- Email: [email protected]
Magnetic and microstructural properties of FeNdB type magnets with additives
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1059-1060
Aim
journal of magnetism
~
and
magnetic materials ELSEVIER
Magnetic and microstructural properties of FeNdB type magnets with additives M. Seeger a,* j. Bauer a G. Rieger a, H. Kronmfiller a, j. Bernardi b, j. Fidler b a MPlfftr Metallforschung, Institutfiir Physik, Heisenbergstr. 1, D-70569 Stuttgart, Germany b Institute of Applied and Technical Physics, T.U. Wien, Wiedner Hauptstr. 8-10, A-1040 Wien, Austria
Abstract The influence of small additions of Ga and Nb on the magnetic and microstructural properties of sintered and melt-spun FeNdB based magnets was analyzed. New intergranular phases and precipitates improve the magnetic decoupling thus enhancing the critical fields /x0Hc up to 2.5 T in melt-spun Fe72NdlvB7.sGa1.sNb2.
The permanent magnetic properties of FeNdB magnets can be favorably influenced by adding small amounts of further elements. These additives either change the intrinsic magnetic properties by substituting Fe or Nd in the hard magnetic FelaNd2 B phase or influence the microstructure, e.g. due to the formation of new intergranular phases. These non-magnetic additives are divided into two classes [1]. The type I is represented by low melting metals like Ga, A1 or Cu and form low-melting new binary and ternary phases whereas the type II additives, e.g. Nb, Mo, V, form high-melting borides and precipitates. We have investigated the influence of Ga and Nb or Mo on the magnetic and microstructural properties in both, melt-spun and sintered FeNdB magnets. Melt-spun samples of the nominal compositions Fe75NdlsB6Ga p Fe72Nd17B7.sGal.sNb2 and Fe73Nd17BTGa 1Mo 2 were prepared using the single roller technique (Vroner = 14-50 m / s ) and sintered magnets of the composition Fe74Nd18B6GalNbl were prepared from hydrogen decrepitated and jet milled powder. Subsequent annealing treatments were carried out with varying annealing temperatures and times in order to obtain maximum room temperature coercive fields. Fig. 1 summarizes the room temperature hysteresis loops of two annealed melt-spun magnets one containing only a type I additive and one containing a type I and a type II additive, respectively, as well as a sintered and annealed magnet also with both types of additives. Regarding the critical field b%ncrit which is characterized by maximum susceptibility, the isotropic melt-spun sample with Ga and Nb (/x0Hcn t = 2.5 T) ex-
ceeds the sample with only Ga additions (2.2 T) by more than 10%. Furthermore the rectangularity of the latter sample is worse indicating a less homogeneous microstructure which leads to a broad field range of the magnetization reversal. This different behavior has been checked by determining the maximum reversible applied field, max /~oHrev , up to which the demagnetization curve behaves completely reversibly. In the sample of composition Fe73NdlTBvGalM02 the quantity /xoHremv ax reaches the value 1.7 T whereas the sample Fe75Nd18B6Ga 1 of Fig. 1 shows irreversible demagnetization processes at significantly smaller field values. The oriented sintered magnet has a critical field of 1.7 T and a remanence of 1.2 T. In the theory of micromagnetism the critical field of a real permanent magnet ].t~0ncrit c a n be described in an extended version of Brown's expression for the nucleation field, /xoH N [2], /..toHcrit = a K a o 2 K 1 / M s - NeffJs.
l
.
email:
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 0 7 4 9 - 7
.
.
.
Fer4Ndl8 B6GalNbl-
melt-spun Fe75Ndis B6Ga
-5 * Corresponding author. Fax: +49-711-689-1932; [email protected].
slnte'red
.
.
.
.
(1)
l
,i
-
j ~ ~
0
\melt-spun Fe72Nd17BT..sGat.sN~
5
/40H IT]
Fig. 1. Room temperature hysteresis loops of different magnets.
