Critical scaling in the cubic helimagnetCu2OSeO3

Abstract

We present a detailed ac susceptibility investigation of the fluctuation regime in the insulating cubic helimagnet Cu 2 OSeO 3 . For magnetic fields μ 0 H 200 mT, and over a wide temperature (T ) range, the system behaves according to the scaling relations characteristic of the classical three-dimensional Heisenberg model. For lower magnetic fields, the scaling is preserved only at higher T and becomes renormalized in a narrow-T range above the transition temperature. Contrary to the well-studied case of MnSi, where the renormalization has been interpreted within the Brazovskii theory, our analysis of the renormalization at H = 0 shows the fluctuation regime in Universality is a concept that lies at the heart of modern physics since it describes the general scaling behavior of widespread physical phenomena within the vicinity of a critical point. In condensed matter physics, universal scaling laws are readily applied to interpret measurements of thermodynamic observables in order to discern the symmetry of the physical properties close to phase transitions. A classic example where these concepts have been extensively tested, both theoretically and experimentally, is the second-order paramagnetic (PM) to ferromagnetic (FM) transition [1]. In general, as the system approaches the critical point, both the size and the number of fluctuations of the relevant order parameter increase. It is also known that if the interactions between the fluctuations are strong enough, this may even alter the order of the phase transition. In a recent comprehensive study, it was proposed [2] that a specific type of renormalization put forward by Brazovskii [3] can be applied to describe the weakly first-order nature of the PM to helimagnetic (HM) transition at zero magnetic field in metallic MnSi [4,5]. In this scenario, the renormalization arises due to the crucial role played by the Dzyaloshinskii-Moriya (DM) interaction which alters the nature of fluctuations close to T HM , and so causes the system to avoid the second-order transition expected within mean field theory. Other studies of the unusual critical behavior in MnSi include recent polarized neutron scattering experiments [4], from which it was proposed that a "skyrmion-liquid" phase exists for a narrow-temperature range (T ∼ 1 K) above T HM . A similar claim was deduced from a Monte Carlo study where an analogy with blue phases in liquid crystals has been established [6].Recently, Cu 2 OSeO 3 was identified as a new compound to display a direct PM to HM transition in zero magnetic field. Cu 2 OSeO 3 crystallizes in the same space group as MnSi (P 213), and has two crystallographically inequivalent Cu sites with a dominant antiferromagnetic interaction between the nearest neighbors [7]. The ratio of Cu ions within the two inequivalent sites is 3:1, giving rise to the formation of the local ferrimagnetic (FiM) 3-up-1-down state [5] which * zivkovic@ifs.hr is then modulated by the DM interaction. Similar as for MnSi, by applying a weak magnetic field close to the ordering tem...

Formation of incommensurate long-range magnetic order in the Dzyaloshinskii-Moriya antiferromagnet Ba2CuGe2O7 studied by neutron diffraction

“…Such a change in curvature was experimentally found in MnSi [11] in addition to the latent heat associated with the weak first-order transition [12,13]. Subsequently, it was demonstrated that other cubic helimagnets with the same space group like Cu 2 OSeO 3 [14] and FeCo 1−x Si x [15] show similar behavior.…”

Magnetic Fluctuations, Precursor Phenomena, and Phase Transition in MnSi under a Magnetic Field

“…This peculiar scattering occurs in a region dominated by precursor phenomena that have been probed with a wide range of experimental techniques [9][10][11][14][15][16]. Similar precursor phenomena have also been found in FeGe [17,18] and the multiferroic Cu 2 OSeO 3 [19][20][21] and are thus of general relevance to chiral magnetism.…”

Dissipation processes in the insulating skyrmion compoundCu2OSeO3

“…At still larger fields the moments become disordered but fieldpolarized. The dashed line extending diagonally into the paramagnetic regime represents a cross-over from such a field-polarized region and a fully disordered paramagnetic state at high temperatures which has been recently shown to follow a classical 3D Heisenberg scaling 14 . The SkL pocket is located within the conical phase, spanning 2 − 3 K just below the order-disorder line between points P 2 and P 3 .…”