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 ....

The non-magnetic and non-Fermi-liquid CaRuO3 is the iso-structural analog of the ferromagnetic (FM) and Fermi-liquid SrRuO3. We show that an FM order in the orthorhombic CaRuO3 can be established by the means of tensile epitaxial strain. The structural and magnetic property correlations in the CaRuO3 films formed on SrTiO3 (100) substrate establish a scaling relation between the FM moment and the tensile strain. The strain dependent crossover from non-magnetic to FM CaRuO3 was observed to be associated with switching of non-Fermi liquid to Fermi-liquid behavior. The intrinsic nature of this strain-induced FM order manifests in the Hall resistivity too; the anomalous Hall component realizes in FM tensile-strained CaRuO3 films on SrTiO3 (100) whereas the non-magnetic compressive-strained films on LaAlO3 (100) exhibit only the ordinary Hall effect. These observations of an elusive FM order are consistent with the theoretical predictions of scaling of the tensile epitaxial strain and the magnetic order in tensile CaRuO3. We further establish that the tensile strain is more efficient than the chemical route to induce FM order in CaRuO3.

The exchange-bias effects in the mosaic epitaxial bilayers of the itinerant ferromagnet (FM) SrRuO3 and the antiferromagnetic (AFM) charge-ordered La0.3Sr0.7FeO3 were investigated. An uncharacteristic low-field positive exchange bias, a cooling-field driven reversal of positive to negative exchange-bias and a layer thickness optimised unusual vertical magnetization shift were all novel facets of exchange bias realized for the first time in magnetic oxides. The successive magnetic training induces a transition from positive to negative exchange bias regime with changes in domain configurations. These observations are well corroborated by the hysteretic loop asymmetries which display the modifications in the AFM spin correlations. These exotic features emphasize the key role of i) mosaic disorder induced subtle interplay of competing AFM-superexchange and FM double exchange at the exchange biased interface and, ii) training induced irrecoverable alterations in the AFM spin structure.

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We have studied the origin of soft-mode hardening in SrTiO3 thin films using broadband terahertz time-domain spectroscopy. We measured the dielectric dispersions in the terahertz region of as-deposited, O2 annealed, and high-temperature annealed SrTiO3 thin films deposited on MgO and La0.3Sr0.7Al0.65Ta0.35O3 substrates. The results show that the ferroelectric soft mode softens dramatically by the high-temperature annealing. We also measure x-ray diffractions and atomic force microscope images and conclude that the hardening of the ferroelectric soft mode in the thin films is determined by the grain size of each crystalline domain which is enlarged by the high-temperature annealing due to remelting.