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Magnetic behaviour of the 3d sublattice in RCo4−xFexB, where R = Gd, Dy, Ho, Er
Physica B 168 (1991) 115-120 North-Holland
Magnetic behaviour of the 3d sublattice R = Gd, Dy, Ho, Er
in RCo,_,Fe,B,
where
Z. Drzazga Institute of Physics,
Silesian
University,
40-&n
Katowice,
Poland
Received 19 July 1990 Revised 28 September 1990
The results of magnetization and magnetic anisotropy measurements for GdCo,_,Fe,B (O
1. Introduction
2. Experimental
An analysis of the magnetic properties of intermetallic rare-earth (R) and transition metal (T) compounds the 3d sublattice contribution is treated similar to that of the isostructural compounds with a nonmagnetic element R (Y, La, Lu). In RCo,B the problem is more complicated. Both the magnetic moment and the magnetic anisotropy of the Co sublattice in the compounds with a nonmagnetic rare earth are quite different from those with a magnetic rare earth, especially with a heavy rare earth [l-4]. The mean cobalt moment is equal to 0.67~~ and lh for YCo,B and GdCo,B, respectively. YCo,B and LuCo,B show the easy plane while GdCo,B has an easy c-axis at low temperatures. Only the introduction of small amounts of iron into GdCo,B changes the easy direction from the c-axis towards the base plane. The purpose of the investigations reported here was to study the influence on magnetic properties of the substitution of Co by Fe in RT,B with a heavy rare earth.
The samples were prepared by arc melting and analysed by methods described previously [5, 61. The RT,B compounds tolerate a considerable amount of iron substitution unlike RT,. The phase boundary depends on the rare earth, and lies for GdCo,_,Fe,B at about x = 2.6 and shifts to higher x values with decreasing atomic radius of R. This is realized in ErFe,B [7]. The magnetic moment was measured by means of a magnetometer with an extracting sample in a magnetic field up to 1.8 T and the temperature ranging from 4.2 K up to T,. Magnetic anisotropy measurements were carried out with a torsion magnetometer in an applied field up to 1.8 T in the temperature range 4.2-300 K. Measurements were made on aligned samples prepared from powder with grain diameters smaller than 10 pm. The easy mangetization direction was determined by X-ray diffraction analysis. X-ray analysis showed axial anisotropy at room temperature for compositions with x = 0, 0.1 and 2.6 and base plane anisotropy for compositions with
0921-4526/91/$03.50 0 1991- Elsevier Science Publishers B.V. (North-Holland)
Z. Drzazga
116
I Magnetic
behaviour
x =0.25, 1, 1.5 and 2. For x=0.15, 0.2, 2.5 the interpretation of the easy c-axis was not definitive due to the noncomplete vanishing of the (h 0 k) reflections. Experimental results of magnetic measurements are presented in figs. l-3 and tables 1 and 2.
3. Results and discussion 3.1. Magnetic moment of 3d sublattice All the studied compounds are ferrimagnetics with the Curie temperature above room temperature. Magnetic behaviour of the 3d sublattice in the presence of the rare earth moment can be analysed for the GdCo,_,Fe,B compounds. The thermal variation of the magnetization for GdCo,_,Fe,B (x = 1, 1.5, 2, 2.6) are plotted in fig. 1. As the concentration of iron increases the Curie temperature increases, while the compensation point shifts towards lower temperatures. This is attributed to the enhancement of the exchange interaction due to the replacement of Co by Fe atoms having larger magnetic moments. The magnetic behaviour of the 3d sublattice appears to be predominantly of band
-5-
Mco ___/ __-<____________------I
I
0
100
200
300
400
500 -
600
700
800
T[Kl
Fig. 1. Temperature dependences of magnetization for GdCo,_,Fe,B. The computed sublattice magnetization of Gd and 3d for x = 2 are also given.
of the RCO,_~F~,B
3d sublattice
type. According to the rigid band model the decrease in 3d electron concentration lowers the Fermi level in such a way that both subbands become unsaturated and the density of states increases causing an increase of the 3d magnetic moment in series of pseudobinary rare-earthcobalt-iron compounds [8]. However, some magnetic properties can be analysed in a localized model. For this purpose the sublattice magnetizations for both Gd as well 3d metals were obtained by solving the following equations involving Brillouin functions in the two-sublattice model at various temperatures: M(T) = ~,(O)B,,(x,)
- WO)BST(%)
>
where xR
=
2(&
‘T
=
gTsT( x
-
(-2(&
l)J(/%‘kT)
PdkT) -
‘)kJnRTMR
+
nTT”T)
?