1060
M. Seeger et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1059-1060
K] denotes the first order anisotropy constant and J~ = /x0Ms is the spontaneous magnetic polarization. The microstructural parameters O~K, a 0 and N e f f a r e due to the non-perfect microstructure. The quantity a K takes into account the reduced crystal anisotropy at the surface of the grains and aq, takes care of the misalignment of the grains, i.e. the angle between easy axis and direction of the demagnetizing field. Neff takes into account the strong local demagnetization effects at the edges and corners of the grains. Because of the non-perfect magnetic decoupling the lowest aq, value, denoted as a ,min, determines the macroscopic demagnetization of the magnet. Therefore Eq. (1) can be written in the form /.ZO H c r i t / J s
=
m i n /,Is -Neff, o~K/xoH N
(2)
rain with p, o H ~ in= a~0rain 2 K 1 / M ~. The quantity o~q, can be calculated from the anisotropy constants K] and K 2 using a formula given in Ref. [3]. Considering only K1, a ~ i" has the value 5.~ The microstructural parameters o~/~ and Nef~ can then be determined by plotting the temperature dependence of /xoH~it according to tzoHcrit(T)/Js(T) vs. min tzoH N ( T ) / J ~ ( T ) . Figs. 2 and 3 show the behavior of /.toHcrit(T) and the evaluation of a x and Neff of a meltspun magnet with both types of additives (Fey2Nd]7B7.sGal.sNb2) before and after the heat treatment as well as a sintered and heat treated magnet. It is obvious that the heat treatment changes the temperature dependence but does not increase the critical field at elevated temperatures. Regarding the microstructural parameters, both increase during the annealing. On the one hand, the quality of the grain surfaces improves and on the other hand, the grains develop sharp edges and corners due to grain growth preferably along the crystallographic directions. The Neff parameter of the sintered magnet is still larger. It should be
cD
. . . .
i
. . . .
I
. . . .
I
. . . .
i
. . . .
I I I melt-spun annealed "~'~"~ melt-spun as-qu. .g-o-o sintered
~r, '~ ~ ' ~
,
i ~
///
, ~f'+ /*//"~" ,0
&
6 I
O
lill K
20(I K
5
I
:3-0 , , , /"
J
0
m.-s.
annealed
~'~
m
043~)
sintered
s
I
as
qu
,
2
aK=l.02
Neff=0.98
~K--092
Neff=0.83
C(K=0.97
Neff= 1.26
I
4 /ZoHNm'n/Js
Fig. 3. The determination of ceK and
N e f f.
noted that in all systems the microstructural parameters are correlated, i.e. the favorable increase of one parameter is coupled with the deteriorating increase of the other one. The results obtained by magnetic measurements agree with microstructural investigations. Melt-spun FeNdBGaNb and FeNdBGaMo magnets show a very fine and homogeneous microstructure with average grain sizes between 20 and 100 nm. In contrast, FeNdBGa magnets exhibit larger grain size gradients and a broader distribution of the size and the shape of the grains. Especially at the free surface side polyhedral grains with sizes up to 1 Ixm can be seen. Nevertheless, the grain size itself has no drastic influence on the critical field. We conclude that the role of type II additives in melt-spun magnets is the formation of nucleation centers thus leading to a finegrained microstructure during the quenching process. The sintered FeNdBGaNb magnets are characterized by rather spherical grains with an average size of about 5 Ixm. Phase analyses have shown that Nb is not and Ga only little soluble in the dO grains. Nb is present in boride grains in the intergranular region and in small precipitates within the dO grains whereas Ga forms the binary Nd3Ga phase and Nds(Fe,Ga) 3 precipitates. Furthermore, after the heat treatment additionally the 8 phase (Nd6Fe13_xGa1+ x (x = 12)) occurs. References
"~-oh
o
. . . .
100
i
200
.
.
.
3
.
300
.
400
.
500
T [K]
Fig. 2. The behavior of/./,0Hcrit(T).
600
[1] J. Fidler, Proc. 7th Int. Symp. Magn. Anisotropy & Coercivity in RE-TM Alloys, ed. B. Street (University of Western Australia, 1992) p. 11. [2] H. Kronmiiller, K.-D. Durst and M. Sagawa, J. Magn. Magn. Mater. 74 (1988) 291. [3] G. Martinek and H. Kronmijller, J. Magn. Magn. Mater. 86 (1990) 177.
