Pulsed laser deposition growth of Heusler half-metallic thin films
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
This thesis presents the growth, physical characterisation and magnetic properties of two thin film Heusler alloys, Co₂MnSi and Co₂MnGa, which were deposited on substrates of MgO and GaAs using the technique known as pulsed laser deposition (PLD). These two materials are classified as half-metallic Heusler alloys, and are predicted to be strongly ferromagnetic at room temperature. These alloys are also predicted to have spin moments 100% polarised in one direction which originates from an energy gap in the minority band at the Fermi level (EF). These attributes make Co₂MnSi and Co₂MnGa ideal for use in spintronic devices. The properties of half-metallic Heusler alloys are very sensitive to crystal structure disorder, which is detrimental to both the alignment of magnetic moments within the film and the level of spin polarisation. Films ordered in the L21 crystal structure are required to observe the full potential of the properties associated with half-metallic behaviour. Growth quality of Co₂MnSi and Co₂MnGa single layer thin films was investigated by varying three main growth parameters: laser fluence, substrate choice and substrate growth temperature. Bi-layer Co₂MnGa thin films were also grown using PLD to determine whether a buffer layer of the same material grown under different conditions improved the deposited crystal structure of the film on top. In-situ Reflection High Energy Electron Diffraction (RHEED) imaging, Scanning Electron Microscope (SEM) images, X-ray Diffraction (XRD) and resistivity measurements using a Physical Property Measurement System (PPMS) were techniques used to characterise the physical properties of the thin films such as growth orientation, quality and atomic arrangement of the films’ crystalline structure. Overall, PLD was used to successfully deposit Co₂MnSi and Co₂MnGa epitaxial thin films on both MgO and GaAs when a minimum substrate growth temperature of 450°C was chosen. Co₂MnSi thin films required a minimum fluence of 4.5 J/cm² for epitaxial growth whereas the crystal quality of Co₂MnGa did not appear to be very sensitive to variations in fluence. XRD analysis suggested all the films grown in this thesis were arranged in the B2 crystal structure, not L21. The general trend of the temperature dependent resistivity of the Co₂MnSi series showed these thin films exhibit ferromagnetic behaviour as the resistivity monotonically increases with temperature. Observing this data with different variations of the temperature dependent resistivity equation confirmed the half-metallic nature of Co₂MnSi. Investigation of the magnetic properties of Co₂MnSi and Co₂MnGa showed a magnetic hysteresis when the films were swept through an applied magnetic field at a constant temperature. The saturation magnetic moment (MS) values at 300K decreased relative to values at 10K by 23% for Co₂MnSi and 34% for Co₂MnGa. Alignment disorder of the magnetic moments occurs as a ferromagnetic material approaches the Curie temperature. Co₂MnGa has a lower Curie temperature of 694K than 985K for Co₂MnSi which resulted in a larger decrease in MS at 300K. The MS values of these Heusler alloys were predicted to be 5.07 μB/f.u. for Co₂MnSi and 4.07 μB/f.u. for Co₂MnGa. The highest MS values determined from the thin films in this thesis were 4.65±0.83 μB/f.u. and 6.02±3.01 μB/f.u. for Co₂MnSi and Co₂MnGa respectively on MgO, and 3.54±1.36 μB/f.u and 3.60±0.77 μB/f.u. for Co₂MnSi and Co₂MnGa respectively on GaAs. Values for Co₂MnSi on MgO agree within uncertainty with the theoretically predicted magnetic moment, whereas Co₂MnSi on GaAs is 3% lower than this value. Values for Co₂MnGa agree for GaAs substrates but not for MgO substrates, this was a direct result of the high uncertainty in the thickness of films deposited on MgO, of up to 50% on films of 20-30nm thickness. For both materials on both types of substrates, there is a strong correlation between high MS values and a minimum growth temperature of 450°C. Higher fluence was also a factor as thicker films produced more accurate values for MS. Including a buffer layer between the substrate and the deposited film in the case of Co₂MnGa films was shown to positively influence the physical properties of the thin film growth. Epitaxial thin films of Co₂MnGa with buffer layers resulted in thin films with lattice parameters closer to the bulk values than the films without buffer layers presented in this thesis. However, the addition of the buffer layer resulted in MS values lower than those of the optimally prepared single layer films grown at 450°C. Future work heavily relies on the growth of thicker films. Some of the films examined in this thesis were 10-20nm thick, much thinner than the target of ~ 100nm. In some cases, the low thickness caused difficulties with determining the properties of the films due to an absence of film peaks from XRD results and a large uncertainty in the thickness determined from Rutherford backscattering (RBS) which resulted in an overestimation of the MS values. Having a more diverse range of substrate growth temperature and fluence value combinations for each substrate choice would also be beneficial by providing more detail about the optimum growth parameters.