α-Fe nanocomposite magnets

Accepted Manuscript Structure and magnetic properties of Sm(Fe,Si)9C/α-Fe nanocomposite magnets Riadh Bez, Karim Zehani, Maria Batuk, Gustaaf Van Tendeloo, Najeh Mliki, Lotfi Bessais PII:

S0925-8388(16)33241-8

DOI:

10.1016/j.jallcom.2016.10.122

Reference:

JALCOM 39289

To appear in:

Journal of Alloys and Compounds

Received Date: 16 May 2016 Revised Date:

9 September 2016

Accepted Date: 15 October 2016

Please cite this article as: R. Bez, K. Zehani, M. Batuk, G. Van Tendeloo, N. Mliki, L. Bessais, Structure and magnetic properties of Sm(Fe,Si)9C/α-Fe nanocomposite magnets, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.122. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Structure and magnetic properties of Sm(Fe,Si)9 C/α-Fe nanocomposite magnets 1a,b

a CMTR, b LMOP

, Karim Zehania , Maria Batukc , Gustaaf Van Tendelooc , Najeh Mlikib , Lotfi Bessaisa

ICMPE, UMR7182, CNRS, Universit´ e Paris Est, 2-8 rue Henri Dunant F-94320 Thiais, France LR99ES17, Facult´ e des Sciences de Tunis, Universit´ e de Tunis El Manar, 2092 Tunis, Tunisia c EMAT, University of Antwerp, B-2020 Antwerp, Belgium

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Riadh Bez

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Abstract

SmFe8.75 Si0.25 C/α-Fe nanocomposites have been successfully synthesized using high energy milling, followed by annealing at 750◦ C. The crystal structure of these compounds was characterized by the Rietveld

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method using powder X-ray diffraction data. By increasing the concentration of Sm, we observed a decrease in the amount of α-Fe phase. The morphology of the samples was determined by scanning and transmission electron microscopy. The average grain size is about 20 nm. The magnetic properties were investigated at room temperature and at 10 K. A ferromagnetic behavior was observed in all samples at both temperatures. An increase of the soft magnetic phase α-Fe induced an increase in the magnetization and a decrease in coercivity. Keywords:

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Nanocomposite, Structural properties, Magnetic properties

1. Introduction

Permanent magnetic materials, containing rare-earth (R) and transition metal (M) elements with re-

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markable magnetic properties, have been a topic of great interest in recent studies both from a fundamental

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and an applied point of view. The Sm-based intermetallics with relatively high Curie temperature, such as

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Sm2 Fe17 , have been considered as potential candidates for use as permanent magnets [1].

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The out-of-equilibrium carbonated Sm(Fe,Si)9 phase with the hexagonal P6/mmm structure, precursor of

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the Sm2 (Fe,Si)17 ordered R-3m phase, is obtained by high energy ball milling. It is known as a hard magnetic

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phase with interesting magnetic properties [2, 3]. Furthermore, the Curie temperatures of the carbonated

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P6/mmm Sm(Fe,Si)9 alloys is systematically higher than the one of the carbonated Sm2 (Fe,Si)17 R-3m series

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[3].

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On the other hand, nanocomposite magnets consisting of a mixture of a hard magnetic phase with high

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coercivity and a soft magnetic phase with high saturation magnetization have an immense potential to ob-

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tain high magnetic performance. These systems are promising for advanced magnetic applications such as, 1 Author

to whom correspondence should be addressed; electronic mail: [email protected] - [email protected]

Preprint submitted to Elsevier

October 15, 2016

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for example, permanent magnets.

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Additional to the predicted high specific energy product, the presence of Fe or other Fe-based phases

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nanocomposite magnets show a higher corrosion resistance and are lower in cost compared to classical rare-

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earth based magnets [4]. It is well-known that optimum exchange coupling depends upon the crystalline

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size of soft magnetic phase, which remains in the order of 20 nm [5].

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Up to date, many systems, including Nd2 Fe14 B/Fe3 B [6], Nd2 Fe14 B/Fe [7, 8] and Sm2 Fe17 Nx /Fe [9] have

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been extensively studied. The usual processing techniques used to synthesize nanocomposite magnets are

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melt-spinning [10, 11], mechanical milling [12], and chemical techniques [13, 14].

