NITED STATES ARTMENT OF MMERCE

BLICATION NBS TECHNICAL NOTE 735

of Magnetic Fields

on Magnetic Storage Media

Used in Computers
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics—Optical Physics—Linac

Radiation^—Nuclear Radiation^—Applied Radiation-—Quantum Electronics'

Electromagnetics^—Time and Frequency'—Laboratory Astrophysics'—Cryogenics'.

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

Government; provides advisory and research services to other Government agencies;

and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Reactor

Radiation—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to pro- mote the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies

upon request. The Institute also monitors NBS engineering standards activities and

provides liaison between NBS and national and international engineering standards

bodies. The Institute consists of the following divisions and offices: Engineering Standards Services—Weights and Measures—Invention and

Innovation—Product Evaluation Technology—Building Research—Electronic Technology—Technical Analysis—Measurement Engineering—Office of Fire Programs.

THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition,

and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader

aspects of the National Measurement System; provides appropriate services to ensure

that the NBS staff has optimum accessibility to the scientific information of the world,

and directs the public information activities of the Bureau. The Office consists of the following organizational units: Office of Standard Reference Data—Office of Technical Information and

Publications—Library—Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

' Located at Boulder, Colorado 80302.
The Effects of Magnetic Fields on

Magnetic Storage Media Used in Computers

Sidney B. Geller

Information Processing Technology Division

Center for Computer Sciences and Technology

National Bureau of Standards

Washington, D.C. 20234

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U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, SBcre^ory

S NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, Acf/ng Director,

Issued July 1972
National Bureau of Standards Technical Note 735

Nat. Bur. Stand. (U.S.), Tech. Note 735, 35 pages (July 1972) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:735). Price 40 cents.

I
»

Contents

Page

1. Introduction 1

2. General Guidelines for Protection Against Damage to

Magnetic Recording Media .,.,.. 2

3. Effects of Magnetic Fields on Recorded Media 3

3.1 Answers to Specific Questions of Interest to

Computer Installation Managers 3

4. Description of Experiments. ,« «.. 12

4.1 Experimental Procedures 12

4.2 Small Bar Magnet Experiments 15

4.3 Large Magnet Experiments and Tests 16

5. Additional Test Results and Comments 25

6. References 29

List of Figures

1. Variations in Pulling Force for Magnets of Different

Weights 7

2. Effect of a Single Bar Magnet on a Recorded Computer

Tape 13

3. Effect of 2 Bar Magnets on a Recorded Computer Tape ... 14

4. Magnetic Field Produced by the Large Test Magnet 17

5. Effect of a Large Magnet in Contact with the Recorded

Tape Reel (Test 1 Chart) 18

6. Measured and Calculated Percentage Decreases in Recorded

Signal Levels 19

7. Effect of a Large Magnet 25.4 mm (1 in) from Recorded

Tape Reel (Test 2 Chart) 20

8. Effect of Mu-metal Shield (Test 3 Chart) 21

9. Effect of a Large Magnet in Contact with Plastic Tape

Case (Test 5 Chart) 22

10. Effect of a Large Magnet in Contact with the Recorded

Tape Reel (Test 6 Chart: 200 frpi) 24

11. Effects of Temperature and Humidity on Magnetic Tapes . . 27

iii
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This study was supported by the National Bureau of Standards

Magnetic Media Program*.

Through Research and Technical Service Project 65OIIII

XV
The Effects of Magnetic Fields on Magnetic

Storage Media Used in Computers

Sidney B. Geller

Experiments have been performed with different types

of magnets to determine their effects on the information

stored on magnetic storage media. The test results that

were obtained with recorded computer tapes are discussed

and guidelines are suggested to computer installation

managers for protecting their recorded tapes ^ disks and

drums against unwarranted erasure by magnetic fields.

The effects of other forms of energy on recorded magnetic

media are briefly considered.

Key words: erasure, magnetic media; erasure, permanent

magnet; erasure, shielding against; erasure, signal

loss due to; magnet fields, erasure with; magnetic media,

computer; magnetic media, information damage; permanent

magnets; permanent magnet, erasure with; shielding,

1. INTRODUCTION

Magnetic storage media have become indispensible to data processing operations in industry and in government. Magnetic tapes,

disks and drums are used to store voluminous quantities of data on both a temporary and an archival basis. Unfortunately, data in

magnetic form can be altered or destroyed by devices that are able

to exert magnetic forces on the surfaces of these media. Such

destructive action can be either intentional or unintentional. The

most effective and insidious of the destructive devices is the simple

permanent magnet which can produce powerful erasing fields and is

easily obtained, carried and concealed. In addition, it requires no external power sources for producing or maintaining its fields.

This first report deals primarily with the effects of permanent

magnets on the recorded information on magnetic computer tapes. The

results of this study are directly applicable to the other magnetic

surface devices such as disks and drums. The study also discusses

both the capabilities and limitations of the permanent magnet and the

electromagnet as destructive devices.

Section 2 of this report presents a number of guidelines to be

used in computer installations for protecting their magnetically

recorded data from intentional or accidental damage.

Section 3 uses a question-and-answer format to discuss topics

that are of interest to the managers of computer installations.

These deal mainly with the effects of magnetic fields on recorded

magnetic computer tapes.
Section 4 describes the experiments that were performed in order

to answer some of the questions that were posed in Section 3. A

Section 5 presents additional test results and comments concerning the effects of other possible sources of damage to magnetic media.

