Md. Motin Seikh - Academia.edu (original) (raw)
Papers by Md. Motin Seikh
Materials advances, 2024
Achievement of B-site cation ordering for LaFe 0.5 Mn 0.5 O 3 , unlike other LaB 0.5 B 0 0.5 O 3 ... more Achievement of B-site cation ordering for LaFe 0.5 Mn 0.5 O 3 , unlike other LaB 0.5 B 0 0.5 O 3 perovskites, has not been reported yet in polycrystalline samples. Thus far, there has been only one report on Fe and Mn ordering in LaMnO 3-LaFeO 3 artificial superlattices constructed on SrTiO 3 using laser MBE methods [Ueda et al. Phys. Rev. B, 1999, 60, R12561.]. The ferromagnetic ordering with the T C around 230 K has been attributed to the Fe 3+ (d 5)-O-Mn 3+ (d 4) superexchange interaction for the superlattice with 1/1 stacking periodicity. Here, for the first time, we report the Fe and Mn ordering at the interface of the La 0.45 Ca 0.55 MnO 3-LaFeO 3 (LCMO-LFO) nanocomposite, achieving a T C of B225 K. It has been also observed that such an ordering is very sensitive to the processing temperature. Processing the composite at 1000 1C leads to complete randomization in the cationic distribution, which results in suppression of the high temperature transition at 225 K observed for 700 and 800 1C heat-treated samples. Highresolution TEM, HAADF, EDX mapping and ED analysis support the interfacial phase formation. This cationic ordering through interfacial reaction is further substantiated by the formation of a well-studied La 2 MnCoO 6 double perovskite obtained from a LaMnO 3-LaCoO 3 nanocomposite by thermal treatment under similar conditions as that of the LCMO-LFO system. Our study revealed that by means of the interface reaction during composite processing, the product phase may dominate the overall property of the system. The temperature-controlled cationic ordering at the interfacial product phase of dominating magnetic properties may provide a novel route to design functional ceramic materials.
Physical Chemistry Chemical Physics, Dec 31, 2023
Journal of Materials Chemistry C, Dec 31, 2023
Co-Published with Indian Institute of Science (IISc), Bangalore, India eBooks, Jul 1, 2008
Journal of Materials Chemistry, Apr 3, 2002
Several methods have been employed to synthesize SiC nanowires. The methods include heating silic... more Several methods have been employed to synthesize SiC nanowires. The methods include heating silica gel or fumed silica with activated carbon in a reducing atmosphere, the carbon particles being produced in situ in one of the methods. The simplest method to obtain b-SiC nanowires involves heating silica gel with activated carbon at 1360 uC in H 2 or NH 3. The same reaction, if carried out in the presence of catalytic iron particles, at 1200 uC gives a-Si 3 N 4 nanowires and Si 2 N 2 O nanowires at 1100 uC. Another method to obtain Si 3 N 4 nanowires is to heat multi-walled carbon nanotubes with silica gel at 1360 uC in an atmosphere of NH 3. In the presence of catalytic Fe particles, this method yields a-Si 3 N 4 nanowires in pure form.
