Spin-Driven Multiferroic Properties of PbMn7O12 ... - ACS Publications

May 27, 2016 - International Center for Materials Nanoarchitectonics (WPI-MANA), ... for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 305-0...
12 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/IC

Spin-Driven Multiferroic Properties of PbMn7O12 Perovskite Alexei A. Belik,*,† Yana S. Glazkova,†,‡ Noriki Terada,§ Yoshitaka Matsushita,§ Alexey V. Sobolev,‡ Igor A. Presniakov,‡ Naohito Tsujii,§ Shigeki Nimori,∥ Kanji Takehana,∥ and Yasutaka Imanaka∥ †

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ‡ Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory, 119992 Moscow, Russia § National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan ∥ Tsukuba Magnet Laboratory, National Institute for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan S Supporting Information *

ABSTRACT: We synthesize PbMn7O12 perovskite under high-pressure (6 GPa) and high-temperature (1373 K) conditions and investigate its structural, magnetic, dielectric, and ferroelectric properties. We find that PbMn7O12 exhibits rich physical properties from interplay among charge, orbital, and spin degrees of freedom and rich structural properties. PbMn7O12 crystallizes in space group R3̅ near room temperature and shows a structural phase transition at TCO = 397 K to a cubic structure in space group Im3;̅ the Im3̅-to-R3̅ transition is associated with charge ordering. Below TOO = 294 K, a structural modulation transition associated with orbital ordering takes place. There are two magnetic transitions with Néel temperatures of TN1 = 83 K and TN2 = 77 K and probably a lock-in transition at TN3 = 43 K (on cooling). There is huge hysteresis on specific heat (between ∼37 and 65 K at 0 Oe), dielectric constant (between ∼20 and 70 K at 0 Oe), and dc and ac magnetic susceptibilities around the lock-in transition. Sharp dielectric constant, dielectric loss, and pyroelectric current anomalies are observed at TN2, indicating that electric polarization is developed at this magnetic transition, and PbMn7O12 perovskite is a spin-driven multiferroic. Polarization of PbMn7O12 is measured to be ∼4 μC/m2. Field-induced transitions are detected at ∼63 and ∼170 kOe at 1.6−2 K; similar high-magnetic field properties are also found for CdMn7O12, CaMn7O12, and SrMn7O12. PbMn7O12 exhibits a quite small magnetodielectric effect, reaching approximately −1.3 to −1.7% at 10 K and 90 kOe. room temperature (RT),24,25 and electric polarization is developed as a result of a magnetic transition.8,26−28 It was reported that giant electric polarization is developed in CaMn7O12 below TN1 = 91 K, and there is another magnetic transition at TN2 = 48 K.8 On the other hand, a polar structure of BiMn7O12 (space group Im)23 at RT suggests that it could be a proper ferroelectric because of the lone electron pair of Bi3+ cations; dielectric anomalies were also observed at magnetic transition temperatures of BiMn7O12.22 Pb2+ is another popular cation with the lone electron pair to induce polar distortions. PbMn7O12 was recently synthesized19 and found to crystallize in space group R3̅ at RT similar to CaMn7O12, CdMn7O12, and SrMn7O12. One magnetic transition was found at 68 K in PbMn7O12 in contrast to CaMn7O12, CdMn7O12, and SrMn7O12, where two magnetic transitions take place,8,18 and no dielectric anomalies and ferroelectric properties were detected.19 Therefore, properties of PbMn7O12 need to be clarified further. In this work, we reinvestigate phase transitions and magnetic, dielectric, and ferroelectric properties of PbMn7O12. We find that PbMn7O12 exhibits rich properties from interplay among charge, orbital, and spin degrees of freedom. We find a structural modulation transition at TOO = 294 K (in addition to the already reported transition at TCO = 397 K),18,19 two magnetic

1. INTRODUCTION A-site ordered quadruple perovskite structure materials, (AA′3)B4O12, have received a great deal of attention for rich interplay among charge, orbital, and spin degrees of freedom.1−4 They show many interesting physical and chemical properties, for example, low-field magnetoresistance,5 heavy Fermion physics,6,7 multiferroic behavior,8,9 and high-performance catalytic activity.10 As the members of the perovskite family, they allow large variations in their chemical compositions,1 for example, A = Na+, Mn2+, Cd2+, Ca2+, Sr2+, Pb2+, R3+ (R = rare earths), or Bi3+; A′ = Cu2+, Mn3+, Co2+,11 or Pd2+;12 and B = Mn3+/4+, Fe3+, Cr3+, Al3+, Ti4+, V4+, Ge4+, Sn4+, Ru4+, Ir4+, Ta5+, Nb5+, Sb5+, etc. As one can see, this particular A-site ordered structure is realized because of the quite different nature and sizes of the A and A′ cations, with the A′ cations being (mostly 3d) transition metals, which can adopt square-planar coordination. When A′ = B = Mn, an interesting family of materials is formed, AMn7O12, which contains Mn4+ and Jahn−Teller Mn3+ cations in different proportions depending on the oxidation state of the A cations. Many members of this family were discovered in the 1970s by Marezio et al.13,14 AMn7O12 perovskites were reported with A = Na+,13−16 Mn2+,17 Cd2+,13,18 Ca2+,8,13 Sr2+,13,18 Pb2+,19 La3+,13,20 Pr3+,21 Nd3+,13 and Bi3+.22,23 Among them, multiferroic properties were found in CaMn7O128 and BiMn7O12;23 however, CaMn7O12 is the so-called spindriven multiferroic because it crystallizes in space group R3̅ at © XXXX American Chemical Society

