Evaluation of Bulk Degaussers for Secure Erasure of Magnetic Media

Presented at the THIC Meeting at the National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder CO 80305-5602

August 21-22, 2007

Fred Spada

University of California, San DiegoCenter for Magnetic Recording Research

9500 Gilman Drive, La Jolla CA 92093-0401Phone: +1-858-534-8675 FAX: +1-858-534-2720

E-mail: fspada@ucsd.edu

228/21/078/21/07

IntroductionBulk degaussing methods expose entire magnetic storage devices or removable magnetic media cartridges to sufficiently strong AC, DC, or pulsed magnetic fields.

Bulk degaussing methods are preferred and sometimes essential for erasure of magnetic media containing unencrypted data, when large quantities of magnetic storage devices must erased, or when erasure must be performed quickly.

Media containing extremely sensitive data must be secure erased, which is defined as erasure so that the magnetic patterns cannot be recovered or reconstructed by any known means.

Bulk erasure methods satisfy this criterion when the recorded magnetic patterns have been erased to the “media noise” level (i.e., there are no bit patterns remaining for recovery)

338/21/078/21/07

Longitudinal Rigid Disk Coercivity Values Between 1990 and 2005

1 From 1990-2000 Rigid Disk Drive Magnetic Head/Media Market and Technology Reports

Many degaussers currently being marketed do not produce fields with sufficient magnitude to achieve secure erasure of modern high coercivity magnetic media.

1995 2000 20051000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

Peripheral Research Projections CMRR Measurements

Dis

k C

oerc

ivity

(Oe)

Year

448/21/078/21/07

The suitability of a bulk degausser for secure erasure of magnetic media depends upon:

- Magnetic properties of the media being erased

- Magnitude, orientation, and distribution of magneticfields within the degaussing chamber

- Physical design characteristics of the degausser that may promote or permit inadvertent improper use (e.g.,placement of media in weak field locations within thedegaussing chamber).

558/21/078/21/07

-10000 -8000 -6000 -4000 -2000 0-1.0x10-4

-8.0x10-5

-6.0x10-5

-4.0x10-5

-2.0x10-5

0.0

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

-2.0x10-8-1.0x10-8

0.01.0x10-82.0x10-83.0x10-84.0x10-85.0x10-86.0x10-87.0x10-88.0x10-89.0x10-81.0x10-71.1x10-71.2x10-7

HcrFWHM= 5330 Oe

Hcrp= 4740 Oe

M (e

mu)

H (Oe)

Hcr= 4670 Oe

dM

/dH

(em

u/O

e)

A. Measure Properties of Magnetic Media

Remanent magnetization behavior and switching field distributiondetermined via:

-VSM, AGM, SQUID (tape and hard disk media; limited tosmall specimen dimensions)

-Polar Kerr Magnetometer (perpendicular disk media; both small specimens and entire disks can be measured)

Experimental Approach

668/21/078/21/071/1

/2000

7/1/20

001/1

/2001

7/1/20

011/1

/2002

7/1/20

021/1

/2003

7/1/20

031/1

/2004

2400

2800

3200

3600

4000

4400

4800

Hc (O

e)

Date on Drive Label

A B C D E F G H I

Glass Plate

Disk DriveManufacturer

CMMR measurements of disk coercivity reveal considerable variation among disk drive manufacturers and product lines, even within a narrow manufacturing period

778/21/078/21/07

B. Measurement of Degausser Field Magnitudes and Distributions

AC and DC magnetic fields are simultaneously measured along three orthogonal directions using a 3-axis Hall probe and 3-channel gaussmeter.

A computer controlled x-y-x robotic stage permits field distributions to be obtained for a wide range of degausser cavity volumes.

These measurements often require partial disassembly of the degausser, and knowledge of electronic circuits that may provide needed trigger signals and special control of degausser operation.

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1. Rigid Disk Media

Use of spin stands for measuring playback signal strength after degaussing is impractical due to narrow recorded track widths in modern disk drives (≤0.15 µm). After removal from the spindle, mechanical tolerances make realignment of the recorded track axis with the spindle axis almost impossible.

Degaussing the entire hard drive magnetically alters/damages the drive motor, recording head, and head control mechanism, yet weak recorded signal patterns may remain on the media for attempted recovery.

