Magnetic and ferromagnetic resonance studies in Co-Cu composite films

Magnetic and ferromagnetic resonance studies in CoCu composite films

R. Krishnan, H. Lassri, M. Seddat, M. Tessier, Sivaraman Guruswamy, and Satyam Sahay

Citation: Journal of Applied Physics 75, 6607 (1994); doi: 10.1063/1.356913 View online: http://dx.doi.org/10.1063/1.356913

View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/75/10?ver=pdfcov Published by the AIP Publishing

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Magnetic and ferromagnetic resonance studies in Co-Cu composite films

R. Krishnan, H. Lassri, M. Seddat, and M. Tessier

Lahoratoire de Magn$tism et Matt?iam Magnitiques, C.N.R.S. 9219.5 Metdon, France Sivaraman Guruswamy and Satyam Sahay

Dtprtttwt of Metallurgical Etlgitweritzg, Utkw+y of Utah, Satt Lake City2 Utah 84lJ2

Co, .=JYu, composite films with 0~x483 have been sputter deposited on to water-cooled glass substrates. Transmission electron microscopy and scanning tunneling microscopy studies reveal clusters whose average size is 2SO ,& Magnetization when corrected to the Co volume is independent of Cu concentration and is equal to that of bulk Co. The in-plane saturation field is about 200 Oe at x=0, starts increasing for x=40, and shows a peak value of 1.2 kOe for x= 60.

Coercivity shows a similar behavior. Out-of-plane perpendicular anisotropy develops with the increase in Cu concentration. The ferromagnetic resonance studies show that (ij the linewidth increases for x,75 and iii) perpendicular anisotropy, probably induced by stress, also increases with s. Annealing up to 300 “C does not produce any noticeable changes but when annealed at 400 “C the perpendicular anisotropy decreases indicating some stress relief.

INTRODUCTION RESULTS AND DISCUSSIONS

Granular solids, consisting of metallic particles embed- ded in a matrix of another metal or oxide material have been studied in the past because of their interesting physical propcrties.l>’ Recently great interest has indeed been aroused in such materials following the exciting work by Berkowitz et al.” and Xiao et a!.,’ who observed large magnetoresis- tance (‘MR) in composite films of Co-Cu and Co-Ag. The origin of this effect is far from being well understood. Sev- eral papers which are basically devoted to the study of MR effects are appearing on these materials. Considerable effort is being devoted to the materials research of these kinds of films in order to optimize the properties for applications as sensors. We have chosen the Co-Cu system for this study and focused our attention on magnetic properties such as magne- tization and anisotropy using ferromagnetic resonance (FMR) techniques. We have also carried out some structural studies using transmission electron microscopy (TEM) and scanning tunneling microscopy @TM).

The planar TEM picture corresponding to the sample with Cu=46 at. % shown in Fig. lt has a magnification of 308 571X. One can observe clusters varying in size from about 160 to 450 /i. The average cluster size averaged over 30 clusters that are reasonably clear is about 250 A. The clusters are separated by grain boundaries. The cluster size varies from location to location even within this micrograph.

This appears to be consistent with STM studies to be de- scribed below.

Surface roughness studied by SThI of several samples showed no significant differences with the film thickness or Cu content. Figure 2 shows the surface roughness of the sample with Cu=66 at. % as a typical case. The top surface appears to be wavy with the average maximum height of about 50 A. The size of such zones is in the range 300450 A although one could see regions of smaller ones. Such shapes and sizes would lead to demagnetization factors which are not easy to calculate as discussed later.

EXPERIMENTAL DETAILS

The M-H loops along the film normal were typical of hard axis ones with no remanence. The in-plane &J-El loops are rectangular and their shapes change with increasing Cu The Co-Cu composite (we prefer this terminology to al-

loyj films were deposited on water-cooled glass substrates by rf sputtering using a 75mm-diam disk of Co and Cu chips as target. Most of the samples were prepared under standard conditions, namely? with a rf power of 80 W, argon pressure pAr of 6 mTorr and a film thickness of 14OO-C200 A, moni- tored by quartz oscillators. Some samples were prepared with rf power of 80 and 100 W, pAr in the range 3-23X 10m3 Torr, and film thickness ranging from 300 to 2600 A.