2„ GENERAL GUIDELINES FOR PROTECTION AGAINST DAMAGE

TO MAGNETIC RECORDING MEDIA

The results of both experience and of the tests that will be

discussed in Sections 3 and 4 provide support for the following

rules-of-thumb for protecting computer installations aginst damage

to their magnetic recording media and the stored magnetic data:

ao When not in use, magnetic tapes (reel or cassette) and disk

packs should be stored in storage cabinets with adequate shielding

qualitieso If the tape reels, tape cassettes and disk packs are stored one foot from the inner walls of the cabinet, no degradation

of recorded signals will be caused by magnets or electromagnetic

devices which are carried into the storage room or are capable of

being used outside of the storage area^ Several manufacturers*

produce both suitable cabinets and shielding material and will supply

quantitative data on their shielding qualities. Additional protection is afforded the magnetic media by the normal ferromagnetic metal

walls found in many computer installations. These reduce the effects

of magnets applied from outside the computer room by two-to-three

orders of magnitudeo Any metal wall to which a magnet will adhere

will provide some shielding against magnetic fields. At the present

time the drum and disk cabinets are normally made of plastic;

however, they could also be made from shielding materials that will

provide the same protection as the previously mentioned storage units.

b, A tape vault should be installed as far away as possible

from all fixed sources of stray magnetic fields such as large

transformers, motors and generators. When necessary, individually

shielded containers are available for tape reels that contain

important data. Shielded containers are particularly valuable for

transporting these reels. The tapes in repository should not be

subjected to temperature extremes, since these are capable of

producing both physical damage as well as signal erasure effects.

(See figure 11).

c. Since a magnet with a field intensity of 1 tesla (10,000

gauss) can be carried in a briefcase or a ladies handbag it is

recommended that measures should be undertaken to prevent the entry

of persons possessing such devices. Magnetic detectors (e.g.,

magnetometers) may be used at close range to apprehend these

individuals o Detectors which will warn of concealed magnets without

*For example, see the "eem" Catalog, United Technical Publications,

Vol. 3, p. 2018 and p. 2088 (1971-72).

2
p

"keepers" are quite inexpensive (less than $1000). Detectors which

will protect against magnets with keepers will probably cost from

$1000 to $5000 and are effective against magnets with 0.002 to 0.01

tesla (20-100 gauss) field intensities,

do It is difficult to retrieve recorded information from a magnetic medium whose timing or clock track signals have been intentionally damaged. This damage can be done with magnetic devices or by interfering with the recorded process as it actually occurs. Devices such as simple spark gap units plugged into wall outlets

or small concealed battery operated transmitters can cause interference with the magnetic recording process as well as with other

computer functions. Therefore, regular searches of the entire

facility should be conducted for such devices and only authorized

persons should be permitted to enter the computer installation.

Co Duplicate copies of all important tapes such as master

tapes and program tapes should be made and kept in a different

secure location.

3. EFFECTS OF MAGNETIC FIELDS ON RECORDED MEDIA

3.1 Answers to Specific Questions of Interest to

Computer Installation Managers

Several (i.e., eight) questions have been asked indicative of

those that confront computer installation managers faced with

guarding against intentional (or unintentional) attempts to use magnetic devices to destroy information recorded on magnetic media.

These questions along with their answers are presented in subsequent

paragraphs.

a. Question 1

Is there an easily understandable relationship between such

factors as magnet size or power, distance, and time? How do these

relate to the process of information destruction on magnetic media?

(1) We can interpret magnet "power" as the ability of a magnet

to exert a force on any object or material that is capable of

reacting to its magnetic field. The most sensitive of these

materials are "ferromagnetic" materials such as iron or the oxides

of iron that are found in the coating of magnetic tapes. These

materials can be made to move physically under the influence of the

magnetic field or in the case of a magnetic tape the microscopic

"domains" in the iron oxide can be affected by the field in such a way

as to produce the magnetized "patterns" in the coating that represent

the desired signals on the tape. Of course, it is possible that the

magnetic field is an undesirable one and, therefore, it alters or destroys the existing tape signal patterns.
(2) NOTE: This report will use the new International System

of Units known as "SI" units. In each case the SI unit will be

followed by the "customary" units in parentheses. The conversion

factors are given in the appendix,

(a) The strength or intensity of a magnetic field is described

in units of "amperes per meter" (oersteds),

(b) The density of the magnetic "flux lines" in the magnetic

field which is also indicative of the strength of the field is

measured in units of "teslas" (gauss). The gauss and the oersted

are usually considered to be equal to each other in value when the

field is measured in air.

(c) The following symbols for SI units will be used:

amperes per meter = A/m

kilograms = kg

meters = m

millimeters = mm

newt on

= N

teslas = T

(3) The relationship of the greatest interest for protecting

tape files is that between the magnet and its distance from the

object upon which it is exerting its force. Generally, magnetic

forces produced by magnets are appreciable only over very short

distances. The field strength of the magnet decreases very rapidly

with distance from the magnet. For example, one permanent "U"

shaped magnet showed a measured field strength of approximately

56,000 A/m (700 oersteds) on a line between its pole faces. At

51 mm (2 inches) from the magnet the field strength dropped to

6800 A/m (85 oersteds) and at 76 mm (3 inches) it dropped to 2600 A/m

(33 oersteds).

A rule-of-thumb that relates the field strength of a magnet

to the distance from the magnet is that the field strength decays

at the rate of approximately

1 1 1 1 X X

«

distance distance distance (distance)-^

It must be stressed that a great ally of the computer installation

manager is the distance between the tape library and the unwanted

magnetic field, A barrier against unauthorized personnel just a few

feet from the tape vault will be extremely effective in preventing

damage from magnetic fields,

A field that is produced by a permanent magnet and the field

that is produced by an electromagnet powered by batteries have the same erasure effects. However, an electromagnet develops its

magnetic field only when it is energized by the batteries and is used
I

I

in applications in which it Is necessary to vary the strength of

the field. For example, in conveying scrap iron it is necessary to

release as well as to lift the metal; therefore, the magnetic field

strength must be variable.

The ability of a magnetic material to resist the erasure of its

recorded information is mostly dependent upon a quality known as "coercivity" which is measured in units of amperes per meter (oersteds).

