1 Introduction

The hexagonal ferrites MFe12O19 (M = Ba, Sr, Pb) which have the magnetoplumbite structure (space group 63/mmc) are important permanent magnetic materials for microwave applications, small motors and magnetic recording applications [1]. Alabanese et al. [2] showed that in cationic-substituted M-type hexagonal ferrites the distribution of metallic ions among the different sub lattices is greatly modified by replacement of Ba with Sr . The permanent magnetic properties of hard ferrites are known to depend on their microstructural characteristics such as grain size, porosity, the presence of second phases and growth anisotropy [3]. High remanence requires a high sintered density and growth anisotropy, whereas high coercivity requires a small grain size [4]. Since the discovery of M-type hexagonal ferrites, extensive research has been carried out to improve their magnetic properties, particularly by the use of cationic substitutions [5, 6]. Some workers have substituted light rare-earth ions (LRE) such as La, Pr and other metal cations for Sr (Ba) and Fe respectively, taking into account of the ionic radius of elements [7].

Ba and Sr hexaferrites have been prepared by various techniques, including co-precipitation, the use of glass–ceramic or organic precursors, sol–gel synthesis, citrate–nitrate synthesis and ceramic processing [8–11]. Conventional solid state reactions for preparing barium and strontium ferrite powders require a high calcining temperature (1,200–1,300 °C) of milled mixtures of ferric oxide with barium or strontium carbonate. Furthermore, the milling process can introduce impurities into the material. Recently the sol–gel auto-combustion (citrate–nitrate) method has aroused interest because of its utilization of the heat released from the reactions of the hydrocarbon species from the chelating agents with the nitrate oxidant, rapidly producing nano-powders in situ. The literature indicates that the use of nano powder improves the sintering ability of the system [12].

The present research determines the optimal conditions for the solid-state synthesis of chromium-doped SrFe12O19 and studies the influence on the magnetic properties of the resulting product of replacing the mixture of conventional sintering additives with varying amounts of nanosized strontium hexaferrite synthesized by the citrate–nitrate reaction.

2 Experimental

All the starting materials were of analytical grade, except the iron oxide which was rolling plant waste from the Mobareke Steel Company, Iran. The chemical analysis of this reactant is shown in Table 1.

Table 1 Chemical analysis of iron oxide
Full size table

Based on the composition of the iron oxide, the molar ratio of iron oxide to strontium oxide was taken as six. The oxides were blended in a polymer jar in a ball mill, using a ball: powder ratio of 11:1, milled at 550 rpm for 1 h. The optimal synthesis temperature required to obtain SrFe12O19 from this mixture was determined by heating samples in an electric furnace at a heating rate of 10 °C/min for 2 h at 1,140–1,220 °C. DTA patterns of this mixture (not shown here) suggest the formation of SrFe12O19 can occur at about 800 °C. The XRD patterns of the products were determined using a Brucker D8 diffractometer with Cu-Kα radiation and a Ni filter. The magnetic properties of all the samples were determined by measuring their hysteresis loops using a model AMT-4 magnetometer (Shuangji Electronic Co, China).

Samples of the SrFe12O19 prepared under the above optimised conditions were doped with varying amounts of Cr2O3 to produce a series of compounds SrCrxFe12-xO19 where x = 0 − 0.5. Before firing these powder mixtures, 1.5 wt% of nano-sized SrFe12O19 prepared as described below was added as a sintering aid, and for comparison, some samples were prepared with 1.5 wt% of a commercial calcium silicate borate sintering aid (Type MN112, Taban Magnetic Material Development Co, Iran). Three types of samples were prepared, one containing only the commercial additive, one containing only the nano-sized SrFe12O19 and one containing 0.75 wt% of each additive. Aqueous slurries of these powder mixtures were blended in steel jars for 8 h with a ball: water: powder ratio of 11:3:1. The slurries were then pressed under a magnetic field of 400–800 A m−1 at a pressure of 100 MPa to form pellets 3 cm diameter × 1 cm high and fired in air at 1,210–1,260 °C for 2 h at a heating rate of 10°C/min.

