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Electronic structures and magnetic properties of iron nitrides
i
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1933-1934 North-Holland i
i
'
Electronic structures and magnetic properties of iron nitrides S. Ishida and K. Kitawatase Department of Physws, Facult), of Science, Kagoshima Unit'ersit)'. Kagoshima 890, ..!apart To investigate the effect of nitrogen atoms :m the magnetic properties of iron atoms, band calculations were carried out for Fe4N and Felt, N ., by the Linearized Mufti;: Tin Orbitais (LMTO) method within the framework of the Local-Spin-Density (LSD) approximation. It was found that the magnetic moments of iron atoms near to nitrogen atoms decrease on the 3c sites of Fe,~N and on the 4e sites of Fet~,N2, while the iron atoms far from r.i~roge~ atoms have gi.'a-~ magr~etic moments on the la sites of Fe.~N and on the 4d sites of Fe~,N,. The structure of Fe4N [I] is of the pcrovskite type with lattice parameter a = 0.3795 nm. Fe4N h,~s two types of iron atoms (Fe(la), Fe(3c)) occupying the la and 3c sites. Shirane et al. [2] reported that the magnetic moments on Fe(la) and Fca ~c) are 3.0 and 2.0tZB and their hyperfinc fields are 34" ai~d 215 kOc, respectively. FeI6N 2 [3] has a bet crystal structure (a = 0.572 nm, c = 0.629 nm) with three types of iron atoms (Fe(4e), Fc(4d), Fe(8h)) occupying the 4e, 4d, and 8h sites. A high saturation magnetic flux density was observed for F e - N film by Kim and Takahashi [4]. They indicated that the high value (2.58 T) is attributed to the formation of Fe~6N, and that the values of the magnetic flux density and the magnetic moment are estimated as 2.83 T and 3 . 0 / x J F e , rcspectively. Mitsuoka c t a l . [5] mcasured the magnetization of F e - N compounds and estimated that of Fc~,N 2 to bc 2.611zJFc. Recently, Komuro ct al. [6] rcportcd that single cD'stal Fcl~,N 2 films can cpitaxially grow on Fc films a~.d they estimated the magnetic flux density of Fc~,N, as _.,',;-3.(I T. Using the LMTO method and the LSD approximation, we calculated the electronic structures of Fe4N and Fel~,N 2 to investigate the effect of nitrogen atoms on the magnetic properties. To describe the wavefunctions, the maximum angular momenta were chosen as lm~,x = 2 for N and Fe. Self-consistency was achieved at 35 k-points ~ r Fe4N and 75 k-points for FcI~,N 2 in tkeir irreduciblc Brillouin zones. The radii of the atomic spheres were choscn so as to preserve chargc neutrality for each atom. The dcnsity-of-statcs (DOS) of fcc Fc and Fc4N is -- COHsiuczcu z~c shown in fig. I. We --'-' .... " .l|ypotlitCtlt;izU .... : - ' Ice " '-'(Fe4N 0) with the same latticc paramctcr as Fc4N to study the effect of nitrogen atoms. Thc full and dashcd curves show the DOS for up and down spins. Comparing the DOS of fcc Fe, F e ( l a ) and Fe(3c), we notice that the hybridization between p states of N and d states of Fc in Fe4N is stronger for Fe(3c) th:m for Fc(la), because Fc(3c) has two nitrogen atoms at 0.190 nm, while Fe(la) is surrounded by 12 Fc(3c) ncarcst
neighbors at 0.296 nm. The obtained magnetic moment and hyperfine field are 3.10/zt~ and 328 kOe for Fe(la) and 1.94/z B and 242 kOe for Fe(3c). These are in good agreement with the experimental values (3.0 + 0.1)/xi~ and (345 _+ 10) kOe for Fe(la) and (2.0 __ 0.1)#! ~ and (220 _+_15) kOe for Fe(3c). In order to explain the difference of these values for fe(1) and Fe(3c), Shirane et ai. [2] considered that Fe(la) and Fe(3c) are in the I
t
'
I
/.,....; '. ,"
N p .~
~ji
1
t
f e ( 3 c ) d-~--'--'~"
-
"
"
~
~
~
":'. 30 E o
,
fe(la)
:
d
[t
30
i-, .
_ _ _
.2 " ".
, " ": ......
:-
.....
":
i
O
-0.6
0.2
Energy(Ryd)
Fig. 1. The DOS curves of the d bands of fcc Fe and the p and d bands of N, Fe(3c) and Fe(la) in Fe,~N. The full ::nO dashed curves are for up and down spins. The Fermi energy is shown by the vertical line.