The higher the coercivity of a material the more difficult it is

either to record onto or to erase information from its magnetic surface.

In order to erase the recorded signal completely it is necessary that

the strength of the erasing field is greater in value than the

coercivity of the medium. For example, the coercivities of typical

magnetic computer tape range in value from approximately 20,000 to

21,000 A/m (250 to 265 oersteds); therefore, any magnetic field

whose peak strength is less than these values cannot erase the signal

entirely from the tape. Commercial equipments that are used

specifically for erasing the recorded data from the tapes will usually

develop magnetic fields that are at least 5 times as great as the

coercivity of the tape. In addition, these fields must extend through

every area of the oxide surface in order to produce complete erasure. From these considerations it can be seen that once the previously

described magnet has been moved to a distance of more than 51 mm

(2 in) from the surface of the tape it is no longer able to significantly damage the tape record,

(4) The relationship between a magnetic field and the length

of the time that it exerts its influence on the recorded medium

is complex. However, it can be stated that if the magnetic field is

strong enough to completely erase the tape data it will do its maximum

damage almost instantaneously. Lesser field strengths produce

erasure and second-order effects more gradually, but no matter how

long a recorded tape remains in either a steady or a varying magnetic

field it cannot be completely erased unless the peak or maximum

value of the field is greater than the coercivity of the tape. The

effect of a field is slightly different depending upon whether it is a slowly varying magnetic (ac) field or a steady value (dc) field.

Slowly varying (ac) fields are produced by devices such as motors,

generators and transformers, while steady fields are produced by

permanent magnets and direct current electromagnets. The devices

are normally designed with shields that tend to keep their ac magnetic

fields from permeating the surrounding environment but some leakage

may occur. The strength of a slowly varying (ac) magnetic field

decreases inversely with distance cubed in the same way as the steady

field frcxn the permanent magnet.

There are some second-order effects that result from subjecting

recorded tapes to relatively weak ac and dc magnetic fields. For

example, a small steady field from a permanent magnet can cause a form of background "noise" to appear on a recorded tape that becomes

greater in intensity as the tape remains in the field for longer

periods of time. While this noise may be undesirable for audio tapes

5
it is of no importance in a digital system. This noise effect is

often caused on magnetic tapes by recording heads that have become

magnetized and behave like ordinary, weak permanent magnets* This

is why recording heads should be demagnetized by equipments

specifically designed for this purpose that are known as "degaussers".

Stray varying (ac) fields can also cause erasure of recorded data in

proportion to the highest or "peak" strength that the field may

attain. A relatively weak ac field can enhance "signal transfer"

or "print -through" effects on a reel of magnetic tape. Print-through

is the process by which the desired recorded signal on one layer of

tape can cause an adjacent layer against which it is wound to develop

a weak version of the same signal pattern. The longer the recorded

tape is subjected to the external ac field, the stronger the "printthrough" signal becomes [l] . Print-through effects are worsened

if the tapes are stored in higher temperature environments.

Print-through, however, is not a significant factor in digital

recordings. Another interesting time dependent effect is that the

longer a recorded tape has been stored the harder it is to erase the

signal. That is, it is easiest to remove the magnetic pattern from

a tape immediately after it has been recorded. If the peak value of an external ac field is strong enough to damage the signal on a tape

then the longer that tape is within the field the more effective

the erasing action will be, A stronger ac field will be more effective

in less time than a weaker ac field, A field strength of approximately

4000 A/m (50 oersteds) will begin to affect the amplitude of a recorded

digital signal. This level, however, will not damage the information

content irretrievably,

(5) The metallurgical and physical design of a permanent magnet

is a very complicated process. There is considerable interaction

among the various parameters such as magnet length, cross-section area,

shape, gap size, chemical composition, etc. As a general rule the

field strength and "pulling force" of a permanent magnet increases

when the volume (or weight) of the magnet increases for a given magnetic

material and shape. Figure 1 shows the power or "pulling force"

exerted by a group of magnets of the same "U" shape but different

weights*. The pulling force is measured in terms of the attraction

of each magnet to a thick piece of iron. Note the rapid decrease in

magnet power with distance from the iron piece. For example, the

pulling force of the 18 kg (40 lbs") horse-shoe shaped magnet

decreased from approximately 2800 N (620 pounds force) to approximately

13 N (3 pounds force) within 76 mm (3 in).

Figures in brackets indicate the literature references at the end of

this paper,

*Alnico VB magnets; Crucible Steel Company of America.
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Distance to Magnet *

Figure 1. Variations in Pulling Force for Magnets of Different

Weights.
The shape of the magnet will affect its pulling power as well as

its ability to produce the greatest magnetic field at a distance.

A long, straight bar magnet will produce a greater field strength

at a distance than the same magnet bent into a "U" shape. However,

the "U" magnet will have greater pulling power.

Modern advances in magnetic materials have produced permanent

magnets with fields that are extremely powerful in spite of their

small size. For example, a new group of cobalt-rare earth materials

such as cobalt- samarium have been discovered which have unusual

magnetic properties [2].

Most of the experimental details from which the answers to

the following questions were derived are given in section 4. It

should be understood that the answers also apply to magnetic

disks and drums when they are subjected to the same conditions.

b. Question 2

Could someone put a couple of little bar magnets, such as we

use to hang papers on steel walls in a plastic tape case, put the case on a tape rack, erase and destroy a significant number of reels

on the rack over a period of months?