The nano-sized SrFe12O19 sintering additive was synthesized by the sol–gel auto-combustion method [13]. Fe(NO3)3.9H2O and Sr(NO3)2 in the molar ratio 11.6:1 were dissolved in a minimum amount of deionised water. Citric acid was then added to chelate the Sr2+ and Fe3+ and the solution was brought to pH 7 by adding aqueous ammonia. The solution was evaporated to dryness by heating at 100 °C on a hot plate with continuous stirring until the mixture formed a viscous brown gel which was then calcined in air at different temperatures up to 1,100 °C for 1 h.

All the products were studied by XRD powder diffraction and SEM, after coating with Au, using a LEO 435 VP electron microscope operated at 20 keV. The B–H loops of all the samples were also determined.

3 Results and discussion

3.1 Optimization of the undoped SrFe12O19 synthesis temperature

The formation temperature of SrFe12O19 is related to the raw material characteristics such as their particle size, surface area and composition, as well as processing parameters such as the type of mixing. The XRD patterns of the present samples fired in the temperature range 1,180–1,210 °C were identical but differences were found in their magnetic properties. An increase in the calcination temperature from 1,180 to 1,210 °C results in an increase in the coercive field from 3,500 to about 4,000 Oe but the remanence and rectangularity ratio remains constant at about 3,600 G and 87%, respectively. Firing temperatures >1,210 °C result in partial sintering, making the samples difficult to mill for subsequent processing. Therefore, samples of SrFe12O19 for subsequent experiments were synthesised at 1,210 °C. The XRD pattern of undoped SrFe12O19 synthesised under these conditions at 1,210 °C is shown in Fig. 1.

Fig. 1
figure 1

XRD powder pattern of undoped SrFe12O19 powder fired at 1,210 °C. All peaks are from SrFe12O19 (JCPDS file no. 33-1340)

Full size image

3.2 Optimization of the SrFe12O19 sintering temperature

The magnetic properties of samples sintered with 1.5 wt% MN112 sintering additive at 1,210–1,260 °C show that increasing the firing temperature produces an increase in the remanence magnetization from 3,975 to 4,130 G, due to increased sintering and decreased porosity. A concomitant decrease in coercive field from 3,518 to 2,882 Oe with increasing sintering temperature is probably related to increased grain growth. Since permanent-magnet applications require both high remanence and coercive field, these results led to the adoption of a sintering temperature of 1,220 °C for subsequent experiments on Cr-doped samples.

3.3 Optimization of the chromium dopant content

XRD patterns of Cr-doped SrCrxFe12-xO19 samples where x = 0−0.5 (Fig. 2) show no change up to x = 0.3, but at x = 0.5 the non-magnetic hematite phase appears. Mossbauer studies [14] indicate that the hexagonal cell of SrFel2Ol9 contains five distinguishable crystallographic Fe sites, three octahedral 12k, 4f2 and 2a, one tetrahedral 4f1 and one bi- pyramidal 2b site. In general, Cr atoms do not occupy tetrahedral sites, ruling out substitution in the 4f1 site and confining the substitution to 10 of the 12 Fe3+ ions in SrFel2O19 [15].

Fig. 2
figure 2

XRD patterns of Cr-doped samples sintered at 1,220 °C with 1.5 wt% MN112 sintering additive. a. 0.1 atom % Cr, B 0.3 atom % Cr, C 0.5 atom % Cr. Peaks marked * are from hematite (JCPDS file no. 33-0664), all other peaks are from SrFe12O19 (JCPDS file no. 33-1340)

Full size image

Since the ionic radii of Cr+3 and Fe+3 are almost equal, the appearance of hematite is unlikely to result from the effect of size but may be due to the change of exchange energy accompanying Cr3+ substitution in the lattice [15].

The magnetic properties of Cr-doped SrCrxFe12−xO19 sintered at various temperatures in the presence of 1.5 wt% MN112 sintering additive are shown in Figs. 3 and 4 as a function of Cr content. Increasing the amount of chromium up to x = 0.5 results in a decrease of remanence magnetization at all temperatures (Fig. 3). This may be related to the electron configuration of Cr3+ which results in a magnetization of 3μB, whereas Fe3+ is 5μB. The presence of Cr3+ in SrCrxFe12−xO19 can significantly affect the magnetic properties; it is reported that the total magnetic moment of SrFe12019 in M-type hexaferrite is 20μB [16]. The magnetic saturation of the hexagonal ferrite lattice can be written [17]:

