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Magnetic anisotropy in ThMn12-type compounds studied by 155Gd Mössbauer spectroscopy
Journal of the Less-Common Metals, 146 (1989) L15 - L18
L15
Letter
Magnetic anisotropy in ThMn&ype “‘Gd Mijssbauer spectroscopy
compounds studied by
M. W. DIRKEN and R. C. THUEL Kamerlingh
Onnes Laboratorium,
2300 RA Leiden
(The Netherlands)
K. H. J. BUSCHOW Philips Resenrch Laboratories,
5600 JA ~jndhoven
{The Nether~nds~
(Received September 6,1988)
The search for novel starting materials for permanent magnet applications has led to the discovery of iron-rich ternary rare earth compounds of the tetragonal ThMnl, structure [I - 31. The formula composition of these compounds can be represented as RFeIz_,M,, where x = 1 for M = Ti and W, and x ~2:2 for M = V, Cr, MO and Si. The crystallographic unit cell (space group 14/mm) contains two formula units. The rare earths atoms occupy a single c~s~~o~aphic position (Xa). The iron and M atoms occupy the position 8i, 8j and 8f. There is a strong preference for the M atoms to occupy the 8i sites when M - Ti, V, W and MO [4 - 61. Results of magnetic measurements and neutron diffraction have shown that the iron sublattice anisotropy favours an easy magnetization direction parallel to the c axis [ 71. The rare earth anisotropy varies with the R component in RFe12-,M,, and can most conveniently be described by means of crystal field theory. The corresponding crystal field Hamiltonian is n.m
where B”, and 0; are the crystal field parameters and the Stevens operators respectively. The lowest-order term B! can be written as where (Yeis the second-order Stevens constant, and (r2) the expectation value of the squared 4f wavefunction radius. Experimental information on the lowest-order term can be obtained from lssGd Mgssbauer spectroscopy. From the quadrupole splitting (QS = ;e’Qq = $eQV,,) one may derive experimktal values of the second-order parameters A! via the relation
v,*
A$+-
40 -Y0022-5088/89/$3.50
(3)
) @ Elsevier ~quoiat~inted
in The Netherlands
L16
where r_ is the Sternheimer anti-shielding factor. For the crystal structure considered, one has Ai = 0 and the asymmetry parameter q equals zero. In the present ~vest~ation we have measured the “‘Gd Mijssbauer spectra of GdFe 10.8Til.2 ad GdWdb. Mossbauer spectra were taken at 4.2 K, using the 86.5 keV resonance of is5Gd. The source was neutron-irradiated SmPds, enriched to 97.7% in ‘54Sm. Data were collected with an intrinsic germanium detector and stored in a microprocessor analyser system. The spectra were analysed by means of a least-squares fitting procedure involving the diagonalization of the full nuclear Ihuniltonian and using a transmission integral. Independently refined variables were the isomer shift (IS), the effective hyperfine field (B,,,), and the quadropole splitting (QS). From the latter quantity, the field gradient tensor V,, was obtained via the relation QS = $eQVzz, using the value Q = 1.59 X 1O-28 m2 given by Cook and Cashion [8] and assuming that V,, < 0 [9]. The linewidths of absorber and source were constrained to 0.25 and 0.40 mm s-i respectively. Experimental results are shown in Fig. 1. From the fits (curves through the data) we obtained the hyperfine parameters listed for these two compounds in Table 1. Included in the table are also results obtained previously on several other compounds of this class of materials [ 91. If one neglects relatively small contributions to the hyperfine field such as the Lorentz field, the demagnetizing field, and the dipolar field, one may write
2
98
s ‘8 .E” 97 e g too i d
98
Velocity (mm/s)
Fig. 1. lssGd Mijssbauer spectra of two ThMnIz-type compounds.
