Thomas Hauet

Bit patterned media based on block copolymer directed assembly with narrow magnetic switching field distribution

Electron-beam (E-beam) directed assembly, which combines the long-range phase and placement registration of e-beam lithography with the sharp dot size and spacing uniformity of block copolymer self assembly, is considered highly promising for fabricating templates that meet the tight magnetic specifications required for write synchronization in bit patterned media magnetic recording systems. In our study, we show that this approach also yields a narrower magnetic switching field distribution (SFD) than e-beam patterning or block copolymer self-assembly alone. We demonstrate that the pattern uniformity, i.e., island diameter and placement distributions are also important for achieving tight magnetic SFDs.

Coercivity tuning in Co/Pd multilayer based bit patterned media

In order to adjust the reversal field of high anisotropy [Co(2.8 A)/Pd(9 A)]8 bit patterned media (BPM), one may increase the individual Co thickness to change the overall anisotropy or alternatively combine the high anisotropy multilayer with a lower anisotropy material, thus creating a heterogeneous laminated system. In the current study, we find that [Co/Pd]8−N[Co/Ni]N hard-soft laminated BPM allows tuning the coercivity while maintaining a narrow normalized switching field distribution (σSFD/HC), whereas homogeneous structures with increased individual Co thickness show a more pronounced increase in σSFD/HC. Possible reasons for the different behavior, such as changes in strain and texture, are discussed.

Role of reversal incoherency in reducing switching field and switching field distribution of exchange coupled composite bit patterned media

Model exchange coupled composite media, namely [Co/Pd]5/Pd(t)/[Co/Ni]3/Co multilayers, deposited onto prepatterned substrates have been investigated for potential use in bit patterned recording. Optimizing the thickness of the Pd interlayer allows decreasing the switching field (HC) and switching field distribution (σSFD) while maintaining thermal stability. The dependence of the remanent coercivity on the external field angle indicates that the gain in HC and σSFD originates from the incoherent magnetization reversal introduced by slightly decoupling the hard and soft multilayer stacks. This tendency is confirmed by further reductions in HC and σSFD, when inserting another Pd interlayer within the hard [Co/Pd]5 stack.

Quantifying perpendicular magnetic anisotropy at the Fe-MgO(001) interface

We show that Fe-MgO interfaces possess strong perpendicular magnetic anisotropy of 1.0 ± 0.1 erg/cm2 in fully epitaxial MgO/V/Fe/MgO(001) and MgO/Cr/Fe/MgO(001) heterostructures. The sign and amplitude of the total anisotropy are quantified as a function of Fe thickness using magnetometry and ferromagnetic resonance. There is a transition from out-of-plane to in-plane anisotropy for 6 Fe monolayers in V/Fe/MgO and only 4 monolayers in Cr/Fe/MgO. A detailed study of the Fe magnetization and effective anisotropy in both systems explains this difference and quantifies the Fe-MgO interface anisotropy.

Magnetic anisotropy modified by electric field in V/Fe/MgO(001)/Fe epitaxial magnetic tunnel junction

Single-crystalline V/Fe(0.7 nm)/MgO(1.2nm)/Fe(20 nm) magnetic tunnel junctions are studied to quantify the influence of an electric field on the Fe/MgO interface magnetic anisotropy. The thinnest Fe soft layer has a perpendicular magnetic anisotropy (PMA), whereas the thickest Fe layer acts as sensor for magnetic anisotropy changes. When electrons are added at the PMA Fe/MgO interface (negative voltage), no anisotropy changes are observed. For positive voltage, the anisotropy constant decreases with increasing bias voltage. A huge 1150 fJ V−1 m−1 anisotropy variation with field is observed and the magnetization is found to turn from out-of-plane to in-plane of the sample with the applied voltage.

Origin of magnetic switching field distribution in bit patterned media based on pre-patterned substrates

Using a combination of synchrotron radiation based magnetic imaging and high-resolution transmission electron microscopy we reveal systematic correlations between the magnetic switching field and the internal nanoscale structure of individual islands in bit patterned media fabricated by Co/Pd-multilayer deposition onto pre-patterned substrates. We find that misaligned grains at the island periphery are a common feature independent of the island switching field, while irregular island shapes and misaligned grains specifically extending into the center of an island are systematically correlated with a reduced island reversal field.

Measurements of the write error rate in bit patterned magnetic recording at 100–320 Gb/in2

We demonstrate a technique for measuring the intrinsic bit-error-rate as a function of write misregistration in bit patterned media. We examine the recording performance at bit densities of 100, 200, and 320 Gigabits per square inch (Gb/in2) and find that the on-track write misregistration margin for error rates below 10−4 is ∼1/4 of a bit length for all three densities. We demonstrate two-dimensional recording at sub 10−4 bit error rate at 100 and 200 Gb/in2 and with a 10−3 bit error rate at 320 Gb/in2.

Nanoscale spintronic oscillators based on the excitation of confined soliton modes

This paper demonstrates how to excite complex soliton modes in nanomagnets with perpendicular to plane magnetic anisotropy driven by the non-uniform injection of a spin-polarized current. We addressed the study toward two different scenarios, in the first the excitation of two rotating bubble/antibubble pairs is predicted, in the second one, by means of the topological density, we characterized the dissipative droplet recently measured as single constrained bubble/antibubble pair. Our results are important for the theoretical understanding of how to control the spatial structure of soliton modes for application in spintronics, magnonics, and domain wall devices.

High-Density Bit Patterned Media: Magnetic Design and Recording Performance

We examine the magnetic properties and recording performance of bit patterned exchange coupled composite (ECC) magnetic media at different bit and island aspect ratios. The ECC media consists of Co/Pd and Co/Ni multilayers whose coupling is controlled using Pd interlayers. We show that this multilayer system can be tuned to provide writeable media with a low switching field distribution for bit patterned magnetic recording. The recording performance of 100 Gb/in2 media shows a sub 1e-4 bit error rate floor and misregistration errors that are well-described by a simple error model.

Influence of ion irradiation on switching field and switching field distribution in arrays of Co/Pd-based bit pattern media

We have used ion irradiation to tune switching field and switching field distribution (SFD) in polycrystalline Co/Pd multilayer-based bit pattern media. Light He+ ion irradiation strongly decreases perpendicular magnetic anisotropy amplitude due to Co/Pd interface intermixing, while the granular structure, i.e., the crystalline anisotropy, remains unchanged. In dot arrays, the anisotropy reduction leads to a decrease in coercivity (HC) but also to a strong broadening of the normalized SFD/HC (in percentage), since the relative impact of misaligned grains is enhanced. Our experiment thus confirms the major role of misorientated grains in SFD of nanodevice arrays.