The composition of the samples was determined by elec- tron probe microanalysis and the fihn thickness with a pro- filometer. Some selected samples were studied by TEM us- ing a JEOL 200 CK microscope and also by STM.

Magnetization and fif-H loops were measured with a vibrating sample magnetometer. FMR was observed at 9.8 GHz with the dc applied field both in the film plane (Hpa,.J

and perpendicular (Hperp) to it. FIG. 1. TEM planar picture of the sample with Cu=ilh. Magnification x0.3x le.

J. Appt. Phys. 75 (IO), 16 May 1994 Q 1994 American Institute of Physics 6607

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FIG. 2. STM surface imag of the sample with Cu=66%. x axis 50 nmidiv,

2 axis 15 nmidiv.

content due to the appearance of perpendicular anisotropy (I$,,). We will discuss the presence of H, when describing the FMR results. No uniaxial anisotropy in the film plane could be detected. Figures 3(aj and 3(b) show two typical results for x=-20 and 60, respectively. Figure 3(b) is typical of a loop with the presence of H, and consequently this leads to an increase in the saturation field H, . Figure 4 shows the Cu content dependence of /is. it seen that it is low and around 200 Oc for ~640 but shows a sudden jump for x > 40 to reach a peak value of about 1.2 kOc for s = 60. The Cu content dependence of the in-plane coercivity H, is also very interesting and somewhat analogous to that of !Z, as shown in Fig. 4. tf, first increases sharply for s>30 and after showing a broad peak it decrease again. The peak value of H, is about 100 Oe. The magnetization, when expressed in terms of the total sample volume CofCu shows a linear decrease as x increases., But since Co and Cu do not form an alloy the magnetization should be expressed in terms of the actual Co volume. In this case. the magnetization of all the samples equals that of bulk Co. A similar conclusion was also reached by Kneller’ on evaporated Co-Cu films. This shows that the Co moment is conserved because the Co at- oms tend to cluster together. The magnetization of the samples was independent of the deposition parameters and the film thickness in the range 300-2500 & but not so for the FMR properties as will be discussed later.

The FMR resonance linewidth AH, as is well known, is a sensitive tool to check the quality of the films. We made a systematic study of AH as a function of the deposition pa- rameters (,rf power and pAr) and film thickness. Changing the rf power from SO to 100 W did not sensibly affect the FMR

A---- -..-- __ ::::>- .,;>

M

&=85ck H,=1.2kCk~

FIG. 3. The in-plane b-i-H loops fur the samplss for (ai Cu=N and (hi btl

at.

%, respectively.

5

.o B

5 s

D

FIG. 4. ‘I%e Cu content dependence of the saturation field H, and the in- plane cocrcivi~y 11,.

properties. AH,,, was generally higher than AH,,, in all the samples. This could arise from an inhomogeneous demagne- tization field in the film plane due to the interspersing of Co and Cu clusters. For a given composition both AHpars and

&, remain almost independent of I)*~ in the range 6-23 mTorr but show a rapid increase by a factor of 7 for p,,=3.5 mTorr. This is due to inhomogeneous line broadening and it can be inferred that very low pti leads to inhomogeneity.

This cannot be due to Ar inclusion which is known to occur only for high Ar pressures. Therefore we attribute this to the inhomogeniety caused by the high kinetic energy of the ada- toms resulting from lesser collisions due to a large mean free path at the fairly low Ar pressure. This leads to more mixing of Co and Cu and the resonance fields are perturbed by the presence of nonmagnetic inclusion of Cu. Also at low argon pressures, plasma instability occurs. Finally the study of the thickness dependence of GUY,,.,, showed that it decreases from 90 to 70 Oe when the film thickness is increased from 300 to 1400 A and then decreases only slightly for X00-&

thick sample. So the following standard conditions of prepa- ration were adopted; p.

x

=6 mTorr, t-f power=80 W, and film thickness = 1400 2 200 . The properties of the sampIes with different Cu content are discussed below.

0 20 40 60 80

cu ( at ?6 ‘1

FIG. 5. TIK Cu content dependence of the perpendicular anisotropy HP amuning a demagnetization field of 4&f. corresponds to after annealing at 405 “C. See the text.

