A new genre of Spin-Transfer Torque (STT) MRAM is proposed, in which
bi-directional writing is achieved using thermoelectrically controlled magnonic
current as an alternative to conventional electric current. The device uses a
magnetic tunnel junction (MTJ), which is adjacent to a non-magnetic metallic
and a ferrite film. This film stack is heated or cooled by a Peltier element
which creates a bi-directional magnonic pulse in the ferrite film. Conversion
of magnons to spin current occurs at the ferrite-metal interface, and the
resulting spin-transfer torque is used to achieve sub-nanosecond precessional
switching of the ferromagnetic free layer in the MTJ. Compared to electric
current driven STT-MRAM with perpendicular magnetic anisotropy (PMA),
thermoelectric STT-MRAM reduces the overall magnetization switching energy by
more than 40% for nano-second switching, combined with a write error rate (WER)
of less than 10-9 and a lifetime of 10 years or higher. The combination of
higher thermal activation energy, sub-nanosecond read/write speed, improved
tunneling magneto-resistance (TMR) and tunnel barrier reliability make
thermoelectric STT-MRAM a promising choice for future non-volatile memory
The unusual electrical and optical properties of graphene make it a promising
candidate for optoelectronic applications. An important, but as yet unexplored
aspect is the role of photo-excited hot carriers in charge and energy transport
at graphene interfaces. Here, we perform time-resolved (~250 fs) scanning
photocurrent microscopy on a tunable graphene pn junction. The ultrafast
pump-probe measurements yield a photocurrent response time of ~1.5 ps at room
temperature increasing to ~4 ps at 20 K. Combined with the negligible
dependence of photocurrent amplitude on environmental temperature this implies
that hot carriers rather than phonons dominate energy transport at high
frequencies. Gate-dependent pump-probe measurements demonstrate that both
thermoelectric and built-in electric field effects contribute to the
photocurrent excited by laser pulses. The relative weight of each contribution
depends on the junction configuration. A single laser beam excitation also
displays multiple polarity-reversals as a function of carrier density, a
signature of impact ionization. Our results enhance the understanding of
non-equilibrium electron dynamics, electron-electron interactions, and
electron-phonon interactions in graphene. They also determine fundamental
limits on ultrafast device operation speeds (~500 GHz) for potential
graphene-based photon detection...
We investigate the spin-dependent electric and thermoelectric properties of
ferromagnetic zigzag-graphyne nanoribbons (ZGNRs) using the density-functional
theory combined with the non-equilibrium Green's function method. A giant
magnetoresistance is obtained in the pristine even-width ZGNRs and can be as
high as 10e6 %. However, for the doped systems, a large magnetoresistance
behavior may appear in the odd-width ZGNRs rather than the even-width ones.
This suggests that the magnetoresistance can be manipulated in a wide range by
the dopants on edges of ZGNRs. Another interesting phenomenon is that in the B-
and N-doped even-width ZGNRs the spin Seebeck coefficient is always larger than
the charge Seebeck coefficient, and a pure-spin-current thermospin device can
be achieved at specific temperatures.
With developing interest in power generation applications of thermoelectrics and the growing influence of advanced materials on thermoelectric device fabrication, there is an increased demand for better understanding of module-level behavior. Likewise, novel module geometries are being explored for higher performance and require sophisticated modeling methods.
In addition to new geometrical design, transport phenomena, such as Thomson heating and contact resistances, aggravate the complexity of modeling thermoelectric modules (TEMs) and thus limit design capability. Typically, these effects are either approximated (or in some cases neglected entirely) with little exploration in to the validity of the underlying assumptions associated with the approximation. As such, standard models are often predicated on assumptions that cannot be made beyond very limited operating regimes. Consequently, most TEM analysis generally utilizes simplistic methods of modeling on a module-level scale, which introduce inaccuracies that must be redressed.
Particularly with larger temperature gradients, typically negligible effects could begin to impact overall system performance. Material property temperature-dependency, combined with leakage effects...
A model of a thermoelectric heat pump driven by a thermoelectric generator with external heat transfer irreversibility is proposed. The performance of the combined thermoelectric heat pump device obeying Newton's heat transfer law is analyzed using the combination of finite time thermodynamics and non-equilibrium thermodynamics. Two analytical formulae for heating load versus working electrical current, and the coefficient of performance (COP) versus working electrical current, are derived. For a fixed total heat transfer surface area of four heat exchangers, the allocations of the heat transfer surface area among the four heat exchangers are optimized for maximizing the heating load and the COP of the combined thermoelectric heat pump device. For a fixed total number of thermoelectric elements, the ratio of the number of thermoelectric elements of the generator to the total number of thermoelectric elements is also optimized for maximizing both the heating load and the COP of the combined thermoelectric heat pump device. The influences of thermoelectric element allocation and heat transfer area allocation are analyzed by detailed numerical examples. The optimum working electrical currents for maximum heating load and maximum COP at different total numbers of thermoelectric elements and different total heat transfer areas are provided...