Suppression of the long-range magnetic order in Pb3(Mn1-xFex)7O15 upon substitution of Fe for Mn
JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS Volume: 342 Pages: 100-107
aKirensky Institute of Physics, Russian Academy of Sciences, Siberian Branch, Krasnoyarsk 660036, Russia
bInstitute of Chemistry and Chemical Technology, Russian Academy of Sciences, Siberian Branch, Krasnoyarsk 660049, Russia
cLaboratory for Neutron Scattering, ETH Zurich and Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
dInstitut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble, Cedex 9, France
Highlights
- • In the first time the single crystals of Pb
- (Mn
- Fe
- )
- O
- were grown.
- • The valence and locations of iron ions in crystal were determined.
- • Suppression of the long-range magnetic order in Pb
- (Mn
- Fe
- )
- O
- was discovered.
Abstract
Structure and magnetic properties of Pb3(Mn1−xFex)7O15 single crystals with х=0–0.2 grown by spontaneous crystallization from solution in melt have been investigated. All the crystals belong to the hexagonal space group P63/mcm. The magnetic properties appeared to be strongly dependent on the iron doping level. At small (х=0.05) dopant concentrations, the value of magnetization and Neel temperature TN decrease insignificantly (TN=70 K). With increasing х, the three-dimensional magnetic ordering does not occur and temperature dependences of magnetization at х≥0.1 exhibit spin-glass-like features in the low-temperature region.
Keywords
- Crystal growth;
- Ferrimagnetism;
- Layered magnetic compounds
1. Introduction
Manganites with mixed-valence of manganese are oxide compounds that have been attractive objects of investigation for the last few decades. Rich variety of their physical properties caused by the interplay of charge, spin, and orbital degrees of freedom and possibility of controlling these properties make these materials interesting for both fundamental research and application [1]. The most systematically studied compounds are the manganites with the perovskite structure R1−xAxMnO3 (R is the rare-earth element and А is Са, Sr, Ba, Pb, etc.). In the perovskite structure, Mn3+ and Mn4+ cations are localized in octahedra joint vertices. This circumstance plays a key role in the picture of the exchange interactions. Mixed-valence manganese oxides with the structures different from perovskite one remain understudied; however, a number of recent studies have been devoted to the materials (for instance, Pb3Mn5V2O16[2] and BaMn3O6[3]) where oxygen octahedra have, as a rule, common edges and form single layers. Being still not clearly understood, various intriguing physical phenomena observed in the doped perovskite-like manganites stimulate the search for other oxide families containing mixed-valence manganese ions with the structure different from the perovskite one.
Of particular interest is the natural mineral zenzenite with the chemical formula Pb3(Fe3+Mn3+)4Mn4+3O15 where Mn3+ ions are partially replaced by Fe3+ ions. Despite the artificial zenzenite was grown long ago [4], its physical properties have not been studied. There is a known work of Bush et al. [5] devoted to the investigation of magnetic and electrical properties of the crystal with the chemical formula Pb3Mn6O13. In our previous studies, we reported data on the magnetic [6], dielectric [7], and calorimetric [8] properties of the Pb3Mn7O15 single crystals with mixed-valence manganese ions. We found anomalies on the temperature dependence of magnetization at Т1=160 K, Т2=70 K, and Т3=25 K, which were consistent with the anomalies on the temperature dependence of specific heat. The temperature dependences of ε′ and ε″ also exhibit anomalies in the temperature range 150–210 K that strongly depend on frequency. The most important questions unanswered by now are where the magnetic phase transitions originate from and how they transform the magnetic structure. There still has been a lack of unified interpretation of anomalies on the temperature dependence of complex permittivity. It is unclear whether there are charge ordering and small-radius polarons and, if there are, whether they are evoked by the stereoactivity of Pb2+ ions.
The crystal structure of Pb3Mn7O15 was described first by Darriet et al. [9] on the basis of the orthorhombic Cmc21 space group. Later, Marsh and Herbstein. [10] reconsidered the structure in the space group Cmcm using the single-crystal data of Darriet et al. [9]. Then, Le Page and Calvert [11] proposed the hexagonal unit cell with the space group P63/mcm. In Holstman's et al. work [4], zenzenite Pb3(Fe3+Mn3+)4Mn4+3O15 also has the hexagonal P63/mcm symmetry. According to the data of our preliminary X-ray diffraction (XRD) measurements on powders prepared by grinding of Pb3Mn7O15 single crystals, most of the observed XRD peaks can be satisfactorily indexed in the hexagonal space group P63/mcm at room temperature [6]. Recently, we carried out additional structural studies on a high-resolution synchrotron in the temperature range 15–295 K [12]. The results appeared surprising: the obtained orthorhombic structure with the space group Pnma was not found in the previous studies on Pb3Mn7O15 [4], [6], [9], [10] and [11]. The thermogravimetric analysis allowed us to determine the oxygen content x=14.93±0.05 in the samples [12]. No structural phase transitions were observed within the investigated temperature range 15–300 K [12]. As we discovered later, upon heating Pb3Mn7O15, the room-temperature orthorhombic Pnma structure transformed first (at Т1=400 K) to a spatially modulated structure and then (at Т2=560 K), to the hexagonal P63/mcm structure [13].
These contradictory structural data might originate from an enhanced sensitivity of the Pb3Mn7O15 structure to the crystal growth conditions or deviations in the synthesis parameters, almost unavoidable at repeatable synthesis. Another possible explanation might be the influence of impurity traces in initial chemical reagents. We grew the crystals with different dopants (Li, Ga, Ge, Ru, etc.) in small concentrations (∼5 at%) and found that small amounts of impurities embedded in Pb3Mn7O15 did not affect its crystal structure. In some samples with impurities, the Neel temperature dropped from 70 to 65 K, which was apparently related simply to the diamagnetic dilution [13]. Doping of Pb3Mn7O15 with 3-d and 4-d ions in amounts of ∼10–30 at% led to the noticeable changes in the structure and magnetic properties, as was shown for Pb3Mn5.5Ni1.5O15[14].