ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 322 (2010) 763–766

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

E-m

journal homepage: www.elsevier.com/locate/jmmm

The effect of urea-to-nitrates ratio on the morphology and magneticproperties of Mn0.8Mg0.2Fe2O4

M.A. Ahmed

Materials Science Laboratory(1), Physics Department Faculty of Science, Cairo University, Giza, Egypt

a r t i c l e i n f o

Article history:

Received 17 June 2009

Received in revised form

14 September 2009Available online 21 October 2009

Keywords:

Mn–Mg nanometric ferrite

Flash auto-combustion

Magnetization

Coercivity

53/$ - see front matter & 2009 Elsevier B.V. A

016/j.jmmm.2009.10.019

ail address: Moala47@hotmail.com

a b s t r a c t

Mn0.8Mg0.2Fe2O4 ferrite was synthesized using flash auto-combustion technique using urea as fuel. The

effect of the urea-to-nitrates ratio was examined and found to affect the samples characteristics. The as-

burnt powder was crystallized in single-phase spinel structure of cubic symmetry. The lattice parameter

was decreased with increase in the urea-to-nitrates ratio (n) while the crystal size increased from 21 to

42 nm with n changing from 6.67 to 10. The coercivity increases while the saturation and remanence

magnetization decreases with increase in n. This was attributed to the disturbance of the spin order as a

result of the surface effects.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Composite oxides with spinel structures (AB2O4) and spacegroup Fd3m are important inorganic metalloid materials, whichare widely used in different fields. They are used not only as heat-resistant pigments, which can be applied to porcelain andceramics [1], but also as gas-sensitive [2], catalytic [3], magneticand wave absorbing materials [4–6].

In recent years, several synthetic methods for preparing highlycrystalline and uniformly sized magnetic nanoparticles of Mnferrite have been reported [7–16]. However, most of thesemethods cannot be applied to a large scale and economicproduction because they require expensive and often toxicreagents, high reaction temperature and long reaction time.Among them, one is the combustion reaction stands out as analternative and highly promising method for the synthesis ofthese ferrites [17,18]. The resulting product is a crystalline dryagglomerated into highly fluffy foam with high chemical homo-geneity and purity.

The goal of this work is to synthesize single-phase Mg-substituted Mn nanoferrite using flash auto-combustion techni-que. Moreover, we are concerned with improving the physico-chemical properties of Mg-substituted Mn ferrite by the variationof the fuel ratio. This was done by examining both structural andmagnetic characterizations.

ll rights reserved.

2. Experimental techniques

Mn0.8Mg0.2Fe2O4 ferrite powder was synthesized by combus-tion reaction, which involves mixing metallic ions that act as anoxidizing reagent with a fuel (urea) that acts as the reducingagent. This redox mixture consisted of managanese nitrateMn(NO3)2 �6H2O, magnesium nitrate Mg(NO3)2 �6H2O, iron ni-trate Fe(NO3)3 �9H2O and urea –CO(NH2)2. All the reagents wereof high degree purity. Stoichiometric ratios of metal nitrates andurea were calculated based on the components’ total oxidizingand reducing coefficients for the stoichiometric balance, so thatthe equivalence ratio was unity and the energy released wasmaximum [19]. Carbon, hydrogen, manganese, magnesium andiron elements were considered as reducing elements whoserespective valences were +4, +1, +2, +2 and +3. Oxygen wasconsidered an oxidizing agent with a valence of �2. The valenceconsidered for nitrogen was 0, because it is an inert element. Thereactants were hand mixed and heated on a magnetic stirrer. Withrising temperature, melting occurred and a dark liquid wasproduced, which on boiling became more viscous. Soon after theviscous liquid began frothing, self ignition took place, leading toan increase in the temperature, appearance of a central point ofincandescence that propagates in swift ripples to walls of thebeaker and evolution of large quantity of gases. The reaction wastoo fast and produced dry, fragile foam, which was transformedinto powder at the slightest touch. This method is quite simple,fast and inexpensive since it does not involve intermediatedecomposition and/or calcining steps. Furthermore, it is easy tocontrol the stoichiometry and crystallite, size which haveimportant influence on the magnetic properties of the ferrite[7–9]. Also, this method exploits an exothermic, usually very rapid

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20 30 40 50 60 70 80

020406080

100

(622

)

(531

)

(620

)

(440

)

(511

)(4

22)

(400

)

(222

)(311

)

n=10

n=9

n=7.7Cou

nts

2Q

n=6.67

(220

)

*

* Fe2O3

Fig. 2. X-ray diffraction patterns of the sample Mn0.8Mg0.2Fe2O4 prepared with

different urea/nitrates ratios (n=6.67, 7.7, 9 and 10).

0.20

M.A. Ahmed / Journal of Magnetism and Magnetic Materials 322 (2010) 763–766764

and self-sustaining chemical reaction between the desired metalsalts and a suitable organic fuel, usually urea. A key feature of thismethod is that the heat required to sustain the chemical reactionis provided by the reaction itself and not by an external source[19,20].

