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Temperature dependent FMR on CoCr-layers
51
Journal of Magnetism and Magnetic Materials 83 (1990) 57-58 North-Holland
TEMPERATURE
DEPENDENT
FMR ON COG-LAYERS
E.M.C.M. REUVEKAMP, A.M. de WI’ITE, J.C. LODDER and H. ROGALLA Umversrry of Twente, Faculty
W.A.M.
AARNINK,
G.J. GERRITSMA,
of Applied Physics, P.O. Box 217, 7500 AE Enschede, The Netherlands
Temperature dependent FMR measurements were performed on two series of rf sputtered films. The FMR measuring temperature could be varied between 77 and 300 K. There is indication of the existence of 2 CoCr phases in the film from ion-milled samples. The perpendicular anisotropy increased with both decreasing temperature and stress-relaxation by removing the substrate. 1. lnhwduction
To obtain information on the magnetic properties of CoCr-layers, particularly concerning stratification as was reported by Mitchell [l] and Stam [2], ferromagnetic resonance was performed. The FMR sample was cooled from 300 to 77 K to study the temperature dependence of the crystal and stress-induced anisotropy. A cylindrical resonance cavity operated in TE,,, mode at 30 GHz was used. A full angle-dependence of the magnetic induction of the FMR spectra was recorded; the angle between the film-normal and the external magnetic induction was varied between roughly - 20 o and 110 O. An as sputtered thickness series, consisting of 50-100-200-300 nm films, were rf sputtered at VRF = 1.6 kV and pa, = 4 x 1O-3 mbar, using an U/19 at% CoCr target, on 0.3 mm Si-substrates. One section of the 300 nm film was ion-milled with 10 keV Ar+ to a second thickness series consisting of 25-50-lOO-150200-250 nm films. To avoid an annealing effect a low Ar+-flux was used; an etch rate of 250 rim/h was achieved. The substrate was removed with a NaOH wet-etch from a second section of the 300 nm. 2. FMR theory and data processing
F [3].
energy
saturation magnetisation, netisation vector, B, magnetic flux-density or induction, 8, p the angle between film-normal and M, B and K the effective perpendicular anisotropy: K=K,+$(X,+X,,)u,, K,
the crystal
(2) anisotropy,
X,,
3. CoCr analysis The main CoCr properties are displayed in table 1. AES showed a constant Co-Cr composition of 81-19 at% over the whole depth for the as sputtered films. Room-temperature angle-dependent resonance inductions are represented in fig. 1. The 100 and 200 nm films showed an intermediate resonance induction behaviour. All films had only one maximum of microwave absorption. In particular the resonance induction for the thickest films did not agree with angle dependent FMR theory with one set of parameters poMs(Q - 1) and y. For all films, including the ion-milled films, it is very well possible to fit all measurements with two sets of fitparameters. The value of poMs( Q - 1) for different thicknesses is given in fig. 2. The gyromagnetic ratios are about y - 1.88 x 10” rad s-’ T-’ (g = 2.14).
Table 1
O-M.B,,
with
striction constants, u,, in-plane stress (u,, > 0 for tension), assuming an in-plane stress only and hexagonal crystallites with the c-axis normal to the film plane. The Simplex fitprocedure, using the high field approximation expression for w,(B,) (ref. [2], p. 61) extended to 3rd order in B-i, is applied to solve the parameters peM,(Q - l), with Q = 2K/poM:, and the gyromagnetic ratio, y, from the measurements. The line width of the absorption is in first-order proportional to the Gilbert damping-parameter (Y from the LandauLifshitz equation (ref. [2], p. 67) and therefore angle-independent as long as a is.
h,
the magneto-
0304-8853/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
CoCr film properties Dektak thickn. d VSM A4, VSM H, Torque MM K Quality factor Q XRD A&,
for the as sputtered 47 487 102 119 0.80 3.36
97 455 98 105 0.81 -
films
211 437 80 99 0.83 3.15
297 (nm) 433 (kAm_‘) 64 (kA m-‘) 95 (kJ rnm3) 0.81 3.83 (“)
E.M.C.M.
58
T=300K
B,(P)
N
Rruvekamp
et (11./ Temperature
LINE
50 “IT 300nm THEORETICAL
0.x +.a -,-
dependent FMR on CoCr-layers MOTH
AB,lpl.
1OOnm x
FIl
300
0
Fig. 1. Room-temperature
angle-dependent
resonance induc-
tion. for the 50 and 300 nm as sputtered.
x.0.
AS SWTTWD
1.
l.O
ION-MILLED
1.11
0
0
l
q
0
0
0
0
+
x
x x
+
0
t
+ 1
I
OO Fig.
