- Email: [email protected]
Structure and magnetic properties of (PrxDy1−x)Fe10.5V1.5 compounds and their nitrides
Journal of
ELSEVIER
Journal of Magnetismand Magnetic Materials 152 (1996) 396-400
magnetic materials
Structure and magnetic properties of (PrxDyl_ x)Fel0.sV1.5 compounds and their nitrides Jun Yang
*, S h e n g z h i D o n g , W e i h u a M a o , Y i n g c h a n g Y a n g
Department of Physics, Peking University, Beijing 100871, China
Received 5 April 1995; revised 8 June 1995
Abstract
The structural and magnetic properties of (PrxDyl_x)Felo.sVi.5 compounds (x = 0 - 0.8) and their nitrides have been investigated by X-ray diffraction and magnetic measurements. It is found that single-phase V-stabilized (PrxDy 1_ x)Felo.sV1.5 compounds can be formed in the Pr content range x = 0-0.8. The substitution of Dy for Pr in the compounds results in the decrease in the lattice parameters and saturation magnetization. The easy magnetization direction at room temperature turns from easy c-axis for x = 0-0.3 to easy plane for x = 0.4-0.8, while the Curie temperatures remain almost unaffected by the substitution of Dy, with Tc values of about 590 K. Upon nitrogenation, the Curie temperature is increased to around 820 K, and the saturation magnetization increases by approximately 20%, while the easy magnetization direction lies along c-axis in the temperature range 0 K to Tc. At low Dy contents, the (PrxDyl_x)FelosV1.sNy compounds have excellent intrinsic magnetic properties favourable for permanent magnet applications.
1. Introduction
It has been found that a rare earth and iron can not form a binary RFe12 phase, and that a third element is necessary to stabilize ternary R(Fe,M)12 phases with the ThMnl2 structure, where M = Ti, V, Mn, Mo, W, Si, etc. [1,2]. Since 1990, it has been reported that interstitial atoms such as H, C and N can be introduced into the R(Fe,M)12 compounds by gas-solid phase reactions [3-5]. Drastic changes in the magnetic properties of the 1:12 compounds have been observed upon nitrogenation. The interstitial nitrogen atoms have not only increased the Tc and saturation magnetization, but also changed the mag-
* Correspondingauthor. Present address: Departmentof Physics, McGill University, 3600 University Street, Montreal, Quebec, Canada H3A 2T8. Email: [email protected];fax: + 1514-398-6526.
netocrystalline anisotropy of the rare earth ions. The easy magnetization direction of R(Fe,M)12Ny compounds lies along c-axis for R = Pr, Nd, Dy, Tb and Ho, but in the basal plane for R = Er and Sm. Many studies have been made on Nd(Fe,M)12 nitrides [610] intending to explore their intrinsic and permanent magnetic properties. Pr(Fe,M)lENy (M = Ti and Mo) were studied [11,12] because the sign of the secondorder Stevens factor a~ is the same for Pr 3÷ and Nd 3+, and their intrinsic magnetic properties indicate that they are also potential candidates for permanent magnets. A coercivity of 6 kOe has been realized by mechanical alloying and subsequent nitriding of Pr(Fe,Mo)12 compounds [13]. For M = V, however, it seems to be more difficult to produce the singlephase PrFel0.sV1.5 compound. In a previous study [10], we showed that DyFel0.sV1.sNy, which has Tc = 8 1 9 K, HA(RT)= 13 T and Ms(RT)= 88.9
0304-8853/96/$15.00 © 1996 ElsevierScience B.V. All rights reserved SSDI 0304-8853(95)00463-7
J. Yang et al. /Journal of Magnetism and Magnetic Materials 152 (1996) 396-400
397 i
e m u / g , could be used to substitute for Nd in the NdFelo.sV1.sNy compound to increase its Tc and H A. In this paper, we try to stabilize PrFelo.5V1.5 compound with small amounts of Dy for Pr, and to study the influence of Dy content and nitrogenation on the structure and the magnetic properties of (Prx Dyl _x)Felo.sV1. 5 compounds.
I
i
~= 0"~
2. Experimental methods Alloys of the stoichiometric compounds (PrxDyl_x)Fel0.sV1. 5 were prepared by arc melting of 99.9% pure materials in a purified argon atmosphere. Additional amounts (3-5 wt%) of rare earth elements were added to compensate for their loss in the preparation process. The ingots were wrapped in a tantalum foil, sealed in a vacuum quartz tube, and then annealed at 800-950°C for one week. Nitrogenation was carried out by passing high-purity nitrogen gas at atmospheric pressure over finely ground powder samples at 450-550°C for 2 h. X-ray diffraction with Cu Kot radiation was used to detect the structure of the phases in the samples. The weight percentages of nitrogenation were determined by chemical analysis. The cylindrical powder samples were aligned in a 10 kOe field and fixed in epoxy resin. The magnetization curves parallel and perpendicular to the orientation direction were measured on aligned samples with fields of up to 70 kOe at 1.5 K by an extracting sample magnetometer. The Curie temperatures were determined from the M - T curves obtained with a vibrating sample magnetometer operating in a field of 1 kOe. Besides the magnetic measurements, X-ray diffraction analyses were performed on aligned samples to determine the direction of easy magnetization.
