Magneto-elastically excitable tag having a reliably deactivatable amorphous alloy for use in a mechanical resonance monitoring system

Amorphous alloys having the formula EQU Fe.sub.a Co.sub.b Ni.sub.c Si.sub.x B.sub.y M.sub.z are employed as monitoring strips for mechanically oscillating tags, for example for anti-theft protection, together with a source of a pre-magnetization field in which the strip is disposed so as to place the strip in an activated state. In the formula, M denotes one or more elements of groups IV through VII of the periodic table, including C, Ge and P, and the constituents in at % meet the following conditions: a lies between 20 and 74, b lies between 4 and 23, c lies between 5 and 50, with the criterion that b+c>14, x lies between 0 and 10, y lies between 10 and 20, and z lies between 0 and 5 with the sum x+y+z being between 12 and 21. These alloys have a resonant frequency associated therewith and when passed through an alternating field whose alternation frequency coincides with the resonant frequency, a pulse having a signal amplitude is produced. These alloys can be deactivated by removing the pre-magnetization field, which causes a change in the resonant frequency and the resulting signal amplitude. These alloys exhibit a change in resonant frequency and signal amplitude due to changes in the orientation of the tag in the earth's magnetic field which is smaller than the change occurring upon removal of the pre-magnetization field, so that the tag can be reliably deactivated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to the employment of an amorphous, 
magnetostrictive alloy in monitoring or identification systems for 
producing magneto-elastic tags that can be deactivated by removing a 
pre-magnetization field. 
2. Description of the Prior Art 
Magneto-elastically excitable tags usually contain monitoring strips that 
are composed of an amorphous alloy with high magnetostriction. PCT 
Application WO 90/03652 discloses the employment of amorphous alloys 
containing nickel in addition to iron for monitoring systems with 
mechanical resonance. 
Alloys with magnetostrictive properties can be employed, for example, in 
identification systems for tags. The magnetostriction is exploited in 
order to place a strip of this alloy into oscillation by means of an 
alternating field acting on this strip. When the alternating field is 
deactivated, the strip, which continues to oscillate, generates magnetic 
field changes that a pick-up coil converts into induced voltage pulses. An 
evaluation of these voltage pulses reveals whether a strip of the 
oscillating material having a specific strip length is contained in the 
field. 
An item can thus be identified by applying a strip of a particular length 
thereto, the length serving as an identifier, or an anti-theft security 
system can also be based on this principle, whereby only magnetized strips 
of a specific length are attached to the goods and the presence of such a 
strip is detected by the coil system for field excitation and for pick-up 
of the magnetic oscillations after every excitation time span. 
When a strip of amorphous magnetostrictive material is exposed to a 
magnetic field, then the magnetostriction causes a change in the length of 
the strip. This dependency, however, is not linear but is dependent on the 
dimensions of the strip and on the size of the magnetic field. When the 
magnetic field is boosted in equal steps given a specific strip, then one 
finds that only small changes in length initially occur, then the changes 
in length become greater with increasing steps of the magnetization boost, 
and then no further change in length ensues upon the occurrence of 
saturation, despite a magnetic field that continues to be boosted in 
steps. 
The effect of this property is that such a strip can be excited to 
mechanical oscillations when it is exposed to a pre-magnetization field 
whose size results in a great change in length given a uniform change of 
the magnetic field. A further effect of the change in length ensuing due 
to the magnetic field is that the length of the strip changes in this 
region without a tensile stress acting on the strip. 
The modulus of elasticity of the material is the determining factor for the 
resonant frequency of the oscillation given mechanical oscillation of a 
strip. The force required for a specific change in length becomes greater 
and the resonant frequency of the oscillating strip becomes higher, as the 
modulus of elasticity increases. An additional change in length, however, 
ensues due to the influence of the magnetic field without a force being 
necessary. The material thus acts as though it had a lower mechanical 
modulus of elasticity than it really has. 
The result is that the resonant frequency given excitation by an 
alternating magnetic field becomes lower with increasing pre-magnetization 
than it is without pre-magnetization. A strip that oscillates at a 
specific resonant frequency with high signal amplitude with a given 
pre-magnetization will oscillate substantially less given excitation with 
the same frequency when the pre-magnetization field is removed, because 
the resonant frequency is thereby boosted and the exciting frequency and 
the resonant frequency no longer coincide. 