The sample was prepared one more time keeping the desiredmolar ratios of the metal nitrates the same and increasing the fuelratio (n=6.67, 7.7, 9 and 10). It is observed that by increasing thefuel to nitrates ratio (n), there was a longer flame time.

Characterization of the prepared powder was carried out by X-ray diffraction using diffractometer model Proker D8 with CuKa

radiation (l=1.5418 A) in a wide range of Bragg’s angle (20–801) atroom temperature. The average particle size (L) was calculatedfrom X-ray line broadening using (3 1 1) peak and Debye–Sherrer’sequation: L=0.89l/b cos y; b is the full-width at half-maximum(FWHM) and l is the wavelength of radiation. The hysteresis andmagnetization measurements were performed using a vibratingsample magnetometer (VSM; 9600-1 LDJ, USA) with a maximumapplied field of 15 kOe at room temperature.

8.45 8.46 8.47 8.48 8.49 8.50 8.510.00

0.05

0.10

0.15

8.4

8.6

8.8a (°A)

a (°

A)

20

30

40

50

L (n

m)

1/L

3. Results and discussion

The decomposition of urea is highly exothermic, aiding thedecomposition of nitrates salts into desired products at a fasterrate with low external energy consumption. Urea is considered asthe best suitable organic fuel because it is available commercially,cheap and generates the highest temperature during combustion.The gases were eliminated once the mixture of metal nitrates andurea was ignited, producing a typical flame as shown in Fig. 1. Thereaction was quite fast and produced dark brown free-flowingpowder that is attracted easily to the magnet. The reaction timedid not exceed 30 s.

X-ray diffraction patterns (Fig. 2) of the sampleMn0.8Mg0.2Fe2O4 prepared with different urea/nitrates ratios(n=6.67, 7.7, 9 and 10) reveal single-phase spinel structure ascompared and indexed with ICDD card no 74-2403. The sampleprepared with n=10 reveals the existence of small peaks of Fe2O3

beside the main phase (spinel structure). The lattice parameterwas computed (Fig. 3(a)) and found to decrease with increase inthe urea/nitrates ratio (n), which indicates more densificationwith enhancement in the grain growth. The lattice parameter ofthe bulk MnFe2O4 is 8.525 A and that of bulk MgFe2O4 is 8.372 A[21]. The remarkable increase in the particle size from 21 nm up to

Fig. 1. Photograph of the flame produced in the preparation of the samples.

6 8 10Urea/nitrates ratio (n)

10

Fig. 3. (a) The variation of the lattice parameter and the crystal size with the urea/

nitrates ratio (n) and (b) the reciprocal of the crystal size versus the lattice

parameter.

42 nm with n is attributed to increase in the reaction temperatureand time (i.e. self combustion) as the fuel ratio increases. This wasobserved as a longer flame time with higher n. It is well knownthat increase in sintering temperature and/or time increases thecrystal size and improves the density of the sample [22]. Fig. 3(b)illustrates increase in the reciprocal of the crystal size (1/L) withthe lattice parameter (a), which points to a surface relaxation inthese samples due to the large surface/volume ratio.

Fig. 4(a–d) shows the room temperature magneticcharacterization (hysteresis plot) of Mn0.8Mg0.2Fe2O4 ferritepowder prepared with different urea/nitrates ratios (n=6.67, 7.7,9 and 10). The plots give nearly the same trend but with differentvalues of Mr, Ms and Hc. From the graphs, one can plot the physicalquantities remanence (Mr), saturation magnetization (Ms), Mr/Ms

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-6000 -3000 0 3000 6000-80

-40

0

40

80

n=6.67

-6000 -3000 0 3000 6000

-40

0

40

n=7.7

-6000 -3000 0 3000 6000-80

-40

0

40

80

n=9

M (e

mu/

g)

H (Oe)

-6000 -3000 0 3000 6000-40

-20

0

20

40

n=10

-400 -200 0 200 400 -400 -200 0 200 400-40

-20

0

20

40

-40

-20

0

20

40

-400 -200 0 200 400-40

-20

0

20

40

-400 -200 0 200 400-20

-10

0

10

20

Fig. 4. (a–d) Hysteresis plots of the sample Mn0.8Mg0.2Fe2O4 prepared with different urea/nitrates ratios (n=6.67, 7.7, 9 and 10)

M.A. Ahmed / Journal of Magnetism and Magnetic Materials 322 (2010) 763–766 765

and coercivity (Hc) versus the urea content (n) as in Fig. 5(a–d).From the data it is clear that the coercivity (Hc) increases withincrease in n with an opposite manner to the variation of Mr andMs. This is in good agreement with the relation HcaKðm0MsÞ

�1; K isthe anisotropy constant and m0 the permeability of free space.Increase in Hc is mainly due to increase in the crystal size from20 nm at n=6.67 to 40 nm at n=10. This could be interpretedaccording to Kittel’s theory [23,24], in which the particle size hasinverse proportionality with HC. As the particle size (L) decreases,the coercivity increases and below a critical diameter (say Ls), theparticle becomes single domain and in this size range thecoercivity has maximum value. When the particle size decreasesbelow the Ls the coercivity starts decreasing according to therelation [25]Hc=g�h/L3/2.

