NanocrystallineMnZnFerrites Synthesized by Co-precipitation and Refluxing Method

HUA Fei1,2, PENG Hui-fen1, YIN Cui-cui1, ZHANG Huan-que3, SUO Qiang-qiang3

( 1.School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China; 2. Department of Fire Protection Engineering, Chinese People’s Armed Police Force Academy, Langfang, Hebei,

China;3. School of Chemical Engineering and Technology，Hebei University of Technology，Tianjin 300130，China）

Abstract:

NanocrystallineMnZn ferrite spinel powders have been synthesized by co-precipitation and refluxing

methodusing ferric chloride (FeCl3·6H2O) ， manganese sulfate (MnSO4·H2O), zinc sulfate (ZnSO4·7H2O),

ammonium oxalate (0.2mol/L)，ammonia (NH3·H2O) and sodium hydroxide (NaOH)as the raw materials. The

effects of the pH value of co-precipitation solution on the crystalline phase formation and magnetic properties

were systematically characterized.XRD analysis shows that all the samples have a pure spinel structure without

any other phases with the crystallite sizes of about 22 nm.The products with saturation magnetization (M s) of

58.628 emu/g were obtained when the pH value of co-precipitation solution and refluxing time are 13 and 7 h,

respectively. MnZn ferrite powder was further calcined at 400~600℃ in nitrogen and reducing atmosphere (99

vol.% CO2+1vol.% H2). All the MnZn ferrite nanoparticles calcined in reducing atmosphere (99 vol.% CO2+1vol.

% H2) possess a simple spinel structure and having a maximum M s value of 188.155 emug/g at temperature of

1100℃and the corresponding crystallite size is 68.1224 nm. But XRD patterns demonstrate that the ferrites

undergo several phase transitions during calcined in nitrogen ant the particle size,M s and coercive force H c

strongly depend on the calcined temperature.

1. Introduction

MnZn ferrites represent the most important group of soft ferrites which have high magnetic permeability,

saturation magnetization, electrical resistivity, low coercivity and power losses[1]are widely used in the field of

electronics and electrics such as deflection yoke rings, computer memory chips, magnetic recording heads,

microwave devices, transducers, and transformers[2]. Apart from these applications, nano-sizedMnZn ferrites are

good candidates for biomedical purposes,including hydrothermal, magnetically guided drug delivery, and

magnetic resonance imaging[3], since they present a high magnetic moment, chemically stable and their surfaces

are very reactive to attach biological molecules[4].

Synthesis of ferrites is usually carried out by the ceramic technique which employs a relatively high

temperatures solids-state reactions between intimately mixed fine powders of component oxides and other salts to

increase the reactivity of components[5-6]. The disadvantages of this conventional method include rather large and

non-uniform particle size and induced impurities, which restrict further improvement of the products [7].In order to

overcome these weaknesses and to meet the requirements for new applications, some wet chemical processes like

co-precipitation [8-10], sol-gel [11-12], hydrothermal method [13], and micro-emulsion process [14-15]et al.have been

considered for production of nanoscale ferrites with excellent magnetic properties.

Among these different synthesis methods, co-precipitation used to prepare uniform magnetic ferrite

nanoparticles with narrow size distribution at a relatively low reaction temperature appears to have attracted much

attention in recent years for its simplicity and productivity[4],and is also widely used for biomedical applications

due to ease of implementation and the need for less hazardous materials and procedures [16]. The key of co-

precipitation involves separation of the nucleation and growth of the nuclei via homogeneous precipitation

reactions [16-18].

