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The observation of body modes and complex modes in single-crystal nickel thin films
Surface Science 2'71 (1992)575-578 North-Holland
s u r f a c e sciellce :i'""""
!
The observation of body modes and complex modes in single-crystal nickel thin films N.H. Ahmad, I. H a m e e d a n d M.A. K a r e e m Physics Department, College of Science, University of Basrah, Basrah, lraq Received 18 June 1990; accepted for publication 3 January 1992
The excitation of body modes and complex modes in (100) single-crystal nickel films epitaxially deposited onto NaCI substrates are reported. Experimental investigations on some of these films reveal the presence of body modes only. Observations on ot~er films indicate the presence of complex modes which exhibit the characteristic feature of migration from within the body mode to the law field side of the main mode as the value of the complex pinning parameter is changed.
I. Introduction Spin waves have been excited in single-crystal nickel thin films deposited on single-crystal substrates of NaC1 and LiF with the field applied in the sample plane [1] and normal to the sample plane [2] with small number of modes observed. No surface modes have been excited in such films. Recently surface modes have been observed in single-crystal nickel films grown on NaF substrates [3] and on NaCI substrates [4]. These modes have been evident in both orientations and interpreted in terms of surface inhomogeneity model of Puzskarski [5]. We wish to report in this work the observation of body modes and complex spin wave modes in (100) single-crystal nickel films epitaxially deposited onto NaCI substrates. The excitation of such complex modes can be accounted for by introducing an imaginary component into the surface parameters [6].
2. Experimental procedu~'e Single-crystal thin films of nickel were prepared by vacuum evaporation onto heated (100) cleavage surfaces of NaCI using the method de-
scribed by Ahmad [7]. The films were 1800 ,~ thick and electron microscopy observations showed them to be good (100) single crystals with a law density of twins. This crystallographic quality was also shown in the plots of resonance field versus azimuthal orientation with the static magnetic field in the film plane since these plots showed four-fold symmetric shift of the resona~,ce iine. An X-band Vm~:m spectrometer was employed to ob.,:erve the spectra using 10O kHz field modulation and a phase sensitive detection system. The measurements were made at room temperature with the field applied normal (0 = 0 °) and parallel (0 = 90°) to the sample plane. Three films were deposited onto NaCI substrates. Two of them gave similar spin wave resonance spectra and the third gave a spectrum with evident complex modes.
3. Results and discussion Fig. 1 shows a spectrum for a perpendicular field config,~r~ion for one of the tw~ samples which gave similar spectra. No complex modes are observed in both parallel and perpendicular field orientations. The largest number of modes is observed when 0 = 0 °. From this spectrum we
C039.6028/92/$05.0(~ ~; !002 - E!sevier Science Publishers B.V. All rights resern'ed
N.H. Ahmed et al. / Body modes and complex modes in Ni films
576
observe that both odd and even modes are excited and that the odd (symmetric) modes are higher in intensity than the even (asymmetric) modes. This implies that asymmetric pinning conditions exist at the two film surfaces. The spectrum is strongly dependent on the angle 0. This implies that the pinning conditions on both s u r f r ' e s could be angle dependent. This dependence may be associated with the film substrate interface anisotropy [3]. It was possible to interpret the spectra in terms of a pinning diagram in p - r space [5]. Various values of a and b have been used to find the best fit to the mode intensities. The computed values for the surface parameters are found to be a = 0.999176 and b = 0.005000. Thus both a and b are less than unity. This implies that the point located by the parameters r and p exists below the line p = 1 and in region of the pinning diagram which corresponds to the absence of surface modes, and lies or,, the left of the circle. Computed intensities of the body modes normalized to the first body mode are shown in fig. 2. It is noted that the envelope of the odd modes is slightly higher than that of the even modes although the decrcase of intensity with mode number n is less rapid for the experimental results. The quadratic dependence of the resonance absorption fields on the mode number of the body modes deviates slightly for the first two modes, The value of D can be computed from
Ni / NaCt ~.=1
/ 6
g
g
5
4
-~
3
g
2
9
.cKsi
Fig. 1. The spin wave first-derivative absorption spectrum in single-crystal nickel thin film grown on NaC! substrate with applit:d magnetic field normal to the film plane.