0312-8853/92/$[)5.00 © 1092 - Elsevier Science Publishers B.V. All rights reserved
1934
S. lshida, K. Kitawatase / Properties of iron nitrMes ,.~---.~---
i~
e
- I.------
- 4 - - ~ '
Np
20
Fel4e) d
,',, i! t'~
: ":~ "-~ "v
Fe(4d) d
,m
-7 3o
l,u
..:
A
....
,
;
;
•
.
,
Fe(8h) d 30
,ii
,,'" i -0.6
-0.4
-0.2 t: 0.0 Energy(Ryd)
0.2
Fig. 2. The DOS curves of the p and d bands of N, Fe(4e), Fe(4d) and Fe(8h) in Fel~,N2. The full and dashed curves are for up and down spins. The Fermi energy is sho~n by the vertical line. 3d74s and 3dS4s states, respectively. That is, charge transfer from N to Fe(3c) was assumed. However, preserving charge neutrality for each atom, we could obtain satisfactory agreement with experimental values. Since the magnetic moment of fcc I:e (Fe4N 0) was calculated as 2.68tz~, the moment in Fe4N is larger on Fe(la) and smaller on Fe(3c) than that ct fcc Fe. It is found from fig. 1 that the decrement in the Fe(3c) moment is attributed to the increment of the d band tail due to the hybridization between p states of N and d states of Fe(3c) and that the effect of nitrogen atoms on the Fe(la) moment (increment) is indirect and through the change of the d bands of the neighbouring Ft.(3c). Fig. 2 shows the DOS of Fel6N2 for the p states of N and the d states of Fe(4e), Fc(4d) and Fe(Sh). Comparing the DOS of the three kinds of iron atoms, wc can see that the main peaks of the DOS are lower for Fe(4e) than for Fe(4d) and intermediate between
those of Fe(4e) and Fe(4d) for Fe(8h) and that the hybridization between the p states of N and the d states of Fe is stronger in the order Fe(4e), Fe(8h), Fe(4d). In order to clarify the influence of nitrogen atoms, we also calculated the DOS of hypothetical Fel6N 0 which has no nitrogen atom on the 2a sites in the unit cell of Fe16N 2. The local DOS of Fet6N 0 (not shown here) are similar to those of Fei~,N 2. However, the following difference is seen. The small peaks located below - 0 . 5 Ry in fig. 2 which are attributed to the hybridization, are not seen for Fe tt, No. The tails of Fe(4e) and Fe(8h) above the Fermi level are high for Fet6N 2 than for Fel6N 0, while that of Fe(4d) is lower. From the change of the tail due to the occupation of the 2a sites by N, we can expect that the magnetic moments on Fe(4e) and Fe(8h) decrease, while those on Fe(4d) increase. In fact, the moments on Fe(4e), Fe(8h) and Fe(4d) are calculated as 1.96, 2.41 and 2.91~a for Fet6N a and 2.35, 2.51 and 2.58~n for FeigN 0, respectively. We also calculated the hyperfine magnetic fields. They are 241, 250 and 337 kOe on Fe(4e), Fe(8h) and Fe(4d) sites, which is not proportional to the own magnetic moment. We divide the contribution into two parts, that is, the contribution from core s electrons (denoted by H c) and that from valence s electrons (denoted by Hv). The values of H c are 196, 247 and 302 kOe and those of H v are 44, 4 and 34 kOe for Fe(4e), Fe(8h) and Fe(4d), respectively. ttc is proportional to the own moment (about - 1 0 0 kOe//x~), but H,, strongly depends on the neighbouring moments. As described above, we found that the d states of iron atoms (Fe(3c) in Fe4N and Fe(4e) and Fe(8h) in Fc ~,N 2) close to nitrogen atoms strongly hybridize with the valence states of the nitrogen atoms and the moment of the iron sites decrease, while the iron atoms (Fe(la) in FeaN and Fe(4d) in Fe~6N 2) are from nitrogen atoms are indi~'cctly influet~ced by the nitrogens through the neighbouring iron atoms and carry giant magnetic moments. References
[1] K.H. Jack, Proc. Roy. Soc. London Ser. A 195 (1948) 34. [2] G. Shirane, W.J. Takei and S.L. Ruby, Phys. Rev. 126 (! 962) 49. [3] K.H. Jack, Proc. Roy. Soc. London Ser. A 208 (1951) 216. [4] T.K. Kim and M. Takahashi, Appl. Phy:;. Lett. 20 (1972) 492. [5] K. Mitsuoka, tt. Miyajima, H. Ino and S. Chikazumi, J. Phys. Soc. Jpn. 53 (1984) 2381. [6] M. Komuro, Y. Kozono, M. Hanazono and Y. Sugita, J. Appl. Phys. 67 (1990) 5126.
