Session BB01: V: Spin Transport and Magnetization Dynamics

Magnetism and spin-charge conversion in epitaxial van der Waals heterostructures

Layered materials are a class of quantum materials with electronic properties of exceptional interest for many domains of solid-state physics. Their crystal structure with van der Waals bonding between unit layers makes it feasible to stabilize single 2D layers and to form a wide range of heterostructures without the constraint of lattice matching. In this family of materials, transition metal dichalcogenides and topological insulators hold great promise for spintronics owing to their large spin-orbit coupling and the locking between the electron spin and momentum [1]. The recent discovery of van der Waals 2D magnets has also opened exciting opportunities to explore low dimensionality magnetism, proximity phenomena in heterostructures and all-van der Waals spin devices [1].

 

While most research on these materials is currently performed with nanoflakes mechanically exfoliated from bulk crystals, molecular beam epitaxy is emerging as a powerful method to grow large-area 2D materials with fine tuning of the composition, control of the thickness down to the 2D limit and ability to fabricate heterostructures with sharp and clean interfaces. I will discuss the specificities and challenges of van der Waals epitaxy and review recent progress in the fabrication of van der Waals materials by this technique, including topological insulators [2], transition metal dichalcogenides [3] and 2D magnets [4]. I will then illustrate the potential of these materials for spintronics with examples of heterostructures in which spin-charge interconversion is implemented, leading to large spin-orbit torques and current-driven magnetization switching. [5]   

Understanding the Spin Current Hermitivity in Ferromagnet Bilayers

The effervescent area of spintronics has produced different fields of research; among them is the field of interface phenomena capable of revealing the most intriguing effects of quantum mechanics. In this work, we study the phenomenon of spin current hermitility in ferromagnet bilayers, which appears in response to the spin Hall effect and represents the anti-polarization of the spin current due to the coupling between interfacial magnetic anisotropies. We performed this observation on permalloy/cobalt bilayers and other ferromagnet bilayers [1].   

Electrical detection of ferromagnetic resonance via rectification effects

Spin rectification effects are widely used to study magnetization dynamics in magnetic materials and spintronic devices. For instance, ferromagnetic resonance (FMR) can be easily detected by measuring a dc photovoltage resulting from the rectification of rf current in a ferromagnet with oscillating magnetization. Here we present an experimental study of the rectification effects produced by FMR in a ferromagnetic (NiFe, Fe) wire. A system of four independent nonmagnetic contact probes was used to supply both rf and dc currents to the wire and measure the resulting photovoltage at different locations in the wire. Our multiprobe system provided a means to separate contributions to the photovoltage from the ferromagnet/nonmagnet contacts and the bulk of the ferromagnet. The contact photovoltage was found to increase approximately linearly with the dc bias applied to the wire. In contrast, the bulk contribution was found to be almost independent of the dc bias. By tuning properties of individual contact probes we were able to change the magnitude of the contact photovoltage and even reverse its sign. Our results highlight the different contributions to photovoltage and the importance of contact properties/nonlinearities for rectification effects in spintronic devices.   

Room temperature low magnetic damping in polycristalline Co2-xMn1+xGe

We studied the static and dynamic magnetic properties of polycristalline Co_2-xMn_1+xGe Heusler thin films by SQUID magnetometry and ferromagnetic resonance. We found that when varying the stoichiometry, the effective magnetic damping can vary up to two orders of magnitude. We relate the variation in magnetic relaxation to the extrinsic mechanisms of damping that depend on the microstructure of the films. Transmission electron microscopy reveals that our samples display periodic planar Mn-O clusters due to deposition method, with the samples richer in Mn having larger grains that pass through the Mn-O planes. We observe an increased non-linear frequency dependence of the FMR linewidth in these samples, which we attribute to the grain to grain two-magnon scattering processes. With decreasing Mn content, the contribution of the two-magnon scattering processes diminishes that we connect with the grain shape and size. Our results suggest a strong connection between the microstructure and extrinsic magnetic damping contributing to the understanding of magnetic relaxation in polycristalline films.   

