Enhancement of coercivity and saturation magnetization of Al3+ substituted M-type Sr-hexaferrites

Accepted Manuscript Enhancement of coercivity and saturation magnetization of Al Sr-hexaferrites

3+

substituted M-type

F. Rhein, R. Karmazin, M. Krispin, T. Reimann, O. Gutfleisch PII:

S0925-8388(16)32463-X

DOI:

10.1016/j.jallcom.2016.08.085

Reference:

JALCOM 38586

To appear in:

Journal of Alloys and Compounds

Received Date: 9 May 2016 Revised Date:

9 August 2016

Accepted Date: 10 August 2016

Please cite this article as: F. Rhein, R. Karmazin, M. Krispin, T. Reimann, O. Gutfleisch, Enhancement 3+ of coercivity and saturation magnetization of Al substituted M-type Sr-hexaferrites, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.08.085. 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.

ACCEPTED MANUSCRIPT

Enhancement of coercivity and saturation magnetization of Al3+ substituted M-type Sr-hexaferrites F. Rhein*+, R. Karmazin*, M. Krispin*, T. Reimann$, O. Gutfleisch+ * Corporate Technology, Siemens AG, Otto Hahn Ring 6, 81739 Munich, Germany Department of SciTec, Ernst-Abbe-University of Applied Sciences Jena, Carl-Zeiss-Promenade 2, 07745 Jena, Germany +

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$

Institute of Materials Science, Technische Universität Darmstadt, Alarich-Weiss-Str. 16, 64287 Darmstadt, Germany

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Abstract

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Hexagonal SrFe12-xAlxO19 (x = 0, 0.2, 1, 2, 4) powders were prepared via mechanochemical activation and subsequently calcined at different temperatures (between 900 °C and 1300 °C). Afterwards the powders were milled by high energy milling and annealed at 1000 °C in NaCl to obtain ultrafine nano-particles. The particle size, measured by scanning electron microscopy (SEM), varied between 50 nm and few micrometers depending on Al3+ substitution and calcination temperature.

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Average crystallite size, determined by X-ray diffraction (XRD), decreases from 330 nm ± 30 nm (x = 0) to 70 nm ± 10 nm (x = 4) by substitution of Fe3+ by Al3+ for optimized calcination temperature of

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1100 °C. Furthermore coercivity measured by SQUID magnetometry increases from 420 kA/m (x = 0) to 970 kA/m (x = 4). A maximum saturation magnetization of 74 Am²/kg (x = 0) was observed. With

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substitution of Fe3+ by Al3+ saturation magnetization decreases monotonously to 28 Am²/kg (x = 4). Annealing in NaCl matrix at lower temperatures compared to calcination temperatures leads to a further increase of coercivity. At the same time saturation magnetization increases for SrFe12-xAlxO19 (x = 0, 1) by NaCl annealing treatment. Additionally, we discuss the initial magnetization curves of SrFe12O19 and SrFe8Al4O19 after different processing steps with respect to the specific reversal mechanism of hexaferrites. The here proposed processing route enables a simultaneous enhancement of coercivity and saturation magnetization as compared to conventional ceramic method [1, 2]. The presented processing route can solve challenges of conventional manufacturing

Email address: [email protected]

ACCEPTED MANUSCRIPT steps towards single domain grains in rare earth free SrFe12-xAlxO19 for higher coercivity and could enable an improved industrial production process. Keywords: hard magnetic materials, Sr-ferrite, mechanochemical processing, annealing treatment,

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nanocrystalline structure, magnetic measurements 1. Introduction

M-type hexagonal ferrites such as BaFe12O19 and SrFe12O19 are of enormous industrial

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importance. In 2012 they were the mostly used permanent magnets accounting for a volume of more than 80 % of the global demand [3, 4]. Hexaferrites are relatively cheap, mechanically and chemically

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stable, corrosion resistant, with low electrical conductivity and possess a relatively large saturation magnetization (Ms) with high intrinsic coercivity (iHc) and magnetic anisotropy field (HA). However, compared to rare earth based permanent magnets the range of applications for electrical engines and generators is limited because of the lower iHc and Br. High coercivity iHc can be achieved by

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synthesis of single domain particles [5], while the critical single domain particle size Dc of SrFe12O19 is approximately 740 nm [6]. A further increase of iHc can be achieved by increasing the magnetic anisotropy for example by addition of cobalt and light rare earth elements like lanthanum, which

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boosts the raw material costs. Alternatively substitution of Fe3+ by Al3+ results in an increase of coercivity iHc and increases magnetic anisotropy field HA (HA = 2αK1/Ms) at a simultaneously decrease

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of saturation magnetization Ms following equation (1) [7,8] iHc

= 2αK1/Ms - βMs/µ0,

eq. (1)

Here, magnetocrystalline anisotropy constant K1 is proportional to the total number of moments per unit cell and can be calculated for SrFe12-xAlxO19 by K1(x) = K1(0)*(24-4x)/24 with K1(0) = 3.57 *106 erg/cm3 for pure SrFe12O19 [9]. The factors α and β depend on the specific reversal mechanism and morphology of hexaferrite particles. The coefficient α is governed by the particle size, for small particles α is increased and leads to an increase of coercivity [7]. The factor β depends on remanence Mr and demagnetization factor N, more precisely on the particle shape, especially in terms of aspect 2

