Anisotropic Thermal Expansion and a Second-Order Charge Order Transition in the Ferrimagnetic Dy₂CuZnMn₄O₁₂ Perovskite with Triple A‑Site Cation Ordering

Dy2CuZnMn4O12 perovskite, belonging to the A-site columnar-ordered quadruple perovskite family with the general composition of A2A′A″B4O12, was prepared by a high-pressure, high-temperature method at 6 GPa and 1500 K. Its crystal structure was studied by synchrotron powder X-ray diffraction between 100 and 800 K. The ideal cation distribution (without antisite disorder) was found to be realized within the sensitivity of the synchrotron X-ray diffraction method. Between 100 and 400 K, it crystallizes in space group Pmmn (no. 59) and has layered charge ordering of Mn3+ and Mn4+ at the B sites. Above 425 K, it crystallizes in space group P42/nmc (no. 137) with one crystallographic B site and an average Mn3.5+ oxidation state. The charge ordering transition (at TCO = 425 K) appears to be of the second order as no anomalies were found on differential scanning calorimetry curves and temperature dependence of the unit cell volume, and the orthorhombic a and b lattice parameters merge gradually. The compound demonstrates anisotropic thermal expansion with the c lattice parameter decreasing with increasing temperature above 280 K. A ferrimagnetic transition occurs at TC = 116 K with an additional, gradual rise of magnetic susceptibilities below 45 K, probably due to increases of the ordered moments of the Dy sublattices

Origin of Magnetization Reversal and Exchange Bias Phenomena in Solid Solutions of BiFeO₃–BiMnO₃: Intrinsic or Extrinsic?

Magnetic properties of BiFe_0.7Mn_0.3O₃ (with a Néel temperature (<i>T</i>_N) of 425 K) and BiFe_0.6Mn_0.4O₃ (with <i>T</i>_N = 350 K) were investigated by magnetic measurements between 5 and 400 K. They crystallize in space group <i>Pnma</i> with the √2<i>a</i>_p × 4<i>a</i>_p × 2√2<i>a</i>_p superstructure (<i>a</i>_p is the parameter of the cubic perovskite subcell) with <i>a</i> = 5.57800(9) Å, <i>b</i> = 15.7038(3) Å, and <i>c</i> = 11.22113(16) Å for BiFe_0.6Mn_0.4O₃. Both compounds show magnetization reversal or negative magnetization phenomena. However, it was found that the magnetization reversal is dependent on magnetic prehistory of a sample and measurement protocols. No magnetization reversal was observed when virgin samples were measured below <i>T</i>_N. Magnetization reversal effects appeared only when the samples were cooled in small magnetic fields from temperatures above <i>T</i>_N or after the samples were magnetized. The exchange bias effect or a shift of isothermal magnetization curves, depending on the measurement conditions, was also observed. The exchange bias changes its sign as a function of temperature and cooling conditions. Our findings allowed us to propose the extrinsic origin (related to sample inhomogeneities) of the magnetization reversal effect in these two compounds

Solid Solutions between BiMnO₃ and BiCrO₃

Solid solutions BiMn_1–<i>x</i>Cr_<i>x</i>O₃ (0 ≤ <i>x</i> ≤ 1) have been prepared at 6 GPa and 1370–1620 K. Their structural properties have been studied with synchrotron X-ray powder diffraction, and their physical properties have been investigated by dc/ac magnetic, specific heat, dielectric, and differential scanning calorimetry measurements. A magnetic phase diagram of BiMn_1–<i>x</i>Cr_<i>x</i>O₃ is established. A phase with orbital ordering observed in BiMnO₃ is suppressed at <i>x</i> > 0.1, accompanied by a drop in the ferromagnetic Curie temperature <i>T</i>_C from 101 K for <i>x</i> = 0 to 76 K for <i>x</i> = 0.15 and sharp changes in the lattice parameters. The <i>T</i>_C value monotonically decreases up to <i>x</i> = 0.3 (with <i>T</i>_C = 53 K). For intermediate compositions with <i>x</i> = 0.4, 0.5, spin-glass magnetic properties are found at 28 and 24 K, respectively. The Néel temperature <i>T</i>_N linearly increases from 36 K for <i>x</i> = 0.6 to 111 K for <i>x</i> = 1.0. A spin-reorientation transition is observed at 61 K for <i>x</i> = 0.9 and 72 K for <i>x</i> = 1.0. Re-entrant spin-glass transitions are also observed for samples with <i>x</i> = 0.3, 0.6, 0.7 by ac susceptibility at low temperatures. At high temperatures, a structural phase transition from <i>C</i>2/<i>c</i> to <i>Pnma</i> symmetry is observed for all compositions with a monotonic change of the phase transition temperature. The magnetic phase diagram from the BiMnO₃-rich side (<i>x</i> ≤ 0.5) resembles a phase diagram of BiMn_1–<i>x</i>Sc_<i>x</i>O₃ solid solutions, indicating that the nature of substituting cations (magnetic or nonmagnetic) is not crucial for doped BiMnO₃