5O"
4O"
~ 20
Fig. 1. X-ray diffraction patterns of (PrxDy l_x)Fel0.SVl. 5 (a) and their nitrides (b) powders for x = 0.5 and 0.8.
4.770 ,~ for x = 0, to a = 8.564 ,~, c = 4.775 A for x = 0.8. When the amount of Pr equals to x = 0.9, a large amount of et-Fe(V) appears in addition to the 1:12 phase. Fig. 1 shows the X-ray diffraction patterns of (PrxDyl_x)Fel0.sV1. 5 compounds with x = 0.5 and 0.8, and Fig. 2 their corresponding thermomagnetic curves (M-T). ThMnl2-type phase was not obtained in the composition range of Pr(Fe~_zVz)12, with z = 0.13-0.21, by annealing in the temperature range 800-1100°C in this study, although PrFellTi and PrFel0.sMoL5 compounds have been obtained by annealing at ll00°C. This difference may be due to the smaller atomic radius of V (1.36 ,~) compared with those of Ti (1.45 ~,) and Mo (1.40 ,~). Upon nitrogenation, the tetragonal structure is retained with an increase in lattice parameters and unit cell volume. Meanwhile, a small amount of et-Fe(V) precipitated during nitrogenation, as can be seen from the X-ray diffraction patterns shown in Fig. 1. The lattice parameters a and c, unit cell volumes V and their relative variations ~ V / V for the compounds
3. Results and discussion 3.1. Phase formation and structure From the X-ray diffraction patterns and thermomagnetic measurements, the (PrxDyl_x)Fel0.sV1. 5 series are identified as single phase, with Pr contents in the range x = 0-0.8. The lattice parameters a and c increase with Pr content from a = 8.483 A, c =
It~ltll
.
.
.
,
.
.
.Igt)'Nm
Fig. 2. Thermomagnetic curves of (PrxDy l_x)Felo.SV1.5 for x = 0.5 and 0.8 (a) and their nitrides (b).
J. Yang et at/Journal of Magnetism and Magnetic Materials 152 (1996) 396-400
398
(PrxDY l_x)Fe10.SV1. 5 and their nitrides, are listed in Table 1. It is obvious that the lattice parameters of the nitrides also increase with the Pr content, as in the original counterparts due to the smaller radius of Dy 3+ than that of Pr 3+ because of lanthanide contraction. Upon nitrogenation, the unit cell volume increases by 1 . 8 - 3 . 8 % compared with their original counterparts. The largest value appears in the Dy-rich nitride compounds, implying that a small amount substitution of Dy for Pr helps to enhance the nitrogen absorption ability. The amounts of nitrogen in these compounds range from 0.5 to 1.0 by chemical analysis.
3.2. Curie temperature and saturation magnetization The Curie temperature Tc and saturation magnetization M s (1.5 K) of the nitrides are summarized in Table 2, and compared with their original counterparts. It can be seen that the Curie temperature of (PrxDyl_x)Fel0.sV1. 5 compounds are all approximately 590 K, indicating that the Curie temperature of DyFel0.sV1. 5 should be nearly the same as that of the PrFel05V1. 5 compound, if it exists. This is also the case in Ti [14] and Mo-stabilized R(Fe,M)12 compounds. Curie temperatures increase by nearly
3.81
0.7 0.8
0.5 0.6 0.7 0.8
98.11 (68.55) 98.55 (72.05) 99.18 (75.54) 97.76 (76.94) 106.93 (79.54) 117.70 (84.34) 124.62 (98.95) 128.41 (109.11) 145.70 (120.86)
819 (590) 820 (588) 828 (583) 818 (585) 820 (590) 825 (595) 820 (590) 818 (595) 815 (595)
20.0
IIc (cone) IIc (cone) IIc (cone)
16.3 14.7
IIc
15.6
(cone)
Ilc
14.4
300 K
IIc (llc)
lie (llc) lie (Nc) lie (lie)
IIc
(ab-plane) Ilc lie (ab-plane)
12.5
IIc
12.7
IIc
(ab-plane) []c lie (ab-plane)
12.5
IIc
13.2
IIc
(ab-plane)
where
8V/V (%)
356.34 (343.26) 353.99 (343.96) 352.59 (344.25) 353.21 (345.04) 353.80 (346.22) 354.30 (347.31) 354.86 (348.50) 355.64 (349.32) 356.73 (349.62)
0.6
0.4
1.5 K
2.37
V (,~3)
4.8027 (4.7695) 4.8098 (4.7747) 4.8010 (4.7733) 4.7809 (4.7748) 4.7878 (4.7743) 4.8004 (4.7754) 4.8032 (4.7725) 4.8096 (4.7713) 4.8136 (4.7726)
0.5
0.3
EMD
(T)
TC = ~[TFe q- (TF2e q- 41RFe)
c (,~)
8.6137 (8.4834) 8.5790 (8.4876) 8.5697 (8.4924) 8.5953 (8.5008) 8.5963 (8.5157) 8.5911 (8.5281) 8.5954 (8.5281) 8.5991 (8.5564) 8.6087 (8.5589)
0.4
0.2
HA
(K)
2.42
a (A)
0
0.3
0.1
Tc
(emu/g)
2.91
x
0.2
0
Ms
