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phys. stat. sol. (a) 102, 181 (1987)
Subject classification: 61.40; 61.80; 76.80; Sl.1; 51.4
Institute of Physics, Jagiellonian University, Cracowl) (a ) , Institute of Physics, Technical University, Cracow (b) , and Institute of Nuclear Physics, Cracow (c )
Studies of Local Structure and Thermal Annealing Processes in Implanted Iron-Gadolinium Alloys by Conversion Electron Mossbauer Spectroscopy BY E. KOLAWA (a)2), T. TYLISZCZAK (b)3), B. D. SAWICKA ( c ) ~ ) , J. A. SAWICKI (a)4), and M. DRWIEGA ( c )
Residence sites of iron atoms implanted in a gadolinium matrix are studied using conversion electron Mossbauer spectroscopy. Samples are obtained by implantation of 57C0+ ions (50 keV; 2 x 1013 cm-2) and 57Fe+ ions (70 keV; 5 x 1015, 10l6, 2 x 10l6, and 4 x 10l6 cm-2) into poly- crystalline gadolinium foils held a t room temperature. The concentration of implanted iron varies from 0.01 to 12 atyo in various samples. The measurements of Mossbauer spectra are made a t temperatures between 4.2 and 800 K. The Mossbauer spectra of as-implanted samples indicate paramagnetic features, and are represented by a superposition of three or four quadrupole-split components. Iron is mostly found as isolated (monomeric) impurity or as aggregates (Fe-Fe dimers). The investigated alloys are unstable and undergo structural relaxation upon heating between 400 and 500 K. At temperatures higher than 500 K, the transformation of the microstruc- ture is observed, associated either with interstitial ordering processes of Fe atoms in Gd matrix or/and formation of some intermetallic Fe-Gd phases. Arguments for the former interpretation, based on Miedema’s model of alloying are presented.
Die von implantierten Eisenatomen in einer Gadoliniummatrix besetzten PlLtze werden mittels Konversionselektronen-MoBbauerspektroskopie untersucht. Die Proben werden durch Implan- tation von 57Co+-Ionen (50 keV; 2 X 1013 und 57Fe+-Ionen (70 keV; 5 x 1015; 10l6; 2 x 10l6 und 4 x 10l6 erne2) in auf Zimmertemperatur gehaltene polykristalline Gadoliniumschichten er- halten. Die Konzentration des implantierten Eisens variiert von 0,Ol bis 12 Atyo in verschiedenen Proben. Die Messungen der MoBbauerspektren erfolgen bei Temperaturen zwischen 4,2 und 800 K. Die MoBbauerspektren der ursprunglich implantierten Proben zeigen paramagnetische Eigen- schaften und werden durch eine uberlagerung von drei oder vier quadrupolaufgespaltenen Kom- ponenten dargestellt. Eisen wird meist als isolierte (monomere) Storstelle oder als Aggregate (Fe-Fe-Dimere) gefunden. Die untersuchten Legierungen sind nioht stabil und durchlaufen eine strukturelle Relaxation nach dem Aufheizen zwischen 400 und 500 K. Bei Temperaturen hoher als 500 K wird eine Transformation der Mikrostruktur beobachtet, die entweder mit Zwischengitter- platzordnungsprozessen der Fe-Atome in der Gd-Matrix oder/und der Bildung einiger inter- metallischer Fe-Gd-Phasen verknupft ist. Argumente fur die zuerst genannte Interpretation, die auf dem Legierungsmodell von Miedema beruhen, werden angegeben.
1. Introduction
Alloying effects due to implantation of transition metal ions into rare earth matrices have been scarcely investigated so far, despite considerable interest in alloys such as
l) P1-30059 Cracow, Poland. 2, Present address: California Institute of Technology, Pasadena CA 91125, USA. 3, Present address: McMaster University, Hamilton, 485 4M1, Canada. 4, Present address: Chalk River Nuclear Laboratories, Chalk River, KOJ 1J0, Canada.
