Anisotropy Field Distribution Research Articles

Electric-field-induced angular dependence of magnetic anisotropy in a FeCo/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure

To understand the distribution of the in-plane magnetic anisotropy under a dc electric field, FeCo films deposited onto Pb(Mg1/3Nb2/3)O3-PbTiO3 (011)-orientated substrates by RF-magnetron sputtering were investigated. Vibrating sample magnetometer was performed and the occurrence of switching was demonstrated of the magnetization easy axis in FeCo films upon applying solely a dc electric field. A theoretical calculation was performed to provide a simplified account of the magnetoelastic contribution to the magnetic anisotropy. Quantification of the angular distribution of the magnetic anisotropy field under various electric fields was obtained, which can contribute to realizing low-loss electric-field-turning devices.

Visualizing magnetocrystalline anisotropy field distribution of patterned triangular L10 FePt nanodots

Measuring magnetocrystalline anisotropy field distribution of patterned magnetic nanodots using a microscopically visualizing way is important for understanding some important magnetic behaviors such as switching field distribution (SFD) of patterned recording media. We present a detailed analysis of the remanent domain structures of L10-FePt triangular nanodots as revealed by high resolution magnetic force microscopy (MFM) with the help of micromagnetic simulation, showing that the domain structure diversity can effectively account for a dot-to-dot variation of the magnetocrystalline anisotropy field. Our method could shed light not only on understanding the fundamental causes of a wider SFD but also on designing future nanostructured magnetic devices.

Magnetic Anisotropy of Maghemite Nanoparticles Probed by RF Transverse Susceptibility

We present radio frequency magnetic transverse susceptibility measurements on γ-Fe2O3 nanoparticles, which yield an estimation of their effective anisotropy constant, Keff as a function of nanoparticle size. The resulting values range from 4 to 8 × 104 erg/cm3, being on the order of the magnetocrystalline anisotropy in bulk maghemite. Keff values increase as the particle diameter increases. Evidences of anisotropy field distribution given by the size distribution in the samples, and interparticle interactions that increase as the particle size increases, are also observed in the TS measurements. The effects of such interparticle interaction overcome those of thermal fluctuations, in contrast with the behavior of other iron oxide particles.

Effects of eddy current and dispersion of magnetic anisotropy on the high-frequency permeability of Fe-based nanocomposites

Fe–Cu–Nb–Si–B microflakes have been prepared by ball milling. The structural, magnetostatic and microwave permeability of the flakes and flake-filled composites have been studied. Two ferromagnetic phases, nanograins and amorphous matrix, are found in the flakes. The Mössbauer study shows that the nanograins are α-Fe3(Si) with D03 superlattice structure. High resolution transmission electron microscopy shows that the nanograins are well dispersed in the matrix. The microwave permeability of composites containing the flakes has been measured. The comparison of the intrinsic permeability of the flakes obtained from the permeability measurements and from the anisotropy field distribution reveals a disagreement in the magnetic loss peak location. It is concluded that the low-frequency loss in the composites is not due to the effect of eddy currents. The low-frequency loss may be attributed to other sources, such as domain wall motion or peculiarities of the magnetic structure of the flakes in the composite.

Measurement of Magnetic Properties Relevant to Heat-Assisted-Magnetic-Recording

Heat-assisted magnetic recording (HAMR) is likely the successor of perpendicular magnetic recording (PMR) technology. The quality of HAMR writing is dominated not by the magnetic field gradient (as in PMR), but by the high thermal gradient generated by the laser heating spot. Knowledge of temperature dependence of magnetic parameters: anisotropy field Hk(T), saturation moment Ms(T) and anisotropy field distribution (δHk) the proximity of Curie temperature is crucial for media development. Here we describe a new system to probe the magnetic properties close to Tc, as well as a semianalytical model to explain the results. The model takes into account the thermal fluctuations; input into the model are experimentally measured data for Ms(T), Hk(T), mean field demagnetization factor, σHk and grain size distribution. In the paper we present two types of data: 1) thermal erasure in the absence of a magnetic field; and 2) thermomagnetic writing, allowing the estimate of the switching field distribution at high temperatures.