Aurivillius Phase Bi₄V₃O₁₂ with d¹ Magnetic Cations, Anisotropic and Negative Thermal Expansion, Multiple Structural Transitions, and Low-Dimensional Magnetism

Aurivillius phases are an important class of inorganic compounds as they often show ferroelectric properties, and some members of this family are used in nonvolatile ferroelectric memories. The majority of Aurivillius phases have nonmagnetic d0 cations in the perovskite block. Bi4Ti3O12 is the best-known and extensively studied compound within this family. Here, using a high-pressure, high-temperature synthesis method, we could successfully prepare a full magnetic analogue, Bi4V3O12, with d1 cations. Bi4V3O12 is unstable in air above about 520 K. However, in an inert atmosphere, Bi4V3O12 demonstrates two first-order reversible structural transitions near 525 and 760 K. The high-temperature prototypical phase is the same in both Bi4V3O12 and Bi4Ti3O12 with tetragonal (T) I4/mmm symmetry and aT = 3.85608(5) Å and cT = 32.6920(8) Å (at 850 K) for Bi4V3O12, while the low-temperature phases are different. Bi4V3O12 shows anisotropic thermal expansion above 300 K and negative volumetric thermal expansion above about 700 K. Magnetic measurements showed a broad maximum near 70 K on magnetic susceptibility, indicating the presence of low-dimensional magnetism with strong antiferromagnetic interactions between V4+ ions with the Curie–Weiss temperature of about −370 K. But no long-range magnetic ordering was found in Bi4V3O12 down to 2 K

Fresh Look at the Mystery of Magnetization Reversal in YVO₃

Phase transitions and detailed magnetic properties of polycrystalline AP-YVO_3.00(1) (prepared at ambient pressure by a conventional solid-state method) and polycrystalline HP-YVO_3.04(1) and HP-YVO_3.05(1) (AP-YVO₃ treated at 6 GPa and 1600 K during 130 and 15 min, respectively) were investigated. The three samples showed a remarkable exchange bias (EB) effect. HP-YVO_3.04 and HP-YVO_3.05 had similar chemical composition, crystallographic parameters, and particle size, but their magnetic properties were qualitatively different. EB was negative at all temperatures in AP-YVO₃ and HP-YVO_3.05, resulting in the absence of magnetization reversal (MR). Positive EB was observed in HP-YVO_3.04 between <i>T</i>_N2 = 71 K and <i>T</i>* = 88 K resulting in MR or negative magnetization between those temperatures. It was demonstrated that polycrystalline HP-YVO_3.04 behaved similar to single crystals of YVO_3+δ. By the careful control of the trapped magnetic field, measurement conditions were found under which no MR occurred in HP-YVO_3.04 at moderate magnetic fields, indicating that MR is not an intrinsic property of YVO_3+δ. A drastic effect of trapped magnetic fields on MR and memory effects were observed. The importance of an “insignificant” anomaly at <i>T</i>_FM = 140 K for MR was suggested. We also suggested that “positive exchange bias”, “defects”, “interfaces”, and “pinning” should be keywords for understanding YVO₃ and probably other perovskite materials with the MR effect

Effects of Oxygen Content on Bi₃Mn₃O_11+δ: From 45 K Antiferromagnetism to Room-Temperature True Ferromagnetism

