Note: Descriptions are shown in the official language in which they were submitted.
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METALLIC GLASS ALLOYS FOR MECHANICALLY
RESONANT MARKER SURVEILLANCE SYSTEMS
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to metallic glass alloys; and more particularly to
metallic glass alloys suited for use in mechanically resonant markers of
article
surveillance systems.
2. Description of the Prior Art
Numerous article surveillance systems are available in the market today to
help identify and/or secure various animate and inanimate objects.
Identification of
personnel for controlted access to limited areas, and securing articles of
merchandise against pilferage are examples of purposes for which such systems
are
employed.
An essential component of all surveillance systems is a sensing unit or
"marker", that is attached to the object to be detected. Other components of
the
system include a transmitter and a receiver that are suitably disposed in an
"interrogation" zone. When the object carrying the marker enters the
interrogation
zone, the functional part of the marker responds to a signal from the
transmitter,
which response is detected in the receiver. The information contained in the
response signal is then processed for actions appropriate to the application:
denial
of access, triggering of an alarm, and the like.
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Several different types of markers have been disclosed and are in use. In
one type, the functional portion of the marker consists of either an antenna
and
diode or an antenna and capacitors forming a resonant circuit. When placed in
an electromagnetic field transmitted by the interrogation apparatus, the
antenna-diode
marker generates harmonics of the interrogation frequency in the receiving
antenna. The detection of the harmonic or signal level change indicates the
presence of the marker. With this type of system, however, reliability of the
marker identification is relatively low due to the broad bandwidth of the
simple
resonant circuit. Moreover, the marker must be removed after identification,
which is not desirable in such cases as antipilferage systems.
A second type of marker consists of a first elongated element of high
magnetic permeability ferromagnetic material disposed adjacent to at least a
second
element of ferromagnetic material having higher coercivity than the first
element.
When subjected to an interrogation frequency of electromagnetic radiation, the
marker generates harmonics of the interrogation frequency due to the non-
linear
characteristics of the marker. The detection of such harmonics in the
receiving coil
indicates the presence of the marker. Deactivation of the marker is
accomplished
by changing the state of magnetization of the second element, which can be
easily
achieved, for example, by passing the marker through a dc magnetic field.
Harmonic marker systems are superior to the aforementioned radio-frequency
resonant systems due to improved reliability of marker identification and
simpler
deactivation method. Two major problems, however, exist with this type of
system: one is the difficulty of detecting the marker signal at remote
distances. The
amplitude of the harmonics generated by the marker is much smaller than the
amplitude of the interrogation signal, liniiting the detection aisle widths to
less than
about three feet. Another problem is the difficulty of distinguishing the
marker
signal from pseudo signals generated by other ferromagnetic objects such as
belt
buckles, pens, clips, etc.
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Surveillance systems that employ detection modes incorporating the
fundamental mechanical resonance frequency of the marker material are
especially
advantageous systems, in that they offer a combination of high detection
sensitivity, high operating reliability, and low operating costs. Examples of
such
systems are disclosed in U.S. Patent Nos. 4,510,489 and 4,510,490 (hereinafter
the
'489 and '490 patents).
The marker in such systems is a strip, or a plurality of strips, of known
length of a ferromagnetic material, packaged with a magnetically harder
ferromagnet (material with a higher coercivity) that provides a biasing field
to
establish peak magneto-mechanical coupling. The ferromagnetic marker material
is
preferably a metallic glass alloy ribbon, since the efficiency of magneto-
mechanical
coupling in these alloys is very high. The mechanical resonance frequency of
the
marker material is dictated essentially by the length of the alloy ribbon and
the
biasing field strength. When an interrogating signal tuned to this resonance
frequency is encountered, the marker material responds with a large signal
field
which is detected by the receiver. The large signal field is partially
attributable to
an enhanced magnetic permeability of the marker material at the resonance
frequency. Various marker configurations and systems for the interrogation and
detection that utilize the above principle have been taught in the '489 and
'490
patents.
