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Domain wall pinning in NdFeB melt-spun magnets
N.I--I
~
Jounialof materials
ELSEVIER
Journal of Magnetism and Magnetic Materials 161 (1996) 235-238
Domain wall pinning in NdFeB melt-spun magnets A. Tanguy a, j. Bras b, j. Degauque b a E.D.F./D.E.R., Les Renardi~res, 77250 Moret sur Loing, France b Laboratoire de Physique des Solides associ[ au C.N.R.S, INSA Toulouse, D£partement Physique, avenue de Rangueuil, 31077 Toulouse Cedex, France
Received 17 August 1995; revised 3 January 1996
Abstract Vanadium addition in melt-spun MQ2 magnets reduces the initial magnetic susceptibility. It also leads to a grain size decrease and to the formation of rich vanadium precipitates. Lorentz microscopy observations show that these precipitates do not pin the domain walls. However, this result could be attributed to the thinned state of the specimen. Furthermore, a similar experience is implemented with a Sm2C017 slide. In this case, a strong pinning effect is observed. In conclusion, the influence of vanadium on magnetic characteristics can only be attributed to the grain size reduction. Keywords: Permanent magnets; NdFeB; SmCo; Rapid solidification; Vanadium; Microstructure; Coercivity; Lorentz microscopy
1. Introduction Many investigations have aimed at improving the coercivity of rare earth based magnets by adding some elements to the basic composition in order to form precipitates [ 1-3]. In the case of SmCo sintered magnets, it has been shown that precipitates which strongly pin the domain walls are obtained, after an appropriate heat treatement, for s o m e Sm(Co,Fe,Cu,M)7_ s alloys with M = Zr, Ti [4]. For such magnets the dominant mechanism of coercivity is domain wall pinning [5]. In the case of NdFeB magnets, such an influence has not been observed. In this work, the interaction of vanadium-rich precipitates with the domain walls, in NdFeB hotpressed magnets, is investigated with the help of Lorentz microscopy. Then, a comparison is made with the pinning effect observed in Sm2Co17.
2. Experiment The ingots are prepared in an argon atmosphere from constitutent elements with at least a 3N purity. The basic composition is: Fes0.1- xNdl39_xB5.4+xVx A10.6, with x = 0 and 2.1. The ingots are induction melted and melt-spun (planar flow casting), in an argon atmosphere, onto a rotating copper wheel with a peripheral velocity of 23.6 m / s . The metallic flakes obtained are manually ground to obtain a coarse powder ( ~ 100 /xm) which is hot-pressed, at 620°C, with a pressure of 380 MPa in order to form MQ2 type magnets. Details are given elsewhere [6]. The sintered Sm(Co,Cu,Fe,Zr)7, 2 permanent magnets (Recoma 28) were supplied by Ugimag. Specimens for transmission electron microscopy (TEM) are prepared from magnets by cutting and
0304-8853f96f$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0304- 8 85 3 ( 9 6 ) 0 0 0 3 0 - 3
A. Tanguy et al. / Journal of Magnetism and Magnetic Materials 161 (1996) 235-238
236
mechanical polishing followed by ion milling. Classical TEM and Lorentz microscopy observations are performed on a Jeol 200 Cx coupled with an EDX microanalysis apparatus. The magnetic contrast is produced by shifting the objective aperture diaphragm (Foucault technique). No specific device is used: the variation of the specimen orientation towards the magnetic objective lens field (600 k A / m , parallel to the electron beam) induces some local changes of magnetic structure [7]. Magnetic properties are measured with an hysteresigraph (LDJ).