230 K to about 820 K upon nitriding. The thermomagnetic curves of (PrxDy I _x)Fel0.sV1.5 compounds and their nitrides for x = 0.5 and 0.8 are shown in Fig. 2. The Curie temperatures of R - F e intermetallic compounds are determined by the F e - F e , R - F e and R - R interactions. In general, the F e - F e interactions are dominant and R - R interactions negligible. In the molecular field approximation [15], the magnetic ordering temperature is given by
Table 1 Lattice parameters a and c, unit cell volume V and its relative variation ~V/V of (PrxDyl_x)Fel0.sV1.5 compounds (data in parentheses) and their nitrides as a function of Pr content x
0.1
Table 2 Saturation magnetization Ms (1.5 K), Curie temperature To and anisotropy field HA (1.5 K), together with the easy magnetization direction (EMD) at 1.5 K and room temperature, of the intermetallic compounds (PrxDyl_x)Fel0.sV1.5 (data in parentheses) and their nitrides
1
2.19 2.01 1.82 1.81 2.03
-~2
,,1/2]
],
TFe = nFe_FeNFe[4S*(S * + 1) / Z 2 / 3 k B ) , TRF o =
(1)
(2)
nR_FelYI(NFoNR)I/2[2S * (S* + 1)) ~/2 X g j ( J a ( J R + 1))l/:lX2/3ka] ,
(3)
with ( g j - 1 ) / g j " 2 ( S * ( S * + 1 ) ) 1 / 2 is the effective moment of iron in the paramagnetic state, and NFe and N R are the numbers of Fe and R atoms per unit volume, g j and JR are the Land6 factor and the total angular moment of the rare earth atoms, respec-
J. Yanget al./Journal of Magnetism and MagneticMaterials 152 (1996)396-400 tively. TEe is the contribution to the Curie temperature by the Fe sublattice and is determined by the nearest-neighbour distance between Fe atoms and the number of Fe coordinations. In the studied series, TFe can be regarded as constant. Therefore, Tar ~, which comes from the R - F e interactions, could be deduced to be invariable according to Eq. (1). Because gj and JR for Dy 3÷ are larger than those for Pr 3÷, so the value of I~Igj(JR(JR + 1)) 1/2 should increase with the amount of Dy in the compounds. Furthermore, (NRNF~) 1/2 should also increase due to the lattice contraction with Dy substitution. Thus, it can be deduced that nR_F: should decrease with increasing Dy according to Eq. (3), indicating that the R - F e interaction in DyFel0.sV1. 5 is smaller than those in (PrxDyl_x)Fel0.sV15 ( x ~ 0 ) ; this is qualitatively consistent with the results obtained in RFe11Ti compounds [16]. It is evident that the spontaneous magnetization increases with the amount of Pr, from 68.55 e m u / g for x = 0 to 120.86 e m u / g for x = 0.8. This may originate from the antiparallel alignment of the moment of Dy 3+ ion with that of Fe, in contrast with the parallel alignment of Pr 3÷ ion. When nitriding the compounds, large increases in the spontaneous magnetizations are achieved. Since the 4f electrons of rare earth atoms are well localized, in principle, the magnetic moments of the rare earth are close to g j J / z B. Accordingly, the effect of increasing the spontaneous magnetizations is related to a modification of Fe 3d electron band due to volume expansion, and the fact that N atoms attract the wavefunctions of R atoms rather than of Fe atoms, and in turn release the Fe atoms from bonding with R atoms [18]. The increases in the Curie temperature and the saturation magnetization upon nitrogenation are believed to be related to the cell volume expansion. Usually, the relatively low Curie temperature of R(Fe,M)12 is explained in terms of distance-dependent exchange interactions between Fe atoms. Because of the short interatomic distance between the Fe neighbours on some sites, such as 8j sites, an antiferromagnetic interaction is assumed to exist in R(Fe,M)12 compounds. Obviously, the lattice expansion obtained by introducing the nitrogen atoms reduces this effect. Moreover, a band structure calculation for Y2Fe17N3 has indicated that the lattice ex-
[321 I
r
I
~)
399 I
i21 i
t
tbl
!
~O:
20
400
50"
I
400
/
40"
I 30 ° 2e
Fig. 3. X-ray diffraction patterns of (PrxDyl_x)Fel0.sV1.5 on non-oriented and aligned samplesfor x = 0.3 (a) and x = 0.4 (b). pansion causes a narrowing of the Fe 3d band with a resulting increase in the saturation moment and the Curie temperature [17].