The coupled dynamics of charge, spin, orbital and lattice degrees of freedom, related to the emer... more The coupled dynamics of charge, spin, orbital and lattice degrees of freedom, related to the emergence of complex phases in transition metal oxides has been a long standing research topic among condensed matter community. This monograph largely emphasis on the experimental investigations of several oxide systems by employing a number of techniques. The synthetic methods include conventional solid state route, single crystal growth using floating zone melting technique and thin films. The various aspects of investigations include electronic and magnetic properties, charge-ordering and electronic phase segregation in rare earth manganites, spin- state transitions in rare earth cobaltites and insulator-metal transitions in magnetite (Fe3O4) and karelianite (V2O3). Besides the measurements of magnetoelectric properties, several spectroscopic techniques are employed to study the various problems. The LSIS HS spin-state transition in cobaltite is explored using infrared and Raman spectroscopy. Brillouin scattering has been successfully utilized to emphasize role of low energy excitations in the strongly correlated electron systems
Physical review, Oct 9, 2018
Sr 2 Ca 2 CoMn 2 O 9 is made of Ising spin chains distributed on a triangular lattice, with antif... more Sr 2 Ca 2 CoMn 2 O 9 is made of Ising spin chains distributed on a triangular lattice, with antiferromagnetic coupling between them. In spite of a unique interchain distance, this compound exhibits two peculiarities making it deviate from the textbook case of geometrical frustration expected in ideal triangular lattices: first, there are three subclasses of chains shifted from each other along the chain direction; second, these chains fall into two categories which differ by some of the intrachain distances between spins. These features are found to confer a character of centered honeycomb lattice on this system of spin chains. Experimental investigation of the magnetism of Sr 2 Ca 2 CoMn 2 O 9 reveals a peculiar pretransitional regime, which is manifested by a frequency-dependent peak of susceptibility. We put this behavior in relation with the formation of short-range one-dimensional ferromagneticlike segments taking place a few degrees above the T N. This phenomenon is itself regarded as a precursor effect of the long-range ordering, induced by the particular topology of the spin system.
Journal of Magnetism and Magnetic Materials, Dec 1, 2016
Metallic nanoparticles exsolved at the surface of perovskite oxides have been recently shown to u... more Metallic nanoparticles exsolved at the surface of perovskite oxides have been recently shown to unlock superior catalytic activity and durability towards various chemical reactions of practical importance. For example, for the CO oxidation reaction, exsolved Ni nanoparticles in oxidised form exhibit site activities approaching those of noble metals. This is of particular interest for the prospect of replacing noble metals with earth-abundant metal/metal oxide catalysts in the automotive exhaust control industry. Here we show that for the CO oxidation reaction, the functionality of exsolved Ni nanoparticles can be further improved when Fe is co-exsolved with Ni, as Fe-Ni alloy nanoparticles, eventually forming mixed oxide nanoparticles. As compared to the Ni nanoparticles, the alloy nanoparticles exhibit higher site activities, greatly improved durability over 170 h of continuous testing and increased tolerance towards sulphur-based atmospheres. Similarly to the single metal nanoparticles, the alloys demonstrate outstanding microstructural stability and high tolerance towards coking. These results open additional directions for tailoring the activity and durability of exsolved materials for the CO oxidation reaction and beyond.
Journal of Physics: Condensed Matter, Jun 4, 2004
Temperature-dependent Brillouin scattering studies have been carried out on La0.77Ca0.23MnO3 acro... more Temperature-dependent Brillouin scattering studies have been carried out on La0.77Ca0.23MnO3 across the paramagnetic insulator-ferromagnetic metal (I-M) transition. The spectra show a surface Rayleigh wave (SRW) and a high velocity pseudo surface acoustic wave (HVPSAW) besides bulk acoustic waves (BAW). The Brillouin shifts associated with SRW and HVPSAW show blue-shifts, where as the frequencies of the BAW decrease below the I-M transition temperature (TC) of 230 K. These results can be understood based on the temperature dependence of the elastic constants. We also observe a central peak whose width is maximum at TC .
Comptes Rendus Chimie, Oct 1, 2018
The very recent studies of two classes of transition metal oxides with a triangular metallic subl... more The very recent studies of two classes of transition metal oxides with a triangular metallic sublattice are reported. The first class of materials concerns the swedenborgite CaBaCo 4 O 7 family, which is shown to be a ferrimagnetic multiferroic with a gigantic-induced electrical polarization, and the second one deals with the spin chain oxides of the Sr 4 Mn 2 CoO 9 type, which have been found to behave as single chain magnets and single ion magnets. These materials that exhibit a potential for applications in the field of memory devices and quantum computation pave the way for future investigations in these systems.
