- Email: [email protected]
57Fe Mössbauer study of Gd2Fe14−xMxC compounds with M Ni, Si, Cu, V
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1001-1002
~H
journal of magnetism and magnetic materials
ELSEVIER
57Fe M6ssbauer
study of Gd2Fe]4_ M C compounds with M = Ni, Si, Cu, V
M. Morariu a, M. Rogalski a M. Valeanu a, N. Plugaru a,* E. Burzo b a Institute of Physics and Technology of Materials, P.O. Box MG-06, Bucharest, Romania b Faculty of Physics, University of Cluj-Napoca, 3400 Cluj-Napoca, Romania
Abstract GdaFea4_xMx C compounds, with M = Ni, Si, Cu or V and x < 1.5 were investigated by 57Fe MSssbauer spectroscopy. A preferential substitution of iron sites by the M elements is evidenced. The hyperfine parameters are discussed in correlation with the data obtained by magnetic measurements.
The R2Fe]4C compounds, where R is a rare-earth or yttrium, crystallize in a tetragonal structure having the P 4 2 / m n m space group symmetry [1,2]. In this lattice, the R atoms are distributed in two crystallographic sites, the Fe atoms in six different positions, and C is located in one type of site. Because of the solid state transformation at high temperatures by which R2Fe]4C decomposes, there are some difficulties in their formation. As result, the R2Fe]4C phases were not so intensively studied as the corresponding boron compounds. Previously, we reported the magnetic properties of Gd2Fe]4 xM~C compounds, with M = Ni, Si, Cu or V [3]. A possible deviation from a random distribution of M elements on iron sites was suggested. To obtain information on the site localization of the substituting elements, M6ssbauer effect measurements were also performed on these systems. The 57Fe M6ssbauer spectra were obtained at room temperature, by using a constant-acceleration Elron spectrometer, and ~7C0 in a rhodium matrix as source. The spectra were fitted by considering five variable parameters for a given sextet: the hyperfine field, H ~ , the quadrupole splitting, QS, the isomer shift, IS, the linewidth, 17, and the relative intensity, I, of the sub-spectra. Relative areas for all the magnetic sextets were constrained in the ratio 3:x:l:l:x:3, where x ranges from 2.0 to 2.1 [4]. Typical 57Fe MSssbauer spectra are shown in Fig. 1 for M = Ni. The assignment of the sextets was based on their relative areas, which correspond to the relative site popula-
tion, as well as on their hyperfine fields, as previously discussed [4-6]. The analysis of the relative intensities of the sub-spectra suggests a preferential substitution of Fe by the M elements. Thus, the Ni atoms replace Fe mostly in 16k 2 sites and to a less extent, 16k~ ones. Si atoms are distributed in the 4c, 8j] and 16k 2 sites, while Cu and V are preferentially located in 16k 2 sites and to a less extent in 16k~ sites. The preferential substitution of iron changes the local environment of Fe sites. The most affected is that of the 8j2 site which has the greatest number of Fe nearest neighbours: 4Fe(16kl), 4Fe(16k2), 3Fe(8ja) and 1Fe(4e). The local environment effects should be reflected also in the distribution of the 57Fe hyperfine fields.
I=
1.o5 t~ . I
Gd2F e~a.mNio.~C
.
.~ .
....
,,, ~.~6
-lO
Gd2Fe13NiC
,~ ,
Fax:
+40-1-3122247;
email:
"
Gd2Fel2
~.
-
•
5Nil s c
..
. .~.
-~ velocity,
* Corresponding author. [email protected].
'
.'
i , ,~.r-. ~ .~"
I
I
.
~ ~.'~ • ,.-. "~"
~
L-~.,
1.42
.
~/ .~" ,
1.15
.
i~ . ; i i : ' ' : { f t
k
,
o
5
lo
mm/s
Fig. 1. 57Fe M6ssbauer spectra for Gd2Fel4 xNix C compounds, at room temperature.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 1 4 9 9 - X
M. Morariu et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1001-1002
1002
Table 1 Hyperfine fields in kOe (_+3kOe) for GdzFe]4 xMx C compounds at 295 K x
16 k I
16 k 2
8 Jl
8 J2
4e
4c
Aver.
