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Hyperfine fields and electronic structures in FeCo and FeNi multilayer systems
Solid State Communications, Vol. 95, No. 3, pp. 127-130, 1995 Copyright 8 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 003%1098/95 $9.50+.00 0038-1098(95)00249-9
Pergamon
HYPER.FlNE FIELDS AND ELECTRONIC STRUCTURES Fe/Co AND Fe/Ni MULTILAYER SYSTEMS Manabu Institute
Takahashi,
for Materials
IN
Xiac I-111and Yoshiyuki Kawazoe
Research,
Tohoku IJniversity,
Sendai 980-77, Japan
(Received 31 January 1995; revised version received 23 March 1995 by T. Tsuzuki)
Using KKR band structure calculation method, hyperfine field distributions and electronic structures in Fe&o3 and FeB/Nia multilayer systems are investigated where all the atoms are sited on bee lattice points. The calculated hyperfine fields at the Fe sites in the interior monolayer-s are enhanced and those at the Fe site in the interfacial monolayers are reduced. It is found that the reduction of transferred hyperfine field (valence electron contribution to the hyperfine fields) results in the reduced hyperfine field at the Fe site in the interfacial monolayers. Core electron contributions are almost proportional to the local spin moment (ok = -99kG/hn) and valence ones are roughly proportional to the total moment within the first two shells of neighbors.
Keywords: A. metals, A. magnetic fiims and multilayers, A. magnetically ordered materials, D. electronic band structure.
1. Introduction
cated on the bee lattice points are investigated by a first principle band structure calculation. The influence of lattice relaxations on the hyperfine fields and local spin moment distributions is also checked. In the next section the geometries and methods used in the present study are briefly mentioned. In section 3 the calculated results are described and the discussions are given in section 4. The last section is devoted to the conclusions.
Magnetic multilayer films have attracted considerable attention from academic and industrial interests, because they show much interesting physical properties useful for magnetooptical recording, e.g. giant magneto resistance and perpendicular anisotropy’, ‘. Among them, magnetic thin films and multilayers made by molecular beam epitaxy (MBE) method have attracted growing re search interests since met&able crystalline phases can be obtained by this method31 4. Nuclear magnetic resonance (NMR) or Mijsbauer spectroscopy study is one of the most powerful methods to obtain the information on electronic, magnetic and crystal structures in solids. However, few studies have been made on the details of hyperfine parameters in such magnetic multilayer systems. Recently, it was shown by Bakkaloglu et al5 that two kinds of Fe sites accompanied with higher and lower hyperfine fields exist in Fe/Co multilayers prepared by magnetron sputtering method. They concluded that the Fe sites accompanied with higher fields are located in the interfacial region and those with lower fields in the interior region. On the other hand, it was pointed out by Gutierrez et al6 that in bee Ni/[Fe(lOO)] multilayers made by MBE there are two kinds of Fe sites like Co/Fe multilayer; the higher field Fe sites are in the interior Fe monolayers and the lower ones in the interfacial Fe monolayers. In this paper hyperfine field distributions in both Fe/Co and Fe/Ni multilayers where all atoms are lo
2. Geometry
and Method
In the present study, we consider multilayers consisting of three Co or Ni monolayers and three Fe monolayers periodically in order to investigate the hyperfine field and local spin moment distributions. All the atoms are assumed to be on regular or distorted bee lattice points. Both [110] and (loo] stacked structures are investigated. The m&in-tin Green function KKR method is used to perform band structure and total energy calculations’. The spin dependent exchange correlation potential pro posed by von Barth and Hedin” is used with Morsi, Janack and Willams (MJW) parameterizationg and nonrelativistic Schrijdinger equations are solved. The hyperfine field is given by the equation Hhf = !fp~ (nt - nl), where nt and rzl are the up and down spin electron densities at nucleus, respectively. The theoretical understanding of hyperfine interactions in metals and metallic alloys and the density functional calculations of the quantities relating to hyperfine interactions have been reviewed by H. Akai et al.” 127
128
Fe/Co AND Fe/Ni MULTILAYER
Table 1: Calculated lattice parameters by total energy minimization. The units for lattice constant and for monolayer thickness are Bohr radius and the lattice constant, respectively.
