The storage materials rely on the principles and

 

The Structural
Properties of Data Storage Materials

 Data storage materials rely on the principles and
properties of magnetism. The magnetic moment in an atom is the result of intrinsic
angular moment and orbital magnetic dipole moment. Magnetic field is generated
by orbiting valence electrons and normally traveling from south to north pole. The
direction of the electrons orbiting within atoms determines the direction of
magnetic field. Two electrons orbiting in opposing direction adds up to zero
magnetic field. However, atoms with unpaired electron spins can make a material
slight magnetic. Atomic electron configurations determine the various types of
magnetic properties. The characteristic of magnetic materials varies based on
the behavior of dipoles, and domains before and after the presence of a
magnetic field. The materials can be categorized into five types of magnetism:
dimagnetism, paramagnetism, ferromagnetism, antiferromagnetism, and
ferrimagnetism. Materials with paired electron spins have zero dipoles. Dimagnetism
occurs when some randomly oriented dipoles are aligned opposite to the externally
applied field in a material with zero dipole. Dimagnetism has a weak magnetic
susceptibility (?) (Piramanayagam, 2012). Magnetic susceptibility is defined as

? =
M/H

where M=magnetization and H= applied magnetic field.

Paramagnetic materials have small positive
magnetic susceptibilities a result of the applied magnetic field and the behavior
of the material’s electron. Paramagnetism describes the production of net
magnetization when a magnetic field is applied on randomly oriented dipoles.
Paramagnetic materials cannot be used for data storage because there is no net
magnetization after the removal of the magnetic field (Wu et. al., 2009). Some magnetic materials behave as a storage center because
they store remnant magnetization. Ferromagnetic and antiferromagnetic materials
have intrinsic magnetic properties. Randomly oriented domains in ferromagnetic
materials have net magnetic moment from unpaired electron spins that are aligned
within domain.  They exhibit strong magnetization
in the presence of external magnetic field, and can retain a portion of the magnetization
after the removal of the magnetic field. When the electron dipoles
spontaneously aligned opposite to each other in antiferromagnetic materials,
there is zero net magnetization. The influence of the applied magnetic field on
antiferromagnetic will result in a weaker magnetization than in a ferromagnetic
material.

The remnants of magnetic field (Br)
left within material after the removal of field can be illustrated by
hysteresis loop as seen in figure 1. The area within the loop represents the
magnetic energy lost after the field is removed. Soft magnetic materials have a
small area inside the loop. Soft magnets can be used for application in
magnetic screening, and power conversion. Hard magnetic materials have a larger
area under the hysteresis loop indicates a high remnant flux density which is
advantageous in data storage applications. Hc is the required energy
needed to completely remove magnetization in a material. The higher value Hc,
the harder it is to demagnetize a material (Wu et. al., 2009). Magnetic materials with the potential for storing large
remnants of magnetic fields can be exploited for storing information.  

Figure 1.  Hysteresis Loop of a Ferromagnetic Material (Wu et. al., 2009)

Ms=
maximum magnetization (saturation magnetization) Hc= coercivity

 

Studies have demonstrated that changes in properties
such as the direction of magnetization, temperature, and grain size can
optimize the storage capacity of magnetic materials. The boundaries of the
domain define the grain boundaries and sizes. The pioneering
magnetic material for data storage by IBM had a diameter of 24in with with only
100Gb/in2 areal density in …(…). Areal density is measure of the volume of
the bit per grain volume. Areal density can be optimized by reducing the volume
of grain because the density is inversely proportional to gain volume. Grain
volume can be reduced by reducing the geometrical dimension of the grains. Low grains
have low volume but higher areal density. Consequently, the grain sizes of
subsequent magnetic materials have grown smaller than the former.  The reduction in grains also improved
magnitude of magnetization that can be stored. There is limit to which the
grain sizes can be reduced for optimization of magnetic data storage materials
without compromising some properties. Magnetic anisotropy describes the energy necessary
different magnetization direction within the domain in a material. When the
grain size is too small, the orientation can be easily reversed by changes in
temperature. This property called superparamagnetism. Superparamagnetic effect
can be prevented if the product of magnetic anisotropy and grain volume is
estimated to be 40-60 times larger than the product of Boltzmann’s constant and
temperature (Bandi?, and Victoria, 2008). Temperature is another factor that
affects magnetization. Curie temperature is the temperature where the saturation
drops to zero. Reducing temperature decrease the magnetic susceptibility.

? =
M/H=C/T

M=magnetization

H=
applied magnetic field

C=
curie material constant

T=
temperature in Kelvin

 

Data can be stored in magnetic materials in a form of
bits and a bit is the smallest unit of data. The polarity of the magnetic field
of each bit can be changed by applied current. The inductive head consists of a
coil which permits current flow and controls production magnetic field (Piramanayagam, 2012).
Current can change the direction of the applied current and thus change the
direction of magnetization of each bit. Traditionally bits were oriented
parallel to medium. A sensor reads the magnetic field emitted from each pole
and converts magnetic information into binary information which can be
interpreted by computing devices. As a magnetic field sensor moves across each
region, a voltage produced is represented as 1 where like-poles meet or 0 when
opposing poles are adjacent (Piramanayagam,
2012). The component that reads or writes the bits
the head

The conventional longitudinal recording technology
faced some limitations as the demand for highly dense data storage increased. Perpendicular
recording technology was developed by Prof. Iwasaki in 1975 properties,
replacing longitudinal recording (Wu et.
al., 2009). Changing the direction in which magnetic data is recorded increased
the storage capacity. It also reduced the distance between bits, but the interference
of neighboring magnetic fields made it difficult for the traditional inductive
head to read each bit. In 1990s, magnetoresistive (MR) and giant
magnetoresistive (GMR) head emerged to replace the inductive head. MR and GMR
consist of a reading and writing component. Magnetoresistive heads sense and
read the high electron flow resistance produced by the collision of electron
spins when external fields are applied (Wu
et. al., 2009; Piramanayagam, 2012).