ACCEPTED MANUSCRIPT ratio W/t (W: width, t: thickness parallel to easy axis). Coercivity iHc decreases when N increases and the particle shape becomes more platelet-like, i.e. the aspect ratio W/t increases [2]. Particle morphology and magnetic properties of SrFe12-xAlxO19 powders depend on the synthesis route. Various attempts of chemical methods have been reported in literature, for example

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hydrothermal [10], sol-gel [9, 11, 12], co-precipitation [13, 14, 15], self-propagating reaction [16, 17] or auto combustion [18, 19] are known. With those methods small particles with high coercivity iHc and low saturation magnetization Ms can be obtained. In contrast, conventional ceramic process of

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solid-state reaction [20, 21] and molten-salt method [22] results in powders with high saturation magnetization Ms (due to a high calcination temperature up to 1300 °C) with decreased coercivity iHc.

iHc

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For example in Ref [1] the synthesis of small particles of approximately 300 nm with high coercivity of 422 kA/m and saturation magnetization Ms of 70.9 Am²/kg by mechanochemical activation of

Fe2O3 and SrCl2 in a Na2CO3 aqueous solution is reported.

In the present work, a low temperature process using mechanochemical activation (MCA) is

high coercivity iHc.

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introduced. A low calcination temperature of 1100 °C enables the synthesis of small particles with

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An advantage of the present synthesis is an easier mechanochemical activation of the raw materials Fe2O3 and SrCO3 in comparison to Na2CO3 aqueous solution method used in [1]. After calcination the

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powder was subsequently milled and annealed at lower temperatures in a NaCl melt to achieve well crystallized small hexaferrite particles with high saturation magnetization Ms above 74 Am²/kg. The phase composition, particle and crystallite sizes, lattice parameters and magnetic properties at each process step were analyzed. Based on the initial magnetization curve the specific reversal mechanisms of different hexaferrite powders are discussed.

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ACCEPTED MANUSCRIPT 2. Experimental 2.1 Synthesis Precursor materials Fe2O3 (Sigma Aldrich, 99 % purity), SrCO3 (Sigma Aldrich, 99% purity) and

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Al2O3 (Riedel-de Haë, pure) were mixed in a stoichiometric ratio. 600 g of the mixed powders were mechanochemical activated in an attritor (VMA DISPERMAT® SL, 5500 rpm) with 0.75 kg ZrO2 balls of 1 mm in diameter for 6 h at room temperature. The powder to isopropanol (milling medium) volume

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ratio was 1:10. After mechanical activation the powder was calcined at different temperatures between 900 °C and 1300 °C with dwell times of 1 h, heating rate of 60 K/h and cooling rate of

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600 K/h. Afterwards the calcined powders were milled in an attritor with 4000 rpm for 3 h (same conditions as the mechanochemical activation). The milled powders were sieved and annealed in presence of NaCl (powder to NaCl weight ratio 1:1) at 1000 °C for 1 h. Finally, the powders were

2.2 Characterization

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washed with deionized water to remove NaCl.

Phase ratios, lattice parameters and crystallite size of different powders were determined by Xray diffraction (XRD) investigations using a D8 Advance (Bruker AXS, CuKα). Scanning electron

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microscopy (SEM, Phillips/FEI XL-30) was used for an analysis of particle morphology and particle size

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distribution. Magnetic properties (coercivity iHc and saturation magnetization Ms) of hexaferrite powders were measured using a SQUID magnetometer (Quantum Design MPMS XL) with a maximum applied field of 5570 kA/m at 300 K. Saturation magnetization Ms was taken as the magnetization at maximum applied field of 5570 kA/m.

3. Results and Discussion 3.1 Activation and calcination

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ACCEPTED MANUSCRIPT Due to the mechanochemical activation a lower calcination temperature, compared to the conventional ceramic process of solid-state reaction, is used to synthesize pure Sr-hexaferrite powders with small particle and crystallite sizes. The XRD pattern of Alx substituted Sr-hexaferrite powders for x = 0, 0.2, 1, 2, 4 after calcination at 1100 °C for 1 h are shown in Fig. 1. The main phase

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is typically the hexagonal ferrite (magnetoplumbite ferrite phase, M-type ferrite) structure (> 95 %) with space group P63/mmc (194). The observed secondary phase (small reflections at 2Ѳ = 32.82° and 46.13°) of SrFe12-xAlxO19 (x = 0, 0.2) can be identified as Sr4Fe6O13 (purple stars, Fig. 1). For larger

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Al content (SrFe12-xAlxO19; x = 2, 4) additional lines are present at 2Ѳ = 20.10°, 28.39° and 29.92° which are typical of SrAl2O4 (green triangles, Fig. 1). Reflections of XRD pattern are broader and

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shifted to higher 2Ѳ values with increasing Al3+ content. To synthesize directly pure M-type hexaferrite powders longer dwell times are necessary, resulting in an increase of particle sizes. At lower calcination temperatures the content of secondary phase increases. Results of determination of crystallite size D and lattice parameters by Rietveld refinement are

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shown in Table 1. Both, crystallite size and lattice parameters tend to decrease with increasing Al3+ content, which is in agreement with literature. The substitution of Fe3+ by Al3+ inhibits grain growth which results in a decrease of crystallite size [9]. In addition a lattice contraction of the unit cell can

