Magnetic study of M-type doped barium ferrite nanocrystalline powders

Abstract

We have studied the static magnetic properties of three different M-type doped barium ferrite compounds prepared by the glass crystallization method. The zero-field-cooled (ZFC) and field-cooled (FC) processes have been recorded at low field and they all show the typical features of a small particle system. The ZFC curves display a broad peak at a temperature T,, which depends on the distribution of particle volumes in the sample. Isothermal magnetization curves M(H) at several temperatures and saturation magnetization M, as a function of temperature have been measured for the Co-Ti sample (BaFe 10,4Coc.8Tic8019). The dependence on temperature of the macroscopic magnetic parameters has been analyzed. The distribution of blocking temperatures is studied from the derivative of the remanent-to-saturation magnetization ratio with respect to temperature and it is fitted to a lognormal distribution, leading to a mean blocking temperature ( TB) = (8 1 f 40) K. The distribution of volumes of the magnetic unit is also obtained from this lltting. The dependence on temperature of the coercive field follows a Tk-law below 35 K. The value of the k exponent is discussed within the scope of two models: (i) the aligned case (k=0.5) and (ii) the random case (k=0.77).

Tunneling magnetoresistance inCo−ZrO2granular thin films

“…However, it is well known that the bulk value is an overestimation of the particle magnetization in fine particle systems, due to strong surface and finite-size effects. 34,35 In our case, around 50-60 % of Co atoms are affected by the lowcoordination number at the surface, giving rise to smaller M S as compared to bulk Co. Consequently, this method leads to an underestimation of the average particle size 34,35 as compared to that obtained from TEM.…”

Interaction effects and energy barrier distribution on the magnetic relaxation of nanocrystalline hexagonal ferrites

“…The fitted values were the following: T B0 ϭ 81 K ͑peak of the distribution͒, T B ϭ 86 K ͑mean blocking temperature͒ and ϭ0.38 ͓half width of the ln(T B ) distribution͔. 11 The same log-normal distribution of volumes was found by fitting the magnetization curves in the SPM regime to a distribution of Langevin function. 27 The low-field susceptibility of an assembly of interacting particles in the SPM regime is expected to be of the form…”

“…f 1 (E) describes the extra contribution to the lowest energy barriers, while f 2 (E) is centered at high energies and describes the contribution of non or weakly interacting particles and/or particles with magnetizing interactions ͑which shifts energy to higher values͒. As p is much lower than 1, the effective distribution of energy barriers f (E) is dominated essentially by f 2 (E) so that the high temperature relaxation measurements, remanent-to-saturation data, 11 and the isothermal magnetization curves in the SPM regime 27 may be accurately described by taking into consideration a single log-normal distribution. The fitted value of 2 is in reasonable agreement with that obtained from thermoremanent data ( ϭ0.38), while T B02 lies in between the peak of the blocking temperature distribution ͑81 K͒ and the maximum of the ZFC (T M ϭ 205 K͒, as found in other particulate systems.…”

“…These have a plateletlike shape and a narrow size distribution. 11,15,17 II. EXPERIMENTAL Nanocrystalline BaFe 10.4 Co 0.8 Ti 0.8 O 19 particles were prepared by the glass crystallization method.…”

“…[7][8][9][10][11][12] From the magnetic point of view, pure M -type barium ferrite BaFe 12 O 19 and related compounds obtained by cationic substitution, display a large variety of magnetic structures, from collinear ferrimagnetism 8,9 to canonic spin-glass-like behavior, 12 which depend on the degree of magnetic frustration induced by cationic substitution. In particular, the BaFe 10.4 Co 0.8 Ti 0.8 O 19 compound seems to be ideal for perpendicular magnetic recording.…”