters such as local anisotropy and the correlation lengths[22]. We propose to apply similar ideas to the nanomaterials. The applica- tion of this random anisotropy model to nanomaterials could be justified as follows. The nanograins due to their low dimension have a lower symmetry in the regions particularly near the sur- face, resulting in a kind of uniaxial anisotropy. As the grains are oriented at random there is no alignment of this axis which then leads to a spread in their direction. This is then analogous to the amorphous materials where the topological disorder leads to a spread in the axis of symmetry. The essential difference of course is that in the amorphous alloys the structural correlation length is of the order of 1 or 2 nm whereas in nonomaterials the grains size is an order of magnitude bigger. This would result in some differ- ences in details and could affect the magnitude of the anisotropy.
We briefly describe below the model we have used. We can Fig. 4.TEM (a) and HRTEM (b) images of cobalt ferrite.
Fig. 5.TEM (a) and HRTEM (b) images of FeCo nanoparticles annealed for 30 min at 873 K.
Fig. 6.TEM (a), HRTEM (b), and electron microdiffractions along [1 1 1] (c) and [1 0 0] (d) axis of FeCo nanoparticles annealed for 4 h at 873 K.
Fig. 7.Hysteresis loops of the cobalt ferrite synthesized by polyol process at 10 K and 300 K.
describe the approach to magnetic saturation by the formula [18–22]:
MðHÞ ¼Ms 1 a2
HþHUþHex
ð Þ2
" #
ð2Þ with
a2¼H2r 15¼ 1
15 2KL
Ms
2
ð3Þ
whereHis the applied magnetic field in (kOe),Msis the saturation magnetization in (emu g1),HUis the coherent anisotropy field,Hex
is the exchange field,Hris the random magnetic field, anda2is a constant which is a function of KL the local anisotropy and Ms. The magnetization curves for all samples are found to fit well Eq.
(2)as shown inFig. 9. The values ofMs, anda2obtained from the fitting at 300 K were used to determineKLusing Eq.(3). Values of
the parameters obtained by this way are displayed onTable 2. We find that the local anisotropy increases from 0.18106erg cm3 for FeCo as synthesized to 0.92106erg cm3for Fe55Co45annealed at 873 K for 4 h, at 300 K.
4.3. Details of electronic structure calculations
For Fe1xCox(x= 45) alloys considered as chemically disordered system we used the appropriate Korringa Kohn Rostoker method within Coherent Potential Approximation (KKR-CPA). For this method, we assume that the Fe site is randomly occupied by both Fe and Co atoms with appropriate 0.55 and 0.45 occupation prob- abilities, respectively. The corresponding code[23]proceeds with an effective potential assumed to be spherically symmetric giving non-overlapping muffin tin spheres around the atoms and constant potential in the interstitial region. For CoFe2O4which is considered as an ordered oxide system, for which the constant potential approximation in the interstitial region is not valid. The appropri- ate FLAPW method [24] is used. Also, it performs the density functional theory (DFT) calculations using the General Gradient Approximation (GGA) where the Kohn–Sham equation and energy functional are evaluated self consistently.
The crystallographic structure is defined in the space group Im- 3m (2 2 9) for Fe55Co45alloy with 2a (0, 0, 0) site randomly shared by Fe and Co. The one of CoFe2O4compound is defined in the space group Fd-3m (2 2 7) with 16c (1/8, 1/8,1/8), 8b (1/2, 1/2, 1/2) and 32e (x= 0.260, x= 0.260,x= 0.260) sites occupied by Co, Fe and O, respectively. Both materials are considered to be in the ferro- magnetic state with cell lattices of 2.8539 Å and 8.4087 Å, respectively.
4.4. Results and discussion
The density of state (DOS) of Fe55Co45is computed using KKK- CPA method[23].
Total density of states (DOS) of Fe55Co45is reported inFig. 10.
Taken as a reference, Fermi level position exhibits no zero DOS, pointing out to metallic character as expected for this alloys type.
Analysis gives evidence of a shift between the band of the major spin and that of the minor spin pointing out to the polarization of these bands as well. This polarization induces a magnetic moment carried by both Fe and Co atoms. Projected DOS on atoms giving the l-decomposed DOS of like-states 3d(Fe) and 3d(Co) are displayed inFig. 10. They specify that total DOS is mainly originat- ing from the 3d(Fe) and 3d(Co) bands contributions in both occu- pied and unoccupied. Computed magnetic moment values of Fe and Co atoms are 2.65lB and 1.89lB respectively. Comparison to magnetic moment values of Fe and Co pure metals, exhibit substantial increase and a slight augmentation for Fe and Co, respectively (seeTable 3). Total magnetic moment is computed as well and found equal to 2.3lB. Total density of states (DOS) deduced from FLAPW calculations of CoFe2O4 is presented in Fig. 11. The main evidence is the absence of the gap at Fermi level EFtaken as energy reference confirming the conductor character of this compound. In addition, the difference in the DOS shape for Fig. 8.Hysteresis loops, at 300 K, for the cobalt ferrite synthesized by polyol
process before and after annealing for different times (above). Saturation magne- tization as function of the FeCo content (below).
