Study on a factor of spin current power generation by neutron scattering
S. Shamoto1), T. U. Ito1), H. Onishi2), H. Yamauchi3), Y. Inamura4), M. Matsuura5), M. Akatsu6),
K. Kodama3), A. Nakao5), T. Moyoshi5), K. Munakata5), T. Ohhara4),
M. Nakamura4), S. Ohira-Kawamura4), Y. Nemoto7), and K. Shibata4)1) : Research Gr. for Nanoscale Structure and Function of Advanced Materials, ASRC, JAEA
2) Research Gr. for Spin-energy Transformation Science, ASRC, JAEA
3) : MSRC, JAEA 4) : J-PARC center JAEA, 5) : CROSS
6) : Dept. of Physics, Niigata Univ. 7) : Grad. Sch. of Sci. and Tech., Niigata Univ.

Spin current is a flow of spin-angular momentum. The spincurrent power generation bases on a revolutionary method thatcan generate electricity in an insulator from the flow of the spincurrent produced by, for example, a temperature gradient. In
other words, the spin current is a flow of magnon. The flow
results in a voltage on the electrodes of platinum via the inverse
spin Hall effect [1]. The power generation has been investigatedusing ferromagnetic materials such as yttrium iron garnet (YIG)
with high efficiency [2]. The composition of the magnetic
material YIG is Y3Fe5O12 with up and down Fe3+ spins, which are
in a ratio of 3 : 2, as shown in Fig. 1. It results in a ferrimagnetic
state. The rotation of these two types of spins is not free in three
dimensions and spins can only rotate in an anticlockwise
direction at low energies as shown in Fig. 1. Such restrictions
come from the crystallographic symmetry of spins. Furthermore,
it was theoretically pointed out that this rotation is closely related
to the spin current of YIG, and if both clockwise andanticlockwise rotations are mixed, the efficiency decreases [3]. In
this YIG, it turns out that these two types of rotational energies
are split by about 30 meV, therefore, only the anticlockwise
rotation in Fig. 1 occurs at low temperatures. Furthermore, themagnitude of the spin current depends on the state number of spinrotations (magnon) which determines the spin current power
generation efficiency. The state number of the magnon can bemeasured as the magnon density of states (MDOS) by inelastic
neutron scattering (INS). However, it has been experimentallydifficult because it is necessary to evaluate the scattering intensity
in an absolute scale, therefore, it has not been directly determinedeven in a popular magnet YIG so far.
The MDOS on YIG has been studied [4] as shown in Fig. 2 by
INS in a wide energy range over three orders of magnitude using
three INS spectrometers, 4SEASONS, AMATERAS, and DNA
in the Material and Life Science Experimental Facility of J-PARCCenter. With this knowledge to derive the MDOS of acomplicated magnetic structure like YIG, we can now apply thismethod to many other substances in the future. The MDOS
obtained here is associated with the rotational motion of the spinsin Fig. 1. The MDOS integrated in energy is related to themagnitude of the spin. It was confirmed that its size matches withthe magnetic moment measured by magnetic susceptibility. The
magnetic moment even in a complicated system, in which thespins may be fluctuating in a certain frequency region, can bedetermined by this method.
Recently, high-efficient spin current power generationmaterials have been searched. The guidelines for a ferrimagnetic
material are as follows.
1. The clockwise and anticlockwise rotational modes do notoverlap.
2. The MDOS in Fig. 2 are high at low energy corresponding
to the working temperature range.

These conditions can be satisfied in inelastic neutron scatteringexperiments. In the future, we will investigate not only magnon
but also phonon effects on the spin current power generation byneutron scattering, because of possible strong interactions
between them.
References[1] K. I. Uchida et al., Nature (London) 455, 778 (2008).
[2] T. Kikkawa et al., Phys. Rev. B 92, 064413 (2015).
[3] J. Barker et al., Phys. Rev. Lett. 117, 217201 (2016).
[4] S. Shamoto, T. U. Ito, H. Onishi, H. Yamauchi et al., Phys.
Rev. B 97, 054429 (2018).

Fig.1. Anticlockwise rotational mode for 5 spins in a YIG
Fig.1. Anticlockwise
modefor 5 This
spinsmodein a YIG
sub-lattice
sub-lattice is rotational
schematicallyshown.
carriesis schematicallymode carriesa spincurrent
current to one
a spin currentshown.
to oneThis
direction.
The spin
direction.
rotationalin Y3Feis thedetermined
5O12ondirectionThis
in Y
is determinedbased
3Fe5O12direction
basedon the crystallographic[3].
crystallographic
symmetry symmetry[3].

Fig. 2. Magnon density of states (DM(E)) of YIG obtained by inelastic
neutron scattering [4], which is proportional to imaginary part ofdynamical spin susceptibility (”(E)). The magnitude is one of
the important factors for spin current power generation.