COST Action CA17123 “Magnetofon”
General Action Meeting and Workshop (on line)
November 23 to 27 2020

Wednesday, 25.11.2020: Ultrafast magneto-electrics 
Chair: Dr. Davide Bossini
9:00 9:40
Prof. Manfred Fiebig, ETH Zurich; “All-optical control of antiferromagnetism
When discovered, antiferromagnetism was considered an interesting variety of nature, yet without any practical value. The emerging field of antiferromagnetic spintronics expresses that this view is about to change. In particular, absence of a macroscopic magnetization in antiferromagnets gives rise to a variety of properties and effects not open to ferromagnets. One of these is the intrinsically faster response to external perturbations. In my talk, I will discuss various aspects of all-optical control in antiferromagnets. First, I will describe how the time-dependent evolution of an antiferromagnetic order parameter can be traced through three-dimensional space. I will then show that damping processes that are negligible in ferromagnets can be used as a lever towards facilitating the all-optical reversal of an antiferromagnetic order parameter. In many systems, antiferromagnetic phases are an indicator of strong electronic correlations in a system, which are themselves of great interest. As an example, I will describe the dynamical interplay of antiferromagnetism and quantum criticality in heavy-fermion compounds. A discussion of dynamical magnetoelectric coupling effects in antiferromagnets will round off the picture.

9:40 10:10
Dr. A. M. Kalashnikova, Ioffe Institute St. Petersburg: “Laser-induced change of indirect magnetoelectric coupling in composite multiferroic CoFeB/BaTiO3

Combining ferromagnetic and ferroelectric layers coupled through a strain transfer across the interface is the promising way to realize a room-temperature magneto-electric multiferroic with tailored properties [1]. Such structures has been successfully used to realize switchable spin-wave waveguides, electrically-switchable magnetic nanoelements etc.
Here we investigate [2] laser-induced magnetization dynamics in composite multiferroic CoFeB/BaTiO3 in order to reveal if the magneto-electric coupling between ferromagnetic layer CoFeB and ferroelectric substrate BaTiO3 mediated by the magneto-elastic effect can be changed by femtosecond laser pulses. We demonstrate that the magneto-elastic parameter of CoFeB can be reduced by ~27 % following the ultrafast laser-induced heating. This results in precession of magnetization detected in experiment. We also find that under specific conditions, the excitation of CoFeB/BaTiO3 by a femtosecond laser pulse may result in precessional switching of magnetization, and discuss how the parameters of the structure need to be tailored to realize reliable laser-induced switching of magnetization. We further discuss that the demonstrated laser-induced change of magneto-elastic parameter effectively reduces the indirect magneto-electric coupling and suggest further steps for exploiting this effect.

  1. T. H. E. Lahtinen, et al., Pattern transfer and electric-field-induced magnetic domain formation in multiferroic heterostructures, Adv. Mater. 23, 3187 (2013).
  2. L. A. Shelukhin et al., Laser-Induced Magnetization Precession in Individual Magnetoelastic Domains of a Multiferroic Co40Fe40B20/BaTiO3 Composite, Phys. Rev. Applied 14, 034061 (2020)
10:10 10:40
Edi Topić, University of Zagreb: “Magnetic order and multiferroicity in hybrid tetrachlorocuprate-organic layered perovskites

