We develop a theory of the magnetic field-induced formation of Skyrmion crystal state in chiral magnets in two spatial dimensions, motivated by the recent discovery of the Skyrmionic phase of magnetization in thin film of Fe0.5Co0.5Si and in the A-phase of MnSi. Ginzburg-Landau functional of the chiral magnet re-written in the CP 1 representation is shown to be a convenient framework for the analysis of the Skyrmion states. Phase diagram of the model at zero temperature gives a sequence of ground states, helical spin → Skyrme crystal → ferromagnet, as the external field B increases, in good accord with the thin-film experiment. In close analogy with Abrikosov's derivation of the vortex lattice solution in type-II superconductor, the CP 1 mean-field equation is solved and shown to reproduce the Skyrmion crystal state.PACS numbers:

We investigated the physical properties of antiperovskite compound SnCFe3 by comprehensive magnetic measurements. The strong irreversibility is clearly observed from zero-field-cooled and field-cooled magnetizations. The peaks of ac susceptibility display strong dependences on the frequency and magnetic field. Both the magnetic relaxation effects and related analysis indicate a typical spin-glass (SG) behavior in SnCFe3. The corresponding characteristic parameters are obtained: the freezing temperature T0=20.3 K, the dynamical exponent zν=9.441, and the flipping time τ0=2.42×10−11 s. Furthermore, the Sn deficiency affects significantly the SG behavior and results in a sharp decrease in T0.

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We report an enhanced negative giant magnetoresistance (GMR) with larger temperature span in Ni-doped antipervoskite compounds GaCMn3−xNix. The observed GMR can peak at ∼75% (at 85 kOe) and exceed 60% (at 50 kOe) over a temperature span of approximate 110 and 50K for x=0.05 and 0.10, respectively. Compared with the parent GaCMn3, the well-enhanced GMR in Ni-doped samples is suggested to be associated with the partially suppressed antiferromagnetic (AFM) ground state, which favors the transition from the high-resistivity AFM state to the low-resistivity canted ferromagnetic state under an external magnetic field.

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