Jayashree Das - Academia.edu (original) (raw)

Papers by Jayashree Das

Journal of Magnetism and Magnetic Materials, 2015

ABSTRACT With a motivation to compare the magnetic property, we synthesised undoped, transition m... more ABSTRACT With a motivation to compare the magnetic property, we synthesised undoped, transition metal (TM) Mn doped and (Mn:Fe) co-doped ZnO ceramics in the compositions ZnO, Zn0.98Mn0.02O and Zn0.96(Mn0.02Fe0.02)O. Systematic investigations on the structural, microstructural, defect structure and magnetic properties of the samples were performed. Low temperature as well as room temperature ferromagnetism has been observed for all our samples, however, enhanced magnetisation at room temperature has been noticed when ZnO is co-doped with Fe along with Mn. Particularly the sample with the composition Zn0.96Mn0.02Fe0.02O showed a magnetisation value more than double of the sample with composition Zn0.98Mn0.02O, indicating long range strong interaction between the magnetic impurities leading to higher ferromagnetic ordering. Raman and PL studies reveal presence of higher defects in form of oxygen vacancy clusters created in the sample due to Fe co doping. PL study also reveals enhanced luminescence efficiency in the co doped sample. Temperature dependent magnetisation study of this sample shows the spin freezing temperature around 39 K indicating the presence of small impurity phase of Mn2−xZnxO3 type. Electron Spin Resonance signal obtained supports ferromagnetic state in the co doped sample. Enhancement of magnetisation is attributed to interactions mediated by magnetic impurities through large number of oxygen vacancies created by Fe3+ ions forming bound magnetic polarons (BMP) and facilitating long range ferromagnetic ordering in the co- doped system.

... Our micro-Raman and X-ray photoelectron studies revealed the existence of these defects which... more ... Our micro-Raman and X-ray photoelectron studies revealed the existence of these defects which are believed to be the origin of the unconventional ferromagnetism (FM) of ZnO bulk ceramic at room temperature. PACS: 70; 71.55.Gs; 75.50.Pp. ...

Physica B-condensed Matter, 2010

Room temperature ferromagnetism (RTFM) with a saturation magnetization (Ms) of 0.025 emu/cm3 has ... more Room temperature ferromagnetism (RTFM) with a saturation magnetization (Ms) of 0.025 emu/cm3 has been observed in single crystal of zinc oxide (ZnO). The presence of intrinsic defects like zinc (Zn) interstitials and oxygen (O) vacancies in single crystal ZnO is established through micro-Raman and photoluminescence studies. The role of intrinsic defects in introduction of RTFM in single crystal ZnO is discussed.

Solid State Communications

Solid State Communications, 2016

Physica B: Condensed Matter, 2012

ABSTRACT This paper reports the paramagnetic behavior of Mn doped ZnO co-doped with rare earth (G... more ABSTRACT This paper reports the paramagnetic behavior of Mn doped ZnO co-doped with rare earth (Gd and Sm) atoms. The formation of secondary rare earth oxides (Gd2O3 and Sm2O3) is confirmed from the X-ray diffraction patterns. The rare earth oxides in the system forbids the grain growth and interconnection between the grains. The weak link between the grains suppresses the long range exchange interaction between the Mn ions and hence, reduces the ferromagnetic ordering. Owing to the large mismatch between ionic radii of rare earth and transition metal atoms inside the matrix, the rare earth element cannot contribute to promote ferromagnetic behavior in Mn doped ZnO, irrespective of their high individual magnetic moments.

Materials Research Bulletin, 2011

Materials Letters, 2011

Enhancement in room temperature ferromagnetism has been observed in Mn doped ZnO, when co-doped w... more Enhancement in room temperature ferromagnetism has been observed in Mn doped ZnO, when co-doped with Ni. In the co-doped system, Ni–O–Ni and Ni–O–Mn carrier mediated exchange interaction dominates the Mn–O–Mn exchange interaction which causes a fair increase in the value of saturation magnetization at room temperature. It is believed that the simultaneous doping of Ni and Mn onto ZnO increases

Journal of Physics and Chemistry of Solids, 2010

This paper reports on multiferroic properties of Ho substituted BiFeO 3 (Bi 1 À x Ho x FeO 3) cer... more This paper reports on multiferroic properties of Ho substituted BiFeO 3 (Bi 1 À x Ho x FeO 3) ceramics. It is observed that for x ¼ 0.15, a prominent ferroelectric loop is seen at 300 K even if the system remains in rhombohedral (R3c) phase without appearance of any observable impurity phases. A well shaped M-H loop is observed at 10 K for x ¼ 0.15. However it showed ferromagnetism, confirming the contribution of Ho 3 + towards enhancement of ferromagnetic properties of BiFeO 3 at 300 K. Suppression of impurity phases of pure BiFeO 3 bulk ceramic favors the reduction of mobile oxygen vacancies and reduces leakage current, due to which ferroelectric properties of BiFeO 3 is enhanced. We argue that Ho substitution at Bi site is likely to suppress the spiral spin modulation and at the same time increase the canting angle, which favors enhanced multiferroic properties. XRD, SEM, magnetization, polarization and chemical bonding analysis measurements were carried out to explain the multiferroic behavior.

