Structural and Magnetic Characterisation of Heusler Alloy Thin Films under Optimised Growth Condition for Spintronic Devices

Spintronic devices have been playing an important role in magnetic storage and memory applications for the last 20 years. For such a trend to continue, it is critical to develop new magnetic materials, in particular, in the context of antiferromagnetic spintronics, materials with high Curie temperatures, large spin polarisations, and low saturation magnetisations. In this study, three Heusler alloys, Mn2VSi, Mn3Ga and Mn3Ge have been investigated and experimental results have been carried out. 80 nm thick polycrystalline Mn2VSi films have been deposited on silicon substrates with an 18 nm silver seed layer and a 3 nm aluminium capping layer using a sputtering system. The best quality film is obtained for 723 K growth. The Mn2VSi thin film is verified to be antiferromagnetic, where an exchange bias is found when a 3 nm ferromagnetic CoFe layer has been deposited on the top of the Mn2VSi layer. The exchange bias is measured to be 34 Oe at 100 K. The blocking and thermal activation temperature of Mn2VSi is estimated to be below 100 K and within a range between 100 K and 448 K, respectively. Polycrystalline Mn3Ga layers with thickness in the range from 3-20 nm were deposited at room temperature. To investigate the onset of exchange-bias, a ferromagnetic Co0.6Fe0.4 layer (3.3-9 nm thick) capped with 5 nm Ta, were subsequently deposited. X-ray diffraction measurements confirm the presence of Mn3Ga (0002) and (0004) peaks characteristic of the D019 antiferromagnetic structure. The 6 nm thick Mn3Ga film shows the largest exchange bias of 430 Oe at 120 K with a blocking temperature of 225 K. The blocking temperature is found to decrease with increasing Mn3Ga thickness. These results in combination with X-ray reflectivity measurements confirm that the quality of the Mn3Ga/Co0.6Fe0.4 interface controls the exchange bias, with the sharp interface with the 6-nm-thick Mn3Ga inducing the largest exchange bias. The magneto-crystalline anisotropy for 6 nm thick Mn3Ga thin film sample is calculated to be Polycrystalline Mn3Ge samples with same stack layer structure as Mn3Ga films were also studied. A growth temperature of 773 K promotes the crystallisation of the 100 nm thick Mn3Ge layer showing a D019 antiferromagnetic structure. The exchange bias cannot be observed due to the potential interlayer diffusion during high temperature sputtering.