and g, = 2, S = 1, nRRnRT, nTT are molecular field coefficients characterizing Gd-Gd, Gd-T and T-T interactions, respectively. It is noteworthy that magnetization curves of GdCo,_,Fe,B compounds (fig. 1) were fitted using the same set of molecular field factors (nGdCd = 4 TIpB, nGdT = 28 T/h, nTT = 127T/h). Table 1 presents the values of T, and the mean magnetic moment of 3d metals as a function of composition. Values of both T, and the 3d moment increase and show a tendency for saturation as the Fe content increases. This suggests that a portion of the Co moment is induced by exchange interactions due to larger magnetic moment of the substituted Fe atoms. It is accepted that the iron moment is virtually independent of the composition in an R(CoFe), system. The study performed on RFe,B compounds by the Mossbauer effect and magnetic measurements showed that the mean value of the iron moment is 1.5~lg [9, lo]. On this assumption the Co moment was determined for the GdCo,_,Fe,B series. Figure 2 presents the magnetic moment
2. Drzazga
Table 1 Magnetic properties of GdCo,_.Fe.B x
I Magnetic behaviour of the RCo,_,Fe,B
compounds.
~(4.2 K)
&W
K, (4.2 K)
(II,)
(A
(lO’J/m’)
1.0 0.9 0.9 1.17 1.35 1.45 1.55
4.3 1.0 -0.9 -4.7 -2.85 - 1.65 0.48
0
505
2.85
0.1 0.25 1 1.5 2 2.6
630 725 770 820
3.4 3.4 2.3 1.6 1.2 0.8
'100
150
117
3d sublattice
ered in analysing the magnetic properties of cobalt in GdCo,_,T, (T = Fe, Ni, Al) [13, 141. The lack of increase of the magnetic moment for the initial concentration of iron can be partially explained by the local environment model [5]. Only the presence of a sufficiently large number of iron atoms as nearest neighbours leads to an increase of the cobalt moment. The transition metal occupies two kinds of sites (2c, 6i) in the CeCo,B-type [16] structure with different types and numbers of neighbouring atoms. A detailed explanation of the mechanism governing the change of the magnetic moment is almost impossible on the basis of purely magnetostatic measurements. Certain information may be derived from Mossbauer effect studies on these GdCo,_,Fe,B compounds and will be discussed in our next paper [ 171. 3.2. Magnetic anisotropy of 3d sublattice
200 Hex(T)
Fig. 2. Variation of the cobalt moment with exchange field at 4.2 K.
I versus the exchange field H,, acting on the Co ion, estimated using the above molecular field parameters. In line with Burzo’s suggestion this curve may be analysed as a type of induced magnetism [l, 111. Initially the magnetic moment of cobalt keeps its value of about lb. At a certain critical value of the exchange field a cobalt magnetic contribution appears (when x > 1). The cobalt moment then increases almost linearly until finally achieving saturation. The proportionality constant V,, determined from ill]
AM,, = V,, AH,, >
The most striking behaviour of these compounds is found for magnetic anisotropy. Figure 3 presents the composition dependence of the anisotropy constant in the GdCo,_,Fe,B series. This anisotropy constant appears to be representative for 3d sublattice magnetic anisotropy on the assumption that the 4f moment of Gd does not give a rare earth contribution to the 6 t
-
-E In
A
41 I
,
I
,’
/’ ,’
(2) -2 -
is equal to -10-‘/1-~/T and is slightly larger than that reported for the Y,_,Gd,Co,B [l] and YCo,_,Fe,B [12] systems. This confirms the marked sensitivity of the cobalt moment to exchange interactions. Summing up, the behaviour of the 3d sublattice in GdCo,_,Fe,B may be interpreted in the model of induced metamagnetism or in the context of collective electron metamagnetism, i.e. models previously consid-
:
1
‘t‘t,
,’
-6 _
,’
P
/I
,’ A’
\\ \
-4 < %_
,’
#I’
,Y’
‘\
,’ *.__’ . ____--I+_____--
_x .’
