29 research outputs found

### Magnetoelastic nature of solid oxygen epsilon-phase structure

For a long time a crystal structure of high-pressure epsilon-phase of solid
oxygen was a mistery. Basing on the results of recent experiments that have
solved this riddle we show that the magnetic and crystal structure of
epsilon-phase can be explained by strong exchange interactions of
antiferromagnetic nature. The singlet state implemented on quaters of O2
molecules has the minimal exchange energy if compared to other possible singlet
states (dimers, trimers). Magnetoelastic forces that arise from the spatial
dependence of the exchange integral give rise to transformation of 4(O2)
rhombuses into the almost regular quadrates. Antiferromagnetic character of the
exchange interactions stabilizes distortion of crystal lattice in epsilon-phase
and impedes such a distortion in long-range alpha- and delta-phases.Comment: 11 pages, 4 figures, Changes: corrected typos, reference to the
recent paper is adde

### High antiferromagnetic domain wall velocity induced by NÃ©el spin-orbit torques

We demonstrate the possibility to drive an antiferromagnetic domain wall at high velocities by fieldlike NÃ©el spin-orbit torques. Such torques arise from current-induced local fields that alternate their orientation on each sublattice of the antiferromagnet and whose orientation depends primarily on the current direction, giving them their fieldlike character. The domain wall velocities that can be achieved by this mechanism are 2 orders of magnitude greater than the ones in ferromagnets. This arises from the efficiency of the staggered spin-orbit fields to couple to the order parameter and from the exchange-enhanced phenomena in
antiferromagnetic texture dynamics, which leads to a low domain wall effective mass and the absence of a Walker breakdown limit. In addition, because of its nature, the staggered spin-orbit field can lift the degeneracy between two 180Â° rotated states in a collinear antiferromagnet, and it provides a force that can move such walls and control the switching of the states

### Laser-driven quantum magnonics and THz dynamics of the order parameter in antiferromagnets

The impulsive generation of two-magnon modes in antiferromagnets by
femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent
longitudinal oscillations of the antiferromagnetic order parameter that cannot
be described by a thermodynamic Landau-Lifshitz approach. We argue that this
dynamics is triggered as a result of a laser-induced modification of the
exchange interaction. In order to describe the oscillations we have formulated
a quantum mechanical description in terms of magnon pair operators and coherent
states. Such an approach allowed us to} derive an effective macroscopic
equation of motion for the temporal evolution of the antiferromagnetic order
parameter. An implication of the latter is that the photo-induced spin dynamics
represents a macroscopic entanglement of pairs of magnons with femtosecond
period and nanometer wavelength. By performing magneto-optical pump-probe
experiments with 10 femtosecond resolution in the cubic KNiF$_3$ and the
uniaxial K$_2$NiF$_4$ collinear Heisenberg antiferromagnets, we observed
coherent oscillations at the frequency of 22 THz and 16 THz, respectively. The
detected frequencies as a function of the temperature ideally fit the
two-magnon excitation up to the N\'eel point. The experimental signals are
described as dynamics of magnetic linear dichroism due to longitudinal
oscillations of the antiferromagnetic vector.Comment: 25 pages, 10 figure

### Evidence of non-degenerated, non-reciprocal and ultra-fast spin-waves in the canted antiferromagnet {\alpha}-Fe2O3

Spin-waves in antiferromagnets hold the prospects for the development of
faster, less power-hungry electronics, as well as new physics based on
spin-superfluids and coherent magnon-condensates. For both these perspectives,
addressing electrically coherent antiferromagnetic spin-waves is of importance,
a prerequisite that has so far been elusive, because unlike ferromagnets,
antiferromagnets couple weakly to radiofrequency fields. Here, we demonstrate
the electrical detection of ultra-fast non-reciprocal spin-waves in the
dipolar-exchange regime of a canted antiferromagnet. Using time-of-flight
spin-wave spectroscopy on hematite (alpha-Fe2O3), we find that the magnon wave
packets can propagate as fast as 30 km/s for reciprocal bulk spin-wave modes
and up to 10 km/s for surface-spin waves propagating parallel to the
antiferromagnetic N\'eel vector. The electrical detection of coherent
non-reciprocal antiferromagnetic spin waves holds makes hematite a versatile
platform where most of the magnonic concepts developed for ferromagnet can be
adapted paving the way for the development antiferromagnetic and
altermagnet-based magnonic devices