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In the present work, we focus on the influence of α-Fe content on the magnetic properties of SmFe8.75 Si0.25 C/αFe nanocomposite magnets.

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2. Experiment

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SmFe8.75 Si0.25 powder samples having a 1:9/α-Fe nanocomposite structure with various volume fractions

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of α-Fe were prepared by varying the Sm compensation: 15%, 20% and 25% during the preparation of the

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parent alloys. A mixture of Sm2 Fe17 , Si(99.99%) and Sm (99.9%) powders was milling in a Fritsch planetary

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mill for 5 h under high-purity argon atmosphere. The ball-to-powder weight ratio was 15/1 and the rotation

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speed was 600 rpm. The obtained powders were wrapped in tantalum foil, were sealed under vacuum in

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silica ampules and further annealed during 30 min at the optimized temperature Ta = 750◦ C. After the

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heat treatment, the samples were quenched directly in water.

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Carbonation is achieved after reaction SmFe8.75 Si0.25 powders with an appropriate amount of anthracene

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(C14 H10 ) powders at 420◦ C under vacuum for 48h. At 420◦ C, the C14 H10 decomposes, releasing hydrogen

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gas which is trapped by small pieces of magnesium placed in the ampule and separated by silica wool. This

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temperature was also chosen not to modify the microstructure of the initial non-carbonated alloys. Released

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carbon atoms then diffused into the compound according to the reaction :

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7R-M + 1/2C14 H10 + 5/2Mg −→ 7R-M-C +5/2MgH2

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The final products were characterized using powder X-ray diffraction (XRD) on a Brucker diffractometer

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with Cu-Kα radiation to determine the crystal structure and identify the present phases. The structure was

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refined using Rietveld method as implemented in the FullProf computer code [15, 16] in the assumption

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of Thompson-Cox-Hastings line profile allowing multiple-phase refinement of each of the coexisting phases.

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The classical goodness-of-fit indicators χ2 and RB have been used. Scanning electron microscopy (SEM) and

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energy dispersive spectroscopy (EDS) were performed on a JEOL scanning electron microscope. Transmis-

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sion electron microscopy (TEM) images were acquired using Philips CM30 and FEI Tecnai G2 microscopes

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operated at 200 kV. Specimens for TEM analysis were prepared by grinding the samples in an agate mortar 2

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with ethanol and depositing few drops of the suspension on a Cu grid covered with a holey carbon layer.

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Magnetic hysteresis M-H measurements were performed using a Physical Properties Measurement System

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(PPMS9) Quantum Design, at T = 10 K and 293 K with a field up to 90 kOe. The M¨ossbauer spectra

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with absorbers containing 10 mg/cm2 of natural iron, were collected at 295 K on a constant acceleration

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spectrometer with 25 mCi57 Co/Rh source. The spectrometer calibration gives a line-width of 0.25 mm/s

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for α-Fe. The spectra were fitted according to the procedure discussed below with estimated errors of ±0:1

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T for hyperfine fields HHF and ±0:005 mm/s for isomer shifts δ and quadrupole shifts 2.

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3. Results and discussion

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3.1. Structural approach

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Fig. 1 shows the XRD pattern of the SmFe8.75 Si0.25 compound, before carbonation, synthesized with

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25% excess of Sm. According to this pattern, the main phase has a CaCu5 type structure with a hexagonal

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P6/mmm symmetry. Notice the absence of the expected intensities of the extra peaks (104, 112 and 024)

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related to the typical structure R-3m which is obtained at higher annealing temperatures above 900◦ C [17].

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Consequently, the structure has been refined on the basis of the vacancy model [18] established previously

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for the over-stoichiometric Sm-Co alloys.

