Terahertz-nanoscale visualization of the microscopic spin-charge architecture of colossal magnetoresistive switching

Abstract

Resolving sub-10 nm spin switching and the associated terahertz (THz) electrodynamics during the colossal magnetoresistance (CMR) transition is a definitive frontier in reaching the fundamental spatial, temporal, and energy-dissipation limits of spin-based microelectronics and quantum logic architectures. Yet, the requirement of simultaneous control of high magnetic field, cryogenic environment, and nanometer-scale resolution has remained an elusive benchmark for terahertz nanoscopy, leaving the obscured nano-scale high-frequency dynamics of these transitions largely unexplored. Here, we overcome these limitations by utilizing a custom-built cryogenic magneto-THz scattering-type scanning near-field optical microscopy (cm-THz-sSNOM) platform to resolve the nanoscale, THz spectroscopic evolution of the magnetic field-driven CMR transition in a manganite single crystal $\text{Pr}_{2/3}\text{Ca}_{1/3}\text{MnO}_{3}$. Our measurements provide a real-space visualization of the local THz conductivity, capturing the moment that magnetic-field-induced spin switching triggers the phase transition from an antiferromagnetic insulator to a ferromagnetic metal. THz nano-imaging, together with an ellipsoidal near-field model, reveals a multi-scale transition initiated by 1-2 nm isolated spin-flip sites at low magnetic fields, which coalesce into $\sim$15~nm conducting regions as the threshold field is approached. These results provide an in situ, previously inaccessible THz real-space view of CMR switching, establishing a general analysis framework for mapping spin-charge-lattice-orbit-coupled dynamics at spatial scales that transcend the nominal sSNOM resolution.

Figures (4)

• Figure 1: Terahertz magneto-nanoscopy of the colossal magnetoresistance (CMR) phase transition.a) Schematic representation of the $\sim$80 $\mu$m tall, $\sim$150 nm diameter Pt-coated AFM probe above the Pr$_{0.67}$Ca$_{0.33}$MnO$_3$ surface, illustrating the detection of sub-resolution conducting and insulating domains. b) Orthorhombic crystal lattice structure of Pr$_{0.67}$Ca$_{0.33}$MnO$_3$Vesta3. c,d) Schematic of the Mn$^{3+}$/Mn$^{4+}$ spin configurations in c) the zero-field charge-ordered insulating state and d) the magnetic-field-induced charge-disordered metallic state. e) Macroscopic magneto-transport phase diagram showing the insulator-to-metal transition boundary as a function of magnetic field and temperature. f) Resistivity as a function of temperature for constant magnetic fields of 0 T (black) and 5 T (red), highlighting the CMR effect upon field-induced melting of the charge-ordered state.
• Figure 2: Magnetic field-driven THz near-field nano-spectroscopy and spectral evolution.a) Integrated THz near-field $S_2$ peak amplitude as a function of increasing magnetic field following zero-field cooling to 29 K. The dashed line illustrates the insulator-to-metal transition (IMT) triggered by AFM-FM spin-switching. b) Representative time-domain THz $S_2$ near-field waveforms recorded at discrete magnetic fields (0 T, 2 T, 3 T, 4 T, and 5 T), capturing the field-induced enhancement of the scattered electric field. c) Normalized THz $S_2$ spectral ratios ($S_2(B)/S_2(0~T)$) revealing the emergence of a Drude-like response and low-frequency enhancement above the threshold field. Grayed regions denote instrumental artifacts arising from tip-cantilever resonances. (Inset) Corresponding raw THz $S_2$ spectra derived via Fourier transform of the time-domain traces in (b).
• Figure 3: Real-space THz nano-imaging and sub-resolution modeling of the CMR transition.a--d) Simultaneous acquisition of a) AFM topography, b) THz $s_1$, c) THz $s_2$, and d) THz $s_3$ near-field signals over a $5 \times 4$$\mu$m$^2$ region at 3.5 T. The topographic mapping confirms a flat surface profile, ensuring that the observed near-field contrast is electronic in origin. e--j) Experimental second-harmonic ($s_2$) THz near-field images as a function of magnetic field (0 T to 4.5 T) on a $4 \times 1$$\mu$m$^2$ global scale. k--p) Corresponding modeled THz near-field images generated via an ellipsoidal dipole model and stochastic spin-flip population, reproducing the experimental signal evolution and spatial homogeneity across the transition.
• Figure 4: Multi-scale domain evolution and statistical validation.a--f) Unfiltered modeled domain structures illustrating the stochastic population of conducting spin-flip sites across the magnetic field-driven transition (0 T to 4.5 T). This simulation reveals the transition's progression from isolated 1--2 nm switching sites at low fields to the formation of $\sim$15 nm percolative regions as the metallic volume fraction increases. g) Intensity histograms comparing the spatial signal distribution of the experimental near-field images (red) and the modeled results (blue). The excellent agreement across all magnetic fields validates the use of the ellipsoidal near-field interaction model to quantify the underlying phase-separated architecture.