The comparative study of the substitution of zinc and lithium for iron in the 114 ferrites, YBaFe4O7 and CaBaFe4O7, shows that these diamagnetic cations play a major role in tuning the competition between ferrimagnetism and magnetic frustration in th
ese oxides. The substitution of Li or Zn for Fe in the cubic phase YBaFe4O7 leads to a structural transition to a hexagonal phase YBaFe4-xMxO7, for M = Li (0.30 < x < 0.75) and for M = Zn (0.40 < x < 1.50). It is seen that for low doping values i.e. x = 0.30 (for Li) and x = 0.40 (for Zn), these diamagnetic cations induce a strong ferrimagnetic component in the samples, in contrast to the spin glass behaviour of the cubic phase. In all the hexagonal phases, YBaFe4-xMxO7 and CaBaFe4-xMxO7 with M = Li and Zn, it is seen that in the low doping regime (x ~ 0.3 to 0.5), the competition between ferrimagnetism and 2 D magnetic frustration is dominated by the average valency of iron. In contrast, in the high doping regime (x ~ 1.5), the emergence of a spin glass is controlled by the high degree of cationic disorder, irrespective of the iron valency.
The study of the ferrites YBaFe4-xGaxO7 shows that the substitution of Ga for Fe in YBaFe4O7 stabilizes the hexagonal symmetry for 0.40 < x < 0.70, at the expense of the cubic one. Using combined measurements of a. c. and d. c. magnetization, we esta
blish that Ga substitution for Fe in YBaFe4O7 leads to an evolution from a geometrically frustrated spin glass (for x = 0) to a cationic disorder induced spin glass (x = 0.70). We also find an intermediate narrow range of doping where the samples are clearly phase separated having small ferrimagnetic clusters embedded in a spin glass matrix. The origin of the ferrimagnetic clusters lies in the change in symmetry of the samples from cubic to hexagonal (and a consequent lifting of the geometrical frustration) as a result of Ga doping. We also show the presence of exchange bias and domain wall pinning in these samples. The cause of both these effects can be traced back to the inherent phase separation present in the samples.
The d.c. magnetization and magnetic relaxation studies of the calcium doped samples, Y0.95Ca0.05BaCo2O5.5 and YBa0.95Ca0.05Co2O5.5, show the existence of a magnetic glass like behaviour in the family of cobaltites for the first time. Our investigatio
ns reveal glass-like arrest of kinetics at low temperature which prevents the system from reaching its magnetic ground state. We show that the low temperature state of these calcium doped phases, which consists of coexisting antiferromagnetic and ferro (or ferri) magnetic phase fractions, can be tuned in a number of ways. Our observations establish that the low temperature state of this oxide is not in thermal equilibrium. The glassy state is formed with the assistance of an external magnetic field, which makes it distinctly different from the more well known metastable state, the spin glass state. The cooling field can tune the fractions of the coexisting phases, and the glass-like state formed at low temperature can also be devitrified by warming the sample. The role of Ca doping in the appearance of these phenomena is discussed in terms of phase separation, involving Co3+ disproportionation into Co4+ ferromagnetic clusters and Co2+ antiferromagnetic clusters.
An oxygen hyperstoichiometric ferrite CaBaFe4O7+delta (delta approx 0.14) has been synthesized using soft reduction of CaBaFe4O8. Like the oxygen stoichiometric ferrimagnet CaBaFe4O7, this oxide also keeps the hexagonal symmetry (space group: P63mc),
and exhibits the same high Curie temperature of 270 K. However, the introduction of extra oxygen into the system weakens the ferrimagnetic interaction significantly at the cost of increased magnetic frustration at low temperature. Moreover, this canonical spin glass (Tg ~ 166 K) exhibits an intriguing cross-over from de Almeida-Thouless type to Gabay-Toulouse type critical line in the field temperature plane above a certain field strength, which can be identified as the anisotropy field. Domain wall pinning is also observed below 110 K. These results are interpreted on the basis of cationic disordering on the iron sites.
The triple magnetic-transport-structural transition versus temperature in three series of 114 cobaltites - Y1-xYbxBaCo4O7, Y1-xCaxBaCo4O7 and Yb1-xCaxBaCo4O7 - has been studied using magnetic, transport and differential scanning calorimetric measurem
ents. The effect of the size mismatch {sigma}2, due to cationic disordering at the Ln sites upon such a transition is shown for the first time in a triangular lattice. We show that increasing <rLn> has an effect of increasing TS dramatically, while the size mismatch {sigma}2 at the Ln sites decreases TS substantially. Moreover, the cationic mismatch at the Ln sites modifies the nature of the hysteretic transition by turning the sharp first order transition seen in the undoped samples into an intermix of first and second order transitions. These results are discussed on the basis of the particular nature of the high temperature form which exhibits a hexagonal close packed structure (space group: P63mc) with respect to the low temperature orthorhombic form (space group: Pbn21), the latter corresponding to a distortion of the former due to a puckering of the kagome layers.