Journal of Materials Chemistry C, 2018
Differently from Ca3CoMnO6 the triangular spin chain oxides Sr4−xCaxMn2CoO9 have been synthesized... more Differently from Ca3CoMnO6 the triangular spin chain oxides Sr4−xCaxMn2CoO9 have been synthesized for a large compositional range, 0 ≤ x ≤ 2.7, allowing the study of the impact of chemical pressure on their magnetic properties.
Journal of Magnetism and Magnetic Materials, Oct 1, 2018
The striking effect of Ti-doping on the magnetic properties of La 0.5 Ca 0.5 MnO 3 nanoparticles ... more The striking effect of Ti-doping on the magnetic properties of La 0.5 Ca 0.5 MnO 3 nanoparticles have been investigated by different methods. The half doped manganites are at the boundary between the ferromagnetic metallic and antiferromagnetic insulating phases. There is a delicate balance between ferromagnetic double exchange and antiferromagnetic superexchange interactions. In half doped La 0.5 Ca 0.5 MnO 3 charge ordered antiferromagnetic state is destabilized below a critical size followed by development of ferromagnetism. We have seen that slight disturbance in magnetic exchange interactions by Ti-substitution (>5%) drastically changes the fraction of ferromagnetic clusters. The ferromagnetic Mn 3+-O-Mn 4+ interaction is suppressed by Mn 3+-O-Ti 4+ type of interactions. For more than 10% Ti doping the surface ferromagnetic character of the nanoparticles is largely weighed down. This suppression of ferromagnetic component by diamagnetic Ti-doping eventually leads to phase separated states with lower magnetization value.
Materials Science And Engineering: B, Dec 1, 2017
Abstract We report the solution combustion synthesis of FeMnO3 as well as its detailed structural... more Abstract We report the solution combustion synthesis of FeMnO3 as well as its detailed structural and magnetic characterizations. The X-ray and electron diffraction studies were utilized for structural characterization. It crystallizes in cubic Ia3 space group with bixbyite structure having cell parameter a = 9.413(4) A. It is observed that FeMnO3 lacks long range ordering and shows spin glass behaviour with a freezing temperature Tg ∼ 33 K. This result is in contradiction with the earlier reports of long range antiferromagnetic and ferrimagnetic ordering. The prevalence of spin glass state has been explained with the most relevant dynamic scaling theory giving the glass transition, critical exponent and characteristic time, respectively, Tg = 33 ± 0.2 K, zν = 5.7 ± 0.5 and τ0 ∼ 10−12 s. The evolution of spin glass state can be ascribed to the structural genesis, where the competing antiferromagnetic and ferromagnetic interaction as well as cationic disordering results in magnetic frustration.
Solid State Communications, May 1, 2009
The effect of calcium doping upon the magnetic and transport properties of the smaller size lanth... more The effect of calcium doping upon the magnetic and transport properties of the smaller size lanthanide cobaltites of the type Ln 1−x Ca x BaCo 2 O 5.50+δ (Ln = Dy and Ho) has been investigated for 0 ≤ x ≤ 0.2. The substitution of Ln by calcium induces a large deterioration of the long-range magnetic ordering in the system finally leading to a paramagnetic state. Such instability in the magnetic ordering could be related to strong bending of the CoO Co bond, the local lattice distortion and the random vacancies in oxygen sublattice. The present results suggest that not only the cobalt valence but also the size of the lanthanide and the local lattice distortion play a crucial role in determining the physical properties of this layered cobaltite.
