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Topic Name: Dartmouth Researchers Find Out Chromium's Hidden Electrical Properties of Magnets: May Useful for Spintronics
Category: Nuclear Magnetic Resonance
Research persons: Yeong-Ah Soh, Ravi Kummamuru
Location: Dartmouth College, United States
Details
Two
Dartmouth researchers have determined that the element chromium displays
electrical properties of magnets in surprising ways. This finding can be used in
the emerging field of “spintronics,” which might someday contribute to new and
more energy efficient ways of processing and storing data.
The study, titled “Electrical effects of spin density wave
quantization and magnetic domain walls in chromium,” will be published in the
April 17 issue of the journal Nature.
Electrons have an intrinsic angular momentum, called spin, in
addition to their electrical charge. In electronics work, it is the charge of
the electron that is used for calculations and transmitting information. In
spintronics, it is the electron spin that is exploited.
“The phenomena that we have discovered are likely to lead to
new applications of chromium,” says Yeong-Ah Soh, the lead researcher on the
paper and an associate professor of physics and astronomy at Dartmouth. She
worked on the study with Ravi Kummamuru, a former post-doctoral research
associate at Dartmouth now at the University of Illinois at Urbana-Champagne.
She goes on to explain that in essence, this indicates that a
simple and well-known element, chromium, displays different electrical
properties on heating and cooling. These differences reflect subtle internal
rearrangements of the electrons and their spins.
In ferromagnets, the kind of common magnet you might see on a
refrigerator, the spins of electrons interact with each other leading to
alignment. In antiferromagnets, however, the interactions between neighboring
electron spins are such that they are opposed. Researchers have long studied the
electrical properties of ferromagnets and the influence of electron spin. Less
attention has been paid, according to Soh and Kummamuru, to the influence of
spin on the electrical properties in antiferromagnets, where it is more
difficult to manipulate, and chromium is special since it is the only simple
element that is an antiferromagnet.
“Antiferromagnets are used in numerous fields: physics,
materials science, and chemistry, and they are increasingly used in technology,
where they are found in the tiny heads that read the data on computer disc
drives,” says Soh. “Our research opens the entire new field of controlled
electrical effects at a slightly-larger-than-quantum scale in antiferromagnets.
The findings show that not only ferromagnets can be used in spintronics; there
is a possibility that antiferromagnets can also be employed to manipulate and
store information.”
Note for Spintronics
Spintronics, also known as magnetoelectronics, is an emerging technology which
exploits the quantum spin states of electrons as well as making use of their
charge state. The electron spin itself is manifested as a two state magnetic
energy system.
The discovery of giant magnetoresistance in 1988 by Albert Fert et al. and Peter
Grünberg et al. independently is considered as the birth of spintronics.
Spintronics describes technology that makes use of the spin state of electrons.
It can provide an extension to electronics.
Electrons exhibit the basic properties of spin, charge, and mass. When the
intrinsic spin of an electron is measured, it is found in one of two spin
states, which we denote as spin up and spin down. Since the Pauli Exclusion
Principle dictates that the quantum-mechanical wave function of two paired
fermions must be antisymmetric, no two electrons can occupy the same quantum
state, implying that an entangled pair of electrons cannot have the same spin.
There is generally a splitting of the spin-up and spin-down energy levels via
the Zeeman effect, so electrons with their spins aligned with an external field
are less energetic than electrons with their spins anti-aligned. Electrons
absorb or emit photons (quanta of electromagnetic energy) to change valence
orbits, and they lose spin coherence by interacting with mutually resonant
photon frequencies, causing the electrons to spin flip by energy transfer,
through mutual spin-orbit coupling, and through photon emission.
In order to make a spintronic device, the primary requirement is to have a
system that can generate a current of spin polarized electrons, and a system
that is sensitive to the spin polarization of the electrons. Most devices also
have a unit in between that changes the current of electrons depending on the
spin states.
The simplest method of generating a spin-polarised current is to inject the
current through a ferromagnetic material. The most common application of this
effect is a giant magnetoresistance (GMR) device. A typical GMR device consists
of at least two layers of ferromagnetic materials separated by a spacer layer.
When the two magnetization vectors of the ferromagnetic layers are aligned, then
an electrical current will flow freely, whereas if the magnetization vectors are
antiparallel then the resistance of the system is higher.
Two variants of GMR have been applied in devices, current-in-plane where the
electric current flows parallel to the layers and
current-perpendicular-to-the-plane where the electric current flows in a
direction perpendicular to the layers.
