13 research outputs found
Tracking the ultrafast motion of an antiferromagnetic order parameter
The unique functionalities of antiferromagnets offer promising routes to
advance information technology. Their compensated magnetic order leads to spin
resonances in the THz-regime, which suggest the possibility to coherently
control antiferromagnetic (AFM) devices orders of magnitude faster than
traditional electronics. However, the required time resolution, complex
sublattice interations and the relative inaccessibility of the AFM order
parameter pose serious challenges to studying AFM spin dynamics. Here, we
reveal the temporal evolution of an AFM order parameter directly in the time
domain. We modulate the AFM order in hexagonal YMnO by coherent
magnon excitation and track the ensuing motion of the AFM order parameter using
time-resolved optical second-harmonic generation (SHG). The dynamic symmetry
reduction by the moving order parameter allows us to separate electron dynamics
from spin dynamics. As transient symmetry reductions are common to coherent
excitations, we have a general tool for tracking the ultrafast motion of an AFM
order parameter.Comment: 5 pages, 4 figure
Efficient spin excitation via ultrafast damping-like torques in antiferromagnets
Damping effects form the core of many emerging concepts for high-speed
spintronic applications. Important characteristics such as device switching
times and magnetic domain-wall velocities depend critically on the damping
rate. While the implications of spin damping for relaxation processes are
intensively studied, damping effects during impulsive spin excitations are
assumed to be negligible because of the shortness of the excitation process.
Herein, we show that, unlike in ferromagnets, ultrafast damping plays a crucial
role in antiferromagnets because of their strongly elliptical spin precession.
In time-resolved measurements, we find that ultrafast damping results in an
immediate spin canting along the short precession axis. The interplay between
antiferromagnetic exchange and magnetic anisotropy amplifies this canting by
several orders of magnitude towards large-amplitude modulations of the
antiferromagnetic order parameter. This leverage effect discloses a highly
efficient route towards the ultrafast manipulation of magnetism in
antiferromagnetic spintronics
Nonlinear optical diode effect in a magnetic Weyl semimetal
Weyl semimetals have emerged as a promising quantum material system to
discover novel electrical and optical phenomena, due to their combination of
nontrivial quantum geometry and strong symmetry breaking. One crucial class of
such novel transport phenomena is the diode effect, which is of great interest
for both fundamental physics and modern technologies. In the electrical regime,
giant electrical diode effect (the nonreciprocal transport) has been observed
in Weyl systems. In the optical regime, novel optical diode effects have been
theoretically considered but never probed experimentally. Here, we report the
observation of the nonlinear optical diode effect (NODE) in the magnetic Weyl
semimetal CeAlSi, where the magnetic state of CeAlSi introduces a pronounced
directionality in the nonlinear optical second-harmonic generation (SHG). By
physically reversing the beam path, we show that the measured SHG intensity can
change by at least a factor of six between forward and backward propagation
over a wide bandwidth exceeding 250 meV. Supported by density-functional theory
calculations, we establish the linearly dispersive bands emerging from Weyl
nodes as the origin of the extreme bandwidth. Intriguingly, the NODE
directionality is directly controlled by the direction of magnetization. By
utilizing the electronically conductive semimetallic nature of CeAlSi, we
demonstrate current-induced magnetization switching and thus electrical control
of the NODE in a mesoscopic spintronic device structure with current densities
as small as 5 kA/cm. Our results advance ongoing research to identify novel
nonlinear optical/transport phenomena in magnetic topological materials. The
NODE also provides a way to measure the phase of nonlinear optical
susceptibilities and further opens new pathways for the unidirectional
manipulation of light such as electrically controlled optical isolators.Comment: 28 pages, 12 figure
Quantum metric nonlinear Hall effect in a topological antiferromagnetic heterostructure
Quantum geometry - the geometry of electron Bloch wavefunctions - is central
to modern condensed matter physics. Due to the quantum nature, quantum geometry
has two parts, the real part quantum metric and the imaginary part Berry
curvature. The studies of Berry curvature have led to countless breakthroughs,
ranging from the quantum Hall effect in 2DEGs to the anomalous Hall effect
(AHE) in ferromagnets. However, in contrast to Berry curvature, the quantum
metric has rarely been explored. Here, we report a new nonlinear Hall effect
induced by quantum metric by interfacing even-layered MnBi2Te4 (a PT-symmetric
antiferromagnet (AFM)) with black phosphorus. This novel nonlinear Hall effect
switches direction upon reversing the AFM spins and exhibits distinct scaling
that suggests a non-dissipative nature. Like the AHE brought Berry curvature
under the spotlight, our results open the door to discovering quantum metric
responses. Moreover, we demonstrate that the AFM can harvest wireless
electromagnetic energy via the new nonlinear Hall effect, therefore enabling
intriguing applications that bridges nonlinear electronics with AFM
spintronics.Comment: 19 pages, 4 figures and a Supplementary Materials with 66 pages, 4
figures and 3 tables. Originally submitted to Science on Oct. 5, 202
Antiferromagnetic metal phase in an electron-doped rare-earth nickelate
Long viewed as passive elements, antiferromagnetic materials have emerged as
promising candidates for spintronic devices due to their insensitivity to
external fields and potential for high-speed switching. Recent work exploiting
spin and orbital effects has identified ways to electrically control and probe
the spins in metallic antiferromagnets, especially in noncollinear or
noncentrosymmetric spin structures. The rare earth nickelate NdNiO3 is known to
be a noncollinear antiferromagnet where the onset of antiferromagnetic ordering
is concomitant with a transition to an insulating state. Here, we find that for
low electron doping, the magnetic order on the nickel site is preserved while
electronically a new metallic phase is induced. We show that this metallic
phase has a Fermi surface that is mostly gapped by an electronic reconstruction
driven by the bond disproportionation. Furthermore, we demonstrate the ability
to write to and read from the spin structure via a large zero-field planar Hall
effect. Our results expand the already rich phase diagram of the rare-earth
nickelates and may enable spintronics applications in this family of correlated
oxides.Comment: 25 pages, 4 figure
Coherent spin dynamics in optically excited antiferromagnets
Antiferromagnetic spintronics emerged in the last decade as a promising approach to overcome limitations of current information technology. Owing to the vanishing net magnetization, antiferromagnetic materials exhibit spin dynamics on sub-picosecond timescales potentially allowing not only for data storage and logic circuit applications that are orders of magnitude faster than their established ferromagnetic counterparts, but also the development of new paradigms for device architectures with greater functionality. Due to a tremendous interest in the realization of antiferromagnet-based devices, the tools for the ultrafast control and manipulation of antiferromagnets are currently being explored.
