- Email: [email protected]
155Gd Mössbauer effect study of Gd2Mn17Cx compounds
Journal of the Less-Common
1SsGd MOSSBAUER
Metals, 168( 1991) 269-216
EFFECT
STUDY OF Cd,Mn,,C,
269
COMPOUNDS
M. W. DIRKEN”, R. C. THIEL”, T. H. JACOBSaab and K. H. J. BUSCHOWa,b “Kamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, The Netherlands bPhilips Research Laboratories, 5600 JA Eindhoven, The Netherlands
(Received July 6, 1990)
Summary
We have prepared ternary carbides of the type Gd,Mn,,C, with x = 0.8, 1.0, 1.5,2.0, 2.2, 2.5 and 2.7 and have studied these compounds by means of X-ray diffraction, a.c. susceptibility measurements and 155Gd Mossbauer spectroscopy. Of these compounds only Gd,Mn,,C,,, adopts the rhombohedral Th,Zn,, structure, the other compounds crystallizing in the hexagonal ThzNi,, structure. The lattice constants are given for all compounds. The a.c. susceptibility measurements showed that the magnetic ordering temperatures of these compounds are fairly low (less than 40 K), with a tendency to decrease with carbon concentration. The ls5Gd Mossbauer spectra showed a broadening and/or splitting that increases with carbon concentration and reflects an increasing contribution from subspectra with a large quadrupole interaction. From fitting the spectra of the rhombohedral compound Gd,Mn,& it was derived that the field gradient is maximal at the nuclei of those atoms for which the interstitial nearest neighbour hole positions are completely filled by carbon atoms.
1. Introduction
The magnetic properties of the iron-rich rare earth compounds of the type R,Fe,, have lower magnetic ordering temperatures than less iron-rich binary rare earth compounds. Also, the crystal-field-induced rare earth sublattice magnetization reaches only moderate values. This has hampered the application of R*Fe,,type compounds as starting materials for permanent magnets. Recent investigations have shown that the magnetic properties of R,Fe,, can be considerably improved by interstitial solution of carbon atoms [l]. In the corresponding ternary intermetallic compounds these carbon atoms partially occupy the interstitial 9e position of the rhombohedral Th,Zn,, structure type [2]. These 9e sites are located close to the rare earth atoms. When fully occupied, the formula composition would correspond to R,Fe,,C,. In practice it proved possible to fill the 9e sites only partially with carbon atoms, the concentrations remaining well below x = 2 in R2Fe1,Cx, even for the heavy rare earth elements. 0022-5088/91/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
270
Experimental information on the effect of the interstitial carbon atoms on the field gradient at the rare earth sites and on the concomitant contributions expected for the crystal field induced anisotropy was obtained by ls5Gd Mijssbauer spectroscopy performed on several compounds of the series Gd,Fe,,C, and TmzFel,C, [3,4]. Substantial differences in local field gradient were found for Gd atoms surrounded by zero, one or two carbon atom nearest neighbours [3]. The limited concentration range prevented us from obtaining experimental information for gadolinium atoms surrounded by three nearest neighbour carbon atoms, the occurrence of such gadolinium atoms being negligible for x = 1.5, assuming a statistical distribution of the carbon atoms over the three available interstitial sites. It was shown by Block and Jeitschko [S] that in R,Mn,,C, considerably larger concentrations of carbon can be reached than in R2Fe1,Cx. For this reason we have extended our investigation to the series Gd,Mn,,C,.
