- Email: [email protected]
Structures and magnetic properties of the nitrides (Nd1-xDyx)TiFe11Ny
Journal of Magnetism and Magnetic Materials 124 (1993) 301-304 North-Holland
Structures and magnetic properties of the nitrides (Ndl_,Dyx)TiFellN, Lin-shu Kong, Lei Cao and Bao-gen Shen State Key Laboratory
of Magnetism,
Institute of Physics, Chinese Academy
of Sciences, Be&g
100080, China
Received 13 May 1992; in revised form 1 December 1992
The rare earth-iron intermetallics (Nd, _,Dy,)TiFe,, (x = 0.0, 0.1, 0.2, 0.3 and 0.4) have been nitrided and the structures and magnetic properties of these nitrides have been investigated. The nitrogen contents y in (Nd,_,Dy,)TiFe,,N, are estimated to be 0.6 for x = 0.0 and 1.2 for x = 0.1, 0.2, 0.3 and 0.4. The anisotropies increase with increasing Dy content and the Curie temperatures are all more than 700 K. It is found that the compounds (Ndt_,Dy,)TiFe,,N, at small Dy concentrations have excellent intrinsic magnetic properties favourable for permanent magnet applications.
1. Introduction
Since the discovery that the magnetic properties of R,Fe,, compounds can be improved by nitrogenation, a worldwide effort has been made to investigate the nitrides of iron-rich rare earth intermetallic compounds [l-4]. It is found that RTiFe,, compounds can be also charged with nitrogen, and that the interstitial nitrogen atoms have a strong effect on the magnetic properties of these compounds [5-81. By introducing nitrogen atoms in RTiFe,,, the Curie temperature as well as the saturation magnetization is strongly enhanced. Moreover, the sign of the R sublattice anisotropy shows a complete reversal. The R sublattice anisotropy changes from an easy plane to an easy c-axis for R elements with the second Stevens coefficient cxJ < 0, such as Nd, Tb, Dy and Ho, and changes from an easy c-axis to an easy plane for R elements with (Ye> 0, such as Sm and Er. As a result, NdTiFe,,N, becomes a potential candidate for permanent magnet applications.
to: Dr Lin-shu Kong, State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China. Correspondence
0304-8853/93/$06.00
In the present work, the effect of the Dy substitution in NdTiFe,,N, compounds has been studied. The possibility of (Nd,_,Dyx)TiFe,,N, for a high-performance permanent magnet is discussed.
2. Experimental The samples of (Nd,_,Dyx)TiFe,, (x = 0.0,0.1, 0.2,0.3 and 0.4) compounds were prepared by arc melting of 99.9% pure primary materials in a purified argon atmosphere. X-ray diffraction showed the samples to be single phase. The ingots were finely ground and charged with purified nitrogen gas. In order to investigate the reaction between samples and nitrogen gas and to determine the proper temperature of nitrogenation, we first observed the variation of the nitrogen pressure with increasing temperature under the condition that the samples and the nitrogen gas are sealed. It was found that a nearly single phase can be obtained when the sample is heated in nitrogen atmosphere with a pressure of about 1 bar at 450-500°C for l-2 h. The concentration of nitrogen in the nitride is estimated by the weight increase.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
302
Lin-shu I&..; ot al. / Properties of the nitrides (Nd I _ x Dy, ) TiFe,, NY
The powder samples fixed in epoxy resin were aligned in a magnetic field of about 10 kOe. Magnetization curves parallel along and perpendicular to the orientation direction were measured on aligned samples with a field up to 70 kOe at 1.5 K and room temperature by an extracting-sample magnetometer. The Curie temperatures were determined from M-T curves measured at a magnetic field of 2 kOe by a vibrating-sample magnetometer.
