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Magnetocrystalline anisotropy of Ni and Mn substituted Nd2Fe14B compounds
Journal of Magnetism and Magnetic Materials 67 (1987) 373-377 North-Holland, Amsterdam
373
M A G N E T O C R Y S T A L L I N E A N I S O T R O P Y OF Ni AND Mn S U B S T I T U T E D Nd2Fel4B COMPOUNDS F. BOLZONI, F. LECCABUE, O. MOZE, L. PARET! and M. SOLZI Istituto MASPEC, via Chiavari 18/a, 43100 Parma, Italy Received 3 February 1987
Measurements of the effects of Ni and Mn substitutions for Fe on the magnetic properties of Nd2Fe14B compounds are reported. The Curie temperature is slightly increased with Ni substitution whilst in the case of replacement of Fe by Mn it is reduced drastically. A monotonic decrease of both the lattice parameters a and c is observed. The saturation magnetization is decreased by both Ni and especially Mn substitutions. The composition dependence of both the reorientation spin transition temperature and the cone angle has been measured. The influence of the 3d metal substitution on the Nd anisotropy has been measured and discussed. The composition dependence of the room temperature anisotropy field values, which is an important figure of merit for permanent magnet applications, decreases slightly in the case of Ni and drastically for Mn substitution. A comparison with the case of Co substitution has been made.
1. Introduction The series of Rare-earth-Transition metal compounds based on the Nd2Fe14B tetragonal structure have recently attracted considerable interest because of their potential as permanent magnets [1,2]. The rare-earth ion is the main source of the magnetic anisotropy. The rare-earth anisotropy which is single ion in origin is determined by the symmetry of the crystal sites where the rare-earth ions are located. Such a symmetry defines the crystalline electric field at the rare-earth ion site and in turn the kind and intensity of the magnetic anisotropy [3]. However the anisotropy of the iron sublattice, the origin of which is less well understood, gives rise to a non-negligible contribution to the overall anisotropy [4,5]. The replacement of Fe with other 3d elements in Nd2Fe~nB, even if the tetragonal structure is maintained, is expected to produce a complex modification of the anisotropy of the system. Two distinct contributions of Co to the anisotropy were indeed evidenced when Fe is substituted by Co in both Pr2FelaB and Nd2Fe14B [6,7]. A direct contribution arises from the different anisotropy properties of the two 3d ions and an indirect one from a modification of
the Nd anisotropy which is possibly due to an alteration of the crystal field experienced by Nd ions [7]. In an attempt to study the influence of Mn and Ni substitutions for Fe on the overall anisotropy of NdEFe14B, a systematic study of the magnetic properties of the compounds Nd 2 Fe14_xTMx B with TM = Mn, Ni and x = 1, 2 and 3 has been performed. In addition a comparison between the anisotropies of both Mn and Ni substituted Y and Nd2Fe14B compounds was carried out. The data referring to the Co substitution [7] are also reported for comparison.
2. Experimental details Polycrystalline samples of nominal composition Nd2Fe14_xTMxB with T M = Mn, Ni and x = 1, 2 and 3 were prepared by melting ultrapure elements in an arc furnace in a purified Ar atmosphere. The samples were subsequently homogenised at 950 ° C in sealed quartz tubes under Ar atmosphere for 7 days. Spherical samples were cut from the buttons. Thermomagnetic analysis (TMA) was used to verify whether the samples were single phase or not and to measure their Curie temperatures. X-ray
0304-8853/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
374
[i Bolzoni et al. / Ni and Mn substituted Nd2Fe/4B compounds
diffraction was used to check the existence of the tetragonal structure and to measure the variation of the lattice parameters with Mn and Ni content. Vibrating sample and pulsed field magnetometry was used in order to measure the saturation magnetization. The unique singular point detection method (SPD) [8] was used to measure the anisotropy field. Powdered samples oriented in a 19 kOe magnetic field and fixed in wax were used for measurements of the initial dc susceptibility perpendicular to the c-axis. This enabled a determination to be made of the easy-axis to easycone spin reorientation transition temperature. The magnitude of the cone angle was obtained by making a polar plot of the magnetization at a fixed field value in a vibrating sample magnetometer.
o~ 12,3 .~.o__ Ni ~ _ M n 12 9
i
8,5
i
i
i
L
i
i
i
=
950 ~ Ni ~ n 930 ~ ~
t
91o t 1 . 3 9 ~
0
2 40 X
1
2 4 X
Fig. 2. Composition dependence of the lattice parameters a and c, unit cell volume V~ and c/a ratio in NdzFel4_~Me, B (C)) and Y2Fe14 xMexB (O) for Me = Ni and Mn.
