The rich physics of A-site-ordered quadruple perovskite manganites AMn₇O₁₂

Perovskite-structure AMnO3 manganites played an important role in the development of numerous physical concepts such as double exchange, small polarons, electron–phonon coupling, and Jahn–Teller effects, and they host a variety of important properties such as colossal magnetoresistance and spin-induced ferroelectric polarization (multiferroicity). A-site-ordered quadruple perovskite manganites AMn7O12 were discovered shortly after, but at that time their exploration was quite limited. Significant progress in their understanding has been reached in recent years after the wider use of high-pressure synthesis techniques needed to prepare such materials. Here we review this progress, and show that the AMn7O12 compounds host rich physics beyond the canonical AMnO3 materials

Competing electronic instabilities in the quadruple perovskite manganite PbMn₇O₁₂

Structural behavior of PbMn_{7}O_{12} has been studied by high resolution synchrotron x-ray powder diffraction. This material belongs to a family of quadruple perovskite manganites that exhibit an incommensurate structural modulation associated with an orbital density wave. It has been found that the structural modulation in PbMn_{7}O_{12} onsets at 294 K with the incommensurate propagation vector ks=(0,0,∼2.08). At 110 K another structural transition takes place where the propagation vector suddenly drops down to a quasicommensurate value ks=(0,0,2.0060(6)). The quasicommensurate phase is stable in the temperature range of 40-110 K, and below 40 K the propagation vector jumps back to the incommensurate value ks=(0,0,∼2.06). Both low temperature structural transitions are strongly first order with large thermal hysteresis. The orbital density wave in the quasicommensurate phase has been found to be substantially suppressed in comparison with the incommensurate phases, which naturally explains unusual magnetic behavior recently reported for this perovskite. Analysis of the refined structural parameters revealed that that the presence of the quasicommensurate phase is likely to be associated with a competition between the Pb^{2+} lone electron pair and Mn^{3+} Jahn-Teller instabilities

BiMn7 O12: Polar antiferromagnetism by inverse exchange striction

Despite extensive research on magnetically induced ferroelectricity there exist relatively few studies on how a preexisting electric polarization affects magnetic order. Given that well-established magnetoelectric coupling schemes can in principle work in reverse, one might anticipate that primary, polar magnetic structures could be uniquely stabilized in ferroelectric crystals, however, this scenario is apparently rare. Here, we show that in ferroelectric BiMn7O12, a pure, polar E-type antiferromagnetic order emerges below T1=59 K, and we present a phenomenological model of trilinear magnetoelectric coupling consistent with Bi3+ lone-pair driven polar distortions uniquely stabilizing the polar antiferromagnetism via modulation of Heisenberg exchange pathways, i.e., inverse exchange striction. In addition, below T2=55 K there occurs large commensurate canting of the E-type structure due to the onset of ferrimagnetic order on a separate crystallographic sublattice that may be exploited for additional magnetoelectric functionality

A plethora of structural transitions, distortions and modulations in Cu-doped BiMn7O12 quadruple perovskites

The presence of strongly competing electronic instabilities in a crystalline material can produce fascinating structural phenomena. For example, the A-site-ordered quadruple perovskite BiMn7O12 hosts both active polar instabilities of the Bi3+ lone pair electrons and Jahn–Teller instabilities of Mn3+ cations that drive the following sequence of phase transformations on cooling, Im-3 > I2/m > Im > P1, corresponding to orbital ordering and polar distortions. Carrier doping by Cu2+ tunes the two instabilities in BiCuxMn7−xO12 solid solutions and significantly complicates the system behavior. The x = 0.05 and 0.1 members show the following sequence of phase transformations on cooling, Im-3 > I2/m > R-1(αβγ)0 > R3(00γ)t, and are examples of materials with the electric dipole helicoidal texture in the ground state and a dipole density wave structure in the intermediate R-1(αβγ)0 phase (Science 2020, 369, 680–684). Here, the detailed behavior of the BiCuxMn7−xO12 solid solutions with x = 0.2–0.8 was investigated by laboratory X-ray, synchrotron X-ray, and neutron powder diffraction between 5 K and 620 K, and differential scanning calorimetry measurements. Nearly every composition (with a step Δx = 0.1) has a unique behavior when considering both the sequence of phase transitions and the presence of incommensurate superstructure reflections. The sequence Im-3 > HT-Immm(t)* > Immm* > LT-Immm(t)* is realized for x = 0.2 and 0.3 (where t denotes pseudo-tetragonal), Im-3 > I2/m* > Immm(t)* – for x = 0.4, Im-3 > I2/m* > I2/m* – for x = 0.5, Im-3 > I2/m* > Im-3 – for x = 0.6 and 0.7, and Im-3 > R-3 > I2/m > Im-3 – for x = 0.8, where asterisks denote the presence of additional incommensurate reflections. Re-entrance of the high-temperature cubic phase was observed at low temperatures for x = 0.6–0.8 suggesting strong competition between the different electronic instabilities. The re-entrant cubic phases have nearly zero thermal expansion

Incommensurate spin-density-wave antiferromagnetism in CeRu2Al2B

The newly discovered Ising-type ferromagnet CeRu2Al2B exhibits an additional phase transition at TN=14.2 K before entering the ferromagnetic ground state at TC=12.8 K. We clarify the nature of this transition through high resolution neutron diffraction measurements. The data reveal the presence of a longitudinal incommensurate spin-density wave (SDW) in the temperature range of TC<T<TN. The propagation vector q∼(0,0,0.148) is nearly temperature independent in this region and discontinuously locks into q=0 at TC. Mean-field calculations of an effective Ising model indicate that the modulated SDW phase is stabilized by a strong competition between ferromagnetic and antiferromagnetic exchange interactions. This makes CeRu2Al2B a particularly attractive model system to study the global phase diagram of ferromagnetic heavy-fermion metals under the influence of magnetic frustration

Magnetic inhomogeneities in the quadruple perovskite manganite [Y_{2-x}Mn_{x}] Mn_{6}O_{12}

A combination of competing exchange interactions and substitutional disorder gives rise to magnetic inhomogeneities in the [Y_{2-x}Mn_{x}]Mn_{6}O-{12}x = 0.23 and 0.16 quadruple perovskite manganites. Our neutron powder scattering measurements show that both the x=0.23 and 0.16 samples separate into two distinct magnetic phases; below T_{1} = 120 ± 10 K the system undergoes a transition from a paramagnetic phase to a phase characterized by short-range antiferromagnetic clusters contained in a paramagnetic matrix, and below T2≈65 K the system is composed of well-correlated long-range collinear ferrimagnetic order, punctuated by short-range antiferromagnetic clusters. A sharp increase in the antiferromagnetic phase fraction is observed below ≈33 K, concomitant with a decrease in the ferrimagnetic phase fraction. Our results demonstrate that the theoretically proposed antiferromagnetic phase is stabilized in the [Y_{2-x}Mn_{x}] Mn_{6}O_{12} manganites in the presence of dominant B-B exchange interactions, as predicted