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Characterization of NdFeB permanent magnet by Mössbauer spectroscopy
Volume 7. number 9, IO
MATERIALS LETTERS
January 1989
CHARACTERIZATION OF Nd-Fe-B PERMANENT MAGNET BY M&SBAUER SPECTROSCOPY Xunyi ZENG, Tao FU Shanghai institute of Metallurgy, Academia Sinica, 865 Chang Ning Road, Shanghai 200050, China
Keyu JIANG, Sihao WUNG and Xielong YANG East China Normal University. 3663 North Zhong Shan Road, Shanghai 200062, China
Received 1 August 1988; in final form 16 November t 988
The reduced remanent magnetizationj, and packing fraction of the hard magnetic phase p are two important parameters fundamental for processing control in the fabrication of anisotropic magnets. “Fe Massbauer spectroscopy is proposed to determine j, of Nd-Fe-B permanent magnet and, in turn, p of Nd>Fe,.,B is deduced from the measured remanent magnetization.
1. introduction
2. Experimental
Permanent magnet materials are characterized by high coercivity iH=and high remanent magnetization B,, and the relevant figure of merit expressing the quality of a permanent magnet is its maximum energy product (BH) ,,,. For anisotropic permanent magnets, it is apparent,
Two samples are used in this work. Both of them are produced by conventional powder metallurgy techniques. Specimen A is prepared in our laboratory and specimen B is a commercial product. The direction of orientation to that of press is perpendicular for the specimen A and parallel for the specimen B. Permanent magnetic properies of the magnets were measured after an impulsive magnetizing field of 60 kOe. The density was determined by Archimedes’ method in water. The absorber used for transmission Mijssbauer spectroscopy was transverse slices cut from the magnets and ground to a thickness of 100 urn. The specimen used for conversion X-ray Mbssbauer spectroscopy (CXMS) was a bulk sample of x 1 mm thickness. A constant acceleration Miissbauer spectrometer was used. y-rays transmit perpendicularly to the surface of the absorber. The source is 57Co (Rh). The Mlissbauer spectra were calibrated with a standard a-Fe foil. The experimental data were least-squares fitted using a computer.
&=4nM,Pj,
f
(1)
where MS is the magnetization of the hard ferromagnetic phase, p is its packing fraction and j, is the reduced remanent magnetization. In fact j~=(cos6),
(2)
where 8 is the angle between &l, and the preferred direction of the magnet. In case ,rjr, is associated with magnetocrystalline anisotropy, (BH) “, = iBf = +( 47rM,)2p2jf ,
(3)
provided & is high enough so that & IS fB,. Thus, for a given type of material, p and j, are important parameters fundamental for processing control. In this Letter, 57Fe Mijssbauer spectroscopy is proposed to find j, of Nd-Fe-B permanent magnet and p is, in turn, deduced through eq. (1). 350
0167-577x/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 7, number 9.10
MATERIALS LETTERS
3. Results and discussion The permanent magnetic properties of the specimens are listed in table 1. It is obvious that specimen A is of higher quality than specimen B. The 57Fe CXMS of specimen A and transmission MS of specimen B are shown in figs. 1 and 2 re-
January I989
spectively. There are detectable contributions from paramagnetic phase. Note also that the intensities of the Am,=0 transitions are different for specimen A and B, indicating different degree of perfection of alignment of NdzFe,,B grains. The well-known relation between the intensity of the second peak (Am,=O) to that of the first (Am,= - 1) is
Table 1 Permanent magnetic properties of specimens A and B Specimen
A B
B, (kG)
,H, We)
(BW, (MG Oe)
P (g/cm’)
12.2
8.05 8.36
35.2 22.2
7.38 7.38
10.0
-4
-2
0
2
4
6
mm/s Fig. 1. “Fe CXMS of specimen A.
:. -. ‘.
