- Email: [email protected]
Spin polarization of Ni overlayers on an Al(100) substrate
Journal of Magnetism and Magnetic Materials 35 (1983) 9-10 North-Holland Publishing Company S P I N P O L A R I Z A T I O N O F Ni O V E R L A Y E R S O N A N AI(100) S U B S T R A T E N. T A T E N O *, T. K A M B A R A **, K.I. G O N D A I R A The University of Electro-Communications, Chofu, Tokyo 182, Japan a n d Y. I S S H I K I Tokyo Metropolitan Industrial Technical Institute, Nishigaoka, Kita.ku, Tokyo 115, Japan
The spin-polarized electronic structures of the two 6-layer (100) slabs, NilA15 and Ni2AI 4 are determined by using the discrete-variational Xa method. The results show that the magnetic moment of the Ni layers in contact with Al is noticeable reduced but not completely suppressed.
1. Introduction The magnetism of surfaces and interfaces of thin Ni layers deposited on a nonmagnetic metallic substrates has been actively investigated. However, the problem of whether the ferromagnetism of Ni in the surface and the interfacce is suppressed or not has not been dearly settled yet. Libermann et al. [1] observed the variation of the ferromagnetic flux with the thickness of a Ni film and found that a few layers were magnetically inert. Bergmann [2] concluded from anomalous Hall effect measurements that a very thin Ni film backed with a normal metal is not ferromagnetic. Meservey et al. [3] have shown by spin-polan'zed tunneling measurements that ferromagnetism is suppressed in films of Ni thinner than three atomic layers when they are in contact with A1 substrate. Cox et al. [4] studied qualitatively the magnetic properties of a film of an itinerant ferromagnet in contact with a normal metal using the Hubbard model Hamiltonian. They showed that the magnetic moment vanished more readily for films with a nearly filled band as in Ni than for those with an approximately half-filled band as in Fe. These results conflict with the spin-polarized photoemission measurement by Pierce and Siegmann [5]. They suggested that Ni becomes ferromagnetic for thickness as low as a monolayer. Furthermore Freeman et al. [6] determined the magnetic properties of Ni overlayers on Cu000) substrate on the basis of self-consistent calculations of the spin-polarized electronic structure. The result is that the magnetic moments on an interface layer of Ni are reduced but do not zero. In order to make clear the magnetic properties of Ni in very thin layers deposited on a normal metal substrate, we determine the spin-polarized electronic structures of the two 6-layer (100) slabs, NilAI 5 and Ni2A14 * Present address: Hitachi Software Engineering Co. Ltd., Kanagawa, Japan **To whom correspondence should be addressed.
using the self-consistent charge discrete variational X a method [7-9]. The calculated results show that the magnetic moment of the Ni films in contact with Al is noticeably reduced but not completely suppressed. 2. Computational method The essential features of the computational techniques used in the present work have been discussed in detail in the previous paper [9]. Here we give only a ~brief description of the part relevant to the present calculation. Two-dimensional Bloch wavefunctions are constructed from a numerical basis set (3d, 4s, 4p for Ni and 3s, 3p for A1) and orthogonalized to core-orbital wavefunctions. The atomic orbitals are obtained by solving the atomic Schri~linger equation with the spherical potential whose radius is 3.0 a.u. and depth is - 1 . 0 a.u.. We adopt the X a approximation for the exchange-correlation potential and take a = 0.7. The eigenvalue equation is solved iteratively at each k point, the matrix elements in it being evaluated numerically by substituting weighted sums of the integrand values at 2400 sample points. The iterative procedure is carried out until the coincidence between the input and output values of the occupation number of the atomic orbitals is attained within 0.05. The occupation number is determined by the Mulliken population analysis method. The integration in k space is made by using a linear analytical triangle scheme based on sampling I0 k points in one eighth of the two-dimensional Brillouin zone. 3. Results and discussion The self-consistent electron populations of 3d, 4s, and 4p orbitals of a Ni atom are listed in table 1 for the NilAl ~ (100) slab in which the distances between atoms are the same as in an fcc A1 crystal. The spin polarization arises only from 3d electrons and is 0.23 which is
