Epitaxial strain and the magnetic properties of canted antiferromagnetic perovskite NaNiF3 thin films

The perovskite crystal structure is known to exhibit a multitude of interesting physical phenomena owing to the intricate coupling of the electronic and magnetic properties to the structure. Fluoroperovskites offer an alternative chemistry to the much more widely studied oxide materials, which may prove advantageous for applications. It is demonstrated here for the first time that the antiferromagnetic perovskite fluoride, NaNiF3, can be synthesized in thin film form. The films were grown via molecular beam epitaxy on SrTiO3 (100) substrates to produce high quality epitaxial films in the thickness range of 5-50 nm. The Pnma structure of the films was confirmed by x-ray diffraction. There was a decrease in the out-of-plane lattice spacing from the bulk value corresponding to a maximum strain of 1.7% in the thinnest film. Canted antiferromagnetism was measured in all films using magnetometry and a negative change in the antiferromagnetic ordering temperature of ΔTN = - 9.1 ± 0.7 K was observed with increasing strain

Plasmon-enhanced electron-phonon coupling in Dirac surface states of the thin-film topological insulator Bi2Se3

Raman measurements of a Fano-type surface phonon mode associated with Dirac surface states (SS) in Bi2Se3 topological insulator thin films allowed an unambiguous determination of the electron-phonon coupling strength in Dirac SS as a function of film thickness ranging from 2 to 40 nm. A non-monotonic enhancement of the electron-phonon coupling strength with maximum for the 8 - 10 nm thick films was observed. The non-monotonicity is suggested to originate from plasmon-phonon coupling which enhances electron-phonon coupling when free carrier density in Dirac SS increases with decreasing film thickness and becomes suppressed for thinnest films when anharmonic coupling between in-plane and out-of-plane phonon modes occurs. The observed about four-fold enhancement of electron-phonon coupling in Dirac SS of the 8 - 10 nm thick Bi2Se3 films with respect to the bulk samples may provide new insights into the origin of superconductivity in this-type materials and their applications

Effect of Mn doping on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3

Transient reflectivity (TR) measured at laser photon energy 1.51 eV from the indirectly intersurface coupled topological insulator Bi2-xMnxSe3 films (12 nm thick) revealed a strong dependence of the rise-time and initial decay-time constants on photoexcited carrier density and Mn content. In undoped samples (x = 0), these time constants are exclusively governed by electron-electron and electron-phonon scattering, respectively, whereas in films with x = 0.013 - 0.27 ultrafast carrier dynamics are completely controlled by photoexcited electron trapping by ionized Mn2+ acceptors and their dimers. The shortest decay-time (~0.75 ps) measured for the film with x = 0.27 suggests a great potential of Mn-doped Bi2Se3 films for applications in high-speed optoelectronic devices. Using Raman spectroscopy exploiting similar laser photon energy (1.58 eV), we demonstrate that due to indirect intersurface coupling in the films, the photoexcited electron trapping in the bulk enhances the electron-phonon interaction strength in Dirac surface states

Magnetoelectric properties of 500 nm Cr2O3 films

The linear magnetoelectric effect was measured in 500 nm Cr2O3 films grown by rf sputtering on Al2O3 substrates between top and bottom thin film Pt electrodes. Magnetoelectric susceptibility was measured directly by applying an AC electric field and measuring the induced AC magnetic moment using superconducting quantum interference device magnetometry. A linear dependence of the induced AC magnetic moment on the AC electric field amplitude was found. The temperature dependence of the magnetoelectric susceptibility agreed qualitatively and quantitatively with prior measurements of bulk single crystals, but the characteristic temperatures of the film were lower than those of single crystals. It was also possible to reverse the sign of the magnetoelectric susceptibility by reversing the sign of the magnetic field applied during cooling through the N\'eel temperature. A competition between total magnetoelectric and Zeeman energies is proposed to explain the difference between film and bulk Cr2O3 regarding the cooling field dependence of the magnetoelectric effect.Comment: accepted at Physical Review