Thus from R, cos2B can be deduced. Here cos’8 is actually a mean value. If 8 is small as in our case, we have cos 8= 1 - f6’. It can be verified that the root mean square of cos 8 is equal to the mean of cos 8, or j,. The packing fraction p of magnetic hardening phase can in turn be evaluated through eq. ( 1) taking 16.2 kG as the 47rM, of Nd2Fe,,B [ 1 1. These data are listed in table 2. The data derived from transmission MS and CXMS agreed with each other. The packing fractions of Nd2FeIqB of these magnets are in agreement with general estimates [ 21. From the density of the specimen, it is easy to show that there is 3% porosity for both magnets, reasonably taking the density of the cast alloy to be 7.60 g/cm’. The remainder of the constituents of the magnet is then oxides, Nd-rich and B-rich phases. The difference in p values for specimens A and B, of course, is resulting from the difference in composition of the magnets. The alignment of Nd2Fe,,B grains of specimen A is relatively perfect but there is still room for improvement. For randomly oriented Nd*Fe,,B grains, j, would be 0.5. Now for specimen B, jr =0.72>0.5, indicating that the Nd2Fe14B grains are partly aligned even though the alignment is poor. The Fe atoms occupy six crystallographically nonequivalent sites in the structure of NdzFe,4B. In addition, there are three pairs of Fe sites with the same Table 2 Data deduced from MS
1
-8
1
I
-6
-4
1
-2 V
I
024
I
I
I
68
(mm/s)
Fig. 2. 57Fetransmission MS of specimen B.
I
Specimen
Geometry of MS
R
Jr
P (Oh)
A
transmission CXMS transmission
0.10 0.14 0.44
0.93 0.90 0.72
81.0 83.7 85.7
B
351
Volume 7, number 9,10
MATERIALS LETTERS
occupancy. Thus the 57Fe Miissbauer spectrum is complex and the results of M~ssbauer studies are confused as to separate the observed spectrum into each sextuplet and to assign it to the corresponding Fe site. There is no difficulty involved here to find jr since our interest is just to find R. The proposed method of evaluation of p here is strai~tfo~ard, while the procedure of quantitative phase analysis of Ma et al. [ 3 ] is rather complicated. It is true that they can differentiate the quantities of other non-magnetic phases, but their method of evaluation involved some significant assumptions.
352
January 1989
References [I ] N.C. Koon, RN. Das, M. Rubinstein and J. Tyson, J. Appl. Phys. 57 (1985) 4091. [2] K. Durst and H. Kronmtiller, in: Proceedings of the 8th International Workshop on RE Magnets and Their Applications, Dayton, Ohio, May 1985, p. 725. [3] R. Ma, Z. Li, J. Ping, Y. Xu and S. Pan, in: Proceedings of the 9th Intemation~ Workshop on RE Magnets and Their Applications, Bad Soden, Federal Republic of Germany, September 1987, Part I, p. 583.
MATERIALS LETTERS
January 1989
CHARACTERIZATION OF Nd-Fe-B PERMANENT MAGNET BY M&SBAUER SPECTROSCOPY Xunyi ZENG, Tao FU Shanghai institute of Metallurgy, Academia Sinica, 865 Chang Ning Road, Shanghai 200050, China
Keyu JIANG, Sihao WUNG and Xielong YANG East China Normal University. 3663 North Zhong Shan Road, Shanghai 200062, China
Received 1 August 1988; in final form 16 November t 988
The reduced remanent magnetizationj, and packing fraction of the hard magnetic phase p are two important parameters fundamental for processing control in the fabrication of anisotropic magnets. “Fe Massbauer spectroscopy is proposed to determine j, of Nd-Fe-B permanent magnet and, in turn, p of Nd>Fe,.,B is deduced from the measured remanent magnetization.
1. introduction
2. Experimental
Permanent magnet materials are characterized by high coercivity iH=and high remanent magnetization B,, and the relevant figure of merit expressing the quality of a permanent magnet is its maximum energy product (BH) ,,,. For anisotropic permanent magnets, it is apparent,
Two samples are used in this work. Both of them are produced by conventional powder metallurgy techniques. Specimen A is prepared in our laboratory and specimen B is a commercial product. The direction of orientation to that of press is perpendicular for the specimen A and parallel for the specimen B. Permanent magnetic properies of the magnets were measured after an impulsive magnetizing field of 60 kOe. The density was determined by Archimedes’ method in water. The absorber used for transmission Mijssbauer spectroscopy was transverse slices cut from the magnets and ground to a thickness of 100 urn. The specimen used for conversion X-ray Mbssbauer spectroscopy (CXMS) was a bulk sample of x 1 mm thickness. A constant acceleration Miissbauer spectrometer was used. y-rays transmit perpendicularly to the surface of the absorber. The source is 57Co (Rh). The Mlissbauer spectra were calibrated with a standard a-Fe foil. The experimental data were least-squares fitted using a computer.