Journal of Alloys and Compounds, Mar 1, 2019
Abstract Here we report synthesis, structure and magnetic properties of low temperature synthesiz... more Abstract Here we report synthesis, structure and magnetic properties of low temperature synthesized nanodimensional La1-xBaxFe0.5Mn0.5O3 (x = 0.25, 0.33 and 0.50) perovskites. All the samples crystallize in cubic Pm-3m space group. The additional reflection spots (110) in electron diffraction (ED) requiring doubling of cubic cell parameter suggests the possible ordering of Fe and Mn in B-site. The magnetic property of the nanostructured samples is associated with the ferromagnetic clusters developed at the surface of nanoparticles. Such clusters disappeared subsequent to high temperature annealing. Importantly, unlike the homologous series La0.5Ca0.5Fe0.5Mn0.5O3 and La0.5Sr0.5Fe0.5Mn0.5O3 or electronically equivalent ferromagnetic LaFe0.5Cr0.5O3 (TC ∼ 120 K), La0.5Ba0.5Fe0.5Mn0.5O3 fails to exhibit any long range ordering. This curious observation can be better rationalized by considering the cationic size disorder parameter (σ2 = Σxiri2 - 2) in A-site rather than the random distribution of Fe and Mn in B-site.
Chemical Physics Letters, Jun 1, 2001
Low-spin (LS) to intermediate-spin (IS) state transitions in crystals of LnCoO3 (Ln=La, Pr and Nd... more Low-spin (LS) to intermediate-spin (IS) state transitions in crystals of LnCoO3 (Ln=La, Pr and Nd) have been investigated by variable temperature infrared spectroscopy. The spectra reveal the occurrence of the transition around 120, 220 and 275 K, respectively, in LaCoO3,PrCoO3 and NdCoO3, at which temperatures the intensities of the stretching and the bending modes associated with the LS state decrease,
Journal of Physics and Chemistry of Solids, 2014
The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo 4 O 7 has been investigate... more The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo 4 O 7 has been investigated. It is shown that the oxides CaBaCo 4 O 7 þ δ and CaBaCo 4 À x O 7 À δ (0 rx r 0.20) synthesized below 1100 1C in air exhibit phase separation, where ferrimagnetic regions with T C $ 56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at $ 14-20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co 3 þ cations, with respect to the ideal formula CaBaCo 2 2 þ Co 2 3 þ O 7. This excess Co 3 þ tends to destroy the ferromagnetic zigzag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples.
Materials advances, 2024
Achievement of B-site cation ordering for LaFe 0.5 Mn 0.5 O 3 , unlike other LaB 0.5 B 0 0.5 O 3 ... more Achievement of B-site cation ordering for LaFe 0.5 Mn 0.5 O 3 , unlike other LaB 0.5 B 0 0.5 O 3 perovskites, has not been reported yet in polycrystalline samples. Thus far, there has been only one report on Fe and Mn ordering in LaMnO 3-LaFeO 3 artificial superlattices constructed on SrTiO 3 using laser MBE methods [Ueda et al. Phys. Rev. B, 1999, 60, R12561.]. The ferromagnetic ordering with the T C around 230 K has been attributed to the Fe 3+ (d 5)-O-Mn 3+ (d 4) superexchange interaction for the superlattice with 1/1 stacking periodicity. Here, for the first time, we report the Fe and Mn ordering at the interface of the La 0.45 Ca 0.55 MnO 3-LaFeO 3 (LCMO-LFO) nanocomposite, achieving a T C of B225 K. It has been also observed that such an ordering is very sensitive to the processing temperature. Processing the composite at 1000 1C leads to complete randomization in the cationic distribution, which results in suppression of the high temperature transition at 225 K observed for 700 and 800 1C heat-treated samples. Highresolution TEM, HAADF, EDX mapping and ED analysis support the interfacial phase formation. This cationic ordering through interfacial reaction is further substantiated by the formation of a well-studied La 2 MnCoO 6 double perovskite obtained from a LaMnO 3-LaCoO 3 nanocomposite by thermal treatment under similar conditions as that of the LCMO-LFO system. Our study revealed that by means of the interface reaction during composite processing, the product phase may dominate the overall property of the system. The temperature-controlled cationic ordering at the interfacial product phase of dominating magnetic properties may provide a novel route to design functional ceramic materials.
