Dr. Berend T. Jonker is the Senior Scientist and Head of the Magnetoelectronic Materials & Devices section in the Materials Science & Technology Division at the Naval Research Laboratory, Washington, DC. His current research focuses on semiconductor spintronics, including electrical spin injection and transport in semiconductors, and the fabrication and development of prototype spintronic devices.
Fabrication and Development of Prototype Spintronic Devices
We selected the Park NX 10 AFM instrument on a cost/performance ratio based on our bid specifications. The specialized scientific instrument equipment market is an international market, we evaluated a number of companies world-wide before selecting Park Systems AFM. Park Systems AFM was identified as the one that would get the highest return for the cost of purchase.
– Dr. Berend Jonker, Sr. Scientist Naval Research Laboratory
What are Magnetoelectronic devices and how are you using them?
My current work involves looking at materials properties and how they interface including magnetic metals. Select materials promise to be thero magnetic meaning they have a non zero phenomenon or a magnetic moment. This is used universally in motors, tools, etc.
We are currently looking at developing material for IT non volitale memory for reprogramming logic, ultra low power consumption for devices and circuits and sensors for drones. These will allow us to leave a device underwater or for it to be taken out into field without carrying batteries. We are focusing on ultra low power consumption devices we can leave anywhere or have delivered by drones.
How do you use microscopy in your research?
Sensors cover a broad area for sonar optical and magnetic sensors. We routinely use SEM, AFM, MFM, and a variety of optical spectroscopy to evaluate materials we grow. We do a lot of material synthesis and have a broad suite of characterization tools.
The Park NX 10 AFM is one of our tools. It was selected in our government procurement program largely due to the cost performance ratio based on our bid specifications and was used in recent research published in ACS Omega on the first demonstration of metallic spin. This offers exciting opportunities for the advancement of sensors and data storage.
Specialized scientific equipment market is an international global market. Our laboratory has a wide array of equipment for measurement including spin-polarized scanning tunneling microscopy (STM) and many others pieces of the most modern equipment available world-wide.
NRL Produces Spin Filtering at Room Temperature with Graphene
An interdisciplinary team of scientists at the U.S. Naval Research Laboratory (NRL) have reported the first demonstration of metallic spin filtering at room temperature using ferromagnet-graphene-ferromagnet thin film junction devices — spin is a fundamental property of electrons, in addition to charge, that can be used to transmit, process and store data.
The spin filtering had been theoretically predicted and previously seen only for high-resistance structures at cryogenic temperatures,” said Dr. Enrique Cobas, principal investigator, NRL Materials Science and Technology Division. “The new results confirm the effect works at room temperature with very low resistance in arrays of multiple devices.
The thin film junctions demonstrated low resistance, and the magnetoresistance characteristic of a spin filter interface from cryogenic temperatures to room temperature. The research team also developed a device model to incorporate the predicted spin filtering by explicitly treating a metallic minority spin channel with spin current conversion, and determined that the spin polarization was at least 80 percent in the graphene layer.
“Graphene is famous for its extraordinary in-plane properties, but we wanted to look at conductivity between stacked graphene sheets and how they interact with other materials,” said Cobas. To do so, NRL researchers developed a recipe to grow large multi-layer graphene films directly on a smooth, crystalline nickel alloy film while retaining that film’s magnetic properties, then patterned the film into arrays of cross-bar junctions. “We also wanted to show we could produce these devices with standard industry tools, not just make one device,” Cobas added.
The spin filtering phenomenon is due to an interaction of the quantum mechanical properties of graphene with those of a crystalline nickel film. When the nickel and graphene structures align, only electrons with one spin can pass easily from one material to the other, an effect termed spin filtering, that results in spin polarization of an electric current.
“There is room for improvement as theory suggests the effect can be increased by an order of magnitude by fine-tuning the number of graphene layers,” said Dr. Olaf van ‘t Erve, research scientist, NRL Materials Science and Technology Division. “However, current models do not include the spin-conversion that happens inside the ferromagnetic contacts. Once we account for those effects, we’re already close to the ideal case of 100 percent spin polarization in the graphene layer, enabling us to revise our device geometry and materials to maximize the effect.”
The result is relevant to next-generation non-volatile magnetic random access memory (MRAM), which uses spin-polarized pulses to flip a magnetic bit from 0 to 1 and vice-versa. It may also find use in future spin logic technologies or as magnetic sensors.
The research results are reported in the November 2, 2016, issue of ACS Nano (DOI 10.1021/acsnano.6b06092). The full research team included Drs. Enrique Cobas, Olaf van ‘t Erve, Shu-Fan Cheng, Konrad Bussman and Berry Jonker from the Materials Science and Technology Division and Drs. James Culbertson and Glenn Jernigan from the Electronics Science and Technology Division at NRL.
Supporting Information ABSTRACT: Single-monolayer transition metal dichalcogenides exhibit exceptionally strong photoluminescence (PL), dominated by a combination of distinct neutral and charged exciton contributions. We show here that the surface charge associated with ferroelectric domains patterned into a lead zirconium titanate film with an atomic force microscope laterally controls the spatial distribution of neutral and charged exciton populations in an adjacent WS2 monolayer. This is manifested by the intensity and spectral composition of the PL measured in air at room temperature from the areas of WS2 over a ferroelectric domain with a polarization dipole pointed either out of the surface plane or into the surface plane. This approach enables spatial modulation of PL intensity and trion/ neutral exciton populations and fabrication of lateral quantum dot arrays in any geometry, with potential applications in nonvolatile optically addressable memory or optical quantum computation.
See complete article at: http://pubs.acs.org/journal/acsodf
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