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1933-1934 North-Holland i
i
'
Electronic structures and magnetic properties of iron nitrides S. Ishida and K. Kitawatase Department of Physws, Facult), of Science, Kagoshima Unit'ersit)'. Kagoshima 890, ..!apart To investigate the effect of nitrogen atoms :m the magnetic properties of iron atoms, band calculations were carried out for Fe4N and Felt, N ., by the Linearized Mufti;: Tin Orbitais (LMTO) method within the framework of the Local-Spin-Density (LSD) approximation. It was found that the magnetic moments of iron atoms near to nitrogen atoms decrease on the 3c sites of Fe,~N and on the 4e sites of Fet~,N2, while the iron atoms far from r.i~roge~ atoms have gi.'a-~ magr~etic moments on the la sites of Fe.~N and on the 4d sites of Fe~,N,. The structure of Fe4N [I] is of the pcrovskite type with lattice parameter a = 0.3795 nm. Fe4N h,~s two types of iron atoms (Fe(la), Fe(3c)) occupying the la and 3c sites. Shirane et al. [2] reported that the magnetic moments on Fe(la) and Fca ~c) are 3.0 and 2.0tZB and their hyperfinc fields are 34" ai~d 215 kOc, respectively. FeI6N 2 [3] has a bet crystal structure (a = 0.572 nm, c = 0.629 nm) with three types of iron atoms (Fe(4e), Fc(4d), Fe(8h)) occupying the 4e, 4d, and 8h sites. A high saturation magnetic flux density was observed for F e - N film by Kim and Takahashi [4]. They indicated that the high value (2.58 T) is attributed to the formation of Fe~6N, and that the values of the magnetic flux density and the magnetic moment are estimated as 2.83 T and 3 . 0 / x J F e , rcspectively. Mitsuoka c t a l . [5] mcasured the magnetization of F e - N compounds and estimated that of Fc~,N 2 to bc 2.611zJFc. Recently, Komuro ct al. [6] rcportcd that single cD'stal Fcl~,N 2 films can cpitaxially grow on Fc films a~.d they estimated the magnetic flux density of Fc~,N, as _.,',;-3.(I T. Using the LMTO method and the LSD approximation, we calculated the electronic structures of Fe4N and Fel~,N 2 to investigate the effect of nitrogen atoms on the magnetic properties. To describe the wavefunctions, the maximum angular momenta were chosen as lm~,x = 2 for N and Fe. Self-consistency was achieved at 35 k-points ~ r Fe4N and 75 k-points for FcI~,N 2 in tkeir irreduciblc Brillouin zones. The radii of the atomic spheres were choscn so as to preserve chargc neutrality for each atom. The dcnsity-of-statcs (DOS) of fcc Fc and Fc4N is -- COHsiuczcu z~c shown in fig. I. We --'-' .... " .l|ypotlitCtlt;izU .... : - ' Ice " '-'(Fe4N 0) with the same latticc paramctcr as Fc4N to study the effect of nitrogen atoms. Thc full and dashcd curves show the DOS for up and down spins. Comparing the DOS of fcc Fe, F e ( l a ) and Fe(3c), we notice that the hybridization between p states of N and d states of Fc in Fe4N is stronger for Fe(3c) th:m for Fc(la), because Fc(3c) has two nitrogen atoms at 0.190 nm, while Fe(la) is surrounded by 12 Fc(3c) ncarcst
neighbors at 0.296 nm. The obtained magnetic moment and hyperfine field are 3.10/zt~ and 328 kOe for Fe(la) and 1.94/z B and 242 kOe for Fe(3c). These are in good agreement with the experimental values (3.0 + 0.1)/xi~ and (345 _+ 10) kOe for Fe(la) and (2.0 __ 0.1)#! ~ and (220 _+_15) kOe for Fe(3c). In order to explain the difference of these values for fe(1) and Fe(3c), Shirane et ai. [2] considered that Fe(la) and Fe(3c) are in the I
t
'
I
/.,....; '. ,"
N p .~
~ji
1
t
f e ( 3 c ) d-~--'--'~"
-
"
"
~
~
~
":'. 30 E o
,
fe(la)
:
d
[t
30
i-, .
_ _ _
.2 " ".
, " ": ......
:-
.....
":
i
O
-0.6
0.2
Energy(Ryd)
Fig. 1. The DOS curves of the d bands of fcc Fe and the p and d bands of N, Fe(3c) and Fe(la) in Fe,~N. The full ::nO dashed curves are for up and down spins. The Fermi energy is shown by the vertical line.
