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Electrical Spin Manipulation in Atoms and Molecules

Periodic Reporting for period 1 - SpinMan (Electrical Spin Manipulation in Atoms and Molecules)

Reporting period: 2016-09-01 to 2018-08-31

In the last 30 years the magnetic data storage industry has enjoyed a steady increase in the storage density, from about 10 Mb/in2 in the early 90’s to the present ~1.0 Tb/in2 - a hundred thousand fold increase. Such progress has been driven by the large customers’ demand for cheap and reliable data storage capacity, which is certainly due to increase at an unprecedented speed in the next few decades. Recent estimates suggest that the worldwide data production in 2025 will reach 22 zettabytes, i.e 22 trillion gigabytes. In fact, nearly any modern human activity requires storing information.

Magnetic data storage is currently the only technology capable of addressing such massive and constantly growing volumes of data, but it faces the problem of shrinking magnetic memory units from the micro to the nano-scale. The ultimate limit for data storage is the atomic one, where every atom magnetic moment can store a single (or multiple) bits. Additionally magnetic atoms have been also proposed as a platforms for quantum computing. The question then becomes how to address and control these magnetic moments.

The only reliable way to probe and switch the magnetism at the atomic level is through an electrical current. In fact typical experiments involve reading the conductance of nano-junctions. Understanding in detail the fine features of these experiments and, even more ambitiously, model these without relying on adjustable parameters is a formidable theoretical challenge. Firstly one has to solve an intrinsic many-body problem. Secondly, this is an electron transport problem and one needs a non-equilibrium description. Finally, the fine details of the system electronic structure play an important role.

The main questions addressed by SpinMan were:

Can one construct a material-specific parameter-free many-body scheme, applicable to real nano-junctions?

Then, can we describe and moreover predict whether and how a magnetic moment can be addressed and controlled at the atomic scale through an electrical current?

Ultimately we achieved these objectives. We indeed developed a many-body method for electron transport through nano-scale systems, which is fully predictive.
The method was applied to gain a deep and general understanding about the interaction of a charge current with magnetic nano-systems across a variety of experimental relevant situations. While in the past the theoretical modelling of such systems had been limited to fitting the parameters of effective models, we were able to relate the macroscopic quantities accessible to measurements to a first-principles atomic level understanding of the physics. Finally, by working in collaboration with the experimental group of Prof. Sebastian Loth (University of Stuttgart, Germany), we discovered that measuring the dynamics of a few-atom magnetic system permitted it to function as a highly sensitive surface-integrated sensor capable of detecting the presence and state of nearby magnetic nano-objects. The ability to sense the magnetic state of individual magnetic nano-objects is a key capability for powerful applications such as the measurement of magnetism in complex structures with nanometer precision.
We developed the basic theoretical framework to describe magnetic nano-systems in and out-of equilibrium including many-particle effects. The framework combines Density Functional Theory and the Non-Equlibrium Green’s Function method with a general embedding scheme, which allows to select a correlated subspace of a system and treat it by means of a number of advanced numerical many-body techniques. Our theoretical framework was implemented in new modules for electronic transport software Smeagol and they will be distributed free for academic use.

The method was applied to two different problems.

Firstly, we simulated a number of recent experiments addressing charge transport through nano-junctions comprising either magnetic layered nano-materials or magnetic molecules. By doing so, we validated the theoretical method and, most importantly, we achieved an atomistic understanding on how an electrical current interacts with magnetic moments. In particular we considered there effects: spin-transfer torque, Kondo effect and inelastic magnetic excitations. STT is an effect, in which a spin-polarized charge current can switch the orientation of the magnetization of a ferromagnetic atomic layer by exchanging angular momentum. The Kondo and magnetic excitations are the two many-body effects commonly employed in experiments to characterise the magnetic properties of molecules.

Secondly, we put forward a conceptually new protocol that allows to sense with unprecedented accuracy a magnetic environment. The experimental collaborators created an atomic spin sensor consisting of three Fe atoms and showed that it could detect nanoscale antiferromagnets through a minute surface-mediated magnetic interaction. We contributed to rationalise this finding by proposing that the coupling, even to an object with no net magnetic moment modifies the transition matrix element between two states of the Fe-atom-based spin sensor and therefore its spin relaxation time. Furthermore we explain the mechanism for the surface mediated magnetic interaction. The sensor can detect nanoscale antiferromagnets at up to three nanometers distance and achieves an energy resolution of 10 micro-electronvolts surpassing the thermal limit of conventional scanning probe spectroscopy.

The results of SpinMan have been published in open-access format on peer-reviewed international journals, while a general review of the theoretical method was included in the book chapter.

The results of SpinMan also acquired a large visibility during the workshop “Theoretical methods in Molecular Spintronics” organised by research fellow in San Sebastian (Spain) from the 17th to the 20th of September 2018.
SpinMan will have a strong scientific impact in research areas such as spintronics and quantum computing and it will boost the academic career of the research fellow.

Spintronics and quantum computing hold great promise for many applications. Notably, spintronics has already impacted our society. In fact the read-heads of hard-disk drives in modern computers exploit one of the prototypical spintronic effects, namely the Tunnel Magneto Resistance across Fe/MgO/Fe junctions. The discovery of this effect represents the best example of the prominent role of theory in the field as it was first predicted by using electronic structure calculations [Butler et al. Phys. Rev. B 63, 054416 (2001)] and it was subsequently demonstrated experimentally [Parkin et al. Nat. Mater. 3, 862, (2004), Yuasa et al. Nat. Mater. 3, 868 (2004)]. The developments of SpinMan have contributed to maintain the importance of state-of-the-art theory in guiding the design and screening of novel nano-materials and devices for technological applications.

SpinMan represented a great opportunity for the research fellow to significantly mature as scientist realising his potential, while gaining international exposure and recognisability. This was achieved thanks to a few factors: international mobility, knowledge transfer from and to the host institution and the possibility to collaborate with researchers that represents the scientific excellence in Europe.