We present a detailed study of the phase diagram surrounding the skyrmion lattice (SkL) phase of Cu 2 OSe 2 O 3 using high-precision magnetic ac susceptibility measurements. An extensive investigation of transition dynamics around the SkL phase using the imaginary component of the susceptibility revealed that at the conical-to-SkL transition a broad dissipation region exists with a complex frequency dependence. The analysis of the observed behavior within the SkL phase indicates a distribution of relaxation times intrinsically related to SkL. At the SkL-to-paramagnet transition a narrow first-order peak is found that exhibits a strong frequency and magnetic field dependence. Surprisingly, very similar dependence has been discovered for the first-order transition below the SkL phase, i.e. where the system enters the helical and conical state(s), indicating similar processes across the order-disorder transition.

The recent discovery of magnetic skyrmion lattices initiated a surge of interest in the scientific community. Several novel phenomena have been shown to emerge from the interaction of conducting electrons with the skyrmion lattice, such as a topological Hall-effect and a spin-transfer torque at ultra-low current densities. In the insulating compound Cu2OSeO3, magneto-electric coupling enables control of the skyrmion lattice via electric fields, promising a dissipation-less route towards novel spintronic devices. One of the outstanding fundamental issues is related to the thermodynamic stability of the skyrmion lattice. To date, the skyrmion lattice in bulk materials has been found only in a narrow temperature region just below the order-disorder transition. If this narrow stability is unavoidable, it would severely limit applications. Here we present the discovery that applying just moderate pressure on Cu2OSeO3 substantially increases the absolute size of the skyrmion pocket. This insight demonstrates directly that tuning the electronic structure can lead to a significant enhancement of the skyrmion lattice stability. We interpret the discovery by extending the previously employed Ginzburg-Landau approach and conclude that change in the anisotropy is the main driver for control of the size of the skyrmion pocket.

We present an experimental investigation of the magnetic structure in a tetramer system SeCuO3 using neutron diffraction and nuclear resonance techniques. We establish a non-collinear, commensurate antiferromagnetic ordering with a propagation vector k = (0, 0, 1). The order parameter follows a critical behavior near TN = 8 K, with a critical exponent β = 0.32 in agreement with a 3D universality class. Evidence is presented that a singlet state starts to form on tetramers at temperatures as high as 200 K, and its signature is preserved within the ordered state through a strong renormalization of the ordered magnetic moment on two non-equivalent copper sites, mCu1 ≈ 0.4 µB and mCu2 ≈ 0.7 µB at 1.5 K. S1 S2 S3 S4 J 11 J 12 J 12 Cu2 Cu1 FIG. 1. (Color online) A single Cu2 -Cu1 -Cu1 -Cu2 tetramer, with shaded CuO4 plaquettes. Below a schematic drawing of spins and exchange interactions between them (see Eq. 1).