3. Results and discussion
Table 1 Lattice parameters a and c, unit cell volume change in unit cell volume upon nitrogenation Compound
a
NdTiFe,, NdTiFe,,N, Nd&%.rTrFetr Nda.sDya.rTiFettN,
Nda.sDye.,TrFer t Nda.sDya.2TiFeItN, Nda.7Dyo,sTiFeII Nda.7Dya.sTiFeItN, Nda.,Dyc,~TiFett Nda.,Dya.4TrFeItN,
V and relative AV/ V
V
AV/
6)
k,
G3’,
(%)
8.5847 8.6358 8.5741 8.6488 8.5764 8.6216 8.5640 8.6215 8.5526 8.6337
4.7894 4.8132 4.7967 4.8290 4.7955 4.8254 4.7916 4.8154 4.7965 4.8247
352.97 358.96 352.63 360.88 352.74 358.69 351.42 357.93 350.85 359.91
V
1.7 2.3 1.7 1.9 2.6
3.1. Phase formation and structure Experiments on NdTiFe,, compounds indicate that NdTiFe,,, unlike the heavy rare earth compounds with the 1: 12 phase, is easy to decompose when it is heated in nitrogen atmosphere. Its proper temperature of nitrogenation is found to be about 450°C. Higher heating temperatures can cause extra cw-Fe impurity phase in the sample. This limits the amount of nitrogen absorption in the NdTiFe,, compound. When Dy is substituted for Nd, the compounds are able to absorb more nitrogen gas with little impurity a-Fe. X-ray powder diffraction patterns indicate that the nitrides (Nd,_,Dy,)TiFe,,N, (x = 0.0, 0.1, 0.2, 0.3 and 0.4) compounds retain the same structure as their parent compounds. They all crystallize in the tetragonal ThMn,,-type structure. The lattice parameters of (Nd,_,Dyx)TiFe,,N, and (Nd,_,Dyx)TiFe,, are listed in table 1. It can be seen that the introduction of nitrogen atoms leads to an increase in the cell volume. The relative volume changes of AV/V are increased by 1.7-2.6%. Since the atomic radius of N atoms is smaller than those of Fe or Ti atoms, the cell volume dilating implies that the N atoms should enter into the lattice interstitially rather than substitutionally. By weighing, the nitrogen content y in (Nd,_,Dyx)TiFe,,N, are estimated to be about 0.6 for x = 0.0 and about 1.2 for X= 0.1, 0.2, 0.3 and 0.4. It is in the further study that whether the nitrogen atoms enter second interstitial sites in the 1: 12 structure.
3.2. Saturation magnetization and Curie temperature The saturation magnetization of (Nd 1_-x Dy,)TiFe,,N, at 1.5 and 293 K, respectively, are shown in table 2. The magnetizations decrease with increasing Dy content except for that of NdTiFe,,N,. The magnetization of NdTiFe,,N, is not as great as expected because the N content y in NdTiFe,,N, is only about 0.6. In the rare earth-iron intermetallics the coupling between the R and Fe moments is such that the spins in all cases are antiparallel. This results in ferromagnetic coupling for the light and ferrimagnetic coupling for the heavy lanthanides. Therefore, the substitution of the light element Nd with the heavy element Dy leads to a decrease in saturation magnetization. The Curie temperatures of (Nd,_,Dyx)TiFe,,N, are all more than 700 K (see table 2).
Table 2 Saturation anisotropy n
0.0 0.1 0.2 0.3 0.4
magnetization a,, Curie field HA of Nd,_,Dy,TiFe,,N,
us (emu/g) T=1.5K
T = 293 K
149.91 157.88 147.53 144.01 135.98
133.47 138.92 132.67 131.179 123.28
temperature
T,
T,(K)
HA (kOe) (T = 293 K)
700 740 745 730 740
8 9 10 10 11
and
Lin-shu Kong et al. / Properties of the nitrides (Nd,_ xDy,)TiFe,tNy
303
tocrystalline anisotropy consists of a contribution of the Fe sublattice and a contribution of the R sublattice for the rare earth-iron compound, and that the R sublattice anisotropy originates from a crystal field induced single-ion anisotropy. The Fe sublattice anisotropy in RTiFe,,N, favours an easy c-axis magnetization. Owing to the sign reversal of the R sublattice anisotropy affected by nitrogen atoms, the anisotropies of the Nd sublattice and the Dy sublattice also have an easy c-axis. Thus, the (Nd , _-xDy,)TiFe, ,N, compounds are expected to have a strong easy axis anisotropy, which can be seen in fig. 1. The anisotropy fields of (Nd, _,Dy,)TiFe,, N, are estimated by extrapolating magnetization curves parallel along and perpendicular to the aligned direction. The results are summarized in table 2. The anisotropy fields increase with increasing Dy content. It is worth noting that the areas between the magnetization curves of two directions increase drastically at small Dy concentrations, which means that high coercivities can be obtained in these compounds.
