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Register a new userMagneto-optic Response of the Metallic Antiferromagnet Fe$_{2}$As to Ultrafast Temperature Excursions
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The linear magneto-optical Kerr effect (MOKE) is often used to probe magnetism of ferromagnetic materials, but MOKE cannot be applied to collinear antiferromagnets (AFs) due to the cancellation of sub-lattice magnetization. Magneto-optical constants that are quadratic in magnetization, however, provide an approach for studying AFs on picosecond time scales. Here, we combine transient measurements of optical reflectivity and birefringence to study the linear optical response of Fe$_{2}$As to small ultrafast temperature excursions. We performed temperature dependent pump-probe measurements on crystallographically isotropic (001) and anisotropic (010) faces of Fe$_{2}$As bulk crystals. We find the largest optical signals arise from changes in the index of refraction along the $z$-axis, i.e. perpendicular to the Neel vector. Both real and imaginary parts of the time-resolved optical birefringence rotation signal approximately follow the temperature dependence of the magnetic heat capacity, as expected if the changes in dielectric constants are dominated by contributions of exchange interactions to the dielectric constant. We conclude that under our experimental conditions, changes in the exchange interaction contribute more strongly to the temperature dependence of the magneto-optic constants than the Voigt effect.
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Magnetocrystalline anisotropy is a fundamental property of magnetic materials that determines the dynamics of magnetic precession, the frequency of spin waves, the thermal stability of magnetic domains, and the efficiency of spintronic devices. We combine torque magnetometry and density functional theory calculations to determine the magnetocrystalline anisotropy of the metallic antiferromagnet Fe$_2$As. Fe$_2$As has a tetragonal crystal structure with the Neel vector lying in the (001) plane. We report that the four-fold magnetocrystalline anisotropy in the (001)-plane of Fe$_2$As is extremely small, ${K_{22}} = - 150~{rm{ J/}}{{rm{m}}^{rm{3}}}$ at T = 4 K, much smaller than perpendicular magnetic anisotropy of ferromagnetic structure widely used in spintronics device. ${K_{22}}$ is strongly temperature dependent and close to zero at T > 150 K. The anisotropy ${K_1}$ in the (010) plane is too large to be measured by torque magnetometry and we determine ${K_1} = -830~{rm{ kJ/}}{{rm{m}}^{rm{3}}}$ using first-principles density functional theory. Our simulations show that the contribution to the anisotropy from classical magnetic dipole-dipole interactions is comparable to the contribution from spin-orbit coupling. The calculated four-fold anisotropy in the (001) plane ${K_{22}}$ ranges from $- 292~{rm{ J/}}{{rm{m}}^{rm{3}}}$ to $280~{rm{ J/}}{{rm{m}}^{rm{3}}}$, the same order of magnitude as the measured value. We use ${K_1}$ from theory to predict the frequency and polarization of the lowest frequency antiferromagnetic resonance mode and find that the mode is linearly polarized in the (001)-plane with $f = $ 670 GHz.
To understand spin interactions in materials of the Cu$_2$Sb structure type, inelastic neutron scattering of Fe$_2$As single crystals was examined at different temperatures and incident neutron energies. The experimental phonon spectra match well with the simulated phonon spectra obtained from density functional theory (DFT) calculations. The measured magnon spectra were compared to the simulated magnon spectra obtained via linear spin wave theory with the exchange coupling constants calculated using the spin polarized, relativistic Korringa-Kohn-Rostoker method in Zhang et al. (2013). The simulated magnon spectra broadly agree with the experimental data although, the energy values are underestimated along the $K$ direction. Exchange coupling constants between Fe atoms were refined by fits to the experimental magnon spectra, revealing stronger nearest neighbor Fe1-Fe1 exchange coupling than previously reported. The strength of this exchange coupling is almost an order of magnitude higher than other exchange interactions despite the three-dimensional nature of the phonon interactions. The lack of scattering intensity at energies above 60 meV makes unconstrained determination of the full set of exchange interactions difficult, which may be a fundamental challenge in metallic antiferromagnets.
By correlating time- and angle-resolved photoemission and time-resolved transverse- magneto- optical Kerr effect measurements, both at extreme ultraviolet wavelengths, we uncover the universal nature of the ultrafast photoinduced magnetic phase transition in Ni. This allows us to explain the ultrafast magnetic response of Ni at all laser fluences - from a small reduction of the magnetization at low laser fluences, to complete quenching at high laser fluences. Both probe methods exhibit the same demagnetization and recovery timescales. We further show that the ultrafast demagnetization in Ni is indeed a magnetic phase transition that is launched within 20 fs, followed by demagnetization of the material within ~200 fs, and subsequent recovery of the magnetization on timescales ranging from 500 fs to >70 ps. We also provide evidence of two competing channels with two distinct timescales in the recovery process, that suggest the presence of coexisting phases in the material.
Metallic antiferromagnets with broken inversion symmetry on the two sublattices, strong spin-orbit coupling and high N{e}el temperatures offer new opportunities for applications in spintronics. Especially Mn$_{2}$Au, with high N{e}el temperature and conductivity, is particularly interesting for real-world applications. Here, manipulation of the orientation of the staggered magnetization,textit{ i.e.} the N{e}el vector, by current pulses has been recently demonstrated, with the read-out limited to studies of anisotropic magnetoresistance or X-ray magnetic linear dichroism. Here, we report on the in-plane reflectivity anisotropy of Mn$_{2}$Au (001) films, which were N{e}el vector aligned in pulsed magnetic fields. In the near-infrared, the anisotropy is $approx$ 0.6%, with higher reflectivity for the light polarized along the N{e}el vector. The observed magnetic linear dichroism is about four times larger than the anisotropic magnetoresistance. This suggests the dichroism in Mn$_{2}$Au is a result of the strong spin-orbit interactions giving rise to anisotropy of interband optical transitions, in-line with recent studies of electronic band-structure. The considerable magnetic linear dichroism in the near-infrared could be used for ultrafast optical read-out of the N{e}el vector in Mn$_{2}$Au.
We reported the first observation of metallic hydrogen (MH) in the low temperature limit at a pressure of ~495 GPa in an article published in Science (1). This transition was first predicted by Wigner and Huntington (WE) over 80 years ago (2) at a pressure of ~25 GPa. In recent decades it became clear that the required pressure for metallization was far greater, in the 400-500 GPa range. Until now the observation of the WE transition in diamond anvil cells (DACs) has been prevented by one problem: the diamonds break before a sufficiently high pressure has been achieved. This has driven the high-pressure community to improve DACs and experimental methods to understand and overcome the conditions that limited the performance of diamonds and the pressure. In our experiment, with increasing pressure, we observed a clear transition from a transparent sample of solid molecular hydrogen to an opaque black sample to a shiny reflective sample of MH, as determined by reflectance measurements. There is no doubt that MH was produced at the highest pressures. Yet there have been criticisms concerning the pressure that was achieved, the possibility that the 50 nm alumina layer, deposited on diamonds to inhibit diffusion of hydrogen, might be transformed to a metal and be responsible for the reflectance, and analysis of the reflectance. Here we respond to the criticisms posted on the condensed matter arXiv by Loubeyre, Occelli, and Dumas (LOD)- arXiv:1702.07192, Eremets and Drozdov (ED)- arXiv:1702.05125, and Goncharov and Struzhkin (GS)- arXiv:1702.04246.
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