Journal of Magnetism and Magnetic Materials 316 (2007) 29–33 Structural and magnetic studies on mechanosynthesized BaFe 12x Mn x O 19 Puneet Sharma a, , R.A. Rocha a , S.N. Medeiros a , B. Hallouche b , A. Paesano Jr. a a Departamento de Fı´sica, Universidade Estadual de Maringa´, Av. Colombo 5790, Maringa´ CEP 87020-900, PR—Brazil b Departamento de Quimica e Fı´sica, UNISC, Sta. Cruz do Sul-RS-Brazil Received 18 October 2006; received in revised form 6 January 2007 Available online 11 April 2007 Abstract Barium hexaferrite powders with manganese substitution were prepared by mechanosynthesis. The structural and magnetic properties were characterized by X-ray diffractometer and vibration sample magnetometer, respectively. XRD patterns were refined by Rietveld method. Preferential site occupation of manganese ion was investigated by room temperature (RT) Mo¨ssbauer measurements. XRD results showed a single-phase barium hexaferrite with some residual hematite. Crystallite size was observed to decrease with substitution amount. Lower saturation magnetization and increased coercivity is observed in substituted samples. RT Mo¨ssbauer measurements showed that manganese ions preferentially occupy 12k, 4f 2 , and 2a sites. r 2007 Elsevier B.V. All rights reserved. PACS: 61.10Nz; 75.50.Gg; 75.60Ej; 76.80. +y Keywords: Barium hexaferrites; Mechanosynthesis; Magnetic properties; Mo¨ssbauer spectroscopy; Rietveld refinement 1. Introduction Barium hexaferrite is isostructural with magnetoplum- bite (PbFe 12 O 19 ) and has been extensively used for permanent magnets and high-density magnetic recording media [1,2]. The hexagonal structure is built up of a cubic block S, having the spinel structure and a hexagonal block R, containing Ba 2+ ion. Five oxygen layers make one molecule and two molecules make one unit cell. In the unit cell, Fe 3+ ions occupy five different crystallographic sites, i.e., tetrahedral (4f 1 ), octahedral (12k, 2a, 4f 2 ), and hexahedral (2b) site of oxygen lattice. In the magnetically ordered state of BaFe 12 O 19 , the 12k, 2a, and 2b sites have their spins parallel to the crystallographic c axis, whereas those of 4f 1 and 4f 2 points are anti-parallel, constituting a net magnetic moment of 40 m B [1]. To tailor the magnetic properties of the barium hexaferrites, Fe 3+ ions are partially substituted by various di, tri, and tetravalent ions, aiming to occupy the spin- down sites and, consequently, to increase the net magne- tization. Nevertheless, if Fe 3+ is substituted by a divalent ion, a simultaneous substitution (e.g., of a Ba 2+ by a trivalent ion) is necessary to satisfy the condition of charge balance [3–5]. Actually, a simple partial Ba 2+ substitution by a trivalent rare earth ion (e.g., La 3+ , Nd 3+ , Sm 3+ ) induces the valence change of Fe 3+ to Fe 2+ and may, depending on the substitutional rare earth ion, enhance the magnetization and/or coercivity [6,7]. Besides that, the simultaneous substitution of divalent–tetravalent ions for Fe 3+ is another approach that satisfies the electronic neutrality criterium [8–11]. Apart from substitutions, much attention has also been devoted to study the properties of barium hexaferrites produced by synthesis processes different from the conventional ceramic method such as chemical routes and high-energy ball milling (HEBM) [4,6,12–14]. In fact, a growing number of articles have reported the substitution of iron and/or barium ions in hexaferrites prepared by mechanical alloying of precursors oxides followed by thermal annealing at moderate temperatures [15,16]. Interestingly, it was reported that substitution effects in ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.03.207 Corresponding author. E-mail address: [email protected] (P. Sharma).