6608 J. Appl. Phys., Vol. 75, No. IO, 15 May 1994 Krishnan et al.

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I t ! 1 I 1 1 I 1 c

0 20 40 60 80

cu ( at % )

FIG. 6. The Cu content dependence of the parallel FMR line width AH,, . Also are shown data for s= 67, for film thicknesses q 300 *I, A 720 I$ 0

I ~011 A, 0 2000 -4, respectively.

The FMR spectra in composite films are to be distin- guished from those on multilayers where oscillatory coupling esiqts between the magnetic layer and where supplementary resonance modes have been observed.” The FMR spectra with H in the film plane showed a single intense mode except in the very few cases where a very small shoulder could be observed at the lower field side which we have neglected for the present discussion. TTowever, the perpendicular spectra of all the samples consisted of an intense mode followed at Tower fidds by two or sometimes, three satellite modes of smaller (--5%,) intensity as compared to the first one. These modes are not standing spin waves as shown by the absence of a quadratic behavior in their field positions and the ran- dom way in which their intensities varied. No clear correla- tion could be established between the spacing of the modes and either the Cu content or the film thickness. Apparently the composite nature of the samples perturbs these modes which could he cigenmodes. From the parallel and perpen- dicular resonance fields we calculated the ,g factor and effec- tive demagnetization 4 rr.&f’ = 4 rr~U -HP, where we have implicitly assumed that the demagnetization factor corre- sponds to that of a thin film. For the calculation we consid- ered only those resonance modes which yielded the R factor close to 2.2 tvpical of Co. From 4rrhf’ one can calculate HP, provided-one knows the value of 4?r. If we consider only the Co clusters which are participating in the resonance then 4r~!V has to be taken as 17.5 kG based on the measured M values. In this case I’!, increases almost linearly with the Cu content as shown in Fig. 5. These HP values are unrea- sonably large because the in-plane loops study shows that the magnetization is essentially in the film plane. One might therefore ask what would be the real demagnetization factor in such composite films which consist of clusters and are not homogeneous. From TEM studies we had inferred that the average cluster size was about 240 A. Considering the films thickness, which is 12OOtiOO A, the aspect ratio is not any-

more that of a thin film. Therefore the demagnetization factor N which creates the internal field N( 4 TM) shotrId be much Tess than one normally used for thin films. Calculation of the exact demagnetization factor is quite complex for the clus- ters with which we are concernedS7 If one assumes as a first approximation, N= 0.5 which is reasonable then the demag- netization field becomes 2rM. In this case the HP values reported in Fig. 5 will have to be halved. These HP values are more reasonable and consistent with the easy plane loops observed. There could be some contribution to HI, from mag- netoelastic energy (crXh) (where the first and second terms represent the stress and the magnetostriction) due to stresses arising from the Cu clusters which surround the Co ones. Of course some stress would also be created by the sputter growth process. Indeed we annealed some samples in a vacuum of about 5X 10d6 Torr for a duration of 3 h and studied the effect on the magnetic properties. No change was observed for annealing temperatures up to 300 “C. However annealing at 405 “C produced some changes in the FMR properties. No change in the magnetization was observed as was expected because the magnetization cannot exceed that of bulk cobalt but a decrease in HP was observed as shown in Fig. 5 for x= 60 at. %. This reduction could be attributed to the relieving of some of the stress hence a reduction in the magnetoelastic anisotropy. Annealing at 405 “C also leads to a small increase in coercivity of about 15%. The coercivity depends on the microstructure which was possibly modified by the annealing treatment. More structural work is needed to interpret this result.

As regards the Cu concentration dependence. of the line- width, the L&l,,, remains constant at 80220 Oe for 17 ~X=S 60 and for x2 60 it increases rapidly to a value of 200 Oe. But AHpara shows a totally different behavior as shown in Fig. 6. It decreases initially with the Cu addition and after showing a broad minimum starts increasing rapidly for x2 60 to reach a value of 300 Oe for x = 82, In the same figure is also shown the linewidth for samples with x= 67, with thicknesses 300, 720, 1200, and 2600 ti, the highest linewidth being for the thinnest sample as mentioned earlier.

More structural studies are needed to understand the above results and are in progress.

We wish to thank Y. Dumond for the determination of composition and Dominique Tmoff for helping with STM studies.

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J. Appl. Phys., Vol. 75, No. IO, 15 May 1994 Krishnan et al. 6609

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