ABSTRACT. The recent Kitaev model (2006) provides an exact model to achieve a quantum spin liquid ground state in a 2D honeycomb lattice system through Ising-like bond-dependent interactions [1]. While first considered as a toy model, a theoretical work from Jackeli and Khaliullin has paved the way towards the realization of Kitaev physics in bulk materials. They first showed that bond-dependent interactions can be achieved through the interplay between crystal field, spin-orbit coupling and bond geometry using 4d and 5d transition metal ions, that exhibit a strong spin-orbit coupling [2]. Since then, a significant amount of experimental works have focused on iridate and ruthenate compounds to find a suitable candidate material [3]. Co2+ ions in an octahedral crystal field stabilize a jeff = 1/2 ground state with an orbital degree of freedom and have been more recently put forward for realizing Kitaev interactions [4], a prediction we have tested by investigating spin dynamics in two cobalt honeycomb lattice compounds, Na2Co2TeO6 and Na3Co2SbO6, using inelastic neutron scattering. We used linear spin wave theory to show that the magnetic spectra can be reproduced with a spin Hamiltonian including a dominant Kitaev nearest-neighbor interaction, weaker Heisenberg interactions up to the third neighbor, and bond-dependent off-diagonal exchange interactions [5]. Beyond the Kitaev interaction that alone would induce a quantum spin liquid state, the presence of these additional couplings is responsible for the zigzag-type long-range magnetic ordering observed at low temperature in both compounds. These results provide evidence for the realization of Kitaev-type couplings in cobalt-based materials, despite hosting a weaker spin-orbit coupling than their 4d and 5d counterparts, and therefore open to new possibilities for future new material prospection.

References : [1] A. Kitaev, Annals of Physics 321, 2-111 (2006) [2] G. Jackeli and G. Khaliullin, Physical Review Letters 102, 017205 (2009) [3] S. M. Winter et al., J. Phys. : Condens. Matter 29, 493002 (2017) [4] H. Liu and G. Khaliullin, Physical Review B 97, 014407 (2018); R. Sano et al., Physical Review B 97, 014408 (2018); H. Liu et al., Physical Review Letters 125, 047201 (2020) [5] M. Songvilay et al., Physical Review B 102, 224429 (2020)

ABSTRACT. The world-wide need for ever-increasing computing performances calls for new technologies for data storage and handling. Most prominently, in new-generation non-volatile magnetic memories the write operation through an external applied magnetic field is no longer possible, due to Moore-ruled scaling down of the magnetic bits: instead, acoustic control of magnetism appears as an appealing route for low-energy-consumption operation, exploiting the magneto-elastic coupling (MEC) in a thin-film planar heterostructure, driven by inverse magnetostriction at micron-size wavelength [1]. Besides the technological relevance, much attention has been recently paid to fundamental investigation on interplay of spin degrees of freedom and coherent phonon excitation, without the intermediation of electronic excitation (see e.g. [2]): this research represents somehow the extreme case, where elastic excitations from the whole Brillouin zone act on the spin dynamics. Many studies on MEC rely on thin ferromagnetic films deposited on top of a properly cut piezoelectric crystal [3]: the elastic waves are generated in the substrate by interdigitated transducers (IDTs), thus making the access to a clean room facility a must. The perturbation of the magnetization is inferred by the attenuation and phase-lag of the elastic waves as detected by a second IDT. Following Janusonis et al. [4], we approached the study in a contactless way, with no need for lithography: the sample is an electron-beam deposited Nickel film (40 nm) on fused silica substrate, with 10 nm SiO2 capping. We implemented a UHV-ready Transient Grating spectroscopy setup, exploiting a 300 fs, high repetition rate pulsed laser at the NFFA-SPRINT laboratory, hosted at FERMI@Elettra in Trieste, Italy. Two 1030 nm pump pulses interfere to generate an intensity grating on the sample, whose local thermal dilation launches coherent surface acoustic waves. A third, time-delayed 515 nm pulse can be used at will either as an acoustic probe, through standard Transient Grating diffraction, or as a magnetic probe, exploiting Faraday rotation. Tuning an external applied magnetic field, we can reach the condition of acoustically-driven ferromagnetic resonance, in which the time-dependent magneto-elastic field balances the Gilbert damping, allowing the spin precession to last long after its intrinsic damping. Using different diffractive optics elements, we can change the acoustic wavevector and frequency, thus finding different points of the ferromagnetic resonance curve (Kittel mode); insight in the thin film properties can be extracted.

