The versatility of Lorentz microscopy for studying nanomagnetic systems

Bio

Trevor is a Lecturer in the Materials and Condensed Matter Physics Group at the University of Glasgow. He obtained his PhD in Material Science at the University of Nottingham, focusing on the transmission electron microscopy (TEM) of magnetic nanoparticles. His research evolved to investigate a range of nanomagnetic processes by combining in-situ TEM methods with Lorentz microscopy techniques. This took place during time spent at Imperial College London, Centre for Electron Nanoscopy (Denmark), Ernst-Ruska Centre (Germany), the University of Glasgow and CEA-LETI in Grenoble (France). Trevor’s primary interests include the functional magnetism within 3D nanostructures, nanoelectronics, magnetotactic bacteria, meteorites, minerals, nanoparticles and thin films.

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 Fe3O4to maghemite (γ-Fe2O3) influences the preservation of remanence of the Earth's magnetic field by Fe3O4. Further, the thermomagnetic behaviour of Fe3O4grains 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 Fe3O4NPs 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].

Sm(CoFeCuZr)7permanent magnets are hard magnets that are commonly used in applications that require elevated temperatures. The high coercivity results from the strong pinning of magnetic domain walls at phase boundaries of the Sm2Co17and SmCo5phases, which require a comprehensive characterization of both the structural and magnetic properties with nanometer resolution. Electron holographic tomography is used to reconstruct the magnetic vector potential and the 3-dimensional magnetization distributions quantitatively in needle-shaped Sm(CoFeCuZr)7magnets. The interplay between the shape and the magnetocrystalline anisotropies stabilized magnetic single and multi domains. The spin structure of domain walls was successfully reconstructed in 3-dimensions and the correlation to the structure was established.

[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).

Figure 1: Reconstructed magnetic induction maps of (a) Fe3O4particles; (b) FeRh; and (c) DMTJs.