3. Results and discussion Classical observations by TEM of NdFeB MQ2 magnets show that (i) about 75% of the crystalline grains have a size smaller than 100 nm; (ii) samples with vanadium contain less grains larger than 1 /xm than those without; (iii) vanadium precipitates preferentially at grain boundaries when the grain size is about 50-70 nm and in the middle of the grains when these are larger than 1 #m. The EDX analyses of the precipitates show that they are vanadium-rich but their small size (10-20 nm) makes the determination of their exact composition difficult. These precipitates have the same morphology as FeVzB 2 ones present in vanadium-doped Ugistab-sintered magnets provided by Ugimag [8]. For melt-spun magnets, the limitation of the grain growth in the presence of vanadium is attributed first to the stabilization of the amorphous phase during the quench, secondly to the formation of precipitates at grain boundaries during the hot compression [6,9]. Magnetic measurements have shown that vanadium additions leads to an improvement of the squareness of the second quadrant curve (Fig. 1). O%V J (T) 0.8 :0,6 . ~- ~ H (kA/m)
2.1%V
" "
0.4 0.2 .0
-
1 /
2
~
-600
-400
-200
//
!-0.2 -0.4 l -O.6
Fig.
1.
First
magnetizing
curves
of
MQ2
F e s o t - rNdl3.9 xB54+aV~Alo.6, x = 0 and x = 2.1.
magnets
0.9 0.8 0.7. 0,6
J (T) i f
o.5
~
0.4 0.3 , 0.2
Fig.
2.
o~v~
~ ~J
....
~/ ~ J /~
0
~
~ j/
2,1%V
....... J~ ~
250
500
Demagnetization
H (kMm)
750 curves
1000 of
MQ2
1250 magnets
Fes0 A _ rNdl3 9_ ~Bs.4+ rV~Al0. 6. x = 0 and x = 2.1.
Moreover, the first magnetizing curves obtained with 0% V and with 2.1% V are very different, especially at low field (Fig. 2). On the basis of Pinkerton's hypothesis about the magnetic behaviour during first magnetization [10], this lower initial susceptibility can be interpreted by the pinning of the domain walls. It can be also attributed to a single domain grain behaviour [6,11]. Indeed, MQ2 magnets are mainly composed of isotropic and single domain grains: the critical size D c of single domain existence in one grain of NdFeB is estimated to be between 150 and 300 nm. So, the purpose of the following is to try to find out why vanadium reduces initial susceptibility: is it only because it decreases the amount of grains which are larger than D~ (such grains contain domain walls at virgin state which can move under low field) or because the precipitates pin domain walls? An answer to this question can be determined by observing the behaviour of magnetic domain walls (DW) by Lorentz microscopy.
3.1. Domain wall motion observations by Lorentz microscopy Previous computations have shown that for the high magnetocristalline anisotropy grains of Nd2Fe~4B, the angle between the direction of the magnetization M and the c-axis [001] does not exceed a few degrees even when the magnetic field H ( ~ 600 k A / m ) of the microscope objective lens is applied perpendicularly to c [7]. Therefore, during the obervations by Lorentz microscopy, the action of the external field on the grain magnetization depends on 0 = (H, c) (Fig. 3). By tilting the specimen, the nucleation and the evolution of magnetic domains and the eventual
A. Tanguy et al. / Journal of Magnetism and Magnetic Materials 161 (1996) 235-238
~,
tiltingaxis
/7 fielddirection parralldto the electronbeam Fig. 3. When 0 varies, the field favours some domains which tend to expand to the detriment of the others.
interactions between domain walls (DW) and precipitates can be visualised in situ. 3.2. NdFeB MQ2 magnets with 2.1% V
237
close to the plane of the slide. Fig. 4a (0 = 85 °) and Fig. 4b (0 = 87 °) show that the domain wall has notably moved for a A(Hcos 0) = 20 k A / m . This shows that the precipitates do not impeded the movement of the domain wall. Therefore, the pinning effect cannot explain the modification of the magnetic behaviour during first magnetization. The behaviour of the domain wall is similar to the one encountered in NdFeB sintered magnets [12] in which the DW appears always when 0 = 90 ° and the precipitates (which are larger than those observed in MQ2) do not pin the DW significantly. However, one may wonder if the unpinning can be due to a reduction of the interaction strength between domain walls and precipitates caused by the thinned state of the slide ( < 1000 A). To eliminate
The domain walls are imaged in some large grains which contain precipitates and whose c-axis lies
Fig. 4. Foucault mode: a DW and vanadium-rich precipitates (P) in FeNdB MQ2 magnet with 2.1% V. (a) 0 = 85°, (b) 0 = 87°: the DW has moved (Fig. 3).