Fig. 3
figure 3

Remanence magnetization of Cr-doped SrFe12O19 sintered with 1.5 wt% MN112 sintering additive as a function of Cr content

Full size image
Fig. 4
figure 4

Coercive field of Cr-doped SrFe12O19 sintered with 1.5 wt% MN112 sintering additive as a function of Cr content

Full size image
$$ {\text{Ms}} =\,6{\text{M}}^{{{\text{Fe}}3 + }}_{{(12{\text{K}})}} - 2{\text{M}}^{{{\text{Fe}}3 + }}_{{(4{\text{f}}2)}} - 2{\text{M}}^{{{\text{Fe}}3 + }}_{{(4{\text{f}}1)}} + {\text{M}}^{{{\text{Fe}}3 + }}_{{(2{\text{b}})}} + {\text{M}}^{{{\text{Fe}}3 + }}_{{(2{\text{a}}2)}} $$
(1)

Under the most favourable conditions where the spins of Cr3+ and Fe3+ are aligned parallel in each site of the crystal structure, the magnetic saturation of the unit cell of chromium-doped hexaferrite is given by:

$$ \begin{aligned} {\text{Ms}} = & [6({\text{xM}}^{{{\text{Fe}}3 + }}_{{(12{\text{K}})}} + \left( {1 - {\text{x}}} \right){\text{ M}}^{{{\text{Cr}}3 + }}_{{(12{\text{K}})}} ] - 2[{\text{M}}^{{{\text{Fe}}3 + }}_{{(4{\text{f}}2}} ) + \left( {1 - {\text{x}}} \right){\text{ M}}^{{{\text{Cr}}3 + }}_{{(4{\text{f}}2)}} ] - 2{\text{M}}^{{{\text{Fe}}3 + }}_{{(4{\text{f}}1)}} \\ & + [{\text{xM}}^{{{\text{Fe}}3 + }}_{{(2{\text{b}})}} + \left( {1 - {\text{x}}} \right){\text{ M}}^{{{\text{Cr}}3 + }}_{{(2{\text{b}})}} ] + [{\text{xM}}^{{{\text{Fe}}3 + }}_{{(2{\text{a}}2)}} + \left( {1 - {\text{x}}} \right){\text{ M}}^{{{\text{Cr}}3 + }}_{{(2{\text{a}}2)}} ] \\ \end{aligned} $$
(2)

In the above equation, x is the mole fraction of crystal sites in the hexaferrite unit cell which are occupied by iron cations. According to above equation, it is evident that addition of chromium will decrease the remanence magnetization.

Figure 4 shows the intrinsic coercivity of Cr-doped samples sintered at various temperatures, as a function of the Cr concentration x up to x = 0.5. The addition of Cr up to x = 0.5 increases the intrinsic coercivity, as expected from Eq. 3 [18]

$$ {\text{Hcj}} = {{\upalpha}}\left( {2{\text{K/Ms}}} \right) $$
(3)

where Ms is the magnetic saturation and K is the magnetic anisotropy. According to Eq. 3, the addition of Cr will bring about a decrease in the remanence magnetization (Br) and saturation magnetization (Ms), causing an increase in the intrinsic coercivity.

These results suggest that a Cr doping content of x = 0.3 produces a product with optimal magnetic properties, and this composition was adopted for the subsequent studies of the effect of nano-sized SrFe12O19 additive.

3.4 Effect of nano-sized SrFe12O19 additions on the magnetic properties of SrCr0.3Fe11.7O19

The synthesis of nano-sized SrFe12O19 powder by citrate–nitrate route is controlled by the ratio of Fe:Sr, the ratio of citrate to the other metal ions and the pH of the system. To obtain monophase SrFe12O19, the ratio of citrate ion to the metal ions was set at 1, with pH about 7 and a Fe:Sr ratio of 11.6:1. The XRD diffraction patterns of the unfired product and samples fired at 700 and 1,100 °C (Fig. 5) show that the optimal calcining temperature is 1,100 °C. At a higher Fe:Sr ratio of about 11.6:1, non-magnetic hematite appears.