The effects of oxygen content on the structural, physical, and chemical properties of Bi3Mn3O11 with KSbO3-type structure have been investigated. It was found that the oxygen content in Bi3Mn3O11+δ can vary over a wide δ range, keeping the same cubic structure (space group Pn3̅, a = 9.12172(5) Å for δ = −0.5, a = 9.13784(8) Å for δ = 0, and a = 9.09863(7) Å for δ = 0.6) and semiconducting properties of the material. At the same time, magnetic properties change from true antiferromagnetic with TN = 45 K for δ = −0.5 to true ferromagnetic with TC = 307 K for δ = +0.6. Bi3Mn3O11 (δ = 0) shows ferrimagnetic-like properties with TC = 150 K and features typical for a re-entrant spin-glass below 30 K. Noticeable changes of the magnetic transition temperature and magnetism in Bi3Mn3O11+δ with δ can be compared with changes of the magnetic and electronic properties of LaMnO3+δ, BiMnO3+δ, high-temperature copper superconductors (e.g., YBa2Cu3O7+δ), and other cuprates. Bi3Mn3O11.6 shows a new record high TC among insulating/semiconducting true ferromagnets. Our results demonstrate that the oxygen content can vary for the same cation composition in KSbO3-type materials, and the oxygen content can be increased up to BiMnO3.867 (Bi3Mn3O11.6)

Effects of Oxygen Content on Bi₃Mn₃O_11+δ: From 45 K Antiferromagnetism to Room-Temperature True Ferromagnetism

The effects of oxygen content on the structural, physical, and chemical properties of Bi3Mn3O11 with KSbO3-type structure have been investigated. It was found that the oxygen content in Bi3Mn3O11+δ can vary over a wide δ range, keeping the same cubic structure (space group Pn3̅, a = 9.12172(5) Å for δ = −0.5, a = 9.13784(8) Å for δ = 0, and a = 9.09863(7) Å for δ = 0.6) and semiconducting properties of the material. At the same time, magnetic properties change from true antiferromagnetic with TN = 45 K for δ = −0.5 to true ferromagnetic with TC = 307 K for δ = +0.6. Bi3Mn3O11 (δ = 0) shows ferrimagnetic-like properties with TC = 150 K and features typical for a re-entrant spin-glass below 30 K. Noticeable changes of the magnetic transition temperature and magnetism in Bi3Mn3O11+δ with δ can be compared with changes of the magnetic and electronic properties of LaMnO3+δ, BiMnO3+δ, high-temperature copper superconductors (e.g., YBa2Cu3O7+δ), and other cuprates. Bi3Mn3O11.6 shows a new record high TC among insulating/semiconducting true ferromagnets. Our results demonstrate that the oxygen content can vary for the same cation composition in KSbO3-type materials, and the oxygen content can be increased up to BiMnO3.867 (Bi3Mn3O11.6)

Effects of Isovalent Substitution in the Manganese Sublattice on Magnetic, Thermal, and Structural Properties of BiMnO₃: BiMn_1-<i>_x</i>M<i>_x</i>O₃ (M = Al, Sc, Cr, Fe, Ga; 0 ≤ <i>x</i> ≤ 0.2)

Solid solutions BiMn1-xMxO3 with M = Al, Sc, Cr, Fe, and Ga and 0 ≤ x ≤ 0.2 were prepared at a high pressure of 6 GPa and 1333−1453 K, and their magnetic, thermal, and structural properties were investigated. The orbital-ordered monoclinic phase of BiMnO3 (phase I) is destroyed by a small percentage of substitution. The M elements can be classified by their ability to destroy phase I in the sequence Ga (x ≈ 0.08) ≈ Fe (x ≈ 0.08) x ≈ 0.04) ≈ Al (x ≈ 0.04) x ≈ 0.02), where phase I is most stable for Ga substitution (up to x ≈ 0.08) and less stable for Sc substitution (up to x ≈ 0.02). The orbital-disordered high-temperature monoclinic phase of BiMnO3 (phase II) is stabilized with larger x. In all cases, a compositional range was found where phases I and II coexist at room temperature. In phase I, the effect of substitution on the ferromagnetic transition temperature is weak (e.g., TC = 102 K for BiMnO3 and TC = 99 K for BiMn0.95Ga0.05O3), but there is a drastic effect on the orbital ordering temperature (e.g., TOO = 474 K for BiMnO3 and TOO = 412 K for BiMn0.95Ga0.05O3). Magnetic susceptibilities of phase I are typical for ferromagnets while, in phase II, ferromagnetic cluster-glass-like behavior is observed. The magnetic transition temperature of phase II (e.g., TC = 70 K for BiMn0.8Ga0.2O3) exhibits a sudden drop compared with that of phase I. The effect of substitution on the structural monoclinic-to-orthorhombic transition is different depending on M (e.g., Tstr = 768 K for BiMnO3, Tstr = 800 K for BiMn0.95Ga0.05O3, and Tstr = 738 K for BiMn0.85Cr0.15O3)

