Spintronics is based on the transport
processes of spin-polarized current between the elements of electronic devices.
The spin current control involves availability of spin current injectors,
control apparatuses of electron spin orientation and elements of a spin current
filtration. Therefore, the main areas of research in spintronics are the search
for new materials with a high degree of spin polarization of the conduction
electrons, studies of the laws of spin relaxation in solids and spin-dependent
transport in spintronics elements, and the development of methods and schemes
for controlling these processes. Unfortunately, in many scientific works,
especially in theoretical studies, idealized structures and models are
considered without taking into account the influence of technological and
constructive factors on the working capacity and the possibility of the
practical realization of the investigated elements and devices of spintronics.
However, in experimental studies it is necessary to have good repeatability of
the measurement results not only on one test sample, but also to obtain these
results on many identical samples.
series manufacture, even more stringent requirements are imposed on the element
or device being manufactured. Such critical requirements are high technical and
operational characteristics, as well as manufacturability of production and the
possibility of using existing processes and production equipment. Therefore,
researchers need to take into account these requirements, and if they do not
conduct detailed experimental studies of the influence of these factors on the
characteristics of the elements and devices under investigation, then they
should conduct at least a comprehensive theoretical analysis of this issue.
Such an approach to scientific research will ensure faster and more qualitative
introduction of research results into practice.
article, we want to describe the main methods and schemes that are used in
constructing spintronics elements and devices, and also give a brief analysis
of the prospects for their introduction into practice. We hope
that our article will be useful not only for researchers but also for the
development of new spintronics elements.
The ideal material for spintronics
should have high conductivity and all conduction electrons in it at room
temperature should have the same polarization, that is, their spins should be
oriented in a direction specified by the magnetic field. Therefore, the search
for materials with a high degree of polarization of electrons is one of their
priority tasks in spintronics. At room temperatures there is a predominant
orientation of electron spins in ferromagnetic metals, magnetic semimetals and
magnetic semiconductors [1-6]. For the latter the spin polarization degree at
the Fermi surface can reach 100%. Currently, spintronics uses mainly
ferromagnetic metals such as Fe, Co and Ni, as well as their alloys. In the
ferromagnetic metals at the Fermi level in the magnetization state, a spin
(exchange) splitting of the electron band is observed (Figure 1): on a zone
with spins directed along the material's magnetization (the lowest band with
minimal energy), and on a zone with spins directed against the magnetization of
the material (upper zone with higher energy). The splitting ? represents a
ferromagnetic gap in the spectrum of electrons of such a metal. In
ferromagnetic 3d-metals (Fe, Co, Ni), as a result of the exchange spreading of
the 3d bands, the zone of electrons with spins "up" and
"down" are filled in differently and have different density of states
N(E) at the Fermi level EF. In the conductivity of ferromagnetic
metals, along with valence electrons (s- and p-type), polarized 3d-electrons
(in any case, their significant part) also take part. Therefore, in electronic
transport in ferromagnetic metals, in the weak electric fields with a field
intensity E<?/e, the prevailing direction of the spin polarization of the
conduction electrons arises. Polarization is due to an imbalance between
electrons with spin "up" and with spin "down". The carrier
retains its polarization of the spin until it collides with another magnetic
atom or interacts with the lattice due to the spin-orbit coupling.
Ferromagnetic properties also have
semiconductors doped with ferromagnetic elements and some semimetals. In
ferromagnetic semiconductors and semimetals in the state of magnetization, the
exchange splitting ? is sufficiently large. The upper spin zone in them is
practically empty and the electron transport in such materials is almost
entirely provided by spin-polarized electrons [2,6] (Figure 1,). Therefore,
ferromagnetic semiconductors and ferromagnetic semimetals are a promising
material for creating spin injectors in spintronics elements.
Figure 1. Scheme of the spin-splitting of the band spectrum caused
by the exchange interaction
are the densities of
electron states with different spin projections (
is the magnetization
vector: b and c – ferromagnetic metal, d – magnetic semimetals.
The magnetic properties of
ferromagnetic semiconductors are caused by the interaction between electrons in
the sp-band of the semiconductor and
by electrons on the d-shell of the
impurity. Such an interaction is one of the forms of the Heisenberg exchange interaction
with the Hamiltonian H=-JsS, where J
is the exchange integral between the spin of the main carriers of the
semiconductor (s) and the transition
element (S). In particular, the s-electron in the conduction band
experiences an s-d exchange interaction
with the energy of the order of one hundred electron-volts Jsd~0.1
eV. The energy of such an interaction is much weaker for the
case of holes in the valence band of a semiconductor. As a result, in the
conduction band of the ferromagnetic semiconductors there is the splitting and
the value of splittinng is proportional to the magnitude of the localized
magnetic moment of the ferromagnetic semiconductors.