182 E. KOLAWA, T. TYLISZCZAK, B. D. SAWICKA, J. A. SAWICKI, and M. DRWIEGA
CoGd or FeGd, and their significance as potential bubble memories and magnetic recording devices (see e.g. 111). The microstructure and properties of iron-gadolinium alloys, which are apparently governed by the dramatic atomic volume mismatch and great electron charge transfer between Fe and Gd constituents, are not clear, espe- cially in the amorphous phase of the system. Because of drastically different atom- ic properties of constituents, amorphous alloys of rare earth metals with tran- sition metals (a-TM-RE) can be formed by co-evaporation or sputtering over vir- tually the entire range from x = 0.1 to 0.9, and over a more limited composition range (rare earth-rich) by rapid quenching from the melt 12 to 51. In our previous papers [6, 71 we have shown that a-TM-RE alloys can also be obtained by ion im- plantation, even a t implanted iron concentrations as low as x = 0.05. Ion bombard- ment produces many vacancies, interstitials, and dislocation loops, and therefore facilitates amorphisation.
I n the present work we performed systematic measurements of Mossbauer spectra of 57Fe implanted in Gd as a function of implanted ion dose, temperature, and iso- chronal thermal treatment. We were able to determine the relative contributions of different iron states in the alloys as well as to follow their temperature dependence and changes after heat treatment under vacuum and in the air. In addition, we attempted to correlate our data with atomic parameters and, in particular, with the Miedema model of alloying.
2. Experimental Procedure 2.1 Sample preparation
The targets were made of high purity polycrystalline gadolinium foils with thicknesses of 0.3 mm and area of 10 x 10 mm2 in each case. Implantation of the stable isotope 57Fe was performed using a mass separator of the Institute of Nuclear Physics in Cracow. Implanted doses were 5 x 1015, 1016, 2 x and4 x l0l6 atoms/cm2, cor- responding to iron concentrations of 1.5, 3, 6, and 12 at%, respectively. One of the samples was made by implantation of radioactive 57C0 (Tip = 270 d) with energy 50 keV. The implantation of W o was performed in Laboratorium Algemene Natuur- kunde, University of Groningen, The Netherlands. The sample was implanted up to the dose of 2 x 1013 atoms/cmZ and contained about 0.01% of Co and Fe impurities, with the activity of the obtained source 2 pCi. All implantations were performed a t room temperature and under vacuum of Torr. Before the implantations the sur- faces of the targets were carefully cleaned with a sharp blade. After implantations the samples were stored in oxygen-free containers. Isochronal thermal annealing8 of samples were conducted under vacuum of Torr in 100 K/30 min or 100 K/2 h steps.
22 The measurements
The sample implanted with radioactive WO was investigated using Mossbauer emis- sion spectroscopy. The spectra were recorded a t room temperature and a t 4.2 K against a single line absorber (sodium ferrocyanide enriched in 57Fe). The spectrum obtained at room temperature is shown in Fig. 1 (top).
The samples implanted with stable 57Fe isotope were studied by conversion electron Mossbauer spectroscopy (CEMS), using techniques described in detail elsewhere [S]. Room temperature spectra were measured with a helium-flow proportional counter, whereas the spectra a t low and a t high temperatures were recorded with the help of channel electron multipliers. The CEMS spectrometer described in [9] was modified
Studies of Local Structure and Thermal Annealing Processes in Fe-Gd Alloys 183
Fig. 1. Mossbauer emission spectrum of W o implanted in Gd matrix a t energy 50 keV and fluence 2 x lOI3 ern+ (top) and conversion electron Mossbauer spectra of 57Fe implanted in Gd matrix a t energy 70 keV and fluences 10l6, 2 x and 4 x 1016cm-2. Measure- ments were carried out a t room temperature
to enable both thermal annealing and measurements a t temperatures up to 800K under vacuum better than Tom, without removing the sample from the vacuum system. The CEMS spectra were taken using a 57CoCr source with the activity of 50 mCi.