Received: March 29, 2016

A

DOI: 10.1021/acs.inorgchem.6b00774 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

on cooling and heating in a frequency range of 100 Hz to 2 MHz (10 points on the logarithmic scale) and magnetic fields of 0−90 kOe starting from cooling curves (the sweep mode was used with a rate of 0.5−1 K/min; applied ac voltage of 0.1 V). Resistivity was extracted from impedance data; for this purpose, data were collected between 0.1 Hz and 2 MHz (40 points on the logarithmic scale) in the settle mode with a step of 5 K (applied ac voltage of 0.1 V; after the temperature was stable, we waited for 10 min before starting measurements). Pyroelectric current measurements were taken with a Keithley 6517B electrometer. Poling in electric fields of ±172 kV/m was performed from 100 to 10 K at 0 Oe (and at 172 kV/m from 100 to 2 K at 90 kOe); at 10 K (or 2 K), an electric field was removed, and electrodes were shortened. We monitored the background current, and measurements (with a heating rate of 7.5 K/min) were started after the background current was below 1 pA for more than 5 min (Figure S31). Temperature and magnetic fields were controlled by a PPMS. Pieces of pellets were used in all magnetic, specific heat, dielectric, and pyroelectric current measurements. The same pellets were used for dielectric and pyroelectric current measurements with the following dimensions: thickness of 1.15 mm and electrode area of 19.6 mm2 for sample 2 and thickness of 0.78 mm and electrode area of 12.0 mm2 for sample 4. Differential scanning calorimetry (DSC) curves of powder samples were recorded on a Mettler Toledo DSC1 STARe system at heating and cooling rates of 5−10 K/min between 130 and 423 K in sealed Al capsules. Three runs were performed to check the reproducibility, and very good reproducibility was observed.

transitions with Néel temperatures of TN2 = 77 K and TN1 = 83 K, and a lock-in transition near TN3 = 43 K with huge hysteresis in some properties. We find that electric polarization is developed below TN2 (and not below TN1) with the polarization value of 4 μC/m2, which is much smaller than polarization values reported in CaMn7O12. Two magnetic field-induced transitions are detected at 1.6 K. Our results indicate that PbMn7O12 perovskite is a spin-driven multiferroic and a very interesting compound.

2. EXPERIMENTAL DETAILS PbMn7O12 samples were prepared from stoichiometric mixtures of Mn2O3, “MnO1.839”, and PbO (99.999%), where “MnO1.839” is a commercial “MnO2” (Alfa Aesar, 99.997%) whose oxygen content was determined to be MnO1.839 (a mixture of Mn2O3 and MnO2). The mixtures were placed in Au capsules and treated at 6 GPa and 1373 K for 2 h (the duration of heating to the desired temperatures was 10 min) in a belt-type high-pressure apparatus. After the heat treatments, the samples were quenched to RT, and the pressure was slowly released. The samples were black dense pellets. Single-phase Mn2O3 was prepared from commercial “MnO2” (99.997%) by being heated in air at 923 K for 24 h. A number of samples were prepared; they are labeled 2, 3-1, 3-2, 4, and N. Even though we used the same nominal synthesis conditions, sample properties were slightly different as shown below. This fact indicates that tiny variations in the chemical composition of PbMn7O12 (caused by different amounts of impurities and variations in real synthesis conditions) have some effects on its properties, but fundamental properties of PbMn7O12 were the same for different samples. X-ray powder diffraction (XRPD) data were collected at RT on a RIGAKU MiniFlex600 diffractometer using Cu Kα radiation (2θ range of 10−80°, step width of 0.02°, and scan speed of 1 deg/min). PbMn7O12 samples contained small amounts of Mn2O3 and Pb3(CO3)2(OH)2 impurities [3.8 and 0.7 wt %, respectively, for sample 2 (Figure S1)]. Low-temperature (from 5 to 300 K) and hightemperate (from 298 to 590 K) XRPD data were measured on a RIGAKU SmartLab instrument using Cu Kα1 radiation (45 kV, 200 mA; 2θ range of 5−120°, step width of 0.01°, and scan speed of 4 deg/min) using a cryostat system and a furnace attachment. XRPD patterns were analyzed and lattice parameters obtained by the Rietveld method using RIETAN-2000.29 Lattice parameters of PbMn7O12 (sample 2) at RT were a = 10.52100(2) Å and c = 6.40946(1) Å (space group R3)̅ .30 Magnetic susceptibilities ( χ = M/H) were measured using SQUID magnetometers [Quantum Design, MPMS-7T(XL) and MPMS-1T] between 2 and 350 K (or 400 K) in different applied magnetic fields under both zero-field-cooled (ZFC) and field-cooled-on-cooling (FCC) conditions. In ZFC measurements, samples were rapidly inserted into magnetometers kept at 10 K, the temperature was set to 2 K, and finally a measurement magnetic field was applied. FCC curves were recorded after ZFC measurements starting from 350 K (or 400 K). In a few cases, field-cooled curves on warming (FCW) were measured after FCC measurements. Isothermal magnetization measurements were taken at 2 K using MPMS-7T; three branches were measured: (1) from 0 to 70 kOe, (2) from 70 to −70 kOe, and (3) from −70 to 70 kOe. High-field M versus H measurements were performed on the NIMS hybrid magnet at 1.6 K from 0 Oe to 251 kOe and from 251 kOe to 0 Oe. Frequency-dependent ac susceptibility measurements at different static magnetic fields (Hdc = 0, 0.01, 0.1, 1, and 10 kOe) were performed with a Quantum Design MPMS-1T instrument from 150 to 2 K and from 2 to 100 K at frequencies ( f) of 1.99, 110, 299.5, and 499 Hz and an applied oscillating magnetic field (Hac) of 5 Oe. The specific heat, Cp, was recorded between 2 and 300 K on cooling and heating at 0−90 kOe by a pulse relaxation method using a commercial calorimeter (Quantum Design PPMS) starting from cooling curves. Dielectric properties were measured using a NOVOCONTROL Alpha-A High Performance Frequency Analyzer between 5 and 300 K

