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Next Generation Memory Devices. Sakhrat Khizroev. Center for Nanoscale Magnetic Devices. Florida International University Miami, Florida, U.S.A. Outline. Background Perpendicular Magnetic Recording Three-dimensional Magnetic Recording Protein-based memory Summary. Background.

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Next GenerationMemoryDevices

Sakhrat Khizroev

Center for Nanoscale Magnetic Devices

Florida International University Miami, Florida, U.S.A.


  • Background

  • Perpendicular Magnetic Recording

  • Three-dimensional Magnetic Recording

  • Protein-based memory

  • Summary


Traditionally, Scaling Laws were followed to advance data storage technologies


At 1 Gbit/in2 information density, bit sizes are: 400 x 1600 nm2

At 100 Gbit/in2 : 40 x 160 nm2

At 1 Tbit/in2 : 13 x 52 nm2

Scaling: Smaller Transducers and Media

Human Hair 75,000nm

Smoke Particle



Flying Height 5 nm

Media 10-100nm

Disk Substrate

At 1 Tbit/in2 information density, Bit Sizes are: 13 x 52 nm2

Superparamagnetic Limit



Bit transition

SNR ~ log(N), N - number of grains per bit

While scaling, need to preserve number of grains per bit to preserve SNR

Grain size is reduced for higher areal densities:

Media Stability

Probability of magnetization reversal due to thermal fluctuations:


Thermally stable media:

Relaxation time =  = 72 sec for KuV/kT=40

 = 3.6x109 years for KuV/kT = 60


If a<aminimum, medium becomes thermally unstable leading to severe deterioration of recorded data over time.

Approaches to avoid superparamagnetic instabilities:

  • Decrease aminimum by increasing KU

  • Increase a by decreasing the number of grains per bit

  • Demagnetization fields in transitions shorten the relaxation time


Patterned Media

Perpendicular Recording*

*S. Khizroevand D. Litvinov, Perpendicular Magnetic Recording, Kluwer Academic Publishers, 2004; ISBN 1-4020-2662-5.

Perpendicular Recording*: Well-defined Anisotropy



In a typical longitudinal recording layer the magnetic anisotropy axes of individual grains are randomly oriented in the plane of the film

2D random medium

In perpendicular recording layer the anisotropy axis is relatively well aligned (<2-4 degrees) perpendicular to the plane of the film

oriented medium

Substantially relaxes the requirements for write field gradients

Can use thicker recording layer - better thermal stability !!!

(increased V in KUV/kBT ratio)

*S. Khizroevand D. Litvinov, Perpendicular Magnetic Recording, Kluwer Academic Publishers, 2004; ISBN 1-4020-2662-5.

Nanoscale Device: Tbit/in2 Recording Transducer*



Bit Sizes: 13 x 52 nm2

*S. Khizroev, D. Litvinov, “Physics of perpendicular recording: writing process,” Appl. Phys. Reviews – Focused Review, JAP95 (9), 4521 (2004).

Gap Versus Fringing Field Writing

Higher areal density media requires higher write fields !!!

In perpendicular recording the write process effectively occurs in the gap (Write Field < 4pMS)

In longitudinal recording the write process is done with the fringing fields (Write Field < 2pMS)

*S. Khizroevand D. Litvinov, Perpendicular Magnetic Recording, Kluwer Academic Publishers, 2004; ISBN 1-4020-2662-5.

FIB to Trim Regular Transducers into Nanoscale Devices*

The most critical step is

to make a probe with Nanoscale dimensions

FIB Etch to Define a Nanoprobe

FIB Deposition

*S. Khizroev, D. Litvinov, FIB Review in Nanotechnology14, R7-15 (2004).

Numerical Calculations*

Modeled Fields (Quantum-mechanical)

Gallium Ion Implantation



*Jointly with Integrated Inc. ,a group at Durham University, UK, and groups at Carnegie Mellon University

FIB-fabricated Nanoscale Transducers*

Longitudinal Transducer

with a 30 nm Width

Perpendicular Transducer

with a 60 nm Width

Note: It takes ~ 10 minutes to make one such device in the University


*S. Khizroev, D. Litvinov, FIB Review in Nanotechnology14, R7-15 (2004).

Control of Gallium Diffusion*

Ion Dose



2 x 106 Ions/cm2



3 x 105 Ions/cm2

NOTE: Although NO texture change is observed through AFM,

substantial magnetic grain change is seen through MFM

*D. Litvinov, E. Svedberg, T. Ambrose, F. Chen, E. Schlesinger, J. Bain, and S. Khizroev, “Ion implantation of magnetic thin-films and nanostructures,” JMMM 277 (3-4), xxx (2004).

