Paramagnetic spins take electrons for a ride, produce electricity from heat

Paramagnetic spins take electrons for a ride, produce electricity from heat
Magnon-electron drag is an advective effect between magnons (waves of precession in the spins of individual atoms & represented as little grey cones) and electrons (green dots). The thermal gradient creates a gradient in the angles of the precession cones, which leads to a flow of magnons that then drags electrons along and creates thermopower. In the paramagnetic state, the local thermal fluctuations of magnetization (i.e. paramagnons) form little packets of magnons. These paramagnons can transfer the momentum they acquire in the thermal gradient to electrons and generate thermopower. In contrast, in a classical paramagnet, magnetic moments on the individual atoms are completely uncorrelated; there is no paramagnon or paramagnon drag thermopower in that case. Credit: Renee Ripley, Ohio State University

An international team of researchers has observed that local thermal perturbations of spins in a solid can convert heat to energy even in a paramagnetic material—where spins weren't thought to correlate long enough to do so. This effect, which the researchers call "paramagnon drag thermopower," converts a temperature difference into an electrical voltage. This discovery could lead to more efficient thermal energy harvesting—for example, converting car exhaust heat into electric power to enhance fuel-efficiency, or powering smart clothing by body heat.

The research team includes scientists from North Carolina State University, the Department of Energy's Oak Ridge National Laboratory (ORNL), the Chinese Academy of Sciences and the Ohio State University.

In solids with magnetic ions (e.g., manganese), thermal perturbations of spins either can align with each other (ferromagnets or antiferromagnets), or not align (paramagnets). However, spins are not entirely random in paramagnets: they form short-lived, short-range, locally ordered structures—paramagnons—which exist for only a millionth of a millionth of a second and extend over only two to four atoms. In a new paper describing the work, the researchers show that despite these shortcomings, even paramagnons can move in a difference and propel along with them, creating paramagnon drag thermopower.

In a proof-of-concept finding, the team observed that paramagnon drag in manganese telluride (MnTe) extends to very high temperatures and generates a thermopower that is much stronger than what electron charges alone can make.

The research team tested the concept of paramagnon drag thermopower by heating lithium-doped MnTe to approximately 250 degrees Celsius above its Néel temperature (34 degrees Celsius) - the temperature at which the spins in the material lose their long-range magnetic order and the material becomes paramagnetic.

"Above the Néel temperature, one would expect the thermopower being generated by the to drop off," says Daryoosh Vashaee, professor of electrical and computer engineering and materials science at NC State and co-corresponding author of the paper describing the work. "However, we didn't see the expected drop off, and we wanted to find out why."

At ORNL the team used neutron spectroscopy at the Spallation Neutron Source to determine what was happening within the material. "We observed that even though there were no sustained spin waves, localized clusters of ions would correlate their spins long enough to produce visible magnetic fluctuations," says Raphael Hermann, a materials scientist at ORNL and co-corresponding author of the paper. The team showed that the lifetime of these spin waves—around 30 femtoseconds—was long enough to enable the dragging of electron charges, which requires only about one femtosecond, or one quadrillionth of a second. "The short-lived spin waves, therefore, could propel the charges and create enough thermopower to prevent the predicted drop off," Hermann says.

"Before this work, it was believed that magnon drag could exist only in magnetically ordered materials, not in paramagnets," says Joseph Heremans, professor of mechanical and aerospace engineering at the Ohio State University and co-corresponding author of the paper. "Because the best thermoelectric materials are semiconductors, and because we know of no ferromagnetic semiconductor at room temperature or above, we never thought before that magnon drag could boost the thermoelectric efficiency in practical applications. This new finding changes that completely; we can now investigate paramagnetic semiconductors, of which there are a lot."

"When we observed the sudden rise of Seebeck coefficient below and near the Néel temperature, and this excess value extended to high temperatures, we suspected something fundamentally related to spins must be involved," says Huaizhou Zhao, a professor at the Chinese Academy of Science in Beijing and co-corresponding author of the paper. "So we formed a research team with complementary expertise which laid the groundwork for this discovery."