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be observed, due to a smaller ionic radius of Al3+ cation (0.535 Å) compared to the ionic radius of Fe3+ (0.64 Å) as reported in [23]. Rietveld refinement results in lattice parameters c = 23.059 ± 0.001 Å

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and a = 5.885 ± 0.001 Å for SrFe12O19 and decreased lattice parameters c = 22.936 ± 0.006 Å and a = 5.848 ± 0.002 Å for SrFe8Al4O19. The ratio of c/a increases from 3.918 ± 0.001 to 3.922 ± 0.002 with increasing Al3+ content. Crystallite size D, determined from X-ray diffraction pattern (Fig. 1), declines from 330 nm ± 30 nm (SrFe12O19) to 70 nm ± 10 nm (SrFe8Al4O19) after calcination at 1100 °C. SEM micrographs of SrFe12-xAlxO19 (x = 0, 1, 2, 4) powders calcined at 1100 °C are shown in Fig. 2 (a) (d). For SrFe12O19 particle size distribution from 89 nm to 1039 nm (median of volume particle size distribution WSEM; ¯ = 396 nm, Table 2) and partial formation of sintering necks can be seen in Fig. 2 (a). 5

ACCEPTED MANUSCRIPT Typical hexagonal platelets in the range of few micrometers are observed for calcination temperatures above 1150 °C (not shown). SEM analysis confirms the inhibition of particle growth due to the substitution of Fe3+ by Al3+. Al3+ substituted SrFe11Al1O19 (Fig. 2 (b)) exhibits a narrower particle size distribution from 90 nm to 890 nm (WSEM; ¯ ~ 230 nm) and sintering necks still exist between

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small particles. Morphology of SrFe10Al2O19 particles are very similar to SrFe8Al4O19 particles, shown in Fig. 2 (c) and Fig. 2 (d). A further decrease of particle sizes is observed with a particle size distribution from

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approximately 50 nm to 830 nm (WSEM; ¯ ~ 200 nm). The measured particles are smaller in thickness and width with less formation of sintering necks. The formation of hexagonal platelets in the range of

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few micrometers starts at calcination temperature of 1200 °C and higher for SrFe8Al4O19 (not shown). The influence of substitution of Fe3+ by Al3+ onto magnetic properties of SrFe12-xAlxO19 (x = 0, 0.2, 1, 2, 4) powders is shown in Fig. 3. Coercivity iHc and saturation magnetization Ms of SrFe12-xAlxO19 powders were measured using a SQUID magnetometer. For SrFe12O19 powder (open squares, Fig. 3)

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coercivity iHc increases up to 420 kA/m after calcination at 900 °C, at higher calcination temperatures coercivity iHc declines to 180 kA/m (Tcalc = 1300 °C) in contrast. Saturation magnetization Ms increases from 70 Am²/kg after calcination at 900 °C to 74 Am²/kg after calcination at 1300 °C. The maximum

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coercivity iHc of 430 kA/m for SrFe11.8Al0.2O19 powder (green circles, Fig. 3) is observed after

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calcination at 1000 °C. At higher calcination temperatures coercivity iHc drops continuously to 220 kA/m (1300 °C). The saturation magnetization Ms is nearly constant 63 Am²/kg. A further increase of coercivity iHc up to 510 kA/m is reached for SrFe11Al1O19 (blue triangles, Fig. 3) after calcination at 1100 °C, at higher calcination temperatures coercivity iHc decreases to 400 kA/m (Tcalc = 1300 °C). The saturation magnetization Ms of SrFe11Al1O19 decreases slightly from 60 Am²/kg (Tcalc = 900 °C) to approximately 55 Am²/kg after calcination at 1300 °C. For SrFe10Al2O19 powders (orange squares, Fig. 3) the coercivity iHc was increased from 260 kA/m to 650 kA/m by increasing calcination temperature from 900 °C to 1200 °C. At higher calcination temperatures of 1300 °C coercivity iHc drops to 530 kA/m (Tcalc = 1300 °C). Saturation magnetization Ms of SrFe10Al2O19 powders decreases 6

ACCEPTED MANUSCRIPT continuously from 53 Am²/kg to 43 Am²/kg with increasing calcination temperature from 900 °C to 1300 °C. Highly Al3+ substituted SrFe8Al4O19 powder (red stars, Fig. 3) exhibits a strong increase of coercivity iHc from 280 kA/m (Tcalc = 900 °C) to 970 kA/m (Tcalc = 1200 °C). However, for calcination temperature of 1300 °C coercivity iHc drops slightly to 940 kA/m. The saturation magnetization

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decreases from 39 Am²/kg to 19 Am²/kg by increasing calcination temperature from 900 °C to 1300 °C.

Obviously, higher Al3+ content inhibits particle growth and phase formation, after calcination at same

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temperature, which results in smaller crystallite sizes and an increase of Al3+ rich secondary phase (Tcalc ≤ 1100 °C). The increase of coercivity can be explained by small particles in the range of the

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critical single domain size Dc and with an increased magnetic anisotropy field HA, which is for Al3+ substituted Sr-hexaferrites experimentally shown and reported in [6]. The authors report an increase of critical single domain size Dc up to 6.1 µm for SrFe8Al4O19 and simultaneously decreased lattice parameters a = 5.78 (0.03) Å and c = 22.76 (0.52) Å which results in coercivity iHc = 1290 kA/m and

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saturation magnetization Ms = 11.8 Am²/kg. In contrast, the lattice parameters in the present work are slightly higher and residues of Fe2O3 and SrAl2O4 can be observed after calcination at 1100 °C, which confirms the assumption that a calcination temperature of 1100 °C is too low to synthesize

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pure SrFe8Al4O19.

