SPINTRONICS
,Introduction:
History and overview of spin electronics, Classes of magnetic materials:
Objectives:
The main objectives of this first lecture are
(a) to provide a history and a broad overview of spin electronics and
(b)to introduce briefly different classes of magnetic materials.
(a). History and overview of spin electronics:
Spin electronics (also termed as spintronics), at the interface between magnetism and
electronics, is a new field of research in multidisciplinary level. The central theme is to
manipulate actively the spin degrees of freedom in solid-state systems. In other words, the basic
concept of spintronics is the manipulation of spin currents, in contrast to mainstream electronics
in which the spin of the electron is mostly ignored. It is well known to the most of the students
in science and engineering disciplines that every elemental particle (electrons, neutrons,
photons and neutrinos, etc.) has a quantum mechanical property called 'spin' and it has a
quantized value (including zero), which can be measured in principle. Adding this spin degree
of freedom in the mainstream of electronics provides new effects, new capabilities and new
functionalities, suitable for various futuristic magnetoelectronic applications.
It is important to know that there are three important factors making the spin of conduction
electrons attractive for future technology, which are: (1) storing information using electron
spin, (2) transformation of stored information (the spin can be transferred as it is attached to
mobile carriers), and (3) the detection of stored information.
• Information can be stored in a system of electron spins because they can be polarized
to either up or down states. To represent a binary digit (bit) state, for example, spin up may
represent a high or '1' and spin down represent the low or '0' logic level.
• The second factor, the ability of information transfer by electron spins, relies on two
facts: (i) electrons are mobile and (ii) electrons have a relatively large spin memory.
Interestingly, conduction electrons "remember'' their spins much longer than they remember
their momentum states.
• Finally, after the transformation of a spin state, it has to be detected. This is performed
by observing the spin polarization either optically (photoexcited spin-polarized electrons and
holes in a semiconductor recombine by emitting circularly polarized light or the electron spins
interact with light and cause a rotation of the light polarization plane) or electronically through
charge-spin coupling (when spin accumulates on the conductor side at the interface of a
conductor and a ferromagnet, a voltage or a current appears. By measuring the polarity of the
voltage or the current, one can predict the spin orientation in the conductor.). In addition, the
possibility of having long spin relaxation time or spin diffusion length in electronic materials
makes spintronics a viable potential technology.
, It is important to note that everybody has already a spintronic device in their desktop/laptop,
since the read heads of the hard disc drives use the giant magnetoresistance (GMR)
phenomenon to read the magnetic information written on the disc. Magnetoresistance (MR) is
a term widely used to mean a change in the electrical resistivity due to the externally applied
magnetic field. GMR [1,2] exploits the influence of the spin of the electrons on the electrical
resistivity in a magnetic multilayer films composed of alternate ferromagnetic (FM) and
nonmagnetic (NM) layers (For example, Fe and Cr). Before the discovery of GMR, the
investigations on the charge and spin of the electrons were usually considered to be independent
of each other and hence little attention was paid to probe a correlation between charge and spin.
However, the influence of the spin on the mobility of the electrons in FM metals, first suggested
by Mott [3], had been experimentally demonstrated and theoretically described in early works
[4] more than ten years before the discovery of GMR.
Nevertheless, how to enhance the MR effect is an attractive challenge and further scope of
progress for scientists in fundamental physics and also for researchers in industries. There are
several strategies to obtain larger GMR effects: (1) taking the current flowing perpendicular to
plane GMR (CPP – GMR), (2) using the tunneling current (Tunneling MR (TMR)), (3) using
a new class of materials called half-metal as the magnetic constituent and (4) using the ballistic
current (Ballistic MR (BMR)). Usually resistance measurements in thin metallic specimens are
carried out in a conventional geometry to use an electric current flowing in the film plane. Such
configuration is called as Current in Plane (CIP) geometry. In contrast, resistance
measurements in the other geometry (i.e., CPP) are very inconvenient for thin metallic films.
However, an enhancement of MR ratio is expected in the CPP geometry compared to the CIP
geometry because the GMR effect is associated with electrons passing through interfaces.
Apparently, realization of GMR was the first step on the road of utilization of the spin degree
of freedom, which triggered the development of the active field of research of spintronics.
Today, this field is extending largely with very promising new axes like the phenomena of spin
transfer, spintronics with semiconductors, molecular spintronics and single-electron
spintronics. In addition, the recent advent of quantum computing has added a new dimension
to spintronics. The spin polarization of a single electron can exist in a coherent superposition
of two orthogonal spin polarizations (i.e., mutually anti-parallel spin orientations) for a
relatively long time without losing the phase coherence, compared to the charge degree of
freedom. Therefore, spin has become the preferred vehicle to host a quantum bit. The potential
application of spin to scalable quantum logic processors has provided a tremendous boost to
the field of spintronics.
(b). Classes of Magnetic Materials:
As the origin of magnetism lies in the orbital and spin motions of electrons and how the
electrons interact with one another, the right way of introducing different types of magnetism
is to demonstrate how the materials respond to an external applied magnetic fields.
Based on the magnetic response, materials can be classified into the following five major
groups: (1) Diamagnetism; (2) Paramagnetism; (3) Ferromagnetism; (4) Ferrimagnetism; and
(5) Antiferromagnetism. While the materials in the first two groups do not exhibit any collective
magnetic exchange interactions and are not magnetically ordered, the next three groups exhibit
long-range magnetic order below a certain temperature, called critical (or Curie/Neel)
temperature. Ferromagnetic and ferrimagnetic materials are generally considered as magnetic
,Introduction:
History and overview of spin electronics, Classes of magnetic materials:
Objectives:
The main objectives of this first lecture are
(a) to provide a history and a broad overview of spin electronics and
(b)to introduce briefly different classes of magnetic materials.