2. Experimental details Various compounds of the type GdzMnl,C, were prepared by arc melting from starting materials of at least 99.9% purity. We used a slight excess of manganese to compensate for losses during melting. After arc melting, the samples were wrapped into tantalum foil, sealed into an evacuated silica tube and annealed for about 3 weeks at 900 “C. After annealing, the samples were cooled quickly to room temperature by breaking the silica tube under water. X-ray diffraction of the quenched samples was performed by means of a Philips X-ray diffractometer PW using Cu Ka radiation in conjunction with a graphite single crystal monochromator. In agreement with earlier investigations [5], we found that the compound with 2 : 17 stoichiometry had not formed when using comparatively low carbon concentrations. In those cases the main phase in the corresponding X-ray diagram was of the tetragonal ThMn,, structure type. In Table 1, the lattice constants are listed for those compositions in which formation of a compound of the type Gd,Mn,,C, had taken place. It can be seen that the
TABLE 1 Structure and lattice constants of various Gd,Mn,,C,-type
0.8
1.0 1.5 2.0 2.2 2.5 2.7
compounds
Structure
a(A)
C(A)
Th,Ni,, Th,Ni,, Th,Ni,, Th,Ni,, Th,Ni,, Th,Ni,, ThJn,,
8.673 8.733 8.764 8.768 8.783 8.776 8.778
8.503 8.486 8.505 8.479 8.543 8.524 12.793
271
crystal structure is of the hexagonal ThzNi,, type for low carbon concentrations, changing to the rhombohedral Th,Zn,, type for high carbon concentrations. The magnetic properties of the Gd,Mn,,C, compounds were studied by means of a.c. susceptibility measurements, using an apparatus based on the mutual inductance technique [6]. A primary coil produced a small oscillating magnetic field with a frequency of 82.9 Hz. Results for three of the compounds investigated are shown in Fig. 1. It follows from the a.c. susceptibility measurements that the magnetic ordering temperatures are fairly low and have a tendency to decrease with carbon concentration. The Miissbauer spectra were taken at 4.2 K (and for Gd,Mn,,C,,, also at 10 K) using the 86.5 keV resonance of gadolinium. The source was SmPd,. Details of the spectrometer have been described elsewhere [3]. A survey of the spectra taken for the various Gd,Mn,,C, compounds at 4.2 K is shown in Fig. 2. It can be seen from the results shown that increasing carbon concentration leads to an increasing broadening and/or splitting of the spectra.
3. Analysis of spectra It was shown in the preceding section that the Gd,Mn,,C, compounds are formed with two different structure types. For low carbon concentrations, the hexagonal Th,Ni,, type is observed. This structure type contains two inequivalent gadolinium positions. For high carbon concentrations, the rhombohedral Th,Zn,,
GdzMniSx X varyq
from
0.8 to 2.7
2.5
6
100
5
c
w A
2
100
2 100
2.2
‘-:k 20
40
100
07 .,I
-6
60 T(K)
-
-4
-2 Velocfty
0
2 (mms
4
‘)
Fig. 1. Temperature dependence of the a.c. susceptibility in several ternary carbides Gd,Mn,,C,. Different vertical scales apply to the three curves shown. Fig. 2. ‘55Gd MGssbauer spectra of a series of ternary carbides of the type Cd&In,&,
6
of the type
taken at 4.2 K.
272
type is found, characterized by only a single crystallographic gadolinium position. The occurrence of two sites in the former compounds makes an interpretation of the corresponding Mossbauer spectra fairly difficult. This is one of the reasons that we restricted our analysis of the spectra to those of the rhombohedral compound Gd,Mn,,C,,,. The other reason is that the hexagonal compounds are less suited for comparison with ls5Gd Mossbauer data obtained on Gd,Fe,,C, compounds, since the crystal structure of the latter is of the rhombohedral type. It was mentioned in the introduction that the formula composition of compounds with fully occupied 9e position corresponds to R,Mn,,C,. In that case each of the rare earth atoms has three nearest neighbour carbon atoms. For partially occupied 9e positions in R,Mn,,C, there are then four possibilities. The rare earth atoms can have IZ= 0, 1, 2 or 3 carbon atom neighbours and the corresponding probabilities can be calculated for each concentration x by means of the formula P[N,n,f]=N!f”(l
-f”)/n!(N-n)!
(1)
where the maximum number of carbon neighbours equals N= 3 and where the fractional 9e site occupancyf(x) equals 0.9 in Gd,Mn,&. The spectra obtained for Gd,Mn,,C,,, at 4.2 and 10 K are shown in more detail in Fig. 3. These spectra have been analysed by means of a least-squares fitting procedure involving the diagonalization of the full nuclear Hamiltonian and using a transmission integral. The independently refined variables were the isomer shift (IS), the effective hyperfine field and the quadrupole splitting (QS) (or the field gradient tensor V,,, obtained via the relation QS =($)eQV,,, using the value by Tanaka et al. [7’]).The line width of absorber and Q= 1.30 x 1O-2s m2 gtven ’ source were constrained to 0.25 and 0.40 mm s-l for the transmission integral.
GdzMmKb.7 10K
_.
-6
-4
-2 Velocity
0
2
4
6
(mm5’)
Fig. 3. lS5Gd MGssbauer spectra of Gd,Mn,,C,,, at 4.2 and 10 K. The solid line through the data points represents a fit, as discussed in the main text.
