Elementary specific spin and orbital moments of ultrathin CoFeB amorphous films on GaAs(100)

Nanoscale CoFeB amorphous films have been synthesized on GaAs(100) and studied with X-ray magnetic circular dichroism (XMCD) and transmission electron microscopy (TEM). We have found that the ratios of the orbital to spin magnetic moments of both the Co and Fe in the ultrathin amorphous film have been enhanced by more than 300% compared with those of the bulk crystalline Co and Fe, and in specifically, a large orbital moment of 0.56*10^-6 B from the Co atoms has been observed and at the same time the spin moment of the Co atoms remains comparable to that of the bulk hcp Co. The results indicate that the large uniaxial magnetic anisotropy (UMA) observed in the ultrathin CoFeB film on GaAs(100) is related to the enhanced spin-orbital coupling of the Co atoms in the CoFeB. This work offers experimental evidences of the correlation between the UMA and the elementary specific spin and orbital moments in the CoFeB amorphous film on the GaAs(100) substrate, which is significant for spintronics applications.

The magnetic amorphous CoFeB alloys have attracted renewed interests for the applications in the next generation spintronics such as magnetic random access memory (MRAM) 1,2,3 and spin field effect transistor (SpinFET) 4,5 . For the development of SpinFET, the structure and magnetic properties of various ferromagnetic (FM) thin films on top of semiconductors (SC) such as GaAs and Si have been extensively studied over the last two decades 6,7,8,9,10 . One of the most interesting discoveries is a uniaxial magnetic anisotropy (UMA) observed in several FM/SC 11,12 when the thickness of the FM layer is reduced down to nanometer scale. For example, the bcc Fe films on GaAs(100) substrates display the UMA from 1.4 nm to 11.5 nm 13 , and for bcc CoFe on GaAs(100), the UMA has been found between 1.1 and 1.7nm 14 . In the crystalline FM/SC systems, the magnetocrystalline anisotropy (MCA) might also change with the reduction of the thickness. 15 Generally, the UMA and MCA have been found to coexist in most of the common FM/SC film systems 16 . To exclude the contribution from MCA, and thus focus on the UMA in the FM/SC film system, an effective method would to be to alloy metalloid material into the ferromagnetic films and to have amorphous magnetic thin films. Approximately 20% Boron alloyed with CoFe compound has been proven desirable.
The additional Boron only slightly reduces the Curie temperature and saturation field while completely destroy its crystallinity 17 . Recent research indeed found that the amorphous CoFeB films deposited on top of GaAs still exhibit the UMA 18, Error! Bookmark not defined.. Several models have been proposed including, bond-orientational anisotropy (BOA) 6,19 , Neel-Taniguchi directional pair-ordering model 20 and random anisotropy model 21 , to explain the origin of the UMA in CoFeB/GaAs. According to the BOA model, a mediumto-long range microstructural anisotropy is responsible for the UMA. The Neel-Taniguchi directional pair-ordering model introduces anisotropy via the dipole-like coupling between individual atom-pairs, leading to anisotropic chemical ordering of near-neighbour atoms in randomly oriented coordination. The random anisotropy model emphasizes the break of the rotational symmetry of the Hamiltonian, which gives rise to the hard magnetic behaviour even in random amorphous magnets. The origin of the UMA has also been suggested as being due to the enhanced spin-orbit coupling and interface interaction 22 , which is controlled by the orbital moment and the crystal lattice 23 .The orbital moment has been found to have a more important role than the spin moment in giving rise to the magnetic anisotropy 28 Structural properties of the grown films were studied by JEOL 2200FS double aberration corrected (scanning) transmission electron microscope (S) TEM. Cross-sectional TEM specimens were prepared using conventional methods that include mechanical thinning and polishing followed by Ar ion milling in order to achieve electron transparency 37 .
The in-plane magnetic hysteresis ( -) loops were measured using a superconducting quantum interference device-vibrating sample magnetometer (SQUID-VSM). As a strong uniaxial anisotropy field ( ) as large as 270 was expected, the VSM measurement was conducted using a maximum magnetic field of 400 to ensure the samples were fully saturated. The samples were measured at angles 0° and 90°, i.e. along the hard and easy axis, respectively.
XMCD measurements were performed at normal incidence to the Ta/CoFeB/GaAs (100) sample in the MAX Lab I1011 station. The XMCD spectra were measured at both positive and negative applied fields 30 . The data was collected by Total Electron Yield (TEY) detector in the analysis chamber under a magnetic field of 2000 . This was the operational limit of the magnet in the station, as the magnetic field has to be set at a relatively low value in order to limit magnet overheating 31 . The magnetization hysteresis loop of the CoFeB film along the out-of-plane direction was measured by a Polar MOKE, and the saturation field was found be to as large as 12000 . It is apparent, from Fig.1(b), that for the out-of-plane direction, the magnetic field used during the XMCD measurements was not sufficient to saturate the sample. It is for this reason that, the spin and orbital moments obtained from the XMCD were scaled up. During this work, all the measurements were performed at room temperature.
where the is saturation magnetization and is the saturation field along the HA.
