Session W50: Spin Dynamics, Topology and Damping

Damping in Iron Thin Films: Recent Experimental Insights

Confirming the origins of Gilbert damping by experiment has remained a challenge for many decades. This is the case even for some of the simplest and most technologically relevant ferromagnetic metals, such as pure Fe. In this presentation, we will discuss our findings that provide crucial – and counterintuitive – insights into the mechanisms of damping in Fe thin films. We will first present evidence for intrinsic “conductivity-like” Gilbert damping, where cleaner epitaxial films can exhibit higher damping at low temperatures [1]. We will also show that room-temperature intrinsic Gilbert damping in polycrystalline Fe is remarkably insensitive to the film microstructure [2]. These findings constitute a path forward in understanding the fundamentals of damping in ferromagnetic metals for practical device applications.

[1] B. Khodadadi et al. Phys. Rev. Lett. 124, 157201 (2020)

[2] S. Wu et al. arXiv:2109.03684   

Magnon Landau Levels and Spin Responses in Antiferromagnets

We study emergent gauge fields produced by gradients of the Dzyaloshinskii-Moriya interaction and propose a model of antiferromagnetic topological insulator of magnons.  This model is featured with spin degenerate Landau levels, akin to the quantum spin Hall system. In the long wavelength limit, the Landau levels induced by the emergent gauge field reflect relativistic physics described by the Klein-Gordon equation. Interestingly, by tuning the induced gauge field, this model uncovers an unconventional Hofstadter's butterfly. Due to the spin degeneracy property of the Landau levels, the system naturally exhibits magnonic spin Nernst response.  Moreover, the physical picture in this model is closely tied to the topological responses in skyrmion and vortex-antivortex crystal phases of AFM insulators. Our studies show that AFM insulators exhibit rich physics associated with topological magnon excitations.   

Elasto-Dynamical Induced Spin and Charge Pumping in Bulk Heavy Metals

     Analogous to the Spin-Hall Effect (SHE), {\it ab initio} electronic structure calculations reveal that acoustic phonons can induce charge (spin) current flowing along (normal to) its propagation direction. Using Floquet approach we have calculated the elastodynamical-induced charge and spin pumping in bulk Pt and demonstrate that: (i) While the longitudinal charge pumping is an intrinsic observable, the transverse pumped spin-current has an extrinsic origin that depends strongly on the electronic relaxation time; (ii) The longitudinal charge current is of nonrelativstic origin, while the transverse spin current is a relativistic effect that to lowest order scales linearly with the spin-orbit coupling strength; (iii) both charge and spin pumped currents have parabolic dependence on the amplitude of the elastic wave.   

How spin and orbital angular momenta are correlated during laser-induced ultrafast demagnetization

It is very well known that there is some degree of correlation

between spin and orbital degrees of freedom during laser-induced

ultrafast demagnetization. What is unknown or less clear is how

they are correlated. The experiments now have accumulated rich

sets of data from different types of materials, from 3d

transition ferromagnets to rare-earth materials.  Here, we use

time-dependent Liouville density functional theory and regular

TDDFT methods to demonstrate clearly that this correlation

exists. Due to spin-orbit coupling, the spin is dragged by the

orbital. In all the six systems that we investigated, we find

this correlation is element-specific. This information is crucial

to understanding demagnetization.  As researchers shift their

views toward phonon-based demagnetization mechanisms, we are

reminded that there are still much more important issues

overlooked so far. If the phonon angular momentum transfer is

important, then it must show up in the spin-orbital correlation

diagram.   

Strain Dependent Magnetocrystalline Anisotropy from First Principles

There has been emerging interest in metallic antiferromagnets since the potential for electrical switching was shown for CuMnAs and Mn$_2$Au. Magnetocrystalline anisotropy (MCA) is essential to understand spin dynamics and strain can be utilized to modify the anisotropy energy.

We used first-principles density functional theory to compute the strain dependence of MCA for Fe$_2$As and Mn$_2$As, which have same magnetic structure as CuMnAs.