In the current thesis, we use time-resolved optical experiments to unravel the coherent spin dynamics in antiferromagnets on their intrinsic timescales. Three consecutive projects form the cornerstones of the present cumulative thesis. In the first project, we achieved for the first time the experimental discrimination of different spin excitation mechanisms. The fundamental understanding of the relevant excitation mechanism constitutes an unprecedented degree of optical control of antiferromagnets. The second project is concerned with probing antiferromagnetic spin dynamics. We show that time-resolved measurements of optical second-harmonic generation provide quantitative access directly to the antiferromagnetic order parameter. In combination with established magneto-optical probes, we track the motion of an antiferromagnetic order parameter in three dimensions. We find that the spin precession during an antiferromagnetic resonance exhibits a pronounced ellipticity, which opens up new routes for the energy efficient control of antiferromagnetic order. Lastly, we show that spin damping during the optical excitation gives rise to an optically induced ferromagnetic spin canting in otherwise fully compensated antiferromagnets. We show that this process, which has so far been neglected for ultrafast optical excitations, can be the dominant spin excitation mechanism in antiferromagnets.
The results of this thesis provide new insights into the optical control and manipulation of antiferromagnets. These key findings are crucial for the development of future antiferromagnetic spintronic devices.
Beyond academic research, the present thesis also constitutes an educational advancement. In an effort to improve our capabilities of teaching X-ray diffraction techniques to undergraduate students, an intuitive and flexible device is presented in the appendix
A perspective on nonlinearities in coherent magnetization dynamics
The recent thrust in ultrafast magnetization dynamics aims at extending spintronic functionalities to terahertz frequencies. Deterministic manipulation of magnetization at the corresponding ultrashort timescales requires minute control not only over the magnetization itself but also the reservoirs it is interacting with. Although the various intricate couplings between spins, phonons, and electrons-all of which are susceptible to ultrashort laser pulses-lead to many (often nonlinear) coupling routes, magnetization-dynamical nonlinearities have remained largely underexplored. In this Perspective, we highlight recent advances and foresee future developments in the rapidly evolving field of nonlinear magnetization dynamics. Given the elementary character of coherent excitations, we put particular emphasis on their nonlinearities. We briefly review theoretical aspects and assess excitation mechanisms to reach the nonlinear regime of magnetic excitations in a broad class of magnetic materials, such as ferromagnets, antiferromagnets, and ferrimagnets. We present an overview of the groundbreaking experiments that showcase the unique insights provided by magnetic nonlinearities. We conclude by discussing open challenges and opportunities that underpin the potential of nonlinear magnetization dynamics for the advancement of spintronics and cavity quantum electrodynamics with spin waves at terahertz frequencies
A perspective on nonlinearities in coherent magnetization dynamics
The recent thrust in ultrafast magnetization dynamics aims at extending spintronic functionalities to terahertz frequencies. Deterministic manipulation of magnetization at the corresponding ultrashort timescales requires minute control not only over the magnetization itself but also the reservoirs it is interacting with. Although the various intricate couplings between spins, phonons, and electrons - all of which are susceptible to ultrashort laser pulses - lead to many (often nonlinear) coupling routes, magnetization-dynamical nonlinearities have remained largely underexplored. In this Perspective, we highlight recent advances and foresee future developments in the rapidly evolving field of nonlinear magnetization dynamics. Given the elementary character of coherent excitations, we put particular emphasis on their nonlinearities. We briefly review theoretical aspects and assess excitation mechanisms to reach the nonlinear regime of magnetic excitations in a broad class of magnetic materials, such as ferromagnets, antiferromagnets, and ferrimagnets. We present an overview of the groundbreaking experiments that showcase the unique insights provided by magnetic nonlinearities. We conclude by discussing open challenges and opportunities that underpin the potential of nonlinear magnetization dynamics for the advancement of spintronics and cavity quantum electrodynamics with spin waves at terahertz frequencies.ISSN:0003-6951ISSN:1077-311
Electronic bulk and domain wall properties in B-site doped hexagonal ErMnO3
Acceptor and donor doping is a standard for tailoring semiconductors. More recently, doping was adapted to optimize the behavior at ferroelectric domain walls. In contrast to more than a century of research on semiconductors, the impact of chemical substitutions on the local electronic response at domain walls is largely unexplored. Here, the hexagonal manganite ErMnO3 is donor doped with Ti4+. Density functional theory calculations show that Ti4+ goes to the B site, replacing Mn3+. Scanning probe microscopy measurements confirm the robustness of the ferroelectric domain template. The electronic transport at both macroscopic and nanoscopic length scales is characterized. The measurements demonstrate the intrinsic nature of emergent domain wall currents and point towards Poole-Frenkel conductance as the dominant transport mechanism. Aside from the new insight into the electronic properties of hexagonal manganites, B-site doping adds an additional degree of freedom for tuning the domain wall functionality