It can be seen from the Fig.1 (a) (100) orientation has obvious larger value.
The hysteresis loop for the CoFeB/GaAs(100) sample along the perpendicular direction measured by a Polar MOKE is included in Fig.1(b), which shows that the perpendicular direction is the hard axis. As mentioned earlier, when making the XMCD measurement along perpendicular direction, the applied magnetic field of 2000 was not large enough to saturate the sample. From the perpendicular loop in figure 1(b), the saturation magnetic field is determined to be 10189 . Comparing the magnetization at 2000 Oe and that at saturation, the data of spin and orbital moments from the XMCD have been scaled up by a factor of 5.09 as included in table I. High resolution cross-sectional TEM image of the structure is shown in Fig.2. The films thicknesses of 3.5 nm and 2 nm for CoFeB and Ta, respectfully, match the growth settings.
The structure of CoFeB film is amorphous starting from the very interface, in contrast to Ref [33] where interfacial crystallinity at the SC/FM interface could not be ruled out. The clear distinction between the Ta and CoFeB can be observed as well as between the CoFeB and GaAs due to single crystal structure of the GaAs substrate and the amorphous state of the CoFeB. Structural TEM analysis suggests that the interface interaction and shape anisotropy related to any possible deformation of the CoFeB layer would play little role on the formation of the UMA.
X-ray absorption spectra (XAS) of the Co and Fe 2 and 3 edges for CoFeB on GaAs (100) are shown in Fig. 3 (a) and (c) respectively, in which + and − are the absorption coefficients under antiparallel and parallel magnetic fields to the photon incident direction. Figure 3 shows the XMCD spectra for the Fe and Co -edges of the CoFeB film. According to XMCD sum rules, the orbital ( ) and spin ( ) magnetic moments and the ratio ( ) of to can be determined from XAS and XMCD spectra by the following equations 30 : Where and are the orbital and spin magnetic moments in units of ⁄ , respectively, and 3 is the 3d electron occupation number of the specific transition metal atom. 3 and 2 denote the integration ranges. 〈 〉 is the expectation value of magnetic dipole and is equal to half of in Hartree atomic units 22 . The spin and orbital moments are also dependent on the d-band hole density in CoFeB and the intensity of the polarized x-ray in XMCD measurement. The value of 3 for Fe and Co in CoFeB sample is controversial. While in Ref [34], the 3 in amorphous film is unknown, Ref [35] gives the values of 3 for Fe and Co of 6.61 and 7.51, respectively, by first-principles calculation, which is the same as the reported values for bulk Fe and Co 30 . In this work, we have used the values of 3 from Ref [34] to calculate the spin and orbital moments of the Co and Fe in the CoFeB film.  Table I   One of the most striking results from Table I is that the spin and orbital moments of the Co atoms are significantly larger than those of the Fe atoms. When considering the value of the , we can see that the Co atoms also has a larger value than that of the Fe atoms. As compared the orbital moment of the crystalline hcp Co, the orbital moment of the Co atom in the CoFeB has been enhanced by more than 370%. This suggests that in the CoFeB (100) amorphous film, the Co atoms at the interface with the GaAs contribute more than Fe to the UMA. Our results indicate that the large UMA observed in the CoFeB(100)/GaAs(100) system comes from the large spin-orbit coupling of the Co atoms.
Spin-orbital coupling is a desired property in terms of the controllability by electric field in spintronic operation. The orbital moment of the Co atoms in the CoFeB/GaAs(100) has been found to be as large as 0.56 µ B , which is the largest orbital moments observed in any amorphous magnetic alloys as far as we know. It is interesting to note from Table 1  In conclusion, we have investigated uniaxial magnetic anisotropy and the elementary specific spin and orbital moments in the CoFeB(100)/GaAs system by magnetization measurement, XMCD measurement and sum rule calculations. The results obtained by VSM measurements confirmed that the UMA can achieve as large as 270 , which is among the largest of the UMA observed in any CoFeB amorphous alloys. XMCD measurements reveal that the UMA is correlated with a strong spin-orbit coupling related to the enhanced orbital to spin moment ratios of both Fe and Co in the CoFeB. More importantly, the spin moment of the Co has been found to remain as large as that of the crystalline hcp Co, and the orbital moments is enhanced by more than 370%, suggesting the dominate contribution of the spin-orbit coupling of the Co atoms to the UMA in the CoFeB(100)/GaAs amorphous film. These results would be useful for understanding the fundamental magnetic properties of the amorphous CoFeB films, which could be important for the applications of this class of materials in the next generation spintronics devices.