Thickness induced uniaxial anisotropy and unexpected four-fold symmetry in Co/SiO2/Si films

Co films with thickness ranging from 20 to 160 nm have been fabricated on SiO2/Si substrates by pulsed laser deposition method (PLD). It was found that that the Co crystal tends to have a structure of bulk hcp Co with the increase of the Co film thickness, and the coercivity of the Co film decreases with increasing film thickness due to the change of the grain size. A uniaxial anisotropy was found in Co films with thickness less than 120 nm, while the Co films with thickness more than 120 nm show an unexpected four-fold anisotropy which is ascribed to the existence of two types (directions) of strongly exchange-coupled hcp Co grains.


I. INTRODUCTION
2][3] In all of these applications, the magnetic anisotropy of the magnetic thin films is of great importance for the determination of the magnetization directions and the reversals in the devices.5][6][7] Besides, magnetic anisotropy of magnetic thin films is related to the microstructure of the films 8,9 as well as the interaction between the films and the substrates through chemical bonding and strain. 102][13] With its native oxide, Co/CoO is also a prototype of the exchange bias effect studying for decades. 14However, due to relatively large magnetocrystalline anisotropy from the hcp structure, the magnetic anisotropy of the Co is much more complicated than those of Fe and Ni, especially in the thin films.The investigation of the relationship between magnetocrystalline anisotropy and film thickness of the cobalt material is vital to the development of the magnetic materials and magnetoelectronic devices.So far, although many studies have been done for Co/Si or Co/Cu/Si multilayer systems, 12,[15][16][17] few attentions have been paid to the magnetic properties of the Co thin films on a SiO 2 covered Si substrate which is widely used in the fabrication of integrated circuits, such as transistors, capacitors and flash-memories.
In this paper, we have investigated the variation of the coercivity and magnetic anisotropy with thicknesses of Co the films grown on SiO 2 /Si substrates.In particular, we found that the anisotropy of the samples transforms from uniaxial to weak four-fold symmetry with the thickness increasing.By fitting the anisotropy curves with a theoretical model, the origin of the four-fold symmetry is proposed to result from the two types (directions) of hcp Co grains which are strongly exchange coupled.

II. EXPERIMENTAL DETAILS
A series of Co films were deposited on SiO 2 /Si substrates at room temperature using pulsed laser deposition system (PLD) with the thickness ranging from 20 nm to 160 nm.The working pressure was 1.8×10 -3 Torr with 12.3 sccm 99.99% Ar flow, and a laser spot energy of 3.0 J/cm 2 was used.The repetition rate of the laser pulse was set to 5 Hz and the deposition rate was about 6 nm per minute.All the samples were capped with 2 nm Au in order to prevent oxidation.The structures of the films were characterized by X-ray diffraction (Philips X'pert Pro) with a Cu-Kα radiation (λ = 1.5418Å).The magnetic properties were measured using Magneto-Optical Kerr Effect (MOKE).

