Cation redistribution in ultrafine zinc ferrite subjected to high energy ball milling

Santhosh D.Shenoy^$, P.A.Joy*, Alok Banerjee⁺ and M.R.Anantharaman^$

^$Department of Physics, Cochin university of Science and Technology, Cochin, 682 022

^*Physical Chemistry Division, National Chemical Laboratory, Pune 411 008

⁺Inter university consortium for DAE Facilities, Khandwa road, Indore-452 017

Zinc ferrite in the micron regime is a normal spinel with no net magnetisation. However, zinc ferrite in the fine regime has been reported to be exhibiting ferrimagnetic properties and has been a subject of intense research recently. Ultrafine zinc ferrite particles (ZnFe₂O₄) with a very small amount of alpha ferric oxide (aFe₂O₃) prepared by low temperature chemical coprecipitation technique were subjected to high energy ball milling under dry conditions. These samples were milled for several hours and subjected to structural and magnetic studies at various stages of milling. XRD analysis indicates that the particles are nanomertric in size. aFe₂O₃ phase was found to be increasing with milling time. Strain calculations indicates a sharp increase in the lattice strain with milling time. Room temperature magnetization curves show a typical superparamagnetic feature for unmilled sample but with milling, hysteresis was observed. The saturation magnetization (M_s) attains a higher value of 16.41 emu/gm for 10 hours milled sample while the initial coprecipitated ZnFe₂O₄ exhibits an M_s of 2.196 emu/gm. AC magnetic susceptibility studies at 133.33Hz and 2 Oe field indicates a blocking temperature (T_B) shift towards higher temperature with milling. Below the blocking temperature the magnetization direction within each nanoparticle aligns along the easy axis of the nanoparticle. But the random arrangement of easy axes of nanoparticles causes the magnetic susceptibility to decrease steadily below the blocking temperature. For unmilled sample the T_B is well below liquid nitrogen temperature while for 10 hours milled sample this rises to around 230⁰K. X-ray diffraction, room temperature VSM and low temperature AC susceptibility studies suggests cation reversal that might have been facilitated by the presence of aFe₂O₃ in the initial sample. The local temperature and pressure developed during high energy ball milling would have contributed to the migration of Zn²⁺ cation from the tetrahedral (A) site to the octahedral (B) site.

SPINEL STRUCTURE

My PhD thesis conclusions

STUDIES ON THE EFFECT OF HIGH ENERGY BALL MILLING ON THE STRUCTURAL, MAGNETIC AND ELECTRICAL PROPERTIES OF SOME NORMAL SPINELS IN THE ULTRAFINE REGIME

Magnetism and magnetic materials have been a fascinating subject for the mankind ever since the discovery of loadstone. Since then, man has been applying this principle of magnetism to build devices for various applications. The discovery of ferrites in the early 20th century and the subsequent theory of ferrimagnetism propounded by Louie Neel resulted in a surge of research activities. The emergence of nanoscience and nanotechnology during the last decade had its impact in the field of magnetism and magnetic materials too. Now, it is common knowledge that materials synthesized in the nanoregime exhibit novel and superlative properties with respect to their coarser sized counterparts in the micron regime. For instance, a ferrite material prepared in the nanoregime will exhibit altogether different magnetic and electrical properties when compared to their cousins in the micron regime.
Spinel ferrites resemble the structure of the naturally occurring mineral MgAl2O4. They are further categorized into two- normal and inverse spinels. Zinc ferrite belongs to the class of normal spinel. Zinc ferrite when prepared in the micron regime by ceramic methods is antiferromagnetic with TN » 10K. Recently many researchers, worldwide, observed anomaly in the magnetic properties of zinc ferrites when they are prepared in the ultrafine regime. Contrary to the earlier beliefs, they exhibited net magnetisation at room temperature. The reason for this anomalous behaviour of nanozinc ferrites is still unknown and many schools of thought exist. One theory is that the cation redistribution on the A and B sites is responsible for the ferrimagnetic ordering found in these nanomaterials. Others believe the existence of superparamagnetic particles and spin clusters being responsible for the enhanced magnetization. Its also reasonable to assume that surface spins or surface magnetism play a very vital role in determining the magnetic properties of these type of materials in the ultrafine regime. So zinc ferrite can be a candidate material for checking any of the hypotheses mentioned above. Moreover, finite size effects on the magnetic properties also play a crucial role in deciding the performance characteristics of a magnetic material when employed as a device. So study of various properties, namely, structural, electrical and magnetic assume significance both from the fundamental and application point of view. With this motivation, a systematic study on ultrafine zinc ferrite is undertaken by synthesizing the zinc ferrite in the fine particle regime by cold coprecipitation methods.
The study of the finite size effect on the structural, electrical and magnetic properties was carried out by systematically reducing the size of the as prepared materials by high-energy ball milling. Both wet milling and dry milling were employed to fine-tune the size. Various analytical tools like X-ray diffraction, Transmission electron microscopy, vibration sample magnetometer, ac magnetic susceptibility, Mössbauer spectrometer, Keithley source measuring unit and impedance analyzer were employed to characterize and evaluate the properties at various stages of preparation. The results are correlated in order to tailor the properties and to propose a plausible mechanism for the anomaly exhibited by zinc ferrites in the ultrafine regime.
Zinc aluminate (ZnAl2O4), widely used catalyst is a nonmagnetic normal spinel and is an ideal template to test the hypothesis of cation redistribution, if any, in the nanoregime. Moreover the finite size effects on the electrical and structural properties are also studied for comparison sake.
XRD pattern of zinc ferrite samples indicate presence of small amount of a-Fe2O3 in all milled samples whose percentage is found to be increasing with milling time. In 10 hours milled sample ZnO phase is also detected suggesting decomposition of ZnFe2O4 to a-Fe2O3 and ZnO. The broad shape of the diffraction peaks replicates the formation of fine particle structure with small crystallite size distribution and existence of strong internal lattice strains, which is introduced during high-energy ball milling. This decrease in particle size as the milling time increases is due to the fact that the kinetic energy generated by the series of collisions among balls is transferred to the zinc ferrite powder. The sudden decrease in grain size is not achieved in this case as in the case of particles prepared by the solid-state reaction method. The reason is that the coprecipitated zinc ferrite, which is the starting material, itself is of nanometer sized before the start of milling. Prolonged milling reduces the lattice strain and increases the grain size. This may be due to the high local temperature and pressure generated during the combustion as a result of the high-energy ball milling and the difficulty in maintaining the very high stress during milling. Lattice parameter reduction observed with milling gave the first hint of a possible cation redistribution induced by milling in these samples.

My MSc project conclusions

Single phasic LiFe samples containing Zn and Ti with and without Bi2O3 have been prepared. The structural parameters have been evaluated and particle is estimated. Dielectric measurements have been carried on these samples at different frequencies and different temperatures. The dielectric constants of the samples lie in the range 20 to 40 except in the case of LiTiFe. LiTiFe without Bi2O3 exhibited a dielectric constant of 1400. These dielectric values are ideal ones for microwave ferrites. The advantage of adding Ti and Zn is to increase the 4pMs values and also to inhibit grain growth.
Investigations involving sac in conjunction with sdc will aid in explaning the dielectric behaviour of these ferrites. Further scope exists to do a systematic study to ascertain the role Bi2O3 in the structural dielectric and electrical properties of these ferrities.