### Antiferromagnet-mediated interlayer exchange: hybridization versus proximity effect

We investigate the interlayer coupling between two thin ferromagnetic (F)
films mediated by an antiferromagnetic (AF) spacer in F*/AF/F trilayers and
show how it transitions between different regimes on changing the AF thickness.
Employing layer-selective Kerr magnetometry and ferromagnetic-resonance
techniques in a complementary manner enables us to distinguish between three
functionally distinct regimes of such ferromagnetic interlayer coupling. The F
layers are found to be individually and independently exchange-biased for thick
FeMn spacers - the first regime of no interlayer F-F* coupling. F-F* coupling
appears on decreasing the FeMn thickness below 9 nm. In this second regime
found in structures with 6.0-9.0 nm thick FeMn spacers, the interlayer coupling
exists only in a finite temperature interval just below the effective N\'eel
temperature of the spacer, which is due to magnon-mediated exchange through the
thermally softened antiferromagnetic spacer, vanishing at lower temperatures.
The third regime, with FeMn thinner than 4 nm, is characterized by a much
stronger interlayer coupling in the entire temperature interval, which is
attributed to a magnetic-proximity induced ferromagnetic exchange. These
experimental results, spanning the key geometrical parameters and thermal
regimes of the F*/AF/F nanostructure, complemented by a comprehensive
theoretical analysis, should broaden the understanding of the interlayer
exchange in magnetic multilayers and potentially be useful for applications in
spin-thermionics.Comment: 14 pages, 9 figure

### Peculiarities of the stochastic motion in antiferromagnetic nanoparticles

Antiferromagnetic (AFM) materials are widely used in spintronic devices as
passive elements (for stabilization of ferromangetic layers) and as active
elements (for information coding). In both cases switching between the
different AFM states depends in a great extent from the environmental noise. In
the present paper we derive the stochastic Langevin equations for an AFM vector
and corresponding Fokker-Planck equation for distribution function in the phase
space of generalised coordinate and momentum. Thermal noise is modeled by a
random delta-correlated magnetic field that interacts with the dynamic
magnetisation of AFM particle. We analyse in details a particular case of the
collinear compensated AFM in the presence of spin-polarised current. The energy
distribution function for normal modes in the vicinity of two equilibrium
states (static and stationary) in sub- and super-critical regimes is found. It
is shown that the noise-induced dynamics of AFM vector has pecuilarities
compared to that of magnetisation vector in ferromagnets.Comment: Submitted to EPJ ST, presented at the 4-th Conference on Statistical
Physics, Lviv, Ukraine, 201

### Antiferromagnetic spintronics

Antiferromagnetic materials are magnetic inside, however, the direction of
their ordered microscopic moments alternates between individual atomic sites.
The resulting zero net magnetic moment makes magnetism in antiferromagnets
invisible on the outside. It also implies that if information was stored in
antiferromagnetic moments it would be insensitive to disturbing external
magnetic fields, and the antiferromagnetic element would not affect
magnetically its neighbors no matter how densely the elements were arranged in
a device. The intrinsic high frequencies of antiferromagnetic dynamics
represent another property that makes antiferromagnets distinct from
ferromagnets. The outstanding question is how to efficiently manipulate and
detect the magnetic state of an antiferromagnet. In this article we give an
overview of recent works addressing this question. We also review studies
looking at merits of antiferromagnetic spintronics from a more general
perspective of spin-ransport, magnetization dynamics, and materials research,
and give a brief outlook of future research and applications of
antiferromagnetic spintronics.Comment: 13 pages, 7 figure

### Spin transport and spin torque in antiferromagnetic devices

Ferromagnets are key materials for sensing and memory applications. In contrast, antiferromagnets which represent the more common form of magnetically ordered materials, have found less practical application beyond their use for establishing reference magnetic orientations via exchange bias. This might change in the future due to the recent progress in materials research and discoveries of antiferromagnetic spintronic phenomena suitable for device applications. Experimental demonstration of the electrical switching and detection of the NÃ©el order open a route towards memory devices based on antiferromagnets. Apart from the radiation and magnetic-field hardness, memory cells fabricated from antiferromagnets can be inherently multilevel, which could be used for neuromorphic computing. Switching speeds attainable in antiferromagnets far exceed those of ferromagnetic and semiconductor memory technologies. Here we review the recent progress in electronic spin-transport and spin-torque phenomena in antiferromagnets that are dominantly of the relativistic quantum mechanical origin. We discuss their utility in pure antiferromagnetic or hybrid ferromagnetic/antiferromagnetic memory devices