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).

Granular films composed of well defined nanometric Co particles embedded in an insulating ZrO 2 matrix were prepared by pulsed laser depositon in a wide range of Co volume concentrations ͑0.15Ͻ x Ͻ 0.43͒. High-resolution transmission electron microscopy ͑TEM͒ showed very sharp interfaces between the crystalline particles and the amorphous matrix. Narrow particle size distributions were determined from TEM and by fitting the low-field magnetic susceptibility and isothermal magnetization in the paramagnetic regime to a distribution of Langevin functions. The magnetic particle size varies little for Co volume concentrations x Ͻ 0.32 and increases as the percolation limit is approached. The tunneling magnetoresistance ͑TMR͒ was successfully reproduced using the Inoue-Maekawa model. The maximum value of TMR was temperatureindependent within 50-300 K, and largely increased at low T, suggesting the occurrence of higher-order tunneling processes. Consequently, the tunneling conductance and TMR in clean granular metals are dominated by the Coulomb gap and the inherent particle size distribution.

The static and dynamic magnetic properties of nanocrystalline BaFe 10.4 Co 0.8 Ti 0.8 O 19 M -type doped barium ferrite were studied in detail to clarify the effect of interactions on the magnetic relaxation of an assembly of small particles. The logarithmic approximation was unable to account for the magnetic relaxation of the sample. Interaction effects were analyzed from the low-field susceptibility, ⌬M plots and the time dependence of thermoremanence, indicating that demagnetizing interactions led to an enhancement of both the relaxation rate at low temperatures and the amount of the lowest energy barriers. It is thus suggested that care should be taken when analyzing thermoremanent data at low temperature, in order not to confuse these experimental findings with the signature of macroscopic quantum tunneling. ͓S0163-1829͑97͒04709-7͔

We report on the occurrence of exchange bias on laser-ablated granular thin films composed of Co nanoparticles embedded in an amorphous zirconia matrix. The deposition method allows one to control the degree of oxidation of the Co particles by tuning the oxygen pressure at the vacuum chamber (from 2 x 10(-5) to 10(-1) mbar). The nature of the nanoparticles embedded in the nonmagnetic matrix is monitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM) CoO(x), with a FM/AFM intermediate regime for which the percentage of the AFM phase can be increased at the expense of the FM phase, leading to the occurrence of exchange bias in particles of about 2 nm in size. For an oxygen pressure of about 10(-3) mbar the ratio between the FM and AFM phases is optimum with an exchange bias field of about 900 Oe at 1.8 K. The mutual exchange coupling between the AFM and FM is also at the origin of the induced exchange anisotropy on the FM leading to high irreversible hysteresis loops, and the blocking of the AFM clusters due to proximity to the FM phase.