Nanoscale FIB Process*

The process how to make Nano-precision patterns with FIB was

shared with a few companies and successfully implemented by:

Carnegie Mellon University, IBM, Seagate, and others

A Part of a Device made in

the Industry before the

process was implemented

Same Device made with

the process implemented



500 nm

500 nm

*S. Khizroev, D. Litvinov, FIB Review in Nanotechnology14, R7-15 (2004).


Media Stack

Glass Substrate



Dynamic Kerr Measurement of the Field from a Nanoscale Transducer*

Kerr-Image Snap-Shots for a SPH Transducer (Near-field Kerr Microscopy)

*These experiments were repeated at Seagate, CMU, and IBM

*D. Litvinov, J. Wolfson, J. Bain, R. White, R. Chomko, R. Chantrell, and S. Khizroev, “Dynamics of perpendicular recording,” IEEE Trans. Magn. 37 (4), 1376-8 ( 2001).


Ion image of a FIB-fabricated and magnetically active 3-nm-long feature

MFM image of recorded nanoscale magnetic "dots"

Perpendicular Recording with Bit Widths of less than 65 nm*



MFM Images of Nanoscale Size Information

~190 ktpi

CoCrPtTa alloy

400 nm

CoB/Pd multilayer

~400 ktpi

130 nm

The FIB-made transducer

Current “state-of-the-art” longitudinal recording is <100ktpi

*S. Khizroev, D. A. Thompson, M. H. Kryder, and D. Litvinov, Appl. Phys. Lett. 81 (12), 2256 (2002); Editor's choice for the Virtual Journal of Nanoscale Science & Technology, Sep 23rd 2002.

Three-dimensional Magnetic Recording

  • Perpendicular Recording promises to defer the superparamagnetic limit to ~ 1 Terabit/in2

  • Heat-Assisted and Patterned Media are still 2-D limited and relatively slow

It is expected that Moore’s law will inevitably reach its limit between 2010 and 2020

 Time to stack multiple active layers on top of each other

3-D Magnetic Recording is a data storage form of 3-D integration

Conventional and 3-D Recording Media

Note: Each cell is 50 x 50 nm2

3-D Magnetic Recording

Lead Ph.D. Graduate Student: Yazan Hijazi, Sakhrat Khizroev

  • The development of 3-D magnetic recording is divided into two phases:

  • Multi-level Recording: not optimally utilized 3-D space

  • Note: Effective areal density increase is by a factor of Log2L (where L is the number of signal levels)

  • 3-D Recording: each magnetic layer is separately addressed

  • Note: Effective areal density increase is by a factor of N (where N is the number of recording layers)

Note: These are not active layers

Note: Each cell is 50 x 50 nm2

Recording Head

The current in the single pole head is varied to vary the recording field

Each recording is performed via two pulses: 1) a cell is saturated and 2) the information is recorded

Simulated Recording Field

Schematics of a Transducer

Playback Head

The playback head is designed to preferably read the vertical field component which is dominant in this case

Stray Field from 3D Medium

Differential Reader Configuration

Electronic Images of FIB-fabricated Transducer

Multi-level Recording on a Continuous Medium

Recording Step 2:

Recording Step 1:

  • Major Disadvantages:

  • Every time a track is recorded into the bottom layer, there are side regions in the top layer in which the earlier recorded information is lost because of the overlapping side region

  • The superparamagnetic limit

Recording Step 3:

Multi-level Recording on a Patterned Medium

FIB-etched Patterned Medium

Patterned Media by Toh-Ming Lu

Note 1: The tilt angle can be controlled via deposition condition

Note 2: The inter-layer separation should be sufficient to break the quantum-mechanical “exchange” coupling

Note: Each cell is ~50 x 50 nm2

Multi-level Recording on a Patterned Medium: Writing

Note 2: The inter-layer separation should be sufficient to break the quantum-mechanical “exchange” coupling

Micromagnetic Simulation Illustrating Two Cases of Interlayer Separation:

a) < 1 nm and b) > 2 nm


M up

M down

Recording Field Profile

Multi-level Recording on a Patterned Medium: Writing

H= -

H1>Hc >H2

Recording Field Profiles in Individual Layers at a Given Current Value



























H4>Hc >H5

H3>Hc >H4

H2>Hc >H3

Multi-level Recording on a Patterned Medium: Playback

Magnetic “Charge” Representation of the Playback Process




Simulated Stray Field from a 3-D Medium at different levels of recording

MFM Images of Two Types of Media

Each cell is ~ 60 x 60 nm2

SNR Limitations

  • Patterned Media (ideally, fabrication technique limited)

  • Electronic noise sources are 10 Ohm GMR Sensor and 0.2 nV/sqrt(Hz) preamp noise over a 500 MHz CTF bandwidth at 1 Gbit/sec

Note 1: Special encoding channels should be used to reduce BER

Note 2: The demagnetization field could be fairly large for some configuration. Special bit encoding should be considered to avoid the unfavorable bit configuration.

