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

Abstract

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 formation 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.al1 on Si (111), followed by similar experiments on several bulk metal surfaces,2,3 semiconductors4,5 and supported thin films.6–8 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 Co2MnSi (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 Conductive Atomic Force Microscopy (CAFM) characteristics from different regions of the affected area. Transport behavior of the formed junctions under CAFM has same elements of spin diode characteristics. These results are explained in analogy with spin polarized electrical transport. Co2MnSi thin films were grown on (110) oriented SrTiO3 substrates with Pulsed Laser Deposition (PLD) technique using KrF excimer laser (λ ≈ 248 nm).25 A typical growth rate of 0.56 Å/s was used to deposit 40 nm thick films at 200°C, which were subsequently annealed at 400, 500 and 600°C for one hour. The crystallographic structure of the films was characterized using a PANalytical X’Pert PRO X-ray diffractometer equipped with a CuKα1 source. Vibrating Sample Magnetometer (VSM) with a maximum field of 1.7 Tesla was used for in-plane magnetization measurements at room temperature. More detail on structural and magnetic characterization are given in supplemental material.26 A Multimode AFM with Nanoscope V controller, (Veeco Ltd, USA) is used for all AFM studies. All three samples were analyzed for their roughness. For electrical breakdown and ring formation studies, a program written in C++ interfaced with the controller is executed in Lithography mode. The metal coated AFM tip was biased with respect to the grounded sample and the experiment was carried out in ambient at a relative humidity of 40%. Lithography was done in contact mode with −12 V of tip bias and tip velocity of 0.2 μm/s. For Kelvin probe force microscopy, a metal coated tip biased at ac- amplitude of 2.5 V and a frequency ≈80 kHz was used in interleave scan, potential mode with a lift height of 50 nm. For local I-V measurements and current mapping, an extended Tunneling Atomic Force Microscopy (TUNA) module, Bruker AXS was used. For conductivity mapping the current sensitivity of the amplifier was set to 10 pA/V and scanned at 3V. Whereas for I-V, point spectroscopy, current sensitivity factor is set to 1nA/V. Scanning electron microscopy (SEM) imaging of the modified CMS region was done using Ziess EO MA10 variable pressure SEM in conjunction with energy dispersive spectroscopy (EDS) facility (Oxford Inca energy 250, 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)–1(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°C annealed films are smooth, 600°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)–1(f) show the ring formation under applied bias between AFM tip and CMS films annealed at 400, 500 and 600°C respectively.