Damir Pajić1, Pavla Šenjug1, Dario Barišić1, Edi Topić2, Mirta Rubčić1
1Department of Physics, 2 Department of Chemistry, Faculty of Science, University of Zagreb, Croatia
Metal-organic compounds with perovskite crystal structure provide a fertile playground for design of the multifunctional materials. Some properties can be mutually dependent, and some orders mutually coupled, paving the way to design of the magnetoelectric multiferroics.
Interesting example of the hybrid layered perovskite multiferroic is ethylammonium tetrachlorocuprate, which consists of the ferromagnetic layers of corner sharing [CuCl4]2− octahedra connected by two layers of polarizable organic ions of C2H5NH3+. Besides the known ferroelectric transition around 245 K, it shows rich magnetic behaviour below 10.5 K, including magnetic anisotropy and transitions between different magnetic states. Although the search for magnetoelectric effect was not successful, a slight change of magnetization with the structural changes above 300 K was observed.
Composition of this metal-organic perovskite was changed and the accompanied changes of the crystal structure and magnetic properties were studied. Up to now, magnetization is measured in DC regime using SQUID magnetometer in temperature range 2–300 K at different magnetic fields up to 5 T. Large impact of geometric and electronic changes of the organic cation structure on magnetic properties was observed in following two groups.
Novel series of the solid-state architectures consisting of tetrachlorocuprate units and ortho-, meta- and para-anisidinium were prepared and in their crystal structure considerable change of geometry was observed: from the discrete square planar tetrachlorocuprate anions in the ortho-anisidinium compound to the Ruddlesden-Popper perovskite phase with slightly distorted layers built from CuCl6 octahedra in the para-anisidinium compound. Spins 1/2 per Cu2+ ion remain in the paramagnetic state down to the lowest temperatures in the ortho-anisidinium tetrachlorocuprate. However, there are transitions to the ferromagnetic state measured at 4.2 K and 9.5 K for the meta-anisidinium and para-anisidinium tetrachlorocuprates, respectively. Their ferromagnetic-like hysteresis loops are very soft, without the observable coercivity.
Surprise happens in difference between the chloroethylammonium tetrachlorocuprate and bromoethylammonium tetrabromocuprate: the first one has antiferromagnetic transition at 7.8 K with metamagnetic transition in fields above 150 Oe, while the second one has ferromagnetic transition at 11 K and broad hysteresis loop with coercivity around 100 Oe at 2 K. Polar order is predicted in theoretical calculations, and magnetoelectric experiments could be challenging.
This research showed that a small change in structure has drastic influence on the geometry and magnetism of the cuprate units. At the moment it seems to us that the 3D metal-organic perovskites have brighter perspective for magnetoelectric coupling than the 2D. If having on mind wide possibility of combining the building blocks with different physical properties like elastic, (super)conducting, optical, polar, magnetic, the rich field of applications can be foreseen for 2D perovskites, including also influence of the the light on the magnetic and electric response. Some of those will be of our future interest.

10:40 11:10
Dr. Matteo Savoini, ETH Zurich: “Multidimensional Excitation of Soft-Phonon Modes,
Soft-mode ferroelectrics are materials with a spontaneous electrical polarization driven predominantly by the condensation of an infrared active vibrational mode to zero frequency. Our aim is to understand whether it is possible to use coherent control of these vibrational modes to control ferroelectric polarization on (sub-)picosecond timescales via multi-dimensional phonon excitation.
Here I will present our work on a model system, GeTe, an ionic semiconductor below TC ~ 500 K, performed using a multi-pulse excitation scheme of the IR- and Raman-active phonon modes. Our preliminary results indicate that this multidimensional approach in the excitation scheme can pave the way towards a much wider exploration of the energy potential landscape, allowing for strong nonlinearities in the measured dynamics.

11:10 11:25
Dr. Mohsen Hafet-Thorbati, TU Dortmund: “Magnetic Blue-Shift of Mott Gaps

A substantial energy gap of charge excitations induced by strong correlations is the characteristic feature of Mott insulators. We study how the Mott gap is affected by long-range antiferromagnetic ordering. Our key finding is that the Mott gap is generically enhanced by the magnetic ordering: a magnetic blue-shift (MBS) occurs. We establish this important fact in a three-dimensional Hubbard model, the paradigm for strongly correlated systems. The MBS can increase the Mott gap by about 70% as the temperature decreases from the Néel
temperature TN to zero.
The coupling between spin and charge degrees of freedom bears the potential to render spin-to-charge conversion possible, serving as a milestone for the development of spintronic and magnonic devices operating on the femtosecond time-scale. In view of these applications, we show that the blue-shift observed upon magnetic ordering in the optical conductivity of the hexagonal MnTe (α-MnTe) can be viewed as an indicator for a MBS of Mott gaps.