Hdemag >> 4Ms

Three-dimensional Recording

Schematic Diagrams of a 3-D Memory Device

Biasing Conductor for Layer Identification during Writing

2-D Recording/Reading Grid (similar to MRAM)

Soft Underlayer

Magnetically-induced Writing

K-th layer is identified

(K-1)-th layer is identified

Note: The current in the biasing conductor is continuously decreased from the maximum to zero to identify individual layers starting from the top to the bottom

Thermally-induced Writing

(jointly with Seagate Research)

Simulation by Roman Chomko

3-D Reading

Different Implementations

  • Active layers: MRAM devices stacked together

  • Magnetic Resonance FM

  • Magnetic Resonance FM

CoCrPtTa alloy

3-D Reading: Magnetic Resonance Force Microscopy

Comparative MFM Images of Atomic-size Information obtained by the Conventional State-of-the-art MFM (left) and the FIU-developed “Smart” Nano-probe


CoCrPtTa alloy

Electron Image of “Smart” Nano-probe(made via FIB)

3-D Reading: Magnetically-induced Reading*

Note 1: Through the variation of the “softness” of the SUL, one can vary the sensitivity field of each cell

Sensitivity Field with a “Free” SUL (red) and “Saturated” SUL (black)

Note 2: Effective physical scanning in the vertical direction is produced via the variation of the “softness” of the SUL. Thus, each layer could be independently addressed

According to the Reciprocity principle, the signal in each cell is given by Expression

*Provisional patent filed with US PTO on August 4th 2004

Parallel Set of Signals at Ibias = 0 (A turn)

Recorded Pattern in Layer 6

Parallel Set of Signals at Ibias = 1.56 (A turn)

Recorded Pattern in Layer 4

Parallel Set of Signals at Ibias = 5.85 (A turn)

Recorded Pattern in Layer 2

Summary on 3-D Magnetic Recording

  • The study of 3-D magnetic recording has been initiated

  • During the last year, the PIs have authored 8 peer-review papers on the underlying physics of magnetic and magneto-thermal recording

  • Specific designs of 3-D magnetic devices have been proposed

  • The university is in the process of filing a patent on the proposed mechanism.


Within two years, demonstrate an experimental prototype of a stable (for at least 50 years at room temperature) 3-D magnetic memory with at least ten recording layers with an effective areal density of at least 1 Terabit/in2 and a data rate faster than 2 Gbit/sec

Protein-based Memory

  • Why Protein?

  • Naturally occurring residues of proteins (Bacteriorhodopsin (bR) mutants) in the form of

  • molecules with a diameter of less than 3 nm (more than 100 times smaller than polymeric

  • material used to DVDs) demonstrate unprecedented thermal stability at room temperature

  • (critical advantage over magnetic storage, correspond to areal densities of much beyond 10

  • Terabit/in2

  • Unprecedented recyclablity of protein medium: it can be rewritten more than 10 million times (more than 1000 times better than CD/DVD)

  • The light-sensitive properties of proteins integrated with the modern semiconductor laser

  • technology provide a relatively straightforward control of recording and retrieving information

  • from the protein media.

  • Much faster time response of protein media (as compared to magnetic media): the time

  • response in the protein media is in the picosecond region (as compared to the nanosecond

  • region in magnetic media)

  • Economical

  • Non-volatile

Wild-life Bacteriorhodopsine (bR) produced by Halobacteria

Salinos del Rio on Lanzarote Island

Schematics of a halobacterial cell and its functional devices

*R. R. Birge, Scientific American, 90-95, March 1995

Protein-based Memory

Goal is to demonstrate the feasibility of recording/storing/retrieving information on/from photochromic proteins at areal densities of above 1 Terabit/in2 and data rates of above 10 Gigahertz.