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Buschow and Van der Goot [18] showed the concentration dependence of the c cell parameter for Co

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superstoichiometric SmCo5 and Sm1−s Co5+2s , for small s values. They observed an increase of the c pa-

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rameter due to the substitution of a Sm atom by a more voluminous pair Co-Co along the c axis. The

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stoichiometric RM5 structure of CaCu5 P6/mmm type (fig. 2) can evolve to a M enriched phase R1−s M5+2s

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where s indicates the number of R substitutions. This evolution results from the substitution of s rare earth

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atoms by s dumbbells M-M for small s values (s=0.03-0.04)[19]. Bessais et al. [20, 21] revised the model

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previously established for small s values and extended it to higher s values. The R atoms are located in the

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wyckoff position 1a (0,0,0) of the hexagonal P6/mmm structure. With decreasing R content 2s transition

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metals substitute the R positions. They constitute the dumbbell pairs occupying the 2e (0,0, Z) site. Due to

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the vacant R position, the initial 2c hexagons shift towards the c axis occupying the positions 6l (X, 2X, 0)

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with X < 31 . The evolution of the crystallographic sites from the rhombohedral to the hexagonal structure

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is described in Table 1. The stoichiometry Sm1−s Co5+2s has also been confirmed for Si substituted systems

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SmFe9−y Siy [2].

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We show in fig. 3, the Rietveld analysis of SmFe8.75 Si0.25 before carbonation synthesized with 25% excess

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of Sm. The result of the structure refinement performed for this sample, shows the presence of a main phase

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(98%) with the hexagonal structure. The lattice parameters are a = 4.9241(4) ˚ A and c = 4.1622(6) ˚ A. A

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minor quantities of Sm2 O3 (about 2%) was also detected.

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3.2. Structure and Microstructural analysis Powder X-ray diffraction patterns of the prepared samples after carbonation for all Sm excess (15%

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sample (A) , 20% sample (B) and 25% sample (C) ) are shown in fig. 4. The diffraction peak at 2θ = 44.6◦

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corresponds to the (1 1 0) crystallographic plane of the cubic α-Fe structure. Note that the intensity of this

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peak decreased with the increase of the amount of the Sm compensation.

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Fig. 5 shows the Rietveld analysis of XRD pattern of samples (A) and (C).

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All the peaks in all patterns can be well indexed on the hexagonal 1:9 structure with space group P6/mmm and the cubic α-Fe structure.

Small amounts of Sm2 O3 are also detected. Sm2 O3 is formed due to the initial excess of samarium during

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the synthesis. It reacts with traces of oxygen present on the reacting powder surface or with air during the

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measurements. It is clear that the relative contribution of both crystalline phases given by the Rietveld

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analysis depends on the amount of Sm. The proportion of α-Fe phase decreases from 30.2% (sample (A))

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to 0.8% (sample (C)) with the increase of the Sm compensation.

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It has been reported in our previous work [3] that the Si prefers to occupy the 3g sites in SmFe9−y Siy

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compounds. Carbon atoms are in the 3f interstitial sites having (1/2, 0, 0) coordinates (fig. 6). Atom

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Wyckoff positions in the P6/mmm hexagonal of the SmFe8.75 Si0.25 C phase are listed in table 2. It is worthy

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to note that these values, which are the same for all three materials, were fixed according to [3]. The isotropic

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Debye-Waller factors were also fixed to 1 ˚ A2 during the refinement.

For sample (B), the Rietveld analysis leads to the unit cell parameters: a = 5.0246(2) ˚ A, c = 4.1951(8)

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˚ A for SmFe8.75 Si0.25 C and a = 2.8702(1) ˚ A for α-Fe. These values are very close to those obtained for the

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same phases in previous work [3, 22].

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The cell parameter of α-(Fe,Si) varies from 2.8616(5) to 2.8646(8) ˚ A and it is smaller than that of pure

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α-Fe (2.869(1) ˚ A) [22, 23]. The grain size of SmFe8.75 Si0.25 C and α-Fe is about 20 nm for all samples as

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determined by the Scherrer method from the XRD patterns. These grain sizes are further confirmed by

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TEM. The structural data for all samples are summarized in Table 3.

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It is well known that the magnetic properties of nanocomposite magnets strongly depend on the microstructure, which affects the exchange coupling between the soft and hard magnetic phases.

Energy dispersive X-ray spectroscopy was performed to check the nominal composition of the particles (fig. 7). No significant oxygen content was detected in the samples, which confirms the XRD results

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TEM was used to examine the microstructure of the final materials. Fig.8 shows, as an example, a bright

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field image of sample (C). The particles are rather homogeneously distributed, with an average grain size

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of 20 nm; this size is in agreement with the grain size determined by XRD. It is worthy to note that such 4

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morphology are, in general, favorable for an enhancement of magnetic properties [24]. High resolution images

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of the different samples A, B and C are shown in fig.9. It confirms that the grains are well crystallized.

