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A comparison of the magnetic anisotropy of R2Fe14B with R2Fe14C compounds (R= Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu)
130
Journal of Magnetism
and Magnetic
Materials
83 (1990) 130-132 North-Holland
A COMPARISON OF THE MAGNETIC ANISOTROPY OF R,Fe,,B WITH R,Fe,,C COMPOUNDS (R = Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu) R. GROSSINGER
a, R. KREWENKA
‘, X.C. KOU’
and K.H.J.
BUSCHOW
’
0 Inst.1: Experimentalphysik, Techn. (/mu. Vtenna, Austria h Inst. of Metal Rex, Acad. Sinica, Shenyang P.R. China ( Phdrps Research Lab., NL-5600 JA Eindhoven, The Netherlands The magnetic properties of the R,Fe,,B are compared with those of the R*Fe,,C (R = rare earth element). Special emphasis is laid on the anisotropy behaviour. The carbides have a lower c lattice parameter. a lower ordering temperature 7; and a generally higher 3d-anisotropy than the borides. The saturation magnetization is of comparable size.
1. Introduction The magnetic properties of R,Fe,,B compounds were extensively studied because of their importance for a new family of cobalt free magnet materials (see e.g. refs. [1,2]). There exists a corresponding series of carbides which attracted attention recently [3,4]. The fact that in a bulk alloy of the composition Dy*Fe,,C and also R,Fe,,X (X = B,C, _,) substantial coercive forces were observed [5,6], indicated that the carbides might be also candidates for the production of permanent magnets. The magnetic anisotropy of the R,Fe,,C system has not yet been studied systematically. It is therefore the aim of the present paper to investigate the anisotropy field as a function of temperature for the uniaxial R,Fe,,C compounds in order to compare the behaviour with that of the corresponding borides. 2. Experimental The carbide samples were prepared from 99.9% pure starting materials by means of arc melting in purified argon gas. After arc melting the samples were wrapped in Ta-foil, sealed into an evacuated quartz tube and
vacuum annealed for several weeks, first at 900°C and subsequently at 800°C. The preparation of the boride samples was given in ref. [7]. The crystallographic and the magnetic characterisation was given in refs. [3,7,8]. The anisotropy field was determined in a pulsed field system by applying the SPD-technique (Singular Point Detection) as described first in ref. [9]. 3. Results All borides as well as the carbide samples were approximately single phase; the amount of impurity phase was about 5%, the impurity phase being either a-Fe or R,Fe,,. The temperature dependence of the anisotropy fields for the borides was already given in ref. [S]. That of the carbides is given in figs. la and b. The Nd,Fe,,C compound exhibits at low temperatures again a spin reorientation, which is accompanied by a FOMP (First Order Magnetization Process) anomaly as was found for the corresponding Nd,Fe,,B compound. The temperature dependence of H, of the carbide compounds with R = Gd and Lu is very similar to that of the borides as was also shown in ref. [lo]. This indicates a comparable anisotropy behaviour of the 3d-lattice. In-
Table 1 Crystallographic R=
and magnetic Ce
T, (K) a (nm) ( (nm) 0 (Am2/kg) EAM (RT) ~“off,
0)
Pr
parameters
of the systems
R,Fe,,C
Nd
Sm
Cd
Tb
DY
Ho
Er
Tm
530 0.881 1.205 169 co/c 9.5
580 0.880 1.195 154 ?
630 0.879 1.189 91.5
585 0.877 1.186 60.5
555 0.8754 1.183 52.5
525 0.874 1.18 54
510 0.873 1.177 62 pi/c 0.1
500 0.872 1.175 90.9 ? 0.4
7
C
3.53
?
?