Table 2
Some magnetic parameters of the cobalt ferrite synthesized by polyol process before and after annealing for different times at 300 K.
Sample Ms(emu g1) Ms(lB) Mr(emu g1) Hc(Oe) Hr(kOe) KL(106erg cm3)
FeCo as synthesized 55 2.27 4.94 99 1.2 0.18
FeCo (30 min) 159 1.62 4.16 129 – –
FeCo (1 h) 156 1.60 5.89 102 – –
FeCo (2 h) 164 1.68 6.72 82 – –
FeCo (4 h) 235 2.40 8.19 76 0.95 0.92
spin up and spin down confirms the magnetic state as expected.
The above results fit with the traditionally picture of the magne- tism of conductor materials.
Details analysis of DOS point out that total DOS is dominated by both 3d bands of Co and Fe contributions close to Fermi level on [4 eV, 2 eV] energy range (seeFigs. 11). Small difference between major spin and minor spin is observed on Oxygen DOS which can be explained in terms of DOS polarization induced by the presence of small magnetic moment on oxygen atoms (Fig. 12).
Magnetic moments carried by atoms are computed as well.Ta- ble 3gathers the resulting magnetic moments as deduced from the differences between the spin up and spin down populations of electron in each atomic sphere and in the interstitial region. As reasonably expected in this material type, magnetic moments localized on Fe and Co are found equal to 2.11lB, and 1.92lB, respectively; whereas the magnetization is estimated to 6.42
lB/f.u. It is worthy noticed that such non integer value points out also to conductor state of our material since for insulating systems
the magnetic moments are considered as integer values. As sur- prising additional evidence, we found 0.58lBas value of intersti- tial magnetism. Such substantial interstitial magnetism clearly reveals the existence of a noticeable spin density out of iron and Co atoms.
5. Conclusion
Polyol medium process followed annealing under argon has demonstrated to be an effective way. It has enabled us to develop a high magnetic moment for the Fe55Co45nanoparticles. The Riet- veld analysis for X-ray diffraction pattern, as well as TEM study, of samples annealed at different times showed an increased of the wt.% of FeCo nanoparticles (bcc structure) up to 99 % for annealing of 4 h. The magnetic measurements have shown a high magnetiza- tion saturation (235 emu g1) and a low coercive field (76 Oe).
We have shown that it is possible to extend the application of random magnetic anisotropy model originally developed for amor- phous alloys to the nanocrystalline materials of cobalt ferrite and FeCo alloys. The model gives a good fit of the experimentalM(H).
In addition, we have determined, for cobalt ferrite and FeCo alloy (obtained after annealed during 4 h at 873 K), some fundamental parameters such as random anisotropy fields and random anisot- ropy constant. We found that the local anisotropy is at least an order of magnitude higher than in the corresponding bulk FeCo Fig. 9.Magnetization curve of the cobalt ferrite synthesized by polyol process
before and after annealing for different times at 300 K.
Fig. 10.Total DOS and the l-decomposed DOS of like-states 3d in Co and Fe of Fe55Co45.
Table 3
Magnetic moments computed from FLAPW calculations.
Sample ltot(lB) lFe(lB) lCo(lB) lO(lB)
Fe55Co45 2.30 2.65 1.89 –
CoFe2O4 6.42 2.11 1.92 0.04
Fig. 11.Total DOS and the l-decomposed DOS of like-states 3d of Co and Fe in CoFe2O4from FLAPW calculations.
Fig. 12.The l-decomposed DOS of like-states 2p(O) from FLAPW calculations.
alloys. First-principles spin-density functional calculations, using FLAPW method, are performed to probe magnetic structure details.
Namely, magnetic moments carried by atoms are computed and magnetization is found equal to experimental data. Moreover, sub- stantial interstitial magnetism is revealed in FeCo2O4compound.
Acknowledgements
This work is mainly supported by the CNRS and the ‘‘Ministére de l’Enseignement Supérieur, de la Recherche Scientifique’’ (LAB MA03) (Tunisia), PHC-Utique (project 11/G 1301), DGRS/CNRS project 12/R 13-01 and CNRS – CNRST project with Morocco.
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