Ferroelectric phase transition and spontaneous electric polarization in CaMn₇O₁₂from first principles

Abstract

The phase transition mechanism and ferroelectric polarization of CaMn 7 O 12 are investigated by using the density functional theory. Our results show that the P3 space group should be the ground-state structure with R3 as the intermediate phase. It is the helicoidal magnetic order that induces the first transition to the R3 phase while the second transition to P3 structure results from soft modes under the constraint of the magnetic structure. The two phase transitions from R3 through R3 to P3 are second order and first order, respectively, which is consistent with experimental observations. Group theoretical analysis shows that the particular domain states and multi-domain structure of the P3 phase match with the observed coexistence of two magnetic modulations below T N2 =48 K. Our calculated electric polarization is also in good agreement with experiment. By analyzing the polarization contributions from both mode and atomic-decomposition viewpoints, we find that the Raman-type distortions give rise to a significant contribution to the total polarization. This unexpected result can be understood through the asymmetric change of the Born effective charges caused by the particular helicoidal magnetic order, which leads to an abnormal infrared character of the purely Raman-active modes.

Ferroelectricity driven by soft phonon and spin order in multiferroic BiMn3Cr4O12

“…It can be seen that all the oxygen atoms suffer from the most significant forces (122‐130 meV/Å). Such remarkable values are comparable with the spin‐induced Hellmann‐Feynman forces in CaMn 7 O 12 system and much higher than that in TbMnO 3 compound . For the Mn and Cr ions, we find moderate Hellmann‐Feynman forces with several tens of meV/Å.…”

“…For the Mn and Cr ions, we find moderate Hellmann‐Feynman forces with several tens of meV/Å. Similar to the case of CaMn 7 O 12 , the rotationally variant spin alignments of Mn and Cr sublattices will inevitably cause charge redistributions of their own and, consequently, affect the adjacent O ions, which therefore results in the noticeable Hellmann‐Feynman forces. The relative magnitudes of F x , F y , and F z components are determined by the particular configurations of atomic positions and the spin orientation.…”

“…According to the existing theoretical and experimental results, it seems that the nonrelativistic exchange striction mechanism is an indispensable part in the type‐II multiferroics possessing large spin‐induced polarization. In addition to HoMnO 3 , the large polarization in CaMn 7 O 12 can be ascribed to the SOC‐triggered exchange striction mechanism . (Some comments about the ferroelectricity in CaMn 7 O 12 : A recent report disputed the ferroelectric transition of CaMn 7 O 12 at T 1 ≈ 87 K and only found a tiny electric polarization below T 2 ≈ 46 K. Considering the impurity‐containing character of the CaMn 7 O 12 ceramic samples, we suspect that the absence of ferroelectricity at T 1 is due to the centrosymmetric configuration of B ‐site Mn 4+ (Mn3) spins, which causes the phase transition path following the sequence as the temperature decreases through T 1 and T 2 .…”

“…Although the PM insulating behavior can be described by the dynamical mean‐field theory (DMFT), this method has not been accomplished in the current VASP package. Instead, the rotationally invariant AFM or ferromagnetic (FM) configurations are often assumed in order to study the lattice dynamics of the high‐symmetric PM reference phase, which has been successfully used in type‐II multiferroics such as R Mn 2 O 5 ( R = Tb, Dy) and CaMn 7 O 12 . In the present work, we consider both the FM state and a particular AFM configuration (denoted as AFMr, of which all the Mn spins are up while all the Cr spins are down) for the paraelectric BMCO.…”

“…Instead, the rotationally invariant AFM or ferromagnetic (FM) configurations are often assumed in order to study the lattice dynamics of the high-symmetric PM reference phase, which has been successfully used in type-II multiferroics such as RMn 2 O 5 (R = Tb, Dy) 30,31 and CaMn 7 O 12 . 32 In the present work, we consider both the FM state and a particular AFM configuration (denoted as AFMr, of which all the Mn spins are up while all the Cr spins are down) for the paraelectric Im3 BMCO. For these two hypothetical magnetic configurations, the calculated structural parameters are listed in Table 1.…”