The spin-torque magnetic diode (STMD) effect [1] is a quadratic rectification effect of the input microwave current I_RF(t) in a magneto-resistive nano-junction, which is commonly observed in a traditional regime of operation of an STMD, when the magnetization of the “free” layer lies in-plane, and when the frequency f_sof the current I_RF(t) is close to the ferromagnetic resonance (FMR) frequency f₀ of the junction. It was demonstrated theoretically in [2] that in an STMD, biased by an out-of-plane static magnetic field, a novel dynamical regime of STMD operation characterized by large-angle out-of-plane magnetization precession can be realized.

It was demonstrated experimentally in [3] that the out-of-plane magnetization precession regime in an STMD predicted in [2] can be realized without any bias magnetic field, if an STMD “free” layerhas a perpendicular magnetic anisotropy.It was further shown in [3] that the developed bias-free STMD provides sufficient dc voltage to power a practical nanodevice − a black phosphorus photosensor.

Here we present an analytical and numerical theory explaining the performance of such a bias-free STMD with perpendicular magnetic anisotropy [4].We show that such a device can operate as a broadband energy harvester capable of converting incident RF power into a DC power with a conversion efficiency of ∼5%.

References

[1] A. Tulapurkar, Y. Suzuki, A. Fukushima et al., Nature., Vol.438, p.339 (2005).

[2] V. Prokopenko, I. N. Krivorotov, E. Bankowski et al., J. Appl. Phys., Vol. 111, p.123904 (2012).

[3] B. Fang, M. Carpentieri, S. Louis et al., Phys. Rev. Appl., Vol. 11, p.014022 (2019).

[4] P. Yu. Artemchuk, O. V. Prokopenko, E. N. Bankowski et al., AIP Advances., Vol. 11, p. 025234 (2021).

Aqueous Zinc ion batteries (ZIB) based on Zn²⁺ intercalation chemistry is gaining attention as large scale energy storage system due to zinc’s high capacity (820 mA h g⁻¹), high abundance and stability along with lower material costs.However, a comprehensive understanding of the principles governing Zn-MnO₂ electrochemistry has not yet been achieved. In particular, the identity of intercalating cationic species (i.e. Zn²⁺ and/or H⁺) and subsequent redox evolution in MnO₂ cathode material is still not clear. Towards understanding the electrochemical changes in the MnO₂ cathode (such as oxidation changes at metal centers) during the battery cycling process, we employed in situ cryogenic x-ray photoelectron spectroscopy (cryo-XPS) technique. Our unique in situ coin cell setup coupled with cryo-XPS characterization uniquely reveals the chemical identity and distribution of active participants in MnO₂ cathode under various charge and discharge conditions without the need for disruptive sample preparations such as solvent washing or Ar sputtering.By preserving the interface, we observed the broadening of Zn 2p core spectra indicating the evolution of Zn-O and Zn-SO₄ bonding environment during long term cycling process (up to 200 cycles).Similarly, the Mn 2p core spectra reveal the emergence of Mn³⁺ from parent Mn⁴⁺ indicating the Zn intercalation induced redox reactions within cathode material.These new molecular-level insights about the intercalation mechanism and subsequent redox state changes will be discussed based on electronic states evolutions within MnO₂ cathodes.

Among chemical mechanical harvesters, a carbon nanotube (CNT) based chemo-mechanical harvester was reported in 2017[1]. The chemo-mechanical harvester showed remarkable harvesting performance of frequency normalized peak power or highest peak power between a few Hz and 600 Hz compared to other types. This harvester used a coiled CNT yarn made by highly twisting a CNT yarn. As stretching the coiled CNT yarn, the density of the coiled CNT yarn increases. When the density of the coiled CNT yarn increases, it is to escape the ions of the electrochemical double layer formed on the surface of the CNT inside the coiled CNT yarn, just like water comes out when a wet towel is pulled out. In this way, the mechanical contraction is converted into electrical energy. In this work, to improve the performance of this harvester, we propose a novel internal structure that can facilitate ion access inside the coiled CNT yarn and maximize the density change when the coiled CNT yarn is stretched[2]. The coiled CNT yarn with a novel internal structure has four times the peark power and more than twice the efficiency compared to the performance reported in 2017[1]. This structure opened up the possibility to further improve the performance of the harvester and it was analyzed by molecular dynamics modeling. Our results are expected to contribute to the implementation of self-powered IoT systems in the ocean.

Ref.

[1] S. H. Kim et. al, "“Harvesting electrical energy from carbon nanotube yarn twist” Science 357, 773-778 (2017)

[2] S. Oh et. al, "Chemo-mechanical energy harvesters with enhanced intrinsic electrochemical capacitance in carbon nanotube yarns” Advanced science accepted (2022)