Tracking is not possible even with functioning drive components because critical header/servo patterns on the disk have been degraded or entirely erased by the degausser fields.

Hard disk media erasure can be monitored ex-situ using a Scanning Magnetoresistance Microscope and a Magnetic Force Microscope.

C. Verification of Erasure

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a. Scanning Magnetoresistance Microscope (SMRM)2

- uses the magnetoresistive read element (AMR, GMR, or TMR) in a rigid disk slider assembly.

-media specimen is mounted on a computer-controlled nanopositioning stage

-the response of the reader is monitored as the media specimen isstepped in the x and y horizontal directions with the slider in contact with the media surface.

2Yamamoto & Schultz, Applied Physics Letters, 69(21), 3263 (1996)

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Disk segment

Nanopositioning stage surface

Mechanical head alignment stage (x-y-θ)

Entire disks or media segments can be mounted on the SMRM stage

11118/21/078/21/07

Mosaic of nine SMRM images of a sector header region on ~2700 Oe longitudinal media ( ~1.5 µm wide AMR sensor)

~400 µm

80 µm

User Data ECC Servo Tracks Track ID Data Preamble Address Mark User Data

Dimensions of a single scan are limited to 100 µm x 100 µm area, but the stationary slider can be mechanically repositioned for scanning adjacent locations on the disk surface.

12128/21/078/21/07

b. Magnetic Force Microscope (MFM)3

measures response of sharp magnetic tip to force gradient produced by stray fieldsemanating from the media surface (figure from ref. 3)

tip is raster scanned over themedia surface (figure from ref. 3)

3Rugar et al,, Journal of Applied Physics, 68(3), 1169 (1990)

13138/21/078/21/07

MFM image obtained close to outside diameter of a ~5000 Oe perpendicular media platter from 160 GB laptop drive

(2006 manufacture date)

User data(random pattern)

30 µm x 30 µm area

14148/21/078/21/07

SMRM image of a 20 MHz pattern on ~2700 Oe longitudinal disk media obtained with a 1.5 µm AMR sensor (after exposure to 2080 Oe peak field in a pulsed field

degausser)

0 5 10 15 20 25 30 350.4072

0.4074

0.4076

0.4078

0.4080

0.4082

Play

back

Vol

tage

(V)

Downtrack (µm)

Down-track line scanof MR output voltage

0.0 0.2 0.4 0.60.0000

0.0001

0.0002

0.0003

0.0004

Spatial FrequencyFF

T A

mpl

itude

FFT of down-trackline scan

16 µm

36µm

c. Rigid Disk Test Media

Specimens having a known, saturated, regular magnetic pattern are required for monitoring erasure to the medium noise floor.

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0.14 µm GMR Sensor(20 MHz pattern on ~2700 Oe disk media)

16 µm

36 µ

m

16 µm

36 µm

0.8 µm/stepcross-track

0.4 µm/stepcross-track

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0000

0.0002

0.0004

0.0006

0.0008

Spatial Frequency

FFT

Am

plitu

de

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0000

0.0002

0.0004

0.0006

0.0008

Spatial Frequency

FFT

Am

plitu

de

• bit resolution and track edge definition are improved with the smaller sensor

• finer cross-track step sizes yield further improvements

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line scan FFT of line scanOriginal Pattern

Partial Erase

0 2000 4000 6000 8000 100000.1170

0.1172

0.1174

0.1176

0.1178

0.1180

0.1182

0.1184

0.1186

Play

back

Am

plitu

de (m

V)

Downtrack (nm)0.0 0.1 0.2 0.3 0.4 0.5

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

Spatial Frequency

FFT

Ampl

itude

0 2000 4000 6000 8000 10000

0.1305

0.1306

0.1307

0.1308

0.1309

0.1310

0.1311

0.1312

0.1313

0.1314

Play

back

Am

plitu

de (m

V)

Downtrack (nm)

0.0 0.1 0.2 0.3 0.4 0.50.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

FFT

Ampl

itude

Spatial Frequency

Down-track alignment of the MR sensor can be problematic with the SMRM

17178/21/078/21/07

Definition of the “down-track” direction becomes somewhat arbitrary after exposing media with a constant frequency pattern in isolated tracks to large degaussing fields… the selected line scan may actually cross several tracks.

