Large magneto-optical Kerr effect induced by collinear antiferromagnetic order

TL;DR

The study tackles the emergence of a giant magneto-optical Kerr effect in a room-temperature collinear antiferromagnet, alpha-Fe2O3, arising from a Tt-symmetry-broken order rather than net magnetization. It combines polar Kerr measurements, symmetry analysis of the magnetic point group, and first-principles DFT+U with Wannier-based linear-response calculations to compute Kerr spectra. The spontaneous Kerr angle reaches ~80 mdeg in the easy-plane AFM state and is well reproduced by calculations with zero net moment, while the easy-axis phase shows no spontaneous Kerr; a large Kerr response per unit canting is observed. The work demonstrates that Tt-symmetry-broken AFMs can exhibit giant Kerr effects with vanishing magnetization, offering a platform for optical readout of up/down spin states and potential optical writing via inverse effects.

Abstract

In modern technology, the optical readout of magnetic information is conventionally achieved by the magneto-optical Kerr effect, i.e., the polarization rotation of reflected light. The Kerr rotation is sensitive to time-reversal symmetry breaking and generally proportional to magnetization, enabling optical readout of the up and down spin states in ferromagnets. By contrast, antiferromagnets with a collinear antiparallel spin arrangement have long been considered inactive to such magneto-optical responses, because of Tt-symmetry (time-reversal T followed by translation t symmetry) and lack of macroscopic magnetization. Here, we report the observation of giant magneto-optical Kerr effect in a room-temperature antiferromagnetic insulator alpha-Fe2O3. In this compound, the up-down and down-up spin states induce the opposite sign of spontaneous Kerr effect, whose Kerr rotation angle turned out to be exceptionally large (~ 80 mdeg, comparable to typical ferromagnets). Our first-principles calculations successfully reproduce both the absolute magnitude and spectral shape of the Kerr rotation and ellipticity with remarkable accuracy, which unambiguously proves that it originates from a Tt-symmetry-broken collinear antiferromagnetic order, rather than magnetization. This compound hosts temperature-dependent transition between easy-plane and easy-axis antiferromagnetic states, and their contrasting behaviors are also investigated in detail. The present results demonstrate that even a simple collinear antiferromagnetic order can induce a giant magneto-optical Kerr effect, and highlight Tt-symmetry-broken antiferromagnets as a promising material platform for highly sensitive optical detection of up-down and down-up spin states.

Figures (4)

• Figure 1: Classification of various types of collinear magnets and spontaneous magneto-optical Kerr effect.a, Comparison of ferromagnet (FM), conventional antiferromagnet (AFM), and TtS-broken AFM. Red arrows represent local magnetic moments. Blue and red circles indicate magnetic and non-magnetic ions, respectively. Magnetic domains A and B are converted into each other by time-reversal operation $\mathcal{T}$. These two domains (i.e., $\uparrow \downarrow$ and $\downarrow \uparrow$) are identical under a translation operation $t$ in conventional AFMs, but not identical in TtS-broken AFMs with appropriately located non-magnetic ions. b, Comparison of spontaneous magneto-optical Kerr effect in FMs and TtS-broken AFMs. In the latter case, the collinear $\uparrow \downarrow$ spin order induces a fictitious magnetic field, which leads to the emergence of Kerr rotation even without magnetization $M$. Gray and blue arrows represent the propagation vector and polarization direction of the light, respectively. The sign of Kerr rotation angle $\theta_{\rm K}$ is opposite between the domains A and B. Note that the spontaneous Kerr effect is not allowed in conventional antiferromagnet, because of its TtS.
• Figure 2: Spontaneous magneto-optical Kerr effect in a collinear antiferromagnet $\alpha$-Fe$_2$O$_3$ at room temperature.a,d, Schematic illustration of the spin arrangements in the easy-plane AFM phase of $\alpha$-Fe$_2$O$_3$. Magnetic domains A ( a) and B ( d) (i.e., the $\uparrow \downarrow$ and $\downarrow \uparrow$ spin states) are converted into each other by time-reversal operation, which are characterized by the opposite sign of fictitious magnetic field and tiny spontaneous magnetization $\Delta M_{\rm cant}$ along the $a$-axis. b,c, Magnetic field dependence of magnetization $M$ and polar magneto-optical Kerr rotation angle $\theta_{\rm K}$ measured with the normal incidence to the $a$-plane surface for $B \parallel a$ at 300 K in the easy-plane AFM phase. The incident light energy $h\nu$ is set as 2.25 eV. Here, the domain A (domain B) is selected by applying the negative (positive) sign of external magnetic field. The definition of $\Delta M_{\rm cant}$ and $\Delta \theta_{\rm K}$ are indicated in b and c, respectively.
• Figure 3: The presence (absence) of spontaneous magneto-optical Kerr effect in the easy-plane (easy-axis) AFM state.a-d, Magnetic field dependence of magnetization $M$ and polar magneto-optical Kerr rotation angle $\theta_{\rm K}$ under $B \parallel a$ at 50 K ( a,b) and 245 K ( c,d). e, $B$-$T$ magnetic phase diagram for $B \parallel a$, where the background color represents the amplitude of $\theta_{\rm K}$. f, Temperature dependence of spontaneous Kerr rotation angle $\Delta\theta_{\rm K}$, obtained by linearly extrapolating the $\theta_{\rm K}$-$B$ curve to $B=0$ (as shown in Fig. 2c). g, Symmetry analysis of various collinear spin arrangements on the $\alpha$-Fe$_2$O$_3$ crystal lattice. Magnetic point group, magnetic structure and corresponding symmetry-imposed shape of optical conductivity tensor $\tilde{\sigma}$ are presented for easy-axis AFM, easy-plane AFM and ferromagnetic (FM) spin states.
• Figure 4: Microscopic origin of spontaneous magneto-optical Kerr effect in the easy-plane AFM phase of $\alpha$-Fe$_2$O$_3$.a, Electronic band structure and density of state (DOS) of $\alpha$-Fe$_2$O$_3$ in the easy-plane AFM state, theoretically calculated based on DFT calculation with on-site Hubbard correlation $U$ = 4.0 eV. The letter symbols represent the positions in Brillouin zone, schematically illustrated in the right panel. b, Experimental energy spectra of Kerr rotation angle $\theta_{\rm K}$ and Kerr ellipticity $\eta_{\rm K}$, measured at 300 K (easy-plane AFM state) with $B \parallel a$ at 1.0 T. c, Theoretical energy spectra of $\theta_{\rm K}$ and $\eta_{\rm K}$ in the easy-plane AFM state with $M=0$, calculated based on the electronic structure in a. d, Full logarithmic plot of magnetization dependence of the polar magneto-optical Kerr rotation $\theta_{\rm K}$ for various magnetic materials. Conventional ferromagnets are located in the yellow shadowed region.