Spintronic plates are used in the field of mass-storage devices; in 2002 IBM
scientists announced that they could compress massive amounts of data into a
small area, at approximately one trillion bits per square inch (1.5 Gbit/mm˛) or
roughly 1 TB on a single sided 3.5" diameter disc. The storage density of hard
drives is rapidly increasing along an exponential growth curve. The doubling
period for the areal density of information storage is twelve months, much
shorter than Moore's Law, which observes that the number of transistors in an
integrated circuit doubles every twenty-four months.
Note for Angular Momentum
In physics, the angular momentum of an object rotating about some reference
point is the measure of the extent to which the object will continue to rotate
about that point unless acted upon by an external torque. In particular, if a
point mass rotates about an axis, then the angular momentum with respect to a
point on the axis is related to the mass of the object, the velocity and the
distance of the mass to the axis.
Angular momentum is important in physics because it is a conserved quantity: a
system's angular momentum stays constant unless an external torque acts on it.
Torque is the rate at which angular momentum is transferred in or out of the
system. When a rigid body rotates, its resistance to a change in its rotational
motion is measured by its moment of inertia. Angular momentum is an important
concept in both physics and engineering, with numerous applications. For
example, the kinetic energy stored in a massive rotating object such as a
flywheel is proportional to the square of the angular momentum. Conservation of
angular momentum also explains many phenomena in sports and nature.
Note for Ferromagnetism
Ferromagnetism is the "normal" form of magnetism with which most people are
familiar, as exhibited in horseshoe magnets and refrigerator magnets. It is
responsible for most of the magnetic behavior encountered in everyday life. The
attraction between a magnet and ferromagnetic material is "the quality of
magnetism first apparent to the ancient world, and to us today," according to a
classic text on ferromagnetism.
Ferromagnetism is defined as the phenomenon by which materials, such as iron, in
an external magnetic field become magnetized and remain magnetized for a period
after the material is no longer in the field. All permanent magnets are either
ferromagnetic or ferrimagnetic, as are the metals that are noticeably attracted
to them.
Historically, the term ferromagnet was used for any material that could exhibit
spontaneous magnetization: a net magnetic moment in the absence of an external
magnetic field. This general definition is still in common use. More recently,
however, different classes of spontaneous magnetisation have been identified
when there is more than one magnetic ion per primitive cell of the material,
leading to a stricter definition of "ferromagnetism" that is often used to
distinguish it from ferrimagnetism. In particular, a material is "ferromagnetic"
in this narrower sense only if all of its magnetic ions add a positive
contribution to the net magnetization. If some of the magnetic ions subtract
from the net magnetization (if they are partially anti-aligned), then the
material is "ferrimagnetic". If the ions anti-align completely so as to have
zero net magnetization, despite the magnetic ordering, then it is an
antiferromagnet. All of these alignment effects only occur at temperatures below
a certain critical temperature, called the Curie temperature (for ferromagnets
and ferrimagnets) or the Néel temperature (for antiferromagnets).
There are a number of crystalline materials that exhibit ferromagnetism (or
ferrimagnetism). The table on the right lists a representative selection of them
here, along with their Curie temperatures, the temperature above which they
cease to exhibit spontaneous magnetization.
Ferromagnetic metal alloys whose constituents are not themselves ferromagnetic
in their pure forms are called Heusler alloys, named after Fritz Heusler.
One can also make amorphous (non-crystalline) ferromagnetic metallic alloys by
very rapid quenching (cooling) of a liquid alloy. These have the advantage that
their properties are nearly isotropic (not aligned along a crystal axis); this
results in low coercivity, low hysteresis loss, high permeability, and high
electrical resistivity. A typical such material is a transition metal-metalloid
alloy, made from about 80% transition metal (usually Fe, Co, or Ni) and a
metalloid component (B, C, Si, P, or Al) that lowers the melting point.
A relatively new class of exceptionally strong ferromagnetic materials are the
rare-earth magnets. They contain lanthanide elements that are known for their
ability to carry large magnetic moments in well-localized f-orbitals.
The property of ferromagnetism is due to the direct influence of two effects
from quantum mechanics: spin and the Pauli exclusion principle. The spin of an
electron, combined with its orbital angular momentum, results in a magnetic
dipole moment and creates a magnetic field. (The classical analogue of
quantum-mechanical spin is a spinning ball of charge, but the quantum version
has distinct differences, such as the fact that it has discrete up/down states
that are not described by a vector; similarly for "orbital" motion, whose
classical analogue is a current loop.) In many materials (specifically, those
with a filled electron shell), however, the total dipole moment of all the
electrons is zero (i.e., the spins are in up/down pairs). Only atoms with
partially filled shells (i.e., unpaired spins) can experience a net magnetic
moment in the absence of an external field. A ferromagnetic material has many
such electrons, and if they are aligned they create a measurable macroscopic
field.
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