Switching Field Distribution Research Articles

Effects of Annealing Temperature on Exchange Spring Behavior of Barium Hexaferrite/Nickel Zinc Ferrite Nanocomposites

(BaFe12O19)1−x/(Ni0.6Zn0.4Fe2O4)x magnetic nanocomposites with various molar ratios (x = 0.0, 0.1, 0.2, 0.3) have been synthesized by a physical mixing method at different temperatures (T = 700°C to 900°C) and their structure, morphology, and magnetic properties investigated using x-ray diffraction analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and vibrating-sample magnetometry. The results prove that their magnetic properties were strongly influenced by the sintering temperature as well as the molar ratio of hard to soft phase. The magnetization and switching field distribution curves for the nanocomposites sintered above 800°C showed a single smooth hysteresis loop, suggesting the existence of the exchange spring phenomenon. The variation of the coercivity, saturation magnetization, and ratio of remanence to saturation magnetization (Mr/Ms) with the hard-to-soft weight ratio can be explained based on the role of exchange and dipole interactions in the nanocomposites. High maximum energy product (BH)max was obtained for the (BaFe12O19)0.9/(Ni0.6Zn0.4Fe2O4)0.1 sample annealed at 800°C.

Characterization of switching field distributions in Ising-like magnetic arrays

The switching field distribution within arrays of single-domain ferromagnetic islands incorpo- rates both island-island interactions and quenched disorder in island geometry. Separating these two contributions is important for disentangling the effects of disorder and interactions in the magnetization dynamics of island arrays. Using sub-micron, spatially resolved Kerr imaging in an external magnetic field for islands with perpendicular magnetic anisotropy, we map out the evolution of island arrays during hysteresis loops. Resolving and tracking individual islands across four different lattice types and a range of inter-island spacings, we extract the individual switching fields of every island and thereby determine the relative contributions of interactions and quenched disorder in the arrays. The width of the switching field distribution is well explained by a simple model comprising the sum of an array-independent contribution (interpreted as disorder-induced), and a term proportional to the maximum field the fully polarized array could exert on a single island. We conclude that disorder in these arrays is primarily a single-island property.

Switching field reduction of a perpendicular magnetic nanodot in a microwave magnetic field emitted from a spin-torque oscillator

We demonstrate microwave-assisted magnetization switching of a perpendicular magnetic nanodot in a microwave stray field from a spin-torque oscillator (STO). The switching field decreases when the STO is operated by applying a current. The switching field reduction is almost the same as that in a microwave magnetic field generated by a signal generator despite the fluctuations of the STO oscillation. The switching field distribution, however, is broader when the STO is used. We also examine the magnetization switching process in the nanosecond region by applying a nanosecond-order pulse current to the STO and measuring the STO signal waveform. The onset of the STO oscillation and subsequent assisted switching occur within a few nanoseconds.

First order reversal curves and intrinsic parameter determination for magnetic materials; limitations of hysteron-based approaches in correlated systems

The generic problem of extracting information on intrinsic particle properties from the whole class of interacting magnetic fine particle systems is a long standing and difficult inverse problem. As an example, the Switching Field Distribution (SFD) is an important quantity in the characterization of magnetic systems, and its determination in many technological applications, such as recording media, is especially challenging. Techniques such as the first order reversal curve (FORC) methods, were developed to extract the SFD from macroscopic measurements. However, all methods rely on separating the contributions to the measurements of the intrinsic SFD and the extrinsic effects of magnetostatic and exchange interactions. We investigate the underlying physics of the FORC method by applying it to the output predictions of a kinetic Monte-Carlo model with known input parameters. We show that the FORC method is valid only in cases of weak spatial correlation of the magnetisation and suggest a more general approach.

