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Unidirectional control of optically induced spin waves (Vol. 48, No. 4)

For future information technologies, the field of magnonics is rapidly emerging. Spin waves ̶collective modes of spin precessions ̶ are promising information carriers in magnonics, as Joule heating is negligible and propagation damping is low. Spatial control of the spin wave is indispensable for future application such as spin-wave switching, spin-wave-assisted recording, and sensing of small magnetic fields. In this article, unidirectional control of optically induced spin waves in a rare-earth iron garnet crystal is demonstrated. We observed the interference of two spin-wave packets with different initial phases generated by circularly polarized light pulses. This interference results in unidirectional propagation if the spin-wave sources are spaced apart at 1/4 of the wavelength of the spin waves and the initial phase difference is set to π/2. The propagating direction of the spin wave is switched by the polarization helicity of the light pulses. Moreover, in a numerical simulation, applying more than two spin-wave sources with a suitable polarization and spot shape, arbitrary manipulation of the spin wave by the phased array method was replicated. This achievement opens up a field of magnetic materials science and explores an alternative sensing technique using magnetic fields.

I. Yoshimine, Y. Y. Tanaka, T. Shimura and T. Satoh, Unidirectional control of optically induced spin waves, EPL 117, 67001 (2017)
[Abstract]

Imaging helps to spot fake ancient daggers (Vol. 48, No. 4)

Combining neutron and X-ray imaging gives clues to how ancient weapons were manufactured

Since the 19^th century, collectors have become increasingly interested in weapons from ancient Asia and the Middle East. In an attempt to fight forged copies, physicists are now adding their imaging power to better authenticate these weapons; the fakes can't resist the investigative power of X-rays combined with neutron imaging. In a study published recently, the authors have demonstrated the usefulness of such a combined imaging approach to help museum curators in their quest to ensure authenticity. They can now reliably tell first-class modern copies of early daggers and swords from authentic ones. In this study, the authors focus on a kris—the distinctive weapon of Malaysia and Indonesia—and a kanjar— a double-edged dagger with a slightly curved blade and a pistol-grip made of metal, ivory, jade or some other hard-stone found e.g. in Persia and India. The authors found the internal structure of the traditional kris examined in this study was inconsistent with descriptions of traditional forging methods to be found in the extant literature, thus suggesting the artefact was a fake. By contrast, the kanjar analysed in the study is most likely to be authentic, as the material distribution in the volume of the blade conforms to traditional metallurgical processes.

F. Salvemini, F. Grazzi, N. Kardjilov, F. Wieder, I. Manke, D. Edge, A. Williams and M. Zoppi, Combined application of imaging techniques for the characterization and authentication of ancient weapons, Eur. Phys. J. Plus 132, 228 (2017)
[Abstract]

Accurate determination of Curie temperature in helimagnet FeGe (Vol. 48, No. 4)

Cubic helimagnet FeGe, the prototype of skyrmion materials near room temperature, has emerged and may impact future information technology. The magnetic entropy change (MEC) of helimagnet FeGe and the close relationship between the MEC and critical exponents of a second-order phase transition were studied. A relatively small MEC under external high magnetic field indicates the coexistence and competition between exchange anisotropy and magneto-crystalline anisotropy, and a stable balance is formed in the precursor region when the applied magnetic field cannot completely transform FeGe into a single magnetic structure phase. Based on the obtained magnetic entropy change and critical exponents, an accurate Curie temperature of helimagnet FeGe under zero magnetic field is confirmed to be 279.1 K, lower than 282 K deduced directly from the derivative magnetic susceptibility and higher than 278.2 K previously reported. So，the accurate determination of Curie temperature is conducive to reconsider the inhomogeneous chiral-spin state and reconstruct the magnetic phase diagram in the precursor region of helimagnet FeG.

L. Xu, H. Han, J. Fan, D. Shi, D. Hu, H. Du, L. Zhang, Y. Zhang and H. Yang, Magnetic entropy change and accurate determination of Curie temperature in single-crystalline helimagnet FeGe, EPL 117, 47004 (2017)
[Abstract]