3. RESULTS 3.1. DSC and XRPD Measurements of PbMn7O12. PbMn7O12 shows strong DSC anomalies near 397 K (on heating) and 392 K (on cooling) due to the R3̅ ↔ Im3̅ structural phase transition.18,19 This phase transition is associated with charge ordering (CO): (PbMn3+3) (Mn3+3Mn4+1)O12 ↔ (PbMn3+3) (Mn3.75+4)O12. Close examinations of the DSC curves reveal additional very weak anomalies near 294 K (Figure 1). These

Figure 1. Fragments of differential scanning calorimetry curves of PbMn7O12 (sample 2) on heating and cooling (10 K/min). All three runs are shown. The secondary axis gives the same enlarged curves (red and blue; the cooling curves are shifted along the y axis by −0.15 W/g) to emphasize anomalies near TOO = 294 K.

anomalies are detected in all PbMn7O12 samples (Figures S2 and S3). Similar anomalies were observed in CdMn7O12 and CaMn7O1218,30 and assigned to structural modulation transitions25,31 related to orbital ordering (OO).26 Laboratory XRPD patterns of PbMn7O12 show no additional reflections below 294 K (Figure S4); however, we can observe very small superstructure reflections by synchrotron XRPD (Figure S4).30 The superstructure reflections can be indexed with the same lattice parameters and space group P3̅ indicating a commensurate B

DOI: 10.1021/acs.inorgchem.6b00774 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

CdMn7O12, the cH parameter slightly increases with a decrease in temperature below TOO.30,32 No anomalies in the lattice parameters are observed at TN1 (or TN2) in PbMn7O12. 3.2. Magnetic Properties of PbMn7O12. Figure 3 gives specific heat data of PbMn7O12 at different magnetic fields.

structural modulation [in comparison with an incommensurate structural modulation found in CaMn7O12 and described by space group R3̅(00γ)0].25,26,30,31 Structural modulation reflections in CaMn7O12 could also be detected only by synchrotron XRPD or neutron powder diffraction.25,31 Therefore, the weak DSC anomalies observed in PbMn7O12 originate from a structural modulation transition with TOO = 294 K.30 The temperature dependence of the lattice parameters in PbMn7O12 is shown in Figure 2. To plot the lattice parameters

Figure 3. Specific heat data (Cp/T vs T) of PbMn7O12 (samples 3-1 and N) at (a) 0 Oe and (b) 70 kOe on cooling and heating. The curves of PbMn7O12 (sample N) are shifted by +0.3 J K−2 mol−1 for the sake of clarity. The inset in panel b gives the Cp/T vs T curves of PbMn7O12 (sample 3-1) at 0 and 70 kOe on cooling. The arrows show the Néel temperatures. See Figures S5−S12 for details.

Two magnetic transitions are detected with Néel temperatures of TN1 = 83 K and TN2 = 77 K; specific heat anomalies almost coincide on cooling and heating. We also detect a kinklike anomaly near TN3 = 43 K, and huge hysteresis is observed near this temperature between ∼37 and ∼65 K. The intensities of the anomalies at TN1 and TN2 and the hysteresis near TN3 are slightly sample-dependent, but the transition temperatures are almost the same. Such huge hysteresis is a fingerprint of a lockin transition, when components of incommensurate propagation vectors stop changing with temperature. Small hysteresis appears in magnetic fields near TN1 and TN2 (Figure 3b and Figures S5−S12). Magnetic susceptibilities of PbMn7O12 are given in Figures 4 and 5. At small magnetic fields (e.g., 100 Oe), the susceptibilities increase sharply at TN1, indicating the development of a weak ferromagnetic moment, and then drop sharply at TN2, indicating the suppression of weak ferromagnetism. The dχ/dT versus T curves clearly exhibit anomalies exactly at TN1 and TN2 (Figure S13). There are some broad anomalies near 70 K on both ZFC and FCC curves, and there are steplike anomalies at TN3 on the FCC curves. The FCC and FCW curves almost