L17
TABLE 1 Hyperfine parameters derived from fits of the r5sGd Mossbauer spectra of various GdFerz_,M, compounds at 4.2 K (the vahzes of Az” were derived from those of V,, by means of eqn. (3)) Compound
GdFe losTir. GdFereVz GdFercMoz GdFerc.sWl.2 GdFeloSiz
IS (mm s-r)
I”T;)*’
BCE (T)
0.24 0.22 0.22 0.23 0.30
17.5 11.4 9.8 12.8 24.6
16.7 21.8 23.4 20.4 8.6
V (ifO17 (50.7) (44.6) (43.0) (46.0) (57.8)
V cme2)
0.9 1.65 1.35 1.6 1.41
A% (Kco-2)
9 (deg)
-79 -144 -118 -140 -123
0 0 0 0 0
where B,,, and BCE represent contributions due to the core polarization and conduction electron polarization respectively. Using B,,,, = -33.2 T and the data listed for Beff in Table 1, one may calculate BCE= 33.2 + Beif. From the hyperfine field data studied so far on a large variety of transition metal intermetallics it may be inferred that generally IBcEl < lB,,,l, which means that Beff < 0. The values for BcE calculated under this assumption have been listed in Table 1. The value listed for GdFelOVz is in good agreement with the values found by Gubbens et al. in several RFe,,V, compounds by 161Dy, ‘@jEr and 169TmMiissbauer spectroscopy [lo]. The major part at BcE is due to the transferred hyperfine field, reflecting the conduction electron polarization due the neighbo~~g magnetic moments. Values of the average iron moments obtained from high-field measurements made on magnetically aligned powders [ll] are given in Table 2. It can be seen that these moment values show differences of about 20%, and hence cannot be used to explain the rather large differences in BCE observed for the different compounds. This is true for the BCE value of GdFe,$i,, in particular. The compound GdFemSiz differs from the other compounds considered in Table 1 in that the non-magnetic atoms do not show a preferential occupation of the 8i site, but rather prefer the Sj and 8f sites. It is doubtful, however, whether this fact can explain the low value of BCE in GdFe,$Si,, since TABLE 2 Crystallographic and magnetic characteristics of several GdFe12-xM, Compound
GdFe 1o.e~r.z GdFereVz GdFe &o2 GdFere.sWr.2 GdFercSiz
a (-1 0.8523 0.8518 0.8581 0.8565 0.8437
0.4783 0.4778 0.4806 0.4777 0.4757
600 616 430 550 610
compounds
Pm
(pn (Fe atom-‘))
(1 - 02)A? (WI-~1
1.9 1.6 1.7 2.1 2.1
-40 -70 -60 -70 -60
L18
a larger fraction of iron atoms occupy the 8i site in this compound, and this site is known to favour a large value of the magnetic moment for the iron atoms occupying these sites [ 41. For completeness we have included in Table 1 the values for BCE calculated under the assumption Beff > 0 (given in parentheses). These values show a much better correlation with the corresponding iron moments than those calculated for the case Beft < 0. Hence the situation Beff > 0 is more likely. The AZ0 values calculated from the V,, values in Table 1 by means of eqn. (3) have been listed in Table 1. The A$ values obtained in this manner cannot be used straightforwardly for calculating the anisotropy constants(if the lowest order term is considered, one has K, = -3/2 Na aJ (r’>(O$). The reason for this is the shielding of the 4f electrons from the crystalline electric field due to the ligands by the outer 5s25p6 electron shells of the rare earth ions. This shielding leads to the introduction of a screening factor (1- u2), which reduces Ai by about half of its value [ 121. Values of (1 - o,)At relevant for anisotropy calculations have been included in Table 2. The compound DyFe,,Ti was studied in detail by Li et al. [ 31. These authors observed that DyFe,,Ti exhibits a spin reorientation from easy plane to easy axis with increasing temperature. From an analysis