The analysis of the spectra was com- plicated because of superposition of several poorly resolved components. The spectra were fitted with the various sets of Lorentzian lines to get the best self- consistent description of all experimental data for both as-implanted and thermally annealed specimens. Some of the obtained spectra are shown in Fig. 1 to 4. The results of the spectra analysis are given in Tables 1 and 2. All isomer shift data are given with respect to a-Fe metal.
velacity irnrnls) - 3. Experimental Results 3.1 Dose dependence
The Mossbauer spectra as a function of the implanted dose are presented in Fig. 1. A consistent deconvolution of all the spectra could be obtained by fitting the spectra with three or four quadrupole-split doublets (D1 to D4). Parameters of these doublets are presented in Table 1. Additionally, another doublet (D5) appears after the high temperature sample annealing, seeTable2. Each doublet presents a different iron state. The identification of different iron states in samples was done on the basis of the values of its Mossbauer parameters and their variation with the implanted dose and annealing treatment. The interpretation was based on our earlier Mossbauer measurements of other rare earth metals implanted with 57Fe ions [S, 71.
The main fraction of iron is represented by the doublet D l with the isomer shift IX = -0.15 mm/s and the quadrupole splitting QS = 0.5 mm/s (Table 1). This frac- tion of iron is interpreted as representing well separated iron atoms in the disordered gadolinium matrix. The relative contribution of this iron fraction (60 to 85%) to the whole iron content is nearly constant in the investigated iron concentration range from 0.01 to 12 atyo.
Tab
le 1
T
he p
aram
eter
s of
the
Mos
sbau
er s
pect
ra o
f W
O an
d 57
Fe im
plan
ted
into
Gd
mat
rix
mea
sure
d at
roo
m t
empe
ratu
re.
D1,
D2,
D3,
an
d D
4 re
fer
to q
uadr
upol
e-sp
lit d
oubl
ets,
as
expl
aine
d in
the
text
. IS
isom
er s
hift
, QS
quad
rupo
le s
plit
ting
, W w
idth
(F
WH
M) o
f re
so-
nant
line
s, R
rel
ativ
e co
ntri
buti
on i
n th
e sp
ectr
al a
rea.
The
IS
shi
ft is
giv
en i
n re
fere
nce
to a
-iro
n -~
~~
D1
D2
D3
D4
IS
QS
W
It
IS
QS
W
R IS
Q
S W
R
IS
QS
W
It
~-~
_
~-
__
__
_
sam
ple
dose
(a
tom
s/cm
2)
-
(mm
ls)
(mm
ls)
(mm
ls)
(%)
(mm
ls)
(mm
ls)
(mm
ls)
(%)
(mm
ls)
(mm
ls)
(mm
ls)
(7"
) (m
ml5
) (m
mls
) (m
mls
) (Y
o)
57C
o:G
d 2
X 10
13
-0.1
3 0.
42
0.55
65
.0
0.74
0.
43
0.79
20
.0
0.50
2.
57
0.94
15
.0
57Fe
:Gd
5 x
1015
-0
.15
0.55
0.
49
80.0
0.
02
1.02
0.
65
1.0
0.48
1.
13
0.78
19
.0
10l6
-0
.15
0.55
0.
48
57.1
-0
.01
1.06
0.
59
41.0
0.
42
1.15
0.
48
1.9
2 X
10l
6 -0
.17
0.51
0.
52
84.4
-0
.05
1.11
0.
30
5.4
0.38
0.
80
0.37
10
.2
4 X
10l
6 -0
.15
0.45
0.
52
85.8
-0
.07
0.99
0.
30
8.0
0.30
0.
75
0.38
6.
1 _
__
Tab
le 2
T
he p
aram
eter
s of
the
Mos
sbau
er s
pect
ra o
f 57
Fe im
plan
ted
into
Gd
mat
rix,
as
a fu
ncti
on o
f th
e te
mpe
ratu
re o
f m
easu
rem
ent.
(For
ex
plan
atio
n re
fer
to T
able
1)
2 x
10'6
78
293
4 x
10'6
78
29
3 40
0 50
0 60
0 70
0
-0.0
7 0.
55
0.54
87
.1
-0.1
7 0.
51
0.52
84
.4
-0.0
7 0.
49
0.51
85
.2
-0.1
5 0.
45
0.52
85
.8
-0.0
3 -
0.68
75
.7
-0.2
7 0.
41
0.45
87
.0
-0.2
9 0.
37
0.49
65
.1
-0.3
8 0.
34
0.45
51
.4
-0.0
2 1.
11
0.31
-0
.05
1.11
0.
30
-0.0
2 1.