273
Owing to the fact that there is only partial occupancy of the 9(e) interstitial neighbour site position by carbon for some of the gadolinium sites, the trigonal site symmetry of the corresponding gadolinium atoms has been broken. This implies that the asymmetry parameter 77for the latter sites is not equal to zero. The fitting procedures have been performed with the constraint that the angle 8 between H,, and V, be either 0” or 90”. Two different sites have been considered, characterized by two or three carbon atoms occupying the adjacent 9(e) positions. The relative abundance of these sites depends on the carbon concentration and was calculated by means of the binomial distribution mentioned above (eqn. 1). The values for the various hyperfine parameters are given in the bottom part of Table 2 together with.
TABLE 2 Data obtained from fitting rssGd Miissbauer spectra of Gd,Fe,,C,and Gd&in,,C,,, in parenthesis represent the experimental error of the last digit of the corresponding
(the figures given values)
Gd2Fe,,C, (measurements at 4.2 K, for all sites 19= 90) x=0.0 number of carbon atoms 0 relative contribution 1.0 21(l) H,, (T ) 4.4( 1) ~z:r(102’Vm-‘) 0.27( 2) IS(mms-‘) A! (Ka;*) -384 x = 0.6 number of carbon atoms relative contribution H,, (T ) 1/;,(102’Vm-2) IS(mms_‘) Ai (Kac2) rl x=1.2 number of carbon atoms relative contribution H,, (T) ~z(102’Vm~2) IS(mms_‘) Ai (Ka;*) 11
0 0.516
21(l)
2 0.097 5(2) j(2) 0.3( 1)
1 0.387
8(l)
4.4( 3) 0.27( 2) L 384 -
12.0 0.33( 2) - 1047 0.4( 2)
-436 -
0 0.229 21 4.4( 3) 0.29 - 384
1 0.464 5.7(5) 12.2(3) 0.36( 2) - 1065 0.4( 2)
2 0.307 10.3( 9) 8.5( 3) 0.32(2) - 742 0.3( 2)
1
2 0.13 15(l) 5.1( 1.0) 0.38( 1) - 455 2.1(3)
Gd,Mn,,C, (measurements at 4.2 K, 8 = 0) x = 2.7 number of carbon atoms 0 relative contribution JO Hz, (T 1 ~z:r(1021Vm-2) IS(mms_‘) At (Ka;*) rl
-0
3 =O
3 --0
3 3.87 24.6(2) 15.1(2) 0.40( 1) - 1318 -
274
the relatiye intensities calculated under the assumption of a binomial distribution function with the fitting procedure. The fits correspond to the solid lines in Fig. 3.
4. Discussion The data obtained from fitting the ls5Gd Mossbauer spectra of Gd,Mn,,C,,, can be compared in Table 2 with data obtained previously for several Gd*Fei,C, compounds of lower carbon concentration. It may be seen from Table 2 that the spectra of GdzFel,C, were fitted with 13= 90”. The reason for this is the presence of a comparatively strong anisotropy of the iron sublattice in the former compounds, which leads to an easy iron sublattice magnetization direction perpendicular to the c axis. Owing to the presence of a strong Gd-Fe exchange interaction, this same moment direction is also adopted by the gadolinium atoms in Gd2Fe1,Cx. These features are absent in Gd,Mn,,C,,, . Our main purpose in studying the ls5Gd Mijssbauer spectra of GdZMni,C, was to obtain experimental information of the second order crystal field parameter A$!at the site associated with rare earth atoms having three nearest carbon atoms. The following relations are commonly employed to derive values of the second order crystal field parameters Ai and AZ from the values of V,, and 7 derived from the Mossbauer spectra A;= - Q/(1 - ym)4
(2)
A;=A’:(
(3)
V,,- V,)/V,,= T,IA;
The quantity yrn is the Sternheimer antishielding factor, for which we used yrn = - 92. The values of At obtained from I’,, for the separate sites in Gd,Mn,,C,,, compounds have been included in Table 2. Relations (2) and (3) are valid, for instance, in a point charge model. However, recently we have shown that there is a large contribution to the field gradient of the valence electrons of the central rare earth ion [8]. Close to the rare earth nucleus the 6p electron density is larger than that of 5d electrons. Hence I’,, may experience a larger influence from the 6p electrons than Ai, so that the validity of eqns. (2) and (3) is restricted. These relations can be used, however, to predict trends, as will be done in the present paper. The Ai values found in GdzFel,C, for the gadolinium site with two nearest neighbour atoms is different for the two samples with x = 0.6 and x = 1.2. Based on arguments of relative intensities it is reasonable to assume that the value listed for the x = 1.2 sample is the most reliable value, as follows also from the experimental error listed for the corresponding values of V,, and 7. In Gd,Mn,,C,,,, the subspectrum belonging to gadolinium atoms with two carbon neighbours is again of relatively low intensity, involving a fairly large experimental error in the value of V,,. Taking account of these inaccuracies in the experimental data, one may state that the A i value pertaining to gadolinium atoms with two carbon neighbours may be somewhat lower in the manganese compound than in the iron compound.