Energy barrier versus switching field for patterned Co80Pt20 alloy and Co/Pt multilayer films

Two Co/Pt multilayer samples have been fabricated with a difference in the number of bilayers, leading to a total magnetic layer thickness of 3 nm and 20 nm. From these films, large arrays of magnetic islands have been patterned using laser interference lithography and ion beam etching. We have investigated the switching field distribution (SFD) of approximately 80 islands and thermal switching field distribution SFDT of individual islands of both samples using the anomalous Hall effect. We compare the results of these measurements with the (SFDT) of a previously investigated alloy with a magnetic layer thickness of 20 nm by comparing the results of over 1000 hysteresis loops of a single weak island and a single strong island. We found that that the energy barrier for the multilayer islands increases with increasing switching field, whereas it was previously found that the energy barrier for the alloy stays constant with varying switching fields. When comparing the two multilayer samples, we observe that the grain size, anisotropy, and switching field distribution are more or less independent on thickness, whereas the switching field at both 0 K and 300 K decreases with film thickness.

Study of grain interactions in perpendicular magnetic recording media using first order reversal curve diagrams

It is demonstrated that, for perpendicular magnetic recording (PMR) media, first order reversal curve analysis can independently measure the grain coercive field distribution (or switching field distribution) and the strength of grain interactions, i.e., the demagnetization (mean-field) factor and the dispersion of grain interaction fields around the mean-field. The coercive field distribution is used to determine the intrinsic anisotropy field distribution of PMR media. The temperature dependence of the demagnetization factor shows that the strength of inter-granular exchange coupling is increasing with increasing medium thickness and it is decreasing with increasing temperature.

Microstructures and perpendicular magnetic properties of Co/Pd multilayers on various metal/MgO seed-layers

We studied the effects of metal/MgO seeds (metal = Ta, Ru, or Pd) on the crystalline structure and perpendicular magnetic properties of Co/Pd multilayers to investigate the possibility of developing a (100) texture with sufficiently high perpendicular anisotropy and small switching field distributions for applications such as patterned media and perpendicular magnetic random access memories. The Pd/MgO or the MgO seed successfully promoted a (100) texture of Co/Pd multilayers. In particular, the Pd/MgO seed developed a strong (100) texture in the Co/Pd multilayer and resulted in perpendicular magnetic anisotropies ∼2 × 106 erg/cm3. On the other hand, the Co/Pd multilayer with the Ta/MgO or the Ru/MgO seed showed a strong (111) texture, inducing a perpendicular magnetic anisotropy higher than that of the (100) textured films. The coercive fields of Co/Pd multilayers with the (111) texture were over 4 kOe and higher than those with the (100) texture, which were ∼2 kOe when they were patterned into 2 × 2 μm2 islands. The switching field distributions of the Co/Pd multilayers with the (100) texture were smaller than those of the Co/Pd multilayers with the (111) texture. Our findings suggest that the Pd/MgO or the MgO seed can be a template suitable for device applications.

Validation of ΔH(M,ΔM)-technique for identification of switching field distributions in the presence of thermal relaxation

The potential of hysteresis loop-based methods for the characterization of granular magnetic materials is investigated in the presence of thermal relaxation effects. Specifically, we study the reliability of the ΔH(M,ΔM)-method to recover the intrinsic switching field distribution in the presence of thermal relaxation. As input data, we use the computational results obtained from kinetic Monte Carlo simulations of interacting Stoner–Wohlfarth particle arrays including the anisotropy field and grain size distributions, and then analyze them using the hysteron-based ΔH(M,ΔM)-method to identify the accuracy limits of this methodology. It is found that the accuracy of the ΔH(M,ΔM)-method is not substantially changed by the presence of thermal relaxation.

Intrinsic defects in perpendicularly magnetized multilayer thin films and nanostructures

Intrinsic magnetic defects in perpendicularly magnetized nanostructures reduce the predictability of device and developing recording technologies. In addition to a distribution of local anisotropy fields, we show that such defects also exhibit variations in local anisotropy axes. The magnetic defects are identified by the application of in-plane and out-of-plane magnetic fields and magnetic force microscopy imaging. Those defects that control magnetization reversal in arrays of patterned Co/Pd multilayers are highly dependent on applied field orientation. The symmetry of the defects with respect to the applied field direction indicates that the anisotropy consists of a canted axis, deviating from the surface normal. Micromagnetic simulations confirm that variations in anisotropy axis can cause a significant change in reversal field depending on the location and orientation of the defects, consistent with experimental results.