1

(1) The length of time that the magnets are near to the

recorded tapes has no relationship to their effectiveness in

obliterating the tape data. If the fields from the magnets are capable of erasing the tape they will do so as soon as the tape is

in their vicinity. In a test these little bar magnets whose field

strengths averaged from approximately 44,000 to 52,000 A/m

(550 to 650 oersteds) were contacted singly and in^pairs against the

flange of a recorded reel of computer tape. Figure 3 in section 4.2

shows that they were able to cause as much as a 357o decrease in the

recorded signal amplitude on the track next to the edge track when

used in pairs. This can be significant for some systems. Question

2 as posed, however, indicates that these magnets are to be placed

into an adjoining plastic tape case. This produces a considerable

"spacing loss" for the magnetic field and, therefore, reduces its

erasing effects by moving the tape into a much weaker part of the

field. It was found that if the tapes as well as the magnets are both enclosed in plastic cases then the erasing effect is negligible

for these bar magnets.

c. Question 3

Could someone put a 51 mm x 102 mm (2 in x 4 in) magnetic picture

hook in his pocket, walk by a tape rack, and destroy a significant

number of tapes in the rack?

(1) The maximum magnetic flux density close to the surface

of each of a group of magnetic picture hooks was measured as 0„045

to 0.05 T (450 to 500 gauss). They had no effect at all on the signal

8
when carried in a pocket in the vicinity of a recorded reel of tape.

One of these magnets was firmly pressed directly against the exposed

flange of a reel of tape. The track next to the edge track experienced an 87o decrease in signal amplitude. The magnetic picture

hooks are less effective than the bar magnets in Question 2, because

their internal array of flat magnets do not tend to produce fields

that are as strong at a distance. The test was repeated with the

tape enclosed in its plastic container and with the magnetic picture

hook strapped to the container for 2 days. No observable signal

loss occurred.

d. Question 4

Could someone attach the picture hook to the rack in an obscure

place and destroy a number of tapes over a period of a month?

(1) This question has been answered by the previous comments.

Time is no element in the destructive process; only the maximum

magnetic field strength and proximity to the tape are important.

The picture hook out of direct contact with the flanges on the reels

of tape is ineffective as a signal erasing device. As a matter of

fact, when a permanent magnet is fastened to a metal rack surface by

magnetic attraction, its fields tend to be "short circuited" or "shielded" by the metal and are even less effective than before at

any distance from the magnet.

e. Question 5

Given a one-story building housing a tape storage area, could

someone place a permanent magnet weighing 18 kg (40 lbs) on the

roof and have any effect on the tapes?

(1) On the basis of the rule-of-thumb that was stated in

section 3.1,(3) the field of a magnet will decrease to approximately

1 X -rr: x

10 10 10 1000

of its field value that existed at 0.305 m (1 ft) from the pole

faces if we move 3.05 m (10 ft) from the magnet. At the present

time there are no permanent magnets available that can completely

erase the information from a magnetic tape that is placed 0.305 m

(1 ft) away from the magnet.

fo Question 6

Given a first floor installation separated from the street by a 6,1 m (20 ft) sidewalk, with the tape racks 30.5 m (100 ft) from the
curb, could someone place an electromagnet (of the type used in

junk yards to load scrap) on its side in a van, connect the magnet

to a series of automobile storage batteries in the van, drive up to the

curb outside the computer room and, in say a minute, wipe out the

entire installation's file and programs?

(1) This experiment was actually simulated at a local scrap

metal (junk) yard on April 2, 1971. Four recorded tapes were placed

into the field of a scrap lifting magnet at various distances: at

0.41 m (1.3 ft), at 1.52 m (5 ft), and at 3.05 m (10 ft). At

0.41 m (1.3 ft) a uniform signal loss of approximately 5% was observed.

At 1.52 m (5 ft) and above no observable loss occurred. According to

the crane operator, the magnet had a lifting capacity of approximately

363 kg (800 lbs) of scrap metal. However, the gap structure of the

lifting magnet is such that there is little magnetic field strength

extending outward from the magnet face. This type of a lifting

magnet is effective primarily upon contact with the material and

cannot perform at a distance as suggested,

(2) Now consider a non-lifting type of electromagnet: a magnetic field forms around any wire through which electric current

flows. This field can be made very powerful by shaping the wire

into the form of a spring-like coil or "solenoid" and increasing the

current flow. However, increased current flow requires more power

from the electrical supply as well as cooling systems for the coils.

Coils that are specifically designed for producing very strong

magnetic fields that are in the range from approximately 8 to 40

million A/m (100,000 to 500,000 oersted) are not designed to produce

fields at a long distance. They are always constructed to produce

concentrated magnetic fields in the center region of the coils and

are used to perform basic scientific experiments on materials that are placed into the most powerful regions of the coil field. For

example, one known* high intensity water-cooled magnet coil which

has approximately a 12 million A/m (150,000 oersted) field near its

center drops to less than approximately 80,000 A/m (1000 oersted)

within 0,305 m (1 ft) of the coil center. This magnet requires a power supply that is capable of supplying at least 3 million watts of

electrical power to the coil. A magnet system such as this is

impossible to transport in any ordinary vehicle. Less magnet power

is required to produce a strong magnetic field if the coil is wound

around an iron "core". However, an iron core limits the maximum

electromagnetic field strength to a value that is well below the

maximum field that can be produced by a coil with an "air" core alone.

The most powerful distant field can be produced by an electromagnet

coil with a straight line shape while a magnet that is bent into

the shape of a "U" or a "horseshoe" develops more intense "local"

or close-up fields but weaker distant fields,

*"Bitter" type magnet at the Naval Research Laboratory High Magnetic

Field facility, Washington, D,C.

10
A low temperature "cryogenic" magnet operating in a "pulsed

field" mode is an exotic and more profitable way to develop extremely

high field intensities with truck portable magnets [3]. Experiments

have not yet been performed to determine their effective erasing

range. However, it may be stated that within the present state-ofthe-art it is not possible to produce a distant field with a mobile

electromagnet in any known form that is capable of erasing the

recorded signal from a magnetic tape at 30,5 m (100 ft) from the source.

g. Question 7

Is there any way to shield the information on a magnetic tape

from an outside magnetic field produced by a magnet or a motor?

