Periodic Reporting for period 1 - SPARTACUS (Scenarios and Principles for Antiferromagnetic Recording: taming spins coherently and ultrafast)
Période du rapport: 2022-11-01 au 2025-04-30
Magnetism offers a natural and cheap means to store digital information. Due to many breakthroughs in fundamental research, magnetic recording technology has been developing in a breath-taking fashion during the last 50 years, presently delivering Terra (10^12) bits per cm2 at less than a nanocent per bit. Aiming to decrease the recording time of a single bit has boosted the interest in fundamental studies of ever-faster magnetization dynamics. While recent developments in photonics enable nearly lossless data transfer with speeds exceeding 1 Tb/s, current magnetic data storage cannot keep up with these data-flow rates and already now data centres are becoming the biggest consumers of electricity world-wide. In principle, two metastable states of a magnet have equal entropies and equal energies and according to (quasi-equilibrium) thermodynamics, a switching between these states can be realized even with zero production of heat. However, in this case, the switching must be a reversible process which takes an infinitely long time . Therefore within thermodynamics, ultrafast and least dissipative magnetic switching seem to be mutually exclusive. Similarly to classical mechanics of an object with velocity v, energy dissipated per unit time as a result of friction is proportional to v^2 and an increase of the velocity is accompanied by even faster increase of dissipations. Hence the modern magnetic data storage faces a fundamental dilemma between the time and energy dissipation for the recording of a single bit. The fundamental obstacles and the need to inspire the next generations of magnetic technology urge to develop radically new approaches for ultrafast and low-dissipative magnetic switching.
Antiferromagnets represent a highly-promising playground for the quest of the fastest and the least-dissipative mechanism of data storage. However, despite the 60-years long search for thermodynamic conjugates to the antiferromagnetic order parameter, efficient means to control antiferromagnetism is still a hot topic and their lack is the main reason that hampers applications of antiferromagnets and further development of antiferromagnetic spintronics, magnonics and data storage, in particular.
The objectives of SPARTACUS are
- to generate the strongest ever ultrafast stimulus for antiferromagnets through constructive interference of several independent channels to control spins (electronic pumping, coherent phononic pumping and direct action of THz magnetic fields);
- to achieve ultrafast switching of spins in antiferromagnets between stable states and thus to challenge the fundamental limits on energy dissipation and speed for writing magnetic bits;
- to demonstrate THz writing rates on antiferromagnetic storage media and thus shift the paradigm of data storage from ferromagnetism to antiferromagnetism.
SPARTACUS will achieve its objectives through fundamental research of the following questions:
1. Which electronic or phononic excitations enable the most efficient, least dissipative and the fastest control of spins in real antiferromagnets?
2. What kind of spin dynamics is triggered by resonantly driving electronic, phononic and magnonic excitations and combinations thereof in real antiferromagnets?
3. What are the critical energy dissipations leading to an increase of entropy during ultrafast switching of antiferromagnets via coherent or/and strongly non-equilibrium states?
4. What is the optimal route to establish the fastest possible and the least dissipative antiferromagnetic switching?
Antiferromagnets represent a highly-promising playground for the quest of the fastest and the least-dissipative mechanism of data storage. However, despite the 60-years long search for thermodynamic conjugates to the antiferromagnetic order parameter, efficient means to control antiferromagnetism is still a hot topic and their lack is the main reason that hampers applications of antiferromagnets and further development of antiferromagnetic spintronics, magnonics and data storage, in particular.
The objectives of SPARTACUS are
- to generate the strongest ever ultrafast stimulus for antiferromagnets through constructive interference of several independent channels to control spins (electronic pumping, coherent phononic pumping and direct action of THz magnetic fields);
- to achieve ultrafast switching of spins in antiferromagnets between stable states and thus to challenge the fundamental limits on energy dissipation and speed for writing magnetic bits;
- to demonstrate THz writing rates on antiferromagnetic storage media and thus shift the paradigm of data storage from ferromagnetism to antiferromagnetism.
SPARTACUS will achieve its objectives through fundamental research of the following questions:
1. Which electronic or phononic excitations enable the most efficient, least dissipative and the fastest control of spins in real antiferromagnets?
2. What kind of spin dynamics is triggered by resonantly driving electronic, phononic and magnonic excitations and combinations thereof in real antiferromagnets?
3. What are the critical energy dissipations leading to an increase of entropy during ultrafast switching of antiferromagnets via coherent or/and strongly non-equilibrium states?
4. What is the optimal route to establish the fastest possible and the least dissipative antiferromagnetic switching?
We have performed a literature study, prepared and published a manuscript on the physics of optical control and detection of the antiferromagnetic Néel vector in altermagnets and beyond (A. V. Kimel, B. I. Ivanov, Th. Rasing, JMMM 598, 172039 (2024)).
We show that the regime of ultrafast toggle switching can be also realized via a mechanism without relying on heat. This new regime of ‘cold’ toggle switching can be observed in ferrimagnets without a compensation point and over an exceptionally broad temperature range. The control of magnetic anisotropy required for the toggle switching exhibits reduced dissipation compared to laser-induced-heating mechanism, however the dissipation and the switching-time are shown to be competing parameters (T. Zalewski et al, Nature Communications 15, 4451 (2024)).
We have studied the effect of metasurfaces on the efficiency of excitation of spin resonances in antiferromagnetic YFeO3 and yttrium iron garnet Y3Fe5O12. In particular, using the magneto-optical Faraday effect as a probe, we experimentally demonstrate that due to the metasurface the electromagnetic field, otherwise described by plane waves, acquires an out-of-plane magnetic field component and resonantly enhances the field at the frequency of the Gd-Fe spin resonance in the ferrimagnet.
