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Interface magnetization of Fe-SiO multilayer film studied by neutron bragg reflection
OO38-1098/78/0408-0095
Solid State Communications, Vol. 26, pp. 95-97. ‘0 Pergamon Press Ltd. 1978. Printed in Great Britain
$02.00/O
INTERFACE MAGNETIZATION OF Fe-SiO MULTILAYER FILM STUDIED BY NEUTRON BRAGG REFLECTION M. Sato and K. Abe The Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan (Received 1 November 1977 by W. Susuki)
Magnetization of Fe metal at the interface to SiO has been studied by measuring the intensity of the first and the second order neutron Bragg reflections on the Fe-SiO multilayer film. From the analysis of the form factor, the magnetically “dead” layer is found to be less than l/l0 atomic layer, if any. Brief comment on the applicability of the multilayer to the neutron polarizer or analyzer is given. THE PROBLEM of the surface magnetization of the ferromagnetic metals has been a subject of many works [l] . Fulde et al. discussed the possibility of the existence of the magnetically “dead” surface layers [2] . Teraoka et al. suggested the enhancement of the surface magnetization or the susceptibility of b.c.c. metals [3] . In order to study the magnetization of Fe metal at the interface to insulator, we have measured the first and the second order neutron Bragg reflections from the artificial Fe-SiO multilayer film. The multilayer was prepared by successive evaporation of Fe and SiO in a vacuum of about 5 x lo-* torr. In the experiment we used three pieces of multilayer films which were aligned almost parallel. They were made by simultaneous evaporation. The mrmber of Fe-SiO bilayer in one multilayer film was 80. These multilayer films were covered with SiO a few-hundred angstroms thick to prevent the oxidation. To avoid the agglomeration of the evaporated materials, the substrate was attached to a Cu plate cooled by liquid nitrogen. During the evaporation process, the thickness of each film was monitored by the thickness gauge with quartz oscillator. The thickness of the bilayers was oonsistent with those determined by the Bragg angle. The period D of the multilayer was 60 A of which two thirds was SiO and the rest was Fe. Figure 1 shows the observed first order neutron Bragg reflection obtained at room temperature by the “0-28” scan with the wave length X = 2.44 A. The open circles are the intensities (Z1)measured in a condition of H 1 K, where H is the applied magnetic field and K is the scattering vector which is perpendicular to the film. The solid circles are the results (‘1,) for H II K. The splitting of the peak is due to the fact that three pieces of the multilayer film were not exactly parallel. Figure 2 shows the results of the second order Bragg reflection. Same abbreviations are used as in Fig. 1. 95
X104n.
1
ZCI
acg.
Fig. 1. The intensity of the first order Bragg reflection. The open circles are the results of H 1 K. The solid circles are those for H IIK:The neutron wave length is 244A. In the present case, the saturation of the magnetization is confirmed by measuring the H-dependence of J,(H). Therefore Z,,includes only the nuclear scattering and Zl is the sum of the nuclear and the magnetic scattering intensities. The reproducibility of these lines has been checked carefully. To extract the information about the surface
Vol. 26, No. 2
INTERFACE MAGNETIZATION OF Fe-SiO MULTILAYER FILM
96
Table I.
A
d = d’
d’=(d-2)Ain(a)
d’=(d--).&in(b)
Observed
1
0.889
1.124
0.987 f 0.012
magnetization, we define the following quantity, A = (~~/r,),l(l,,/l,), , where 1 and 2 express the order of the Bragg reflection. The value of A is sensitive to the difference between the form factors corresponding to the magnetic and nuclear distributions on the Fe films. A becomes unity only when the both distributions are the same. The experimentally observed value of A is shown in Table 1 with the calculated values, which were obtained by using the magnetically dead and the magnetically enhanced layer models shown in (a) and (b) of Fig. 3, respectively. In these models, the thickness of the “anomalous” layer is chosen to be 1 A (5 D,/2 where D,, is the thickness of one atomic layer) and the magnetization in the anomalous layer is assumed to be zero and
1 oH+K l
II//K
twice of the bulk one for models (a) and (b), respectively. The calculated values of A are known to be insensitive to the irregularity and the smearing of the interface of the Fe and the SiO films. They are sensitive only to the amount of the total change in the magnetization at the interface. The deviation of the observed value of A. However, simple estimation shows that this obtained by the model calculations and it can be concluded that the interface magnetization of Fe to SiO is neither dead nor enhanced within the error of less than D,JlO. There is a possibility that the adsorption of the residual gases in the evaporation chamber at the interface of Fe and SiO films [4,5] gives an effect on the value of A. However simple estimation shows that this effect is comparable with that caused by the “enhanced” layer of D,JlO. Therefore it can be concluded that the interface magnetization is not dead within the error of - D,,/lO. The increase of the magnetization at the surface suggested by some authors [3,6] does not exist for our Fe-SiO system. This multilayer has a possibility to be used as a polarizer or an analyzer of neutron beam. With this multilayer the peak reflectivity was about 13% for the ’ polarized beam, the effective mosaicness, which caused by the nonzero AD/D, was more th 10’ and the polarization was 99.8%. For the purpo I of making a polarizer we can choose the thickness of Fe as D/2 and we expect the higher reflectivity than the present one. More detailed studies are in progress.
