The StructuralProperties of Data Storage Materials Data storage materials rely on the principles andproperties of magnetism.
The magnetic moment in an atom is the result of intrinsicangular moment and orbital magnetic dipole moment. Magnetic field is generatedby orbiting valence electrons and normally traveling from south to north pole. Thedirection of the electrons orbiting within atoms determines the direction ofmagnetic field. Two electrons orbiting in opposing direction adds up to zeromagnetic field. However, atoms with unpaired electron spins can make a materialslight magnetic. Atomic electron configurations determine the various types ofmagnetic properties. The characteristic of magnetic materials varies based onthe behavior of dipoles, and domains before and after the presence of amagnetic field. The materials can be categorized into five types of magnetism:dimagnetism, paramagnetism, ferromagnetism, antiferromagnetism, andferrimagnetism.
Materials with paired electron spins have zero dipoles. Dimagnetismoccurs when some randomly oriented dipoles are aligned opposite to the externallyapplied field in a material with zero dipole. Dimagnetism has a weak magneticsusceptibility (?) (Piramanayagam, 2012).
Magnetic susceptibility is defined as ? =M/Hwhere M=magnetization and H= applied magnetic field. Paramagnetic materials have small positivemagnetic susceptibilities a result of the applied magnetic field and the behaviorof the material’s electron. Paramagnetism describes the production of netmagnetization when a magnetic field is applied on randomly oriented dipoles.
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Paramagnetic materials cannot be used for data storage because there is no netmagnetization after the removal of the magnetic field (Wu et. al., 2009).
Some magnetic materials behave as a storage center becausethey store remnant magnetization. Ferromagnetic and antiferromagnetic materialshave intrinsic magnetic properties. Randomly oriented domains in ferromagneticmaterials have net magnetic moment from unpaired electron spins that are alignedwithin domain. They exhibit strong magnetizationin the presence of external magnetic field, and can retain a portion of the magnetizationafter the removal of the magnetic field. When the electron dipolesspontaneously aligned opposite to each other in antiferromagnetic materials,there is zero net magnetization. The influence of the applied magnetic field onantiferromagnetic will result in a weaker magnetization than in a ferromagneticmaterial. The remnants of magnetic field (Br)left within material after the removal of field can be illustrated byhysteresis loop as seen in figure 1.
The area within the loop represents themagnetic energy lost after the field is removed. Soft magnetic materials have asmall area inside the loop. Soft magnets can be used for application inmagnetic screening, and power conversion. Hard magnetic materials have a largerarea under the hysteresis loop indicates a high remnant flux density which isadvantageous in data storage applications. Hc is the required energyneeded 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 largeremnants 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 propertiessuch as the direction of magnetization, temperature, and grain size canoptimize the storage capacity of magnetic materials. The boundaries of thedomain define the grain boundaries and sizes. The pioneeringmagnetic material for data storage by IBM had a diameter of 24in with with only100Gb/in2 areal density in …(…). Areal density is measure of the volume ofthe bit per grain volume.
Areal density can be optimized by reducing the volumeof grain because the density is inversely proportional to gain volume. Grainvolume can be reduced by reducing the geometrical dimension of the grains. Low grainshave low volume but higher areal density. Consequently, the grain sizes ofsubsequent magnetic materials have grown smaller than the former. The reduction in grains also improvedmagnitude of magnetization that can be stored. There is limit to which thegrain sizes can be reduced for optimization of magnetic data storage materialswithout compromising some properties.
Magnetic anisotropy describes the energy necessarydifferent magnetization direction within the domain in a material. When thegrain size is too small, the orientation can be easily reversed by changes intemperature. This property called superparamagnetism.
Superparamagnetic effectcan be prevented if the product of magnetic anisotropy and grain volume isestimated to be 40-60 times larger than the product of Boltzmann’s constant andtemperature (Bandi?, and Victoria, 2008). Temperature is another factor thataffects magnetization. Curie temperature is the temperature where the saturationdrops to zero. Reducing temperature decrease the magnetic susceptibility. ? =M/H=C/TM=magnetizationH=applied magnetic fieldC=curie material constantT=temperature in Kelvin Data can be stored in magnetic materials in a form ofbits and a bit is the smallest unit of data. The polarity of the magnetic fieldof each bit can be changed by applied current. The inductive head consists of acoil which permits current flow and controls production magnetic field (Piramanayagam, 2012).
Current can change the direction of the applied current and thus change thedirection of magnetization of each bit. Traditionally bits were orientedparallel to medium. A sensor reads the magnetic field emitted from each poleand converts magnetic information into binary information which can beinterpreted by computing devices. As a magnetic field sensor moves across eachregion, a voltage produced is represented as 1 where like-poles meet or 0 whenopposing poles are adjacent (Piramanayagam,2012). The component that reads or writes the bitsthe headThe conventional longitudinal recording technologyfaced some limitations as the demand for highly dense data storage increased.
