Intergranular Magnetostatic Interactions Research Articles

The effect of the interface on magnetic properties of perovskite-spinel nanocomposites

In this research, perovskite-spinel nanocomposite with formula (x)(Cuo.3Mn0.7Fe2O4)- (1-x)(La0.9o Bi0.10 FeO3); 0 ≤ x ≤ 1, were prepared by the auto-combustion method. A single phase of orthorhombic and cubic structure for La0.9oBi0.10FeO3(LBFO); Cuo.3Mn0.7Fe2O4(CMFO) respectively have been proved for two parents and nanocomposites without any further secondary phase using XRD analysis. High-resolution transmission electron microscopy (HRTEM) images reveal nearly spherical nanoparticles for the parents and agglomerated for nanocomposites. Magnetic hysteresis loops demonstrate ferromagnetic (FM) behavior for the spinel compound and nanocomposite, where LBFO possesses G-type antiferromagnetic (AFM). Competition between FM and AFM phases lead to enhancement in magnetic parameters such as saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc). The magnetic properties at low temperatures show the decrease in magnetization and coercivity with increasing the temperature while the magnetic exchange bias HEB was increased until T=200 K and then decreased. The existence of intergranular magnetostatic interactions between the LBFO and CMFO interact at the interface of nanocomposites and play a decisive role in strain energy, exchange energy, and magnetic anisotropy

The effect of the thickness ratio of magnetic layers on the microstructure and magnetic properties of (CoCrPt)97.5Nb2.5/Co75Cr13Pt12/Cr thin films

A Co75Cr13Pt12 intermediate magnetic layer was deposited between the (CoCrPt)97.5Nb2.5 upper magnetic layer and chromium underlayer by the magnetron sputtering technique. The effect of the thickness ratio of two magnetic layers and post-annealing treatment on the microstructure and magnetic properties of the films were investigated. The magnetic characteristics of the films were obtained by magnetic force microscopy, hysteresis loops and switching field distribution curves. Although annealing had no significant effect on the layer roughness, it rotated the easy axis of the magnetic layer towards the film plane, thereby enhanced the coercivity. The results showed an impressive effect of the magnetic intermediate layer thickness and post-annealing on the improving of coercivity through isolation of the magnetic grains. Formation of a non-magnetic halo around the magnetic grains reduced the inter-granular magnetostatic interaction.

Effective medium theory of magnetization reversal in magnetically interacting particles

We report an effective medium theory of magnetization reversal and hysteresis in magnetically interacting particles, where the intergranular magnetostatic interaction is accounted for by an effective medium approximation. We introduce two dimensionless parameters, /spl lambda/ and h/sub 0/, that completely characterize the hysteresis in a ferromagnetic polycrystal when the grain size is much larger than the exchange length so that the exchange coupling can be ignored. The competition between the anisotropy energy and the intergranular magnetostatic energy is measured by /spl lambda/, while the competition between the anisotropy energy and Zeeman's energy is measured by h/sub 0/. The hysteresis loop, magnetostatic energy density, and anisotropy energy density calculated by using this theory agrees well with micromagnetic simulations. The calculations also reveal that the subnucleation field switching due to the magnetic field fluctuation is important when the magnet is not very hard, and that has been accounted for by a probability-based switching model.

Remanence enhancement in magnetically interacting particles

In this paper, we report an effective-medium theory on the remanence of magnetically interacting particles to demonstrate the effect of intergranular magnetostatic interactions on the remanence enhancement of materials, which agrees excellently with micromagnetic simulations. A dimensionless parameter \ensuremath{\lambda} measuring the competition between anisotropy energy and magnetostatic energy is defined, which completely characterizes the remanence of magnets if the exchange coupling is negligible, appropriate when the grain size is 10 nm or larger. Three distinct regimes were observed: $\ensuremath{\lambda}<0.1$ for hard magnets, where anisotropy energy dominates and little remanence enhancement is observed; $0.1<\ensuremath{\lambda}<1$ for intermediate magnets where up to 50% remanence enhancement is observed due to the increased intergranular magnetostatic energy; and $\ensuremath{\lambda}>1$ for soft magnets, where the dominance of magneto-static energy leads to much reduced remanence in the materials.

Micromagnetic study of the intrinsic loop squareness S* in perpendicular media

Micromagnetic simulations have been performed to deshear the remanent hysteresis loops in perpendicular media. Results demonstrate that the intrinsic loop squareness S* increases with increasing the intergranular exchange couplings and with decreasing the intergranular magnetostatic interactions. However, within the range of parameters examined, changes of the magnetic film thickness and the medium-soft magnetic underlayer spacing exhibit negligible effect on S*. The applied field angle has also been found to exhibit insignificant effect on S* and Neff. Therefore, intergranular interactions are the major factors to determine S* and Neff. To deshear the loop, an effective demagnetization factor Neff was found applicable. Empirical formulas have been fitted to relate S* and Neff to media parameters.

