Fast domain wall motion in the vicinity of the angular momentum compensation temperature of ferrimagnets
Antiferromagnetic spintronics is an emerging research field which aims to utilize antiferromagnets as core elements in spintronic devices. A central motivation towards this direction is that antiferromagnetic spin dynamics is expected to be much faster than its ferromagnetic counterpart.
However, experimental investigations of antiferromagnetic spin dynamics have remained absent, mainly because of the magnetic field immunity of antiferromagnets.
By employing rare earth–3d-transition metal ferrimagnets, researchers in the Department of Physics at KAIST have recently successfully demonstrated the fast field-driven antiferromagnetic spin dynamics.
A magnetic domain wall (DW) is a boundary that divides oppositely directed magnetic domains and has received significant attention because its motion can be utilized to a shift register-based memory devices.
To compete with other technologies, attaining ultra-high speed DW motion is a prerequisite.
However, a velocity breakdown due to the angular precession of DW, referred to as the Walker breakdown, generally limits the functional performance in ferromagnet-based DW devices.
Such precessional motion is unavoidable in ferromagnets: it originates from the intrinsic spin angular momentum.
Antiferromagnets are another type of magnet, in which two sub-lattices are directed in opposite directions.
Since the total angular momentum of antiferromagnets is zero, it has been predicted that the angular precession of DW would be suppressed in antiferromagnets, leading to a faster DW speed.
This important notion further motivates the current field of antiferromagnetic spintronics.
However, the experimental demonstration of fast antiferromagnetic DW dynamics has been missing, because the field-driven antiferromagnetic DW motion is impossible due to the field-immunity of antiferromagnets.
Recently, a research team of the Physics department at KAIST, led by Prof. Kab-Jin Kim, has discovered that the seemingly impossible field-driven antiferromagnetic DW dynamics can be obtained in ferrimagnets containing both rare earth (RE) 3d-transition metal (TM) elements.
In “RE–TM” ferrimagnets, the spins of two inequivalent sub-lattices are coupled antiferromagnetically.
Because of different Landé g-factors between the RE and TM elements, these ferrimagnets have two special temperatures: the magnetization compensation temperature, TM, at which the two magnetic moments cancel each other; the angular momentum compensation temperature, TA, at which the net angular momentum vanishes.
At TA, the spin dynamics becomes purely antiferromagnetic, because the time evolution of the state of a magnet is governed by the commutation relation of the angular momentum, not of the magnetic moment.
On the other hand, a finite net magnetic moment at TA enables for an investigation of the field-driven dynamics.
Therefore, one can expect that the field-driven antiferromagneic spin dynamics is possible at TA. To demonstrate this idea, the research team has performed the DW motion experiment in GdFeCo ferrimagnets, in which Gd and FeCo are coupled antiferromagnetically.
They found that the DW speed is significantly enhanced on approaching TA, reaching a maximum speed as high as 2 km/s at TA, which is the much faster than that in the ferromagnet.
These studies highlight that that the fast field-driven antiferromagnetic spin dynamics is achievable at T = TA of ferrimagnets.
The researchers also show a fast DW speed near room temperature by tuning TA, opening a possibility for ultra-high-speed device operation at room temperature.
Furthermore, a sharp and narrow peak of DW speed at TA provides a simple but accurate method to determine TA, which has not previously been possible.
These results thus will provide important keys for the future realization of antiferromagnetic spintronics.
This study was performed in collaboration with researchers from Kyoto University, Nihon University, Korea University and UCLA. The results were recently published in Nature Materials in an advanced online publication (September 25, 2017)
Website: https://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4990.html (paper link)
* lab webpage : http://time.kaist.ac.kr/