Research Group for Spin-Energy Transformation ScienceGroup Leader: Eiji Saitoh
Members : Michiyasu Mori, Hiroaki Onishi, Jun’ichi Ieda, Mamoru Matsuo, Bo Gu, Zhuo Xu,Yuichi Ohnuma, Satoru Okayasu, Masao Ono, Hiroyuki Chudo, Yudai Ogata, Ryo Takahashi,
Masaki Imai

Spin is the angular momentum and the source of magnetization.
Its rotational motion is limited in one direction under a magnetic
field. Our strategy is to utilize this special nature of spin toenhance energy efficiency, which is the key for sustainability ofour society. Spin Seebeck effect, spin motive force, rectificationof heat flow are our main targets. In addition, the special natureof spin leads to a new principle to control mechanical rotationand vibration. In this fiscal year, the Barnet effect has beenstudied in several rear-earth metals [1]. This subject is explainedin Research Highlights. Other two interesting results are statedbelow.
Spin Seebeck Effect by Spinon Spin Current
Spin Seebeck effect is observed as thermoelectric generation
using a device of metal and magnet films. So far various
ferromagnets have been examined as the magnetic films.
Temperature gradient applied to the device produces
magnetically polarized current (spin current) in the ferromagnet.
The spin current is injected into the metallic film and isconverted into the charge current by the inverse spin Hall effect.
In a usual insulating ferromagnet, the spin current is carried bymagnons. On the other hand, low-dimensional quantum spin
systems do not show long-ranged order and their magnetic
excitations are described by spinons, which are topologicalexcitations in the spin liquid state, i.e., highly fluctuating stateof spins. What is the difference between magnons and spinons?
We successfully observe the spin Seebeck effect by the spinons
using the device of Pt and Sr2CuO3. Figure 1 (a) shows
schematic figure of quantum spin chain composed of spin-1/2,
which is localized on Cu surrounded by four oxygens. Figure 1(b) shows our device geometry. The injected spin current Is in Pt
is converted to the charge current and leads to voltage V
perpendicular to Is. As expected, V is observed only when atemperature gradient ∇T is parallel to the spin chain as shownin Fig. 1 (c). This is the evidence of spinon spin Seebeck effect.
In particular, V is enhanced around 20 K, where the spinon
picture works well. Surprisingly the sign of voltage is negativeand is opposite to the spin Seebeck effect using usual
ferromagnet. The sign difference is attributed to a symmetry ofspinon band.
Spin Current in a Half-metallic Ferromagnet
Another way to generate a spin current is injection of a charge
current into a ferromagnetic metal, in which the charge current
is magnetically polarized. However, the charge current is always
accompanied by the Joule heating. Hence, the spin current
without charge current is called pure spin current and isexpected to be a potential candidate as spin injector of magnetic
memories. In this case, the key is the polarization offerromagnetic metal defined by the ratio of n↑-n↓ and n↑+n↓with densities of up-spin n↑ and down-spin n↓. Idealistically, n↑=1 and n↓=0 or vice versa is the best to effectively generate
the spin current. This is called half-metallic ferromagnet (HMF)
as shown in Fig. 2 (a), in which the up-spin band is metallic andthe down-spin band is insulating. However, the pure spin current
is impossible in Fig. 2 (a), since the pure spin current iscomposed of angular momentum 1. Here, we remember that theHFM is ferromagnet and has the Goldstone-mode associatedwith the spinwave. This spin fluctuation must exist in the HFM
and leads to the smeared gap as shown in Fig. 2 (b). This isapparently the correlation effect of electrons in the HFM. We
study the spin Hall effect in the HFM using Kubo formula
assuming some extrinsic mechanism called side-jump and skew
scatterings. It is found that the spin Hall conductivity (SHC) isproportional to T2/3. This SHC sensitive to a temperature T canbe a tool to study the minority-spin state, which is rather
difficult to detect by other methods.
References[1] Y. Ogata et al., Appl. Phys. Lett. 110, 072409 (2017).
[2] D. Hirobe et al., Nature Physics 13, 30–34 (2017).
[3] Y. Ohnuma et al., Phys. Rev. B 94, 184405 (2016).

(b)(a)

Fig.1 (a) Schematic figureof spin chain in Sr2CuO3.
(b) Schematic figure oflongitudinal spin Seebeck
device. By the inverse spin
Hall effect, the spincurrent Is generated by atemperature gradient isconverted to the electricfield ESHE ∝ σ × Is. (c)ObservedvoltageV
divided by magnitude ofmagnetic field B for
temperature gradient ∇ Tparallel(blue)
andperpendicular (green) to
the spin chain.

(c)Fig.2 Schematic figures of density of states in a halfmetallic ferromagnet; (a) without spin fluctuations and (b)with spin fluctuations [3].