04
0.31
-0
.0
0.92
0.
35
-0.1
2 0.
95
0.34
-0
.18
0.94
0.
32
3.5
0.47
0.
69
0.36
9.
3 5.
4 0.
38
0.80
0.
37
10.2
7.8
0.44
0.
70
0.45
8.
0 8.
0 0.
30
0.75
0.
48
6.1
13.9
0.
14
0.92
0.
34
10.4
8.
2 0.
09
0.76
0.
32
4.7
0.0
-
--
0.0
-0
.47
0.99
0.
31
34.9
0.
0 _
__
0.0
-0
.53
0.96
0.
35
48.6
Studies of Local Structure and Thermal Annealing Processes in Be-Gd Alloys 185
The second major iron fraction is the doublet D2. The parameters of this doublet, IS = 0 mm/s and QS = 0.95 to 1.1 mm/s, can be ascribed to iron dimers. The isomer shift is similar to that for pure iron. Doublet D2 is absent for the sample with the lowest iron content, i.e. the dose 2 x 1013 atoms/cm2. D2 is the second major fraction (= 40%) for the sample with 10l6 atoms/cm2, and for other samples it increases with increasing concentration from 1% for low iron concentration to about 87" a t high iron concentrations.
Two other components are present in the spectra, namely doublets D3 and D4, but their fractional contributions are much smaller than those of doublets D1 and D2. The parameters of these doublets indicate that they represent fractions of iron in ionic form, D3 for Fe3+ and D4 for Fe2+. The presence of iron in ionic states can be connected with iron atoms in the neighborhood of oxygen atoms and probably in- corporated in an oxidized surface layer of samples. For the sample with the dose of 2 x 1013 atoms/cm2 the parameters of the doublets D3 and D4 are somewhat different from the parameters obtained for other samples (Table 1) and are close to the param- eters for the garnet GdFe,O, [lo]. Thus, in the case of low-dose samples the doublets
1 I I I I I 1 -3 -2 -1 0 I 2 3
velocity ImrnJsl -
D3 and D4 can be interpreted differently as representing the fraction of iron in a compound of type GdFe,O,, probably also present on the surface of the sample.
3.2 Temperature dependence
The sample implanted with the dose of 4 x 10l6 atomslcm2 was measured over a temperature range from 78 to 700K. Mossbauer spectra are presented in Fig. 2 and their parameters are given in Table 2 .
The spectrum measured a t 78 K is simi- lar t o the room temperature spectrum and indicates no magnetic orderingin the sam- ple. The Mossbauer spectrum measured a t 400 K is very different : i t is a single, broad line ( W= 0.7 mm/s), with the isomer shift of about 0 mm/s. At higher temperatures (4500 K), this effect disappears and the spectrum resembles again the pattern characteristic of non-annealed samples.
Fig. 2. The Mossbauer (CEMS) spectra of the 4 x 1016atoms/cm2 sample Fe in Gd sample measured at different temperatures from 78 to 700 K as indicated. E = 70 keV
186 E. KOLAWA, T. TYLISZCZAK, B. D. SAWICKA, J. A. SAWICKI, and M. DRWIEGA
The observed effects can be interpreted in terms of the structural relaxation which occurs often during an annealing of amorphous alloys below the crystallization tem- perature [ l l , 121. Such a structural relaxation was observed for example by Nishihara et al. [ l l ] for the spectrum of amorphous YFe, prepared by rf sputtering. In a-YFe, the collapse of the hyperfine pattern to a broad, single line occured also around 400 K. In our case, a fine structure attributed to the growth of the crystalline state was observed a t approximately 600 K (Fig. 2), and the changes in the spectra measured above this temperature were irreversible. The Mossbauer spectra a t 600 and 700 K consist of two components: D1 doublet and a new D5 doublet, whose fractional contribution grows with the increase of the temperature of measurements.