Figure 2. Temperature dependence of the lattice parameters and volume (V for the R3̅ and P3̅ phases and 1.5V for the Im3̅ phase) in PbMn7O12 (sample 3-2). For the cubic phase, aH = aC√2 and cH = aC × 0.5√3, where aC is the lattice parameter of the Im3̅ phase.

of the R3̅ and Im3̅ phases on one figure, we transform the cubic lattice parameter, aC, to the trigonal ones (in the hexagonal setting), aH and cH, as aH = aC√2 and cH = aC × 0.5√3. The aH and cH parameters show significant changes at TCO, while the unit cell volume exhibits a small jump on heating. A small jump in the volume was also observed in SrMn7O12 at TCO.18 Below TOO, the cH parameter of PbMn7O12 is almost independent of temperature, while no anomalies are detected in the aH parameter or unit cell volume. In the case of CaMn7O12 and C

DOI: 10.1021/acs.inorgchem.6b00774 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 5. (a) ZFC, FCC, and FCW dc magnetic susceptibility curves of PbMn7O12 (sample 2) at 10 kOe. (b) Inverse FCC curves ( χ−1 vs T) of PbMn7O12 (samples 2, 3-1, 3-2, and N) at 100 Oe and 70 kOe. The parameters (μeff and θ) of the Curie−Weiss fit of the FCC 70 kOe data (line) between 200 and 390 K are given. The inset in panel a shows the dependence of TN1 and TN2 on the ionic radius (in 12-fold coordination) in AMn7O12 (A = Cd, Ca, Sr, or Pb).18

Figure 4. (a) ZFC and (b) FCC dc magnetic susceptibility (χ = M/H) curves of PbMn7O12 (samples 2, 3-1, 3-2, and N) at 100 Oe. Arrows show the Néel temperatures. See Figures S13 and S14 for details.

coincide with each other between 2 and 43 K, while the ZFC and FCW curves coincide with each other above 70−75 K (Figure 5a and Figure S14). Similar features were observed in CdMn7O12;18 they are also fingerprints of lock-in transitions. The inverse FCC magnetic susceptibilities (at 70 kOe) between 200 and 390 K are fit by the Curie−Weiss equation χ (T ) = μeff 2 N[3kB(T − θ )]−1

Isothermal magnetization data of PbMn7O12 at different temperatures are given in Figure 8 and Figures S18−S20. There is very weak hysteresis near the origin, suggesting that a weak ferromagnetic moment is not suppressed completely. There is also a field-induced transition at ∼65 kOe. High-field magnetization measurements reveal an additional field-induced transition from ∼170 kOe (Figure 9d). Similar behavior is found in other members of the family, in CdMn7O12, CaMn7O12, and SrMn7O12 (Figure 9). This fact suggests common high-field properties of AMn7O12 (A = divalent cations). It is interesting to note that the first magnetization curve (measured from 0 to 70 kOe on MPMS-7T) is outside of the hysteresis loop (obtained during measurements from 70 to −70 kOe and from −70 to 70 kOe) in all the samples (insets of Figure 9 and Figure S20). This behavior is observed regardless of temperature, the sample insertion procedure (rapidly inserted into a magnetometer or cooled slowly from temperatures above TN1), or the sign of a trapped field (the bottom inset of Figure 8). Because pellets were used in the measurements, particle movements cannot explain this behavior. It might be caused by the difference in initial states between the first and third magnetization curves. AMn7O12 has a very complicated groundstate magnetic structure below TN2.8,33 Therefore, (partially) irreversible field-induced changes in a magnetic structure or a

(1)

where μeff is the effective magnetic moment, N is Avogadro’s number, kB is Boltzmann’s constant, and θ is the Weiss constant. We obtain the following values: μeff = 12.793(4) μB, and θ = −30.0(2) K; the former is close to the expected value of 12.61 μB for six Mn3+ ions and one Mn4+ ion. At 400 K, there are drops in inverse magnetic susceptibility because of the structural R3̅-to-Im3̅ transition. Figures 6 and 7 depict ac susceptibilities of PbMn7O12. They basically follow the dc susceptibilities with sharp peaks on both χ′ versus T and χ″ versus T curves near TN1, smaller peaks near TN2, broad frequency-independent anomalies near 70 K, and steplike anomalies near TN3. The dc field has strong effects on both real and imaginary parts of ac susceptibilities suppressing and, at the same time, making clearer different anomalies at TN1, TN2, and TN3. For example, at Hdc = 1 kOe, anomalies on the χ″ versus T curves disappear at TN1, leaving a very sharp anomaly at TN2 (Figure 7 and Figures S15−S17). Note that ac susceptibilities show additional anomalies below 10 K, which match with frequency-dependent anomalies on the dielectric constant (see below). D

DOI: 10.1021/acs.inorgchem.6b00774 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 7. ac susceptibilities of PbMn7O12 (sample 3-1) in different static magnetic fields (Hdc): (a) the real part ( χ′ vs T) in the logarithmic scale and (b) the imaginary part ( χ″ vs T). Secondary axes give details of some curves on the linear scale. Measurements were performed on cooling from 150 to 2 K using an ac field with the amplitude Hac = 5 Oe and frequency f = 110 Hz. See Figures S15−S17 for details.