1
2
/’
, /’
/’
/d’
1 0
3
4
-x Fig. 3. Composition dependence of the anisotropy constant of GdCo,_=Fe,B and LuCo,_,Fe,B. (Data of LuCo_,Fe,B are taken from ref. [3].) K,
118
Z. Drzazga
I Magnetic behaviour of the RCo,_,Fe,B
magnetic anisotropy. At low temperatures the macroscopic constant K, obtained from measurements is close to the microscopic parameter KT derived from the relation taking into account a canting of the sublattice moments due to competition between the energies of inter-sublattice exchange and sublattice anisotropy [ 181. GdCo,B shows a positive anisotropy constant. Only introduction of a small quantity of iron [4] changes the easy direction of magnetization from the c-axis to the base plane. With increasing iron content the first anisotropy constant changes sign at about x = 0.15, then passes through a minimum and again changes sign at about x = 2.5. This composition dependence shows a tendency analogous to that of YCo,_,Fe,B and LuCo,_,Fe,B [3] although the range of the easy base plane is smaller than in Y and Lu compounds. This confirms the fact that K,(x) in RT,B is quite different from that of 15 compounds in which a maximum is observed [19]. The anisotropy of the 3d sublattice is not yet fully understood. However, NMR and Mossbauer experiments on the intermetallic compounds [20-241 indicate that the anisotropy of the 3d sublattice is related to the crystal-field and spin-orbit interaction. NMR results show Co anisotropy contributions differing both in sign and and magnitude from those of the crystallographic nonequivalent sites (c, g or i) of 3d atoms. It was found that 2c sites in YCo, and YCo,B make positive contribution to the magnetic anisotropy whereas the 3g sites in RCo, and 6i in YCo,B give negative contributions. The competition between the negative and positive contributions could be the cause of the anomalous temperature dependence of the anisotropy constant in YCo,B [2] and GdCo,B [18]. Therefore, the model of the individual site anisotropy of 3d ions [25] can be applied to analyse the behaviour of magnetic anisotropy in GdCo,_,Fe,B compounds. The first anisotropy constant can be expressed as follows: K,(x) = K,(O) +
2fzcAK,,,, + 6fi AK,,,, 7
(3)
where K,(O) is the overall anisotropy constant of the cobalt compound, AK,,i = K,, - K,, is the
3d sublattice
difference between the two individual site contributions of iron and cobalt at the given site i, fzc, fei are the occupancy factors of the iron in the 2c and 6i sites in the CeCo,B type structure. Assuming the occupancy factors to be the same as in ThCo,_, Fe, [26] as in YCo,_,Fe,B and LUCO4-,Fe,B, the experimental data for K,(x) could be fitted to eq. (3) giving values of parameters AK,, = -10.4 x lo5 J/m3 and AK,, = 58.6 x lo5 J/m’. These parameters have larger absolute values than in the Y and Lu compounds and are quite different from those of the 1:5 system. It follows that cobalt and iron in RT,B have individual site contributions of the same sign unlike most rare-earth-cobalt-iron compounds which have opposite signs for these contributions [25, 271. Values of the occupancy factors i can also be estimated by the Mossbauer method. Mossbauer studies on the GdCo,_,Fe,B samples show a more complicated distribution of the iron [17]. At low Fe content the dumbbells and d sites appear mostly to be occupied. Then a strong preference to occupy one site is observed. Filling of this site to above 90% occurs for x = 0.25. At large Fe content a binomial distribution of the B atoms must be taken into account. Another point of ambiguity in the interpretation of the spectra is the assignment of the lines [9, 10, 281. It follows from our analysis [17] that Fe atoms prefer to occupy the 6i sites, which is in contradiction to the suggestion by Gross et al. [29] but it confirms the tendency of Fe to occupy 3g sites (corresponding to 6i) as predicted by means neutron diffraction. The opposite sign of values of AK,,, found for 1:5 and 1:4:B systems can be caused by a different electron configuration of 3d ions with a different sign of the corresponding Steven’s factor, or by a different sign of the crystal-field parameter. In most intermetallic compounds the configurations 3d8 for Co and 3d’ (3d6) for Fe with opposite signs of the Steven’s factor are usually taken [25]. Thus iron and cobalt give different individual site contributions to the overall anisotropy. In RCo,_,Fe,B, the magnetic moments of Co as well as Fe are reduced in comparison with RCo,_,Fe,, i.e. 1~~ and 1.5&;
Z. Drzazga
I Magnetic behaviour of the RCo,_,Fe,B
1.7& and 2.2b, respectively). These lower magnetic moments of the 3d ions in RT,B can be related to the configurations 3d9 and 3d8 having the same sign of Steven’s factors. In consequence the same sign for the individual site contributions for cobalt and iron is obtained. The Co contributions to the anisotropy in Y-Co and Gd-Co systems have been calculated within the point charge model [30-321. However, the results depend on the relative magnitude and sign of the point charges used. Particularly, it refers to the boron and 3d sites [33, 34, 21. Using the point charge model for 3d electrons in metallic systems seems to be doubtful. The problem requires the deeper studies.
3.3. Fe influence on magnetic properties in RCo,Fe,B
Table 2 gives the magnetic parameters for RCo,Fe,B with R= Dy, Ho, Er. The mean magnetic moment of the 3d metal in these compounds, estimated from the resultant magnetic moment with the assumption that the rare earth moment is close to the free ion value, approaches the value of 1.5&, as in RFe,B. In Fe-substituted RCo,B compounds with the other rare earths, an increase of the Co moment is also observed [35]. As the magnetic anisotropy in RCo,Fe,B is considered the contributions originating from both the 3d and R sublattice must be taken into account. The rare earth anisotropy is related to crystal-field effects in the intermetallic compounds. The crystal-field with exchange-field
Table 2 Magnetic properties of RCo,Fe,B
compounds.