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
10
N. Tateno et aL / Spin polarization of Ni overlayers on AI(IO0)
Table 1 Self-consistent atomic-orbital population of majority (1') and minority (J,) spin electrons and spin-polarization of 3d electrons (#d) in Ni layers for the monolayer (100) film, the NilAl 5 (100) slab, and the Ni2AI 4 (100) slab Orbital
Mono layer
Ni l AI ~
Surface of Ni2A14
Interface of Ni2A14
3d 1" 3d J, 4s 1" 4s J, 4p 1" 4p ~, #d
4.67 3.91 0.49 0.46 0.24 0.22 0.76
4.53 4.30 0.35 0.33 0.26 0.25 0.23
4.52 4.07 0.48 0.47 0.25 0.22 0.45
4.55 4.36 0.33 0.31 0.23 0.22 0.19
0./-~ 0.50 0.60
0.10
0 . 2 0 0.30
0.40
0.50
0.60 0.70
ENERGY(AU)
ENERGY(A U ) 0.00 0.10 0.20 0.30
Majority Spin
Ill
-0.40 -0.30 -0.20 -0.10 0.00
very small compared with the value, 0.6, for a bulk Ni crystal. The local density of states for the Ni layer in the N i l A I 5 (100) slab is shown in fig. 1. In order to investigate the origin of the reduction of the spin polarization in the Ni overlayer, we calculated the electronic structure of a monolayer Ni film whose atomic separations are the same as in an fcc AI crystal. The density of stes of the film is shown in fig. 2. The electron populations of each valence orbital are also listed in table 1. It is seen from comparing fig. 1 with fig. 2 that the width of the narrow d band increases and the exchange splitting between the majority and minority spin d bands decreases by the backing of a Ni layer with AI layers. The population of the majority spin 3d orbitals decreases slightly by the backing, while the population of the minority spin 3d orbitals increases noticeably by the backing. The disappearance of the minority spin d hole comes not from the electron transfer from the AI substrate to the Ni layer but from the variation of electronic configuration of Ni atoms, which is due to the
.0.50-0.&0-0.30-0.20-0.10
IlJl I
Fig. 2. Density of states of majority-spin and minority-spin electrons of the monolayer Ni(100) film. raising of the 4s band through mixing with the 3s and 3p bands of AI. The self-consistent electron populations of 3d, 4s, and 4p orbitals of a Ni atom in the surface Ni layer and in the interface Ni layer are listed in table 1 for the N i 2 A I 4 (100) slab in which all of the atomic distances are the same as in fcc A1 except that the distance between the surface and interface Ni layers is that of a bulk Ni crystal. The electron populations in the interface Ni layer are quite similar to those in the Ni layer of the Ni IA15 slab, while the populations in the surface Ni layer are similar to those in the surface layer of a nine-layer Ni(100) slab [10]. A similar result has been obtained from the Ni2CusNi2 (100) slab [6]. There exists a tendency of the electron populations in Ni overlayres on a normal metal substrate such that the spin polarization in the interface Ni layer is reduced noticeably .but the influence of the substrate hardly appears upon the spin polarization of the Ni layers above the interface.
0.70
References
Majority Spin
-ll
Minority Spin
Fig. 1. Local density of states of majority-spin and minority-spin electrons for the Ni layer in the NiiAl 5 (100) slab.
[1] L. Liebermann, J. Clinton, D.M. Edwards and J. Mathon, Phys. Rev. Lett. 25 (1970) 232. [2] G. Bergmann, Phys. Rev. Lett. 41 (1978) 264. [3] R. Meservey, P.M. Tedrow and V.R. Kalvey, J. Appl. Phys. 52 (1981) 1617. [4] B.N. Cox, R.A. Tahir-Kh¢li and R.J. Elliott, Phys. Rev. B20 (1979) 2864. [5] D.T. Pierce and H.C. Siegmann, Phys. Rev. B9 (1974) 4035. [6] A.J. Freeman, D.S. Wang and H. Krakauer, J. Appl. Phys. 53 (1902) 1997. [7] D.E. Ellis and G.S. Painter, Phys. Rev. B2 (1970) 2887. [8] C.S. Wang and A.J. Freeman, Phys. Rev. B19 (1979) 793. [9] G. Yokoyama, N. Hirashita, T. Oguchi, T. Kambara and K.I. Gondaira, J. Phys. FII (1981) 1643. [10] C.S. Wang and A.J. Freeman, Phys. Rev. B21 (1980) 4585.