line scan FFT of line scan

0 2000 4000 6000 8000 10000

0.1182

0.1183

0.1184

0.1185

0.1186

0.1187

Play

back

Am

plitd

ue (m

V)

Downtrack (nm)0.0 0.1 0.2 0.3 0.4 0.5

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

FFT

Ampl

itude

This problem is not encountered with preamble-type magnetic patterns…

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Down-track alignment is less critical with specimens having a preamble-type pattern over the entire disk surface

Isolated tracks (72.5 MHz @ 7200 RPM)

radius = 25 mm radius = 42 mm

“Stitched” adjacent tracks (65 MHz @ 7200 RPM)

24 µm

1.25 µm0.03 µm down-track steps0.05 µm cross-track steps

radius = 25 mm radius = 42 mm

19198/21/078/21/07

MFM phase images of a preamble-like magnetic patternrecorded on ~5000 Oe perpendicular disk media

10mmfrom

disk i.d.

15mmfrom

disk i.d.

20 mmfrom

disk i.d.

25 mmfrom

disk i.d.

20208/21/078/21/07

d. Rigid Disk Erasure Experiment

Production media with highest available coercivity (Hc~5000- 5300 Oe) and having pre-recorded saturated patterns of known frequency (65-90 MHz) are placed in a drive body at the normal platter positions.

Reassembled drives with all original components are placed in the degausser chamber and subjected to either incomplete or complete degauss cycles.

FFT amplitude of the MR playback response, and 2 dimensional power spectral density plots of the MFM images are compared with thoseproduced by the original saturated pattern.

21218/21/078/21/07

SMRM and MFM images are obtained at severaldifferent radii on each test specimen

Inside Radius

Outside RadiusDisk Fragment

24 µm

1.25µm

22228/21/078/21/07

2. Tape Media

Tape capstan

Recording and playback head

a. MicroPhysics THT tape tester can be used for recording and playback of test patterns on a variety of tapes (various formats, widths). This instrument requires a loop of tape having a specific, fixed length.

Computer-controlled

micro-drive for stepping

head position

23238/21/078/21/07

0 2 4 6 8 10

-60

-50

-40

-30

-20

-10

0For PM2000

After Degauss Before Degauss Noise Level

Play

Bac

k Si

gnal

(dB

m)

Frequency (MHz)

THT tester results for a Gen1 LTO tape (Hc ~1900 Oe) before and after erasure in Permanent Magnet Degausser A (prototype - never introduced as product)

24248/21/078/21/07

b. Modified LTO tape drives offer the following advantages for tape erasure studies:

- erasure levels can be measured over any segment within the tape volume contained in the cassette

- tape cassettes can be studied in their original form, as handled in real operating environments

- OEM heads/electronics are used for recording/playback

Disadvantage – OEM drives require the necessary proprietary microcode for drive control

outside

middle

inside

25258/21/078/21/07

LTO Gen 1 drive with disabled servo control and tape edge guiding was previously used for erasure studies of LTO Ultrium I (~1900 Oe ) and Ultrium II (~ 2200 Oe) tape media.

Edge guiding is no longer possible with the smaller recorded track widths and “shingle writing” used in Gen 3 and Gen 4 drives (media Hc ~2600-2800 Oe).

Alternative approach: with disabled servo control, monitor erasure of the tape media’s factory-recorded servo patterns using the servo readers in the OEM recording head.Ferrofluid images of one servo band and a portion of one data band in LTO I (left) and LTO III (right) tapes4

4 Images courtesy of IBM Corp.

Servo pattern

Data band

Human hair

26268/21/078/21/07

1

2

The servo reader in LTO drive’s recording head can be locked in the center of a chosen servo band by adjusting the timing between the 4-4 and 4-5 servo bursts before disabling the servo control. This permits erasure to be monitored across tape width.

Servo bands

Head Positions

27278/21/078/21/07

MFM can also be used to verify erasure of servo bands but measurements are practical only for limited portions of tape

MFM images of 80 µm x 80 µm areas of LTO servo pattern in virgin tape media

28288/21/078/21/07

III. Verification of Degausser Efficacy(Rigid Disk Media)

A rotating, decaying field within a plane parallel to the degausser chamber floor results from vector addition of out-of-phase magnetic fields produced by precisely timed high

current discharges through two orthogonal coil assemblies.