A considerable number of studies were also focused on the ferrite powder synthesis via an oxalate-based

process. Only few studies were devoted to the preparation of MnZn ferrite and most of them use Fe (Ⅲ) as rare

material. For example, Angermann et al.[19] reveal that precipitation at 20℃ results in the formation of β-oxalate

with orthorhombic structure and monoclinic α-oaxlate is obtained when performed at 90℃ for the first time. And

decomposition at 650℃, the ferritization could be completely was confirmed by Ghodake et al. [20]. Fritsch et al.[21]

have pointed out the manganese rich compounds (x≥1.5) have a complex structure that can be cubic, tetragonal, or

a mixture of both these two phase which indicates a lack of miscibility existing in the Fe 3O4-Mn3O4 phase diagram

at low temperature.

In this communication, the thermal decomposition of oxalates is replaced by refluxing method after co-

precipitation using Fe (Ⅲ) as rare materials.The structure of the MnZn ferrites particles depend on the precipitation

conditions. In order to overcome the oxidation of ferrite under low temperature, the phase transition during

400~1200℃under nitrogen and reducing atmosphere (99 vol.% CO2+1vol.% H2) was also discussed.

2. Experimental

MnZn ferrite nanoparticles were prepared by the co-precipitation and refluxing method using ferric chloride

(FeCl3·6H2O) ， manganese sulfate (MnSO4·H2O), zinc sulfate (ZnSO4·7H2O), sodium hydroxide (NaOH),

ammonium oxalate (0.2mol/L)，ammonia (NH3·H2O) as the raw materials. All the materials had analytical purity

and used without further purification. The required metal salts and sodium hydroxide were dissolved in deionized

water separately to form aqueous solutions.

First, ammonium oxalate and ammonia were added into ferric chloride solution under constant magnetic

stirring at room temperature.Then the solution of manganese sulfate and zinc sulfate in required molar ratio were

added to the above solution when the pH value turns to be 4. Ammonium oxalate and ammonia were continued to

add into the above solution till the pH value equal 8.5, then sodium hydroxide was added to the solution until pH

value 13. The pH value of the solution was continually monitored. The intermediate product obtained by above

co-precipitation process was then transferred to a reflux system of self-made where the transformation of metal

oxalates and hydroxide into nano-ferrites took place upon heating at 100℃ for up to 7 h. The nanoscaleMnZn

ferrites were then obtained by washing four times with distilled water and dried at 80℃for 7 h. The co-

precipitation powder was calcined at 400~1200℃.

X-ray diffraction (XRD, Bruker, AXS) with Cu-k α, having λ=1.5406 Å for all samples was carried out at room

temperature and calcined at different temperatures to determine the phase structure and crystalline size. The

saturation magnetization (M s) and intrinsic coercive force (H c)of the nanoparticles were measured at room

temperature by a vibrating sample magnetometer (VSM, LakeShore-7400).X Ray Fluorescence spectrometer

（XRF） technique was used to determine the elemental compositions of prepared ferrite powders. Thermos

Gravimetric Analyzer( TGA-DTA, TA, SDT-DTA-2960) up to 1200℃ in N2 at the heating rate of 10℃min-1 was

used to study thermal decomposition of the final as-prepared metal complex.

3. Results and discussion

The Mn-Zn-ferriteparticles precipitated at room temperature are dark brown powder which is systematically

studied. Fig.1a shows XRD patterns for final products prepared under various co-precipitation pH values and the

same refluxing time of 7 h. It shows that all the samples have a pure spinel structure (JCPDS 74-2402) without

any other phases being detected,all the peaks of the samples can be clearly indexed to the eight major peaks of the

spinel ferrites, which are (220), (311), (222), (400), (422), (511), (440) and (533) planes of cubic unit cell,

corresponding to spinel structure of Mn0.8Zn0.2Fe2O4, and indicates that pure nanosizedMnZn ferrite can be

successfully synthesized at low temperature by co-precipitation and refluxingprocess. This is in contrast to

conventional ceramic route in which spinel phase is obtained at high temperature (900~1300℃) by employing the

calcination and sintering processes[2]. Similar observations have been reported in an earlier study by Meng et al.[22],

who used ferrous sulfate (FeSO4·7H2O), sodium hydroxid (NaOH) and ammonia (NH3·H2O) as the raw materials.

