AVS2017 Session MI+SA-WeM: Controlling Magnetism in Oxides and Multiferroics and Chirality in Spin Transport and Magnetism (cont.)

The coexistence of electric polarization and magnetization in multiferroic materials provides great opportunities for realizing magnetoelectric coupling, including electric field control of magnetism, or vice versa, through a strain mediated magnetoelectric coupling in layered magnetic/ferroelectric multiferroic heterostructures [1-9]. Strong magnetoelectric coupling has been the enabling factor for different multiferroic devices, which however has been elusive, particularly at RF/microwave frequencies. In this presentation, I will cover the most recent progress on new integrated multiferroic materials and devices for sensing, and from power to mm-wave electronics. Specifically, we will introduce magnetoelectric multiferroic materials, and their applications in different devices, including: (1) ultra-sensitive magnetometers based on RF NEMS magnetoelectric sensors with picoTesla sensitivity for DC and AC magnetic fields, which are the best room temperature nano-scale magnetometers; (2) novel ultra-compact multiferroic antennas immune from ground plane effect with f200µm × 1µm or l₀/600 in size, -18dBi gain, self-biased operation and 1~2% voltage tunable operation frequency; and (3) novel GHz magnetic and multiferroic inductors with a wide operation frequency range of 0.3~3GHz, and a high quality factor of close to 20, and a voltage tunable inductance of 50%~150%. At the same time, I will also demonstrate other voltage tunable multiferroic devices, including tunable isolating bandpass filters, tunable bandstop filters, tunable phase shifters, etc. These novel integrated multiferroic devices show great promise for applications in compact, lightweight and power efficient sensing, power, RF, microwave and mm-wave integrated electronics.

Understanding spin-selective interactions between electrons and chiral molecules is critical to elucidating the prospective significance of electron spin in biological processes. We report the visualization of spin‑dependent charge transport in microscale‑patterned, self‑assembled monolayers of double‑stranded DNA on ferromagnetic substrates using fluorescence microscopy. Patterned DNA arrays provide background regions in every measurement to quantify the substrate magnetization-dependent fluorescence due to the chiral‑induced spin selectivity effect. Fluorescence quenching of photoexcited dye molecules bound within DNA duplexes is dependent upon the rate of charge separation/recombination upon photoexcitation and efficiency of DNA‑mediated charge transfer to the surface. Here, the latter process is modulated with an external magnetic field to switch the magnetization orientation of the underlying ferromagnetic substrates. Using this experimental technique, we are investigating molecular parameters that can be manipulated to influence the magnitude of the spin selectivity effect in DNA arrays to assess candidly the potential of chiral assemblies for organic spintronics. In particular, we are monitoring the influence of heavy metal species that are incorporated predictably within DNA duplexes to change the strength of molecular spin-orbit coupling as a result of the heavy atom effect.

Nanocrystalline Fe₃O₄ thin films were grown by adopting two different reduction approaches (1) vacuum annealing (2) wet H₂ annealing . While vacuum annealed films shows Verwey transition with lower resistivity compared to the bulk Fe₃O₄, the same are not observed in electric transport properties of wet H₂ annealed films. However, this transition was clearly seen in the temperature dependence of magnetization of both sets of Fe₃O₄ thin films. This seems to indicate that the both electric transport and magnetization are independent processes; it’s just coincidence to happen at same place of Verwey transition at 120 K in Fe₃O₄. Different electric transport properties in both reductions treated Fe₃O₄ films could be ascribed to different grain sizes/grain boundary volumes, inhomogeneities and presence of residual atomic-H at grain boundaries emanating from complex reductions treatments.

[1] C A F Vaz, F J Walker, C H Ahn, and S Ismail-B, J. Phys.: Condens. Matter 27 (2015) 123001.

[2] C. A. F. Vaz, J. Hoffman, Y. Segal, J.W. Reiner, R. D. Grober, Z. Zhang, C. H. Ahn, and F. J. Walker, PRL 104, 127202 (2010).

[3] M.-A. Husanu and C.A.F. Vaz, Spectroscopic characterisation of multiferroic interfaces. In C. Cancellieri and V. N. Strocov (editors), Spectroscopy of TMO interfaces. Springer-Verlag, 2017 (forthcoming).

The interaction of electrons with helical molecules attains growing interest due to a spin selectivity in electron transmission. Experiments on self-assembled monolayers of double stranded DNA [1] and oligopeptides [2,3] indicated a very efficient spin filtering behavior of the molecules at room temperature.

In present experiments enantiopure M- and P-heptahelicene molecules are evaporated onto different metal single crystal surfaces. The molecules arrange themselves to a highly ordered monolayer [4,5]. Samples are then irradiated with λ = 213nm laser radiation to generate photoelectrons from the substrate. These electrons are transmitted through the heptahelicene layer and analyzed with regard to their average longitudinal spin orientation by Mott scattering. The sign of the spin polarization depends on the helicity of the enantiomer. The effect of the heptahelicene on the spin orientation seems to be independent on the substrate.

References

[1] Göhler, B.; Hamelbeck, V.; Markus, T. Z.; Kettner, M.; Hanne, G. F.; Vager, Z.; Naaman, R.; Zacharias, H., Science 2011, 331, 894.

[2] Ray, K.; Ananthavel, S. P.; Waldeck, D. H.; Naaman, R. Science 1999, 283, 814.

[3] Kettner, M.; Göhler, B.; Zacharias, H.; Mishra, D.; Kiran, V.; Naaman, R.; Fontanesi, C.; Waldeck, D. H.; Sęk, S.; Pawłowski, J.; Juhaniewicz, J. J.Phys.Chem. C 2014, 119, 26.

[4] Fasel, R.; Cossy, A.; Ernst, K.-H.; Baumberger, F.; Greber, T.; Osterwalder, J. J. Chem. Phys. 2001, 115, 2.

[5] Seibel, J.; Parschau, M.; Ernst, K.-H., J.Phys.Chem. C 2014, 118, 29135

The yield of electrons transmitted through chiral molecules can depend on the electron’s spin; chiral molecules can therefore act a spin filters. This effect is referred to as chirality-induced spin selectivity (CISS). Previous experiments have e.g. been performed with monolayers of double-stranded DNA adsorbed on gold [1] and silicon [2] substrates. In this contribution, we present results of our spin-resolved photoemission experiments performed at room temperature. The samples consist of self-assembled monolayers of helical molecules – various types of double-stranded peptide nucleic acid (PNA) – on polycrystalline gold surfaces. The samples are irradiated by a laser at λ = 213nm to generate photoelectrons from the gold substrate which are then transmitted through the adsorbed monolayer. Subsequently, the electrons are analyzed by a Mott polarimeter. We found longitudinal spin polarizations of −6% for PNA and +25% for γ-PNA. The results indicate that the adsorbed molecules act as a spin filter.

[1] B. Göhler et al., Science 331, 894 (2011)

[2] M. Kettner et al., Adv. Mater. Interfaces 3, 1600595 (2016)