STRUCTURAL, MAGNETIC PROPERTIES AND EXCHANGE-COUPLING BEHAVIOR OF MAGNETICALLY HARD/SOFT SrFe 12 O 19 /Co NANOCOMPOSITE PRODUCED BY THE HIGH ENERGY BALL-MILLING METHOD

: The high energy ball-milling (HEBM) procedure has been used for the preparation of magnetically hard/soft (1-x )SrFe 12 O 19 / x Co nanocomposite (with x = 0.1, 0.2 and 0.3). Effects of aging temperatures (800 - 1000 °C) on microstructure, morphology and magnetic properties have been studied. X-ray diffraction spectrum indicates the existence of two phases: the hexagonal SrFe 12 O 19 and Co after HEBM process. Scanning electron microscopy was used to characterize the morphology, size and elemental composition of the synthesized nanocomposites. Thermal gravimetric analysis/differential scanning calorimetry were used to estimate the phase transition temperatures of the nanocomposites. The magnetic characterization was carried out via vibrating sample magnetometer at room temperature for the nanocomposite that showed the magnetically single-phase behavior. With increasing the content of Co, the coercivity ( H c ) of the nanocomposites is decreased. Specific saturation magnetization ( σ s ) is clearly increased with increasing of aging temperatures until reached the maximum ( σ s = 110 Am 2 /kg) with x = 0.1 aged at 1000 °C. The nanocomposite with x = 0.1 aged at 800 °C has enhancement in the squareness ratio ( σ r /σ s ) that reached to 10.2% increment in comparison with the single phase SrFe 12 O 19 . The HEBM process and synthesized SrFe 12 O 19 /Co nanocomposite can be regarded as a suitable technique for preparing hard magnetic nanomaterial for permanent magnets.


INTRODUCTION
Hard/soft magnetic nanocomposites (NCs) have aroused great interest due to the tremendous progress in their magnetic properties [1].By optimizing the composition, the aging temperature and the appropriate ratio of hard/soft magnetic phases, the structure and magnetic performances of these NCs can be enhanced [2,3].Hard/soft magnetic NCs were produced using the electrospinning method [4], sol-gel procedure [5] and high energy ball milling (HEBM) process [6,7].At the HEBM method, there is a noticeable a "kink" or "bee-waist" at the demagnetization loops, suggesting decoupling between hard and soft phases due to the very large softphase grains [7].In this study, hard/soft (1-x)SrFe12O19/xCo (x = 0.1, 0.2, and 0.3) NCs materials have been synthesized using the HEBM method, with an effort to significantly increase the hard /soft exchange-coupling.The effects of the mass ratio of the hard/soft phases and the aging temperature on the morphology, microstructure and magnetic performances of the hard/soft NCs are discussed.

EXPERIMENTAL
Hard/soft (1-x)SrFe12O19/xCo (with x = 0.1, 0.2 and 0.3) NCs was synthesized by the HEBM process.Commercially SrFe12O19 and Co powders of magnetic properties (Hc = 125, 16.7 kA/m, σs = 57.41,165 Am 2 /kg and σr/σs = 0.441, 0.073), respectively, were mixed with 2 ml of acetone in a stainless-steel ball mill vial.The Aktivator 2S planetary ball mill was used for milling at a speed of 800 rpm.The sample was milled for 6 hours at room temperature with mass ratio of a ball to powder 10/1.An analogous synthesis process was used to produce other mass ratio composite precursors.Finally, the (1-x)SrFe12O19/xCo composite precursor was aged at 800, 900, and 1000 °C for 2 h, respectively.A Netzsch STA 409 PC/PG thermogravimetric analyser was used for differential scanning calorimetry (DSC)/thermal gravimetric analysis (TGA) measurement.The structure characterization of NCs was carried out by a DRON-4 diffractometer using Co-Kα radiation.The morphology and elemental analysis of the powders was achieved using a Bruker Quantax 200 energy dispersive X-ray spectroscopy (EDX).Magnetic measurements were done at room temperature using a vibrating sample magnetometer (VSM-250) under an applied magnetic field up to 1600 kA/m.