Yb
Lu 495 0.8709 1.171 133 C
3.1
Explanatron: a,c: lattice constants according to refs. [4,5]; p,,M\: saturation induction at room temperature [5]; 4: saturation magnetization at 4.2 K [5]; EAM (RT): easy axis of magnetization at room temperature; c: c-axis; pl: easy plane; co: cows; ?: not yet clear; co/c: spin reorientation from co (low temp.) to c (high temp.); p,,HA: anisotropy field at room temperature. 0304-8853/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
R. Grijssinger et al. / Anisotropy *Ha
of R I Fe,, B and R 2 Fe,,C
131
compounds
,.Ha (T)
(Tl
L_ b
80
260
440
T iKi
0 620
80
260
R,Fe,, C; R=Gd temperature dependence of the anisotropy field of the samples R,Fe,,C; R = ‘I%(X), Dy (+). the temperature where a FOMP was observed. Fig. 1. (a) Temperature
dependence
of the anisotropy
field of the samples
teresting are also the Er- and Tm compounds. Er,Fe,,B and Tm,Fe,,B show a spin reorientation from the plane to the c-axis occurring at 325 and 315 K, respectively. In Er,Fe,,C a similar spin reorientation was observed at 308 K. Only above 308 K HA measurements using the SPD-technique were possible in this compound. In the case of Tm,Fe,,C, H,(T) shows at T’ = 312 K a drastic change of the slope. This might be explained assuming that the easy axis of magnetization of Tm,Fe,,C lies in the easy plane below T’ and in the c-axis above T’. The singularity observed below T’ is therefore attributed to a uniaxial Tm,Fe,,C,. The existence of a 2/17 impurity phase in the sample was proved by X-ray diffraction also. The H,(T) curves of the compounds R,Fe,,C with the heavy rare earths R = Tb, Dy, Ho were detectable above room temperature only due to their fast increase with decreasing temperature. In table 1 the main crystallographic and magnetic parameters of the borides and of the carbides are surveyed.
440
T ii\1
620
(0). Lu (A). Er (x). Tm (+); (b) Ho (A). Nd (0). The arrow indicates
iv) The anisotropy fields of the carbides are always larger. The reduction of the magnetic moments could be a possible explanation for the decrease of T, in the carbides. More basically the key to understand the differences between R,Fe,,B and R,Fe,,C might be the decrease of the lattice constant in the c direction. Pressure experiments performed on Nd,Fe,,B gave a negative value dT,/dp of (- 26.5 k 1) K/GPa [ll]. The reduction of c in the carbides should therefore cause a decrease of T,. According to an explanation given by ref. [lo] the second order anisotropy constant of the 3d-sublattice (K,) should be proportional to the factor M,’ (1 - a(c/a)) 2, where a is a constant which is for this structure close to 0.52. Regarding table 1 the reduction of the c/a ratio is evident for all carbides. Based on these data one finds that the factors for Gd,Fe,,B and Gd,Fe,,C are 123 and 77, respectively. The difference is more than 40% which is a little too large. However, it points into the right direction. i.e. it looks as if the reduction of the c parameter causes the increase of the magnetic anisotropy of the 3d sublattice.
4. Discussion Looking on the data as given in table 1 and comparing them with that of the corresponding R,Fe,,B compound (see refs. _ 17.81) the following trends are visible: 9 The carbides have always a lower Curie temperature compared to the borides. ii) The a lattice parameter is always nearly the same: however, the c parameter is always smaller for the carbides than for the borides. iii) The saturation moments at 4.2 K are similar, although, according to ref. [lo] they are smaller in the carbides than in the borides.
References Croat, J.F. Herbst. R.W. Lee and F.E. Pinkerton. J. Appl. Phys. 55 (1984) 2078. M. Sagawa. S. Fujimura. N. Togawa. H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. F.R. de Boer. Huang Ying-kai. Zhang Zhi-dong. D.B. de Mooij and K.H.J. Buschow. J. Magn. Magn. Mat. 72 (1988) 167. A.T. Pedziwiatr, W.E. Wallace and E. Burzo, J. Magn. Magn. Mat. 59 (1986) L179. N.C. Liu and H.H. Stadelmaier. Mater. Lett. 4 (1986) 377.
[II J.J. VI [31
[41 [51
132
R. Grikinger
et al. / Anisotropy
[6] N.C. Liu. H.H. Stadelmaier and G. Schneider, J. Appl. Phys. 61 (1987) 3574. [7] S. Sinnema, R.J. Radwanski, J.J.M. Franse, D.B. de Mooij and K.H.J. Buschow, J. Magn. Magn. Mat. 44 (1984) 333. [8] R. G&singer, X.K. Sun, R. Eibler, K.H.J. Buschow and H.R. Kirchmayr, J. de Phys. 46 (1985) C6-221.
of R _,Fe,, B and R 2 Fe,,C
compounds
[9] G. Asti and S. Rinaldi, J. Appl. Phys. 45 (1974) 3600. [IO] X.C. Kou, X.K. Sun, Y.C. Chuang, T.S. Zhao. R. GrGssinger and H.R. Kirchmayr. to be published. [ll] J. Kamarad, Z. Arnhold and J. Schneider, J. Magn. Magn. Mat. 67 (1987) 29.