0 20 40

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

Mag

netic

Fie

ld (O

e)

Time (ms)

X-Field (parallel to cavity height) Y-Field (parallel to cavity width) Z-Field (parallel to cavity depth)

Degausser B

Note axis label convention

1. Pulsed Field Degaussers

29298/21/078/21/07

Stress tests show that the peak field magnitudes and the decay profiles of both the Y- and Z- magnetic field components in Degausser B are essentially

unchanged after ~23 hours of continuous, repetitive operation

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X-Field(height direction)

Y–Field(width direction)

Z–Field(depth direction)

Peak field distributions on a horizontal plane 0.75” above the chamber floor of “Degausser B” (approximate platter position in degausser cavity)

Rear of Chamber

Door Figure aspect ratios intentionally distorted

Gauss

31318/21/078/21/07

Erasure of ~4200 Oe longitudinal media in Degausser B

0 500 1000 1500 2000

0.0000

0.0001

0.0002

0.0003

0.0004

FF

T A

mpl

itude

Degauss Setting

Inside Media Noise Floor

0 500 1000 1500 2000

0.0000

0.0001

0.0002

0.0003

0.0004

FFT

Am

plitu

de

Degauss Setting

Outside Media Noise Floor

0 500 1000 1500 2000

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005 Inside Media Noise Floor

FFT

Am

plitu

de

Degauss Setting0 500 1000 1500 2000

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005 Outside Media Noise Floor

FFT

Am

plitu

de

Degauss Setting

Erasure of ~5000 Oe longitudinal media in Degausser B

Note:“Degauss

Setting” scales linearly with

field, but is notfield

magnitude

32328/21/078/21/07

0

30

60

90

120

150

180

210

240

270

300

330

02000400060008000

10000120001400016000

02000400060008000

10000120001400016000

Initial Post Stress

Mag

netic

Fie

ld (G

auss

)Polar Plot

For certain critical applications, complete erasure must be achieved well below the peak degausser output (e.g., the pre-recorded pattern on ~5000 Oe media was still recognizable at 75% peak output of “Degausser B”).

Higher peak fields produced by “Degausser C” provide a greater safety margin for ensuring erasure of sensitive data on ~5000 Oe longitudinal media:

33338/21/078/21/07

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.58

7

6

5

4

3

2

1

0

Width (inches)

Dep

th (i

nche

s)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.58

7

6

5

4

3

2

1

0

Width (inches)

Dep

th (i

nche

s)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.58

7

6

5

4

3

2

1

0

Width (inches)

Dep

th (i

nche

s)

X-Field(vertical direction)

Y – Field(width direction)

Z – Field(depth direction)

Rear of Chamber

Door

Peak field distributions on a horizontal plane 0.64” above the chamber floor of Degausser C

Figure aspect ratios intentionally distorted

Gauss

Hard DriveRegion

Hard DriveRegion

Hard DriveRegion

34348/21/078/21/07

Erasure of ~4200 Oe longitudinal media in Degausser C

0 500 1000 1500 2000

0.00000

0.00002

0.00004

0.00006

0.00008

0.00010

0.00012

0.00014 Inside Media Noise Floor

FFT

Am

plitu

de

Degauss Setting0 500 1000 1500 2000

0.00000

0.00002

0.00004

0.00006

0.00008

0.00010

0.00012

0.00014 Outside Media Noise Floor

FFT

Am

plitu

de

Degauss Setting

Erasure of ~5000 Oe longitudinal media in Degausser C

0 500 1000 1500 2000

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

Inside Media Noise Floor

FFT

Am

plitu

de

Degauss Setting0 500 1000 1500 2000

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025 Outside Media Nosie Floor

FFT

Am

plitu

de

Degauss Setting

Note:“Degauss

Setting” scales linearly with

field, but is notfield

magnitude

35358/21/078/21/070 500 1000 1500 2000

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025 Outside Media Nosie Floor

FFT

Am

plitu

de

Degauss Setting

The SMRM and MFM produce similar results

36368/21/078/21/07

Pulsed Field Degausser D

This degausser employs only one solenoid coil and produces a single field pulse parallel to depth direction (Z-Field). A small permanent magnet assembly

is located near the front of the degaussing chamber.