Ideal storage materials have low noise to signal
ratio, thermal stability and high dense bits. The shrinkage of bit length,
track width, disk grain size and thickness, fly height and gap by s factor can increase
areal density (Wu et. al., 2009). Further
reduction in size is limited because it decreases performance by reducing the
amplitude of signal to noise ratio, head to disk gap, thermal stability,
changing electronic and magnetic properties (Wu
et. al., 2009). Alternatives reading/writing heads and new magnetic materials make
it possible to improve the signal to noise ratio, the thermal stability,
readability and writability as the grain sizes are reduced to nanometers. The
development of heat-assisted magnetic head optimized SNR, thermal stability and
writability using anisotropic material with smaller grains by bringing it below
5nm away from the recording medium (Piramanayagam, 2012). The thermal heat is
used to reduce magnetic anisotropy (Bandi?, and Victoria,
2008). A protective lubricant over the magnetic medium prevent data loss
during sudden contact with the head during eat-assisted magnetic recording
(HAMR) (Piramanayagam, 2012; Shiroishi et. al., 2009).  

The volume of magnetic bits can be increased by
increasing the bit boundary with nonmagnetic materials or voids by procedures
like lithography.  Patterned media
emerged in the race to find a medium with high areal density and minimize the
effect of superparamagnetism for a higher data storage.  It removes magnetic transition noise and
increases the volume and anisotropy of each bit. Bit patterned media is coupled
with heat-assisted recording can support areal density about 5Tb/in2
but quite a low areal density in the absence of heat assisted recording
(Shiroishi et. al., 2009).  There are
numerous techniques used for the fabrication of patterned media. The first
method is E-beam Lithography (EBL) illustrated in figure 2.  EBL fabrication process consist of sputtering
electrons on 30nm thick PMMA is layered (positive resist) or Hydrogen Silsesquioxane
(HSQ) negative resist layer over Si substrate, removing the positive/negative
mask layer and then etching the patterned Si substrate (Wu et. al., 2009; Choi et.al., 2012). EBL can be used to
make a medium with circular or square-like bits as small as 10-20nm (Choi
et.al., 2012). EBL pattern with 1/N period can increase the density by N2
(Shiroishi et. al., 2009). In nanoimprinting lithography (NIL), is a
nanopatterned mould is inserted into a substrate layered with a positive resist
by mechanical pressure or UV exposure followed by the removal of he positive
resist (Choi et.al., 2012). A patterned medium by NIL has 25nm islands 70nm
apart while alternative process, interference lithography, only produced 30nm
array 100nm apart (Wu et.
al., 2009). Patterned medium by lithographic processes are slow and expensive.
Over millions of pounds to change current media to bit patterned media (Shiroishi
et. al., 2009).

Figure 2.
Patterned Media Fabricated by EBL process (Choi et.al., 2012)

 

            Templated growth is
another process of making patterned media. It uses a film with closely packed
pores to be used a mask for electron beam deposition of magnetic material. The
small pores can produce smaller magnetic islands for data storage.  The film can be fabricated from anodizing
aluminum film and the pore size can be controlled by voltage, current, density
or pH. Another film is from di-block copolymer that can self segregate and
create a pattern (Choi et.al., 2012). A third process, Ion implantation, can be
illustrated in figure 3.

Figure 4.
Patterned media by Ion implantation process (Choi et.al., 2012)

 

There are some studies exploring the ways to optimize
storage capacity of magnetic materials. Since there is a limit to grain
refinement of magnetic materials, some are some studies are exploring ways to
pack more information while keeping the grain size at a nanoscale. The new type
of materials is termed super lattice-like (SLL) structures. Nanoscale change in
electrical pulse induces a phase change and materials that have this property
are said to have phase change memory. An example is a super lattice-like
structure produced by spraying an alternating layer of 6nm thick crystalline
material like Sb4Te and 4nm thick thermal stable amorphous GaSb film
totaling 120nm total thickness. A minimum electric pulse 10ns can be used to
move between the three semi-stable resistance (Lu et. al., 2016). Lu et. al.
(2016) discovered that SLL structures have the potential to have multilevel
storage capacity in the difference phases.FePt nanoparticle is a another SLL
structure and multiphase material could be used to get areal densities of
10Tbit/in2 (Wang, 2008). FePt is created by gas-phase condensation technique
in which a metal evaporates from sputter metallic source to condense into a
chemically ordered particles after it deposits on a cold substrate. However,
the main problem with are long-range order patterning, surface roughness of the
medium and the alignment of easy axis (Wang, 2008).

Another recording technology is shingled writing recording
(SWR). SWR technique maximimises storage to 2 Tb/in2 by sequential overlapping
tracks because inarrowers the width of erase band (Shiroishi, et. al., 2009).
However the drawback of SWR is prevuoisly written data have to erased in the
reverse order sequentialy to update a single data the rewritten (Shiroishi, et.
al., 2009). With two dimensional magnetic recording (TDMR), SWR achiveds the
highest areal density possible which is arounf 100 Tb/in2. TDMR
employs a multiple 2-D array head to scan while substrating the interference with
the overlapping regions to gain complete information (Shiroishi, et. al., 2009).
It is currently a concept

 

 

Figure 5. sheet resistance vs,
temperature(Lu et. al., 2016)

 

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