Intrinsic Switching Field Distribution Research Articles

Switching field distribution of ultradense arrays of single-crystalline magnetic nanowires

Ultradense arrays of magnetic nanoelements present considerable interest for extending areal densities in magnetic recording media, provided that they display high switching fields and corresponding low standard deviations. Here, we report the switching field distribution of bottom–up synthesized single-crystalline vertical Co nanowires self-organized in 2D hexagonal superlattices. The combined shape and Co hexagonal compact magnetocrystalline anisotropies in individual nanowires of diameter as small as 6 nm define a robust perpendicular magnetic anisotropy despite important interactions in superlattices of 10 × 1012 NWs/in2. Using quantitative analysis of temperature-dependent first-order reversal curves, we capture the switching field distribution in this dipolar-coupled perpendicularly magnetized nanomagnets. First, the interwire dipolar interactions are treated separately and show a dominant mean field character with temperature independent amplitudes that scale with the nanowire packing fraction. Then, the intrinsic switching field distribution, namely, independent of interwire interactions, is determined as a function of temperature in the 5–300 K range. The mean value and deviation are both found to be driven by the intrawire dipolar interaction and the temperature-dependent uniaxial magnetocrystalline anisotropy, but of smaller amplitudes than those expected from bulk behavior. With coercive fields ranging between 0.3 and 0.8 T, the switching field deviations relative to coercivity reach 20%, which is a moderate value regarding pitch arrays as small as 8 nm.

Switching Field Distribution in BN/FePtCAg/MgTiON and FePtCAg/MgTiOBN Films.

BN is the currently required segregant for perpendicular FePt media. We found that BN can be diffused from the MgTiOBN intermediate layer during a high temperature process. The FePtCAg film sputtered on MgTiOBN layers illustrates higher perpendicular magnetocrystalline anisotropy (Ku) (1.43 × 107 erg/cm3) and coercivity (normal to film surface) (17 kOe) at 350 K compared to BN/FePtCAg/MgTiON film. From the microstructure, the FePtCAg film shows the granular structure on the MgTiOBN intermediate layer, but parts of the irregular FePt grains are agglomerated and partially separated in the matrix, with grains size being, on average, 26.7 nm. Cross-sectional imaging showed that the FePt grains have a truncated pyramid shape with a lower wetting angle, which is influenced by the surface energy of MgTiOBN. BN segregation at FePt grains or boundaries is still not clear. Using the electron energy loss spectrum (EELS), we found that part of the BN atoms were clearly observed in the FePt lattice and iron-boride oxide was indexed in the x-ray photoelectron spectroscopy (XPS) spectra. To determine the effects of BN segregant (from capping layer or intermediate layer) on the magnetic switching behavior of FePtCAg film, the intrinsic-(ΔHint = 6.17 kOe, 6.54 kOe) and extrinsic- (ΔHext = 0.80 kOe, 0.39 kOe) switching field distribution (SFD) were measured by plotting saturated major- and unsaturated minor- hysteresis loops to evaluate the crystal orientation and microstructure (grains volume and distribution) for BN/FePtCAg/MgTiON and FePtCAg/MgTiOBN films, respectively. The main contribution of intrinsic SFD is the c-axis misalignment for the BN/FePt/MgTiON sample; however, the dispersed magnetic anisotropy has a higher input to intrinsic SFD for FePtCAg/MgTiOBN/CrRu film.

FORC-Diagram Analysis for a Step-like Magnetization Reversal in Nanopatterned Stripe Array.

The fabrication approach of a magnonic crystal with a step-like hysteresis behavior based on a uniform non-monotonous iron layer made by shadow deposition on a preconfigured substrate is reported. The origin of the step-like hysteresis loop behavior is studied with local and integral magnetometry methods, including First-Order Reversal Curves (FORC) diagram analysis, accompanied with magnetic microstructure dynamics measurements. The results are validated with macroscopic magnetic properties and micromagnetic simulations using the intrinsic switching field distribution model. The proposed fabrication method can be used to produce magnonic structures with the controllable hysteresis plateau region’s field position and width that can be used to control the magnonic crystal’s band structure by changing of an external magnetic field.

Temperature Dependence and Microstructure Effects on Magnetic Properties of FePt(B, Ag, C) Film.