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3.3. Magnetic properties 3.3.1. Room temperature measurements

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Fig. 10 shows the hysteresis loops measured at room temperature of the three investigated samples.

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These samples exhibit hard magnetic behaviour. The saturation magnetization Ms and the anisotropy

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constant K were deduced using the saturation approach law [25] :
M (H) = Ms 1 − Ha2

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K Ms

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where a =

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The magnetic properties at room temperature are summarized in Table 4. We see that the magnetic

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properties of the SmFe8.75 Si0.25 C/α-Fe nanocomposites vary with the proportion of the different magnetic

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phases. With the increase of the α-Fe content, the magnetization increases, however, it occurs at the expense

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of the decrease in coercivity.

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A similar tendency has been observed by N.V. Rama Rao et al. on SmCo5 /Fe composite [26]. R.

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Ficher et al. [27] performed micromagnetism calculation on the Nd2 Fe14 B/α-Fe composite and confirmed

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theoretically that the increase of the soft magnetic phase α-Fe induces an increase in magnetization and a

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decrease in coercivity.

The maximum of specific energy product (BH)max = 13.1 MGOe (103.3 kJ/m3 ) is obtained for the sample

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(C). This value of (BH)max is higher than the reported value of 10.7 MGOe observed for Sm2 Fe15 Si2 C/α-Fe

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melt spun ribbons [28].

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3.3.2. Low temperature measurements

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To study the difference between the magnetic properties at room temperature and at low temperature,

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hysteresis loops were also measured at 10 K (see Fig. 11.) In contrast to the loops measured at room temperature, the demagnetization curves measured at 10 K

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contain a shoulder at low field and the two magnetic phases become decoupled. This two-step demagnetiza-

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tion behavior which appears at 10 K but not at room temperature can be explained by the fact that the soft

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and hard magnetic phases are optimally coupled when the size of the soft magnetic α-Fe grains is roughly p twice the width of a domain wall (δh =Π Ah /Kh ) of the hard magnetic grains [29], where Ah and Kh are

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the exchange and anisotropy constants of the hard phase, respectively. δh gets smaller with decreasing tem-

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perature due to the increase of the anisotropy constants, as a consequence the optimal grain size is shifted

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to smaller values and the majority of the α-Fe grains become partly or even completely decoupled from the

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neighbouring hard magnetic grains.

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A similar behavior was observed by Liu et al. [30] in PrCo3.5 /Co nanocomposites. They studied the vari-

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ation of the form of the hysteresis loop as a function of temperature from 300 to 5 K. They showed that

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at low temperature the hysteresis cycle had a shoulder while this was not the case at room temperature.

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They explained this phenomenon by the decoupling of the two magnetic phases, soft and hard, due to the

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increase of anisotropy at low temperatures.

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3.4. M¨ ossbauer spectra analysis

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The atomic arrangement in the structure is rather complex due, on the one hand, to the existence of

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three crystallographic sites and, on the other hand, to the statistical distribution of Si and 2e dumbbell

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atoms. Consequently, the experimental spectra result from the superposition of numerous sextets. Various

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sets of hyperfine parameters might lead to a good reproduction of the data but the choice of the solution is

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dependent on consistent physical models supported by other techniques or justified by pertinent theoretical

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considerations.

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The SmFe8.75 Si0.25 C M¨ ossbauer spectra were fitted with five sextets. The site assignment of the hyper-

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fine parameters observed for the crystallographically distinct iron sites in SmFeSiC was ruled by two aspects.

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On the one hand, the correlation between the isomer shift and the Wigner-Seitz cell (WSC) volume, calcu-

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lated using crystallographic data derived from the Rietveld refinement : the larger the WSC volume, the

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larger the isomer shift.

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On the other hand, the hyperfine field sequence is based on the near-neighbor environments of the four iron

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sites.

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For the 1:9C samples, five broadened sextets gave a good agreement with the experimental spectra, consis-

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tent with the relative Fe population 2e, 3g, 6l. In the last step of the refinement, the closer isomer shift values

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were averaged and fixed, the other parameters were liberated. The highest field sextet is unambiguously

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assigned to the dumbbell 2e family in perfect agreement with the expected population value. The remaining

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sextets are assigned to the 3g and 6l sites.

