Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-03-04
2001-10-09
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
Reexamination Certificate
active
06299702
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic amorphous alloys and to a method of annealing these alloys in a magnetic field. The present invention is also directed to amorphous magnetostrictive alloys for use in a magnetomechanical electronic article surveillance system. The present invention furthermore is directed to a magnetomechanical electronic article surveillance system employing such a marker, as well as to a method for making the amorphous magnetostrictive alloy and a method for making the marker.
2. Description of the Prior Art
It is well known from Chikazumi, Physics of Magnetism (Robert E. Krieger Publishing Company, Malbar, Fla.) chapter 17, p. 359 ff. (1964), for example, that most ferromagnetic alloys exhibit a uniaxial anisotropy when they are heat-treated in a magnetic field whereby the induced magnetic easy axis is parallel to the direction of the annealing field or, more generally, parallel to the domain magnetization during annealing. The aforementioned Chikazumi text gives an example for the magnetization curve of a permalloy (crystalline Fe-Ni alloy) sample measured in a direction perpendicular to the induced magnetic easy axis. Chikazumi notes that in this case the magnetization takes place through a rotation of each magnetic domain giving rise to a linearly ascending magnetization curve.
Luborsky et al., “Magnetic Annealing of Amorphous Alloys”, IEEE Trans. on Magnetics MAG-11, p. 1644-1649 (1975) give an early example for magnetic field annealing of amorphous alloys. They transversely field-annealed amorphous Fe
40
Ni
40
P
14
B
6
alloy strips in a magnetic field of 4 kOe which was oriented across the ribbon width, i.e. perpendicular to the ribbon axis and in the ribbon plane. After a 2 hrs. treatment at 325° C. and subsequent cooling of 50 deg/min and 0.1 deg/min, for example, they found a hysteresis loop with virtually vanishing remanence and linear dependence of the magnetization versus the applied field up to ferromagnetic saturation which occurs when the applied field equals or exceeds the induced anisotropy field. The authors attributed their observation to the fact that the magnetic field annealing induces a magnetic easy axis transverse to the ribbon direction and that upon applying a magnetic field the magnetization changes by rotation out of this easy axis.
Actually amorphous metals are particularly sensitive to magnetic field annealing owing to the absence of magneto-crystalline anisotropy as a consequence of their glassy non-periodic structure. Amorphous metals can be prepared in the form of thin ribbons by rapidly quenching from the melt which allows a wide range of compositions. Alloys for practical use are basically composed of Fe, Co and/or Ni with an addition of about 15-30 at % of Si and B (Ohnuma et al., “Low Coercivity and Zero Magnetostriction of Amorphous Fe—Co—Ni System Alloys” Phys. Status Solidi (a) vol. 44, pp. K151 (1977)) which is necessary for glass formation. The virtually unlimited miscibility of the transition metals in the amorphous state yields a large versatility of magnetic properties. According to Luborsky et al., “Magnetic Anneal Anisotropy in Amorphous Alloys”, IEEE Trans. on Magnetics MAG-13, p. 953-956 (1977) and Fujimori “Magnetic Anisotropy” in F. E. Luborsky (ed) Amorphous Metallic Alloys, Butterworths, London, pp. 300-316 (1983) alloy compositions with more than one metal species are particularly susceptible to the magnetic field anneal treatment. Thus, the magnitude of the induced anisotropy K
u
can be varied by choice of the alloy composition as well as by appropriate choice of the annealing temperature and time to range from a few J/m
3
up to about 1 kJ/m
3
. Accordingly the anisotropy field which is given by H
K
=2 K
u
/J
s
(cf. Luborsky et al., “Magnetic Annealing of Amorphous Alloys”, IEEE Trans. on Magnetics MAG-11, p. 1644-1649 (1975); J
s
is the saturation magnetization) and which, for a transversely field-annealed material, defines the field up to which the magnetization varies linearly with the applied field before reaching saturation, can be varied from values well below 1 Oe up to values of approximately H
k
≈25 Oe.
The linear characteristics of the hysteresis loop and the low eddy current losses both associated with transversely field-annealed amorphous alloys are useful in a variety of applications such as transformer cores, for example (cf. Herzer et al, “Recent Developments in Soft Magnetic Materials”, Physica Scripta vol T24, p 22-28 (1988)). Another field of application where transversely annealed amorphous alloys are particularly useful makes use of their magnetoelastic properties which is described in more detail in the following.
Becker et al., Ferromagnetismus (Springer, Berlin), ch. 5, pp. 336 (1939) or Bozorth, Ferromagnetism (d. van Nostrand Company, Princeton, N.J.) ch. 13, p 684 ff (1951) explain in their textbooks that the magnetostriction associated with rotation of the magnetization vector is responsible for the fact that in ferromagnetic materials Young's modulus changes with the applied magnetic field, which is usually referred to as the &Dgr;E effect.
Consequently U.S. Pat. No. 3,820,040and Berry et al. “Magnetic annealing and Directional Ordering of an Amorphous Ferromagnetic Alloy”, Physical Reviews Letters, vol. 34, p. 102-1025 (1975) realized that an amorphous Fe-based alloy, when transversely field annealed, exhibits a &Dgr;E effect two orders of magnitude larger than for crystalline iron. They attributed this striking difference to the lack of magnetocrystalline anisotropy in the amorphous alloy, which allows a much greater response to the applied stress by magnetization rotation. They also demonstrated that annealing in a longitudinal field largely suppresses the &Dgr;E effect since in this condition the domain orientations are not susceptible to stress-induced rotation. In the Berry 1975 et al. article it is recognized that the enhanced &Dgr;E effect in amorphous metals provides a useful means to achieve control of the vibrational frequency of an electromechanical oscillator with the help of an applied magnetic field.
The possibility to control the vibrational frequency by an applied magnetic field was found to be particularly useful in European Application 0 093 281 for markers for use in electronic article surveillance (EAS). The magnetic field for this purpose is produced by a magnetized ferromagnetic strip (bias magnet) disposed adjacent to the magnetoelastic resonator, with the strip and resonator being contained in a marker or tag housing. The change in effective magnetic permeability of the marker at the resonance frequency provides the marker with signal identity. This signal identity can be removed by-chanding the resonant frequency by means of the applied field. Thus the marker can, for example, be deactivated by degaussing the bias magnet, which removes the applied magnetic field, and thus changes the resonant frequency appreciably. Such systems originally (cf. European Application 0 093 281, and Application PCT WO 90/03652) used markers made of amorphous ribbons in the “as prepared” state which also can exhibit an appreciable &Dgr;E effect owing to uniaxial anisotropies associated with production-inherent mechanical stresses.
U.S. Pat. No. 5,469,140 discloses that the application of transverse field annealed amorphous magnetomechanical elements in electronic article surveillance systems removes a number of deficiencies associated with the markers of the prior art which use as prepared amorphous material. In an example, this patent describes a linear behavior of the hysteresis loop up to an applied field of at least about 10 Oe. This linear behavior associated with the transverse field annealing avoids the generation of harmonics which can produce undesirable alarms in other types of EAS systems (i.e., harmonic systems). Such interference with harmonic systems actually is a severe problem with the aforementioned magneto-elastic markers of the prior art, due the non-linear hysteresis loo
Schiff & Hardin & Waite
Sheehan John
Vacuumschmelze GmbH
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