1.2 Magnetic recording crisis and challenges

Magnetic recording is one of the most important examples of working nanotechnology. Here we will mostly be concerned with the hard disk magnetic storage. The magnetic information in the hard disk is stored in ``bits'' which are patterns of magnetization in a magnetizable material to store data. The present-day bit size is of the order of $100 nm$.

The magnetic recording started more than one hundred years ago. In 1888 Oberlin Smith suggested the possibility of magnetic recording using cotton threads in which steel dust would be suspended. Ten year later, in 1898, Valdemar Poulsen recorded the sound of his own voice in a steel wire extended between two walls, inclined in order to allow a small magnet to slide down the wire with constant velocity. While the electromagnet went down the wire the sound was recorded and it was replayed changing the magnet by a telephone earphone. This experiment originated the patent of the Telegraphone the same year. Posteriorly, in 1956 IBM created the IBM 305 RAMAC the first computer that incorporated a hard drive, the IBM 350. This hard disk had a total capacity of $5 Mbytes$ consisting of $50$ disks with an areal density of $0.002 Mb/in^2$.

Figure 1.2: Diagram showing the differences between longitudinal and perpendicular magnetic recording (from D. Weller of Seagate Research).

Figure 1.3: Historical evolution of the bit density in magnetic recording media (from Hitachi).

During the recent past the data density stored on rigid disk media (see Fig. 1.2), which is the highest density format, has been increasing at the rate of 60% per annum, following an exponential law equivalent to the Moore law for the integration of transistors in a chip (see Fig. 1.3). In the nineties and due to the introduction of advanced GMR spin-valve heads this rate of advance has increased to 100% per annum and in the last years this growth rate is slowed down. Until several years ago the evolution of magnetic recording was based on the ``scaling'' principle: the dimensions of recording heads, head-media separation, magnetic grain size were proportionally reduced in order to achieve higher areal density. Today the situation is different. Scaling is no longer applicable. These days, a good understanding of basic physics of recording phenomena is a necessity for an engineer working in magnetic recording. It is pertinent at this time to enquire as to where the fundamental physical limitations of magnetic recording may lie. In this context there are several principle areas of interest: the first of these is limitations to data rate. These are concerned with the fundamental physics of the maximum rate at which a magnetic moment may reverse from one direction to the other. The theoretical calculation of these limits is complex and not well understood. Secondly, and of principle concern, it is the limit to the density at which information can be stored in a magnetic thin film. This latter limitation is based on the signal to noise ratio and also on the question of the thermal stability of increasingly small written bits.

Thermal stability of a bit of information is of critical importance particularly as bits are made smaller and media are made thinner. In conventional magnetic recording, the medium is a granular film and a bit consists of several ( $N=500 \div 1000$) almost non-interacting magnetic grains. Signal to noise considerations are extremely complex and derive from factors such as the shape of bits and cross-talk between neighboring bits or even neighboring tracks but from simple statistical estimation the signal-to-noise ratio (SNR) is proportional to $\sqrt{N}$ [Mallinson 91]. Therefore, the number of grains included in a bit can not be reduced in order to preserve SNR and, consequently, the increasing bit density implies a reduction of the grain size. However, a reduction in the grain size leads to a reduction in the energy barrier $KV$, separating two magnetization states, where $K$ is the anisotropy constant and $V$ the grain volume, which determines the thermal stability of the written information. When the energy barrier is comparable to the thermal energy (see Fig. 1.4), the magnetization becomes unstable and the inversion of the magnetization by thermal fluctuations is likely to occur. This effect is known as superparamagnetism and the corresponding limitation of the density as superparamagnetic limit. Essentially, values of $KV/k_BT>60$ ($k_B$ is the Boltzmann's constant and $T=300 K$ is the temperature) are required to ensure the long-term stability of written information. According to this, H.Charap established that the maximum achievable density, considering a stability of at least tens years for the stored information, was $40 Gb/in^2$[Charap 97]. This limit has been shortly overcome after this prediction, showing the necessity of more realistic modeling of the superparamagnetic behavior. Additionally, after the overrun of this limit, the challenge of the magnetic recording industry is to surpass the density of $1 Tb/in^2$ within a thermal stable medium.