In one particularly useful system, the marker material is excited into
oscillations by pulses, or bursts, of signal at its resonance frequency
generated by
the transmitter. When the exciting pulse is over, the marker material will
undergo
damped oscillations at its resonance frequency, i.e., the marker material
"rings
down" following the termination of the exciting pulse. The receiver "listens"
to the
response signal during this ring down period. Under this arrangement, the
surveillance system is relatively immune to interference from various radiated
or
power line sources and, therefore, the potential for false alarms is
essentially
eliminated.
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A broad range of alloys have been claimed in the '489 and '490 patents as
suitable for marker material, for the various detection systems disclosed.
Other
metallic glass alloys bearing high permeability are disclosed in U.S. Patent
No.
4,152,144.
A major problem in use of electronic article surveillance systems is the
tendency for markers of surveillance systems based on mechanical resonance to
accidentally trigger detection systems that are based an alternate technology,
such
as the harmonic marker systems described above: The non-linear magnetic
response of the marker is strong enough to generate harmonics in the alternate
system, thereby accidentally creating a pseudo response, or "false" alarm. The
importance of avoiding interference among, or "pollution" of, different
surveillance
systems is readily apparent. Consequently, there exists a need in the art for
a
resonant marker that can be detected in a highly reliable manner without
polluting
systems based on alternate technologies, such as harmonic re-radiance.
SUMMARY OF INVENTION
The present invention provides magnetic alloys that are at least 70% glassy
and, upon being annealed to enhance magnetic properties, are characterized by
relatively linear magnetic responses in a frequency regime wherein harmonic
marker systems operate magnetically. Such alloys can be cast into ribbon using
rapid solidification, or otherwise formed into markers having magnetic and
mechanical characteristics especially suited for use in surveillance systems
based on
magneto-mechanical actuation of the markers. Generally stated the glassy metal
alloys of the present invention have a composition consisting essentially of
the
formula Coa Feb Ni,; Md B. Sif Cg, where M is selected from molybdenum and
chromium and "a", "b", "c", "d", "e", "f' and "g" are in atom percent, "a"
ranges
from about 40 to about 43, "b" ranges from about 35 to about 42 and "c" ranges
from about 0 to about 5, "d" ranges from about 0 to about 3, "e" ranges from
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about 10 to about 25 , "f' ranges from about 0 to about 15 and "g" ranges from
about 0 to about 2. Ribbons of these alloys, when mechanically resonant at
frequencies ranging from about 48 to about 66 kHz, evidence relatively linear
.magnetization behavior up to an applied field exceeding 8 Oe as well as the
slope
of resonant frequency versus bias field close to or exceeding the level of
about 400
Hz/Oe exhibited by a conventional mechanical-resonant marker. Moreover,
voltage amplitudes detected at the receiving coil of a typical resonant-marker
system are higher for the markers made from the alloys of the present
invention
than those of the existing resonant marker. These features assure that
interference
among systems based on mechanical resonance and harmonic re-radiance is
avoided
The metallic glasses of this invention are especially suitable for use as the
active elements in markers associated with article surveillance systems that
employ
excitation and detection of the magneto-mechanical resonance described above.
