Ultrafast coherent lattice dynamics coupled to spins in the van der Waals antiferromagnet FePS3
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Date
2023
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Abstract
2D materials, like the antiferromagnetic van der Waals semiconductors FePS3 studied
in this work, open up new possibilities for technological applications due to the
unique interaction of their magnetization with electronic, optical, and mechanical
properties. Furthermore, they provide the potential to study magnetism and
magnetization dynamics in reduced dimensions. Up do date, the coherent control of
the magnetization of these materials has barely been studied. Our research addresses
this gap by using ultrashort light pulses. In this context, time-resolved studies can
give an insight into the evolution of the light-induced dynamics, which essentially
require a dedicated experimental setup.
In this thesis, we present a comprehensive study on the development and application
of a table-top laser setup designed for magneto-optical pump-probe experiments and
adaptable for the investigation of microscopic samples. The system employs two
optical parametric amplifiers, with a tunable photon-energy range of 0.5 eV - 3.5 eV
for both the pump and the probe beam. Remarkable is the high pump amplitude
modulation rate at 50 % of the laser repetition rate, realized via the integration of
an electro-optical modulator, blocking every second pump pulse. Combined with a
high-frequency digitizer, performing single pulse detection, our system can achieve a
high sensitivity, down to 50 µdeg of the probe polarization rotation. The setup can
apply magnetic fields of up to ±9 T, and voltages in the kV regime while providing
a temperature control between 4 K-420 K.
The functionality of the setup’s systems is demonstrated by performing static Kerrrotation
and ultrafast demagnetization measurements in a cobalt single crystal as a
function of the most important experimental parameters.
The major part of this thesis is dedicated to our studies on a coherent optical
lattice mode of terahertz frequency triggered by femtosecond laser pulses in the
antiferromagnetic van der Waals semiconductor FePS3 . This specific 3.2 THz phonon
mode shows a close relation to the antiferromagnetic order, as it vanishes above the
Néel temperature and hybridizes with a magnon mode in the presence of a magnetic
field. We investigate it as a function of sample temperature, probe polarization,
excitation photon energy and externally applied magnetic fields. The resonant
excitation of a crystal-field split electronic ..-.. transition efÏciently pumps the
displacive excitation process of the mode, while the magnetic linear dichroism is
identified as the magneto-optical effect, which reflects the phonon mode in the probe
rotation. By applying magnetic fields of up to 9 T we can generate and observe
the coherent hybridized phonon-magnon mode, thus exploiting the hybridization to
excite coherent spin-dynamics. Furthermore, we investigate the coherent phonons
in the bulk form of FePS3 and in an exfoliated flake with a thickness of 380 nm.
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Keywords
Phonon, Ultrafastmagnetism, Pulsed laser, FePS3, 2D-magnetsim