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Heusler alloy films for nano-spintronic devices


Department of Electronic Engineering

About the Project

Heusler-alloy films are theoretically predicted to show 100% spin polarisation and is therefore ideal to be used in spintronic devices [1],[2]. Four key requirements for the device applications are identified to be large giant magnetoresistance (GMR), large tunnelling magnetoresistance (TMR), large spin-transfer torque and fast spin resonance. These requirements can be achieved by utilising the fundamental properties of the Heusler alloys, such as atomic substitution, generalised Slater-Pauling behaviour, crystalline ordering, half-metallicity, low damping constant, high Curie temperature, good lattice matching and large activation volume. To date the main obstacles for the Heusler-alloy films to be used in spintronic devices are their (i) high crystallisation temperature, (ii) interfacial atomic disordering and (iii) small activation volume. Here, we have investigated these properties for both epitaxial and polycrystalline films and have found a favourable crystallisation orientation to lower the ordering temperature by inducing a two-dimensional growth. We have demonstrated the effect of interfacial dusting to maintain the crystalline ordering from atomic diffusion by annealing. We have also established the above requirements can be controlled by the competition between the structural and magnetic volume, the latter of which can be defined as activation volume. In all cases, the polycrystalline films have found to be advantageous over the epitaxial ones due to their strain-free growth with controlled grain size. We anticipate that the optimised polycrystalline films can be used in the next generation hard disk read heads and magnetic random access memory cells [3].

The devices will be grown using our high vacuum sputtering system in the Department of Physics. The devices will then be patterned in the state-of-the-art electron beam lithography system in Leeds, where we have 25% share. This system can pattern 7 nm feature with stitching and overlay accuracy of <1 and <7 nm, respectively. The successful devices will be measured by our transport measurement setup between 4K and 350K.

We have been active in the field of spintronics with many collaborators within the UK, EU and worldwide. It is preferable that the candidate has some hands-on experience on thin film growth in an ultrahigh vacuum and/or nanometric scale device characterisation. PhD students are normally given an opportunity to present their work at the annual domestic Magnetism conference and at least once in an international conference, such as the International Conference on Magnetism or Magnetism and Magnetic Materials.

This project is partially supported by JST-CREST programme for the development of a new ferromagnetic material for over 1,000% tunnelling magnetoresistance using machine learning.

Entry requirements:
Candidates should have (or expect to obtain) a minimum of a UK upper second class honours degree (2.1) or equivalent in Electronic and Electrical Engineering, Physics, Computer Science, Mathematics, Music Technology or a closely related subject.

How to apply:
Applicants should apply via the University’s online application system at https://www.york.ac.uk/study/postgraduate-research/apply/. Please read the application guidance first so that you understand the various steps in the application process.

Funding Notes

This is a self-funded project and you will need to have sufficient funds in place (eg from scholarships, personal funds and/or other sources) to cover the tuition fees and living expenses for the duration of the research degree programme. Please check the Electronic Engineering website View Website for details about funding opportunities at York.

References

[1] C. Felser and A. Hirohata (Ed.), Heusler Alloys (Springer, Berlin, 2015).
[2] A. Hirohata et al., Spin 4, 1440021 (2014).
[3] A. Hirohata and K. Takanashi, J. Phys. D: Appl. Phys. 47, 193001 (2014).

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