Studies on Structural and Magnetic Properties in Co / Sm Multilayers

Co/Sm multilayer films with structure of 20 [Co (x nm)/Sm (1.2 nm)] where x = 1.1, 2.2 and 4.2 nm and 20[Co (4.2 nm)/Sm (x nm)] where x=1.2 nm to 7.5 nm were fabricated using dc magnetron sputtering. Each multilayer film consisted of 20 bilayers of Co layers with various thicknesses sandwiched with Sm layers. The application of low angle X-ray diffraction measurements to the characterization of these multilayers is described. The periodic layered structure with sharp interfaces was observed for all multilayer films. The measured magnetization values are lower than the values calculated in terms of the nominal concentration of cobalt in the multilayers. This implies significant “mixing” at small film thickness. The formation of a high magneto crystalline anisotropy of CoSm alloy at the interfaces, as a result of interdiffusion between Co and Sm layers was considered to be responsible for the increase of the coercivity for Co/Sm multilayer.


INTRODUCTION
In recent year, cobalt based alloys in the form of thin films and thin film multilayer structures consist of alternate magnetic and non magnetic layers of manometer thickness have attracted much attention due to their magnetic properties.One important application of such films is as high-density magnetic recording media.Many such alloys are based on the CoCr binary system with often multinary additions of such elements as Pt, Ta and B. An alternative binary system is that of CoRe alloys (Re-rare earth), and in particular the CoSm system.This allows, in principle and in equilibrium, the formation of a series of CoSm compounds with properties that reflect those of the compounds formed in bulk CoSm permanent magnets namely Co 17 Sm 2 , Co 5 Sm and Co 7 Sm 2 .There is also the additional interest in this system in that CoSm alloys can be deposited in the amorphous phase at Sm above about 20 atomic %.
According to (Murdock et al., 1992), magnetic recording media for high-density magnetic data storage with low noise require a material consisting of small and magnetically isolated grains.In small grain sizes of about 10 nm or below, high magneto crystalline anisotropy is needed in order to avoid thermal fluctuation that tends to destabilize the magnetization of the recorded bits (1997).
Early attempts to grow CoSm alloys in the form of thin films exhibiting large coercivity values were carried out by (Theuerer et al., 1969;Gronou et al., 1983), reported that CoSm films with high coercivity had been prepared by flash evaporation on glass substrates.They suggested that these films could be used as a highdensity magnetic recording medium.Some recording experiments have been investigated, for example, by (Velu & Lambeth,1992;Velu et al., 1994), However, the growth characteristics of CoSm alloys including the concentration range (1982), the epitaxial relation between CoSm alloy films and underlayer materials, (1998; 1991), interaction effects (1998; 2000), and the magnetic switching volume (1995), suggest that improved magnetic properties of CoSm films could be obtained by utilization and optimisation of appropriate deposition conditions.
Several methods have been used in growing CoSm thin films.They include sputtering, evaporation and plasma spraying.Thin films with controlled geometry and surface smoothness as well as composition can be easily fabricated by the sputtering method (1991).It is commonly observed that sputter deposition of CoSm thin films below 600 0 C usually produces alloys with an amorphous structure (1987).The amorphous structure of these alloys may limit their anisotropy and thus their coercivity.A higher value of coercivity can be simply achieved by introducing the crystalline state in the CoSm alloy by deposition with elevated substrate temperature or by post annealing of as-deposited films.
With regard to multilayer films, which are composed usually of ferromagnetic and non-magnetic thin layers, the interfacial mixing may exhibit novel magnetic properties.(1989; 1990) Work on multilayer films in which the nonmagnetic layer is composed of a rare earth and metalloid elements has been carried out for ferromagnetic materials forming the ferromagnetic layers.It was reported by Petford-Long et al., (1990) that interfacial mixing exists for the Co/W structure that affects the magnetic properties.
This paper is concerned with the investigation of magnetic and structural properties of Co/Sm multilayer films.This study is driven by fundamental interest and by the need to design materials with magnetic properties which might be suited for high density magnetic recording media.Multilayer films of alternatively deposited ferromagnetic and nonmagnetic layers are also proving to be important for understanding the fundamental physics of interacting magnetic films.

