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R. N. Bhowmik

Publications and source records attributed to R. N. Bhowmik.

32 records · Page 2Linked to original sources
arXiv2011

Synthesis and study of alpha-Fe1.4Ga0.6O3: An advanced Ferromagnetic Semiconductor

We report the synthesis of alpha-Fe1.4Ga0.6O3 compound and present its structural phase stability and interesting magnetic, dielectric and photo-absorption properties. In our work Ga doped alpha-Fe2O3 samples are well stabilized in alpha phase (rhombohedral crystal structure with space group R3C). Properties of the present composition of Ga doped alpha-Fe2O3 system are remarkably advanced in comparison with recently most studied FeGaO3 composition. At room temperature the samples are typical soft ferromagnet, as well as direct band gap semiconductor. Dielectric study showed low dielectric loss in the samples with large enhancement of ac conductivity at higher frequencies. Optical absorption in the visible range has been enhanced by 4 to 5%. This composition has exhibited large scope of tailoring room temperature ferromagnetic moment and optical band gap by varying grain size and non-ambient (vacuum) heat treatment of the as prepared samples by mechanical alloying.

cond-mat.mtrl-sci↗
arXiv2010

Evidence of Ferrimagnetism in Ferromagnetic La0_{67}Ca0_{33}MnO_3 nanoparticle

The present report is dedicated to show that ferromagnetic La0.67Ca0.33MnO3 (LCMN) particles can be better described in the framework of ferrimagnetic model. To confirm the ferrimagnetic signature in ferromagnetic LCMN particles, the temperature dependence of the inverse of magnetic susceptibility in the paramagnetic state of the samples was taken as a tool of data analysis. The observed ferrimagnetism is understood as an effect of of the core-shell spin structure in LCMN particles.

cond-mat.mes-hall↗
arXiv2009

Reproducibility of room temperature ferromagnetism in Zn0.95Mn0.05O and its understanding

The present work reproduces the room temperature ferromagnetism by doping magnetic Mn atoms in diamagnetic ZnO. The ferromagnetic ordering is extended up to 640 K in mechanical milled Zn0.95Mn0.05O samples. The bulk and nanocrystalline samples are stabilized in hexagonal crystal structure with space group p63mc. The grain size and lattice strain of the samples, estimated from XRD spectrum using Williamson-Hall plot, showed a significant variation with milling time. Surface structure (morphology, distribution of grains and elements) is observed to be reasonably good from SEM picture and EDX spectrum at room temperature. The ferromagnetic ordering in bulk, milled and alloyed samples is primarily due to the diffusion of Mn(2+) ions into the lattice sites of ZnO. The enhancement of magnetic moment and ferromagnetic ordering temperature with reducing the grain size is discussed in terms of the existing theoretical predictions and experimental works. The role of mechanical milling induced disorder for the enhancement of room temperature ferromagnetism in Zn0.95Mn0.05O (dilute magnetic semiconductor) is also highlighted.

cond-mat.str-el↗
arXiv2008

Disorder Effects in La substituted ferrimagnetic Ca2FeMoO6 double perovskite

Ca2-xLaxFeMoO6 double perovskite with La concentration x = 0 to 0.6 was synthesized using solid state sintering route. The standard techniques of XRD, SEM and EDX were applied to characterize the material. Crystal structure of the samples was stabilized in monoclinic phase with space group P2I/n and lattice expansion was indicated with the increase of x. The increase of La concentration gradually suppressed the coexisting minor secondary phase in the material and simultaneously, EDX results indicated the accommodation of more Mo atoms in the crystal structure. A significant modification in the surface morphology of the material was noted from adhesive type surface for x = 0 to brittle type surface with more grain boundary contributions for La doped samples. We understand a significant change in magnetic properties (appearance of cluster glass component, reduction of magnetic moment and indication of higher TC) and in electrical properties (reduction of metallic character) in terms of enhanced internal disorder in the material, introduced due to La doping in double perovskite structure.

cond-mat.mtrl-sci↗
arXiv2008

Magnetic Response of NiFe2O4 nanoparticles in polymer matrix

We report the magnetic properties of magnetic nano-composite, consisting of different quantity of NiFe2O4 nanoparticles in polymer matrix. The nanoparticles exhibited a typical magnetization blocking, which is sensitive on the variation of magnetic field, mode of zero field cooled/field cooled experiments and particle quantity in the matrix. The samples with lower particle quantity showed an upturn of magnetization down to 5 K, whereas the blocking of magnetization dominates at lower temperatures as the particle quantity increases in the polymer. We examine such magnetic behaviour in terms of the competitive magnetic ordering between core and surface spins of nanoparticles, taking into account the effect of inter-particle (dipole-dipole) interactions on nanoparticle magnetic dynamics.