Figure 1.4: Energy of a single domain particle under applied field. The reversal modes and its corresponding energy barriers are indicated. From Ref. [Weller 99].

At present longitudinal magnetic recording systems are the basis of all low cost high-density information storage systems. Toshiba was the first company to fabricate a perpendicular recording based hard drive in $2004$ [Toshiba 04]. In January 2006, Seagate Technology [Seagate 06] began shipping its first laptop sized, 2.5 inch hard drive using perpendicular recording technology, the Seagate Momentus 5400.3. At this time the majority of its hard disk storage devices utilizes the new technology. In October 2007 Seagate Technology announced a new record of magnetic recording density of 421 Gb (gigabits) per square inch. The company announced the results of a magnetic recording demonstration that used perpendicular recording heads and media created with currently available production equipment. The difference between longitudinal and perpendicular recording is the orientation of the anisotropy of media grains. Fig. 1.2 shows schematically the difference between both recording solutions. In the case of longitudinal recording the grains magnetization is lying in the plane of the magnetic medium. When the medium is magnetized by the recording head, the average magnetization is pointing in the down-track direction (see Fig 1.2). When perpendicular head and media are used, the media anisotropy is oriented perpendicular to the thin film and its magnetization is pointing either ``up'' or ``down''.

The advantages of the perpendicular recording system over the longitudinal recording are multiple: (i) Higher thermal stability can be achieved by small in-plane grain diameter with cylindrical grain structure. (ii) A vertical pole head in a recording media with a soft underlayer can generate twice the field of longitudinal recording head. This allows writing higher coercivity medium, further decreasing grain size and maintaining media thermal stability. (iii) The read-back signal amplitude from perpendicular medium with soft underlayer is larger compared with equivalent longitudinal medium, improving signal-to-noise ratio. (iv) Perpendicular media grains are strongly oriented. This results in smaller medium noise and a sharper recorded transition. (v) The demagnetization field in the perpendicular medium is small at the transition region. This allows writing narrower magnetic transitions and improves thermal stability of high density data. The most optimistic scenarios predicted that the optimized perpendicular system may achieve a factor of 4-5 over the best longitudinal areal density.

Figure 1.5: The magnetocrystalline anisotropy versus the square of the saturation magnetization for different magnetic materials. Extracted from Ref. [Weller 99].

Figure 1.6: The different requirements composing the magnetic recording ``trilemma''.

The introduction of perpendicular recording technology comes to ease the superparamagnetic problem. However, this solution only delays the problem. On the other hand, clearly, a reduction in the grain size can be compensated for by an increase in the magnetocrystalline anisotropy constant $K$. Fig. 1.5 shows the magnetocrystalline anisotropy of different materials as compiled by D. Weller [Weller 99]. The saturation magnetization is also indicated, due to the destabilizing role of the magnetostatic interaction in thin films. The material used for current recording media is the $CoPtCr$, in which the $Cr$ is segregated in the grain boundaries, reducing the interaction between grains. The two materials with highest magnetocrystalline anisotropy constant are $FePt$ and $SmCo$. Nevertheless, the $SmCo$ alloys present problems of oxidation of the $Sm$ [Kardelky 05], which effectively reduces the potentiality as material for magnetic recording, due to the deterioration of the performance of the medium with time. The $L1_0$ chemically ordered $FePt$ alloy, which is less susceptible to oxidation, has attracted much attention, and is considered, together with $CoPt$ as the most promising candidate for a recording medium of new generation. The $FePt$ has been successfully produced in several forms: epitaxially grown single-crystal films [Farrow 96], chemically synthesized nanoparticles [Sun 00] (SOMA ``self-organized magnetic array'' project) and isolated island particles [Takahashi 04]. On the other hand, stable, high perpendicular anisotropy media with no size dispersion non-interacting grains is still a challenge.