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SrFe12-xAlxO19 (x = 0, 1, 2, 4) powders, calcined at 1100 °C, are used for the following milling and annealing experiments. For these powders a small particle size, below the critical single domain size, and a phase purity ≥ 95 % was achieved. Lower calcination temperatures result in a lower fraction of Sr-hexagonal phase, higher calcination temperatures lead to particle growth above the critical single domain size compared to calcination at 1100 °C.

3.2 Milling and annealing XRD on 3 h milled powders shows partial degradation of SrFe12-xAlxO19 (x = 0, 1, 2, 4) phase and an increase of secondary phases such as Fe2O3, Sr4Fe6O13 and SrAl2O4. The XRD pattern, displayed 7

ACCEPTED MANUSCRIPT in Fig. 4, exhibits broader peaks. Annealing at 1000 °C in NaCl promotes recrystallization of SrFe12xAlxO19

(x = 0, 4) phases without formation of secondary phases. The secondary phase of SrAl2O4

(cyan circles, Fig. 4) vanishes after annealing treatment. Rietveld refinement results in a crystallite size of 51 ± 1 nm.

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SEM and EDX (not shown) analyses confirm the partial degradation of Sr-hexaferrite phase after milling. Particles of SrFe12O19 after 3 h milling are presented in Fig. 2 (e). They are smaller, in the range from 45 nm to 808 nm (WSEM; ¯ = 262nm), irregular shaped with surface defects and less

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agglomerated, compared to SrFe12O19 particles after calcination. After annealing treatment (Fig. 2 (g)), the particle size increases (WSEM; ¯ = 284 nm), the surfaces are smoother and the particles

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appear more cubic, along with a reduced aspect ratio of WSEM; ¯ /tSEM;¯ = 2.04 and with less sintering necks compared to SrFe12O19 particles after calcination and milling.

Fig. 2 (f) shows the SrFe8Al4O19 particles after the milling process. Particles are edged, irregular with surface defects and strong agglomerated. After annealing treatment, the particle surface is rounded

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with simultaneously degradation of surface defects, as shown in Fig. 2 (h). The particles seem smaller (33 nm - 480 nm, WSEM; ¯ = 154 nm), flatter and more agglomerated compared to SrFe8Al4O19 particles after calcination (Fig. 2 (d)). A quantitative analysis of aspect ratio for SrFe12-xAlxO19 (x= 2, 4)

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powders is significantly affected by errors. Just a few particles are perpendicularly orientated in

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which case the thickness t can be determined by SEM micrographs (Table 2). Magnetic properties of SrFe12-xAlxO19 (x = 0, 1, 2, 4) powders after calcination, 3 h milling and subsequent annealing are shown in Fig. 5 (a) and (b). Coercivity iHc of SrFe12O19 powder (open squares Fig. 5 (a)) decreases from 370 kA/m to 190 kA/m by high energy milling, after annealing it rises up to 430 kA/m. Similar trend is observed for saturation magnetization Ms, which decreases by milling from 71.5 Am²/kg to 55 Am²/kg and increases to 74 Am²/kg after annealing treatment. Values have been extracted from M-H hysteresis loops as displayed in Fig. 5 (c) at applied field of 5570 kA/m. SrFe11Al1O19 powder (blue triangles) exhibits same behavior like SrFe12O19 powder, coercivity

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ACCEPTED MANUSCRIPT iHc

drops after milling from 510 kA/m to 250 kA/m and increases after annealing up to 520 kA/m.

Saturation magnetization Ms decreases from 58.5 Am²/kg to 53.5 Am²/kg and was increased by annealing treatment to 61 Am²/kg. For highly Al3+ substituted SrFe12-xAlxO19 (x = 2, 4) powders a larger increase of coercivity iHc and a

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slight decrease of saturation magnetization Ms after annealing treatment, compared to magnetic properties after calcinations is observed. Coercivity iHc of SrFe10Al2O19 (orange squares, Fig. 5 (a) and (b)) decreases from 570 kA/m to 330 kA/m by milling and rises up to 650 kA/m. While saturation

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magnetization Ms shows only a slight decrease from 44.5 Am²/kg to 41.5 Am²/kg after milling and a slight increase to 43 Am²/kg by annealing treatment. For SrFe8Al4O19 powder (red stars, Fig. 5 (a) and

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(b)) coercivity iHc drops from 730 kA/m to 370 kA/m after milling, subsequent annealing treatment results in a strong increase up to 780 kA/m. Saturation magnetization decreases continuously from 28 Am²/kg to 25.5 Am²/kg after milling to 23 Am²/kg after annealing.

The XRD, SEM and SQUID magnetometer measurements confirm a partial degradation of particles

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and of the hexaferrite phase, respectively, after milling. The induced surface defects, observed with SEM (Fig. 2 (e) and (f)), and partial degradation of hexaferrite phase, confirmed by XRD, are reasons for reduced magnetic properties after milling process. Drops of saturation magnetization Ms can be

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explained by a partial destroyed hexaferrite phase and amorphous structures. Furthermore

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destruction of particles, inducing of stress and surface defects can decrease coercivity iHc of hexaferrite particles.