Other uses may be found in sensors utilizing magneto-mechanical actuation and
its
related effects and in magnetic components requiring high magnetic
permeability.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood and further advantages will
become apparent when reference is made to the following detailed description
of
the preferred embodiments of the invention and the accompanying drawings in
which:
Fig. 1(a) is a schematic representation of the magnetization curve taken
along the length of a existent resonant marker, where B is the magnetic
induction
and H is the applied magnetic field;
Fig. 1(b) is a schematic representation of the magnetization curve taken
along the length of the marker of the present invention, where Ha is a field
above
tl which B saturates;
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Fig. 2 is a schematic representation of signal profile detected at the
receiving coil depicting mechanical resonance excitation, termination of
excitation
at time t = to and subsequent ring-down, where Vo and V1 are the signal
amplitudes at the receiving coil at t = to and t = ti (1 msec after ta ),
respectively;
and
Fig. 3 is a schematic representation of the mechanical resonance frequency,
fr , and response signal , V1 , detected in the receiving coil at 1 msec after
the
termination of the exciting ac field as a function of the bias magnetic field,
Hb,
where Hbl and Hb2 are the bias fields at which V, is a maximum and fT is a
minimum, respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, there are provided magnetic
metallic glass alloys that are characterized by relatively linear magnetic
responses in
the frequency region where harmonic marker systems operate magnetically. Such
alloys evidence all the features necessary to meet the requirements of markers
for
surveillance systems based on magneto-mechanical actuation. Generally stated
the
glassy metal alloys of the present invention have a composition consisting
essentially of the formula Coa Fee Ni,- Ma Be Sif Cg, where M is selected from
molybdenum and chromium and "a", "b", "c", "d", "e", 'f' and "g" are in atom
percent, "a" ranges from about 40 to about 43, "b" ranges from about 35 to
about
42 and "c" ranges from about 0 to about 5, "d" ranges from about 0 to about 3,
"e" ranges from about 10 to about 25 , "f' ranges from about 0 to about 15 and
"g" ranges from about 0 to about 2. The purity of the above compositions is
that
found in normal commercial practice. Ribbons of these alloys are annealed with
a
magnetic field applied across the width of the ribbons at elevated
temperatures for
a given period of time. Ribbon temperatures should be below its
crystallization
temperature and the heat-treated ribbon needs to be ductile enough to be cut
up.
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The field strength during the annealing is such that the ribbons saturate
magnetically along the field direction. Annealing time depends on the
annealing
temperature and typically ranges from about a few minutes to a few hours. For
commercial production, a continuous reel-to-reel annealing furnace may be
preferred. In such cases, ribbon travelling speeds may be set at between about
0.5
and 12 meter per minute. The annealed ribbons having, for example, a length of
about 38 mm, exhibit relatively linear magnetic response for magnetic fields
up to
or more than 8 Oe applied parallel to the marker length direction and
mechanical
resonance in a range of frequencies from about 48 kHz to about 66 kHz. The
linear magnetic response region extending to the level of more than 8 Oe is
sufficient to avoid triggering most of the harmonic marker systems. The
annealed
ribbons at lengths shorter or longer than 38 mm evidence higher or lower
mechanical resonance frequencies than 48-66 kHz range.
Ribbons having mechanical resonance in the range from about 48 to 60 kHz
are preferred. Such ribbons are short enough to be used as disposable marker
materials. In addition, the resonance signals of such ribbons are well
separated
from the audio and commercial radio frequency ranges.
Most metallic glass alloys that are outside of the scope of this invention
typically exhibit nonlinear magnetic response regions below about 8 Oe level.
Resonant markers composed of these alloys accidentally trigger, and thereby
pollute, many article detection systems of the harmonic re-radiance variety.
There are a few metallic glass alloys outside of the scope of this invention
that do show linear magnetic response for an acceptable field range. These
alloys,
however, contain high levels of molybdenum or chromium, resulting in increased
raw material costs and reduced ribbon castability owing to the higher melting
temperatures. The alloys of the present invention are advantageous, in that
they
afford, in combination, extended linear magnetic response, improved mechanical
resonance performance, good ribbon castability and economy in production of
usable ribbon.
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Apart from the avoidance of the interference among different systems, the
markers made from the alloys of the present invention generate larger signal
amplitudes at the receiving coil than conventional mechanical resonant
markers.
This makes it possible to reduce either the size of the marker or increase the
detection aisle widths, both of which are desirable features of article
surveillance systems.
Examples of metallic glass alloys of the invention include C042 Fe4o B11 Si7,
CO 42 Fe40 B12 Si6 ,C042 Fe40 B13 Si5 , C042 Fe40 B14 S14 , C042 Fe4o B15 Si3
, C042
Fe40 B16 Si2, C042 Fe4o B17 Si1 , C042 Fe4o B13 Si3 C2, C04o Fe40 Ni2 B13 Si5
, C040
Fe38 N14 B13 SiS, C041 Fe40 M01 B13 Si5 , C041 Fe38 M03 B13 Si5, C041 Fe40 Cr1
B13
SI5 , C041 Fe38 Cr3 B 13 Si5 , and Co43 Fe35 Ni3 B 13 S14 C2 w herein
subscripts are in
atom percent.