275
The experimental error in V,, is fairly low for the site with three carbon neighbours in GdzMn,,Cz,, which is due to the large intensity of the corresponding subspectrum. Looking again at the experimental uncertainties, one may state that At for the site with three carbon neighbours is at least twice as large as the A i value of the site with two carbon neighbours. It is furthermore reasonable to assume that the same relationship holds in Gd2Fe1,Cx, the main effect being due to the presence of the carbon nearest neighbours at the corresponding gadolinium sites in both cases. Focussing our attention now on the series Gd2Fel,C,, one has the following interesting behaviour of A !j. The parameter Afj for gadolinium atoms with no carbon neighbours is only of modest magnitude. The effect of one carbon neighbour more than doubles the absolute value of A!. The simultaneous effect of two carbon neighbours is seen to be lower than that of a single carbon neighbour but the effect of three carbon neighbours is again very strong, the absolute value of At being estimated to be about four times larger than the value without carbon neighbours. In all these cases there is no change in sign of A:. Concluding, from the results obtained in the present investigations it may be derived that the electric field gradient at the gadolinium nuclei in Gd2Fe1,Cx depends strongly on the number of interstitial neighbour atoms. It reaches a maximum for full occupation of the nearest neighbour interstitial site, but is fairly large also in the case of one interstitial nearest neighbour carbon atom. Within the limited validity of eqns. (2) and (3) an analogous behaviour is expected for-the second-order crystal field parameter A!. This means that the crystal field induced anisotropy of the rare earth sublattice may reach fairly high values for full or almost full occupation of the nearest neighbour interstitial hole position. Unfortunately this situation cannot be reached in the R2Fe1,Cx series. However, it can be reached in compounds of the type R2Fe1,Nx, the existence of which was first found for R = Sm, Y [9] and subsequently also for the remainder of rare earth elements [lo]. If the effect of the nitrogen atoms in these compounds is comparable to that of carbon atoms, one may expect fairly large anisotropy contributions of the rare earth sublattice. Preliminary room temperature measurements of this anisotropy field in Sm,Fe,,N, show this to be the case, indeed [9]. ls5Gd Mossbauer spectroscopy measurements on Gd,Fe,,N, are currently being undertaken to study further the effect of interstitial nitrogen atoms on V,, in these interstitial compounds.
References 1 D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 142 (1988) 349. 2 R. B. Helmholdt and K. H. J. Buschow, .I. Less-Common Met., 144 (1988) L33. 3 M. W. Dirken, R. C. Thiel, L. J. de Jongh, T. H. Jacobs and K. H. J. Buschow, J. Less-Common Met., 155 (1989) 339. 4 P. C. M. Gubbens, A. M. van der Kraan, T. H. Jacobs and K. H. J. Buschow, J. Magn. Magn. Mater., 80(1989)265. 5 G. Block and W. Jeitschko, J. Solid&ate Chem., 7U( 1987) 271. 6 A. F. Deutz, R. Hulstman and F. J. Kranenburg, Rev. Sci. Instrum., 60 (1989) 113.
276 7 Y. Tanaka, 0. B. Laubacher, R. M. Stoffen, E. B. Shera, H. D. Wohlfarth and M. v. Hoehn, Phys. Lert., 108B (1982) 8. 8 R. Coehoorn, K. H. J. Buschow, M. W. Dirken and R. C. Thiel, Phys. Rev., B42 (1990) 4645. 9 J. M. D. Coey and Hong Sun, J. Magn. Magn. Muter.,87(1990) L251. 10 K. H. J. Buschow, D. B. de Mooij, R. Coehoom, C. de Waard and T. H. Jacobs, J. Mugn. Magn. Mater., 92(1990) L35.