3. Results and discussion X-ray diffraction measurements verified that the tetragonal 2 : 1 4 : 1 phase was retained for all the samples under examination. In all the samples a small trace of a-Fe ( < 2%) was detected by T M A whilst in addition in the case of Ni compounds with x = 2 and 3 some amount of 2 : 1 7 phase ( = 2%) was also observed. The composition dependence of the Curie tern-
perature for both Ni and Mn substituted Nd 2Fe]4 B compounds compared with those of Co substituted compounds is displayed in fig. 1. It can be seen that Ni substitution gives rise to a slight increase in Tc whilst there is a strong linear decrease in the case of Mn substitution. It is clear that beyond the substitution of 25% of Fe atoms by Mn atoms the system is no longer magnetic at room temperature.
500
200
400
o~ 175
%
E
300 (..) 0% ~¢a 200
~:~ 150
\ 125
100 0
0
I
I
1
2
I
X
3
Fig. l. Composition dependence of the Curie temperature T~ in Nd2Fe14_~,Me~ B for Me = Ni (O), Mn (A) and Co (11).
100
0
, 1
, 2
X
3
Fig. 3. Composition dependence of the saturation magnetization o~ at 77 K in NdzFe14 xMexB for Me = Ni (O), Mn (A) and Co (i).
F. Bolzoni et al. / Ni and Mn substituted Nd2FeI4B compounds
The composition dependence of the lattice parameters unit cell volume and c/a ratio for Ni and Mn substituted NdzFe]4B compounds is shown in fig. 2. The unit cell parameters can be seen to decrease linearly. For comparison, the same data for Ni and Mn substituted YzFe]aB compounds is also displayed in fig. 2 [11]. In the case of Mn substituted Nd and Y compounds there is an increase in the c/a ratio. The opposite is observed in Co substituted compounds [7]. An intermediate situation is observed with Ni substitution where the c/a ratio increases for Y compounds and decreases for those of Nd. This could possibly be related to different preferential site occupation of different 3d atoms in the three
375
Z E
U
60
P tO O
,~, 40
20
0
0
I
t
I
1
2
3
X
cases.
Fig. 5. Composition variation of the total (11) and N d (o) anisotropy in Nd2Fe14_xNixB at T / T c = 0.45.
Similar behaviour to that of the Mn substituted Y2Fe14B compounds has also been observed in the case of Er2Fe14_xMnxB compounds [9] for x > 3. The composition dependence of the saturation magnetization os at 77 K for Ni, Mn and Co compounds is shown in fig. 3. It can be seen that Ni substitution gives rise to a substantial decrease in o~. Such a decrease is more marked in the case of Mn, particularly for x > 1. The temperature dependence of the anisotropy field H a for Ni and Mn compounds was measured by the SPD technique. At low temperatures the SPD measurements revealed the presence of a first-order magnetization process (FOMP) [10].
Due to the presence of the FOMP and in order to take into account the effects of the Curie temperature variation, the composition dependence of the anisotropy field for both Ni and Mn compounds is reported in fig. 4 at a constant T/T~ ratio of 0.45 (since for this ratio there is no FOMP and a normal approach to saturation magnetization is observed). It can be seen that H a decreases linearly in the case of Ni substitution whilst for Mn an increase is observed. Taking into account the fact that in the case of both Ni and Mn compounds the saturation magnetization os decreases
150
60 100
J
P
J
-
-
•
tO
oT.-
0 -!-m
50
4O
20
0
0
[
I
l
1
2
3
X Fig. 4. Composition variation of the anisotropy field H a in Nd2Fe14_xMexB for Me = Ni (O) and M n (A) at T I T c = 0.45.
