Posts

Sagnac interferometry for high-sensitivity optical measurements of spin-orbit torque

We develop an ultrasensitive magneto-optic method to quantify spin-orbit torques (SOT) based on a modified Sagnac magneto-optic Kerr effect (MOKE) interferometry ( ). The high sensitivity of Sagnac interferometry permits for the first time optical quantification of spin-orbit torque from small-angle magnetic tilting of samples with perpendicular magnetic anisotropy (PMA). We find significant disagreement between Sagnac measurements and simultaneously-performed harmonic Hall (HH) measurements of spin-orbit torque on Pt/Co/MgO and Pd/Co/MgO samples with PMA. The Sagnac results for PMA samples are consistent with both HH and Sagnac measurements for the in-plane geometry, so we conclude that the conventional analysis framework for PMA HH measurements is flawed. We suggest that the explanation for this discrepancy is that although magnetic-field induced magnetic tilting in PMA samples can produce a strong planar Hall effect, when tilting is instead generated by spin-orbit torque it produces negligible change in the planar Hall signal. This very surprising result demonstrates a flaw in the most-popular method for measuring spin-orbit torques in PMA samples, and represents an unsolved puzzle in understanding the planar Hall effect in magnetic thin films.

– Karimeddiny… Ralph, Luo et al. Under review at PNAS. Arxiv: 2109.13759

Opto-valleytronic spin injection in MoS₂/graphene hybrid spin valves

We report the first coherent spin transfer across a vdW interface based on monolayer MoS₂/graphene hybrid spin valves. This is achieved by exciting spin-valley populations in MoS₂ using a circularly polarized laser beam, and then detecting an electrical spin signal using a ferromagnetic electrode (Co) with a high-quality MBE grown tunneling barrier (MgO). We observe that the magnitude and direction of the spin polarization are controlled by both helicity and photon energy, which is consistent with the unique spin-polarized band structure of monolayer MoS₂. We also perform Hanle spin precession to precisely extract the photon-to-spin conversion efficiency of 5%. This work opens the door for many opportunities using light to read and write coherent spins in vdW heterostructures. One follow-up work is a collaboration between Penn State and me to use circularly polarized light to detect magnetization at a topological insulator/magnetic insulator interface.

– Luo, et al. Nano Lett. 17, 3877–3883 (2017)

– Pillsbury…Luo et al. In prep.

Discovery of room-temperature intrinsic monolayer ferromagnet in epitaxial Manganese Selenide film by molecular beam epitaxy (MBE)

My colleague and I achieve the first experimental realization of large-area room-temperature monolayer intrinsic ferromagnetism in epitaxial Manganese Selenide films grown by MBE. Magnetic and structural characterizations show that in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe₂) monolayer, while for thicker films it originates from a combination of vdW MnSe₂ and/or interfacial magnetism of α-MnSe(111). SQUID measurements of monolayer MnSe_x films on both GaSe and SnSe₂ base layers show a ferromagnetic ordering with a large saturation magnetization of ∼4 Bohr magnetons per Mn.

– O’Hara…Luo et al. Nano Lett. 18, 3125-3131 (2018)

Ultrafast imaging of spin-valley dynamics in monolayer transition metal dichalcogenides (TMDs) and their heterostructures

We develop and employ ultrafast time-resolved Kerr rotation microscopy with ~150 fs temporal and sub-micron spatial resolutions to image spin-valley polarizations in monolayer TMDs and their heterostructures. In monolayer CVD-grown WS₂, we reveal a highly-complex spatial dependence that we attribute to competing roles of dark trions, strains, and localized states. We also show that long-lived (> 5 ns) spin-valley polarization is robust to an in-plane magnetic field up to 700 mT, indicative of spins and valleys that are stabilized with a strong intrinsic spin-orbit field. In monolayer p-type WSe₂/graphene heterostructures, we discover a strong reduction of spin-valley lifetime (~ 5 ps) due to spin transfer into graphene. A key piece of evidence is a non-zero photocurrent detected in the heterostructure, which suggests that 5 ps is the rate at which spin-polarized holes leaving WSe₂, as oppose to dephasing in WSe₂.

– McCormick….Luo et al. 2D Mater. 5, 011010 (2018)

– Newburger, Luo, et al. In prep.