3.3. Magnetocrystalline anisotropy The data related to the magnetocrystalline anisotropy are also listed in Table 2. The easy magnetization lies along c-axis for x < 0.3, but in the basal plane for x > 0.4 in the host compounds at room temperature, as shown by the X-ray diffraction patterns on aligned samples in Fig. 3. Because both Pr 3+ and Dy 3+ ions have negative second-order Stevens factors, a j, the easy magnetization direction of the R 3÷ ions should lie in the basal plane in the original compounds due to the negative sign of second-order coefficient of the crystal field A20. In contrast, the Fe sublattice tends to align along the c-axis in these compounds. The competition between the R and Fe sublattices results in the easy magnetization changes with Pr content, implying that there should exist a spin reorientation at lower temperatures for x < 0.3 in the (PrxDyl_x)Felo.sVl. 5 compounds. Upon nitrogenation, the easy magnetization direction lies along c-axis in the temperature range from 0 K to Tc in the studied Pr content range. The magnetocrystalline field decreases with the Pr content in the nitride compounds. Fig. 4 shows the magnetization curves measured parallel and perpendicular to the alignment direction at 1.5 K of (PrxDyl_x)Feto.sV1.sNy for x = 0.5 and 0.8, from which the H A values of 12.5 and 13.2 T were
400
J. Yang et al. /Journal of Magnetism and Magnetic Materials 152 (1996) 396-400 160
: o"
::
the lattice parameters, Curie temperature and saturation magnetization of the compounds are increased, and the easy magnetization direction is turned into an easy c-axis. At small Dy concentrations, the (PrxDyl_x)Feto.sV1.sNy compounds have excellent intrinsic magnetic properties that would be favourable for permanent magnet applications.
..
÷
• -
0
Io
I
5O
I
•
•
•
X=
•
•
*
x:
I
50
0•5
0.8
I
"TO
H(kOe)
Fig. 4. Magnetization curves of (PrxDYl_x)Fel0.sVl.sNy measured at 1.5 K along and perpendicular to the orientation direction for x = 0.5 and 0.8.
obtained, respectively. Considering the large saturation magnetization of 145.7 e m u / g and Curie temperature Tc = 815 K, (Pr0.sDy0.2)Fel0.sVl.sNy can be considered as a potential candidate for permanent magnet applications. The change in the magnetocrystalline anisotropy originates from the effect of the interstitial nitrogen on the crystal field interaction at rare earth sites. The nitrogen atoms occupy the 2b sites, which possess the same I 4 / m m m point symmetry as the rare earth ions. Using a single-ion model and considering not only the rare earth ions, but also the nitrogen ions as ligands, we calculated the crystal field• The calculation shows that the contribution from neighbouring nitrogen ions to the crystal field is positive and large, while neighbouring rare earth ions contribute negative but small interactions, and thus make the sign of A20 become positive.
4. Conclusions We have systematically studied the phase formation of Dy-substituted (PrxDyl_x)Fel05V15 compounds and the effects of Dy content on their structure and magnetic properties. We also compared the variations in structure and magnetic properties of the compounds before and after nitrogenation. It is found that single-phase 1:12 compound can be obtained with Pr contents up to x = 0.8; the lattice parameters and saturation magnetization increase with the Pr content; the easy magnetization direction at room temperature changes from easy c-axis to easy plane between x = 0.3 and 0.4; while their Curie temperatures remain at around 590 K. Upon nitrogenation,
Acknowledgements This work was supported by the National Science Foundation and the Open Magnetism Laboratory of the Chinese Academy of Science.
