Formation of double ring patterns on Co2MnSi Heusler alloy thin film by anodic oxidation under scanning probe microscope

Double ring formation on Co2MnSi (CMS) films is observed at electrical breakdown voltage during local anodic oxidation (LAO) using atomic force microscope (AFM). Corona effect and segregation of cobalt in the vicinity of the rings is studied using magnetic force microscopy and energy dispersive spectroscopy. Double ring forma-tion is attributed to the interaction of ablated material with the induced magnetic field during LAO. Steepness of forward bias transport characteristics from the unperturbed region of the CMS film suggest a non equilibrium spin contribution. Such mesoscopic textures in magnetic films by AFM tip can be potentially used for memory storage applications.

Formation of nanoscale oxide patterns on metal and semiconductor surfaces by oxidation using a conductive scanning probe under atomic force microscope (AFM) is termed as Local anodic oxidation (LAO). This phenomena was first observed by Dagata et.al [1] on Si (111), followed by similar experiments on several bulk metal surfaces [2,3], semiconductors [4,5] and supported thin films. [6][7][8] All these experiments have suggested that the electrochemical reaction between the AFM tip and the surface under water meniscus is very crucial for the formation of oxide patterns. [9,10] This phenomena is also observed under reactive conditions with a suitable organic meniscus instead of a water meniscus. [11] Corona effect along the patterns during LAO was reported by few groups. [12,13] Corona formation was believed to be the effect of the lateral diffusion of the oxyanions (OH − ) under high percentage of humidity. For higher bias, a different mechanism where a transient shock wave assisted ion spreading of OH − is proposed. [14] Intermixing of elements in a bilayer system of GaAs/AlAs has also been observed during LAO. [15] This is supposed to be due to the electric breakdown of thin semiconducting and dielectric films under anodic voltages greater than 10 V. [16] This process is very stochastic and is explained by different mechanisms and simulations. Dielectric breakdown is commonly studied for metal-insulator-metal configurations to measure dielectric strength. Such studies have generated considerable interest for resistive random access memory devices. Many groups have demonstrated the reproducibility of forming reversible metallic filaments at soft dielectric breakdown leading to conductive paths which have great potential for high density storage. [17,18] Recently the possibility of inducing local magnetic anisotropy using relativistic energy and high frequency e-beam has generated considerable interest for spin electronics and memory applications. [19] Half metallic-ferromagnet full-Heusler alloy Co 2 MnSi (CMS) is a promising spintronic material due to its high Curie temperature ( 985 K) [20,21] and theoretically predicted 100% spin polarization of conduction electrons. [22] But experimentally measured degree of polarization is ≈ 60%. [23] For thin films of these materials, half metallicity is very sensitive to the nature of surface and interface. It is demonstrated that Mn-Mn termination retains the half-metallicity where as Co or Mn-Si termination leads to mixing of spin sub bands. [24] In this paper, we report our results on AFM tip based anodic oxidation of CMS and the effect of induced magnetic field on pattern formation during dielectric breakdown. We explain how the Oersted field generated is quite enough in magnitude to rearrange the material, changing the properties of the heusler alloy locally. Till now the Oersteds field generated during LAO has not been given much importance as most of the studies were done on non magnetic semiconductors and metal surfaces. This work was initiated with the objective of creating planar nanostructures of CMS films under AFM to study planar tunnel junctions and magneto transport in nanowires of this half metallic compound. The process of anodic oxidation in AFM, however, leads to the formation of interesting ring structures.
This paper describes our studies of the topography, chemical composition and mechanism of formation of such rings. We explain how the observed ring formation is different from the general corona effect seen during LAO at high voltages and explain the same by monitoring Oxford instruments). CMS-Co junction was formed under AFM by engaging Co coated AFM tip onto CMS film. For magnetic imaging, Co/Cr coated AFM tip is magnetized and used in interleave scan with a lift height of 30 nm. Figure 1(a-c) shows AFM topographic images of all three films, CMS 400, CMS 500 and CMS 600 respectively, before and after the application of bias. From these micrographs we conclude that while surfaces of 400 and 500 0 C annealed films are smooth, 600 0 C annealing leads to roughening due to extensive crystal growth. The average roughness of these films is 0.1 nm, 0.2 nm and 0.5 nm respectively. Figure 1(d-f) show the ring formation under applied bias between AFM tip and CMS films annealed at 400, 500 and 600 0 C respectively.
CMS 400 forms the double ring with some asymmetry and the outer diameter is of the order of 5 µm. Along with the ring formation, a spread of the material in the vicinity of the modified region which is identified as a corona effect is seen on these films. Whereas CMS 600 forms highly irregular shaped features, often it was found difficult to modify the CMS 600 surface. CMS 500 surface consistently gives a highly symmetric double ring structure with least spread to the surroundings. No corona effect is observed for CMS 500 and 600 films. Figure 2 shows a cross sectional profile across the diameter of one such ring produced on CMS 500 surface. The double ring formation is highly symmetrical as seen in Fig 2(a).
The diameter of the outer ring as measured from line profile shown in Fig. 2(b), is 1.1 µm  Fig. 3(a). In KPFM, the contact potential difference (CPD) betweem the metal tip and sample surface is estimated based on Fermi level equilibrium model. [27] This is the amount of voltage required to nullify the electrostatic force generated between two materials due to their work function difference. The lower CPD in the modified region as seen in Fig.