3.3 Thermal annealing studies
The annealing studies were performed for Gd samples implanted with iron a t doses of 1016 and 2 x l O l 6 atoms/cm2. The annealings were performed under vacuum of 10-6 Torr, in isochronal 100 K steps from 370 to 770 K, for 2 h in each step. After each step of annealing the Mossbauer was taken a t room temperature. Additionally, the sample with 4 x 1016atoms/cm-2 was measured a t various temperatures, 400, 500,600, and 700 K. Because each measurement lasted about 24 h, such measurements correspond to the long thermal treatment, so that, to check for changes, after each high temperature measurement the sample was cooled down to room temperature and the Mossbauer spectrum was taken. Fig. 2 shows the spectra of the 4 x l0l6 atoms/cm2 sample as a function of measuring temperature, and Fig. 3 and 4 show the spectra obtained a t room temperature after annealing treatment of the samples.
The first observation which can be made on the basis of thermal annealing studies is that the fraction ascribed to doublet D1 decreases upon annealing in all investigated samples. Thus, iron in monomeric state is unstable in all samples, but it occurs up to about 600 K. For the samples implanted with doses of 2 x l0l6 and 4 x 10le atoms/cm2, the fractional contribution of the D2 fraction (attributed to iron dimers) also decreases upon sample annealing and vanishes after about 500 to 600 K annealing. This indi- cates that iron in Gd matrix has no tendency to form iron aggregates, a t least in the highly damaged (amorphous) state of the Ye-Gd system. A different behaviour of doublet D2 was observed for annealing of the sample implanted with a low dose (10ls atoms/cm2). In this case the relative contribution of iron dimers increased with increase of the annealing temperature (Fig. 3) ; this means that iron dimers are stable forms in the low-dose implanted gadolinium, and that there is a tendency of iron aggregation into dimers a t low iron concentrations.
For all investigated samples a drastic change takes place after annealing a t tem- peratures above 500 t o 600 K. The spectra consist now of the doublet D1 and a new component, the doublet D5. The case of the sample 1016 atoms/cmZ is somewhat dif- ferent: D2 and D3 are present instead of D1, but also here the doublet D5 appears as will be discussed below. The parameters of this new component D5 are similar in all cases ( I S = -0.3 mm/s, QS = 1 mm/s) but very different from the parameters of D l to D4 for samples non-annealed or annealed a t lower temperatures. This identifies D5 as a completely new form of iron. The value of the isomer shift for D5 indicates to a high electron density a t iron nuclei, while the small linewidth ( W = 0.3 mmls) shows that this iron state in Gd is well defined. The relative con- tribution of doublet D5 grows with increasing annealing temperature. We think
Studies of Local Structure and Thermal Annealing Processes in Fe-Gd Alloys 187
velocity (mm!s) -
Fig. 3
I I I I I I I -3 -2 -1 0 I 2 3
velocity Imm 1s) __c
Fig. 4
Fig. 3. The Mossbauer (CEMS) spectra of 2 x 10l6 atoms/cm2 Fe in Gd ( E = 70 keV) measured a t room temperature after subsequent isochronal annealing5 in vacuum during 2 h, a t temper- atures as indicated. Bottom: the spectrum obt.ained after heating in air
Fig. 4. The Mossbauer (CEMS) spectra of the 4 X 10l6 atoms/cm2 Fe in Gd sample measured a t room temperature after subsequent annealing in vacuum during 48 h a t temperatures as indicated
that this D5 doublet should be interpreted as a fraction of iron found in a well defined place in the recrystallized gadolinium matrix (see Discussion).
The Gd sample implanted with dose 2 x 10l6 atoms/cm2 was annealed a t 690 K in air, and the' obtained spectrum is displayed in Fig. 3. The Mossbauer spectrum obtained after this annealing consists only of the doublet D3. This proves that the doublet D3 represents a fraction of iron in oxidized form, Fe3+. The Mossbauer param- eters of such a sample closely resemble the parameters of amorphous Fe-Gd garnet, studied recently by Schultes et al. [13].
188 E. KOLAWA, T. TYLISZCZAK, B. D. SAWICK~, J. A. SAWICKI, and M. DRW-IEGA
4. Discussion
The implanted Fe-Gd system has appeared t be a complicated case, indicating several iron states and having tendencies of structural transformations a t relatively low tem- peratures. Despite of difficulties in analysis and interpretation of the Mossbauer spectra, our results may contribute to the understanding of structural aspects in amorphous TM-RE alloys. We will discuss some of them in more detail below.
