Influence of La doping on the properties ofSrBa hexagonal ferrites Pham Quang Niema, Nguyen Chaua,*, Nguyen Hoang Luonga, Dang Le Minhb a Center for Materials Science, National University
Trang 1Influence of La doping on the properties of
SrBa hexagonal ferrites Pham Quang Niema, Nguyen Chaua,*, Nguyen Hoang Luonga, Dang Le Minhb
a
Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam b
Department of Solid State Physics, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam
Abstract
A series of SrBa ferrites with composition (Sr0.75Ba0.25)1x(La2O3)x/2 5.3Fe2O3 has been prepared by the conventional ceramic technology The size of the crystallites corresponds to that of a single magnetic domain Doping
of La not only enhances the coercivity of these materials but also improves the remanence The reasons for improving the hard magnetic properties of the investigated ferrites are discussed
r2002 Elsevier Science B.V All rights reserved
Keywords: Magnetic oxides; Hard magnetic materials; Fine particles; Hexagonal ferrites
The high quality of hard magnetic materials is
determined by three main parameters of the
demagnetization curve in the second quadrant:
the coercive field HC; the remanence Br and the
convexcoefficient Z ¼ Bd Hd=ðBHÞmax: The
magnetization process of hard ferrites consisting
of single magnetic domain particles is governed by
the rotation process of the domain moments In
that case the coercive field can be expressed as
follows:
HC¼ aK1
Is
þ bðN1 N2ÞIsþ clst
Is
where a; b; c are numerical constants, N1 and N2
are demagnetization factors along two
perpendi-cular directions, lS is the magnetostriction and t
the mechanical strain
The first term in expression (1) corresponds to
the contribution of the magnetocrystalline
aniso-tropy of the material, the second one is given by the shape anisotropy of the crystalline particles (single magnetic domains) and the third one originates from the action of the elastic mechanical deformation
In fact, the first term plays the decisive role for creating a high coercivity, the second term can take part of several tens of a percent and the last one can only contribute several hundreds of Gauss
to HC For that reason, almost all authors have concentrated their investigations to enhance the magnetic anisotropy by partly substituting Me or
Fe in the hexagonal ferrite MeO 6Fe2O3 by various elements including rare-earth elements Recently, the substitution of small amounts of SrO
by La2O3 in the hexagonal Sr ferrites has been found to lead to an evident improvement of the magnetic parameters[1–5]
In this paper we present our study of the influence of La doping on the structure and properties of a SrBa hexagonal ferrite
*Corresponding author Tel./fax: +81-4-858-9496.
E-mail address: chau@cms.edu.vn (N Chau).
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 5 8 - 1
Trang 2Raw materials of Fe2O3, SrCO3, BaCO3, La2O3
and SiO2 with high purity have been used as
starting materials The investigated ferrite system
has the following composition: (Sr0.75Ba0.25)1x
(La2O3)x/2 5.3Fe2O3 where x ¼ 0:00; 0.02, 0.04,
0.06 The ratio (SrO, BaO): Fe2O3=1:5.3 was
chosen according to Refs [6,7] Powders were
mixed and milled in a vibrating ball mill, pressed in
pellets and presintered at 12501–12601C for 1–2 h
and then milled in the vibrating-ball mill with
milling times varying from 24 to 48 h Powder
samples were isotropically and anisotropically
pressed at pressures of 3 and 0.3 t/cm2,
respec-tively, and then sintered at 1250–12601C for 1–
1.5 h in air
The experimental techniques employed in our
investigations include: thermal analysis (by SDT
2960, TA-Instruments), crystallographic-structure
analysis (by X-ray diffractometer D 5005, Bruker),
particle-size measurements (by Mastersizer
Micro-plus Ver 2–17, Malvern), microstructure analysis
(by scanning electron microscope (SEM) 5410 LV,
Jeol) and magnetic measurements (by
vibrating-sample magnetometer (VSM) DMS 880, Digital
Measurement Systems)
From the DSC analysis of the powder mixtures
we observe that at around 8501C there is a thermal
decomposition of SrCO3 and BaCO3 In this
temperature region, the ferritization reaction is
strongly enhanced Moreover, the temperature at
which the ferritization reaction stops increases
with increase of the La doping, namely 11191C,
11271C, 11431C and 11621C corresponding with
x ¼ 0; 0.02, 0.04 and 0.06, respectively
The X-ray diffraction patterns (Fig 1) show
that the samples are with hexagonal structure
Their lattice parameters are collected in Table 1
From this table one can see that the lattice
parameter c slightly increases whereas the
para-meter a is somewhat fluctuating
The particle size analysis shows that after
milling for 48 h in the vibrating-ball mill, the
particles are reduced in dimension with increasing
La doping in the sample
sample with x ¼ 0:02 taken along the axis
perpen-dicular to the preferred magnetization direction
We can see from this figure that almost all the
grains have the size of a single magnetic domain (o1.3 mm) Moreover, there is a high degree of orientation of the particles in the anisotropic pressing The role of SiO2in our sample is to limit the grain growth
aniso-tropic sample with x ¼ 0:06 measured parallel and perpendicular to the preferred magnetization
2-Theta Scale
x = 0.00
x = 0.02
x = 0.04
x = 0.06
Fig 1 X-ray diffraction patterns of (Sr0.75Ba0.25)1x (La2O3)x/2 5.3Fe2O3 samples for different x-values.