SYSTEMS
Vol. 95, No. 3
Table 3: Calculated hyperfine field(kG) in Co3 /Fea and Nia /Fes multilayers. The values in brackets are experimental one@. Co3P3
a dx-x bulk a
Cos/[bccFez(lOO)] Nis/[bccFea(lOO)] 5.27 5.28 0.48a 0.50a bccFe 5.34 bccCo 5.27 bccNi 5.24
WOI
~W
Fe1
-272
-268
-252(-303)
-260(-304)
Fe2 Xl
-283 -205
-286 -215
-302(-352) -128
-279(-347) -121
-189 bccCo -180
-100 bccNi -64
x2
Since a notable lattice relaxation has experimentally been observed in Fe/Ni multilayer& ‘*, in order to es timate the influence of lattice relaxation on the hyperfine field, lattice relaxation is taken into account. There are many adjustable geometrical parameters to obtain the most stable structure in multilayer systems. In the present calculation, we restrict the number of adjustable parameters. At first, esch atom is put on the bee lattice points whose lattice constant is obtained by total energy minimization of bulk bee Fe. After the unit cell volumes are relaxed, the distance dx-x (X=Co, Ni) between two X monolayers and dpe-X between Fe and X monolayers are relaxed with the relation dFe-x = dx-x kept and the distance dFe-Fe fixed. 3. Results Table 1 shows the lattice parameters obtained by the present total energy minimizations of Cos/Fes and Nis/Fes multilayem. It is observed that the thickness of Co monolayer on the bee Fe (100) surface is almost the same as dlzml which is the distance between two (100) planes in bee lattice. On the other hand, the thickness of Ni monolayer decreases from dlzsq. The local spin moment distributions in the Fes/Cos and Fes/Nis multilayers and in the bulk bee Fe, Co and Ni are shown in Table 2. In Table 2, the lattice pa rametem are fixed at the values for bee lattice. After the lattice parameters are altered, the spin moments at interfacial Fe, interior Fe, interfacial Ni and interior Ni sites are changed to 2.49, 2.27, 0.62 and 0.35 (pg), respectively. The local spin moments at the Fe sites are enhanced from the bulk bee Fe. fipecially, the local moments of the interfacial Fe are strongly enhanced and those of the Co and Ni sre also enhanced from their bulk values. In the Ni layer, the spin moment largely varies from monolayer to monolayer comparing with the Co
Table 2: Calculated
spin moment(pg) (in Cos /Fe3 and Nig /Fez multilayers stacked on bee [lOO] and [llO] directions. Fe1 and Xl are interfacial atoms and Fe2 and X2 are interior atoms.
CodFe3
WI Fe1 Fe2 Xl x2
2.52 2.33 1.73 1.75 bccFe 2.22 fccNi 0.59
Nis/Fea
WI 2.45 2.34 1.75 1.75 bccCo 1.69
WOl 2.55 2.30 0.66 0.46 bccNi 0.48
-197 bccFe -242
w
-80
layer. In the Ni layer the difference of spin moment between the interfacial and interior Ni sites is about 0.2pn, while in the Co layer the difference is not found. Table 3 shows the hyperfine field distributions in the Fe3/Co3 and Fes/Nis multilayers and in the bulk bee Fe, Co and Ni. In both Fe/Co and Fe/Ni multilayer systems, the hyperfine fields at the Fe site in the interfacial monolayers are reduced from the values at the interior Fe site. The hyperfine fields at the Co and Ni sites are enhanced from their bulk values. Especially, those at the sites in the interfacial monolayers are most strongly enhanced. The difference of the hyperfine field values between the interfacial and interior Fe sites is larger in the [loo] stacked layer than the Ill01 stacked one for Fe/Ni multilayers and it is little smaller in the [loo] stacked layer than the (1101 stacked one for the Fe/Co case. Core and valence electron contributions to the hyperfine fields( Hrc and Hh) are separately shown in Table 4. Hrc is large at the site where the local spin moment is large. Hh is influenced by the neighboring atoms; it is found from Table 4 that if an atom is surrounded by the atoms having large spin moments, it becomes large. After the lattice parameters are altered, the values of interfacial Fe, interior Fe, interfacial Ni and interior Ni sites are changed to -242, -298, -119 and -98 (kG), respectively. The change of the hyperfine field is at most about 1T between before and after the lattice parame tern are altered. It is noticed that the calculated hyperfine fields are about 20% smaller than the experimental values. Thii is caused by the fact that the local spin density approximation for exchange-correlation potentials are used. As far as relative values and trends are concerned, the re sults are consistent with the experiments. Figure 1 shows the partial densities of states at the atoms in the interfscial and interior monolayers for the present multilayers. The partial DOS at the interfacial Table 4: Calculated core and valence electron contributions(kG) to the hypefine field shown at table3.
