First! Super spin Seebeck effect based on chiral phonon excitation

Schematic diagram of the spin Cybek effect of chiral phonon excitation: When a temperature gradient is applied to a two-dimensional layered organic-inorganic hybrid perovskite material implanted by chiral cations, the spin current driven by chiral phonons can be observed in the thin layer of metal in contact with it.

On February 14, 2023, Beijing time, the team of Professor Jun Liu and Professor Dali Sun of North Carolina State University, the team of Professor Wei You of the University of North Carolina at Chapel Hill, and Professor Zhang Lifa, Professor Zhou Jun and Professor Li Xiao of Nanjing Normal University published a paper entitled “Chiral-phonon-activated spin Seebeck effect” in the journal Nature Materials New research.

This achievement reports a new method for achieving efficient spin flow excitation based on non-magnetic chiral materials, which breaks through the limitation that spin devices must rely on magnetic materials in the past, and realizes the direct conversion of thermal energy into spin current through chiral phonons in non-magnetic materials, which provides an important new idea for new spintronic devices with low design cost, simple preparation, energy saving and wide application.

The corresponding authors of the paper are Zhang Lifa, Wei You, Dali Sun, and Jun Liu; Kyunghoon Kim, Eric Vetter, Liang Yan, and Cong Yang are co-first authors of the paper.

Spin is an important property of electrons, which can be used as an information carrier to manufacture spintronic devices and realize data storage, communication and computing. Compared to conventional electronics, spintronics devices can significantly reduce energy consumption and limit device failure due to heat generation. Spin Seebeck devices are an important branch of spintronic devices, which can use thermal energy to generate spin current, thereby activating other spintronic devices in the circuit, so spin Seebeck devices have broad application prospects in information transmission, storage, new energy development and waste heat utilization. However, the spin current generated by conventional means is weak, and other complex control methods such as ferromagnetic contact and external magnetic field are required, which greatly hinders the development of electronic devices.

Recently, the research team observed for the first time the chiral phonon-driven spin current in a two-dimensional layered organic-inorganic hybrid perovskite system implanted with chiral cations (as shown in Figure 1), and the measured spin Cybek effect was more significant than previously reported. This is due to the introduction of chiral organic cations that breaks the spatial inversion symmetry of the material, even if the decentralization of the left- and right-handed circularly polarized phonon modes is lifted in the absence of an external magnetic field, exhibiting non-zero phonon angular momentum at temperature gradients. When the chiral system comes into contact with a thin layer of adjacent metal, the phonon angular momentum is transferred to the adjacent metal film, injecting an unequilibrium spin current. This series of results shows that chiral phonons have great application value in spin thermoelectronics, and also provide a new method for spin manipulation in non-magnetic materials.

Figure 1: Schematic diagram of the spin Seebeck effect, spin Seebeck effect with chiral phonon activation, and experimental measurements.

The research team used ultrafast laser pulses to irradiate non-magnetic two-dimensional chiral organic-inorganic hybrid perovskite materials, induce transient temperature gradients, excite chiral phonons in the system, and then generate non-equilibrium spin currents in adjacent metal thin layers (Figure 2). Using time-resolved magneto-optical Kerr measurement equipment, the experimental team accurately detected the transient spin current in the adjacent non-magnetic conductor copper, and found that the phase of the spin current had a one-to-one correspondence with the chiral ligand (i.e., S-type or R-type) of the chiral perovskite film, and the duration was as long as 4 nanoseconds.

Figure 2: Chiral material surface temperature difference and generated transient spin current.

In addition, in order to confirm the spin characteristics of the generated spin current, the research team plated a 15nm-thick magnetic NiFe alloy on a copper film, and magnetized it by an inclined external magnetic field, so that its magnetization direction was partially perpendicular to the chiral phonon-activated spin current. Due to the self-rotational shift moment, the NiFe layer absorbs spin current, generating a precession of magnetization strength, a physical process clearly demonstrated by the time-resolved magneto-optical Kerr effect measurement system (Figure 3).

Figure 3: Autorotational shift moment from chiral phonon-activated spin Seebeck effect.

The research team further realized the linear adjustment of the chiral phonon-activated spin current by adjusting the emission power and modulation frequency of the laser (as shown in Figure 4). According to the measurement results, the corresponding chiral phonon-activated spin Seebeck coefficient is about 104 A/(Km). This chiral phonon-activated spin Seebeck coefficient is orders of magnitude higher than the conventional spin Seebeck effect coefficient in most reported magnetic materials.

Figure 4: Laser power-dependent properties, laser modulation frequency-dependent properties, and magnetic field-dependent properties of the autorotational moment.(Source: Science Network)

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