Electronic Theses and Dissertations





Document Type


Degree Name

Master of Science




Materials Science

Committee Chair

Sanjay R Mishra

Committee Member

Muhammad S Jahan

Committee Member

Thang Ba Hoang


M-type hexaferrite, MeFe12O19, (Me = Ba, Sr, Pb) finds wide industrial applications due to its high saturation magnetization, coercivity, and electrical resistivity. The magnetic and electrical properties of hexaferrites are often altered to meet the application needs. The alteration of hexaferrite properties is possible via substitution of either Sr/Ba/Pb or Fe with magnetic, non-magnetic, or rare-earth ions. Earlier, our lab reported Al3+ Sr-hexaferrite with a very high coercivity of 18 kOe and saturation magnetization 9 emu/g, but with a concomitant reduction in magnetization. This obviously leads to the search of either synthesis technique or material where enhancement of coercivity is possible without scarifying magnetization of the magnet.In the present study, we have adapted the auto-combustion technique to prepare pure-phase doped hexaferrite (Sr1-x/12Mx/12Fe12-xAlxO19, M = Ca2+, Ba2+, and La3+) to enhance coercivity greater than 18 kOe while maintaining a high saturation magnetization. Our strategy involves substitution of Sr2+ with Ca2+, Ba2+, or La3+ and Fe3+ with Al3+ non-magnetic ion. Following the synthesis, a detail structural, magnetic, and Mössbauer study was undertaken to understand the structure-property relationship in doped hexaferrite compounds. The lattice parameters a and c, unit cell volume, and grain size reduction was observed with the doping in all compounds. The lattice parameter variation is in accordance with the differences in ionic radii of the parent compound and the guest ions, as Ca2+-Al3+ substitution shows highest lattice contraction. The magnetic parameters extracted from the magnetization vs. field measurement shows that a record coercivity of 24.1kOe is observed forSr0.67Ca0.33Fe8Al4O19 sample with Ms = 14emu/g. The saturation magnetization of SrFe12-xAlxO19, Sr1-x/12Cax/12Fe12-xAlxO19, Sr1-x/12Bax/12Fe12-xAlxO19, and Sr1-x/12Lax/12Fe12-xAlxO19 decreased at the rate of about 20.4%, 20.7%, 20.8%, and 21.7% with x, respectively. The Curie temperature also shows approximately a linear reduction, at the rate of about 12.7% with x for all compounds. The Mossbauer spectroscopy shows that Al3+ prefer to occupy the 4f1site. The hyperfine field show reduction due to magnetic dilution effect while the reduction in isomer shift value occurs with the decrease in s-electron density at the iron nucleus due to lattice contraction. The quadrupole shift value of 2b site showed a considerable reduction in all compounds, which could affect the magneto-crystalline anisotropy of the compound. From the detailed experimental study it is concluded that (1) All samples show reduction in magnetization with doping due to magnetic dilution effect, (2) Al3+ substitution occurs at 4f1 Fe3+ site for all samples calcined at 1200oC, (3) a high coercivity value up to 24 kOe with saturation magnetization value of 14 emu/g is achieved for Ca2+-Al3+ compound, (4) among Ca2+, Ba2+ and La3+ doping, Ca2+ has maximum influence on coercivity enhancement, (5) combined effect of grain refinement and magneto-crystalline anisotropy could contribute to the observed high coercivity value of the compound, (6) comparatively high saturation magnetization value is observed for x =4 samples, which could result from a critical balance between super-exchange interaction and lattice distortion, (7) Ca2+-Al3+ and Ba2+ -Al3+ doping maintains high magnetization while La2+-Al3+ brings greater reduction in magnetization value due to charge compensation effect, (8) Hopkinson peak was observed in all samples because of transition of ferrimagnetic to superparamagnetic phases of nanoparticles, and (9) the Curie temperature for all samples shows reduction with the substitution due to weakening of the super-exchange interaction strength upon lattice distortion.


Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.