New principle for significant electric-power saving in magnetic memories:
topology-based manipulation of magnets in spintronics

Y. Araki1), J. Ieda1), M. Yamanouchi2), T. Sakai2), T. Uemura2) and H. Ohta2)
1): Spin-energy science Gr., JAEA 2): Hokkaido Univ.

Magnetic memories store information by associating a binary
number (0 or 1) with the magnetization direction (North or Southpole) (Fig. 1 (a)). Developing magnetic memories with higher
operation speed and denser integrability requires us to reduce theelectric power consumption in manipulating magnetization
within nanometer-size domains. For this purpose, the techniquecalled “spin-transfer torque (STT)” has been commonly used fora decade or so, which manipulates the magnetization direction byapplying an electric current (Fig. 1(b)). However, current
injection in metals causes the Joule heating due to the electricresistivity. The waste of energy by the Joule heating is inevitablein the STT, which becomes a crucial problem in developing
highly integrated magnetic memories. To solve this problem, wehave carried out theoretical [1] and experimental [2] studies, anddiscovered a new mechanism for manipulating magnetization
with significantly lower power consumption relying on theprocess free from electric resistivity.
The key to this power-saving mechanism is the “topology”.
Topology is the character of the quantum-mechanical structure ofelectrons in materials. When an electric voltage is applied, anelectron having a nontrivial topological structure moves
perpendicular to the voltage. This behavior, known as the“anomalous velocity”, is unaffected by the electric resistivity anddoes not cause power consumption. In our theoretical work, wefound a new mechanism for manipulating magnetization with low
power consumption that is enabled by the anomalous velocity
from the topology [1]. We coined a term the “topologicalHall torque (THT)” for this mechanism, which can be
induced by applying an electric voltage instead of a current (Fig.
1(c)). In comparison with the conventional STT, the electric

Fig. 1 (a) Magnetic memories store digital information (0 or 1) bymanipulating the direction of magnetization (up or down, shownby red arrows). (b) The conventional technique called “spintransfer torque” uses an electric current to manipulatemagnetization, but it suffers from electric power consumption. (c)We discovered a new mechanism “topological Hall torque” basedon the electron topology, to manipulate magnetization with low
power consumption by applying an electric voltage.

current needed for manipulating magnetization by the THT
mechanism is about 1/100 times smaller. This means that theTHT can significantly reduce the electric power consumption bya factor of 1/10,000.
We verified our theoretical proposal of the THT by theexperimental measurement of a metallic ferromagnet, SrRuO3 [2].
We injected an electric current in the system and measured theinduced torque acting on the magnetization at various
temperatures. The efficiency of the torque per current was solarge that the theoretical estimation from the conventional STT
cannot be applied, and moreover, it also showed a nonmonotonic
two-peak structure depending on the temperature. Using atheoretical model calculation, we have clarified that both themagnitude and temperature dependence of the measured
efficiency can be understood in terms of the THT (Fig. 2). The
THT in this system is described by the anomalous velocity arisingfrom the topological structure known as “Weyl points” in theelectronic bands of SrRuO3.
Our new findings of the topology-based principle formagnetization manipulation indicate a decisive role of topology
in spintronics. By selecting and designing materials with a strong
topological structure of electrons, our idea of the THT will be
helpful in developing future magnetic memories operating withsignificantly less power consumption.
These works were partially supported by KAKENHI (No.
19H05622) and the Leading Initiative for Excellent YoungResearchers (LEADER).
References[1] Y. Araki and J. Ieda, Phys. Rev. Lett. 127, 277205 (2021).
[2] M. Yamanouchi, Y. Araki, T. Sakai, T. Uemura, H. Ohta, andJ. Ieda, Sci. Adv. 8, eabl6192 (2022).

Fig. 2 Schematic pictures of the experimental verification of the newly
found principle. The energy bands of electrons in SrRuO3 show
the topological structure called “Weyl points” (top panel). By
measuring the efficiency for manipulating magnetization in thismaterial, it showed the two-peak structure depending on thetemperature (red line in the bottom panel). This behavior isconsistent with the estimation by our theory of the topologicalHall torque based on the Weyl point structure (blue line in thebottom panel).