One of the most important parameters in
ferromagnetic semiconductors is the Curie temperature Tc. The value of Tc
increases usually with increasing the concentration of doping magnetic atoms.
However, the magnetic impurity atoms form inhomogeneous clusters at large
concentrations [3, 4]. It is possible to obtain a higher concentration of
magnetic impurities in thin semiconductor films, but the value of Tc in such materials is not
much higher than room temperature. Another the important parameter of the
magnetic semiconductors is the degree of polarization P of the carriers in in the conduction band. The value of P increases with the ratio of the band
splitting to the Fermi energy. At low temperatures, the value of P can approach P =100%, but the conductivity of the ferromagnetic semiconductor
decreases with decreasing temperature. These features of magnetic
semiconductors are their main deficiency when used in spintronics.
More sustained performance has the
ferromagnetic semimetals, which include some ferromagnetic oxides and Heusler
alloys [2,3,7]. The Heusler ferromagnetic alloys include a combination of three
metals and most of them have a sufficiently high Curie temperature and good
conductivity. Many semi-Heusler alloys (NiMnSb,
PtMnSb) and full-Heusler alloys (Co2MnSi,
Co2MnGe, Co2MnSn, Fe2MnSi) have a high degree
of spin polarization of conduction electrons. However, the Heusler
ferromagnetic alloys are complex alloys, which complicates the technology of
obtaining thin films from these alloys.
The high degree of spin polarization of
conduction electrons and the high Curry temperature have ferromagnetic oxides (CrO2 and Fe3O4) ,
and double perovskites (Sr2FeReO6)
[9-11]. However, ferromagnetic oxides have poor conductivity, and chrome
dioxide is difficult to obtain due to the complexity of the phase diagram of
coexistence. Manganites and double perovskites have quite good conductivity,
but their production is a rather complex process.
High degree of spin
polarization can be received also in
the conducting metals which have the strong spin-orbital
interaction. In the heavy metals (for instance Pt, Ta), the spin
polarization occurs through the spin-induced transverse deflection (relative to
the electric current) and in opposite directions electrons with opposites spin
projections, known as the spin-Hall effect [12-15]. In this case, the
spin-orbit interaction generates pure spin currents transversally to the input
electric current, as it is represented in Figure 2 .
second type, the spin-orbit interaction generates the spin polarization nearby
the interface which is caused by spin splitting of an
electron band spectrum (known as the Rashba effect) [16-18]. In real systems,
the size of the Rashba spin splitting (Figure 2)  depends on the
asymmetry of the electronic wave function near heavy nuclei. The size of the
spin splitting is highly sensitive to the detailed atomic structure at the
interface or surface.
Figure 2. The
dispersions of the two-dimensional electron gas previous to (on the left) and
after (on the right) action of the Rashba spin-orbit interaction causing the
spin-splitting along of planar momentum
is the Rashba
is the offset away
of the spectral
curves relative to the initial
position. The spin Hall effect of
the spin-orbit-conversion of the input electric current
in the transverse spin
with the spin
polarization denoted by arrows along the
axis in the
two-layered magnetic nanostructure(on the right). The latter is composed of the bottom layer made of
heavy normal metal (
) and the strong spin-orbit interaction and the upper
ferromagnetic layer (
). The spin current
exerts on the dynamics of magnetization
of the magnetic layer,
is external magnetic
Obviously, in the production of
spintronic elements the problem concerning the adaptation of using materials to
existing microelectronic technology arises. The materials of spintronics should
not change their properties under the influence of humidity, pressure,
temperature and be integrated with modern microelectronics and be
"technological", for example, allowing the application of the ohmic
contact. The majority of known semimetals are complex compounds, and for their
practical use it is necessary to conduct works on working out of manufacturing
techniques of thin films.
necessary to understand, that the devices of spintronics can replace the
similar devices of microelectronics in rather limited areas of technology. First of all, it is the area of
registration and processing of small constant and super-high-frequency
electromagnetic signals. For such
signals elements of spintronics based on metallic
nanofilms compared to the semiconductor elements of microelectronics possess a
lower degree of noise, high thermal stability factor and can be used in a
higher frequency range. The elements of spintronics can compete, in our
opinion, with the elements of semiconductor microelectronics in the small-size
magnetic field sensors, in the devices of high-speed non-kinematic data
recording and the devices for generating the electromagnetic radiation in the
gigahertz and terahertz frequency range. Separately, we should note the
prospect of using the elements of spintronics in devices of special and
The basic material
for creation the elements of spintronics are multilayered heterogeneous
magnetic nanofilms which are the electrodes in these elements. To control the
spin current in the elements of spintronics, it is necessary to change the
magnetization direction of the control magnetic electrode. This can be done
with the help of a magnetic field of electric current, which flows through the
special electrodes. However, such the method is ineffective for creating a
sufficiently strong magnetic field in the local nano- and even the microregion.