Fig. 5. Foucault mode: Recoma 28 sintered magnet; 0 = 90 °. The DW is pinned on the platelets. (a) The decentering of the aperture diaphragm leads to light the upper magnetic domain. (b) The aperture diaphragm is decentred in the opposite direction; it leads to light the lower magnetic domain.
238
A. Tanguy et al. / Journal of Magnetism and Magnetic Materials 161 (1996) 235-238
this doubt, we have carried out the same experiment with sintered SmzCot7. Indeed, whereas SmC% is described as a nucleation type magnet, the coercivity mechanisms of the Sm2C017 is attributed by all the authors to the pinning of the DW 3.3. Sintered
Sm2Co17 magnets
Fig. 5a and Fig. 5b show a typical microstructure of sintered Sm2Co17 magnets according to Strnat [4] and Hadjipanayis [13]: a network of very small cells of a rhombohedrical matrix phase ( 2 / 1 7 ) which are separated and often completely surrounded by a thin boundary phase (Sm(Co,Cu) 5 type). The cells have linear dimension of about 100-200 nm and the cell walls are typically 5 - 2 0 nm thick. In addition, Fig. 5a shows two domains, imaged by Foucault mode, separated by a domain wall. The contrast is reversed (Fig. 5b) by decentering the aperture diaphragm in the direction opposite to the one used for Fig. 5a. In Fig. 5a and Fig. 5b, the domain wall lies along the boundary phase platelets. This configuration does not move even with a tilt variation of 45 ° which corresponds to a A ( H c o s 0 ) = 400 k A / m . This experiment clearly shows that when a strong pinning effect exists, it is not eliminated at the thinned state.
4. Conclusion In melt-spun NdFeB MQ2 magnets, the pinning of the domain walls by the vanadium-rich precipitates is not responsible for the low susceptibility
observed at low field during the first magnetization. Only the smaller size of the grain, when some vanadium is added, can explain the reduction of the susceptibility.
Acknowledgements The authors would like to thank M. Fagot (University Toulouse 3) for her help in electronic microscopy; A. Chamberod (CEN Grenoble) and D. Binesti (EDF-Moret sur Loing) for their helpful cooperation; F. Vial (Ugimag-Saint Pierre d'Allevard) for the provision of the sintered magnets. This work is supported by Electricit~ de France.
References [1] [2] [3] [4] [5]
E. Henig, Proc. CEAM, September 1990, p. 287. M. Sagawa, P. Tenaud, IEEE Trans. Magn. 26 (1990) 1942. R. Grossinger, X.C. Kou, IEEE Trans Magn. 26 (1990) 1957. K.J. Stmat, J. Magn. Magn. Mater. 100 (1991) 38. J.D. Livingston and D.L. Martin, J, Appl. Phys. 40 (1977) 1350. [6] A. Tanguy, Thesis, INSA, Toulouse, France (1994). [7] J. Bras, J. Degauque and M. Fagot, J. Microsc. Spectrosc. Electron. 13 (1988) 439. [8] K. Biyadi, Thesis, INSA, Toulouse, France (1992). [9] A. Tanguy, D. Binesti, J. Bras, J. Degauque, M. Fagot, A. Chamberod and F. Vanoni, J. de Phys. IV, C3, Suppl. J. de Phys. 1II, 2 (1992). [10] F.E. Pinkerton, Mater. Res. Soc. Syrup. Proc. 96 (1987). [11] X.C. Kou, W.J. Qiang, H. Kronmiiller and L. Schultz, J. Appl. Phys. 74 (1993) 6791. [12] J. Bras, K. Biyadi, M. Fagot and J. Degauque, J. de Phys. IV, C3, Suppl. J. de Phys. III, 2 (1992) C3-113. [13] G.C. Hadjipanayis, J. Appl. Phys. 55 (1984) 2091.
