Highlighted paper: Spin-phonon and magnetostriction phenomena in CaMn7O12 helimagnet probed by Raman spectroscopy

Investigation with participation of researchers and students of the Physics Graduate Program (Prof. Alejandro P. Ayala e Bruno Sousa) in collaboration with UFMA and Universidad de La Coruna contributes to the understanding of the mechanisms involved on magnetically-induced ferroelectricity.

The scientific paper with the participation of members of the Brazilian material research community highlighted this month is: Spin-phonon and magnetostriction phenomena in CaMn7O12 helimagnet probed by Raman spectroscopy. Nonato, A.; Araujo, B.S.; Ayala, AP; Maciel, AP; Yanez-Vilar, S.; Sanchez-Andujar, M.; Senaris-Rodriguez, MA; Paschoal, CWA. Applied Physics Letters 105, 222902 (2014); DOI: 10.1063/1.4902234.

By means of a study based mainly on Raman spectroscopy technique, researchers from Brazil, in collaboration with scientists from Spain, advanced the understanding of the mechanisms involved in the generation of magnetically induced ferroelectricity (electrical polarization that occurs in some materials with magnetic spiral ordering, even when not under the action of electric fields) in the compound CMO. CMO, whose formula is CaMn7O12, is a ceramic oxide of perovskite structure, which presents both, low temperature ferroelectricity and antiferromagnetism.

Besides to contribute to the advances of fundamental research, the paper, which was recently published in the journal Applied Physics Letters (APL), opens the possibilities for the creation of new materials whose polarization can be controlled by means of magnetic fields. Such materials could be applied, for example, to new, faster, and energy-efficient spintronic devices for data storage.

The study was developed by the PhD student Ariel Nonato Almeida de Abreu Silva, supervised by Carlos William de Araujo Paschoal, professor of the physics department of Federal University of Maranhão (UFMA), where he leads a research group on dielectric and vibrational properties. “The idea arose from the search for multiferroic and magnetoelectric materials that allow to control the electric polarization through substitutions,” says Professor Paschoal, who signs the article along with seven other researchers. According to him, the CMO was chosen because it presents a rich phase diagram (magnetic, structural and orbital ordering) and because it is unique in the mechanisms that generate ferroelectricity from its magnetic properties.

Among the peculiarities of CMO, there is a magnetic transition, occurring at 90 K (about -180 ° C), in which the compound passes from the paramagnetic phase to the antiferromagnetic, inducing a giant ferroelectricity.

In the investigation, Ariel and his supervisor analyzed in detail the Raman spectra of CMO samples at various temperatures (from 300 K, about 26 ° C to 10 K, about -263 ° C) to investigate the Collective vibrations of the atoms of the crystalline lattice (phonons) and their relation to the magnetic order. Among other results, they were able to prove that, at 90 K, the phonons exhibited an unusual behavior due to the coupling with the magnetic order.

“The main contribution of this work was to help in the understanding about how phonons couple with magnetic ordering in CaMn7O12 (CMO). This is undoubtedly a great step that allows us to advance in the understanding of the origin of the induced electrical polarization in the CMO, which still is reason of great discussion in the literature “, says Paschoal.

The experimental work of this study began with the synthesis of the samples, which was carried out at the University of  Coruña (Spain), where Ariel was performing a “sandwich period” under the supervision of Professor Maria Antonia Señaris Rodriguez. Subsequently, at the University of Santiago de Compostela, a series of magnetic measurements were carried out. Finally, the measurements of Raman spectroscopy were carried out in the Laboratory of Light Scattering of the Federal University of Ceará (UFC), in collaboration with Professor Alejandro Pedro Ayala, and in the Physics Department of UFMA, in the Laboratory of Vibrational Spectroscopy and Impedance (LEVI).

The research was funded by Brazilian federal agencies (CNPq and CAPES) and state agencies (FUNCAP and FAPEMA, from Ceará and Maranhão, respectively) and entities in Europe.


Source: SBPMAT (http://sbpmat.org.br/artigo-em-destaque-fonos-acoplados-a-ordem-magnetica-na-origem-da-ferroeletricidade/)