166
7 Opto-spintronics
manipulation has created high impact for future spin-based electronics devices and
quantum computation. Femtosecond optical excitation gives raise a fundamental
question. The question is whether magnetization reversal would be possible, which is
faster than within half a precessional period? These questions may not play a decisive
role at larger time scale and for equilibrium states, but it becomes very important when
magnetically ordered spins, the electron system and the lattice become dynamically
isolated.
Such investigations on ultrafast magnetization dynamics are technologically relevant where there is a lag between manipulating speed and data storing, creating a
so-called ultrafast technology gap. The same thing is also applicable in spintronics
as in, for example, MRAM devices. Therefore, the investigation of the fundamental
as well as practical speed limits of magnetization direction are surely of great importance. Logical bits designated by ‘ones’ and ‘zeros’ are stored in magnetic memory
devices by assigning the magnetization vector either ‘up’ or ‘down’. Conventionally, magnetic bit is recorded by reversing the magnetization on the application of
magnetic field. Intuitively, one would expect that switching could be infinitely fast,
limited only by the attainable strength and shortness of the magnetic field pulse.
However, recent experiments show that deterministic magnetization reversal does
not take place if the magnetic field pulse is shorter than two picoseconds. Could
optical pulses be an alternative?
7.4 Laser Pulse and Its Impact on a Magnetic System
Demonstration of ultrafast demagnetization of a Ni film by a 60 femtosecond optical
laser pulse triggers the emergence of laser-based controlling of magnetization (Beaurepaire et al. 1996). Findings were also confirmed by subsequent experiments and
opened the possibility of light generated coherent magnetic precession, laser-induced
spin reorientation or even modification of magnetic structure on a time scale of one
picosecond or less. However, despite the reporting of many interesting experimental
results, the ultrafast optical manipulation of magnetism is still not clearly understood.
The excitation with fs laser pulse puts a magnetic medium into a non-equilibrium state
where the conventional macro-spin approximation fails and the magnetic phenomena
cannot be explained thermodynamically. At shorter time scales (sub-picosecond),
the exchange interaction is viewed as time-dependent phenomenon. All these issues
seriously complicate the understanding of this problem and give raise many questions.
What are the roles of different kinds of interactions, namely, spin–orbit interaction, spin–lattice interaction and electron–lattice interactions in the ultrafast light
controlled magnetism? How does the band structure of electron affect the laserassisted changes in magnetic order? A thorough investigation may answer these
questions, because light controlled magnetism has been observed in dielectrics,
metals and also in semiconductors. As magnetism is related to angular momentum,
then demagnetization is concerned with fundamental question of the conservation
and transfer of angular momentum. Different types of angular momentum transfer
7 Opto-spintronics
manipulation has created high impact for future spin-based electronics devices and
quantum computation. Femtosecond optical excitation gives raise a fundamental
question. The question is whether magnetization reversal would be possible, which is
faster than within half a precessional period? These questions may not play a decisive
role at larger time scale and for equilibrium states, but it becomes very important when
magnetically ordered spins, the electron system and the lattice become dynamically
isolated.
Such investigations on ultrafast magnetization dynamics are technologically relevant where there is a lag between manipulating speed and data storing, creating a
so-called ultrafast technology gap. The same thing is also applicable in spintronics
as in, for example, MRAM devices. Therefore, the investigation of the fundamental
as well as practical speed limits of magnetization direction are surely of great importance. Logical bits designated by ‘ones’ and ‘zeros’ are stored in magnetic memory
devices by assigning the magnetization vector either ‘up’ or ‘down’. Conventionally, magnetic bit is recorded by reversing the magnetization on the application of
magnetic field. Intuitively, one would expect that switching could be infinitely fast,
limited only by the attainable strength and shortness of the magnetic field pulse.
However, recent experiments show that deterministic magnetization reversal does
not take place if the magnetic field pulse is shorter than two picoseconds. Could
optical pulses be an alternative?
7.4 Laser Pulse and Its Impact on a Magnetic System
Demonstration of ultrafast demagnetization of a Ni film by a 60 femtosecond optical
laser pulse triggers the emergence of laser-based controlling of magnetization (Beaurepaire et al. 1996). Findings were also confirmed by subsequent experiments and
opened the possibility of light generated coherent magnetic precession, laser-induced
spin reorientation or even modification of magnetic structure on a time scale of one
picosecond or less. However, despite the reporting of many interesting experimental
results, the ultrafast optical manipulation of magnetism is still not clearly understood.
The excitation with fs laser pulse puts a magnetic medium into a non-equilibrium state
where the conventional macro-spin approximation fails and the magnetic phenomena
cannot be explained thermodynamically. At shorter time scales (sub-picosecond),
the exchange interaction is viewed as time-dependent phenomenon. All these issues
seriously complicate the understanding of this problem and give raise many questions.
What are the roles of different kinds of interactions, namely, spin–orbit interaction, spin–lattice interaction and electron–lattice interactions in the ultrafast light
controlled magnetism? How does the band structure of electron affect the laserassisted changes in magnetic order? A thorough investigation may answer these
questions, because light controlled magnetism has been observed in dielectrics,
metals and also in semiconductors. As magnetism is related to angular momentum,
then demagnetization is concerned with fundamental question of the conservation
and transfer of angular momentum. Different types of angular momentum transfer