of their data, they determined the value of A: to be equal to -65 K aop2, compared with -40 K ao-’ found by us. Given the uncertainties associated with the two procedures used to determine the A! values, and taking account of the uncertainty associated with the factor (1 - u2) used in Table 2, one may still speak of a satisfactory agreement. At any rate, the values of (1- o,)A$! are much lower than those found in hard magnetic materials of the type RCo,, R2Fer4B, R,Fe& and RzCo14B [9]. Our results also show that the rare earth contribution to the magnetocrystalline anisotropy in RFe12_xM, compounds is almost the same for M = V, MO, W and Si, but is significantly smaller in the titanium compounds. 1 D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 136 (1988) 207; Philips J. Res., 42 (1987) 246. 2 K. Ohashi, Y. Tawara, R. Osugi, J. Sukurai and Y. Komura, J. Less-Common Met., 139 (1988) Ll. 3 H.-S. Li, B.-P. Hu and J. M. D. Coey, Solid State Commun., 66 (1988) 133. 4 R. B. Helmholdt, J. J. M. Vleggaar and K. H. J. Buschow, J. Less-Common Met., 138 (1988) Lll. 5 0. Moze, L. Pareti, M. Solzi and W. I. F. David, Solid State Commun., 66 (1988) 465. 6 R. H. Helmholdt, J. J. H. Vleggaar and K. H. J. Buschow, J. Less-Common Met., 144 (1988) 209. 7 F. R. de Boer, Y.-K. Huang, D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 135 (1987) 199. 8 D. Cook and J. D. Cashion, Hyperfine Interactions, 5 (1978) 479. 9 K. H. J. Buschow, D. B. de Mooij, M. Brouha, H. H. A. Smit and R. C. Thiei, IEEE Trans. Magn., 24 (1988) 1611. 10 P. C. M. Gubbens, A. M. v.d. Kraan and K. H. J. Buschow, in C. Herget, H. Kronmiiller and R. Poerschke (eds.), Proc. Vfh Int. Symp. on Magn. Anisotropy, Badsoden, 198 7, Deutsche Physikahsche Geselischaft. 11 F. R. de Boer, R. Verhoef and K. H. J. Buschow, J. Mugn. Magn. Mater., in the press. 12 D. L. Ulrich and R. G. Barnes, Phys. Reu., I64 (1967) 428.
L15
Letter
Magnetic anisotropy in ThMn&ype “‘Gd Mijssbauer spectroscopy
compounds studied by
M. W. DIRKEN and R. C. THUEL Kamerlingh
Onnes Laboratorium,
2300 RA Leiden
(The Netherlands)
K. H. J. BUSCHOW Philips Resenrch Laboratories,
5600 JA ~jndhoven
{The Nether~nds~
(Received September 6,1988)
The search for novel starting materials for permanent magnet applications has led to the discovery of iron-rich ternary rare earth compounds of the tetragonal ThMnl, structure [I - 31. The formula composition of these compounds can be represented as RFeIz_,M,, where x = 1 for M = Ti and W, and x ~2:2 for M = V, Cr, MO and Si. The crystallographic unit cell (space group 14/mm) contains two formula units. The rare earths atoms occupy a single c~s~~o~aphic position (Xa). The iron and M atoms occupy the position 8i, 8j and 8f. There is a strong preference for the M atoms to occupy the 8i sites when M - Ti, V, W and MO [4 - 61. Results of magnetic measurements and neutron diffraction have shown that the iron sublattice anisotropy favours an easy magnetization direction parallel to the c axis [ 71. The rare earth anisotropy varies with the R component in RFe12-,M,, and can most conveniently be described by means of crystal field theory. The corresponding crystal field Hamiltonian is n.m
where B”, and 0; are the crystal field parameters and the Stevens operators respectively. The lowest-order term B! can be written as where (Yeis the second-order Stevens constant, and (r2) the expectation value of the squared 4f wavefunction radius. Experimental information on the lowest-order term can be obtained from lssGd Mgssbauer spectroscopy. From the quadrupole splitting (QS = ;e’Qq = $eQV,,) one may derive experimktal values of the second-order parameters A! via the relation