0.08
. D i/1
g o.o6 .E (p > ",S O . 0 4 m
0.02
i I
6
5
4
3
n
2
Fig. 2. Computed intensities of the bulk modes normalized to the first bulk mode for 0 = 0° spectrum.
the slope of the line connecting the computed values of k 2 for the body modes of the 0 = 0 ° spectrum and the corresponding resonance magnetic fields (fig. 3). The points are fitted to a straight line with the exception of the first two modes. The exchange constant D so obtained can be compared with that found experimentally and with other published data; see table 1. The absence of surface modes might be accounted for as due to vanishing of certain interactions existing at the interface, responsible for the excitation of these modes. As previously p:oposed [8], the type of surface anisotropy responsible for the excitation of these modes is due to a surface oxide layer on the free surface. A super exchange interaction between a nickel atom and adjacent substrate atoms may exist at the Ni/NaC1 interface [3]. Fig. 4 shows a series of spectra for the third film in which the applied magnetic field is rotated about an axis parallel to the sample plane. The spectrum consists of a broad asymmetric mode when the magnetic field is perpendicular to the film surface (0 = 0°), but at 0 = 2 ° two low-intensity high-field lines are observed, which at 0 = 6° have merged to give a weak distorted line. However at 0 > 6 ° the spectrum clemly shows again two weak high-field lines, Further increase of 0 causes the separation between the two weak lines to increase and at O---- 15° the main line and the
N.H. Ahrned et al. / Body modes and comph.~ modes in Ni fihns
577
KG)
0 f l J
1 1
6
0
15
/
!
I
I
I
5
10
15
20
I
36
,,,i
25 k 2 (Rod) 2
.------30
301(10 - s
38
Fig. 3. Bulk-mode resonance fields against computed values of k for the 0 = 0 ° spectrum. 40
low-intensity line (of higher energy) have merged to give a broad and distorted line. This weak line emerges at the low-field side of the main line when 0 = 20 °. The intensity of the other weak line (of lower energy) decreases monotonically with increasing resonance field but at 20 ° the separation of the two weak lines becomes maximum. Further increase in O causes the weak line (of lower energ)) to move towards lower fields and its intensity to decrease to the extent that when O = 90 ° (magnetic field in the plane of the sample) only one line is observed. This sequence of changes can be visualized in a stick diagram (fig. 5) in which the heights of the lines are approximately proportional to the estimated line intensities. Main feature of figs. 4 and 5 is the emergence of two weak lines of energy lower than that of the main body mode and a movement of a line from the high-field side of the main body mode to the low field side and with Table 1 Exchange constant for nickel at room temperature Reference
D (10 -~'' erg cm =)
Structure of the film
Present work A h m a d and Kareem [4] Mitra and Whiting [3] Rusov [1]
49 ± 3 56_+3 47 +_5 74 + 3
Single Single Si,,gle Single
cl3.,,tal clystal cr}.'slai crystal
90
Fig. 4. Spin wave resonance spectra fl~r different orientations of the applied magnetic field. The dominant mode has been aligned in the spectra to show the relative motion of thc resonance lines.
increasing intensity which is always less than that of the main body modc. Thc relative motion of these lines and the variation of their intcnsitics with position indicates that one or two of tile lines must be a localized mode [5]. h may be possible to interpret this sequence of changes if the surface parameters are real. Surface modes are possible if one of the surface parameters or both are made to vary such that the two modes change in both position and intensity until the two modes become equal to each other. It is not certain that equalization occurs at 0 = 6° since at this angle the two modes have merged to give one weak distorted iine. Another possibility R~r the surfao: parameter.,, to bc rcal occurs if one of them is made to va~3' such that at values of O > 6 ° the weak line (of higher energy) shifts towards the left, and at certain well defined value,; of the surface parameters the surface mode becomes a space mode and
578
N.H. Ahmed et aL / Bo@ modes and complex" modes in Ni films
D
D
_L ~3=
(ii) The weak high-field line (of lower energy) may be QLSM and as 0 varies from 0 ° to 40* this mode continues shifting towards lower values of magnetic field with gradually increasing intensity. If ~ is allowed to have a-certain fixed value with t ~ 0, then any decrease in t causes the energy of the QLSM to increase, thus shifting the mode towards lower fields eventually appearing on the low-field side of the body mode position. This is consistent with the sequence of the observed spectra. If q~ is assumed to have a small value and is allowed to increase, the value of t changes and consequently the intensity may change in a similar way as that of a complex mode. Thus the intensity would be expected to increase as it shifts towards lower fields. The origin of a complex parameter could result from a random surface potential [10] or from defect centers randomly induced by the high-energy electron bombardment used to promote nucleation [9].
