Observation of spin wave resonance in Ni thin films after
adsorption of oxygen
Citation for published version (APA):
Janssen, M. M. P. (1970). Observation of spin wave resonance in Ni thin films after adsorption of oxygen. Journal of Applied Physics, 41(1), 399-402. https://doi.org/10.1063/1.1658354
DOI:
10.1063/1.1658354
Document status and date: Published: 01/01/1970
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JOURNAL OF APPLIED PHYSICS VOLUME 41, NUMBER 1 JANUARY 1970
Observation of Spin Wave Resonance in Ni Thin Films after Adsorption of Oxygen*
M. M. P. JANSSENt
Center for Surface and Coatings Research, Lehigh University, Bethlehem, Pennsyh'ania 18015
(Received 22 May 1969)
Ni thin films were deposited in ultrahigh vacuum (UHV) onto soft glass substrates. Film thicknesses ranged from 74 to 837 A. On films in UHV, only the uniform precession mode was observed during the micro-wave experiments. Admission of gases such as Hz, HzO, Oz, NzO, and air caused a lowering of the resonance field of the uniform precession mode. This effect was interpreted earlier as a relief of compressive stress related to surface tension. After adsorption of gases the films were virtually stress free. In the case of Oz, NzO, and air admission a standing spin wave resonance (SWR) mode (p = 1) slowly developed in films thicker than 400 A after the lowering of the resonance field of the uniform precession mode was observed. From the fact that in UHV and in the presence of Hz and H20 no SWR was found, while development of the
first mode was slow in oxygen-containing atmosphere and was thought to be related to the growth of a NiO layer, it may be concluded that spin wave excitation is made possible in this case as a result of surface spin pinning due to ferromagnetic-antiferromagnetic exchange coupling (Meiklejohn and Bean) rather than surface anisotropy (Nee!). The surface spin pinning was found to be weak but the intensity of the first mode increased rapidly with film thicknesses above 400 A. On the basis of the surface spin pinning model proposed by Kittel the exchange constant A was assigned an average value of 0.74XlO-6 erg/cm.
INTRODUCTION
The vacuum system in which the films were prepared
and measured has been described elsewhere.l Ni thin
films (area 8XSO mm) were evaporated onto extremely well-degassed soft glass substrates (Corning 0211, alkali borosilicate, thickness 0.3 mm) at 2S-3SoC. The vacuum during evaporation was in the 10 9 Torr range, deposition rates varied from 40 to 100 A/min. The nickel source consisted of a well-degassed W wire coated with Ni (99.99S%), mounted approximately 80 mm from the substrate and parallel to it. Using this arrangement, the maximum film thickness reached was 837 A. After evaporation the vacuum dropped to
values below 2X 10-10 Torr. It is believed that no
appreciable gas adsorption takes place under these conditions. Ferromagnetic resonance spectra of the films in UHV were taken as soon as the films were prepared. During measurements the film plane was perpendicular to the static magnetic field. The effects of gas adsorption on the resonance spectra were studied by admission of gases to the system via a fine control valve. During all experiments the films were at room
temperature. H2 and O2 (impurities less than 2 ppm)
and air were dried via a liquid-nitrogen trap; H20
and N20 were cleaned by repeated freezing and pump-ing.
In a previous studyl.2 films prepared and measured
in UHV were found to be in a state of compressive intrinsic stress. This was concluded from the fact that the values of the resonance fields (HRJ..) were more * This research was supported by the Advanced Research Projects Agency, Department of Defense, through the U.S. Office of Naval Research under Contract No. Nonr. 610(09).
t Present address: Laboratory for Physical Chemistry, Uni-versity of Technology, Eindhoven, Netherlands.
1 M. M. P. Janssen, J. App\. Phys. (to be published). 2 M. M. P. Janssen, J. App\. Phys. 40, 3055 (1969).
than 9200 G. Adsorption of O2, N20, H2, H20, and
air caused stress relief. [For a stress-free film the resonance equation of the uniform precession mode
is w/y=HRJ..-47rM •. Taking M.=490 emu/cm8, w=
27rv (v=9.3X109 secl
) and 'Y=2.807rgNiX106 (gNi=
2.18) yields HRJ..= 9200 G.] These observations were explained by means of the surface tension and a simple model was proposed to which the observed resonance fields agreed quantitatively. Clearly separated from the stress relief effect was the appearance of a spin wave resonance mode in the spectra of films thicker than
400 A in atmospheres containing O2 and N20; the
latter effect is reported in detail in this paper.