It has been shown that recorded information on a magnetic tape

can be shielded from the effects of a magnetic field by keeping the

tapes at a distance from a source such as a permanent magnet or motor. It is also possible to shield the tape further by placing

it either into an individual container or into a vault whose surfaces

are made of ferromagnetic "shielding" material [4], These materials

are good conductors of both electricity and magnetic "flux" and

will short-circuit the path of the magnetic field. That is, most of

the magnetic field energy is absorbed in the shielding material

and very little passes through it into the air on the opposite side.

The shield is usually fabricated from some ferrous or ferrite

material in a number of different physical forms such as sheets,

rolls and coatings of various thicknesses. Hypernick, Permalloy,

Mu-metal, Netic and Co-netic are examples of shielding materials

for magnetic fields. A multi- layer shielding structure can be

devised by using the proper ccxnbinations of materials and air space.

For example, Netic shielding has been designed for reducing the

strength of very powerful magnetic fields while Co-Netic is designed

to absorb weaker magnetic fields. Therefore, a layer of Netic can

be placed close to the outside of the tape enclosure in order to

absorb most of the incoming magnetic field. This can be followed by

an air space, and then a layer of Co-Netic close to the interior wall

of the enclosure will absorb the remaining field.

Figure 7 in section 4.3 shows the effect of a large permanent

magnet on a recorded reel of magnetic tape. When the magnet is

placed 38 mm (1.5 in) from the tape surface a 15% signal loss is

sustained on the beginning footage. When a piece of 1,3 mm (0,050 in)

thick Mu-metal shielding is then placed between the tape reel and the

magnet there is no observable signal loss as shown in figure 8 in

section 4.3.

A potential source of unwanted magnetic fields such as a large

motor, generator or transformer that is near the tape installation

should also be enclosed in shielding materials. The shield layer

or layers should be at least 6,4 mm (0,25 in) or more away from the

11
device and should surround it completelyo Whenever possible, the

initial placement of the tape repository should avoid sources of

unwanted fields.

There are several organizations whose primary function is the

production, design and engineering of shielded environments* It is

worthwhile idea to consider using their services,

h. Question 8

Are there magnets available that are much more powerful than

the little office bar or picture hook, magnets but that are still

easy to conceal and to carry about?

Yes, there are<, The tests that were performed with a concealable

magnet which weighed approximately 0,80 kg (1.75 lbs) and had a maximum center field strength of approximately 56,000 A/m (700 oersteds)

indicated that they can be extremely effective even when they contact

the outer edges of the tape reel as it is stored in a vertical

position. Figure 5 in section 4.3 shows that the signal on the first

210 m (700 ft) of a recorded tape was badly damaged. The broad dark

trace on the left side of the chart represents a significant loss of

signal strength. The dark trace becomes progressively thinner as the signal loss decreases with distance into the tape reel and ends

up at its normal undamaged signal level.

The results of a number of tests that were performed with this

magnet are described in Section 4,3.

4. DESCRIPTION OF EXPERIMENTS

4.1 Experimental Procedures

In each of the following experiments computer tapes were

first recorded with continuous pulse trains and were then subjected

to the magnetic fields from different permanent magnets at

varying distances. The resulting peak signal levels were then

measured in the NBS Magnetic Media Measurement Laboratory with the

NBS reference tape signal amplitude measurement system [5], The

signal peak outputs from various read head tracks were read from the

tape on an IM 2401-VI tape transport at a speed of 2.86 meters

per second (112,5 inches per second) and were recorded by a strip

chart recorder as shown in figure 2. In some cases the chart speeds

were varied from slow (1 mm/second) to fast (125 mm/second) in

order to display fine signal detail. For example, in figure 2 and 3

each drop in signal level coincides with a single rotation of the

tape reel. This drop occurs at the positions of the tape surface

closest to the point at which the magnet touched the reel flange.

At the slow chart speeds of 1 mm/second all of the drops in signal

level merge into a solid trace. At this slow speed each vertical

12
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14
chart line represents a passage of approximately 14,3 m (47 ft) of

tape. Each horizontal line represents a 1% variation in signal level.

4.2 Small Bar Magnet Experiments

a. Experiments were performed to determine whether the signal

levels of stored data on a reel of magnetic tape can be altered or destroyed with typical office type 25 mm x 3,2 mm x 3.2 mm

(1 in X 0„13 in x 0.13 in) Alnico bar magnets and 51 mm x 102 mm

(2 in X 4 in) picture hooks. The field strengths of the bar magnets

and hooks were measured with a Hall probe-gaussmeter combination.

The bar magnets had maximum field intensities of approximately

44,000 to 52,000 A/m (550 to 650 oersteds) and the picture hooks had

maximum intensities of approximately 36,000 to 40,000 A/m (450 to

500 oersteds). These fields are greater than the typical computer

tape coercivities which range from approximately 20,000 to 21,000

A/m (250 to 265 oersteds). Therefore, it is only the spacing loss

produced by the reel flange thickness plus the spacing of the data

bit position from the inside flange surface that prevent even worse

obliteration of the recorded signals.

Figure 2 shows the peak signal levels on tracks A (Track N0o2)

and track B (Track No, 5) of a computer tape which has been recorded

with a 180 kHz pulse train*. This is a 3200 frpi (NRZI) signal.

The end of a single bar magnet had been pressed against the side of

the plastic reel flange on the track A side of the tape reel

immediately after recording. The center of track A was spaced

approximately 4,2 mm (0,16 in) from the pole of the magnet as it

contacted the flange. Track B is near the centerline of the tape and

is at most 4,3 mm (0,17 in) from the center of track A, The signal

levels shown in figure 2 were measured during the first "read-afterwrite" pass. Track A displays a peak signal level drop of 18% and

track B displays a 2% drop. The average overall tape signal peak

level had decreased by 37o from the first "read-while-write" pass

(not shown) due to the normal demagnetization effects not related to

the bar magnets.

b. Figure 3 shows the effects on the same recorded signal

that are now produced by the contact of 2 bar magnets whose

magnetic fields have been forced outward by placing their "like"

poles togethero This effectively increases the volume of the magnet.

