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Ultrafast photo-magnetic recording in transparent medium
A. Stupakiewicz, K. Szerenos, D. Afanasiev, A. Kirilyuk, A. V. Kimel
TL;DR
The paper investigates ultrafast magnetization control in a garnet film while limiting laser-induced heating. It uses polarization-dependent light–matter interaction to switch magnetic domains and observes picosecond dynamics with a small temperature increase.
Problem
Ultrafast magnetization control must be achieved while keeping laser-induced heating far below the temperature needed to reach the Curie temperature.
Method
The approach uses polarization-dependent light–matter interaction in Co-substituted yttrium iron garnet to control magnetization between domains with opposite My.
Results
20 ps is the characteristic rise time of the magnetization-precession signal, while the estimated temperature increase is 1.25 K.
Takeaways & Limitations
Polarization-dependent excitation provides a mechanism for steering magnetization in the garnet while keeping the temperature increase at least two orders of magnitude below that required to reach the Curie temperature.
Abstract
from arXiv · showhide
Finding a conceptually new way to control the magnetic state of media with the lowest possible production of heat and simultaneously at the fastest possible time-scale is a new challenge in fundamental magnetism [1-4] as well as an increasingly important issue in modern information technology [5]. Recent results demonstrate that exclusively in metals it is possible to switch magnetization between metastable states by femtosecond circularly polarized laser pulses [6-8]. However, despite the record breaking speed of the switching, the mechanisms in these materials are directly related to strong optical absorption and laser-induced heating close to the Curie temperature [9-12]. Here we report about ultrafast all-optical photo-magnetic recording in transparent dielectrics. In ferrimagnetic garnet film a single linearly polarized femtosecond laser pulse breaks the degeneracy between metastable magnetic states and promotes switching of spins between them. Changing the polarization of the laser pulse we deterministically steer the net magnetization in the garnet, write "0" and "1" magnetic bits at will. This mechanism operates at room temperature and allows ever fastest write-read magnetic recording event (< 20 ps) accompanied by unprecedentedly low heat load (< 6 J/cm3).
METHODS
The methods combine single-pulse magneto-optical imaging, time-resolved pump–probe measurements, and symmetry analysis to characterize photo-magnetic switching in YIG:Co. They quantify the induced precession, optical absorption, deposited heat, and polarization-dependent switching mechanism.
- Materials: The study uses 7.5 µm-thick YIG:Co films with measured magnetic, crystallographic, and damping parameters at room temperature.The film was grown by liquid phase epitaxy and has saturation magnetization 4πMS=90 G, Néel temperature 445 K, and Gilbert damping α=0.2.
- Single-pulse imaging: Single-pulse magneto-optical imaging uses 50 fs pump pulses, Faraday-rotation contrast, and images acquired about 10 ms after excitation.Domains are visualized with a polarizing microscope, analyzer, and CCD camera; before–after images reveal photo-magnetic changes.
- Time-resolved imaging: Time-resolved single-shot imaging uses 40 fs, 800 nm probe pulses and synchronized shutter–camera triggering to isolate one pump pulse.The delay generator and reduced amplifier repetition rate exclude excitation by more than one pump pulse; delays span 50 fs to 1 ns.
- Thermal estimate: At 1300 nm, using Imin=34 mJ cm-2 and absorption a=0.12, the deposited heat is estimated as 6 J cm-3 and the temperature increase as ΔT=1.25 K.The calculated temperature rise is at least two orders of magnitude below that required to reach the Curie temperature.
- Magnetization dynamics: The induced precession has a characteristic rise time of about 20 ps and reaches maximum signal at about 60 ps.Opposite phases across domain boundaries and cancellation at walls support coherent magnetization rotation rather than domain-wall motion.
- Magnetization dynamics: The precession amplitude follows sin(2φ), is largest for orthogonal <100>-type polarizations, and increases linearly with pump fluence.These measurements characterize the polarization-dependent photomagnetic effective field in YIG:Co.
- Symmetry analysis: The symmetry analysis models light–matter coupling with fourth-rank tensor terms and derives a polarization-dependent energy contribution that selects the sign of My.For reduced point-group symmetry, the simplified expression is WL(E,M)=A(ExEx*MxMy - EyEy*MxMy); [100] and [010] polarization favor opposite magnetic states.