Thermal energy barrier distribution measurements in perpendicular media

dc demagnetized remanent curves at different applied field time durations have been applied to measure the thermal energy barrier distribution of two sets of perpendicular alloy media. Sharrock’s formula is applied at all levels of percentages of magnetization switching. The measured energy barrier distribution width decreases with increasing intergranular exchange coupling and with reducing intergranular magnetostatic interactions. The results agree well with micromagnetic simulations.

Effect of intergranular interactions on thermal energy barrier distribution in perpendicular media

Micromagnetic simulations have been performed to simulate dynamic hysteresis loops in perpendicular media. Thermal energy barrier distributions have been calculated. For a fixed percentage of magnetization switched, a linear variation between the scaled applied field Ha(t)/HK and ln(f0t)1/2 is found. Without including intergranular magnetostatic interactions, intergranular exchange coupling reduces the thermal energy barrier distribution width, compared to the physical volume distribution width. However, the magnetostatic interactions increase the energy barrier width substantially. The increase is significantly reduced at smaller magnetostatic interactions. For the effective volume of 50% magnetization switched (V50), the results show V50=〈V2〉 almost independent of intergranular magnetostatic and exchange interactions.

Correlation between activation volume and intergranular magnetic interaction in longitudinal media

We have investigated the correlation between the activation grain diameter derived from activation volume and the intergranular magnetic interaction, in CoCrPt-based thin film media fabricated by an ultra clean sputtering process. We found that the activation grain diameter depends on the physical grain diameter and intergranular exchange interaction, but is independent of intergranular magnetostatic interaction (4/spl pi/M/sub s//H/sub k//sup grain/). The activation grain diameter normalized by the physical grain diameter, which corresponds with the normalized coercivity (H/sub c//H/sub k//sup grain/) and microstructure, indicate the character of the intergranular exchange interaction in the media with various compositions. It is also clarified that the media noise can be characterized by the activation grain diameter together with the intergranular magnetostatic interaction.

Effect of intergranular exchange on thermal energy barrier distribution in longitudinal magnetic recording

Micromagnetic simulations have been performed to examine thermal energy barrier distributions in longitudinal recording. Results show that intergranular exchange reduces the energy barrier distribution width compared to the width of the physical volume distribution, which agrees well with experimental measurements. The rms average energy barrier is almost unchanged by the interactions. Intergranular magnetostatic interactions also decrease the thermal energy barrier width but only for either small exchange or small volume distributions. These results indicate that a measurement of energy barrier width reduction can be used to estimate the intergranular exchange of a typical longitudinal medium.

Simulation of intergranular interaction in sintered magnets

The different methods of the representations of the particles in the process of modelling the intergranular magnetostatic interaction are considered. The representation of grains (or groups of the grains if the latters are separated with boundaries pervious for domain walls) in sintered magnets by uniformly magnetized polyhedrons is proposed. In the three-dimensional model considered, the multi-domain grains correspond to the particles with the magnetization lower than the saturation value. It is shown that the correctness of this representation can be verified for nucleation-type magnets by simulating the initial magnetization curve. The fitting in the case of the representation by uniformly magnetized cubes is much better than that in the case of the conventional representation by point dipoles.

Simulation of intergranular interaction in sintered magnets

Different methods of the representation of the particles in the process of modelling the intergranular magnetostatic interaction are considered. The representation of grains (or groups of the grains if they are separated with boundaries pervious for domain walls) in sintered magnets by uniformly magnetized polyhedrons is proposed. In the three-dimensional model considered the multi-domain grains correspond to the particles with magnetization lower than the saturation value. It is shown that the correctness of this representation can be verified for nucleation-type magnets by simulating the initial magnetization curve. The fitting in the case of the representation by uniformly magnetized cubes is much better than that in the case of the conventional representation by point dipoles.

Magnetostatic interaction in nucleation-type magnets

The problem of taking into account the intergranular magnetostatic interaction in the process of modelling the nucleation-type sintered magnets magnetization reversal is discussed. The calculations of magnetization curves, minor loops and magnetization returns are carried out by means of a model in which the interaction is represented by a field that fluctuates about the average internal field. Comparison with experiment permits us to separate the specific macroscopic displays of the intergranular magnetostatic interaction in nucleation-type sintered magnets having various coercivities. It is found that, besides the initial magnetization curve and the thermal remagnetization, the interaction displays a nature of minor loops after dc-demagnetization in the case of the magnets having a coercivity of about 20 kOe and an increase of the magnetization return in the case of the magnets having a coercivity of about 10 kOe.