Laser pulse induced ultrafast spin current through the antiferromagnetic insulator in Pt/CoO/FeCoB

Abstract

Spin current transmission in antiferromagnetic materials is one of the intriguing topics in antiferromagnetic spintronics. The antiferromagnetic order can mediate the transmission of the spin current in the form of propagating magnons. Several groups have reported this phenomenon in varieties of antiferromagnetic materials, such as NiO, CoO, Fe2O3, FeMn, and IrMn. Most of the investigations have so far been conducted at frequencies much lower than the resonant frequency of the antiferromagnetic materials. In order to explore the physics of the phenomenon, it is crucial to extend the investigation to the THz range, at which antiferromagnets typically have a good susceptibility to electromagnetic field, because this frequency domain embraces the antiferromagnetic resonant frequencies. A suitable method for investigating antiferromagnetic dynamics could be a relatively recently developed technique, where THz radiation is generated with an ultrashort laser pulse in a heavy metal (HM) / ferromagnetic metal (FM) bilayer structure [1,2].In this work [3], we investigated laser stimulated THz emission from Pt/CoO/FeCoB and explored the sub-picosecond pulsed spin current transmission through the antiferromagnetic CoO interlayer. We also particularly look into the polarization of the THz electromagnetic wave and reveal that the latter is influenced by the Néel vector in the antiferromagnets.Figure 1 shows the schematic illustration of the measurement set up. Magnetic field of 100 mT was applied to <110> direction and the CoO Néel vector initially remains in one of the [100] directions, which is an easy axis of CoO. The field cooling process then invokes a twisting of the CoO magnetic moment. The THz electromagnetic wave with x and y components and the polarization angle φ was detected by using two wire grid polarizers. The polarization of the emitted THz wave is parallel to the electric field Ec induced in the Pt layer. The polarization of the THz wave essentially tells us the polarization direction of the spin current injected into the HM layer through the antiferromagnetic layer.Figures 2(a) and 2(b) show the y component of the THz wave, Eypeak, and φ as a function of temperature. Eypeak decreases with decreasing temperature regardless of CoO layer thickness dCoO, which is associated with the dependence of the sheet conductivity of the films and the inverse spin Hall conductivity of the Pt layer [4]. On the other hand, one can notice that φ apparently increases at lower temperature and has an onset at around 200K and 300K for dCoO=2.0 and 5.0 nm, respectively, which coincide with the blocking temperature for these samples.Assuming the polarity of spin density of the ultrafast spin current induced in the FM layer is same direction as magnetization in FeCoB layer, the variations of φ for dCoO=2.0 and 5.0 nm below the blocking temperature indicates that the spin current carried by the THz magnon experiences a non-uniform scattering background, which preferentially scatters the spin orthogonal to the Néel vector gradually rotating away from the axis of magnetization in FeCoB layer. This observation is consistent with previous reports on the spin current experiments in exchange biased systems where the spin current impedance was modified by an accommodation of Néel vector twisting [5].In summary, we found that sub-picosecond pulsed spin current induced by the femtosecond laser pulse can transmit through antiferromagnetic CoO layers. Our results not only demonstrate the picosecond magnon spin current transmission, but also the picosecond interaction of the THz magnons with the Néel vector in the antiferromagnet.This work was partially supported by JSPS KAKENHI Grant (Nos. 17H04924, 17H05181, H1803787, 19K21972, and 26103004), the Center for Spintronics Research Network (CSRN), and the Center for Science and innovation in Spintronics (CSIS). This work was also partly promoted by the Collaborative Research Program of the Institute for Chemical Research, Kyoto University. One of the authors (YS) acknowledges the Graduate Program in Spintronics (GP-Spin) at Tohoku University. **