17 research outputs found
Single shot ultrafast all optical magnetization switching of ferromagnetic Co/Pt multilayers
In a number of recent experiments, it has been shown that femtosecond laser
pulses can control magnetization on picosecond timescales, which is at least an
order of magnitude faster compared to conventional magnetization dynamics.
Among these demonstrations, one material system (GdFeCo ferromagnetic films) is
particularly interesting, as deterministic toggle-switching of the magnetic
order has been achieved without the need of any symmetry breaking magnetic
field. This phenomenon is often referred to as all optical switching (AOS).
However, so far, GdFeCo remains the only material system where such
deterministic switching has been observed. When extended to ferromagnetic
systems, which are of greater interest in many technological applications, only
a partial effect can be achieved, which in turn requires repeated laser pulses
for full switching. However, such repeated pulsing is not only energy hungry,
it also negates the speed advantage of AOS. Motivated by this problem, we have
developed a general method for single-shot, picosecond timescale, complete all
optical switching of ferromagnetic materials. We demonstrate that in
exchange-coupled layers of Co/Pt and GdFeCo, single shot, switching of the
ferromagnetic Co/Pt layer is achieved within 7 picoseconds after irradiation by
a femtosecond laser pulse. We believe that this approach will greatly expand
the range of materials and applications for ultrafast magnetic switching.Comment: 11 pages, 3 figures, supplementary material
The role of electron and phonon temperatures in the helicity-independent all-optical switching of GdFeCo
Ultrafast optical heating of the electrons in ferrimagnetic metals can result
in all-optical switching (AOS) of the magnetization. Here we report
quantitative measurements of the temperature rise of GdFeCo thin films during
helicity-independent AOS. Critical switching fluences are obtained as a
function of the initial temperature of the sample and for laser pulse durations
from 55 fs to 15 ps. We conclude that non-equilibrium phenomena are necessary
for helicity-independent AOS, although the peak electron temperature does not
play a critical role. Pump-probe time-resolved experiments show that the
switching time increases as the pulse duration increases, with 10 ps pulses
resulting in switching times of ~sim 13 ps. These results raise new questions
about the fundamental mechanism of helicity-independent AOS.Comment: 18 pages, 6 figures and supplementary material
Optical Switching in Tb/Co-Multilayer Based Nanoscale Magnetic Tunnel Junctions
Magnetic tunnel junctions (MTJs) are elementary units of magnetic memory
devices. For high-speed and low-power data storage and processing applications,
fast reversal by an ultrashort laser pulse is extremely important. We
demonstrate optical switching of Tb/Comultilayer-based nanoscale MTJs by
combining optical writing and electrical read-out methods. A 90 fs-long laser
pulse switches the magnetization of the storage layer (SL). The change in
magnetoresistance between the SL and a reference layer (RL) is probed
electrically across the tunnel barrier. Single-shot switching is demonstrated
by varying the cell diameter from 300 nm to 20 nm. The anisotropy,
magnetostatic coupling, and switching probability exhibit cell-size dependence.
By suitable association of laser fluence and magnetic field, successive
commutation between high-resistance and low-resistance states is achieved. The
switching dynamics in a continuous film is probed with the magneto-optical Kerr
effect technique. Our experimental findings provide strong support for the
growing interest in ultrafast spintronic devices.Comment: total pages 22, Total figure
Picosecond Spin Orbit Torque Switching
Reducing energy dissipation while increasing speed in computation and memory
is a long-standing challenge for spintronics research. In the last 20 years,
femtosecond lasers have emerged as a tool to control the magnetization in
specific magnetic materials at the picosecond timescale. However, the use of
ultrafast optics in integrated circuits and memories would require a major
paradigm shift. An ultrafast electrical control of the magnetization is far
preferable for integrated systems. Here we demonstrate reliable and
deterministic control of the out-of-plane magnetization of a 1 nm-thick Co
layer with single 6 ps-wide electrical pulses that induce spin-orbit torques on
the magnetization. We can monitor the ultrafast magnetization dynamics due to
the spin-orbit torques on sub-picosecond timescales, thus far accessible only
by numerical simulations. Due to the short duration of our pulses, we enter a
counter-intuitive regime of switching where heat dissipation assists the
reversal. Moreover, we estimate a low energy cost to switch the magnetization,
projecting to below 1fJ for a (20 nm)^3 cell. These experiments prove that
spintronic phenomena can be exploited on picosecond time-scales for full
magnetic control and should launch a new regime of ultrafast spin torque
studies and applications.Comment: Includes article + supplementary information. Latest version uses
full name of the first author. Nature Electronics (2020
Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films
Ultrafast control of the magnetization in ps timescales by fs laser pulses
offers an attractive avenue for applications such as fast magnetic devices for
logic and memory. However, ultrafast helicity-independent all-optical switching
(HI-AOS) of the magnetization has thus far only been observed in Gd-based,
ferrimagnetic amorphous (\textit{a}-) rare earth-transition metal
(\textit{a}-RE-TM) systems, and a comprehensive understanding of the reversal
mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic
\textit{a}-GdTbCo thin films, from x = 0 to x = 18, and
elucidate the role of Gd in HI-AOS in \textit{a}-RE-TM alloys and multilayers.