Physical Chemistry Chemical Physics, Dec 31, 2023
Journal of Materials Chemistry C, Dec 31, 2023
Co-Published with Indian Institute of Science (IISc), Bangalore, India eBooks, Jul 1, 2008
Journal of Materials Chemistry, Apr 3, 2002
Several methods have been employed to synthesize SiC nanowires. The methods include heating silic... more Several methods have been employed to synthesize SiC nanowires. The methods include heating silica gel or fumed silica with activated carbon in a reducing atmosphere, the carbon particles being produced in situ in one of the methods. The simplest method to obtain b-SiC nanowires involves heating silica gel with activated carbon at 1360 uC in H 2 or NH 3. The same reaction, if carried out in the presence of catalytic iron particles, at 1200 uC gives a-Si 3 N 4 nanowires and Si 2 N 2 O nanowires at 1100 uC. Another method to obtain Si 3 N 4 nanowires is to heat multi-walled carbon nanotubes with silica gel at 1360 uC in an atmosphere of NH 3. In the presence of catalytic Fe particles, this method yields a-Si 3 N 4 nanowires in pure form.
The coupled dynamics of charge, spin, orbital and lattice degrees of freedom, related to the emer... more The coupled dynamics of charge, spin, orbital and lattice degrees of freedom, related to the emergence of complex phases in transition metal oxides has been a long standing research topic among condensed matter community. This monograph largely emphasis on the experimental investigations of several oxide systems by employing a number of techniques. The synthetic methods include conventional solid state route, single crystal growth using floating zone melting technique and thin films. The various aspects of investigations include electronic and magnetic properties, charge-ordering and electronic phase segregation in rare earth manganites, spin- state transitions in rare earth cobaltites and insulator-metal transitions in magnetite (Fe3O4) and karelianite (V2O3). Besides the measurements of magnetoelectric properties, several spectroscopic techniques are employed to study the various problems. The LSIS HS spin-state transition in cobaltite is explored using infrared and Raman spectroscopy. Brillouin scattering has been successfully utilized to emphasize role of low energy excitations in the strongly correlated electron systems
Physical review, Oct 9, 2018
Sr 2 Ca 2 CoMn 2 O 9 is made of Ising spin chains distributed on a triangular lattice, with antif... more Sr 2 Ca 2 CoMn 2 O 9 is made of Ising spin chains distributed on a triangular lattice, with antiferromagnetic coupling between them. In spite of a unique interchain distance, this compound exhibits two peculiarities making it deviate from the textbook case of geometrical frustration expected in ideal triangular lattices: first, there are three subclasses of chains shifted from each other along the chain direction; second, these chains fall into two categories which differ by some of the intrachain distances between spins. These features are found to confer a character of centered honeycomb lattice on this system of spin chains. Experimental investigation of the magnetism of Sr 2 Ca 2 CoMn 2 O 9 reveals a peculiar pretransitional regime, which is manifested by a frequency-dependent peak of susceptibility. We put this behavior in relation with the formation of short-range one-dimensional ferromagneticlike segments taking place a few degrees above the T N. This phenomenon is itself regarded as a precursor effect of the long-range ordering, induced by the particular topology of the spin system.
Journal of Magnetism and Magnetic Materials, Dec 1, 2016
Metallic nanoparticles exsolved at the surface of perovskite oxides have been recently shown to u... more Metallic nanoparticles exsolved at the surface of perovskite oxides have been recently shown to unlock superior catalytic activity and durability towards various chemical reactions of practical importance. For example, for the CO oxidation reaction, exsolved Ni nanoparticles in oxidised form exhibit site activities approaching those of noble metals. This is of particular interest for the prospect of replacing noble metals with earth-abundant metal/metal oxide catalysts in the automotive exhaust control industry. Here we show that for the CO oxidation reaction, the functionality of exsolved Ni nanoparticles can be further improved when Fe is co-exsolved with Ni, as Fe-Ni alloy nanoparticles, eventually forming mixed oxide nanoparticles. As compared to the Ni nanoparticles, the alloy nanoparticles exhibit higher site activities, greatly improved durability over 170 h of continuous testing and increased tolerance towards sulphur-based atmospheres. Similarly to the single metal nanoparticles, the alloys demonstrate outstanding microstructural stability and high tolerance towards coking. These results open additional directions for tailoring the activity and durability of exsolved materials for the CO oxidation reaction and beyond.
