Research Group for Spin-Energy Transformation ScienceGroup Leader: Eiji Saitoh
Members: Michiyasu Mori, Hiroaki Onishi, Jun’ichi Ieda, Koki Nakata, Kei Yamamoto,Yasufumi Araki, Satoru Okayasu, Masao Ono, Hiroyuki Chudo, Kazuya Harii, Masaki Imai

We are searching for new pathways to transform various formsof energy such as heat, magnetic and mechanical ones into
electricity by means of spintronics. The conversion from heat toa magnetically polarized current (spin current) is applied to finda spin on-off switch as reported below in the first subject. Togenerate a spin current, detailed information on magnetic
excitation in various materials is necessary. For a new candidateof a spin current generator, the magnetic phase diagram of afrustrated spin ladder is introduced. In addition, some interesting
results observed by a mechanical rotation are reported in the
research highlight of our group [1].
Spin Colossal Magnetoresistance in an Antiferromagnetic
insulator [2]One of the important components in electronics is a transistor
based on an on-off switch of charge current. A spin current is aflow of angular momentum. By using thespin current as analternative to the charge current, the Joule heating can be
reduced particularly in an insulator. However, it is not easy torealize an on-off switch of spin current. Colossal
magnetoresistance (CMR) is a large change in electricalconductivity induced by a magnetic field in the vicinity of ametal–insulator transition. We demonstrated an analogous spin
effect near the Néel temperature, TN = 296 K, of theantiferromagnetic insulator Cr2O3. By applying a temperature
gradient in Y3Fe5O12(YIG)/Cr2O3/Pt tri-layer (see the inset ofFig. 1 (a)), a spin current is injected from YIG to Pt throughCr2O3 and converted to a charge current, which is observed as avoltage, VSSE. This phenomenon is called the spin Seebeck effect(SSE). As shown in Fig. 1 (a), VSSE suddenly drops to zero at TN.
If Cr2O3 layer was absent, i.e., in YIG /Pt bi-layer, VSSE isFig.1 The temperature dependence of VSSE (a) in the YIG/Cr2O3/Pttri-layer and (b) in a YIG/Pt bi-layer. The inset is a schematic pictureof our device. The temperature gradient, ∇T, is along the z direction,and the external magnetic field (H) is along the y direction. The
magnetic insulator, YIG, is used as a spin source to inject aspin
current Jsin into the Cr2O3. TA transmitted spin current Jsout throughthe Cr2O3 is detected as VSSE in the Pt layer. The symbols, e↓- and e↑, indicate a flow of electrons.

almost constant around TN as shown in Fig. 1 (b). The
transmittance of spin current is defined by the ratio of VSSE inYIG/Cr2O3/Pt tri-layer to that in YIG/Pt. Therefore, our resultmeans that the spin-current transmission can be modulated by
up to about 500%. Above TN, the spin current is carried bycorrelations of the paramagnetic moments in Cr2O3, while below
TN, the spin current cannot propagate, since the magnetization in
Cr2O3 is perpendicular to the polarization of the injected spincurrent. In fact, this spin version of CMR can be controlled bychanging the direction of a magnetic field. Our results have thepotential to simplify the design of fundamental spintronicscomponents, such as enabling the realization of spin-current
switches or spin-current-based memories.
Magnetic Phase diagram of a Frustrated Spin Ladder [3]In the first subject of this report, the spin current is carried bymagnons generated in YIG. A magnetic field generally leads to agap in a dispersion relation of magnons and suppresses a spincurrent at low temperatures. On the other hand, another type ofquasi-particle called triplon is generated in the frustrated spinladder of BiCu2PO6. A triplon is a triplet excitation and isstabilized in amagnetic field. Since it has a well-defineddispersion relation, there is a possibility to be used in the SSE
instead of YIG. To utilize the triplons, we studied the magnetic
phase diagram of a frustrated spin ladder shown in Fig. 2 (right).
The phase diagram has not been determined so far since afrustrated spin system is essentially a many-body problem and isdifficult to be solved analytically. Then, we have numericallysolved a Hamiltonian composed of three types of antiferromagnetic Heisenberg interactions as assigned in Fig. 2(right). As a result, it is found that the triplon has a gap inenergy when the magnetization is set to 0, 1/3, 1/2, and 2/3.
Those appear in the shaded areas of the phase diagram in Fig. 2(left). The three types of interactions, J1, J2, and J ⊥ , can bechanged by substituting elements, e.g., partial substitution of P
by As, and/or applying pressure. It is expected that such a gapengineering will provide a new source for the progress of spincurrent generators.

Fig. 2 The magnetic phase diagrams of a frustrated spin ladder at themagnetization 0, 1/3, 1/2, and 2/3. In the shaded areas, themagnetization curve shows a magnetic plateau, where the magnetic
excitation has a gap.

References[1] M. Imai et al., Appl. Phys. Lett. 113, 052402 (2018).
[2] Z. Qiu et al., Nature Materials 17, 577-580 (2018).
[3] T. Sugimoto et al., Phys. Rev. B 97, 144424 (2018).