0312-8853/92/$[)5.00 © 1092 - Elsevier Science Publishers B.V. All rights reserved
1934
S. lshida, K. Kitawatase / Properties of iron nitrMes ,.~---.~---
i~
e
- I.------
- 4 - - ~ '
Np
20
Fel4e) d
,',, i! t'~
: ":~ "-~ "v
Fe(4d) d
,m
-7 3o
l,u
..:
A
....
,
;
;
•
.
,
Fe(8h) d 30
,ii
,,'" i -0.6
-0.4
-0.2 t: 0.0 Energy(Ryd)
0.2
Fig. 2. The DOS curves of the p and d bands of N, Fe(4e), Fe(4d) and Fe(8h) in Fel~,N2. The full and dashed curves are for up and down spins. The Fermi energy is sho~n by the vertical line. 3d74s and 3dS4s states, respectively. That is, charge transfer from N to Fe(3c) was assumed. However, preserving charge neutrality for each atom, we could obtain satisfactory agreement with experimental values. Since the magnetic moment of fcc I:e (Fe4N 0) was calculated as 2.68tz~, the moment in Fe4N is larger on Fe(la) and smaller on Fe(3c) than that ct fcc Fe. It is found from fig. 1 that the decrement in the Fe(3c) moment is attributed to the increment of the d band tail due to the hybridization between p states of N and d states of Fe(3c) and that the effect of nitrogen atoms on the Fe(la) moment (increment) is indirect and through the change of the d bands of the neighbouring Ft.(3c). Fig. 2 shows the DOS of Fel6N2 for the p states of N and the d states of Fe(4e), Fc(4d) and Fe(Sh). Comparing the DOS of the three kinds of iron atoms, wc can see that the main peaks of the DOS are lower for Fe(4e) than for Fe(4d) and intermediate between
those of Fe(4e) and Fe(4d) for Fe(8h) and that the hybridization between the p states of N and the d states of Fe is stronger in the order Fe(4e), Fe(8h), Fe(4d). In order to clarify the influence of nitrogen atoms, we also calculated the DOS of hypothetical Fel6N 0 which has no nitrogen atom on the 2a sites in the unit cell of Fe16N 2. The local DOS of Fet6N 0 (not shown here) are similar to those of Fei~,N 2. However, the following difference is seen. The small peaks located below - 0 . 5 Ry in fig. 2 which are attributed to the hybridization, are not seen for Fe tt, No. The tails of Fe(4e) and Fe(8h) above the Fermi level are high for Fet6N 2 than for Fel6N 0, while that of Fe(4d) is lower. From the change of the tail due to the occupation of the 2a sites by N, we can expect that the magnetic moments on Fe(4e) and Fe(8h) decrease, while those on Fe(4d) increase. In fact, the moments on Fe(4e), Fe(8h) and Fe(4d) are calculated as 1.96, 2.41 and 2.91~a for Fet6N a and 2.35, 2.51 and 2.58~n for FeigN 0, respectively. We also calculated the hyperfine magnetic fields. They are 241, 250 and 337 kOe on Fe(4e), Fe(8h) and Fe(4d) sites, which is not proportional to the own magnetic moment. We divide the contribution into two parts, that is, the contribution from core s electrons (denoted by H c) and that from valence s electrons (denoted by Hv). The values of H c are 196, 247 and 302 kOe and those of H v are 44, 4 and 34 kOe for Fe(4e), Fe(8h) and Fe(4d), respectively. ttc is proportional to the own moment (about - 1 0 0 kOe//x~), but H,, strongly depends on the neighbouring moments. As described above, we found that the d states of iron atoms (Fe(3c) in Fe4N and Fe(4e) and Fe(8h) in Fc ~,N 2) close to nitrogen atoms strongly hybridize with the valence states of the nitrogen atoms and the moment of the iron sites decrease, while the iron atoms (Fe(la) in FeaN and Fe(4d) in Fe~6N 2) are from nitrogen atoms are indi~'cctly influet~ced by the nitrogens through the neighbouring iron atoms and carry giant magnetic moments. References
[1] K.H. Jack, Proc. Roy. Soc. London Ser. A 195 (1948) 34. [2] G. Shirane, W.J. Takei and S.L. Ruby, Phys. Rev. 126 (! 962) 49. [3] K.H. Jack, Proc. Roy. Soc. London Ser. A 208 (1951) 216. [4] T.K. Kim and M. Takahashi, Appl. Phy:;. Lett. 20 (1972) 492. [5] K. Mitsuoka, tt. Miyajima, H. Ino and S. Chikazumi, J. Phys. Soc. Jpn. 53 (1984) 2381. [6] M. Komuro, Y. Kozono, M. Hanazono and Y. Sugita, J. Appl. Phys. 67 (1990) 5126.