0
_
0
_
I
I
I
1
2
3
X
Fig. 6. Composition variation of the total (11) and N d (e) anisotropy in N d 2 F e a 4 _ x M n x B at T/T~ = 0.45.
376
F. Bolzoni et al. / Ni and Mn substituted Nd2Fej4B compounds
with increasing composition a clearer idea of the behaviour of the anisotropy can be obtained by calculating the anisotropy constants Kj + 2 K 2 + 3K 3 = HaMJ 2 . Furthermore a comparison of this quantity for Y compounds (metal sublattice anisotropy [11]), makes it possible to separate the effects of Ni and Mn substitution on the rare-earth ion anisotropy. The composition dependence of both total and Nd anisotropy constants K 1 + 2 K 2 + 3K 3 are reported in figs. 5 and 6 for Ni and Mn substitution, respectively, at T / T c = 0.45. The Nd anisotropy does not appear to be affected by Mn substitution. It can clearly seen that the increase of H a observed with increasing Mn content is simply due to the decrease of os. In fact the total anisotropy is linearly reduced upon addition of Mn. In contrast the N d anisotropy in Ni substituted compounds appears to be markedly affected. The linear reduction in the overall anisotropy can be almost completely ascribed to the decrease in the N d anisotropy. Similar behaviour has been observed in Co substituted compounds. (On the other hand, as observed in the case of Co compounds [7], a larger effect of the metal sublattice on the Nd anisotropy should be expected at high substitutions.) The composition dependence of the anisotropy
75
\
o
\
\
so
\,
A
25
0
I
0
1
m
J _ _ _ _
2
I
3 X
Fig. 7. Variation of the anisotropy field H~ at 293 K with Ni (e), Mn (J0 and Co (E) content in Nd2Fe14_xMe~B compounds.
125
20
100 ~ . v.
~- 15
75
k,-
X
10
50
5® 0 0
® 2..
t __lJ
1
2
3 x
25
___.l
0
1
2~
1
2
3
X
0
Fig. 8. (a) Composition variation of the cone angle 0 k at 77 K in Nd2Fel4 xMe~B for Me = Ni (e), Mn (A) and Co (ll); (b) composition variation of the spin reorientation transition temperature T~r in N d 2 F e 1 4 _ , M e x B for Me = Ni (e), Mn (A) and
Co (m). field at 293 K (fig. 7) gives some indication of the usefulness of Ni and Mn substitutions as compared with Co for permanent magnet applications. A slight decrease of H a is observed in the case of Ni substitution whilst the decrease in H a is dramatic in the case of Mn compounds. In the latter case the decrease has to be completely attributed to the reduction of T~ with Mn substitution. It is well known that Nd2Fe14B undergoes a spin-reorientation transition from easy-axis to easy-cone at 135 K. The temperature at which the transition takes place is decreased with all the substitutions and the rate of decrease is higher in going from Co, Ni to Mn substitution (fig. 8b). The value of the cone angle 0 c at 77 K is reduced in the same manner (fig. 8a). Furthermore the temperature at which the F O M P transition commences is decreased in all cases. Some disagreement between the present data and work by other authors on Mn compounds regarding the values of the Curie temperature and the spin-reorientation transition temperature [12,13] are most likely due to fluctuations in composition which are possible in Mn compounds. This is related to the very high vapour pressure of Mn at the melting point. The same problem has also arisen in the case of Mn substituted Y compounds [11].
F. Bolzoni et al. / Ni and Mn substituted N d eFe I 4 B compounds
If some kind of competition between different anisotropy contributions can be invoked as a mechanism for the observed FOMP and spin reorientation transition, the present results clearly indicate that the substitution of Fe with Co, Ni and Mn markedly reduces the strength of such a mechanism. It should also be noted that in contrast to Co and Ni the dramatic effects of Mn on the spin-reorientation transition and the cone angle must be mostly attributed to the weakening of the magnetic interactions which is evidenced by a strong decrease in Tc (at a constant T / T c ratio the Nd anisotropy is not affected by Mn substitution (fig. 6)). This indicates the presence of a different preferential substitution in the case of Mn compounds. Indeed recent neutron powder diffraction measurements on Mn substituted Er2FelnB compounds indicate that there is a site preference for Mn atoms at the J2 crystallographic sites [14] whilst in the case of Co substituted compounds there is a site preference for the Fe atoms at J2 sites [15]. Taking into account the effects of the temperature we conclude that the resultant anisotropy of Nd2Fe14 B is reduced by Ni and Co substitution whilst it is not affected by Mn substitution in the composition range under consideration here. On the other hand, due to the different variation of T~ with Co, Ni and Mn substitution the room temperature values of the anisotropy field is slightly affected by Co and Ni and drastically reduced by Mn substitution.
