Terahertz magnonics in antiferromagnetic iron oxides

Leenders, Ruben and Mikhaylovskiy, Rostislav and Jamison, Steven and Kartvelishvili, Vakhtang (2024) Terahertz magnonics in antiferromagnetic iron oxides. PhD thesis, Lancaster University.

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Abstract

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.