174 research outputs found
Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet SrFeO
We report a neutron scattering study of the magnetic order and dynamics of
the bilayer perovskite SrFeO, which exhibits a temperature-driven
metal-insulator transition at 340 K. We show that the Fe moments adopt
incommensurate spiral order below K and provide a
comprehensive description of the corresponding spin wave excitations. The
observed magnetic order and excitation spectra can be well understood in terms
of an effective spin Hamiltonian with interactions ranging up to third
nearest-neighbor pairs. The results indicate that the helical magnetism in
SrFeO results from competition between ferromagnetic
double-exchange and antiferromagnetic superexchange interactions whose
strengths become comparable near the metal-insulator transition. They thus
confirm a decades-old theoretical prediction and provide a firm experimental
basis for models of magnetic correlations in strongly correlated metals.Comment: PRL, in pres
Strong coupling between magnetic and structural order parameters in SrFe2As2
X-ray and Neutron diffraction as well as muon spin relaxation and M\"ossbauer
experiments performed on SrFeAs polycrystalls confirm a sharp first
order transition at ,K corresponding to an orthorhombic phase
distortion and to a columnar antiferromagnetic Fe ordering with a propagation
vector (1,0,1), and a larger distortion and larger size of the ordered moment
than reported for BaFeAs. The structural and the magnetic order
parameters present an remarkable similarity in their temperature dependence
from down to low temperatures, showing that both phenomena are intimately
connected. Accordingly, the size of the ordered Fe moments scale with the
lattice distortion when going from SrFeAs to BaFeAs.
Full-potential band structure calculations confirm that the columnar magnetic
order and the orthorhombic lattice distortion are intrinsically tied to each
other.Comment: 10 pages, 4 figure
High Magnetic Field Behaviour of the Triangular Lattice Antiferromagnet, CuFeO_2
The high magnetic field behaviour of the triangular lattice antiferromagnet
CuFeO_2 is studied using single crystal neutron diffraction measurements in a
field of up to 14.5 T and also by magnetisation measurements in a field of up
to 12 T. At low temperature, two well-defined first order magnetic phase
transitions are found in this range of applied magnetic field (H // c): at
H_c1=7.6(3)/7.1(3) T and H_c2=13.2(1)/12.7(1) T when ramping the field up/down.
In a field above H_c2 the magnetic Bragg peaks show unusual history dependence.
In zero field T_N1=14.2(1) K separates a high temperature paramagnetic and an
intermediate incommensurate structure, while T_N2=11.1(3) K divides an
incommensurate phase from the low-temperature 4-sublattice ground state. The
ordering temperature T_N1 is found to be almost field independent, while T_N2
decreases noticeably in applied field. The magnetic phase diagram is discussed
in terms of the interactions between an applied magnetic field and the highly
frustrated magnetic structure of CuFeO_2Comment: 7 pages, 8 figures in ReVTeX. To appear in PR
Lattice Instability and Competing Spin Structures in the Double Perovskite Insulator Sr2FeOsO6
The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of
spin structures that differ in the spin sequence of ferrimagnetic iron - osmium
layers along the tetragonal c-axis. Neutron powder diffraction experiments,
57Fe M\"ossbauer spectra, and density-functional theory calculations suggest
that this behavior arises because a lattice instability resulting in
alternating iron-osmium distances fine-tunes the balance of competing exchange
interactions. Thus, Sr2FeOsO6 is an example for a double perovskite, in which
the electronic phases are controlled by the interplay of spin, orbital, and
lattice degrees of freedom.Comment: 8 Pages, 3 Figure
Magnetization reversal driven by electron localization-delocalization crossover in the inverse spinel Co2VO4
Oblique triangular antiferromagnetic phase in CsCuCoCl
The spin-1/2 stacked triangular antiferromagnet CsCuCoCl with
undergoes two phase transitions at zero field. The
low-temperature phase is produced by the small amount of Co doping. In
order to investigate the magnetic structures of the two ordered phases, the
neutron elastic scattering experiments have been carried out for the sample
with . It is found that the intermediate phase is identical to
the ordered phase of CsCuCl, and that the low-temperature phase is an
oblique triangular antiferromagnetic phase in which the spins form a triangular
structure in a plane tilted from the basal plane. The tilting angle which is
42 at K decreases with increasing temperature, and becomes
zero at K. An off-diagonal exchange term is proposed as the
origin of the oblique phase.Comment: 6 pages, 7 figure
Local structure determination in helimagnetic Co8Zn8Mn4 xFex
We have carried out a structural and magnetic characterization of Co8Zn8+y Mn4−y−x Fex by means of
bulk techniques, powder neutron scattering and Mössbauer spectroscopy. From our data, we derive
the main magnetic parameters and carry out a detailed structural characterization.Weobserve that
disorder is present in our samples in various forms and that this seems to be generic for this class of
materials. However, the disorder appears to have little impact on the character of the magnetic phases,
and thus seems to be of no relevance for the skyrmionic phases established for these types of materials
Characterization of a new small cell lung cancer (SCLC) cell line STP54 derived from a metastatic bioptate of a combined type of SCLC with Non-SCLC component.
EBAG9 controls CD8(+) T cell memory formation responding to tumor challenge in mice
Insight into processes that determine CD8(+) T cell memory formation has been obtained from infection models. These models are biased toward an inflammatory milieu and often employ high avidity CD8(+) T cells in adoptive transfer procedures. It is unclear whether these conditions mimic the differentiation processes of an endogenous repertoire that proceed upon non-inflammatory conditions prevailing in premalignant tumor lesions. We examined the role of cytolytic capacity on CD8(+) T cell fate decisions when primed by tumor cells or by minor histocompatibility antigen-mismatched leukocytes. CD8(+) memory commitment was analyzed in Ebag9-deficient mice that exhibit an enhanced tumor cell lysis. This property endowed Ebag9(-/-) mice with extended control of Tcl-1 oncogene-induced chronic lymphocytic leukemia progression. In Ebag9(-/-) mice, an expanded memory population was obtained for anti-HY and anti-SV40 T antigen-specific T cells, despite unchanged effector frequencies in the primary response. By comparing the single-cell transcriptomes of CD8(+) T cells responding to tumor cell vaccination, we found differential distribution of subpopulations between Ebag9(+/+) and Ebag9(-/-) T cells. In Ebag9(-/-) cells, these larger clusters contained genes encoding transcription factors regulating memory cell differentiation, along with anti-apoptotic gene functions. Our findings link EBAG9-controlled cytolytic activity and the commitment to the CD8(+) memory lineage
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