The removal of the pre-magnetization field also results in a change of the 
magnetic field now only results in a relatively slight change in the 
length of the strip, so that the signal height also significantly 
decreases without pre-magnetization field. 
Together, the two factors cause a mechanical oscillation of the strip to be 
suppressed upon removal of the pre-magnetization field. It is thus 
possible to deactivate an anti-theft security strip composed of this 
material by removing the pre-magnetization field. 
This is achieved in the case of anti-theft security strips by, for example, 
a demagnetizing magnet connected to the strip. In other systems wherein 
the pre-magnetization field is in part generated by a coil in the 
examination area, the oscillation can be suppressed by turning off this 
pre-magnetization field. 
Despite these relationships, a reliable deactivation within the same time 
required for a reliable response cannot always be achieved with known 
pre-magnetization techniques. The reason for this is that the earth's 
magnetic field also acts on the monitoring strip in addition to the 
pre-magnetization field, and build-up of tolerances that influence the 
resonant frequency must be taken into account given mass production of 
monitoring strips. 
When the monitoring strip is rotated in the earth's magnetic field, this 
will increase the pre-magnetization at one end region or strip-half and 
decrease it in the other. This results in a natural fluctuation of the 
resonant frequency. The monitoring apparatus, however, must then be set 
such that these fluctuations of the resonant frequency do not lead to the 
failure to generate an alarm under proper circumstances, while still 
insuring that the resonant frequency changes to such an extent upon 
removal of the pre-magnetization field is certain that an alarm can no 
longer be triggered. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to specify an alloy for employment 
as monitoring strip in monitoring or identification systems that responds 
reliably under true alarm-causing conditions but which can be reliably 
deactivated. This requires an alloy which exhibits only a relatively 
slight change in its resonant frequency caused by superimposing the 
earth's magnetic field on the pre-magnetization field, but exhibits a 
considerable change in its resonant frequency when the pre-magnetization 
field is removed. 
The above object is achieved in accordance with the principles of the 
present invention in an amorphous alloy having the formula Fe.sub.a 
Co.sub.b Ni.sub.c Si.sub.x B.sub.y M.sub.z employed as monitoring strips 
for mechanically oscillating tags, for example for anti-theft protection, 
together with a source of a pre-magnetization field in which the strip is 
disposed so as to place the strip in an activated state. In the formula, M 
denotes one or more elements of groups IV through VII of the periodic 
table, including C, Ge and P, and the constituents in at % meet the 
following conditions: a lies between 20 and 74, b lies between 4 and 23, c 
lies between 5 and 50, with the criterion that b+c&gt;14, x lies between 0 
and 10, y lies between 10 and 20, and z&lt;5 with the sum x+y+z being between 
12 and 21. These alloys have a resonant frequency associated therewith and 
when passed through an alternating field whose alternation frequency 
coincides with the resonant frequency, a pulse having a signal amplitude 
is produced. These alloys can be deactivated by removing the 
pre-magnetization field, which causes a change in the resonant frequency 
and the resulting signal amplitude. These alloys exhibit a change in 
resonant frequency and signal amplitude due to changes in the orientation 
of the tag in the earth's magnetic field which is smaller than the change 
occurring upon removal of the pre-magnetization field, so that the tag can 
be reliably deactivated. 
In addition, the invention achieves other objects that are important, in 
particular, for monitoring systems for anti-theft protection. In an 
anti-theft monitoring system that is based on mechanical resonance, the 
monitoring strip is deactivated by demagnetizing a magnet attached to the 
strip. Monitoring strips which are thus connected to the goods, however, 
could undesirably trigger a false alarm in a monitoring zone operating 
according to the harmonics method. In such monitoring systems, monitoring 
strips are discovered in an examination field by evaluating the harmonics 
of the exciting alternating field that they generate. 