D
..... L_L_I____
I .... x
4°
0°
6 °
D D
k
11
12"
10*
l 15 °
il D
20° Dll C
30*
C
I
34* D
36*
D
I
D
I
I
38*
40*
90 °
Fig. 5. Stick diagram to visualize the sequence of changes of the spectra. The heights of the lines approximately indicate the relative intensities. (D) Dominant mode, (C) complex mode.
further on an equalization effect takes place [5]. This characteristic feature is not observed experimentally. If the surface parameters are assumed to be complex then the complex spin wave modes of the quasi-localized character (QLSM) can be excited [6]. The energy and damping of such modes of wave vector k = s + it is of the form x - 2 cos s cosh t and 2 sin s sinh t respectk, ely, where s = q~, the phase of the complex pinning parameter. Its value determines the location of the QLSM within the body mode. Now two possibilities need to be discussed: (i) when 0 varies from 0 ° to 6 °, the weak high-field iili'll~
't~%,.all I l l l ~ l l l i , . ' l
%,~11%.. I t...~)) /
ill(.I,~'
II,Jt~
~i,~bdll
II
(~Jl.llli,,,.l
(;.ILO
0
becomes larger than 6 ° this mode shifts towards the left migrating from withSn the body-mode manifold to the low-field side of the main mode.
Acknowledgement We are indebted to Dr. J.S.S. Whiting for his interest in this work.
References [1] G.I. Rusov, Soy. Phys. Solid State 11 (1969) 96. [2] N.H. Ahmad, M. Prutton and J.S.S. Whiting, Proc. R. Soc. London A 328 (1972) 49. [3] D.P. Mitra and J.S.S. Whiting, J. Phys. F 8 (1978) 2401. [4] N.H. Ahmad and M.A. Kareem, Physica B, to be published. [5] H. Puszkarski, Prog. Surf. Sci. 9 (1979) 191. [6] H. Puszkarski, Solid State Comnmn. 33 (1980) 757. [7] N.H. Ahmad, PhD Thesis, University of York (1972). [8] P.E. Wigen, T.S. Stakelon, H. Puszkarski and P. Yen, AlP Conf. Proc. 29 (1976) 670. [9] A.A. Hussain and J.S,S. Whiting, J. Phys. F 16 (1986) 1127. [It)] S. Seizer, N. Majlis and H. ['uszkar~ki, Phys. Rev. B 29 (1984) 517.
s u r f a c e sciellce :i'""""
!
The observation of body modes and complex modes in single-crystal nickel thin films N.H. Ahmad, I. H a m e e d a n d M.A. K a r e e m Physics Department, College of Science, University of Basrah, Basrah, lraq Received 18 June 1990; accepted for publication 3 January 1992
The excitation of body modes and complex modes in (100) single-crystal nickel films epitaxially deposited onto NaCI substrates are reported. Experimental investigations on some of these films reveal the presence of body modes only. Observations on ot~er films indicate the presence of complex modes which exhibit the characteristic feature of migration from within the body mode to the law field side of the main mode as the value of the complex pinning parameter is changed.
I. Introduction Spin waves have been excited in single-crystal nickel thin films deposited on single-crystal substrates of NaC1 and LiF with the field applied in the sample plane [1] and normal to the sample plane [2] with small number of modes observed. No surface modes have been excited in such films. Recently surface modes have been observed in single-crystal nickel films grown on NaF substrates [3] and on NaCI substrates [4]. These modes have been evident in both orientations and interpreted in terms of surface inhomogeneity model of Puzskarski [5]. We wish to report in this work the observation of body modes and complex spin wave modes in (100) single-crystal nickel films epitaxially deposited onto NaCI substrates. The excitation of such complex modes can be accounted for by introducing an imaginary component into the surface parameters [6].
2. Experimental procedu~'e Single-crystal thin films of nickel were prepared by vacuum evaporation onto heated (100) cleavage surfaces of NaCI using the method de-
scribed by Ahmad [7]. The films were 1800 ,~ thick and electron microscopy observations showed them to be good (100) single crystals with a law density of twins. This crystallographic quality was also shown in the plots of resonance field versus azimuthal orientation with the static magnetic field in the film plane since these plots showed four-fold symmetric shift of the resona~,ce iine. An X-band Vm~:m spectrometer was employed to ob.,:erve the spectra using 10O kHz field modulation and a phase sensitive detection system. The measurements were made at room temperature with the field applied normal (0 = 0 °) and parallel (0 = 90°) to the sample plane. Three films were deposited onto NaCI substrates. Two of them gave similar spin wave resonance spectra and the third gave a spectrum with evident complex modes.