0
283
309
283
345
274
247
296
Ni
0.5 1.0 1.5
285 280 271
311 306 292
287 276 270
350 344 329
324 318 311
299 293 294
248 251 229
296 288 285
Si
0.5 1.0 1.5
273 243 241
298 272 266
275 239 242
345 319 300
318 289 285
298 272 267
240 221 202
286 263 258
Cu
0.5 1.0
287 290
310 316
288 290
354 354
331 336
295 295
258 256
297 296
V
0.5 1.0
278 227
300 252
278 230
341 291
317 271
298 264
241 204
290 243
M
The Hhf values, at room temperature, are given in Table 1. The data for the parent compound are taken from Ref. [6]. Generally, the hyperfine fields decrease when increasing the contents of substituting elements. Greater changes of Hhf values are found in samples with vanadium substitution. As a result of preferential occupation of the substitution elements, a distribution of the hyperfine fields was found in the case of 8J2 site. Therefore, this subspectrum was analysed by splitting it into two sublattices, with different intensities (Table 1). The values of the isomer shifts are generally small and negative, suggesting that charge transfers are not important in these systems. An exception is encountered for GdzFe13VC, which shows relatively large IS. The largest IS values, in absolute magnitude, for all the compounds, correspond to the Fe(4e) sites, as previously determined [4,5]. The QS values seem not to be affected by the substituting elements. The low Curie points of RzFe14C phases are ascribed to the presence of negative exchange interactions between Fe(8jl)-Fe(16k 2) and Fe(8jl)-Fe(8jl) atoms, due to their distances, dye_re, smaller than ~ 2.45 A [1]. The present M6ssbauer results show that the M elements are located mainly at these iron sites. Consequently, a decrease of the 2.1
\ . 1.9
:~
1.7 0 Cu X
1.5 0.5
I
I.S
Fig. 2, Composition dependences of the mean iron moments determined at 4.2 K by magnetic measurements and at room temperature from 5VFe hypeffine field values.
negative exchange interactions, leading to an increase of the Tc values is expected. Greater Tc values, as effect of substitutions, were evidenced in R2Fe]4 xMxC compounds for M = Ni, Si, Cu by magnetic measurements [3]. For vanadium substituted samples, a decrease of the Curie temperatures was observed. To obtain additional data on this matter we analysed the composition dependences of the magnetizations. The mean iron moments were determined from 57Fe hyperfine fields, assuming a hyperfine interaction constant Hhf/MFe = (147 + 2) k O e / / x B [5] and are plotted in Fig.2. A large decrease of the Mve values is found for x > 0.5, determining an additional decrease of the Tc values and thus, counteracting the effects due to iron site preferential occupancy. We conclude that M = V, Ni, Cu and Si elements in Gd2Fe14 xMx C compounds substitute iron sites mainly involved in negative exchange interactions. As result, the Curie temperatures increase, as evidenced in compounds with M = Ni, Cu and Si. The decrease of Tc values in vanadium substituted compounds is attributed to a higher decrease of the mean iron magnetic moments than those found in samples with Ni, Cu or Si.
References [1] E. Burzo, Landolt-B/Smstein Handbook, Vol. 19i2 (Springer, Berlin, 1992) p. 232. [2] K.H.J. Buschow, Rep. Prog. Phys. 54 (1991) 1123. [3] E. Burzo, J. Laforest, N. Plugaru, M. Valeanu and L. Stanciu, IEEE Trans. Magn. 30 (1994) 625. [4] G.J. Long and F. Grandjean, in: Supermagnets, Hard Magnetic Materials, eds. G.J. Long, F. Grandjean (Kluwer Academic, Dordrecht, 1991) p. 355. [5] E. Burzo, M. Morariu, M.S. Rogalski and A.T. Pedziwiatr, Hyperfine Interactions 50 (1989) 701. [6] G.J. Long, R. Kulasekere, O.A. Pringle, F. Grandjean and K.H.J. Buschow, J. Magn. Magn. Mater. 117 (1992) 239.