Anisotropy Distribution of FePt Nanoparticles With Twinned Structures

The existence of the five-fold twinned structures was observed in 3.4 nm FePt nanoparticles, and other n-fold structures were also been found in magnetic nano-grains in recording media. An analytical model, following the spirit of the Stoner-Wohlfarth model, is brought up to study the anisotropy field distribution of FePt or other magnetic nanoparticles with twinned or n-fold structures, based on simple statistical calculations of twin regions' volumes. It is found that the nanoparticle has an effective uniaxial anisotropy, and the average anisotropy field is substantially reduced compared to that of the single crystal. The calculated anisotropy distributions are also compared with the empirical distribution functions used in micromagnetic simulations.

The high-frequency soft magnetic properties of FeCoSi/MnIr/FeCoSi trilayers

The dynamic properties of as-deposited FeCoSi/MnIr/FeCoSi trilayers, prepared by RF magnetron sputtering method, have been investigated in the frequency range from 100 MHz to 9 GHz. The effective anisotropy fields of the top and bottom CoFeSi and Gilbert constant ( α) are analyzed using the Landau–Lifshitz–Gilbert equation. The experimental result implies that the f FMR can be easily tuned up to 8.2 GHz by controlling the CoFeSi thickness and unidirectional anisotropy. The extracted α decreases as the increasing of exchange bias field, which behavior can be explained as the suppressing of the anisotropy field distribution by exchange bias. Therefore, the exchange biased CoFeSi/MnIr/CoFeSi trilayers with the high ferromagnetic resonance frequency and low loss properties are promising for the high-frequency application.

Investigation on the magnetic anisotropy of particulate and thin film recording media

Some aspects of the magnetic anisotropy of fine particles and thin films, employed in longitudinal recording media, are studied. The analysis is carried out through the measurement of perpendicular magnetization and the calculation of the second derivative, which allows to obtain the distribution of anisotropy fields. In assemblies of α-Fe metal particles, where shape anisotropy prevails, the evolution of the anisotropy field distribution with packing is studied. In Ba ferrite particles, where both magnetocrystalline and shape anisotropy are present and comparable, the effect of the ionic substitutions is analyzed. In thin film media, a comparison between magnetic layers with different composition (ternary and quaternary Co-based alloy) is carried out. In all materials, the relevance of the analysis of the whole distribution is underlined. From the anisotropy results, information on the magnetization switching is drawn.

Capacitance Enhanced Synchronization of Pairs of Spin-Transfer Oscillators

Macro-spin simulations have been performed to study the synchronization of pairs of serially connected spin torque oscillators in parallel with a capacitor. The sensitivity to the STO process variation, which is modeled by the anisotropy field distribution for different STO samples, can be dramatically decreased by the coupling of a capacitor in the circuit. There exists an optimum capacitance where the multi-STO synchronized state develops the highest robustness.

Permeability and losses in ferrites from dc to the microwave regime

Fluxmetric and transmission line methods have been combined in the wideband characterization of Mn–Zn and Ni–Zn soft ferrites. The real μ′(f) and imaginary μ″(f) permeability components and the energy loss W(Jp,f) have been obtained in ring samples, for defined values of the peak polarization Jp up to 1 GHz. The role of an increasing dc magnetic field 0≤Hdc⊥≤140 kA/m transverse to the ring plane on the frequency dependence of μ′(f), μ″(f), and W(Jp,f) has also been investigated. Permeability and energy losses are interpreted applying the statistical theory of losses to the case where spin damping is the leading cause of the magnetic energy dissipation. The separate roles of domain wall processes and rotations are consequently put in evidence and predicted by associating them to relaxation and ferromagnetic resonance effects, respectively. The rotational susceptibility is obtained, as a solution of the linear Landau–Lifshitz–Gilbert equation, averaged over a distribution of effective anisotropy fields and the loss is calculated accordingly.