X-ray powder diffraction
Figures (1a, b, and c) show the diffraction peaks of the composition, confirming the presence of M-type hexaferrite SrFe12O19 (SFO) as the hard phase and CoFe2O4 spinel ferrite (CFO) as the soft phase.Figure 1d indicates the existence of two phases: the hexagonal SrFe12O19 and Co after HEBM process.As the concentration and aging temperature increase, the diffraction peaks of CFO gradually increase [5].There is no metal Co peak in the spectrum, indicating that Co ions are located in the crystal sites of the SFO structure and form a CFO phase [8].At 800 °C, the presence of the Co3O4 phase is related to an incomplete aging reaction.When the aging temperature is increased, the occurrence of a high proportion of CFO phase.
The decrease in the relative intensity of the peaks may be due to the substituted ions replacing the occupied ions at the lattice sites.The Co ion preferably occupies the 4f2 and 2a positions of the Fe ion octahedron [9].As shown in Table 1, according to XRD data analysis, the lattice parameters (a and c) and the unit cell volume (Vcell) are calculated.Compared with the parent structure (a = 0.5881 nm and c = 2.3052 nm [8], the value of the lattice parameter of the SFO phase is increased from a = 2.3003 to 2.3076 nm; c = 0.5873 to 0.5887 nm with an increase in Co content from 0.1 to 0.3 wt% and increasing aging temperature from 800 to 1000 °C.This increase is attributed to the fact that the ionic radius of the cation Co 2+ (0.074 nm) is larger than that of Fe 3+ (0.064 nm) [8].Due to the increase in a and c, the Vcell also increased from 0.6872 to 0.6926 nm 3 .Table 1 shows the increase of average crystallite size <D>, as the aging temperature of all samples was increased from 800 to 1000 °C, demonstrates that grain growth arises during the aging process.

Differential scanning calorimetry
The DSC/TGA analysis of 0.9SrFe12O19/0.1CoNCs heated at a rate of 10 °C/min was performed.As illustrated in Figure 2, the DSC curve shows a strong endothermic peak near 902 °C.This is due to the transition from Co3O4 to CoO and the formation of CFO spinel.The small endothermic peak from 900 to 1100 °C may be due to decomposition of a small amount of SrFe12O19 and the formation of SrFeO3.From the TGA curve, the 2.2% weight loss is mainly caused by the decomposition of Co3O4 into CoO and SrFe12O19 into SrFeO3 [10].
May26-28, 2021, Brno, Czech Republic, EU  substitution of Sr(Fe,Co)12O19.From the smooth demagnetization curve of NCs, an exchange coupled between the magnetically hard/soft phases can be observed [5].
This can be attributed to the large crystal grains, it is easy to cause domain wall movement, thus reducing Hc [11].In addition, as the aging temperature increases, the volume fraction of CFO increases and Hc decreases owing to the fact that the Hc of SFO/CFO NCs is not as large as that of SFO ferrite.
The σs of (1-x) SrFe12O19/xCo (x = 0.1, 0.2 and 0.3) increases as the calcination temperature increases.In addition, as shown in Figure 5b, the σs of NCs varies nonlinearly with x.Herein, the 0.9SrFe12O19/0.1CoNCs powder, aged at 1000 °C, has the highest σs value (110 Am 2 /kg).This can be ascribed to the exchange coupled between the magnetically hard/soft phases.Furthermore, the magnetic moment of Co 2+ (3 μB) is not as large as that of Fe 3+ (5 μB), so replacing Fe 3+ with Co 2+ instead of 4f2 will increase the σs of the composite material [12].The 0.7SrFe12O19/0.3CoNCs, aged at 800 °C, has the lowest σs value (65 Am 2 /kg).As the Co substitution increases, the tetrahedral-octahedral exchange interaction between Fe 3+ and O 2-ions weakens, so the magnetic moment value of the system decreases [13].
Remanence (σr) of (1−x)SrFe12O19/xCo increases with the increase in aging temperature.As shown in Figure 5c, the σr trend of (1-x) SrFe12O19/xCo decreases as x increases.Due to the exchange coupled between the magnetically hard/soft phases, σr will be generated in the prepared nanocomposite powder.3) was successfully prepared using the HEBM process.The maximum magnetic properties (Hc = 214.9kA/m, σs = 82.68Am 2 /kg and σr/σs = 0.489) were achieved for 0.9SrFe12O19/0.1CoNCs, aged at 800 °C.Excessive Co and higher aging temperature will cause defects in the main hard magnetic SrFe12O19 phase and increase the amount of CoFe2O4 soft magnetic phase, thereby reducing the magnetic properties.
Future work will focus on the synthesis of different composites with lower cost and higher performance.

Figure 2 Figure 3
Figure 2 DSC/TGA curves for the formation of CoFe2O4

Table 1
Sample composition x, the volume fraction of phases and XRD parameters (the lattice parameter (a, c), the cell volume (Vcell) and average crystallite size <D> of SrFe12O19 in (1−x)SrFe12O19/xCo NCs aged at 800, 900 and 1000 °C