Altermagnetic Superconducting Diode Effect in Mn$_{3}$Pt/Nb Heterostructures

TL;DR

Altermagnetic order can split spin bands without net magnetization, enabling a superconducting diode effect (SDE) in Nb without external fields. The authors fabricate Nb films proximitized by Mn3Pt in two altermagnetic phases, T1 and T2, and probe I–V characteristics across temperatures and perpendicular magnetic fields. They observe a strong zero-field SDE in T1 with diode efficiencies up to about 50% near Tc, while T2 shows a substantially weaker yet present SDE, both enhanced by Bz and accompanied by field- and temperature-dependent interference features in the superconducting state. This work establishes Mn3Pt/Nb heterostructures as a platform for magnetization-free, tunable SDEs, shedding light on altermagnetic superconductivity and motivating further exploration of non-collinear altermagnetic materials for dissipationless spintronics.

Abstract

Compensated magnetic orders that can split the spin-degeneracy of electronic bands have become a very active field of research. As opposed to spin-orbit coupling, the splitting resulting from these "altermagnets" is not a small relativistic correction and, in contrast to ferromagnets, not accompanied by a net magnetization and large stray fields. In particular, the theoretical analysis of the interplay of altermagnetism and superconductivity has taken center stage, while experimental investigations of their coexistence remain in their infancy. We here study heterostructures consisting of Nb thins films interfaced with the $T_1$ and $T_2$ phases of Mn$_3$Pt. These non-collinear magnetic states can be thought of as descendants from the same altermagnetic order in the absence of spin-orbit coupling. We demonstrate the non-trivial impact on the superconducting state of Nb, which exhibits a zero-field superconducting diode effect, despite the compensated ($T_2$) and nearly-compensated ($T_1$) magnetic order; the diode efficiencies can reach large values (up to 50$\%$). The diode effect is found to be highly sensitive to the form of the magnetic order, illustrating its potential as a symmetry probe. The complex magnetic field and temperature dependence hint at a rich interplay of multiple contributing mechanisms. Our results define a new materials paradigm for dissipationless spintronics and magnetization-free diode functionality, while motivating further exploration of non-collinear altermagnetic superconductors.

Figures (3)

• Figure 1: Noncollinear magnetism in Mn$_3$Pt and superconducting Nb films interface: (a) Schematic representation of the spin configurations in cubic Mn$_3$Pt: $T_1$ phase exhibits an all-in/all-out noncollinear spin arrangement on the Mn sublattice, while the $T_2$ phase shows a head-to-tail configuration. Illustration of the Mn$_3$Pt(111)/Nb(110) heterostructure epitaxially grown on a Al$_2$O$_3$(0001), with field applied perpendicular to the film. (b) X-ray diffraction patterns of a 25 nm Nb film and $T_1$ ($T_2$)-Mn$_3$Pt (15 nm)/Nb (25 nm) bilayer. (c) Constant-current STM topography (100 nm $\times$ 50 nm) obtained on $T_1$-Mn$_3$Pt(111)/Nb(110)/Al$_2$O$_3$(0001) and $T_2$-Mn$_3$Pt(111)/Nb(110)/Al$_2$O$_3$(0001) (bottom panel, 90 nm $\times$ 40 nm) films. The measurements were performed under tunneling conditions of I$_t$ = 0.2 nA, U$_{\text{bias}}$ = 0.5 V and I$_t$ = 1 nA, U$_{\text{bias}}$ = 1 V respectively. (d,e) Magnetic field dependence of Hall resistivity ($\rho_{xy}^{\text{AHE}}$) for (d) $T_1$-Mn$_3$Pt/Nb and (e) $T_2$-Mn$_3$Pt/Nb bilayers. We observe nonvanishing Berry curvature-induced large anomalous Hall response in $T_1$-Mn$_3$Pt and negligible Hall signal in $T_2$-Mn$_3$Pt. This represents strong phase dependence of anomalous Hall resistivity in Mn$_3$Pt. (f) Magnetic susceptibility ($\chi$) versus temperature near the superconducting transition temperature ($T_c$), alongside the temperature-dependent longitudinal resistivity ($\rho_{xx}$) of both Nb film and $T_1$-Mn$_3$Pt/Nb heterostructure. No significant change in $T_c$ is observed between the $T_1$ and $T_2$ phases.
• Figure 2: Giant SDE in $T_1$-Mn$_3$Pt/Nb: (a) Current-voltage (I-V) characteristics of $T_1$-Mn$_3$Pt/Nb measured in zero magnetic field over a temperature range of 2.5 - 7.5 K. Optical microscope image of Hall bar strip (inset) of Mn$_3$Pt/Nb heterostructure, in which scale bar denotes 40 $\mu$m. (b) Temperature dependence of absolute value of nonreciprocal critical current and corresponding diode efficiency at zero field. We observe pronounced hysteresis in I-V curves, while the SDE peaks slightly below $T_c$ and vanishes at superconducting transition temperature. Diode efficiency reaches its maximum around 33% at 6.5 K in absence of any external field. (c) Demonstration of zero-field supercurrent rectification at 2.5 K via periodic switching between superconducting and normal conducting states by changing polarity of applied currents between $I_c^{+}$ and $\left| I_c^{-} \right|$. (d) I-V characteristics of $T_1$-Mn$_3$Pt measured under different out-of-plane magnetic fields ($B_z$). Note that SDE does not depend on the polarity of magnetic field. (e) Contour map showing evolution of differential resistance versus critical current and magnetic field at 4.5 K. The plot highlights oscillatory modulation of critical supercurrents with magnetic field, indicating of interference effects arising from superconducting order parameter. (f) Evolution of diode efficiency with magnetic field ($B_z$) at different temperatures. The data reveals that the SDE is further amplified by the application of external magnetic fields, with the enhancement being strongly temperature dependent.
• Figure 3: Weak SDE in $T_2$-Mn$_3$Pt/Nb: (a) Zero-field I-V characteristics of $T_2$-Mn$_3$Pt/Nb measured across temperature range from 2.5 K to 6.5 K showing a reduced rectification with negligible asymmetric critical currents. (b) Temperature dependence of positive and negative critical currents along with corresponding zero-field diode efficiency, plotted as a function of temperature. (c) Variation of critical currents with application of magnetic field at 4.5 K.(d) Magnetic field dependence of I-V characteristics for $T_2$-Mn$_3$Pt/Nb at 4.5 K. (e) Symmetric dV/dI mapping as a function of current bias and magnetic field at 4.5 K. (f) Diode efficiency at different temperatures as a function of magnetic field, which is non-zero as expected by symmetry for the $T_2$ state but much weaker than in the $T_1$-Mn$_3$Pt/Nb counterpart.