Review Article: FePt heat assisted magnetic recording media

Heat-assisted magnetic recording (HAMR) media status, requirements, and challenges to extend the areal density (AD) of magnetic hard disk drives beyond current records of around 1.4 Tb/in.2 are updated. The structural properties of granular high anisotropy chemically ordered L10 FePtX-Y HAMR media by now are similar to perpendicular CoCrPt-based magnetic recording media. Reasonable average grain diameter ⟨D⟩ = 8–10 nm and distributions σD/D ∼ 18% are possible despite elevated growth temperatures TG = 650–670 °C. A 2× reduction of ⟨D⟩ down to 4–5 nm and lowering σD/D < 10%–15% are ongoing efforts to increase AD to ∼4 Tb/in.2. X = Cu ∼ 10 at. % reduces the Curie temperature TC by ∼100 K below TC,bulk = 750 K, thereby lowering the write head heat energy requirement. Multiple FePtX-Y granular layers with Y = 30–35 vol. % grain-to-grain segregants like carbides, oxides, and/or nitrides are used to fully exchange decouple the grains and achieve cylindrical shape. FePt is typically grown on fcc MgO (100) seedlayers to form well oriented FePt (002). A FePt lattice parameter ratio c/a ∼0.96 and high chemical order S > 0.90 result in magnetic anisotropy KU ∼ 4.5 × 107 erg/cm3, and only 25% below the FePt single crystal value KU = 6.6 × 107 erg/cm3 has been achieved in 7–8 nm diameter grains. Switching field distributions depend on anisotropy field (HK) distributions, which are currently of the order of ΔHK/HK ∼ 10% (ΔHK ∼ 10–12 kOe, HK ∼ 10–11 T) at room temperature. High thermal conductivity heat sink layers, including Ag, Au, Cu, and Cr, are used to optimize the cooling rate and maximize the down- and cross-track thermal gradient, which determines the achievable track density.

Direct measurement of single-dot coercivity and statistical analysis of switching field distribution in bit-patterned media using scanning hard-X-ray nanoprobe

To investigate the possible sources of the switching field distribution in bit-patterned media, we applied a scanning hard-X-ray nanoprobe technique based on X-ray magnetic circular dichroism spectroscopy to an array of Co-Pt dots with the typical diameters of 200 nm. Element-specific magnetization curves at the Pt L3 edge were measured for individual dots isolated from each other, and the magnetization switching field (coercivity) values were determined for more than 100 individual dots. To assess the effect of dot diameters as a possible source of switching field distribution, a statistical analysis of the coercivity values and dot diameters measured for many dots was performed. The resulting switching field distribution had a mean of ⟨HSW⟩=1.80 kOe and a standard deviation of σHSW = 0.64 kOe. The relative deviation of σHSW/⟨HSW⟩=36% was not in good agreement with the relative dispersion in a dot diameter of σD/⟨D⟩=2.7%, and no clear correlation between the coercivity and dot diameter was observed. These results may suggest other possible sources of switching field distribution than dot diameter, such as dispersion in the c-axis orientation and in magnetocrystalline anisotropy.

Measurement of magnetic property of FePt granular media at near Curie temperature

The characterization of the magnetic switching behavior of heat assisted magnetic recording (HAMR) media at near Curie temperature (Tc) is important for high density recording. In this study, we measured the magnetic property of FePt granular media (with room temperature coercivity ~25kOe) at near Tc with a home built HAMR testing instrument. The local area of HAMR media is heated to near Tc by a flat-top optical heating beam. The magnetic property in the heated area was in-situ measured by a magneto-optic Kerr effect (MOKE) testing beam. The switching field distribution (SFD) and coercive field (Hc) of the FePt granular media and their dependence on the optical heating power at near Tc were studied. We measured the DC demagnetization (DCD) signal with pulsed laser heating at different optical powers. We also measured the Tc distribution of the media by measuring the AC magnetic signal as a function of optical heating power. In a summary, we studied the SFD, Hc of the HAMR media at near Tc in a static manner. The present methodology will facilitate the HAMR media testing.

An extension of the Néel-Brown model for systems with multiple switching pathways

TheNéel-Brown model is the most widely accepted model for the description of magnetization reversal by thermal excitation. This model predicts a decreasing average switching field and an increasing width ΔH of switching field distribution as the temperature is increased, and has been found to hold good on several occasions. However, for a few classes of systems, the temperature dependence of ΔH shows the opposite trend, and so far no satisfactory explanation exists. We present here an experimental study of switching field statistics of permalloy (Ni80Fe20) thin films on Si(100) grown by pulsed laser ablation. It was seen that the sample deviates from the Neel-Brown behavior in the manner described above. We performed calculations based on a natural extension of the Néel-Brown model, which incorporated multiple reversal pathways characterized by a Gaussian distribution of coercive fields. Calculations based on this model for different values of the width parameter σHSW show two distinct kinds of behavior. At low values of σHSW, the total width ΔH is limited by thermal broadening according to the traditional Neel-Brown expression. This regime is characterized by an increasing ΔH with temperature. For high σHSW, the broadening is dominated by σHSW, which masks thermal broadening. In this regime, ΔH decreases with increasing temperature. Whereas the experimentally observed temperature dependence of the average switching field was found to be in good agreement with this model, qualitative agreement with regard to the temperature dependence of ΔH could be observed only for relaxation times lower than ~10−40s, which is much smaller than Néel-Brown relaxation times (10−9–10−19s) usually encountered in the literature.