~
Jounialof materials
ELSEVIER
Journal of Magnetism and Magnetic Materials 161 (1996) 235-238
Domain wall pinning in NdFeB melt-spun magnets A. Tanguy a, j. Bras b, j. Degauque b a E.D.F./D.E.R., Les Renardi~res, 77250 Moret sur Loing, France b Laboratoire de Physique des Solides associ[ au C.N.R.S, INSA Toulouse, D£partement Physique, avenue de Rangueuil, 31077 Toulouse Cedex, France
Received 17 August 1995; revised 3 January 1996
Abstract Vanadium addition in melt-spun MQ2 magnets reduces the initial magnetic susceptibility. It also leads to a grain size decrease and to the formation of rich vanadium precipitates. Lorentz microscopy observations show that these precipitates do not pin the domain walls. However, this result could be attributed to the thinned state of the specimen. Furthermore, a similar experience is implemented with a Sm2C017 slide. In this case, a strong pinning effect is observed. In conclusion, the influence of vanadium on magnetic characteristics can only be attributed to the grain size reduction. Keywords: Permanent magnets; NdFeB; SmCo; Rapid solidification; Vanadium; Microstructure; Coercivity; Lorentz microscopy
1. Introduction Many investigations have aimed at improving the coercivity of rare earth based magnets by adding some elements to the basic composition in order to form precipitates [ 1-3]. In the case of SmCo sintered magnets, it has been shown that precipitates which strongly pin the domain walls are obtained, after an appropriate heat treatement, for s o m e Sm(Co,Fe,Cu,M)7_ s alloys with M = Zr, Ti [4]. For such magnets the dominant mechanism of coercivity is domain wall pinning [5]. In the case of NdFeB magnets, such an influence has not been observed. In this work, the interaction of vanadium-rich precipitates with the domain walls, in NdFeB hotpressed magnets, is investigated with the help of Lorentz microscopy. Then, a comparison is made with the pinning effect observed in Sm2Co17.
2. Experiment The ingots are prepared in an argon atmosphere from constitutent elements with at least a 3N purity. The basic composition is: Fes0.1- xNdl39_xB5.4+xVx A10.6, with x = 0 and 2.1. The ingots are induction melted and melt-spun (planar flow casting), in an argon atmosphere, onto a rotating copper wheel with a peripheral velocity of 23.6 m / s . The metallic flakes obtained are manually ground to obtain a coarse powder ( ~ 100 /xm) which is hot-pressed, at 620°C, with a pressure of 380 MPa in order to form MQ2 type magnets. Details are given elsewhere [6]. The sintered Sm(Co,Cu,Fe,Zr)7, 2 permanent magnets (Recoma 28) were supplied by Ugimag. Specimens for transmission electron microscopy (TEM) are prepared from magnets by cutting and
0304-8853f96f$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0304- 8 85 3 ( 9 6 ) 0 0 0 3 0 - 3
A. Tanguy et al. / Journal of Magnetism and Magnetic Materials 161 (1996) 235-238
236
mechanical polishing followed by ion milling. Classical TEM and Lorentz microscopy observations are performed on a Jeol 200 Cx coupled with an EDX microanalysis apparatus. The magnetic contrast is produced by shifting the objective aperture diaphragm (Foucault technique). No specific device is used: the variation of the specimen orientation towards the magnetic objective lens field (600 k A / m , parallel to the electron beam) induces some local changes of magnetic structure [7]. Magnetic properties are measured with an hysteresigraph (LDJ).