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ARTICLE IN PRESS
0304-8853/$
doi:10.1016
�CorrespE-mail a
Journal of Magnetism and Magnetic Materials 316 (2007) 29–33
www.elsevier.com/locate/jmmm
Structural and magnetic studies on mechanosynthesizedBaFe12�xMnxO19
Puneet Sharmaa,�, R.A. Rochaa, S.N. Medeirosa, B. Halloucheb, A. Paesano Jr.a
aDepartamento de Fısica, Universidade Estadual de Maringa, Av. Colombo 5790, Maringa CEP 87020-900, PR—BrazilbDepartamento de Quimica e Fısica, UNISC, Sta. Cruz do Sul-RS-Brazil
Received 18 October 2006; received in revised form 6 January 2007
Available online 11 April 2007
Abstract
Barium hexaferrite powders with manganese substitution were prepared by mechanosynthesis. The structural and magnetic properties
were characterized by X-ray diffractometer and vibration sample magnetometer, respectively. XRD patterns were refined by Rietveld
method. Preferential site occupation of manganese ion was investigated by room temperature (RT) Mossbauer measurements. XRD
results showed a single-phase barium hexaferrite with some residual hematite. Crystallite size was observed to decrease with substitution
amount. Lower saturation magnetization and increased coercivity is observed in substituted samples. RT Mossbauer measurements
showed that manganese ions preferentially occupy 12k, 4f2, and 2a sites.
r 2007 Elsevier B.V. All rights reserved.
PACS: 61.10Nz; 75.50.Gg; 75.60Ej; 76.80. +y
Keywords: Barium hexaferrites; Mechanosynthesis; Magnetic properties; Mossbauer spectroscopy; Rietveld refinement
1. Introduction
Barium hexaferrite is isostructural with magnetoplum-bite (PbFe12O19) and has been extensively used forpermanent magnets and high-density magnetic recordingmedia [1,2]. The hexagonal structure is built up of a cubicblock S, having the spinel structure and a hexagonal blockR, containing Ba2+ ion. Five oxygen layers make onemolecule and two molecules make one unit cell. In the unitcell, Fe3+ ions occupy five different crystallographic sites,i.e., tetrahedral (4f1), octahedral (12k, 2a, 4f2), andhexahedral (2b) site of oxygen lattice. In the magneticallyordered state of BaFe12O19, the 12k, 2a, and 2b sites havetheir spins parallel to the crystallographic c axis, whereasthose of 4f1 and 4f2 points are anti-parallel, constituting anet magnetic moment of 40 mB [1].
To tailor the magnetic properties of the bariumhexaferrites, Fe3+ ions are partially substituted by variousdi, tri, and tetravalent ions, aiming to occupy the spin-
- see front matter r 2007 Elsevier B.V. All rights reserved.
down sites and, consequently, to increase the net magne-tization. Nevertheless, if Fe3+ is substituted by a divalention, a simultaneous substitution (e.g., of a Ba2+ by atrivalent ion) is necessary to satisfy the condition of chargebalance [3–5]. Actually, a simple partial Ba2+ substitutionby a trivalent rare earth ion (e.g., La3+, Nd3+, Sm3+)induces the valence change of Fe3+ to Fe2+ and may,depending on the substitutional rare earth ion, enhance themagnetization and/or coercivity [6,7]. Besides that, thesimultaneous substitution of divalent–tetravalent ions forFe3+ is another approach that satisfies the electronicneutrality criterium [8–11].Apart from substitutions, much attention has also been
devoted to study the properties of barium hexaferritesproduced by synthesis processes different from theconventional ceramic method such as chemical routesand high-energy ball milling (HEBM) [4,6,12–14]. In fact, agrowing number of articles have reported the substitutionof iron and/or barium ions in hexaferrites prepared bymechanical alloying of precursors oxides followed bythermal annealing at moderate temperatures [15,16].Interestingly, it was reported that substitution effects in
ARTICLE IN PRESSP. Sharma et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 29–3330
mechanosynthesized barium hexaferrite significantly differfrom other methods, conventional or chemicals, in terms oftheir structural and magnetic properties [17].
In this sense, in this work, manganese-substitutedbarium hexaferrites were prepared by mechanosynthesisand their structural and magnetic properties investigated.The choice of manganese as a substitutional for iron isaccounted for the divalent, trivalent, and tetravalent ionicstates in which the cation is found as, for instance, in themanganese oxides (i.e., MnO, Mn2O3, Mn3O4, and MnO2)[18]. In addition, recent investigations revealed that insome ferrimagnetic oxides, including barium hexaferrites,manganese retains the trivalent state [19]. Therefore, itcould be assumed that manganese substitution does notrequire any charge balance. On other hand, the ionic radiusof Mn3+ is also comparable to the ionic radius of Fe3+ ion(0.69A), what may favor the substitution up to higherlevels.
