A.S. Lileev et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 504–506
505
Fe(A2) + amorphous
amorphous phase.
phase - FeO(B1) + Fe(A2) +
several sextets with practicallyequal isomer shifts
(dE0:10 mm/s) representing a component exhibiting a
hyperfine field distribution (180 kOeoHhf o390 kOe).
This contribution maybe attributed to the amorphous
phase. The considerable difference of d compared to the
one of a-Fe (dE À 0:15 mm/s) permits to assume that
this phase is a solid solution of oxygen in iron. The
Mo.ssbauer spectrum of a powder obtained after 3 h
milling of mixture B is presented in Fig. 2. Assuming
that the Debye–Waller factors are the same in all the
above-mentioned phases, the relative amount of the
phases was calculated from the Mo.ssbauer data. The
results were in satisfactoryagreement with the X-ray
data (Table 1).
The variations of the phase composition were
observed after 0.5 h milling of the powders. The final
state of FeO+a-Fe+amorphous phase with average
size of crystallites, determined from X-ray-line broad-
ening, of 15–20 nm was reached after 3 h milling of
mixture A and after 2 h milling of mixture B. As an
example the X-raypattern of mixture B after 3 h milling
is presented in Fig. 1. One should note that the average
particle size after milling (determined byScanning
Electron Microscopy) was 0.3–0.4 mm. Thus, one can
assume that macroparticles containing nanocrystallites
of Fe and FeO as well as amorphous phase, which are
formed as a result of solid state transformations, were
produced at the milling process.
According to X-rayinvestigations, the annealing of
the milled powders led to dissociation of FeO and the
amorphous phase into a-Fe+Fe3O4 (H1.1). The average
crystallite size was about 20 nm. For example, the
powder obtained bythe milling of the mixture A
contained after annealing 1971% of a-Fe and
8172% of Fe3O4. In this case the analysis of the
Mo.ssbauer spectra confirmed completelythe results of
the X-rayphase composition determination. Note that
the annealing helps to increase the remanence (Br), the
energyproduct ( ðBHÞmax) and to retain the high coercive
force. For example, the mixture B after the milling and
annealing contained 3772 vol% of a-Fe, 6372 vol% of
Fe3O4 and had the following properties: IHc ¼ 56 kA/m,
Br ¼ 0:48 T, ðBHÞmax ¼ 9 kJ/m3.
The abundance of a-Fe and FeO in the milled
powders depended on Fe content in the starting mixture,
but the amount of the amorphous phase was practically
the same in both mixtures (about 30 vol%, Table 1). As
a result of the milling the intrinsic coercive force of the
powders increased up to 45–50 kA/m.
57Fe Mo.ssbauer spectra recorded at 300 K could be
decomposed into
a
sextet with hyperfine field
Hhf ¼ 330 kOe, which was attributed to a-Fe and two
doublets with isomer shift (relative to 57CoRh at 300 K)
dE0:81 mm/s, which maybe attributed to FeO [4]. Note
(
that the lattice parameter of the FeO phase (aE4:28 A)
points to a considerable oxygen enrichment in compar-
ison to the stoichiometric composition. In addition, the
Mo.ssbauer spectra were fitted with a superposition of
99
96
93
-12
-8
-4
0
4
8
12
velocity (mm/s)
Fig. 2. 57Fe Mo.ssbauer spectrum of mixture B after 3 h milling.
Fig. 1. X-raydiffraction pattern of mixture B after 3 h milling
(Co Ka radiation).
Table 1
Phase composition of the mixtures A and B (vol%)
Method
Mixture A
Mixture B
a-Fe
FeO
Amorphous phase
a-Fe
FeO
Amorphous phase
X-raydiffraction analysis
Mo.ssbauer spectroscopy14
9 72
58712
33714
3072
4273
2875
77
9
32
49
19
Lileev