XRD peaks of all samples are broaden (Fig. 1a), especial at pH=12, and the peak width decrease with the increase

of pH value, which indicates that the mean crystalline size of as-synthesized ferrites is fine and gradually increase

with increase pH value until 13. The average crystallite size of the samples estimated from half-widths of theXRD

(311) peaks using the well-known Debye-Scherrer’s equation

D=0.94 λβcosθ

(1)

Where, λ is wavelength of X-ray, θ is Bragg’s angle and β is full width at half the maxima (FWHM), which were

22 nm for the sample prepared at the optimized pH value of 13. All hysteresis loops exhibit very small coercive

force field which can be explained by the very small particle size of all samples. When the critical particle size for

MnZn ferrites shrink down to less than 10 nm, superparamagnetism will be occurs and the coercibity will decrease

to zero[23]. Fig. 1b shows that the saturation magnetizationM s strongly depends on the pH value and increase with

increasing pH value, which contributes to the improved crystallization, and a rather high saturation magnetization

of 58.628 emu/g has been achieved in the MnZn ferrite nanoparticles, which is a rather higher value in recent

years[24].

However, this high saturation magnetization is still significantly lower than that of bulk ferrite for the nano-size

effect[25].Formation of dead layer (magnetically inactive layer [26]) on the surface, existence of random canting of

particle surface spins, non-saturation effect due to random distribution of particle size, deviation from the normal

cation distribution/cation site disorder, presence of adsorbed water also result in the reduction of magnetic

properties of the reduction in size of the nanosized particle[4,26-28]. The reduction is size of the particles was

explained by considering magnetically inactive layer on the surface of particles [29]. Assuming the thickness, t, of

the inactive layer as a constant, the magnetization of the particle to first order is given as Eq. (2)

M s=M s(bulk)(1−6 t /D)(2)

Here, D is the particle diameter. Amiri[30]considered that the canted spin or spin glass like layer at the surface of

the nanoparticles, which rises due to the larger fraction of surface to volume atoms in small particles.

(a) (b)

Fig.1 XRD patterns (a) and magnetic hysteresis loops (b) for the samples

as synthesized by co-precipitation and refluxing method at various co-precipitation pH values.

The composition MnZn ferrite samples as-prepared for metal ions Mn, Zn and Fe was determined by XRay

Fluorescence spectrometer（XRF）technique are given in Table 1.

Table 1 Composition of elements of MnZnferrite samples as-prepared

Elements Composition (wt%) Estimated chemical composition

Targeted XRF

Mn 17.49 16.12

Mn0.73Zn0.24Fe2.1O4-δZn 5.55 6.24

Fe 49.79 46.63

The thermal behavior of the nanocrystallineMnZn ferrite during heating in N2 was monitored by the thermal

analysis (Fig. 2). The mainly mass loss of the MnZn ferrites samples as-prepared occurred during RT to 800 ℃

(about 8%), below 200℃，the corresponding mass loss is mainly of planar water and crystallization water, which

accounts for about half of the total mass loss and the corresponding exothermic peak of DSC curve is wide.

Nearly 500℃ the minority of metal oxalate and hydrates decomposition [19]. In addition, the TG curve also have a

tiny mass loss when the temperature higher than 960℃, this may be related to the zinc evaporation above its

melting point (950℃). At high sintering temperature, the heating analysis reaction, ZnO→Zn+1/2O2, speeds up.

The proposed reaction leads to Zn volatilization and consequently inner stress of material increase [31], which

could not be ignored, for minimum zinc loss and well homogenized internal structure with absence of Fe 2O3plays

a crucial role to attain higher saturation magnetization[2].

Fig.2 TG and DTA curves of a nanocrystalline MnZn ferrite powder heated in N2 with 10 ℃/min.