It is important that goods with magneto-elastically excitable tags that 
have already been deactivated do not trigger an alarm. This is achieved 
with the inventive tags by setting a flat magnetization loop without 
remanence discontinuities by a thermal treatment in a transverse magnetic 
field,

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Investigations of known amorphous alloys that are used for tags which 
resonate in a magnetic field were undertaken. The following were measured: 
the change of the resonant frequency as a consequence of the earth's 
magnetic field .DELTA.fr(H) in kHz given a change in the position of the 
monitoring strip; the frequency change fr(0)-fr(H) in kHz that arises when 
the resonant frequency is measured with and then without pre-magnetization 
field; the signal voltage U in mV that is a criterion for the amplitude of 
the signal to be evaluated; and the decay time t.sub.R in ms, i.e. the 
time that passes after excitation of a monitoring strip of the alloy with 
resonant frequency until the signal induced by the mechanical oscillation 
has decayed to one-tenth of the original value. A field strength in the 
range H=400-800 A/m, as is also typically used in this context, was 
employed for the pre-magnetizing magnetic field in the exemplary 
embodiments. The positional change of the monitoring strip in the earth's 
magnetic field was simulated by a reduction of or boost in this 
preomagnetizing field by 40 A/m (amount of the earth's field strength). 
Further, a consecutive test number (No.) for the individual exemplary 
alloys and the cobalt content in weight % are recited in the table. 
The following Table 1 contains values for two different, known alloys. The 
alloy under tests Nos. 1 and 2 is known from the aforementioned PCT 
Application WO 90/03652, whereas the alloy under test No.3 is mentioned in 
German Utility Model 9412456. The tests with respect to Nos. 1 and 2 were 
undertaken with the same alloy. The alloy in the manufactured state was 
investigated as test No. 1 and the alloy under Nos. 2 and 3 was 
investigated after a thermal treatment with which a linear, flat loop was 
set. 
TABLE 1 
__________________________________________________________________________ 
Co .DELTA.fr(H) 
fr(0)-fr(H) 
U t.sub.R 
No. 
Composition (at %) 
Wt % 
(kHz) 
(kHz) (mV) 
(ms) 
__________________________________________________________________________ 
1 Fe-40 Ni-38-Mo-4 B-18 
-- 0.34-0.72 
1.94-3.51 
150 
4 
2 Fe-40 Ni-38-Mo4 B-18 
-- 1.04-1.65 
6.55-6.84 
20 1.3 
3 Fe-39.5 Co-39.5 Si-6 B-15 
47.9 
0.79-1.44 
2.43-4.92 
220 
5.2 
__________________________________________________________________________ 
One can see that test No.1 has a variation of the resonant frequency due to 
the earth's magnetic field .DELTA.fr below 1 kHz dependent on the specimen 
investigated, whereas the change in the resonant frequency given removal 
or addition of a pre-magnetization field (fr(0)-fr(H)) exhibits a change 
in the resonant frequency of more than 1.94. The signal amplitude at 150 
mV, and the oscillation duration also suffice for utilization in 
monitoring systems with mechanically oscillating tags. 
The disadvantage of this alloy, however, is that discontinuous changes in 
remanence occur in the magnetization loop, which can trigger a false alarm 
when a security tag having such a monitoring strip is conducted through a 
monitoring field of an anti-theft security system that exploits the 
harmonics generated by an alternating field for detection. 
The values recited under No. 2 arise with the same alloy after a thermal 
treatment for achieving a flat, linear loop. One can see that the 
dependency on the earth's magnetic field has become significantly greater 
since the fluctuations in the resonant frequency given a change in 
position of the strip lie above 1 kHz. Even though the separation from the 
change in the resonant frequency given removal of the pre-magnetization 
field, at over 6 kHz, is adequate such a band is not very well suited for 
mechanically oscillating tags--especially because of the low signal 
amplitude of 20 mV and the short decay time of the oscillating strip. 
Like the alloy under No. 1, the alloy under No. 3 of Table 1 again exhibits 
an adequately low fluctuation of the resonant frequency dependent on the 
earth's magnetic field and also lies in a good usable range in terms of 
the other values, namely the change in the resonant frequency given 
removal of the pre-magnetization field signal voltage and the decay time. 
The replacement of nickel with cobalt, however, has resulted in a cobalt 
content of 47.9 by weight percent in the alloy, so that this alloy 
presents economic disadvantages for mass employment because of the 
relatively high price of cobalt. 
It was inventively recognized that amorphous alloys that contain iron as 
well as cobalt and nickel and whose metalloid part (Si, B) does not exceed 
certain values can have the following properties: 
1. a high signal amplitude and a long lasting signal after the exciting 
field is turned off; 
2. a linear, flat characteristic of the magnetization loop in order to 
avoid false alarms in other security systems; 
3. a low dependency of the resonant frequency on the pre-magnetizing field 
strength (earth's field); 
4. a reliable deactivation of the mechanical oscillator upon removal of the 
pre-magnetizing field due to adequate change of the resonant frequency and 
signal amplitude; 
5. low raw material costs due to an optimally low Co content; 
6. a ductility after thermal treatment that allows a bending of the strip 
to a diameter of less than 2 mm without the signal amplitude being 
subsequently significantly deteriorated. 
