We demonstrate a method to create arbitrary terahertz (THz) polarization profiles by exploiting the magnetic field-dependent emission process of a spintronic source. As a proof-of-concept, we show that by applying a specific magnetic field pattern to the source, it is possible to generate a quadrupole-like THz polarization profile. Experimental measurements of the electric field at the focus of the THz beam revealed a polarity flip in the transverse profile of the quadrupole-like mode with a resulting strong, on-axis longitudinal component of 17.7 kV cm−1. This represents an order of magnitude increase in the longitudinal component for the quadrupole-like profile compared to a linear polarization, showing an example of how the magnetic field patterning of a spintronic source can be exploited to obtain desirable THz polarization properties. This unique ability to generate any desired THz polarization profile opens up possibilities for schemes such as rotatable polarization spectroscopy and for efficient mode coupling in various waveguide designs. Furthermore, the strong longitudinal fields that can be generated have applications in areas including intra-subband spectroscopy of semiconductors, non-diffraction limited THz imaging, and particle-beam acceleration.

This is a repository copy of Patchiness of ion-exchanged mica revealed by DNA binding dynamics at short length scales.

Antiferromagnets have considerable potential as spintronic materials. Their dynamic properties include resonant modes at frequencies higher than can be observed in conventional ferromagnetic materials. An alternative to single-phase antiferromagnets are synthetic antiferromagnets (SAFs), engineered structures of exchange-coupled ferromagnet/nonmagnet/ferromagnet trilayers. SAFs have significant advantages due to the wide-ranging tunability of their magnetic properties and inherent compatibility with current device technologies, such as those used for Spin-transfer-torque magnetic random-access memory production. Here we report the dynamic properties of fully compensated SAFs using broadband ferromagnetic resonance and demonstrate resonant optic modes in addition to the conventional acoustic (Kittel) mode. These optic modes possess the highest zero-field frequencies observed in SAFs to date with resonances of 18 and 21 GHz at the first and second peaks in antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, respectively. In contrast to previous SAF reports that focus only on the first RKKY antiferromagnetic coupling peak, we show that a higher optic mode frequency is obtained for the second antiferromagnetic coupling peak. We ascribe this to the smoother interfaces associated with a thicker nonmagnetic layer. This demonstrates the importance of interface quality to achieving high-frequency optic mode dynamics entering the subterahertz range.

Raman microspectroscopy was used to measure compressive strain within epitaxial graphene (EG) grown on the carbon-terminated SiC(0001¯) face as a function of annealing time for a growth temperature of 1400 °C. A maximum strain of −0.5% was seen at the longest time of 55 min. This differs from the −0.9% expected for strain caused by cooling from the growth temperature due to the differential thermal contraction between the SiC and EG layer, despite good agreement between this model and data on EG on SiC(0001). We suggest that this is due to the different EG bonding mechanisms on the two SiC faces.

Magnetostrictive amorphous FeSiB and FeGaSiB thin films, thickness 50nm have been grown by the co-sputtering-evaporation technique with a range of Ar pressure (4 -8 bar) to control the Ga percentage within the films and study their effect on the magnetic, structural and magnetostriction properties. By x-ray diffraction, it was found that all the films had an amorphous structure and the only peaks present were for Si substrate. Using a magneto-optical Kerr effect (MOKE) magnetometer, it found that, for the FeSiB films, the anisotropy field (Hk) increased slowly as the pressure increased, while for the FeGaSiB films, the saturation field (Hs) ≈ 4000 A/m for all pressures. For both the film sets, the coercive field (Hc) was less than 800 A/m. The magnetostriction constants (s) of the FeSiB thin films increased with increasing pressure. While for the FeGaSiB films, the magnetostriction constant decreased with increasing the sputtering gas pressure, with the maximum = 11.4 ppm, at the lowest pressure 4 bar. Thus it was determined that the addition of Ga atoms reduced the intrinsic stress within the films, while maintaining the amorphous morphology.

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The exchange stiffness constant is recognized as one of the fundamental properties of magnetic materials, though its accurate experimental determination remains a particular challenge. In thin films, resonance measurements exploiting perpendicular standing spin waves (PSSWs) are increasingly used to extract this parameter, typically through a determination of the first-order PSSW mode. Here, we present a systematic study of multiple PSSW modes in NiFe films, where both the sample thickness and the cap layer material are varied. The results show that a simple analysis based on the Kittel rigid pinning model yields an exchange stiffness constant that varies with thickness, mode number, and capping layer material. This finding is clearly inconsistent with physical expectation that the exchange stiffness constant of a material is single valued for a particular set of thermodynamic conditions. Using a more general exchange boundary condition, we show, through a comprehensive set of micromagnetic simulations, that a dynamic pinning mechanism originally proposed by Wigen is able to reproduce the experimental results using a single value of Aex. Our findings support the utility of short wavelength, higher order PSSWs to determine the Aex of thin films and show that the value of Aex obtained has a weak dependency on the material immediately adjacent to the magnetic layer.