Aim
journal of magnetism
~
and
magnetic materials ELSEVIER
Magnetic and microstructural properties of FeNdB type magnets with additives M. Seeger a,* j. Bauer a G. Rieger a, H. Kronmfiller a, j. Bernardi b, j. Fidler b a MPlfftr Metallforschung, Institutfiir Physik, Heisenbergstr. 1, D-70569 Stuttgart, Germany b Institute of Applied and Technical Physics, T.U. Wien, Wiedner Hauptstr. 8-10, A-1040 Wien, Austria
Abstract The influence of small additions of Ga and Nb on the magnetic and microstructural properties of sintered and melt-spun FeNdB based magnets was analyzed. New intergranular phases and precipitates improve the magnetic decoupling thus enhancing the critical fields /x0Hc up to 2.5 T in melt-spun Fe72NdlvB7.sGa1.sNb2.
The permanent magnetic properties of FeNdB magnets can be favorably influenced by adding small amounts of further elements. These additives either change the intrinsic magnetic properties by substituting Fe or Nd in the hard magnetic FelaNd2 B phase or influence the microstructure, e.g. due to the formation of new intergranular phases. These non-magnetic additives are divided into two classes [1]. The type I is represented by low melting metals like Ga, A1 or Cu and form low-melting new binary and ternary phases whereas the type II additives, e.g. Nb, Mo, V, form high-melting borides and precipitates. We have investigated the influence of Ga and Nb or Mo on the magnetic and microstructural properties in both, melt-spun and sintered FeNdB magnets. Melt-spun samples of the nominal compositions Fe75NdlsB6Ga p Fe72Nd17B7.sGal.sNb2 and Fe73Nd17BTGa 1Mo 2 were prepared using the single roller technique (Vroner = 14-50 m / s ) and sintered magnets of the composition Fe74Nd18B6GalNbl were prepared from hydrogen decrepitated and jet milled powder. Subsequent annealing treatments were carried out with varying annealing temperatures and times in order to obtain maximum room temperature coercive fields. Fig. 1 summarizes the room temperature hysteresis loops of two annealed melt-spun magnets one containing only a type I additive and one containing a type I and a type II additive, respectively, as well as a sintered and annealed magnet also with both types of additives. Regarding the critical field b%ncrit which is characterized by maximum susceptibility, the isotropic melt-spun sample with Ga and Nb (/x0Hcn t = 2.5 T) ex-
ceeds the sample with only Ga additions (2.2 T) by more than 10%. Furthermore the rectangularity of the latter sample is worse indicating a less homogeneous microstructure which leads to a broad field range of the magnetization reversal. This different behavior has been checked by determining the maximum reversible applied field, max /~oHrev , up to which the demagnetization curve behaves completely reversibly. In the sample of composition Fe73NdlTBvGalM02 the quantity /xoHremv ax reaches the value 1.7 T whereas the sample Fe75Nd18B6Ga 1 of Fig. 1 shows irreversible demagnetization processes at significantly smaller field values. The oriented sintered magnet has a critical field of 1.7 T and a remanence of 1.2 T. In the theory of micromagnetism the critical field of a real permanent magnet ].t~0ncrit c a n be described in an extended version of Brown's expression for the nucleation field, /xoH N [2], /..toHcrit = a K a o 2 K 1 / M s - NeffJs.
l
.
email:
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 0 7 4 9 - 7
.
.
.
Fer4Ndl8 B6GalNbl-
melt-spun Fe75Ndis B6Ga
-5 * Corresponding author. Fax: +49-711-689-1932; [email protected].
slnte'red
.
.
.
.
(1)
l
,i
-
j ~ ~
0
\melt-spun Fe72Nd17BT..sGat.sN~
5
/40H IT]
Fig. 1. Room temperature hysteresis loops of different magnets.