50
100 150 FILM THlCKNESS lnml
2w
I
,
250
300
-
2. Material parameter p,,M,(Q - 1) for different thicknesses, as sputtered and ion-milled films.
film
The y-values for the perpendicular and the in-plane field fit were almost the same for thin films (50 nm) and differ about 4% for the 300 nm films, indicating the
B,(Pl
FOR
3 TEMPERATURES.
100nm i: q 0
-.-
300 K 1LL K 70
K
THEORETICAL
”
0
”
1’
30
”
I3 ldegreesl
’
60 -
”
90
4. Angle-dependent line width at 77 and sputtered 100 nm film.
1
”
300
120
K, as
/I
0
0
Fig.
8
K
78 K
0
-[
F‘IT
5
existence of more than one CoCr phase in the thicker films. At lower temperatures a typical example of changed angle-dependent resonance induction behaviour is shown in fig. 3. At 77 K the values of paM,(Q - 1) were 0.036 and - 0.074 for, respectively, the perpendicular and the in-plane field fit. This meant that the film had an increased perpendicular anisotropy. The cause for this has to be the increased crystal anisotropy, as is well known for pure cobalt crystals at lower temperature. The increase is opposed by an increased in-plane tension o,,, originated by a differing thermal expansion between the CoCr-film and the substrate, because of negative X,, X, [4]. FMR on the 300 nm CoCr film with removed substrate showed an increased perpendicular anisotropy, as expected. The value of pa M,( Q - 1) for the perpendicular and in-plane fit were, respectively, -0.027 and -0.039, indicating a stress contribution to the anisotropy of about - 15 to - 20%. The line width measured did not agree with theory, assuming an angle-independent (Y.Typical angle-dependent line widths are shown in fig. 4. It can be concluded that the line width (and consequently a) is strongly angle-dependent, probably caused by the columnar morphology of sputtered CoCr-films. Futher investigations on this point are needed. References
0
Fig. 3. Angle-dependent peratures,
30
B Idegrees)
60
-
90
resonance induction for different as sputtered 100 nm film.
120
tem-
[l] J.O. Artman, J. Appl. Phys. 61 (1987) 3137. [2] M.T.H.C.W. Stam, Thesis University of Twente, The Netherlands (1989). [3] J. Smit and H.G. Beljers, Philips Res. Rep. 10 (1955) 113. [4] Y. Hoshi, M. Matsuoka. M. Naoe and S. Yamanaka, IEEE Trans. on Magn. MAG-20 (1984) 797.
Journal of Magnetism and Magnetic Materials 83 (1990) 57-58 North-Holland
TEMPERATURE
DEPENDENT
FMR ON COG-LAYERS
E.M.C.M. REUVEKAMP, A.M. de WI’ITE, J.C. LODDER and H. ROGALLA Umversrry of Twente, Faculty
W.A.M.
AARNINK,
G.J. GERRITSMA,
of Applied Physics, P.O. Box 217, 7500 AE Enschede, The Netherlands
Temperature dependent FMR measurements were performed on two series of rf sputtered films. The FMR measuring temperature could be varied between 77 and 300 K. There is indication of the existence of 2 CoCr phases in the film from ion-milled samples. The perpendicular anisotropy increased with both decreasing temperature and stress-relaxation by removing the substrate. 1. lnhwduction
To obtain information on the magnetic properties of CoCr-layers, particularly concerning stratification as was reported by Mitchell [l] and Stam [2], ferromagnetic resonance was performed. The FMR sample was cooled from 300 to 77 K to study the temperature dependence of the crystal and stress-induced anisotropy. A cylindrical resonance cavity operated in TE,,, mode at 30 GHz was used. A full angle-dependence of the magnetic induction of the FMR spectra was recorded; the angle between the film-normal and the external magnetic induction was varied between roughly - 20 o and 110 O. An as sputtered thickness series, consisting of 50-100-200-300 nm films, were rf sputtered at VRF = 1.6 kV and pa, = 4 x 1O-3 mbar, using an U/19 at% CoCr target, on 0.3 mm Si-substrates. One section of the 300 nm film was ion-milled with 10 keV Ar+ to a second thickness series consisting of 25-50-lOO-150200-250 nm films. To avoid an annealing effect a low Ar+-flux was used; an etch rate of 250 rim/h was achieved. The substrate was removed with a NaOH wet-etch from a second section of the 300 nm. 2. FMR theory and data processing
F [3].
energy
saturation magnetisation, netisation vector, B, magnetic flux-density or induction, 8, p the angle between film-normal and M, B and K the effective perpendicular anisotropy: K=K,+$(X,+X,,)u,, K,
the crystal
(2) anisotropy,
X,,
3. CoCr analysis The main CoCr properties are displayed in table 1. AES showed a constant Co-Cr composition of 81-19 at% over the whole depth for the as sputtered films. Room-temperature angle-dependent resonance inductions are represented in fig. 1. The 100 and 200 nm films showed an intermediate resonance induction behaviour. All films had only one maximum of microwave absorption. In particular the resonance induction for the thickest films did not agree with angle dependent FMR theory with one set of parameters poMs(Q - 1) and y. For all films, including the ion-milled films, it is very well possible to fit all measurements with two sets of fitparameters. The value of poMs( Q - 1) for different thicknesses is given in fig. 2. The gyromagnetic ratios are about y - 1.88 x 10” rad s-’ T-’ (g = 2.14).
