Michael Harder

Spin Pumping in Electrodynamically Coupled Magnon-Photon Systems

We use electrical detection, in combination with microwave transmission, to investigate both resonant and nonresonant magnon-photon coupling at room temperature. Spin pumping in a dynamically coupled magnon-photon system is found to be distinctly different from previous experiments. Characteristic coupling features such as modes anticrossing, linewidth evolution, peculiar line shape, and resonance broadening are systematically measured and consistently analyzed by a theoretical model set on the foundation of classical electrodynamic coupling. Our experimental and theoretical approach paves the way for pursuing microwave coherent manipulation of pure spin current via the combination of spin pumping and magnon-photon coupling.

Electrical detection of magnetization dynamics via spin rectification effects

The purpose of this article is to review the current status of a frontier in dynamic spintronics and contemporary magnetism, in which much progress has been made in the past decade, based on the creation of a variety of micro- and nano-structured devices that enable electrical detection of magnetization dynamics. The primary focus is on the physics of spin rectification effects, which are well suited for studying magnetization dynamics and spin transport in a variety of magnetic materials and spintronic devices. Intended to be intelligible to a broad audience, the paper begins with a pedagogical introduction, comparing the methods of electrical detection of charge and spin dynamics in semiconductors and magnetic materials respectively. After that it provides a comprehensive account of the theoretical study of both the angular dependence and line shape of electrically detected ferromagnetic resonance (FMR), which is summarized in a handbook formate easy to be used for analyzing experimental data. We then review and examine the similarity and differences of various spin rectification effects found in ferromagnetic films, magnetic bilayers and magnetic tunnel junctions, including a discussion of how to properly distinguish spin rectification from the spin pumping/inverse spin Hall effect generated voltage. After this we review the broad applications of rectification effects for studying spin waves, nonlinear dynamics, domain wall dynamics, spin current, and microwave imaging. We also discuss spin rectification in ferromagnetic semiconductors. The paper concludes with both historical and future perspectives, by summarizing and comparing three generations of FMR spectroscopy which have been developed for studying magnetization dynamics.

Study of the cavity-magnon-polariton transmission line shape

We experimentally and theoretically investigate the microwave transmission line shape of the cavity-magnon-polariton (CMP) created by inserting a low damping magnetic insulator into a high quality 3D microwave cavity. While fixed field measurements are found to have the expected Lorentzian characteristic, at fixed frequencies the field swept line shape is in general asymmetric. Such fixed frequency measurements demonstrate that microwave transmission can be used to access magnetic characteristics of the CMP, such as the field line width ΔH. By developing an effective oscillator model of the microwave transmission we show that these line shape features are general characteristics of harmonic coupling. At the same time, at the classical level the underlying physical mechanism of the CMP is electrodynamic phase correlation and a second model based on this principle also accurately reproduces the experimental line shape features. In order to understand the microscopic origin of the effective coupled oscillator model and to allow for future studies of CMP phenomena to extend into the quantum regime, we develop a third, microscopic description, based on a Green’s function formalism. Using this method we calculate the transmission spectra and find good agreement with the experimental results.

Spin pumping in strongly coupled magnon-photon systems

We experimentally investigate magnon polaritons arising in ferrimagnetic resonance experiments in a microwave cavity with a tunable quality factor. To this end, we simultaneously measure the electrically detected spin pumping signal and the microwave reflection (the ferrimagnetic resonance signal) of a yttrium iron garnet (YIG)/platinum (Pt) bilayer in the microwave cavity. The coupling strength of the fundamental magnetic resonance mode and the cavity is determined from the microwave reflection data. All features of the magnetic resonance spectra predicted by first principle calculations and an input-output formalism agree with our experimental observations. By changing the decay rate of the cavity at constant magnon-photon coupling rate, we experimentally tune in and out of the strong coupling regime and successfully model the corresponding change of the spin pumping signal and microwave reflection. Furthermore, we observe the coupling and spin pumping of several spin wave modes and provide a quantitative analysis of their coupling rates to the cavity.

Nondestructive two-dimensional phase imaging of embedded defects via on-chip spintronic sensor

A microwave near field phase imaging technique has been achieved by an on-chip spintronic sensor. The sensor directly rectifies a microwave field into a dc voltage signal by employing the spintronic principle, in which the relative phase between microwave electric and magnetic fields plays an important role. By manipulating the relative phase, the sensor can nondestructively detect embedded defects of subwavelength size.