Journal of Physics: Condensed Matter, Jun 4, 2004
Temperature-dependent Brillouin scattering studies have been carried out on La0.77Ca0.23MnO3 acro... more Temperature-dependent Brillouin scattering studies have been carried out on La0.77Ca0.23MnO3 across the paramagnetic insulator-ferromagnetic metal (I-M) transition. The spectra show a surface Rayleigh wave (SRW) and a high velocity pseudo surface acoustic wave (HVPSAW) besides bulk acoustic waves (BAW). The Brillouin shifts associated with SRW and HVPSAW show blue-shifts, where as the frequencies of the BAW decrease below the I-M transition temperature (TC) of 230 K. These results can be understood based on the temperature dependence of the elastic constants. We also observe a central peak whose width is maximum at TC .
Comptes Rendus Chimie, Oct 1, 2018
The very recent studies of two classes of transition metal oxides with a triangular metallic subl... more The very recent studies of two classes of transition metal oxides with a triangular metallic sublattice are reported. The first class of materials concerns the swedenborgite CaBaCo 4 O 7 family, which is shown to be a ferrimagnetic multiferroic with a gigantic-induced electrical polarization, and the second one deals with the spin chain oxides of the Sr 4 Mn 2 CoO 9 type, which have been found to behave as single chain magnets and single ion magnets. These materials that exhibit a potential for applications in the field of memory devices and quantum computation pave the way for future investigations in these systems.
Journal of Materials Chemistry C, 2018
Differently from Ca3CoMnO6 the triangular spin chain oxides Sr4−xCaxMn2CoO9 have been synthesized... more Differently from Ca3CoMnO6 the triangular spin chain oxides Sr4−xCaxMn2CoO9 have been synthesized for a large compositional range, 0 ≤ x ≤ 2.7, allowing the study of the impact of chemical pressure on their magnetic properties.
Journal of Magnetism and Magnetic Materials, Oct 1, 2018
The striking effect of Ti-doping on the magnetic properties of La 0.5 Ca 0.5 MnO 3 nanoparticles ... more The striking effect of Ti-doping on the magnetic properties of La 0.5 Ca 0.5 MnO 3 nanoparticles have been investigated by different methods. The half doped manganites are at the boundary between the ferromagnetic metallic and antiferromagnetic insulating phases. There is a delicate balance between ferromagnetic double exchange and antiferromagnetic superexchange interactions. In half doped La 0.5 Ca 0.5 MnO 3 charge ordered antiferromagnetic state is destabilized below a critical size followed by development of ferromagnetism. We have seen that slight disturbance in magnetic exchange interactions by Ti-substitution (>5%) drastically changes the fraction of ferromagnetic clusters. The ferromagnetic Mn 3+-O-Mn 4+ interaction is suppressed by Mn 3+-O-Ti 4+ type of interactions. For more than 10% Ti doping the surface ferromagnetic character of the nanoparticles is largely weighed down. This suppression of ferromagnetic component by diamagnetic Ti-doping eventually leads to phase separated states with lower magnetization value.