Compound
DyCo,Fe,B HoCo,Fe,B ErCo,Fe,B ‘) Measured at 20 K
650 590 570
119
3d sublattice
model predicts large magnetocrystalline anisotropy in RT,B [18]. The easy direction of magnetization for a rare earth is determined by the Steven’s factor. The anisotropy of the 3d sublattice is smaller by nearly two orders (table 1 and 2). The negative contribution of 3d anisotropy for x = 2 can be too small to cause a change in the easy magnetization direction in ErCo,Fe,B. There is no evidence of spin reorientation in the studied temperature range. It is interesting to compare the magnitude of magnetic anisotropy in RCo,Fe,B and RCo,B (table 2). The anisotropy constant obtained at 4.2 K is relevant to the rare earth anisotropy because the 3d sublattice anisotropy can be negligible at low temperatures. It should be noticed that the value of the anisotropy constant for RCo,Fe,B compounds are markedly lower than in RCo,B. However, it follows from the analysis performed in ref. [18] that the increase of exchange interaction should not lead to the decrease of the magnetic anisotropy of the rare earth sublattice in RCo,Fe,B on the assumption that iron does not change the crystal-field parameter. Nevertheless, substituting Fe atoms for Co modified the crystal structure. For Fe-rich compounds a binomial interchange of sites between Fe and B was found by means of Mossbauer measurements [28, 171. A disordered replacement of small boron atoms by larger 3d atoms can cause local deformation of the highly symmetrical site of the R ion. In consequence the anisotropy of the rare earth sublattice can be reduced. A strong decrease of magnetic anisotropy due to deformation of the crystal structure was proved for RCO~+~ compounds [36, 371.
The values in parentheses
are relevant to RCo,B [18].
$;
~(4.2 K) (A%)
A3d) (PA
K,(4.2 K) ( lo6 J/m3)
easy axis at3OOK
-285 -196 -158
4.0 4.1 2.4
1.5 1.5 1.65
-.- \ A_.<, -8.0 3 (-13.5)
I c (plane) I L. r ,..L-, A. \~,nUc,
120
Z. Drzazga
I Magnetic
behaviour
4. Summary Introduction of iron into RCo,B compounds markedly influences their magnetic properties. The increase of the 3d sublattice moment and of the Curie temperature is observed. The cobalt moment in the studied compounds is sensitive to the enhancement of the exchange interaction. The 3d sublattice anisotropy shows an anomalous composition dependence which is different from those in most of the intermetallics with rare-earth elements (YCo,_,Fe,, YCo,_,Fe,, Y,Co,,_,Fe,B). In Fe-rich RCo,B compounds the decrease of the rare earth magnetocrystalline anisotropy can be caused partly by a deformation of the crystal structure due to binomial distributions of the boron and 3d metal atoms.
Acknowledgements The author wishes to thank Mgr. A. Winiarska for performing the X-ray measurements. This work is partly supported by the Polish Academy of Science.
References [II E. Burzo,
I. Creanga and M. Ursu, Solid State Commm. 64 (1987) 585. PI T.T. Dung, N.P. Thuy, N.M. Hong and T.D. Hien, Phys. Stat. Sol. (a) 106 (1988) 201. (31 N.M. Hong, J.J.M. Frame, N.P Thuy and T.D. Hien, J. de Phys. 49 (1988) C8-545. K. Borgiel and T. Mydlarz, [41 Z. Drzazga, A. Winiarska, J. Magn. Magn. Mat. 83 (1990) 155. and T. Mydlarz, Proc. 5th VI Z. Drzazga, A. Winiarska Int. Symp. on Magnetic Anisotropy in Coercive R-T Alloys, Bad Soden (1987) p. 297. A. Winiarska and F. Stein, J. LessPI Z. Drzazga, Common Met. 153 (1989) L21. [71 S.Y. Jiang, W.E. Wallace and E. Burzo, J. LessCommon Met. 119 (1986) LS.
