B. L. Patil

Structural, IR and magnetisation studies on La3+ substituted copper ferrite

Abstract The ferrites of the composition CuLa 2x Fe 2−2x O 4 prepared by ceramic method exhibited spinel phases for X upto 0.4 whereas for x > 0.4 the mixed phases of spinel and pervoskite were observed. For spinel phase x x > 0.4 the lattice constants of spinel phase increase and remain constant. The pervoskite phase has been indentified to be that of La 2 CuO 4 . The compositional variation of magnetisation shows decrease of n B with the increase of x. However, the samples with x =0.6 do not exhibit net magnetisation. The Curie temperature shows decrease with x for upto 0.4 while for x =0.4 Curie temperature increases and remains invariant. This constant value of T C is nearly equal to the Curie point for pure copper ferrite. The variation of magnetisation has been discussed on the basis of AB interaction and migration of Cu, Fe to A site from B site. La goes to B site only. The I.R. spectra reveal two extra prominent bonds for x =0.4. The change in the force constants is explained by considering the oxygen vacancies and La ions on B site.

Electrical properties of Si4+ substituted copper ferrite

The electrical resistivity, ϱ, and Seebeck coefficient, α, for the system Cu1+xSixFe2−2xO4 (where x = 0.05, 0.1, 0.15, 0.2 and 0.3) have been studied as a function of temperature. Temperature variation of the resistivity exhibits two breaks. Each break is associated with a change in activation energy. The conduction process at low temperature is governed by the reaction CuA1++ CuA2+→CuA2++ CuA1+. However, at higher temperature, it is due to intersite cation exchange and reoxidation such as CuA2++ FeB3+→ CuB2++ FeA3+. Measurement of the Seebeck coefficient, α, from room temperature to 800 K reveals n-type conduction for the sample with x= 0.05, while the measurements for other samples show p-type conduction for lower temperatures and n-type conduction at higher temperatures. The activation energies in the paramagnetic region are found to be less than those in the ferrimagnetic region.

Conduction mechanism in Sn4+ substituted copper ferrite

Thermoelectric power (α) and electrical resistivity (ρ) are reported for the system Cu1+xSnxFe2−2xO4 (wherex=0·05, 0·1, 0·15, 0·2 and 0·3) from room temperature to 800 K. The compositions withx=0·05 and 0·2 exhibitn-type conduction while the compositions withx=0·1 and 0·15 showp- ton-type conduction change after 423 K. The conduction at low temperature (i.e. < 400 K) is due to impurities, while at higher temperature (i.e. > 400 K), it is due to polaron. Hopping conduction phenomenon for the present system has been explained on the basis of localized model of electrons. Additional localization may arise due to Sn4+ + Fe2+ stable pairs at B-site and Cu1+ + Fe3+ pair at A-site.

The electrical properties of Ge4+ substituted copper ferrite

The electrical resistivity (ρ) and thermoelectric power (α) were measured in the temperature range from 300 K to 750 K on the system Cu1+xGexFe2−2xO4 with x=0.05, 0.1, 0.15, 0.2 and 0.3. The dependence of electrical properties with temperature showed three regions. In the first region the conduction is due to impurities whereas in the second and third it is due to polaron hopping. The temperature dependence ofα andρ suggests that the transport properties measured may be interpreted on the basis of polaron hopping. The activation energy in paramagnetic region is found to be less than that in ferrimagnetic region. This behaviour therefore can be stated to be anomalous.

Bulk magnetic properties of tetravalent ion substituted Cu-ferrite

The lattice parameter,a, increases with the increase of Ti4+ substitution while it decreases with the increase of Ge4+ substitution in Cu1+xAxFe2−2xO4 compounds [A=Ti4+, Ge4+Sn4+]. On substitution of Sn4+,a does not vary. CuFe2O4 exhibits single domain (SD) behaviour that changes to multi domain (MD) type on Ti4+ substitution. On Ge4+ substitution initially all the compositions exhibit SD behaviour while forx>0.2 they show MD behaviour. All Sn4+ substituted compositions exhibit SD behaviour. The Curie temperature decreases on addition of Ti4+, Sn4+ and Ge4+. The magnetic momentnB (77 K) initially increases and then decreases with the substitution of Ti4+, Ge4+, and Sn4+. This is explained by cation distribution and reduction of Fe3+ ions.

Stability of phases in copper ferrite

Abstract Phase stabilities in copper ferrite are studied with substitution of Ti and Ge tetravalent ions. In Ti-substituted samples (Cu1+x Ti x Fe2x O4) the parent tetragonal structure is retained up to x=0.15, and for higher content of Ti it changes to cubic. In the case of Ge-substituted samples (Cu1+x Ge x Fe2–2x O4), the cubic structure is observed for 0.0<x≤0.2, and the tetragonal structure reverts for higher contents of Ge. The origin of the distortion lies in square covalent bond formation of copper ions at the octahedral site. The observed results are explained on the basis of cation distribution and covalent bond formation of copper and germanium ions.

Tetravalent titanium addition to copper ferrite electrical properties

Electrical resistivity of the ferrites Cu1 + xTixFe2 - 2xO4 (x equals 0 to 0.5 insteps 0.05) has been measured in the temperature region 300 K to 700 K. It is seen that the plots of log (Rho) Vs 1/T exhibit a linear relationship throughout the temperature range. The conduction mechanism in these ferrites is explained on the basis of the hopping mechanism. The conduction in the low-temperature region (< 370 K) is due to impurities defects and interstitials, etc. In the high-temperature region (i.e., > 400 K), it is mainly due to hopping of small polarons. A sharp deviation from linearity is observed around 520 K which is related to the transition temperature close to tetragonal to cubic transformation. The compositional variation of resistivity increases slowly at lower content of Ti < 0.2 and monotonously for higher content > 0.3. The slight decrease in resistivity at x equals 0.2 is related to structural change from tetragonal to cubic. The variation of resistivity with Tix is attributed to the hindering of the mechanism for x < 0.2 and for x > 0.3 to impurities, in homogenous distribution of cations in the spinel.

Electrical properties of Cd0.3Ni0.7+xMnxFe2O4 ferrites

Electrical resistivity of Cd0.3Ni0.7 + xMnxFe2 - 2xO4 ferrites has been measured in the temperature range from 300 K to 1000 K. Three distinct regions have been observed in log (rho) Vs 103/T curves for all the samples. This is explained on the basis of the hopping mechanism. The conduction in the first region is due to impurity charge carriers. In the second and third regions it is influenced by the order-disorder hopping mechanism. The variation of resistivity with Mn concentration shows an increase in resistivity up to x equals 0.10 and decreases with further increase of x. The increase in resistivity is due to stable bond formation of Fe2 + Mn+3 at B site which hinders the Verway mechanism. The decrease in resistivity is attributed to the formation of Ni3+, Mn3+ clusters and Mn3O4 + Mn2O3 impurity phases. Additional trapping due to local Jahn-Teller distortion around Mn3 and Mn4 also plays an important role in these materials.