Ultrafast and Distinct Spin Dynamics in Magnetic
Alloys and Heterostructures
I. Radu1, T. A. Ostler2* et al.
1Helmholtz
Zentrum, Berlin, Germany
2The University of York, York, UK
1. Introduction
4. Incorporating Laser Heating
Although the next generation of ultrahigh density magnetic data storage
will rely on tuned material properties, such as, Curie temperature or
magnetic anisotropy, realized on alloys consisting of several elements,
theoretical treatment of their magnetization dynamics has so far assumed
equilibrium between their sublattices. Such a picture fails to describe
processes that occur on the sub-picosecond timescale, such as
femtosecond laser excitation.
Linearly polarised light has the effect of generating heat in the magnetic
material. To incorporate the laser heating into the model we require a well
defined temperature, which will vary with time. The model we are using
allows time variations of the temperature via the stochastic process. We use
a two-temperature model that defines a temperature associated with
delocalized electrons and the phonons of the system[3]:
Using a model of atomistic spin dynamics and X-ray magnetic circular
dichroism (XMCD) experiments, we show magnetization processes on the
sub-picosecond timescale vary depending on the magnetic moments and
the exchange interaction that couples them.
2. Atomistic Model
Our model solves a set of coupled Landau-Lifshitz-Gilbert equations for an
(3D) ensemble of spin:
5. Results
Numerical Simulations
The Hamiltonian of the system contains only Heisenberg (nearest
neighbour) exchange and anisotropy:
The effective field in the LLG equation is then written,
,
with,
, being the (stochastic) thermal term. The thermal term has the
properties:
Experimental Measurements
3. Disordered Moment Model
Using our model of atomistic spin dynamics we modelled a ferrimagnetic
(GdFeCo[1,2]) and a ferromagnetic material (NiFe) with two distinct species
Both simulations and experiments show different demagnetization times
for different species for both ferromagnetically and antiferromagnetically
coupled alloys.
6. Conclusions
We predict that the
demagnetization time
scales with the atomic
magnetic moment based
on our model:
Supported experimentally
for a range of materials.
References:
[1] – Radu et al. Nature 472, 205-208 (2011).
[2] – Ostler et al. Phys. Rev. B 84, 024407 (2011).
[3] – Chen et al. Int. Journ. Heat and Mass Trans. 49, 307-316 (2006).