of the RCo,_,Fe,B
3d sublattice
(81 K.H.J. Buschow, Rep. Prog. Phys. 40 (1977) 1179. [9] H.M. van Noort, D.B. de Mooij and K.H.J. Buschow, J. Less-Common Met. 111 (1985) 87. [lo] P.P. Vaishnava, C.W. Kimball, A.M. Umarji, S.K. Malik and G.K. Shenoy, J. Magn. Magn. Mat. 49 (1985) 286. [ll] E. Burzo, Solid State Cornmun. 20 (1976) 569. [12] E. Burzo, N. Plugaru, V. Pop and I. Creanga, Phys. Stat. Sol. (a) 113 (1989) K253. [13] E. Burzo, Rev. Roum. Phys. 23 (1978) 689. 1141 E. Burzo, J. Less-Common Met. 77 (1981) 251. [15] V. Jaccarino, L.R. Walker, Phys. Rev. Lett. 15 (1965) 295. [16] Yu. B. Kuz’ma, N.S. Bilonizhko, Kristallografija 18 (1973) 710. [17] E. Popiel, Z. Drzazga and A. Winiarska, to be published. [18] Z. Drzazga, J. Magn. Magn. Mat. 89 (1990). [19] F. Rothwarf, H.A. Leupold, J. Greedan, W.E. Wallace and D.K. Das, Int. J. Magn. 4 (1973) 267. [20] R.L. Streever, Phys. Rev. B 19 (1979) 2704. [21] K. Inomata, Phys. Rev. B23 (1981) 2076. [22] Cz. Kapusta, N. Spiridis, H. Figiel, J. Magn. Magn. Mat. 83 (1990) 157. [23] P.C. Gubbens and K.H.J. Buschow, Phys. Stat. Sol. (a) 34 (1976) 729. [24] J.M. Alameda, D. Givord, C. Jeandy, H.S. Li, Q. Lu and J.L. Oddou, J. de Phys. 46 (1985) 1581. [25] J.J.M. Franse, N.P. Thuy and N.M. Hong, J. Magn. Magn. Mat. 72 (1988) 361. [26] J. Laforest and J.S. Shah, IEEE Trans. Magn. MAG-9 (1973) 217. [27] N.P. Thuy, J.J.M. Franse, N.M. Hong and T.D. Hien, J. de Phys. 49 (1988) C8-531. [28] G. Zouganelis, A. Kostikas, A. Simopoulos and D. Niarchos, J. Magn. Magn. Mat. 75 (1988) 91. [29] Y. Gros, F. Hartmann-Bouton, C. Meyer, M.A. Fremy, P. Tenaud and P. Auric, J. de Phys. 49 (1988) C8-543. [30] K. Inomata, Phys. Rev. B23 (1981) 2076. [31] B. Szpunar and P.A. Lindgard, J. Phys. F9 (1979) L55. [32] Z. Kakol and H. Figiel, Phys. Stat. Sol. (b) 138 (1986) 151. [33] H. Oesterreicher, F.T. Parker and M. Misroch, Appl. Phys. 12 (1977) 287. [34] H.H.A. Smit, R.C. Thiel and K.H.J. Buschow, J. Phys. F 18 (1988) 295. [35] S.Y. Jiang, W.E. Wallace and E. Burzo, J. Magn. Magn. Mat. 61 (1986) 257. [36] J. Deportes, D. Givord, J. Schweizer and F. Tasset, IEEE Trans. Magn. MAG-12 (1976). [37] J. Schweizer and F. Tasset, J. Phys. F 10 (1980) 2799.
Magnetic behaviour of the 3d sublattice R = Gd, Dy, Ho, Er
in RCo,_,Fe,B,
where
Z. Drzazga Institute of Physics,
Silesian
University,
40-&n
Katowice,
Poland
Received 19 July 1990 Revised 28 September 1990
The results of magnetization and magnetic anisotropy measurements for GdCo,_,Fe,B (O
1. Introduction
2. Experimental
An analysis of the magnetic properties of intermetallic rare-earth (R) and transition metal (T) compounds the 3d sublattice contribution is treated similar to that of the isostructural compounds with a nonmagnetic element R (Y, La, Lu). In RCo,B the problem is more complicated. Both the magnetic moment and the magnetic anisotropy of the Co sublattice in the compounds with a nonmagnetic rare earth are quite different from those with a magnetic rare earth, especially with a heavy rare earth [l-4]. The mean cobalt moment is equal to 0.67~~ and lh for YCo,B and GdCo,B, respectively. YCo,B and LuCo,B show the easy plane while GdCo,B has an easy c-axis at low temperatures. Only the introduction of small amounts of iron into GdCo,B changes the easy direction from the c-axis towards the base plane. The purpose of the investigations reported here was to study the influence on magnetic properties of the substitution of Co by Fe in RT,B with a heavy rare earth.
The samples were prepared by arc melting and analysed by methods described previously [5, 61. The RT,B compounds tolerate a considerable amount of iron substitution unlike RT,. The phase boundary depends on the rare earth, and lies for GdCo,_,Fe,B at about x = 2.6 and shifts to higher x values with decreasing atomic radius of R. This is realized in ErFe,B [7]. The magnetic moment was measured by means of a magnetometer with an extracting sample in a magnetic field up to 1.8 T and the temperature ranging from 4.2 K up to T,. Magnetic anisotropy measurements were carried out with a torsion magnetometer in an applied field up to 1.8 T in the temperature range 4.2-300 K. Measurements were made on aligned samples prepared from powder with grain diameters smaller than 10 pm. The easy mangetization direction was determined by X-ray diffraction analysis. X-ray analysis showed axial anisotropy at room temperature for compositions with x = 0, 0.1 and 2.6 and base plane anisotropy for compositions with
0921-4526/91/$03.50 0 1991- Elsevier Science Publishers B.V. (North-Holland)
Z. Drzazga
116
I Magnetic
behaviour
x =0.25, 1, 1.5 and 2. For x=0.15, 0.2, 2.5 the interpretation of the easy c-axis was not definitive due to the noncomplete vanishing of the (h 0 k) reflections. Experimental results of magnetic measurements are presented in figs. l-3 and tables 1 and 2.