Increasing Tb content results in increasing perpendicular magnetic anisotropy
and coercivity, without modifying magnetization density, and slower
remagnetization rates and higher critical fluences for switching but still
shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on
the two-temperature model reproduce these results qualitatively and predict
that the lower damping on the RE sublattice arising from the small spin-orbit
coupling of Gd (with ) is instrumental for the faster dynamics and lower
critical fluences of the Gd-rich alloys. Annealing
\textit{a}-GdTbCo leads to slower dynamics which we argue
is due to an increase in damping. These simulations strongly indicate that
acounting for element-specific damping is crucial in understanding HI-AOS
phenomena. The results suggest that engineering the element specific damping of
materials can open up new classes of materials that exhibit low-energy,
ultrafast HI-AOS
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Investigation and Control of Ultrafast Magnetic Phenomena
Spintronic devices have shown a lot of promise in low power and non-volatile memory applications. However, conventional spintronic devices are limited by the speed of equilibrium magnetization reversal. For more than two decades, the field of ultrafast magnetism, wherein magnetic processes in (sub)picosecond timescales are triggered by the ultrafast non-equilibrium heating of magnetic thin films with femtosecond laser pulses, has provided us with the tantalizing prospect of controlling magnetism in unprecedentedly fast timescales. This dissertation will detail the research conducted over the last 6 years in understanding ultrafast magnetic phenomena, and in controlling and integrating them with conventional spintronic processes to realize fast, non-volatile spintronic devices.The first part of the dissertation will focus on work done to understand the fundamental limitations of some spintronic and ultrafast magnetic phenomena. This will include experiments on detecting the current induced spin accumulation due to the spin-orbit effects in heavy metals directly on the heavy metal surface using an optical technique called the magnetization-induced second harmonic generation (MSHG). Insight into the dynamics and timescales of current induced spin accumulation in conventional spin-orbit torque (SOT) devices gained from these experiments will help understand the speed limitations of such devices. The dissertation then focuses on the ultrafast helicity-independent all-optical switching (HI-AOS) in ferrimagnetic GdFeCo and GdTbCo alloys. These experiments shed a light on the underlying mechanism of such a process, and unravel the complex interplay of exchange coupling, elemental damping and other parameters in ultrafast magnetization switching events. The upper limit for the pulse duration of optical excitation that triggers HI-AOS, which has important implications when it comes to integrating these processes on-chip, is also studied.The second part of the dissertation will introduce ways to build up on the experimental results of the first part, thereby moving towards the integration of ultrafast magnetic phenomena into conventional spintronic devices. Experiments performed to extend the ultrafast HI-AOS capabilities of GdFeCo to Co/Pt multilayers by controlling the exchange interaction between these two films are presented. This is of technological significance because HI-AOS had thus far only been reported in Gd-based ferrimagnetic films, which are not very attractive for device integration due to their ferrimagnetic nature. Co/Pt multilayers, on the other, are ferromagnetic and are well suited for application in spintronic devices. Then, the ultrafast control of magnetism by picosecond heat current and electrical current pulses will be introduced. Finally, the dissertation will present recent results on demonstrating the deterministic spin-orbit torque switching of a Co/Pt ferromagnet by short, 6 ps electrical pulses
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Investigation and Control of Ultrafast Magnetic Phenomena
Spintronic devices have shown a lot of promise in low power and non-volatile memory applications. However, conventional spintronic devices are limited by the speed of equilibrium magnetization reversal. For more than two decades, the field of ultrafast magnetism, wherein magnetic processes in (sub)picosecond timescales are triggered by the ultrafast non-equilibrium heating of magnetic thin films with femtosecond laser pulses, has provided us with the tantalizing prospect of controlling magnetism in unprecedentedly fast timescales. This dissertation will detail the research conducted over the last 6 years in understanding ultrafast magnetic phenomena, and in controlling and integrating them with conventional spintronic processes to realize fast, non-volatile spintronic devices.The first part of the dissertation will focus on work done to understand the fundamental limitations of some spintronic and ultrafast magnetic phenomena. This will include experiments on detecting the current induced spin accumulation due to the spin-orbit effects in heavy metals directly on the heavy metal surface using an optical technique called the magnetization-induced