WOI 2.46 2.29 0.62 0.51 hcpCo 1.58
Ni3F-3
PW
Fe1 Fe2 Xl X2
CoJ[bccFes(lOO)] valence core -255 -17 -234 -49 -179 -26 -182 -14
Ni&mFe3(100)1 core -257 -230 -72 -51
valence
6 -72 -57 -49
Vol. 95, No. 3
Fe/Co AND Fe/Ni MULTILAYER
0
-0.5
II
I
0
-0.5
-wy(Wd)
Energy(W)
Figure 1. Partial density of states at interfacial atoms in Xs/Fes multilayers. Energy is measured from Fermi level in Rydberg.
sites are roughly similar to those of F&o and Fe-Ni dilute all~ys’~* 14. The interfscial Co site partial DOS is similar to the Co impurity site partial DOS in the iron host even around the Fermi level. However, there is a noticeable difference between the Ni site partial DOS in Fe/Ni multilayer system and the Ni impurity in Fe host around the Fermi level. In the Fe/Ni layer case, the peak of the minority-spin DOS near the Fermi level sinks under the Fermi level, though it is located just on the Fermi level for the case of Ni impurity in the Fe host13. This leads to the conclusion that the spin moment of the Ni atom in the interface is not ss large as that of the Ni impurity in Fe host. 4. Discussion In the present calculation it was found that the Ni layers on bee Fe (100) surface prefer to bet structure with c/a less than one. In the Co layer on the same surface, the monolayer thickness does not significantly change from the values of the bee structure within the present optimization. Because of using muffin-tin ap proximation and the restricted optimization, we can not determine the precise values of the lattice parameters. However, at least we can mention that the spin moment and hyperfine field distributions are not sensitive to the lattice parameters as far ss their tendencies are concerned. The amount of change of hyperfine fields be tween before and after the lattice parameters are altered is within about 1T and the distributions are not significantly affected. We also checked that the dependency of the hyperfme distributions on dFe_Fe and it was found that even if dFe-Fe changes about lo%, the distributions are not seriously affected. The enhancement of spin moment at the Fe site in the interfacial layers is due to the similar reason to that concerning to the FeCo(Ni) al10ys’~* 14. Comparing the partial density of state of bulk bee Fe, the local majorityspin DOS at Fe site becomes more filled and the weight of the minority-spin DOS below the fermi energy be comes smaller. In the Ni layers the difference of spin
SYSTEMS
129
moment between the interfacial and interior Ni sites is about 0.2,.&B, while in the Co layers such difference is not found. It indicates that there are some differences on the local spin moment distributions between Co and Ni layers epitaxially grown on Fe surfaces; in the Ni layers the length of the local spin moment is varying from layer to layer and in the Co layers it is almost constant. Hyperflne fields can be decomposed to core and v& lence electron contributions ss mentioned in the previous section. Valence &electron spin polarization leads to core local polarization around nuclei through exchange interaction with core electrons and the core polarization causes Hk. Therefore, Ha increases with the growth of the local spin moments. Hfv is influenced by not only local spin polarization at the noticed site but also spin polarizations at neighboring atoms. At the Fe site in the interior monolayem, Hh is large because the Fe is surrounded by strongly polarized Fe atoms, and at the Fe site in the interfacial monolayem it is small because of lack of the neighboring Fe sites. At the interfacial Fe site of the Fe/Ni multilayer systems, the reduction of Hh is larger than that of the Fe/Co multilayer because the Ni sites have smaller local spin moments than the Co sites. These evidences result that the hyperfine field at the inter-facial Fe site in the Fe/Co and Fe/Ni multilayers is reduced. Although only Fes/X3 multilayers are investigated, it is expected that the Fe