CANCELLED: Group Seminar 2022/23

Abstract:

In order to better understand the magnetic behaviour of natural minerals or functional performance of modern spintronic devices, it is often necessary to investigate the underlying processes on the nano-scale. Transmission electron microscopy (TEM) allows atomic spatial resolution imaging and combining in-situ TEM experiments with Lorentz techniques like electron holography or differential phase contrast (DPC) imaging allows for imaging of magnetisation in nanostructures whilst under the influence of external stimuli, e.g. gas atmospheres, biasing, temperature, etc. In this context, several examples of the use of in-situ TEM and magnetic imaging will be presented.

Fe₃O₄ is the most magnetic naturally occurring mineral on Earth, carrying the dominant magnetic signature in rocks and providing a critical tool in palaeomagnetism. The oxidation of Fe₃O₄ to maghemite (γ-Fe₂O₃) influences the preservation of remanence of the Earth's magnetic field by Fe₃O₄. Further, the thermomagnetic behaviour of Fe₃O4 grains directly affects the reliability of the magnetic signal recorded by rocks. Through combining electron holography with environmental TEM, in situ heating and liquid cell TEM, the effects of oxidation ^[1] and temperature ^[2] (Fig. 1a) on the magnetic behaviour of Fe₃O₄ NPs are visualised successfully, as well as the magnetism within hydrated magnetotactic bacteria ^[3].

Equiatomic iron-rhodium (FeRh) has attracted much interest due to its magnetostructural transition from its antiferromagnetic (AF) to ferromagnetic (FM) phase and is considered desirable for potential application in a new generation of novel nanomagnetic or spintronic devices. Several scanning TEM techniques are performed to visualise the localised chemical, structural and magnetic properties of a series of FeRh samples. The quantitative evolution of the growth and co-existence of AF and FM phases in the FeRh structure are observed directly during in-situ heating using DPC imaging ^[4,5] (Fig. 1b).

Perpendicular shape anisotropy (PSA) and double magnetic tunnel junctions (DMTJs) offer practical solutions to downscale spin-transfer-torque Magnetic Random-Access Memory (STT-MRAM) beyond 20 nm technology nodes. The methodology for the systematic transfer of individual SST-MRAM nano-pillars to image their magnetic configurations directly using off-axis electron holography is presented ^[6]. The improved phase sensitivity through stacking of electron holograms can be used to image subtle variations in DMTJs (Fig. 1c) and the thermal stability of <20 nm PSA-STT-MRAM nano-pillars during in situ heating. ^[7]

References:

[1] T. P. Almeida et al., Nature Communications 5, 5154 (2014).

[2] T. P. Almeida et al., Science Advances 2, e1501801 (2016).

[3] T. Prozorov at al., Journal of the Royal Society: Interface 14, 20170464 (2017).

[4] T. P. Almeida et al. Scientific Reports 7, 17835 (2017).

[5] T. P. Almeida et al., Physical Review Materials, 4(3), 034410 (2020).

[6] T. P. Almeida et al., APL Materials, 10, 061104 (2022).

[7] T. P. Almeida et al., Nano Letters, 22, 4000–4005 (2022).