3. Results and discussion Classical observations by TEM of NdFeB MQ2 magnets show that (i) about 75% of the crystalline grains have a size smaller than 100 nm; (ii) samples with vanadium contain less grains larger than 1 /xm than those without; (iii) vanadium precipitates preferentially at grain boundaries when the grain size is about 50-70 nm and in the middle of the grains when these are larger than 1 #m. The EDX analyses of the precipitates show that they are vanadium-rich but their small size (10-20 nm) makes the determination of their exact composition difficult. These precipitates have the same morphology as FeVzB 2 ones present in vanadium-doped Ugistab-sintered magnets provided by Ugimag [8]. For melt-spun magnets, the limitation of the grain growth in the presence of vanadium is attributed first to the stabilization of the amorphous phase during the quench, secondly to the formation of precipitates at grain boundaries during the hot compression [6,9]. Magnetic measurements have shown that vanadium additions leads to an improvement of the squareness of the second quadrant curve (Fig. 1). O%V J (T) 0.8 :0,6 . ~- ~ H (kA/m)
2.1%V
" "
0.4 0.2 .0
-
1 /
2
~
-600
-400
-200
//
!-0.2 -0.4 l -O.6
Fig.
1.
First
magnetizing
curves
of
MQ2
F e s o t - rNdl3.9 xB54+aV~Alo.6, x = 0 and x = 2.1.
magnets
0.9 0.8 0.7. 0,6
J (T) i f
o.5
~
0.4 0.3 , 0.2
Fig.
2.
o~v~
~ ~J
....
~/ ~ J /~
0
~
~ j/
2,1%V
....... J~ ~
250
500
Demagnetization
H (kMm)
750 curves
1000 of
MQ2
1250 magnets
Fes0 A _ rNdl3 9_ ~Bs.4+ rV~Al0. 6. x = 0 and x = 2.1.
Moreover, the first magnetizing curves obtained with 0% V and with 2.1% V are very different, especially at low field (Fig. 2). On the basis of Pinkerton's hypothesis about the magnetic behaviour during first magnetization [10], this lower initial susceptibility can be interpreted by the pinning of the domain walls. It can be also attributed to a single domain grain behaviour [6,11]. Indeed, MQ2 magnets are mainly composed of isotropic and single domain grains: the critical size D c of single domain existence in one grain of NdFeB is estimated to be between 150 and 300 nm. So, the purpose of the following is to try to find out why vanadium reduces initial susceptibility: is it only because it decreases the amount of grains which are larger than D~ (such grains contain domain walls at virgin state which can move under low field) or because the precipitates pin domain walls? An answer to this question can be determined by observing the behaviour of magnetic domain walls (DW) by Lorentz microscopy.
3.1. Domain wall motion observations by Lorentz microscopy Previous computations have shown that for the high magnetocristalline anisotropy grains of Nd2Fe~4B, the angle between the direction of the magnetization M and the c-axis [001] does not exceed a few degrees even when the magnetic field H ( ~ 600 k A / m ) of the microscope objective lens is applied perpendicularly to c [7]. Therefore, during the obervations by Lorentz microscopy, the action of the external field on the grain magnetization depends on 0 = (H, c) (Fig. 3). By tilting the specimen, the nucleation and the evolution of magnetic domains and the eventual
A. Tanguy et al. / Journal of Magnetism and Magnetic Materials 161 (1996) 235-238
~,
tiltingaxis
/7 fielddirection parralldto the electronbeam Fig. 3. When 0 varies, the field favours some domains which tend to expand to the detriment of the others.
interactions between domain walls (DW) and precipitates can be visualised in situ. 3.2. NdFeB MQ2 magnets with 2.1% V
237
close to the plane of the slide. Fig. 4a (0 = 85 °) and Fig. 4b (0 = 87 °) show that the domain wall has notably moved for a A(Hcos 0) = 20 k A / m . This shows that the precipitates do not impeded the movement of the domain wall. Therefore, the pinning effect cannot explain the modification of the magnetic behaviour during first magnetization. The behaviour of the domain wall is similar to the one encountered in NdFeB sintered magnets [12] in which the DW appears always when 0 = 90 ° and the precipitates (which are larger than those observed in MQ2) do not pin the DW significantly. However, one may wonder if the unpinning can be due to a reduction of the interaction strength between domain walls and precipitates caused by the thinned state of the slide ( < 1000 A). To eliminate
The domain walls are imaged in some large grains which contain precipitates and whose c-axis lies
Fig. 4. Foucault mode: a DW and vanadium-rich precipitates (P) in FeNdB MQ2 magnet with 2.1% V. (a) 0 = 85°, (b) 0 = 87°: the DW has moved (Fig. 3).