3. Results and discussion Fig. 1 shows a spectrum for a perpendicular field config,~r~ion for one of the tw~ samples which gave similar spectra. No complex modes are observed in both parallel and perpendicular field orientations. The largest number of modes is observed when 0 = 0 °. From this spectrum we
C039.6028/92/$05.0(~ ~; !002 - E!sevier Science Publishers B.V. All rights resern'ed
N.H. Ahmed et al. / Body modes and complex modes in Ni films
576
observe that both odd and even modes are excited and that the odd (symmetric) modes are higher in intensity than the even (asymmetric) modes. This implies that asymmetric pinning conditions exist at the two film surfaces. The spectrum is strongly dependent on the angle 0. This implies that the pinning conditions on both s u r f r ' e s could be angle dependent. This dependence may be associated with the film substrate interface anisotropy [3]. It was possible to interpret the spectra in terms of a pinning diagram in p - r space [5]. Various values of a and b have been used to find the best fit to the mode intensities. The computed values for the surface parameters are found to be a = 0.999176 and b = 0.005000. Thus both a and b are less than unity. This implies that the point located by the parameters r and p exists below the line p = 1 and in region of the pinning diagram which corresponds to the absence of surface modes, and lies or,, the left of the circle. Computed intensities of the body modes normalized to the first body mode are shown in fig. 2. It is noted that the envelope of the odd modes is slightly higher than that of the even modes although the decrcase of intensity with mode number n is less rapid for the experimental results. The quadratic dependence of the resonance absorption fields on the mode number of the body modes deviates slightly for the first two modes, The value of D can be computed from
Ni / NaCt ~.=1
/ 6
g
g
5
4
-~
3
g
2
9
.cKsi
Fig. 1. The spin wave first-derivative absorption spectrum in single-crystal nickel thin film grown on NaC! substrate with applit:d magnetic field normal to the film plane.
0.08
. D i/1
g o.o6 .E (p > ",S O . 0 4 m
0.02
i I
6
5
4
3
n
2
Fig. 2. Computed intensities of the bulk modes normalized to the first bulk mode for 0 = 0° spectrum.
the slope of the line connecting the computed values of k 2 for the body modes of the 0 = 0 ° spectrum and the corresponding resonance magnetic fields (fig. 3). The points are fitted to a straight line with the exception of the first two modes. The exchange constant D so obtained can be compared with that found experimentally and with other published data; see table 1. The absence of surface modes might be accounted for as due to vanishing of certain interactions existing at the interface, responsible for the excitation of these modes. As previously p:oposed [8], the type of surface anisotropy responsible for the excitation of these modes is due to a surface oxide layer on the free surface. A super exchange interaction between a nickel atom and adjacent substrate atoms may exist at the Ni/NaC1 interface [3]. Fig. 4 shows a series of spectra for the third film in which the applied magnetic field is rotated about an axis parallel to the sample plane. The spectrum consists of a broad asymmetric mode when the magnetic field is perpendicular to the film surface (0 = 0°), but at 0 = 2 ° two low-intensity high-field lines are observed, which at 0 = 6° have merged to give a weak distorted line. However at 0 > 6 ° the spectrum clemly shows again two weak high-field lines, Further increase of 0 causes the separation between the two weak lines to increase and at O---- 15° the main line and the
N.H. Ahrned et al. / Body modes and comph.~ modes in Ni fihns
577
KG)
0 f l J
1 1
6
0
15
/
!
I
I
I
5
10
15
20
I
36
,,,i
25 k 2 (Rod) 2
.------30
301(10 - s
38
Fig. 3. Bulk-mode resonance fields against computed values of k for the 0 = 0 ° spectrum. 40
low-intensity line (of higher energy) have merged to give a broad and distorted line. This weak line emerges at the low-field side of the main line when 0 = 20 °. The intensity of the other weak line (of lower energy) decreases monotonically with increasing resonance field but at 20 ° the separation of the two weak lines becomes maximum. Further increase in O causes the weak line (of lower energ)) to move towards lower fields and its intensity to decrease to the extent that when O = 90 ° (magnetic field in the plane of the sample) only one line is observed. This sequence of changes can be visualized in a stick diagram (fig. 5) in which the heights of the lines are approximately proportional to the estimated line intensities. Main feature of figs. 4 and 5 is the emergence of two weak lines of energy lower than that of the main body mode and a movement of a line from the high-field side of the main body mode to the low field side and with Table 1 Exchange constant for nickel at room temperature Reference
D (10 -~'' erg cm =)
Structure of the film
Present work A h m a d and Kareem [4] Mitra and Whiting [3] Rusov [1]
49 ± 3 56_+3 47 +_5 74 + 3
Single Single Si,,gle Single
cl3.,,tal clystal cr}.'slai crystal
90
Fig. 4. Spin wave resonance spectra fl~r different orientations of the applied magnetic field. The dominant mode has been aligned in the spectra to show the relative motion of thc resonance lines.