The main fraction of iron in all investigated Gd samples before annealing is repre- sented by the doublet D1. This fraction is usually as high as 80 to 90%, in a few cases lower, 60 to 70%. Since Fe-Gd alloys have a tendency to be amorphous, we assume that this main fraction of iron locates in an amorphous host atom environ- ment. Such an assumption seems to be justified by the value of the quadrupole splitting and by the line broadening, typical for amorphous specimens. It is not sur- prising that the disorder introduced by the implanted energetic iron ion in the Gd matrix is stabilized by the Fe impurity in the host after the implantation process is over. This fraction of iron represents in various samples slightly different iron states, depending on the iron concentration in the sample. For the sample with iron concentration below the solid solubility limit of Fe in Gd (2 x 1013 atoms/cm2) the doublet D1 has the smallest quadrupole splitting, QS = 0.4 mmls. In this case Fe atoms seem to be distributed a t random in the weakly disturbed Gd matrix and the QS value of the D1 doublet is in good agreement with the value of QS for sub- stitutional Fe sites in the Gd hexagonal lattice. In samples with iron concentrations in the range from 1.5 to 3%, Fe atoms occupy only sites with amorphized local sur- roundings because the Fe concentration is not high enough to ensure amorphization of the entire implanted layer. In this case the matrix lattice keeps its overall struc- ture, but it is highly perturbed. The QS value of doublet D1, QS = 0.55 mm/s, the largest among all the cases, can be explained by the distortion of the Gd lattice pro- ducing electric field gradients larger than for low iron concentrations and also slightly larger than for high Fe concentrations. For the highest Fe content (6 to 12%) we deal with completely amorphous Fe-Gd alloys. I n this range of iron concentration one can easily obtain Fe-Gd amorphous alloys by sputtering or splat-cooling [5], and thus the probability of formation of amorphous alloys by ion implantation is very high I16 to 181. I n the spectra of these samples the QS value of the doublet D1, QS = 0.45 mm/s, is smaller than for medium Fe concentrations, which agrees with the fact that the complete amorphization of the implanted layer leads to an increase in the local symmetry around iron impurities. The fraction represented by the doublet D1 has, for all samples, a large negative isomer shift, IS = -0.15 mm/s, which agrees well with the value calculated for Fe atoms in amorphous environments, using the Miedema and van der Woude model [19]. The case of amorphous environment is simple because iron atoms occupy unstrained positions and there is no contribution to I S from a volume mismatch [el. The detailed analysis of the isomer shifts in im- planted Fe-RE alloys in terms of the Miedema and van der Woude model, as well as in simple terms of atomic volumes of the host matrices, was given by Sawicka et al. in [6] and [7].
The second fraction observed in most of the samples is the doublet D2. We ascribe this fraction to iron dimers (oligomers) which seem to be justified by the I S value close to metallic iron, the value of the quadrupole splitting (QS = 1 mm/s), and by the tendency of the increase of this fraction with the increase of the iron concentra- tion. In the lowest dose sample (2 x 1013 atoms/cm2) this fraction is not observed, and its very small contribution is seenin the next dose sample (1 % in 5 x 1015 atoms/cm2). Bt these iron concentrations the probability of dimer formation is very small [20];
Studies of Local Structure and Thermal Annealing Processes in Fe-Gd Alloys 189
0.37; and 15% for the two cases, respectively. The D2 fraction is very large (w 400/) for the dose 10l6 atoms/cm2. For this case, however, the probability of formation of dimers, and larger precipitates (oligomers), evaluated from the binomial distribution (1) is only 30%. For the high dose samples (2 x 10le and 4 x l0l6 atoms/cmz) the fractional contribution of the D2 fraction is again very low, only about 7%. This is understood in the following way: a completely amorphized Gd matrix does not restrict the solubility limit of iron impurities and thus iron atoms remain in a strain-free surroundings and iron dimers (oligomers) are not forced to be formed.