The spin-polarized electronic structures of the two 6-layer (100) slabs, NilA15 and Ni2AI 4 are determined by using the discrete-variational Xa method. The results show that the magnetic moment of the Ni layers in contact with Al is noticeable reduced but not completely suppressed.
1. Introduction The magnetism of surfaces and interfaces of thin Ni layers deposited on a nonmagnetic metallic substrates has been actively investigated. However, the problem of whether the ferromagnetism of Ni in the surface and the interfacce is suppressed or not has not been dearly settled yet. Libermann et al. [1] observed the variation of the ferromagnetic flux with the thickness of a Ni film and found that a few layers were magnetically inert. Bergmann [2] concluded from anomalous Hall effect measurements that a very thin Ni film backed with a normal metal is not ferromagnetic. Meservey et al. [3] have shown by spin-polan'zed tunneling measurements that ferromagnetism is suppressed in films of Ni thinner than three atomic layers when they are in contact with A1 substrate. Cox et al. [4] studied qualitatively the magnetic properties of a film of an itinerant ferromagnet in contact with a normal metal using the Hubbard model Hamiltonian. They showed that the magnetic moment vanished more readily for films with a nearly filled band as in Ni than for those with an approximately half-filled band as in Fe. These results conflict with the spin-polarized photoemission measurement by Pierce and Siegmann [5]. They suggested that Ni becomes ferromagnetic for thickness as low as a monolayer. Furthermore Freeman et al. [6] determined the magnetic properties of Ni overlayers on Cu000) substrate on the basis of self-consistent calculations of the spin-polarized electronic structure. The result is that the magnetic moments on an interface layer of Ni are reduced but do not zero. In order to make clear the magnetic properties of Ni in very thin layers deposited on a normal metal substrate, we determine the spin-polarized electronic structures of the two 6-layer (100) slabs, NilAI 5 and Ni2A14 * Present address: Hitachi Software Engineering Co. Ltd., Kanagawa, Japan **To whom correspondence should be addressed.
using the self-consistent charge discrete variational X a method [7-9]. The calculated results show that the magnetic moment of the Ni films in contact with Al is noticeably reduced but not completely suppressed. 2. Computational method The essential features of the computational techniques used in the present work have been discussed in detail in the previous paper [9]. Here we give only a ~brief description of the part relevant to the present calculation. Two-dimensional Bloch wavefunctions are constructed from a numerical basis set (3d, 4s, 4p for Ni and 3s, 3p for A1) and orthogonalized to core-orbital wavefunctions. The atomic orbitals are obtained by solving the atomic Schri~linger equation with the spherical potential whose radius is 3.0 a.u. and depth is - 1 . 0 a.u.. We adopt the X a approximation for the exchange-correlation potential and take a = 0.7. The eigenvalue equation is solved iteratively at each k point, the matrix elements in it being evaluated numerically by substituting weighted sums of the integrand values at 2400 sample points. The iterative procedure is carried out until the coincidence between the input and output values of the occupation number of the atomic orbitals is attained within 0.05. The occupation number is determined by the Mulliken population analysis method. The integration in k space is made by using a linear analytical triangle scheme based on sampling I0 k points in one eighth of the two-dimensional Brillouin zone. 3. Results and discussion The self-consistent electron populations of 3d, 4s, and 4p orbitals of a Ni atom are listed in table 1 for the NilAl ~ (100) slab in which the distances between atoms are the same as in an fcc A1 crystal. The spin polarization arises only from 3d electrons and is 0.23 which is