Figure 6. Real parts of the ac susceptibility ( χ′ vs T) of (a) PbMn7O12 (sample 3-1) and (b) PbMn7O12 (sample N) on the logarithmic scale. The insets show imaginary parts of the ac susceptibility ( χ″ vs T). Measurements were performed on cooling from 150 to 2 K at a zero static magnetic field using an ac field with the amplitude Hac = 5 Oe and frequencies ( f) of 2, 110, 300, and 500 Hz.

domain structure are possible, especially when the fact that the hysteresis loop is not completely “closed” between −70 and 70 kOe is taken into account. 3.3. Dielectric Constant and Pyroelectric Current Measurements of PbMn7O12. Dielectric constant and dielectric loss of PbMn7O12 as functions of temperature and a magnetic field are shown in Figure 10, Figure 11, and Figures S21−S30. We observe very sharp dielectric anomalies at TN2 (=77 K), and no anomalies are detected at TN1. Sharp anomalies are also detected on dielectric loss from ∼74 kHz; at lower frequencies, dielectric loss anomalies are probably hidden by the sharp increase in dielectric loss due to relaxation processes. A broad anomaly is also found near TN3 on the cooling curves at a zero magnetic field, with huge hysteresis on heating. At magnetic fields, anomalies near TN2 on cooling become quite broad, while sharp anomalies are still observed on heating (Figure 10 and Figures S24, S25, S27, and S29). There are also drops in the dielectric constant below 10 K (Figure 10) and strong frequency dependence in all magnetic fields (the inset of Figure 10 and Figures S23 and S25−S28). The drops in the dielectric constant at low temperatures were also observed in other compounds, CdMn7O12, CaMn7O12, and SrMn7O12.18 PbMn7O12 exhibits a quite small magnetodielectric effect, reaching about −1.3 to −1.7% at 10 K and 90 kOe (Figure 11). We observe three main contributions to the measured pyroelectric current (Figure 12). After the heating process is

Figure 8. M vs H curves of PbMn7O12 (samples 2 and N) at 2 K. The top inset gives details of the same curves near the origin. Measurements were performed from 70 to −70 kOe and from −70 to 70 kOe using an MPMS-7T. f.u. indicates formula unit. The bottom inset shows M vs H curves of PbMn7O12 (sample 2) at 20 K; the sample was cooled from 110 K (above TN1) to 20 K in a positive trapped field (confirmed by a superconducting sample), and three branches were measured on MPMS-7T: (1) from 0 to 70 kOe (green squares), (2) from 70 to −70 kOe, and (3) from −70 to 70 kOe (blue triangles). E

DOI: 10.1021/acs.inorgchem.6b00774 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 9. High-field M vs H curves of (a) CaMn7O12, (b) SrMn7O12, (c) CdMn7O12 (sample N), and (d) PbMn7O12 (sample 2). Measurements were taken at 1.6 K from 0 Oe to 251 kOe (black curves) and from 251 kOe to 0 Oe (red curves). Brown lines are drawn to guide the eye to emphasize kinks at high magnetic fields. The insets show the same high-field M vs H curves together with M−H curves measured on an MPMS-7T at 2 K. Three branches were measured on the MPMS-7T: (1) from 0 to 70 kOe (green squares), (2) from 70 to −70 kOe, and (3) from −70 to 70 kOe (blue triangles). See Figures S18−S20 for details.

same temperature. To obtain polarization, current should be integrated over time. It is obvious that the second (extrinsic) contribution has the largest area, and it significantly overlaps with the third intrinsic contribution. Because of the overlap of extrinsic and intrinsic contributions, it is difficult to separate these two contributions (independent of the presence or absence of the first contribution). Therefore, we integrate the current just in the vicinity of TN2 assuming a smooth background around TN2; this procedure results in a polarization of 4 μC/m2. When we change the sign of the poling electric field, the measured current looks symmetrical up to ∼85 K (Figure 12); this fact gives additional support to the idea that the broad peak near 93 K is extrinsic.