A very interesting behaviour of the Mossbauer spectra was observed for the sample with 4 x 10l6 atoms/cm2. At 400 K the spectrum changes its characteristic pattern of two broad unresolved structures and collapses into a single broad line. This line can be deconvoluted as consisting of a major single line, and two much smaller doublet fractions; D2 and D3. The collapse of a D1 fraction into a single line can be ex- plained as a structural relaxation effect. Such effect was observed [ll, 12, 21, 221 for amorphous substances, just below the temperatures a t which a recrystallization occurs. The lack of quadrupole splitting suggests that the relaxation goes through a quite symmetric, perhaps cubic atomic arrangement. The single line fraction has a broadened linewidth ( W = 0.7 mm/s) which is explained bytheincrease in the variety of atomic configurations around iron impurities when a metastable amorphous phase relaxes into a more stable state during annealing just below the crystallization tem- perature.
The crystallization process of the implanted layers in all our samples seems to occur a t temperatures of about 500 to 600 K. After annealing of the samples a t these temperatures, the measured spectra change and a new doublet (D5) appears, with I S = x -0.3 mm/s and QH = = 1 mm/s. This iron fraction increases with the increase in the annealing temperature, a t the cost of the fraction of iron atoms located in interstitial sites of the hexagonal Gd matrix rebuilt in the annealing process. It is most probable that the D5 doublet represents some intermetallic compound. However, another interpretation of this doublet is also possible, namely, that it represents the fraction of iron atoms located in interstitial sites of the hexagonal Gd matrix. Such interpretation needs further confirmation which cannot be given by the Mossbauer effect data only, but can be given, for instance, by EXAFS measure- ments or ion channeling studies. The recrystallization of the implanted layer into the interstitial Fe-Gd solid solution would agree with the predictions of the Miedema model [23] that Fe atoms prefer interstitial sites in rare earth matrices. Some additional arguments to support the model of Fe interstitials will be discussed below.
The Gd hexagonal lattice has two kinds of interstitial sites : tetrahedral and octa- hedral. The radii of octahedral and tetrahedral gaps in Gd are ro = 0.74 x 10-1 nm and rt = 0.41 x 10-1 nm, respectively. The atomic radius of Be is 0.126 nm [24] and the probability that iron atoms, with Miedema’s assumption of volume contraction [23], are in octahedral gaps is much higher than in tetrahedral ones.
The resonant absorption effect measured a t room temperature for the Gd sample implanted with a dose of 4 x l O l a atoms/cmZ was two times larger when measured just after implantation than when measured after annealing a t 770 K (Fig. 1 and 4). Thus, in the region from which conversion electrons are emitted (x 100 nm), the number of atoms decreased by a factor of two. Thus, one can assume that in the probed layer of the sample annealed a t 770 K the iron concentration is about 4%. The coordination number of iron atoms on octahedral sites in the hexagonal Gd lattice is 6. The probability, P(n, z), that an iron atom would have as nearest neigh-
190 E. KOLAWA, T. TYLISZCZAK, B. D. SAWICKA, J. A. SAWICKI, and M. DRWIEGA
bors n Fe atoms can be calculated from the formula
P(n, 2) = (;) Xn(1 - 2 y - n ,
where n = 0 for iron monomers, n = 1 for dimers, etc. and x is the mean iron concen- tration. Thus, the probability of octahedral iron monomers a t 4% concentration of iron in Gd matrix is 78%, which is in very good agreement with the value of 78% determined from the Mossbauer spectrum for the sample annealed a t 770K. This implies that iron atoms in octahedral sites are be surrounded by only Gd neigh- bours.
In terms of the Miedema and van der Woude model [19] the isomer shift of the Fe impurity completely surrounded by host atoms is described by the sum of I#max (isomer shift value for unstrained dilute limit) and a volume-mismatch term
(2) I f l c a l c = I s m a x + I s V a l .
The volume-mismatch contribution, ISvo1, arises in diluted crystalline solid solutions, where it is necessary to take into account the change in the isomer shift when the volumes of the iron atoms change due to the difference in sizes between octahedral gaps in Gd matrix and iron atoms. ISvo1 can be derived from elastic continuum con- siderations [19],
where V& is the volume of the octahedral site in Gd lattice; Vre is the atomic vol- ume of iron [25]; B F e and B G d are bulk moduli for Fe and Gd, respectively; (a In IS/a In V)pe = 1.33 mm/s [26].