3. Results and discussion 3.1. Magnetic moment of 3d sublattice All the studied compounds are ferrimagnetics with the Curie temperature above room temperature. Magnetic behaviour of the 3d sublattice in the presence of the rare earth moment can be analysed for the GdCo,_,Fe,B compounds. The thermal variation of the magnetization for GdCo,_,Fe,B (x = 1, 1.5, 2, 2.6) are plotted in fig. 1. As the concentration of iron increases the Curie temperature increases, while the compensation point shifts towards lower temperatures. This is attributed to the enhancement of the exchange interaction due to the replacement of Co by Fe atoms having larger magnetic moments. The magnetic behaviour of the 3d sublattice appears to be predominantly of band
-5-
Mco ___/ __-<____________------I
I
0
100
200
300
400
500 -
600
700
800
T[Kl
Fig. 1. Temperature dependences of magnetization for GdCo,_,Fe,B. The computed sublattice magnetization of Gd and 3d for x = 2 are also given.
of the RCO,_~F~,B
3d sublattice
type. According to the rigid band model the decrease in 3d electron concentration lowers the Fermi level in such a way that both subbands become unsaturated and the density of states increases causing an increase of the 3d magnetic moment in series of pseudobinary rare-earthcobalt-iron compounds [8]. However, some magnetic properties can be analysed in a localized model. For this purpose the sublattice magnetizations for both Gd as well 3d metals were obtained by solving the following equations involving Brillouin functions in the two-sublattice model at various temperatures: M(T) = ~,(O)B,,(x,)
- WO)BST(%)
>
where xR
=
2(&
‘T
=
gTsT( x
-
(-2(&
l)J(/%‘kT)
PdkT) -
‘)kJnRTMR
+
nTT”T)
?
and g, = 2, S = 1, nRRnRT, nTT are molecular field coefficients characterizing Gd-Gd, Gd-T and T-T interactions, respectively. It is noteworthy that magnetization curves of GdCo,_,Fe,B compounds (fig. 1) were fitted using the same set of molecular field factors (nGdCd = 4 TIpB, nGdT = 28 T/h, nTT = 127T/h). Table 1 presents the values of T, and the mean magnetic moment of 3d metals as a function of composition. Values of both T, and the 3d moment increase and show a tendency for saturation as the Fe content increases. This suggests that a portion of the Co moment is induced by exchange interactions due to larger magnetic moment of the substituted Fe atoms. It is accepted that the iron moment is virtually independent of the composition in an R(CoFe), system. The study performed on RFe,B compounds by the Mossbauer effect and magnetic measurements showed that the mean value of the iron moment is 1.5~lg [9, lo]. On this assumption the Co moment was determined for the GdCo,_,Fe,B series. Figure 2 presents the magnetic moment
2. Drzazga
Table 1 Magnetic properties of GdCo,_.Fe.B x
I Magnetic behaviour of the RCo,_,Fe,B
compounds.
~(4.2 K)
&W
K, (4.2 K)
(II,)
(A
(lO’J/m’)
1.0 0.9 0.9 1.17 1.35 1.45 1.55
4.3 1.0 -0.9 -4.7 -2.85 - 1.65 0.48
0
505
2.85
0.1 0.25 1 1.5 2 2.6
630 725 770 820
3.4 3.4 2.3 1.6 1.2 0.8
'100
150
117
3d sublattice
ered in analysing the magnetic properties of cobalt in GdCo,_,T, (T = Fe, Ni, Al) [13, 141. The lack of increase of the magnetic moment for the initial concentration of iron can be partially explained by the local environment model [5]. Only the presence of a sufficiently large number of iron atoms as nearest neighbours leads to an increase of the cobalt moment. The transition metal occupies two kinds of sites (2c, 6i) in the CeCo,B-type [16] structure with different types and numbers of neighbouring atoms. A detailed explanation of the mechanism governing the change of the magnetic moment is almost impossible on the basis of purely magnetostatic measurements. Certain information may be derived from Mossbauer effect studies on these GdCo,_,Fe,B compounds and will be discussed in our next paper [ 171. 3.2. Magnetic anisotropy of 3d sublattice
200 Hex(T)
Fig. 2. Variation of the cobalt moment with exchange field at 4.2 K.
