We have performed Raman scattering measurements on bismuth ferrite (BiFeO3) nanoparticles and studied both magnetic and lattice modes. We reveal strong anomalies between 140 K and 200 K at the frequency of magnon and E(LO1), E(TO1), and A1(LO1) phonon modes. These anomalies are related to a surface expansion and are enhanced for nanoparticle sizes approaching the spin cycloidal length. These observations point out the strong interplay between the surface, the lattice, and the magnetism for sizes of BiFeO3 nanoparticles close to cycloid periodicity.
Most multiferroic materials with coexisting ferroelectric and magnetic order exhibit cycloidal antiferromagnetism with wavelength of several nanometers. The prototypical example is bismuth ferrite (BiFeO3 or BFO), a room-temperature multiferroic considered for a number of technological applications. While most applications require small sizes such as nanoparticles, little is known about the state of these materials when their sizes are comparable to the cycloid wavelength. This work describes a microscopic theory of cycloidal magnetism in nanoparticles based on Hamiltonian calculations. It is demonstrated that magnetic anisotropy close to the surface has a huge impact on the multiferroic ground state. For certain nanoparticle sizes the modulus of the ferromagnetic and ferroelectric moments are bistable, an effect that may be used in the design of ideal memory bits that can be switched electrically and read out magnetically. arXiv:1812.08297v4 [cond-mat.mtrl-sci]
The magnetoelectric coupling, i.e., cross-correlation between electric and magnetic orders, is a very desirable property to combine functionalities of materials for next-generation switchable devices. Multiferroics with spin-driven ferroelectricity presents such a mutual interaction concomitant with magneto-and electro-active excitations called electromagnons. TbMnO 3 is a paradigmatic material in which two electromagnons have been observed in the cycloidal magnetic phase. However, their observation in TbMnO 3 is restricted to the cycloidal spin phase and magnetic ground states that can support the electromagnon excitation are still un-1 arXiv:1901.00919v1 [cond-mat.str-el] 3 Jan 2019 der debate. Here, we show by performing Raman spectroscopy measurements under pressure that the lower-energy electromagnon (4 meV) disappears when the ground state enters from a cycloidal phase to an antiferromagnetic phase (E-type). On the contrary, the magnetoelectric activity of the higher-energy electromagnon (8 meV) increases in intensity by one order of magnitude. Using microscopic model calculations, we demonstrate that the lowerenergy electromagnon, observed in the cycloidal phase, originates from a higher harmonic of the magnetic cycloid, and we determine that the symmetric exchange-striction mechanism is at the origin of the higher-energy electromagnon which survives even in the E-type phase. The colossal enhancement of the electromagnon activity in TbMnO 3 paves the way to use multiferroics more efficiently for generation, conversion and control of spin waves in magnonic devices.
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