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MagTalks 2024

by SiM and PhysikUP

University of Porto, Portugal

10 April 2024

 

Students in Magnetism (SiM) by the IEEE Magnetics Society brings together both undergraduate and graduate students in magnetism through activities and events, while supporting magnetism-related educational events. This year, PhysikUP and SiM collaborates to bring you a set of lectures presented by the distinguished members of the DFA to showcase the various intriguing research in the broad field of magnetics within the department. Don't miss this opportunity to explore the vast world of magnetism!

At a Glance

After a brief introduction by PhysikUP and Ricardo Pinto of SiM, there will be six presentations aimed at showcasing the various lines of research in magnetism, at the DFA. Each lecture is scheduled to be approximately 15 minutes, followed by 5 minutes of Q&A between the speaker and the audience. Between the third and forth presentations, there will be a coffee break of 20 minutes.  

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Speakers

Rui Vilarinho

On the possible short-range magnetic ordering in TbMnO3 paramagnetic phase

Rare-earth orthomanganites (RMnO3) and orthoferrites (RFeO3) have renewed the scientific interest due to their remarkable magnetic properties, spin-reorientation transitions and, more recently, by the discovery of low-temperature ferroelectricity and multiferroicity in some of them [1]. While in the RMnO3, the Mn3+ spins present a TN typically below 50 K, into magnetically ordered phases that strongly depend on the rare-earth, in the RFeO3, the Fe3+ spins have a TN above 600 K into a common canted AFM phase [1].

Despite being heavily studied materials, there are recent reports of a deviation to the Curie-Weiss law in the paramagnetic phase of TbMnO3 well above TN (between 150 and 200 K), accompanied by a negative thermal expansion of the c-axis, and anomalies on the optical phonon energies measured by THz, FTIR and Raman spectroscopies [2]. This magnetostructural effect has been interpreted on the basis of short-range magnetic ordering and/or crystal-field excitations of the Tb3+ cation.
 
In this work, we study both TbMnO3 and TbFeO3, showing that similar anomalies on the magnetization, lattice parameters and phonon energies, to those observed in the paramagnetic phase of TbMnO3, also occur in TbFeO3 at the same temperatures, well within its antiferromagnetic phase [3]. Combining neutron powder diffraction obtained at ILL and Raman spectroscopy, we show this magnetostructural effect is a result of shift of oxygen positions below 200 K, which further increase the octahedra rotation angle by 0.15º in TbFeO3. We also show that in other RMnO3 and RFeO3 such anomalies are not found.
 
To understand whether short-range magnetic ordering or crystal-field excitations underlie this phenomenon, we performed muon spin spectroscopy on TbMnO3 at PSI. For this, we take advantage of using the muons to measure the local magnetic fields through the Knight shift, at 1 Å distance from the oxygen position. Our results show that, although the local magnetic field increases below 200 K, the Knight shift is perfectly scaled by a paramagnetic law down to TN = 40 K. This result allows us to conclude that no short-range order occurs in the paramagnetic phase of TbMnO3 that would explain the observed magnetostructural effect. We then suggest that in fact an interplay between the oxygen position and the crystal-field excitation levels of the Tb3+ occurs, leading to a further deformation of the crystallographic structure, which then affects the macroscopic magnetic properties [3].
 
[1] T. Kimura et al., Nature 426, 6962 (2003); E. Bousquet et al., JPCM 28, 123001 (2016)
[2] K. Berggold et al., PRB 76, 094418 (2007); D. O'Flynn JPCM 26, 256002 (2014);
[3] R. Vilarinho et al., Scientific Reports 12, 9697 (2022) R. Vilarinho et al., PRB 108, 174401 (2023)

João Horta Belo

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Magnetic Refrigeration and Magnetocaloric Materials at IFIMUP

Currently, Heating/Cooling, Ventilation and Air Conditioning (HVAC) devices rely on > 150 years old gas expansion/compression technology and account for about 20 % of the overall electricity use worldwide and over 10 % of current global greenhouse gas emissions. As such, this industry is a major contributor to global climate change, and therefore the need to improve HVAC devices is urgent.

Room-temperature magnetic refrigeration is one of the most promising alternative technologies because in addition to the absence of environmentally/health harmful gases, it is expected to be more energy-efficient, require less maintenance, have longer lifespan, and produce less noise than vapour compression refrigeration. In IFIMUP we have been developing research on magnetic refrigeration and on their active materials – the magnetocaloric materials – for over 20 years now. In this talk I´ll introduce you to the main concepts and cover some of the most interesting and recent results obtained in our group, from materials first-order phase transition kinetics to a brand new magnetocaloric effect that hopefully will help cool down vaccines.

João Ventura

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On spintronics magnetic sensors

Magnetic sensors are critical components in modern technology, enabling devices to detect and measure magnetic fields with precision. These sensors play a pivotal role in a wide range of applications, from navigation systems in smartphones and vehicles to monitoring and controlling industrial processes.

In particular, Spintronics sensors leverage the intrinsic spin of electrons and its associated magnetic moment to detect magnetic fields with exceptional sensitivity. This approach marks a shift from traditional charge-based electronics, opening new avenues for developing magnetic sensors that are not only more energy-efficient but also capable of operating at lower power levels and with greater data storage capabilities. At the heart of spintronics sensors are structures such as the spin valve or magnetic tunnel junction, which exhibit large changes in electrical resistance in response to small magnetic fields, enabling precise detection and measurement. These advancements in spintronics are not just enhancing the performance of existing applications like hard drives and memory devices but are also paving the way for revolutionary applications in quantum computing and advanced medical imaging technologies.