started from 10 K at 0 Oe, the absolute value of the current rapidly increases from the background level of ∼0.5 to 5 pA (Figure S31), and the current is almost constant from 10 to ∼60 K; this is the first contribution. We observe a very broad negative anomaly (at a positive poling field) centered at 75 K and a very broad positive anomaly centered at 93 K, the second contribution, and we detect a sharp negative peak at TN2 = 77 K, the third contribution. No detectable anomalies are found near TN1. As shown in many works, very broad symmetrical anomalies are extrinsic and originate from thermally stimulated current (the second contribution).18,34,35 We believe that the first contribution also originates from a thermally stimulated current of a different origin. Its origin is not known at the moment, but the strong frequency dependence of the dielectric constant and dielectric loss below 10 K (Figure 10) gives support to the presence of such conductivity/relaxation processes. The strong frequency dependence of the dielectric constant and loss is usually observed at 100−300 K in (mixed-valent) transition metal perovskites depending on their conductivity. In PbMn7O12, we observe such typical features from ∼80 K at 100 Hz to 130 K at 2 MHz, and the temperature−frequency dependence of the maximum of dielectric loss follows the Arrhenius law with activation energies of 0.210−0.235 eV (Figure S21). The sharp pyroelectric current anomaly should be intrinsic because it coincides with TN2 and because sharp dielectric constant and loss anomalies are also observed at the

4. DISCUSSION We observe sharp anomalies in the dielectric constant, dielectric loss, and pyroelectric current in PbMn7O12 at TN2. A combination of these observations gives strong support to the idea that ferroelectric polarization is developed in PbMn7O12, and the polarization is a result of magnetic ordering. Our results prove that PbMn7O12 is a spin-driven multiferroic. However, there is a significant difference compared with the reported results for CaMn7O12.9 First, the polarization value of PbMn7O12 is much smaller that that of CaMn7O12.8,27,28 Second, the measurable polarization appears below TN2 in PbMn7O12 in comparison with the reported appearance of polarization below TN1 F

DOI: 10.1021/acs.inorgchem.6b00774 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 11. Magnetic field dependence of the magnetodielectric ratio {εMD = [ε(H) − ε(0)]/ε(0) × 100} of PbMn7O12 at various temperatures and a fixed frequency f = 665 kHz: (a) sample 2 and (b) sample 4.

Figure 10. (a) Temperature dependence of the dielectric constant of PbMn7O12 (sample 2) at 0 and 90 kOe on cooling and heating at one frequency f = 665 kHz. The vertical arrows mark the Néel temperatures (TN). The inset shows the frequency dependence at 0 Oe on cooling. (b) Temperature dependence of dielectric loss at different frequencies at 0 Oe on cooling. See Figures S21−S30 for details.

in CaMn7O12. We show that polarization on the order of a few microcoulombs per square meter can be detected even when resultant pyroelectric current overlaps with noticeable thermally stimulated current, but our results do not exclude the appearance of polarization below TN1 if the polarization value is much smaller than a few microcoulombs per square meter. In the previous work,19 no dielectric anomalies were found in PbMn7O12 probably because weak dielectric anomalies were hidden by relaxation processes, which result in a significant increase in the dielectric constant at low temperatures. In our samples, a significant increase in the dielectric constant starts at higher temperatures (Figure 10 and Figures S21 and S22), allowing the detection of anomalies at TN2. The total resistivity of our sample was ∼20 times larger than that of ref 19 between 130 and 300 K (Figure S32) and continued to increase exponentially (with an activation energy of 0.2 eV) in comparison with a saturation behavior observed in ref 19. A detailed impedance analysis is provided in Figures S33−S38. The intrinsic dielectric constant of PbMn7O12 (measured at frequencies above ∼100 kHz) slightly increases with a decrease in temperature below ∼120 K. A very similar temperature dependence of the dielectric constant was observed in CdMn7O12,18 CaMn7O12,18,36 and SrMn7O12;18 this fact suggests that such a

Figure 12. Results of pyroelectric current measurements for PbMn7O12 (sample 2).9 Poling was performed from 100 to 10 K under ±172 kV/m at 0 Oe and from 100 to 2 K under 172 kV/m at 90 kOe. The top inset shows the resultant polarization after the integration of the pyroelectric current near the sharp anomalies at TN2 = 77 K. The bottom inset shows an enlarged portion of pyroelectric current near the sharp anomalies. See Figure S31 for details.

temperature dependence is a general feature of the AMn7O12 (A = Cd, Ca, Sr, or Pb) family. Decreases in the dielectric G

DOI: 10.1021/acs.inorgchem.6b00774 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

that PbMn7O12 is a spin-driven multiferroic below TN2 with polarization of ∼4 μC/m2. Field-induced transitions are detected from ∼63 and 170 kOe at 1.6−2 K.

