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Longitudinal spin excitations and magnetic anisotropy in antiferromagnetically ordered BaFe2As2  [PDF]
Chong Wang,Rui Zhang,Fa Wang,Huiqian Luo,L. P. Regnault,Pengcheng Dai,Yuan Li
Physics , 2013,
Abstract: We report on a spin-polarized inelastic neutron scattering study of spin waves in the antiferromagnetically ordered state of BaFe2As2. Three distinct excitation components are identified, with spins fluctuating along the c-axis, perpendicular to the ordering direction in the ab-plane, and parallel to the ordering direction. While the first two "transverse" components can be described by a linear spin-wave theory with magnetic anisotropy and inter-layer coupling, the third "longitudinal" component is generically incompatible with the local moment picture. It points towards a contribution of itinerant electrons to the magnetism already in the parent compound of this family of Fe-based superconductors.
Bi-collinear antiferromagnetic order in the tetragonal $α$-FeTe  [PDF]
Fengjie Ma,Wei Ji,Jiangping Hu,Zhong-Yi LU,Tao Xiang
Physics , 2008, DOI: 10.1103/PhysRevLett.102.177003
Abstract: By the first-principles electronic structure calculations, we find that the ground state of PbO-type tetragonal $\alpha$-FeTe is in a bi-collinear antiferromagnetic state, in which the Fe local moments ($\sim2.5\mu_B$) are ordered ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe square lattice. This bi-collinear order results from the interplay among the nearest, next nearest, and next next nearest neighbor superexchange interactions $J_1$, $J_2$, and $J_3$, mediated by Te $5p$-band. In contrast, the ground state of $\alpha$-FeSe is in the collinear antiferromagnetic order, similar as in LaFeAsO and BaFe$_2$As$_2$.
Measurement of Coherent Polarons in the Strongly Coupled Antiferromagnetically Ordered Iron-Chalcogenide Fe1.02Te using Angle-Resolved Photoemission Spectroscopy  [PDF]
Zhongkai Liu,Ruihua He,Donghui Lu,Ming Yi,Yulin Chen,Makoto Hashimoto,Robert G. Moore,Sung-Kwan Mo,Elizabeth A. Nowadnick,Jin Hu,Tijiang Liu,Zhiqiang Mao,Thomas P. Devereaux,Zahid Hussain,Zhi-Xun Shen
Physics , 2012, DOI: 10.1103/PhysRevLett.110.037003
Abstract: The nature of metallicity and the level of electronic correlations in the antiferromagnetically ordered parent compounds are two important open issues for the iron-based superconductivity. We perform a temperature-dependent angle-resolved photoemission spectroscopy study of Fe1.02Te, the parent compound for iron chalcogenide superconductors. Deep in the antiferromagnetic state, the spectra exhibit a "peak-dip-hump" line shape associated with two clearly separate branches of dispersion, characteristics of polarons seen in manganites and lightly-doped cuprates. As temperature increases towards the Neel temperature (T_N), we observe a decreasing renormalization of the peak dispersion and a counterintuitive sharpening of the hump linewidth, suggestive of an intimate connection between the weakening electron-phonon (e-ph) coupling and antiferromagnetism. Our finding points to the highly-correlated nature of Fe1.02Te ground state featured by strong interactions among the charge, spin and lattice and a good metallicity plausibly contributed by the coherent polaron motion.
Microwave-assisted chemical reduction routes for direct synthesis of Fe–Pt nanoparticles in ordered face centered tetragonal L10 phase
S. Acharya,K. Singh
Applied Nanoscience , 2011, DOI: 10.1007/s13204-011-0012-5
Abstract: In the present work, microwave-assisted chemical reduction route has been explored for the direct synthesis of face centered tetragonal (fct) L10 phase of Fe–Pt nanoparticles. Effects of microwave power and irradiation time on the growth process were investigated. Using this facile and high yield technique we could tune particle size from 7 to 17 nm. The as-prepared Fe–Pt NPs were observed in ordered fct L10 phase without any post-synthesis treatment. The particle size and magnetic properties of the as-prepared Fe–Pt were found to be very sensitive to the microwave irradiation power, while influence of exposure time was insignificant. The hysteresis measurements were performed at room temperature (300 K) to study magnetic properties of as-synthesized Fe–Pt as a function of crystallite size. All specimens of Fe–Pt were found to exhibit ferromagnetic behavior at room temperature. Coercivity and saturation magnetization were observed to be decreasing with diminishing particle size. The microwave-assisted route is found to be a simple technique for direct synthesis of metal alloys and may prove to be a potential tool of high density data storage materials such as Fe–Pt.
Investigation of the physical properties of the tetragonal CeMAl4Si2 (M = Rh, Ir, Pt) compounds  [PDF]
N. J. Ghimire,F. Ronning,D. J. Williams,B. L. Scott,Yongkang Luo,J. D. Thompson,E. D. Bauer
Physics , 2014, DOI: 10.1088/0953-8984/27/2/025601
Abstract: The synthesis, crystal structure, and physical properties studied by means of x-ray diffraction, magnetic, thermal and transport measurements of CeMAl$_{4}$Si$_{2}$ (M = Rh, Ir, Pt) are reported, along with the electronic structure calculations for LaMAl$_{4}$Si$_{2}$ (M = Rh, Ir, Pt). These materials adopt a tetragonal crystal structure (space group P4/mmm) comprised of BaAl$_4$ blocks, separated by MAl$_2$ units, stacked along the $c$-axis. Both CeRhAl$_{4}$Si$_{2}$ and CeIrAl$_{4}$Si$_{2}$ order antiferromagnetically below $T_{N1}$=14 and 16 K, respectively, and undergo a second antiferromagnetic transitition at lower temperature ($T_{N2}$=9 and 14 K, respectively). CePtAl$_{4}$Si$_{2}$ orders ferromagnetically below $T_C$ =3 K with an ordered moment of $\mu_{sat}$=0.8 $\mu_{B}$ for a magnetic field applied perpendicular to the $c$-axis. Electronic structure calculations reveal quasi-2D character of the Fermi surface.