Strong and tunable spin lifetime anisotropy in dual-gated bilayer graphene towards graphene-based spin logic applications  

We discover a strong and highly-tunable spin-lifetime anisotropy with a long out-of-plane spin lifetime up to 7.8 ns at 100 K in dual-gated bilayer graphene. This demonstrates the successful manipulation of spins in graphene by electrically-controlled spin-orbit fields, which is unexpected due to graphene’s weak intrinsic spin-orbit coupling (∼12 μeV). Near the charge neutrality point, we show that the application of a perpendicular electric field opens a band gap and generates an out-of-plane spin-orbit field that stabilizes out-of-plane spins against spin relaxation, leading to a large spin-lifetime anisotropy (defined as the ratio between out-of-plane and in-plane spin lifetime) up to ∼12 at 100 K. This work establishes the high electrical tunability of graphene-based spintronic devices, which also builds on our earlier work of developing current-based spin detection in graphene spin valves for circuit integrations.

– Xu…Luo et al. Phys. Rev. Lett. 121, 127703 (2018), Editors’ Suggestion.

– Wen…Luo et al. J. of Appl. Phys. 115, 17B741 (2014)

Control of Schottky barriers in monolayer MoS₂ transistors with ferromagnetic contacts towards MoS₂ spin hall transistors 

This highly-cited work (>300, Google Scholar) establishes the first viable approach to control the Schottky barrier height in monolayer MoS₂/ferromagnet contacts, thus achieving low contact resistance. This is a significant step towards harvesting the spin properties of MoS₂ for spintronic devices such as spin Hall transistors. We achieve this by inserting a high-quality thin MgO oxide barrier grown by MBE to alleviate the conductance mismatch while minimizing carrier depletion near the contacts. Later on, an alternative method using TiO₂ insertion has been shown by another group (ACS Nano 2014,8, 476-482).

– Chen…Luo et al. Nano Lett. 13, 3106–3110 (2013).

Discovery of vdW photothermoelectric effect in atomic layer heterojunctions

We recently reported the first observation of a highly-tunable vdW photothermoelectric effect in dual-gated MoS­₂/graphene monolayers. In stark contrast to photovoltaic effects arising from excitonic absorption in MoS₂, the vdW photothermoelectric effect originates from photoexcitation of hot electrons in graphene and thermoelectric transport across the vdW junction. A multiple-polarity switching of photocurrent as a function of junction bias and carrier density is captured by our DFT-calculated heterostructure Seebeck coefficient. Systematic studies of photoconductance reveal the vdW photothermoelectric effect as the dominant mechanism for photocurrent generation at room temperature, as opposed to excitonic absorption. This work opens up the exciting realm of spin caloritronics based on vdW heterostructures.

– Luo et al. Nat. Mater. under review.

MBE growth of large-area high-quality epitaxy thin films for spintronic, magnetic, and photovoltaic device applications

Epitaxial germanane (GeH) is predicted to have a band structure similar to graphene but with a large spin-orbit gap at K points of Brillouin zone. We achieve MBE synthesis of high-quality epitaxial GeH on CaGe₂ substrates and subsequent electrochemical delamination to transfer GeH onto arbitrary substrates while keeping the high-quality X-ray diffraction and photoluminescence signatures unchanged. This work establishes a widely-applicable method to transfer any as-grown MBE vdW thin film onto an arbitrary substrate for device integration. In another work, we grow controlled pristine and cobalt-doped (2%, 4%, 6%, and 8%) TiO₂ thin films on LaAlO₃(100) substrate by MBE. Magnetic measurements show room-temperature ferromagnetism in all thin films and suggest that growth conditions, oxygen vacancies, and a bound magnetic polaron model account for the magnetic properties. Photoconductivity microscopy confirms the photosensitive nature of TiO­₂ for photovoltaic solar cell applications.

– Amamou…Luo et al. 2D Mater. 2, 035012 (2015)

–  Naseem… Luo et al. Appl. Surf. Sci. 493, 691-702 (2019)

In October 2019, I started my postdoc fellowship position at Cornell University working with Prof. Dan Ralph (primary), Prof. Kin Fai Mak, and Prof. Jie Shan (secondary), with a focus on spin transfer torque in vdW and topological hybrid systems.

Here’s my Cornell Presidential Postdoctoral fellowship and Honorary Kavli Fellowship info.

Here’s the departmental news from Ohio State: https://physics.osu.edu/news/kelly-luo-awarded-prestigious-cornell-presidential-postdoctoral-fellowship

CV_Yunqiu Kelly Luo_20220411_web

What an honor! I’m very excited to be awarded a Presidential Fellowship by the OSU Graduate School.  The Presidential Fellowship is the most prestigious award given by the Graduate School. Read more here on the our Physics Department Homepage!

Learn more about OSU Presidential Fellowship.