SWR in Ni thin films was earlier observed by Kimura
and Nose.3 Their films were prepared on unheated
glass substrates under vacuum conditions of 4X 10-5
Torr; observations were made in air. A 2000-A thick-ness film showed 4 SWR peaks. The average value of the exchange coupling constant A was calculated to
be 0.75X10-6 erg/em.
Lykken et al.4 found SWR in permalloy thin films
in the vacuum in which the films had been prepared
(approximately 1X10-7 Torr) as well as in air. They
concluded that spin pinning exists in a modest vacuum. EXPERIMENTAL RESULTS
The results obtained on 16 films ranging in thickness
from 74 to 837 A are summarized in Table 1. Films
as prepared and measured in UHV show decreasing resonance fields with increasing film thickness, in accordance with the surface tension model that was mentioned before. In the resonance spectra of these ultraclean films only the uniform precession mode
3 R. Kimura and H. Nose, J. Phys. Soc. Japan 17, 604 (1962). 4 G. I. Lykken, W. L. Harman, and E. N. Mitchell, J. App\. Phys. 37, 3353 (1966).
400 M. M. P. JANSSEN TABLE I. Experimental results. Films as Mter exposure
prepared and toHzorHzO
measured in UHV (10--100 Torr) Mter ey:,osure to O2, air, or NzO
Film H_l Hpoel
(atmosp eric pressure, long time)
thickness (t) (not (not (I_dIp...,)
(1) Hp...,· observed) Hp-'J observed) H p..., H p_1 Hp...,-IIp-l co time tlH_b tlHp-l Ac
74 10400 8870 280 113 10 270 9110 220 118 10 150 9160 200 156 9880 9150 250 204 9780 9240 140 267 9600 9260 160 402 9 410 9160 7220 1940 0.019 210 300 0.78 445 9440 9180 7860 1320 0.031 200 320 0.65 462 9380 9200 7940 1260 0.018 220 240 0.67 477 9 470 9260 (Hz) 9200 7880 1320 0.020 190 310 0.75 524 9 510 9270 (Hz) 9260 8140 1120 0.046 150 310 0.77 550 9 510 9290 (H2O) 9260 8250 1010 0.052 150 240 0.76 574 9400 9260 8390 870 0.070 210 230 0.71 657 9 380 9230 8530 700 0.090 190 170 0.75 808 9 350 9250 8760 490 0.245 150 160 0.79 837 9400 9260 (H2O) 9240 8780 460 0.230 160 170 0.80
.. H =resonance field (HRJ.) in G. tion points of the absorption curve.
b Linewidth, defined as the field separation in gauss between the inflec- • Exchange constant in 10 .... erg/em. average value 0.74 XlO-' erg/em.
(p=
0) was found. After exposure to H2 (477 and 524A
films) and H20 (550 and 837A
films) the resonance fields shifted towards lower values (stress relief) but no SWR modes appeared. When O2 was admitted to these films the resonance fields showed a small additional shift and a SWR mode (labeledp=
1) slowly developed. Labelingp=
1 of the appearing SWRmode is justified in view of the fact that the surface spin pinning model is accepted for the interpretation of the results of this study. In this model the
p=
1 mode will be the: strongest when spin pinning exists at both interfaces. From earlier workl it can becon-cluded that oxidation takes place at both the film/gas and film/substrate interfaces, while this study shows
film thickness 445A film thickness 837 A
(0) UHV (o) UHV
'" 20 mio ,x1<,-6
oxygen~J\/
to"[l
9160(c) 00 time in air
f-
-14
-I
f-5.5 kG 10.5 kG 55 kG (a) 9240/r
~-j
10.5 kG (b)FIG. 1. Examples of ferromagnetic resonance spectra. (a) Development of weak spin wave mode (relatively thin illm); (b) development of stronger spin wave mode (thicker illm).