[1] W. Yang and H. Schmidt, Appl. Phys. Rev. 8, 021304 (2021) [2] D. Afanasiev et al. Nat. Mater. 20(5), 607-611 (2021) [3] M. Weiler et al. Phys. Rev. Lett. 106, 117601 (2011) [4] J. Janušonis et al. Sci. Rep. 6.1, 1-10 (2016)

ABSTRACT. Advanced deposition, e.g. nanolithographic, techniques are now capable of tailoring well designed and assembled nanomagnetic particles. The latter, called here macrospins as being treated in a continuum approximation, behave analogously to single spins mainly due to their shape anisotropy. The present study concerns the impact of the geometry of the single nanoparticle as well as the arrangement of the particles in finite-length chains on the reversal mechanisms under variable magnetic field. It will be shown that an appropriate arrangement and end conditions ensure repeatable and tuneable reversal sequences so that the particular intermediate configuration may storage and/or encrypt information.

ABSTRACT. Single-domain magnetic crystals typically show either one of the ferro-, ferri- or antiferromagnetic ordering types or the long-wavelength modulation of one of these structures. Based on high-field measurements of the magnetization and the field dependence of the magnetic resonances in the far-infrared region, we identified an exotic magnetic phase in multiferroic Co2Mo3O8. Namely, alternating crystallographic layers of Co spins host collinear antiferro- and asymmetrically canted ferrimagnetic orderings, respectively. Due to the high characteristic magnetic fields, direct observation of the magnetic structure via neutron scattering is not realistic. Thus, far-infrared spectroscopy in magnetic field provides a unique tool to validate model predictions.

ABSTRACT. Recent widespread interest in pyrophosphate materials intensified studies on various compounds containing transition metals, like Cu or Fe. The copper pyrophosphate compound is a subject of increasing interest because of its materials science applications [1], that has universal structures with a wide range of behavior. This material also exhibit enhanced electrical and photonic properties. We performed comprehensive theoretical and experimental studies in order to characterize and identify the nanocrystalline copper pyrophosphate confined in mesoporous silica. The electronic and crystal structure were optimized within the density functional theory implemented in the VASP software [2] with the strong electron interactions in the 3d states on copper atoms and van der Waals corrections included in calculations. The relaxed lattice parameters and atomic positions agree very well with the results of the diffraction measurements for nanocrystalline copper pyrophosphates embedded inside SBA-15 silica pores. The obtained Mott insulating state with the energy gap of around 3 eV shows the antiferromagnetic order with magnetic moments on copper atoms (0.8μB) that is compatible with the experimental studies. The phonon dispersion relations were obtained to study the dynamical properties of copper phosphate and the element-specific atomic vibrations were analyzed using the partial phonon density of states. The calculated Raman spectrum is consistent with the experimental data obtained for the nanocrystals. References [1] M. M. Rao, Materials Research Bulletin, 1991, 26, 813-819. [2] G. Kresse, J. Furthmuller, 1996. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54, 11169.

ABSTRACT. Exerting control over quantum materials is one of the main goals in condensed matter physics. Oxide interfaces and surfaces have emerged as a versatile platform for material design, where new fundamental properties can be generated by assembling condensed matter at the atomic scale. Light plays a pivotal role in this scientific exploration. Probing materials with light reveals the collective excitations and the energy landscapes that underpin correlated dynamics. Recently we have come to the realisation that light not only reveals the organisation of condensed matter, it can also unlock new properties and promote phase transitions. The overarching goal of the field is to control macroscopic material properties, paving the way to new scientific insights and future emerging technologies.

Resonant ultrafast excitation of infrared-active phonons is a powerful technique with which to control the electronic properties of materials that leads to remarkable phenomena such as light-induced superconductivity, switching of ferroelectric polarization and ultrafast insulator-to-metal transitions. We will discuss how light-driven phonons can be utilized to coherently manipulate macroscopic magnetic states. Intense mid-infrared electric field pulses tuned to resonance with a phonon mode of the archetypical antiferromagnet DyFeO3 induce ultrafast and long-living changes of the fundamental exchange interaction between rare-earth orbitals and transition metal spins. Non-thermal lattice control of the magnetic exchange, which defines the stability of the macroscopic magnetic state, allows us to perform picosecond coherent switching between competing antiferromagnetic and weakly ferromagnetic spin orders. In this talk, the potential of resonant light excitation for the manipulation of ferroic order on ultrafast timescales will be emphasised [1-4].

