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Antiferromagnetic spintronics investigated by quantum sensing techniques

Periodic Reporting for period 1 - ASIQS (Antiferromagnetic spintronics investigated by quantum sensing techniques)

Reporting period: 2019-01-01 to 2020-12-31

An increasing interest in the antiferromagnets was boosted in 2016 by the discovery of electric Néel vector manipulation, which is of a great promise for data storage technologies. In antiferromagnets, information can be stored within antiferromagnetic domains, which can be controlled by spin-orbit torques and read by magnetoresistance effect. Since this pioneering discovery, a plethora of materials controlled by different mechanisms based on electric current have been proposed and demonstrated. Interestingly, due to the difficulty to directly image the antiferromagnetic state, some of the strongest reported effects remain controversial. However, while many laboratories focused on the development of these materials and schemes for data storage, research on synthetic antiferromagnets for computational purposes stayed out of the spotlight. This is caused mainly by the fact that such a functionality was not demonstrated even in the conventional ferromagnets. The discovery of chiral coupling in thin ferromagnetic films with tailored anisotropies obtained in the course of this project opened avenues to antiferromagnetically couple neighboring out-of-plane magnetic regions via the interfacial Dzyaloshinskii-Moriya interaction and realize all-electric magnetic logic devices.

At the start of this project, we identified the possibility to build logic gates employing the chiral coupling effect in antiferromagnetically coupled regions of ferromagnetic films, which in combination with the spin-orbit-torques offer a unique way of all-electric control. The core of our logic-circuits is a NOT gate, which comprises two out-of-plane magnetized regions, which serve also as domain wall conduits, which are chirally antiferromagnetically coupled via a narrow in-plane magnetized region. The polarity of a domain wall (e.g. up-down) electrically driven across such a region can be therefore inverted (to down-up). This concept has been extended towards more complex logic circuits and functionalities.

While the domain wall logic has been shown in prototypical films based on Co, other attractive materials would enable further functionalities such as reduced electric currents or higher domain wall mobilities or their facilitated implementation into existing industrial processes. In the second part of the project, we have therefore developed a scheme based on asymmetric domain wall motion, which can be used to quantify the coupling in any material. We have shown that an equally efficient coupling mechanism can be realized in CoB or GdCoB thin films.

In the third part of the project, we have explored the possibility to establish a strong coupling in out-of-plane magnetized ferrimagnets. These are especially interesting near their compensation temperature where they become effectively antiferromagnetic and thus offer similar attractive properties as antiferromagnets. Crucially, they can be imaged by conventional laboratory techniques. Similarly to the spatially patterned anisotropies in the ferromagnets, we have spatially patterned material properties by means of ion irradiation so that regions with compensation temperatures below and above room temperatures were created. This, so far, revealed a strong novel coupling mechanism allowing to antiferromagnetically couple neighboring ferrimagnetic regions, which can be embedded into logic circuits. This work can potentially offer unprecedented functionalities.
The results obtained during the project cover three main topics:

1. The demonstration of in-memory computation using chirally coupled synthetic antiferromagnets;
2. The development of a versatile method to measure chiral coupling;
3. The demonstration of strong coupling in compensated ferrimagnets.

1. The results obtained for the first topic pertain to the demonstration of the current-driven domain wall logic. Our concept for chiral magnetic domain-wall logic takes advantage of the fast current-driven domain wall motion and strong chiral coupling in Pt/Co/Al heterostructures. Kerr microscopy, magnetic force microscopy and scanning transmission electron microscopy were employed to directly track the magnetic state evolution following the stimuli of electric current pulses in the developed devices. The devices encompass NOT, NAND and NOR logic gates, which were used to build XOR and full-adder logic gates. These gates can be used to build any complex logic circuit used in arithmetic logic unit of each computer processor. These experiments were also complemented by all-electric measurements to demonstrate full-functionality aiming towards applications. Micromagnetic simulations have been employed to confirm the concepts and to provide deeper insight into the dynamic-coupling mechanism.

Our work constitutes a significant breakthrough in the field of magnetic recording since it opens up the possibilities to use the magnetic storage devices also as processors. (Z. Luo, et al., Nature 579, 214–218 (2020); Hrabec, et al., Applied Physics Letters 117, 130503 (2020)).

Further dissemination was achieved through:
- Invited talk at international conference (RIEC, Sendai, Japan), regular talk at international conference (Skymag, Paris (2020), cancelled due to Covid) and by invited seminars: Ceitec, Brno (2019); Czech Academy of Science; Prague (2020); Laboratoire de Physique des Solides, Orsay (2020).
- Outreach events at TecDays to the high school students
- Press releases.
- A scientific illustration featured on the Cover page of Swiss bulleting ‘SPG Mitteilungen’ (2019).

2. The results on this topic return to the need to quantify the developed chiral coupling in thin magnetic films. This is an essential prerequisite to develop new materials and further optimize the coupling strength. We have created a system using a domain wall biased by the electric chiral coupling where we evaluate the asymmetry of the field- and current-driven domain wall dynamics. By doing so, we have extended the family of potential magnetic films to low-pinning materials. (Z. Liu, et al., under preparation (2021)).

3. The results on this topic concern the demonstration of strong coupling mechanism in compensated ferrimagnetic (antiferromagnetic) films. The ultimate goal is to develop highly energy efficient logic devices inspired by those developed in 1) while taking the advantage of fast antiferromagnetic dynamics and immunity to the parasitic magnetic fields. To achieve the coupling, we have used the possibility to tailor the properties of thin ferrimagnets by means of He irradiation. By doing so, we have mainly modified the compensation temperature, i.e. the temperature at which the ferrimagnets becomes effectively antiferromagnets. This work, which yielded very promising first results, will be continued beyond the duration of the ASIQS project.
Impact of the research:
- The discovery of the two novel coupling mechanism used in topic 1 and in 3 opens the possibility to design artificial lateral structures based on nearest-neighbor antiferromagnetic coupling. We expect that the discovery of the couplings will stimulate fundamental research as well as work towards technologically-relevant applications.
- The demonstration of the electric driven domain wall logic has a direct impact on the magnetic storage industry since it significantly expands the possibilities of the device functionality. This is supported by the submitted patent application: EP20161352.8 (2020).
Chirally coupled nanomagnets (Photo: Markus Fischer, Paul Scherrer Institut)