Examples of inventive alloys are recited below in Table 2 under Nos. 4 
through 26: 
TABLE 2 
__________________________________________________________________________ 
Co .DELTA.fr(H) 
fr(0)-fr(H) 
U t.sub.R 
No. 
Composition (at %) 
Wt. % 
(kHz) 
(kHz) 
(mV) 
(ms) 
__________________________________________________________________________ 
4 Fe-49 Co-6 Ni-27 Si-2 B-16 
7.2 0.78-1.39 
2.41-5.16 
151 
5.3 
5 Fe-47 Co-10 Ni-25 Si-2 B-16 
12.0 
0.69-1.22 
2.11-4.52 
187 
6.0 
6 Fe-41 Co-10 Ni-31 Si-2 B-16 
12.0 
0.51-0.93 
1.54-3.31 
137 
6.2 
7 Fe-36 Co-10 Ni-36 Si-2 B-16 
11.9 
0.58-1.05 
1.82-3.88 
172 
6.6 
8 Fe-31 Co-10 Ni-41 Si-2 B-16 
13.7 
0.65-1.17 
2.01-4.31 
185 
5.6 
9 Fe-51.5 Co-11.5 Ni-20 Si-1 B-16 
13.7 
0.56-1.02 
1.72-3.70 
157 
6.0 
10 Fe-41.5 Co-11.5 Ni-30 Si-1 B-16 
13.7 
0.41-0.74 
1.25-2.69 
129 
10.2 
11 Fe-49.5 Co-13 Ni-20 Si-1.5 B-16 
15.6 
0.53-0.94 
1.57-3.41 
173 
9.2 
12 Fe-44.5 Co-13 Ni-25 Si-1.5 B-16 
15.5 
0.47-0.83 
1.39-3.01 
158 
7.9 
13 Fe-39.5 Co-13 Ni-30 Si-1.5 B-16 
15.5 
0.41-0.74 
1.23-2.66 
139 
8.5 
14 Fe-34.5 Co-13 Bi-35 Si-1.5 B-16 
15.4 
0.38-0.71 
1.15-2.43 
120 
9.9 
15 Fe-29.5 Co-13 Ni-40 Si-1.5 B-16 
15.4 
0.45-0.80 
1.36-2.93 
141 
7.4 
16 Fe-28 Co-13 Ni-41 Si-2 B-16 
15.4 
0.59-1.06 
1.79-3.86 
167 
6.4 
17 Fe-44 Co-16 Ni-22 Si-2 B-16 
19.2 
0.39-0.72 
1.16-2.51 
145 
9.9 
18 Fe-40 Co-16 Ni-26 Si-2 B-16 
19.1 
0.44-0.80 
1.34-2.89 
160 
9.7 
19 Fe-28 Co-16 Ni-38 Si-2 B-16 
19.0 
0.45-0.81 
1.32-2.87 
139 
7.3 
20 Fe-40.5 Co-20.5 Ni-20 Si-3 B-16 
24.6 
0.38-0.71 
1.12-2.46 
147 
10.3 
21 Fe-51 Co-21 Ni-10 Si-2 B-16 
25.2 
0.58-1.03 
1.75-3.75 
203 
7.7 
22 Fe-46 Co-21 Ni-15 Si-2 B-16 
25.2 
0.43-0.78 
1.30-2.83 
123 
7.1 
23 Fe-43 Co-21 Ni-18 Si-2 B-16 
25.1 
0.37-0.68 
1.10-2.38 
115 
9.0 
24 Fe-41 Co-21 Ni-20 Si-2 B-16 
25.1 
0.31-0.59 
0.93-2.00 
100 
11.9 
25 Fe-41 Co-21 Ni-20 Si-1 B-17 
25.2 
0.31-0.56 
0.89-1.94 
108 
12.1 
26 Fe-40.6 Co-21 Ni-20 Si-2.5 B-16 
26.2 
0.36-0.67 
1.13-2.40 
103 
9.8 
__________________________________________________________________________ 
The exemplary alloys were subjected to a thermal treatment in a magnetic 
transverse field for setting the flat, linear loop. Typical annealing 
temperatures were from 280.degree. through 440.degree. C. The annealing 
times were in the range from a few seconds through several hours. The 
exact thermal treatment causes a typical range of variation for the 
quantities .DELTA.fr(H) and fr(H)-fr(0) that is indicated in the above 
table. The investigations were implemented for strips having a length of 
40 mm. The typical resonant frequencies were in the range of 50-60 kHz. 
