37 research outputs found
Low temperature heat capacity of Fe1−xGax alloys with large magnetostriction
The low temperature heat capacity Cp of Fe1−xGax alloys with large magnetostriction has been investigated. The data were analyzed in the standard way using electron (γT) and phonon (βT3) contributions. The Debye temperature ΘD decreases approximately linearly with increasing Ga concentration, consistent with previous resonant ultrasound measurements and measured phonon dispersion curves. Calculations of ΘD from lattice dynamical models and from measured elastic constants C11,C12, and C44 are in agreement with the measured data. The linear coefficient of electronic specific heat γ remains relatively constant as the Ga concentration increases, despite the fact that the magnetoelastic coupling increases. Band structure calculations show that this is due to the compensation of majority and minority spin states at the Fermi level
Effect of carbon addition on the single crystalline magnetostriction of Fe-X (X = Al and Ga) alloys
The effect of carbon addition on the magnetostriction of Fe–Ga and Fe–Al alloys was investigated and is summarized in this study. It was found that the addition of carbon generally increased the magnetostriction over binary alloys of Fe–Ga and Fe–Al systems. The formation of carbide in the Fe–Ga–C alloys with a composition near D03 phase region decreased the magnetostriction drastically. Fe–Al–C and Fe–Ga–C alloys responded differently to thermal treatments; the magnetostriction in the quenched Fe–Al–C alloys is equal to or slightly lower than that of the slow cooled as is observed in binary Fe–Al alloy; in contrast, the magnetostriction is generally higher in quenched Fe–Ga–C alloys than slow cooled condition, consistent with the behavior of binary alloys of Fe–Ga. A significant increase in magnetostriction between 25% and 165% depending on the phase region in Fe–Ga–C alloys by quenching was observed in the A2+D03 two-phase region and D03 single phase region
Interplay between Fe and Nd magnetism in NdFeAsO single crystals
The structural and magnetic phase transitions have been studied on NdFeAsO
single crystals by neutron and x-ray diffraction complemented by resistivity
and specific heat measurements. Two low-temperature phase transitions have been
observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142
K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe
moments order AFM in the well-known stripe-like structure in the (ab) plane,
but change from AFM to ferromagnetic (FM) arrangement along the c direction
below T* = 15 K accompanied by the onset of Nd AFM order below T_Nd = 6 K with
this same AFM configuration. The iron magnetic order-order transition in
NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis
exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and
PrFeAsO.Comment: revised; 4 pages, 3 figures; accepted for publication in Phys. Rev.
Two-dimensional magnetic interactions in LaFeAsO
Inelastic neutron scattering measurements demonstrate that the magnetic interactions in antiferromagnetic LaFeAsO are two dimensional. Spin-wave velocities within the Fe layer and the magnitude of the spin gap are similar to the AFe2As2 based materials. However, the ratio of interlayer and intralayer exchange is found to be less than ∼10−4 in LaFeAsO, very similar to the cuprates, and ∼100 times smaller than that found in AFe2As2 compounds. The results suggest that the effective dimensionality of the magnetic system is highly variable in the parent compounds of the iron arsenides and weak three-dimensional interactions may limit the maximum attainable superconducting Tc
Controlling Magnetic Order, Magnetic Anisotropy, and Band Topology in Semimetals and
Neutron diffraction and magnetic susceptibility studies show that
orthorhombic single-crystals of topological semimetals and undergo three
dimensional C-type antiferromagnetic (AFM) ordering of the Mn moments at
and K, respectively, significantly lower than that
of the parent SrMnSb with K. Magnetization versus applied
magnetic field (perpendicular to MnSb planes) below exhibits slightly
modified de Haas van Alphen oscillations for the Zn-doped crystal as compared
to that of the parent compound. By contrast, the Cu-doped system does not show
de Haas van Alphen magnetic oscillations, suggesting that either Cu
substitution for Mn changes the electronic structure of the parent compound
substantially, or that the Cu sites are strong scatterers of carriers that
significantly shorten their mean free path thus diminishing the oscillations.
Density functional theory (DFT) calculations including spin-orbit coupling
predict the C-type AFM state for the parent, Cu-, and Zn-doped systems and
identify the -axis (i.e., perpendicular to the Mn layer) as the easy
magnetization direction in the parent and 12.5% of Cu or Zn substitutions. In
contrast, 25% of Cu content changes the easy magnetization to the -axis
(i.e., within the Mn layer). We find that the incorporation of Cu and Zn in
SrMnSb tunes electronic bands near the Fermi level resulting in different
band topology and semi-metallicity. The parent and Zn-doped systems have
coexistence of electron and hole pockets with opened Dirac cone around the
Y-point whereas the Cu-doped system has dominant hole pockets around the Fermi
level with a distorted Dirac cone. The tunable electronic structure may point
out possibilities of rationalizing the experimentally observed de Haas van
Alphen magnetic oscillations
Magnetic structure of Dy3+ in hexagonal multiferroic DyMnO3
Element specific x-ray resonant magnetic scattering (XRMS) investigations were undertaken to determine the magnetic structure of the multiferroic compound, hexagonal DyMnO3. In the temperature range from 68 K down to 8 K the Dy3+ moments are aligned and antiferromagnetically correlated in the c direction according to the magnetic representation Γ3. The temperature dependence of the observed intensity can be modeled assuming the splitting of ground-state doublet crystal-field levels of Dy3+ by the exchange field of Mn3+. XRMS together with magnetization measurements indicate that the magnetic representation is Γ2 below 8 K