190
8 Terahertz Spintronics
But the main drawback of metal Thz emitter is their poor bandwidth. It is only around
3 THz.
In this situation, it is important to mention that all previously stated THz emitters
solely exploit the advantages of the charge but not the spin of the electron and deliver
emission spectra with substantial gaps. Low-cost and easy-to-use emitters have to be
devised for extensive and extended use of THz spectroscopy. A promising approach is
spintronic terahertz emitters, based on a few nanometer thick metallic heterostructure
composed of magnetic and non-magnetic materials. The rapid advances in the field
of spintronics and femtomagnetism reveal that the electron spin has opened up an
entirely new and promising technology for generation, detection and control of spin
currents in metals and insulators. In fact, combining the spintronic and photonic
properties of magnetic materials, novel THz source has already been reported which
is operated by −1 nJ laser pulses from a compact, high-repetition-rate femtosecond
laser oscillator. Choosing proper spintronic material and optimizing its geometrical
parameters one can realize a single device that may exhibit many advantages like
large bandwidth, low pump power, easy operation, scalability and low cost. Spinbased THz emitters can be of two types; (i) femtosecond laser driven and (ii) spin
current driven. Now, we will discuss about different laser-driven spin-based THz
emitters in detail.
8.5.1 Metallic Spintronic THz Emitter (MSTE)
The new exciting phenomena like spin-dependent Seebeck effect (SDSE), the inverse
Spin Hall Effect (ISHE) and the Spin Seebeck effect (SSE) have made spintronic
devices more competent towards the efficient generation, transport and detection of
spin currents (Werake et al. 2011; Saitoh et al. 2006). These effects have already been
discussed in Sect. 2.12. The key role is played by ferromagnetic (FM)/non-magnetic
(NM) bilayer for the emission THz radiation. Various types of conventional magnetic
materials and binary alloys are used to make THz source (Kampfrath et al. 2013;
Li et al. 2017; Wu et al. 2016; Freimuth et al. 2015; Bocklage 2017; Ganichev
et al. 2002; Yang et al. 2016; Olejník et al. 2018; Walowski and M. Meunzenberg,
2016; Spintronics for Next Generation Innovative 2015; Kampfrath 2018; Seifert
et al. 2017; Lendinez et al. 2019). Some of the promising alloys are: ferrimagnetic
magnetite (Fe 3 O 4 ), (anti)ferromagnetic iron rhodium (FeRh) and the ferrimagnetic
alloys dysprosium cobalt (DyCo 5 ) and gadolinium iron (Gd 24 Fe 76 ). A schematic
structure and basic principle of operation of bilayer THZ emitter are shown in Fig. 8.4.
Basic Principle of Operation:
A femtosecond laser that emits ultrashort pulse hits the ferri-/ferromagnetic (FM)
and the non-magnetic (NM) nanostructure. Subsequently, it stimulates the electrons
in the ferromagnetic film and brings them in non-equilibrium state. In FM material,
majority (spin-up) and minority (spin-down) spin channels have two different transport properties. Hence, a strong spin current is created and flows into the adjacent
8 Terahertz Spintronics
But the main drawback of metal Thz emitter is their poor bandwidth. It is only around
3 THz.
In this situation, it is important to mention that all previously stated THz emitters
solely exploit the advantages of the charge but not the spin of the electron and deliver
emission spectra with substantial gaps. Low-cost and easy-to-use emitters have to be
devised for extensive and extended use of THz spectroscopy. A promising approach is
spintronic terahertz emitters, based on a few nanometer thick metallic heterostructure
composed of magnetic and non-magnetic materials. The rapid advances in the field
of spintronics and femtomagnetism reveal that the electron spin has opened up an
entirely new and promising technology for generation, detection and control of spin
currents in metals and insulators. In fact, combining the spintronic and photonic
properties of magnetic materials, novel THz source has already been reported which
is operated by −1 nJ laser pulses from a compact, high-repetition-rate femtosecond
laser oscillator. Choosing proper spintronic material and optimizing its geometrical
parameters one can realize a single device that may exhibit many advantages like
large bandwidth, low pump power, easy operation, scalability and low cost. Spinbased THz emitters can be of two types; (i) femtosecond laser driven and (ii) spin
current driven. Now, we will discuss about different laser-driven spin-based THz
emitters in detail.
8.5.1 Metallic Spintronic THz Emitter (MSTE)
The new exciting phenomena like spin-dependent Seebeck effect (SDSE), the inverse
Spin Hall Effect (ISHE) and the Spin Seebeck effect (SSE) have made spintronic
devices more competent towards the efficient generation, transport and detection of
spin currents (Werake et al. 2011; Saitoh et al. 2006). These effects have already been
discussed in Sect. 2.12. The key role is played by ferromagnetic (FM)/non-magnetic
(NM) bilayer for the emission THz radiation. Various types of conventional magnetic
materials and binary alloys are used to make THz source (Kampfrath et al. 2013;
Li et al. 2017; Wu et al. 2016; Freimuth et al. 2015; Bocklage 2017; Ganichev
et al. 2002; Yang et al. 2016; Olejník et al. 2018; Walowski and M. Meunzenberg,
2016; Spintronics for Next Generation Innovative 2015; Kampfrath 2018; Seifert
et al. 2017; Lendinez et al. 2019). Some of the promising alloys are: ferrimagnetic
magnetite (Fe 3 O 4 ), (anti)ferromagnetic iron rhodium (FeRh) and the ferrimagnetic
alloys dysprosium cobalt (DyCo 5 ) and gadolinium iron (Gd 24 Fe 76 ). A schematic
structure and basic principle of operation of bilayer THZ emitter are shown in Fig. 8.4.
Basic Principle of Operation:
A femtosecond laser that emits ultrashort pulse hits the ferri-/ferromagnetic (FM)
and the non-magnetic (NM) nanostructure. Subsequently, it stimulates the electrons
in the ferromagnetic film and brings them in non-equilibrium state. In FM material,
majority (spin-up) and minority (spin-down) spin channels have two different transport properties. Hence, a strong spin current is created and flows into the adjacent