[1] J.R. Hortensius, D. Afanasiev, M. Matthiesen, R. Leenders, R. Citro, A.V. Kimel, R.V. Mikhaylovskiy, B.A. Ivanov, A.D. Caviglia Coherent spin-wave transport in an antiferromagnet Nature Physics (2021) arXiv:2105.05886

[2] E. Persky, N. Vardi, A. M. R. V. L. Monteiro, T. C. van Thiel, H. Yoon, Y. Xie, B. Fauqué, A. D. Caviglia, H. Y. Hwang, K. Behnia, J. Ruhman & B. Kalisky Non-universal current flow near the metal-insulator transition in an oxide interface Nature Communications 12, 3311 (2021) arXiv:2103.08658

[3] D. Afanasiev, J.R. Hortensius, M. Matthiesen, S. Mañas-Valero, M. Šiškins, M. Lee, E. Lesne, H.S.J. van der Zant, P.G. Steeneken, B.A. Ivanov, E. Coronado, A.D. Caviglia Controlling the anisotropy of a van der Waals antiferromagnet with light Science Advances 7, eabf3096 (2021) arXiv:2010.05062

[4] D. Afanasiev, J.R. Hortensius, B.A. Ivanov, A. Sasani, E. Bousquet, Y.M. Blanter, R.V. Mikhaylovskiy, A.V. Kimel, A.D. Caviglia Ultrafast control of magnetic interactions via light-driven phonons Nature Materials 20, 607 (2021) arXiv:1912.01938

ABSTRACT. In the first part of this review we will describe electric field tuning of polar phonons. The most sensitive to the electric field is the ferroelectric soft mode in SrTiO3 and KTiO3, but we will demonstrate that also higher-frequency phonons in these materials are sensitive to the electric field, because the polar phonons are mutually coupled in perovskites.

The phonons are usually not sensitive to magnetic fields, but if some magnetic material exhibits strong spin phonon coupling, tuning of the phonon frequencies with magnetic field can be measured. This we will demonstrate in infrared spectra of EuTiO3. Moreover, in antiferromagnetic phase of this material, the ferromagnetic resonance shifts from microwave to terahertz range, if magnetic field rises to 7 T. Since the ferromagnetic resonance is influenced only by external magnetic field applied perpendicularly to magnetic polarization of THz radiation, index of refraction of EuTiO3 ceramics exhibits a strong anisotropy in the magnetic field, although dielectric permittivity is isotropic.

Magnons are generally excited by the magnetic component of the electromagnetic radiation and therefore they contribute in magnetic permeability. Nevertheless, if a dynamic magnetoelectric coupling is allowed, spin waves can be excited also by the electrical component of the THz radiation and therefore these magnons called electromagnons can contribute also in permittivity. We will demonstrate how to distinguish magnons and electromagnons in the THz spectra of room-temperature multiferroics with Y- and Z-type hexaferrite crystal structures. Since the electromagnons are activated by the magnetic exchange striction in conical magnetic structure of hexaferrites and this structure disappears in magnetic field above 2 T, electromagnons also disappear in higher magnetic field. On the other hand, ferromagnetic resonance shifts again from microwave in the THz region when the magnetic field exceeds 4 T. Ferromagnetic resonance is also tunable with external electric field, which can be used for accurate determination of value of magnetoelectric coupling. Possibility of AC and DC electric field tuning of electromagnons will be also discussed.

ABSTRACT. In recent times, the role of three-dimensionality in the propagation properties of spin waves has been increasingly at the center of research interest. We investigate the fundamental properties emerging from a progressive undulation of the film plane, from smooth (2D) to rippled (3D). The geometric undulation (which acts as a constraint for the magnetization texture) is taken along a single direction, is periodic and the period is constant, while the amplitude (which is the differential maximum height with respect to the film thickness) is gradually increased from 0 to 60nm. We find that the undulated systems display at once properties both of a continuous ferromagnetic film and a discretized medium. Depending if the spin wave oscillates parallel or perpendicular to the film undulation direction, and if the external field is applied parallel or perpendicular to the undulation direction, the spin wave dispersion shows peculiar regularities determined by the third dimension (the transverse one), which are tunable through geometric design or magnetization changes. We study the characteristic modification of the internal effective field and link it to the resulting spin wave profile and frequency. We discuss the invariance of the spin wave group velocity with respect to even major changes in the undulation, when the field is applied perpendicular to the undulation direction. Finally we address a potential dual band activity, namely the possibility of a simultaneous propagation of two independent spin wave signals, with separated frequency bands and disjoint oscillation regions.

