Research Group for Mechanical Control of Materials and Spin Systems
Group Leader: Eiji Saitoh
Members: Satoru Okayasu, Masao Ono, Hiroyuki Chudo, Kazuya Harii, Ryo Takahashi,Hiroshi Yasuoka

The research objectives of Mechanical Control of Materials
and Spin System are to develop new methods for controllingspin currents by combining electron spins and mechanicalrotation, and/or by coupling spins and nuclear magnetic
resonance (NMR) techniques, and to expand a new frontier ofspintronics. Spintronics is a subject of great current interest thatwill be essential in next-generation electronics. Spintronics
exploits an electronic charge and spin simultaneously so as toproduce novel functional and electronic devices. A central
concept of the spintronics is a flow of an electronic angularmomentum called spin current. In our research group, amechanical angular momentum and nuclear angular momentum
are focused on as new angular momentum source interactingwith spin currents.
Barnett effect of gadolinium in a paramagnetic state
The Barnett effect is a phenomenon that a rotating object ismagnetized. The effect was discovered in 1915 [1, 2]. We report
the first observation of the Barnett effect in a paramagnetic state
by mechanically rotating a gadolinium. We developed amagnetic measurement setup comprised of a high-speed rotation
system and a fluxgate magnetometer for the measurement (Fig.
1 left panel). The right panel of Fig. 1 shows the rotationalfrequency dependence of the magnetization of the gadolinium
sample at 300±0.5 K and that of a blank capsule. We estimate
the magnetization of the rotating sample, M Ω , from the strayfield measured by the fluxgate magnetic sensor using a dipolemodel. We find that the magnetization is proportional to therotational frequency and its polarity changes with the rotation
direction. For the blank capsule, no rotation frequency anddirection dependence are observed. Thus, the magnetizationarises from the rotating Gd sample.

Nuclear magnetic resonance (NMR) for a I=1/2 system in an
external magnetic ﬁeld and mechanical rotation
NMR signals are measured under the synchronous rotation ofa sample and a detector coil where the rotation axis isperpendicular to the external ﬁeld. We found that NMR line ofspin 1/2 nuclei system (31P in H 3 PO 4 ) splits into two lines
despite the system having a single Zeeman level separation andthe splitting is given by twice the angular velocity of the rotation(Fig. 2). We analytically calculated the motion of a nuclear spinin mechanical rotation based on quantum mechanics, andshowed that the line splitting originates from double precession
in the nuclear spin motion and that the splitting is determined byonly the angular velocity of the rotation without any material
parameters. The result implies that measuring the line splitting
can be utilized for precise determination of mechanical rotation
frequency [3].

Fig. 2 Resonance spectra at various angular velocities. The redlines are the experimental results. The origin is defined as
the center frequencies of the lines. The black triangles
indicate resonance peak positions. The unconventionalline splitting appears at Ω/2π = 1.5 and 3 kHz.

Fig.1 (Left panel) The experimental setup for observation of theBarnett effect.
(Right panel) Rotational frequency dependence of themagnetization observed at 300±0.5K for the Gd sample
and the blank capsule. Each data point is averaged overthree measurements with the error bar in the standard
deviation 1σ including the rotational frequency fluctuation.

References[1] S. J. Barnett, Phys. Rev. 6, 239-270 (1915).
[2] S. J. Barnett, Rev. Mod. Phys. 7, 129 (1935).
[3] K. Harii et al., Jpn. J. Appl. Phys. 54, 050302 (2015).