Study of spin-wave propagation properties in ferrimagnetic insulator TmIG thin films

Spin wave propagation in materials exhibiting perpendicular magnetic anisotropy (PMA) has garnered significant attention due to its potential for enabling novel and energy-efficient spintronic devices [1]. Thulium iron garnet (TmIG), magnetic insulator with PMA [2], is a compelling candidate for spintronic applications due to its unique magnetic properties and spin transport characteristics [2]. In this work, we employ broadband electrical detection method to measure spin-wave (SW) propagation in thin TmIG films (thickness: 2 – 35 nm) grown by pulsed laser deposition on gadolinium-gallium-garnet (GGG) and substituted GGG (SGGG) substrates to measure SW propagation length and SW velocity [3]. We perform ferromagnetic resonance spectroscopy on TmIG thin films grown on GGG and sGGG to study effects of thickness on magnetic damping and to characterize PMA. We further examine the propagation properties of Damon–Eshbach spin-waves (DESWs) and isotropic magnetostatic forward volume spin-waves (MSFVSWs) in TmIG/GGG and TmIG/SGGG thin films. We finally outline future experiments relevant for advanced data processing and quantum magnonic applications. [1] A.V. Chumak, et al., Nat. Phys. 11(6):453 (2015). [2] C.N. Wu, et al., Sci. Rep. 8, 11087 (2018). [3] Timalsina, et al., arXiv:2310.06188 (2023).   

Observation of Long-Distance Coherence in Critically-Driven Cavity Magnonics

Developing quantum networks necessitates using a beam of light to coherently administer distant resonators, a process that often encounters the obstacle of dissipation-induced decoherence. Here, we discover a long-distance coherence in cavity magnonics operating in the linear regime. By locally setting the cavity near critical coupling to traveling photons, non-local magnon-photon coherence is established via strong coupling mediated by photons travelling over a 2-meter distance. The coupling strength oscillates anomalously with the photon phase, exhibiting twice the period as conventional photon-mediated couplings. Our work shows the potential of using critical phenomena for harnessing long-distance coherence in networks of distributed systems. The observed anomalous features reveal the tip of an iceberg of photon-mediated coupling in magnetic systems.   

Two new experiments to probe nuclear moments in atoms

In our previous NMR work, we found an exact form

for the nuclear wave function that easily allows for transitions

between arbitrary states. This was for a magnetic

field B(t) = B₀ z +B₁[x cos(ωt)+ y sin(ωt)], where

B₁ << B₀. We now extend this technique to consider the

interaction of that magnetic field with both the nuclear

and electronic states of an atom. The atomic nuclear

and electronic interactions have the effective bare Hamiltonian

Η₀ = Ω_nI_z ⊗ 1_e + Ω_e1_z ⊗ J_e, where Ω_n,Ω_e are

the Rabi expressions for the nucleus and electrons, respectively.

The model can be solved using standard perturbation

theory, since the exact wave functions for the

nucleus for spin I and the electrons of total angular momentum

J are used. Results for the electric quadrupole

moment and the magnetic octupole moment for general

I, J are found, and some figures for (I, J) = (1, 1/2) and

(3/2, 1/2) are presented   

Monte Carlo simulations of imaginary time Liouvillian dynamics in the mixed field Ising model.

Under generic Hamiltonian dynamics, governing the evolution of interacting many-body systems, local operators are expected to rapidly evolve in time, resulting in complex structures of operator strings. Understanding the properties of this operator growth process has attracted significant research interest, especially in relation to thermalization in quantum systems. Here, we present a Monte Carlo scheme which enables probing operator growth by sampling the imaginary time Liouvillian dynamics. Compared to more conventional quantum Monte Carlo schemes, our configuration space comprises local operators occupying a D+1 dimensional lattice instead of wavefunctions. Transitions between operator strings are captured by the action of the Liouvillian. Crucially, our algorithm is free of the numerical sign problem for the mixed field quantum Ising model at infinite temperature, allowing for numerically exact calculations. Our findings support the recently proposed "Operator Growth Hypothesis", predicting a high-frequency exponential tail for generic local spectral functions. In particular, we resolve subtle logarithmic corrections in 1D and a crossover in the high-frequency decay rates in 2D. Extension of our approach to other models and observables will be discussed.