Fast domain wall motion in the vicinity of the angular momentum compensation temperature of ferrimagnets

Abstract

Antiferromagnetic spintronics is an emerging research field which aims to utilize antiferromagnets as core elements in spintronic devices 1,2 . A central motivation toward this direction is that antiferromagnetic spin dynamics is expected to be much faster than ferromagnetic counterpart because antiferromagnets have higher resonance frequencies than ferromagnets 3 . Recenttheories indeed predicted faster dynamics of antiferromagnetic domain walls (DWs) than ferromagnetic DWs 4-6 . However, experimental investigations of antiferromagnetic spin dynamics have remained unexplored mainly because of the immunity of antiferromagnets to magnetic fields. Furthermore, this immunity makes field-driven antiferromagnetic DW motion impossible despite rich physics of field-driven DW dynamics as proven in ferromagnetic DW studies. Here we show that fast field-driven antiferromagnetic spin dynamics is realized in ferrimagnets at the angular momentum compensation point TA. Using rare-earth-3d-transition metal ferrimagnetic compounds where net magnetic moment is nonzero at TA, the field-driven DW mobility remarkably enhances up to 20 km s −1 T −1 . The collective coordinate approach generalized for ferrimagnets 7 and atomistic spin model simulations 6,8 show that this remarkable enhancement is a consequence of antiferromagnetic spin dynamics at TA. Our finding allows us to investigate the physics of antiferromagnetic spin dynamics and highlights the importance of tuning of the angular momentum compensation point of ferrimagnets, which could be a key towards ferrimagnetic spintronics.Encoding information using magnetic DW motion is essential for future magnetic memory devices, such as racetrack memories 9,10 . High-speed DW motion is a key prerequisite for making the racetrack feasible. However, velocity breakdown due to the angular precession of DW, referred to as the Walker breakdown 11 , generally limits the functional performance in ferromagnet-based DW devices.Recently, it was reported that the DW speed boosts up significantly in antiferromagnets due to the suppression of the angular precession 4-6 . However, the immunity of antiferromagnets to magnetic fields yields notorious difficulties in creating, manipulating, and detecting antiferromagnetic DWs, compared to ferromagnetic ones. One possibility to avoid these difficulties is offered by the synthetic

Current-induced domain-wall motion in patterned nanowires with various Gd–Fe compositions for magneto-optical light modulator applications

“…Kim et al have already shown that the DW velocity driven by magnetic field was significantly higher at the AMC point than that for the MC with transition metal rare-earth alloy (Gd-Fe-Co), and the AMC composition was transition metal (Co-Fe) richer than the MC composition. 27) It is quite reasonable to explain our results using the same argument, even though the driving force is spin-transfer torque generated by current.…”

Unveiling domain wall dynamics of ferrimagnets in thermal magnon currents: Competition of angular momentum transfer and entropic torque

“…The conceptual idea behind Eqs. (8) and (9) is that the dynamics of a FI can be viewed as an effective FM with angular momentum given by l fu -a model which has recently been employed in a similar fashion by Kim et al [49] for field-driven FI-DW motion. Such a model should be valid for the low wall velocities in thermal gradients; although some deviations might occur close to T A since we do not take into account inertial effects proportional toZ, andΦ [48,66].…”

“…Thus, very close to this point we expect some deviations in the wall velocity and precession which we derive from the ferromagnetic model here in this section. Though qualitatively, this singular behavior accelerates the spin dynamics around T A and is the reason why FIs with angular momentum compensation points are becoming so relevant for applications and functionalities related to the speed of the spin dynamics [49,51,54]. Additionally, while the micromagnetic exchange stiffness and its temperature dependence for FMs and AFMs is somehow known [71], the specifics of A eff in FIs remains an open problem-especially at elevated temperatures.…”

“…So-called "pure" AFMs, such as NiO for instance, in which there is no net-magnetization in bulk, require more sophisticated means of excitation [47,48]. FIs can be seen as a generalization of both systems, FMs and AFMs, since one may selectively tune the relevant magnetic properties by modifying for instance the sample temperature or composition [49,50]. This allows for an enhanced control of the ferromagnetic-or antiferromagnetic-like character of the spin dynamics and enables to potentially exploit the characteristically fast spin-dynamics of an AFM [49][50][51], while at the same time one can easily manipulate them by using magnetic fields, and measure it by conventional detection methods such as the magneto-optical Kerr effect (MOKE) or x-ray magnetic circular dichroism (XMCD).…”

“…the properties of thermal magnon currents strongly depend on the underlying microscopic spin structure [52,53]. Thus, DW motion in FI driven by temperature gradients has been scarcely investigated so far [6] and previous works on DW motion in FIs (and synthetic AFMs) were focused on more controllable stimuli, such as electric currents [51,54] and magnetic fields [49].…”