Preparation and magnetic studies in amorphous Tbx-Fe1−xAl2O3 multilayers

Journal of the Less-Common

Metals, 145 (1988)

613 - 619

PREP~ATION AND ~AGNE~C SPIES Tb,-Fe1 _ w/Al2O3 MULTILAYERS*

613

IN POMPOUS

R. KRISHNAN, M. PORTE and M. TESSXER Laboratoire

de Magne’tisme, CNRS 92195 Meudon

C&de% (France)

J. P. VITTON and Y. LE CARS Kodak Pathe’, Centre de Recherches,

71102 Chalon-sur-Saline

(France)

(Received May 13,1988)

summary We have prepared multilayered films of amorphous Tb,-Fe 1_ X/_A1203 and studied their magnetic properties. Samples were prepared by sequential r.f. sputtering at an r,f. power of 100 W and at argon pressures of 6 and 10 mTorr. Water-cooled glass substrates were used. The alumina layer thickness was kept constant at 4 nm and that of the Tb-Fe layer varied in the range from 3 to 100 nm. M-H and polar Kerr loops were measured and torque magnetometry was used to obtain both magnetization M and the uniaxial anisotropy I(, . The samples prepared at both of the argon pressures gave similar results. For a Tb-Fe layer thickness d < 4 nm, M shows a strong increase indicating some partial oxidation of terbium. For d > 6 nm, M remains constant. K, appears even for d = 3 nm but it attains a maximum value only for d > 6 nm. Although K, is high, the coercivity H, is still small for d = 6 nm. This indicates that H, and K, are not related in a simple way. The fact that & increases with d is understood in terms of contributions arising from short- and medium-age order whereas, for d < 6 nm, local random anisotropy of terbium is present. Finally, the presence of some inhomogeneities is indicated by the Kerr loops although they are rectangular for samples with d > 5 nm.

1. production Amorphous films of rare earth-transition metal alloys such as Tb-Fe and Tb-Fe-Co are of interest from both the fundamental and the application points of view. As disordered systems they offer rich possibilities for exploring some fundamental aspects in magnetism such as anisotropy and the spin structure. From the practical point of view it is now well established *Paper presented at the Symposium on the Preparation and Properties of Metastable Alloys at the E-MRS Spring Meeting, Strasbourg, May 31 - June 2,1988. 0022-5088/88/$3.50

0 Elsevier Sequoiai~inted

in The Netherlands

614

that these alloys could be used for magneto-optical information storage [ 1,21. The main characteristics of these films, generally obtained by sputter deposition, are the presence of a uniaxial anisotropy KU which makes the film normal the easy axis of magnetization, a rectangular hysteresis loop and coercitivities on the order of a few kilooesteds [ 1 - 41. Multilayer (ML) films are now of current research interest and efforts are being devoted to obtaining new properties by modulating the thickness of the sublayers [ 51. These multilayers consist of single metals, either single crystal or polycrystalline, and for the system to be magnetic at least one of the metal layers should be magnetic [6 J. However, recently reports have also been published on systems based on amorphous layers also [ 71. We describe here our work on amorphous Tb,-Fe1 _r/Al2O, MLs. This work was motivated by the following. The presence of a uniaxial anisotropy in an amorphous film has been a puzzle. It was proposed that in the case of rare earth metals such as terbium and dysprosium with strong spin-orbit coupling a strong random anisotropy arises. Each local rare earth site has its own symmetry axis and hence an easy axis of magnetization but these axes are oriented at random in space. However, the average of this is not zero as it should be in the case of a perfect amorphous material but has a resultant which is imposed by the growth process [S]. Several other mechanisms have also been proposed amongst which one can mention the following. Exchange anisotropy due to coupling between two ferromagnetic regions via another region which is ant~e~om~netic [9] or bond orientational effects which occur during the process of film deposition and which result in an anisotropic distribution of first neighbours for the rare earth metal [lo] could give a non-zero resultant. Thus it turns out that in addition to local random anisotropy one could expect other contributions arising from chemical short-range order and medium-range order. Finally, some contribution could also arise from the interplay between stresses and the m~netostriction, particularly for those rare earths with large magnetostriction. Therefore we decided to study the magnetic properties in amorphous in Tb-Fe films as a function of film thickness for very thin layers. As it is not possible to characterize easily one single ultrathin layer we prepared mult~ayers so as to have enough material. The separator layer was chosen as A120, because it is optically transparent and generally used as a dielectric layer to protect the magnetic layer from any oxidation by exposure to air. Furthermore, in multilayers one would also expect interesting effects to arise from materials that are formed as a result of diffusion at the interfaces. We describe here our results on samples with two different compositions. 2. Experimental details The ML samples were prepared by sequential deposition by diode r.f. sputtering. We used 2400 unit (Leybold-Heraeus) fitted with a turbo-