4. Conclusion Measurements of the effects of Ni and Mn substitution on the magnetic properties of Nd 2 Fe14B reveal that the Curie temperature is slightly increased with Ni substitution whilst in the case of Mn it is drastically reduced; the replacement of 25% of Fe by Mn results in a non ferromagnetic order at room temperature. The saturation magne-
377
tization is decreased in both cases, the reduction being dramatic for Mn substitution. The effects of Ni on the Nd anisotropy are such that it acts to decrease it whilst Mn appears not to affect the Nd anisotropy. This further indicates the presence of a preferential substitution which is different for Mn, Ni and Co compounds.
Acknowledgement The authors acknowledge the support of the Commission of the European Communities within the CEAM Project of the Stimulation Programme.
References [1] J.F. Herbst, J.J. Croat, F.E. Pinkerton and W.B. Yelon, Phys. Rev. B 29 (1984) 4176. [2] D. Givord, H.S. Li and J.M. Moreau, Solid State Commun. 50 (1984) 497. [3] D. Givord, H.S. Li and R. Perrier de la Bathie, Solid State Commun. 51 (1984) 857. [4] S. Hirosawa and M. Sagawa, Solid State Commun. 54 (1985) 335. [5] F. Bolzoni, F. Leccabue, L. Pareti, J.L. Sanchez and A. Deriu, J. Magn. Magn. Mat. 54-57 (1986) 595. [6] F. Bolzoni, J.M.D. Coey, J. Gavigan, D. Givord, O. Moze, L. Pareti and T. Viadieu, J. Magn. Magn. Mat. 65 (1987) 123. [7] F. Bolzoni, F. Leccabue, O. Moze, L. Pareti, M. Solzi and A. Deriu. J. Appl. Phys. 61 (June 1987) in press. [8] G. Asti and S. Rinaldi, J. Appl. Phys. 45 (1974) 3600. [9] G.P. Meisner and C.D. Fuerst, IEEE Trans. Magn. MAG22 (1986) 744. [10] L. Pareti, F. Bolzoni and O. Moze, Phys. Rev. B 11 (1985) 7604. [11] L. Pareti, M. Solzi, F. Bolzoni, O. Moze and R. Panizzieri, Solid State Commun. 61 (1987) 761. [12] M. Jurczyk and W.E. Wallace, IEEE Trans. Magn. MAG22 (1986) 755. [13] M.Q. Huang, E.B. Boltich, W.E. Wallace and E. Oswald, J. Less-Common Metals 124 (1986) 55. [14] C.D. Fuerst, G.P. Meisner, F.E. Pinkerton and W.B. Yelon, private communication (1986). [15] J. Herbst and W.B. Yelon, private communication (1986).
373
M A G N E T O C R Y S T A L L I N E A N I S O T R O P Y OF Ni AND Mn S U B S T I T U T E D Nd2Fel4B COMPOUNDS F. BOLZONI, F. LECCABUE, O. MOZE, L. PARET! and M. SOLZI Istituto MASPEC, via Chiavari 18/a, 43100 Parma, Italy Received 3 February 1987
Measurements of the effects of Ni and Mn substitutions for Fe on the magnetic properties of Nd2Fe14B compounds are reported. The Curie temperature is slightly increased with Ni substitution whilst in the case of replacement of Fe by Mn it is reduced drastically. A monotonic decrease of both the lattice parameters a and c is observed. The saturation magnetization is decreased by both Ni and especially Mn substitutions. The composition dependence of both the reorientation spin transition temperature and the cone angle has been measured. The influence of the 3d metal substitution on the Nd anisotropy has been measured and discussed. The composition dependence of the room temperature anisotropy field values, which is an important figure of merit for permanent magnet applications, decreases slightly in the case of Ni and drastically for Mn substitution. A comparison with the case of Co substitution has been made.