Fig 2 SEM picture of an anisotropic (Sr0.75Ba0.25)1x (La2O3)x/2 5.3Fe2O3 sample with x ¼ 0:02 along the plane perpendicular to the preferred direction.
Table 1 Lattice constants of (Sr0.75Ba0.25)1x(La2O3)x=2 5.3Fe2O3 samples
a ( ( A) 5.881 5.875 5.879 5.880
c ( ( A) 23.058 23.062 23.064 23.070
Trang 3direction We see that along the ‘‘hard’’ axis the
magnetization is far from saturation and that the
squareness coefficient of the hysteresis loop is
much higher for the ‘‘easy’’ axis
The hysteresis loop parameters of the
aniso-tropic samples are collected inTable 2 From this
table we suggest that the orienting magnetic field
in the pressing process was too low (in our
experiment it was 10 kOe) to achieve a complete
orientation of the ferrite particles FromTable 2it
seems that doping of La in our samples not only
improves the coercivity and magnetization but the
maximum energy product is also enhanced due to
the increase of the squareness S (and therefore
increase of the convexcoefficient) We also can
conclude fromTable 3that doping of La leads to
an increase of the hysteresis loop parameters of
isotropic samples For the isotropic ferrite (Sr0.75Ba0.25)0.94(La2O3)0.03 5.3Fe2O3, the energy product (BH)max reaches the rather high value of 1.34 MG Oe
Substitution of La for (Sr, Ba) leads to the following reaction:
ðSr; BaÞ2þþFe3þ -La
Sr;Ba La3þþFe2þ: ð2Þ The formed Fe2+ ions possibly are located at the 4f1, 4f2positions This leads to an increase of the total magnetization This conclusion agrees with results from the temperature dependence of the saturation magnetization of these samples
Fig 4
As we can see from Tables 2 and 3, at using small amounts of La2O3 (o6 mol%) as a sub-stitute for (SrO, BaO) and at applying a traditional ceramic technology, the magnetic properties of hexagonal (Sr, Br) ferrites have been improved As explained in Refs [4,5], the appropriate La2O3
amount and the small SiO2 doping contribute to the creation of needle shape particles with the size
of a single magnetic domain, leading to an increase
of the magnetization as well as the magnetic anisotropies (both magnetocrystalline anisotropy and shape anisotropy of the particles) All these
Fig 3 Hysteresis loops of an anisotropic (Sr0.75Ba0.25)1x
(La2O3)x/2 5.3Fe2O3 sample with x ¼ 0:06 measured parallel
and perpendicular to the preferred direction.
Table 3 Characteristics of demagnetization curves of isotropic ferrite samples (sintering temperature 12501C)
Sample x ¼ 0:00 x ¼ 0:02 x ¼ 0:04 x ¼ 0:06
Br(kG) 1.96 2.07 2.23 2.01
B H C (kOe) 1.98 2.12 2.11 2.09 (BH)max (MG Oe) 1.03 1.22 1.34 1.09
Table 2
Characteristics of hysteresis loops of anisotropic samples (sintering temperature 12601C)
j H C (Oe) 1926 1886 3513 3500 3089 2987 3371 3243
a M is magnetization measured at 13.5 kOe.
Trang 4factors see Eq (1) contribute to the hard-magnetic
properties of the studied ferrites
Measurements of the electrical properties of the
samples show that the decrease of the specific
resistance in the doped sample is related to the
enhancement of the conductivity due to the
formation of Fe2+ions in the octahedral sublattice
when La3+ions are substituted for (Sr, Ba)2+(see
Eq (2)) In fact, doping of La leads to a
strong decrease of the resistance of the samples
(x ¼ 0; r ¼ 3:2 105O cm; x ¼ 0:0220:06; r ¼
5 10321:8 103O cm) From this result it could
be concluded that substitution of La for (SrBa) leads to a change of valence of Fe3+ions to Fe2+ ions at the 4f2position in the octahedral sublattice
Acknowledgements
We express our sincere thanks to the National Program for Fundamental Research for financial support
References
[1] H Yamamoto, M Nagakura, H Tarada, IEEE Trans Magn 26 (1990) 1144.
[2] H Taguchi, T Takeishi, K Suwa, K Masuzawa, Y Minachi, J Phys IV (France) 7 (1997) C1-311.
[3] M Sagawa, H Nagate, T Wantanabe, O Itatazi, J Phys.
IV (France) 7 (1997) C1-307.
[4] N.K Dung, N Chau, B.T Cong, D.L Minh, N.X Phuc,
J Phys IV (France) 7 (1997) C1-313.
[5] N.K Dung, N Chau, B.T Cong, D.L Minh, Proceedings
of the Third International Workshop on Materials Science (IWOMS’99) Hanoi, November 2–4, 1999, p 357 [6] N Chau, J Science of the Hanoi University, Phys 3 (1990) 51.
[7] C.Z Srig, D Bonnengerg, K.A Hempel, P Karduck, H.J Kloor, Ch Saner, J Phys (France) 7 (1997) C1-315.
0
20
40
60
80
10 0
x = 0.02
x = 0.00
x = 0.04
x = 0.06
T (K) Fig 4 Thermomagnetic curves of isotropic (Sr0.75Ba0.25)1x
(La2O3)x/2 5.3Fe 2O3 samples for different x-values (sintered at
12501C, maximum applied field 13.5 kOe).