178 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
Emergence of ferroelectricity at the morphotropic phase boundary of ultrathin BiFeO
We demonstrate the robustness of polarization in ultrathin compressive
strained BiFeO single layers and heterostructures during epitaxial
thin-film growth. Using in-situ optical second harmonic generation (ISHG), we
explore the emergence of ferroelectric phases at the strain-driven morphotropic
phase boundary in the ultrathin regime. We find that the epitaxial films grow
in the ferroelectric tetragonal (T-) phase without exhibition of a critical
thickness. The robustness of this high-temperature T-phase against
depolarizing-field effects is further demonstrated during the growth of
capacitor-like (metal|ferroelectric|metal) heterostructures. Using
temperature-dependent ISHG post-deposition, we identify the thickness-dependent
onset of the monoclinic distortion in the T-matrix and trace the signature of
the subsequent emergence of the strain-relaxed rhombohedral-like monoclinic
phase. Our results show that strain-driven T-phase stabilization in BiFeO
yields a prominent candidate material for realizing ultrathin ferroelectric
devices.Comment: 5 pages, 3 figure
Current-induced switching of YIG/Pt bilayers with in-plane magnetization due to Oersted fields
We report on the switching of the in-plane magnetization of thin yttrium iron
garnet (YIG)/Pt bilayers induced by an electrical current. The switching is
either field-induced and assisted by a dc current, or current-induced and
assisted by a static magnetic field. The reversal of the magnetization occurs
at a current density as low as ~A/cm and magnetic fields of ~T, two orders of magnitude smaller than in ferromagnetic metals,
consistently with the weak uniaxial anisotropy of the YIG layers. We use the
transverse component of the spin Hall magnetoresistance to sense the magnetic
orientation of YIG while sweeping the current. Our measurements and simulations
reveal that the current-induced effective field responsible for switching is
due to the Oersted field generated by the current flowing in the Pt layer
rather than by spin-orbit torques, and that the switching efficiency is
influenced by pinning of the magnetic domains
Electrostatic topology of ferroelectric domains in YMnO
Trimerization-polarization domains in ferroelectric hexagonal YMnO were
resolved in all three spatial dimensions by piezoresponse force microscopy.
Their topology is dominated by electrostatic effects with a range of 100 unit
cells and reflects the unusual electrostatic origin of the spontaneous
polarization. The response of the domains to locally applied electric fields
explains difficulties in transferring YMnO into a single-domain state. Our
results demonstrate that the wealth of non-displacive mechanisms driving
ferroelectricity that emerged from the research on multiferroics are a rich
source of alternative types of domains and domain-switching phenomena
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
Ultrafast Modification of the Polarity at LaAlO/SrTiO Interfaces
Oxide growth with semiconductor-like accuracy has led to atomically precise
thin films and interfaces that exhibit a plethora of phases and functionalities
not found in the oxide bulk material. This yielded spectacular discoveries such
as the conducting, magnetic or even superconducting LaAlO/SrTiO
interfaces separating two prototypical insulating perovskite materials. All
these investigations, however, consider the static state at the interface,
although studies on fast oxide interface dynamics would introduce a powerful
degree of freedom to understanding the nature of the LaAlO/SrTiO
interface state. Here we show that the polarization state at the
LaAlO/SrTiO interface can be optically enhanced or attenuated within
picoseconds. Our observations are explained by a model based on charge
propagation effects in the interfacial vicinity and transient polarization
buildup at the interface
Fermi volume evolution and crystal field excitations in heavy-fermion compounds probed by time-domain terahertz spectroscopy
We measure the quasiparticle weight in the heavy-fermion compound
CeCuAu () by time-resolved THz spectroscopy for
temperatures from 2 up to 300\,K. This method distinguishes contributions from
the heavy Kondo band and from the crystal-electric-field satellite bands by
different THz response delay times. We find that the formation of heavy bands
is controlled by an exponentially enhanced, high-energy Kondo scale once the
crystal-electric-field states become thermally occupied. We corroborate these
observations by temperature-dependent dynamical mean-field calculations for the
multi-orbital Anderson lattice model and discuss consequences for quantum
critical scenarios.Comment: Published version, 6 pages (including references), 5 figures,
Supplemental Material (2 pages) adde
Long-range order in arrays of composite and monolithic magneto-toroidal moments
Magneto-toroidal order, also called ferrotoroidicity, is the most recently
established type of ferroic state. It is based on a spontaneous and uniform
alignment of unit-cell-sized magnetic whirls, called magneto-toroidal moments,
associated with a macroscopic toroidization. Because of its intrinsic
magnetoelectric coupling, this new ferroic state could be useful in the
development of spintronic devices. We exploit two-dimensional periodic arrays
of magnetostatically coupled nanomagnets as model systems for the investigation
of long-range magneto-toroidal order. We present two pathways promoting this
order, namely (i), structures comprising a ring of uniformly magnetized
sub-micrometer-sized bar magnets and (ii), structures in which each magnetic
building block itself hosts a magnetic vortex. For both cases calculations of
the magnetic-dipole interaction and micromagnetic simulations reveal the
conditions for the formation of spontaneous magneto-toroidal order. We confirm
this order and the formation of magneto-toroidal domains in our arrays by
magnetic force microscopy. We identify the presence of two types of domain-wall
states emerging from the competition of two intrinsic microscopic couplings.
Our work not only identifies the microscopic conditions promoting spontaneous
magneto-toroidal order but also highlights the possibility tailor mesoscale
magnetic arrays towards elusive types of ferroic order.Comment: 20 pages, 7 figure
Nonreciprocal second harmonic generation in a magnetoelectric material
Nonreciprocal devices that allow the light propagation in only one direction
are indispensable in photonic circuits and emerging quantum technologies.
Contemporary optical isolators and circulators, however, require large size or
strong magnetic fields because of the general weakness of magnetic light-matter
interactions, which hinders their integration into photonic circuits. Aiming at
stronger magneto-optical couplings, a promising approach is to utilize
nonlinear optical processes. Here, we demonstrate nonreciprocal magnetoelectric
second harmonic generation (SHG) in CuB2O4. SHG transmission changes by almost
100% in a magnetic-field reversal of just 10 mT. The observed nonreciprocity
results from an interference between the magnetic-dipole- and
electric-dipole-type SHG. Even though the former is usually notoriously smaller
than the latter, it is found that a resonantly enhanced
magnetic-dipole-transition has a comparable amplitude as non-resonant
electric-dipole-transition, leading to the near-perfect nonreciprocity. This
mechanism could form one of the fundamental bases of nonreciprocity in
multiferroics, which is transferable to a plethora of magnetoelectric systems
to realize future nonreciprocal and nonlinear-optical devices.Comment: 21 pages, 4 figure
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