The magnetization behavior characterized by a B-H curve is shown in Fig.
1(a) for a conventional mechanical resonant marker, where B is the magnetic
induction and H is the applied field. The overall B-H curve is sheared with a
non-
linear hysteresis loop existent in the low field region. This non-linear
feature of the
marker results in higher harmonics generation, which triggers some of the
harmonic marker systems, hence the interference among different article
surveillance systems.
The definition of the linear magnetic response is given in Fig. 1(b). As a
marker is magnetized along the length direction by an external magnetic field,
H,
the magnetic induction, B, results in the marker. The magnetic response is
relatively linear up to H. , beyond which the marker saturates magnetically.
The
quantity Ha depends on the physical dimension of the marker and its magnetic
anisotropy field. To prevent the resonant marker from accidentally triggering
a
surveillance system based on harmonic re-radiance, H, should be above the
operating field intensity region of the harmonic marker systems.
The marker material is exposed to a burst of exciting signal of constant
amplitude, referred to as the exciting pulse, tuned to the frequency of
mechanical
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resonance of the marker material. The marker material responds to the exciting
pulse and generates output signal in the receiving coil following the curve
leading
to V. in Fig. 2. At time to , excitation is terminated and the marker starts
to ring-
down, reflected in the output signal which is reduced from V. to zero over a
period
of time. At time tl , which is 1 msec after the termination of excitation,
output
signal is measured and denoted by the quantity V 1. Thus V 1/ V. is a measure
of
the ring-down. Although the principle of operation of the surveillance system
is
not dependent on the shape of the waves comprising the exciting pulse, the
wave
form of this signal is usually sinusoidal. The marker material resonates under
this
excitation.
The physical principle governing this resonance may be summarized as
follows: When a ferromagnetic material is subjected to a magnetizing magnetic
field, it experiences a change in length. The fractional change in length,
over the
original length, of the material is referred to as magnetostriction and
denoted by the
symbol X. A positive signature is assigned to a, if an elongation occurs
parallel to
the magnetizing magnetic field.
When a ribbon of a material with a positive magnetostriction is subjected to
a sinusoidally varying external field, applied along its length, the ribbon
will
undergo periodic changes in length, i.e., the ribbon will be driven into
oscillations.
The external field may be generated, for example, by a solenoid carrying a
sinusoidally varying current. When the half-wave length of the oscillating
wave of
the ribbon matches the length of the ribbon, mechanical resonance results. The
resonance frequency f,. is given by the relation
fr = (1/2L)(E/D) -5,
where L is the ribbon length, E is the Young's modulus of the ribbon, and D is
the
density of the ribbon.
Magnetostrictive effects are observed in a ferromagnetic material only
when the magnetization of the material proceeds through magnetization
rotation.
No magnetostriction is observed when the magnetization process is through
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magnetic domain wall motion. Since the magnetic anisotropy of the marker of
the
alloy of the present invention is induced by field-annealing to be across the
marker
width direction, a dc magnetic field, referred to as bias field, applied along
the
marker length direction improves the efficiency of magneto-mechanical response
from the marker material. It is also well understood in the art that a bias
field
serves to change the effective value for E, the Young's modulus, in a
ferromagnetic material so that the mechanical resonance frequency of the
material
may be modified by a suitable choice of the bias field strength. The schematic
representation of Fig. 3 explains the situation further: The resonance
frequency, fr ,
decreases with the bias field, Hb, reaching a minimum,
(fr),,,;,,,at Hb2. The signal response, V, , detected , say at t = ti at the
receiving coil,
increases with Hb , reaching a maximum, Vm , at Hbl. The slope, dfr /dHb, near
the
operating bias field is an important quantity, since it related to the
sensitivity of the
surveillance system.