Materials Science And Engineering: B, Dec 1, 2017
Abstract We report the solution combustion synthesis of FeMnO3 as well as its detailed structural... more Abstract We report the solution combustion synthesis of FeMnO3 as well as its detailed structural and magnetic characterizations. The X-ray and electron diffraction studies were utilized for structural characterization. It crystallizes in cubic Ia3 space group with bixbyite structure having cell parameter a = 9.413(4) A. It is observed that FeMnO3 lacks long range ordering and shows spin glass behaviour with a freezing temperature Tg ∼ 33 K. This result is in contradiction with the earlier reports of long range antiferromagnetic and ferrimagnetic ordering. The prevalence of spin glass state has been explained with the most relevant dynamic scaling theory giving the glass transition, critical exponent and characteristic time, respectively, Tg = 33 ± 0.2 K, zν = 5.7 ± 0.5 and τ0 ∼ 10−12 s. The evolution of spin glass state can be ascribed to the structural genesis, where the competing antiferromagnetic and ferromagnetic interaction as well as cationic disordering results in magnetic frustration.
Solid State Communications, May 1, 2009
The effect of calcium doping upon the magnetic and transport properties of the smaller size lanth... more The effect of calcium doping upon the magnetic and transport properties of the smaller size lanthanide cobaltites of the type Ln 1−x Ca x BaCo 2 O 5.50+δ (Ln = Dy and Ho) has been investigated for 0 ≤ x ≤ 0.2. The substitution of Ln by calcium induces a large deterioration of the long-range magnetic ordering in the system finally leading to a paramagnetic state. Such instability in the magnetic ordering could be related to strong bending of the CoO Co bond, the local lattice distortion and the random vacancies in oxygen sublattice. The present results suggest that not only the cobalt valence but also the size of the lanthanide and the local lattice distortion play a crucial role in determining the physical properties of this layered cobaltite.
Journal of Alloys and Compounds, Mar 1, 2019
Abstract Here we report synthesis, structure and magnetic properties of low temperature synthesiz... more Abstract Here we report synthesis, structure and magnetic properties of low temperature synthesized nanodimensional La1-xBaxFe0.5Mn0.5O3 (x = 0.25, 0.33 and 0.50) perovskites. All the samples crystallize in cubic Pm-3m space group. The additional reflection spots (110) in electron diffraction (ED) requiring doubling of cubic cell parameter suggests the possible ordering of Fe and Mn in B-site. The magnetic property of the nanostructured samples is associated with the ferromagnetic clusters developed at the surface of nanoparticles. Such clusters disappeared subsequent to high temperature annealing. Importantly, unlike the homologous series La0.5Ca0.5Fe0.5Mn0.5O3 and La0.5Sr0.5Fe0.5Mn0.5O3 or electronically equivalent ferromagnetic LaFe0.5Cr0.5O3 (TC ∼ 120 K), La0.5Ba0.5Fe0.5Mn0.5O3 fails to exhibit any long range ordering. This curious observation can be better rationalized by considering the cationic size disorder parameter (σ2 = Σxiri2 - 2) in A-site rather than the random distribution of Fe and Mn in B-site.
Chemical Physics Letters, Jun 1, 2001
Low-spin (LS) to intermediate-spin (IS) state transitions in crystals of LnCoO3 (Ln=La, Pr and Nd... more Low-spin (LS) to intermediate-spin (IS) state transitions in crystals of LnCoO3 (Ln=La, Pr and Nd) have been investigated by variable temperature infrared spectroscopy. The spectra reveal the occurrence of the transition around 120, 220 and 275 K, respectively, in LaCoO3,PrCoO3 and NdCoO3, at which temperatures the intensities of the stretching and the bending modes associated with the LS state decrease,
Journal of Physics and Chemistry of Solids, 2014
The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo 4 O 7 has been investigate... more The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo 4 O 7 has been investigated. It is shown that the oxides CaBaCo 4 O 7 þ δ and CaBaCo 4 À x O 7 À δ (0 rx r 0.20) synthesized below 1100 1C in air exhibit phase separation, where ferrimagnetic regions with T C $ 56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at $ 14-20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co 3 þ cations, with respect to the ideal formula CaBaCo 2 2 þ Co 2 3 þ O 7. This excess Co 3 þ tends to destroy the ferromagnetic zigzag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples.