Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P38: Terahertz Spintronics and Demagnetization DynamicsFocus Live
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Sponsoring Units: GMAG DMP FIAP Chair: Kyusup Lee, Natl Univ of Singapore |
Wednesday, March 17, 2021 3:00PM - 3:12PM Live |
P38.00001: Operating ferrimagnet as terahertz antiferromagnetic spin-torque oscillator Hantao Zhang, Mingda Guo, Ran Cheng Ultrafast dynamics is a great promise of antiferromagnetic (AFM) spintronics, but AFM materials are hard to detect due to vanishing magnetization. A plausible alternative is to use ferrimagnets (FIMs) that can be operated as AFM with high frequency while detected as simple ferromagnets. We investigate how the terahertz spin-torque oscillator previously proposed in AFM can be implemented in FIMs without losing its essential merits. We find that anti-damping torques can trigger spontaneous oscillation of the exchange mode in FIMs, resembling the terahertz AFM spin dynamics. While sustaining an auto-oscillation of AFM requires dynamic feedback, an FIM oscillator can stabilize on its own. This unique feature remarkably simplifies the experimental realization. |
Wednesday, March 17, 2021 3:12PM - 3:24PM Live |
P38.00002: Terahertz spin-to-charge conversion by interfacial skew scattering in magnetic metal bilayers Oliver Gueckstock, Lukas Nadvornik, Martin Gradhand, Tom Sebastian Seifert, Genaro Bierhance, Martin Wolf, Mehran Vafaee, Joel Cramer, Georg Woltersdorf, Ingrid Mertig, Gerhard Jakob, Mathias Klaeui, Tobias Kampfrath An efficient spin-to-charge current conversion (S2C) is important for the detection and generation of spin currents in spin-based electronics [1]. Interfaces of heterostructures are known to have a marked impact on this process [2]. Here, we study ultrafast S2C at interfaces by terahertz (THz) emission spectroscopy [3-4] in a model system: F|N bilayers consisting of thin metal films of a ferromagnetic layer F and nonmagnetic layer N with strong and weak spin-orbit coupling (SOC). Varying the interface composition allows us to drastically change the amplitude and even invert the polarity of the THz charge current that is induced by S2C close to the interface. Remarkably, when N is a material with weak SOC, we find a dominant interface contribution to the ultrafast S2C. Symmetry arguments and first-principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin-polarized electrons at interface imperfections. Our results paves the towards designing S2C by interfacial skew scattering. |
Wednesday, March 17, 2021 3:24PM - 3:36PM Live |
P38.00003: Terahertz spin dynamics in antiferromagnetic Mn2Au Yannic Behovits, Alexander Chekhov, Stanislav Bodnar, Martin Jourdan, Mathias Klaeui, Tobias Kampfrath In antiferromagnets, the intrinsic terahertz (THz) magnon resonances are expected to enable pathways to high-speed spin information processing. In antiferromagnetic CuMnAs and Mn2Au, switching of the Néel vector has been demonstrated by using pulsed electrical currents and free-space THz pulses [1-3]. The switching was attributed to the Néel spin-orbit torque (NSOT), which is proportional to the current. However, the underlying spin dynamics have not yet been studied on their natural, i.e. sub-picosecond, time scales. |
Wednesday, March 17, 2021 3:36PM - 3:48PM Live |
P38.00004: Terahertz Magnon Polaritons in YFeO3 in Pulsed High Magnetic Fields Andrey Baydin, Kenji Hayashida, Nicolas Marquez Peraca, Takuma Makihara, Fuyang Tay, Xinwei Li, Xiaoxuan Ma, Zuanming Jin, Wei Ren, Guohong Ma, Gary T Noe, Ikufumi Katayama, Jun Takeda, Hiroyuki Nojiri, Dmitry Turchinovich, Shixun Cao, Motoaki Bamba, Junichiro Kono Antiferromagnetic materials are promising for spintronic applications due to faster spin dynamics than ferromagnets. YFeO3 is a canted antiferromagnet that supports quasi-antiferromagnetic and quasi-ferromagnetic magnon modes in the terahertz (THz) frequency range. Here, we investigate magnon polaritons arising from coupling between THz photons and the quasi-ferromagnetic magnon mode as a function of magnetic field up to 30 T applied along the c-axis using single-shot THz time-domain spectroscopy. We observe a peak splitting whose magnitude increases with the applied magnetic field. We developed a microscopic theoretical model that explains the data without any fitting parameters. The thickness dependence of the splitting at a given magnetic field shows a smooth crossover from a magnon regime to a magnon-polariton regime. |
Wednesday, March 17, 2021 3:48PM - 4:00PM Live |
P38.00005: Same driving force for laser-induced ultrafast demagnetization and THz spin transport Reza Rouzegar, Liane Brandt, Lukas Nadvornik, Alexander Chekhov, David Reiss, Oliver Gueckstock, Martin Wolf, Tom Sebastian Seifert, Piet brouwer, Georg Woltersdorf, Tobias Kampfrath We investigate the driving force of two phenomena that follow femtosecond laser excitation: ultrafast magnetization quenching and spin transport. Two model systems are considered here: (A) a single ferromagnetic (F) metal layer (e.g. Co) and (B) a F|N bilayer consisting of a ferromagnetic (F) and nonmagnetic (N) layer (e.g. Pt). First, laser-induced demagnetization of F is measured by detecting the magnetic-dipole radiation emitted by the dynamically quenched F magnetization [1]. Second, laser-induced spin transport in F|N leads to an in-plane charge current in N due to the inverse spin Hall effect. This current is measured by detecting the concomitantly emitted THz electric-dipole radiation [2]. We observe that, apart from a global scaling factor, the emitted THz pulse follows the same temporal dynamics for both the F and F|N system. This remarkable agreement arises because (i) optically triggered demagnetization in F layer and spin transport in F|N are driven by the same force; (ii) The spin current is only a small perturbation of the spin dynamics in the isolated F layer. |
Wednesday, March 17, 2021 4:00PM - 4:12PM Live |
P38.00006: Ultrafast demagnetization of ferromagnetic iron following optical vs terahertz excitation Alexander Chekhov, Yannic Behovits, Julius Heitz, Christian Denker, David Reiß, Martin Wolf, Martin Weinelt, Piet brouwer, Markus Muenzenberg, Tobias Kampfrath Ultrafast laser-induced demagnetization of ferromagnets is of fundamental relevance for ultrafast spintronics. An important open question is how angular-momentum transfer from the electron spins to the crystal lattice depends on the shape (nonthermal vs thermal) of the electron distribution directly after excitation [1]. To tackle this issue, we perform a direct comparison of magnetization dynamics of an iron thin film triggered by an ultrashort optical pump pulse (photon energy 3 eV, strongly nonthermal perturbation) and a terahertz pump pulse (4 meV, quasi-thermal perturbation) [2]. By varying the polarity of the magnetic field and the THz pump field, we are able to extract the signals related solely to the demagnetization process in the regime linear in the pump fluence. Our results show that on time scales of 100 fs and slower, the dynamics of ultrafast demagnetization doesn’t rely on the initial distribution of the excited electrons. This observation is consistent with an analytical model, in which the driving force of the magnetization change is proportional to the transient excess energy of the electronic system, independent of the precise shape of the electron distribution. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P38.00007: Ultrafast demagnetization times in ferromagnetic alloys Guoping Zhang, Yihua Bai, Thomas George
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Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P38.00008: Effect of demagnetization factors on spin current transport PO-HSUN WU, Ying-Ting Chan, Tzu-Chao Hung, Yi-Hui Zhang, Danru Qu, Tien-Ming Chuang, C. L. Chien, Ssu Yen Huang Spintronics has evolved from exploiting spin-polarized current to pure spin current. Interestingly, many spin current phenomena were first reported not in thin films but in bulk specimens. The specimen shape and associated demagnetizing field can significantly impact the spin current phenomenon. However, this important point has not been considered in previous studies. In this work [1], we demonstrated the strong influence of the demagnetizing field on the spin current transports, including the spin Seebeck effect, spin Hall magnetoresistance, and planar Hall resistance, in Pt/YIG. The calculated effective demagnetizing factors were closely related to the thickness- and width-dependent anomalous plateau behaviors. Furthermore, through magnetic force microscopy and angular-dependent PHE measurements, we observe the evolution of the magnetic domains with an abrupt 90° magnetic rotation. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P38.00009: Terahertz photoconductivity and photocarrier dynamics in graphene–mesoporous silicon nanocomposites Defi Junior Jubgang Fandio We investigate the photocarrier and charge transport dynamics in graphene-mesoporous silicon nanocomposites using optical pump-terahertz probe technique. These nanocomposites were made from a free-standing mesoporous silicon membrane covered with graphene-like coating. The temporal decay of the differential transmission data reproduced using a bi-exponential function show decay times of 74 ps and 730 ps in the uncoated mesoporous film. These decay times were significantly reduced to 5 ps and 25 ps in graphene-mesoporous silicon nanocomposites, respectively. A capture/recombination model was also proposed to describe the decay dynamics in these nanomaterials. The complex frequency-dependent photoconductivity data were well reproduced with a modified Drude-Smith model that considers diffusive restoring currents [1]. Our results show that the effective mobility is preserved in all the nanocomposites despite the increase in carrier confinement with graphene deposition temperature. The relatively high mobility and low photocarrier recombination time in these nanocomposites make them good candidates for the fabrication of pulsed terahertz devices. