Joint research project

Advanced characterization methods for the study of rare-earth single-ion magnets on oxide substrates

Project leaders
Valerio Bellini, Jindrich Kolorenc
Agreement
REPUBBLICA CECA - CAS (ex AVCR) - Czech Academy of Sciences
Call
CNR/CAS triennio 2019-2021 2019-2021
Department
Physical sciences and technologies of matter
Thematic area
Physical sciences and technologies of matter
Status of the project
New

Research proposal

Small clusters of magnetically active transition metals (iron, cobalt ...) constitute the elemental base for magnetic recording at the present time. Single rare-earth (RE) atoms may represent the ultimate limit in the process of downsizing magnetic memories, their large magnetic anisotropy and long spin coherence possibly leading to the realization of single ion magnets (SIM). The recent observation of an extremely long spin relaxation time of up to 1500 s at 10 K for a Ho atom on MgO/Ag(100) [Science 352, 318 (2016)] has attracted much interest in the scientific community. The extraordinary magnetic stability results from a symmetry-protected magnetic ground state and from the decoupling of the Ho spin from the underlying metal by an oxide barrier, which reduces decoherence thanks to its mechanical stiffness and insulating character. Furthermore, the spin of the Ho atoms can be readily manipulated (read and written) by an STM tip [Nature 543, 226 (2017)], highlighting the potential for information storage. Following similar ideas, some of us have recently demonstrated long relaxation times (about 100 s) and magnetic remanence also for individual Dy atoms on SrTiO3.
Another promising approach is to embed the RE ions within a molecular environment. In fact, synthetic chemistry offers an amazing possibility of creating bulk quantities of perfectly identical molecular objects, which can be integrated in devices by bottom-up approaches [Nature Mater. 7, 179 (2008); Nature Mater. 4, 335 (2005)]. The family of RE-based metallo-organic molecules RE-Pc2 [Angew. Chem. Int. Ed. 44, 2931 (2005)] has shown high structural stability and magnetic integrity [J. Am. Chem. Soc. 132, 11900 (2010)], which allow these molecules to be interfaced with metallic surfaces and electrodes [Nano Lett. 10, 3364 (2008)] and coupled to magnetically active layers, e.g. Ni, Co, [Phys. Rev. Lett. 107, 177205 (2011)] by a super-exchange mechanism via the Pc ligand, as recently suggested by some of us [ACS Nano 10, 9353 (2016)]. Moreover, high-resolution AFM/STS images of TbPc2 and Tb2Pc3 molecules deposited on Ag have been recently achieved, as demonstrated also by the Czech experimental unit [arXiv:1805.03913v1 (2018)]. Most interestingly, the opening of the hysteresis loop at 3 K for a TbPc2 molecule deposited on the MgO substrate [Adv. Mater. 28, 5195 (2016)] has been observed.
From the theoretical point of view, simulation of RE ions is not trivial due to the very localized 4f orbitals that host the RE spins. The initial characterization of these magnetic systems can be performed by density-functional theory (DFT) using the projector augmented-wave (PAW) method [Phys. Rev. B 50, 17953 (1994)] in conjunction with exchange-correlation functionals that go beyond the local density approximation (LDA), for instance taking into account in a static approximation the electron correlations as in the LDA+U [Phys. Rev. B 48, 16929 (1993)]. Such strategy has been applied to RE clusters [Sci. Rep. 6, 19676 (2016)] and adatoms [Nature 503, 242 (2013)], as well as to RE-Pc2 molecules [Sci. Rep. 6, 21740 (2016)], and it is able to supply only a qualitative picture of the magnetic properties of these systems. In order to properly describe the magnetic ground-state multiplet and the anisotropy energy, more refined methods have to be used. For small isolated molecules, quantum chemistry methods [J. Chem. Phys. 137, 064112 (2012); Phys. Rev. Mater. 2, 024405 (2018)] can be employed. For adatoms and molecules adsorbed on surfaces, these methods are prohibitively demanding and hence methods based on the dynamical mean-field theory are usually applied. The Czech team has developed a method referred to as LDA+ED that combines DFT with the exact diagonalization of the Anderson impurity model, and applied it to investigate isolated Sm and Nd atoms on free-standing graphene [Phys. Rev. B 94, 125113 (2016)], Ho atoms on platinum substrates [Sci. Rep. 7, 2751 (2017)], and Dy atoms on Ir substrates [J. Magn. Magn. Mater. 454, 61 (2018)].
In this project we propose a combined theoretical/experimental characterization of the structural, electronic and magnetic properties of RE single-ion magnets. We will use advanced theoretical methods based on the DFT, that go beyond the standard LDA+U approximation, such as the LDA+ED approach. Experimentally, the samples will be prepared by molecular beam epitaxy, by evaporating the RE atoms and molecules onto oxide substrates. The magnetic characterization will be carried out by means of X-ray circular dichroism experiments, at several European synchrotron radiation facilities, and the interplay with the structural properties revealed by microscopic and spectroscopic analysis.
The originality of this project grounds on the complementary skills of the researchers of the CNR and CAS teams, and on the combined methodology proposed. The theoretical section comprises two units [Dr. Bellini (CNR-NANO) in Modena, Dr. Kolorenc and Dr. Shick (CAS-IOP) in Prague], the former with expertise in the simulation of solid state and molecular systems in experimental conditions, using state-of-the-art methodologies and codes, the latter more experienced in the development of novel advanced theoretical methods to account for strong electron correlation effects, combining DFT and many-body techniques. The experimental section of the team comprises the CNR-ISM unit in Trieste (Dr. Barla), with strong expertise in resonant X-ray absorption and scattering experiments (he designed the BOREAS beam line installed at the ALBA synchrotron), and two junior researchers, i.e. Dr. Bonizzoni (CNR-NANO in Modena) working in the low-temperature lab run by Prof. Affronte, equipped with cryostats reaching down to 300 mK, laser (400-700nm) and microwave (0-26GHz) sources, and Dr. Stetsovych (CAS-IOP in Prague) working in the group lead by Dr. Jelinek, where advanced (with sub-molecular resolution) Scanning Probe (AFM/STM) experiments are carried out.

Research goals

The goal of this proposal is to perform research on the frontline topic of the exploration of the unique electronic and magnetic properties of the rare-earth elements at the nanoscale. The very recent discovery of the single-RE-atom based data storage shows their potential for ultra-high-density storage media and opens up new opportunities for achieving the magnetic storage density above 100 Tbit per squared inch.
We aim to establish a network between Italian and Czech groups active in the field of RE-based magnetic nano-systems. The goal is to step forward in the understanding of the magnetic properties of RE adatoms and RE-based molecules adsorbed on insulating substrates (SrTiO3/MgO), benefitting from the complementary skills of the involved researchers.
We expect to improve the current state-of-the-art theoretical methods, combining density-functional theory (DFT) with correlated impurity models, which will allow us to accurately treat the strong spin-orbit and correlation effects that characterize the magnetic properties of RE single-ion magnets. Furthermore, the collaboration with the experimental members of the Italian and Czech teams will lead to a multidisciplinary approach merging diverse fields of expertise.
The final objective of this project is to assemble a critical mass of scientists working on these relevant scientific topics, and to lay the foundations for a prolonged cooperation especially in terms of further national/international projects.

Last update: 28/03/2024