Fig. 1 : The 4 magnetic configurations of multilevel dots with a perpendicular magnetic layer (green) and an in-plane magnetic layer (gray).
Spintec proposes a method to greatly increase the data storage density in hard disk drives. The potential of this technique has been demonstrated experimentally.
The ever-increasing need for computing power also implies an increase of the data volume being used and stored. Presently, the goal in hard disk drives is to reach storage densities of the range of one terabit per square inch. In all magnetic media, the elementary information bit is stored in the form of the magnetized state of a magnetic dot. The magnetization can be written by applying a magnetic field, and read by measuring the perpendicular component of the stray field. The write and read operations are performed by a magnetic read head, which converts a current into a magnetic field (write), and a magnetic field into a voltage (read).
To increase the density the element dot size must be reduced. The physical challenge is the thermal stability of the magnetic element, which becomes unstable as dot size becomes closer to the so-called super-paramagnetic limit. Several approaches are being investigated to solve this problem of keeping a reasonable signal-to-noise ratio together with an acceptable write power. Discrete media is one possible way, with each patterned dot storing just one bit. The difficulty is to achieve such dots in a way which can be reproduced. Spintec has proposed the use of a pre-patterned silicon (Si) substrate using standard microelectronic fabrication processes. The substrate is first patterned to realize an array of Si pillars. The magnetic layers are then deposited on top of these pillars.
This approach also allows for the attainment of multi-level dots: each dot being capable of having more than two different magnetic configurations, each dot storing more than one bit. To do so, two magnetic layers are stacked on the pillar. The originality of Spintec’s approach is to have one layer with perpendicular magnetization and a second one with in-plane magnetization (Fig.1). The stray fields of the 4 magnetic configurations are different, and allow coding 2 bits per dot. The value of the magnetic field at the center of the pillar is the same in configurations A(0,1) and C(1,1), but different at the edges. The magnetic read head, being sensitive to the perpendicular component of the stray field, sees the sequence of stray fields when moving over the dot. The two states can thus be distinguished because moving from a dot in state D(1,0) to C(1,1) results in a ‘odd’ magnetic field profile, while going from D(1,0) to A(1,0) the magnetic field profile is ‘even’ (Fig. 2). The readout is done in a differential way, using a first dot in a well-known reference state. Compared to a configuration with two perpendicular layers, this approach allows for a better signal-to-noise level and a better use of the available storage surface, since part of the information is stored in the edges of the dot. It also prevents the magnetic instability of two perpendicular magnetizations being in opposite directions. The information is written applying first a High field to set the hard layer direction and, afterwards, a lower field to set the soft layer direction without affecting the other one.
Fig. 3 : MFM image of the patterned media surface. Shown left is the topography image and the magnetic image to the right. The 4 magnetic configurations illustrated in Fig. 1 are clearly seen in dots A, B, C, D.
Such devices have been fabricated and characterized through magnetic force microscopy (MFM) to map the sample surface (Fig.3). These measurements confirm that the technique can correctly identify the 4 possible configurations of the magnetic dots. The increase in hard disk real density can be achieved without reducing the dot size.
Maj : 18/02/2014 (955)