Restoration of tetragonal $C_4$ symmetry coexistent with filamentary superconductivity in the pressure induced intermediate phase in the iron-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$  [PDF]
Yan Zheng,Pok Man Tam,Jianqiang Hou,Anna E. B?hmer,Thomas Wolf,Christoph Meingast,Rolf Lortz
Physics , 2015,
Abstract: The hole doped Fe-based superconductors Ba$_{1-x}$A$_x$Fe$_2$As$_2$ (where A=Na or K) show a particular rich phase diagram. It was observed that an intermediate re-entrant tetragonal phase forms within the orthorhombic antiferromagnetically-ordered stripe-type spin density wave state above the superconducting transition [S. Avci et al., Nature Comm. 5, 3845 (2014), A. E. B\"ohmer et al., arXiv:1412.7038v2]. A similar intermediate phase was reported to appear if pressure is applied to underdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ [E. Hassinger et al., Phys. Rev. B 86, 140502(R) (2012)]. Here we report data of the electric resistivity, Hall effect, specific heat, and the thermoelectrical Nernst and Seebeck coefficients measured on a Ba$_{0.85}$K$_{0.15}$Fe$_2$As$_2$ single crystal under pressure up to 5.5 GPa. The data reveals a coexistence of the intermediate phase with filamentary superconductivity. The Nernst coefficient shows a large signature of nematic order that coincides with the stripe-type spin density wave state up to optimal pressure. In the pressure-induced intermediate phase the nematic order is removed, thus confirming that its nature is a re-entrant tetragonal phase.
Observation of Josephson coupling through an interlayer of antiferromagnetically ordered chromium  [PDF]
M. Weides,M. Disch,H. Kohlstedt,D. E. Buergler
Physics , 2009, DOI: 10.1103/PhysRevB.80.064508
Abstract: The supercurrent transport in metallic Josephson tunnel junctions with an additional interlayer made up by chromium, being an itinerant antiferromagnet, was studied. Uniform Josephson coupling was observed as a function of the magnetic field. The supercurrent shows a weak dependence on the interlayer thickness for thin chromium layers and decays exponentially for thicker films. The diffusion constant and the coherence length in the antiferromagnet were estimated. The antiferromagnetic state of the barrier was indirectly verified using reference samples. Our results are compared to macroscopic and microscopic models.
Antiferromagnetically coupled alternating spin chains  [PDF]
Adolfo E. Trumper,Claudio Gazza
Physics , 2001, DOI: 10.1103/PhysRevB.64.134408
Abstract: The effect of antiferromagnetic interchain coupling in alternating spin (1,1/2) chains is studied by mean of a spin wave theory and density matrix renormalization group (DMRG). In particular, two limiting cases are investigated, the two-leg ladder and its two dimensional (2D) generalization. Results of the ground state properties like energy, spin gap, magnetizations, and correlation functions are reported for the whole range of the interchain coupling $J_{\perp}$. For the 2D case the spin wave results predict a smooth dimensional crossover from 1D to 2D keeping the ground state always ordered. For the ladder system, the DMRG results show that any $J_{\perp}>0$ drives the system to a gapped ground state. Furthermore the behaviour of the correlation functions closely resemble the uniform spin-1/2 ladder. For $J_{\perp}$ lower than 0.3, however, the gap behaves quadratically as $\Delta\sim0.6 J^2_{\perp}$. Finally, it is argued that the behaviour of the spin gap for an arbitrary number of mixed coupled spin chains is analogous to that of the uniform spin-1/2 chains.
Electron interaction with domain walls in antiferromagnetically coupled multilayers  [PDF]
F. G. Aliev,R. Schad,A. Volodin,K. Temst,C. Van Haesendonck,Y. Bruynseraede,I. Vavra,V. K. Dugaev,R. Villar
Physics , 2003, DOI: 10.1209/epl/i2003-00597-2
Abstract: For antiferromagnetically coupled Fe/Cr multilayers the low field contribution to the resistivity, which is caused by the domain walls, is strongly enhanced at low temperatures. The low temperature resistivity varies according to a power law with the exponent about 0.7 to 1. This behavior can not be explained assuming ballistic electron transport through the domain walls. It is necessary to invoke the suppression of anti-localization effects (positive quantum correction to conductivity) by the nonuniform gauge fields caused by the domain walls.
Magnetic properties of antiferromagnetically coupled CoFeB/Ru/CoFeB  [PDF]
N. Wiese,T. Dimopoulos,M. Ruhrig,J. Wecker,H. Bruckl,G. Reiss
Physics , 2005, DOI: 10.1016/j.jmmm.2004.11.539
Abstract: This work reports on the thermal stability of two amorphous CoFeB layers coupled antiferromagnetically via a thin Ru interlayer. The saturation field of the artificial ferrimagnet which is determined by the coupling, J, is almost independent on the annealing temperature up to more than 300 degree C. An annealing at more than 325 degree C significantly increases the coercivity, Hc, indicating the onset of crystallization.
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