Application of the anisotropy field distribution method to arrays of magnetic nanowires

The applicability of the anisotropy field distribution method and the conditions required for an accurate determination of the effective anisotropy field in arrays of magnetic nanowires have been evaluated. In arrays of magnetic nanowires that behave as ideal uniaxial systems having only magnetostatic contributions to the effective anisotropy field, i.e., shape anisotropy and magnetostatic coupling, the method yields accurate values of the average anisotropy field at low-moderate dipolar coupling and accuracy decreases as wire packing increases. When an additional non-negligible magnetocrystalline anisotropy is present, the method is less accurate, as shown for the case of hcp Co nanowires.

Fabrication challenges for patterned recording media

Lithographically patterned recording media are one of the approaches to achieving Tb/in 2 and beyond recording densities. This will require fabrication of sub-10 nm discrete magnetic islands covering a full disk with tight spacing and size distributions and a narrow switching field distribution. To become an economically successful technology, this will need to be achieved with high throughput and low cost. The technology to fabricate such patterned media will need to be developed, and may require innovative solutions such as self-assembly and nanoimprinting, along with improved magnetic thin films for achieving high anisotropy and narrow switching field distributions.

Mössbauer study of ferromagnetic metallic glasses irradiated by swift heavy ions at temperatures 100 and 300 K

The effect of swift heavy ion irradiation on ferromagnetic metallic glasses Fe40Ni38Mo4B18 and Fe78Si9B13 has been studied. The ion beams used are 100 MeV 127I and 180 MeV 197Au. The specimens were irradiated at fluences ranging from 3 × 1012 to 1.5 × 1014 ions/cm2. The irradiations have been carried out at temperatures 100 and 300 K. The magnetic moments are sensitive towards the irradiation conditions such as irradiation temperature and stopping power of incident ion beam. The irradiation-induced effects have been monitored, by using Mossbauer spectroscopy. The modifications in magnetic anisotropy and hyperfine magnetic field distributions, as an effect of different irradiation temperature as well as different stopping power have been discussed. After irradiation, all the samples remain amorphous and magnetic anisotropy considerably changes from its original in-plane direction. The results show enhancement in magnetic anisotropy in the specimen irradiated at 100 K, as compared to that of irradiated at 300 K. It is expected that at low temperature, the stresses produced in the material would remain un-annealed, compared to the samples irradiated at room temperature and therefore, the modification in magnetic anisotropy would be enhanced. A distribution of hyperfine magnetic field, of the samples irradiated at low temperature, show a small but distinct peak at ∼ 11 Tesla, indicating Fe-B pairing.

Investigation of magneto-anisotropy field for system of particles with near superparamagnetic volume

We obtained the temperature dependence for low-field boundary of the anisotropy field distribution in a system of barium hexaferrite nanocrystals in the temperature range from 300 to 700 K. We treated the experimental data taking into account the influence of thermal fluctuations on the anisotropy field and the transition of particles into the paramagnetic state, stimulated by external magnetic field. We showed that the dependence under consideration is formed by particles of different volume, which increased from 3.5×10 −18 to 40×10 −18 cm 3 while the particles lost their magnetic stability with the temperature growth.

Effect of orientation on the thermal stability in advanced metal particulate tapes

The effects of the degree of particle orientation on the normalized magnetization decay and activation volume (Vact) were investigated for advanced data recording tapes prepared from ultrafine metal particulate (MP) composite. In this study, the mean volume of particles (Vphy) for advanced MP tapes varied between 1.7 and 5.3×10−18cm3, inclusive of a surface oxide layer. In MP tape with a larger Vphy (=5.3×10−18cm3), increasing the orientation ratio (OR) for improved recording characteristics was found to decrease the normalized magnetization decay. However, the OR values had little effect on the normalized magnetization decay of MP tape with a smaller Vphy (=1.7×10−18cm3). This may be attributed to the existence of a few particles approaching the theoretical superparamagnetic limit of 0.7×10−18cm3. In order to decrease the normalized magnetization decay, it is particularly important to increase Hc, which improves the anisotropy constant and the distribution of anisotropy field (HA) for advanced data recording tapes with very small Vphy. The value of Vact in the low reverse field, which is a major factor affecting media noise, decreased as OR increased regardless of Vphy. Also, Vact of both tapes decreased as the reverse field decreased in the range of 1.2kOe or less. Vact of an assembly of ultrafine particles was dependent on the distributions of HA and volume of particles, also the value of rotational hysteresis integral that related to the deterioration in the mechanism of magnetization reversal giving rise to an incoherent rotation.