1. Introduction The series of Rare-earth-Transition metal compounds based on the Nd2Fe14B tetragonal structure have recently attracted considerable interest because of their potential as permanent magnets [1,2]. The rare-earth ion is the main source of the magnetic anisotropy. The rare-earth anisotropy which is single ion in origin is determined by the symmetry of the crystal sites where the rare-earth ions are located. Such a symmetry defines the crystalline electric field at the rare-earth ion site and in turn the kind and intensity of the magnetic anisotropy [3]. However the anisotropy of the iron sublattice, the origin of which is less well understood, gives rise to a non-negligible contribution to the overall anisotropy [4,5]. The replacement of Fe with other 3d elements in Nd2Fe~nB, even if the tetragonal structure is maintained, is expected to produce a complex modification of the anisotropy of the system. Two distinct contributions of Co to the anisotropy were indeed evidenced when Fe is substituted by Co in both Pr2FelaB and Nd2Fe14B [6,7]. A direct contribution arises from the different anisotropy properties of the two 3d ions and an indirect one from a modification of
the Nd anisotropy which is possibly due to an alteration of the crystal field experienced by Nd ions [7]. In an attempt to study the influence of Mn and Ni substitutions for Fe on the overall anisotropy of NdEFe14B, a systematic study of the magnetic properties of the compounds Nd 2 Fe14_xTMx B with TM = Mn, Ni and x = 1, 2 and 3 has been performed. In addition a comparison between the anisotropies of both Mn and Ni substituted Y and Nd2Fe14B compounds was carried out. The data referring to the Co substitution [7] are also reported for comparison.
2. Experimental details Polycrystalline samples of nominal composition Nd2Fe14_xTMxB with T M = Mn, Ni and x = 1, 2 and 3 were prepared by melting ultrapure elements in an arc furnace in a purified Ar atmosphere. The samples were subsequently homogenised at 950 ° C in sealed quartz tubes under Ar atmosphere for 7 days. Spherical samples were cut from the buttons. Thermomagnetic analysis (TMA) was used to verify whether the samples were single phase or not and to measure their Curie temperatures. X-ray
0304-8853/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
374
[i Bolzoni et al. / Ni and Mn substituted Nd2Fe/4B compounds
diffraction was used to check the existence of the tetragonal structure and to measure the variation of the lattice parameters with Mn and Ni content. Vibrating sample and pulsed field magnetometry was used in order to measure the saturation magnetization. The unique singular point detection method (SPD) [8] was used to measure the anisotropy field. Powdered samples oriented in a 19 kOe magnetic field and fixed in wax were used for measurements of the initial dc susceptibility perpendicular to the c-axis. This enabled a determination to be made of the easy-axis to easycone spin reorientation transition temperature. The magnitude of the cone angle was obtained by making a polar plot of the magnetization at a fixed field value in a vibrating sample magnetometer.
o~ 12,3 .~.o__ Ni ~ _ M n 12 9
i
8,5
i
i
i
L
i
i
i
=
950 ~ Ni ~ n 930 ~ ~
t
91o t 1 . 3 9 ~
0
2 40 X
1
2 4 X
Fig. 2. Composition dependence of the lattice parameters a and c, unit cell volume V~ and c/a ratio in NdzFel4_~Me, B (C)) and Y2Fe14 xMexB (O) for Me = Ni and Mn.
3. Results and discussion X-ray diffraction measurements verified that the tetragonal 2 : 1 4 : 1 phase was retained for all the samples under examination. In all the samples a small trace of a-Fe ( < 2%) was detected by T M A whilst in addition in the case of Ni compounds with x = 2 and 3 some amount of 2 : 1 7 phase ( = 2%) was also observed. The composition dependence of the Curie tern-
perature for both Ni and Mn substituted Nd 2Fe]4 B compounds compared with those of Co substituted compounds is displayed in fig. 1. It can be seen that Ni substitution gives rise to a slight increase in Tc whilst there is a strong linear decrease in the case of Mn substitution. It is clear that beyond the substitution of 25% of Fe atoms by Mn atoms the system is no longer magnetic at room temperature.