I versus the exchange field H,, acting on the Co ion, estimated using the above molecular field parameters. In line with Burzo’s suggestion this curve may be analysed as a type of induced magnetism [l, 111. Initially the magnetic moment of cobalt keeps its value of about lb. At a certain critical value of the exchange field a cobalt magnetic contribution appears (when x > 1). The cobalt moment then increases almost linearly until finally achieving saturation. The proportionality constant V,, determined from ill]
AM,, = V,, AH,, >
The most striking behaviour of these compounds is found for magnetic anisotropy. Figure 3 presents the composition dependence of the anisotropy constant in the GdCo,_,Fe,B series. This anisotropy constant appears to be representative for 3d sublattice magnetic anisotropy on the assumption that the 4f moment of Gd does not give a rare earth contribution to the 6 t
-
-E In
A
41 I
,
I
,’
/’ ,’
(2) -2 -
is equal to -10-‘/1-~/T and is slightly larger than that reported for the Y,_,Gd,Co,B [l] and YCo,_,Fe,B [12] systems. This confirms the marked sensitivity of the cobalt moment to exchange interactions. Summing up, the behaviour of the 3d sublattice in GdCo,_,Fe,B may be interpreted in the model of induced metamagnetism or in the context of collective electron metamagnetism, i.e. models previously consid-
:
1
‘t‘t,
,’
-6 _
,’
P
/I
,’ A’
\\ \
-4 < %_
,’
#I’
,Y’
‘\
,’ *.__’ . ____--I+_____--
_x .’
1
2
/’
, /’
/’
/d’
1 0
3
4
-x Fig. 3. Composition dependence of the anisotropy constant of GdCo,_=Fe,B and LuCo,_,Fe,B. (Data of LuCo_,Fe,B are taken from ref. [3].) K,
118
Z. Drzazga
I Magnetic behaviour of the RCo,_,Fe,B
magnetic anisotropy. At low temperatures the macroscopic constant K, obtained from measurements is close to the microscopic parameter KT derived from the relation taking into account a canting of the sublattice moments due to competition between the energies of inter-sublattice exchange and sublattice anisotropy [ 181. GdCo,B shows a positive anisotropy constant. Only introduction of a small quantity of iron [4] changes the easy direction of magnetization from the c-axis to the base plane. With increasing iron content the first anisotropy constant changes sign at about x = 0.15, then passes through a minimum and again changes sign at about x = 2.5. This composition dependence shows a tendency analogous to that of YCo,_,Fe,B and LuCo,_,Fe,B [3] although the range of the easy base plane is smaller than in Y and Lu compounds. This confirms the fact that K,(x) in RT,B is quite different from that of 15 compounds in which a maximum is observed [19]. The anisotropy of the 3d sublattice is not yet fully understood. However, NMR and Mossbauer experiments on the intermetallic compounds [20-241 indicate that the anisotropy of the 3d sublattice is related to the crystal-field and spin-orbit interaction. NMR results show Co anisotropy contributions differing both in sign and and magnitude from those of the crystallographic nonequivalent sites (c, g or i) of 3d atoms. It was found that 2c sites in YCo, and YCo,B make positive contribution to the magnetic anisotropy whereas the 3g sites in RCo, and 6i in YCo,B give negative contributions. The competition between the negative and positive contributions could be the cause of the anomalous temperature dependence of the anisotropy constant in YCo,B [2] and GdCo,B [18]. Therefore, the model of the individual site anisotropy of 3d ions [25] can be applied to analyse the behaviour of magnetic anisotropy in GdCo,_,Fe,B compounds. The first anisotropy constant can be expressed as follows: K,(x) = K,(O) +
2fzcAK,,,, + 6fi AK,,,, 7
(3)
where K,(O) is the overall anisotropy constant of the cobalt compound, AK,,i = K,, - K,, is the
3d sublattice
difference between the two individual site contributions of iron and cobalt at the given site i, fzc, fei are the occupancy factors of the iron in the 2c and 6i sites in the CeCo,B type structure. Assuming the occupancy factors to be the same as in ThCo,_, Fe, [26] as in YCo,_,Fe,B and LUCO4-,Fe,B, the experimental data for K,(x) could be fitted to eq. (3) giving values of parameters AK,, = -10.4 x lo5 J/m3 and AK,, = 58.6 x lo5 J/m’. These parameters have larger absolute values than in the Y and Lu compounds and are quite different from those of the 1:5 system. It follows that cobalt and iron in RT,B have individual site contributions of the same sign unlike most rare-earth-cobalt-iron compounds which have opposite signs for these contributions [25, 271. Values of the occupancy factors i can also be estimated by the Mossbauer method. Mossbauer studies on the GdCo,_,Fe,B samples show a more complicated distribution of the iron [17]. At low Fe content the dumbbells and d sites appear mostly to be occupied. Then a strong preference to occupy one site is observed. Filling of this site to above 90% occurs for x = 0.25. At large Fe content a binomial distribution of the B atoms must be taken into account. Another point of ambiguity in the interpretation of the spectra is the assignment of the lines [9, 10, 281. It follows from our analysis [17] that Fe atoms prefer to occupy the 6i sites, which is in contradiction to the suggestion by Gross et al. [29] but it confirms the tendency of Fe to occupy 3g sites (corresponding to 6i) as predicted by means neutron diffraction. The opposite sign of values of AK,,, found for 1:5 and 1:4:B systems can be caused by a different electron configuration of 3d ions with a different sign of the corresponding Steven’s factor, or by a different sign of the crystal-field parameter. In most intermetallic compounds the configurations 3d8 for Co and 3d’ (3d6) for Fe with opposite signs of the Steven’s factor are usually taken [25]. Thus iron and cobalt give different individual site contributions to the overall anisotropy. In RCo,_,Fe,B, the magnetic moments of Co as well as Fe are reduced in comparison with RCo,_,Fe,, i.e. 1~~ and 1.5&;
Z. Drzazga
I Magnetic behaviour of the RCo,_,Fe,B
1.7& and 2.2b, respectively). These lower magnetic moments of the 3d ions in RT,B can be related to the configurations 3d9 and 3d8 having the same sign of Steven’s factors. In consequence the same sign for the individual site contributions for cobalt and iron is obtained. The Co contributions to the anisotropy in Y-Co and Gd-Co systems have been calculated within the point charge model [30-321. However, the results depend on the relative magnitude and sign of the point charges used. Particularly, it refers to the boron and 3d sites [33, 34, 21. Using the point charge model for 3d electrons in metallic systems seems to be doubtful. The problem requires the deeper studies.