4. Conclusion
0
10
20
20
40
50
60
70
H (kOe)
Fig. 1. Magnetization curves of (Nd,_,Dy,)TiFe,,N, with applied fields parallel (0) and perpendicular (v ) to the aligned direction.lSK:o------oar r------~;293K:.-. or V-T.
It is difficult to get a good phase of nitrides NdTiFe,,N, due to the decomposition in the nitrogenation process. By substituting Nd with Dy, the compounds can absorb more nitrogen gas with little impurity a-Fe, and the anisotropy fields are improved. The compounds (Nd, _,Dy,)TiFe,,N, at small Dy concentrations are promising for high-performance permanent magnets.
Acknowledgements
They are higher than that of Nd,Fe,,B to those of Sm,Fe,,N, compounds. 3.3. Magnetocrystalline
and close
anisotropy
Figure 1 shows the magnetization curves measured on aligned samples of (Nd,_,Dy,)TiFe,,N, compounds at 1.5 and 293 K with applied fields parallel and perpendicular to the aligned direction, respectively. It is known that the magne-
This work was supported by the National Science Foundation of China and the award of a Post-Doctoral Fellowship.
References [ll J.M.D. Coey and Hong Sun, J. Magn. Magn. Mater. 87 (1990) L251.
304
Lin-shu Kong et al. / Properties of the nitrides (Nd, _ x Dy,) TiFe,, N,
121 Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan and S.L. Ge, Proc. 11th Int. Workshop on Rare-Earth Magnets and Their Applications, vol. 2, Pittsburgh, PA, October 1990, p. 190. [3] K.H.J. Buschow, R. Coehoorn, D.B.de Mooij, K. de Waard and T.H. Jacobs, J. Magn. Magn. Mater. 92 (1990) L25. [4] K. Schnitzke, L. Schultz, J. Wecker and M. Katter, Appl. Phys. Lett. 57 (1990) 2853. [S] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan, Y.T. Hou, S.
Huang and L. Yang, J. Less-Common 37. (61 Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Appl. Phys. Lett. 58 (1991) 2042. [7] J.M.D. Coey, Hong Sun and D.P.F. Magn. Mater. 101 (1990) 310. [8] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Solid State Commun. 78 (1991) 317.
Metals
170 (1991)
Pan and S.L. Ge, Hurley,
J. Magn.
Pan and S.L. Ge,
Structures and magnetic properties of the nitrides (Ndl_,Dyx)TiFellN, Lin-shu Kong, Lei Cao and Bao-gen Shen State Key Laboratory
of Magnetism,
Institute of Physics, Chinese Academy
of Sciences, Be&g
100080, China
Received 13 May 1992; in revised form 1 December 1992
The rare earth-iron intermetallics (Nd, _,Dy,)TiFe,, (x = 0.0, 0.1, 0.2, 0.3 and 0.4) have been nitrided and the structures and magnetic properties of these nitrides have been investigated. The nitrogen contents y in (Nd,_,Dy,)TiFe,,N, are estimated to be 0.6 for x = 0.0 and 1.2 for x = 0.1, 0.2, 0.3 and 0.4. The anisotropies increase with increasing Dy content and the Curie temperatures are all more than 700 K. It is found that the compounds (Ndt_,Dy,)TiFe,,N, at small Dy concentrations have excellent intrinsic magnetic properties favourable for permanent magnet applications.
1. Introduction
Since the discovery that the magnetic properties of R,Fe,, compounds can be improved by nitrogenation, a worldwide effort has been made to investigate the nitrides of iron-rich rare earth intermetallic compounds [l-4]. It is found that RTiFe,, compounds can be also charged with nitrogen, and that the interstitial nitrogen atoms have a strong effect on the magnetic properties of these compounds [5-81. By introducing nitrogen atoms in RTiFe,,, the Curie temperature as well as the saturation magnetization is strongly enhanced. Moreover, the sign of the R sublattice anisotropy shows a complete reversal. The R sublattice anisotropy changes from an easy plane to an easy c-axis for R elements with the second Stevens coefficient cxJ < 0, such as Nd, Tb, Dy and Ho, and changes from an easy c-axis to an easy plane for R elements with (Ye> 0, such as Sm and Er. As a result, NdTiFe,,N, becomes a potential candidate for permanent magnet applications.
to: Dr Lin-shu Kong, State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China. Correspondence
0304-8853/93/$06.00
In the present work, the effect of the Dy substitution in NdTiFe,,N, compounds has been studied. The possibility of (Nd,_,Dyx)TiFe,,N, for a high-performance permanent magnet is discussed.
