Giant anomalous Nernst signal in the antiferromagnet YbMnBi2

A large anomalous Nernst effect (ANE) is crucial for thermoelectric energy conversion applications because the associated unique transverse geometry facilitates module fabrication. Topological ferromagnets with large Berry curvatures show large ANEs; however, they face drawbacks such as strong magnetic disturbances and low mobility due to high magnetization. Herein, we demonstrate that YbMnBi2, a canted antiferromagnet, has a large ANE conductivity of ~10 A m−1 K−1 that surpasses large values observed in other ferromagnets (3–5 A m−1 K−1). The canted spin structure of Mn guarantees a non-zero Berry curvature, but generates only a weak magnetization three orders of magnitude lower than that of general ferromagnets. The heavy Bi with a large spin–orbit coupling enables a large ANE and low thermal conductivity, whereas its highly dispersive px/y orbitals ensure low resistivity. The high anomalous transverse thermoelectric performance and extremely small magnetization make YbMnBi2 an excellent candidate for transverse thermoelectrics.

The chiral Hall effect in canted ferromagnets and antiferromagnets

The anomalous Hall effect has been indispensable in our understanding of numerous magnetic phenomena. This concerns both ferromagnetic materials, as well as diverse classes of antiferromagnets, where in addition to the anomalous and recently discovered crystal Hall effect, the topological Hall effect in noncoplanar antiferromagnets has been a subject of intensive research in the past decades. Here, we uncover a distinct flavor of the Hall effect emerging in generic canted spin systems. We demonstrate that upon canting, the anomalous Hall effect acquires a contribution which is sensitive to the sense of imprinted vector chirality among spins. We explore the origins and basic properties of corresponding chiral Hall effect, and closely tie it to the symmetry properties of the system. Our findings suggest that the chiral Hall effect and corresponding chiral magneto-optical effects emerge as useful tools in characterizing an interplay of structure and chirality in complex magnets, as well as in tracking their chiral dynamics and fluctuations.

Charge-to-Spin Interconversion in Low-Symmetry Topological Materials

The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Here, we show that the reduced symmetry of strong spin-orbit coupling materials such as MoTe2 or WTe2 enables a new form of canted spin Hall effect (SHE), characterized by large and robust in-plane spin polarizations, which gives rise to an unprecedented charge-to-spin interconversion effect. Through quantum transport calculations on realistic device geometries, including disorder, we found long spin diffusion lengths (λs) and a gate tunable charge-to-spin interconversion efficiency with an upper value reaching θxy ≈ 80%. The SHE figure of merit λsθxy ∼ 1–50 nm, can significantly exceed values of conventional SHE materials, and stems from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour. Specific guidelines for unambiguous experimental confirmation are proposed, paving the way towards exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and band topology can govern the intrinsic SHE, and how they may be exploited to broaden the range and efficiency of spintronic functionalities.

Мёссбауэровские исследования состава и магнитной структуры нанокомпозитов Fe-=SUB=-3-=/SUB=-O-=SUB=-4-=/SUB=-/γ-Fe-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- типа ядро-оболочка во внешнем магнитном поле (Часть 2)

The results of Mössbauer studies of the composition and magnetic structure of Fe3O4 / -Fe2O3 nanoparticles placed in an external magnetic field with a strength of 1.8 kOe, which is a continuation of the work [A.S. Kamzin, I.M. Obaidat, A.A. Valliulin, V.G. Semenov, I.A. Al-Omari. FTT No. 10/2020]. It is shown that the thickness of the maghemite (-Fe2O3) shell can be changed by the synthesis conditions. It was found that on the surface of the maghemite (-Fe2O3) shell in the Fe3O4 / -Fe2O3 nanocomposites there is a layer in which the magnetic moments are not oriented collinearly to the moments located in the depth of the shell, i.e., there is a canted spin structure. An intermediate layer in the spin-glass state is formed between the core and the shell. The data obtained on the structure of core / shell particles are important for understanding the properties of nanocomposites, which are of great interest for applications in various fields, including biomedicine.

Свойства синтезированных методом пиролиза ультразвуковой аэрозоли наночастиц MgFe-=SUB=-2-=/SUB=-O-=SUB=-4-=/SUB=- для биомедицинских применений

We present the data of studies on the structure, phase states, and magnetic properties of magnetic nanoparticles (MNPs) of magnesium ferrite spinel (MgFe_2O_4), synthesized by ultrasonic aerosols pyrolysis. Primary single-phase MNPs with an average size of 9.6, 11.5, and 14.0 nm, synthesized from precursors at concentrations of 0.06, 0.12, and 0.24 M, respectively, agglomerate into tightly aggregated spherical particles (secondary particles) with sizes of 206, 300, and 340 nm, respectively. Primary particles inside the spheres do not interact with each other and are in a superparamagnetic state. There is a layer on the surface of the particles, the magnetic structure of which differs from the structure of the inner part of the MNP; this is explained by the formation of a canted spin structure or a spin glass state in the surface layer of the MNPs. MgFe_2O_4 nanospheres obtained from a precursor at a concentration of 0.06 M are most promising as valid sources of heat in magnetic hyperthermia therapy.

Search for canted spin arrangement in Er2−xTbxFe14B with Mössbauer spectroscopy

The materials studied were polycrystalline compounds Er2−xTbxFe14B (x = 0.1, 0.2, 0.3, 0.4) which crystallize in a tetragonal lattice and display a variety of spin arrangements. The compounds have been measured with 57Fe Mössbauer spectroscopy over the temperature range 80–320 K in order to investigate the spin reorientation processes. Each compound was studied in a wide temperature range, with precise Mössbauer scanning in the vicinity of the transition. The set of spectra obtained for a given compound was analyzed using simultaneous fitting procedure to investigate the influence of the transition on the shape of the spectra. The fitting program was specified to analyze the transition according to the ‘two state model’: spins flip abruptly from initial angle to final arrangement (90° angle). Obtained results suggest that spin reorientation process cannot be described using only the mentioned above model. Additional computer simulations based on the Yamada–Kato model were conducted to determine temperature range and the type of spin alignments in the vicinity of the transition. These theoretical results supported by spectra analysis suggest the existence of intermediate (canted) spin arrangements in the studied compounds. The spin arrangement diagram was constructed.