Structural and Magnetic Properties of Cobalt Implanted Diamond-like Carbon Films Synthesized by Ion Beam Methods
Authors:   , Sridhar Gupta Prasanth Gupta 
Published: November 2021

<p>Novel methods to synthesize and modify the structure, composition, magnetic behavior and electrical conductivity of thin films are of tremendous importance for fabrication of nano-devices. This research focuses on the application of ion beam methods to produce diamond-like carbon thin films and incorporate magnetic nanostructures within them for potential spintronic applications. Diamond-like carbon films were synthesized by ion beam deposition and were implanted with magnetic ions at low energy to induce magnetic order in near-surface region of the thin film. This research work focuses on the exotic ion-solid interactions that occur between the energetic ions and the base matrix and their subsequent effects on the structure, atomic distribution and magnetic behavior of the implanted films. The results are of significant interest to both, ion beam community and spintronics industry.   Diamond-like carbon (DLC) is an amorphous carbon material that contains significant fraction of its carbon bonded in sp³ hybridization. DLC films were produced by high energy ion beam deposition, specifically by both, direct ion beam deposition and mass selective ion beam deposition. Raman spectroscopy revealed that DLC films deposited by molecular ion beams C₃H₆⁺ with 5 keV deposition energy has 30 - 50 % of its carbon bonded in sp³ hybridization. The sp³ content was observed to decrease with increase in deposition energy. The hydrogen content in the deposited films was measured to be 25 - 30 at.% by resonant nuclear reaction analysis. The selection of deposition energy and ion species was observed to have strong influence on the thin film properties.   Cobalt was chosen as the preferred magnetic ion for doping diamond-like carbon thin films. Co was implanted at low energy (30 keV) and at room temperature. The implantation fluence (incident ions per unit area) was varied from 0.8-20 x10¹⁶ atoms.cm⁻². The implantation current density was limited to 5 μA.cm⁻² to prevent any bulk heating effects. Monte-Carlo simulations suggest the implantation profile to be single Gaussian with a projected range of ∼37 nm. High resolution Rutherford backscattering spectrometry (HR-RBS) however showed that the Co distribution varied from a unimodal distribution at low fluences (< 1:5 x 10¹⁶ atoms.cm⁻²) to an asymmetric bimodal distribution at high fluence (7 x 10¹⁶ atoms.cm⁻²). Furthermore, a steady state condition was reached at an implantation fluence of (12 x 10¹⁶ atoms.cm⁻²) which is not expected from the simulations. Cross-sectional TEM imaging and corresponding fast Fourier transform analysis reveals that the implanted Co atoms precipitate into cobalt carbide nanoparticles. At high fluences, the nanoparticles accumulate at two regions : (i) in the immediate near-surface region (nanoparticles as large as 5 nm) (ii) at the projected range of implanted ions (large density of ~ 2 nm nanoparticles) in agreement with the measured Co distribution.  The bimodal distribution along with the nanoparticle formation can only be explained by the occurrence of non-ballistic processes such as precipitation and localized diffusion during Co implantation. These processes are enhanced by energy deposited during collision cascades, relaxation of thermal spikes, and defects formed during ion implantation. A key factor responsible for the manifestation of these effects, is the presence of hydrogen in the base matrix.  Cobalt implantation was carried out on amorphous carbon films (lacking hydrogen) at the same implantation conditions. HR-RBS and TEM measurements on these films show that Co assumes a unimodal distribution when hydrogen is absent in the base matrix. Resonant nuclear reaction analysis shows that ion implantation leads to massive redistribution of hydrogen atoms in the DLC films. Raman spectroscopy further indicate that ion implantation causes graphitization of DLC films. These results show that the energetic ion interactions in DLC matrix are unique and lead to interesting elemental distribution and structural effects.  Cobalt containing nanoparticles in the DLC films lead to room-temperature magnetic order. Magnetic measurements were carried out using a magnetic property measurement system employing a superconducting quantum interference device. The temperature dependence of saturated moment observed from the magnetic measurements of low fluence samples resembles the behavior of a spin glass material. This is explained by the structural disorder present in the magnetic nanoparticles. Increasing the implantation fluence, reduces the spin glass contribution and leads to formation of ordered magnetic phases within the nanoparticles. This is confirmed by both the TEM data and observation of ferromagnetic contribution in addition to spin glass behavior in the temperature dependent magnetization measurements of high fluence sample. Low field magnetic measurements further reveal that the nanoparticles exhibit superparamagnetic behavior. Interestingly, the measurements also indicate presence of dipole-dipole interaction between the nanoparticles.  As deposited DLC films were observed to be electrically insulating. Co implantation resulted in the formation of metallic nanoparticles which is measured to have increased the conductivity of DLC by nearly six order of magnitude. Measurement of electrical conductivity as a function of temperature indicates the dominant conduction mechanism to arise from the tunneling between the large disordered metallic nanoparticles found in the near surface region. Since the particles are randomly oriented, the electrons can tunnel through different pathways and the overall behavior is observed to have a power law dependence over the measurement temperature. At higher temperature, an additional contribution is observed to arise from the smaller nanoparticles positioned deeper inside the implanted DLC films. This contribution can be semiconducting or metallic in nature and is dependent on the implanted Co concentration. The overall conduction behavior of Cobalt implanted DLC is modeled as a network of two parallel resistors with each resistor arm, representing the contribution from the larger and smaller nanoparticles respectively.  This research overall shows that ion implantation into DLC can harness the effects of collision cascades, thermal spikes and defect formation for producing magnetic nanoparticles with unique elemental distributions. The implanted DLC films exhibit novel magnetic and transport properties which can ultimately lead to the development of spintronic devices.  Key results from this research are published in physics journals such as Applied Physics Letters, Journal of Physics: Applied Physics D, Nuclear Instruments and Methods:B which are the relevant journals for this research work.</p>