11:25 11:40
Fabio Formisano, Radboud University Nijmegen: “Femtosecond magneto-optics of EuO

A laser pulse is probably the shortest stimulus in condensed-matter physics. Aiming to understand magnetization dynamics at ever shorter timescale, one unavoidably faces the question about effects of light on the strongest interaction in magnetism – exchange spin-spin interaction – and the role of this effect in optical control of magnetism. Trying to answer these questions, one realizes the most fundamental problem of ultrafast magnetism: in a strongly non-equilibrium state, prepared by ultrafast laser excitation, the (dynamics) exchange coupling between the spins (and thus the origin of magnetism) is poorly understood.
Europium oxide (EuO) is a ferromagnetic semiconductor with the Curie Temperature Tc = 69 K. Starting from the discovery of a rather unique combination of transport properties of semiconductor and ferromagnetic order in EuO [1][2], this material has been a subject of intense research. It also gives an interesting playground for fundamental studies of ultrafast optical control of magnetism [3][4][5].
Herein, we report on the photoinduced dynamics of the magneto-optical Faraday effect and the transmittivity of ferromagnetic semiconducting phase of EuO. Excitation with 8-fs laser pulses launches significantly different dynamics of the Faraday effect compared to magnetic refraction. It is argued that the effects are dynamic probes of the magnetization and the exchange interaction in the material which have distinctly different dynamics at the sub-100 fs time scale.

  1. B. T. Matthias, R. M. Bozorth, and J. H. Van Vleck, Phys. Rev.Lett. 7, 160 (1961).
  2. A. Mauger and C. Godart, Phys. Rep. 141, 51 (1986).
  3. F. Liu et al., Phys. Rev. Lett. 108, 257401, (2012).
  4. M. Matsubara et al., Phys. Rev. B, 86, 195127 (2012).
  5. R. R. Subkhangulov et al., Scientific Reports volume 4, 4368 (2014).


11:40 11:55
Dr. Christian Tzschaschel, Harvard University: “Efficient spin excitation via ultrafast damping-like torques in antiferromagnets

Damping effects form the core of many emerging concepts for high-speed spintronic applications. Important characteristics such as device switching times and magnetic domain-wall velocities depend critically on the damping rate. While the implications of spin damping for relaxation processes are intensively studied, damping effects during impulsive spin excitations are assumed to be negligible because of the shortness of the excitation process. Here, we show that, unlike in ferromagnets, ultrafast damping plays a crucial role in antiferromagnets because of their strongly elliptical spin precession. In time-resolved measurements, we find that ultrafast damping results in an immediate spin canting along the short precession axis. The interplay between antiferromagnetic exchange and magnetic anisotropy amplifies this canting by several orders of magnitude towards large-amplitude modulations of the antiferromagnetic order parameter. This leverage effect discloses a highly efficient route towards the ultrafast manipulation of magnetism in antiferromagnetic spintronics.

11:55 12:10
Pavla Šenjug, University of Zagreb: “Magnetoelectric multiferroic guanidinium copper(II) formate

Multiferroic guanidinium copper(II) formate, [C(NH2)3][Cu(HCOO)3], belongs to the family of metal organic perovskites with the formula ABX3. It crystallizes in the orthorombic space group Pna21 and consists of copper – formate 3D framework with guanidinium cations in its nearly cubic cavities. Due to the Jahn-Teller effect of Cu2+ ion, the octahedral coordination is elongated, producing the framework of Cu-formate chains in the c direction, where Cu2+ ions inside the chain are linked with short bonds and Cu ions between the chains with the long bonds.
We have studied magnetic properties of [C(NH2)3][Cu(HCOO)3] using MPMS5 SQUID magnetometer. Temperature dependence of magnetization showed a broad peak centered at 45 K which can be ascribed to antiferromagnetic order in the Cu-formate chains. Below 4.6 K the sharp rise of magnetization indicated a formation of 3D spin canted antiferromagnetic long range order. Hysteresis loops at the temperatures below 4.6 K showed a sharp increase of magnetization for the fields lower than 50 Oe. Anisotropy could be seen in both M(T) and M(H) measurements. The effect of electric field on the magnetization was observed at temperatures below 4.6 K as a difference in magnetization curves measured with and without the applied electric field, pointing to the existence of substantial magnetoelectric coupling.
Search for the explanations of observed magnetic and magnetoelectic behaviour is in progress considering the interesting crystal structure of this compound. Wishing for better understanding we are collaborating with other scientists doing the complementary research such as the ab-initio calculations and torque anisotropy measurements. It would also be interesting to do electron paramagnetic resonance experiments and optical measurements in order to see if there is magnetoelectric response at much higher frequencies making it possible choice for the application as a fast magnetoelectric.


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