Recently, a number of new methods of controlling the spin current have been
proposed. The most interesting and practically perspective methods are: the switching of control nanoelectrode (the free
electrode) under the action of the magnetic field
of a spin-polarized current injected from the magnetic pinned reference electrode
[19-22], the thermomagnetic switching of control nanoelectrode (the weak free
electrode) in the multilayers magnetic structure with the temperature-variable
interlayer exchange interaction between the magnetic high-coercivity layer (the
pinned reference electrode) and magnetic low-coercivity layer (the weak free
electrode) , the
thermomagnetic switching of control nanoelectrode due to the exchange
interaction in the antiferromagnetic layer with two magnetic sublattices ,
the switching of the magnetic layer under the
action of the magnetic field of spins in the spin Hall effect [12-15]
and the switching of the magnetic layer due to
the Rashba effect [17, 18].
The switching of the control nanoelectrode by a spin
current occurs due to the injection of spin-polarized electrons into this
electrode, which create a control magnetic field (Figure 4). The
injected spin-polarized current or the spin current in combination with an s-d-exchange interaction can result in
the electric-driven magnetization dynamics in the form of the magnetization
switching or magnetization precession. With a large value of such the
nonequilibrium magnetic field, the control electrode reverses magnetization
along the direction of the magnetic field given by the spin current. A great
quality of this method is the possibility of locally switch of the
conductibility not only in the microstructures but also in the nanostructures
with the help of short nanopulses of the electric current. The main shortcoming
is the inability to reverse switch in a structure with two magnetic nanolayers
when the polarity of the electric pulse is changed. For this, it is necessary
to create the magnetic structures with three magnetic nanolayers (Figure 3),
which have a different coercive force [21, 22].
3. Scheme of generation of
nonequilibrium magnetic field in the nonmagnetic nanolayer (N) (at the left)
under the injection of the spin-polarized electrons from ferromagnetic
nanolayer (F) and the
scheme of the rerecording cell (at the right) under the spin current: l –
ferromagnetic electrode (pinned electrode),
with coercive force H12
- barrier nanolayer, 3 - ferromagnetic electrode (free electrode), with
coercive force H3, 4 -
ferromagnetic electrode (pinned electrode), with coercive force H4
magnetic nanostructures based on the heavy normal metal and magnetic layers the
spin current, caused by the spin-Hall effect, enters into the magnetic layer
exerting the torque on magnetization and its switching or precession. The
features of the mentioned electric- controlled magnetization dynamics are
determined by the electron and magnetic structures of the nanolayers and
interfaces of the magnetic nanostructures. The mentioned magnetic
nanoctructures are viable whereas they are characterized by low threshold
magnitudes of the input current density required for manipulation by magnetic
states at a high thermal stability. Heavy metals with the enough strong
spin-orbit coupling can provide a giant spin Hall effect generating spin
current intense enough to
induce efficient spin-transfer-torque switching of ferromagnets, thereby
providing a new approach for controlling magnetic devices that can be superior
to existing technologies. In this case, the spin-torque-switching of both
out-of-plane and in-plane magnetized layers of can be realized, that permits
eliminate the main obstacles currently impending the development of magnetic
memory and non-volatile spin logic technology. The schemes of the corresponding
magnetic nanostructure and the magnetic member are depicted in Figure 4 .
switching of the magnetic layer under the action of the magnetic field of the
spins produced by the spin-Hall effect has a good prospect of practical use in
the elements of spintronics. This method allows one to simply change the
direction of the effective magnetic spins, by changing the direction of the
electric current, and also to obtain sufficiently strong magnetic fields due to
the transmission of a large current. However, it is unlikely to find
applications in the manufacture of microchips due to the complexity of the
design and manufacturing technology. On the other hand, on the basis of this
effect, good results have already been obtained in the generation of
high-frequency electromagnetic radiation due to the oscillation of the magnetic
moment in magnetic films . Spin torque oscillator, based on the
magnetization precession, has attracted much attention as a future
nanocommunication device because it can produce a large emission power (P>1
µW), a high quality factor (k>103), a high oscillation frequency
(F>1 GHz). In spin
Hall oscillators are
used generally the magnetic nanolayers from magnetic materials with great
values of magnetization and small coercive force that increases magnitude of signal. However in such
layers it is difficult to receive rotation of the magnetic moment with the
frequency even 1 terahertz. The use of ferrimagnetic layers in the spin Hall's
oscillators makes it possible to increase the generation frequency to ten
terahertz and maybe up to a
Figure 4. Schematic of the two-layered
magnetic nanostructure with the electric-driven magnetization via the effect of
the spin Hall generation of the spin current which enters into the magnetic
is the electric
current vector, red and green arrows denote spin directions,
is the torque on the
magnetization due to the Oersted field and the
spin-transform-torque from the spin Hall effect (on the left). Schematic of the
spin-hall magnetic member with red out based on the tunnel magnetoresistance
effect (on the right).
