Influence of antisymmetric exchange interaction on quantum tunneling of magnetization in a dimeric molecular magnet Mn6

We present magnetization measurements on the single molecule magnet Mn6, revealing various tunnel transitions inconsistent with a giant-spin description. We propose a dimeric model of the molecule with two coupled spins S=6, which involves crystal-field anisotropy, symmetric Heisenberg exchange interaction, and antisymmetric Dzyaloshinskii-Moriya exchange interaction. We show that this simplified model of the molecule explains the experimentally observed tunnel transitions and that the antisymmetric exchange interaction between the spins gives rise to tunneling processes between spin states belonging to different spin multiplets.Comment: 5 pages, 4 figure

Laser-induced Precession of Magnetization in GaMnAs

We report on the photo-induced precession of the ferromagnetically coupled Mn spins in (Ga,Mn)As, which is observed even with no external magnetic field applied. We concentrate on various experimental aspects of the time-resolved magneto-optical Kerr effect (TR-MOKE) technique that can be used to clarify the origin of the detected signals. We show that the measured data typically consist of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in the sample.Comment: 4 pages, 5 figure

Highly anisotropic interlayer magnetoresistance in ZrSiS nodal-line Dirac semimetal

We instigate the angle-dependent magnetoresistance (AMR) of the layered nodal-line Dirac semimetal ZrSiS for the in-plane and out-of-plane current directions. This material has recently revealed an intriguing butterfly-shaped in-plane AMR that is not well understood. Our measurements of the polar out-of-plane AMR show a surprisingly different response with a pronounced cusp-like feature. The maximum of the cusp-like anisotropy is reached when the magnetic field is oriented in the $a$-$b$ plane. Moreover, the AMR for the azimuthal out-of-plane current direction exhibits a very strong four-fold $a$-$b$ plane anisotropy. Combining the Fermi surfaces calculated from first principles with the Boltzmann's semiclassical transport theory we reproduce and explain all the prominent features of the unusual behavior of the in-plane and out-of-plane AMR. We are also able to clarify the origin of the strong non-saturating transverse magnetoresistance as an effect of imperfect charge-carrier compensation and open orbits. Finally, by combining our theoretical model and experimental data we estimate the average relaxation time of $2.6\times10^{-14}$~s and the mean free path of $15$~nm at 1.8~K in our samples of ZrSiS.Comment: 8 pages, 4 figure

Star-forming galaxies versus low- and high-excitation radio AGN in the VLA-COSMOS 3GHz Large Project

We study the composition of the faint radio population selected from the VLA-COSMOS 3GHz Large Project, a radio continuum survey performed at 10 cm wavelength. The survey covers the full 2 square degree COSMOS field with mean $rms\sim2.3$ $\mu$Jy/beam, cataloging 10,899 source components above $5\times rms$. By combining these radio data with UltraVISTA, optical, near-infrared, and Spitzer/IRAC mid-infrared data, as well as X-ray data from the Chandra Legacy, and Chandra COSMOS surveys, we gain insight into the emission mechanisms within our radio sources out to redshifts of $z\sim5$. From these emission characteristics we classify our souces as star forming galaxies or AGN. Using their multi-wavelength properties we further separate the AGN into sub-samples dominated by radiatively efficient and inefficient AGN, often referred to as high- and low-excitation emission line AGN. We compare our method with other results based on fitting of the sources' spectral energy distributions using both galaxy and AGN spectral models, and those based on the infrared-radio correlation. We study the fractional contributions of these sub-populations down to radio flux levels of $\sim$10 $\mu$Jy. We find that at 3 GHz flux densities above $\sim$400 $\mu$Jy quiescent, red galaxies, consistent with the low-excitation radio AGN class constitute the dominant fraction. Below densities of $\sim$200 $\mu$Jy star-forming galaxies begin to constitute the largest fraction, followed by the low-excitation, and X-ray- and IR-identified high-excitation radio AGN.Comment: 7 pages, 3 figures, The many facets of extragalactic radio surveys: towards new scientific challenges, Bologna 20-23 October 201

Spin Tunneling and Phonon-assisted Relaxation in Mn12-acetate

We present a comprehensive theory of the magnetization relaxation in a Mn12-acetate crystal in the high-temperature regime (T>1 K), which is based on phonon-assisted spin tunneling induced by quartic magnetic anisotropy and weak transverse magnetic fields. The overall relaxation rate as function of the longitudinal magnetic field is calculated and shown to agree well with experimental data including all resonance peaks measured so far. The Lorentzian shape of the resonances, which we obtain via a generalized master equation that includes spin tunneling, is also in good agreement with recent data. We derive a general formula for the tunnel splitting energy of these resonances. We show that fourth-order diagonal terms in the Hamiltonian lead to satellite peaks. A derivation of the effective linewidth of a resonance peak is given and shown to agree well with experimental data. In addition, previously unknown spin-phonon coupling constants are calculated explicitly. The values obtained for these constants and for the sound velocity are also in good agreement with recent data. We show that the spin relaxation in Mn12-acetate takes place via several transition paths of comparable weight. These transition paths are expressed in terms of intermediate relaxation times, which are calculated and which can be tested experimentally.Comment: 18 pages, 22 EPS figures, REVTe