Magnetic studies on conventionally prepared bariumhexaferrites with manganese substitution at high substitu-tion levels were reported previously by Obradors et al. [20].It was observed that high substitution levels destroy theGorter collinear magnetic structure [21], converts thesymmetry from hexagonal to triclinic and significantlyreduces the saturation magnetization.
Aiming to understand the effects of the manganesesubstitution in mechanosynthesized barium hexaferrites,solid solutions of the type BaFe12�xMnxO19 were preparedin the range 0pxp0.50 and characterized by X-raydiffraction (XRD), Mossbauer spectroscopy, and magne-tization measurements.
Fig. 1. X-ray diffractograms for the BaFe12O19 (a) and BaFe11.5Mn0.5O19
(b) samples. The inserts show the discrepancy between experimental value
and Fit.
2. Experimental
BaFe12�xMnxO19 samples with X ¼ 0.1, 0.2, 0.3, 0.4,and 0.5 were prepared by HEBM for 30 h in freeatmosphere, using laboratory grade compounds BaCO3,Fe2O3, and Mn2O3, in a planetary mill with hardened steelvial and balls. The ball-to-powder ratio was 10:1 and thespeed rotation was 300 rpm. Further, the milled powderswere annealed at 1050 1C for 2 h, also in free atmosphere,using a resistance furnace. The heating and cooling rateswere 5 1C/min.
The phase characterization of powders was carried out ina SHIMADZU-6000 X-ray diffractometer. Diffractionintensity was measured in the range 251p2yp751, with astep of 0.021 for 1 s. The XRD patterns of some heat-treated samples were refined by the Rietveld method(FULLPROF SUITE-2000 software).
Mossbauer characterizations were performed in thetransmission geometry, using a conventional spectrometeroperating in a constant acceleration mode, with the gammarays provided by a 57Co(Rh) source. The Mossbauerspectrum was analyzed with a non-linear least-squareroutine, with Lorentzian line shape.
The magnetic properties of randomly pressed powderswere measured in a vibrating sample magnetometer, with amaximum field of 1.5 T.
3. Results and discussion
The X-ray diffractograms of the non-substituted andX ¼ 0.50 substituted barium hexaferrite samples are shownin Fig. 1. For both samples, all major peaks correspond tothe barium hexaferrite but a minor presence of hematitecan also be verified. The residual amount of hematite, asobtained by the Rietveld analyses, was less than onepercent in all samples. As previously reported, a fraction ofhematite generally remains unexhausted in hexaferritesprepared with the exact composition of 12 transition metalions [13]. No peaks for the Mn2O3 phase were observed inany of the samples.The crystallite sizes, lattice parameters, and R-factors
obtained by Rietveld refinement are given in Table 1. Thelattice parameters are almost constant within the composi-tion range, though a gradual decrease in crystallite size isobserved. As expected, the cell parameters were not
ARTICLE IN PRESSP. Sharma et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 29–33 31
influenced for a low substitution level because of thecomparable ionic radius of Mn3+ and Fe3+ ion.
The refined atomic positions for the BaFe12O19 andBaFe11.5Mn0.5 samples are shown in Table 2. Thedifference in atomic position parameter values resultingfrom the substitution of Mn3+ for Fe3+ ion is much morevisible in the z-position of the ions, though some oxygenions also changed their y–coordinate. This is consequenceof the columnar structure of the unitary cell that allows formore freedom in the c-direction to accommodate thesubstituting ions.
Fig. 2 shows the room temperature (RT) Mossbauerspectra of for the non-substituted and X ¼ 0.5 substitutedsamples. The spectra were fitted with five discrete sextets,corresponding to the five crystallographic sites of thehexaferrite structure (i.e., 12k, 4f1, 4f2, 2a, and 2b). For theX ¼ 0 sample, the subspectral areas were constrainedaccording to the iron site population (i.e., 6:2:2:1:1) butwere free to vary for the X ¼ 0.5 sample. For simplifica-tion, no component for residual hematite was super-imposed, considering its small fraction. Table 3 presentsthe hyperfine parameters and the site occupation numbersfor the iron sites (NFe), as obtained from the Mossbauerrelative areas and specific concentrations.