Al3+ rich powders, such as SrFe12-xAlxO19 (x = 2, 4), exhibit a lower decrease of saturation magnetization Ms after milling, probably based on smaller particles and crystallite sizes after calcination. The applied 3 h milling process has a lower influence on particle and crystallite sizes, which are in the dimension of approximately 200 nm. SEM analysis, shown in Fig. 2 (f), confirms this assumption. The particle surfaces are more irregular, but the size of milled particles is in the same order of magnitude as that of the particles after calcination. For SrFe12O19 the milling process results

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ACCEPTED MANUSCRIPT in irregular particle shape with surface defects as well, but the particles are significant smaller (Fig. 2 (e)) compared to the calcined particles (Fig. 2 (a)). Our results are consistent with earlier studies on high energy milled Sr-hexaferrites, which show that magnetic properties are strongly affected by milling processes [5, 24].

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Annealing treatment at 1000 °C in NaCl melt improves diffusion processes and therefore a recrystallization of the hexaferrite phase after milling. XRD measurements confirm the improved recrystallization process, which results in pure hexaferrite phase without presence of secondary

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phases. Furthermore a lower annealing temperature of 1000 °C compared to 1100 °C calcination temperature enables synthesis of smaller particles with a low particle aspect ratio of WSEM; ¯ /tSEM;¯ ~

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2 , especially for powders with low Al3+ content, confirmed by SEM analysis. The recrystallization of small particles and the annealing of surface defects, verified by SEM (Fig. 2 (g) and (h)), improve the magnetic properties of hexaferrite powders compared to magnetic properties after calcination. Saturation magnetization of pure SrFe12O19 and low Al3+ substituted SrFe11Al1O19 powders increase by

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milling and subsequently annealing based on an improved recrystallization process. Saturation magnetization Ms of highly substituted Al3+ powder decreases based on the improved substitution of Fe3+ by Al3+ and therefore decreased magnetic moment.

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The influence of processing steps on magnetization behavior of different SrFe12-xAlxO19 was

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investigated. M-H hysteresis loops of SrFe12O19 powder and SrFe8Al4O19 powder, measured by SQUID magnetometer after calcination at 1100 °C, 3h milling and subsequently annealing, as shown in Fig. 6. Differences of initial magnetization curves after several process steps can be identified. For both powders the magnetic permeability (slope of initial magnetization curves) decreases after milling and annealing compared to the calcined powder in the range from 0 to 250 kA/m. A detailed physical interpretation is given in the following discussion.

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ACCEPTED MANUSCRIPT Fig. 6 shows the M-H hysteresis loops with initial magnetization curves of SrFe12O19 and SrFe8Al4O19 powders, which are measured to understand magnetic reversal mechanisms and the influence of milling and annealing onto domain wall motion processes after different process steps. There are two different types of coercivity mechanisms for particles larger than the critical single domain size, i)

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nucleation such as in SrFe12O19, Nd2Fe14B or SmCo5 and ii) pinning such as in Sm2Co17 [7]. For nucleation type magnets the saturation magnetization Ms is reached quickly, at fields much lower than the coercive field, triggered by easily moving domain walls. A high coercivity is caused by

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hindered nucleation of reversed domains [7]. The magnetic susceptibility in the initial magnetization curve of pinning type magnets is much lower; due to pinning at precipitations or defects the existing

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domain walls are immobile. A high field close to coercivity iHc is necessary to unpin the domain walls and to reach saturation magnetization Ms. Also a pinning of domain walls at the surface of grains in nucleation type magnets, such as SrFe12O19, has been discussed in literature [7]. For single domain particles a reversal of the magnetization by coherent rotation of all magnetic moments exists, which

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exhibits a high coercivity iHc. One could also state that in this case nucleation- and pinning-type processes converge and a distinction between them is not useful. In our work we observe a steep slope of the initial magnetization curve until the applied field amounts approximately 250 kA/m,

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which points to a nucleation-type mechanism. For comparison a hysteresis loop of SrFe12O19 powder calcined at 1300 °C is shown in Fig. 6 (a) (black line). In this case only large multi domain particles

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exist which exhibit nucleation mechanism. The slope of initial magnetization curve of SrFe12O19 powder calcined at 1100 °C is flatter above 200 kA/m which suggests that at this point a second coercivity mechanism sets in, more precisely the coherent rotation of magnetic moments for single domain particles.

Due to the milling process the content of single domain particles and pinning centers such as surface defects increases. The destruction of large particles with multi domain behavior results in a lower magnetic permeability (Fig. 6 green triangles), compared to the initial magnetization curve after calcination. The magnetic properties decrease based on the partial phase degradation, discussed

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ACCEPTED MANUSCRIPT above. After annealing the magnetic properties of SrFe12O19 are improved, based on the lower annealing temperature compared to calcination temperature, and result in a higher content of particles smaller than the critical single domain size, which results in a flatter initial magnetization curve (lower permeability, Fig. 6 blue squares).

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Additionally the magnetic anisotropy field HA of a highly substituted Al3+ powder such as SrFe8Al4O19 increases compared to unsubstituted SrFe12O19 which results in higher coercivity and an even flatter slope of initial magnetization curve for single domain particles (Fig. 6 (b)). The saturation

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magnetization decreases due to the improved recrystallization process and substitution of Fe3+ by

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Al3+ by annealing treatment.