Switching field distribution of exchange coupled ferri-/ferromagnetic composite bit patterned media

We investigate the switching field distribution and the resulting bit error rate of exchange coupled ferri-/ferromagnetic bilayer island arrays by micromagnetic simulations. Using islands with varying microstructure and anisotropic properties, the intrinsic switching field distribution is computed. The dipolar contribution to the switching field distribution is obtained separately by using a model of a triangular patterned island array resembling 1.4 Tb/in2 bit patterned media. Both contributions are computed for different thicknesses of the soft exchange coupled ferrimagnet and also for ferromagnetic single phase FePt islands. A bit patterned media with a bilayer structure of FeGd(5 nm)/FePt(5 nm) shows a bit error rate of 10−4 with a write field of 1.16 T.

Magnetostatic Interactions in Self-Assembled CoxNi1-xFe2O4/BiFeO3 Multiferroic Nanocomposites.

Self-assembled vertically aligned oxide nanocomposites consisting of magnetic pillars embedded in a ferroelectric matrix have been proposed for logic devices made from arrays of magnetostatically interacting pillars. To control the ratio between the nearest neighbor interaction field and the switching field of the pillars, the pillar composition CoxNi1-xFe2O4 was varied over the range 0 ≤ x ≤ 1, which alters the magnetoelastic and magnetocrystalline anisotropy and the saturation magnetization. Nanocomposites were templated into square arrays of pillars in which the formation of a "checkerboard" ground state after ac-demagnetization indicated dominant magnetostatic interactions. The effect of switching field distribution in disrupting the antiparallel nearest neighbor configuration was analyzed using an Ising model and compared with experimental results.

Weak dipolar interaction between CoPd multilayer nanodots for bit-patterned media application

The morphology and magnetic performance of CoPd multilayer nanodots have been investigated by a combination of DC sputtering, anodic aluminum oxide(AAO) template method and Ar ion etching. The nanodots exhibit a normal switching field distribution(SFD) of 17%, which is comparative to Bit-patterned media (BPM) made by e-Beam. It is found that the first-order reverse curve (FORC) data of CoPd nanodots have a good match with the mean-field model, which shows dipolar interaction contributes as small as 8.4% to the total SFD. Magnetic force microscope(MFM) imaging at a certain location confirms the FORC results by observing the effect of dipolar interaction directly. Our CoPd multilayer nanodots can be a good candidate for BPM application.

Superior bit error rate and jitter due to improved switching field distribution in exchange spring magnetic recording media.

We report two effects that lead to a significant reduction of the switching field distribution in exchange spring media. The first effect relies on a subtle mechanism of the interplay between exchange coupling between soft and hard layers and anisotropy that allows significant reduction of the switching field distribution in exchange spring media. This effect reduces the switching field distribution by about 30% compared to single-phase media. A second effect is that due to the improved thermal stability of exchange spring media over single-phase media, the jitter due to thermal fluctuation is significantly smaller for exchange spring media than for single-phase media. The influence of this overall improved switching field distribution on the transition jitter in granular recording and the bit error rate in bit-patterned magnetic recording is discussed. The transition jitter in granular recording for a distribution of Khard values of 3% in the hard layer, taking into account thermal fluctuations during recording, is estimated to be a = 0.78 nm, which is similar to the best reported calculated jitter in optimized heat-assisted recording media.

