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Detailed investigation of Ca$_{2}$CoSi$_{2}$O$_{7}$ was performed in its low-temperature magnetoelectric state combining neutron diffraction with magnetization measurements on single crystals. The crystal and magnetic structures well below the antiferromagnetic transition temperature of TN≈5.7 K were determined using neutron diffraction. Neutron diffraction data imply no structural phase transition from 10 K down to 2.5 K and are well described within the orthorhombic space group P21212 with a 3×3×1 supercell compared with the high-temperature unmodulated state (tetragonal space group P¯421m). We found that in zero magnetic field the magnetic space group is P212′12′ with antiferromagnetic order along the [100] or [010] axes for two types of 90∘ twin domains, while neighboring spins along the [001] axis are ordered ferromagnetically. A noncollinear spin arrangement due to small canting within the ab plane is allowed by symmetry and leads to the existence of the tiny spontaneous magnetization below TN. The ordered moment with a magnitude of about 2.8 μB/Co$_{2}$+ at 2.5 K lies in the ab plane. Distinct differences between the magnetic structure of Ca$_{2}$CoSi$_{2}$O$_{7}$ as compared to those of Ba$_{2}$CoGe$_{2}$O$_{7}$ and Sr$_{2}$CoSi$_{2}$O$_{7}$ are discussed.

We have shown recently that both the magnetization and the magnetoresistance of electrodeposited Co-Cu Cu multilayers can be decomposed by assuming the presence of both ferromagnetic (FM) and superparamagnetic (SPM) regions in the magnetic layers. In the present work, for two selected samples, one with a large SPM and another one with a large FM contribution to the giant magnetoresistance, low temperature magnetic and magnetoresistance measurements were performed in order to reveal the evolution of the FM and SPM terms with temperature. The average apparent magnetic moment of the SPM regions deduced from the two sets of data showed a good agreement. The role of electrochemical processes in the formation of the SPM regions is discussed. An attempt has also been made to elaborate on some models for the spatial distribution of the constituent elements (Co and Cu) leading to the occurrence of SPM regions. The results are discussed also in the framework of interacting SPM regions.

Nonlinear transport in the low temperature vortex glass state of single crystal Bi_{2}Sr_{2}CaCu_{2}O_{8} has been investigated over long times (up to two weeks) with fast current pulses driven along the ab plane. It is found that at low temperature and high magnetic field both zero field cooled (ZFC) and field cooled (FC) samples relax towards the same equilibrium state, which is much closer to the original ZFC state than to the FC state. The implication of this FC metastability for the vortex phase diagram is discussed in the context of previous measurements., latex ageg.tex, 5 files, 4 figures, 5 pages [SPEC-S01/041], submitted to Phys. Rev. Lett. Text changes