Perpendicularrecording 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 increasedthe storage capacity. It also reduced the distance between bits, but the interferenceof neighboring magnetic fields made it difficult for the traditional inductivehead to read each bit.
In 1990s, magnetoresistive (MR) and giantmagnetoresistive (GMR) head emerged to replace the inductive head. MR and GMRconsist of a reading and writing component. Magnetoresistive heads sense andread the high electron flow resistance produced by the collision of electronspins when external fields are applied (Wuet. al., 2009; Piramanayagam, 2012).Ideal storage materials have low noise to signalratio, 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 increaseareal density (Wu et. al.
, 2009). Furtherreduction in size is limited because it decreases performance by reducing theamplitude of signal to noise ratio, head to disk gap, thermal stability,changing electronic and magnetic properties (Wuet. al., 2009).
Alternatives reading/writing heads and new magnetic materials makeit possible to improve the signal to noise ratio, the thermal stability,readability and writability as the grain sizes are reduced to nanometers. Thedevelopment of heat-assisted magnetic head optimized SNR, thermal stability andwritability using anisotropic material with smaller grains by bringing it below5nm away from the recording medium (Piramanayagam, 2012). The thermal heat isused to reduce magnetic anisotropy (Bandi?, and Victoria,2008). A protective lubricant over the magnetic medium prevent data lossduring 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 byincreasing the bit boundary with nonmagnetic materials or voids by procedureslike lithography.
Patterned mediaemerged in the race to find a medium with high areal density and minimize theeffect of superparamagnetism for a higher data storage. It removes magnetic transition noise andincreases the volume and anisotropy of each bit. Bit patterned media is coupledwith heat-assisted recording can support areal density about 5Tb/in2but quite a low areal density in the absence of heat assisted recording(Shiroishi et. al., 2009). There arenumerous techniques used for the fabrication of patterned media.
The firstmethod is E-beam Lithography (EBL) illustrated in figure 2. EBL fabrication process consist of sputteringelectrons on 30nm thick PMMA is layered (positive resist) or Hydrogen Silsesquioxane(HSQ) negative resist layer over Si substrate, removing the positive/negativemask layer and then etching the patterned Si substrate (Wu et. al., 2009; Choi et.al., 2012). EBL can be used tomake a medium with circular or square-like bits as small as 10-20nm (Choiet.
al., 2012). EBL pattern with 1/N period can increase the density by N2(Shiroishi et. al.
, 2009). In nanoimprinting lithography (NIL), is ananopatterned mould is inserted into a substrate layered with a positive resistby mechanical pressure or UV exposure followed by the removal of he positiveresist (Choi et.al., 2012). A patterned medium by NIL has 25nm islands 70nmapart while alternative process, interference lithography, only produced 30nmarray 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 (Shiroishiet. al.
, 2009).Figure 2.Patterned Media Fabricated by EBL process (Choi et.al., 2012) Templated growth isanother process of making patterned media. It uses a film with closely packedpores to be used a mask for electron beam deposition of magnetic material. Thesmall pores can produce smaller magnetic islands for data storage.
The film can be fabricated from anodizingaluminum film and the pore size can be controlled by voltage, current, densityor pH. Another film is from di-block copolymer that can self segregate andcreate a pattern (Choi et.al., 2012).
A third process, Ion implantation, can beillustrated in figure 3.Figure 4.Patterned media by Ion implantation process (Choi et.
al., 2012) There are some studies exploring the ways to optimizestorage capacity of magnetic materials. Since there is a limit to grainrefinement of magnetic materials, some are some studies are exploring ways topack more information while keeping the grain size at a nanoscale.
The new typeof materials is termed super lattice-like (SLL) structures. Nanoscale change inelectrical pulse induces a phase change and materials that have this propertyare said to have phase change memory. An example is a super lattice-likestructure produced by spraying an alternating layer of 6nm thick crystallinematerial like Sb4Te and 4nm thick thermal stable amorphous GaSb filmtotaling 120nm total thickness. A minimum electric pulse 10ns can be used tomove between the three semi-stable resistance (Lu et. al.
, 2016). Lu et. al.(2016) discovered that SLL structures have the potential to have multilevelstorage capacity in the difference phases.FePt nanoparticle is a another SLLstructure and multiphase material could be used to get areal densities of10Tbit/in2 (Wang, 2008). FePt is created by gas-phase condensation techniquein which a metal evaporates from sputter metallic source to condense into achemically ordered particles after it deposits on a cold substrate. However,the main problem with are long-range order patterning, surface roughness of themedium 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 overlappingtracks because inarrowers the width of erase band (Shiroishi, et. al., 2009).
However the drawback of SWR is prevuoisly written data have to erased in thereverse order sequentialy to update a single data the rewritten (Shiroishi, et.al., 2009). With two dimensional magnetic recording (TDMR), SWR achiveds thehighest areal density possible which is arounf 100 Tb/in2.
TDMRemploys a multiple 2-D array head to scan while substrating the interference withthe 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)
