Ferromagnetic resonance evidence for a surface ferromagnetic layer in Co-Cr thin films

Journal of Magnetism and Magnetic Materials 54-57 (1986) 1689-1690

1689

F E R R O M A G N E T I C R E S O N A N C E EVIDENCE FOR A S U R F A C E F E R R O M A G N E T I C LAYER IN Co-Cr

THIN

M.L. COFIELD,

FILMS C.F. BRUCKER,

J.S. G A U

a n d J.T. G E R A R D

Research Laboratories, Eastman Kodak Company, Roehester, N Y 14650, USA

We studied the ferromagnetic resonance spectra, at 9.8 and 35.2 GHz microwave frequencies, of electron-beam-evaporated Co-Cr magnetic thin films with multiple resonances and found evidence for a surface ferromagnetic layer by chemically etching the samples. The temperature dependence of the line widths of samples with a bulk and surface resonance suggests that the relaxation mechanism in the surface layer is of the impurity type and probably involves the presence of some Co in an oxidized state. The line width of the surface layer is nearly frequency independent, whereas the bulk resonance line widths are frequency dependent with one resonance whose properties suggest incomplete saturation at 9.8 GHz. The symbols have their usual meanings. Multiple resonance structure can arise in F M R spectra if different volume elements of the sample have different magnetization or anisotropy properties, perhaps due to compositional, morphological, or crystallographic variations in magnetic properties with thickness. The value of the effective anisotropy H~ rf, equal to intrinsic magnetocrystalline anisotropy plus demagnetization anisotropy, can be obtained from perpendicular and parallel reson a n c e data:

The magnetic properties of C o - C r thin films are extensively studied, for applications in perpendicular recording. Magneto-optic Kerr-effect measurements of coercivity a n d r e m a n a n c e near the film surface give values different from those of bulk magnetization measurements [1]. Data from VSM d e p t h profiling show a change in magnetic properties with depth of etching [2]. However, bulk magnetic measurements are not very sensitive to details a b o u t magnetic inhomogeneities that could arise from compositional or structural variations across the thickness of the sample. Ferromagnetic resonance ( F M R ) is a sensitive technique that can provide detailed information a b o u t the magnetic properties of thin films, including magnetic inhomogeneity a n d surface magnetic effects. Some indications of inhomogeneities in sputtered C o - C r films have been reported from F M R data [3]. We report results from F M R studies of some evaporated C o - C r magnetic thin films a n d identify a resonance from a surface ferromagnetic layer. Samples were prepared by electron-beam evaporation from an alloy target with nominal composition Co~oCr20. The samples were 0.3-0.5 ~ m thick and were on a polyimide substrate. The substrate temperature was in the range 110-160°C, and no systematic correlation with F M R properties was observed. The spectra were o b t a i n e d on an IBM I n s t r u m e n t s spectrometer at 9.8 G H z between 100 and 300 K. One sample was also investigated at 35.2 G H z at room temperature. The spectra consist of two or three distinct resonances when the applied field is perpendicular to the film surface at 9.8 GHz. W h e n the field is parallel to the film surface, either a single resonance or two resonances are observed (table l). The resonance equations for the field applied perpendicular a n d parallel to the film surface are given by

We report the data in table I from our measurements. The p r o m i n e n t resonance modes at applied fields of 8 kOe or greater have geff= 2.2-2.3 at 9.8 GHz. These values agree well with data for pure Co films a n d some other Co-alloy films [4,5]. These resonances have negative H~ it, meaning the easy magnetization direction is in the plane of the sample. The b r o a d resonance near 5 - 6 kOe applied field in the parallel a n d perpendicular spectra of some samples at 9.8 G H z has unusual resonance characteristics. First, the resonance line width was b r o a d and asymmetrical. Second, the analysis of the parallel and perpendicular

I ~/~'1 = H ± + H K - 4 c r M

(1)

21.7

(2)

* Behavior is unusual at 9.8 GHz; H,~ff= 2 K u / M ~ -4'rrM s.

~,

I w / 7 1 = [ H I I ( H I I - H K + 4"rrM,)] 1/2-

0304-8853/86/$03.50

H~qff = 1[ -HI[ - 2 H ± + (4HI, H ± +5HII)2'1/2]].

(3)

Table 1 Summary of resonance data at 9.8 GHz Cr (%)

Hit (Oe)

Hi (Oe)

4"rrMs (G)

H~"if (kOe)

geft

19.2

1481 1481 1418 1418 5817 1300 1300 5785 1445 5932

8249 8781 8383 8967 6079 8474 9626 6010 7685 5151

6510

- 5.12 - 5.55 5.29 - 5.77 * - 5.50 - 6.45 * - 4.70 *

2.23 2.16 2.26 2.19

19.5 20.5

© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.