ABSTRACT. Magnonics is a research field complementary to spintronics, in which the quanta of spin waves (magnons) replace electrons as information carriers, promising less energy dissipation. The development of ultrafast nanoscale magnonic logic circuits calls for new tools and materials to generate coherent spin waves with frequencies as high, and wavelengths as short, as possible. Antiferromagnets can host spin waves at THz frequencies and are therefore seen as a future platform for the fastest and the least dissipative transfer of information. However, the generation of short-wavelength coherent propagating magnons in antiferromagnets has so far remained elusive. Here we report the efficient emission and detection of a nanometer-scale wavepacket of coherent propagating magnons in antiferromagnetic DyFeO3 using ultrashort pulses of light. The subwavelength confinement of the laser field due to large absorption creates a strongly non-uniform spin excitation profile, enabling the propagation of a broadband continuum of coherent THz spin waves. The wavepacket features magnons with detected wavelengths down to 125 nm that propagate with supersonic velocities of more than 13 km/s into the material. The long-sought source of coherent short-wavelength spin carriers demonstrated here opens up new prospects for THz antiferromagnetic magnonics and coherence-mediated logic devices at THz frequencies.

ABSTRACT. Comprehensive phonon dynamical studies have recently identified francisite (Cu3Bi(SeO3)2O2Cl) as perhaps the closest known realisation of a proper, one-dimensional, Kittel-like antiferroelectric crystal. Whereby, an internal antiferro dipolar order is identified by the condensation of an anti-polar phonon mode at ~115 K. On top of this, the system also hosts a kagome-like lattice of Cu S =1/2 sites exhibiting strong magnetic frustration. The frustration is ultimately lifted by a weak Dzyaloshinskii-Moriya interaction promoting an antiferromagnetic order below ~25 K. In this talk I will give an overview of the experimental work describing the antiferro dielectric and magnetic order of francisite and explore the potential for multiferroic behavior going forward.

ABSTRACT. Lead zirconate (PbZrO3) is considered the prototypical antiferroelectric material with an antipolar ground state. Yet, several experimental and theoretical works hint at a partially polar behaviour in this compound, indicating that the polarization may not be completely compensated. In this work we propose a simple ferrielectric structure for this lead zirconate. First-principles calculations reveal this state to be more stable than the commonly accepted antiferroelectric phase at low temperatures, possibly up to room temperature, suggesting that PbZrO3 may not be antiferroelectric at ambient conditions. We discuss the implications of our discovery, how it can be reconciled with experimental observations and how the ferrielectric phase could be obtained in practice.

ABSTRACT. The ground state of a system of dipoles located on a cubic lattice, which are affected only by a long-range dipolar interaction, consists of fully anti-ferroelectrically arranged dipoles [1]. In almost all real materials, short-range interactions predominate over dipolar interactions. An example is water, whose molecules have a large electric dipole moment, but also a strong covalent interaction of hydrogens, which suppresses the (anti)-ferroelectric order. But when water is confined in the structural channels of the beryl single crystal so that the water molecules are far enough away to prevent hydrogen bonding, then dipolar interactions play a major role. In this case, we can observe the anti-ferroelectric arrangement of water molecules at low temperatures [2]. The beryl lattice forms a matrix for the dipoles of water molecules. The lattice is hexagonal in the ab-plane stacked in the c-direction [3]. The final ground state is formed by dipoles arranged ferroelectrically in the ab-plane and anti-ferroelectrically in the c-direction. The dipoles can rotate along the hexagonal axis c at a six-well potential. In experiments we can see signs of arrangement, but not a transition to an ordered phase at non-zero temperatures [3]. There are two factors that can suppress the (anti)-ferroelectric order. The first is quantum tunneling across the local six-well potential barrier, and the second is that water molecules are not present in all sites (channels) of the beryl crystal, so there are voids that disrupt the long-range order. Because water molecules cannot move within the matrix, the disorder given by voids is of type quenched. We study the phase transition of a given system under the above conditions using the mean field approximation and Monte-Carlo simulations. Both approaches confirm the prediction of the ferroelectric arrangement and also its suppression due to both of the above circumstances. We show quantitatively the share of both types of suppression.

[1] Luttinger J.M.,Tisza L..PhysRev1946;70:954.[18] [2] Belyanchikov M. A. et al. Dielectric ordering of water molecules arranged in a dipolar lattice, Nature Communications 11, 3927 (2020). [3] Gorshunov, B. P. et al. Incipient ferroelectricity of water molecules confined to nano-channels of beryl. Nat. Commun.7, 12842 (2016)