Synthesis of high Coercivity SrFe12O19 Powders
R. Rajashekar1,2, Ravi Gautam1, D. Prabhu1, *, R. Gopalan1
1
Centre for Automotive Energy Materials, ARCI, Chennai 600 113, India
School of Electronics Engineering, Vellore Institute of Technology, Vellore 632 014, India
*
Corresponding author’s e-mail:dprabhu@arci.res.in, Tel.: +91-44-66632811; Fax: +91-44-6632806
2
Abstract
The paper reports the high coercivity of 6.3 kOe
achieved in SrFe12O19 prepared using co-precipitation
technique. The magnetic properties are found to be
sensitive to the calcination temperature.
Keywords: coercivity, thermo-magnetization, Curie
temperature,
Introduction
Ferrite magnet (Sr/Ba ferrite) is the most
widely used magnet owing to its cost effectiveness.
Though the predicted theoretical limit of coercivity is ~
6.7 kOe, currently the coercivity of the best commercial
magnets are ~ 5 kOe with a remanance of ~4.2 kG
achieved by addition of La or Co. These additions
increase the price of these high grade magnets. The aim
of this present study is to develop SrFe12O19 powders
with coercivity greater than 6 kOe without any
additives.
Experimental
Samples were prepared using the coprecipitation technique with Sr and Fe nitrate with 1:6.4
molar ratio. The samples were calcined at various
temperatures & the various phases present in the sample
were determined using X-ray diffraction technique
(Rigaku, Smart lab, Japan). The magnetic properties
were measured using Vibrating Sample Magnetometer
(Microsense Model EV9, USA). The microstructure of
the calcined samples was observed using a Scanning
Electron Microscope (SEM, Quanta).
magnetization. XRD patterns revealed the presence of
two phases namely SrFe12O19 and Fe2O3. The relatively
low magnetization measured for 900 oC is due to the
larger volume fraction of Fe2O3 as reflected by relative
intensity ratio (ISrFe12O19/IFe2O3) of 1.3. On increasing
the calcination temperature to 1100 oC, an enhancement
in the volume fraction of SrFe12O19 is observed with
relative intensity increasing to 2 and no change in
relative intensity ratio was observed for sample calcined
at 1200 oC which clearly explains the observed trend in
magnetization. Thermo-magnetization measurements
also confirmed the presence of Fe2O3 as determined
from two Curie temperatures measured corresponding to
SrFe12O19 (750 K) and Fe2O3 (948 K).
Conclusion
High coercive SrFe12O19 (Hc = 6.3 kOe) were
prepared using co-precipitaion technique. The presence
of Fe2O3 phase is found to lower the magnetization.
Efforts to reduce Fe2O3 by varying molar ratio and
duration of calcination is expected to yield high grade
SrFe12O19 powders
Results and Discussion:
Figure 1 shows the M vs H curves of the
calcined samples. The sample calcined at 900 oC
measured the highest coercivity of 6.3 kOe with a
magnetization of 41 emu/g.
The coercivity
progressively decreased to 5.6 and 4.4 kOe on
increasing the calcination temperature to 1100 and
1200 oC. On the other hand the magnetization increased
from 41 to 49 emu/g for the sample calcined at 1100 0C.
No enhancement in magnetization was observed when
the sample was calcined at 1200 oC. XRD diffraction
studies clearly reflected the observed trend in
Fig. 1: M vs H curves of the calcined
SrFe12O19showing the high coercivity and
enhancement of M with calcination temperature
siReferences
[1] H.F. Lu, R.Y. Honga, H.Z. Li, “Influence of
surfactants on co-precipitation synthesis of strontium
ferrite”, J. Alloys and Comp. 509, (2011), pp. 10127