38 research outputs found
Disorder induced phase segregation in La2/3Ca1/3MnO3 manganites
Neutron powder diffraction experiments on La2/3Ca1/3MnO3 over a broad
temperature range above and below the metal-insulator transition have been
analyzed beyond the Rietveld average approach by use of Reverse Monte Carlo
modelling. This approach allows the calculation of atomic pair distribution
functions and spin correlation functions constrained to describe the observed
Bragg and diffuse nuclear and magnetic scattering. The results evidence phase
separation within a paramagnetic matrix into ferro and antiferromagnetic
domains correlated to anistropic lattice distortions in the vicinity of the
metal-insulator transition.Comment: 3 pages, 4 figures. Submitted to Phys. Rev. Lett. Figure 1 replace
Exploration of Antiferromagnetic CoO and NiO using Reverse Monte Carlo Total Neutron Scattering Refinement
The atomic and magnetic structures of CoO and NiO have been probed using reverse Monte Carlo refinements of neutron total scattering data. The results obtained show that the known magnetic structure for NiO can be recovered by the reverse Monte Carlo process starting from random spin configurations, but it is insensitive to the spin direction in the {111} ferromagnetic planes. Refinements of the magnetic structure of CoO starting from random spin configurations result in collinear or non-collinear magnetic structure, consistent with those reported by other techniques. Starting from an ordered collinear spin structure for CoO and NiO leads to different results than when starting from a random arrangement of spins, evidence for configurational bias that highlights the need to take care when selecting a starting model for reverse Monte Carlo refinements of magnetic structures
On the origin of neutron magnetic scattering in anti-site disordered Sr2FeMoO6 double perovskites
Anti-site disordering in Sr2FeMoO6 double perovskites (containing Mo atoms at
Fe positions, and viceversa) has recently been shown to have a dramatic
influence in their magnetic and magnetotransport properties. In the present
study, two polycrystalline Sr2FeMoO6 samples showing different degrees of
anti-site disorder (a nominally 'ordered' sample with 70% of cationic ordering
and a nominally 'disordered' sample with 18% of cationic ordering) have been
examined by magnetic measurements and neutron powder diffraction (NPD)
techniques in the 15-500K temperature range. Our main finding is that the
'disordered' sample exhibits a strong magnetic scattering (noticeable even at
500K), comparable to that displayed by the 'ordered' one below TC= 415 K. For
the 'disordered' sample, the magnetic scattering exhibited on low angle Bragg
positions, is not to be ascribed to a (non-existent) ferrimagnetic ordering:
our results suggest that it originates upon naturally-occurring groups of Fe
cations in which strong antiferromagnetic (AFM) Fe-O-Fe superexchange
interactions are promoted, similar to those existing in the LaFeO3 perovskite.
These Fe groups are not magnetically isolated, but coupled by virtue of Fe-O-Mo
AFM interactions, which maintain the long-range coherence of this AFM
structure. Susceptibility measurements confirm the presence of AFM interactions
below 770 K.Comment: 30 pages, 11 figures, to be published in PR
Macrophage-Specific Chemokines Induced via Innate Immunity by Amino Acid Copolymers and Their Role in EAE
The random amino acid copolymer poly(Y,E,A,K)n (Copaxone®) is widely used in multiple sclerosis treatment and a second generation copolymer poly(Y,F,A,K)n with enhanced efficacy in experimental autoimmune encephalomyelitis in mice has been described. A major mechanism through which copolymers function to ameliorate disease is the generation of immunosuppressive IL-10-secreting regulatory T cells entering the CNS. In addition, the antigen presenting cell to which these copolymers bind through MHC Class II proteins may have an important role. Here, both CCL22 (a Th2 cell chemoattractant) in large amounts and CXCL13 in much smaller amounts are shown to be secreted after administration of YFAK to mice and to a smaller extent by YEAK parallel to their serum concentrations. Moreover, bone marrow-derived macrophages secrete CCL22 in vitro in response to YFAK and to higher concentrations of YEAK. Strikingly, these chemokines are also secreted into serum of MHC Class II −/− mice, indicating that an innate immune receptor on these cells also has an important role. Thus, both the innate and the adaptive immune systems are involved in the mechanism of EAE amelioration by YFAK. The enhanced ability of YFAK to stimulate the innate immune system may account for its enhanced efficacy in EAE treatment
Correspondence address:
Natalizumab treatment in multiple sclerosis: marked decline of chemokines and cytokines in cerebrospinal flui
Magnetic short-range order in the new ternary phase Mn8Pd15Si7
A new compound, Mn8Pd15Si7, is reported to crystallize in a face centered cubic unit cell of dimension a = 12.0141(2) Ã…, space groupFm3Ì„m, and can thus be classified as a G-phase. The crystal structure was studied by single crystal X-ray diffraction, X
Effectiveness of first generation disease-modifying therapy to prevent conversion to secondary progressive multiple sclerosis
Background: The use of disease-modifying therapies (DMTs) in multiple sclerosis (MS) has been associated with reduced relapse rates and accumulation of disability. However, studies examining impact of DMT on risk of transition to secondary progressive MS (SPMS) leveraging population-based nationwide data are still rare. Here, we determine the population incidence of conversion to SPMS using two consecutive nation-wide cohorts, one immediately before and one after the introduction of DMT in Sweden. Methods: We included two consecutive population cohorts of relapsing-remitting MS (RRMS) from the Swedish national MS register for the periods 1975–1994 (n = 2161), before DMT availability, and 1995–2011 (n = 3510), in which DMTs, mainly first generation DMT (injectables), became available and eventually were used by 70% of patients. We explored the risk of transition to SPMS as a calendar year function encompassing the two cohorts. In addition, we determined the incidence of transition to SPMS through age strata below and above 50 years in untreated and treated patient subgroups. Results: The risk of conversion to SPMS (adjusted for current age, current time since onset, calendar year and sex) was significantly lower in the second compared with the first population cohort (hazard ratio 0.58; CI 0.48, 0.70). The risk of SPMS conversion per calendar year decreased by 2.6% annually (p < 0.001) after 1995. The risk of SPMS conversion increased with age until age 50. Thereafter, it was unchanged or decreased among those with early MS onset age (<35 years), but continued to increase with onset at higher age, with similar trends in treated and untreated subgroups. Conclusion: The incidence of SPMS conversion significantly decreased at the population level after introduction of first generation DMTs by 1995. DMT efficiency was confirmed by a downward turn of the annual trajectory of the risk of SPMS conversion after 1995. An onset age determined pattern of variable SPMS incidence in higher age appeared in both treated and untreated strata. While first generation DMT delayed conversion to SPMS, their long-term effect was only moderate