SPIN WAVE RESONANCE IN Ni FILMS 401
TABLE II. Effect of exposure time to oxygen on the relative intensity of the spin wave resonance mode p= 1.
Film thickness
(t) (A)
445
524
837 • First exposed to H •• next to air.
b First exposed to H ,0, next to air.
10 min
o·
O.OO9a Ob 20 min 0.0070that oxidation leads to surface spin pinning. The resonance fields after exposure to oxygen of all but
the 74 and 113
A
thickness films were 9200±60 G,indicating that these films were practically stress free. The two thinnest films exhibited lower resonance fields after oxygen adsorption in comparison to all other films, which effect can be attributed to dis-continuity of the films. The relatively large linewidths indicate a slightly inhomogeneous residual stress
distribution. The development of a SWR mode in O2 ,
N20, and air was noticed with all films thicker than
400
A.
The latter development could be observed atO2 pressures as low as lXlO-u Torr. The resonance
spectra are illustrated in Fig. 1 for 445- and 837-1
thickness films. The relative intensity of the spin
wave mode
p=
1, I_I/Ir-O' increased slowly withexposure time to oxygen (see Table II). (For the
intensity I, the height of the signal in derivative
rep-resentation was taken.) The values of Ipo-I/Ip=O after
very long exposure time to air are plotted in Fig. 2 as a function of film thickness. The spacings of the
modes
p=O
andp=
1 increased with decreasing filmthickness, in accordance with the resonance condition for the surface spin pinning model
wh=H
RJ.-47rM.+(2A/M.)k2, (1)where A is the exchange coupling constant, k IS the
(!.M)
!p=o 00 time 0.20 0.10l
.0 o 200 400 600 800 1000 ~ FILM THICKNESS (Angstrom)FIG. 2. Relative intensity of spin wave mode p = 1 after very long exposure to air as a function of film thickness.
(1_/1_) 30 min 40 min 0.0130 0.0170 0.014 0.010 0.014 • In 1 X 10--' Torr 0 •• d In air.
50 min ex> time 0.0180 0.031d 0.026 0.046d 0.230d
wave vector which is equal to P7r/t in which
t
is thefilm thickness and
p is the mode number. While the
SWR modes were still developing, the spacings of the
modes
p=l
andp=O
were observed to be constant.Using Eq. (1) the exchange constant A was calculated
for all films; the average value for A was found to be
0.74X 10-6 erg/em, which is in excellent agreement
with the value reported by Kimura and Nose. DISCUSSION AND CONCLUSIONS For the interpretation of spin wave resonance in magnetic thin films, two different Il).odels are being used, the surface spin pinning model and the volume
inhomogeneity mode1.5 As indicated before, the
sur-face spin pinning model proposed by Kittel6 has been
adopted to explain the results of his study. The main reason for this is that surface effects, which are believed to play the most important role in this work, are the key to the interpretation via the surface spin pinning model. Using the surface spin pinning model, there is no need for assuming a nonuniform distribution of the magnetization normal to the film plane. Such an assumption is a requirement for the volume inhomo-geneity model but is unlikely to be valid for films prepared in URV onto well-outgassed substrates as used in this study.
As pinning mechanisms for the surface spin pinning
model the surface anisotropy7 and exchange anisotropy8
have been proposed. Both mechanisms are based on the anisotropic environment of the surface atoms. Surface anisotropy is present at clean or contaminated surfaces while exchange anisotropy exists as a result of the presence of antiferromagnetic material on the surface of ferromagnetic material (for example, NiO
on Ni). It appears that on the basis of this study a
distinction between the two mechanisms becomes
evident: From the fact that clean films in UHV do not
show SWR modes, it may be concluded that the surface anisotropy cannot be responsible for the excitation of SWR and does not lead to surface spin pinning. Changes in the surface anisotropy, as will occur on
chemisorp-• C. F. Kooi, P. E. Wigen, M. R. Shanabarger, and J. V. Kerri-gan, J. App!. Phys. 35, 791 (1964).
6 C. Kittel, Phys. Rev.
no,
1295 (1958). 7 L. Neel, J. Phys. Radium IS, 225 (1954).8 W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 (1956) .