site in the setond monolayer has the largest hyperfine field and, 8s the width of Fe layers are increasing, the hyperiine fields at the more interior Fe sites approach to the values of bulk bee Fe. At the interior Co or Ni sites Hh is small and at the interfacial site it is large because it is enhanced by the strongly polarized Fe atoms located next to the interfacial Co or Ni sites. Figures 2 and 3 show the dependency of Hfe on the atomic moment and of H,, on the total moment within the first two shells of neighbors for Fe3/Cos and Fe3/Ni3 multilayers. Hfc is proportional to the local spin mo ment and one can write Hfc = (r~i,‘~. The coefficient os are almost the same for Fe, Co and Ni atoms (i.e., cr N -99(kG/pn)). HI, are roughly proportional to the total local moments within the first two shells of neighbors, however, it is hard to express them with the equai l5, because the data points shows tion HrV =BCipi+r only weak linearity. It looks that the 5,.&Bchange of to tal moment within first two shells of neighbors roughly corresponds to 20 N 50kG change of Hk, Hs, at Fe site changes more than 70kG depending on the environment in the systems investigated in the present study. 5. Conclusion In all the investigated cases the calculated hyperfme fields at the Fe site in the interfacial layers are lower and they are higher at the Fe site in the interior layers. At the Fe site in the interfacial monolayers, the reduction of Hlv is larger than the enhancement of Hfe. Totally, the hyperfine fields at the interfacial Fe sites become lower than the interior Fe site. At the Fe site in the interior monolayers, especially, in the second monolayens, because of the enhanced local spin moment and the
Fe/Co AND Fe/Ni MULTILAYER
130 -5O-
++
A . x +
++ z y
-lOO-
20
Legend FeInFalCo FeInFwNl CoinFdCc NIIn Fe/N1
X + A v
s e g S t 8
Vol. 95, No. 3
SYSTEMS Legend CoinFalCo Ni In Fe/N1 Fein Fe/Co Fein Fe/N1
‘I X Xv
-150-
A
.
+
X X
x
\
-2OO-
-250
+
+
4 +
*It I
0
I
0.5
I
I
1.0 1.5 2.0 Local Spin Moment (pe)
T
I
I
2.5
3.0
10
40 Total Moment%in
First Two%ells
(PB)
Figure 2. The dependence of core electron contributions on the local spin moment. The proportional coefficient Q is -99kG/pB
Figure 3. The dependence of valence electron contributions on the total spin moment within the first two shells.
neighboring atoms with large spin moments, both Hrc and HrV are enhanced and these result in the enhanced hyperfine fields. Hr, is almost proportional to the local spin moment and proportional constants hardly depend on the atom species Fe, Co and Ni. The dependency of Hr, on the total spin moment within the two she&s of neighbors are not simple, though it looks roughly proportional.
AcknowledgementsThe authors would like to thank Prof H.Akai for his useful advice about the KKR Green function band structure calculation method. We also acknowledge the continuous support by crew of Supercomputing Center of the Institute for Materials Research, Tohoku University for usage of the HITAC S-3800 system.
References
1) P.
J. H. Bloemen, M. T. Johnson, M. T. H. V&n de Vorst, J. J. de Vries, R. Jungblut, J. ann de Stegge, A. Reiders, and W. J. M. de Jonge, Phys. Rev. B72, 764 (1994). P. Shi and P. M. Levy, Phys. Rev. B49, 15159 (1994).
2) Z.
3) C. J. Robinson, T. Suzuki & C. M. FaIco, &ten’& for Magneto-Optic Data &age, vol. 150, Materials Research Sot., Pittsburgh, (1989). 4) B. T. Jonker, J. P. Hermans and E. E. Marinero, Growth, Characterization, and Properties of ubathin Magnetic Films and M&layers, vol. 151, Materials Researchs Sot., Pittsburgh, (1989). 6. F. Bakkaloglu, M. F. Thomas, R. J. Pollard, and P. J. Grundy, J. Magn. Mag. Mat. 125, 221 (1993). C. G. Gutierrez, M. D. Qiu, H. Tang, and J. C. Walker, J. Magn. Magn. Mater. 93, 369 (1991). H. Akai, B. Drittler, Series in Solid-State
and P. H. Dederichs, Springer Science 103 “Molecular Dynamics Similations”, F. Yonezawa (Ed.), p. 177. Springer-Verlag (1992).