A FePt(B, Ag, C) granular film was formed from post-annealed B4C(1.0 nm)/FePt(Ag, C) layers at a substrate temperature of 470 °C for 2 min. The 6 nm thick FePt(B, Ag, C) film demonstrates high perpendicular magnetic anisotropy (Ku = 2.83 × 107 erg/cm3 at 100 K) and out-of-plane coercivity (Hc = 38.0 kOe at 100 K). The Ku and out-of-plane Hc are respectively increased from 38% and 46% between 350 K and 50 K. The sample with a thickness of 8 nm also shows a similar trend for magnetic properties; however, the tiny magnetization kink which may come from rare Fe-B or disordered FePt grains was observed in the easy axis loop. The intrinsic (ΔHint = 12.6 kOe) and extrinsic switching field distribution (ΔHext = 1.62 kOe) were characterized by major and minor loops to correlate the microstructural grains. The coupled FePt grains grown on a single MgTiON grain were observed in a high-resolution transmission electron microstructure (HRTEM) image. This small intergranular exchange coupling was defined by estimating the magnetic cluster size (46.6 nm) from ΔHext and the average grains size (28.2 nm) from TEM images. The temperature dependence of coercivity was fitted to further understand the magnetization reversal process. The lower microstructural parameter was evidenced in the imperfect grain morphology.

Structure, magnetic and thermal properties of FePt–C–BN granular films for heat assisted magnetic recording

The structure, magnetic and thermal properties of (FePt0.78–C0.22)–BN granular films with various BN volume concentrations are systematically studied. Compared to the FePt–C film without BN doping, the grain size distribution of the FePt grains is reduced by 50% with BN component is 10 vol.%, and the coercivity and the intrinsic switching field distributions of the film are increased 3 times and narrowed by 35%, respectively, when BN component is 6 vol.%. The improvement in magnetic properties is attributed to the enhanced chemical ordering of the FePt L10 phase. Based on the 3ω method, it is found that the relative cross-plane thermal conductivity of the FePt–C–BN layer is more correlated to the grain size distribution of the film, which decreases to the minimum value at 10 vol.% BN. Our results indicate that a certain amount of BN doping can improve the comprehensive performance of the FePt–C–BN granular films, making it a promising candidate for heat assisted magnetic recording.

Thermally nucleated magnetic reversal in CoFeB/MgO nanodots

Power consumption is the main limitation in the development of new high performance random access memory for portable electronic devices. Magnetic RAM (MRAM) with CoFeB/MgO based magnetic tunnel junctions (MTJs) is a promising candidate for reducing the power consumption given its non-volatile nature while achieving high performance. The dynamic properties and switching mechanisms of MTJs are critical to understanding device operation and to enable scaling of devices below 30 nm in diameter. Here we show that the magnetic reversal mechanism is incoherent and that the switching is thermally nucleated at device operating temperatures. Moreover, we find an intrinsic thermal switching field distribution arising on the sub-nanosecond time-scale even in the absence of size and anisotropy distributions or material defects. These features represent the characteristic signature of the dynamic properties in MTJs and give an intrinsic limit to reversal reliability in small magnetic nanodevices.

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.

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.

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.

Magnetostatic nearest neighbor interactions in a Co48Fe52 nanowire array probed by in-field magnetic force microscopy

The magnetization behavior of nanowires embedded in an array is influenced by the sum of the dipolar fields produced by all surrounding nanowires. These magnetostatic interactions largely modify the array properties and thus complicate the reconstruction of the ensemble averaged behavior of the individual nanowires, such as the intrinsic switching field distribution. Simply correcting the shearing of the hysteresis in a mean-field approach does not account for the locally fluctuating demagnetizing field, which originates from the individual magnetization configuration in the close surrounding of each nanowire. We present an in-field Magnetic Force Microscopy study of electrochemically produced Co48Fe52 nanowires, in which the influence of the magnetic nearest neighbor configuration on the switching behavior of the individual embedded nanowires is clearly detected. Based on this finding, a statistical evaluation method of nearest neighbor histograms is proposed, which potentially allows to judge the strength of the local magnetostatic interactions against the magnitude of the intrinsic switching field distribution.

Bit patterned media optimization at 1 Tdot/in2 by post-annealing

We report on the fabrication of 1 Tdot/in2 bit patterned media with high coercivity (HC) and narrow intrinsic switching field distribution (iSFD) based on nanoimprint from a master pattern formed by e-beam guided block copolymer assembly onto a carbon hard mask and subsequent pattern transfer via etching into a thin CoCrPt perpendicular anisotropy recording layer. We demonstrate that an additional vacuum annealing step after pattern transfer into the CoCrPt layer and after Carbon hard mask removal not only yields recovery from undesired damage of the island edges, but actually transforms the islands into a magnetically more favorable compositional phase with higher HC, lower iSFD/HC, and three-fold increased thermal stability. Energy filtered transmission electron microscopy analysis reveals that the diffusion of Cr from the island cores to the periphery of the islands during post-annealing is responsible for the transformation of the magnetic bits into a more stable state.

