Exceptional sign changes of the nonlocal spin Seebeck effect in antiferromagnetic hematite

Abstract

Low-power spintronic devices based on the propagation of pure magnonic spin currents in antiferromagnetic insulator materials offer several distinct advantages over ferromagnetic components including higher-frequency magnons and a stability against disturbing external magnetic fields. In this work, we make use of the insulating antiferromagnetic phase of iron oxide, the mineral hematite α-Fe 2 O 3 to investigate the long-distance transport of thermally generated magnonic spin currents. We report on the excitation of magnons generated by the spin Seebeck effect, transported both parallel and perpendicular to the antiferromagnetic easy-axis under an applied magnetic field. Making use of an atomistic hematite toy model, we calculate the transport characteristics from the deviation of the antiferromagnetic ordering from equilibrium under an applied field. We resolve the role of the magnetic order parameters in the transport, and experimentally we find significant thermal spin transport without the need for a net magnetization.

Spin Seebeck Effect: Sensitive Probe for Elementary Excitation, Spin Correlation, Transport, Magnetic Order, and Domains in Solids

“…The ℓm estimation may be affected by the locally distributed ∇T , which is discussed in References (95,96). So far, nonlocal SSEs are reported in several magnets such as ferrimagnetic YIG (94,95,96,97,98,99,100,101,102,103,104,105), Gd3Fe5O12 (106), Tm3Fe5O12 (107), NiFe2O4 (108), MgAl0.5Fe1.5O4 (109), antiferromagnetic Cr2O3 (111,112), α-Fe2O3 (37,110), NiO (34), YFeO3 (113), BiFeO3 (114), and 2D layered magnets MnPS3 (115,116) and CrBr3 (117).…”

“…This prohibits magnon transmission from YIG to Cr2O3, and thus sharply suppresses the LSSE voltage below TN, being responsible for the observation (125). In 2021, Ross et al (37) demonstrated the nonlocal SSE mediated by the Néel vector n in α-Fe2O3 under the external B parallel to the easy-axis and the attached metal (Pt) strips (Figure 5b, subpanel i), which allows to exclude significant contributions from the net magnetization vector (37). In the easy-axis phase of α-Fe2O3, close to but below the Morin transition temperature (TM = 240 K) such as 210 K, the nonlocal SSE voltage is initially constant around zero at a low B, but shows a sharp negative dip to positive peak transition across a critical field, Bcr, where n reorientates perpendicular to B (Figure 5b, subpanel ii).…”

“…in Figure 2 in the previous review article (2). Recent updates include ferromagnetic EuO (15), two-dimensional (2D) ferromagnetic Cr2Si2Te6, Cr2Ge2Te6 (16,17), ferrimagnetic garnet ferrites R3Fe5O12 (R = Eu, Tb, Dy, Tm) (18,19,20), Y3−xRxFe5O12 with R being 14 rare-earth elements from La to Lu (except for Pm) (21), Lu2Bi1Fe4Ga1O12 (22), spinel ferrites ZnFe2O4 (23), γ-Fe2O3 (24), LiFe5O8 (25), Ni0.65Zn0.35Al0.8Fe1.2O4 (26), Mg 0.5−δ Mn0.5Fe2O4 (27), Y-type hexagonal ferrites Ba2Co2Fe12O22, Ba2Zn2Fe12O22 (28), orthorhombic ferrimagnetic ε-Fe2O3 (29), molecular-based ferrimagnetic Cr II [Cr III (CN)6] (30), various antiferromagnetic (AF) insulators such as NiO (31,32,33,34), FeF2 (35), α-Fe2O3 (36,37), MnCO3 (38), α-Cu2V2O7 (39), SrFeO3 (40), SrMnO3 (41), DyFeO3 (42), and other intriguing materials including a chiral helimagnet Cu2OSeO3 with a skyrmion lattice phase (43,44) and quantum magnets Sr2CuO3 (45,46), CuGeO3 (47), Pb2V3O9 (48), LiCuVO4 (49). In particular, the ferrimagnetic insulator YIG has been essential (1,2), as it exhibits the lowest magnetic damping, high Curie temperature (TC ∼ 560 K), and high resistivity and also is a playground to reveal the role of magnon polarization in SSEs (see Section 6).…”

Influence of low-energy magnons on magnon Hanle experiments in easy-plane antiferromagnets

“…Micrometer long spin transport has first been observed in easy-axis AFIs in similiar devices [8]. Apart from SHE-induced magnons, the transport of thermally generated magnonic spin currents via the spin Seebeck effect has also been reported [27,28].…”

Observation of nonreciprocal magnon Hanle effect

“…At the same time, the spin Hamiltonians used to describe hematite in the literature [30], for example in the original article by Moriya [58], could be oversimplified and might have missed such a nonreciprocity stemming from the crystal structure. To examine this potential origin, atomistic spin modeling of hematite taking into account its exact crystal structure is desirable and, hopefully, will be motivated by our findings [59,60].…”