Effects of Isovalent Substitution in the Manganese Sublattice on Magnetic, Thermal, and Structural Properties of BiMnO₃: BiMn_1-<i>_x</i>M<i>_x</i>O₃ (M = Al, Sc, Cr, Fe, Ga; 0 ≤ <i>x</i> ≤ 0.2)

Solid solutions BiMn1-xMxO3 with M = Al, Sc, Cr, Fe, and Ga and 0 ≤ x ≤ 0.2 were prepared at a high pressure of 6 GPa and 1333−1453 K, and their magnetic, thermal, and structural properties were investigated. The orbital-ordered monoclinic phase of BiMnO3 (phase I) is destroyed by a small percentage of substitution. The M elements can be classified by their ability to destroy phase I in the sequence Ga (x ≈ 0.08) ≈ Fe (x ≈ 0.08) x ≈ 0.04) ≈ Al (x ≈ 0.04) x ≈ 0.02), where phase I is most stable for Ga substitution (up to x ≈ 0.08) and less stable for Sc substitution (up to x ≈ 0.02). The orbital-disordered high-temperature monoclinic phase of BiMnO3 (phase II) is stabilized with larger x. In all cases, a compositional range was found where phases I and II coexist at room temperature. In phase I, the effect of substitution on the ferromagnetic transition temperature is weak (e.g., TC = 102 K for BiMnO3 and TC = 99 K for BiMn0.95Ga0.05O3), but there is a drastic effect on the orbital ordering temperature (e.g., TOO = 474 K for BiMnO3 and TOO = 412 K for BiMn0.95Ga0.05O3). Magnetic susceptibilities of phase I are typical for ferromagnets while, in phase II, ferromagnetic cluster-glass-like behavior is observed. The magnetic transition temperature of phase II (e.g., TC = 70 K for BiMn0.8Ga0.2O3) exhibits a sudden drop compared with that of phase I. The effect of substitution on the structural monoclinic-to-orthorhombic transition is different depending on M (e.g., Tstr = 768 K for BiMnO3, Tstr = 800 K for BiMn0.95Ga0.05O3, and Tstr = 738 K for BiMn0.85Cr0.15O3)

Effects of Oxygen Content on Bi₃Mn₃O_11+δ: From 45 K Antiferromagnetism to Room-Temperature True Ferromagnetism

The effects of oxygen content on the structural, physical, and chemical properties of Bi3Mn3O11 with KSbO3-type structure have been investigated. It was found that the oxygen content in Bi3Mn3O11+δ can vary over a wide δ range, keeping the same cubic structure (space group Pn3̅, a = 9.12172(5) Å for δ = −0.5, a = 9.13784(8) Å for δ = 0, and a = 9.09863(7) Å for δ = 0.6) and semiconducting properties of the material. At the same time, magnetic properties change from true antiferromagnetic with TN = 45 K for δ = −0.5 to true ferromagnetic with TC = 307 K for δ = +0.6. Bi3Mn3O11 (δ = 0) shows ferrimagnetic-like properties with TC = 150 K and features typical for a re-entrant spin-glass below 30 K. Noticeable changes of the magnetic transition temperature and magnetism in Bi3Mn3O11+δ with δ can be compared with changes of the magnetic and electronic properties of LaMnO3+δ, BiMnO3+δ, high-temperature copper superconductors (e.g., YBa2Cu3O7+δ), and other cuprates. Bi3Mn3O11.6 shows a new record high TC among insulating/semiconducting true ferromagnets. Our results demonstrate that the oxygen content can vary for the same cation composition in KSbO3-type materials, and the oxygen content can be increased up to BiMnO3.867 (Bi3Mn3O11.6)