2. Experimental The samples of (Nd,_,Dyx)TiFe,, (x = 0.0,0.1, 0.2,0.3 and 0.4) compounds were prepared by arc melting of 99.9% pure primary materials in a purified argon atmosphere. X-ray diffraction showed the samples to be single phase. The ingots were finely ground and charged with purified nitrogen gas. In order to investigate the reaction between samples and nitrogen gas and to determine the proper temperature of nitrogenation, we first observed the variation of the nitrogen pressure with increasing temperature under the condition that the samples and the nitrogen gas are sealed. It was found that a nearly single phase can be obtained when the sample is heated in nitrogen atmosphere with a pressure of about 1 bar at 450-500°C for l-2 h. The concentration of nitrogen in the nitride is estimated by the weight increase.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
302
Lin-shu I&..; ot al. / Properties of the nitrides (Nd I _ x Dy, ) TiFe,, NY
The powder samples fixed in epoxy resin were aligned in a magnetic field of about 10 kOe. Magnetization curves parallel along and perpendicular to the orientation direction were measured on aligned samples with a field up to 70 kOe at 1.5 K and room temperature by an extracting-sample magnetometer. The Curie temperatures were determined from M-T curves measured at a magnetic field of 2 kOe by a vibrating-sample magnetometer.
3. Results and discussion
Table 1 Lattice parameters a and c, unit cell volume change in unit cell volume upon nitrogenation Compound
a
NdTiFe,, NdTiFe,,N, Nd&%.rTrFetr Nda.sDya.rTiFettN,
Nda.sDye.,TrFer t Nda.sDya.2TiFeItN, Nda.7Dyo,sTiFeII Nda.7Dya.sTiFeItN, Nda.,Dyc,~TiFett Nda.,Dya.4TrFeItN,
V and relative AV/ V
V
AV/
6)
k,
G3’,
(%)
8.5847 8.6358 8.5741 8.6488 8.5764 8.6216 8.5640 8.6215 8.5526 8.6337
4.7894 4.8132 4.7967 4.8290 4.7955 4.8254 4.7916 4.8154 4.7965 4.8247
352.97 358.96 352.63 360.88 352.74 358.69 351.42 357.93 350.85 359.91
V
1.7 2.3 1.7 1.9 2.6
3.1. Phase formation and structure Experiments on NdTiFe,, compounds indicate that NdTiFe,,, unlike the heavy rare earth compounds with the 1: 12 phase, is easy to decompose when it is heated in nitrogen atmosphere. Its proper temperature of nitrogenation is found to be about 450°C. Higher heating temperatures can cause extra cw-Fe impurity phase in the sample. This limits the amount of nitrogen absorption in the NdTiFe,, compound. When Dy is substituted for Nd, the compounds are able to absorb more nitrogen gas with little impurity a-Fe. X-ray powder diffraction patterns indicate that the nitrides (Nd,_,Dy,)TiFe,,N, (x = 0.0, 0.1, 0.2, 0.3 and 0.4) compounds retain the same structure as their parent compounds. They all crystallize in the tetragonal ThMn,,-type structure. The lattice parameters of (Nd,_,Dyx)TiFe,,N, and (Nd,_,Dyx)TiFe,, are listed in table 1. It can be seen that the introduction of nitrogen atoms leads to an increase in the cell volume. The relative volume changes of AV/V are increased by 1.7-2.6%. Since the atomic radius of N atoms is smaller than those of Fe or Ti atoms, the cell volume dilating implies that the N atoms should enter into the lattice interstitially rather than substitutionally. By weighing, the nitrogen content y in (Nd,_,Dyx)TiFe,,N, are estimated to be about 0.6 for x = 0.0 and about 1.2 for X= 0.1, 0.2, 0.3 and 0.4. It is in the further study that whether the nitrogen atoms enter second interstitial sites in the 1: 12 structure.