The peak signal in track A has sustained a 357o decrease in signal

amplitude, track B an 87o decrease and now even track C (not shown)

whose center is approximately 8o2 mm (0,32 in) away from the center

of track A has sustained a 37o loss in signal amplitude. Tests with

the NBS drop-out measuring device did not detect signal level losses

below these levels.

*The tracks are numbered in accordance with USA Standard Recorded

Magnetic Tape for Information Interchange (800 CPI, NRZI)-USASI

X3. 22-1967.

3^3
Additional tests that were run with aluminum flange reels displayei

equivalent effects since aluminum is not ferromagnetic and does not

shield the tape. These signal losses are dangerously close to

producing information losses in practical systems where a drop-out

is typically a 50% decrease from some nominal system level.

4o3 Large Magnet Experiments and Tests

ao Serious attempts at the large scale destruction of tape

libraries would probably be made with powerful magnets that could

be concealed in the pocket or purse. Also, since magnetic tape

reels are normally stored next to each other in a vertical position,

the magnet would tend to be pressed in the vicinity of the outer

flange edges rather than against the inaccessible sides of the reel

flanges. The following series of tests were performed with the

concealable magnet that was referred to in Question 8, The plot of

the magnetic field strength in the region surrounding the magnet

poles is shown in figure 4 as the field lines enter the tape pack.

Only one side of the field is shown; the mirror image of that field

is on the other side of the centerline. There are magnetic field

density regions close to the pole face edges that are as high as 0o25 T (2500 gauss). These do not reach the tape and are not shown

in the figure. The field strength lines are shown in terms of

both lOOOX A/m and oersteds as they exist within the tape reel upon

direct magnet-to-reel contact. The field strength at each plotted

position was found to be the same when the space between magnet and

measuring probe was filled either with air or with layers of magnetic

tape.

bo The 730 m (2400 ft) long tapes used in the following tests were pre-recorded at 3200 frpi bit density. In each case 13 mm

(0o50 in) must be added in order to determine the magnet-to-tape

distance since the outermost layer of tape begins this far in from

the outer edge of the reel flange:

(1) Test 1: Figure 5. The magnet was contacted edge-on

against a recorded reel of tape that was protected only by a wraparound container. The erasing effectiveness of the magnetic field

as a function of distance into the tape can be seen by the gradual

variation in the tape peak signal level. The heavy dark area represents the drop in signal level to a value more than 40% below

the nominal system level. Note that the information has been badly

damaged on approximately the first 210 m (700 ft) of tape. Figure 6

shows a plot of "7o Signal Decrease" versus "Distance Into Reel"

obtained from figure 5. For example, at a distance of approximately

46 mm (1.8 in) in from the outside edge of the reel flange (the

position of the magnet poles) towards the center of the tape reel

the signal has been decreased by 5. 5%. This point is approximately

490 (1600 ft) in from the beginning of the tape. The erasing field

strength and the signal loss at each distance along the tape can

16
TAPE REEL FLANGE

Figure 4. Magnetic Field Produced by the Large Test Magnet,

17
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be derived from figures 4 and 60 It is of interest to note that

the curve that is fitted to the measured data in figure 6 is

proportional to 1 . This indicates that the signal

(distance)-^

strength on this tape decreased at an inverse cube rate in the same manner as a distant magnetic field.

The following tests 2 through 4 were each performed with

a wrap-around container:

(2) Test 2: Figure 7. The magnet was moved approximately

25 mm (1 in) away from the reel edge. The maximum signal level

loss on the beginning tape footage was approximately 11°L.

(3) Test 3: Figure 8, The magnet was moved back approximately

25 mm (1 in) from the reel edge and a piece of Mu-metal 1,3 mm

(0,050 in) thick was placed between the tape and the magnet in

contact with the edge of the reel. There was no measurable signal

loss. However, when the magnet was brought into direct contact

with the Mu-metal (that is, zero air space) the first 61 m (200 ft)

of recorded tape signal was badly damaged. This indicated that the

Mu-metal was saturated and its "permeability" was greatly decreased

by the strong magnetic field,

(4) Test 4: The magnet was moved back 51 mm (2 in) from the

reel edge. There was no observable signal loss. This shows the

value of distance as a shield. The trace is the same as figure 8,

(5) Test 5: Figure 9, The reel of tape was placed into a typical full-sized plastic container. This produces an additional

13 mm (0,5 in) end air space between the magnet and the reel. The

signal was reduced by at most 507o at the beginning of the tape.

The recovery of the signal level to above the 45% loss level occurred

at the 79 m (260 ft) mark. In the case of the direct raagnet-to-reel

contact of Test 1 this recovery level was not reached until close to

the 210 m (700 ft) mark.

(6) Test 6: Figure 10, The reel was pre-recorded at 200 frpi

and placed into a wrap-around container. The general effect was

similar to that in Test 1 except for the erratic trace which, could

have been caused by the angle of the magnet.

All of these tests were performed with the magnet positioned as shown in figure 5, The magnet is in the "easy" magnetizing position

and will produce the maximum signal damage at a given distance. If

the magnet is rotated 90 degrees into the "transverse" position its erasure effect is reduced from a 45% or greater signal loss over the

first 210 m (700 ft) of tape to the same loss only over the first

76 m (250 ft).

23
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24
Tests such as these suggest that it may be of benefit to use

the standard 267 mm (10,5 in) diameter computer tape reel with a reduced amount of tape footageo This would move the beginning of

the recorded tape away from the outside reel edge. Alternately,

it may be possible to use standard 730 m (2400 ft) long computer tapes

with the important information recorded nearest to the inside hub.