Journal of Magnetism
and Magnetic
Materials
83 (1990) 130-132 North-Holland
A COMPARISON OF THE MAGNETIC ANISOTROPY OF R,Fe,,B WITH R,Fe,,C COMPOUNDS (R = Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu) R. GROSSINGER
a, R. KREWENKA
‘, X.C. KOU’
and K.H.J.
BUSCHOW
’
0 Inst.1: Experimentalphysik, Techn. (/mu. Vtenna, Austria h Inst. of Metal Rex, Acad. Sinica, Shenyang P.R. China ( Phdrps Research Lab., NL-5600 JA Eindhoven, The Netherlands The magnetic properties of the R,Fe,,B are compared with those of the R*Fe,,C (R = rare earth element). Special emphasis is laid on the anisotropy behaviour. The carbides have a lower c lattice parameter. a lower ordering temperature 7; and a generally higher 3d-anisotropy than the borides. The saturation magnetization is of comparable size.
1. Introduction The magnetic properties of R,Fe,,B compounds were extensively studied because of their importance for a new family of cobalt free magnet materials (see e.g. refs. [1,2]). There exists a corresponding series of carbides which attracted attention recently [3,4]. The fact that in a bulk alloy of the composition Dy*Fe,,C and also R,Fe,,X (X = B,C, _,) substantial coercive forces were observed [5,6], indicated that the carbides might be also candidates for the production of permanent magnets. The magnetic anisotropy of the R,Fe,,C system has not yet been studied systematically. It is therefore the aim of the present paper to investigate the anisotropy field as a function of temperature for the uniaxial R,Fe,,C compounds in order to compare the behaviour with that of the corresponding borides. 2. Experimental The carbide samples were prepared from 99.9% pure starting materials by means of arc melting in purified argon gas. After arc melting the samples were wrapped in Ta-foil, sealed into an evacuated quartz tube and
vacuum annealed for several weeks, first at 900°C and subsequently at 800°C. The preparation of the boride samples was given in ref. [7]. The crystallographic and the magnetic characterisation was given in refs. [3,7,8]. The anisotropy field was determined in a pulsed field system by applying the SPD-technique (Singular Point Detection) as described first in ref. [9]. 3. Results All borides as well as the carbide samples were approximately single phase; the amount of impurity phase was about 5%, the impurity phase being either a-Fe or R,Fe,,. The temperature dependence of the anisotropy fields for the borides was already given in ref. [S]. That of the carbides is given in figs. la and b. The Nd,Fe,,C compound exhibits at low temperatures again a spin reorientation, which is accompanied by a FOMP (First Order Magnetization Process) anomaly as was found for the corresponding Nd,Fe,,B compound. The temperature dependence of H, of the carbide compounds with R = Gd and Lu is very similar to that of the borides as was also shown in ref. [lo]. This indicates a comparable anisotropy behaviour of the 3d-lattice. In-
Table 1 Crystallographic R=
and magnetic Ce
T, (K) a (nm) ( (nm) 0 (Am2/kg) EAM (RT) ~“off,
0)
Pr
parameters
of the systems
R,Fe,,C
Nd
Sm
Cd
Tb
DY
Ho
Er
Tm
530 0.881 1.205 169 co/c 9.5
580 0.880 1.195 154 ?
630 0.879 1.189 91.5
585 0.877 1.186 60.5
555 0.8754 1.183 52.5
525 0.874 1.18 54
510 0.873 1.177 62 pi/c 0.1
500 0.872 1.175 90.9 ? 0.4
7
C
3.53
?
?
