Universal negative magnetoresistance in antiferromagnetic metals caused by symmetry breaking of electron wave functions

Abstract

Layered van der Waals crystals of topologically non-trivial and trivial semimetals with antiferromagnetic (AFM) ordering of magnetic sublattice are known to exhibit a negative magnetoresistance that is well correlated with AFM magnetization changes in a magnetic field. This effect is reported in several experimental studies with EuFe2As2, EuSn2As2, EuSn2P2, etc., where the resistance decreases quadratically with field by about 5% up to the spin-polarization field. Although this effect is well documented experimentally, its theoretical explanation is missing up to date. Here, we propose a theoretical mechanism describing the observed magnetoresistance that is inherent in AFM metals and is based on violation the binary T2 symmetry. It is almost isotropic to the field and current directions, contrary to the known mechanisms such as giant magnetoresistance and chiral anomaly. The proposed intrinsic mechanism of magnetoresistance is strong in a wide class of the layered AFM-ordered semimetals. The theoretically calculated magnetoresistance is qualitatively consistent with experimental data for crystals of various composition.

Figures (6)

• Figure 1: Schematic picture of the EuSn$_2$As$_2$ lattice structure (adapted from Ref. golov_JMMM_2022). Magenta arrows show Eu-atoms magnetization direction in the A-type AFM ordered state. For more detail on the Eu magnetic moments orientation, see Supplementary Note 3. Horizontal arrow denotes the lattice spacing $c\approx 2.64$nm along $z$-axis arguilla_InChemFront_2017chen_ChPhysLet_2020PRB_tbp.
• Figure 2: Wave function (WF) magnitude distribution along the crystal $z$-axis for the lowest-energy quantum state [Eq. (\ref{['psipmWF']})] in a double-well potential, modeling two AFM sublattices.a Schematic color illustration of the spin-up (red) and spin-down (blue) wave function distribution along the $z$-axis in the half-cell of the $c/2$-size. $c_b$ denotes the distance between the two spin-split WF maxima. Color intensity represents the WF magnitude. Red circles show Eu atoms, and arrows -- their magnetization direction in the AFM state. b Schematic picture of the fourth power $\psi ^{4}(z)$ of electron WF given by Eq. (\ref{['psipmWF']}), entering the scattering rate in Eq. (\ref{['tau']}). The asymmetry parameter $\gamma \equiv E_{ex}/2t_0 =0$ (green solid line), $\gamma=-0.4$ (dashed red line) and $\gamma=0.4$ (dash-dotted blue line).
• Figure 3: Calculated negative magnetoresistance.a Magnetoresistance curve given by Eqs. (\ref{['drho']}) and (\ref{['RMzB']}) at $E_{ex0}/2t_z =0.23$ and $[\omega_c\tau (H=H_{\mathrm{sf}})]^2 =0.2$. b The maximal possible relative value of the proposed NIMR effect as a function of $\gamma_0= E_{ex0}/2t_{0}$, plotted using Eqs. (\ref{['dI']}) or (\ref{['drho']}). It saturates at $50$% for $\gamma_0\gg 1$, when resistivity drops by half.
• Figure 4: Experimental data on magnetoresistance and magnetization of EuSn$_2$As$_2$ bulk crystal.a Examples of the normalized magnetoresistance $R(H)/R(0))$ vs normalized field $(H/H_{\rm sf})$ for two orientations of the bias current, in-plane $R_{ab}$ and perpendicular to the plane $R_c$, and for two magnetic field directions, $\boldsymbol{H}\|(ab)$ and $\boldsymbol{H}\|c$. Data are taken at $T=2-3$K. b Magnetization $M(H)$ dependences for two field orientations, $\boldsymbol{H}\|ab$ and $\boldsymbol{H}\|c$. The nonlinearity of $M(\boldsymbol{H}\|ab)$ in low fields is related with spin canting and spin-flop PRB_tbp. Vertical arrows depict $H_{\rm sf}$ value for two field orientations.
• Figure 5: Calculated spin-polarized electron density distribution. The yellow (turquoise) isosurfaces correspond to fixed differences of charge density with spin along (against) the $a$ axis of AFM ordering of Eu spins. The neighboring SnAs layers have opposite spin polarization of electrons at the Fermi level thus substantiating our model.