References [1] Y.C. Yang, Acta Metall. Sinica 17 (1981) 355 (in Chinese). [2] K.H.J. Buschow, J. Appl. Phys. 63 (1988) 3130. [3] S. Obbade, S. Miraglia, D. Fruchart, M. Pre, P. l'Heritier and A. Ballet, C. R. Acad. Sci. Paris 307 (1988) 889. [4] J.M.D. Coey, H. Sun and D.P.F. Hurly, J. Mang. Magn. Mater. 101 (1991) 310. [5] Y.C. Yang, X.D. Zhang, S.L. Ge, L.S. Kong, Q. Pan, Y.T. Hou, S. Huang and L. Yang, in: Proc. 6th. Int. Symp. on Magnetic Anisotropy and Coercivity in RE-TM Alloys, ed. S.G. Sankar (Carnegie-Mellon University, Pittsburgh, 1990) p. 19. [6] Y.Z. Wang, G.C. Hadjipanayis, A. Kim, D.J. Sellymer and W.B. Yelon, J. Magn. Magn. Mater. 104 (1992) 1132. [7] M. Endoh, K. Nakamura and H. Mikami, IEEE Trans. Magn. 28 (1992) 2563. [8] W. Gong and G.C. Hadjipanayis, IEEE Trans. Magn. 28 (1992). [9] Y.C. Yang, Q. Pan, X.D. Zhang, C.L. Yang, S.L. Ge and B.F. Zhang, J. Appl. Phys. 74 (1992) 2723. [10] J. Yang, S.Z. Dong, B.P. Cheng and Y.C. Yang, J. Appl. Phys. 75 (1994) 3013. [11] H. Fuji, in: Proc. 2rid ISPM, ed. Shougong Zhang, Beijing, July 1992, p. 624. [12] Y.C. Yang, Q. Pan, X.D. Zhang, J. Yang, M.H. Zhang and S.L. Ge, Appl. Phys. Lett. 61 (1992) 2723. [13] Q. Pan, X.D. Zhang, B.P. Cheng and Y.C. Yang, J. Appl. Phys. 75 (1994) 5441. [14] X.C. Kou, T.S. Zhao, R. Grossinger, H.R. Kirchmayr, X. Li and F.R. de Boer, Phys. Rev. B 47 (1993) 3231. [15] Beloritzky, E.A. Fremy, J.P. Gavigan, D. Givord and H.S. Li, J. Appl. Phys. 61 (1987) 3971. [16] B.P. Hu, H.S. Li, J.P. Gavigan and J.M.D. Coey, J. Phys.: Condens. Matter 1 (1989) 755. [17] S.S. Jaswal, W.B. Yelon, G.C. Hadjipanayis, Y.Z. Wang and D.J. Sellmyer, Phys. Rev. Lett. 67 (1991) 644. [18] A. Sakuma, J. Phys. Soc. Jpn. 61 (1992) 4119.
ELSEVIER
Journal of Magnetismand Magnetic Materials 152 (1996) 396-400
magnetic materials
Structure and magnetic properties of (PrxDyl_ x)Fel0.sV1.5 compounds and their nitrides Jun Yang
*, S h e n g z h i D o n g , W e i h u a M a o , Y i n g c h a n g Y a n g
Department of Physics, Peking University, Beijing 100871, China
Received 5 April 1995; revised 8 June 1995
Abstract
The structural and magnetic properties of (PrxDyl_x)Felo.sVi.5 compounds (x = 0 - 0.8) and their nitrides have been investigated by X-ray diffraction and magnetic measurements. It is found that single-phase V-stabilized (PrxDy 1_ x)Felo.sV1.5 compounds can be formed in the Pr content range x = 0-0.8. The substitution of Dy for Pr in the compounds results in the decrease in the lattice parameters and saturation magnetization. The easy magnetization direction at room temperature turns from easy c-axis for x = 0-0.3 to easy plane for x = 0.4-0.8, while the Curie temperatures remain almost unaffected by the substitution of Dy, with Tc values of about 590 K. Upon nitrogenation, the Curie temperature is increased to around 820 K, and the saturation magnetization increases by approximately 20%, while the easy magnetization direction lies along c-axis in the temperature range 0 K to Tc. At low Dy contents, the (PrxDyl_x)FelosV1.sNy compounds have excellent intrinsic magnetic properties favourable for permanent magnet applications.
1. Introduction
It has been found that a rare earth and iron can not form a binary RFe12 phase, and that a third element is necessary to stabilize ternary R(Fe,M)12 phases with the ThMnl2 structure, where M = Ti, V, Mn, Mo, W, Si, etc. [1,2]. Since 1990, it has been reported that interstitial atoms such as H, C and N can be introduced into the R(Fe,M)12 compounds by gas-solid phase reactions [3-5]. Drastic changes in the magnetic properties of the 1:12 compounds have been observed upon nitrogenation. The interstitial nitrogen atoms have not only increased the Tc and saturation magnetization, but also changed the mag-
* Correspondingauthor. Present address: Departmentof Physics, McGill University, 3600 University Street, Montreal, Quebec, Canada H3A 2T8. Email: [email protected];fax: + 1514-398-6526.
netocrystalline anisotropy of the rare earth ions. The easy magnetization direction of R(Fe,M)12Ny compounds lies along c-axis for R = Pr, Nd, Dy, Tb and Ho, but in the basal plane for R = Er and Sm. Many studies have been made on Nd(Fe,M)12 nitrides [610] intending to explore their intrinsic and permanent magnetic properties. Pr(Fe,M)lENy (M = Ti and Mo) were studied [11,12] because the sign of the secondorder Stevens factor a~ is the same for Pr 3÷ and Nd 3+, and their intrinsic magnetic properties indicate that they are also potential candidates for permanent magnets. A coercivity of 6 kOe has been realized by mechanical alloying and subsequent nitriding of Pr(Fe,Mo)12 compounds [13]. For M = V, however, it seems to be more difficult to produce the singlephase PrFel0.sV1.5 compound. In a previous study [10], we showed that DyFel0.sV1.sNy, which has Tc = 8 1 9 K, HA(RT)= 13 T and Ms(RT)= 88.9
0304-8853/96/$15.00 © 1996 ElsevierScience B.V. All rights reserved SSDI 0304-8853(95)00463-7
J. Yang et al. /Journal of Magnetism and Magnetic Materials 152 (1996) 396-400
397 i
e m u / g , could be used to substitute for Nd in the NdFelo.sV1.sNy compound to increase its Tc and H A. In this paper, we try to stabilize PrFelo.5V1.5 compound with small amounts of Dy for Pr, and to study the influence of Dy content and nitrogenation on the structure and the magnetic properties of (Prx Dyl _x)Felo.sV1. 5 compounds.
