|General direction of my research is Theory
of Magnetic Phenomena and Statistical Physics, including
list allows viewing papers that belong to any of these groups separately.
At the time being, my hot topics are random-field and
random-anisotropy magnets, fronts of
tunneling in molecular magnets, and collective
- Current topics:
- random-field and random-anisotropy
- relaxation and decoherence, including
spin-lattice interaction, superradiance and phonon bottleneck
- magnetomechanical effects in nanomagnets
- quantum transitions including Landau-Zener
- spin tunneling in molecular magnets,
including fronts of tunneling
- magnetic deflagration
- structure and dynamics of small magnetic particles,
including internal spin waves
- collapse of skyrmions in ferro- and
- Older topics:
- magnetic phase transitions
- low-dimensional magnetism and surface magnetis
- relaxation of spin waves
- domain-wall motion
Theoretical arguments of 70th by Imry & Ma, Larkin, and Chudnovsky predict that
whatever small random field or random anisotropy would destroy a long-range
order in magnetic and other system (vortex lattice in superconductors,
charge-density waves, etc.). Using modern computer power and advanced numerical
algorithms, we are carrying out an extensive investigation of ordering in
different models described by n-component classical spins in d
dimensions. Contrary to theoretical expectations, we have found that initially
ordered systems only partially disorder if the system supports singularities
(vortices and vortex lines, hedgehogs) or smooth topological structures (kinks,
skyrmions). The former exist for n £ d,
and necessarily arize in the completely disordered Imry-Ma state, as follows
from our new topological argument. As creating singularities cost energy, the
Imry-Ma state is not the ground state of the system and the system remains
partially ordered. The case n = d + 1 is a marginal case. Although
the IM state is the grounds state, the system cannot reach it by relaxation
because of conservation of the topological charge (e.g., of skyrmions). We have
published a big paper on the 3d random-field xy model (n = 2) [Phys.
Rev. B 88, 224418 (2013)] and posted a Letter containing the above general
arguments illustrated by numerical results [arXiv].
Here is the info page of the
project containing animations.
Pinned vortex lines in the 3d random-field xy
I am especially interested in the so-called "PhononBottleneck"
problem in the spin-lattice relaxation that has not recieved a proper analytical
treatment since 1941. While first steps have been done in Spring 2006 and
two papers have been published [Phys.
Rev. B 75, 094409 (2007) and
Rev. B 77, 024429 (2008)], a lot of new
efforts are due, both on the way of analytics and numerical calculations
for quantum and classical models. A possible competition between the phonon
bottleneck (that tends to suppress spin relaxation) and the
Rev. Lett. 89, 157201 (2002)] and
Rev. Lett. 93, 257205 (2004)] superradiance (that tends to
increase spin relaxation) is an exciting question to be investigated.
The phonon bottleneck: Emitted phonons are being reabsorbed by
spins and thus spin-lattice relaxation slows down dramatically
Right now the so-called Landau-Lifshitz-Bloch (LLB) equation of motion for
the magnetization at finite temperatures, derived in 90th [see, e.g.,
Rev. B 55, 3050–3057 (1997)], becomes the best
candidate to be applied to the processes of thermal magnetic recording. The LLB equation describes both transverse and longitudinal relaxation
of the magnetization vector. My colleagues and I are further developing
the aspects of the LLB equation that are important for industrial applications
in magnetic information storage.
LLB equation captures dynamics of the magnetization length and can be used to
describe the effect of heating in fast magnetization reversal
We are continuing working on the beautiful parameter-free spin-phonon
interaction that in the rotating lattice
frame has the form as simple as -W·S [Phys.
Rev. B 72, 094426 (2005)]. This very basic interaction that allows to obtain
some new model-independent results in spin relaxation and rederive some old
results in a simple way, has confused a number of theorists.
Another hot topic
is the magnetic burning or deflagration
that recently was experimentally observed by Myriam Sarachik's group at
the City College of the CUNY and by Javier Tejada's group at the University
of Barcelona. Although the first and simplest theory of the magnetic burning
has been recently published (Suzuki
et al, PRL 2005) that were a lot of questions to be clarified such as
the role of spin tunneling and the ignition of the magnetic avalanch.
Here is the
support page with animations and link to our first paper on magnetic
Rev. B 76, 054410-(13) (2007).
Magnetic deflagration (burning): Decay of a metastable state triggered by the
Continuing the research on magnetic
deflagration, we discovered fronts of spin tunneling in molecular magnets [Phys.
Rev. Lett. 102, 097206-(4) (2009),
Phys. Rev. B 80, 014406-(11) (2009)]. This is the so-called "cold" or
quantum deflagration. In contrast to the standard deflagration, here the
parameter controlling the escape rate of magnetic molecules out of the
metastable state is not temperature but the dipolar field produced by the
magnetic molecules themselves. This self-consistent dipolar field can bring the
system on or off tunneling resonance. We have shown that the system of magnetic
molecules self-organizes in such a way that the molecules are on resonance in a
broad region along the propagation direction, thus facilitating tunneling and
motion of the front.
"Cold" or quantum magnetic deflagration: Decay of the metastable state
via resonance quantum transition controlled by the dipolar field
student Reem Jaafar and further with
undergraduate student Saaber Shoyeb we incorporated both thermal and
quantum effects in our generalized theory of magnetic deflagration in 1d,
Rev. B 81, 180401(R) (2010) and
Rev. B 85, 094403 (2012)]. Current work is being done on the full 3d
latest of my areas of interest is magnetomechanical effects in nanomagnets
related to conservation of the angular momentum. Recently E. M. Chudnovsky and I
have shown that the problem of spin tunneling in a rigid nanoparticle has a
beautiful analytical solution. If the particle's moment of inertia is below some
critical value, the spin cannot tunnel and localizes in one of the up/down
article of our PHD student Reem Jaafar considering a single magnetic
molecule rotating between conducting leads has been chosen for a feature in
Europhysics News and has been featured in
Lehman Today. A new type of spin decoherence arises from interaction of an
embedded spin with a torsional cantilever,
Rev. X 1, 011005 (2011).
Spin tunneling transitions excite torsional cantilever
very new topic is skyrmion collapse in 2d magnets that occurs because the
discreteness of the lattice. With our PhD student Liufei Cai we have calculated
analytically and numerically the lifetime of skyrmions in both ferro- and
Rev. B 86, 024429 (2012). The results can be relevant in high-temperature
superconductivity since all these materials have planes of magnetic atoms.
1) Skyrmion and a gas of emitted spin waves; 2) Spin waves carry away the
remaining energy of a collapsed skyrmion. Vertical axis: Mz.
These are my main current external collaborations:
Chudnovsky, Lehman College, CUNY - collective
spin-phonon relaxation, spin tunneling
Schilling, University of Mainz, Germany - Landau-Zener
Kachkachi, University of Perpignan, France - small
Dr. Oksana Chubykalo, Institute of Material Science, Madrid, Spain -
Look at the Stoner-Wohlfarth's astropyramid
Recognition Award for Research - 2010 - My prepared speech