Spintronics and nanomagnetism
In the nanomagnetism laboratory the focus is on nanostructured magnetic objects, such as thin films and multilayers, patterned magnetic films and nanoparticles. All these systems are interesting because of their novel magnetic behaviour emerging from the competition between different energy terms due to their low-dimensionality. New and extremely complex phenomena are observed: spin waves, vortices and skyrmions, magnetic interactions arising from spatial symmetry breaking (Dzyaloshinskii-Moriya interaction) and so on.
Spintronics, an emerging technology based on nanomagnetic effects, relies on the manipulation of the spin degree-of-freedom in the solid state. The field of spintronics responds to the increasing demand of miniaturisation and employs nanotechnology to realise devices where the spin is coupled to electric, thermal or magnetic effects.
The magnetic force microscope (MFM) in the nanomagnetism laboratory allows to image surface magnetic domain structure varying magnetic field and / or the temperature. In this way, the magnetic domain pattern displacement as a function of applied magnetic field can be acquired. By using this capability, an experimental procedure based on the acquisition and analysis of images at different magnetic field values has been developed to measure the magnetisation process and, as a consequence, the hysteresis loop of a single nanostructure. An example is shown in the figure where two different MFM images of a single Ni80Fe20 nanodot produced by electron beam lithography (800 nm diameter) are reported. The magnetisation is organised into a typical vortex structure. The two nanodots are characterised by opposite chirality whose direction can be controlled and varied by measuring local hysteresis loops using the magnetic force microscopy technique.
The generation and detection of spin currents is one of the key issues in spintronics and may be realised in several ways: by the transport of the electron spin in metals; by the motion of a ferromagnetic domain wall or of topological magnetic objects in nanostructures (like vortices and skyrmions) and by the transmission of spin waves in ferromagnets.
Several spintronics systems are studied at INRIM, namely:
- heterostuctures containing magnetic/non magnetic high-spin-orbit layers (e.g. Co/Pt and Ta/CoFeB). These structures show inversion symmetry breaking and the Dzyaloshinskii-Moriya interaction (DMI), acting as an anisotropic exchange interaction between neighbouring spins. This interaction gives rise to protected magnetic configurations such as domain walls with a fixed chirality and skyrmions, small soliton-like compact magnetic objects. INRIM is active in the research for the determination of the DMI constant whose knowledge is crucial for future spintronic applications.
- magnetic tunnel junctions, which are already widely applied in read heads in hard disks and storage elements in magnetic random access memories (MRAM). Their functioning is based on the spin-transfer-torque caused by a spin current tunneling from one magnetic layer to another through a non magnetic insulating layer. INRIM investigates the magnetization dynamics (e.g. of magnetic vortices) under the effect of important parameters as defects or heat.
- junctions of a metallic layer with a strong spin Hall effect (e.g. Pt) and an insulating ferromagnet (e.g. YIG). For these bilayers, the spin current can be generated in one of the layers and transmitted through the interface in the other one, giving rise to variety of novel physical effects. INRIM is active in both the experimental and theoretical investigation of spin Seebeck, spin Peltier, spin pumping and spin Hall torque effects arising in these configurations and representing promising spintronic devices.
- devices for spin wave measurements in magnetic films (e.g. Permalloy), investigated experimentally in time and frequency domain to understand the generation and propagation of spin currents.
High-sensitivity SQUID magnetometry techniques are currently used to study the magnetisation process in metallic magnetic nanoparticles, core-shell nanocomposite structures and dispersed in polymeric and nanoporous matrix used for biomedical and environmental applications. Magnetic interactions which have been shown to vary with composition, morphology and aggregation are studied and analysed by analytical models. As an example, in the figure the magnetisation curves of iron oxides nanoparticles (with diameter of about 6 nanometers) measured after cooling in presence and absence of a magnetic field are shown.