(a). History and overview of spin electronics:
Spin electronics (also termed as spintronics), at the interface between magnetism and
electronics, is a new field of research in multidisciplinary level. The central theme is to
manipulate actively the spin degrees of freedom in solid-state systems. In other words, the basic
concept of spintronics is the manipulation of spin currents, in contrast to mainstream electronics
in which the spin of the electron is mostly ignored. It is well known to the most of the students
in science and engineering disciplines that every elemental particle (electrons, neutrons,
photons and neutrinos, etc.) has a quantum mechanical property called 'spin' and it has a
quantized value (including zero), which can be measured in principle. Adding this spin degree
of freedom in the mainstream of electronics provides new effects, new capabilities and new
functionalities, suitable for various futuristic magnetoelectronic applications.
It is important to know that there are three important factors making the spin of conduction
electrons attractive for future technology, which are: (1) storing information using electron
spin, (2) transformation of stored information (the spin can be transferred as it is attached to
mobile carriers), and (3) the detection of stored information.
• Information can be stored in a system of electron spins because they can be polarized
to either up or down states. To represent a binary digit (bit) state, for example, spin up may
represent a high or '1' and spin down represent the low or '0' logic level.
• The second factor, the ability of information transfer by electron spins, relies on two
facts: (i) electrons are mobile and (ii) electrons have a relatively large spin memory.
Interestingly, conduction electrons "remember'' their spins much longer than they remember
their momentum states.
• Finally, after the transformation of a spin state, it has to be detected. This is performed
by observing the spin polarization either optically (photoexcited spin-polarized electrons and
holes in a semiconductor recombine by emitting circularly polarized light or the electron spins
interact with light and cause a rotation of the light polarization plane) or electronically through
charge-spin coupling (when spin accumulates on the conductor side at the interface of a
conductor and a ferromagnet, a voltage or a current appears. By measuring the polarity of the
voltage or the current, one can predict the spin orientation in the conductor.). In addition, the
possibility of having long spin relaxation time or spin diffusion length in electronic materials
makes spintronics a viable potential technology.
, It is important to note that everybody has already a spintronic device in their desktop/laptop,
since the read heads of the hard disc drives use the giant magnetoresistance (GMR)
phenomenon to read the magnetic information written on the disc. Magnetoresistance (MR) is
a term widely used to mean a change in the electrical resistivity due to the externally applied
magnetic field. GMR [1,2] exploits the influence of the spin of the electrons on the electrical
resistivity in a magnetic multilayer films composed of alternate ferromagnetic (FM) and
nonmagnetic (NM) layers (For example, Fe and Cr). Before the discovery of GMR, the
investigations on the charge and spin of the electrons were usually considered to be independent
of each other and hence little attention was paid to probe a correlation between charge and spin.
However, the influence of the spin on the mobility of the electrons in FM metals, first suggested
by Mott [3], had been experimentally demonstrated and theoretically described in early works
[4] more than ten years before the discovery of GMR.
Nevertheless, how to enhance the MR effect is an attractive challenge and further scope of
progress for scientists in fundamental physics and also for researchers in industries. There are
several strategies to obtain larger GMR effects: (1) taking the current flowing perpendicular to
plane GMR (CPP – GMR), (2) using the tunneling current (Tunneling MR (TMR)), (3) using
a new class of materials called half-metal as the magnetic constituent and (4) using the ballistic
current (Ballistic MR (BMR)). Usually resistance measurements in thin metallic specimens are
carried out in a conventional geometry to use an electric current flowing in the film plane. Such
configuration is called as Current in Plane (CIP) geometry. In contrast, resistance
measurements in the other geometry (i.e., CPP) are very inconvenient for thin metallic films.
However, an enhancement of MR ratio is expected in the CPP geometry compared to the CIP
geometry because the GMR effect is associated with electrons passing through interfaces.
Apparently, realization of GMR was the first step on the road of utilization of the spin degree
of freedom, which triggered the development of the active field of research of spintronics.
Today, this field is extending largely with very promising new axes like the phenomena of spin
transfer, spintronics with semiconductors, molecular spintronics and single-electron
spintronics. In addition, the recent advent of quantum computing has added a new dimension
to spintronics. The spin polarization of a single electron can exist in a coherent superposition
of two orthogonal spin polarizations (i.e., mutually anti-parallel spin orientations) for a
relatively long time without losing the phase coherence, compared to the charge degree of
freedom. Therefore, spin has become the preferred vehicle to host a quantum bit. The potential
application of spin to scalable quantum logic processors has provided a tremendous boost to
the field of spintronics.
(b). Classes of Magnetic Materials:
As the origin of magnetism lies in the orbital and spin motions of electrons and how the
electrons interact with one another, the right way of introducing different types of magnetism
is to demonstrate how the materials respond to an external applied magnetic fields.
Based on the magnetic response, materials can be classified into the following five major
groups: (1) Diamagnetism; (2) Paramagnetism; (3) Ferromagnetism; (4) Ferrimagnetism; and
(5) Antiferromagnetism. While the materials in the first two groups do not exhibit any collective
magnetic exchange interactions and are not magnetically ordered, the next three groups exhibit
long-range magnetic order below a certain temperature, called critical (or Curie/Neel)
temperature. Ferromagnetic and ferrimagnetic materials are generally considered as magnetic