Yb
Lu 495 0.8709 1.171 133 C
3.1
Explanatron: a,c: lattice constants according to refs. [4,5]; p,,M\: saturation induction at room temperature [5]; 4: saturation magnetization at 4.2 K [5]; EAM (RT): easy axis of magnetization at room temperature; c: c-axis; pl: easy plane; co: cows; ?: not yet clear; co/c: spin reorientation from co (low temp.) to c (high temp.); p,,HA: anisotropy field at room temperature. 0304-8853/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
R. Grijssinger et al. / Anisotropy *Ha
of R I Fe,, B and R 2 Fe,,C
131
compounds
,.Ha (T)
(Tl
L_ b
80
260
440
T iKi
0 620
80
260
R,Fe,, C; R=Gd temperature dependence of the anisotropy field of the samples R,Fe,,C; R = ‘I%(X), Dy (+). the temperature where a FOMP was observed. Fig. 1. (a) Temperature
dependence
of the anisotropy
field of the samples
teresting are also the Er- and Tm compounds. Er,Fe,,B and Tm,Fe,,B show a spin reorientation from the plane to the c-axis occurring at 325 and 315 K, respectively. In Er,Fe,,C a similar spin reorientation was observed at 308 K. Only above 308 K HA measurements using the SPD-technique were possible in this compound. In the case of Tm,Fe,,C, H,(T) shows at T’ = 312 K a drastic change of the slope. This might be explained assuming that the easy axis of magnetization of Tm,Fe,,C lies in the easy plane below T’ and in the c-axis above T’. The singularity observed below T’ is therefore attributed to a uniaxial Tm,Fe,,C,. The existence of a 2/17 impurity phase in the sample was proved by X-ray diffraction also. The H,(T) curves of the compounds R,Fe,,C with the heavy rare earths R = Tb, Dy, Ho were detectable above room temperature only due to their fast increase with decreasing temperature. In table 1 the main crystallographic and magnetic parameters of the borides and of the carbides are surveyed.
440
T ii\1
620
(0). Lu (A). Er (x). Tm (+); (b) Ho (A). Nd (0). The arrow indicates
iv) The anisotropy fields of the carbides are always larger. The reduction of the magnetic moments could be a possible explanation for the decrease of T, in the carbides. More basically the key to understand the differences between R,Fe,,B and R,Fe,,C might be the decrease of the lattice constant in the c direction. Pressure experiments performed on Nd,Fe,,B gave a negative value dT,/dp of (- 26.5 k 1) K/GPa [ll]. The reduction of c in the carbides should therefore cause a decrease of T,. According to an explanation given by ref. [lo] the second order anisotropy constant of the 3d-sublattice (K,) should be proportional to the factor M,’ (1 - a(c/a)) 2, where a is a constant which is for this structure close to 0.52. Regarding table 1 the reduction of the c/a ratio is evident for all carbides. Based on these data one finds that the factors for Gd,Fe,,B and Gd,Fe,,C are 123 and 77, respectively. The difference is more than 40% which is a little too large. However, it points into the right direction. i.e. it looks as if the reduction of the c parameter causes the increase of the magnetic anisotropy of the 3d sublattice.
4. Discussion Looking on the data as given in table 1 and comparing them with that of the corresponding R,Fe,,B compound (see refs. _ 17.81) the following trends are visible: 9 The carbides have always a lower Curie temperature compared to the borides. ii) The a lattice parameter is always nearly the same: however, the c parameter is always smaller for the carbides than for the borides. iii) The saturation moments at 4.2 K are similar, although, according to ref. [lo] they are smaller in the carbides than in the borides.
References Croat, J.F. Herbst. R.W. Lee and F.E. Pinkerton. J. Appl. Phys. 55 (1984) 2078. M. Sagawa. S. Fujimura. N. Togawa. H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. F.R. de Boer. Huang Ying-kai. Zhang Zhi-dong. D.B. de Mooij and K.H.J. Buschow. J. Magn. Magn. Mat. 72 (1988) 167. A.T. Pedziwiatr, W.E. Wallace and E. Burzo, J. Magn. Magn. Mat. 59 (1986) L179. N.C. Liu and H.H. Stadelmaier. Mater. Lett. 4 (1986) 377.
[II J.J. VI [31
[41 [51
132
R. Grikinger
et al. / Anisotropy
[6] N.C. Liu. H.H. Stadelmaier and G. Schneider, J. Appl. Phys. 61 (1987) 3574. [7] S. Sinnema, R.J. Radwanski, J.J.M. Franse, D.B. de Mooij and K.H.J. Buschow, J. Magn. Magn. Mat. 44 (1984) 333. [8] R. G&singer, X.K. Sun, R. Eibler, K.H.J. Buschow and H.R. Kirchmayr, J. de Phys. 46 (1985) C6-221.
of R _,Fe,, B and R 2 Fe,,C
compounds
[9] G. Asti and S. Rinaldi, J. Appl. Phys. 45 (1974) 3600. [IO] X.C. Kou, X.K. Sun, Y.C. Chuang, T.S. Zhao. R. GrGssinger and H.R. Kirchmayr. to be published. [ll] J. Kamarad, Z. Arnhold and J. Schneider, J. Magn. Magn. Mat. 67 (1987) 29.