Fig. 5
figure 5

XRD patterns of nano-sized SrFe12O19 prepared by the citrate–nitrate method. a as-synthesised, b calcined at 700 °C, c calcined at 1,100 °C. Peaks marked * are from hematite (JCPDS file no. 33-0664), peaks marked filled diamond are from SrFe12O19 (JCPDS file no. 33-1340), peaks marked filled circle are from maghemite (JCPDS file no. 39-1346) and peaks marked filled triangle are from magnetite (JCPDS file no.19-0629)

Full size image

The particle size of the nano-sized SrFe12O19 powder thus synthesized was calculated from the Scherrer Eq. 4 to be about 35 nm [15].

$$ {\text{d(nm)}} = {\text{k}}\lambda /\beta \cos \theta $$
(4)

where λ is the X-ray wavelength, β is the half peak width, θ is the Bragg angle and k a constant, taken as 0.89. The high surface area and magnetic properties of this powder suggest that it should be a useful additive to enhance the magnetic properties of SrFe12O19 ceramics, and it was therefore used in the present studies on the SrCr0.3Fe11.7O19 materials developed in the previous section, with and without the use of MN112 sintering additive.

Table 2 compares the magnetic properties of the initial SrFe12O19 powder, the sintered product doped with the optimal 0.3 mol Cr, and this doped material sintered with 1.5 wt% of nano-sized SrFe12O19, with the results for the sample containing 1.5 wt% sintering additive for comparison. Table 2 shows that sintering SrCr0.3Fe11.7O19 with nano-sized SrFe12O19 additive alone produces little difference in remanence magnetization from the sample sintered without any additives, but the coercive field is significantly decreased by the presence of the nano-additive. The use of MN112 sintering additive decreases the remanence magnetization of undoped SrCr0.3Fe11.7O19 but significantly increases the coercive field; this effect is even more pronounced in the sample containing equal proportions of MN112 and nano- SrFe12O19 (Table 2), indicating that this combination of additives produces the best practical magnetic material.

Table 2 Comparison of the magnetic properties of the various samples
Full size table

The reasons for these results can be seen from the SEM microstructures of these samples (Fig. 6). The sample containing only MN112 (Fig. 6b) is seen to be better sintered than the sample containing only nano-sized SrFe12O19, which shows a greater degree of grain growth (Fig. 6a). This enhanced grain growth is the probable explanation for the decreased coercivity in this sample. Use of the two additives together combines the best properties of each, producing the sample with a superior coercive field and therefore the best magnetic properties for practical applications. The two hysteresis loops (B–H and J-H) of this optimized SrCr0.3Fe11.7O19 sample sintered with a combination of 0.75 wt% MN112 sintering additive and 0.75 wt% nano-sized SrFe12O19 are shown in Fig. 7. The flux density (B) is the constant multiple of the magnetic field (H) added to the magnetization (J).So compared to the J-H loop, the B–H curve has a higher right shoulder.

Fig. 6
figure 6

SEM micrographs of SrCr0.3Fe11.7O19 sintered at 1,220 °C, a with 1.5 wt% nano-sized SrFe12O19, b with 1.5 wt% MN112 sintering additive, c with 0.75 wt% nano-sized SrFe12O19 + 0.75 wt% MN112 sintering additive

Full size image
Fig. 7
figure 7

Hysteresis loop of optimised SrCr0.3Fe11.7O19 sample sintered at 1,220 °C with 0.75 wt% nano-sized SrFe12O19 + 0.75 wt% MN112 sintering additive

Full size image

4 Conclusions

Samples of SrFe12O19 were doped with 0.3 atom % Cr3+ and sintered with the addition of 1.5 wt% nano-sized SrFe12O19 in conjunction with a commercial calcium silicate borate sintering aid. The optimum conditions for the development of the magnetic properties of these samples was determined to be 1,220 °C with a 0.3 atom % Cr3+ content; higher Cr contents lead to the appearance of non-magnetic hematite. The addition of nano-sized SrFe12O19 alone leads to enhanced grain growth, with deleterious effects on the magnetic properties, particularly the important coercive field value, whereas the use of 1.5 wt% of commercial sintering additive alone promotes greater sintering with a significant enhancement of the coercive field. The combination of 0.75 wt% of each additive produces the best magnetic properties in these Cr-doped samples (a remanence magnetization of 3,687 G, a coercive field of 4,129 Oe and a rectangularity ratio of 84%).