All of the cited examples have a linear loop, a high signal amplitude above 
100 mV, a decay time of a few ms, and a frequency scatter .DELTA.fr&lt;1 kHz 
as well as an adequately high frequency change of fr(0)-fr(H)&gt;1 kHz after 
demagnetization can be achieved. After thermal treatment, further, the 
alloys have the ductile behavior necessary for further processing. 
Examples 4-17 are especially advantageous since they have a Co part 
clearly below 20% by weight and thus achieve the desired properties with a 
low cost of raw materials. 
For illustration, the following Table 3 shows a few compositions that do 
not achieve the object of the invention: 
TABLE 3 
__________________________________________________________________________ 
Co .DELTA.fr(H) 
fr(0)-fr(H) 
U t.sub.R 
No. 
Composition (at %) 
Wt. % 
(kHz) 
(kHz) (mV) 
(ms) 
__________________________________________________________________________ 
27 Fe-41.4 Ni-41.5 Si-1 B-16 
-- 1.38-2.33 
4.63-9.68 
144 
2.4 
28 Fe-52 Ni-30 Si-2 B-16 
-- 1.25-2.19 
4.14-8.72 
192 
2.9 
29 Fe-57 Ni-25 Si-2 B-16 
-- 1.49-2.56 
5.04-10.52 
203 
2.8 
30 Fe-65 Co-18 Si-1 B-16 
21.7 
1.42-2.39 
4.72-9.82 
178 
2.8 
31 Fe-61.5 Co-21.5 Si-1 B-16 
25.8 
0.98-1.76 
3.12-6.64 
195 
3.9 
32 Fe-47.4 Co-31.6 Si-2 B-19 
39.0 
1.01-1.75 
3.31-6.93 
197 
4.1 
33 Fe-24 Co-55 Si-6 B-15 
66.0 
0.47-0.84 
1.45-3.12 
197 
7.9 
34 Fe-29 Co-27 Ni-27S Si-1 B-16 
31.8 
0.10-0.19 
0.26-0.58 
37 17.1 
35 Fe-27 Co-27 Ni-27 Si-3 B-16 
32.2 
0.16-0.31 
0.46-1.02 
65 23.7 
__________________________________________________________________________ 
Nos. 27-32 have a Co content that is clearly reduced compared to the prior 
art (No. 3) but usually exhibit a frequency scatter .DELTA.fr(H) clearly 
above 1 kHz. 
No. 33 in fact exhibits a slight frequency scatter, but this was only 
possible by increasing the Co content compared to the prior art. The 
increased costs of the alloy are accordingly disadvantageous. 
Examples 34 and 35 comprise a lower Co content and less frequency scatter 
compared to the prior art (No. 3), however, the frequency change after 
demagnetization is too small (&lt;1 kHz). The low signal amplitude is also 
disadvantageous. 
Alloys are thus especially suitable that, for example due to a thermal 
treatment, exhibit a .DELTA.fr of less than 1 kHz, preferably less than 
0.8 kHz, and a change of the resonant frequency fr(0)-fr(H)&gt;1 kHz, 
preferably 1.2 kHz. The thermal treatment for setting a flat loop 
preferably occurs at a temperature from 250.degree. through 450.degree. C. 
for a time from 2 through 60 seconds. The short treatment time makes it 
possible for the ribbon to be thermally treated in throughput production 
with the use of a throughput furnace before being wound into reels. 
Following the thermal treatment in the throughput furnace, the ribbon can 
be immediately cut to the desired length for the monitoring strip. 
Typical dimensions of the strip for the intended application are a length 
of 30 through 50 mm, a width of 0.5 through 25 mm and a thickness of the 
amorphous ribbon in the range from 15 through 40 .mu.m. The decay time 
should be longer than 3 msec, and the pre-magnetization field typically 
lies in a range from 400 through 800 A/m. Advantageous resonant 
frequencies lie in a range from 50 to 60 kHz. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventor to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of his contribution to the art.