1SsGd MOSSBAUER
Metals, 168( 1991) 269-216
EFFECT
STUDY OF Cd,Mn,,C,
269
COMPOUNDS
M. W. DIRKEN”, R. C. THIEL”, T. H. JACOBSaab and K. H. J. BUSCHOWa,b “Kamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, The Netherlands bPhilips Research Laboratories, 5600 JA Eindhoven, The Netherlands
(Received July 6, 1990)
Summary
We have prepared ternary carbides of the type Gd,Mn,,C, with x = 0.8, 1.0, 1.5,2.0, 2.2, 2.5 and 2.7 and have studied these compounds by means of X-ray diffraction, a.c. susceptibility measurements and 155Gd Mossbauer spectroscopy. Of these compounds only Gd,Mn,,C,,, adopts the rhombohedral Th,Zn,, structure, the other compounds crystallizing in the hexagonal ThzNi,, structure. The lattice constants are given for all compounds. The a.c. susceptibility measurements showed that the magnetic ordering temperatures of these compounds are fairly low (less than 40 K), with a tendency to decrease with carbon concentration. The ls5Gd Mossbauer spectra showed a broadening and/or splitting that increases with carbon concentration and reflects an increasing contribution from subspectra with a large quadrupole interaction. From fitting the spectra of the rhombohedral compound Gd,Mn,& it was derived that the field gradient is maximal at the nuclei of those atoms for which the interstitial nearest neighbour hole positions are completely filled by carbon atoms.
1. Introduction
The magnetic properties of the iron-rich rare earth compounds of the type R,Fe,, have lower magnetic ordering temperatures than less iron-rich binary rare earth compounds. Also, the crystal-field-induced rare earth sublattice magnetization reaches only moderate values. This has hampered the application of R*Fe,,type compounds as starting materials for permanent magnets. Recent investigations have shown that the magnetic properties of R,Fe,, can be considerably improved by interstitial solution of carbon atoms [l]. In the corresponding ternary intermetallic compounds these carbon atoms partially occupy the interstitial 9e position of the rhombohedral Th,Zn,, structure type [2]. These 9e sites are located close to the rare earth atoms. When fully occupied, the formula composition would correspond to R,Fe,,C,. In practice it proved possible to fill the 9e sites only partially with carbon atoms, the concentrations remaining well below x = 2 in R2Fe1,Cx, even for the heavy rare earth elements. 0022-5088/91/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
270
Experimental information on the effect of the interstitial carbon atoms on the field gradient at the rare earth sites and on the concomitant contributions expected for the crystal field induced anisotropy was obtained by ls5Gd Mijssbauer spectroscopy performed on several compounds of the series Gd,Fe,,C, and TmzFel,C, [3,4]. Substantial differences in local field gradient were found for Gd atoms surrounded by zero, one or two carbon atom nearest neighbours [3]. The limited concentration range prevented us from obtaining experimental information for gadolinium atoms surrounded by three nearest neighbour carbon atoms, the occurrence of such gadolinium atoms being negligible for x = 1.5, assuming a statistical distribution of the carbon atoms over the three available interstitial sites. It was shown by Block and Jeitschko [S] that in R,Mn,,C, considerably larger concentrations of carbon can be reached than in R2Fe1,Cx. For this reason we have extended our investigation to the series Gd,Mn,,C,.