Summarizing the above, a ribbon of a positively magnetostrictive
ferromagnetic material, when exposed to a driving ac magnetic field in the
presence
of a dc bias field, will oscillate at the frequency of the driving ac field,
and when
this frequency coincides with the mechanical resonance frequency, fT, of the
material, the ribbon will resonate and provide increased response signal
amplitudes.
In practice, the bias field is provided by a ferromagnet with higher
coercivity than
the marker material present in the "marker package".
Table I lists typical values for Vm , Hbl, (fr ),,;,, and Hb2 for a
conventional
mechanical resonant marker based on glassy Feao Ni38 Moa B18. The low value of
Hb2, in conjunction with the existence of the nonlinear B-H behavior below
Hb2,
tends to cause a marker based on this alloy to accidentally trigger some of
the
harmonic marker systems, resulting in interference among article surveillance
systems based on mechanical resonance and harmonic re-radiance..
TABLE I
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Typical values for V. , Hbl ,(f, ),r,;,, and Hb2 for a conventional mechanical
resonant marker based on glassy Fe4o Ni38 Mo4 B18 . This ribbon at a length of
38.1 mm has mechanical resonance frequencies ranging from about 57 and 60 kHz.
V mV Hj Oe (f ; (kHz) Hb Oe
150-250 4-6 57-58 5-7
Table II lists typical values for H,, Vm, Hbt, (fr),,,;1, , Hb2 and df, /dHb
Hb for
the alloys outside the scope of this patent. Field-annealing was performed in
a
continuous reel-to-reel furnace on 12.7 mm wide ribbon where ribbon speed was
from about 0.6 m/min. to about 1.2 m/min.
TABLE II
Values for H,, Vm, Hb1, (fr),,,;,, , Hb2 and dfr /dHb taken at Hb = 6 Oe for
the
alloys outside the scope of this patent. Field-annealing was performed in a
continuous reel-to-reel furnace where ribbon speed was from about 0.6 m/min.
to
about 1.2 m/min and ribbon temperature was about 380 C. The annealing field
was about 1.4 kOe applied across the ribbon width.
Comoosition (at.%) H5 (oe) Vm(myl Flbi (Oel (f]min (kHz Hp2 (Oe) df t /dHb(I-
iz1Oe)
A. C042 Fe3s Mos Bn Sis 11 70 4.5 59 7.5 900
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Alloy A shows not only an unacceptable magnetomechanical resonance responses,
but contains a high level of molybdenum, resulting in increased raw material
costs
and reduced ribbon castability.
The following examples are presented to provide a more complete
understanding of the invention. The specific techniques, conditions,
materials,
proportions and reported data set forth to illustrate the principles and
practice of
the invention are exemplary and should not be construed as limiting the scope
of
the invention.
EXAMPLES
Example 1: Co-Fe-B-Si-C Metallic Glasses
1. Sample Preparation
Glassy metal alloys in the Co-Fe-B-Si-C series, designated as samples No.
1 to 8 in Table III and IV, were rapidly quenched from the melt following the
techniques taught by Narasimhan in U.S. Patent No. 4,142,57 i.
All casts were made in an inert
gas, using 100 g melts. The resulting ribbons, typically 25 m thick and about
12.7
mm wide, were determined to be free of significant crystallinity by x-ray
diffractometry using Cu-ICa radiation and differential scanning calorimetry.
Each
of the alloys was at least 70 % glassy and, in many instances, the alloys were
more
than 90 % glassy. Ribbons of these glassy metal alloys were strong, shiny,
hard
and ductile.
The ribbons were cut into small pieces for magnetization, magnetostriction,
Curie and crystallization temperature and density measurements. The ribbons
for
magneto-mechanical resonance characterization were cut to a length of about
38.1
mm and were heat treated with a magnetic field applied across the width of the
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ribbons. The strength of the magnetic field was 1.1 or 1.4 kOe and its
direction
was varied between 75 and 90 with respect to the ribbon length direction.