500
200
400
o~ 175
%
E
300 (..) 0% ~¢a 200
~:~ 150
\ 125
100 0
0
I
I
1
2
I
X
3
Fig. l. Composition dependence of the Curie temperature T~ in Nd2Fe14_~,Me~ B for Me = Ni (O), Mn (A) and Co (11).
100
0
, 1
, 2
X
3
Fig. 3. Composition dependence of the saturation magnetization o~ at 77 K in NdzFe14 xMexB for Me = Ni (O), Mn (A) and Co (i).
F. Bolzoni et al. / Ni and Mn substituted Nd2FeI4B compounds
The composition dependence of the lattice parameters unit cell volume and c/a ratio for Ni and Mn substituted NdzFe]4B compounds is shown in fig. 2. The unit cell parameters can be seen to decrease linearly. For comparison, the same data for Ni and Mn substituted YzFe]aB compounds is also displayed in fig. 2 [11]. In the case of Mn substituted Nd and Y compounds there is an increase in the c/a ratio. The opposite is observed in Co substituted compounds [7]. An intermediate situation is observed with Ni substitution where the c/a ratio increases for Y compounds and decreases for those of Nd. This could possibly be related to different preferential site occupation of different 3d atoms in the three
375
Z E
U
60
P tO O
,~, 40
20
0
0
I
t
I
1
2
3
X
cases.
Fig. 5. Composition variation of the total (11) and N d (o) anisotropy in Nd2Fe14_xNixB at T / T c = 0.45.
Similar behaviour to that of the Mn substituted Y2Fe14B compounds has also been observed in the case of Er2Fe14_xMnxB compounds [9] for x > 3. The composition dependence of the saturation magnetization os at 77 K for Ni, Mn and Co compounds is shown in fig. 3. It can be seen that Ni substitution gives rise to a substantial decrease in o~. Such a decrease is more marked in the case of Mn, particularly for x > 1. The temperature dependence of the anisotropy field H a for Ni and Mn compounds was measured by the SPD technique. At low temperatures the SPD measurements revealed the presence of a first-order magnetization process (FOMP) [10].
Due to the presence of the FOMP and in order to take into account the effects of the Curie temperature variation, the composition dependence of the anisotropy field for both Ni and Mn compounds is reported in fig. 4 at a constant T/T~ ratio of 0.45 (since for this ratio there is no FOMP and a normal approach to saturation magnetization is observed). It can be seen that H a decreases linearly in the case of Ni substitution whilst for Mn an increase is observed. Taking into account the fact that in the case of both Ni and Mn compounds the saturation magnetization os decreases
150
60 100
J
P
J
-
-
•
tO
oT.-
0 -!-m
50
4O
20
0
0
[
I
l
1
2
3
X Fig. 4. Composition variation of the anisotropy field H a in Nd2Fe14_xMexB for Me = Ni (O) and M n (A) at T I T c = 0.45.
0
_
0
_
I
I
I
1
2
3
X
Fig. 6. Composition variation of the total (11) and N d (e) anisotropy in N d 2 F e a 4 _ x M n x B at T/T~ = 0.45.
376
F. Bolzoni et al. / Ni and Mn substituted Nd2Fej4B compounds
with increasing composition a clearer idea of the behaviour of the anisotropy can be obtained by calculating the anisotropy constants Kj + 2 K 2 + 3K 3 = HaMJ 2 . Furthermore a comparison of this quantity for Y compounds (metal sublattice anisotropy [11]), makes it possible to separate the effects of Ni and Mn substitution on the rare-earth ion anisotropy. The composition dependence of both total and Nd anisotropy constants K 1 + 2 K 2 + 3K 3 are reported in figs. 5 and 6 for Ni and Mn substitution, respectively, at T / T c = 0.45. The Nd anisotropy does not appear to be affected by Mn substitution. It can clearly seen that the increase of H a observed with increasing Mn content is simply due to the decrease of os. In fact the total anisotropy is linearly reduced upon addition of Mn. In contrast the N d anisotropy in Ni substituted compounds appears to be markedly affected. The linear reduction in the overall anisotropy can be almost completely ascribed to the decrease in the N d anisotropy. Similar behaviour has been observed in Co substituted compounds. (On the other hand, as observed in the case of Co compounds [7], a larger effect of the metal sublattice on the Nd anisotropy should be expected at high substitutions.) The composition dependence of the anisotropy
75
\
o
\
\
so
\,
A
25
0
I
0
1
m
J _ _ _ _
2
I
3 X
Fig. 7. Variation of the anisotropy field H~ at 293 K with Ni (e), Mn (J0 and Co (E) content in Nd2Fe14_xMe~B compounds.