In this study a quantitative analysis of volume fraction of nucleation mechanism, pinning mechanism and coherent rotation of all magnetic moments is not possible as several reversal mechanisms overlap. Magnetic Force Microscopy (MFM) measurements are planned to investigate the influence

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Conclusions

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of milling and annealing process onto hexaferrite particles and grains in detail.

(1) Hexagonal SrFe12-xAlxO19 (x = 0, 0.2, 1, 2, 4) powders were successfully synthesized via a

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mechanochemical activation process, resulting in single phase powder after calcination at temperatures above 1100 °C.

(2) Substitution of Fe3+ by Al3+ leads to an increase of coercivity iHc for calcination above 1000 °C and inhibits particle and crystallite growth. (3) The presented milling process exhibits a strong decrease of magnetic properties, especially for lower Al3+ substituted powders. (4) Subsequent annealing in NaCl melt at 1000 °C promotes the formation of pure M-type hexagonal ferrites with small crystallite sizes, especially for high Al3+ substituted Sr hexaferrites. 12

ACCEPTED MANUSCRIPT (5) Furthermore milling and annealing treatment improves simultaneously coercivity iHc and saturation magnetization Ms of SrFe12-xAlxO19 (x = 0, 1) compared to conventional ceramic process of solid-state reaction.

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(6) The increase of coercivity is explained by the increase of single domain particles which is based on analysis of initial magnetization curves. Acknowledgement

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The authors wish to thank Dr. Thomas Berthold for the SEM analysis. This work was supported by

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the German Federal Ministry of Education and Research (grant no 03X3582 - KomMa).

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[10] M. Jean, V. Nachbaur, J. Bran, J.M. Le Breton, Synthesis and characterization of SrFe12O19 powder obtained by hydrothermal process, Journal of Alloys and compounds 496.1 (2010): 306-312.

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[11] C. Sürig, K.A. Hempel, D. Bonnenberg, Formation and microwave absorption of barium and strontium ferrite prepared by sol-gel technique, Applied physics letters 63.20 (1993): 2836-2838. [12] Q. Fang, Y. Liu, P. Yin, X. Li, Magnetic properties and formation of Sr ferrite nanoparticle and Zn, Ti/Ir substituted phases, Journal of magnetism and magnetic materials 234.3 (2001): 366-370.

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[13] M.J. Iqbal, M.N. Ashiq, Physical and electrical properties of Zr–Cu substituted strontium hexaferrite nanoparticles synthesized by co-precipitation method, Chemical Engineering Journal 136.2 (2008): 383-389.

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[14] M.M. Hessien, M.M. Rashad, K. El-Barawy, Controlling the composition and magnetic properties of strontium hexaferrite synthesized by co-precipitation method, Journal of magnetism and magnetic

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materials 320.3 (2008): 336-343. [15] Z. Zi, Y. Sun, X. Zhu, Z. Yang, J. Dai, W. Song, Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles, Journal of Magnetism and Magnetic Materials 321.9 (2009): 1251-1255. [16] M.V. Kuznetsov, Q.A. Pankhurst, I.P. Parkin, Self propagating high-temperature synthesis of chromium substituted magnesium zinc ferrites Mg0.5 Zn0.5 Fe2–x CrxO4 (0≤ x≤ 1.5), Journal of Materials Chemistry 8.12 (1998): 2701-2706.

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ACCEPTED MANUSCRIPT [17] I. Parkin, G. Elwin, A. Komarov, Q. Bui, Q. Pankhurst, L. BarquÝn, Y. Morozov, Convenient, low energy routes to hexagonal ferrites MFe12O (M= Sr, Ba) from SHS reactions of iron, iron oxide and MO2 in air, Journal of Materials Chemistry 8.3 (1998): 573-578. [18] H. Luo, B.K. Rai, S.R. Mishra, V.V. Nguyen, J.P. Liu, Physical and magnetic properties of highly

Magnetism and Magnetic Materials 324.17 (2012): 2602-2608.

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aluminum doped strontium ferrite nanoparticles prepared by auto-combustion route, Journal of

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[19] Y.P. Fu, C.H. Lin, Fe/Sr ratio effect on magnetic properties of strontium ferrite powders synthesized by microwave-induced combustion process, Journal of alloys and compounds 386.1

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(2005): 222-227.

[20] A. Arab, M.R. Mardaneh, M.H. Yousefi, Investigation of magnetic properties of MnZn-substituted strontium ferrite nanopowders prepared via conventional ceramic technique followed by a high energy ball milling, Journal of Magnetism and Magnetic Materials 374 (2015): 80-84.

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[21] M.M. Hessien, M.M. Rashad, K. El-Barawy, I.A. Ibrahim, Influence of manganese substitution and annealing temperature on the formation, microstructure and magnetic properties of Mn–Zn ferrites,

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Journal of Magnetism and Magnetic Materials 320.9 (2008): 1615-1621. [22] S.D. Kim, J.S. Kim, Magnetic properties of Sr-ferrites synthesized in molten (NaCl+ KCl) flux,

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Journal of magnetism and magnetic materials 307.2 (2006): 295-300. [23] H. Luo, B.K. Rai, S.R. Mishra, V.V. Nguyen, J.P. Liu, Physical and magnetic properties of highly aluminum doped strontium ferrite nanoparticles prepared by auto-combustion route, Journal of Magnetism and Magnetic Materials 324.17 (2012): 2602-2608. [24] Z. Jin, W. Tang, J. Zhang, H. Lin, Y. Du, Magnetic properties of isotropic SrFe12O19 fine particles prepared by mechanical alloying, Journal of magnetism and magnetic materials 182.1 (1998): 231237.