Fig. 5. Foucault mode: Recoma 28 sintered magnet; 0 = 90 °. The DW is pinned on the platelets. (a) The decentering of the aperture diaphragm leads to light the upper magnetic domain. (b) The aperture diaphragm is decentred in the opposite direction; it leads to light the lower magnetic domain.
238
A. Tanguy et al. / Journal of Magnetism and Magnetic Materials 161 (1996) 235-238
this doubt, we have carried out the same experiment with sintered SmzCot7. Indeed, whereas SmC% is described as a nucleation type magnet, the coercivity mechanisms of the Sm2C017 is attributed by all the authors to the pinning of the DW 3.3. Sintered
Sm2Co17 magnets
Fig. 5a and Fig. 5b show a typical microstructure of sintered Sm2Co17 magnets according to Strnat [4] and Hadjipanayis [13]: a network of very small cells of a rhombohedrical matrix phase ( 2 / 1 7 ) which are separated and often completely surrounded by a thin boundary phase (Sm(Co,Cu) 5 type). The cells have linear dimension of about 100-200 nm and the cell walls are typically 5 - 2 0 nm thick. In addition, Fig. 5a shows two domains, imaged by Foucault mode, separated by a domain wall. The contrast is reversed (Fig. 5b) by decentering the aperture diaphragm in the direction opposite to the one used for Fig. 5a. In Fig. 5a and Fig. 5b, the domain wall lies along the boundary phase platelets. This configuration does not move even with a tilt variation of 45 ° which corresponds to a A ( H c o s 0 ) = 400 k A / m . This experiment clearly shows that when a strong pinning effect exists, it is not eliminated at the thinned state.
4. Conclusion In melt-spun NdFeB MQ2 magnets, the pinning of the domain walls by the vanadium-rich precipitates is not responsible for the low susceptibility
observed at low field during the first magnetization. Only the smaller size of the grain, when some vanadium is added, can explain the reduction of the susceptibility.
Acknowledgements The authors would like to thank M. Fagot (University Toulouse 3) for her help in electronic microscopy; A. Chamberod (CEN Grenoble) and D. Binesti (EDF-Moret sur Loing) for their helpful cooperation; F. Vial (Ugimag-Saint Pierre d'Allevard) for the provision of the sintered magnets. This work is supported by Electricit~ de France.
References [1] [2] [3] [4] [5]
E. Henig, Proc. CEAM, September 1990, p. 287. M. Sagawa, P. Tenaud, IEEE Trans. Magn. 26 (1990) 1942. R. Grossinger, X.C. Kou, IEEE Trans Magn. 26 (1990) 1957. K.J. Stmat, J. Magn. Magn. Mater. 100 (1991) 38. J.D. Livingston and D.L. Martin, J, Appl. Phys. 40 (1977) 1350. [6] A. Tanguy, Thesis, INSA, Toulouse, France (1994). [7] J. Bras, J. Degauque and M. Fagot, J. Microsc. Spectrosc. Electron. 13 (1988) 439. [8] K. Biyadi, Thesis, INSA, Toulouse, France (1992). [9] A. Tanguy, D. Binesti, J. Bras, J. Degauque, M. Fagot, A. Chamberod and F. Vanoni, J. de Phys. IV, C3, Suppl. J. de Phys. 1II, 2 (1992). [10] F.E. Pinkerton, Mater. Res. Soc. Syrup. Proc. 96 (1987). [11] X.C. Kou, W.J. Qiang, H. Kronmiiller and L. Schultz, J. Appl. Phys. 74 (1993) 6791. [12] J. Bras, K. Biyadi, M. Fagot and J. Degauque, J. de Phys. IV, C3, Suppl. J. de Phys. III, 2 (1992) C3-113. [13] G.C. Hadjipanayis, J. Appl. Phys. 55 (1984) 2091.