402 M. M. P. JANSSEN
tion of H2 or H20, and formation of a nickel hydride
layer (diamagnetic) or adsorbed H20 layer does not
induce SWR either. Kooi el al.9 found that permalloy
films reduced in H2 show indeed weak pinning at the
reduced surface. From the results of this study it is
clear that SWR does occur after oxidation (02, air,
N20), at room temperature. The appearance of SWR
is a slow process, indicating that build-up of an
anti-ferromagnetic NiO layer (10
1
or more) is needed toachieve sufficient pinning. Presence of only a monolayer
of oxygen does not lead to measurable effects. to From
9 C. F. Kooi, W. R. Holmquist, P. E. Wigen, and
J.
T. Doherty,J. Phys. Soc. Japan 17, 599 (1962).
10 See Table II (445 A film); after 10 min at 1 X 10-6 Torr O2
at least an adsorbed monolayer will be present on the surface.
this it may be concluded that the ferromagnetic-antiferromagnetic exchange anisotropy only leads to surface spin pinning and excitation of spin wave re'sonance in this case. The surface spin pinning is relatively weak as may be seen from the fact that in
films thinner than 400
1
no SWR was found, whilethe intensity of the appearing SWR mode for films
thicker than 400
1
increases rapidly with film thickness(Fig. 2).
ACKNOWLEDGMENT
The author is indebted to Dr. H. Leidheiser, Tr.,
Director of the Center for Surface and Coatings Research, for many helpful and encouraging remarks.
JOURNAL OF APPLIED PHYSICS VOLUME 41, NUMBER 1 JANUARY 1970
Mechanical Properties of Copper Films*
H. LEWHEISER, JR., t AND BILLY W. SLOOPEVirginia Institute for Scientific Research, Richmond, Virginia 23226 and Virginia Commonwealth University, Richmond, Virginia 23220
(Received 5 May 1969; in final form 1 August 1969)
The mechanical properties of polycrystaIline copper films, 70o-5000-A thick, were measured using the bulge technique. TlJ,e breaking strength increased with decreasing film thickness, the greatest increase oc-curring below 1000 A. The equivalent bulk breaking strength was found to be 61000 Ib/in2• Young's modulus was independent of film thickness and had the normal value 18X 106Ib/in2•
The mechanical properties of thin films have been reviewed' and questions of film strength and Young's Modulus as a function of film thickness discussed. This paper presents the results of an experimental investiga-tion of the breaking strength and Young's Modulus of vacuum-evaporated polycrystalline copper films in the
700-sooo-1
thickness range. Palatnik el al.2 found thatfor copper films 13-1S0-,u thick the strength was
independent of thickness although about twice that for
cold-worked copper. Lawley and Schuster3 reported the
strength independent of thickness for rolled copper foils 2-S3-,u thick. Oding and Aleksanyan,4 however, found that for vacuum-evaporated copper films greater than 1-,u thick the strength decreased by a factor of two over
* Supported by the United States Office of Naval Research.
t Present address: Lehigh University, Bethlehem, Pa.
1 R. W. Hoffman, Physics of Thin Films, G. Hass and R. E.
Thun, Eds. (Academic Press Inc., New York, 1966), Vol. 3, p. 211.
2 L. S. Palatnik and A. 1. Il'enskii, Sov. Phys.-Solid State 3,
2053 (1962); Sov. Phys.-Dokl. 7,832 (1963).
3 A. Lawley and S. Schuster, Trans. AIME 230, 27 (1964).
• 1. A. Oding and 1. T. Aleksanyan, Sov. Phys.-Dokl. 8, 818 (1964) .
the l.S-4.6-,u thickness range. No results on breaking strength and Young's Modulus have been reported on thinner copper films.
Experimental techniques used for measuring mechan-ical properties of thin films have also been reviewed.' In this investigation the bulge test was used whereby the film sample is mounted over a hole such that a differential pressure applied to the film produces a deflection. The experimental data of applied pressure and film deflection must be related to stress and strain.
It can be shown5 that for a film of thickness I, bulged by
a pressure
p,
the stress is given bySo=p R o/2t, (1)
where So and Ro are the pole stress and radius,
respec-tively. It is of significance that Eq. (1) is independent
of the stress-strain mechanism. If the shape of the
bulged film is paraboloid (or approximately spherical) the pole radius is
(2)