  • Problems with Protein Media:

  • Early proteins were unstable (Solved with discovery of bacteriorhodopsin)

  • Polymers, on which protein structures are made, are less stable than proteins themselves

  • It is not trivial to immobilize proteins in 3-D

  • Holographic methods are not perfected for ultra-high densities (far from competing with magnetic)

  • Approach (2-D Single Molecule Level instead of 3-D) is

  • to take advantage of the 2-D stability of BR media to record on one surface at a single-molecule level or/and use a stack of layers to record in 3-D and

  • take advantage of the most advanced nanoscale recording system – so called heat-assisted magnetic recording (HAMR) based on the near-field optical recording transducer

Data Recording/Retrieval in Protein-Based Storage

Thermal Cycle with Two Stable States

Intermediate 2

Intermediate 1





State A

State B

State A

State B

Recording Mechanism: Two photon processes

Fig. Writing digital 1. Transition A  B.

Two photon absorption causes transition to intermediate state, which then relax to the second stable state B.

Cascade two photon absorption.

Note: Using two photon and other nonlinear processes makes possible remote writing digital information inside optical media volume. It is applicable for nonvolatile multi-layered optical memory.

Earlier Proposed Protein Memory*

Parallel Data Access (page by page via positioning of the green light)


  • Optics never could record high densities

  • 3-D media are not trivial to immobilize

*R. R. Birge, Scientific American, 90-95, March 1995

The Proposed Solution to Demonstrate the Feasibility of Protein Based Storage

  • All the above-described methods of recording/retrieving data are quite complicated and it is hard to see whether they will be implemented and if yes, when. In fact, so far no physical demonstration of ultra-high density recording has been made!

  • The PIs propose

  • first, to use a bit-by-bit 2-D type of recording to demonstrate the feasibility

  • of the protein-based storage (it is trivial to immobilize 2-D media);

  • then, to apply one of the available parallel data recording/retrieving

    mechanism (e.g. holographic).

To accomplish this goal, the PIs use the transducer design earlier developed for heat-assisted magnetic recording (HAMR)*. HAMR is the most advanced recording mechanism proposed so far. The PIs have pioneered one of the most efficient design of the transducer for HAMR

*T. McDaniel, W. Challener, “Issues in heat-assisted perpendicular recording,” IEEE Trans. Magn.39 (4), 1972-9 (2003).

Novel Recording Transducer for Areal Densities Above 1 Terabit/in2

Note: Focused ion beam (FIB) is used to fabricate “apertureless” transducers (with aperture dimensions of less than 100 nm << than the wavelength)*

Electron Image of FIB-fabricated Apertureless Transducer

Air-bearing-surface (ABS) view of laser diode with a thin layer of Al with FIB-etched "C" shape aperture

< 90 nm

In-house made

*F. Chen, A. Itagi, J. A. Bain, D. D. Stancil, and T. E. Schlesinger, “Imaging of optical field confinement in ridge waveguides fabricated on very-small-aperture laser,” Appl. Phys. Lett. 83 (16), 3245 (2003).

Two-Dimensional Protein Media

  • Easy to fabricate*

  • Naturally stable

Optical spectra of a gelatin-mixed BR film in two states, the ground state and one of the intermediate M states*

AFM Image of a 2-D Pattern with a 2.4-nm Period

The decay absorption signal in the excited M-state measured at a wavelength of 410 nm

Note: Patented approach to immobilize proteins Into stable thin-film recording media (H. Arjomandi, V. Renugopalakrishnan)

*A gelatin-mixed bR film under study was fabricated by Lars Lindvold

*The spectra were recorded with a Varian CARY 50 spectrophotometer.

Experimental setup to record and read information on/from proteins

Custom-made Near-field System built around Aurora-3 by DI

Schematic Diagram

Note: The modular structure of the system allows simultaneously using more than one (currently, up to four) sources (red to blue lasers, UV lamps) to conduct photons through a fiber to the sample in the near-field regime. In addition, as described below, the system will allow implementing diode lasers assembled right at the air bearing surfaces (ABS) of the recording probes attached to the SPM’s cantilever.

Early Results: Reading Tracks from Photochromic BR Media

Near-field Optical Readback Signal

Narrowest track is ~ 100 nm

* The signal is the absorbed power in the detector system in the reflection mode


100 Gbit/in2

0. End to Longitudinal Recording

  • 1.Perpendicular Recording

  • 2. Use smaller Grains&Deal with Write Field Problem (~10x gain)

    • Heat Assisted Magnetic Recording (HAMR)

      • E.g. high anisotropy 3 nm FePt grains

  • 3. Single Grain per Bit Recording combined with HAMR (~5x gain)

    • Patterned Media

      4. 3-D Magnetic Recording

      5. Protein-based Memory (Single-Molecule Recording)

1 Tbit/in2

10 Tbit/in2

50 Tbit/in2

Ultimate Recording Density > 50 Tbit/in2 conceivable

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