Table 1
Structural parameters for the BaFe12�xMnxO19 samples, refined from the
Rietveld analysis
X Crystal
size (nm)
Lattice
parameters
R factor
a c Rp Rwp Rexp C2
0.0 57.8 5.89 23.218 13.7 17.6 15.7 1.25
0.10 51.5 5.891 23.217 9.11 13.6 12.1 1.25
0.20 50.2 5.892 23.218 8.9 13.7 12.1 1.28
0.30 51.0 5.891 23.217 8.9 13.4 12.0 1.24
0.40 50.4 5.892 23.221 7.8 11.8 10.20 1.18
0.50 49.6 5.892 23.219 8.6 13.3 12.06 1.18
Table 2
Atomic positions for the BaFe12O19 and BaFe11.5Mn0.5O19 samples, as obtain
Atom Site BaFe12O19
x y z
Ba(1) 2d 2/3 1/3 1/4
Fe(1) 2a 0 0 0
Fe(2) 2b 0 0 0
Fe(3) 4f1 1/3 2/3 0
Fe(4) 4f2 1/3 2/3 0
Fe(5) 12k 0.1686(8) 0.3373(5) 0
O(1) 4e 0 0 0
O(2) 4f 1/3 2/3 0
O(3) 6h 0.1821(3) 0.3129(1) 1/4
O(4) 12k 0.1564(7) 0.3129(1) 0
O(5) 12k 0.5026 0.0043(4) 0
In general, the fitted parameters are in good agreementwith previously reported Mossbauer data for bariumhexaferrites [22,23]. In the present work, no significant
ed from the Rietveld analysis
BaFe11.5Mn0.5O19
x y z
2/3 1/3 1/4
0 0 0
.2585(2) 0 0 0.2575(5)
.0269(3) 1/3 2/3 0.0271(3)
.1905(9) 1/3 2/3 0.1905(5)
.8916(5) 0.1686(8) 0.3373(5) 0.8917(3)
.1497(2) 0 0 0.1517(6)
.9426(2) 1/3 2/3 0.9442
0.1821(3) 0.3551(3) 1/4
.0520(9) 0.1564(7) 0.3119(4) 0.0520(6)
.1493(5) 0.5026 0.0090(1) 0.14976
Fig. 2. RT Mossbauer spectra for the BaFe12O19 and BaF11.5Mn0.5O19
samples.
ARTICLE IN PRESS
Table 3
Hyperfine parameters and iron site occupation (NFe) for the iron sites in
BaFe12O19 and BaFe11.5Mn0.5O19 samples at RT
Sample Site Bhf (T) IS (mm/s) QS (mm/s) G (mm/s) NFe
BaFe12O19 12k 41.4 0.40 0.40 0.35 6
4f1 49.0 0.25 0.25 0.29 2
4f2 51.6 0.27 0.16 0.28 2
2a 50.4 0.25 0.09 0.25 1
2b 40.5 0.21 2.15 0.46 1
BaFe11.5Mn0.5O19 12k 41.0 0.24 0.40 0.39 5.80
4f1 48.3 0.16 0.20 0.28 1.97
4f2 51.1 0.26 0.17 0.26 1.89
2a 49.9 0.26 0.09 0.24 0.90
2b 40.2 0.18 2.13 0.45 0.94
35
40
45
50
55
60
65
0.5
4.4
4.5
4.6
4.7
4.8
4.9
5.0
5.1
5.2
5.3
-0.1 0.0 0.1 0.3 0.4 0.5 0.6
48
50
52
54
5
6
7
8
9
10
11
12
MM
F (
em
u/g
)C
oerc
ivity (
kO
e)
Cry
talli
te s
ize (
nm
)
Mn substitution
Net B
hf (
T)
Mn substitution
Mn Substitution (x)
0.40.30.20.10.0
0.0 0.50.40.30.20.1
58
56
0.2
Fig. 4. Magnetization at 15 kOe field, MMF, (a) and coercivity (b), as a
function of the manganese content, X, for the BaFe12�xMnxO19 samples.
The insets in (a) and (b) show the variation of the net hyperfine magnetic
field and crystallite size, respectively.
