X-Ray Microdiffraction Allows Direct Study of Antiferromagnetic Domain Evolution in Chromium
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face of a light-emitting diode to form a pin-printed optical-sensor array and integrated light source (PPOSAILS). Work on true distributed sensor arrays and biosensor arrays is now under investigation in their laboratory. YUE HU
Organic-Based Magnet Mn(TCNE)x · y(CH2Cl2) Shows Photoinduced Magnetization D.A. Pejakovic and colleagues at the Ohio State University and the University of Utah have reported reversible photoinduced magnetization (PIM) in an organicbased magnet, Mn(tetracyanoethylene)x · y(CH2Cl2) (x ~ 2, y ~ 0.8). At low temperatures, the PIM lasts for several days after illumination. As described in the February 4 issue of Physical Review Letters, for dc magnetization (M) measurements, the polycrystalline material was dispersed in mineral oil and sealed in a quartz capsule. At 5 K, illumination with 2.54-eV argon laser light in a 10-Oe magnetic field increased M. After illumination, M initially increased due to sample cooling and then decreased by about 0.5% in 60 h, suggesting a PIM lifetime of >106 s. The ac susceptibility (χ ac) of about 0.05 mg of the material applied in a thin layer on adhesive tape increased substantially below Tc = 75 K after illumination with 2.7-eV argon laser light for 60 h at 90 K. In the range of 13–18 K, χ′ increased more than 50%, and χ′′ increased by more than a factor of 4. The researchers suggest that the increase originates in altered spin organization and/or spin-spin coupling. After heating to 200 K and cooling to 5 K, about 5% of the increase in χ′ remained. However, heating to above 250 K returned the material to the original state. After illumination with 2.54-eV light for 10 min, the UV–vis–NIR absorption spectrum shows photoinduced absorption in the 1.5–2.4-eV and 3.1–3.8-eV regions. After illumination, 2.41-eV line excitation can reduce the effects. The researchers suggest that PIM results from a photoinduced electronic transition into a metastable state that enhances charge transfer. The change in the IR absorption spectrum was also measured after 10 min of illumination at 2.54 eV. Again, the photoinduced effect is partially reduced after illumination with the 2.41-eV line. The changes in the IR spectrum suggest the presence of a lattice distortion occurring when a fraction of electrons relax from the π* level. The researchers speculate that the lattice distortion plays a key role in stabilizing the state with enhanced magnetization. ELIZABETH SHACK 288
X-Ray Microdiffraction Allows Direct Study of Antiferromagnetic Domain Evolution in Chromium The evolution of antiferromagnetic domains in chromium during a “spin-flip” transition has been observed by x-ray microdiffraction imaging. A team of researchers from Lucent Technologies, NEC Research Institute, and Argonne National Laboratory report in the February 8 issue of Science that the transition begins at the walls between domains with orthogonal modulation vectors and progresses inward during cooling. In contrast to ferromagnets, in which the spin polarization alone characterizes the magnetic domains, the des
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