8) U.
von Barth (1972).
9) V.
L. Moruzzi,
Calculated
and L. Hedin,
3. F. Janak,
Electron
Properties
J. Phys.
C5,
1629
and A. R. Williams, of Metals, Perga-
mon, New York, (1978).
10) H.
Akai, M. Akai, S. Bliigel, B. Drittler, H. Ebert, K. Terakura, R. Zeller, and P. H. Dederichs, Prog. Theor. Phys. Suppl. No. 101, 11 (1990).
11) M.
D. Wieczorek, D. J. Keavney, D. F. Storm and J. C. walker, J. Magn. Magn. Mater. 121, 34 (1993).
12) Z.
Q. Wang, Y. S. Li, F. Jona and P. M. Marcus, Solid State Commun. 61, 623 (1987).
13) P. H. Dederichs, R. Zeller, H. Akai, and H. Ebert, J. Magn. Mug. Mat. 100,241 (1991). 14) B. Drittler, N. Stefanou, S. Bliigel, and P. H. Dederichs, Phys. Rev. B40, 8203 (1989). 15) Le Dang Khoi, P. Veillet Phys. F4, 2310 (1974).
and I. A. Campbell,
J.
Pergamon
HYPER.FlNE FIELDS AND ELECTRONIC STRUCTURES Fe/Co AND Fe/Ni MULTILAYER SYSTEMS Manabu Institute
Takahashi,
for Materials
IN
Xiac I-111and Yoshiyuki Kawazoe
Research,
Tohoku IJniversity,
Sendai 980-77, Japan
(Received 31 January 1995; revised version received 23 March 1995 by T. Tsuzuki)
Using KKR band structure calculation method, hyperfine field distributions and electronic structures in Fe&o3 and FeB/Nia multilayer systems are investigated where all the atoms are sited on bee lattice points. The calculated hyperfine fields at the Fe sites in the interior monolayer-s are enhanced and those at the Fe site in the interfacial monolayers are reduced. It is found that the reduction of transferred hyperfine field (valence electron contribution to the hyperfine fields) results in the reduced hyperfine field at the Fe site in the interfacial monolayers. Core electron contributions are almost proportional to the local spin moment (ok = -99kG/hn) and valence ones are roughly proportional to the total moment within the first two shells of neighbors.
Keywords: A. metals, A. magnetic fiims and multilayers, A. magnetically ordered materials, D. electronic band structure.
1. Introduction
cated on the bee lattice points are investigated by a first principle band structure calculation. The influence of lattice relaxations on the hyperfine fields and local spin moment distributions is also checked. In the next section the geometries and methods used in the present study are briefly mentioned. In section 3 the calculated results are described and the discussions are given in section 4. The last section is devoted to the conclusions.
Magnetic multilayer films have attracted considerable attention from academic and industrial interests, because they show much interesting physical properties useful for magnetooptical recording, e.g. giant magneto resistance and perpendicular anisotropy’, ‘. Among them, magnetic thin films and multilayers made by molecular beam epitaxy (MBE) method have attracted growing re search interests since met&able crystalline phases can be obtained by this method31 4. Nuclear magnetic resonance (NMR) or Mijsbauer spectroscopy study is one of the most powerful methods to obtain the information on electronic, magnetic and crystal structures in solids. However, few studies have been made on the details of hyperfine parameters in such magnetic multilayer systems. Recently, it was shown by Bakkaloglu et al5 that two kinds of Fe sites accompanied with higher and lower hyperfine fields exist in Fe/Co multilayers prepared by magnetron sputtering method. They concluded that the Fe sites accompanied with higher fields are located in the interfacial region and those with lower fields in the interior region. On the other hand, it was pointed out by Gutierrez et al6 that in bee Ni/[Fe(lOO)] multilayers made by MBE there are two kinds of Fe sites like Co/Fe multilayer; the higher field Fe sites are in the interior Fe monolayers and the lower ones in the interfacial Fe monolayers. In this paper hyperfine field distributions in both Fe/Co and Fe/Ni multilayers where all atoms are lo
2. Geometry
and Method
In the present study, we consider multilayers consisting of three Co or Ni monolayers and three Fe monolayers periodically in order to investigate the hyperfine field and local spin moment distributions. All the atoms are assumed to be on regular or distorted bee lattice points. Both [110] and (loo] stacked structures are investigated. The m&in-tin Green function KKR method is used to perform band structure and total energy calculations’. The spin dependent exchange correlation potential pro posed by von Barth and Hedin” is used with Morsi, Janack and Willams (MJW) parameterizationg and nonrelativistic Schrijdinger equations are solved. The hyperfine field is given by the equation Hhf = !fp~ (nt - nl), where nt and rzl are the up and down spin electron densities at nucleus, respectively. The theoretical understanding of hyperfine interactions in metals and metallic alloys and the density functional calculations of the quantities relating to hyperfine interactions have been reviewed by H. Akai et al.” 127
128
Fe/Co AND Fe/Ni MULTILAYER
Table 1: Calculated lattice parameters by total energy minimization. The units for lattice constant and for monolayer thickness are Bohr radius and the lattice constant, respectively.