Oersted Field and Spin Current Effects on Magnetic Domains in [Co/Pd]15Nanowires

An out-of-plane Oersted field produced from a current-carrying Au wire is used to induce local domain formation in wires made from [Co/Pd]15 multilayers with perpendicular anisotropy. A 100 ns pulsed current of 56-110 mA injected into the Au wire created a reverse domain size of 120-290 nm in a Co/Pd nanowire on one side of the Au wire. A Biot-Savart model was used to estimate the position dependence of the Oersted field around the Au wire. The shape, size and location of the reversed region of Co/Pd was consistent with the magnitude of the Oersted field and the switching field distribution of the unpatterned film. A current density of 6.2 x 10^11 A m-2 in the Co/Pd nanowire did not translate the domain walls due to low spin transfer efficiency, but the Joule heating promoted domain growth in a field below the coercive field.

The role of the crystal orientation (c-axis) on switching field distribution and the magnetic domain configuration in electrodeposited hcp Co–Pt nanowires

In this report, Co–Pt nanowires (NWs) were produced via potentiostatic electrodeposition into commonly used commercial ordered-alumina and disordered-polycarbonate membranes with similar pore diameters (≈200 nm). The pore diameter of the membranes and the deposition conditions were chosen such that the Co–Pt NWs fabricated into both membranes had a hexagonal close packed (hcp) crystal structure with a crystallographic texturing of the c-axis in the direction perpendicular to the NWs’ long axis; this effect was more pronounced in the alumina membranes. Due to the local fluctuation in electrodeposition conditions (pore diameter, pore shape), we have found a small variation in the c-axis orientations in the plane perpendicular to the NWs’ long axis. Magnetic characterizations suggested that there is uniaxial anisotropy perpendicular to the Co–Pt NWs’ long axis and the small variation in the orientation of the hcp c-axis plays an important role in the switching-field distribution and the magnetic domain structure of the Co–Pt NWs. First order reversal curves (FORCs) revealed week magnetostatic interactions between Co–Pt NWs, thus suggesting that the different pore alignments are not influencing much the magnetic properties in both membranes. The micromagnetic simulation revealed that the transverse-stripe (TS) and longitudinal stripe (LS) domains are energetically most favorable structures in such NWs. This study accentuates the influence of the crystal orientation (c-axis) of the high-anisotropy materials on their functional magnetic properties and thus is of great importance for the fabrication of nanodevices based on such NWs.

Geometric control of the magnetization reversal in antidot lattices with perpendicular magnetic anisotropy

While the magnetic properties of nanoscaled antidot lattices in in-plane magnetized materials have widely been investigated, much less is known about the microscopic effect of hexagonal antidot lattice patterning on materials with perpendicular magnetic anisotropy. By using a combination of first-order reversal curve measurements, magnetic x-ray microscopy, and micromagnetic simulations we elucidate the microscopic origins of the switching field distributions that arise from the introduction of antidot lattices into out-of-plane magnetized GdFe thin films. Depending on the geometric parameters of the antidot lattice we find two regimes with different magnetization reversal processes. For small antidots, the reversal process is dominated by the exchange interaction and domain wall pinning at the antidots drives up the coercivity of the system. On the other hand, for large antidots the dipolar interaction is dominating which leads to fragmentation of the system into very small domains that can be envisaged as a basis for a bit patterned media.

Magnetization reversal of FePt based exchange coupled composite media

Microstructure and magnetic properties of L10-ordered FePt-C nanogranular films capped with A1-disordered FePt layers with various thicknesses were investigated. After the growth of 9.5 nm thick L10-FePt-C granular layer on a MgO(001) single crystalline substrate at a substrate temperature of Ts = 600 °C, FePt capping layers with thicknesses, tFePt, ranging from 2.7 to 8.6 nm were deposited at Ts = 400 °C. The coercivity of the L10-FePt-C/A1-FePt exchange coupled composite (ECC) media decreased from 4.9 to 1.4 T with increasing tFePt. Evaluation of energy barrier, Eb, using the Sharrock equation indicated no degradation of thermal barrier in spite of the large reduction in the coercivity. The switching field distribution of the ECC media is significantly reduced from 23% for the single layer FePt-C granular film to 8% for the ECC media with tFePt = 6.5 nm. Micromagnetic simulations showed the magnetization reversal of ECC media with tFePt = 2 nm capping layer occurs by the coherent rotation while domain wall pinning becomes dominant for tFePt ≥ 5 nm. The switching field distribution (SFD) of the ECC media is substantially decreased for tFePt = 2 nm. Low coercivity with high Eb and improved SFD observed from the ECC media makes them promising as high areal density recording media for both perpendicular magnetic recording (PMR) and heat assisted magnetic recording (HAMR).