Journal of Magnetism and Magnetic Materials, 2010

This paper reports that defect driven magnetism can be obtained at room temperature by optimizing... more This paper reports that defect driven magnetism can be obtained at room temperature by optimizing metal ion concentration in bismuth ferrite (BFO) following our novel slow step solid state sintering route. We observed a clean signature of enhanced multiferroic behavior in Gd doped bismuth ferrite (Gd-BFO) bulk ceramics at room temperature (RT). Bismuth rich iron deficient Gd-BFO ceramics were prepared by solid state route through slow step sintering schedule at 850 1C. At particular composition, (Bi 1.2 Gd 0.1 Fe 0.8 O 3), this materials completely transform from rhombohedral R3c to orthorhombic Pn2 1 a space group. We emphasized that excess bismuth is expected to act as point defects and occupy interstitials positions, which in turn interact by oxygen vacancies. These defects are likely to promote defect driven ferromagnetism in BFO system. Incorporation of Gd in presence of excess bismuth in BFO enhanced both spin and electric polarization at room temperature. We also infer that Gd substitution in BFO is likely to suppress spiral spin modulation, which also favors ferromagnetism in Gd-BFO.

Journal of Magnetism and Magnetic Materials, 2011

Mn-doped ZnO samples having composition Zn 1 À x Mn x O (x ¼ 0.02, 0.04 and 0.05) were synthesize... more Mn-doped ZnO samples having composition Zn 1 À x Mn x O (x ¼ 0.02, 0.04 and 0.05) were synthesized by solid state reaction technique with varying concentration of Mn from 0.02 to 0.05. Evidence of room temperature ferromagnetism was observed only in the composition Zn 0.98 Mn 0.02 O sintered at 500 1C. Our XRD pattern confirms the presence of Mn 3 O 4 impurity phase in all the Zn 1 À x Mn x O samples with the exception of Zn 0.98 Mn 0.02 O. We emphasize that the appearance of Mn 3 O 4 phase in the system forbids the exchange type of interaction between the Mn ions and suppresses the ferromagnetism in all the Mn overdoped Zn 1 À x Mn x O (x 4 0.02) system. SEM microstructure study also supports the interruption of exchange type of interaction inside the system with the increase in Mn concentration in the sample. Interestingly, for this particular composition, Zn 0.98 Mn 0.02 O sintered at 500 1C, glassy ferromagnetism type of transition is observed at low temperature. This type of transition is attributed to the formation of the oxides of Mn clusters at low temperature.

Journal of Alloys and Compounds, 2011

A clean signature of room temperature multiferroicity in Bi 1.2 Gd 0.1 Fe 0.8 O 3 ceramics is obs... more A clean signature of room temperature multiferroicity in Bi 1.2 Gd 0.1 Fe 0.8 O 3 ceramics is observed. Samples were prepared by slow step sintering technique at 850 • C. Impurity peaks normally present in original BiFeO 3 are completely suppressed with the increase of Bi concentration. Apart from this, structural transformation took place from R3 C to Pn2 1 a space group and a clear orthorhombic grain growth habit is seen by SEM microstructure. Suppression of impurity phases favors the reduction of mobile oxygen vacancies and hence reduces leakage current for which ferroelectric properties of Bi 1.2 Gd 0.1 Fe 0.8 O 3 are enhanced. We argue that the addition of Gd is likely to suppress the spiral spin modulation and at the same time increase the canting angle which favors the enhanced ferromagnetism. Excess Bi is expected to act as point defects and occupy interstitial positions which in turn surrounded by oxygen vacancies and is likely to promote defect driven ferromagnetism with enhanced spin and electric polarization. This finding encourages further revaluation of long-discounted BiFeO 3 for multiferroics research.

Abstract We observed that ZnO bulk ceramics prepared by solid state reaction route at 1300 C exhi... more Abstract We observed that ZnO bulk ceramics prepared by solid state reaction route at 1300 C exhibited unexpected room temperature ferromagnetic (RTFM) property. In the absence of any magnetic impurity the cause of room temperature ferromagnetic signal in the undoped ...

With a motivation to compare the magnetic property, we synthesised undoped, transition metal (TM)... more With a motivation to compare the magnetic property, we synthesised undoped, transition metal (TM) Mn doped and (Mn:Fe) co-doped ZnO ceramics in the compositions ZnO, Zn 0.98 Mn 0.02 O and Zn 0.96 (Mn 0.02 Fe 0.02 )O. Systematic investigations on the structural, microstructural, defect structure and magnetic properties of the samples were performed. Low temperature as well as room temperature ferromagnetism has been observed for all our samples, however, enhanced magnetisation at room temperature has been noticed when ZnO is co-doped with Fe along with Mn. Particularly the sample with the composition Zn 0.96 Mn 0.02 Fe 0.02 O showed a magnetisation value more than double of the sample with composition Zn 0.98 Mn 0.02 O, indicating long range strong interaction between the magnetic impurities leading to higher ferromagnetic ordering. Raman and PL studies reveal presence of higher defects in form of oxygen vacancy clusters created in the sample due to Fe co doping. PL study also reveals enhanced luminescence efficiency in the co doped sample. Temperature dependent magnetisation study of this sample shows the spin freezing temperature around 39 K indicating the presence of small impurity phase of Mn 2 À x Zn x O 3 type. Electron Spin Resonance signal obtained supports ferromagnetic state in the co doped sample. Enhancement of magnetisation is attributed to interactions mediated by magnetic impurities through large number of oxygen vacancies created by Fe 3 þ ions forming bound magnetic polarons (BMP) and facilitating long range ferromagnetic ordering in the co-doped system.