~H
journal of magnetism and magnetic materials
ELSEVIER
57Fe M6ssbauer
study of Gd2Fe]4_ M C compounds with M = Ni, Si, Cu, V
M. Morariu a, M. Rogalski a M. Valeanu a, N. Plugaru a,* E. Burzo b a Institute of Physics and Technology of Materials, P.O. Box MG-06, Bucharest, Romania b Faculty of Physics, University of Cluj-Napoca, 3400 Cluj-Napoca, Romania
Abstract GdaFea4_xMx C compounds, with M = Ni, Si, Cu or V and x < 1.5 were investigated by 57Fe MSssbauer spectroscopy. A preferential substitution of iron sites by the M elements is evidenced. The hyperfine parameters are discussed in correlation with the data obtained by magnetic measurements.
The R2Fe]4C compounds, where R is a rare-earth or yttrium, crystallize in a tetragonal structure having the P 4 2 / m n m space group symmetry [1,2]. In this lattice, the R atoms are distributed in two crystallographic sites, the Fe atoms in six different positions, and C is located in one type of site. Because of the solid state transformation at high temperatures by which R2Fe]4C decomposes, there are some difficulties in their formation. As result, the R2Fe]4C phases were not so intensively studied as the corresponding boron compounds. Previously, we reported the magnetic properties of Gd2Fe]4 xM~C compounds, with M = Ni, Si, Cu or V [3]. A possible deviation from a random distribution of M elements on iron sites was suggested. To obtain information on the site localization of the substituting elements, M6ssbauer effect measurements were also performed on these systems. The 57Fe M6ssbauer spectra were obtained at room temperature, by using a constant-acceleration Elron spectrometer, and ~7C0 in a rhodium matrix as source. The spectra were fitted by considering five variable parameters for a given sextet: the hyperfine field, H ~ , the quadrupole splitting, QS, the isomer shift, IS, the linewidth, 17, and the relative intensity, I, of the sub-spectra. Relative areas for all the magnetic sextets were constrained in the ratio 3:x:l:l:x:3, where x ranges from 2.0 to 2.1 [4]. Typical 57Fe MSssbauer spectra are shown in Fig. 1 for M = Ni. The assignment of the sextets was based on their relative areas, which correspond to the relative site popula-
tion, as well as on their hyperfine fields, as previously discussed [4-6]. The analysis of the relative intensities of the sub-spectra suggests a preferential substitution of Fe by the M elements. Thus, the Ni atoms replace Fe mostly in 16k 2 sites and to a less extent, 16k~ ones. Si atoms are distributed in the 4c, 8j] and 16k 2 sites, while Cu and V are preferentially located in 16k 2 sites and to a less extent in 16k~ sites. The preferential substitution of iron changes the local environment of Fe sites. The most affected is that of the 8j2 site which has the greatest number of Fe nearest neighbours: 4Fe(16kl), 4Fe(16k2), 3Fe(8ja) and 1Fe(4e). The local environment effects should be reflected also in the distribution of the 57Fe hyperfine fields.
I=
1.o5 t~ . I
Gd2F e~a.mNio.~C
.
.~ .
....
,,, ~.~6
-lO
Gd2Fe13NiC
,~ ,
Fax:
+40-1-3122247;
email:
"
Gd2Fel2
~.
-
•
5Nil s c
..
. .~.
-~ velocity,
* Corresponding author. [email protected].
'
.'
i , ,~.r-. ~ .~"
I
I
.
~ ~.'~ • ,.-. "~"
~
L-~.,
1.42
.
~/ .~" ,
1.15
.
i~ . ; i i : ' ' : { f t
k
,
o
5
lo
mm/s
Fig. 1. 57Fe M6ssbauer spectra for Gd2Fel4 xNix C compounds, at room temperature.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 1 4 9 9 - X
M. Morariu et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1001-1002
1002
Table 1 Hyperfine fields in kOe (_+3kOe) for GdzFe]4 xMx C compounds at 295 K x
16 k I
16 k 2
8 Jl
8 J2
4e
4c
Aver.
