MnZn ferrite is one of the most important soft ferrites, which present high magnetic permeability, high saturation magnetization, high resistivity, low coercivity, low power losses and so on [1]. It is widely used in many fields, such as deflection yoke rings, computer memory chips, magnetic recording heads, microwave devices, transducers, transformers and so on [2]. Additionally, nano-sized MnZn ferrite is a good candidate for biomedical purposes, including magnetically guided drug delivery and magnetic resonance image [3], because of its high magnetic moment, good chemical stability and reactive surfaces when attaching to biological molecules [4].

Ferrite is usually synthesized by the conventional ceramic technique, where high temperature is needed in order for enough solid-state reactions between raw materials, large and inhomogeneous particles, together with some impurities, greatly restrict magnetic properties of the products [5]-[7]. Therefore, wet chemical synthesis like co-precipitation [8]-[10], sol-gel [11] [12], hydrothermal method [13], and micro-emulsion process [14] [15] is expected for the production of ferrites with excellent magnetic properties. Among these methods, co-precipi- tation process is often used to the preparation of the homogeneous ferrite nanoparticles [4]. During this process, oxalate is generally used as a precipitant to prepare the ferrite powder. Angerman et al. [16] reported that β-oxalate with orthorhombic structure was formed when precipitating at room temperature, and that monoclinic α-oxalate was obtained at 90˚C. Those oxalates could be completely transformed to ferrite when decomposition at 650˚C [17]. On the other hand, Fritsch et al. [18] found that Mn-riched ferrite (Fe_3−xMn_x□3δ/4O_4+δ) with x > 1.5 presented complex structures like cubic, tetragonal or a mixture of them. This phenomenon should be attributed to the lack of miscibility at low temperature in the Fe₃O₄-Mn₃O₄ system. In this paper, nanocrystaline MnZn ferrite was directly synthesized at room temperature by using the co-precipitation method, followed by a refluxing process. We investigated structures of the products and their phase transitions during heating under different atmosphere.

Figure 1 shows XRD patterns of as-synthesized samples, of which XRD peaks match well with those of the spinel MnZn ferrite (JCPDS No.74-2402). Almost no other peaks are found in the XRD patterns. Those results suggest that the samples prepared at different pH values are pure MnZn ferrite with spinel structure. In addition, apparent widening in the XRD peaks indicates a nano-scale crystallit size in those samples, and the narrower XRD peaks suggest that the crystals enlarge with an increase in the pH values (about 17 nm at pH of 12, and 25 nm at pH of 13).

Figure 2 shows the measured room-temperature hysteresis loops of the samples in Figure 1. Upon increasing pH values in the co-precipitation solutions, the saturation magnetization, M_s, apparently increase. And the

maximum M_s of 58.6 emu/g is obtained in the sample at a pH of 13. This M_s value is almost the highest one reported recently [5] [19]-[22], is about one times larger than that of pH = 12. Similar results were reported by Narasimhan et al. [23].

It is known that magnetism of the powdered MnZn ferrite is tightly related to its crystal size. Szczygiel et al. [24] found that superparamagnetism occurred in the MnZn ferrite powder with its crystal size less than 10 nm. Therefore, decrease in M_s with decrease in pH value should be attributable to the decrease in the crystallite size of the samples in the present research.

It is reported that MnZn ferrite cannot be directly obtained by using oxalate as precipitant during the co-pre- cipitation process, and that heat treatment at certain temperature is generally needed to precursor [16] [22]. Results obtained in the present research not only simplify the preparation process of the MnZn ferrite, but also improve its quality because inclusion is easily produced during the heat treatment.

Figure 3 shows thermal analysis curves of the sample prepared at pH of 13. Total mass loss of the sample is about 8% during heating between room temperature and 1000˚C, and the sharp mass loss occurring below 400˚C should be attributable to evaporation of the adsorbed water and the crystallized water contained in the sample. No apparent endothermal or exothermal peaks are found in the DTA curves. Those results further prove that the samples prepared in this research are pure MnZn ferrite.

The MnZn ferrite is generally used in the bulk state, and then a sintering process is needed for the powdered one. However, the MnZn ferrite is reported to easily decompose according to Equation (2) in a heated state

under the oxidation atmosphere [2], and precipitation of Fe₂O₃ greatly deteriorates its magnetic properties.

Accordingly, we investigate phase transitions and variation in magnetic properties of the prepared samples during calcinations. Figure 4 shows XRD patterns of the calcinated samples under different atmosphere. XRD patterns of the uncalcinated ones are also presented in this figure for comparison. Under certain atmosphere, XRD peaks of the sample become stronger in intensity and narrower in width, followed by a gradual color variation form brown to dark brown, with an increase in the calcination temperature. It is noteworthy that some XRD peaks attributable to Fe₂O₃, concomitant with a reddish color in sample, appear in the XRD pattern of the sample calcinated at 800˚C under N₂ atmosphere. Further calcination at high temperature results in disappearance of those XRD peaks. Precipitation of some Fe₂O₃ from the MnZn ferrite may result from a little oxygen existing in the N₂ atmosphere. In contrast, the CO₂ + H₂ atmosphere is favorable to remain stability of the MnZn ferrite.

The measured hysteresis loops of the samples corresponding to those in Figure 4 are shown in Figure 5. Their magnetization nearly saturates at the maximum field of 20 kOe, the magnetic parameters like M_s, M_r and H_c calculated according to Figure 5 are presented in Table 1. Decrease in M_s with an increase in the calcination temperature below 800˚C under N₂ atmosphere should be related to decomposition of the MnZn ferrite or precipitation of non-magnetic Fe₂O₃, shown in Figure 4(a). Solution of the Fe₂O₃ into the MnZn ferrites at 1000˚C results in a sharp increase in M_s from 27.1 to 80.3 emu/g. On the other hand, the M_s gradually increases with an increase in the calcination temperature below 1100˚C for the samples calainated under CO₂ + H₂ atmosphere, concomitant with a decrease in H_c. The highest M_s of 188.2 emu/g is obtained for the sample, with the crystallite size of about 68.1 nm, calcinated at 1000˚C. This is consistent with growth in crystallite size of samples during the calcination. The sharp decrease in M_s at 1200˚C is perhaps related to evaporation of some elements like Zn at high temperate. The M_s of the samples under CO₂ + H₂ is larger than that of N₂at same calcinations temperature.

MnZn ferrite nanoparticles could be directly prepared by using co-precipitation and then refluxing process. Crystallite size of the obtained samples increased with an increase in pH value of the co-precipitation solution, and that the crystallite size of about 25 nm was obtained for the sample at a pH of 13. This sample also showed the maximum M_s of 58.6 emu/g, which was about one times larger than that of 12 (pH value). Calcination to the obtained samples resulted in an enlargement in their crystal size and an improvement in magnetic properties with an increase in temperatures. The samples calcinated in CO₂ + H₂ atmosphere presented good stability, and the maximum M_s of 188.2 emu/g was obtained for the sample heated at 1100˚C. Unfortunately, precipitation of some Fe₂O₃ at 800˚C suggested poor stability of the nanocrystalline MnZn ferrite in N₂ atmosphere.

This work was supported by Tianjin Research Program of Application Foundation and Advanced Technology (major project) under a contract No.15JEZDJC31000.

Fei Hua,Cuicui Yin,Huanque Zhang,Qiangqiang Suo,Xin Wang,Huifen Peng, (2015) Direct Preparation of the Nanocrystalline MnZn Ferrites by Using Oxalate as Precipitant. Journal of Materials Science and Chemical Engineering,03,23-29. doi: 10.4236/msce.2015.312005