M. García del Muro

Magnetic study of M‐type doped barium ferrite nanocrystalline powders

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 TM, which depends on the distribution of particle volumes in the sample. Isothermal magnetization curves M(H) at several temperatures and saturation magnetization Ms as a function of temperature have been measured for the Co‐Ti sample (BaFe10.4Co0.8Ti0.8O19). 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〉=(81±40) K. The distribution of volumes of the magnetic unit is also o...

Interface effects in the magneto-optical properties of Co nanoparticles in dielectric matrix

The authors present a study of the optical and magneto-optical properties of Co nanoparticles embedded in two amorphous dielectric matrices with different refractive indices such as ZrO2 and Al2O3. The nanostructured films were prepared by pulsed laser deposition, and the morphology and structure were studied by different characterization techniques. The optical and magneto-optical (MO) properties of the Co inside the nanoparticles differ from those of the bulk material; in particular, a decrease in the MO constants is found. These properties are found to depend on the nanoparticle size and on the dielectric matrix, due to the different nanoparticle-matrix interfaces appearing in both cases.

Static magnetic properties of nanocrystalline Co-Ti doped barium ferrite BaFe/sub 12-2x/Co/sub x/Ti/sub x/O/sub 19/ (x=0.8)

We have studied the static magnetic properties of nanocrystalline Co-Ti doped barium ferrite BaFe/sub 10.4/Co/sub 0.8/Ti/sub 0.8/O/sub 19/. The zero-field-cooled (ZFC) and field-cooled (FC) processes show the typical features of a small particle system. The dependence on temperature of the coercive field is discussed within the scope of an aligned and random assembly of particles. The distribution of blocking temperatures is obtained from the remanent-to-saturation magnetization ratio, leading to the mean volume of the magnetic unit (/spl ap/(6/spl plusmn/3)/spl middot/10/sup 4/ /spl Aring//sup 3/), in agreement with the mean particle volume determined by TEM (/spl ap/1.1/spl middot/10/sup 5/ /spl Aring//sup 3/). Remanence curves are used to derive the anisotropy field distribution and the interparticle magnetic interaction. >

The effect of quenching rate on the nanocrystallization of amorphous FeCuNbSiB

Abstract Amorphous ribbons with composition Fe73.5Cu1Nb3Si13.5B9 were produced by planar flow casting using different values of the wheel velocity, within the range 20–32 m/s. In order to understand the effect of the quenching rate on the subsequent degree of nanocrystallization, a detailed characterisation of as-cast and annealed ribbons at 500 and 540°C has been performed using Mossbauer spectroscopy and X-ray diffraction.

Magnetic properties of nanocrystalline barium hexaferrite powders: anisotropy field and interaction effects

Abstract The anisotropy field H a of nanocrystalline barium hexaferrite powders is determined and the influence of interaction between the particles is investigated. The value of H a is found to be extremely high, which may be explained by surface effects. The powders show positive interaction in the superparamagnetic regime (high temperatures) and negative interaction in the magnetic stable regime (low temperatures).

Size mediated control of the optical and magneto-optical properties of Co nanoparticles in ZrO2

We present a study of the optical and magneto-optical MO properties of Co nanoparticles embedded in ZrO2 in the spectral range from 1.4 to 4.3 eV. The nanostructured films were prepared by pulsed laser deposition in a wide range of Co nanoparticle concentrations varying from 20% to 80%. For Co concentration lower than x0.45 the size of the nanoparticles was found to remain almost constant D2.5 nm, whereas it increases above it. Differences are found between the optical and MO constant of the Co nanoparticles and those of continuous Co films. Those differences are associated with size effects of the intraband contribution inside the nanoparticles.

Nanostructural origin of the ac conductance in dielectric granular metals: The case study of Co20(ZrO2)80

The authors show which is the nanostructure required in granular Co20(ZrO2)80 thin films to produce an ac response such as the one that is universally observed in a very wide variety of dielectric materials. A bimodal size distribution of Co particles yields randomly competing conductance channels which allow both the thermally assisted tunneling through small particles and capacitive conductance among larger particles that are further apart. A model consisting on a simple cubic random resistance-capacitor network describes quantitatively the experimental results as functions of temperature and frequency, and enables the determination of the microscopic parameters controlling the ac response of the samples.

T⋅ln(t/τ0) scaling approach and fluctuation field analysis in interacting particulate systems

The time dependence of the magnetization in samples having a different degree of interparticle interactions is analyzed. The selected compound was a nanocrystalline powder of the composition BaFe10.4Co0.8Ti0.8O19 with and without an admixture of nanosized SiO2 particles. Two different approaches were considered for the analysis of the thermally activated demagnetization: the T⋅ln(t/τ0) scaling of the thermomagnetic relaxation in zero applied field, and the classical fluctuation field and activation volume analysis. The energy barrier distribution obtained from the former approach shows that the occurrence of larger demagnetizing interactions leads to a relative enhancement of the lowest energy barriers with respect to the largest barriers. Activation volumes are found to increase with demagnetizing interactions and the leading demagnetizing mechanism appears to shift from an individual particle mode (coherent or nucleation) to a collective one. Both analyses are suggested to be taken into account in order...

THE EFFECT OF MAGNETIC INTERACTION IN BARIUM HEXAFERRITE PARTICLES

The effect of interactions on the magnetic relaxation of nanocrystalline hexagonal barium hexaferrite BaFe10.4Co0.8Ti0.8O19 is discussed. We had previously shown that according to the T ln(t/τ0) scaling, an enhancement of the lowest-energy barriers was detected when demagnetizing interactions were dominant. Also, the Henkel plots obtained for particles of about 10 nm of mean diameter showed that the overall interactions were demagnetizing. In the present work, we have modified the interactions by milling the particles with a nanosized SiO2 powder. Dipolar interactions are modified by breaking the particle aggregates. The observed overall interactions resulted to be also demagnetizing for the milled sample. The time dependence of the magnetization was analyzed according to two different procedures: the fluctuation field and activation volume analysis and the T ln(t/τ0) scaling. Activation volumes were found to increase with demagnetizing interactions and the leading demagnetizing mechanism appeared to shif...

Magnetic relaxation and superparamagnetism in nanocrystalline ferrites

Abstract The validity of the T ln( t / τ 0 ) scaling procedure has been tested by independent determination of the energy barrier distribution f ( E ) from high temperature magnetization curves, and τ 0 by analysing the in-phase component of the ac susceptibility. The effect of interparticle interactions on f ( E ) is discussed. It is shown that the derivative of the master curve M versus T ln( t / τ 0 ) leads to the energy barrier distribution.