MATERIALS AND METHODS
The Co/Sm multilayer films were deposited onto Si (100) substrates by dc magnetron sputtering with a sputtering pressure of 4 x 10 -8 mbar of argon gas and the deposition rates were about 0.21 nm/s and 0.31 nm/s for Co and Sm respectively.The periodic structure i.e., sharp interfaces, in the samples of the samples was studied by small angle Xray diffraction.The data obtained were processed with special computer program which is called XREALM software (1988).Layer thicknesses were determined by sputtering rate and from the low angle X-ray diffraction patterns.The magnetic properties of the samples were studied by alternating gradient force magnetometer (AGFM) and vibration sample magnetometer (VSM).High-resolution transmission electron microscopy (HRTEM) has been used to analyse the microstructure of some of the as-deposited samples.

RESULT AND DISCUSSION
Low Angle X-Ray Diffraction Profiles.The low angle x-ray diffraction technique was used to investigate the structure of Co/Sm multilayer films.Figure 1 shows the low angle diffraction patterns for multilayer films with structure of 20[Co(x nm)/Sm(1.2nm)] where x = 1.1, 2.2, and 4.2.Computer simulations of the data, perform using XREALM softwar (1988) for multilayer films are included for comparison.In the simulated reflectivity profile, variables such as the layer thickness of the Co and Sm films, the roughness of the substrate and the layers, and the relative densities of substrate and films, can be adjusted so that one can obtain the closest fit to the experimental data.It can be seen that almost all the reflection peak positions of the multilayer films correspond to those in the simulated reflectivity profile.From the simulated reflectivity profile analysis, for example, the multilayer film with x=1.1 nm, it was found that the thickness of periodic bilayer was 2.22 nm, which corresponds to the thickness of Co and Sm layers of 1.10 nm and 1.12 nm respectively.
From Figure 1, the multilayer films show several orders of diffraction peaks, indicating the periodic nature of the sample.For the multilayer film with x = 1.1 nm, finite size oscillations (Kiessig fringes) persist to 5.5 degrees, implying that the overall surface of the multilayer film is smooth.However, for x = 2.2, and 4.2 nm finite size oscillations are not very clear, suggesting that the surface of the films is not smooth.With increasing Co layer thickness, diffraction peaks shift to a smaller value of angle of incidence suggesting that the bilayer thickness is increased.From the position of low angle XRD peaks shown in Figure 1, the thickness of the periodic layer (bilayer) for all samples could be calculated.Table 1 shows the bilayer thickness for nominal, simulated and calculated (low angle XRD).cm 3 and 350 emu/cm 3 and for x=4.2nm are 1090 emu/ cm 3 and 1020 emu/cm 3 .This implies significant mixing at small film thickness.

Magnetic Properties
The interaction effect given by δM for the three multilayers is shown in Figure 3.For the multilayer film with x = 1.1 nm the δM shows a broad positive and then negative peak.This could be due to exchange coupling over an extended field range between the Co layers, but there is also a strong dipolar coupling which becomes obvious at larger fields and moderates the exchange coupling because the Co layers are close enough for significant magnetostatic dipolar coupling.
The boardening of the positive δM is due to the interlayer dipolar coupling in the multilayer film, which prevents any sharp cooperative reversal (2001).This result is consistent with the lower loop squareness, Figure 2(a).
For multilayer films with x =2.2 and 4.2 nm the interaction effect (δM) shows a very sharp positive peak.given by Figure 4 are also well agree with the nominal deposition values.From the simulated reflectivity pattern analysis, it was found, as an example, that the thickness of a periodic bilayer was 8 nm, which corresponds to the thickness of Co and Sm layers of 4.2 nm and 3.8 nm respectively.From low angle XRD measurement, it was found that the thickness of that periodic bilayer was estimated from the position of Bragg peaks as about 7.9 nm.Table 2 shows in comparison the excellent agreement between nominal, simulated and experimental bilayer thickness of the multilayers.
However, it can be seen that at higher bilayer thicknesses        Another feature that can be noticed from this picture is the long "waviness" of the layers which decreases further away from the substrate, indicating that the flatness of the layers is also increasing but the sharpness of the interfaces becomes less defined with distance from the substrate.The electron diffraction pattern (inserted in The samples were divided into two groups.The first group of samples takes the form of 20[Co (x nm)/Sm (1.2 nm)].Each multilayer film consisted of 20 bilayers of Co layers with various thicknesses sandwiched with Sm layers of 1.2 nm thick.The second group of multilayer films has the form of 20[Co (4.2 nm)/Sm (x nm)] in which the samples have the same Co layer thickness of 4.2 nm and the Sm layer thickness is progressively increased from 1.2 nm to 7.5 nm.Again, each multilayer film consists of 20 bilayers of 4.2 nm of Co layers sandwiched with Sm of various thicknesses.