second harmonic generation (MSHG). Insight into the dynamics and timescales of current induced spin accumulation in conventional spin-orbit torque (SOT) devices gained from these experiments will help understand the speed limitations of such devices. The dissertation then focuses on the ultrafast helicity-independent all-optical switching (HI-AOS) in ferrimagnetic GdFeCo and GdTbCo alloys. These experiments shed a light on the underlying mechanism of such a process, and unravel the complex interplay of exchange coupling, elemental damping and other parameters in ultrafast magnetization switching events. The upper limit for the pulse duration of optical excitation that triggers HI-AOS, which has important implications when it comes to integrating these processes on-chip, is also studied.The second part of the dissertation will introduce ways to build up on the experimental results of the first part, thereby moving towards the integration of ultrafast magnetic phenomena into conventional spintronic devices. Experiments performed to extend the ultrafast HI-AOS capabilities of GdFeCo to Co/Pt multilayers by controlling the exchange interaction between these two films are presented. This is of technological significance because HI-AOS had thus far only been reported in Gd-based ferrimagnetic films, which are not very attractive for device integration due to their ferrimagnetic nature. Co/Pt multilayers, on the other, are ferromagnetic and are well suited for application in spintronic devices. Then, the ultrafast control of magnetism by picosecond heat current and electrical current pulses will be introduced. Finally, the dissertation will present recent results on demonstrating the deterministic spin-orbit torque switching of a Co/Pt ferromagnet by short, 6 ps electrical pulses
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RKKY Exchange Bias Mediated Ultrafast All‐Optical Switching of a Ferromagnet
The discovery of ultrafast helicity-independent all-optical switching (HI-AOS), as well as picosecond all-electrical switching of a ferrimagnet, has inspired the ultrafast spintronics community to explore ultrafast switching of a ferromagnet to achieve practical ultrafast storage and memory devices. Two explored mechanisms of HI-AOS of a ferromagnet in ferromagnet-ferrimagnet heterostructure are: a) exploiting the indirect exchange coupling with and b) injection of non-local spin current originated from a switching ferrimagnet. In this manuscript, exchange mediated HI-AOS of a Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange coupled “[Co/Pt]-multilayers/Pt spacer/CoGd” heterostructure is demonstrated. The authors have measured layer-resolved static magnetic properties, single-shot HI-AOS, and magnetization dynamics of the ferromagnetic Co/Pt multilayers (MLs), that are ferromagnetically or antiferromagnetically coupled with ferrimagnetic CoGd layers. Time-resolved magnetization dynamics reveal a 3.5 ps switching time of the Co/Pt MLs, which is the fastest switching of a ferromagnet reported to date. Employing an extended microscopic three-temperature model, the temporal dynamics of the exchange coupled ferromagnet–ferrimagnet heterostructure are simulated, qualitatively and quantitatively explaining the experimental switching phenomena. This work experimentally as well as theoretically establishes the mechanism of exchange mediated all-optical switching of ferromagnet-ferrimagnet heterostructures, which can be integrated with a magnetic tunnel junction for efficient reading after ultrafast energy-efficient switching
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Progress toward picosecond on-chip magnetic memory
We offer a perspective on the prospects of ultrafast spintronics and opto-magnetism as a pathway to high-performance, energy-efficient, and non-volatile embedded memory in digital integrated circuit applications. Conventional spintronic devices, such as spin-transfer-torque magnetic-resistive random-access memory (STT-MRAM) and spin-orbit torque MRAM, are promising due to their non-volatility, energy-efficiency, and high endurance. STT-MRAMs are now entering into the commercial market; however, they are limited in write speed to the nanosecond timescale. Improvement in the write speed of spintronic devices can significantly increase their usefulness as viable alternatives to the existing CMOS-based devices. In this article, we discuss recent studies that advance the field of ultrafast spintronics and opto-magnetism. An optimized ferromagnet-ferrimagnet exchange-coupled magnetic stack, which can serve as the free layer of a magnetic tunnel junction (MTJ), can be optically switched in as fast as ∼3 ps. Integration of ultrafast magnetic switching of a similar stack into an MTJ device has enabled electrical readout of the switched state using a relatively larger tunneling magnetoresistance ratio. Purely electronic ultrafast spin-orbit torque induced switching of a ferromagnet has been demonstrated using ∼6 ps long charge current pulses. We conclude our Perspective by discussing some of the challenges that remain to be addressed to accelerate ultrafast spintronics technologies toward practical implementation in high-performance digital information processing systems