Moreover, associated with an increase in $K$ is a consequent increase in the anisotropy field of the medium, given by $H_k=2K/M_s$ with $M_s$- the saturation magnetization value. This leads to increasing medium coercivity and the requirement of larger write fields. Unfortunately, the maximum field of the writing heads is bounded by the current technology and this poses a difficulty to the use of high anisotropy fields, since it is impossible to write in them. This constitutes a well known magnetic recording trilemma (see Fig. 1.6). At the present time the research centers of magnetic recording are actively working searching for new solutions for the future. In the following we mention different solutions that have been suggested:

• Heat assisted magnetic recording.

  HAMR is a Seagate-based acronym for Heat-Assisted Magnetic Recording. Alternatively, Hitachi uses an acronym TAR (``Temperature-assisted recording''). It describes a technology that magnetically records data on high-stability media such as an FePt alloy, using laser thermal assistance to first heat the material. These materials can store single bits in a much smaller area without being limited by the same superparamagnetic effect that limits the current technology used in hard disk storage. The anisotropy constant decreases with increasing temperature at a faster rate than the magnetization, leading to a reduction in the anisotropy field and coercivity with increasing temperature. The idea is to heat with a laser spot in the bit during the writing process as shown in Fig. 1.7, achieving low coercive fields. Posteriorly and after the cooling of the bit, the storing of the magnetization is at room temperature, when the material presents high anisotropy values and, correspondingly, large thermal stability. HAMR could increase the limit of magnetic recording by more than a factor of 100. This could result in storage capacities as great as 50 Tera bits per square inch. Seagate believes it can produce 300 Tera bit (37.5 Tera byte) Hard disk drives using HAMR technology [Wired 07].

• Patterned media.

  Patterned media is a technology that allows to record data in a uniform array of magnetic grains, storing one bit per grain, as opposed to regular hard-drive technology, where each bit is stored in a few hundred magnetic grains. The media consists of a periodic array of discrete magnetic elements either prepared artificially by different lithography techniques or self-organized spontaneously. Each element is a bit that is almost isolated from other elements but the magnetization inside the bit is strongly exchange coupled, compared to the conventional recording media. Therefore, the corresponding energy barrier is larger that the corresponding for the equivalent bit in conventional media and the thermal stability is improved. Another advantage of patterned media is the elimination of the transition noise between bits since the bits are completely separated. The major obstacle to smaller bit size using lithography is finding a low cost means of making media. One of the recent possibilities, already used by Toshiba in their laboratory demonstration is the pre-patterned media for hard disks in which different recording tracks are separated by trenches using imprinted lithography.

  One of the most promising proposals is the combination of $FePt$ SOMA with HAMR.

• Tilted media.

  The coercivity of a single domain particle presents a minimum at an applied field angle of $45^\circ$ while the thermal stability is preserved. Therefore, a medium in which the grains are tilted with this angle will allow less switching field and larger area density. However, the design of the reading heads is very complicated since the magnetization will be inclined with respect to the film normal.

Other proposals are related to the use of technologies different to hard-disk one which by their grown possibilities approach the characteristics of hard disk media. For example, a new type of magnetic storage, called MRAM, is being produced that stores data in arrays of magnetic bits and reads the state of the bits using the GMR effect on the bits. Its advantage is non-volatility, low power usage, and good shock robustness. However, with storage density and capacity orders of magnitude smaller than the hard disk one, MRAM is a nice application for situations where small amounts of storage with a need for very frequent updates ($10^{15}$ writes) are required. Other proposals can be also found in Chapters 3 and 5.

The research for future magnetic recording media is a very challenging task that includes material science and simulation of the performance of the material. The work of this thesis has been carried out in collaboration with Seagate Research, which has also provided me with a four year studentship.

Figure 1.7: Diagram showing a writing head for Heat Assisted Magnetic Recording (HAMR) (from D. Weller of Seagate Research)