cond-mat.mtrl-sci↗
arXiv2008

Structural phase stability and Magnetism in Co2FeO4 spinel oxide

We report a correlation between structural phase stability and magnetic properties of Co2FeO4 spinel oxide. We employed mechanical alloying and subsequent annealing to obtain the desired samples. The particle size of the samples changes from 25 nm to 45 nm. The structural phase separation of samples, except sample annealed at 9000C, into Co rich and Fe rich spinel phase has been examined from XRD spectrum, SEM picture, along with EDAX spectrum, and magnetic measurements. The present study indicated the ferrimagnetic character of Co2FeO4, irrespective of structural phase stability. The observation of mixed ferrimagnetic phases, associated with two Curie temperatures at TC1 and TC2 (>TC1), respectively, provides the additional support of the splitting of single cubic spinel phase in Co2FeO4 spinel oxide.

cond-mat.mtrl-sci↗
arXiv2008

Magnetic Modulation in Mechanical Alloyed Cr1.4fe0.6o3 Oxide

We have synthesized Cr1.4Fe0.6O3 compound through mechanical alloying of Cr2O3 and Fe2O3 powders and subsequent thermal annealing. The XRD spectrum, SEM picture and microanalysis of EDAX spectrum have been used to understand the structural evolution in the alloyed compound. The alloyed samples are matching to rhombohedral structure with R3C space group. The observation of a modulated magnetic order confirmed a systematic diffusion of Fe atoms into the Cr sites of lattice structure. A field induced magnetic behaviour is seen in the field dependence of magnetization data of the annealed samples. The behaviour is significantly different from the mechanical alloyed samples. The experimental results provided the indications of considering the present material as a potential candidate for opto-electronic applications.

cond-mat.mtrl-sci↗
arXiv2008

Lattice disorder and Ferromagnetism in La0.67Ca0.33MnO3 nanoparticle

We study the ferromagnetism of La0.67Ca0.33MnO3 in bulk polycrystalline, nanocrystalline and amorphous phase. The structural change from crystalline phase to amorphous phase exhibited a systematic decrease of TC(paramagnetic to ferromagnetic transition temperature) and spontaneous magnetization (MS). The experimental results suggested few more features, e.g., appearance of large magnetic irreversibility in the temperature dependence of magnetization, lack of magnetic saturation at high magnetic field, blocking of magnetization below TB, and enhancement of coercivity. In addition, the magnetic phase transition near to TC has changed from first order character in bulk sample to second order character in nanocrystalline and amorphous samples. We understand the observed magnetic features as the effects of decreasing particle size and increasing magnetic (spin- lattice) disorder. We noted that magnetic dynamics of amorphous samples is distinctly different from the nanocrystalline samples. The ferromagnetism of amorphous samples are comparable with the properties of reported amorphous ferromagnetic nanoparticles. We also demonstrate the effect of disorder shell in controlling the dynamics of ferromagnetic cores.

cond-mat.mtrl-sci↗
arXiv2006

Unconventional relaxation in antiferromagnetic CoRh$_2$O$_4$ nanoparticles

Magnetic relaxation in antiferromagnetic CoRh$_2$O$_4$ nanoparticles is investigated at 2 K by cooling the sample from a temperature (70 K) well above the antiferromagnetic ordering temperature at 27 K, following zero field cooled (ZFC) and field cooled (FC) process. In ZFC process, the sample at 2 K is subsequently followed by magnetic field on and off sequences, whereas in FC process the cooling field is made off during measurement of remanent magnetization as a function of time. The experiments suggest an unconventional relaxation behaviour in the system, as an effect of increasing surface exchange anisotropy with decreasing the size of antiferromagnetic nanoparticles.

cond-mat.mtrl-sci↗
arXiv2005

Lattice expansion and non-collinear to collinear ferrimagnetic order in MnCr$_2$O$_4$ nanoparticle