"Spins enable a new paradigm in thermoelectricity by alleviating the fundamental tradeoffs imposed by Pauli exclusion on electrons," Vashaee says. "Just as in the discovery of the spin-Seebeck effect, which led to the new area of spincaloritronics, where the spin angular momentum is transferred to the electrons, both the spin waves (i.e., magnons) and the local thermal fluctuations of magnetization in the paramagnetic state (i.e., paramagnons) can transfer their linear momentum to electrons and generate thermopower."

The research appears in Science Advances.

Explore further

A 50-year quest to isolate the thermoelectric effect is now over: Magnon drag unveiled

More information: "Paramagnon drag in high thermoelectric figure of merit Li-doped MnTe" Science Advances (2019).
Journal information: Science Advances

Citation: Paramagnetic spins take electrons for a ride, produce electricity from heat (2019, September 13) retrieved 14 October 2019 from
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Sep 14, 2019
Electrons do not spin. They orbit. Note: an electron is the center of an E Field and a point or location does not spin. If so, how? Logic! Please.

Sep 14, 2019

Is a form of magnetism
whereby some materials
are weakly attracted
by an externally applied magnetic field
and form internal
Induced magnetic fields in the direction of the applied magnetic field

In contrast with this behaviour
diamagnetic materials
are repelled by magnetic fields
and form induced magnetic fields
in the direction opposite to that
of the applied magnetic field

Paramagnetism is due
to the presence of unpaired electrons
in the material
so all atoms
with incompletely filled atomic orbital's
are paramagnetic
Due to their spin
unpaired electrons
have a magnetic dipole moment
and act like tiny magnets
An external magnetic field
causes the electrons' spins
to align parallel to the field
Causing a net attraction

A List of Paramagnetic Atoms
Lithium (Li)
Oxygen (O)
Sodium (Na)
Magnesium (Mg)
Aluminium (Al)
Potassium (K)
Calcium (Ca)
Scandium (Sc)
Titanium (Ti)
Vanadium (V)
Manganese (Mn)
Rubidium (Rb)


Sep 14, 2019
This Magic of Paramagnetism converts heat directly to electrical current

Local thermal perturbations of spins in a solid
can convert heat to energy
in a paramagnetic material
where spins weren't thought to correlate long enough to do so.
paramagnon drag thermo power
converts temperature difference into electrical voltage
this discovery
thermal energy harvesting
has its use's
Converting car exhaust heat into electric power

Sep 14, 2019
Electric cars charge there inefficient batteries with fossil fuels indirectly

By converting car exhaust to electrical power
as this is 33% of the energy from this petrol burnt
which literally goes up in smoke
combined with
this 33% of heat lost
in thermal heat lost in this car's engine block
in other words
if this 66% of heat lost
is converted to electricity
Apart from drastically reducing fuel consumption for the same horse power

When this is achieved in improving piston engines thermal efficiency by up to 66%
no electric car will touch this internal combustion engine with a candle
in other words
as electric cars charge there inefficient batteries with fossil fuels indirectly
and petrol cars burn fossil fuels directly
Electric cars are toast

Sep 14, 2019
This Paramagnon Thermo Power

When petrol cars
come with, Paramagnon Thermo Power
making savings up to 66%
making these petrol cars
achieve 100s of miles to the gallon
these internal combustion engines
can drive electrical generators
combined with the electricity generated by Paramagnon Thermo Power
as only a small battery is needed
to store this energy

For as this car is using this heat lost directly
there is none spare, so 1000lb batteries are not needed

As these 1000lb batteries are these Electric Cars Down Fall

This Paramagnon Thermo Power fossil fuel power plant
has electric motors
that we do not have to worry, if there battery is charged, up or not
our Paramagnon Thermo Power cars
Are 1000lbs lighter, than conventional electric cars

All Thanks to this, Paramagnon Thermo Power!

Sep 14, 2019
However, Gran...
What will be the heat to electricity conversion efficiency?

Sep 14, 2019
Convert car exhaust to electricity? How stupid can we get? Got to be many better uses for this.....

Sep 16, 2019
Electrons do not spin. They orbit.

Electrons have the intrinsic property of spin AND the extrinsic property of orbit (around atoms/molecules). I suspect you don't understand the physics.

Sep 25, 2019
There's always a gremlin in the works

Whydening Gyre> However, Gran...
What will be the heat to electricity conversion efficiency?

This is experimental
Initially, it is going to be expensive and very low

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