&=4nM,Pj,
f
(1)
where MS is the magnetization of the hard ferromagnetic phase, p is its packing fraction and j, is the reduced remanent magnetization. In fact j~=(cos6),
(2)
where 8 is the angle between &l, and the preferred direction of the magnet. In case ,rjr, is associated with magnetocrystalline anisotropy, (BH) “, = iBf = +( 47rM,)2p2jf ,
(3)
provided & is high enough so that & IS fB,. Thus, for a given type of material, p and j, are important parameters fundamental for processing control. In this Letter, 57Fe Mijssbauer spectroscopy is proposed to find j, of Nd-Fe-B permanent magnet and p is, in turn, deduced through eq. (1). 350
0167-577x/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 7, number 9.10
MATERIALS LETTERS
3. Results and discussion The permanent magnetic properties of the specimens are listed in table 1. It is obvious that specimen A is of higher quality than specimen B. The 57Fe CXMS of specimen A and transmission MS of specimen B are shown in figs. 1 and 2 re-
January I989
spectively. There are detectable contributions from paramagnetic phase. Note also that the intensities of the Am,=0 transitions are different for specimen A and B, indicating different degree of perfection of alignment of NdzFe,,B grains. The well-known relation between the intensity of the second peak (Am,=O) to that of the first (Am,= - 1) is
Table 1 Permanent magnetic properties of specimens A and B Specimen
A B
B, (kG)
,H, We)
(BW, (MG Oe)
P (g/cm’)
12.2
8.05 8.36
35.2 22.2
7.38 7.38
10.0
-4
-2
0
2
4
6
mm/s Fig. 1. “Fe CXMS of specimen A.
:. -. ‘.
Thus from R, cos2B can be deduced. Here cos’8 is actually a mean value. If 8 is small as in our case, we have cos 8= 1 - f6’. It can be verified that the root mean square of cos 8 is equal to the mean of cos 8, or j,. The packing fraction p of magnetic hardening phase can in turn be evaluated through eq. ( 1) taking 16.2 kG as the 47rM, of Nd2Fe,,B [ 1 1. These data are listed in table 2. The data derived from transmission MS and CXMS agreed with each other. The packing fractions of Nd2FeIqB of these magnets are in agreement with general estimates [ 21. From the density of the specimen, it is easy to show that there is 3% porosity for both magnets, reasonably taking the density of the cast alloy to be 7.60 g/cm’. The remainder of the constituents of the magnet is then oxides, Nd-rich and B-rich phases. The difference in p values for specimens A and B, of course, is resulting from the difference in composition of the magnets. The alignment of Nd2Fe,,B grains of specimen A is relatively perfect but there is still room for improvement. For randomly oriented Nd*Fe,,B grains, j, would be 0.5. Now for specimen B, jr =0.72>0.5, indicating that the Nd2Fe14B grains are partly aligned even though the alignment is poor. The Fe atoms occupy six crystallographically nonequivalent sites in the structure of NdzFe,4B. In addition, there are three pairs of Fe sites with the same Table 2 Data deduced from MS
1
-8
1
I
-6
-4
1
-2 V
I
024
I
I
I
68
(mm/s)
Fig. 2. 57Fetransmission MS of specimen B.
I
Specimen
Geometry of MS
R
Jr
P (Oh)
A
transmission CXMS transmission
0.10 0.14 0.44
0.93 0.90 0.72
81.0 83.7 85.7
B
351
Volume 7, number 9,10
MATERIALS LETTERS
occupancy. Thus the 57Fe Miissbauer spectrum is complex and the results of M~ssbauer studies are confused as to separate the observed spectrum into each sextuplet and to assign it to the corresponding Fe site. There is no difficulty involved here to find jr since our interest is just to find R. The proposed method of evaluation of p here is strai~tfo~ard, while the procedure of quantitative phase analysis of Ma et al. [ 3 ] is rather complicated. It is true that they can differentiate the quantities of other non-magnetic phases, but their method of evaluation involved some significant assumptions.
352
January 1989
References [I ] N.C. Koon, RN. Das, M. Rubinstein and J. Tyson, J. Appl. Phys. 57 (1985) 4091. [2] K. Durst and H. Kronmtiller, in: Proceedings of the 8th International Workshop on RE Magnets and Their Applications, Dayton, Ohio, May 1985, p. 725. [3] R. Ma, Z. Li, J. Ping, Y. Xu and S. Pan, in: Proceedings of the 9th Intemation~ Workshop on RE Magnets and Their Applications, Bad Soden, Federal Republic of Germany, September 1987, Part I, p. 583.