Understanding the Nature of Ultrafast Polarization Dynamics of Ferroelectric Memory in the Multiferroic BiFeO₃

Abstract

The vast majority of data-storage devices are based on ferroelectric or magnetic materials. [1,2] The magnetic-memory effect is derived from the spin order, while the ferroelectric memories are based on the spontaneous polarization of electric dipoles. In the quest for superior efficiencies, the operating speed of the devices based on these memories has become a major focus of research. The time needed for the memory parameter to respond to ultrafast electric, magnetic, or optical stimuli is vital to determine its operating speed. [3][4][5] The ultrafast dynamics of spin (magnetic) memory over a picosecond time scale upon femtosecond excitation is well established. [3][4][5][6][7][8][9][10][11] Upon femtosecond excitation, the spin reorientation or the ultrafast demagnetization in ferromagnetic and antiferromagnetic materials occurs in the time scale of 1-2 ps, which suggests that when subjected to ultrafast stimuli, the magnetic memory can be manipulated at an exceptionally high speeds. However, the analogous situation of the ultrafast functionality of spontaneous polarization in ferroelectric/multiferroic materials remains unknown.The use of terahertz (THz) radiation-based spectroscopic techniques [12][13][14][15][16][17][18] has uncovered phenomena in a wide range of materials, such as semiconductors, [15,16] strongly correlated electron systems, [11][12][13][14][15] magnetic films, [9][10][11] and biological molecules.[19] The emission of THz radiation provides a direct measure of transient change in electric/magnetic fields over a picosecond time scale. [9][10][11][12][13][14][15][16][17] In any medium, THz emission according to the Maxwell wave equation is governed bywhere E, eô, m, and t are the electric field, electric susceptibility of free space, magnetic permeability, and time, respectively. In this relationship, THz emission can occur through a time-varying current density (J) and/or polarization (P). The polarization can be decomposed into P ¼ P S þ P NL , which indicates that THz emission can be due to time-varying spontaneous polarization P S (such as partial/complete depolarization) and/or optical rectification in non-linear medium (P NL ). [12,13,16,17] While the latter is a well-established source of THz emission, [13,16] the effect of P S is not known. Terahertz emission due to P S is of great importance, as it is a direct means of probing the ultrafast polarization dynamics of ferroelectric memories. [20] It should also provide a better understanding of the time scale at which the electric dipoles in the spontaneous polarized state are accessible for read or write operations. This time scale is crucial in determining the operating speed of ferroelectric memory devices, thus paving the way to improve their efficiency and ultimately resulting in their widespread adoption in various technological applications. To realize this, it is essential to utilize a ferroelectric system with a large P S . A choice popular with many researchers is the room-temperature ferroelectric antiferromagnet, BiFeO 3 ....

Structural dependent ultrafast electron-phonon coupling in multiferroic BiFeO3 films

“…9 From the applied point of view in photonics, it has been proposed that terahertz radiation from BFO films can be used in ferroelectric domain imaging and optical readout of the state of ferroelectric memories. 10,11 In addition, due to high optical transparency and large third order optical nonlinearities, BFO films are promising for applications in ultrafast photonic devices. 12,13 No matter the structural dependent emission of THz radiation or the nonlinear photonic devices, a detailed understanding of the interaction of electron and lattice over a picosecond time scale is an indispensable issue, 14 which is decisive for determining the functional properties of materials.…”

Strain modulated transient photostriction in La and Nb codoped multiferroic BiFeO3 thin films

“…12, 13 Rana et al reported that the ultrafast depolarization of ferroelectric order causes the structural dependent terahertz radiation in BFO. 16 Talbayev et al used the terahertz spectroscopy to interpret the spectrum of long-wavelength magnetic resonance modes of BFO crystal. 17 Coupling of light with mechanical degrees of freedom in BFO was reported by Kundys et al, the light-induced wavelength dependent size change of BFO single crystal can be understood from photostriction effect, a superposition of photovoltaic effect and electrostrictive effect.…”

Polarization imaging of imperfect m-plane GaN surfaces

“…In previous studies, we have revealed that the THz radiation from ferroelectric materials reflected the direction and amount of spontaneous polarization. 16 Photoexcited carriers accelerated by the electric field induced by spontaneous polarization generate THz radiation polarized in the same direction as the spontaneous polarization.…”