Spintronics by “straintronics”: Superferromagnetism with electric-field induced strain
The cones represents the magnetization of the nanoparticles. In the absence of electric field (strain-free state) the size and separation between particles leads to a random orientation of their magnetization, known as superparamagnetism © HZB
When an electric field is applied, the strain induced on the BaTiO3 substrate is transferred to the nanoparticles forcing their realignment along a common direction, known as superferromagnetism. © HZB
Data storage in today’s magnetic media is very energy consuming. Combination of novel materials and the coupling between their properties could reduce the energy needed to control magnetic memories thus contributing to a smaller carbon footprint of the IT sector. Now an international team led by HZB has observed at the HZB lightsource BESSY II a new phenomenon in iron nanograins: whereas normally the magnetic moments of the iron grains are disordered with respect each other at room temperature, this can be changed by applying an electric field: This field induces locally a strain on the system leading to the formation of a so-called superferromagnetic ordered state.
Switching magnetic domains in magnetic memories requires normally magnetic fields which are generated by electrical currents, hence requiring large amounts of electrical power. Now, teams from France, Spain and Germany have demonstrated the feasibility of another approach at the nanoscale: “We can induce magnetic order on a small region of our sample by employing a small electric field instead of using magnetic fields”, Dr. Sergio Valencia, HZB, points out.
Ferroelectric substrate with magnetic nanoparticles on top
The samples consist of a wedge-shaped polycrystalline iron thin film deposited on top of a BaTiO3 substrate. BaTiO3 is a well-known ferroelectric and ferroelastic material: An electric field is able to distort the BaTiO3 lattice and induce mechanical strain. Analysis by electron microscopy revealed that the iron film consists of tiny nanograins (diameter 2,5 nm). At its thin end, the iron film is less than 0,5 nm thick, allowing for “low dimensionality” of the nanograins. Given their small size, the magnetic moments of the iron nanograins are disordered with respect to each other, this state is known as superparamagnetism.
BESSY II: Mapping the magnetic order
At the X-PEEM-Beamline at BESSY II, the scientists analysed what happens with the magnetic order of this nanograins under a small electric field. “With X-PEEM we can map the magnetic order of the iron grains on a microscopic level and observe how their orientation changes while in-situ applying an electric field”, Dr. Ashima Arora explains, who did most of the experiments during her PhD Thesis. Their results show: the electrical field induced a strain on BaTiO3, this strain was transmitted to the iron nanograins on top of it and formerly superparamagnetic regions of the sample switched to a new state. In this new state the magnetic moments of the iron grains are all aligned along the same direction, i.e. a collective long-range ferromagnetic order known as superferromagnetism.
From spintronics to straintronics
The experiments were performed at a temperature slightly above room temperature. ”This lets us hope that the phenomenon can be used for the design of new composite materials (consisting of ferroelectric and magnetic nanoparticles) for low-power spin-based storage and logic architectures operating at ambient conditions”, Valencia says.
Controlling nanoscale magnetic bits in magnetic random access memory devices by electric field induced strain alone, is known also as straintronics. It could offer a new, scalable, fast and energy efficient alternative to nowadays magnetic memories.
Published in Physical Review Materials (2019): Switching on Superferromagnetism
Arora, L. C. Phillips, P. Nukala, M. Ben Hassine , A.A. Ünal, B. Dkhil, Ll. Balcells, O. Iglesias, A. Barthélémy, F. Kronast, M. Bibes, and S. Valencia
DOI: 10.1103/PhysRevMaterials.3.024403
- Porous Radical Organic framework improves lithium-sulphur batteriesA team led by Prof. Yan Lu, HZB, and Prof. Arne Thomas, Technical University of Berlin, has developed a material that enhances the capacity and stability of lithium-sulphur batteries. The material is based on polymers that form a framework with open pores (known as radical-cationic covalent organic frameworks or COFs). Catalytically accelerated reactions take place in these pores, firmly trapping polysulphides, which would shorten the battery life. Some of the experimental analyses were conducted at the BAMline at BESSY II.
- Metallic nanocatalysts: what really happens during catalysisUsing a combination of spectromicroscopy at BESSY II and microscopic analyses at DESY's NanoLab, a team has gained new insights into the chemical behaviour of nanocatalysts during catalysis. The nanoparticles consisted of a platinum core with a rhodium shell. This configuration allows a better understanding of structural changes in, for example, rhodium-platinum catalysts for emission control. The results show that under typical catalytic conditions, some of the rhodium in the shell can diffuse into the interior of the nanoparticles. However, most of it remains on the surface and oxidises. This process is strongly dependent on the surface orientation of the nanoparticle facets.
- Shedding light on insulators: how light pulses unfreeze electronsMetal oxides are abundant in nature and central to technologies such as photocatalysis and photovoltaics. Yet, many suffer from poor electrical conduction, caused by strong repulsion between electrons in neighboring metal atoms. Researchers at HZB and partner institutions have shown that light pulses can temporarily weaken these repulsive forces, lowering the energy required for electrons mobility, inducing a metal-like behavior. This discovery offers a new way to manipulate material properties with light, with high potential to more efficient light-based devices.