40 research outputs found
Microscopic theory of Bose–Einstein condensation of magnons at room temperature
A quantised spin wave – magnon – in magnetic films can undergo Bose- Einstein condensation into two energetically degenerate lowest-energy quan- tum states with non-zero wave vectors ±kBEC. This corresponds to two in- terfering condensates forming spontaneously in momentum space. Brillouin Light Scattering studies for a microwave-pumped film with sub-micrometer spatial resolution experimentally confirm the existence of the two wave- functions and show that their interference results in a non-uniform ground state of the condensate with the density oscillating in space. Moreover, fork dislocations in the density fringes provide direct experimental evidence for the formation of pinned half quantum vortices in the magnon condensate. The measured amplitude of the density oscillation implies the formation of a non-symmetric state that corresponds to non equal occupation of two en- ergy minima. We discuss the experimental findings and consider the theory of magnon condensates which includes, to leading order, the contribution from the non-condensed magnons. The e↵ect of the non-condensed magnon cloud is to increase the contrast of the asymmetric state and to bring about the experimental measurements
Nanowire Spin Torque Oscillator Driven by Spin Orbit Torques
Spin torque from spin current applied to a nanoscale region of a ferromagnet
can act as negative magnetic damping and thereby excite self-oscillations of
its magnetization. In contrast, spin torque uniformly applied to the
magnetization of an extended ferromagnetic film does not generate
self-oscillatory magnetic dynamics but leads to reduction of the saturation
magnetization. Here we report studies of the effect of spin torque on a system
of intermediate dimensionality - a ferromagnetic nanowire. We observe coherent
self-oscillations of magnetization in a ferromagnetic nanowire serving as the
active region of a spin torque oscillator driven by spin orbit torques. Our
work demonstrates that magnetization self-oscillations can be excited in a
one-dimensional magnetic system and that dimensions of the active region of
spin torque oscillators can be extended beyond the nanometer length scale.Comment: The link to the published version is
http://www.nature.com/ncomms/2014/141205/ncomms6616/full/ncomms6616.htm
Chiral charge pumping in graphene deposited on a magnetic insulator
We demonstrate that a sizable chiral charge pumping can be achieved at room
temperature in graphene/Yttrium Iron Garnet (YIG) bilayer systems. The effect,
which cannot be attributed to the ordinary spin pumping, reveals itself in the
creation of a dc electric field/voltage in graphene as a response to the
dynamic magnetic excitations (spin waves) in an adjacent out-of-plane
magnetized YIG film. We show that the induced voltage changes its sign when the
orientation of the static magnetization is reversed, clearly indicating the
broken spatial inversion symmetry in the studied system. The strength of effect
shows a non-monotonous dependence on the spin-wave frequency, in agreement with
the proposed theoretical model.Comment: 8 pages, 5 figure
Generation of coherent spin-wave modes in Yttrium Iron Garnet microdiscs by spin-orbit torque
Spin-orbit effects [1-4] have the potential of radically changing the field
of spintronics by allowing transfer of spin angular momentum to a whole new
class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed
that by depositing Platinum (Pt, a normal metal) on top of a 1.3 m thick
Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer
spin angular momentum through the interface between these two different
materials. The outstanding feature was the detection of auto-oscillation of the
YIG when enough dc current was passed in the Pt. This finding has created a
great excitement in the community for two reasons: first, one could control
electronically the damping of insulators, which can offer improved properties
compared to metals, and here YIG has the lowest damping known in nature;
second, the damping compensation could be achieved on very large objects, a
particularly relevant point for the field of magnonics [6,7] whose aim is to
use spin-waves as carriers of information. However, the degree of coherence of
the observed auto-oscillations has not been addressed in ref. [5]. In this
work, we emphasize the key role of quasi-degenerate spin-wave modes, which
increase the threshold current. This requires to reduce both the thickness and
lateral size in order to reach full damping compensation [8] , and we show
clear evidence of coherent spin-orbit torque induced auto-oscillation in
micron-sized YIG discs of thickness 20 nm
Degenerate and non-degenerate parametric excitation in YIG nanostructures
We study experimentally the processes of parametric excitation in microscopic
magnetically saturated disks of nanometer-thick Yttrium Iron Garnet. We show
that, depending on the relative orientation between the parametric pumping
field and the static magnetization, excitation of either degenerate or
non-degenerate magnon pairs is possible. In the latter case, which is
particularly important for applications associated with the realization of
computation in the reciprocal space, a single-frequency pumping can generate
pairs of magnons whose frequencies correspond to different eigenmodes of the
disk. We show that, depending on the size of the disk and the modes involved,
the frequency difference in a pair can vary in the range 0.1-0.8 GHz. We
demonstrate that in this system, one can easily realize a practically important
situation where several magnon pairs share the same mode. We also observe the
simultaneous generation of up to six different modes using a fixed-frequency
monochromatic pumping. Our experimental findings are supported by numerical
calculations that allow us to unambiguously identify the excited modes. Our
results open new possibilities for the implementation of reciprocal-space
computing making use of low damping magnetic insulators.Comment: 18 pages, 4 figure
Spin wave confinement
This book presents recent scientific achievements in the investigation of magnetization dynamics in confined magnetic systems. Introduced by Bloch as plane waves of magnetization in unconfined ferromagnets, spin waves currently play an important role in the description of very small magnetic systems ranging from microelements, which form the basis of magnetic sensors, to magnetic nano-contacts. The spin wave confinement effect was experimentally discovered in the 1990s in permalloy microstripes. The diversity of systems where this effect is observed has been steadily growing since then, an
Magnonics: From Fundamentals to Applications
Spin waves (and their quanta magnons) can effectively carry and process information in magnetic nanostructures. By analogy to photonics, this research field is labelled magnonics. It comprises the study of excitation, detection, and manipulation of magnons. From the practical point of view, the most attractive feature of magnonic devices is the controllability of their functioning by an external magnetic field. This book has been designed for students and researchers working in magnetism. Here the readers will find review articles written by leading experts working on realization of magnonic devices