I worked at the Lawrence Berkeley National Laboratory (and UC Berkeley) as a post-doctoral researcher in the lab of Prof. Jeffrey Bokor from January, 2020 to April, 2023. I studied the magnetization switching dynamics in ferromagnet and ferrimagnetic thin films using ultrafast optical and electrical excitation. I used time-resolve and depth-resolve MOKE spectroscopy techniques to study the magnetization dynamics and electrical characterization. I worked with Python for measurement and automation to carry out different projects in our lab. I was particularly interested in the switching dynamics of GdCo alloy and Co/Tb multilayer and inspect the effect of the Tb in the observed switching mechanism. I was also involved in studying magnetization dynamics with X-rays at large-scale synchrotron facilities.
Analysis of Ultrafast Magnetization Switching in Exchange Coupled Ferromagnets
Magnetization switching in ferromagnets has so far been limited to the current-induced spin–orbit–torque effects. Recent observation of helicity-independent all-optical magnetization switching (HI-AOS) in an exchange-coupled ferromagnet–ferrimagnet (FM-FEM) heterostructures expanded the range and applicability of such ultrafast heat-driven magnetization switching. Here we report the element-resolved HI-AOS dynamics of such an exchange-coupled system, using a modified microscopic three-temperature model. We have studied the effect of (i) the Curie temperature of the FM, (ii) FEM composition, (iii) the long-range Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange-coupling strength, and (iv) the absorbed optical energy on the element-specific time-resolved magnetization dynamics. The phase-space of magnetization illustrates how the RKKY coupling strength and the absorbed optical energy influence the switching time. Our analysis demonstrates that the threshold switching energy depends on the composition of the FEM and the switching time depends on the Curie temperature of the FM as well as RKKY coupling strength. This simulation anticipates new insights into developing faster and more energy-efficient spintronics devices.
“Analysis of Ultrafast Magnetization Switching in Exchange Coupled Ferromagnets” D. Polley, J. Chatterjee, H. Jang, and J. Bokor. Journal of Magnetism and Magnetic Materials; 470, 170680 (2023).
Ultrafast all-optical switching of a ferromagnet-ferromagnet coupled structure
Helicity-independent all-optical switching of ferri-magnet alloys have been first observed in 2011. Recently scientists are trying to develop an opto-MTJ structure using the optically switchable ferrimagnetic alloy the free layer of the MTJ. But, the spin-polarization of the optically switchable ferrimagnet alloys is very low which leads to a small TMR across the MTJ structure. Here, we have studied the ultrafast optical switching of a highly spin-polarized (ferromagnet) material which is magnetically exchange coupled (RKKY interaction) with a low spin-polarized ferrimagnet. Our experiment reveals that the exchange-coupled ferromagnet (Co/Pt) can be switched as fast as ~3.5 ps. We developed a theoretical analysis based on a microscopic three-temperature model to explain the observed switching dynamics and the evolution of switching timescale as a function of RKKY exchange coupling and absorbed laser energy.
''RKKY Exchange Bias Mediated Ultrafast All‐Optical Switching of a Ferromagnet'' J. Chatterjee, D. Polley, A. Pattabi, H. Jang, S. Salahuddin, and J. Bokor; Advanced Functional Materials 32, 2107490, (2022).
A perspective on picosecond magnetic memory
We have reviewed some of the recent interesting developments in the field of ultrafast magnetic switching from an application point of view. We have discussed the current obstacles to this technology and some probable solutions.
"Progress toward picosecond on-chip magnetic memory'' D. Polley, A. Pattabi, J. Chatterjee, S. Mondal, K. Jhuria, J. Gorchon, and J. Bokor; Applied Physics Letters 120, 140501, (2022).
Picosecond Spin-Orbit Torque Induced Coherent Magnetization Switching in a Ferromagnet
In this article, we have experimentally studied the ultrafast spin-orbit torque (SOT)-induced magnetization dynamics in a ferromagnet using a ~9 ps current pulse and observed ultrafast magnetization switching in ~70 ps. The commonly observed SOT-driven switching is the ~ns timescale is dominated by a domain-wall driven mechanism and is affected by the stochastic domain incubation time. We have triggered the ultrafast and coherent nature of the switching by the ultrafast damping-like torque in addition to the ultrafast thermal anisotropy torque induced by the ~9 ps current pulse. A macroscopic simulation model by solving a modified LLG (LLB) equation in the presence of thermal anisotropy and ultrafast damping-like and field-like torque excellently reproduces the experimental results. The demonstration of coherent magnetization switching in an unprecedented timescale will trigger future studies in obtaining energy-efficient and ultrafast magnetization switching.
“Picosecond Spin-Orbit Torque Induced Coherent Magnetization Switching in a Ferromagnet”, D. Polley, A. Pattabi, A. Rastogi, K. Jhuria, E. Diaz, H. Singh, A. Lemaitre, M. Hehn, J. Gorchon and J. Bokor (arXiv:2211.08266)