However, these precautions will be nullified if the magnet has access

to the side of the tape reel flangeo The field plot in figure 4 suggests that side flange contact with this 0„80 kg (1,75 lb) magnet would

be devastating.

5. ADDITIONAL TEST RESULTS AND COMMENTS

a. All of the tests that were performed were evaluated in

terms of the signal peak level changes that were caused by permanent

magnet fields. A test was also performed to determine whether any

detrimental waveshape distortions occurred due to these dc fields.

In order to do this experiment, a cassette tape was recorded at 1600

frpi and then subjected to a double bar magnet contact at its side

flange. The tape was then read back at 127 ram/second (5 inches /second)

This is sufficiently slow so that the individually reproduced

waveshapes could be stored and displayed on the tube of a storage

oscilloscope at both the nominal 100% level and the damaged level.

The signals with the reduced levels appeared once per revolution

(see section 4,1) and were approximately 44% below the nominal

level. However, the signal waveshapes were relatively undistorted

and their recovery as information would not be affected by distortion

phenomena. Only the reduction in signal amplitude appears likely

to be the cause of information losses, particularly in an "amplitude"

detection type of computer tape reproduce system,

b. There were no erasure tests run with a slowly varying ac magnetic field; however, the information derived from the permanent

magnet tests is useful for both cases. The "peak rule" for ac magnetic fields holds. That is, the recorded signal on the tape will

be affected mostly by the maximum or peak value that the ac field

achieves. Once the tape record is subjected to that maximum value it

will suffer an irreversible loss of signal. As the field goes towards zero value it is similar to the process of withdrawing the permanent

magnet. However, since the ac field then reverses its direction at

a 60 Hz rate it will eventually tend to have a more destructive

and randomizing effect on the signal content. It is as though the

permanent magnet poles were turned back and forth through 180° at

this rate. However, the peak value of the ac field requires approximately the same intensity value as the dc field if it is to produce

complete signal destruction.

Co Although the pertinent tests were not performed at NBS

the following information of interest has been gathered:

25
.1 (1) Microwave electromagnetic fields are ineffective for

magnetic tape signal destruction unless the tape is extremely close

to the antenna. This is because the magnetic component of the field

loses its strength very rapidly with distance from the antenna.

In one series of tests groups of recorded tapes were placed in

various positions at a distance of approximately 460m(1500 ft) from

a 5 million watt^ pulsed radar antenna. They remained there for

27 days and were then subjected to a number of rigorous tests ^\\

including signal-to-noise, print-through and signal amplitude. ^H

Result: No detrimental effects were observed [6].

(2) Nuclear radiation is ineffective as a magnetic tape signal

destroying force except, of course, where physical destruction of the

tape backing or oxide can occur. The electromagnetic field

associated with a nuclear burst is not sufficiently strong to produce

signal erasure, although its irradiation effects may cause damage

to the oxides or backing [?].

(3) Initial information to date indicates that the magnetic

field emanating from the airport metal and gun detector systems are too weak to cause destruction of the signal on a recorded computer

tape. For example, one such system produces a field intensity of

less than 80 A/m (1 oersted) [8]. It is important that the designers

of these detection systems be cognizant of the difficulties which

may accrue if they design units with more powerful searching fields.

Otherwise it may be necessary to develop alternate handling and

trafficking methods for recorded magnetic tapes.

(4) Available information on the effects of lightning strokes

on recorded magnetic data indicates that the tape must be within

3.05 m (10 ft) of the center of the bolt in order to sustain

observable damage and within 0„91 m (3 ft) before severe erasure

occurs. The current at the center of the bolt is approximately 10

amperes [9].

(5) Excessive temperatures can cause signal loss especially

as the Curie temperature of the iron oxide in the tape is approached.

In fact, there is a method of recording on magnetic tape known as "thermoremanent" or "Curie Point" recording in which the oxide is

heated to approximately 675° C. Near this temperature a very small

magnetic field can either magnetize or demagnetize a tape [l].

However, the base material of the magnetic tape will undergo physical

damage and deformation at much lower temperatures. Figure 11 shows

the effects of temperature and humidity on various base materials.

This figure has been reproduced from the Ampex "Trends" Application

Engineering Bulletin No. 2., July 1963.

d. Since the characteristics of the magnetic media surfaces

as well as the recording mode and the record/reproduce system

philosophy will all affect the extent of the damage that a given

device can accomplish it is felt that a study of different methods

for recovering information from damaged tapes and for designing

26
TEMPERATURE,

+ 212 MYLAR USEFUL LIFE DECREASES SHARPLY

»

RELATIVE HUMIDITY. PER CENT

lOO-

+ 175 MYLAR TENSILE STRENGTH

AND TENSILE MODULUS

DROP SHARPLY

i WEAR

I RATE

INCREASES

CELLULOSE

ACETATE

"CUPPING"

INCREASES

50

IDEAL FOR

ARCHIVAL

STORAGE

-+140 +130

LAYER-TO-LAYER ADHESION

AND MYLAR SHRINKAGE BEGIN

BINDER-PLASTICIZER LOSS

ACCELERATES

CELLULOSE ACETATE FILM EMBRITTLES WITH

PROLONGED EXPOSURE

PLASTICIZER

LOSS

ACCELERATES

"SAFE"

RANGE.

MYLAR-BASE

+80-

' +70

}{

IDEAL FOR

ARCHIVAL STORAGE

TAPE

30-

PROBABLE

ELECTRO-STATIC

CHARGING

ZONE

"SAFE"

RANGE

CELLULOSE

ACETATE-BASE

TAPE

I 4o75

I *TM for Dupont Polyester Film

Figure 11. Effects of Temperature and Humidity on Magnetic Tapes.