Only at high temperatures, the spinel-type ferrites in the system Mn-Zn-Fe-O in air can be stable and could

decompose into a mixture ofhaematite(α-Fe2O3) and a Fe-poor spinel during cooling[28]according to Eq. (3)[2].

MnxZn1-xFe2O4+x/4 O2 →（1-x）ZnFe2O4+x/2MnO3 +XFe2O3 (3)

The presence of nonmagnetic phase hematite (α-Fe2O3) deteriorates saturation magnetization of sintered product.

In order to overcome the occurrence of the additional phase and to elucidate the series of phase transformations,

different atmosphere were adopted. Samples of the MnZn ferrites as-prepared were calcined at different

temperatures 400, 600, 800,1000, and 1200℃for 4 h and quenched from that temperature in partial pressure of

oxygen (N2) and reducing atmosphere (99vol.%CO2+1vol.%H2) , respectively. The XRD patterns and magnetic

hysteresis loops for the products are shown in Fig.4 and Fig.5.

Samples were calcined in nitrogen atmosphere to avoid oxidation of spinel phase (Fig. 4a). At low temperatures

(below 600℃) the nanocrystalline ferrite powder loses some weight due to removal of adsorbates. The X-ray

diffractogram of the powder calcined at 600℃ still exhibits reflections of a single-phase spinel, but the XRD

peaks of all these samples are broaden, which shows that calcined at rather low temperatures could not contribute

to the grain growth and crystallization of ferrite particles. Between 600~1000℃,the dark brown powders turned

reddish in the presence of nitrogen that was again the indication of ferric oxide (iron III) phase as shown in XRD

results (Fig. 4a). The XRD patterns are characterized by haematite(α-Fe2O3) as main additional phase in

combination with a majority spinel ferrite, despite the mass loss (3%)the same as low temperature range. Mn and

Fe elements have varying valencies in ferrites, and the oxidation and reduction reactions occur in MnZn ferrites

depending on the oxygen partial pressure in the environment and the heat treatment temperature [1]. MnZn ferrites

decomposition pressure is lower than the oxygen partial pressure in air, when the annealing temperature is low;

the oxygen partial pressure is higher than the ferrite decomposition pressureleading to the decomposition reaction

in ferrites [2].

MnFe2O4 →MnOx +Fe2O3 (4)

Angermann pointed that a fraction of the Mn ions in the defect spinel has an oxidation state higher than +2 and

that reduction of Mn3+ seems to set in at 500℃[28] and the reduction of Mn ions to Mn2+ in that temperature range

has also been reported for spinels of Mn-Fe-O system[32]. A similar series of transitions was reported for the

oxidation of MnZn ferrite at 670℃ in air results in formation of α-Fe2O3 due to oxidation of MnZn ferrite by

Mathur[33] and Ramesh[34]. Fe2O3 and Mn2O3 cannot be dissolved in the cubic spinel structure MnZn ferrite due to

their body-centered cubic (BCC) structure, only precipitate from the main phase of MnZn ferrite as additional

phases and bring considerable damage to the magnetic properties of ferrites. These results revealed that the

formation of hematite phase in ferrite could not be avoided when sintered in nitrogen alone at ≤1000℃。With

annealing temperature increase, the inner oxygen pressure in the samples increasesand when the inner oxygen

pressure in samples is higher than the environment oxygen partial pressure, MnZn ferrites are formed again

according to acombination reaction[35]:

ZnFe2O4 + MnO3 + Fe2O3 →MnZn ferrite(4)

The corresponding XRD patterns of the samples quenched from 800 to 1200℃ demonstrate the theory above, that

the dulplex-phase microstructure (MnZn ferrite and α-Fe2O3)which were formed near 800℃ again react to form a

single MnZn ferrite phase,α-Fe2O3dispears and the powder color go back to dark brownish. The thermal analysis

performed by heating the spinel ferrite in a reducing atmosphere (99 vol.% CO2+1vol.% H2) (Fig. 2 ) also verifies

the mass of the sample remains constant from 1000℃up to 1200℃, which suggests that the phase transitions no

longer been observed in N2above 1000℃in reducing atmosphere.