I
i
~= 0"~
2. Experimental methods Alloys of the stoichiometric compounds (PrxDyl_x)Fel0.sV1. 5 were prepared by arc melting of 99.9% pure materials in a purified argon atmosphere. Additional amounts (3-5 wt%) of rare earth elements were added to compensate for their loss in the preparation process. The ingots were wrapped in a tantalum foil, sealed in a vacuum quartz tube, and then annealed at 800-950°C for one week. Nitrogenation was carried out by passing high-purity nitrogen gas at atmospheric pressure over finely ground powder samples at 450-550°C for 2 h. X-ray diffraction with Cu Kot radiation was used to detect the structure of the phases in the samples. The weight percentages of nitrogenation were determined by chemical analysis. The cylindrical powder samples were aligned in a 10 kOe field and fixed in epoxy resin. The magnetization curves parallel and perpendicular to the orientation direction were measured on aligned samples with fields of up to 70 kOe at 1.5 K by an extracting sample magnetometer. The Curie temperatures were determined from the M - T curves obtained with a vibrating sample magnetometer operating in a field of 1 kOe. Besides the magnetic measurements, X-ray diffraction analyses were performed on aligned samples to determine the direction of easy magnetization.
5O"
4O"
~ 20
Fig. 1. X-ray diffraction patterns of (PrxDy l_x)Fel0.SVl. 5 (a) and their nitrides (b) powders for x = 0.5 and 0.8.
4.770 ,~ for x = 0, to a = 8.564 ,~, c = 4.775 A for x = 0.8. When the amount of Pr equals to x = 0.9, a large amount of et-Fe(V) appears in addition to the 1:12 phase. Fig. 1 shows the X-ray diffraction patterns of (PrxDyl_x)Fel0.sV1. 5 compounds with x = 0.5 and 0.8, and Fig. 2 their corresponding thermomagnetic curves (M-T). ThMnl2-type phase was not obtained in the composition range of Pr(Fe~_zVz)12, with z = 0.13-0.21, by annealing in the temperature range 800-1100°C in this study, although PrFellTi and PrFel0.sMoL5 compounds have been obtained by annealing at ll00°C. This difference may be due to the smaller atomic radius of V (1.36 ,~) compared with those of Ti (1.45 ~,) and Mo (1.40 ,~). Upon nitrogenation, the tetragonal structure is retained with an increase in lattice parameters and unit cell volume. Meanwhile, a small amount of et-Fe(V) precipitated during nitrogenation, as can be seen from the X-ray diffraction patterns shown in Fig. 1. The lattice parameters a and c, unit cell volumes V and their relative variations ~ V / V for the compounds
3. Results and discussion 3.1. Phase formation and structure From the X-ray diffraction patterns and thermomagnetic measurements, the (PrxDyl_x)Fel0.sV1. 5 series are identified as single phase, with Pr contents in the range x = 0-0.8. The lattice parameters a and c increase with Pr content from a = 8.483 A, c =
It~ltll
.
.
.
,
.
.
.Igt)'Nm
Fig. 2. Thermomagnetic curves of (PrxDy l_x)Felo.SV1.5 for x = 0.5 and 0.8 (a) and their nitrides (b).
J. Yang et at/Journal of Magnetism and Magnetic Materials 152 (1996) 396-400
398
(PrxDY l_x)Fe10.SV1. 5 and their nitrides, are listed in Table 1. It is obvious that the lattice parameters of the nitrides also increase with the Pr content, as in the original counterparts due to the smaller radius of Dy 3+ than that of Pr 3+ because of lanthanide contraction. Upon nitrogenation, the unit cell volume increases by 1 . 8 - 3 . 8 % compared with their original counterparts. The largest value appears in the Dy-rich nitride compounds, implying that a small amount substitution of Dy for Pr helps to enhance the nitrogen absorption ability. The amounts of nitrogen in these compounds range from 0.5 to 1.0 by chemical analysis.