Table 1
O-M.B,,
with
striction constants, u,, in-plane stress (u,, > 0 for tension), assuming an in-plane stress only and hexagonal crystallites with the c-axis normal to the film plane. The Simplex fitprocedure, using the high field approximation expression for w,(B,) (ref. [2], p. 61) extended to 3rd order in B-i, is applied to solve the parameters peM,(Q - l), with Q = 2K/poM:, and the gyromagnetic ratio, y, from the measurements. The line width of the absorption is in first-order proportional to the Gilbert damping-parameter (Y from the LandauLifshitz equation (ref. [2], p. 67) and therefore angle-independent as long as a is.
h,
the magneto-
0304-8853/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
CoCr film properties Dektak thickn. d VSM A4, VSM H, Torque MM K Quality factor Q XRD A&,
for the as sputtered 47 487 102 119 0.80 3.36
97 455 98 105 0.81 -
films
211 437 80 99 0.83 3.15
297 (nm) 433 (kAm_‘) 64 (kA m-‘) 95 (kJ rnm3) 0.81 3.83 (“)
E.M.C.M.
58
T=300K
B,(P)
N
Rruvekamp
et (11./ Temperature
LINE
50 “IT 300nm THEORETICAL
0.x +.a -,-
dependent FMR on CoCr-layers MOTH
AB,lpl.
1OOnm x
FIl
300
0
Fig. 1. Room-temperature
angle-dependent
resonance induc-
tion. for the 50 and 300 nm as sputtered.
x.0.
AS SWTTWD
1.
l.O
ION-MILLED
1.11
0
0
l
q
0
0
0
0
+
x
x x
+
0
t
+ 1
I
OO Fig.
50
100 150 FILM THlCKNESS lnml
2w
I
,
250
300
-
2. Material parameter p,,M,(Q - 1) for different thicknesses, as sputtered and ion-milled films.
film
The y-values for the perpendicular and the in-plane field fit were almost the same for thin films (50 nm) and differ about 4% for the 300 nm films, indicating the
B,(Pl
FOR
3 TEMPERATURES.
100nm i: q 0
-.-
300 K 1LL K 70
K
THEORETICAL
”
0
”
1’
30
”
I3 ldegreesl
’
60 -
”
90
4. Angle-dependent line width at 77 and sputtered 100 nm film.
1
”
300
120
K, as
/I
0
0
Fig.
8
K
78 K
0
-[
F‘IT
5
existence of more than one CoCr phase in the thicker films. At lower temperatures a typical example of changed angle-dependent resonance induction behaviour is shown in fig. 3. At 77 K the values of paM,(Q - 1) were 0.036 and - 0.074 for, respectively, the perpendicular and the in-plane field fit. This meant that the film had an increased perpendicular anisotropy. The cause for this has to be the increased crystal anisotropy, as is well known for pure cobalt crystals at lower temperature. The increase is opposed by an increased in-plane tension o,,, originated by a differing thermal expansion between the CoCr-film and the substrate, because of negative X,, X, [4]. FMR on the 300 nm CoCr film with removed substrate showed an increased perpendicular anisotropy, as expected. The value of pa M,( Q - 1) for the perpendicular and in-plane fit were, respectively, -0.027 and -0.039, indicating a stress contribution to the anisotropy of about - 15 to - 20%. The line width measured did not agree with theory, assuming an angle-independent (Y.Typical angle-dependent line widths are shown in fig. 4. It can be concluded that the line width (and consequently a) is strongly angle-dependent, probably caused by the columnar morphology of sputtered CoCr-films. Futher investigations on this point are needed. References
0
Fig. 3. Angle-dependent peratures,
30
B Idegrees)
60
-
90
resonance induction for different as sputtered 100 nm film.
120
tem-
[l] J.O. Artman, J. Appl. Phys. 61 (1987) 3137. [2] M.T.H.C.W. Stam, Thesis University of Twente, The Netherlands (1989). [3] J. Smit and H.G. Beljers, Philips Res. Rep. 10 (1955) 113. [4] Y. Hoshi, M. Matsuoka. M. Naoe and S. Yamanaka, IEEE Trans. on Magn. MAG-20 (1984) 797.