a)
kd
QM
%I
d
(28dcg.1 ?I
4 I
5 I
6 I
Fig. 2. The intensity of the second order Bragg reflection. The same abbreviations are used as those in Fig. 1. B.C. indicates the observed background line.
b)
-d’
Fig. 3. The models for the magnetization @I) distributions. (a) Magnetically “dead” layer model, (b) enhanced magnetic moment model. The form of &(a IMl), the scattering length density, is shown by the heavy lines, and shape of Fe film is indicated by the light ones.
Vol. 26, No. 2
INTERFACE MAGNETIZATION OF Fe-SiO MULTILAYER FILM
Acknowledgements - The authors are indebted to Profs. K. Hirakawa and Y. Endoh for their useful discussions.
They thank Dr. K. Murata and Mr. K. Nomura for their kind assistance during the present work, and Dr. 0. Shimomura and H. Oyanagi for allowing the use of their vacuum evaporating system. One of the authors (M.S.) is also indebted to the financial support from the Sakkokai Foundation during the present work.
REFERENCES 1.
For example, GRADMANN U.,Appl. Whys. 3,161 (1973); many references are therein.
2.
FULDE P., LUTHER A. &WATSON R.E., Whys.Rev. B&440 Phys. Rev. BS,440(1973).
3.
TERAOKA Y. & KANAMORI J.,Proc. ht. Conf. Whys. lhnsition Metals, Tront (1977).
4.
HAYWARD D.O. & TRAPNELLB.M.W., Chemisorption. Butterworths London (1964).
5.
CASWELL H.L., IBMJoumaZ, Apr. 130 (1960).
6.
SHINJO T., HINE S. & TAKADA T.,Proc. 7th ht. Vat. Cbngr. & 3rd Znt. Cbnf Solid Surfaces, Vienna (1977).
97
Solid State Communications, Vol. 26, pp. 95-97. ‘0 Pergamon Press Ltd. 1978. Printed in Great Britain
$02.00/O
INTERFACE MAGNETIZATION OF Fe-SiO MULTILAYER FILM STUDIED BY NEUTRON BRAGG REFLECTION M. Sato and K. Abe The Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan (Received 1 November 1977 by W. Susuki)
Magnetization of Fe metal at the interface to SiO has been studied by measuring the intensity of the first and the second order neutron Bragg reflections on the Fe-SiO multilayer film. From the analysis of the form factor, the magnetically “dead” layer is found to be less than l/l0 atomic layer, if any. Brief comment on the applicability of the multilayer to the neutron polarizer or analyzer is given. THE PROBLEM of the surface magnetization of the ferromagnetic metals has been a subject of many works [l] . Fulde et al. discussed the possibility of the existence of the magnetically “dead” surface layers [2] . Teraoka et al. suggested the enhancement of the surface magnetization or the susceptibility of b.c.c. metals [3] . In order to study the magnetization of Fe metal at the interface to insulator, we have measured the first and the second order neutron Bragg reflections from the artificial Fe-SiO multilayer film. The multilayer was prepared by successive evaporation of Fe and SiO in a vacuum of about 5 x lo-* torr. In the experiment we used three pieces of multilayer films which were aligned almost parallel. They were made by simultaneous evaporation. The mrmber of Fe-SiO bilayer in one multilayer film was 80. These multilayer films were covered with SiO a few-hundred angstroms thick to prevent the oxidation. To avoid the agglomeration of the evaporated materials, the substrate was attached to a Cu plate cooled by liquid nitrogen. During the evaporation process, the thickness of each film was monitored by the thickness gauge with quartz oscillator. The thickness of the bilayers was oonsistent with those determined by the Bragg angle. The period D of the multilayer was 60 A of which two thirds was SiO and the rest was Fe. Figure 1 shows the observed first order neutron Bragg reflection obtained at room temperature by the “0-28” scan with the wave length X = 2.44 A. The open circles are the intensities (Z1)measured in a condition of H 1 K, where H is the applied magnetic field and K is the scattering vector which is perpendicular to the film. The solid circles are the results (‘1,) for H II K. The splitting of the peak is due to the fact that three pieces of the multilayer film were not exactly parallel. Figure 2 shows the results of the second order Bragg reflection. Same abbreviations are used as in Fig. 1. 95
X104n.
