The effect of magnetic interaction in barium hexaferrite particles

Abstract

The effect of interactions on the magnetic relaxation of nanocrystalline hexagonal barium hexaferrite BaFe 10.4 Co 0.8 Ti 0.8 O 19 is discussed. We had previously shown that according to the T ln(t/ 0 ) 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 SiO 2 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/ 0 ) scaling. Activation volumes were found to increase with demagnetizing interactions and the leading demagnetizing mechanism appeared to shift from an individual particle mode for the unmilled sample to a collective one for the milled sample. The second approach showed larger relaxation rates at short times for the milled sample. The effective energy barrier distribution obtained from the scaling suggested that demagnetizing interactions increased in the milled sample, which led to an enhancement of the amount of the lowest-energy barriers. © 1997 American Institute of Physics. ͓S0021-8979͑97͒57608-0͔M -type barium hexaferrite has been widely studied because of its technological applicability. In the last years many efforts have been devoted to the development of synthesis procedures that leads to controlled-size particles, and to the knowledge of its basic properties. The study of the magnetic properties of barium hexaferrite nanoparticles showed that interparticle interactions can affect their magnetic behaviour. The selected system has previously shown demagnetizing interactions.1 In the present work, we have modified the interactions by milling the particles with a nanosized SiO 2 powder. Dipolar interactions are modified by breaking the particle aggregates. According to the results from Ido et al., 2 the resultant interaction will depend on the way the particles were grouped. Barium hexaferrite particles tend to pile up, leading to stacks, where interactions are magnetizing, although particle clusters are also observed, where interactions are demagnetizing. The effect of interactions on the magnetic relaxation has been analyzed in terms of fluctuation field and activation volume. To complete the analysis and the interpretation of the results, the T ln͑t/ 0 ͒ scaling of the time dependence of the thermoremanence ͑which provides the energy barrier distribution͒ have been done. Nanocrystalline BaFe 10.4 Co 0.8 Ti 0.8 O 19 particles, with a mean particle diameter of about 10 nm, were prepared by using the glass crystallization method.3 Two different samples were prepared compacting the nanocrystalline powder by cold pressing ͑in order to a...

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

“…Using the measured value for M FC /H at 300 K and the corresponding bulk saturation magnetization for this material ͓ M b ͑300 K)ϭ317 emu/cm 3 techniques 23,27 were also smaller than those obtained by TEM ͑105 nm 3 ) and XRD ͑90 nm 3 ), as expected due to surface magnetic effects. 10 Two different primary curves concerning the field dependence of the remanence were measured: 29 ͑1͒ the isothermal remanent magnetization curve m r (H)ϭM r (H)/M r (H max ), which was obtained measuring the remanence from the initially demagnetized state and taking the sample through progressively increasing loops; and ͑2͒ the dc demagnetizing remanence curve m d (H)ϭM d (H)/M d (H max ), which was obtained measuring the remanence by progressively increasing demagnetization in a previously saturated sample.…”

“…Concerning particle clusters, such as, for example, quasispherical aggregates, the overall configuration favors demagnetization. [20][21][22][23] According to Eq. ͑3͒, ͉dm d /dH͉ϭ2dm r /dH.…”

“…We note that the cell parameters of the hexagonal unit cell of BaFe 12 That degree of preferential orientation is a consequence of the diameter of the (001) face being much larger than the platelet thickness. Interactions were expected to be magnetizing among particles within the same stack 20,22,23 and demagnetizing among the stacks and within particle clusters. [21][22][23] Both types of interactions are always present in barium ferrites but one is dominant.…”

“…Interactions were expected to be magnetizing among particles within the same stack 20,22,23 and demagnetizing among the stacks and within particle clusters. [21][22][23] Both types of interactions are always present in barium ferrites but one is dominant.…”

Normalization factors for magnetic relaxation of small-particle systems in a nonzero magnetic field

“…1 To characterize the time dependence of the magnetization they analyze the relaxation rate, also called magnetic viscosity, S, as a function of the external parameters. When plotted as a function of the magnetic field it is possible to study the variation of the energy barriers with the field, 2 the interaction effects among the magnetic entities, 3,4 and the switching field distribution 5,6 among other magnetic properties. When plotted as a function of the temperature for a given field it gives information about the reversal mechanisms in films and small-particle systems 7 and it has been proposed as a way to test the possibility of observing macroscopic quantum tunneling ͑MQT͒ effects at low enough temperatures.…”