constant at low temperatures were also observed for all the members, CdMn7O12 (at 13−21 K depending on samples),18 CaMn7O12 (at ∼10 K),18,36 SrMn7O12 (at ∼8 K),18 and PbMn7O12 (at ∼10 K). The origin of dielectric constant decreases is not clear. In the case of PbMn7O12, a strong frequency dependence is found below 10 K on dielectric constant and loss (Figure 10). This fact suggests the existence of some dielectric relaxation processes at such low temperatures. Very weak dielectric constant anomalies are observed in PbMn7O12 at TN2 = 77 K (Figure 10a). This fact is in agreement with a quite small polarization of ∼4 μC/m2 we measured in this compound, but intrinsic polarization of PbMn7O12 should be larger because we integrated current just near TN2 and ignored all intrinsic current below TN2. However, the precise measurement of polarization in PbMn7O12 (by the pyroelectric current method) is not possible because intrinsic pyroelectric current between 10 and ∼75 K cannot be separated from extrinsic effects. In CdMn 7 O 12 , 18 CaMn 7 O 12 , 18,36 and SrMn7O12,18 no detectable dielectric anomalies were found at their Néel temperatures at a zero magnetic field. Therefore, the intrinsic polarization of these compounds is expected to be much smaller than that of PbMn7O12.9 We provide strong evidence of two magnetic transitions in PbMn7O12 at TN2 = 77 K and TN1 = 83 K in comparison with one magnetic transition at 68 K found in the previous work.19 The existence of two magnetic transitions in PbMn7O12 agrees well with the observation of two magnetic transitions in CdMn7O12,18 CaMn7O12,8,18,27 and SrMn7O12 (inset of Figure 5).18 We also obtain indirect evidence that a lock-in transition takes place below TN3 = 43 K during cooling. Huge hysteresis on the specific heat and dielectric constant and between FCC and FCW magnetic susceptibility curves can be reasonably explained only by such a lock-in transition, when components of an (incommensurate) propagation vector stop changing with temperature upon cooling at 43 K and start changing again upon heating at much higher temperatures. We also observe two field-induced transitions in PbMn7O12 (at 1.6 K) and nonequilibrium field-induced behavior, when the first magnetization curve is located outside of a hysteresis loop (Figures 8 and 9). Therefore, PbMn7O12 exhibits rich magnetic properties. We find two structural phase transitions in PbMn7O12. A transition at TCO = 397 K to a cubic structure with space group Im3̅ has already been reported in the literature.19 The second transition takes place at TOO = 294 K.30 Laboratory XRPD patterns show no additional reflections indicating that a structural modulation is quite weak. In CaMn7O12, additional reflections from a structural modulation also could be detected only by synchrotron XRPD or neutron diffraction.25,26,31 However, the lattice parameters extracted from laboratory XRPD data show some changes at both TCO and TOO, confirming the structural phase transitions. Therefore, PbMn7O12 shows rich structural properties.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00774. Details of XRPD, DSC, specific heat, magnetic, and dielectric measurements and impedance analysis (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work partially was supported by World Premier International Research Center Initiative (WPI Initiative, MEXT, Japan), JSPS KAKENHI (Grant 15H05433), and the Russian Foundation for Basic Research (RFBR 14-03-00768).

■

REFERENCES

(1) Vasil’ev, A. N.; Volkova, O. S. Low Temp. Phys. 2007, 33, 895− 914. (2) Dong, S.; Liu, J.-M.; Cheong, S.-W.; Ren, Z. F. Adv. Phys. 2015, 64, 519−626. (3) Gilioli, E.; Ehm, L. IUCrJ 2014, 1, 590−603. (4) Yamada, I. J. Ceram. Soc. Jpn. 2014, 122, 846−851. (5) Zeng, Z.; Greenblatt, M.; Subramanian, M. A.; Croft, M. Phys. Rev. Lett. 1999, 82, 3164−3167. (6) Meyers, D.; Middey, S.; Cheng, J.-G.; Mukherjee, S.; Gray, B. A.; Cao, Y. W.; Zhou, J.-S.; Goodenough, J. B.; Choi, Y. S.; Haskel, D.; Freeland, J. W.; Saha-Dasgupta, T.; Chakhalian, J. Nat. Commun. 2014, 5, 5818. (7) Riegg, S.; Widmann, S.; Meir, B.; Sterz, S.; Gunther, A.; Buttgen, N.; Ebbinghaus, S. G.; Reller, A.; Krug von Nidda, H.-A.; Loidl, A. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 115149. (8) Johnson, R. D.; Chapon, L. C.; Khalyavin, D. D.; Manuel, P.; Radaelli, P. G.; Martin, C. Phys. Rev. Lett. 2012, 108, 067201. (9) Terada, N.; Glazkova, Y. S.; Belik, A. A. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 155127. (10) Yagi, S.; Yamada, I.; Tsukasaki, H.; Seno, A.; Murakami, M.; Fujii, H.; Chen, H. G.; Umezawa, N.; Abe, H.; Nishiyama, N.; Mori, S. Nat. Commun. 2015, 6, 8249. (11) Ovsyannikov, S. V.; Zainulin, Y. G.; Kadyrova, N. I.; Tyutyunnik, A. P.; Semenova, A. S.; Kasinathan, D.; Tsirlin, A. A.; Miyajima, N.; Karkin, A. E. Inorg. Chem. 2013, 52, 11703−11710. (12) Shiro, K.; Yamada, I.; Ikeda, N.; Ohgushi, K.; Mizumaki, M.; Takahashi, R.; Nishiyama, N.; Inoue, T.; Irifune, T. Inorg. Chem. 2013, 52, 1604−1609. (13) Bochu, B.; Chenavas, J.; Joubert, J. C.; Marezio, M. J. Solid State Chem. 1974, 11, 88−93. (14) Marezio, M.; Dernier, P. D.; Chenavas, J.; Joubert, J. C. J. Solid State Chem. 1973, 6, 16−20. (15) Prodi, A.; Gilioli, E.; Gauzzi, A.; Licci, F.; Marezio, M.; Bolzoni, F.; Huang, Q.; Santoro, A.; Lynn, J. W. Nat. Mater. 2004, 3, 48−52. (16) Prodi, A.; Daoud-Aladine, A.; Gozzo, F.; Schmitt, B.; Lebedev, O.; van Tendeloo, G.; Gilioli, E.; Bolzoni, F.; Aruga-Katori, H.; Takagi, H.; Marezio, M.; Gauzzi, A. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 180101. (17) Ovsyannikov, S. V.; Abakumov, A. M.; Tsirlin, A. A.; Schnelle, W.; Egoavil, R.; Verbeeck, J.; Van Tendeloo, G.; Glazyrin, K. V.;