v,*
A$+-
40 -Y0022-5088/89/$3.50
(3)
) @ Elsevier ~quoiat~inted
in The Netherlands
L16
where r_ is the Sternheimer anti-shielding factor. For the crystal structure considered, one has Ai = 0 and the asymmetry parameter q equals zero. In the present ~vest~ation we have measured the “‘Gd Mijssbauer spectra of GdFe 10.8Til.2 ad GdWdb. Mossbauer spectra were taken at 4.2 K, using the 86.5 keV resonance of is5Gd. The source was neutron-irradiated SmPds, enriched to 97.7% in ‘54Sm. Data were collected with an intrinsic germanium detector and stored in a microprocessor analyser system. The spectra were analysed by means of a least-squares fitting procedure involving the diagonalization of the full nuclear Ihuniltonian and using a transmission integral. Independently refined variables were the isomer shift (IS), the effective hyperfine field (B,,,), and the quadropole splitting (QS). From the latter quantity, the field gradient tensor V,, was obtained via the relation QS = $eQVzz, using the value Q = 1.59 X 1O-28 m2 given by Cook and Cashion [8] and assuming that V,, < 0 [9]. The linewidths of absorber and source were constrained to 0.25 and 0.40 mm s-i respectively. Experimental results are shown in Fig. 1. From the fits (curves through the data) we obtained the hyperfine parameters listed for these two compounds in Table 1. Included in the table are also results obtained previously on several other compounds of this class of materials [ 91. If one neglects relatively small contributions to the hyperfine field such as the Lorentz field, the demagnetizing field, and the dipolar field, one may write
2
98
s ‘8 .E” 97 e g too i d
98
Velocity (mm/s)
Fig. 1. lssGd Mijssbauer spectra of two ThMnIz-type compounds.
L17
TABLE 1 Hyperfine parameters derived from fits of the r5sGd Mossbauer spectra of various GdFerz_,M, compounds at 4.2 K (the vahzes of Az” were derived from those of V,, by means of eqn. (3)) Compound
GdFe losTir. GdFereVz GdFercMoz GdFerc.sWl.2 GdFeloSiz
IS (mm s-r)
I”T;)*’
BCE (T)
0.24 0.22 0.22 0.23 0.30
17.5 11.4 9.8 12.8 24.6
16.7 21.8 23.4 20.4 8.6
V (ifO17 (50.7) (44.6) (43.0) (46.0) (57.8)
V cme2)
0.9 1.65 1.35 1.6 1.41
A% (Kco-2)
9 (deg)
-79 -144 -118 -140 -123
0 0 0 0 0
where B,,, and BCE represent contributions due to the core polarization and conduction electron polarization respectively. Using B,,,, = -33.2 T and the data listed for Beff in Table 1, one may calculate BCE= 33.2 + Beif. From the hyperfine field data studied so far on a large variety of transition metal intermetallics it may be inferred that generally IBcEl < lB,,,l, which means that Beff < 0. The values for BcE calculated under this assumption have been listed in Table 1. The value listed for GdFelOVz is in good agreement with the values found by Gubbens et al. in several RFe,,V, compounds by 161Dy, ‘@jEr and 169TmMiissbauer spectroscopy [lo]. The major part at BcE is due to the transferred hyperfine field, reflecting the conduction electron polarization due the neighbo~~g magnetic moments. Values of the average iron moments obtained from high-field measurements made on magnetically aligned powders [ll] are given in Table 2. It can be seen that these moment values show differences of about 20%, and hence cannot be used to explain the rather large differences in BCE observed for the different compounds. This is true for the BCE value of GdFe,$i,, in particular. The compound GdFemSiz differs from the other compounds considered in Table 1 in that the non-magnetic atoms do not show a preferential occupation of the 8i site, but rather prefer the Sj and 8f sites. It is doubtful, however, whether this fact can explain the low value of BCE in GdFe,$Si,, since TABLE 2 Crystallographic and magnetic characteristics of several GdFe12-xM, Compound