3(b) is a clear indication that negative charge is induced in the modified region and its origin
is the dielectric breakdown of CMS films. [28] The pattern formed is a clear signature of dielectric breakdown. As crystallinity of films improve with T A , the threshold for electrical breakdown also increases due to reduction in number of traps. [29] This agrees well with the trend observed in Fig. 1 in comparison with X-ray diffraction data shown in Fig. S1. Hence CMS 600 film of high crystalline quality is unlikely to undergo easy electrical breakdown where as CMS 400 can be ablated easily and the material spread to the surroundings. However, the formation of symmetric ring features on CMS films is very distinct and intriguing. To understand any contribution from the magnetic nature of these films and interaction of the ablated material with the magnetic field generated by the conducting tip, the ring and their surroundings formed on CMS 400 thin film were mapped for their magnetic nature by magnetic force microscopy (MFM). Energy dispersive spectroscopy (EDS) was also done for elemental analysis of various regions of the ring. Both these results unequivocally showed that the distributed material is magnetic in nature and mostly rich in Co. Figure 4 summarizes the results of MFM and EDS analysis of the ring and its corona. The AFM topographic image and the MFM image at a lift height of 30 nm, collected from the same area are displayed in Fig. 4(a) and 4(b) respectively. where I S is the saturation current, B is Schottky barrier height, δP is the non equilibrium spin polarization in the n region, v is applied voltage, A is area, and A ⋆ is Richardson's constant. The current map in Fig. 5(a) agrees well with the I-V characteristics below 3 V from Region 2 and Region 3 as shown in the inset of Fig. 5(b). Difference in the steepness of I-V characteristics in the forward bias from both regions above 3 V gives an indication that the current from Region 3 has a contribution not only from charge drift but also from spin, making it behave like a Spin diode. [32] The transport characteristics in forward bias is similar to the current injection in spin diode, where the I-V characteristics are expressed as [33]: where P p is equilibrium spin polarization in the p region. When unpolarized current is passed across a ferromagnetic semiconductor, the current becomes spin-polarized. [34,35] Electrons which are spin polarized in this material could show a strong interaction with the applied current. Transverse spin current has been recently demonstrated experimentally in GaAs thin films, [36] and in AlGaAs quantum wells. [37] The steeper I-V characteristics for bare region compared to modified region indicates the spin contribution in the transport and loss of half metallicity of the modified region due to electrical breakdown.
Double ring structures were formed on Co 2 MnSi films via electric breakdown, using conducting AFM tip. These rings were analyzed using magnetic force microscopy and energy dispersive spectroscopy. MFM of the rings suggest that the dispersed material forming a corona is magnetic and EDS confirmed that the material is rich in Co. The ring formation is explained in terms of the interaction of ablated material with the magnetic field produced by the conducting AFM tip. Local I-V spectroscopy using Co tip on the ring and its sur-