Giant orbital magnetoresistance in the antiferromagnet CoO driven by dynamic orbital angular momentum interaction

Abstract

Recent predictions of orders of magnitude larger orbital current effects compared to spin currents have attracted significant interest. However, the full potential of giant orbital currents remains to be fully harnessed, since so far, the orbital currents need to be converted into spin currents before they can interact with the static magnetization that is dominated by spin angular momentum in conventional magnets. By using a magnet dominated by orbital angular momentum, we demonstrate a more than fifty-fold enhancement in orbital Hall magnetoresistance in CoO/Cu*, compared to conventional CoO/Pt. This is found to be driven by a unique interaction between dynamic orbital angular momentum from surface oxidized Cu* (i.e., the orbital current) and the static orbital angular momentum which constitutes the magnetic moments in the antiferromagnetic insulator CoO. A distinctive scattering mechanism for orbital currents at the CoO interface leads to a sign reversal in orbital magnetoresistance in CoO/Cu* compared to CoO/Pt. Our results show how by using orbital angular momentum-dominated materials such as CoO, we can harness the benefits of giant orbital currents that have not been possible using conventional spin-dominated magnets, for orbitronics-based devices, offering unprecedented energy efficiency for operations of antiferromagnets that combine ultimate stability with THz dynamics.

Figures (9)

• Figure 1: Magnetic structure of CoO and experimental geometry. (A) Illustration of the magnetic and crystallographic structure of CoO with in-plane magneto-crystalline anisotropy, showing the Néel vector easy axis along the [$\bar{1}10$] axis. The Co and O atoms are represented by blue and black solid circles, respectively. The red and green arrows denote the spin and orbital angular momenta of each Co atom, respectively, pointing in opposite directions satoh2017excitation. (B) Schematic of the transverse MR measurement scheme with electrical contacts in the CoO/Cu* structure, including the magnetic field sweep direction $[1\bar{1}0]$ shown by a green arrow.
• Figure 2: Spin and orbital Hall magnetoresistance. Magnetic field-sweep measurements on (A) CoO(5nm)/Pt(2nm) and (B) CoO(5nm)/Cu*(6nm) samples, respectively. Both samples exhibit a spin-flop transition between 7.5T - 9T. The change in transverse resistance corresponding to the two perpendicular Néel vector orientations is indicated by the red arrow for SMR in A and OMR in B. The measurements are performed at $T = 150K$. (C) The difference in transverse resistance, normalized with the longitudinal resistance for two perpendicular Néel vector states at zero field as a function of temperature. The top panel presents the results for CoO/Cu* (in yellow), while the bottom panel shows the results for CoO/Pt (in blue). (D) The difference in transverse resistance for two perpendicular Néel vector states at zero field as a function of Cu-layer thickness in CoO/Cu* (t). The measurements were performed at a constant temperature of $T = 150K$.
• Figure 3: External field dependence of MR. The change in transverse MR for CoO(5nm)/Cu*(6nm) and CoO(5nm)/Pt(2nm) as a function of applied magnetic field at $T=200K$.
• Figure S1: Structural properties of CoO thin films grown on MgO(001) substrate. (a) $\theta-2\theta$ XRD scan showing the (001) alignment of the CoO($5nm$)/Cu($6nm$) film on MgO(001) substrate. (b) XRD rocking-curves of CoO(002) and MgO(002) planes on top of the $\theta-2\theta$ scan (grey) from panel (a).
• Figure S2: Interaction of spin and orbital currents with spin-dominated AFM $\mathbf{\mathrm{\alpha-Fe_2O_3}}$. Longitudinal resistance of hematite/ (a) Pt and (b) Cu* bilayers at $175K$ in the easy-axis phase for a magnetic field applied along the current direction.