Magnetic recording on nanostructures

The inductive write element records the data in vertical magnetization patterns. The information is then read back with the giant magnetoresistive (GMR) or tunneling magnetoresistive (TMR) read element by measuring the stray magnetic field from the transitions between regions of opposite magnetization. Finally, a signal processing unit transforms the analog readback signal into a stream of data bits.

The fine microstructure of the granualar recording media as shown in the TEM-image below allows writing and storing transitions at almost any location and at high linear densities with the ultimate density limited by the grain size. As illustrated in the inset of the TEM-image the transitions follow the grain boundaries, which provide strong pinning sites for the magnetic transitions. Finally, a thin carbon overcoat and a lubricant layer protect the media from oxidation and physical damage as a result of physical contact with the head.

To increase data-storage densities while maintaining acceptable performance, designers have shrunk the media's grain diameters and decreased the thickness of the media as well as the physical dimensions of the read/write heads. The resulting smaller grain volume V makes them increasingly susceptible to thermal fluctuations, which decreases the signal sensed by the drive's read/write head. If the signal reduction is great enough, data could be lost in time due to this superparamagnetic effect.

Theoretical and experimental analyses arrive at the thermal stability condition K_uV > 55k_BT to maintain sufficient signal stability over at least five years of data storage. This criterion sets an effective limit on the minimum grain volume and was the basis for the predictions that areal densities would be limited by thermal activation of the grains. In the presence of these thermal effects, a variety of material and system changes can be implemented to lessen their impact. Thermal stability is improved by reduction of crystallographic defects, by improving epitaxy, and by materials with higher anisotropy for the recording media, where the increase in K_u counters the reduction in V to maintain the thermal stability.

The latter is currently achieved by increasing the Pt content in modern CoPtCr-SiO₂ media. Other media candidates with high anisotropy are the tetragonal L1₀ phases of FePt and CoPt, artificially multilayered materials and rare earths compounds. Nevertheless, following this path also increases the write field requirements, since the write field H_w ~ K_u / M_s, where M_s is the saturation magnetization of the recording media. The required write field improvements were traditionally achieved by design changes in the write head and the use of materials with higher saturation magnetization as the write poles. However, modern write poles already consist of low-anisotropy materials with saturation magnetization density approaching the highest reported value, which will limit the maximum achievable write field.

It is becoming increasingly clear that, to avoid limitations to areal growth rates arising from thermal instabilities in future high-areal-density products, significant improvements of the recording medium must be achieved. New thin film materials or architectures are needed that ease the write field requirements at high areal densities and are more thermally stable than conventional media. These new concepts include:

Patterned media

The creation of lithographic magnetic nanostructures or “patterned media” for single-bit data storage is recognized as a promising approach to overcome the areal density limit for magnetic recording imposed by superparamagnetism in conventional magnetic media.
As an approach to extend magnetic storage densities we want to investigate patterns of magnetic nanostructures with perpendicular magnetic anisotropy. Each nanostructure can be used for storing a single bit of information. Being single domain, the bit size can be reduced compared to conventional storage media without losing thermal stability.

BPM implies a formation of an ordered at a large scale 2D-array of magnetic nanostructures with a size of about 10 nm and period below 25 nm. Hard magnetic alloys (i.e. FePt) have to be used to satisfy the criteria for long time stability of the stored information bit [1]. However, reduction of a size of nanostructures below certain critical one results in a modification of the magnetization reversal mechanism from nucleation and domain wall propagation (small coercive field) to coherent rotation of magnetization in a single-domain nanostructure. In the latter case, the coercive field required to reverse the magnetization of a bit is equal to anisotropy field (~11 T for a single-domain FePt nanostructure). This field is substantially larger compared to the write field provided by a recording head (< 2 T) [2]. This discrepancy is known as a writeability issue. There are two approaches suggested to reduce the switching field of the magnetic nanostructure: (i) heat assisted magnetic recording (HAMR) and (ii) exchange coupled composite (ECC) media. HAMR relies on the local heating of the magnetic nanostructure close to the Curie temperature before writing event [3]. In contrast, magnetic bit in the ECC media concept [4, 5] consists of two magnetic components: soft and hard parts. In this composite, the thermal stability is given by the hard component, while the switching field gets substantially reduced due to the early nucleation of the reverse domain in the soft layer resulting in a torque at the interface between hard and soft layer influencing the nucleation field of the hard layer. Both approaches have potential for implementation in the future generation of magnetic storage devices.

Fig. 1: Perpendicular magnetic nanostructure arrays (period 100nm).

Fig. 2: Magnetic images of self-assembled particle arrays after Co/Pd film deposition (particle size 310 nm).

Fig. 3: Schematic of the magnetic recording process on patterned perpendicular media using a conventional longitudinal recording head.

Fig. 4: Top: AFM image showing a CoCrPt island row with a period of 103 nm which has been patterned by FIB lithography. Bottom: Corresponding MFM image after in-phase writing revealing alternating magnetization orientations of the written islands. (b) MFM image taken after sequentially writing of four equidistant islands in a dense island array (period: 103 nm).

Percolated media