10 research outputs found
Effects of Ferromagnetic Magnetic Ordering and Phase Transition on the Resistivity of Spin Current
It has been shown experimentally a long time ago that the magnetic ordering
causes an anomalous behavior of the electron resistivity in ferromagnetic
crystals. Phenomenological explanations based on the interaction between
itinerant electron spins and lattice spins have been suggested to explain these
observations. We show by extensive Monte Carlo simulation that this behavior is
also observed for the resistivity of the spin current calculated as a function
of temperature () from low- ordered phase to high- paramagnetic phase
in a ferromagnet. We show in particular that across the critical region, the
spin resistivity undergoes a huge peak. The origin of this peak is shown to
stem from the formation of magnetic domains near the phase transition. The
behavior of the resistivity obtained here is compared to experiments and
theories. A good agreement is observed.Comment: 7 pages, 3 figures, accepted, to appear in J. Appl. Phy
Spin Resistivity in Frustrated Antiferromagnets
In this paper we study the spin transport in frustrated antiferromagnetic FCC
films by Monte Carlo simulation. In the case of Ising spin model, we show that
the spin resistivity versus temperature exhibits a discontinuity at the phase
transition temperature: an upward jump or a downward fall, depending on how
many parallel and antiparallel localized spins interacting with a given
itinerant spin. The surface effects as well as the difference of two degenerate
states on the resistivity are analyzed. Comparison with non frustrated
antiferromagnets is shown to highlight the frustration effect. We also show and
discuss the results of the Heisenberg spin model on the same lattice
Spin transport in magnetic multilayers
We study by extensive Monte Carlo simulations the transport of itinerant
spins travelling inside a multilayer composed of three ferromagnetic films
antiferromagnetically coupled to each other in a sandwich structure. The two
exterior films interact with the middle one through non magnetic spacers. The
spin model is the Ising one and the in-plane transport is considered. Various
interactions are taken into account. We show that the current of the itinerant
spins going through this system depends strongly on the magnetic ordering of
the multilayer: at temperatures below (above) the transition temperature
, a strong (weak) current is observed. This results in a strong jump of
the resistance across . Moreover, we observe an anomalous variation,
namely a peak, of the spin current in the critical region just above . We
show that this peak is due to the formation of domains in the temperature
region between the low- ordered phase and the true paramagnetic disordered
phase. The existence of such domains is known in the theory of critical
phenomena. The behavior of the resistance obtained here is compared to a recent
experiment. An excellent agreement with our physical interpretation is
observed. We also show and discuss effects of various physical parameters
entering our model such as interaction range, strength of electric and magnetic
fields and magnetic film and non magnetic spacer thicknesses.Comment: 8 pages, 17 figures, submitted to J. Phys.: Cond Matte
The study of optical gain for terahertz quantum cascade laser using density matrix method
Terahertz (THz) quantum cascade lasers (QCL) are currently increasing in popularity. It is expected to become the main source of emerging terahertz radiation technology and applications. However to produce the device within the application specification is costly and time consuming. This is because the manufacturing process of the superlattice growth and the device processing and testing are long and expensive processes. Thus a prediction tool is needed to overcome the problems in designing and producing THz QCL within the needed optical expectation. The density matrix method is used to calculate the performance of this device electronically and optically. The result obtained was compared to the experimental result conducted by previous researchers. The calculation result showed that the gain is 20 cmâ1 when the population inversion occurs at threshold current density of 400 A cm-2. Meanwhile a negative gain or loss occurs below 350 A cm-2. As a conclusion, it is demonstrated that this method has a capability to explain the transport phenomena as well as to predict the performance of the THz QCL device design
Temperature Dependence of the Spin Resistivity in Ferromagnetic Thin Films
11 pages, 18 figures, submitted for publicationInternational audienceThe magnetic phase transition is experimentally known to give rise to an anomalous temperature-dependence of the electron resistivity in ferromagnetic crystals. Phenomenological theories based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. In this paper, we show by extensive Monte Carlo (MC) simulation the behavior of the resistivity of the spin current calculated as a function of temperature () from low- ordered phase to high- paramagnetic phase in a ferromagnetic film. We analyze in particular effects of film thickness, surface interactions and different kinds of impurities on the spin resistivity across the critical region. The origin of the resistivity peak near the phase transition is shown to stem from the existence of magnetic domains in the critical region. We also formulate in this paper a theory based on the Boltzmann's equation in the relaxation-time approximation. This equation can be solved using numerical data obtained by our simulations. We show that our theory is in a good agreement with our MC results. Comparison with experiments is discussed
Theoretical and experimental operating wavelength of GaAs/Al
IR photodetectors based on GaAs/Al0.25Ga0.75As multiquantum wells
(QWIP) grown by molecular beam epitaxy (MBE) are studied. The envelop
function formalism is used to determine the theoretical intersubband
transition energies. The electronic states are calculated in both parabolic
and non parabolic cases. IR spectroscopy transmission is used as the
experimental technique to evaluate the optical absorption. The measures are
made at 77 K for incidence at both 45° and Brewster angles geometries.
The last experimental results compare well with the theoretical ones and
correspond to 10â12Â m operating wavelength