Enhanced transverse electron transport via disordered composite formation

Abstract

Transverse electron transport in magnetic materials - manifested in effects such as the anomalous Hall and Nernst effects - holds promise for spintronic and thermoelectric applications. While recent advances have focused on enhancing such transport through topological single crystals via intrinsic mechanisms linked to Berry curvature, practical limitations remain due to their mechanical fragility and narrow material scope. Here, we demonstrate a distinct approach for transverse transport enhancement based on composite formation. Using both theoretical modeling and experiments, we show that disordered mixtures of two ferromagnetic materials can exhibit significantly stronger transverse electron deflection than either constituent alone. This enhancement originates from meandering electron pathways created by the disordered mixture of two materials and does not rely on long-range crystalline order. The identified requirements for this mechanism can be broadly satisfied across different material systems, offering a universal and tunable strategy to engineer large transverse responses in structurally robust platforms.

Figures (19)

• Figure 1: (a) Schematic of 2D network model describing the physical mixture of material A (amorphous, yellow square sites) and B (crystalline, cyan square sites). (b) Each site $\alpha$ is connected to its nearest neighbor sites through four wires $i(=1, 2, 3, 4)$. (c) Three panels illustrate current flow generated by the voltage choice, $V_1^\alpha >0$ and $V_2^\alpha=V_3^\alpha=V_4^\alpha=0$ for the three limiting cases of $(G_{\rm S}^\alpha, G_{\rm A}^\alpha, c^\alpha)$. (d) The results of the network model. Red symbols denote $\bar{\gamma}_{yx}$ and the red-shaded area represents the range of $\gamma_{yx}$ fluctuation with 1.5 times standard deviation. Blue symbols represent $\gamma_{yx}$ for the homogeneous network (right inset). Half-filled green diamond and purple square represent $\gamma_{yx}$ at $P^{(\text{B})}$= 50% for the island and stripe networks (upper-left and lower-left insets), respectively. (e) $\gamma_{yx}^{(\text{A})}$-$\gamma_{yx}^{(\text{B})}$ diagram showing the enhancement condition. Each combination ($\gamma_{yx}^{(\text{A})}$, $\gamma_{yx}^{(\text{B})})$ is marked sky-blue if the enhancement occurs and beige otherwise. Here, $\gamma_{xx}^{({\rm A})} < \gamma_{xx}^{({\rm B})}$ is assumed.
• Figure 2: Meandering current paths originated from domain geometry. (a) Islands of material A (amorphous, yellow) embedded in material B background (crystalline, cyan). (d) Reverse configuration. The red arrows in (a) and (d) indicate the direction of net longitudinal flux $J_{xx}$. The color in (b) and (e) shows the local voltage profiles for (a) and (d), respectively, while (c) and (f) display the transverse components of the voltage profiles (see End Matter). Red arrows in (b-c) and (e-f) denote current paths distorted by the embedded islands. (g) Schematic of TT enhancement via the composite formation.
• Figure 3: SEM color-mapping images of crystalline phases in (a) as-cast, (b) $T_{a}=673$ K, (c) $T_{a}=723$ K, and (d) $T_{a}=973$ K samples. TEM images and corresponding diffraction patterns of (e) amorphous (as-cast), (f, g) partially crystallized ($T_{a}=723$ K), and (h) poly-crystalline ($T_{a}=973$ K) states. (f) and (g) were obtained at amorphous and crystalline regions of the same $T_{a}= 723$ K sample.
• Figure 4: (a) Longitudinal electrical conductivity ($\sigma_{xx}$) and (b) anomalous Hall conductivity ($\sigma_{yx}$) as a function of annealing temperature ($T_{a}$). (c) Anomalous Nernst coefficient ($S_{yx}$) and (d) anomalous Nernst conductivity ($\alpha_{yx}$). All properties were measured at $T=300$ K. The symbols $a$ and $c$ denote the amorphous and crystalline phases, respectively, while $a + c$ denotes their coexistence.
• Figure 5: (Color online) Calculated (a) $\gamma_{xx}$ and (b) $\gamma_{yx}$ for moderate contrast in transport properties (see End Matter). Red symbols denote (a) $\bar{\gamma}_{xx}$ and (b) $\bar{\gamma}_{yx}$, whereas green diamonds represent (a) $\gamma_{xx}$ and (b) $\gamma_{yx}$ for the island structure at $P^{(\text{B})}=50\%$ and modified structures from the island structure for $P^{(\text{B})} \neq 50\%$.