5. CONCLUSION In conclusion, we prepared PbMn7O12 perovskite under highpressure (6 GPa) and high-temperature (1373 K) conditions and investigated its detailed structural properties over a wide temperature range and its detailed magnetic, dielectric, and ferroelectric properties. We found two magnetic transitions in PbMn7O12 with Néel temperatures TN2 = 77 K and TN1 = 83 K and a lock-in transition at TN3 = 43 K. We observed two structural transitions at TCO = 397 K and TOO = 294 K. We proved H

DOI: 10.1021/acs.inorgchem.6b00774 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Hanfland, M.; Dubrovinsky, L. Angew. Chem., Int. Ed. 2013, 52, 1494− 1498. (18) Glazkova, Y. S.; Terada, N.; Matsushita, Y.; Katsuya, Y.; Tanaka, M.; Sobolev, A. V.; Presniakov, I. A.; Belik, A. A. Inorg. Chem. 2015, 54, 9081−9091. (19) Locherer, T.; Dinnebier, R.; Kremer, R. K.; Greenblatt, M.; Jansen, M. J. Solid State Chem. 2012, 190, 277−284. (20) Prodi, A.; Gilioli, E.; Cabassi, R.; Bolzoni, F.; Licci, F.; Huang, Q.; Lynn, J. W.; Affronte, M.; Gauzzi, A.; Marezio, M. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 085105. (21) Mezzadri, F.; Calicchio, M.; Gilioli, E.; Cabassi, R.; Bolzoni, F.; Calestani, G.; Bissoli, F. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 014420. (22) Imamura, N.; Karppinen, M.; Motohashi, T.; Fu, D.; Itoh, M.; Yamauchi, H. J. Am. Chem. Soc. 2008, 130, 14948−14949. (23) Mezzadri, F.; Calestani, G.; Calicchio, M.; Gilioli, E.; Bolzoni, F.; Cabassi, R.; Marezio, M.; Migliori, A. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 100106R. (24) Bochu, B.; Buevoz, J. L.; Chenavas, J.; Collomb, A.; Joubert, J. C.; Marezio, M. Solid State Commun. 1980, 36, 133−138. (25) Slawinski, W.; Przenioslo, R.; Sosnowska, I.; Bieringer, M.; Margiolaki, I.; Suard, E. Acta Crystallogr., Sect. B: Struct. Sci. 2009, 65, 535−542. (26) Perks, N. J.; Johnson, R. D.; Martin, C.; Chapon, L. C.; Radaelli, P. G. Nat. Commun. 2012, 3, 1277−1282. (27) Zhang, G.; Dong, S.; Yan, Z.; Guo, Y.; Zhang, Q.; Yunoki, S.; Dagotto, E.; Liu, J.-M. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 174413. (28) Sannigrahi, J.; Chattopadhyay, S.; Dutta, D.; Giri, S.; Majumdar, S. J. Phys.: Condens. Matter 2013, 25, 246001. (29) Izumi, F.; Ikeda, T. Mater. Sci. Forum 2000, 321−324, 198−205. (30) Belik, A. A.; Glazkova, Y. S.; Katsuya, Y.; Tanaka, M.; Sobolev, A. V.; Presniakov, I. A. J. Phys. Chem. C 2016, 120, 8278−8288. (31) Slawinski, W.; Przenioslo, R.; Sosnowska, I.; Bieringer, M. J. Phys.: Condens. Matter 2010, 22, 186001. (32) Przenioslo, R.; Sosnowska, I.; Suard, E.; Hewat, A.; Fitch, A. N. Phys. B 2004, 344, 358−367. (33) Johnson, R. D.; Khalyavin, D. D.; Manuel, P.; Bombardi, A.; Martin, C.; Chapon, L. C.; Radaelli, P. G. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 180403. (34) De, C.; Ghara, S.; Sundaresan, A. Solid State Commun. 2015, 205, 61−65. (35) Ngo, T. N. M.; Adem, U.; Palstra, T. T. M. Appl. Phys. Lett. 2015, 106, 152904. (36) Kadlec, F.; Goian, V.; Kadlec, C.; Kempa, M.; Vaněk, P.; Taylor, J.; Rols, S.; Prokleška, J.; Orlita, M.; Kamba, S. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 054307.

I

DOI: 10.1021/acs.inorgchem.6b00774 Inorg. Chem. XXXX, XXX, XXX−XXX