(Courtesy Ampex Corporation)

27
system philosophies that minimize information loss would be highly

valuable„

(1) For example, the surface of the magnetic recording media

usually consists of either small "ferromagnetic" particles that are held in a paint-like "binder" or of a thin layer or film of

ferromagnetic material which has been "plated" onto the surface.

The RAMAC and most randcxn access disks as well as all computer tapes are examples of media having "particulate" magnetic surfaces,

FASTRA.ND drums and video files are examples of the film type of

magnetic recording surfaces. In either case it is possible to

design the surface of the medium to be more or less resistant to

erasure by unwanted fields. For example, a resistant medium can be fabricated with a very thin, very high coercivity magnetic surface.

There is, however, considerable interaction between coercivity and

temperature for certain types of films [10], Therefore, the surface

design must take into consideration the characteristics of the

system and the environment in which the media will be used,

(2) Most of the methods that are used for reproducing the data

from computer tapes fall into the categories of either "amplitude"

or "peak" detection [ll]. The amplitude detection process senses

the presence of a pulse for a binary "1" and the absence of a pulse

for a binary "0" information. In order to suppress the effects of

system noise the amplitude detection method usually requires that

the incoming signal shall achieve some threshold level. If the

signal that is reproduced from a tape does not reach the required

level it is lost as information. In the peak detection process

signal cross-over points control the information output. Since

initial test results indicate that the field of a permanent magnet

primarily causes the loss of signal amplitude, it is felt that a peak detection method is more resistant to information loss on damaged

computer tapes,

(3) The resistance of magnetic media to information loss is

also influenced by the encoding scheme that is used. For example,

phase recording withstands the effects of unwanted magnetic fields

better than amplitude recording since it is easier and less expensive

to recover the information on this type of encoding scheme.

Techniques exist for recovery of signal frcm magnetic media onto

which significant levels of noise have been introduced, A finding

in this regard is that NRZI recordings are less susceptible to

"damage" than many other recording techniques, ;

The contributions made by Dr, Ruth M, Davis to sections 2 | and 5d are gratefully acknowledged.

28
6. REFERENCES

[l] Mee, C. D. ^ The Physics of Magnetic Recording, Chapters 3 and

4, p. 80 and p. 134, North Holland Publishing Co., Amsterdam,

(1964).

[2] Becker, J. J., Permanent magnets. Scientific American, 223 , No. 6, 92-100, (1970).

[3] Parkinson, D<, H. , Mulhall, B. E., The Generation of High

Magnetic Fields, Plenum Press, New York (1967).

[4] Robinson, J. G. , Fundamentals of EMI shielding, ElectroTechnology 21, No. 6, 36-39, (1966).

[5] Geller, S. B. , Calibration of NBS secondary standard magnetic

tapes (Computer Amplitude Reference) SRM 3200 using the

reference tape amplitude measurement "Process A" Model II,

NBS Special Publication 260-29, (1971).

[6] Byrd, V., Effects of radar signals on recorded magnetic tapes,

NASA Technical Note, Private Communication, (1967).

[7] Computer Talk, The handling and storage of computer tape.

Magnetic Products Division 3M, _1, No, 1, (1968) <,

[8] Monahan, J. S., Airline metal detector inspections. Applications

Bulletin No. 4, Memorex Corp., (1971).

[9] Pear, C, B. , Magnetic Recording in Science and Industry,

Chapter 3, 106, Reinhold Publishing Corpo , New York, (1967).

[10] Lazzari, No P. et al. Experimental studies used in contact

recording on chromium cobalt films, IEEE Transactions on Magnetics,

5, No, 4, 955-959, (1960).

[11] Bycer, B. B. , Digital Magnetic Tape Recording: Principles and

Computer Applications, Chapter 5, 110-118, Hayden Book Co„ , N<,Y.,

(1965).

[12] ASTM Metric Practice Guide, Handbook 102, U.S. Department of

Commerce, National Bureau of Standards, (1967).

29
APPENDIX

The conversion factors which were used to relate the "customary"

system of units to the new International System of Units (SI) are as follows:

foot converts to 0.3048 meters

gauss converts to 0.0001 teslas

inch converts to 25.4 millimeters

oersted converts to 79.57747 amperes per meter

pound converts to 0.4535924 kilograms

pound (force) converts to 4.448 newtons

Reference [12] was used as a guide for handling the numerical

data in this report.

The expression magnet "power" has been used in section 3.1. a. only as a descriptive term and is not technically meaningful. The

ability of a magnet to exert a force on a responsive material is a function of the intensity or strength of its external magnetic field.

(a) 1 (b) 1 (c) 1 (d) 1 (e) 1 (f) 1

30
FORM NBS-114A (1-711

1. PUBLICATION OR REPORT NO. NBS TN-7352. Gov't Accession No.

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

The Effects of Magnetic Fields on Magnetic Storage Media

Used in Computers

5. Publication Date July 1972

6. Performing Or^nization Code

7. AUTHOR(S)

Sidney B. Geller

$. Performing Organization

10. Project/Task/Work Unit No. 6501111

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Same as No . 9

13. Type of Report & Period Covered Final FY '71

14. %onsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) .Experiments have been performed with different types of magnets to

determine their effects on the information stored on magnetic storage media.

The test results that were obtained with recorded computer tapes are discussed and guidelines are suggested to computer installation managers

for protecting their recorded tapes ^ disks and drums against unwarranted

erasure by magnetic fields. The effects of other forms of energy on recorded

magnetic media are briefly considered.

17. KEY WORDS (Alphabetical order, separated by semicolons) Erasure , magnetic media; erasure,

permanent magnet; erasure, shielding against; erasure, signal loss due to; magnet

fields, erasure with; magnetic media, ccmputer; magnetic media, information damage;

18. AVAILABILITY STATEMENT permanent magnets ; permanent

magnet, erasure with;

S] UNLIMITED. shielding.

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TO NTIS.

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35

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