Some
of the ribbons were heat-treated under tension ranging from zero to about 7.2
kg/mm2 . The speed of the ribbon in the reel-to-reel annealing furnace was
changed from about 0.5 meter per minute to about 12 meter per minute.
2. Characterization of magnetic and thermal properties
Table III lists saturation induction (BS ), saturation magnetostriction (, ),
crystallization temperature (T. )of the alloys. Magnetization was measured by
a
vibrating sample magnetometer, giving the saturation magnetization value in
emu/g
which is converted to the saturation induction using density data. Saturation
magnetostriction was measured by a strain-gauge method, giving in 10-6 or in
ppm.
Curie and crystallization temperatures were measured by an inductance method
and
a differential scanning calorimetry, respectively.
TABLE III
Magnetic and thermal properties of Co-Fe-B-Si-C glassy alloys. Curie
temperatures of these alloys are above the crystallization temperatures and
are not
listed.
No. Composition (at."/o) B esla a.. (oym) T, ( C)
Co Fe B Si C
1 42 40 11 7 - 1.56 26 451
2 42 40 12 6 - 1.55 26 456
3 42 40 13 5 - 1.55 25 454
4 42 40 14 4 - 1.55 25 454
5 42 40 15 3 - 1.55 25 454
6 42 40 16 2 - 1.55 25 452
7 42 40 17 1 - 1.55 25 452
8 42 40 13 3 2 1.57 26 442
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Each marker material having a dimension of about 3 8.1 mmx 12.7mmx20 m
was tested by a conventional B-H loop tracer to measure the quantity H. and
then
was placed in a sensing coil with 221 turns. An ac magnetic field was applied
along the longitudinal direction of each alloy marker with a dc bias field
changing
from 0 to about 20 Oe. The sensing coil detected the magneto-mechanical
response of the alloy marker to the ac excitation. These marker materials
mechanically resonate between about 48 and 66 kHz. The quantities
characterizing
the magneto-mechanical response were measured and are listed in Table IV for
the
alloys listed in Table III.
TABLE IV
Values of H, , Vm , Hbi (f r),,;1, Hb2 and dfr /dHb taken at Hb = 6 Oe for
the alloys of Table III heat-treated at 375 C for 15 min in a magnetic field
of about
1.4 kOe applied perpendicular to the ribbon length direction (indicated by
asterisks). Alloys No. 1, 2 and 8 were field annealed in a reel-to-reel
annealing
furnace at 380 C with a ribbon speed of about 0.6 m/mimute with a magnetic
field
of about 1.4 kOe applied perpendicular to the ribbon direction.
Alloy No. H. (Oe) Vm (mlva H, ,(Oe i0 df. /dHb (Hz/Oe)
1 20 415 8.0 53.5 17.0 610
2 20 350 9.0 52.3 16.2 620
3* 21 330 7.5 50.8 18.5 470
4* 20 375 9.0 50.5 17.2 540
5* 21 320 8.5 51.3 18.7 420
6* 21 320 8.8 51.5 18.5 490
7* 20 330 8.5 51.0 18.2 550
8 20 325 9.0 54.8 17.0 550
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All the alloys listed in Table IV exhibit H. values exceeding 8 Oe, which
make them possible to avoid the interference problem mentioned above. Good
sensitivity ( dfT /dHb ) and large response signal ( V. ) result in smaller
markers for
resonant marker systems.
The quantities characterizing the magneto-mechanical resonance of the
marker material of Table III heat-treated under different annealing conditions
are
summarized in Table V.
TABLE V
Values of V. , HbI ,(f,),,,iõ , Hb2, dfi/dHb taken at Hb =6 Oe for Alloy No. 3
of Table III heat-treated under different conditions in a reel-to-reel
annealing
furnace. The annealing field direction was perpendicular to the ribbon length
direction.
