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Atomistic spin dynamics and spectroscopic investigation of spin-induced magnetoelectric multiferroic materials

Periodic Reporting for period 1 - EMAGICS (Atomistic spin dynamics and spectroscopic investigation of spin-induced magnetoelectric multiferroic materials)

Reporting period: 2019-05-06 to 2021-05-05

If 60 years took us to get from the room-sized computer of the 1960’s to the extremely powerful smartphones of the 2020s, we are just on the threshold of the era of electrically controllable nanoscale storage, which will lead to thinner, lighter and flexible devices, with significantly high and sustainable energetic efficiencies and huge capacities.

Multiferroics (MFs) are materials that can combine at least two primary ferroic properties: ferromagnetism, ferroelectricity and ferroelasticity. In the case of magnetoelectric (ME) MFs, coupling between ferroelectricity and ferromagnetism occurs.The use of MFs can provide electrically written and magnetically read devices, thus faster, low-energy consumption and with a non-destructive magnetic read operation. Particularly for the case of spintronics memories, the presence of the ME effect in MFs may increase the number of logic states from 2 to 4 (or 8), due to the additional binary state emerging from such multifunctionality. Most importantly, the ability to manipulate the magnetization by electric fields leads to simple, cost-effective and energetically sustainable technological strategies. An even more promising route to design efficient future hybrid devices is the use of the dynamical ME effect, where the order parameters of magnetization and polarization are not static, but oscillatory. Very often, in this dynamical regime, elementary excitations called electromagnons emerge, as “carriers” of such dynamical ME coupling. These spin excitations can be tuned by external magnetic and/or electric fields, thus promoting the modulation of the index of refraction by both static fields and electromagnetic radiation.

EMAGICS project aims at a clearer understanding of the magnetoelectric (ME) multiferroic (MF) properties of novel materials, from the macroscopic to the quantum microscopic level, encompassing a series of advanced experimental and computational techniques. The acronym “EMAGICS” stands for “Electromagnonics”, a field that studies the dynamical coupling between the electronic and magnetic properties of ME materials. Merging theory and experiment is the optimal approach for achieving a better understanding of such exotic and sophisticated physical concepts. EMAGICS has employed a combination of first principles calculations and atomistic spin dynamics, together with experimental spectroscopic investigation, for a series of new ME MF materials.

The main research objectives of EMAGICS are: (RO1) to address the origin of the ME coupling in the polar antiferromagnets (AFM) Ni-based tellurates, (RO2) to propose new spin-induced ME MF systems, and (RO3) to synthesise and experimentally investigate the static and dynamical ME effects in these new compounds.
The scientific part of EMAGICS project consists of 3 primary tasks: A) theoretical calculations, B) material synthesis, and C) experimental characterization and investigation.

RO1 was fully achieved for Ni3TeO6, famous for its colossal ME effect, by a series of DFT electronic and magnetic structure calculations, DFT magnetocrysalline anisotropy calculations, DFT exchange interactions calculations, implementation of the above to atomistic spin dynamics simulations, and DFT lattice dynamics calculations. The results reproduce the experimental values previously observed. The latter DFT results were used in atomistic spin dynamics simulations, for the calculation of the magnetic phase transition and magnon dispersion relation, which agree with the experimentally observed excitations reported previously by Skiadopoulou et.al. (Figure 1). DFT lattice dynamics calculations were performed using the magnetic ground state of Ni3TeO6, resulting in phonon frequencies in good agreement with experiments (Figure 2).
RO2 and RO3 were fully achieved for Ni2CoTeO6 and NiCo2TeO6 compounds, by a series of DFT electronic and magnetic structure calculations, synthesis, ME properties and spin and lattice dynamics experiments. Single crystals and polycrystalline pellets of Ni2CoTeO6 and NiCo2TeO6 were successfully synthesized. The magnetic structures of both compounds correspond to an incommensurate AFM helical spin order (Figure 3). Ni2CoTeO6 and NiCo2TeO6 evidence an interplay of magnetic and dielectric properties (Figure 4). Lattice (phonons) and spin (magnons) dynamics experiments reveal several spin excitations, highly sensitive to external magnetic fields, as well as phonons coupled to magnons (Figures 5 and 6).
In summary, EMAGICS succeeded at proposing a theoretical explanation of the underlying quantum level mechanisms of the ME coupling in Ni3TeO6. In addition, the new MF compounds Ni2CoTeO6 and NiCo2TeO6 were proposed, synthesized and experimentally investigated.

Part of the results above were published in Physical Review B. A second manuscript is under preparation from the spin dynamics calculations. More details on EMAGICS project can be found on the website www.emagics.eu.

The work done within EMAGICS project was discussed during the ABINIT workshop, and presented at the international conference INTERMAG21. The importance and impact of ME materials and electromagnonics in our society were promoted in a series of outreach activities, among which a presentation of a 3D atomic and magnetic interactive model at PROBE research pop-up festival, organized by TCD and Dublin Science Gallery. The fellow also took part in online workshops with primary school students in Dublin, organized by AMBER.
The main target of EMAGICS was to unveil the underlying mechanisms that lead to the enhancement of the ME MF properties. Great progress was made towards this complicated task, by establishing a DFT derived model Hamiltonian for the parent structure of Ni3TeO6, synthesizing the new compounds Ni2CoTeO6 and NiCo2TeO6, and proving their ME properties.

The newly synthesized compounds during EMAGICS project presented enhanced ME properties. However, the long-range incommensurate helical spin order observed by the experiments, complicates substantially the DFT calculations. In order to tackle this problem, our future plan is to perform atomistic spin dynamics calculations with test parametrizations for the model Hamiltonian, aiming at reproducing the experimentally observed data. Moreover, employing Machine Learning methods for designing new compounds is an alternative. Exploitation of existing databases, due to the popularity of DFT for the past 20-30 years, by the use of Deep Learning algorithms is a powerful tool for predicting new compounds.

The computational tools used in EMAGICS, DFT+U electronic and magnetic calculations, in combination with lattice and spin dynamics calculations, may be applied and adjusted for a series of polar AFMs with similar properties. Theoretical predictions undoubtedly assist in avoiding complicated experiments required for such studies. DFT is known for its ability to immensely reduce the calculation of required data from sextillion TB to just a few MB, thus being a cost-effective and time-consuming method for the study of a series of properties within a variety of fields.

Finally, the enhanced dynamical ME properties of the new compounds pave the way for designing Terahertz devices, a field that offers a wide range of applications, like Terahertz imaging, tomography and detectors, in fields such as biomedicine, security control, history of arts and archaeology.
Antiferromagnetic phase transition and magnon dispersion relation
Lattice dynamics calculations of Ni3TeO6
Spin dynamics in Ni2CoTeO6 and NiCo2TeO6
Magnetic and dielectric properties of Ni2CoTeO6 and NiCo2TeO6
Lattice dynamics in Ni2CoTeO6 and NiCo2TeO6
Magnetic spin structures of the polar antiferromagnets Ni-based Tellurates