125
20
100 ~ . v.
~- 15
75
k,-
X
10
50
5® 0 0
® 2..
t __lJ
1
2
3 x
25
___.l
0
1
2~
1
2
3
X
0
Fig. 8. (a) Composition variation of the cone angle 0 k at 77 K in Nd2Fel4 xMe~B for Me = Ni (e), Mn (A) and Co (ll); (b) composition variation of the spin reorientation transition temperature T~r in N d 2 F e 1 4 _ , M e x B for Me = Ni (e), Mn (A) and
Co (m). field at 293 K (fig. 7) gives some indication of the usefulness of Ni and Mn substitutions as compared with Co for permanent magnet applications. A slight decrease of H a is observed in the case of Ni substitution whilst the decrease in H a is dramatic in the case of Mn compounds. In the latter case the decrease has to be completely attributed to the reduction of T~ with Mn substitution. It is well known that Nd2Fe14B undergoes a spin-reorientation transition from easy-axis to easy-cone at 135 K. The temperature at which the transition takes place is decreased with all the substitutions and the rate of decrease is higher in going from Co, Ni to Mn substitution (fig. 8b). The value of the cone angle 0 c at 77 K is reduced in the same manner (fig. 8a). Furthermore the temperature at which the F O M P transition commences is decreased in all cases. Some disagreement between the present data and work by other authors on Mn compounds regarding the values of the Curie temperature and the spin-reorientation transition temperature [12,13] are most likely due to fluctuations in composition which are possible in Mn compounds. This is related to the very high vapour pressure of Mn at the melting point. The same problem has also arisen in the case of Mn substituted Y compounds [11].
F. Bolzoni et al. / Ni and Mn substituted N d eFe I 4 B compounds
If some kind of competition between different anisotropy contributions can be invoked as a mechanism for the observed FOMP and spin reorientation transition, the present results clearly indicate that the substitution of Fe with Co, Ni and Mn markedly reduces the strength of such a mechanism. It should also be noted that in contrast to Co and Ni the dramatic effects of Mn on the spin-reorientation transition and the cone angle must be mostly attributed to the weakening of the magnetic interactions which is evidenced by a strong decrease in Tc (at a constant T / T c ratio the Nd anisotropy is not affected by Mn substitution (fig. 6)). This indicates the presence of a different preferential substitution in the case of Mn compounds. Indeed recent neutron powder diffraction measurements on Mn substituted Er2FelnB compounds indicate that there is a site preference for Mn atoms at the J2 crystallographic sites [14] whilst in the case of Co substituted compounds there is a site preference for the Fe atoms at J2 sites [15]. Taking into account the effects of the temperature we conclude that the resultant anisotropy of Nd2Fe14 B is reduced by Ni and Co substitution whilst it is not affected by Mn substitution in the composition range under consideration here. On the other hand, due to the different variation of T~ with Co, Ni and Mn substitution the room temperature values of the anisotropy field is slightly affected by Co and Ni and drastically reduced by Mn substitution.
4. Conclusion Measurements of the effects of Ni and Mn substitution on the magnetic properties of Nd 2 Fe14B reveal that the Curie temperature is slightly increased with Ni substitution whilst in the case of Mn it is drastically reduced; the replacement of 25% of Fe by Mn results in a non ferromagnetic order at room temperature. The saturation magne-
377
tization is decreased in both cases, the reduction being dramatic for Mn substitution. The effects of Ni on the Nd anisotropy are such that it acts to decrease it whilst Mn appears not to affect the Nd anisotropy. This further indicates the presence of a preferential substitution which is different for Mn, Ni and Co compounds.
Acknowledgement The authors acknowledge the support of the Commission of the European Communities within the CEAM Project of the Stimulation Programme.
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