Sara Freitas

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Biomedical applications of magnetism

Magnetism plays a crucial role in biomedical sciences, offering innovative applications ranging from diagnostics to advanced therapies. This talk explores the synergy between magnetism and nanotechnology, providing an in-depth look at the latest achievements and exciting promises in the biomedical field. Magnetic resonance imaging, widely used in medical imaging, continues to evolve with the development of next-generation magnetic nanomaterials, enabling more precise and sensitive diagnoses of a variety of medical conditions.

Furthermore, nanotechnology has propelled the development of magnetic drug delivery systems, offering highly targeted and effective therapies for specific diseases. This lecture provides a comprehensive overview of the biomedical applications of magnetism, highlighting how integration with nanotechnology is shaping the future of medicine and fostering significant advances in patient quality of life.

Gleb Kakazei

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Towards Spin-Wave Computing

A disturbance in the local magnetic order can propagate in a magnetic material in the form of a wave. This wave was first predicted by Bloch in 1929 and was named a spin wave because it is related to a collective excitation of the electron spin system in magnetically ordered solid bodies. The quanta of spin waves are referred to as magnons. The field of science that refers to information transport and processing by spin waves is known as magnonics.
 

Nowadays spin waves are attracting considerable attention as data carriers in novel computing devices, just like electrons being data carriers in conventional electronics. Magnon-based computing is a broad field that includes, for example, the processing of Boolean digital data, unconventional approaches, such as neuromorphic computing, and quantum magnonics aiming to utilize entangled magnon states to process information. The field is very young and is still located primarily in the academic domain, rather than in the engineering or manufacturing stage. The novelty of the field allows for broad exploitation of the underlying physical phenomena for a broad range of applications. Moreover, magnonics is strongly coupled to other fields of modern physics and technology, such as material science, nanotechnology, spintronics, photonics, semiconductor electronics, physics of superconductors, quantum optics, and electronics.
 

At IFIMUP we are working on different components of spin wave computing devices. One of the biggest challenges is to fabricate simple and miniature phase-controlling elements with broad tunability. Recently, we successfully realize such spin-wave phase shifters upon a single nanogroove milled by a focused ion beam in a Co–Fe microsized magnonic waveguide. By varying the groove depth and the in-plane bias magnetic field, we continuously tune the spin-wave phase and experimentally evidence a complete phase inversion [1].
 

Another important task is to develop materials with low spin-wave propagation losses. Very recently, using theoretical prediction that in ferromagnetic metallic films the largest contributions to damping arise from the outermost atomic layers, in collaboration with Durham University we fabricated Co thin films, where the upper and lower few atomic layers were locally modified by doping [2]. The result was astonishing - damping value in synthetically modified films reduced by more than an order of magnitude in comparison with the reference Co one. Now we are working on the maximization of the discovered effect.
 

Finally, we are searching for new generators of spin waves. Magnetics vortexes in submicron circular elements are known to be effective generators in sub-GHz range. Recently we demonstrated a possibility to achieve magnetic vortex state in hybrid nanostructure, in which a soft magnetic nanodot is placed inside the hole in a hard magnetic layer with perpendicular magnetization, providing only dipolar coupling between subsystems. Now we are working on the possibility to create a radial vortex in a much easier achievable technologically and more suitable for practical applications structure - three-layered circular dot that consists of soft layer/non-magnetic layer/hard layer. Micromagnetic simulations demonstrate the possibility to reach operation frequencies up to 7 GHz in the absence of external magnetic field.
 

[1] V. Dobrovolskiy,..., G.N. Kakazei, ACS Appl. Mater. Interfaces 11, 17854 (2019)

[2] Azzawi, A. Umerski, L.C. Sampaio, S.A. Bunyaev, G.N. Kakazei, D. Atkinson, APL Materials 11, 081108 (2022).

[3] R.V. Verba,…, G.N. Kakazei, Phys. Rev. B 101, 064429 (2020)

Mafalda Moreira

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WHY QUANTUM MATTERS: PUTTING A SPIN ON FLEXIBLE TECHNOLOGY

A eletrónica de spin (spintronics) é um ramo da física em rápido crescimento que busca explorar o spin do eletrão como uma forma de codificar, transmitir e guardar informação, através da manipulação da magnetização dos materiais. Este shift no paradigma da computação promete dispositivos mais rápidos, eficientes e não-voláteis à nanoescala.

No entanto, avanços significativos são necessários para atender às exigências da indústria tecnológica de reduzir o tamanho, peso e potência dos dispositivos, enquanto adota a flexibilidade (SWAP-F). Os materiais quânticos, como o grafeno, isoladores topológicos, ou interfaces de Rashba, oferecem novas estratégias para melhorar a resposta destas novas soluções tecnológicas, aproveitando os efeitos quânticos complexos e exóticos que encotramos nestes materiais. Nesta sessão, vamos discutir de forma particular, as aplicações dos isoladores topológicos (Nobel da Física 2016) e as propriedades únicas que tornam estes materiais um verdadeiro playground para fenómenos impulsionados pelo spin.

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