3.2. Saturation magnetization and Curie temperature The saturation magnetization of (Nd 1_-x Dy,)TiFe,,N, at 1.5 and 293 K, respectively, are shown in table 2. The magnetizations decrease with increasing Dy content except for that of NdTiFe,,N,. The magnetization of NdTiFe,,N, is not as great as expected because the N content y in NdTiFe,,N, is only about 0.6. In the rare earth-iron intermetallics the coupling between the R and Fe moments is such that the spins in all cases are antiparallel. This results in ferromagnetic coupling for the light and ferrimagnetic coupling for the heavy lanthanides. Therefore, the substitution of the light element Nd with the heavy element Dy leads to a decrease in saturation magnetization. The Curie temperatures of (Nd,_,Dyx)TiFe,,N, are all more than 700 K (see table 2).
Table 2 Saturation anisotropy n
0.0 0.1 0.2 0.3 0.4
magnetization a,, Curie field HA of Nd,_,Dy,TiFe,,N,
us (emu/g) T=1.5K
T = 293 K
149.91 157.88 147.53 144.01 135.98
133.47 138.92 132.67 131.179 123.28
temperature
T,
T,(K)
HA (kOe) (T = 293 K)
700 740 745 730 740
8 9 10 10 11
and
Lin-shu Kong et al. / Properties of the nitrides (Nd,_ xDy,)TiFe,tNy
303
tocrystalline anisotropy consists of a contribution of the Fe sublattice and a contribution of the R sublattice for the rare earth-iron compound, and that the R sublattice anisotropy originates from a crystal field induced single-ion anisotropy. The Fe sublattice anisotropy in RTiFe,,N, favours an easy c-axis magnetization. Owing to the sign reversal of the R sublattice anisotropy affected by nitrogen atoms, the anisotropies of the Nd sublattice and the Dy sublattice also have an easy c-axis. Thus, the (Nd , _-xDy,)TiFe, ,N, compounds are expected to have a strong easy axis anisotropy, which can be seen in fig. 1. The anisotropy fields of (Nd, _,Dy,)TiFe,, N, are estimated by extrapolating magnetization curves parallel along and perpendicular to the aligned direction. The results are summarized in table 2. The anisotropy fields increase with increasing Dy content. It is worth noting that the areas between the magnetization curves of two directions increase drastically at small Dy concentrations, which means that high coercivities can be obtained in these compounds.
4. Conclusion
0
10
20
20
40
50
60
70
H (kOe)
Fig. 1. Magnetization curves of (Nd,_,Dy,)TiFe,,N, with applied fields parallel (0) and perpendicular (v ) to the aligned direction.lSK:o------oar r------~;293K:.-. or V-T.
It is difficult to get a good phase of nitrides NdTiFe,,N, due to the decomposition in the nitrogenation process. By substituting Nd with Dy, the compounds can absorb more nitrogen gas with little impurity a-Fe, and the anisotropy fields are improved. The compounds (Nd, _,Dy,)TiFe,,N, at small Dy concentrations are promising for high-performance permanent magnets.
Acknowledgements
They are higher than that of Nd,Fe,,B to those of Sm,Fe,,N, compounds. 3.3. Magnetocrystalline
and close
anisotropy
Figure 1 shows the magnetization curves measured on aligned samples of (Nd,_,Dy,)TiFe,,N, compounds at 1.5 and 293 K with applied fields parallel and perpendicular to the aligned direction, respectively. It is known that the magne-
This work was supported by the National Science Foundation of China and the award of a Post-Doctoral Fellowship.
References [ll J.M.D. Coey and Hong Sun, J. Magn. Magn. Mater. 87 (1990) L251.
304
Lin-shu Kong et al. / Properties of the nitrides (Nd, _ x Dy,) TiFe,, N,
121 Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan and S.L. Ge, Proc. 11th Int. Workshop on Rare-Earth Magnets and Their Applications, vol. 2, Pittsburgh, PA, October 1990, p. 190. [3] K.H.J. Buschow, R. Coehoorn, D.B.de Mooij, K. de Waard and T.H. Jacobs, J. Magn. Magn. Mater. 92 (1990) L25. [4] K. Schnitzke, L. Schultz, J. Wecker and M. Katter, Appl. Phys. Lett. 57 (1990) 2853. [S] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan, Y.T. Hou, S.
Huang and L. Yang, J. Less-Common 37. (61 Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Appl. Phys. Lett. 58 (1991) 2042. [7] J.M.D. Coey, Hong Sun and D.P.F. Magn. Mater. 101 (1990) 310. [8] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Solid State Commun. 78 (1991) 317.
Metals
170 (1991)
Pan and S.L. Ge, Hurley,
J. Magn.
Pan and S.L. Ge,