GdFe 1o.e~r.z GdFereVz GdFe &o2 GdFere.sWr.2 GdFercSiz
a (-1 0.8523 0.8518 0.8581 0.8565 0.8437
0.4783 0.4778 0.4806 0.4777 0.4757
600 616 430 550 610
compounds
Pm
(pn (Fe atom-‘))
(1 - 02)A? (WI-~1
1.9 1.6 1.7 2.1 2.1
-40 -70 -60 -70 -60
L18
a larger fraction of iron atoms occupy the 8i site in this compound, and this site is known to favour a large value of the magnetic moment for the iron atoms occupying these sites [ 41. For completeness we have included in Table 1 the values for BCE calculated under the assumption Beff > 0 (given in parentheses). These values show a much better correlation with the corresponding iron moments than those calculated for the case Beft < 0. Hence the situation Beff > 0 is more likely. The AZ0 values calculated from the V,, values in Table 1 by means of eqn. (3) have been listed in Table 1. The A$ values obtained in this manner cannot be used straightforwardly for calculating the anisotropy constants(if the lowest order term is considered, one has K, = -3/2 Na aJ (r’>(O$). The reason for this is the shielding of the 4f electrons from the crystalline electric field due to the ligands by the outer 5s25p6 electron shells of the rare earth ions. This shielding leads to the introduction of a screening factor (1- u2), which reduces Ai by about half of its value [ 121. Values of (1 - o,)At relevant for anisotropy calculations have been included in Table 2. The compound DyFe,,Ti was studied in detail by Li et al. [ 31. These authors observed that DyFe,,Ti exhibits a spin reorientation from easy plane to easy axis with increasing temperature. From an analysis of their data, they determined the value of A: to be equal to -65 K aop2, compared with -40 K ao-’ found by us. Given the uncertainties associated with the two procedures used to determine the A! values, and taking account of the uncertainty associated with the factor (1 - u2) used in Table 2, one may still speak of a satisfactory agreement. At any rate, the values of (1- o,)A$! are much lower than those found in hard magnetic materials of the type RCo,, R2Fer4B, R,Fe& and RzCo14B [9]. Our results also show that the rare earth contribution to the magnetocrystalline anisotropy in RFe12_xM, compounds is almost the same for M = V, MO, W and Si, but is significantly smaller in the titanium compounds. 1 D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 136 (1988) 207; Philips J. Res., 42 (1987) 246. 2 K. Ohashi, Y. Tawara, R. Osugi, J. Sukurai and Y. Komura, J. Less-Common Met., 139 (1988) Ll. 3 H.-S. Li, B.-P. Hu and J. M. D. Coey, Solid State Commun., 66 (1988) 133. 4 R. B. Helmholdt, J. J. M. Vleggaar and K. H. J. Buschow, J. Less-Common Met., 138 (1988) Lll. 5 0. Moze, L. Pareti, M. Solzi and W. I. F. David, Solid State Commun., 66 (1988) 465. 6 R. H. Helmholdt, J. J. H. Vleggaar and K. H. J. Buschow, J. Less-Common Met., 144 (1988) 209. 7 F. R. de Boer, Y.-K. Huang, D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 135 (1987) 199. 8 D. Cook and J. D. Cashion, Hyperfine Interactions, 5 (1978) 479. 9 K. H. J. Buschow, D. B. de Mooij, M. Brouha, H. H. A. Smit and R. C. Thiei, IEEE Trans. Magn., 24 (1988) 1611. 10 P. C. M. Gubbens, A. M. v.d. Kraan and K. H. J. Buschow, in C. Herget, H. Kronmiiller and R. Poerschke (eds.), Proc. Vfh Int. Symp. on Magn. Anisotropy, Badsoden, 198 7, Deutsche Physikahsche Geselischaft. 11 F. R. de Boer, R. Verhoef and K. H. J. Buschow, J. Mugn. Magn. Mater., in the press. 12 D. L. Ulrich and R. G. Barnes, Phys. Reu., I64 (1967) 428.