[Paper Review] Optical Stabilization of Fluctuating High Temperature Ferromagnetism in YTiO$_3$

Ankit S. Disa, Jonathan B. Curtis+10 more|arXiv (Cornell University)|Nov 26, 2021

Magnetic and transport properties of perovskites and related materials43 references4 citations

TL;DR

This study demonstrates that ultrafast optical excitation of a 9 THz oxygen rotational mode in YTiO₃ stabilizes high-temperature ferromagnetism, achieving complete magnetic saturation and transient ferromagnetism up to T_neq > 80 K—nearly three times the thermodynamic Curie temperature (T_c = 27 K). The effect arises from light-induced, mode-selective lattice dynamics that modify orbital polarization and split degeneracy in Ti t2g levels, enabling metastable, non-equilibrium magnetic order.

ABSTRACT

In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic, and chemical constraints. Here, we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO$_3$, a material that exhibits only partial orbital polarization, an unsaturated low-temperature magnetic moment, and a suppressed Curie temperature, $T_c$ = 27 K. The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to $T_{neq} >$ 80 K, nearly three times the thermodynamic transition temperature. First-principles and model calculations of the nonlinear phonon-orbital-spin coupling reveal that these effects originate from dynamical changes to the orbital polarization and the makeup of the lowest quasi-degenerate Ti $t_{2g}$ levels. Notably, light-induced high temperature ferromagnetism in YTiO$_3$ is found to be metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities.

Motivation & Objective

• To overcome the suppression of high-temperature ferromagnetism in YTiO₃ due to electronic degeneracy and strong fluctuations.
• To explore non-equilibrium control of magnetic order using ultrafast optical excitation.
• To identify specific lattice modes that can stabilize ferromagnetic order beyond the thermodynamic T_c.
• To understand the microscopic origin of light-induced magnetic enhancement via nonlinear phonon coupling.
• To demonstrate the feasibility of engineering metastable, functional magnetic states in quantum materials using all-optical methods.

Proposed method

• Ultrafast mid-infrared laser pulses excite specific lattice modes in YTiO₃, selectively driving a 9 THz oxygen rotational mode.
• Time-resolved magneto-optical Kerr effect (TR-MOKE) measures transient magnetic order and its temperature dependence.
• First-principles calculations of nonlinear phonon coupling and orbital polarization changes are used to model the mechanism.
• Model Hamiltonians incorporating Ti t2g orbital splitting and spin-orbit coupling are employed to explain the observed magnetic enhancement.
• Non-equilibrium dynamics are simulated to assess the lifetime and stability of the light-induced ferromagnetic state.
• The system's response is analyzed under varying excitation fluence and temperature to isolate the role of the 9 THz mode.

Experimental results

Research questions

• RQ1 Can ultrafast optical excitation stabilize ferromagnetism in YTiO₃ beyond its thermodynamic Curie temperature?
• RQ2 Which specific lattice mode is most effective in enhancing and stabilizing magnetic order?
• RQ3 What is the microscopic mechanism linking lattice dynamics to changes in orbital polarization and magnetic moment?
• RQ4 How long does the optically induced ferromagnetic state persist, and is it metastable?
• RQ5 Can nonlinear phonon coupling in a correlated oxide lead to functional, non-equilibrium magnetic phases?

Key findings

• Excitation of the 9 THz oxygen rotational mode induces a transient ferromagnetic state with a non-equilibrium Curie temperature T_neq > 80 K, exceeding the thermodynamic T_c = 27 K by nearly threefold.
• Complete magnetic saturation is achieved in the low-temperature regime upon selective excitation of the 9 THz mode, indicating full alignment of magnetic moments.
• The enhancement of ferromagnetism is attributed to light-induced splitting of the degenerate Ti t2g orbitals, reducing electronic frustration and stabilizing spin polarization.
• First-principles calculations confirm that nonlinear phonon coupling drives a transient reconfiguration of orbital occupancy, favoring ferromagnetic alignment.
• The optically stabilized ferromagnetic state persists for several nanoseconds, indicating metastability and potential for functional applications.
• The effect is mode-selective: other lattice modes do not produce comparable magnetic enhancement, highlighting the importance of resonant excitation.