[Paper Review] Giant phonon-induced effective magnetic fields in 4$f$ paramagnets

Dominik M. Juraschek, Prineha Narang|arXiv (Cornell University)|Jul 21, 2020

Magnetic and transport properties of perovskites and related materials61 references4 citations

TL;DR

This paper proposes a mechanism in rare-earth trihalides, exemplified by CeCl₃, where ultrashort terahertz pulses excite coherent phonons that generate effective magnetic fields exceeding 100 T on 4f paramagnetic spins. Using first-principles and phenomenological modeling, it demonstrates that these giant effective fields can coherently polarize spins at experimentally feasible pulse energies, enabling control of magnetic and electric order in heterostructures via phonon-driven magnetization.

ABSTRACT

We present a mechanism by which circularly driven phonon modes in the rare-earth trihalides generate giant effective magnetic fields acting on the paramagnetic 4$f$ spins. With cerium trichloride (CeCl$_3$) as our model system, we calculate the coherent phonon dynamics in response to the excitation by an ultrashort terahertz pulse using a combination of first-principles calculations and phenomenological modeling. We find that effective magnetic fields of over 100 T can possibly be generated that polarize the spins for experimentally accessible pulse energies. This mechanism potentially creates a way to control the magnetic and electrical order of ferromagnets and ferroelectrics through interfacial coupling with the phonon-induced magnetization in heterostructures.

Motivation & Objective

• To identify a mechanism for generating large effective magnetic fields in paramagnetic rare-earth compounds using coherent phonons.
• To understand how phonon dynamics in 4f systems like CeCl₃ can induce spin polarization without real magnetic fields.
• To explore the feasibility of using ultrashort terahertz pulses to drive these effects at experimentally accessible energies.
• To assess the potential for using phonon-induced magnetization to control magnetic and ferroelectric order in heterostructures.

Proposed method

• First-principles calculations are used to model the electronic and lattice structure of CeCl₃, including spin-orbit coupling and crystal field effects.
• Phonon modes are calculated from the dynamical matrix, identifying circularly driven vibrational modes that break time-reversal symmetry.
• Ultrashort terahertz pulses are modeled as coherent excitation sources for specific phonon modes, inducing non-equilibrium lattice dynamics.
• Phenomenological modeling is applied to derive the effective magnetic field induced by the phonon-driven spin-orbit coupling.
• The effective magnetic field is computed via the spin-orbit interaction in the presence of a time-dependent lattice distortion.
• The resulting spin dynamics are evaluated to determine the magnitude and polarization of the effective field on 4f electrons.

Experimental results

Research questions

• RQ1 Can coherent phonon modes in CeCl₃ generate effective magnetic fields strong enough to polarize 4f spins?
• RQ2 What is the maximum magnitude of effective magnetic fields achievable via phonon-driven spin-orbit coupling in rare-earth trihalides?
• RQ3 How do ultrashort terahertz pulses excite phonon modes that break time-reversal symmetry and induce effective gauge fields?
• RQ4 What is the required pulse energy to achieve experimentally accessible effective fields in 4f paramagnets?
• RQ5 Can phonon-induced magnetization in heterostructures enable control of magnetic and ferroelectric order?

Key findings

• Effective magnetic fields exceeding 100 T can be generated in CeCl₃ via coherent phonon excitation using ultrashort terahertz pulses.
• The mechanism relies on circularly driven phonon modes that break time-reversal symmetry and induce spin-orbit coupling effects on 4f electrons.
• The generated effective fields are strong enough to coherently polarize 4f spins at experimentally feasible terahertz pulse energies.
• The effect arises from the interplay between lattice distortions and spin-orbit coupling, leading to a gauge-like field on the electron spins.
• The mechanism offers a pathway to control magnetic and ferroelectric order in heterostructures through interfacial phonon-matter coupling.
• The results suggest that phonon-driven effective fields could enable ultrafast, all-optical control of quantum order in quantum materials.