Research Group for Condensed Matter Theory
Group Leader : Michiyasu Mori
Members : Hiroaki Onishi, Katsunori Kubo, Hiroto Adachi, Bo Gu, Jun'ichi Ieda,Mamoru Matsuo, Takanori Sugimoto, Zhuo Xu, Yuta Yamane, Yuichi Ohnuma

We study new functional materials emerging from theelectrons’ internal degrees of freedom (spin, charge and orbital)
and the electron correlation. New principles and new functionsof devices are also our important targets. In this fiscal year, wefound a new principle of magnetic power inverter by magnetic
domain wall motion [1]. This makes it possible to generate ACvoltages from DC magnetic fields. Furthermore, we proposedthat domain wall motion in a perpendicularly magnetizednanowire could lead to a high stability of spinmotive force
under strong magnetic fields [2]. Details of these two results aresummarized in Research Highlights. Progresses on a
ferromagnetic Josephson junction with magnetic domain walland spin Seebeck effect in ferrimagnets are reported below.
Towards precise measurement of oscillatory domain wall by
ferromagnetic Josephson junction
Nano-scale magnetic materials are extensively studied forspintronics devices due to many advantages such as enhanced
operation speed, low power consumption, and high integrationof memory cell. Non-volatile memory using a magnetic domain
wall (DW) is one example of such devices, and many studies are
devoted to control the DW. Hence, it is worth developing amethod for precise measurement of DW motion.
One example of precise measurements is the Josephson effect.
Under irradiation of microwaves to the Josephson junction, in
which two superconductors (SCs) are separated by an insulator,
the current-voltage (I-V) curve shows step structures at V =n(ℏ/2e)ν with microwave frequency ν, integer n, and the ratio of
the Plank constant and the elementary charge ℏ/e. This structure,
which is called Shapiro step, is adopted to the voltage standard
around the world, since the voltage can be determined in theorder of 10−9 accuracy by the precise values of ν and ℏ/e. Hence,
one can say that the Josephson junction has potential for precise
measurement in principle.
We considered the ferromagnetic Josephson junction (FJJ) as
shown in Fig. 1, in which a ferromagnet with a magnetic
domain wall is inserted between two SCs [3]. The I-V curve istheoretically derived and is found to show stepwise structures
only when the DW oscillates in the FW. The voltage stepappears at V = n(ℏ/2e)ωDW with the DW frequency ωDW. Since
the voltage is determined in the order of 10−9 accuracy, ourresult provides a method to measure the oscillatory DW more

precisely than the present accuracy. This result can be applied tonot only the oscillation but also more general dynamics of theDW. Those possibilities will be examined in a forthcoming
work.
Spin Seebeck effect in antiferromagnets and compensatedferrimagnets
Electric power generation by a temperature gradient (Seebeckeffect) is one of important issues in condensed matter physics,
since such a thermoelectric device possesses fascinating
properties necessary for future energy resources. The spin
Seebeck effect (SSE) can also generate electricity, more
importantly, by a simple setup of device such as a metal on topof a ferromagnet with a temperature gradient (See Fig. 2). In
general understanding, the spin polarized current (spin current)
is injected from the ferromagnet to the metal by the temperature
gradient, and then, the inverse spin Hall effect converts the spincurrent into the usual charge current. In this way, one can obtainelectricity from thermal energy in ferromagnets by the SSE.
Here, one question arises: Does the SSE occur in an
antiferromagnet and a ferrimagnet? A certain class offerrimagnets has a compensation temperature, at which themagnetization disappears even below a ferromagnetic transition
temperature.
We theoretically showed that the SSE vanishes inantiferromagnets, whereas it always survives in ferrimagnets
even at the compensation temperature, despite the absence of its
saturation magnetization or total spin [4]. It is found that nondegeneracy of magnetic excitations (magnons) in low energy is
the origin for the persistence of the SSE in the ferrimagnets. The
compensation temperature is useful for constructing a spincurrent device, since the leakage magnetic-flux is suppressedand does not disturb the spin and charge currents. Such asituation will be realized in Er3Fe5O12 as an example.

Fig.2 Schematic of the device setup for the spin Seebeck effect(SSE). The spin polarized current (spin current) is injectedfrom the ferromagnet to the metal, where the spin current isconverted to the charge current by the inverse spin Hall effect.
The blue arrows in the ferromagnet indicate the precessing
magnetic moment.

Fig.1 Schematic of the ferromagnetic Josephson junction, which iscomposed of two superconductors (SCs) separated by aferromagnet (FM) with a magnetic domain wall (DW). Thecurrent (I) goes through the junction as denoted by arrows.
The red arrows in the FM indicate the magnetic moment.

References[1] J. Ieda et al., Appl. Phys. Lett. 100, 152402 (2012).
[2] Y. Yamane et al., Appl. Phys. Lett. 100, 162401 (2012).
[3] S. Hikino et al., Appl. Phys. Lett. 100, 152402 (2012).
[4] Y. Ohnuma et al., Phys. Rev. B 87, 014423 (2013).