2. Experimental details Various compounds of the type GdzMnl,C, were prepared by arc melting from starting materials of at least 99.9% purity. We used a slight excess of manganese to compensate for losses during melting. After arc melting, the samples were wrapped into tantalum foil, sealed into an evacuated silica tube and annealed for about 3 weeks at 900 “C. After annealing, the samples were cooled quickly to room temperature by breaking the silica tube under water. X-ray diffraction of the quenched samples was performed by means of a Philips X-ray diffractometer PW using Cu Ka radiation in conjunction with a graphite single crystal monochromator. In agreement with earlier investigations [5], we found that the compound with 2 : 17 stoichiometry had not formed when using comparatively low carbon concentrations. In those cases the main phase in the corresponding X-ray diagram was of the tetragonal ThMn,, structure type. In Table 1, the lattice constants are listed for those compositions in which formation of a compound of the type Gd,Mn,,C, had taken place. It can be seen that the
TABLE 1 Structure and lattice constants of various Gd,Mn,,C,-type
0.8
1.0 1.5 2.0 2.2 2.5 2.7
compounds
Structure
a(A)
C(A)
Th,Ni,, Th,Ni,, Th,Ni,, Th,Ni,, Th,Ni,, Th,Ni,, ThJn,,
8.673 8.733 8.764 8.768 8.783 8.776 8.778
8.503 8.486 8.505 8.479 8.543 8.524 12.793
271
crystal structure is of the hexagonal ThzNi,, type for low carbon concentrations, changing to the rhombohedral Th,Zn,, type for high carbon concentrations. The magnetic properties of the Gd,Mn,,C, compounds were studied by means of a.c. susceptibility measurements, using an apparatus based on the mutual inductance technique [6]. A primary coil produced a small oscillating magnetic field with a frequency of 82.9 Hz. Results for three of the compounds investigated are shown in Fig. 1. It follows from the a.c. susceptibility measurements that the magnetic ordering temperatures are fairly low and have a tendency to decrease with carbon concentration. The Miissbauer spectra were taken at 4.2 K (and for Gd,Mn,,C,,, also at 10 K) using the 86.5 keV resonance of gadolinium. The source was SmPd,. Details of the spectrometer have been described elsewhere [3]. A survey of the spectra taken for the various Gd,Mn,,C, compounds at 4.2 K is shown in Fig. 2. It can be seen from the results shown that increasing carbon concentration leads to an increasing broadening and/or splitting of the spectra.
3. Analysis of spectra It was shown in the preceding section that the Gd,Mn,,C, compounds are formed with two different structure types. For low carbon concentrations, the hexagonal Th,Ni,, type is observed. This structure type contains two inequivalent gadolinium positions. For high carbon concentrations, the rhombohedral Th,Zn,,
GdzMniSx X varyq
from
0.8 to 2.7
2.5
6
100
5
c
w A
2
100
2 100
2.2
‘-:k 20
40
100
07 .,I
-6
60 T(K)
-
-4
-2 Velocfty
0
2 (mms
4
‘)
Fig. 1. Temperature dependence of the a.c. susceptibility in several ternary carbides Gd,Mn,,C,. Different vertical scales apply to the three curves shown. Fig. 2. ‘55Gd MGssbauer spectra of a series of ternary carbides of the type Cd&In,&,
6
of the type
taken at 4.2 K.
272
type is found, characterized by only a single crystallographic gadolinium position. The occurrence of two sites in the former compounds makes an interpretation of the corresponding Mossbauer spectra fairly difficult. This is one of the reasons that we restricted our analysis of the spectra to those of the rhombohedral compound Gd,Mn,,C,,,. The other reason is that the hexagonal compounds are less suited for comparison with ls5Gd Mossbauer data obtained on Gd,Fe,,C, compounds, since the crystal structure of the latter is of the rhombohedral type. It was mentioned in the introduction that the formula composition of compounds with fully occupied 9e position corresponds to R,Mn,,C,. In that case each of the rare earth atoms has three nearest neighbour carbon atoms. For partially occupied 9e positions in R,Mn,,C, there are then four possibilities. The rare earth atoms can have IZ= 0, 1, 2 or 3 carbon atom neighbours and the corresponding probabilities can be calculated for each concentration x by means of the formula P[N,n,f]=N!f”(l
-f”)/n!(N-n)!
(1)
where the maximum number of carbon neighbours equals N= 3 and where the fractional 9e site occupancyf(x) equals 0.9 in Gd,Mn,&. The spectra obtained for Gd,Mn,,C,,, at 4.2 and 10 K are shown in more detail in Fig. 3. These spectra have been analysed by means of a least-squares fitting procedure involving the diagonalization of the full nuclear Hamiltonian and using a transmission integral. The independently refined variables were the isomer shift (IS), the effective hyperfine field and the quadrupole splitting (QS) (or the field gradient tensor V,,, obtained via the relation QS =($)eQV,,, using the value by Tanaka et al. [7’]).The line width of absorber and Q= 1.30 x 1O-2s m2 gtven ’ source were constrained to 0.25 and 0.40 mm s-l for the transmission integral.
GdzMmKb.7 10K
_.
-6
-4
-2 Velocity
0
2
4
6
(mm5’)
Fig. 3. lS5Gd MGssbauer spectra of Gd,Mn,,C,,, at 4.2 and 10 K. The solid line through the data points represents a fit, as discussed in the main text.