615

molecular pump. For Tb-Fe we used a composite target and a disc of alumina for the dielectric layer. The starting pressue was (1 - 2) X lo-’ Torr. All samples were deposited with an r.f. power of 100 W and at two different argon pressures PAr of 6 and 10 mTorr onto water-cooled glass substrates. The film thickness was monitored by a quartz oscillator which had already been calibrated. The Tb-Fe layer thickness d was varied in the range 3 - 10 nm whereas that of Al*O, was kept constant at 4 nm, which is enough to prevent any exchange coupling between the magnetic layers, allowing it just to act as a spacer. The total number of bilayers was in the range 10 - 15. All the samples were finally given a protective coating of A1203 15 nm thick before the vacuum was broken. Magnetization M and M-H loops were determined at room temperature using a vibrating sample magnetometer (VSM). Torque measurements were made, using a home-made torque magnetometer, as a function of angle 8 in a plane normal to that of the film and fields of 1 kOe < H < 12.5 kOe. Measurements were taken at 6” intervals. The data were analysed in order to obtain M and the uniaxial anisotropy Ku. Polar Kerr rotation 0x loops were also determined with an He-Ne laser. The film composition was obtained from a thicker single-layer sample prepared under the same conditions by inductively coupled plasma (ICP) analysis. The composition profile was studied by secondary ion mass spectrometry (SIMS) using a CAMECA IMS 4F instrument. The magnetic parameters such as M and the coercivity H, obtained by the different methods agree well within the experimental error.

3. Results and discussion The compositions of the samples prepared at PAr values of 6 mTorr and 10 mTorr are TbZ2Fe7s (sample A) and TbzoFeso (sample B) respectively. Therefore it can be seen that sample A which is richer in terbium has a compensation temperature T,,, close to room temperature whereas for sample B for which the terbium content is lower Tcmp is lower. This is an important aspect to remember for understanding the results because M and H, diverge strongly near T,,,. Figure 1 shows the composition depth profile for sample B with d = 8 nm. The oscillations in the iron concentration indicate the periodicity and one can see eight layers. The damping of the intensity as the substrate is approached is an extrinsic effect and is caused by the technique of analysis. The width of the iron peak is roughly equal to 8 nm. The large peak for aluminium near the air interface arises from the protective alumina coating. This result is indeed quite encouraging. As the magnetization is measured along the film normal, which corresponds to the easy axis, saturation is easily attained for fields smaller than 10 kOe. Figure 2 shows the d dependence of M obtained from VSM measurements for the two samples. For both of them M becomes independent of d for d > 6 nm but for thinner films M shows a strong increase. This increase in M is attributed to the preferential oxidation of some fraction

I

I

I

\

4

6

6

10

*

t Id

Fig. 1. Iron depth profile for sample B (TbzoFesO) with d = 8 nm obtained by SIMS. The origin corresponds to the film surface.

I

0

10

5

dTb-Fe

c

I4

Fig. 2. The Tb-Fe layer thickness d dependence of the saturation magnetization. Fig. 3. The Tb-Fe layer thickness d dependence of the coercivity.

of the terbium. This oxidation could be due to the chemical reaction of terbium with the sputtered A120, still remaining in the chamber (because of the sequential sputtering). It is quite possible that this oxidation occurs for thicker samples also but is not effectively seen from their magnetization data. From the known M dependence on terbium content [4] we can estimate the effective terbium concentration and it turns out that, for sample B for d = 4 nm, the effective terbium content would be 19 at.%. The d dependence of the coercivity H, for the two samples is shown in Fig. 3. For both samples, H, increases by almost by a factor of 7 as d increases from about 3 to 7 nm and then saturates. However, for sample A the values of H, are much higher because here Z’Compis closer to room temperature. However, it is interesting to see that the relative increase in H, as d increases is about the same for the two samples and thereby it is seen that no specific influence arises from TCanP. Now let us discuss the torque measurements in these samples. Figure 4 shows the torque curve for sample A with d = 5 nm. A well-defined uniaxial anisotropy is clear. Also, one notices a large rotational hysteresis which is

617

Fig. 4. Torque curve for sample A (Tb2zFe,s)

with a’ = 5 nm.