Magnetic force microscopy study of the switching field distribution of low density arrays of single domain magnetic nanowires

In the present work, we report on the in situ magnetic force microscopy (MFM) study of the magnetization reversal in two-dimensional arrays of ferromagnetic Ni80Fe20 and Co55Fe45 nanowires (NW) with different diameters (40, 50, 70, and 100 nm) deposited inside low porosity (P < 1%) nanoporous polycarbonate membranes. In such arrays, the nanowires are sufficiently isolated from each other so that long range dipolar interactions can be neglected. The MFM experiments performed for different magnetization states at the same spot of the samples are analysed to determine the switching field distribution (SFD). The magnetization curves obtained from the MFM images are relatively square shaped. The SFD widths are narrower compared to those obtained for high density arrays. The weak broadening of the curves may be ascribed to the NW intrinsic SFD. The influence of diameter and composition of the ferromagnetic NW is also investigated.

Influence of bias voltage on the switching behaviour of CoCrPt–SiO2 perpendicular recording media

A systematic investigation of the influence of negative substrate bias on the microstructure and magnetic switching behaviour of CoCrPt : SiO2 perpendicular recording media is performed by varying the negative bias voltage from 0 to 600 V during the deposition of a bottom Ru (or Ru : TiO2SiO2) intermediate layer. The microscopic structures and the surface morphology are studied by a transmission electron microscope and an atomic force microscope. The crystallographic properties are studied by an x-ray diffractometer. Our observation shows that high biasing is at work to favour the formation of low-energy hexagonal close packed (0 0 2) planes, resulting in high crystallinity of Ru. Addition of oxide to the Ru intermediate layer leads to an increase in grain segregation, which results in a small grain diameter, but broad standard deviation and large surface roughness. Utilization of a bias voltage on the Ru oxide intermediate layer successfully improves the crystal texture of CoCrPt : SiO2 and induces high Hc, a narrow intrinsic switching field distribution and low Hcr45/Hcr0, while maintaining isolated grains and small grain size.

The effect of capped layer thickness on switching behavior in perpendicular CoCrPt based coupled granular/continuous media

A systematic investigation of magnetic switching behavior of CoCrPt based capped media (perpendicularly coupled granular/continuous (CGC) media consisting of granular CoCrPt:SiO2TiO2Ta2O5/capped CoCrPt(B)) is performed by varying the thickness of the capped layer from 0 to 9nm. The microscopic structures of CGC media with different thickness of capped layer are examined by transmission electron microscope. We find out that CoCrPt magnetic grains are separated by nonmagnetic oxide grain boundaries. Grain size and grain boundary are about 8.9nm and 2nm, respectively. The nonmagnetic oxide grain boundaries in the granular layer do not disappear immediately at the interface between the granular and capped layers. The amorphous grain boundary phase in the granular layer propagates to the top surface of the capped layer. After capping with the CoCrPt(B) layer, the grain size at the surface of CGC structure increases and the grain boundary decreases. Both coercivity and intergranular exchange coupling of the CGC media are investigated by Polar magneto-optic Kerr effect magnetometer and alternating gradient force magnetometer. Although Hc apparently decreases at thicker capped layer, no obvious variation of macroscopic switching field distribution (SFD/Hc) is observed. We separate intrinsic switching field distribution from intergranular interactions. The investigation of reduced intrinsic SFD/Hc and increased hysteresis loop slope at coercivity, suggests that improvement of absolute switching field distribution (SFD) is caused by both strong intergranular exchange coupling and uniform grain size. Micromagnetic simulation results further verify our conclusion that the capped layer in CGC media is not uniformly continuous but has some granular nature. However, grains in the CoCrPt(B) capped layer is not absolutely isolated, strong exchange coupling exists between grains.

Effects of grain microstructure on magnetic properties in FePtAg-C media for heat assisted magnetic recording

L10 FePt has been proposed as a magnetic medium suitable for heat assisted magnetic recording, owing to its high magnetocrystalline anisotropy and moderate Curie temperature. Carbon has previously been used as a segregant creating small magnetically decoupled grains by sputter deposition at high temperature. Here, we deposit granular, segregated FePt films having a high degree of chemical order, coercive fields of 4.8 T, high remanence, and average grain size of 7.2 nm with 17% size distribution. Magnetic characterization reveals a large intrinsic switching field distribution of 15 kOe. We incorporate the results obtained by transmission electron microscopy, magnetometry, and x-ray diffraction into a micromagnetic model to better understand the origin of the intrinsic switching field distribution. We find that the dominant contributions to the observed magnetic properties are (i) a grain-size dependent distribution in anisotropy field and (ii) small angle variations in perpendicular anisotropy axis orientation. The presence of small superparamagnetic grains, contributions originating from the grain size distribution, as well as from grains with longitudinal or randomly oriented anisotropy axis are found to play a relatively small role. The results help understand the observed magnetization loops used to evaluate FePt media.