We showed that a coherent magnonic state can substantially change the properties of an antiferromagnet, enabling a new nonlinear path of controlling spins by a pair of THz pulses. The effect is analogous to electronic or ionic Raman scattering, but involves exclusively magnonic excitations and can be thus called magnonic Raman scattering or THz-mediated magnon-magnon coupling. Our work shows that although the efficient control of antiferromagnetism in thermodynamic equilibrium is still a challenge, the problem can be solved by pushing antiferromagnets into a nonequilibrium state where the susceptibility of spins to an external magnetic field is boosted (T. G. H. Blank et al, Phys. Rev. Lett. 131, 096701(2023)).
We showed that a single-cycle terahertz electric field triggers in the topological antiferromagnet MnBi2Te4 strongly anharmonic lattice dynamics, which initiates a light-mediated interaction between otherwise noninteracting phonons. (T. G. H. Blank et al, Phys. Rev. Lett. 131, 026902 (2023)).
We reported on a new regime of magnon-phonon dynamics, which in the vicinity of the Fermi resonance condition, facilitates a mutual, anharmonic energy exchange between magnons and phonons. (T, W. J. Metzger et al, Nature Communications 15, 5472 (2024)).
We showed that the spontaneous magnetization gained temporarily by means of the ultrafast Barnett effect, through the resonant excitation of circularly polarized optical phonons in a paramagnetic substrate, can be used to permanently reverse the magnetic state of a heterostructure mounted atop the said substrate. With the handedness of the phonons steering the direction of magnetic switching, the ultrafast Barnett effect offers a selective and potentially universal method for exercising ultrafast non-local control over magnetic order (C. S. Davies et al, Nature 628, 540–544 (2024)).
We show that the regime of ultrafast toggle switching can be also realized via a mechanism without relying on heat. This new regime of ‘cold’ toggle switching can be observed in ferrimagnets without a compensation point and over an exceptionally broad temperature range. The control of magnetic anisotropy required for the toggle switching exhibits reduced dissipation compared to laser-induced-heating mechanism, however the dissipation and the switching-time are shown to be competing parameters (T. Zalewski et al, Nature Communications 15, 4451 (2024)).
We have studied the effect of metasurfaces on the efficiency of excitation of spin resonances in antiferromagnetic YFeO3 and yttrium iron garnet Y3Fe5O12. In particular, using the magneto-optical Faraday effect as a probe, we experimentally demonstrate that due to the metasurface the electromagnetic field, otherwise described by plane waves, acquires an out-of-plane magnetic field component and resonantly enhances the field at the frequency of the Gd-Fe spin resonance in the ferrimagnet.
We showed that a coherent magnonic state can substantially change the properties of an antiferromagnet, enabling a new nonlinear path of controlling spins by a pair of THz pulses. The effect is analogous to electronic or ionic Raman scattering, but involves exclusively magnonic excitations and can be thus called magnonic Raman scattering or THz-mediated magnon-magnon coupling. Our work shows that although the efficient control of antiferromagnetism in thermodynamic equilibrium is still a challenge, the problem can be solved by pushing antiferromagnets into a nonequilibrium state where the susceptibility of spins to an external magnetic field is boosted (T. G. H. Blank et al, Phys. Rev. Lett. 131, 096701(2023)).
We showed that a single-cycle terahertz electric field triggers in the topological antiferromagnet MnBi2Te4 strongly anharmonic lattice dynamics, which initiates a light-mediated interaction between otherwise noninteracting phonons. (T. G. H. Blank et al, Phys. Rev. Lett. 131, 026902 (2023)).
We reported on a new regime of magnon-phonon dynamics, which in the vicinity of the Fermi resonance condition, facilitates a mutual, anharmonic energy exchange between magnons and phonons. (T, W. J. Metzger et al, Nature Communications 15, 5472 (2024)).
We showed that the spontaneous magnetization gained temporarily by means of the ultrafast Barnett effect, through the resonant excitation of circularly polarized optical phonons in a paramagnetic substrate, can be used to permanently reverse the magnetic state of a heterostructure mounted atop the said substrate. With the handedness of the phonons steering the direction of magnetic switching, the ultrafast Barnett effect offers a selective and potentially universal method for exercising ultrafast non-local control over magnetic order (C. S. Davies et al, Nature 628, 540–544 (2024)).
- Unique experimental apparatus allowing pumping of antiferromagnets with multiple laser pulses in an exceptionally broad spectral range has been designed, installed and presently tested.
- The concept of the Fermi resonance heavily studied in molecular physics unexpectedly appeared to be relevant for our studies of spin dynamics in antiferromagnets (T. W. J. Metzger et al, Nature Communications 15, 5472 (2024).
- T. G. H. Blank et al , Phys. Rev. Lett. 131, 096701 (2023) significantly advances the field beyond the state of the art showing, in particular, that spin excitations in antiferromagnets are intrinsically non-linear and THz control of spins with ultrashort pulses can easily result in the situations, when 1+1>2
- The concept of the Fermi resonance heavily studied in molecular physics unexpectedly appeared to be relevant for our studies of spin dynamics in antiferromagnets (T. W. J. Metzger et al, Nature Communications 15, 5472 (2024).
- T. G. H. Blank et al , Phys. Rev. Lett. 131, 096701 (2023) significantly advances the field beyond the state of the art showing, in particular, that spin excitations in antiferromagnets are intrinsically non-linear and THz control of spins with ultrashort pulses can easily result in the situations, when 1+1>2