Annealing Temperature: 320 C Applied Field: 1.1 kOe
Ribbon Speed Tension V, H. Sfc2~ Hu ~s /dHb
(m/minute) (kg/mmZ) (mV) (Oe) (kHz) (Oe) (Hz/Oe)
0.6 0 290 7.2 52.6 16.5 620
0.6 7.2 410 7.2 52.9 16.0 740
2.1 0 290 6.8 52.5 14 800
2.1 7.2 355 6.0 51.9 14 820
Annealing Temperature: 360 C Applied Field: 1.4 kOe
Ribbon Speed Tension Vm H (f ~ H~ ff r /b
(m/minute) (kg/mm2) (mV) (Oe) (kHz) (Oe) (Hz/Oe)
0.6 0 330 8.0 517 16.5 550
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0.6 2.1 390 7.9 52.5 16.5 620
0.6 7.2 410 7.4 52.2 16.3 620
Annealing Temperature: 440 C Applied Field: 1.1 kOe
Ribbon Speed Tension V, H Sf ~ Hb~ df ~/dHb
(m/minute) (kg/mm2) (mV) (Oe) (kHz) (Oe) (Hz/Oe)
9.1 0 410 6.0 51.5 14.0 900
9.1 1.4 440 6.4 51.6 13.0 780
6.1 0 340 6.4 51.3 14.8 830
6.1 1.4 460 6.3 51.6 13.0 750
3.0 0 320 6.0 51.8 14.6 780
3.0 1.4 430 6.0 51.9 13.7 840
The most noticeable effect is the increase of the signal amplitude when the
marker material is heat-treated under tension.
25 Example 2: Co-Fe-Ni-Mo/Cr/-B-Si-C Metallic Glasses
Glassy metal alloys in the Co-Fe-Ni-Mo/Cr/-B-Si-C system were prepared
and characterized as detailed under Example 1. Table VI lists chemical
compositions, magnetic and thermal properties and Table VII lists quantities
characterizing mechanical resonance responses of the alloys of Table VI.
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TABLE VI
Magnetic and thermal properties of low cobalt containing glassy alloys. T,
is the first crystallization temperature.
Allov No. Composition (at.%) B
Co Fe Ni Mo Cr B Si C (Tesla) (ppm) ( C)
1 41 40 - 1 - 13 5 - 1.51 24 463
2 41 38 - 3 - 13 5 - 1.34 20 467
3 41 40 - - 1 13 5 - 1.51 24 460
4 41 38 - - 3 13 5 - 1.37 20 463
5 40 40 2 - - 13 5 - 1.53 27 456
6 43 35 3 - - 13 4 2 1.50 19 468
7 40 38 4 - - 13 5 1.50 23 456
TABLE VII
Values of Ha , V,,, , Hbl ,( fr ),,,iõ , Hb2 and dfT /dHb taken at Hb = 6 Oe
for
the alloys listed in Table VI heat-treated at 380 C in a reel-to-reel
annealing
furnace with a ribbon speed of about 0.6 m/minute and an applied field of 1.4
kOe
applied perpendicular to the ribbon length direction.
Alloy No. H, (Oe) V m HbL (Oe) (f ~ Hz) Hk_,(Oe) dk /dHHb !Hz/Oe)
1 18 400 8.0 52.3 15.3 730
2 14 270 6.0 56.3 12.4 940
3 18 330 8.5 52.6 16.5 720
4 16 320 7.3 53.9 13.8 860
5 20 250 8.0 54.7 15.2 590
CA 02217722 1997-10-08
WO 96/32731 PCT/US96/05090
-18-
6 19 330 8.2 56.7 16.0 500
7 20 420 9.3 53.8 16.4 500
All the alloys listed in Table VII exhibit H. values exceeding 8 Oe, which
make them possible to avoid the interference problems mentioned above. Good
sensitivity ( dfr /dHb ) and large magneto-mechanical resonance response
signal (
V. ) result in smaller markers for resonant marker systems.
Having thus described the invention in rather full detail, it will be
understood that such detail need not be strictly adhered to but that further
changes
and modifications may suggest themselves to one skilled in the art, all
falling within
the scope of the invention as defined by the subjoined claims.