-15 -10 -5 0 5 10 15
-80
-60
-40
-20
0
20
40
60
80
Ma
gn
etiza
tio
n (
em
u/g
)
x=0.0
x=0.50
Coercivity (kOe)
Fig. 3. RT magnetization curves as a function of the applied field for the
BaFe12O19 and BaFe11.5Mn0.5O19 samples.
P. Sharma et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 29–3332
variations in the hyperfine parameters were observed,comparing the values obtained for both samples. However,a systematic weakening in the hyperfine magnetic fields isverified for the substituted sample. This is attributed to thenon-magnetic Mn3+ ion replacing the Fe3+ ion, whichmay reduce the intra- or inter-sublattice exchange interac-tions.
Comparing the values of NFe for all the five crystal-lographic sites, it is apparent that Mn3+ ion has preferenceto occupy the 12k, 4f2, and 2a sites though someoccupation for 2b site is also observed. Aside the differenceregarding the substitution levels, this is contrary topreviously reported neutron diffraction results, whichindicated that Mn ion avoids the 2b site [24].
Fig. 3 shows the M–H curves for two selected bariumhexaferrite samples. A lower saturation magnetization anda higher coercivity are evident in the Mn-substitutedpowder.
Fig. 4(a) illustrates the effect of manganese substitutionon the RT magnetization at 15 kOe field, MMF, for theBaFe12�xMnxO19 samples. As the substitution levelincreases, the magnetization shows some oscillation thoughthe general tendency is of reducing. The net hyperfine
magnetic field, i.e., the averaged ‘‘vector sum’’ of the fivehyperfine magnetic fields pondered by the subspectralareas, showed as an inset in the figure, presents a similarbehavior. The explanation is addressed to the fact thatmagnetization depends on the individual magnetic mo-ments, which, in turn, are proportional to the hyperfinemagnetic fields existing in the phase.
Fig. 4(b) shows the coercivity, Hc, as a function of themanganese concentration. It can be seen that coercivityincreases almost linearly with the substitution amount. It isa well-known fact that coercivity depends, among others,upon the crystallite size and magnetocrystalline anisotropy[25]. However, the magnetocrystalline anisotropy is notinfluenced by manganese substitution [18] and, therefore,the increase of coercivity should be related to the reductionin crystallite size. The inset included in the figure explicitlyreveals this inverse proportionality. Possible mechanisms
ARTICLE IN PRESSP. Sharma et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 29–33 33
for reduction in crystallite size are the reduction in cellparameters, what did not happen, or a grain pinningeffect [26,27]. The latter could be due to the mechano-chemical reduction of the Mn2O3 precursor to ultrafine particles of MnO, caused by the HEBM followed byheat treatment. The substantially higher ionic radii ofMn2+ ion in MnO as compared to Fe3+, ruled out thepossibility of substitution for Fe3+ during the inter-diffusion process. Therefore, these free MnO ultra fineparticles probably would exist along the grain boundaryand inhibiting the grain growth in the hexaferritephase. The absence of MnO peaks in the diffractionpatterns is explained by its small fraction, probably in theppm order. It is worthy of saying that a grain pinningeffect by Mn2+ was previously reported for the cases ofthin films, magnetic alloys, and other electro-ceramics[28–30]. Another possible explanation for enhancementin coercivity is the super exchange between Fe–Mn–Feions, which could align Fe3+ ion anti-ferromagnetically.This anti-ferromagnetic coupling between Fe3+ ions couldact as hindrance to demagnetize the Mn-substitutedbarium hexaferrite powders, which results in highercoercivity.
4. Conclusions
Mn3+ -substituted barium hexaferrites were first suc-cessfully prepared by mechanosynthesis. Within theworked concentration range, the substitution did not affectthe lattice parameters but produced smaller crystallites.The Mossbauer results revealed that Mn3+ ion preferablyoccupies the 12k, 4f2, and 2a sites. A decrease of E10% inthe maximum magnetization with a corresponding increasein the coercivity of E13% was observed within the usedcomposition range. The lower magnetization and highercoercivity are attributed to the reduction of the averagemagnetic moment of the lattice and to the crystallite sizerefinement, respectively.
Acknowledgements
P. Sharma would like to thank to CNPq for thefellowship grant (Bolsa PDJ).
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