The out-of-plane MCA of both materials is about 100 times larger than in-plane MCA. We then applied bi-axial strain on Mn$_2$As and uni-axial strain on Fe$_2$As to simulate directional strain in thin films. From this we show a tendency to decrease MCA under tension. Reduction of out-of-plane MCA was 3\% for 0.3\% strained Mn$_2$As and 30\% for 2\% strained Fe$_2$As. For Fe$_2$As, 2\% uni-axial strain distorted tetragonal unit cell into orthorhombic and the in-plane MCA became 100 times larger compared to the unstrained MCA.

Further investigation with larger range of strain would map the correlation between strain and energy to decide general amount of strain that can optimize MCA for device applications in the studied materials.   

Spin Manipulation through A Molecular Junction Based on Nuclear Berry Curvature Effect

Nuclear Berry curvature effects arise from going beyond the Born-Oppenheimer approximation and lead to pseudo-magnetic fields for nuclear motion. When a spin-orbit coupling is considered, different pseudo-magnetic fields must be applied to nuclear wavepackets associated with different electronic spins. We calculate electronic and spin currents through a molecular junction by utilizing a nonequilibrium Green's function approach, taking into account the electronic friction tensor and the relevant pseudo-magnetic fields. In the large voltage limit, we report significant spin polarization with decaying and oscillating signatures. These results are consistent with a magnetic AFM CISS experiment.   

Localized excitation of magnetic fields using ultrashort optical pulses

The interaction of optical pulses with magnetic materials has lately attracted a great deal of attention due to its potential applications, notably, the all- optical helicity dependent magnetic recording [1]. The experimental evidence readily shows that this interaction gives rise to a magnetic torque [2] whose nature has been the subject of much debate and discussion[3].

In this work we examine the interaction of fs pulses with a heavy metal in a measurement that is based the Hall effect. Short circularly polarized optical pulses are focused on a thin film of Pt that is patterned for a Hall measurement. An optically induced Hall signal is observed even in the absence of an external magnetic field indicating the generation of a localized magnetic field.

Our work opens a new avenue to explore the inner workings of this elusive interaction.  

 

[1]          C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, and T. Rasing, "All-Optical Magnetic Recording with Circularly Polarized Light", Physical Review Letters 99, 047601 (2007).

[2]          A. Capua, C. Rettner, S.-H. Yang, T. Phung, and S. S. P. Parkin, "Ensemble-averaged Rabi oscillations in a ferromagnetic CoFeB film", Nature Commun. 8, 16004 (2017).

[3]          A. Kirilyuk, A. V. Kimel, and T. Rasing, "Ultrafast optical manipulation of magnetic order", Reviews of Modern Physics 82, 2731 (2010).   

Probing magnetic anisotropy and spin-reorientation transition in 3D antiferromagnet single crystal Ho0.5Dy0.5FeO3|Pt using spin Hall magnetoresistance

Rare-earth orthoferrites (REFeO₃), are 3D antiferromagnets (AFM) that exhibit a weak ferromagnetism originating from slight canting of the spin moments and display a variety of spin reorientation transitions in the magnetic field (H)-temperature (T) parameter space. We present spin Hall magnetoresistance (SMR) studies on a b-plate of single crystal Ho_0.5Dy_0.5FeO₃|Pt hybrid, carried out by rotating H in the ac-plane (α-scan) at various T down to 11 K. At 300 K (H ≥ 800 Oe) in Γ₄(G_x, A_y, F_z) phase, SMR vs. α yielded a highly skewed curve with a sharp change, accompanied by a rotational hysteresis around a-axis. Notably, α-scans (H < 800 Oe) on the pre-pinned domain (±F_z domain) exhibited an anomalous sinusoidal signal of periodicity 360°. Low-T SMR (H = 2.4 kOe) resulted in a weakening of the anisotropy possibly due to the T-evolution of Fe-RE exchange coupling. Besides, below 25 K the SMR modulation showed an abrupt change around c-axis, marking the presence of Γ₂(F_x, C_y, G_z) phase. We have employed a simple Hamiltonian and computed SMR, to examine the observed SMR modulations. Our SMR studies not only serve as an effective probe for magnetic anisotropy and spin reorientation but also, highlight the potential of Ho_0.5Dy_0.5FeO₃ for future AFM spintronic devices.