The magnetization (both dc and ac) of pyrochlore Eu 2 Ti 2 O 7 have been investigated. Ac susceptibility (χ / &χ // ) measurements reveal a new unusual spin freezing below 35K for pure Eu 2 Ti 2 O 7 . Frequency dependence of these ac χ // peak positions indicate slow spin relaxation near this temperature and it follows the Arrhenius formulasuggesting a thermally activated relaxation process. The origin of this spin freezing has been attributed essentially to a single ion process which is associated to Eu 3+ spin relaxation. Non-magnetic dilution by Y 3+ ions also confirms the single ion freezing.
Effects of the tensile and compressive epitaxial strain and the crystallographic orientations on the structural and magnetic properties of (Bi0.9La0.1)2FeCrO6 (BLFCO) films were studied. The BLFCO (001) films (30 nm and 70 nm) were deposited on various single crystal substrates having lattice mismatch with the film in the range of −4.16% to +7.2%. We find that a pronounced ferromagnetic order manifests in the coherently strained films compared to that in the partially strained films. The saturation magnetic moment exhibits dissimilar effects on the type of the lattice mismatch: the coherent tensile strain is less favorable than the coherent compressive strain for the magnetic order in these films. We further establish that the ferromagnetic order exhibits maximum magnetic moment for (111)-oriented and minimum for (110)-oriented coherently strained BLFCO epitaxial films.
Abstract:We demonstrate here an efficient THz source with low electrical power consumption. We have increased the maximum THz radiation power emitted from SI-GaAs based photoconductive emitters by two orders of magnitude. By irradiating the SI-GaAs substrate with Carbon-ions up to 2 m deep, we have created lot of defects and decreased the life time of photo-excited carriers inside the substrate. Depending on the irradiation dose we find 1 to 2 orders of magnitude decrease in total current flowing in the substrate, resulting in subsequent decrease of heat dissipation in the antenna. This has resulted in increasing maximum cut-off of the applied voltage across Photo-Conductive Emitter (PCE) electrodes to operate the device without thermal breakdown from ~35 V to > 150 V for the 25 m electrode gaps. At optimum operating conditions, carbon irradiated (10 14 ions/cm 2 ) PCEs give THz pulses with power about 100 times higher in comparison to the usual PCEs on SI-GaAs and electrical to THz power conversion efficiency has improved by a factor of ~ 800.Electromagnetic radiations having frequencies in Tera-hertz (THz) range (1THz = 10 12 Hz) are not so easy to generate [1] .But due to its applications in security imaging, bio-sensing, chemical identification, material characterization etc., there is high demand of high power THz sources, particularly sources which can generate short THz pulses with broadband spectrum. Till now, photoconductive emitters (PCEs) are known to be the best sources for high power THz pulse generation. Improving the efficiency of THz pulse sources with better designs or material, is one of the major goals of ongoing research in this field. There have been several attempts to increase the THz emission from these sources by modifying the electrical and optical properties of the semiconducting substrate [2] , design of electrodes [3] and patterning the active area of PCE in between the two electrodes [4] .In THz PCEs newly photo generated charge carriers (electron-hole pairs) via laser pulse of width less than 100 fs gets accelerated under already applied electric field and this sudden jump in number of free carriers and their acceleration gives sudden rise in the current. This sudden rise in current in pico-second time domain is responsible for THz pulse emission. In the case, where the semiconductor has carrier life time of less than a pico-second like LT-GaAs, current falls down to the dark level within picoseconds as electron hole pairs recombine with each other. Such materials are useful for the generation of bipolar THz pulses. In semiconductors like SI-GaAs which has carrier life time of more than 50 ps, fall in current takes relatively much longer time. Since electric field of the emitted THz pulse , where J(t)
Photoconductive antennas (PCAs) are among the most conventional devices used for emission as well as detection of terahertz (THz) radiation. However, due to their low optical-to-THz conversion efficiencies, applications of these devices in out-of-laboratory conditions are limited. In this paper, we report several factors of enhancement in THz emission efficiency from conventional PCAs by coating a nano-layer of dielectric (TiO2) on the active area between the electrodes of a semi-insulating GaAs-based device. Extensive experiments were done to show the effect of thicknesses of the TiO2 layer on the THz power enhancement with different applied optical power and bias voltages. Multiphysics simulations were performed to elucidate the underlying physics behind the enhancement of efficiency of the PCA. Additionally, this layer increases the robustness of the electrode gaps of the PCAs with high electrical insulation as well as protect it from external dust particles.
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