Spin phases of the helimagnetic insulator Cu2OSeO3 probed by magnon heat conduction

Abstract

We report studies of thermal conductivity as functions of magnetic field and temperature in the helimagnetic insulator Cu2OSeO3 that reveal novel features of the spin-phase transitions as probed by magnon heat conduction. The tilted conical spiral and low-temperature skyrmion phases, recently identified in small-angle neutron scattering studies, are clearly identified by sharp signatures in the magnon thermal conductivity. Magnon scattering associated with the presence of domain boundaries in the tilted conical phase and regions of skyrmion and conical-phase coexistence are identified.The cubic chiral magnets (MnSi, FeGe, Cu 2 OSeO 3 ) have attracted considerable attention for their complex variety of non-collinear spin phases that include spin modulations with long periods (many lattice spacings) and topological skyrmion phases. Their similar and rich magnetic phase diagrams are dictated by their common noncentrosymmetric cubic lattice symmetry and a hierarchy of competing energy scales (e.g. exchange, Dzyaloshinsky-Moriya, magnetocrystalline anisotropy, Zeeman). Recently two new spin phases, low-temperature skyrmion and "titled conical spiral," were identified in the insulating compound Cu 2 OSeO 3 by small-angle neutron scattering (SANS) [1,2]. These novel phases, arising at low temperature and relatively high magnetic field, reflect competing Zeeman and anisotropy energies that lead to surprising spin textures and a re-orientation of the long-period spin modulation direction.Here we report that field dependent thermal conductivity measurements are a particularly sensitive probe of the spin phase transitions in Cu 2 OSeO 3 because of the compound's unprecedentedly large magnon heat conductivity [3]. Rather little is known experimentally about magnons in ferro-or ferri-magnets from heat transport, their scattering or its dependency on spin textures, whether longrange ordered or not. Such information has increased in its importance and relevance with the surging interest in spintronic and magnonic device applications [4,5]. Cu 2 OSeO 3 is an ideal material for investigating these characteristics because the spin and lattice systems are weakly coupled as evidenced by very low spin-lattice damping [6,7] and by mean-free paths for both magnons and phonons as large as 0.3 mm below 2K [3]. These conditions ensure that energy (e.g. from a heater in thermal conductivity measurements) is transferred from the lattice to the spin system, but is weak enough so the contributions from phonons (κ L ) and magnons (κ m ) are approximately additive [3,8], κ ≃ κ L + κ m .Our measurements reveal a complete suppression of the magnon heat flux in the tilted conical phase along the 110 directions that we attribute to strong magnon scattering by tilt domain boundaries. This observation raises the prospect of exploiting this configuration of heat flux and applied field in a field-controllable spin-current switch. The low-temperature skyrmion phase, characterized by long-range skyrmion lattice order, supports maximum magnon heat conduct...

Spin Seebeck effect in Cu2OSeO3 : Test of bulk magnon spin current theory

“…κ(H) exhibits a step-like increase at the H-C spin-phase transition, a plateau within the C phase, and another step-like increase at the C-FP phase boundary. Similar features for various orientations of heat flow and applied field have been reported in prior studies [11,23]. S LSSE (H) exhibits a small maximum at fields below the H-C phase transition, presumably associated with partial reorientation of the three 100 -oriented helical domains, established in zero-field cooling.…”

“…Given the high thermal conductivity of Cu 2 OSeO 3 [11] and desire to maximize length along the inverse spin-Hall field ([112]), long, thin parallelepiped specimens were necessary, leading to large demagnetization factors (N ∼ 0.75) and some nonuniformity of the applied field; we report external field values here. Extensive prior measurements of M (H) and κ(H) on crystals [11,23] with small N re- veal consistent, coincident signatures of the spin-phase transitions that are employed here to identify the phase boundaries from κ(H) data.…”

“…Prior work demonstrates that field dependent changes in κ are entirely magnonic [11,23]. Separation of κ L and κ m is possible for T 1.2 K where the high-field condition E H ≫ k B T is met (E H = gµ B H, the Zeeman energy) and thus spin-wave excitations are depopulated (gapped).…”

Violation of the magnonic Wiedemann-Franz law in the strong nonlinear regime

“…By manipulating the applied magnetic field, magnon and phonon thermal conductivities can be distinguished experimentally [40] because the former does depend strongly on the field whereas the latter does not. In fact, in Ref.…”

“…In fact, in Ref. [40], it has been experimentally shown that thermal conductivities of phonons and magnons in a magnetic insulator can be separately characterized at low temperatures T ≤ O(1)K. For this reason, we focus only on magnon thermal conductivity in our work.…”