III. RESULTS AND DISCUSSION
Figure 1 shows the X-ray diffraction pattern for Co films with different thicknesses.The inset gives the 2θ scan in the range from 25 o -40 o with all samples.In Co films with thickness less than 40 nm, only a broaden peak with very weak intensity was detected due to the small grain size.One sharp (004) peak can be detected in those Co films with thickness more than 60 nm, which indicates all of the Co films are highly textured along the c axis.The peak positions are identified with Lorentz fitting to be 2θ = 32.88 o , 33.07 o , 33.17 o , 33.33 o corresponding to the films with the thickness of 80, 120, 140 and 160 nm, respectively.The peak intensity becomes stronger with the thickness increasing due to the better crystallinity and larger grain sizes.The Co crystal tends to have a structure of bulk hcp Co with a 2θ diffraction angle of about 33.98 o for Co (004).Besides, the (004) peak shifts slightly towards higher angle with the thickness increasing which indicates the lattice constant becomes smaller with the thickness increasing accordingly.
In order to investigate the relationship between the magnetic anisotropy and thickness in the Co/SiO 2 /Si films, a series of hysteresis loops were acquired by the longitudinal MOKE measurements.Figure 2 shows the typical hysteresis loops along the easy and hard axes for the films with different thicknesses.It can be seen that the anisotropy is gradually enhanced while the films get thicker according to the increased difference of Mr/Ms along the easy and hard axes.Figure 3    is larger than the interacted exchange range, the dependence of coercivity H C with the grain size can be described by H C ∝ 1/D. 18The atomic force microscopy (AFM) was used to observe the changes of Co grain size.Figure 4 is the typical AFM images of 60 nm and 160 nm Co films.The lateral grain size for the 60 nm thickness Co sample is about 100 nm and the grain size gets larger within the thicker films, which is definitely larger than the exchange interaction range between the Co grains (The largest exchange interaction range between Co particles was reported to be 7 nm 19 ).This is related to the growth mechanism of the PLD films.At the beginning of the deposition process, small clusters are nucleated and then grow to form islands.After that, those islands grow larger and larger to merge as a continuous film.These results strongly suggest that the Co film growth on the SiO 2 /Si substrate follows the mode of Volmer-Weber or island in the early stage during the deposition due to the fact that the deposited atoms should be more strongly bound to each other rather than to the substrate, 20 especially when the crystal structure of the film is much more mismatched with the substrate.
To demonstrate the thickness dependent anisotropy more clearly, the angular variation of M r /M s for different film thicknesses is given in figure 5.The 20 nm Co film shows an in-plane isotropy, which can be explained by the amorphous Co films grown on the Si substrate with the amorphous SiO 2 layer on the top.The in-plane anisotropy of the film becomes more obvious with the increasing film thickness.The anisotropy of Co films grown on SiO 2 /Si substrate has been reported by Carvalho, et al. 21They believed that the existence of uniaxial anisotropy might be attributed to the shadowing effect due to the fact that the deposition beam is slightly away from the normal incidence direction during the deposition of Co films.The oblique angles of incidence during deposition of the magnetic films might actually generate a stress on Co films which leads to the formation of the uniaxial anisotropy.Our Co films grown on SiO 2 /Si were fabricated with a rotated substrate, so the oblique angles of incidence cannot induce the stress anisotropy.The enhancement of the uniaxial anisotropy with the increasing thickness is not due to the stress effect.The magnetic anisotropy mainly includes magnetocrystalline anisotropy, stress anisotropy and shape anisotropy. 22In general, the magnetic shape anisotropy becomes more apparent as the film thickness increases due to the demagnetizing field, which will lead to the magnetic moments arranging from in-plane to out-plane.However, the theoretical analysis does not match the experimental phenomena that in-plane anisotropy is enhanced with the increasing thickness which means the demagnetization is not the main case in our samples.So the magnetocrystalline anisotropy should make the major contribution to the uniaxial anisotropy whose easy axis is parallel to <0001> direction (c axis) of hcp Co.For the samples with thicknesses of 140 nm and 160 nm Co films, the unexpected four-fold symmetry instead of the six-fold symmetry was observed with the c axis of hcp Co standing normal to the surface of the samples or the stable twofold symmetry with c axis of hcp Co lying parallel along the surface.The in plane four-fold anisotropy was reported in the previous work with the system of Co/GaAs(001) 23 and in the Co film grown on sapphire 10 and it was also observed in the Co/Cu multilayers grown on Si (001) substrate. 17But it has never been found in the Co films grown on the oxidized Si substrate and it is dramatically induced by the thickness of Co films.M. Shima et al studied the Co/Cu multilayers epitaxial electrodeposited on Si (001) substrate and they suggested that the four-fold symmetry in the multilayers in the plane with a 45 o rotation with respect to the Si substrate. 17 that the interface interaction of Co atoms and O atoms on the sapphire substrate leads to the weak four-fold anisotropy.However, for our samples, a two-fold anisotropy was detected when the Co film thickness was less than 120 nm, while a weak four-fold anisotropy was observed when the Co film thickness was more than 120 nm.According to the results of E. Gu, et al., 23 the epitaxial Co films grown on GaAs(001) showed the similar four-fold symmetry, which could be explained in terms of the oriented hcp Co grains.Those hcp Co grains are strongly exchange coupled to each other that the sum of magnetization vector is coherent over the area.The energy of crystal anisotropy can be written: Where K 1 , K 2 and K 3 represent the first order, the second order and the third order anisotropy, respectively, and θ 1 represents the angle between the magnetization vector and the c axis of the Co hcp structure.If there are two types of hcp Co grains, the anisotropy energy could be written as (we neglect the higher order anisotropy constant): where a 1 and a 2 are the occupy fractions about the two types of grains, θ 1 , θ 2 are the angles between the magnetization of the film and c axis of the two types of grains respectively, whose directions are thought to be vertical to each other.We can rewrite the equation (2) as 23 where ϕ represents the angle between magnetization vector and one of the easy axis of the Co grain.According to equation ( 3), the first term will give rise to the four-fold symmetry determined by K 01 , while the second term can lead to the usual two-fold symmetry determined by K 02 , For thin Co films with thickness below 120 nm, K 01 /K 02 should be small and the two-fold symmetry is dominant.When the film thickness increases, K 01 /K 02 increases and the four-fold anisotropy becomes dominant and appears in the thick films.By using Eq. ( 2 (we neglect the 20 nm Co samples since it is isotropic).This indicates that the two types of grain can be much more common as the films get thicker, According to the above analysis, we believe that the thickness induced in-plane four-fold anisotropy should result from the two types of hcp Co grains which are strongly exchange coupled in the Co films.