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The proportion of α-Fe phase deduced from the M¨ossbaeur spectra analysis is about 43%, 30%, and

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27% for samples A, B and C respectively. The difference between the proportion of α-Fe deduced from the

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M¨ossbaeur spectra analysis and those calculated by Rietveld analysis for X-ray diffraction is justified by the

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fact that the Lamb-M¨ ossbaeur absorption factor is more important for α-Fe than for SmFeSiC phase.

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4. Conclusion

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Novel nanocomposites SmFe8.75 Si0.25 C/α-Fe were produced by the method of high energy milling, fol-

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lowed by annealing at 750◦ C. We used the Rietveld analysis to determine the proportion of phases in all

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samples. We showed that the proportion of α-Fe phase decreases from 30.2% (sample (A)) to 0.8% (sam-

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ple (C)) respectively with the increase of the Sm compensation. Microstructural studies revealed that the

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nanograins are crystalline with an average size of 20 nm. With increasing α-Fe content, the saturation mag-

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netization and the remanence increase while the coercivity decreases. We showed that these nanocomposite

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exhibit hard magnetic behavior at room temperature. A partial decoupling effect of the two magnetic phases

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soft and hard appears at low temperature.

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Acknowledgements

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This work is main supported by the CNRS and the ”Minist`ere de l’Enseignement Sup´erieur, de la

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Recherche Scientifique” (LR99ES17) (Tunisia), PHC-Utique (Project 11/G 1301) and PHC-Maghreb (Project

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15MAG07). The authors acknowledge the French SIE doctoral school of the University Paris Est for its

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support.

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L. Bessais, K. Younsi, S. Khazzan, and N. Mliki. Intermetallics, 997:1004, 2011. C. Djega-Mariadassou, L. Bessais, and A. Nandra. Phys. Rev. B, 68:024406, 2003. L. Bessais, C. Djega-Mariadassou, A. Nandra, M. D. Appay, and E. Burzo. Phys. Rev. B, 69:064402, 2004. Bai Yang, Bao Gen Shen, Tong Yun Zhao, and Ji Rong Sun. Materials Science and Engineering B, 145:11, 2007. R. Skomski. J. Phys. Condens. Matter, 15:841, 2003. R. Coehoorn, D.B Mooij, and C. Waard. J. Magn. Magn. Mater, 80:101, 1989. L. Withanawasam, A.S. Murphy, G.C. Hadjipanayis, and R.F.Krause. J. Appl. Phys, 76:7605, 1994. A. Manaf, R.A. Buckley, and H.A. Davis. J. Magn. Magn. Mater, 128:302, 1993. J. Ding, P.G. McComick, and R. Street. J. Magn. Magn. Mater, 124:1, 1993. W.C.Chang, S.H.Wang, S.J.Chang, and M.Y.Tsa. Journal of Materials Science and Technology, 16:102, 2000. Z.Q.Jin, K.H.Chen, J.Li, H.Zeng, and S.F.Cheng. Acta Materialia, 52:2147, 2004. Z. Chen, Y. Zhang, and G.C. Hadjipanayis. J. Magn. Magn. Mater, 219:178, 2000. L.Zhang and Z.Li. Journal of Alloys and Compounds, 469:422, 2009. H.Zeng, J.Li, Z.L.Wang, J.P.Liu, and S.Sun. NanoLetters, 4:187, 2004. H.M Rietveld. J Appl Crystallogr, 2:65, 1969. J. Rodriguez-Carvajal, MT. Fernandez-Diaz, and JL. Martinez . J. Phys. Condens. Matter, 3:3215, 1991. C. Djega-Mariadassou, L. Bessais, A. Nandra, J. M. Greneche, and E. Burzo. Phys. Rev. B, 65:014419, 2001. K.H.J. Buschow and A.S.V. der Goot. J. Less-Common Met., 14:323, 1968. D. Givord, J. Laforest, J. Schweizer, and F. Tasset. J. Appl. Phys., 50:2008, 1979. L. Bessais, E. Dorolti, and C. Dj´ ega-Mariadassou. App. Phys. lettres, 87:192503, 2005. C. Djega-Mariadassou and L. Bessais. J. Magn. Magn. Mater, 210:81, 1999. M. Phejar, V. Paul-Boncour, and L. Bessais. Intermetallics, 18:2301, 2010. E.P Elsukov, G.N Konygin, and V.E Porsev. Fiz. Met. Mettalloved, 105:152, 2008. R. Hawig E.F. Kneller. IEEE Trans. Magn., 27:3558, 2001. L. N´ eel. J. Phys. Radium, 9:184, 1948. N.V. Rama Rao, R. Gopalana, M. Manivel Raja, V. Chandrasekaran, D. Chakravarty, R. Sundaresan, R. Ranganathan, and K. Hono. J.Magn.Magn.Mater, 312:252, 2007. R. Fischer, T. Schrefl, H. Kronmuller, and J. Fidler. J. Magn. Magn. Mater, 150:329, 1995. Hong wei Zhang, Shao ying Zhang, Bao gen Shen, and Chin Lin. J. Appl.Phys, 85:8, 1999. Eric E. Fullerton, J.S. Jiang, and S.D. Bader. J.Magn.Magn.Mater, 200:392, 1999. J. P. Liu, R. Skomski, Y. Liu, and D. J. Sellmyer. J. Appl. Phys, 87:9, 2000.