273
Owing to the fact that there is only partial occupancy of the 9(e) interstitial neighbour site position by carbon for some of the gadolinium sites, the trigonal site symmetry of the corresponding gadolinium atoms has been broken. This implies that the asymmetry parameter 77for the latter sites is not equal to zero. The fitting procedures have been performed with the constraint that the angle 8 between H,, and V, be either 0” or 90”. Two different sites have been considered, characterized by two or three carbon atoms occupying the adjacent 9(e) positions. The relative abundance of these sites depends on the carbon concentration and was calculated by means of the binomial distribution mentioned above (eqn. 1). The values for the various hyperfine parameters are given in the bottom part of Table 2 together with.
TABLE 2 Data obtained from fitting rssGd Miissbauer spectra of Gd,Fe,,C,and Gd&in,,C,,, in parenthesis represent the experimental error of the last digit of the corresponding
(the figures given values)
Gd2Fe,,C, (measurements at 4.2 K, for all sites 19= 90) x=0.0 number of carbon atoms 0 relative contribution 1.0 21(l) H,, (T ) 4.4( 1) ~z:r(102’Vm-‘) 0.27( 2) IS(mms-‘) A! (Ka;*) -384 x = 0.6 number of carbon atoms relative contribution H,, (T ) 1/;,(102’Vm-2) IS(mms_‘) Ai (Kac2) rl x=1.2 number of carbon atoms relative contribution H,, (T) ~z(102’Vm~2) IS(mms_‘) Ai (Ka;*) 11
0 0.516
21(l)
2 0.097 5(2) j(2) 0.3( 1)
1 0.387
8(l)
4.4( 3) 0.27( 2) L 384 -
12.0 0.33( 2) - 1047 0.4( 2)
-436 -
0 0.229 21 4.4( 3) 0.29 - 384
1 0.464 5.7(5) 12.2(3) 0.36( 2) - 1065 0.4( 2)
2 0.307 10.3( 9) 8.5( 3) 0.32(2) - 742 0.3( 2)
1
2 0.13 15(l) 5.1( 1.0) 0.38( 1) - 455 2.1(3)
Gd,Mn,,C, (measurements at 4.2 K, 8 = 0) x = 2.7 number of carbon atoms 0 relative contribution JO Hz, (T 1 ~z:r(1021Vm-2) IS(mms_‘) At (Ka;*) rl
-0
3 =O
3 --0
3 3.87 24.6(2) 15.1(2) 0.40( 1) - 1318 -
274
the relatiye intensities calculated under the assumption of a binomial distribution function with the fitting procedure. The fits correspond to the solid lines in Fig. 3.
4. Discussion The data obtained from fitting the ls5Gd Mossbauer spectra of Gd,Mn,,C,,, can be compared in Table 2 with data obtained previously for several Gd*Fei,C, compounds of lower carbon concentration. It may be seen from Table 2 that the spectra of GdzFel,C, were fitted with 13= 90”. The reason for this is the presence of a comparatively strong anisotropy of the iron sublattice in the former compounds, which leads to an easy iron sublattice magnetization direction perpendicular to the c axis. Owing to the presence of a strong Gd-Fe exchange interaction, this same moment direction is also adopted by the gadolinium atoms in Gd2Fe1,Cx. These features are absent in Gd,Mn,,C,,, . Our main purpose in studying the ls5Gd Mijssbauer spectra of GdZMni,C, was to obtain experimental information of the second order crystal field parameter A$!at the site associated with rare earth atoms having three nearest carbon atoms. The following relations are commonly employed to derive values of the second order crystal field parameters Ai and AZ from the values of V,, and 7 derived from the Mossbauer spectra A;= - Q/(1 - ym)4
(2)
A;=A’:(
(3)
V,,- V,)/V,,= T,IA;
The quantity yrn is the Sternheimer antishielding factor, for which we used yrn = - 92. The values of At obtained from I’,, for the separate sites in Gd,Mn,,C,,, compounds have been included in Table 2. Relations (2) and (3) are valid, for instance, in a point charge model. However, recently we have shown that there is a large contribution to the field gradient of the valence electrons of the central rare earth ion [8]. Close to the rare earth nucleus the 6p electron density is larger than that of 5d electrons. Hence I’,, may experience a larger influence from the 6p electrons than Ai, so that the validity of eqns. (2) and (3) is restricted. These relations can be used, however, to predict trends, as will be done in the present paper. The Ai values found in GdzFel,C, for the gadolinium site with two nearest neighbour atoms is different for the two samples with x = 0.6 and x = 1.2. Based on arguments of relative intensities it is reasonable to assume that the value listed for the x = 1.2 sample is the most reliable value, as follows also from the experimental error listed for the corresponding values of V,, and 7. In Gd,Mn,,C,,,, the subspectrum belonging to gadolinium atoms with two carbon neighbours is again of relatively low intensity, involving a fairly large experimental error in the value of V,,. Taking account of these inaccuracies in the experimental data, one may state that the A i value pertaining to gadolinium atoms with two carbon neighbours may be somewhat lower in the manganese compound than in the iron compound.