3.2. Curie temperature and saturation magnetization The Curie temperature Tc and saturation magnetization M s (1.5 K) of the nitrides are summarized in Table 2, and compared with their original counterparts. It can be seen that the Curie temperature of (PrxDyl_x)Fel0.sV1. 5 compounds are all approximately 590 K, indicating that the Curie temperature of DyFel0.sV1. 5 should be nearly the same as that of the PrFel05V1. 5 compound, if it exists. This is also the case in Ti [14] and Mo-stabilized R(Fe,M)12 compounds. Curie temperatures increase by nearly
3.81
0.7 0.8
0.5 0.6 0.7 0.8
98.11 (68.55) 98.55 (72.05) 99.18 (75.54) 97.76 (76.94) 106.93 (79.54) 117.70 (84.34) 124.62 (98.95) 128.41 (109.11) 145.70 (120.86)
819 (590) 820 (588) 828 (583) 818 (585) 820 (590) 825 (595) 820 (590) 818 (595) 815 (595)
20.0
IIc (cone) IIc (cone) IIc (cone)
16.3 14.7
IIc
15.6
(cone)
Ilc
14.4
300 K
IIc (llc)
lie (llc) lie (Nc) lie (lie)
IIc
(ab-plane) Ilc lie (ab-plane)
12.5
IIc
12.7
IIc
(ab-plane) []c lie (ab-plane)
12.5
IIc
13.2
IIc
(ab-plane)
where
8V/V (%)
356.34 (343.26) 353.99 (343.96) 352.59 (344.25) 353.21 (345.04) 353.80 (346.22) 354.30 (347.31) 354.86 (348.50) 355.64 (349.32) 356.73 (349.62)
0.6
0.4
1.5 K
2.37
V (,~3)
4.8027 (4.7695) 4.8098 (4.7747) 4.8010 (4.7733) 4.7809 (4.7748) 4.7878 (4.7743) 4.8004 (4.7754) 4.8032 (4.7725) 4.8096 (4.7713) 4.8136 (4.7726)
0.5
0.3
EMD
(T)
TC = ~[TFe q- (TF2e q- 41RFe)
c (,~)
8.6137 (8.4834) 8.5790 (8.4876) 8.5697 (8.4924) 8.5953 (8.5008) 8.5963 (8.5157) 8.5911 (8.5281) 8.5954 (8.5281) 8.5991 (8.5564) 8.6087 (8.5589)
0.4
0.2
HA
(K)
2.42
a (A)
0
0.3
0.1
Tc
(emu/g)
2.91
x
0.2
0
Ms
230 K to about 820 K upon nitriding. The thermomagnetic curves of (PrxDy I _x)Fel0.sV1.5 compounds and their nitrides for x = 0.5 and 0.8 are shown in Fig. 2. The Curie temperatures of R - F e intermetallic compounds are determined by the F e - F e , R - F e and R - R interactions. In general, the F e - F e interactions are dominant and R - R interactions negligible. In the molecular field approximation [15], the magnetic ordering temperature is given by
Table 1 Lattice parameters a and c, unit cell volume V and its relative variation ~V/V of (PrxDyl_x)Fel0.sV1.5 compounds (data in parentheses) and their nitrides as a function of Pr content x
0.1
Table 2 Saturation magnetization Ms (1.5 K), Curie temperature To and anisotropy field HA (1.5 K), together with the easy magnetization direction (EMD) at 1.5 K and room temperature, of the intermetallic compounds (PrxDyl_x)Fel0.sV1.5 (data in parentheses) and their nitrides
1
2.19 2.01 1.82 1.81 2.03
-~2
,,1/2]
],
TFe = nFe_FeNFe[4S*(S * + 1) / Z 2 / 3 k B ) , TRF o =
(1)
(2)
nR_FelYI(NFoNR)I/2[2S * (S* + 1)) ~/2 X g j ( J a ( J R + 1))l/:lX2/3ka] ,
(3)
with ( g j - 1 ) / g j " 2 ( S * ( S * + 1 ) ) 1 / 2 is the effective moment of iron in the paramagnetic state, and NFe and N R are the numbers of Fe and R atoms per unit volume, g j and JR are the Land6 factor and the total angular moment of the rare earth atoms, respec-
J. Yanget al./Journal of Magnetism and MagneticMaterials 152 (1996)396-400 tively. TEe is the contribution to the Curie temperature by the Fe sublattice and is determined by the nearest-neighbour distance between Fe atoms and the number of Fe coordinations. In the studied series, TFe can be regarded as constant. Therefore, Tar ~, which comes from the R - F e interactions, could be deduced to be invariable according to Eq. (1). Because gj and JR for Dy 3÷ are larger than those for Pr 3÷, so the value of I~Igj(JR(JR + 1)) 1/2 should increase with the amount of Dy in the compounds. Furthermore, (NRNF~) 1/2 should also increase due to the lattice contraction with Dy substitution. Thus, it can be deduced that nR_F: should decrease with increasing Dy according to Eq. (3), indicating that the R - F e interaction in DyFel0.sV1. 5 is smaller than those in (PrxDyl_x)Fel0.sV15 ( x ~ 0 ) ; this is qualitatively consistent with the results obtained in RFe11Ti compounds [16]. It is evident that the spontaneous magnetization increases with the amount of Pr, from 68.55 e m u / g for x = 0 to 120.86 e m u / g for x = 0.8. This may originate from the antiparallel alignment of the moment of Dy 3+ ion with that of Fe, in contrast with the parallel alignment of Pr 3÷ ion. When nitriding the compounds, large increases in the spontaneous magnetizations are achieved. Since the 4f electrons of rare earth atoms are well localized, in principle, the magnetic moments of the rare earth are close to g j J / z B. Accordingly, the effect of increasing the spontaneous magnetizations is related to a modification of Fe 3d electron band due to volume expansion, and the fact that N atoms attract the wavefunctions of R atoms rather than of Fe atoms, and in turn release the Fe atoms from bonding with R atoms [18]. The increases in the Curie temperature and the saturation magnetization upon nitrogenation are believed to be related to the cell volume expansion. Usually, the relatively low Curie temperature of R(Fe,M)12 is explained in terms of distance-dependent exchange interactions between Fe atoms. Because of the short interatomic distance between the Fe neighbours on some sites, such as 8j sites, an antiferromagnetic interaction is assumed to exist in R(Fe,M)12 compounds. Obviously, the lattice expansion obtained by introducing the nitrogen atoms reduces this effect. Moreover, a band structure calculation for Y2Fe17N3 has indicated that the lattice ex-
[321 I
r
I
~)
399 I
i21 i
t
tbl
!
