We report comprehensive small angle neutron scattering measurements complemented by ac susceptibility data of the helical order, conical phase, and Skyrmion lattice phase (SLP) in MnSi under uniaxial pressures. For all crystallographic orientations uniaxial pressure favors the phase for which a spatial modulation of the magnetization is closest to the pressure axis. Uniaxial pressures as low as 1 kbar applied perpendicular to the magnetic field axis enhance the Skyrmion lattice phase substantially, whereas the Skyrmion lattice phase is suppressed for pressure parallel to the field. Taken together we present quantitative microscopic information on how strain couples to magnetic order in the chiral magnet MnSi.

Chiral magnets are promising materials for the realisation of high-density and low-power spintronic memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii-Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278 K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray scattering and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain α intr = 0.0036 ± 0.0003 at 310 K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics.

We report an experimental study of the emergence of non-trivial topological winding and longrange order across the paramagnetic to skyrmion lattice transition in the transition metal helimagnet MnSi. Combining measurements of the susceptibility with small angle neutron scattering, neutron resonance spin echo spectroscopy and all-electrical microwave spectroscopy, we find evidence of skyrmion textures in the paramagnetic state exceeding 10 3Å with lifetimes above several 10 −9 s. Our experimental findings establish that the paramagnetic to skyrmion lattice transition in MnSi is well-described by the Landau soft-mode mechanism of weak crystallization, originally proposed in the context of the liquid to crystal transition. As a key aspect of this theoretical model, the modulation-vectors of periodic small amplitude components of the magnetization form triangles that add to zero. In excellent agreement with our experimental findings, these triangles of the modulation-vectors entail the presence of the non-trivial topological winding of skyrmions already in the paramagnetic state of MnSi when approaching the skyrmion lattice transition. I. MOTIVATIONA pre-requisite for the definition of topological magnetic textures is the presence of a continuous magnetization field with a finite amplitude in space and time. An example par excellence of such textures are magnetic skyrmions, representing topologically nontrivial whirls of this magnetization field [1]. The notions of topological winding and topological stability of such skyrmions are only meaningful when the magnetization is sufficiently smooth on local scales. This condition may be readily satisfied in systems exhibiting long-range magnetic order for temperatures far below the transition temperature T c . In contrast, changes of the topological properties require that the magnetization is capable of vanishing on short length and time scales. The associated microscopic mechanisms underlying the transition of skyrmions into different types of conventional long-range magnetic order have been explored in a large number of theoretical and experimental studies [2][3][4][5][6][7][8].A major unresolved question concerns, in contrast, the formation of skyrmion lattice order when starting from a state that is essentially paramagnetic and dominated by an abundance of fluctuations such that the local magnetization, on a coarse-grained level, practically vanishes on average [9][10][11][12][13][14][15]. This alludes to the question whether topologically non-trivial characteristics exist already in * a paramagnetic state, and how they may be accounted for in the framework of the present-day classification of phase transitions [16][17][18]. It connects also with the relevance of non-trivial topological properties in the search for novel electronic properties of solids, e.g., at quantum phase transitions [19,20].Magnetic skyrmions are ideally suited to address this question. However, they are typically portrayed from either one of two seemingly contrary points of view. On the one hand...

We report neutron scattering and AC magnetic susceptibility measurements of the 2D spin-1/2 frustrated magnet BaCdVO(PO4)2. At temperatures well below T N ≈ 1K, we show that only 34% of the spin moment orders in an up-up-down-down strip structure. Dominant magnetic diffuse scattering and comparison to published µsr measurements indicates that the remaining 66% is fluctuating. This demonstrates the presence of strong frustration, associated with competing ferromagnetic and antiferromagnetic interactions, and points to a subtle ordering mechanism driven by magnon interactions. On applying magnetic field, we find that at T = 0.1 K the magnetic order vanishes at 3.8 T, whereas magnetic saturation is reached only above 4.5 T. We argue that the putative high-field phase is a realization of the long-sought bond-spin-nematic state.In the search for new states of matter, the realization of a spin-nematic state -a quantum version of a liquid crystal -has proved an enduring but elusive goal [1]. Of particular interest is the bond spin nematic (BSN), believed to exist in spin-1/2 materials with competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions [1,2]. This state is remarkable in that it combines the long-range entanglement characteristic of a quantum spin liquid [3][4][5][6], with a nematic order that breaks spin-rotational symmetry, while preserving both translational-and time-reversal symmetry [1,2]. As a consequence, the BSN state does not produce a static internal magnetic field, making it difficult to observe in experiment [5,[7][8][9]. Nonetheless, there is now a wellestablished scenario for BSN order ocurring through the condensation of bound pairs of magnons in high magnetic field [2, 10-21], cf. Fig. 1, and a number of promising candidate materials where this may occur [22][23][24][25][26].Of particular note is BaCdVO(PO 4 ) 2 , one of a family of square-lattice, spin-1/2 vanadates [27][28][29][30][31][32]. Early measurements of the heat capacity identified a phase transi-tion with T N ≈ 1 K, for fields H ≤ 3.5 T, while the magnetization was found to saturate for a field H > ∼ 4 T [31]. These results were interpreted in terms of a model with 1 st -neighbor exchange J 1 ≈ −3.6 K and 2 nd -neighbor exchange J 2 ≈ 3.2 K [31], for which the low-field ordered state would be a canted AFM with propagation vector q sq = (1/2, 0) [33][34][35]. Subsequent thermodynamic measurements have extended the magnetic phase diagram of BaCdVO(PO 4 ) 2 , filling in the gaps at high field, and identifying a new low-temperature phase bordering the saturated state [36], precisely where one might expect a BSN to occur [2,10,[14][15][16][18][19][20][21]. This is an exciting development, since the range of fields involved, 4 < ∼ H < ∼ 5 T [36], is much lower than in other candidate systems [22][23][24][25], making it accessible to a wider range of experimental techniques. Despite this progress, the nature of the low-field phase in BaCdVO(PO 4 ) 2 , the form of its magnetic interactions, and the possibility of a BSN in high f...

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