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ACCEPTED MANUSCRIPT Fig. 1 X-ray diffraction patterns of SrFe12-xAlxO19 powders after calcination at 1100 °C for different concentrations. As secondary phase Sr4Fe6O13 is identified in the SrFe12-xAlxO19 (x = 0, 0.2) patterns. Furthermore a small amount of SrAl2O4 is observed for SrFe12-xAlxO19 (x = 2, 4) patterns. Symbols indicate the characteristic peak positions of the corresponding phases.

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Fig. 2 SEM micrographs of hexaferrite powders SrFe12-xAlxO19 (a) x = 0, (b) x = 1, c) x = 2, d) x = 4 after calcination at 1100 °C, milled powder of SrFe12O19 (e) and SrFe8Al4O19 (f) and annealed powder of

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SrFe12O19 (g) and SrFe8Al4O19 (h).

Fig. 3 iHc (a) and Ms (b) of SrFe12-xAlxO19 powder measured at 300 K depending on calcination

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temperatures.

Fig. 4 X-ray diffraction patterns of SrFe12-xAlxO19 (x= 0, 4) powders after calcination at 1100 °C, high energy milling (HEM) 3h and NaCl-salt annealing treatment 1 h at 1000 °C. Fig. 5 iHc (a) and Ms (b) of SrFe12-xAlxO19 powder measured at 300 K after calcination at 1100 °C, high

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energy milling (HEM) 3h and NaCl-salt annealing treatment 1 h at 1000 °C. Saturation magnetization Ms was taken as the magnetization at maximum applied field of 5570 kA/m. (c) full M-H hysteresis

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loops of SrFe12O19 after calcination at 1100 °C, milling 3h and after annealing. Fig. 6 (a) M-H Hysteresis loops of SrFe12O19 after calcination at 1100 °C, milling 3h and after annealing

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treatment with NaCl salt at 1000 °C. (b) M-H Hysteresis loops of SrFe8Al4O19 after calcination at 1100 °C, milling 3h and after annealing treatment with NaCl at 1000 °C.

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ACCEPTED MANUSCRIPT SrFe12O18 PDF 84-1531

SrFe12-xAlxO19

Sr4Fe6O13 PDF 78-2403

Tcalc.=1100 °C

SrAl2O4 PDF 34-0379

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x=0 x = 0.2 x=1

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intensity (a. u.)

Fe2O3 PDF 89-0599

x=2

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x=4

18 21 24 27 30 33 36 39 42 45 48 51 54 57 60

2Θ (°)

Fig. 1 X-ray diffraction patterns of SrFe12-xAlxO19 powders after calcination at 1100 °C for different

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concentrations. As secondary phase Sr4Fe6O13 is identified in the SrFe12-xAlxO19 (x = 0, 0.2) patterns. Furthermore a small amount of SrAl2O4 is observed for SrFe12-xAlxO19 (x = 2, 4) patterns. Symbols

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indicate the characteristic peak positions of the corresponding phases.

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(b)

(a)

ACCEPTED MANUSCRIPT

1 µm

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1 µm

(d)

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(c)

1 µm

1 µm

(e)

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(f)

(h)

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(g)

1 µm

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1 µm

1 µm

1 µm

Fig. 2 SEM micrographs of hexaferrite powders SrFe12-xAlxO19 (a) x = 0, (b) x = 1, c) x = 2, d) x = 4 after

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ACCEPTED MANUSCRIPT calcination at 1100 °C, milled powder of SrFe12O19 (e) and SrFe8Al4O19 (f) and annealed powder of

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SrFe12O19 (g) and SrFe8Al4O19 (h).

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ACCEPTED MANUSCRIPT (a)

(b) 1000

SrFe11.8Al0.2O19

60

SrFe11Al1O19 SrFe10Al2O19

700

50

Ms [Am²/kg]

SrFe8Al4O19

600

i

500 400

40 30

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800

Hc [kA/m]

70

SrFe12O19

900

SrFe12O19

SrFe11.8Al0.2O19

20

SrFe11Al1O19

300 10

SrFe10Al2O19 SrFe8Al4O19

200 0

850

900

950

1000 1050 1100 1150 1200 1250 1300 1350

850

900

950

1000 1050 1100 1150 1200 1250 1300 1350

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T [°C]

T [°C]

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Fig. 3 iHc (a) and Ms (b) of SrFe12-xAlxO19 powder measured at 300 K depending on calcination

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temperatures.

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ACCEPTED MANUSCRIPT

SrFe12-xAlxO19

SrFe12O18 PDF 84-1531 Sr4Fe6O13 PDF 78-2403 SrAl2O4 PDF 34-0379

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x=0 x = 0 HEM 3 h

x = 0 HEM 3 h annealed x=4

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intensity (a. u.)

Fe2O3 PDF 89-0599

x = 4 HEM 3 h annealed

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x = 4 HEM 3 h

18 21 24 27 30 33 36 39 42 45 48 51 54 57 60

2Θ (°)

Fig. 4 X-ray diffraction patterns of SrFe12-xAlxO19 (x= 0, 4) powders after calcination at 1100 °C, high

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energy milling (HEM) 3h and NaCl-salt annealing treatment 1 h at 1000 °C.