~O:
20
400
50"
I
400
/
40"
I 30 ° 2e
Fig. 3. X-ray diffraction patterns of (PrxDyl_x)Fel0.sV1.5 on non-oriented and aligned samplesfor x = 0.3 (a) and x = 0.4 (b). pansion causes a narrowing of the Fe 3d band with a resulting increase in the saturation moment and the Curie temperature [17].
3.3. Magnetocrystalline anisotropy The data related to the magnetocrystalline anisotropy are also listed in Table 2. The easy magnetization lies along c-axis for x < 0.3, but in the basal plane for x > 0.4 in the host compounds at room temperature, as shown by the X-ray diffraction patterns on aligned samples in Fig. 3. Because both Pr 3+ and Dy 3+ ions have negative second-order Stevens factors, a j, the easy magnetization direction of the R 3÷ ions should lie in the basal plane in the original compounds due to the negative sign of second-order coefficient of the crystal field A20. In contrast, the Fe sublattice tends to align along the c-axis in these compounds. The competition between the R and Fe sublattices results in the easy magnetization changes with Pr content, implying that there should exist a spin reorientation at lower temperatures for x < 0.3 in the (PrxDyl_x)Felo.sVl. 5 compounds. Upon nitrogenation, the easy magnetization direction lies along c-axis in the temperature range from 0 K to Tc in the studied Pr content range. The magnetocrystalline field decreases with the Pr content in the nitride compounds. Fig. 4 shows the magnetization curves measured parallel and perpendicular to the alignment direction at 1.5 K of (PrxDyl_x)Feto.sV1.sNy for x = 0.5 and 0.8, from which the H A values of 12.5 and 13.2 T were
400
J. Yang et al. /Journal of Magnetism and Magnetic Materials 152 (1996) 396-400 160
: o"
::
the lattice parameters, Curie temperature and saturation magnetization of the compounds are increased, and the easy magnetization direction is turned into an easy c-axis. At small Dy concentrations, the (PrxDyl_x)Feto.sV1.sNy compounds have excellent intrinsic magnetic properties that would be favourable for permanent magnet applications.
..
÷
• -
0
Io
I
5O
I
•
•
•
X=
•
•
*
x:
I
50
0•5
0.8
I
"TO
H(kOe)
Fig. 4. Magnetization curves of (PrxDYl_x)Fel0.sVl.sNy measured at 1.5 K along and perpendicular to the orientation direction for x = 0.5 and 0.8.
obtained, respectively. Considering the large saturation magnetization of 145.7 e m u / g and Curie temperature Tc = 815 K, (Pr0.sDy0.2)Fel0.sVl.sNy can be considered as a potential candidate for permanent magnet applications. The change in the magnetocrystalline anisotropy originates from the effect of the interstitial nitrogen on the crystal field interaction at rare earth sites. The nitrogen atoms occupy the 2b sites, which possess the same I 4 / m m m point symmetry as the rare earth ions. Using a single-ion model and considering not only the rare earth ions, but also the nitrogen ions as ligands, we calculated the crystal field• The calculation shows that the contribution from neighbouring nitrogen ions to the crystal field is positive and large, while neighbouring rare earth ions contribute negative but small interactions, and thus make the sign of A20 become positive.
4. Conclusions We have systematically studied the phase formation of Dy-substituted (PrxDyl_x)Fel05V15 compounds and the effects of Dy content on their structure and magnetic properties. We also compared the variations in structure and magnetic properties of the compounds before and after nitrogenation. It is found that single-phase 1:12 compound can be obtained with Pr contents up to x = 0.8; the lattice parameters and saturation magnetization increase with the Pr content; the easy magnetization direction at room temperature changes from easy c-axis to easy plane between x = 0.3 and 0.4; while their Curie temperatures remain at around 590 K. Upon nitrogenation,
Acknowledgements This work was supported by the National Science Foundation and the Open Magnetism Laboratory of the Chinese Academy of Science.
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