SYSTEMS
Vol. 95, No. 3
Table 3: Calculated hyperfine field(kG) in Co3 /Fea and Nia /Fes multilayers. The values in brackets are experimental one@. Co3P3
a dx-x bulk a
Cos/[bccFez(lOO)] Nis/[bccFea(lOO)] 5.27 5.28 0.48a 0.50a bccFe 5.34 bccCo 5.27 bccNi 5.24
WOI
~W
Fe1
-272
-268
-252(-303)
-260(-304)
Fe2 Xl
-283 -205
-286 -215
-302(-352) -128
-279(-347) -121
-189 bccCo -180
-100 bccNi -64
x2
Since a notable lattice relaxation has experimentally been observed in Fe/Ni multilayer& ‘*, in order to es timate the influence of lattice relaxation on the hyperfine field, lattice relaxation is taken into account. There are many adjustable geometrical parameters to obtain the most stable structure in multilayer systems. In the present calculation, we restrict the number of adjustable parameters. At first, esch atom is put on the bee lattice points whose lattice constant is obtained by total energy minimization of bulk bee Fe. After the unit cell volumes are relaxed, the distance dx-x (X=Co, Ni) between two X monolayers and dpe-X between Fe and X monolayers are relaxed with the relation dFe-x = dx-x kept and the distance dFe-Fe fixed. 3. Results Table 1 shows the lattice parameters obtained by the present total energy minimizations of Cos/Fes and Nis/Fes multilayem. It is observed that the thickness of Co monolayer on the bee Fe (100) surface is almost the same as dlzml which is the distance between two (100) planes in bee lattice. On the other hand, the thickness of Ni monolayer decreases from dlzsq. The local spin moment distributions in the Fes/Cos and Fes/Nis multilayers and in the bulk bee Fe, Co and Ni are shown in Table 2. In Table 2, the lattice pa rametem are fixed at the values for bee lattice. After the lattice parameters are altered, the spin moments at interfacial Fe, interior Fe, interfacial Ni and interior Ni sites are changed to 2.49, 2.27, 0.62 and 0.35 (pg), respectively. The local spin moments at the Fe sites are enhanced from the bulk bee Fe. fipecially, the local moments of the interfacial Fe are strongly enhanced and those of the Co and Ni sre also enhanced from their bulk values. In the Ni layer, the spin moment largely varies from monolayer to monolayer comparing with the Co
Table 2: Calculated
spin moment(pg) (in Cos /Fe3 and Nig /Fez multilayers stacked on bee [lOO] and [llO] directions. Fe1 and Xl are interfacial atoms and Fe2 and X2 are interior atoms.
