77 research outputs found
Tuning the magnetic and structural phase transitions of PrFeAsO via Fe/Ru spin dilution
Neutron diffraction and muon spin relaxation measurements are used to obtain
a detailed phase diagram of Pr(Fe,Ru)AsO. The isoelectronic substitution of Ru
for Fe acts effectively as spin dilution, suppressing both the structural and
magnetic phase transitions. The temperature of the tetragonal-orthorhombic
structural phase transition decreases gradually as a function of x. Slightly
below the transition temperature coherent precessions of the muon spin are
observed corresponding to static magnetism, possibly reflecting a significant
magneto-elastic coupling in the FeAs layers. Short range order in both the Fe
and Pr moments persists for higher levels of x. The static magnetic moments
disappear at a concentration coincident with that expected for percolation of
the J1-J2 square lattice model
Correlation between polyhedral distortions and phase transitions in spinel FeMnO
Spinel compounds ABX consist of both tetrahedral (AX) and
octahedral (BX) environments with the former forming a diamond lattice
and the latter a geometrically frustrated pyrochlore lattice. Exploring the
fascinating properties and their correlations with structural features is
critical in understanding these materials. FeMnO has been reported
to exhibit one structural transition and two successive magnetic transitions.
Here, we report the polyhedral distortions and their correlations to the
structural and two magnetic transitions in FeMnO by employing the
high-resolution neutron powder diffraction. While a large trigonal distortion
is found even in the high-temperature cubic phase, the first-order
cubic-tetragonal structural transition associated with the elongation of both
tetrahedra and octahedra along the axis occurs at 750 K,
driven by the Jahn-Teller effect of the orbital active B-site Mn cation.
A strong magnetoelastic coupling is unveiled at K as
manifested by the appearance of N\`{e}el-type collinear ferrimagnetic order, an
anomaly in both tetrahedral and octahedral distortions, as well as an anomalous
decrease of the lattice constant and a weak anomaly of . Upon cooling
below K, it evolves to a noncollinear ferrimagnetic order
with a canting of half B-site / spins in the pyrochlore
lattice, which is a unique magnetic order among spinels. Such a noncollinear
order induces modifications of the O-B-O bond angles in the octahedra without
affecting much the bond lengths of the tetrahedra/octahedra. Our study
indicates that FeMnO is a wonderful platform to unveil interesting
magnetic order and to investigate their correlations to polyhedral distortions
and lattice.Comment: 28 pages, 10 figures, submitted for publicatio
Superconductivity in Co-doped LaFeAsO
Here we report the synthesis and basic characterization of LaFe1-xCoxAsO for
several values of x. The parent phase LaFeAsO orders antiferromagnetically (TN
~ 145 K). Replacing Fe with Co is expected to both electron dope the system and
introduce disorder in the FeAs layer. For x = 0.05 antiferromagnetic order is
destroyed and superconductivity is observed at Tconset = 11.2 K. For x = 0.11
superconductivity is observed at Tc(onset) = 14.3 K, and for x = 0.15 Tc = 6.0
K. Superconductivity is not observed for x = 0.2 and 0.5, but for x = 1, the
material appears to be ferromagnetic (Tc ~ 56 K) as judged by magnetization
measurements. We conclude that Co is an effective dopant to induce
superconductivity. Somewhat surprisingly, the system appears to tolerate
considerable disorder in the FeAs planes.Comment: 19 pages, 9 figure
Noncollinear spin structure in Fe3+xCo3−xTi2 (x = 0, 2, 3) from neutron diffraction
Neutron powder diffraction has been used to investigate the spin structure of the hard-magnetic alloy Fe3+xCo3−xTi2 (x = 0, 2, 3). The materials are produced by rapid quenching from the melt, they possess a hexagonal crystal structure, and they are nanocrystalline with crystallite sizes D of the order of 40 nm. Projections of the magnetic moment onto both the crystalline c axis and the basal plane were observed. The corresponding misalignment angle exhibits a nonlinear decrease with x, which we explain as a micromagnetic effect caused by Fe-Co site disorder. The underlying physics is a special kind of random-anisotropy magnetism that leads to the prediction of 1/D1/4 power-law dependence of the misalignment angle on the crystallite size
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