3.3. Fe influence on magnetic properties in RCo,Fe,B
Table 2 gives the magnetic parameters for RCo,Fe,B with R= Dy, Ho, Er. The mean magnetic moment of the 3d metal in these compounds, estimated from the resultant magnetic moment with the assumption that the rare earth moment is close to the free ion value, approaches the value of 1.5&, as in RFe,B. In Fe-substituted RCo,B compounds with the other rare earths, an increase of the Co moment is also observed [35]. As the magnetic anisotropy in RCo,Fe,B is considered the contributions originating from both the 3d and R sublattice must be taken into account. The rare earth anisotropy is related to crystal-field effects in the intermetallic compounds. The crystal-field with exchange-field
Table 2 Magnetic properties of RCo,Fe,B
compounds.
Compound
DyCo,Fe,B HoCo,Fe,B ErCo,Fe,B ‘) Measured at 20 K
650 590 570
119
3d sublattice
model predicts large magnetocrystalline anisotropy in RT,B [18]. The easy direction of magnetization for a rare earth is determined by the Steven’s factor. The anisotropy of the 3d sublattice is smaller by nearly two orders (table 1 and 2). The negative contribution of 3d anisotropy for x = 2 can be too small to cause a change in the easy magnetization direction in ErCo,Fe,B. There is no evidence of spin reorientation in the studied temperature range. It is interesting to compare the magnitude of magnetic anisotropy in RCo,Fe,B and RCo,B (table 2). The anisotropy constant obtained at 4.2 K is relevant to the rare earth anisotropy because the 3d sublattice anisotropy can be negligible at low temperatures. It should be noticed that the value of the anisotropy constant for RCo,Fe,B compounds are markedly lower than in RCo,B. However, it follows from the analysis performed in ref. [18] that the increase of exchange interaction should not lead to the decrease of the magnetic anisotropy of the rare earth sublattice in RCo,Fe,B on the assumption that iron does not change the crystal-field parameter. Nevertheless, substituting Fe atoms for Co modified the crystal structure. For Fe-rich compounds a binomial interchange of sites between Fe and B was found by means of Mossbauer measurements [28, 171. A disordered replacement of small boron atoms by larger 3d atoms can cause local deformation of the highly symmetrical site of the R ion. In consequence the anisotropy of the rare earth sublattice can be reduced. A strong decrease of magnetic anisotropy due to deformation of the crystal structure was proved for RCO~+~ compounds [36, 371.
The values in parentheses
are relevant to RCo,B [18].
$;
~(4.2 K) (A%)
A3d) (PA
K,(4.2 K) ( lo6 J/m3)
easy axis at3OOK
-285 -196 -158
4.0 4.1 2.4
1.5 1.5 1.65
-.- \ A_.<, -8.0 3 (-13.5)
I c (plane) I L. r ,..L-, A. \~,nUc,
120
Z. Drzazga
I Magnetic
behaviour
4. Summary Introduction of iron into RCo,B compounds markedly influences their magnetic properties. The increase of the 3d sublattice moment and of the Curie temperature is observed. The cobalt moment in the studied compounds is sensitive to the enhancement of the exchange interaction. The 3d sublattice anisotropy shows an anomalous composition dependence which is different from those in most of the intermetallics with rare-earth elements (YCo,_,Fe,, YCo,_,Fe,, Y,Co,,_,Fe,B). In Fe-rich RCo,B compounds the decrease of the rare earth magnetocrystalline anisotropy can be caused partly by a deformation of the crystal structure due to binomial distributions of the boron and 3d metal atoms.
Acknowledgements The author wishes to thank Mgr. A. Winiarska for performing the X-ray measurements. This work is partly supported by the Polish Academy of Science.
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