thermomagnetic switching in a magnetic structure with a temperature-variable
interlayer exchange interaction between a magnetic layer with a high coercive
force and a magnetic layer with a low coercive force has a weak prospect of the
practical use in spintronics not only because it is difficult to localize
heating and cooling in the local the micro and nanozone, it is more
difficult to realize high-frequency switching back and forth. The switching of the magnetic layer under the action of
the Rashba effect can be used in low-power spintronics elements, in which
magnetic materials are made of magnetic semiconductors.
The thermomagnetic switching of the
antiferromagnetic layer due to the exchange interaction requires a very high
rate of heating by ultrashort pulses also requires strict control of the
heating temperature. This method is used only for switching of ferrimagnetic
films by femtosecond laser pulses and can not be used in spintronics elements.
It is necessary to notice,
that passing the spin-polarized
current through a row of the regular tunnel structures with two oppositely
magnetized layers it is possible to receive effective generation of
high-frequency radiation. The analog of such structure is the semiconductor
cascade laser. Such sources of the radiation work on the basis of the
electron transitions between the spin levels which are split in a magnetic
field. The energy of spin
splitting is in magnetic
materials is in area E=0,05-0,001 eV,
what corresponds to the terahertz area of frequencies of electromagnetic
radiation. The population inversion on spin
levels can be reached through a pumping of the spin polarized
electrons on the top spin-split level.
The device scheme, in which radiation
on spin transitions is received, is presented in Figure 5.
Figure 5. Schematic
of the double magnetic structure composed of the needle-like nonmagnetic
metallic electrode contacting with the magnetic film.
is the external
magnetic field determining the spin-splitting and the radiation frequency (upwardly). Schematic
of spin-flip transitions accompanied by photon emission (below). Magnetization
M and applied field H are antiparallel, majority electrons in the ferromagnetic
F are injected into the normal metal N, leading to an inverse population of the
Zeeman levels in N. A spontaneous or stimulated spin-flip transition emits a
photon with frequency corresponding to the spin-splitting.
point contacts between high anisotropy ferromagnetic SmCo5 and normal metal Cu
are used to achieve a strong spin-population inversion in the contact core.
Subjected to microwave irradiation in resonance with the Zeeman splitting in
Cu, the inverted spin-population relaxes through stimulated spin-flip photon
emission, detected as peaks in the point contact resistance. Resonant spin-flip
photon absorption is detected as resistance minima, corresponding to sourcing
the photon field energy into the electrical circuit. The illustration of the
spin-flip photon emission processes, for a majority ferromagnetic injector,
such as SmCo5, the electrons injected into the normal metal (N) have their
magnetic moment along the magnetization M of the ferromagnetic (F). With the
external field applied anti-parallel to M, as in Fig. 3a, the electrons
injected into the normal metal populate the high-energy Zeeman level there,
which leads to an inverse population of the two spin-split levels. This inverse
population is not in equilibrium but maintained by the bias current through the
PC. Intrinsic spin relaxation in such weak spin-orbit nonmagnetic metals as Cu
is relatively slow, so spin-flip relaxation through emission or absorption of
photons can be dominant. 
is clear, that in the ferromagnetic metals the magnitude of terahertz radiation
signal should be small however in the semimetals it is possible to expect
useful the increase of power of such terahertz signal. The practical realization of the source of the THz
radiation based on the viable and technological materials (including magnetic
semimetals) and using the effect of the spin-orbit generation of spin current
and gaining corresponding prototypes of the mentioned sources of THz radiation
have very good perspective.
In conclusion we wish to
tell, that scientific analysis and our conclusions resulted in given article
are not invariable and unquestionable. The development of technics and
technology should and will lead to their change. However, we hope that our
analysis corresponds generally level of modern scientific and technological
works. We hope also, that our article will be useful to young
researchers that it emphasizes
importance of the comprehensive approach at studying of
any physical phenomenon, and also emphasizes importance of the practical use of
the received results.
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