increasing intensity which is always less than that of the main body modc. Thc relative motion of these lines and the variation of their intcnsitics with position indicates that one or two of tile lines must be a localized mode [5]. h may be possible to interpret this sequence of changes if the surface parameters are real. Surface modes are possible if one of the surface parameters or both are made to vary such that the two modes change in both position and intensity until the two modes become equal to each other. It is not certain that equalization occurs at 0 = 6° since at this angle the two modes have merged to give one weak distorted iine. Another possibility R~r the surfao: parameter.,, to bc rcal occurs if one of them is made to va~3' such that at values of O > 6 ° the weak line (of higher energy) shifts towards the left, and at certain well defined value,; of the surface parameters the surface mode becomes a space mode and
578
N.H. Ahmed et aL / Bo@ modes and complex" modes in Ni films
D
D
_L ~3=
(ii) The weak high-field line (of lower energy) may be QLSM and as 0 varies from 0 ° to 40* this mode continues shifting towards lower values of magnetic field with gradually increasing intensity. If ~ is allowed to have a-certain fixed value with t ~ 0, then any decrease in t causes the energy of the QLSM to increase, thus shifting the mode towards lower fields eventually appearing on the low-field side of the body mode position. This is consistent with the sequence of the observed spectra. If q~ is assumed to have a small value and is allowed to increase, the value of t changes and consequently the intensity may change in a similar way as that of a complex mode. Thus the intensity would be expected to increase as it shifts towards lower fields. The origin of a complex parameter could result from a random surface potential [10] or from defect centers randomly induced by the high-energy electron bombardment used to promote nucleation [9].
D
..... L_L_I____
I .... x
4°
0°
6 °
D D
k
11
12"
10*
l 15 °
il D
20° Dll C
30*
C
I
34* D
36*
D
I
D
I
I
38*
40*
90 °
Fig. 5. Stick diagram to visualize the sequence of changes of the spectra. The heights of the lines approximately indicate the relative intensities. (D) Dominant mode, (C) complex mode.
further on an equalization effect takes place [5]. This characteristic feature is not observed experimentally. If the surface parameters are assumed to be complex then the complex spin wave modes of the quasi-localized character (QLSM) can be excited [6]. The energy and damping of such modes of wave vector k = s + it is of the form x - 2 cos s cosh t and 2 sin s sinh t respectk, ely, where s = q~, the phase of the complex pinning parameter. Its value determines the location of the QLSM within the body mode. Now two possibilities need to be discussed: (i) when 0 varies from 0 ° to 6 °, the weak high-field iili'll~
't~%,.all I l l l ~ l l l i , . ' l
%,~11%.. I t...~)) /
ill(.I,~'
II,Jt~
~i,~bdll
II
(~Jl.llli,,,.l
(;.ILO
0
becomes larger than 6 ° this mode shifts towards the left migrating from withSn the body-mode manifold to the low-field side of the main mode.
Acknowledgement We are indebted to Dr. J.S.S. Whiting for his interest in this work.
References [1] G.I. Rusov, Soy. Phys. Solid State 11 (1969) 96. [2] N.H. Ahmad, M. Prutton and J.S.S. Whiting, Proc. R. Soc. London A 328 (1972) 49. [3] D.P. Mitra and J.S.S. Whiting, J. Phys. F 8 (1978) 2401. [4] N.H. Ahmad and M.A. Kareem, Physica B, to be published. [5] H. Puszkarski, Prog. Surf. Sci. 9 (1979) 191. [6] H. Puszkarski, Solid State Comnmn. 33 (1980) 757. [7] N.H. Ahmad, PhD Thesis, University of York (1972). [8] P.E. Wigen, T.S. Stakelon, H. Puszkarski and P. Yen, AlP Conf. Proc. 29 (1976) 670. [9] A.A. Hussain and J.S,S. Whiting, J. Phys. F 16 (1986) 1127. [It)] S. Seizer, N. Majlis and H. ['uszkar~ki, Phys. Rev. B 29 (1984) 517.