CodFe3
WI Fe1 Fe2 Xl x2
2.52 2.33 1.73 1.75 bccFe 2.22 fccNi 0.59
Nis/Fea
WI 2.45 2.34 1.75 1.75 bccCo 1.69
WOl 2.55 2.30 0.66 0.46 bccNi 0.48
-197 bccFe -242
w
-80
layer. In the Ni layer the difference of spin moment between the interfacial and interior Ni sites is about 0.2pn, while in the Co layer the difference is not found. Table 3 shows the hyperfine field distributions in the Fe3/Co3 and Fes/Nis multilayers and in the bulk bee Fe, Co and Ni. In both Fe/Co and Fe/Ni multilayer systems, the hyperfine fields at the Fe site in the interfacial monolayers are reduced from the values at the interior Fe site. The hyperfine fields at the Co and Ni sites are enhanced from their bulk values. Especially, those at the sites in the interfacial monolayers are most strongly enhanced. The difference of the hyperfine field values between the interfacial and interior Fe sites is larger in the [loo] stacked layer than the Ill01 stacked one for Fe/Ni multilayers and it is little smaller in the [loo] stacked layer than the (1101 stacked one for the Fe/Co case. Core and valence electron contributions to the hyperfine fields( Hrc and Hh) are separately shown in Table 4. Hrc is large at the site where the local spin moment is large. Hh is influenced by the neighboring atoms; it is found from Table 4 that if an atom is surrounded by the atoms having large spin moments, it becomes large. After the lattice parameters are altered, the values of interfacial Fe, interior Fe, interfacial Ni and interior Ni sites are changed to -242, -298, -119 and -98 (kG), respectively. The change of the hyperfine field is at most about 1T between before and after the lattice parame tern are altered. It is noticed that the calculated hyperfine fields are about 20% smaller than the experimental values. Thii is caused by the fact that the local spin density approximation for exchange-correlation potentials are used. As far as relative values and trends are concerned, the re sults are consistent with the experiments. Figure 1 shows the partial densities of states at the atoms in the interfscial and interior monolayers for the present multilayers. The partial DOS at the interfacial Table 4: Calculated core and valence electron contributions(kG) to the hypefine field shown at table3.
WOI 2.46 2.29 0.62 0.51 hcpCo 1.58
Ni3F-3
PW
Fe1 Fe2 Xl X2
CoJ[bccFes(lOO)] valence core -255 -17 -234 -49 -179 -26 -182 -14
Ni&mFe3(100)1 core -257 -230 -72 -51
valence
6 -72 -57 -49
Vol. 95, No. 3
Fe/Co AND Fe/Ni MULTILAYER
0
-0.5
II
I
0
-0.5
-wy(Wd)
Energy(W)
Figure 1. Partial density of states at interfacial atoms in Xs/Fes multilayers. Energy is measured from Fermi level in Rydberg.
sites are roughly similar to those of F&o and Fe-Ni dilute all~ys’~* 14. The interfscial Co site partial DOS is similar to the Co impurity site partial DOS in the iron host even around the Fermi level. However, there is a noticeable difference between the Ni site partial DOS in Fe/Ni multilayer system and the Ni impurity in Fe host around the Fermi level. In the Fe/Ni layer case, the peak of the minority-spin DOS near the Fermi level sinks under the Fermi level, though it is located just on the Fermi level for the case of Ni impurity in the Fe host13. This leads to the conclusion that the spin moment of the Ni atom in the interface is not ss large as that of the Ni impurity in Fe host. 4. Discussion In the present calculation it was found that the Ni layers on bee Fe (100) surface prefer to bet structure with c/a less than one. In the Co layer on the same surface, the monolayer thickness does not significantly change from the values of the bee structure within the present optimization. Because of using muffin-tin ap proximation and the restricted optimization, we can not determine the precise values of the lattice parameters. However, at least we can mention that the spin moment and hyperfine field distributions are not sensitive to the lattice parameters as far ss their tendencies are concerned. The amount of change of hyperfine fields be tween before and after the lattice parameters are altered is within about 1T and the distributions are not significantly affected. We also checked that the dependency of the hyperfme distributions on dFe_Fe and it was found that even if dFe-Fe changes about lo%, the distributions are not seriously affected. The enhancement of spin moment at the Fe site in the interfacial layers is due to the similar reason to that concerning to the FeCo(Ni) al10ys’~* 14. Comparing the partial density of state of bulk bee Fe, the local majorityspin DOS at Fe site becomes more filled and the weight of the minority-spin DOS below the fermi energy be comes smaller. In the Ni layers the difference of spin
