Ultrafast Néel vector switching

Abstract

We predict ultrafast switching in a chiral anti-ferromagnet that occurs at femtosecond times, nearly 5 orders of magnitude faster than the torque induced nanosecond switching previously observed. The physical mechanism, quite different from that which drives slow switching, involves the creation of massive effective magnetic fields by ultrafast spin current injection. Identifying these fields as key to femtosecond rotation, we establish simple practical rules for their maximisation with wide applicability to all magnetised materials. Employing state-of-the-art time-dependent density-functional theory and using the example of chiral magnet, Mn$_3$Sn, we induce ultrafast rotation enough to drive the switching of magnetic order between the six possible non-collinear ground states. We further demonstrate the possibility of undoing this switching by subsequent injection of oppositely polarized spin current. Our findings place chiral anti-ferromagnets as a materials platform for femtosecond Néel-vector switching, opening a route towards the manipulation of magnetic matter at ultrafast times.

Figures (3)

• Figure 1: Schematic illustration of Mn$_3$Sn lattice and magnetic moments. (a) Primitive unit cell of Mn$_3$Sn, with boundaries indicated by solid black lines. The Mn sites form a kagome pattern with two layers, denoted L1 and L2. (b) The six ground state magnetic configurations, each separated by $\pi/3$ rotation of the local moments, with the magnetic ordering vector, $\mathbf{L}$.
• Figure 2: Ultrafast rotation of local moments by spin current. (a) Time-dependent amplitude of spin current potential, with polarisation vector out of plane. (b,c,d) The calculated time evolution of the magnetisation vector, shown for each of the three distinct sites. Note that a small out-of-plane ($m_{\rm z}$) magnetization develops. Initial, (e), and final, (f), local moment vectors; in each case the second inequivalent layers is shown as and the faded sites. After the spin current the local moments are rotated approximately 60 degrees. (g) Effective spin-dependent electric field for spin current potential shown in (a). (h) Effective magnetic field, calculated from in-kagome-plane magnetisation dynamics.
• Figure 3: Dependence of rotation on current amplitude and polarization. (a) Rate of rotation versus spin current potential (over fixed duration). The relationship is quadratic, as made clear by the plot of the square root of the rotation rate which is visibly linear. (b) Rate of rotation versus polarisation ($P$) of the spin current potential. We define polarization as $P=\frac{\mathcal{A}^{\rm +z}-\mathcal{A}^{\rm -z}}{\mathcal{A}^{\rm +z}+\mathcal{A}^{\rm -z}}\approx \frac{J^{\rm +z}-J^{\rm -z}}{J^{\rm +z}+J^{\rm -z}}$, where ${\pm z}$ indicates spin polarization perpendicular to the kagome plane such that $J^{\rm +z}$ is the current density of ${\rm +z}$ spin electrons and $\mathcal{A}$ is a current potential. For polarisation up to 1 $\mathcal{J}^{-z}$ is varied for fixed $\mathcal{J}^{+z}$ while for polarisation above 1 the total spin current is fixed while the charge current is varied. The rate of rotation is highest for equal charge and spin current ($P=1.0$), and it goes to zero for pure charge currents ($P=0.0$) and for pure spin currents ($P=\infty$, not shown). (c) Rate of rotation versus 'spin purity' $=(\mathcal{A}-A)/(\mathcal{A}+A)$.