where g and h are constants. Below a certain diameter (Lp), thecoercivity becomes almost zero due to the thermal effects, whichare strong enough to spontaneously demagnetize a previouslysaturated assembly of particles. The relatively large values of Ms inthe range 35–62 emu/g might be due to the high degree ofcrystallization and uniform morphologies. Fig. 5(b) shows that thesaturation magnetization has the lowest value at n=10. This wasexplained due to non-collinear induced spin structure and wasenhanced by the smallest lattice parameter at n=10 (Fig. 3(a)). Thesmall value of the saturation magnetization (0.4 emu/g) of thehematite, as a secondary phase, is too small with respect to that ofthe investigated samples, so it can be neglected. Thus, it is not too

significant in the decrease of the saturation magnetization due tothe small intensity (amount) of the phase. In other words, thesmallest magnetization at this urea/nitrates ratio can be explainedby the spin canting, which is not a surface effect but it is a finitesized one. The spin disorder on the surface layer has no significantcontribution here due to the relative large crystal size.

Previous work [26] shows that polycrystalline ferrites consistof different magnetic domains (Weiss domain); the magnetizationin such ferrite increases reversibly with magnetic field due todomain wall motion and then irreversibly due to domain wallrotation. The transition of the spin direction from one Weissdomain to another did not take place abruptly, since that wouldcost too much exchange energy. Instead, there is a gradualrotation of the spin vectors. The domain wall energy is estimatedby so=Kdo, where do is the wall thickness. do=(2kBTC/aK)1/2 [26],where TC is the Curie temperature, kB is Boltzman constant and a

is the lattice parameter. From the reported values of TC in Table 1,one clearly sees that there is no appreciable change in the Curiepoint of the investigated ferrite in the crystal size range20–40 nm. Since the samples under investigation have the samecompositions, i.e. the same order of anisotropy, the domain wallthickness will increase as the lattice parameter decreases(Fig. 3(a)). This could be interpreted as follows: increase indomain wall thickness will impede domain wall movement andmulti-domain structure (MD) is favored. The exchange interactionwill tend to make the wall as thick as possible, which is opposed

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6 8 10

8

12

Mr (

emu/

g)

Urea/nitratesratio (n)

45

60

Ms

(em

u/g)

0.18

Mr/M

s

40

50

Hc

(Oe)

Fig. 5. (a–d) The remanence (Mr), saturation magnetization (Ms), Mr/Ms and

coercivity (Hc) versus the urea/nitrates ratio (n).

Table 1Values of the crystal size, Curie temperature and Bohr magneton as a function of

urea/nitrates ratio for the sample Mn0.8Mg0.2Fe2O4.

Urea content (n) Particle size

L (nm) 70.5

Curie temp.

TC (K) 71

Bohr

Magneton mB

6.67 21.4 538 2.4770.09

7.7 24.2 541 2.2470.09

9 40.6 543 2.4970.09

10 41.8 543 1.3470.05

M.A. Ahmed / Journal of Magnetism and Magnetic Materials 322 (2010) 763–766766

by the crystal energy. For the sample with n=6.67, largest latticeparameter is obtained i.e., smallest domain wall thickness.However, one could declare that the magnetic field requires tosaturate the moments will be increased. In other words, smallfield values could not facilitate the unification of the spindirections inside the different ferrite domains, which is areasonable explanation of the small decrement of Ms with thecrystal size and increase in the coercivity. Meanwhile, higher fieldvalues are needed to demagnetize the samples. The surface of theparticle may behave differently and modify the magneticproperties in nanoparticles [20]. In these materials the superexchange interaction occurs through O2� ions. Absence of oxygenion from the surface results in broken exchange bonds betweenFe3 + ions, which induces surface spin disorder. This significantlymodifies the magnetic properties in nanoparticles where surfaceto volume ratio is high. The values of Bohr magneton arecalculated from mB=(mol.wt�Ms/5585) [26]. The calculatedvalues in Table 1 are smaller than those reported for thesubstituted Mn ferrite [27–31] due to disturbance in the spin

order due to surface effects. These surface effects dominate themagnetic properties of fine particles since decreasing the particlesize increases the ratio of surface spins to the total number ofspins. The magnetic behavior of the particle surface differs fromthat corresponding to the core, because of the distinct atomiccoordination, compositional gradients, concentration and natureof the defects present in both regions. Thus, the core usuallydisplays a spin canting and arrangement similar to that of the bulkmaterial.

4. Conclusion

At the end of this work, we can conclude that flash auto-combustion method is a successful way of producing single-phasespinel nanoferrite of the required size range. This is limited to therange of no10. This simple method results in nanometric ferritewith exclusive magnetic properties suitable for a wide range ofapplications in one step and in a reaction time not exceeding 40 s.Based on the results of the above study and by increasing urea/nitrates ratio, the lattice parameter was decreased and the particlesize increased from 21 nm at n=6.67 up to 42 nm at n=10. Also,the saturation and remanent magnetization were decreased whilethe coercivity was increased. One could select the appropriate fuelcontent to proper applications according to the particle size andrelated magnetic properties of the ferrite.

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