Terahertz magnonics in antiferromagnetic iron oxides
Creators: name: Leenders, Ruben
Publisher: name: Lancaster University
Year: 2024

description: Antiferromagnets, magnetic materials with antiparallel spin ordering and
 therefore lacking magnetic moment have for a long time been studied for
 academic interest only. However, the realization that antiferromagnets
 possess intrinsic magnetic resonance frequencies in the Terahertz range,
 which is orders of magnitude higher than the Gigahertz ferromagnetic
 resonance frequencies, has recently renewed the interest for
 antiferromagnets for applications in energy efficient data storage and
 processing.
 
 
 Moreover waves of spin precession, or magnons, have been proposed as new
 methods for wave-based computing. The miniaturization of such potential
 technological devices requires the spin waves to have nanometer scale
 
 wavelengths, which has proven to be challenging to achieve in
 anitferromagnets.
 
 
 In this thesis, we will study the ultrafast spin dynamics and magnons in a
 specific class of antiferromagnetic iron oxides, the orthoferrites, RFeO3,
 where R is a rare-earth element. These antiferromagnets possess a weak
 ferromagnetic moment due to the canting of the antiparallel spins. 
 
 
 After introducing the field of ultrafast magnetism and magnonics and
 associated concepts in Chapter 1, in the first part of this thesis, we
 will describe how the challenge of generating
 
 nanoscale spin waves can be overcome by exciting a confined region of
 spins near the sample face. We will show how strongly absorbed laser
 pulses will generate a propagating broad-band wavepacket of spin waves.
 
 
 In Chapter 2, we will introduce the basic concepts behind the experiments
 performed in this work. We will introduce the principle of ultrafast
 pump-probe spectroscopy experiments that can be used to measure such spin
 waves, and describe the design of the setup that allows us to drive the
 spin dynamics with intense Terahertz pulses.
 
 
 In Chapter 3, a thorough theoretical description of the technique to
 launch propagating broadband wavepackets of magnons will be given.
 Additionally, we will model the detection of these generated packets of
 spin waves acts in Magneto-Optical Kerr Effect experiments. We find that
 through the emergence of the Brillouin condition, by the appropriate
 choice of the wavelength of the probe pulse, we can select the detected
 wavenumber component of the wave packet, resulting in a probe wavelength
 dependent frequency observed in the
 
 experiment. 
 
 
 In Chapter 4, we proceed to the experiment and search for the spin wave
 packets in HoFeO3. We will show that by exciting the spin dynamics with
 high energy photons above the bandgap energy, we can launch such
 propagating packets of spin waves. We find the theoretically predicted
 dependence of the detected magnon frequency on the wavelength of the probe
 light, and find that we excite a broad range of components of the spin
 wave packet.
 
 
 In Chapter 5, we build upon the experiment in Chapter 4, and study how the
 propagating spin waves can be controlled. We find that we can achieve a
 nonlinear control of the spin waves by introducing a second pump pulse.
 From theoretical calculations, we show that the coupling between the
 propagating magnon and photon acts as an additional nonlinear torque on
 the spins. We will see that this nonlinear torque allows for the
 conversion of the low frequency uniform precession mode of the spins into
 the higher frequency and higher
 
 wavenumber modes of the propagating spin wave packet.
 
 
 In Chapter 6, we will study the spin dynamics in ErFeO3 and TmFeO3 induced
 by intense THz pulses. Despite the magnetic similarities of these
 materials, the spin dynamics shows a very different trend at the Spin
 Reorientation Transition temperatures. In ErFeO3, we observe an unexpected
 giant enhancement of the amplitude, whereas in TmFeO3, this amplitude is
 suppressed. We will show that this difference in the dynamics can be
 attributed to the effect of the coupling between the iron spins and
 rare-earth ions.
 
 
 Finally, in Chapter 7, we will conclude our findings, and provide a
 concise outlook that shows that the intense THz setup is not only suitable
 for the study of antiferromagnetic oxides, but can also be used to study
 metallic thin films. We will demonstrate this with a short summary of
 experimental data measured in the FeRh, which is an antiferromagnet at
 room temperature and exhibits a phase transition to the ferromagnetic
 phase at high temperatures.