Effect of cyclization of N-terminal glutamine and carbamidomethyl-cysteine (residues) on the chromatographic behavior of peptides in reversed-phase chromatography

N-terminal loss of ammonia is a typical peptide modification chemical artifact observed in bottom-up proteomics experiments. It occurs both in vivo for N-terminal glutamine and in vitro following enzymatic cleavage for both N-terminal glutamine and cysteine alkylated with iodoacetamide. In addition to a mass change of -17.03 Da, modified peptides exhibit increased chromatographic retention in reversed-phase (RP) HPLC systems. The magnitude of this increase varies significantly depending on the peptide sequence and the chromatographic condition used. We have monitored these changes for extensive sets (more than 200 each) of tryptic Gln and Cys N-terminated species. Peptides were separated on 100 Å pore size C18 phases using identical acetonitrile gradient slopes with 3 different eluent compositions: 0.1% trifluoroacetic acid; 0.1% formic acid and 20 mM ammonium formate at pH 10 as ion-pairing modifiers. The observed effect of this modification on RP retention is the product of increased intrinsic hydrophobicity of the modified N-terminal residue, lowering or removing the effect of ion-pairing formation on the hydrophobicity of adjacent residues at acidic pHs; and possibly the increased formation of amphipathic helical structures when the positive charge is removed. Larger retention shifts were observed for Cys terminated peptides compared to Gln, and for smaller peptides. Also the size of the retention increase depends on the eluent conditions: pH 10≪trifluoroacetic acid

Control of the Magnon–Photon Coupling

A systematic study of the electrodynamic coupling between the ferromagnetic resonance (FMR) and the cavity mode of a microwave cavity is presented. The nature of the FMR and cavity modes, described by their resonance position and line width, is measured and compared with a classical model based on the Landau-Lifshitz-Gilbert equation and an RLC circuit, which explicitly implement Faradays and Ampères laws. The relationship between the coupling gap, which can be accessed experimentally, and the coupling strength, which is a fundamental property of the FMR/cavity system, is discussed on the basis of this model. A straightforward method is experimentally developed to control the coupling gap. The capability of using such a tunable cavity-FMR coupling in combination with spin pumping could provide us with new technologies that utilize the light-matter interaction to control spin current in cavity spintronics.

Cavity Mediated Manipulation of Distant Spin Currents Using a Cavity-Magnon-Polariton

Using electrical detection of a strongly coupled spin-photon system comprised of a microwave cavity mode and two magnetic samples, we demonstrate the long distance manipulation of spin currents. This distant control is not limited by the spin diffusion length, instead depending on the interplay between the local and global properties of the coupled system, enabling systematic spin current control over large distance scales (several centimeters in this work). This flexibility opens the door to improved spin current generation and manipulation for cavity spintronic devices.

Spin dynamical phase and antiresonance in a strongly coupled magnon-photon system

We experimentally studied a strongly coupled magnon-photon system via microwave transmission measurements. An antiresonance, i.e., the suppression of the microwave transmission, is observed, indicating a relative phase change between the magnon response and the driving microwave field. We show that this antiresonance feature can be used to interpret the phase evolution of the coupled magnon-microwave system and apply this technique to reveal the phase evolution of magnon dark modes. Our work provides a standard procedure for the phase analysis of strongly coupled systems, enabling the phase characterization of each subsystem, and can be generally applied to other strongly coupled systems.

Topological properties of a coupled spin-photon system induced by damping

We experimentally examine the topological nature of a strongly coupled spin-photon system induced by damping. The presence of both spin and photonic losses results in a non-Hermitian system with a variety of exotic phenomena dictated by the topological structure of the eigenvalue spectra and the presence of an exceptional point (EP), where the coupled spin-photon eigenvectors coalesce. By controlling both the spin resonance frequency and the spin-photon coupling strength we observe a resonance crossing for cooperativities above one, suggesting that the boundary between weak and strong coupling should be based on the EP location rather than the cooperativity. Furthermore we observe dynamic mode switching when encircling the EP and identify the potential to engineer the topological structure of coupled spin-photon systems with additional modes. Our work therefore further highlights the role of damping within the strong coupling regime, and demonstrates the potential and great flexibility of spin-photon systems for studies of non-Hermitian physics.