1
ZCI
acg.
Fig. 1. The intensity of the first order Bragg reflection. The open circles are the results of H 1 K. The solid circles are those for H IIK:The neutron wave length is 244A. In the present case, the saturation of the magnetization is confirmed by measuring the H-dependence of J,(H). Therefore Z,,includes only the nuclear scattering and Zl is the sum of the nuclear and the magnetic scattering intensities. The reproducibility of these lines has been checked carefully. To extract the information about the surface
Vol. 26, No. 2
INTERFACE MAGNETIZATION OF Fe-SiO MULTILAYER FILM
96
Table I.
A
d = d’
d’=(d-2)Ain(a)
d’=(d--).&in(b)
Observed
1
0.889
1.124
0.987 f 0.012
magnetization, we define the following quantity, A = (~~/r,),l(l,,/l,), , where 1 and 2 express the order of the Bragg reflection. The value of A is sensitive to the difference between the form factors corresponding to the magnetic and nuclear distributions on the Fe films. A becomes unity only when the both distributions are the same. The experimentally observed value of A is shown in Table 1 with the calculated values, which were obtained by using the magnetically dead and the magnetically enhanced layer models shown in (a) and (b) of Fig. 3, respectively. In these models, the thickness of the “anomalous” layer is chosen to be 1 A (5 D,/2 where D,, is the thickness of one atomic layer) and the magnetization in the anomalous layer is assumed to be zero and
1 oH+K l
II//K
twice of the bulk one for models (a) and (b), respectively. The calculated values of A are known to be insensitive to the irregularity and the smearing of the interface of the Fe and the SiO films. They are sensitive only to the amount of the total change in the magnetization at the interface. The deviation of the observed value of A. However, simple estimation shows that this obtained by the model calculations and it can be concluded that the interface magnetization of Fe to SiO is neither dead nor enhanced within the error of less than D,JlO. There is a possibility that the adsorption of the residual gases in the evaporation chamber at the interface of Fe and SiO films [4,5] gives an effect on the value of A. However simple estimation shows that this effect is comparable with that caused by the “enhanced” layer of D,JlO. Therefore it can be concluded that the interface magnetization is not dead within the error of - D,,/lO. The increase of the magnetization at the surface suggested by some authors [3,6] does not exist for our Fe-SiO system. This multilayer has a possibility to be used as a polarizer or an analyzer of neutron beam. With this multilayer the peak reflectivity was about 13% for the ’ polarized beam, the effective mosaicness, which caused by the nonzero AD/D, was more th 10’ and the polarization was 99.8%. For the purpo I of making a polarizer we can choose the thickness of Fe as D/2 and we expect the higher reflectivity than the present one. More detailed studies are in progress.
a)
kd
QM
%I
d
(28dcg.1 ?I
4 I
5 I
6 I
Fig. 2. The intensity of the second order Bragg reflection. The same abbreviations are used as those in Fig. 1. B.C. indicates the observed background line.
b)
-d’
Fig. 3. The models for the magnetization @I) distributions. (a) Magnetically “dead” layer model, (b) enhanced magnetic moment model. The form of &(a IMl), the scattering length density, is shown by the heavy lines, and shape of Fe film is indicated by the light ones.
Vol. 26, No. 2
INTERFACE MAGNETIZATION OF Fe-SiO MULTILAYER FILM
Acknowledgements - The authors are indebted to Profs. K. Hirakawa and Y. Endoh for their useful discussions.
They thank Dr. K. Murata and Mr. K. Nomura for their kind assistance during the present work, and Dr. 0. Shimomura and H. Oyanagi for allowing the use of their vacuum evaporating system. One of the authors (M.S.) is also indebted to the financial support from the Sakkokai Foundation during the present work.
REFERENCES 1.
For example, GRADMANN U.,Appl. Whys. 3,161 (1973); many references are therein.
2.
FULDE P., LUTHER A. &WATSON R.E., Whys.Rev. B&440 Phys. Rev. BS,440(1973).
3.
TERAOKA Y. & KANAMORI J.,Proc. ht. Conf. Whys. lhnsition Metals, Tront (1977).
4.
HAYWARD D.O. & TRAPNELLB.M.W., Chemisorption. Butterworths London (1964).
5.
CASWELL H.L., IBMJoumaZ, Apr. 130 (1960).
6.
SHINJO T., HINE S. & TAKADA T.,Proc. 7th ht. Vat. Cbngr. & 3rd Znt. Cbnf Solid Surfaces, Vienna (1977).
97