IV. CONCLUSION
In conclusion, we have fabricated the Co films with different thicknesses on the amorphous SiO 2 /Si substrates.XRD pattern showed Co films gradually becomes (004) textured hcp Co phase with the increasing thickness, and the uniaxial anisotropy is enhanced due to the increase of the magnetocrystalline anisotropy.The unexpected four-fold symmetry was found when the thickness of Co is over 140 nm, and the relative ratio between the in-plane uniaxial and four-fold anisotropy is dramatically affected by the thickness of the film.The thickness induced four-fold symmetry can be explained by the strongly exchange coupled two types of hcp Co grains whose ratio varies by the increasing thickness.
Figure1shows the X-ray diffraction pattern for Co films with different thicknesses.The inset gives the 2θ scan in the range from 25 o -40 o with all samples.In Co films with thickness less than 40 nm, only a broaden peak with very weak intensity was detected due to the small grain size.One sharp (004) peak can be detected in those Co films with thickness more than 60 nm, which indicates all of the Co films are highly textured along the c axis.The peak positions are identified with Lorentz fitting to be 2θ = 32.88 o , 33.07 o , 33.17 o , 33.33 o corresponding to the films with the thickness of 80, 120, 140 and 160 nm, respectively.The peak intensity becomes stronger with the thickness increasing due to the better crystallinity and larger grain sizes.The Co crystal tends to have a structure of bulk hcp Co with a 2θ diffraction angle of about 33.98 o for Co (004).Besides, the (004) peak shifts slightly towards higher angle with the thickness increasing which indicates the lattice constant becomes smaller with the thickness increasing accordingly.In order to investigate the relationship between the magnetic anisotropy and thickness in the Co/SiO 2 /Si films, a series of hysteresis loops were acquired by the longitudinal MOKE measurements.Figure2shows the typical hysteresis loops along the easy and hard axes for the films with different thicknesses.It can be seen that the anisotropy is gradually enhanced while the films get thicker according to the increased difference of Mr/Ms along the easy and hard axes.Figure3plots the
) to fit the M r /M s -θ curves of the different Co films, values of |K 01 /K 02 | = 0.231, 0.225, 0.235, 2.98, 6.63 were obtained for Co films with thicknesses of 60, 80 120, 140 and 160 nm, respectively.If one assumes that the anisotropy of our Co films originates from the two types of grains, the values of K 1 and K 2 are of 4.3×10 6 ergs/cm 3 and 1.2×10 6 ergs/cm 3 , respectively.It is possible to estimate the fraction numbers a 1 /a 2 of the two types of Co grains, which are 0.029, 0.015, 0.037, 0.864, 0.936 for the Co films with the increasing thickness