37378/21/078/21/07

Degausser D does not securely erase 5000 Oe longitudinal Media

Before Degauss Cycle After Degauss Cycle

(note different color scale ranges)

38388/21/078/21/07

0 1 2 3 4 5 6 7 8 9 10 110.0

0.5

1.0

0 1 2 3 4 5 6 7 8 9 10 11

3500

4000

4500

5000

5500

6000

6500

7000

7500

Z-Field Magnitude at 0.67" above Bottom Tray

Z Fi

eld

Mag

nitu

de (G

auss

)

Width (inches)

Hard Drive Region

Other MediaRegion

Gau

ss

Width (inches)Hei

ght (

inch

es)

0450900135018002250270031503600405045004950540058506300675072007650810085509000

0 1 2 3 4 5 6 7 8 90.00.51.0

0 1 2 3 4 5 6 7 8 96500

7000

7500

8000

8500

9000

9500

10000Z-Field Magnitude at .772" above Bottom Magnet

Z Fi

eld

Mag

nitu

de (G

auss

)

Width (inches)

Hard Drive Region

Magnet Frame

Magnet

Gau

ss

Width (inches)Hei

ght (

inch

es)

0650130019502600325039004550520058506500715078008450910097501040011050117001235013000

0 1 2 3 4 5 6 7 80.00.51.01.52.0

0 1 2 3 4 5 6 7 8

10600

10800

11000

11200

11400

11600

11800

12000

12200

Z-Field Magnitude at 0.656" above Bottom Tray

Z Fi

eld

Mag

nitu

de (G

auss

)

Width (inches)

Hard Drive Region

Gau

ssWidth (inches)H

eigh

t (in

ches

)

101001022310345104681059010713108351095811080112031132511448115701169311815119381206012183123051242812550

0 1 2 3 4 5 6 7 8 9 10 11 120.00.51.0

0 2 4 6 8 10 12

4000

5000

6000

7000

8000

9000

10000

11000

12000

Width (inches)

Z Fi

eld

Mag

nitu

de (G

auss

)

Z-Field Magnitude at .62" above Bottom Magnet

Gau

ss

Width (inches)

Hei

ght (

inch

es)

060012001800240030003600420048005400600066007200780084009000960010200108001140012000

Left-to-Right Field Distribution Within Vertical Plane Through Maximum Field Locationin Various Prototype and Production Permanent Magnet Assemblies

Deg

auss

erE

Deg

auss

erF

Deg

auss

erG

Deg

auss

erH

2. Permanent Magnet Degaussers

39398/21/078/21/07

0 1 2 3 4 5 6 7 80.00.10.20.30.40.50.60.70.80.91.0

0 1 2 3 4 5 6 7 8 9

-8000

-6000

-4000

-2000

0

2000

4000

Z-Fi

eld

Mag

nitu

de (G

auss

)

Cavity Depth (Inches)

Z-Field Magnitude at 0.67" above Bottom Tray

Cavity Depth (Inches)

Cav

ity H

eigh

t (In

ches

)

Gau

ss

Front Panel

Front-to-Back Variation of Field Along the Center Width of Degausser E(field polarities are inverted in this figure)

40408/21/078/21/07

Many Degaussers Do Not Meet the Secure Erasure Criterionfor Modern Magnetic Media

MFM comparison of actual data tracks in operating drives (longitudinal media) before and after degaussing in an undisclosed degausser

Media From Reference Drive(not degaussed) Media From Degaussed Drive

41418/21/078/21/07

Ray Descoteaux, Senior Electronic Technician

Jung-Il Hong, Post-Doctoral Researcher/Project SpecialistYun Jun Tang, Project Scientist

Peter Lee, (B.S., Electrical Engineering, graduated)Brendan O’Shea, (B.S., Engineering Physics, graduated)Hamid Toolloei, (Computer Science & Engineering, M.S., graduated)Peter Shchupak, (Electrical Engineering, M.S., graduated)

Jordan Gosselin, undergraduate student (Physics)Lynn Greiner, undergraduate student (Electrical Engineering)Bryan Lettner, undergraduate student (Physics)

ACKNOWLEDGEMENTS