Figure 1. STM of synthesized GNRs

Figure 2. Fabricated GNRFET

Chemically synthesized graphene nanoribbons (GNRs) present an attractive alternative to carbon nanotubes as high performance semiconducting channel material. Because of their deterministic growth process, these GNRs possess identical electronic properties en masse and, unlike other GNRs, lack rough edges that potentially reduce mobility and transport.

Our group develops the techniques required to integrate these GNRs into electronic devices, enabling us to measure and characterize their electronic response directly through transport and optical spectroscopy. Working directly with chemists and physicists, we are developing new synthesis methods and precursors that improve device performance and yield, while also developing fabrication techniques that enable electrical probing of molecular heterojunctions at the nanometer scale.

Figure 1. Schematic of the coplanar strip line device and characterization of an electrical pulse (figure extracted from Ref. [2]).

Figure 2. Single-shot optical and electrical switching of ferrimaget, GdFeCo (figure extracted from Ref. [2]).

An ultrafast laser pulse can switch the magnetization of magnetic materials in picosecond (ps) timescales, which far exceeds the current data writing speed of hard disk drives by orders of magnitude. However, implementing an ultrafast laser in device applications has limitations, and instead on-chip electrical manipulation is more desired from an application perspective.

Our group has demonstrated that a ps-long electrical current pulse can be generated using a photoconductive switch, and this current pulse can drive ultrafast magnetization dynamics in different magnetic materials [1, 2]. We have demonstrated ultrafast magnetization reversal using either fs laser pulses or ps electrical pulses in ferrimagnetic [2,3] and ferromagnetic [4] materials. Our current research projects involve ultrafast switching of ferromagnetic or ferrimagnetic materials using a spin-orbit torque [5]; ultrafast switching of ferro/ferri multilayers and ferrimagnetic nano-dots [6].

For scientific understanding of ultrafast dynamics, we study the roles of non-equilibrium among heat carriers, spin current, and temperature, and design efficient materials and devices for ultrafast switching.

[1] R. Wilson et al., Phys. Rev. B 96, 045105 (2017)
[2] Y. Yang et al., Sci. Adv. 3, e1603117 (2017)
[3] J. Gorchon et al., Phys. Rev. B 94, 184406 (2016)
[4] J. Gorchon et al., Appl. Phys. Lett. 111, 042401 (2017)
[5] J. Gorchon et al., arXiv:1912.01377 (2019)
[6] A. El-Ghazaly et al., Appl. Phys. Lett. 114, 232407 (2019)

Figure 1 a. Schematic of multiferroic heterostructure.

b. Finite element analysis (FEA) simulation result
showing magnetic onion state in a Ni ring.

c-f. Electrically cycling PMN-PT between DW rotation
between 0.0 and 0.8 MV/m to rotate DWs in Ni rings, observed by PEEM.

Figure 2. Comparison between the piezo-electric strain distribution in the PMN-PT and the magnetic reorientation of Ni micro-squares observed by PEEM (on the right), at E = 0.66 MV/m. The colored frames indicate different reorienting electric fields.

The electric control of magnetism in the microscale has intrigued many researchers for the past century. Until recently the primary method for controlling on-chip magnetic fields required passing electrical currents through a wire. Unfortunately, miniaturizing components and devices that use this control method results in large energy losses and undesirable heating at the small scale. Seeking an alternative, low-power method to electrically control solid state magnetic properties, fundamental research efforts are underway to study nanoscale multiferroic heterostructures [1]. Multiferroic materials exhibit coupling between the intrinsic magnetic, electric, and elastic order parameters; for example, in some types of these materials the magnetic structures can be controlled at the nanoscale (10-100 nm) level with an electric field, not current.

Our group has demonstrated the rotation of magnetic domain walls (DWs) along the circumference of a magnetic ring triggered solely by an applied electric field in a multiferroic heterostructure consisting of a piezoelectric substrate (PMN-PT) and ferromagnetic Ni rings (Figure 1) [2]. We have further proved the strong influence of the electric-field-induced strain distribution in the piezoelectric substrate on the switching of the magnetic micro-structures (Figure 2) [3]. Recently, we have proposed a new multiferroic system with a composite ferromagnetic layer which can potentially offer much richer magnetoelectric properties with the possibility to tune such properties by tailoring the magnetic layer composition [4].

For measuring voltage-induced strains in the piezoelectric substrate and for imaging the reorientation of the magnetization in the magnetic microstructures, we use X-ray microdiffraction and photoemission electron microscopy (PEEM) techniques.

[1] R. Ramesh and N. A. Spaldin, Nat. Mater. 6, 21 (2007)
[2] H. Sohn et al., ACS Nano 9, 4814 (2015)
[3] R. Lo Conte et al., Nano Lett. 18, 1952 (2018)
[4] Z. Xiao et al., ACS Appl. Mater. Interfaces 12, 6752 (2020)