275
The experimental error in V,, is fairly low for the site with three carbon neighbours in GdzMn,,Cz,, which is due to the large intensity of the corresponding subspectrum. Looking again at the experimental uncertainties, one may state that At for the site with three carbon neighbours is at least twice as large as the A i value of the site with two carbon neighbours. It is furthermore reasonable to assume that the same relationship holds in Gd2Fe1,Cx, the main effect being due to the presence of the carbon nearest neighbours at the corresponding gadolinium sites in both cases. Focussing our attention now on the series Gd2Fel,C,, one has the following interesting behaviour of A !j. The parameter Afj for gadolinium atoms with no carbon neighbours is only of modest magnitude. The effect of one carbon neighbour more than doubles the absolute value of A!. The simultaneous effect of two carbon neighbours is seen to be lower than that of a single carbon neighbour but the effect of three carbon neighbours is again very strong, the absolute value of At being estimated to be about four times larger than the value without carbon neighbours. In all these cases there is no change in sign of A:. Concluding, from the results obtained in the present investigations it may be derived that the electric field gradient at the gadolinium nuclei in Gd2Fe1,Cx depends strongly on the number of interstitial neighbour atoms. It reaches a maximum for full occupation of the nearest neighbour interstitial site, but is fairly large also in the case of one interstitial nearest neighbour carbon atom. Within the limited validity of eqns. (2) and (3) an analogous behaviour is expected for-the second-order crystal field parameter A!. This means that the crystal field induced anisotropy of the rare earth sublattice may reach fairly high values for full or almost full occupation of the nearest neighbour interstitial hole position. Unfortunately this situation cannot be reached in the R2Fe1,Cx series. However, it can be reached in compounds of the type R2Fe1,Nx, the existence of which was first found for R = Sm, Y [9] and subsequently also for the remainder of rare earth elements [lo]. If the effect of the nitrogen atoms in these compounds is comparable to that of carbon atoms, one may expect fairly large anisotropy contributions of the rare earth sublattice. Preliminary room temperature measurements of this anisotropy field in Sm,Fe,,N, show this to be the case, indeed [9]. ls5Gd Mossbauer spectroscopy measurements on Gd,Fe,,N, are currently being undertaken to study further the effect of interstitial nitrogen atoms on V,, in these interstitial compounds.
References 1 D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 142 (1988) 349. 2 R. B. Helmholdt and K. H. J. Buschow, .I. Less-Common Met., 144 (1988) L33. 3 M. W. Dirken, R. C. Thiel, L. J. de Jongh, T. H. Jacobs and K. H. J. Buschow, J. Less-Common Met., 155 (1989) 339. 4 P. C. M. Gubbens, A. M. van der Kraan, T. H. Jacobs and K. H. J. Buschow, J. Magn. Magn. Mater., 80(1989)265. 5 G. Block and W. Jeitschko, J. Solid&ate Chem., 7U( 1987) 271. 6 A. F. Deutz, R. Hulstman and F. J. Kranenburg, Rev. Sci. Instrum., 60 (1989) 113.
276 7 Y. Tanaka, 0. B. Laubacher, R. M. Stoffen, E. B. Shera, H. D. Wohlfarth and M. v. Hoehn, Phys. Lert., 108B (1982) 8. 8 R. Coehoorn, K. H. J. Buschow, M. W. Dirken and R. C. Thiel, Phys. Rev., B42 (1990) 4645. 9 J. M. D. Coey and Hong Sun, J. Magn. Magn. Muter.,87(1990) L251. 10 K. H. J. Buschow, D. B. de Mooij, R. Coehoom, C. de Waard and T. H. Jacobs, J. Mugn. Magn. Mater., 92(1990) L35.