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P38.00010: Spin-current across the metamagnetic transition in FeRh films as measured with THz emission spectroscopy Yinchuan Lv, Hilal Saglam, Soho shim, Jonathan Gibbons, Axel F Hoffmann, Nadya Mason, Fahad Mahmood THz emission from heterostructures of ferromagnetic (FM) and noble metal (NM) layers is attributed to spin-charge conversion at the FM/NM interface, yet similar effects in bare magnetic thin films are not fully understood. Here we perform THz emission spectroscopy on thin films of FeRh, which is well known to undergo an antiferromagnetic to ferromagnetic phase transition around room temperature. The emitted THz field is observed to have a strong temperature dependence and a power-law dependence on the incident pump pulse fluence. Combined with the knowledge of the spin Hall angle, we extract a temperature dependent spin current across the metamagnetic transition. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P38.00011: Terahertz Emission from Inverse Spin Hall Effect in β-W Ralph Romero, Ramesh C Budhani, Norman Armitage Here we demonstrate THz emission from a magnetic/heavy metal bilayer of CoFe/β-W deposited on a magnetic spinel MgAl2O4 substrate. It is believed the mechanism which generates the transient charge currents that are responsible for radiating the THz pulse is the inverse spin hall effect, where a propagating spin current is converted into a transverse charge current. The inverse spin hall effect occurs due to the presence of strong spin-orbit coupling in the heavy metal layer. β-W has been shown to have the largest spin hall angle (a measure of the spin current to charge current conversion efficiency) of all elemental metals. We show the peak THz electric field scales with β-W layer thickness and pump fluence, generating THz at fluences as low as 2mW. We are also able to flip the amplitude of the THz field by switching the magnetization of the CoFe layer, thus switching sign of the majority spin species. From the perspective of applications, these bilayers are attractive as their cost is dominated by the substrate and they are truly capable of generating broadband THz (0.5 - 2 THz) radiation. From a fundamental perspective, THz emission can be a platform for non-destructive probes of materials which exhibit effects due to Berry curvature, such as the spin hall effect. |
Wednesday, March 17, 2021 5:12PM - 5:48PM Live |
P38.00012: Electrically detected spin currents generated by sub-terahertz microwaves Invited Speaker: Jing Shi Spin current generation, transport, detection, and conversion are essential ingredients of spintronics. Similar to coherently generated spin currents in ferromagnets, spin currents in antiferromagnets (AFM) can also be generated by a coherent drive and electrically detected via the inverse spin Hall effect (ISHE) using heavy metals. In uniaxial AFM, spin currents are carried by the two eigen-modes, i.e., left- and right-handed (LH and RH) magnon modes, with the frequencies typically in the terahertz regime. Cr2O3, a uniaxial AFM, has the Neel temperature of 308 K. Its easy axis is directed along the c-axis of the corundum lattice. At zero magnetic field, the resonance frequency is 165 GHz for both modes. In the presence of a magnetic field along the c-axis, the RH (LH) mode frequency linearly increases (decreases) with the field. With the excitation frequency of 240 GHz, the AFM resonance occurs only for the RH mode at ~ 2.7 T. In our AFM spin pumping experiments (1), we have successfully demonstrated spin current generation in both Cr2O3/Pt and Cr2O3/Ta heterostructure devices at this AFM resonance condition. The ISHE voltage in either device reverses the sign at the resonance field in the opposite direction. In addition, the polarity of the ISHE voltages detected by Pt and Ta, two heavy metals with opposite spin Hall angles, is opposite to each other. These facts confirm the pure spin current nature generated at the AFM resonance. We have also observed another resonance peak at the spin-flop transition (~ 6 T) and yet another resonance feature for the net induced magnetic moment at a higher field (~ 10.5 T). At the AFM resonance, we observed a sign change in the ISHE voltage at ~45 K as the temperature is varied. Discussions of the unexpected temperature dependence will be presented in the context of the coherent and incoherent magnon thermalization process. |
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