Magnetic transitions in CaMn7O12 : Raman observation of spin-phonon couplings

Abstract

The quadruple perovskite Calcium manganite (CaMn7O12) is a multiferroic material in which a giant magnetically-induced ferroelectric polarization has been reported, making it potentially very interesting for magnetoelectric applications. Here, we report the Raman spectroscopy study on this compound of both the phonon modes and the low energy excitations from 4 K to room temperature. A detailed study of the Raman active phonon excitations shows that three phonon modes evidence a spin-phonon coupling at TN2 50 K. In particular, we show that the mode at 432 cm −1 associated to Mn(B)O6 (B position of the perovskite) rotations around the [111] cubic diagonal is impacted by the magnetic transition at 50 K and its coupling to the new modulation of the Mn spin in the (a,b) plane. At low energies, two large low energy excitations are observed at 25 and 47 cm −1 . The first one disappears at 50 K and the second one at 90 K. We have associated these excitations to electro-magneto-active modes.

Signatures of magnetostriction and spin-phonon coupling in magnetoelectric hexagonal15R−BaMnO3

“…The symmetry assignment of the modes is done based on our polarization-dependent Raman spectra ( Figure S5 in supplemental material [27]), SYMMODES-Bilbao Crystallographic Server [32], and previous reports [26,33]. The spectrum at 80 K presents an additional number of modes as compared to the one collected at 300 K, implying the possible association with a change in the crystallographic and/or magnetic symmetry of BaMnO3 at low temperatures [34][35][36][37]. To get a better understanding of the phonons, we have collected Raman spectra systematically in the temperature range of 80-800 K and analysed their evolution by fitting with Lorentzian multifunction.…”

“…Emergence of new modes in Raman spectra is possible below magnetic transition due to various reasons such as concurrent magnetic and structural phase transition [54], spinexcitations including one-magnon or two-magnon Raman processes [35,55], or magnetostriction [36,37]. Since phonons and magnetic excitations exhibit contrasting behaviour under external magnetic fields, we have investigated magnetic-field-dependence of the Raman spectrum to shed light on the origin of the new modes.…”

Pressure-induced charge ordering transition in CaMn7O12

“…Structure and properties of CaMn 7 O 12 have been widely studied as a function of temperature [6,[11][12][13]. It undergoes a first-order phase transition in the temperature range 400-460 K as evidence by phase separation [14].…”

“…Such a phase transition is controlled by a charge ordering [5] concomitant with the symmetry equivalence of the Mn2 and Mn3 sites. Upon heating, the distortion of the Mn 3+ O 6 octahedra (Mn2 site) does not change in the mixed-phase region [14] around 440 K. At low temperatures CaMn 7 O 12 undergoes a series of phase transitions associated with the structural modulations that affect the Mn-O bond lengths [15], induce orbital ordering [2,3], and produce the onset of incommensurate magnetic ordering closely connected with the structural modulation [2,13,15]. On the other hand, the behavior of CaMn 7 O 12 upon compression has not been studied.…”