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[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

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Table 1: The evolution of the well known R-3m rhombohedral sites to the new out-of-equilibrium the P6/mmm hexagonal ones

New hexagonal phase 1a(0, 0, 0) 2e(0, 0, z) 6l(x, 2x, 0) 1 3 of 3g(1/2, 0, 1/2) 1 3 of 3g(1/2, 0, 1/2)

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−→ −→ −→ −→ −→

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Rhombohedral 6c(0, 0, z) 6c(0, 0, z) 18f(x, 0, 0) 9d(1/2, 0, 1/2) 18h(x, x, z)

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x 0 0 0.289 1/2 1/2

y 0 0 0.578 0 0

z 0 0.288 0 1/2 0

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Sm(1a) Fe(2e) Fe(6l) Fe(3g) C(3f)

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Table 2: Atom Wyckoff positions in the P6/mmm hexagonal of the SmFe8.75 Si0.25 C phase

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Table 3: Structural results deduced from the Rietveld fit of XRD patterns of (A), (B) and (C) samples. D is the grain size. wt% is the phase abundance. χ2 and RB the Rietveld factors

(B)

a(˚ A) 5.0242(1) 2.8684(5)

c(˚ A) 4.1953(4)

5.0246(2) 2.8702(1)

4.1951(8)

5.0244(5)

4.1950(2)

wt% 67.5 30.2 2.3 82.1 14.8 3.1 95.1 0.8 4.1

D(nm) 19 22

χ2 1.31

1.25

2.44

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1.58

3.65

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RB 3.27

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Phase SmFe8.75 Si0.25 C α-Fe Sm2 O3 SmFe8.75 Si0.25 C α-Fe Sm2 O3 SmFe8.75 Si0.25 C α-Fe Sm2 O3

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Sample (A)

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Table 4: Magnetic properties at room temperature .

Mr (emu/g) 72 66 61

Hc (kOe) 7.9 9.7 12.6

K (MJ/m3 ) 5.8 6.7 6.9

(BH)max (kJ/m3 - MGOe) 76.7 - 9.7 84.1 - 10.6 103.3 - 13.1

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Ms (emu/g) 145 128 119

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Sample (A) (B) (C)

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Table 5: Magnetic properties at 10K.

Mr (emu/g) 74 69 64

Hc (kOe) -

K (MJ/m3 ) 7.8 8.1 8.3

(BH)max (kJ/m3 ) -

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Ms (emu/g) 148 134 121

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Sample (A) (B) (C)

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2e 31.5 0.062 0.020 20.68

3g 22.9 0.045 -0.015 13.74

6l 25.8 0.021 -0.106 13.65

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µ0 HHF δ Q WSC volume

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Table 6: M¨ ossbaeur hyperfine parameter for the sample (A), as an example, at room temperature. Hyperfine field, µ0 HHF (T); isomer shift, δ (mm/s); quadrupole interaction, 2 (mm/s); and the WSC volumes (˚ A). Linewidth = 0.32 mm/s.