0
283
309
283
345
274
247
296
Ni
0.5 1.0 1.5
285 280 271
311 306 292
287 276 270
350 344 329
324 318 311
299 293 294
248 251 229
296 288 285
Si
0.5 1.0 1.5
273 243 241
298 272 266
275 239 242
345 319 300
318 289 285
298 272 267
240 221 202
286 263 258
Cu
0.5 1.0
287 290
310 316
288 290
354 354
331 336
295 295
258 256
297 296
V
0.5 1.0
278 227
300 252
278 230
341 291
317 271
298 264
241 204
290 243
M
The Hhf values, at room temperature, are given in Table 1. The data for the parent compound are taken from Ref. [6]. Generally, the hyperfine fields decrease when increasing the contents of substituting elements. Greater changes of Hhf values are found in samples with vanadium substitution. As a result of preferential occupation of the substitution elements, a distribution of the hyperfine fields was found in the case of 8J2 site. Therefore, this subspectrum was analysed by splitting it into two sublattices, with different intensities (Table 1). The values of the isomer shifts are generally small and negative, suggesting that charge transfers are not important in these systems. An exception is encountered for GdzFe13VC, which shows relatively large IS. The largest IS values, in absolute magnitude, for all the compounds, correspond to the Fe(4e) sites, as previously determined [4,5]. The QS values seem not to be affected by the substituting elements. The low Curie points of RzFe14C phases are ascribed to the presence of negative exchange interactions between Fe(8jl)-Fe(16k 2) and Fe(8jl)-Fe(8jl) atoms, due to their distances, dye_re, smaller than ~ 2.45 A [1]. The present M6ssbauer results show that the M elements are located mainly at these iron sites. Consequently, a decrease of the 2.1
\ . 1.9
:~
1.7 0 Cu X
1.5 0.5
I
I.S
Fig. 2, Composition dependences of the mean iron moments determined at 4.2 K by magnetic measurements and at room temperature from 5VFe hypeffine field values.
negative exchange interactions, leading to an increase of the Tc values is expected. Greater Tc values, as effect of substitutions, were evidenced in R2Fe]4 xMxC compounds for M = Ni, Si, Cu by magnetic measurements [3]. For vanadium substituted samples, a decrease of the Curie temperatures was observed. To obtain additional data on this matter we analysed the composition dependences of the magnetizations. The mean iron moments were determined from 57Fe hyperfine fields, assuming a hyperfine interaction constant Hhf/MFe = (147 + 2) k O e / / x B [5] and are plotted in Fig.2. A large decrease of the Mve values is found for x > 0.5, determining an additional decrease of the Tc values and thus, counteracting the effects due to iron site preferential occupancy. We conclude that M = V, Ni, Cu and Si elements in Gd2Fe14 xMx C compounds substitute iron sites mainly involved in negative exchange interactions. As result, the Curie temperatures increase, as evidenced in compounds with M = Ni, Cu and Si. The decrease of Tc values in vanadium substituted compounds is attributed to a higher decrease of the mean iron magnetic moments than those found in samples with Ni, Cu or Si.
References [1] E. Burzo, Landolt-B/Smstein Handbook, Vol. 19i2 (Springer, Berlin, 1992) p. 232. [2] K.H.J. Buschow, Rep. Prog. Phys. 54 (1991) 1123. [3] E. Burzo, J. Laforest, N. Plugaru, M. Valeanu and L. Stanciu, IEEE Trans. Magn. 30 (1994) 625. [4] G.J. Long and F. Grandjean, in: Supermagnets, Hard Magnetic Materials, eds. G.J. Long, F. Grandjean (Kluwer Academic, Dordrecht, 1991) p. 355. [5] E. Burzo, M. Morariu, M.S. Rogalski and A.T. Pedziwiatr, Hyperfine Interactions 50 (1989) 701. [6] G.J. Long, R. Kulasekere, O.A. Pringle, F. Grandjean and K.H.J. Buschow, J. Magn. Magn. Mater. 117 (1992) 239.