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ACCEPTED MANUSCRIPT (a)

(b) 900

80

800

70

700

60

400 300

SrFe12O19

30 20 10

SrFe10Al2O19

100

SrFe8Al4O19

0

Calcination 1100°C

0 HEM 3h

Annealing 1h

60 40

-20 -40

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HEM 3h

Annealing 1h

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M [Am²/kg]

20 0

SrFe8Al4O19

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SrFe10Al2O19 Calcination 1100°C

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(c)

SrFe12O19

SrFe11Al1O19

SrFe11Al1O19

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200

40

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Ms [Am²/kg]

50

500

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i

Hc [kA/m]

600

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SrFe12O19 1100 °C SrFe12O19 1100 °C milled 3h SrFe12O19 1100 °C annealed

0

2000

4000

6000

H [kA/m]

Fig. 5 iHc (a) and Ms (b) of SrFe12-xAlxO19 powder measured at 300 K after calcination at 1100 °C, high energy milling (HEM) 3h and NaCl-salt annealing treatment 1 h at 1000 °C. Saturation magnetization Ms was taken as the magnetization at maximum applied field of 5570 kA/m. (c) M-H hysteresis loops of SrFe12O19 after calcination at 1100 °C, milling 3h and after annealing.

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(a)

80

Tcalc= 1100 °C 60 40

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M [Am²/kg]

20 0 -20 -40

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SrFe12O19

SrFe12O19 milled 3h

-60

SrFe12O19 annealed

-1000

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SrFe12O19 1300 °C

-80 -2000

0

1000

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H [kA/m]

(b)

30

Tcalc= 1100°C

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20

0

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M [Am²/kg]

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SrFe8Al4O19

-20

-30 -2000

SrFe8Al4O19 milled 3h SrFe8Al4O19 annealed -1000

0

1000

2000

H [kA/m] Fig. 6 (a) M-H hysteresis loops of SrFe12O19 after calcination at 1100 °C, milling 3h and after annealing treatment with NaCl salt at 1000 °C. (b) M-H hysteresis loops of SrFe8Al4O19 after calcination at 1100 °C, milling 3h and after annealing treatment with NaCl at 1000 °C.

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ACCEPTED MANUSCRIPT Table 1 Lattice parameters a and c, c/a ratio, and crystallite size D of different SrFe12-xAlxO19 powders (x = 0, 0.2, 1, 2, 4) after calcination at 1100 °C 1h and for x = 0 and x = 4 after milling 3h and annealing at 1000 °C for 1 h in a NaCl melt.

c/a (Å) 3.918±0.001 3.917±0.001 3.917±0.002 3.919±0.001 3.918±0.001 3.919±0.001 3.922±0.002 3.922±0.002 3.926±0.002

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D [nm] 330±10 66±1 437±13 310±10 260±10 90±4 70±4 38±1 51±1

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c (Å) 23.059±0.001 23.054±0.002 23.058±0.001 23.048±0.001 23.014±0.001 22.989±0.004 22.936±0.006 22.941±0.006 22.900±0.006

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0 0 HEM3h 0 annealed 0.2 1 2 4 4 HEM3h 4 annealed

a (Å) 5.885±0.001 5.886±0.001 5.887±0.001 5.881±0.001 5.873±0.001 5.866±0.001 5.848±0.002 5.849±0.002 5.834±0.002

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x

ACCEPTED MANUSCRIPT Table 2 Particle size distribution (WSEM,min, WSEM,max), median of volume particle diameter distribution WSEM; ¯ , median of volume particle thickness distribution tSEM;¯ and aspect ratio WSEM; ¯ /tSEM;¯ measured by SEM, and Ms and iHc values of different SrFe12-xAlxO19 powders (x = 0, 0.2, 1, 2, 4) after calcination at 1100 °C 1h and for x = 0 and x = 4 after milling 3h and annealing at 1000 °C for 1 h in a

WSEM; ¯ /tSEM;¯ 2.55 2.28 2.04 2.274 1.820 3.15 2.319 2.2

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iHc

Ms [Am²/kg] 71.5±0.7 55.2±0.6 74.1±0.7 69.4±0.7 58.4±0.6 44.5±0.4 28.5±0.3 25.6±0.3 22.9±0.2

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tSEM;¯ [nm] 116 115 139 102 106 72 94 70

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WSEM; ¯ WSEM,max [nm] [nm] 1039 296 808 262 682 284 890 232 576 193 826 227 620 218 482 154

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x WSEM,min [nm] 0 89 0 HEM3h 45 0 annealed 127 0.2 1 87 2 54 4 83 4 HEM3h 67 4 annealed 33

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NaCl melt.

[kA/m]

372±4 187±2 430±4 394±4 507±5 573±6 729±7 370±4 780±8

ACCEPTED MANUSCRIPT

SrAlxFe12-xO19 (x = 0 - 4) were synthesized via a mechanochemical activation




Deterioration of magnetic properties by milling depends on Al content




A subsequent annealing treatment in NaCl melt at 1000 °C promotes recrystallization




Annealing treatment in NaCl melt at 1000 °C increas es magnetic properties




Coercivity is explained by single domain particles and correlated to initial curve

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