The calculated value is IXvo l = -0.12 mm/s. The term ISmax = -0.18 mm/s for Be in Gd is taken from [7]. Using (2) we find for iron atoms on octahedral sites of the Gd lattice IXcalc = -0.30 mm/s. This calculated result is in very good agreement with experimental data: The doublet D5 arising from iron in octahedral sites has I# in the range -0.27 to -0.31 mm/s (see Table 2).
The quadrupole splitting of doublet D5 is about 1 mm/s which is about three times larger than the value calculated for substitutional sites in the hexagonal Gd lattice [14, 151. We calculated the lattice gradient Vatt on octahedral sites of the Gd lattice using the expression
n
F: Vfj = C qk(3x~xkj - d S ; j ) / r i , (4)
where q k is the charge of atom k and xkl, xk2, xk3 are its Cartesian coordinates. The summation was carried out for 50 coordination spheres. The value of the lattice gradient on octahedral sites inGd was Viatt = -0.0238 x nm3/e. Similarly, the estimated value of the lattice gradient on substitutional sites in Gd was Vilatt = = +O.OOS x low3 nm3/e. Thus the lattice gradient in octahedral site of the Gd lattice is about three times larger than the lattice gradient on substitutional sites in Gd. However, the observed QS of doublet D5 is about 1 mm/s and it is much lar- ger than the calculated quadrupole splitting from the lattice gradient on octahedral sites in Gd. From this it appears likely that the electronic EFG makes a crucial contribution to the quadrupole splitting of iron on octahedral sites of the Gd ma- trix.
Studies of Local Structure and Thermal Annealing Processes in Pe-Gd Alloys 191
Another interpretation of the doublet D5 iron states is the possibility of the inter- metallic Gd,Fe phase formation during crystallization of the implanted layers. Vincze et al. [26] investigated the crystallization effects in the amorphous Zr,Fe alloy. dfter annealing the amorphous Zr,,Fe,, sample a t 1000 K for 2 h they observed in the Moss- bauer spectrum a doublet connected with the orthorhombic Zr,Fe intermetallic phase. The parameters of this doublet ( I S = -0.319 mm/s, QS = 0.91 mm/s) are very similar to the parameters of doublet D5 in our work (see Table 2 ) after reduction of the I8 data to room temperature. The similarity of these parameters is probably due to the fact that the nearest neighbor configuration around iron atoms on octahedral sites of the Gd matrix is very similar t o those in orthorhombic RE,Fe phases. However, since the composition of Gd and Fe atoms in the studied samples is very different from Gd,Fe and we observed crystallizabion a t low temperatures (500 to 600 K), the structure is an interstitial Fe-Gd solid solution, in agreement with the Miedema predictions [23], rather than an intermetallic Gd,Fe phase.
It is worthwhile to notice that the ion-implanted alloys usually closely resemble the alloys obtained by fast alloying techniques, such as co-evaporation, co-sputtering, and splat-quenching. The parameters of Mossbauer spectra of implanted and rapidly alloyed systems are nearly the same a t corresponding concentrations. Such simi- larity was recently reported for the Fe-B and Fe-Zr systems [27], and earlier in several other cases [6].
5. Summary
The Mossbauer spectra of as-implanted FeGd samples indicate the presence of several different arrangements of iron atoms in the host, suggesting a complicated nature of atomic disorder in the rare earth matrix. The analysis of the data reveals that the main iron fraction immediately after implantation are iron atoms in an amorphous Gd matrix environment, and the other iron states are iron dimers and iron atoms associated with oxygen. The isochronal thermal annealing shows that the implanted structure is very unstable and undergoes structural relaxation a t 400 to 500K, whereas heating a t 500 to 600 K in vacuum produces a new crystalline phase. The experimental isomer shift of the quadrupole doublet which describes this new phase is in good agreement with the value of the isomer shift calculated from the Miedema- van der Woude model. Annealing a t 700K in the presence of oxygen causes the formation of an amorphous oxidized film, most probably with the composition of the GdF,O, garnet.
The alloys studied indicate a considerable electron charge transfer towards the iron nuclei, which is due to a drastic volume and electronegativity mismatch of both alloy constituents.
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(Received June 9, 1986; in revised form March 26, 1987)