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
10
N. Tateno et aL / Spin polarization of Ni overlayers on AI(IO0)
Table 1 Self-consistent atomic-orbital population of majority (1') and minority (J,) spin electrons and spin-polarization of 3d electrons (#d) in Ni layers for the monolayer (100) film, the NilAl 5 (100) slab, and the Ni2AI 4 (100) slab Orbital
Mono layer
Ni l AI ~
Surface of Ni2A14
Interface of Ni2A14
3d 1" 3d J, 4s 1" 4s J, 4p 1" 4p ~, #d
4.67 3.91 0.49 0.46 0.24 0.22 0.76
4.53 4.30 0.35 0.33 0.26 0.25 0.23
4.52 4.07 0.48 0.47 0.25 0.22 0.45
4.55 4.36 0.33 0.31 0.23 0.22 0.19
0./-~ 0.50 0.60
0.10
0 . 2 0 0.30
0.40
0.50
0.60 0.70
ENERGY(AU)
ENERGY(A U ) 0.00 0.10 0.20 0.30
Majority Spin
Ill
-0.40 -0.30 -0.20 -0.10 0.00
very small compared with the value, 0.6, for a bulk Ni crystal. The local density of states for the Ni layer in the N i l A I 5 (100) slab is shown in fig. 1. In order to investigate the origin of the reduction of the spin polarization in the Ni overlayer, we calculated the electronic structure of a monolayer Ni film whose atomic separations are the same as in an fcc AI crystal. The density of stes of the film is shown in fig. 2. The electron populations of each valence orbital are also listed in table 1. It is seen from comparing fig. 1 with fig. 2 that the width of the narrow d band increases and the exchange splitting between the majority and minority spin d bands decreases by the backing of a Ni layer with AI layers. The population of the majority spin 3d orbitals decreases slightly by the backing, while the population of the minority spin 3d orbitals increases noticeably by the backing. The disappearance of the minority spin d hole comes not from the electron transfer from the AI substrate to the Ni layer but from the variation of electronic configuration of Ni atoms, which is due to the
.0.50-0.&0-0.30-0.20-0.10
IlJl I
Fig. 2. Density of states of majority-spin and minority-spin electrons of the monolayer Ni(100) film. raising of the 4s band through mixing with the 3s and 3p bands of AI. The self-consistent electron populations of 3d, 4s, and 4p orbitals of a Ni atom in the surface Ni layer and in the interface Ni layer are listed in table 1 for the N i 2 A I 4 (100) slab in which all of the atomic distances are the same as in fcc A1 except that the distance between the surface and interface Ni layers is that of a bulk Ni crystal. The electron populations in the interface Ni layer are quite similar to those in the Ni layer of the Ni IA15 slab, while the populations in the surface Ni layer are similar to those in the surface layer of a nine-layer Ni(100) slab [10]. A similar result has been obtained from the Ni2CusNi2 (100) slab [6]. There exists a tendency of the electron populations in Ni overlayres on a normal metal substrate such that the spin polarization in the interface Ni layer is reduced noticeably .but the influence of the substrate hardly appears upon the spin polarization of the Ni layers above the interface.
0.70
References
Majority Spin
-ll
Minority Spin
Fig. 1. Local density of states of majority-spin and minority-spin electrons for the Ni layer in the NiiAl 5 (100) slab.
[1] L. Liebermann, J. Clinton, D.M. Edwards and J. Mathon, Phys. Rev. Lett. 25 (1970) 232. [2] G. Bergmann, Phys. Rev. Lett. 41 (1978) 264. [3] R. Meservey, P.M. Tedrow and V.R. Kalvey, J. Appl. Phys. 52 (1981) 1617. [4] B.N. Cox, R.A. Tahir-Kh¢li and R.J. Elliott, Phys. Rev. B20 (1979) 2864. [5] D.T. Pierce and H.C. Siegmann, Phys. Rev. B9 (1974) 4035. [6] A.J. Freeman, D.S. Wang and H. Krakauer, J. Appl. Phys. 53 (1902) 1997. [7] D.E. Ellis and G.S. Painter, Phys. Rev. B2 (1970) 2887. [8] C.S. Wang and A.J. Freeman, Phys. Rev. B19 (1979) 793. [9] G. Yokoyama, N. Hirashita, T. Oguchi, T. Kambara and K.I. Gondaira, J. Phys. FII (1981) 1643. [10] C.S. Wang and A.J. Freeman, Phys. Rev. B21 (1980) 4585.