Comparing with the samples calcinedin reducing atmosphere (99 vol.% CO2+1vol.% H2), no additional phase

was observed during 600℃ to 1200℃in the XRD pattern, which illustrated that the reducing atmosphere

successfully restrained α-Fe2O3 from occurrence (Fig.3b). It is worth mentioning that the powders color appear

reddish-brown calcined at 600℃and 800℃in reducing atmosphere without any additional phase of α-Fe2O3

should be discussed later.

(a) (b)

Fig.3 XRD patterns of the nanocrystalline MnZn ferrite calcined at different temperatures for 4 h

in N2 (a) and reducing atmosphere (99 vol.% CO2+1vol.% H2) (b).

The hysteresis loops of the ferrite powders obtained by co-precipitation and refluxing method and calcined at

400~1200℃in different atmosphere as a function of magnetic field is shown in Fig.4.The magnetization of all

samples nearly saturated at the maximum field of the measurements (20kOe). The saturation magnetization M s

increases with the crystallite size of the ferrite particles (Table 2). The non-zero coercivityH c increasing with

particle size in the N2 samples, which indicates that in these ferrite powders the majority of the particles have a

size above the critical size for superparamagnetism. For example the critical size of single domain of

nanosizeNiFe2O4nearly 40nm and the critical size of superparamagnetism is 16 nm[36]. But this discipline does not

apply to the (99 vol.% CO2+1vol.% H2) ones. With the temperature increase, the particle grows continually and

reaching a stable size at 1100℃ of about 68.1224nm and a rather high saturation magnetization M s=188.155

emug/g, which is the most highest value in recent years.

(a) (b)

Fig.4 Hysteresis loops of the nanocrystalline MnZn ferrite calcined at different temperatures for 4 h

in N2(a) and reducing atmosphere (99 vol.% CO2+1vol.% H2) (b)

Table 2 Magnetic properties of MnZn ferrites as-preparedcalcined at 400℃~1200℃ in N2 and (99 vol.% CO2+1vol % H2).

atmosphereCalcined

Temperature T(℃)

Saturation

Magnetization M s

(emu/g)

CoercivityH c(Oe)

Retentivity

Magnetization M r

(emu/g)

Crystallite

size（nm）N2 400 43.924 118.427 7.333 32.8824

N2 600 35.658 80.134 4.046 18.5379

N2 800 27.068 58.417 2.161 26.0145

N2 1000 80.305 135.130 11.376 52.8075

N2 1200 87.237 123.583 9.830 81.6177

99 vol.% CO2+1vol.% H2 600 73.108 195.330 14.086 31.7003

99 vol.% CO2+1vol.% H2 800 90.922 175.660 17.230 33.5045

99 vol.% CO2+1vol.% H2 1000 90.791 128.115 8.989 63.4159

99 vol.% CO2+1vol.% H2 1100 180.155 128.575 18.858 68.1224

99 vol.% CO2+1vol.% H2 1200 92.550 126.130 8.986 69.2662

4. Conclusions

The effects of pH value on the structural and magnetic properties of MnZn ferrite nanoparticle, prepared by the

co-precipitation and refluxing method, have been investigated. It was found that the degree of crystallization and

average crystal size improved with the increase of the pH value, and the optimized pH value for MnZn ferrite

system is 13. At the suitablecalcining temperature of 1100 in a reducing atmosphere (99 vol.% CO℃ 2+1vol.%

H2), the purity, crystallization process of ferrites and the saturation magnetization were promoted and the crystal

size keep stable at even higher calcining temperature. But phase transition occurred at 800 in nitrogen was also℃

pointed in the experiment results which should be pay more attention to.

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