We report magnetic behaviour of MnCr$_2$O$_4$, which belongs to a special class of spinel, known as chromite. Bulk MnCr$_2$O$_4$ shows a sequence of magnetic states, which follows paramagnetic (PM) to collinear ferrimagnetic (FM) state below T$_C$ $\sim$ 45 K and collinear FM state to non-collinear FM state below T$_S$ $\sim$ 18 K. The non-collinear spin structure has been modified on decreasing the particle size, and magnetic transition at T$_S$ decreases in nanoparticle samples. However, ferrimagnetic order is still dominating in nanoparticles, except the observation of superparamagnetic like blocking and decrease of spontaneous magnetization for nanoparticle. This may, according to the core-shell model of ferrimagnetic nanoparticle, be the surface disorder effect of nanoparticle. The system also show the increase of T$_C$ in nanoparticle samples, which is not consistent with the core-shell model. The analysis of the M(T) data, applying spin wave theory, has shown an unusual Bloch exponent value 3.35 for bulk MnCr$_2$O$_4$, which decreases and approaches to 1.5, a typical value for any standard ferromagnet, with decreasing the particle size. MnCr$_2$O$_4$ has shown a few more unusual behaviour. For example, lattice expansion in nanoparticle samples. The present work demonstrates the correlation between a systematic increase of lattice parameter and the gradual decrease of B site non-collinear spin structure in the light of magnetism of MnCr$_2$O$_4$ nanoparticles.

cond-mat.mtrl-sci↗
arXiv2003

Static magnetic response of clusters in Co_{0.2}Zn_{0.8}Fe_{1.95}Ho_{0.05}O_{4} spinel oxide

Earlier investigation of Co_{0.2}Zn_{0.8}Fe_{1.95}Ho_{0.05}O_{4} spinel has shown the existence of "super-ferromagnetic " clusters containing Fe^{3+} and Ho^{3+} ions along with small size clusters of Fe^{3+} ions (Bhowmik et al, J. Magn. Magn. Mater. {247}, 83 (2002)). Here, We report the static magnetic response of these clusters. The experimental data suggests some interesting magnetic features, such as, enhancement of magnetization; re-entrant magnetic transitions with paramagnetic to ferromagnetic state below 225 K and ferromagnetic to spin glass state below 120 K; appearance of field induced ferromagnetism. We also observe an unusual maximum in the thermoremanent magnetization (TRM) vs temperature data. Our measurements suggest that this unusuality in TRM is related to the blocking of "super-ferromagnetic" clusters ,out of the ferromagnetic state, along their local anisotropy axis.

cond-mat.soft↗
arXiv2002

Particle size effects in the antiferromagnetic spinel CoRh$_2$O$_4$

We report the particle size dependent magnetic behaviour in the antiferromagnetic spinel CoRh2O4. The nanoparticles were obtained by mechanical milling of bulk material, prepared under sintering method. The XRD spectra show that the samples are retaining the spinel structure. The particle size decreases from 70 nm to 16 nm as the milling time increases from 12 hours to 60 hours. The magnetic measurements suggest that the antiferromagnetic ordering at T$_N$ $\approx$ 27K exists in bulk as well as in nanoparticle samples. However, the magnitude of the magnetization below T$_N$ increases with decreasing particle size.

cond-mat.dis-nn↗
arXiv2002

Magnetic enhancement of Co$_{0.2}$Zn$_{0.8}$Fe$_2$O$_4$ spinel oxide by mechanical milling

We report the magnetic properties of mechanically milled Co$_{0.2}$Zn$_{0.8}$Fe$_2$O$_4$ spinel oxide. After 24 hours milling of the bulk sample, the XRD spectra show nanostructure with average particle size $\approx$ 20 nm. The as milled sample shows an enhancement in magnetization and ordering temperature compared to the bulk sample. If the as milled sample is annealed at different temperatures for the same duration, recrystallization process occurs and approaches to the bulk structure on increasing the annealing temperatures. The magnetization of the annealed samples first increases and then decreases. At higher annealing temperature ($\sim$ 1000$^{0}$C) the system shows two coexisting magnetic phases {\it i.e.}, spin glass state and ferrimagnetic state, similar to the as prepared bulk sample. The room temperature Mössbauer spectra of the as milled sample, annealed at 300$^{0}$C for different durations (upto 575 hours), suggest that the observed change in magnetic behaviour is strongly related with cations redistribution between tetrahedral (A) and octahedral (O) sites in the spinel structure. Apart from the cation redistribution, we suggest that the enhancement of magnetization and ordering temperature is related with the reduction of B site spin canting and increase of strain induced anisotropic energy during mechanical milling.

cond-mat.soft↗