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

Spintronics is a subject of great current interest that will beessential in next-generation electronics. Spintronics exploits anelectronic charge and spin simultaneously so as to produce
novel function and electronic property 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.
Observation of Barnett fields in solids by nuclear magnetic
resonance
Angular momentum conversion between spins andmechanical rotation is a promising candidate of a mechanism for
micro driving devices. Fundamental phenomena of the angularmomentum conversion are the Einstein-de Haas effect (bodyrotation due to its magnetizing) [1], and the Barnett effect(magnetizing by body rotation) [2]. The Barnett effect implies
that an effective magnetic field arises in a rotating body. The
emergent field B, called Barnett field, can be derived
theoretically from rotational coordinate transformation into thebody-fixed frame. B acting on a particle is linearly depends on
the angular velocity of rotation :,
where m, q, and g are mass, charge and g-factor of the particle.
To measure the Barnett field by nuclear magnetic resonance(NMR) method, the detection has to be done on the rotating
frame same as the body. The reason for this is that, if there arerelative velocity between the signal detector and the body(signal emitter), an extrinsic NMR frequency shift arises fromthe relative velocity (rotational Doppler effect). To overcome

Figure 1 shows the schematic illustration of theexperimental assembly. The assembly comprises twocomponents: the stationary coil placed along external field B0
and connected to an NMR spectrometer, and a high-speed rotor
consisting of a cylindrical capsule in which a specially arranged
tuning circuit is installed. This circuit is composed of two smallcoils placed perpendicularly and connected in series, and a small
capacitor. One of the two coils is arranged parallel to thestationary coil to establish a coupling by a mutual inductance
between the tuning circuit and the stationary coil (coupling coil).
The other, the sample coil, holds a sample inside. The RF fieldin the coupling coil is transmitted to the sample coil andgenerates an oscillating RF field to induce an NMR signal.
Under this configuration, the sample coil rotates at exactly thesame angular velocity as the sample. The rotor is put inside thestationary coil and, during measurements, it is rotated up to|/2|=10 kHz.
In Fig. 2, we plot the 115In NMR spectra at various values of
the angular velocity . Clearly, the NMR frequency increases
linearly with . Furthermore, by reversing the rotation direction,
the direction of the NMR shift is also reversed; thus, the sign ofB is reversed. The sign of the g-factor of 115In is known to bepositive; that is, the nuclear magnetic moment is parallel to its
angular momentum. Next we measured the shifts for nuclei withnegative g-factors. From the NMR spectra for 29Si having
negative g-factor, the direction of the NMR shift is clearly
opposite to that for 115In, indicating that the emergent Barnett
field is opposite in direction to that for 115In. These results meanthat the nuclei feel additional magnetic field to the external field.
It is a direct evidence for the existence of Barnett field.

Fig. 2 NMR spectra for positive and negative g-factors. Spectra for(A) 115In and (B) 29Si NMR obtained at various angular velocities.
Fig. 1 An illustration of the experimental assembly.

the difficulty, we developed a new detection method in NMR,
and directly measured the Barnett field [3]. The detection on therotating frame was realized by the newly developed tuning
circuit that consists of a sample and detection coil both installedin the same rotor.

References[1] A. Einstein and W. J. de Haas, Verh. Dtsch. Phys. Ges. 17,152 (1915).
[2] S. J. Barnett, Phys. Rev. 6, 239 (1915).
[3] H. Chudo et al., Appl. Phys. Eepress 7, 063004 (2014).