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New spintronic components based on antiferromagnetic materials

Théophile Chirac 1
1 LNO - Laboratoire Nano-Magnétisme et Oxydes
SPEC - UMR3680 - Service de physique de l'état condensé, IRAMIS - Institut Rayonnement Matière de Saclay
Abstract : Current magnetic memory devices are reaching their physical limits in terms of stability, speed and power consumption as the race to miniaturization intensifies. The emergent research field of spintronics studies the collective behavior of spins in matter and their interplay at interfaces, to find new avenues in terms of materials, architectures and stimulation sources. A particularly promising group of materials are the antiferromagnets. These abundant magnetically ordered materials are naturally stable, robust, ultra-fast and compatible with insulator electronics. Indeed, most transition metal oxide compounds are antiferromagnetic insulators, have resonance in the terahertz range and flop fields of tens of teslas. They can also be semi-metals, metals, semiconductors, superconductors or multiferroics. This thesis focuses on two antiferromagnets: nickel oxide (NiO) and bismuth ferrite (BiFeO₃). NiO is the archetypical antiferromagnet at ambient temperature with a simple crystalline structure. Using dynamical atomistic simulations, I show that this compound can be the elemental brick of a three state memory device controlled by currently available pulses of spin currents, with a picosecond response time. The simulations also explain the formation of chiral structures in BiFeO₃, a ferroelectric antiferromagnet with magnetoelectric coupling between the two orders. In a second part, antiferromagnetic domains in BiFeO₃ are experimentally observed using second harmonic generation of light, with a sub-micron spatial resolution. Antiferromagnetic domains of BiFeO₃ are then excited by an intense femtosecond laser pulse, and the dynamics of the two coupled orders (antiferromagnetism and ferroelectricity) is studied with a sub-picosecond time resolution. Finally, the injection of spin current in an antiferromagnet such as BiFeO₃ or NiO is envisioned by characterizing the spin bursts generated by ultrafast laser-induced demagnetization of adjacent ferromagnetic layers.
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Théophile Chirac. New spintronic components based on antiferromagnetic materials. Condensed Matter [cond-mat]. Université Paris-Saclay, 2019. English. ⟨NNT : 2019SACLS482⟩. ⟨tel-02615261⟩

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