M?ssbauer and X-ray diffraction investigation of nanocrystalline Fe-O alloys

Article abstract of DOI:10.1016/S0304-8853(02)01124-1

X-ray and M?ssbauer investigations were carried out on powders produced by milling of Fe₂O₃ + α-Fe mixtures in a high-energy ball mill and subsequent low-temperature annealing. The nanocrystalline composite alloys obtained as a result of the milling, contained FeO and α-Fe with an average crystallite size of 15-20nm as well as an amorphous phase. Alloys subjected to subsequent annealing contained, however, only α-Fe + Fe₃O₄ with an average crystallite size of about 20nm. Unlike the starting materials the produced powders had properties, which are characteristic for hard magnetic materials.

Full text of DOI:10.1016/S0304-8853(02)01124-1

Journal of Magnetism and Magnetic Materials 258–259 (2003) 504–506
.
Mossbauer and X-raydiffraction investigation
of nanocrystalline Fe–O alloys
A.S. Lileeva, Yu. D. Yagodkina,*, M. Reissnerb, W. Steinerb
a Moscow State Institute of Steel and Alloys, Leninsky Prospekt 4, 119991 Moscow, Russia
b Vienna University of Technology, Wiedner Hauptstrabe 8-10, A-1040 Vienna, Austria
Abstract
X-rayand M o.ssbauer investigations were carried out on powders produced bymilling of Fe 2O3+a-Fe mixtures in a
high-energy ball mill and subsequent low-temperature annealing. The nanocrystalline composite alloys obtained as a
result of the milling, contained FeO and a-Fe with an average crystallite size of 15–20 nm as well as an amorphous
phase. Alloys subjected to subsequent annealing contained, however, only a-Fe+Fe3O4 with an average crystallite size
of about 20 nm. Unlike the starting materials the produced powders had properties, which are characteristic for hard
magnetic materials. r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Nanocrystalline alloys; Fe–O powders; Mo.ssbauer spectroscopy; X-ray diffraction analysis
1. Introduction
balls. The longest milling times were 3 h. According to
the estimation of Shelehov et al. [2] at the applied
conditions the milling process power was about 0.05 W/
g. Computer-controlled diffractometers were used for X-
rayexamination of the samples. Phase composition and
crystallite sizes were determined by a reduced Rietveld
method [3]. In addition, the phase composition of the
powders was investigated by 57Fe Mo.ssbauer spectro-
scopyat room temperature. The magnetic properties
were measured at room temperature in maximum fields
of 1300 kA/m with an accuracyof 3% using a vibrating
sample magnetometer.
It is possible to manufacture nanocrystalline compo-
site Fe–O alloys by milling of Fe+Fe2O3 mixtures in a
high-energyball mill and subsequent low-temperature
annealing [1]. Unlike the starting materials the produced
powders had properties (intrinsic coercive force of 40–
50 kA/m and a remanence up to 0.6 T) which are
characteristic for hard magnetic materials. The aim of
the present paper was the determination of phase
composition and structural characteristics of the phases
present in the milled powders byX-rayand M o.ssbauer
investigations to gain a better understanding of the
appearance of the hard magnetic properties.
3. Results and discussion
According to the X-raydiffraction analsyis, the
starting powders contained Fe2O3 phase (with D5.1-
type lattice) and a-Fe (with A2-type lattice). They had a
coarse-grain structure and a low intrinsic coercive
force (IHco4 kA/m). Treatment in the high-energy
2. Experimental
Powders of Fe2O3+25% a-Fe (mixture A) and
Fe2O3+50% a-Fe (mixture B) were chosen as starting
materials.
ball mill led to
a change of the powder phase
The treatment was carried out using an AGO-2U
planetaryball mill and sealed vials with hardened steel
composition in the following way(crsytal structure
types of the detected phases are indicated in parenth-
esis):
Fe2O3(D5.1)+Fe(A2)-Fe2O3(D5.1)+Fe3O4
*Corresponding author. Fax: +7-095-230-8007.
E-mail address: yag52@mail.ru (Yu.D. Yagodkin).
(H1.1)+FeO(B1)+Fe(A2)-Fe3O4(H1.1)+FeO(B1)+
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 1 2 4 - 1

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

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A.S. Lileev et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 504–506
4. Conclusions
References
[1] Yu.D. Yagodkin, A.S. Lileev, A.I. Salimon, S.A. Jaleel,
Program and Absracts of First Seeeheim Conference on
Magnetism, Seeheim, Germany, 2001, p. 200.
[2] E.V. Shelehov, V.V. Tcherdynntsev, L.Yu. Pustov, S.D.
Kaloshkin, I.A. Tomilin, J. Metastable, Nanocrystalline
Mater. 8 (2000) 603.
As a result of high-energyball milling of Fe O3 and
2
Fe mixtures nanocrystalline composite alloys containing
FeO, a-Fe and the amorphous phase were obtained. The
average size of crystallites was 15–20 nm. The amor-
phous phase is a solid solution of oxygen in iron.
However, the alloys subjected to milling and subsequent
annealing contained only a-Fe and Fe3O4 with average
crystallite size of about 20 nm. Unlike the starting
materials, the produced powders had properties, which
are characteristic for hard magnetic materials.
[3] E.V. Shelehov, T.A. Sviridova, Metalloved. Term. Obrab.
Met. 8 (2000) 16.
[4] V.I. Goldanskii, R.H. Herber (Eds.), Chemical Application
of Mo.ssbauer Spectroscopy, Academic Press, London, NY,
1968.