SYSTEMS
129
moment between the interfacial and interior Ni sites is about 0.2,.&B, while in the Co layers such difference is not found. It indicates that there are some differences on the local spin moment distributions between Co and Ni layers epitaxially grown on Fe surfaces; in the Ni layers the length of the local spin moment is varying from layer to layer and in the Co layers it is almost constant. Hyperflne fields can be decomposed to core and v& lence electron contributions ss mentioned in the previous section. Valence &electron spin polarization leads to core local polarization around nuclei through exchange interaction with core electrons and the core polarization causes Hk. Therefore, Ha increases with the growth of the local spin moments. Hfv is influenced by not only local spin polarization at the noticed site but also spin polarizations at neighboring atoms. At the Fe site in the interior monolayem, Hh is large because the Fe is surrounded by strongly polarized Fe atoms, and at the Fe site in the interfacial monolayem it is small because of lack of the neighboring Fe sites. At the interfacial Fe site of the Fe/Ni multilayer systems, the reduction of Hh is larger than that of the Fe/Co multilayer because the Ni sites have smaller local spin moments than the Co sites. These evidences result that the hyperfine field at the inter-facial Fe site in the Fe/Co and Fe/Ni multilayers is reduced. Although only Fes/X3 multilayers are investigated, it is expected that the Fe site in the setond monolayer has the largest hyperfine field and, 8s the width of Fe layers are increasing, the hyperiine fields at the more interior Fe sites approach to the values of bulk bee Fe. At the interior Co or Ni sites Hh is small and at the interfacial site it is large because it is enhanced by the strongly polarized Fe atoms located next to the interfacial Co or Ni sites. Figures 2 and 3 show the dependency of Hfe on the atomic moment and of H,, on the total moment within the first two shells of neighbors for Fe3/Cos and Fe3/Ni3 multilayers. Hfc is proportional to the local spin mo ment and one can write Hfc = (r~i,‘~. The coefficient os are almost the same for Fe, Co and Ni atoms (i.e., cr N -99(kG/pn)). HI, are roughly proportional to the total local moments within the first two shells of neighbors, however, it is hard to express them with the equai l5, because the data points shows tion HrV =BCipi+r only weak linearity. It looks that the 5,.&Bchange of to tal moment within first two shells of neighbors roughly corresponds to 20 N 50kG change of Hk, Hs, at Fe site changes more than 70kG depending on the environment in the systems investigated in the present study. 5. Conclusion In all the investigated cases the calculated hyperfme fields at the Fe site in the interfacial layers are lower and they are higher at the Fe site in the interior layers. At the Fe site in the interfacial monolayers, the reduction of Hlv is larger than the enhancement of Hfe. Totally, the hyperfine fields at the interfacial Fe sites become lower than the interior Fe site. At the Fe site in the interior monolayers, especially, in the second monolayens, because of the enhanced local spin moment and the
Fe/Co AND Fe/Ni MULTILAYER
130 -5O-
++
A . x +
++ z y
-lOO-
20
Legend FeInFalCo FeInFwNl CoinFdCc NIIn Fe/N1
X + A v
s e g S t 8
Vol. 95, No. 3
SYSTEMS Legend CoinFalCo Ni In Fe/N1 Fein Fe/Co Fein Fe/N1
‘I X Xv
-150-
A
.
+
X X
x
\
-2OO-
-250
+
+
4 +
*It I
0
I
0.5
I
I
1.0 1.5 2.0 Local Spin Moment (pe)
T
I
I
2.5
3.0
10
40 Total Moment%in
First Two%ells
(PB)
Figure 2. The dependence of core electron contributions on the local spin moment. The proportional coefficient Q is -99kG/pB
Figure 3. The dependence of valence electron contributions on the total spin moment within the first two shells.
neighboring atoms with large spin moments, both Hrc and HrV are enhanced and these result in the enhanced hyperfine fields. Hr, is almost proportional to the local spin moment and proportional constants hardly depend on the atom species Fe, Co and Ni. The dependency of Hr, on the total spin moment within the two she&s of neighbors are not simple, though it looks roughly proportional.
AcknowledgementsThe authors would like to thank Prof H.Akai for his useful advice about the KKR Green function band structure calculation method. We also acknowledge the continuous support by crew of Supercomputing Center of the Institute for Materials Research, Tohoku University for usage of the HITAC S-3800 system.
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