Fig. 5. Torque curve for sample B (TbzoFeso) with d = 8 nm. Fig. 6. The Tb-Fe layer thickness d dependence of Ku for sample B (TbzoFeso).

normally seen in such samples [ 41. This arises because the applied field is smaller than the anisotropy field. Another interesting torque curve is obtained for sample B with d = 6 nm and is shown in Fig. 5. Although the overall symmetry is the same as in Fig. 4 one notices the appearance of small furbelow (a small bump) at 13= 909 This would indicate the presence of a small in-plane anisotropy (KU < 0) in some region in the sample which has been slightly oxidized. The values of KU and their d dependences were similar for the two samples and we show the result for sample B in Fig. 6. A small uniaxial anisotropy of the order of 0.7 X lo6 erg cme3 is already present for the thin Tb-Fe layer of 4 nm thickness (and for 3 nm in sample A) but it increases to a value of 1.2 X lo6 erg cmW3for d > 7 nm and remains

618

constant for thicker layers. Considering the weak dependence of K, on the terbium content [4], we are led to conclude that this variation in anisotropy does not arise from the slight variations in the terbium content in the samples. We therefore suppose that the anisotropy for the 4 run thick layer basically arises from the local random anisotropy of terbium atoms. The further increase in KU which occurs as d is increased then could arise from the other contributions we have mentioned such as those arising from the short- and medium-range order. Indeed a layer thickness of 3 or 4 nm appears to be too small for other contributions to become prominent and some minimum layer thickness is needed for them to build up. Our experiments show that the results obtained for two different argon pressures are quite similar. Also, our results on KU agree with those reported for Tb-Fe/ SiOz multilayers by Togami et al. [ 71 for similar Tb-Fe layer thicknesses. It is well known that strong anisotropy gives rise to large coercivities. However, our studies indicate that they are not related in a simpfe fashion. For instance, although one attains a saturation in KU values for d values close to 6 nm, the coercivity is still increasing. There are some quantitative differences between the two compositions, however. In any case the saturation in ;Fi, is attained only for thicker Tb-Fe layers. For sample B this thickness would correspond to about ‘7 or 8 nm. Therefore it is suggested that H, is controlled also by other parameters such as the microstructure of the film. Figures 7 and 8 show the 8x-H loops for sample A for d = 3 nm and d = 5 nm respectively for which KU = 0.7 X lo6 and 1.3 erg cmD3X lo6

-10 t

Fig. 7. Polar Kerr loop for sample A (Tb22Fe,8)

with d = 3 nm.

Fig. 8. Polar Kerr loop for sample A (Tb22Fe78) with d = 5 nm.

619

erg cmm3. The rectangularity is better developed for the sample with d = 5 nm. However, one notices for this sample some structures in the loop. This could arise from some magnetic inhomogeneities and may be a combination of different loops. We also noticed that the Kerr loop taken from the substrate side did not always correspond to that taken from the film surface side. This again would indicate that the samples are not quite homogeneous from a chemical point of view. For instance, as indicated by torque curves some layers are more oxidized than others. As the total film thickness is of the order of 50 nm or so, the light is transmitted through the sample and what is observed could be a complex effect arising from both Kerr and Faraday magneto-optical effects. In conclusion we have prepared amorphous Tb-Fe/Al*O, MLs and studied the variation in the properties as a function of the Tb-Fe layer thickness. It is seen that although the uniaxial anisotropy appears for fairly thin layers (about 4 nm) the coercivity attains its maximum value only for thicker films, thereby showing that these two parameters are not simply related. The Kerr loops for some samples show the presence of magnetic inhomogeneities. It is clear that more experiments are needed before one can obtain fully homogeneous samples. Also it would be interesting to study films containing gadolinium, for which there is no orbital moment, in order to understand the individual contribution from the local anisotropy and from the short- and medium-range order effects. Work is being planned in this direction.

Acknowledgment We thank Mrs. Grattepain for the SIMS profile analysis.

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