1060
M. Seeger et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1059-1060
K] denotes the first order anisotropy constant and J~ = /x0Ms is the spontaneous magnetic polarization. The microstructural parameters O~K, a 0 and N e f f a r e due to the non-perfect microstructure. The quantity a K takes into account the reduced crystal anisotropy at the surface of the grains and aq, takes care of the misalignment of the grains, i.e. the angle between easy axis and direction of the demagnetizing field. Neff takes into account the strong local demagnetization effects at the edges and corners of the grains. Because of the non-perfect magnetic decoupling the lowest aq, value, denoted as a ,min, determines the macroscopic demagnetization of the magnet. Therefore Eq. (1) can be written in the form /.ZO H c r i t / J s
=
m i n /,Is -Neff, o~K/xoH N
(2)
rain with p, o H ~ in= a~0rain 2 K 1 / M ~. The quantity o~q, can be calculated from the anisotropy constants K] and K 2 using a formula given in Ref. [3]. Considering only K1, a ~ i" has the value 5.~ The microstructural parameters o~/~ and Nef~ can then be determined by plotting the temperature dependence of /xoH~it according to tzoHcrit(T)/Js(T) vs. min tzoH N ( T ) / J ~ ( T ) . Figs. 2 and 3 show the behavior of /.toHcrit(T) and the evaluation of a x and Neff of a meltspun magnet with both types of additives (Fey2Nd]7B7.sGal.sNb2) before and after the heat treatment as well as a sintered and heat treated magnet. It is obvious that the heat treatment changes the temperature dependence but does not increase the critical field at elevated temperatures. Regarding the microstructural parameters, both increase during the annealing. On the one hand, the quality of the grain surfaces improves and on the other hand, the grains develop sharp edges and corners due to grain growth preferably along the crystallographic directions. The Neff parameter of the sintered magnet is still larger. It should be
cD
. . . .
i
. . . .
I
. . . .
I
. . . .
i
. . . .
I I I melt-spun annealed "~'~"~ melt-spun as-qu. .g-o-o sintered
~r, '~ ~ ' ~
,
i ~
///
, ~f'+ /*//"~" ,0
&
6 I
O
lill K
20(I K
5
I
:3-0 , , , /"
J
0
m.-s.
annealed
~'~
m
043~)
sintered
s
I
as
qu
,
2
aK=l.02
Neff=0.98
~K--092
Neff=0.83
C(K=0.97
Neff= 1.26
I
4 /ZoHNm'n/Js
Fig. 3. The determination of ceK and
N e f f.
noted that in all systems the microstructural parameters are correlated, i.e. the favorable increase of one parameter is coupled with the deteriorating increase of the other one. The results obtained by magnetic measurements agree with microstructural investigations. Melt-spun FeNdBGaNb and FeNdBGaMo magnets show a very fine and homogeneous microstructure with average grain sizes between 20 and 100 nm. In contrast, FeNdBGa magnets exhibit larger grain size gradients and a broader distribution of the size and the shape of the grains. Especially at the free surface side polyhedral grains with sizes up to 1 Ixm can be seen. Nevertheless, the grain size itself has no drastic influence on the critical field. We conclude that the role of type II additives in melt-spun magnets is the formation of nucleation centers thus leading to a finegrained microstructure during the quenching process. The sintered FeNdBGaNb magnets are characterized by rather spherical grains with an average size of about 5 Ixm. Phase analyses have shown that Nb is not and Ga only little soluble in the dO grains. Nb is present in boride grains in the intergranular region and in small precipitates within the dO grains whereas Ga forms the binary Nd3Ga phase and Nds(Fe,Ga) 3 precipitates. Furthermore, after the heat treatment additionally the 8 phase (Nd6Fe13_xGa1+ x (x = 12)) occurs. References
"~-oh
o
. . . .
100
i
200
.
.
.
3
.
300
.
400
.
500
T [K]
Fig. 2. The behavior of/./,0Hcrit(T).
600
[1] J. Fidler, Proc. 7th Int. Symp. Magn. Anisotropy & Coercivity in RE-TM Alloys, ed. B. Street (University of Western Australia, 1992) p. 11. [2] H. Kronmiiller, K.-D. Durst and M. Sagawa, J. Magn. Magn. Mater. 74 (1988) 291. [3] G. Martinek and H. Kronmijller, J. Magn. Magn. Mater. 86 (1990) 177.