6450 5830 5500

2.35 2.20 2.34

1690

M.L. Cofield el al. / Surface ferromagnetic &ver in Co - Cr

spectra of this c o m p o n e n t with eqs. (3) and (4) gives an unreasonably low value of gCff= 1.18. Therefore, we investigated one sample at a microwave frequency of 35.2 GHz, for possible clarification. The resonance features changed little, except that the broad resonance at low field in the 9.8 G H z spectra had a more symmetrical line shape and a smaller line width at 35 G H z (table 2). The spectrum with the applied field parallel to the film surface was a single mode at 35.2 GHz, in contrast to the same spectrum at 9.8 GHz, where two modes were observed. These data were interpreted according to eq. (3): results are given in table 2. The difference in the resonance behavior of these films with frequency is likely due to incomplete magnetic saturation at 9.8 GHz. In a ferromagnetic specimen that is not saturated at resonance, the magnetization is not parallel to the applied field, and so-called domain resonance p h e n o m e n a can occur, which can lead to line broadening. Our results for the resonance behavior of these films at 9.8 and 35.2 G H z are consistent with this interpretation. We have performed some chemical etching experiments to better understand the origin of the various resonance modes. Fig. l details the results. One mode was very sensitive to the etchant, so that after a few minutes it had completely disappeared from the spectrum. We propose that this resonance is due to a surface ferromagnetic layer produced from air oxidation of the film surface. This view is supported by F M R data taken between 100 and 300 K on a specimen of the same sample used for the etching studies. Results are summarized in fig. 2. The F M R line width of a film with good metallic properties is not very sensitive to temperature. In contrast, a strong variation in line width with temperature has been observed for oxidized surface layers in some YCo 3 and G d C o magnetic thin films [4,5]. In fact, our t e m p e r a t u r e - d e p e n d e n t line-width data are reminiscent of those reported for the YC% magnetic thin films, whose data were interpreted according to an impurityinduced relaxation mechanism involving the presence of some Co: + ions in the surface layer [6]. The F M R resonance line width of a ferromagnetic

"Fable 2 Comparison of spectra at 9,8 and 35.2 GHz FMR mode v 1 2 3

tt~,n'

Hl

(kOe)

(Oe)

Line width AH±

g,.ff (Oe)

35.2 OHz 3.74 5.32 6.19

v = 9.78 GHz 1 - 1.50 2

5.50

3

- 6.45

15000 17250 18450

1000 325 250

2.22 2.11 2.05

6010 8474 9626

= 2000 388 100

1.18 2.35 2.20

Before

Etch

[ ~ l

y-

After × 6

~

Etch

v

I

HappI

= 9.8 G H z

I

l

6.3kOe

Surface mode

l

I

7.3kOe

l

Bulk mode

I

8.1kOe

-

l

89kOe

I

l

9.7kOe

Fig. 1. Results of chemical etching in = 5 N HCI solution. A surface-layer resonance is observed. i I

IOO

i

i

i

i

,

• Surlace • Bulk

9OO

v = 94GHz 7001 500 300

Q

I00 i UO

i

1

i

150

i

i

i

i

i

200 Ternp (K)

I

I

1

i

I

250

i

I

I

I

I 3

oo

Fig. 2. Temperature dependence of the resonance line widths of the surface and bulk modes identified in fig. 1. metal should increase with increasing frequency, according to the Landau-Lifschitz relaxation mechanism [7]. The line width of the bulk resonance at the highest value of the applied field increases as expected at 35.2 GHz. The line width of the low-field resonance is much smaller at 35.2 G H z than at 9.8 GHz, in accord with the idea of domainlike p h e n o m e n a at the lower frequency. The line width of the mode identified as a surface layer is nearly frequency independent, a further indication of intrinsic differences in the magnetic properties of this layer. [11 M. Abe, K. Shono, K. Kobayashi, M. Gomi and S. Namura, J. Appl. Phys. Japan 21 (1982) L22. [2] W.G. Haines, IEEE Trans. Magn. MAG-20 (1984) 812. [3] P.V. Mitchell, A. Layadi, N.S. VanderVen and J.O. Artmann, J. Appl. Phys. 57 (1985) 3976. {4] G. Suran, S. Prasad, H. Jouve and R. Meyer, J. Appl. Phys. 50 (1979) 1617. [5] G. Suran, R. Krishnan, S. Prasad, H. Jouve and R. Meyer, 1EEE Trans. Magn. MAG-16 (1980) 1342. [6] G. Suran, R. Krishnan, J. Sztern, H. Jouve and R. Meyer, J. Magn. Magn. Mat. 7 (1978) 178. [7] R.F. Soohoo, Magnetic Thin Fihns (Wiley, New York, 1964).