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(301)

(202)

(211)

(201)

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(101)

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75

80

(degree)

AC C

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D

Figure 1: X-ray diffraction pattern of SmFe8.75 Si0.25 , before carbonation, synthesized with 25% excess of Sm

14

SC

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AC C

EP

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D

M AN U

Figure 2: The CaCu5 type structure . Blue spheres represent Ca atoms, red and pink - Cu atoms. Wyckoff positions are indicated

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(200)

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1 SmFe

Si

8.75

0.25

2 Sm O

(220)

(113)

RI PT

3

(301)

(202)

(211)

(201)

(002)

(110)

Intensity (a.u)

(101)

2

SC

1

2

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70

75

M AN U

25

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85

(degree)

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Figure 3: Rietveld analysis for X-ray diffraction pattern of SmFe8.75 Si0.25 before carbonation synthesized with 25% excess of Sm

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α−Fe

RI PT

Intensity (a.u)

(A)

(B)

25

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40

45

50

55

angle 2θ (deg)

60

SC

(C)

65

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75

80

AC C

EP

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Figure 4: XRD patterns of SmFe8.75 Si0.25 C samples with 15%(A), 20%(B), and 25%(C) extra Sm compensation

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Sample (A)

1 SmFe8.75Si0.25C

RI PT

Intensity (a.u)

2 α-Fe 3 Sm2O3

1 2 3

1 SmFe8.75Si0.25C

SC

Sample (C)

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M AN U

2 α-Fe 3 Sm2O3

45

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75

1 2 3

80

85

Angle 2q (deg)

AC C

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Figure 5: Observed (dots) and calculated (solid line) XRD patterns of sample (A) and (C). Vertical bars represent the positions of the Bragg reflections. The observed-calculated difference is depicted at the bottom of the figures

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M AN U

SC

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AC C

EP

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D

Figure 6: Local environment for the 3f carbon site in the SmFe9 C compound (structure type P6 / mmm)

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RI PT

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AC C

EP

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D

M AN U

SC

Figure 7: EDX spectra of samples A (left) and C (right)

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M AN U

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AC C

EP

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D

Figure 8: TEM image of sample (C)

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Figure 9: HR TEM images of samples (A) (B) and (C)

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150

150

50

50

M(emu/g)

100

0

0

-50

-50

-100

-100

300 K -60

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-150 -70

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-60

H(kOe)

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H(kOe)

150

Sample (C)

M AN U

100

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M(emu/g)

-50

SC

-150 -70

300 K

0

-50

-100

300 K

-60

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-40

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-10

D

-150 -70

0

10

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70

H(kOe)

EP

TE

Figure 10: Hysteresis loops of studied composites measured at room temperature

AC C

M(emu/g)

100

RI PT

Sample (B)

Sample (A)

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70

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150

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M(emu/g)

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0

0

-50

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-100

10 K -150 -90 -80 -70 -60 -50 -40 -30 -20 -10 0

10 K

-150 -90 -80 -70 -60 -50 -40 -30 -20 -10 0

SC

10 20 30 40 50 60 70 80 90

H(kOe)

10 20 30 40 50 60 70 80 90

H(kOe)

150

Samlpe (C)

M AN U

100

50

M(emu/g)

RI PT

Sample (B) 100

0

-50

-100

10 K

D

-150 -90 -80 -70 -60 -50 -40 -30 -20 -10 0

10 20 30 40 50 60 70 80 90

H(kOe)

EP

TE

Figure 11: Hysteresis loops of studied composites measured at 10 K

AC C

M(emu/g)

Sample (A) 100

24

RI PT

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SC

Megacount

(A)

M AN U

(B)

(C)

-8

-6

-4

-2

0

2

4

6

8

D

Source Velocity (mm/s)

AC C

EP

TE

Figure 12: Room temperature M¨ ossbauer spectra of SmFe8.75 Si0.25 C samples A, B and C.

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TE D

M AN U

SC

RI PT

• SmFe8.75Si0.25C/a-Fe was prepared by high energy ball milling. • The proportion of a-Fe decreases with the increase of Sm compensation. • The nanograins are crystalline with an average size of 20 nm.