Towards the next generation data storage technology: Ultrafast optical control of magnetic materials

The study of all-optical control of materials mainly involves with exploiting interactions amongst the fundamental reservoirs of energy and angular momentum in materials: charge, spin and lattice. The interaction of femtosecond laser pulse with magnetic material triggers transfer of energy from the electrons into spin and lattice systems. The developments in various time-resolved pump-probe techniques allowed us to explore such interactions between these reservoirs, in metals, that occur at sub-picosecond to picosecond timescales. Investigating the interactions at ultrashort timescales is a key to obtain control over many phenomena such as ultrafast demagnetization, laser-induced magnetization reversal (all-optical switching), super-diffusive spin-transport, and even laser-induced magnetic phase-transition. My research goal is to explore mechanisms involving the aforementioned phenomena in advanced magnetic materials suitable for energy efficient spintronic technologies operating at terahertz (THz) frequencies. 

In my talk, I will address potential mechanisms involving the helicity (circular polarization)-independent and helicity-dependent all-optical switching (AOS) processes in both ferrimagnetic GdFeCo and ferromagnetic Co/Pt thin film systems. It has been demonstrated, in metallic magnets, that the loss of spin ordering to the laser excitation, i.e., ultrafast demagnetization triggers a flow of spin-polarized current oscillating at terahertz frequencies, which will eventually be converted to THz charge current when a heavy metal layer is placed next to the magnetic layer. I will present the laser-driven THz-signals, in Co/Pt bilayers, and their sensitivity to varied roughness, crystal structure, and intermixing at the interface. Finally, I will discuss the timescales of laser-induced phase-transition from antiferromagnetic to ferromagnetic phase in FeRh. Excitation of antiferromagnetic FeRh/Pt bilayer by femtosecond laser pulses results in an efficient emission of nearly single cycle THz pulse being an evidence for laser-induced changes of the magnetization in FeRh. The study based on double pump THz-emission spectroscopy technique reveals that the laser-induced phase-transition emerges at sub-nanosecond timescale, the speed that yields a further control over phase-transition by means of both temperature and applied magnetic field. 

BIO: Rajasekhar Medapalli is a postdoctoral researcher at the Center for Memory and Recording Research, University of California San Diego. His research mainly focuses on the all-optical control of (magnetic-)materials at nanometer length scales. He also fabricates rare-earth thin films based spintronic devices suitable for terahertz spintronics. Dr. Medapalli received his Ph.D. in Physics from Radboud University Nijmegen in 2014 focusing on efficient optical control of magnetization dynamics in metallic alloys. Prior to this, he received his M.Tech degree (Nanotechnology) from Amity University in 2009 and M.Sc degree (Physics) from Andhra University in 2007.   

References:
1)	“Efficiency of ultrafast laser-induced demagnetization in GdxFe100-X-YCoY alloys”,  R. Medapalli et al.,,  Phys. Rev. B 86, 054442 (2012). 
2)	“Multiscale dynamics of helicity-dependent all-optical magnetization reversal in ferromagnetic Co/Pt multilayers”, R. Medapalli et al., Physical Review B 96, 224421 (2017), 
3)	Accumulative Magnetic Switching of Ultrahigh-Density Recording Media by Circularly Polarized Light”, Y. K. Takahashi, R. Medapalli et al., Phys. Rev. Applied 6, 054004 (2016). 
4)	“THz emission from Co/Pt bilayers with varied roughness, crystal structure, and interface intermixing”, G. Li*, R. Medapalli* et al., submitted to the journal of Physical Review Materials.