Statistical magnetometry on isolated NiCo nanowires and nanowire arrays: a comparative study

The first-order reversal curve (FORC) method can be used to extract information about the interaction and switching field distribution of ferromagnetic nanowire arrays, yet it remains challenging to acquire reliable values. Within ordered pores of anodic alumina templates we electrochemically synthesize eight different NixCo1−x samples with x varying between 0.05 and 1. FORC diagrams are acquired using vibrating sample magnetometry. By dissolving the template and using the magneto-optical Kerr effect, we measure the hysteresis loops of up to 100 different and isolated nanowires for each sample to gain precise information about the intrinsic switching field distribution. Values of the interaction field are extracted from a deshearing of the major hysteresis loop. We present a comparative study between all methods in order to evaluate and reinforce current FORC theory with experimental findings.

A study on the extent of exchange coupling between (Ba0.5Sr0.5Fe12O19)1−x(CoFe2O4)x magnetic nanocomposites synthesized by solgel combustion method

One step citrate gel combustion method followed by high temperature annealing was employed for preparing (Ba0.5Sr0.5Fe12O19)1−x(CoFe2O4)x (x=0.1, 0.2, and 0.3) composite ferrite powders. The powders were subjected to annealing at 800°C in order to decisively study the phase evolution of the combined hard and soft ferrites. Thermogravitry (TGA)/differential scanning calorimetry (DSC) analysis exhibited three stages of decomposition in the precursor gels combined with an exothermic peak at 210°C. X-ray diffraction (XRD) analysis confirmed that the diffraction peaks were perfectly indexed to the hexagonal magnetoplumbite structure of Ba0.5Sr0.5Fe12O19 and the cubic spinel structure of CoFe2O4. Fourier transform infrared spectroscopy (FT-IR) analysis for the samples showed a Co–O stretching vibration accompanied with Co–O–Co or Fe–O–Fe bands at 1220cm−1. The morphology of the samples were examined by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The crystallinity of a selected sample was evaluated by using the high resolution transmission electron microscope (HR-TEM) and selected area electron diffraction (SAED) pattern. It confirmed the presence of planes comprising the hard and soft phases in the synthesized nanocomposites. The magnetic parameters like saturation magnetization MS, remanent magnetization MR, squareness ratio SR, coercivity HC and magnetic moment µB were evaluated using hysteresis by employing vibrating sample magnetometer (VSM). Maximum HC of 4.7kOe and MS of 60.4emu/g were obtained for (Ba0.5Sr0.5Fe12O19)0.9(CoFe2O4)0.1. Switching field distribution curves were analysed by using the demagnetization curve. The exchange coupling between the hard and soft phases were analysed by the dM/dH plots and it indicated the exchange coupling first increased with the increase in the concentration of spinels and then decreased. The possible comparison of exchange coupling between the hard and soft phases has been discussed in detail.

Origin of open recoil curves in L10-A1 FePt exchange coupled nanocomposite thin film

Mixed phase FePt systems with intergranular coupling may be looked upon as natural exchange spring systems. The coupling strength between the soft and hard phase in these systems can be analyzed using recoil curves. However, the origin of open recoil curves depicting the breakdown of exchange coupling or anisotropy variation in hard phase is still an ambiguity and requires an in-depth analysis. In order to investigate this, an analysis of the recoil curves for L10–A1 FePt nanocomposite thin films of varying thickness have been performed. The switching field distribution reveals that the maximum of openness of recoil curve is directly proportional to the amount of uncoupled soft phase present in the system. The coupling between the hard and soft phase is also found to increase with the thickness of the film. Monte Carlo simulations on a model three dimensional array of interacting nanomagnetic grains provide further insight into the effect of inter granular exchange interactions between the soft and hard phases.

Determination of the electric field induced anisotropy change in sub-100 nm perpendicularly magnetized devices

We measure the voltage or electric field (EF) modulated change in anisotropy using two methods on the same nanometer sized device: 1) Directly using the area of the hard axis magnetization loop and 2) Indirectly using the switching field distribution method. Both methods yield similar values of efficiency. With the indirect method, the efficiency derived from the thermal stability was found to be more consistent than that from the anisotropy field. Our data also suggests that memory devices that rely solely on EF effects may benefit from larger device sizes.