Intrinsic Switching Field Distribution of Arrays of Ni80Fe20 Nanowires Probed by In Situ Magnetic Force Microscopy

The progress of magnetization reversal of weakly packed ferromagnetic Ni80Fe20 nanowire arrays of different diameters (40, 50, 70 and 100 nm) electrodeposited in polycarbonate membranes was studied by magnetic force microscopy (MFM). For such a low packing density of nanomagnets, the dipolar interactions between neighbouring wires can be neglected. The intrinsic switching field distribution has been extracted from in situ MFM images and its width was found to be considerably smaller than for densely packed nanowire arrays.

Self consistent measurement and removal of the dipolar interaction field in magnetic particle assemblies and the determination of their intrinsic switching field distribution

Using low density arrays of bistable magnetic nanowires as a model dipolar system, it is shown that the dipolar interaction field coefficient can be measured from the remanence curves as well as from other functions of the isothermal remanent magnetization and the DC demagnetization remanence obtained as an affine transformation of the Wohlfarth relation. Based on mean field arguments, these measurements are used to subtract and remove the contribution of the configuration dependent dipolar interaction field from the major loop and remanence curves. The corrected remanence curves are first used to obtain the intrinsic switching field distribution of the nanowire array and then to validate this approach showing that they yield results consistent with the Wohlfarth relation for an assembly of noninteracting particles, thus providing a self-consistent procedure to verify the measured values of the interaction field and its removal from the measurements.

Recording performance and thermal stability in perpendicular media with enhancement of grain isolation as well as magnetic anisotropy field

Magnetic clustering, thermal stability, and recording performance on perpendicular media with multilayered magnetic anisotropy field (Hk)-gradient CoPtCr-oxide/Cap layers with various Ru-oxide layer thicknesses (tRu-oxide) on top of Ru/NiW layers are investigated. With increasing tRu-oxide from 0 to 1.3 nm, Hc and Hs are enhanced but Hn is reduced. Magnetic correlation length (Dn) extracted from a set of major and minor loops significantly decreases but intrinsic switching field distribution remains unaffected. A short-time switching field (Ho) proportional to Hk increases linearly while KuV/kT remains unchanged. Similar KuV/kT is explained by compensation of the reduced Dn with the enhanced Ku induced by a thin Ru-oxide. However, thermal decay rate degrades from 0.06 to 0.32%/decade, which correlates well with Hn. Similar values of initial minor loop slopes indicate no change in magnetic switching behavior. A 1 nm-thick Ru-oxide layer as a well-defined granular template significantly improves recording performance: narrower MCW at 10 T by 8 nm and higher SNR at 2 T by 1.4 dB are observed even at lower OW by 8 dB compared to the media without Ru-oxide. All the recording parameters as a function of Dn correlate well.

Extraction of short time intrinsic switching field distributions in perpendicular media

The effects of thermal fluctuations are characterized in granular perpendicular media of two different grain sizes. We extract short time switching fields Ho and intrinsic switching field distributions and find that after correcting for thermal effects and the demagnetizing fields, the switching field distributions are almost independent of grain size, in contrast to the long time scale behavior with broader distributions observed for the small grain media. In addition, the shape anisotropy fields of the magnetic clusters are shown to decrease the observed Ho and are more significant as grain size is reduced.

Study of magnetization reversal of Co/Pd bit-patterned media by micro-magnetic simulation

Bit-patterned media based on a single-bit-per-island may be a promising candidate for perpendicular magnetic recording at the Tb/in2 level because they could provide a lower noise and higher density. The understanding of magnetization reversal processes in such patterned media is important. In this work, the range of single domain island size based on Co/Pd bit-patterned media was determined. Demagnetization effect, dipolar interactions and switching field distribution (SFD) for bit-patterned media were quantitatively studied by the simulation based on Landau-Lifshitz-Gilbert equation. The total hysteresis loops and SFD were comparable with the experiment ones. The SFD increased from 2σ=1.2kOe (as the calculated intrinsic SFD) to the experimental value of 1.9kOe due to dipolar interactions which is in a good agreement with the experimental results (2.0kOe). Optimized patterned structure with a minimized SFD and maximized data storage densities was found to have an island size of 10nm and islands separation of 20nm. The calculated ratio of SFD/Hc (Hc: the coercivity) is 9.2%, which is below the threshold of 10% for 1Tb/in2 pattern media.