Figure 1 .
Figure 2b.The coercivity of this multilayer decreases
Figure 1.The thicknesses of the Co and Sm layers

Figure 5
Figure 5 shows a highly magnified micrograph of this multilayer film.Within the Co layers some crystalline grains are observed.These crystalline grains are a few nanometres (1-2 nm) in size (nanocrystallites) while the Sm layers appear to be amorphous.The diffuse structure of the Sm layers and the interfaces is especially obvious towards the top of the micrograph and away from the substrate.The Effect of Sm Layer Thickness.Low Angle X-ray Diffraction Profiles.Figure 6 shows a set of low angle x-ray diffraction patterns for multilayer films with a structure of 20[Co (4.2 nm)/Sm (x nm)] where x ranges from 1.2 to 7.5 nm.Computer simulation of the data for x=1.2 nm is included for comparison.From the figure,

Figure 7 .
Figure 7. Magnetization of 20[Co (4.2 nm)/Sm (x nm)] multilayers as a function of Sm layer thickness; the red line represents theoretical values calculated from equation (2).

Figure 8
Figure 8 is a plot of the coercivity of these multilayer films as a function of Sm layer thickness.It can be seen that the coercivities of the multilayer films increases when Sm layer thicknesses increase.The increase in coercivity with increasing samarium layer thickness can be interpreted as being due to surface roughness or mixing, which increases when the Sm layer thickness is increased.This phenomenon leads to the formation a Co 100-x Sm x alloy at the interface between Co and Sm layers.This effect is supported by the results shown in the low angle XRD measurement shown in Figure 6 in which the diffraction peaks become slightly broader as Sm layer thickness is increased.It also mirrors the coercivity behaviour in Figure 8 where a relative preponderance of cobalt leads to lower coercivities.

Figure 9
Figure 9 shows typical hysteresis loops measured at room temperature for as-deposited multilayer films.As the Sm layer thickness is increased the shape of the hysteresis loop changes from high (square-like) to low loop squareness.The hysteresis loop of the multilayer films with Sm layer thickness of 1.2 nm is nearly square with a coercivity of 84 Oe.Above this

Figure 10 )
Figure 10) from this multilayer film reveals two broad rings associated with an amorphous structure.The first diffuse ring comes from Sm spacings, which can be clearly seen in the electron diffraction pattern of a planar sample (Figure 10).The second broad ring exhibits faint arc shaped reflections indicating some texture in the cobalt component of the multilayer.

Figure 11 Figure 11 .
Figure 11 shows a highly magnified micrograph of this multilayer film.The brighter Sm rich layer has a thickness that is smaller than its nominal thickness (3.8 nm).The Co layers contain 3-6 nm size nanocrystals with similar lattice fringes as indicated by arrows while the Sm layers appears to be amorphous as can be clearly seen.Obviously, the size of nanocrystallities has increased for thicker Co layers compared to multilayer with lower Co layer thickness, as shown in the HRTEM micrograph Figure 5.