METHOD OF ACHIEVING A CONTROLLED STEP CHANGE IN THE MAGNETIZATION LOOP OF AMORPHOUS ALLOYS

Abstract: A magnetic theft detection system includes a glassy metal alloy strip having a value of magnetostriction near zero. The strip has been annealed to produce a step change in the magnetization versus applied field behavior (B-H loop) thereof, and has a composition consisting essentially of the formula: (Co Fe)100-x (Si B)x where 20 </= x </= 23 and 15.4 </= Co/Fe </= 15.9 and 7.5 </= B/Si </= 9. Annealing of the metal alloy strip in an oxidizing atmosphere causes the formation of a surface oxide followed by a distinctive crystalline Co-layer with thickness in the range of 1 to 2 mu m. The thickness of the crystalline Co-layer determines the value of the threshold magnetic field and is controlled by the annealing condition and the as cast surface chemistry and structure.

Description: METHOD OF ACHIEVING A CONTROLLED STEP CHANGE IN THE MAGNETIZAΗON LOOP OF AMORPHOUS ALLOYS BACKGROUND OF THE INVENTION Field Of The Invention: This invention relates to a deactivatable electronic article surveillance system marker having a step change in the magnetic flux thereof, and more particularly to a model for the physical mechanism ofthe marker, and the processing conditions and chemistry required to create a controlled step change in the markers' magnetization behavior. 2. Description Of The Prior Art: Electronic article surveillance (EAS) systems in which magnetic markers detect the presence of articles within an interrogation zone are well known in the art. These systems utilize soft magnetic materials having low coercivity (Hc), low magnetocrystalline anisotropy(K), low magnetostriction (λ) and high permeability (μ), to induce a signal high in harmonic content in the presence of an applied magnetic field. Unique harmonics, reradiated by these materials, commends their use as magnetic markers to identify objects under surveillance, as disclosed in US patent 4,298,862. Harmonic tags have been developed that are composed of materials having a "Perminvar" type loop, as described in US patents 4,823,113 and 4,938,267. When these soft magnetic alloys are annealed below the Curie temperature(Tc) in a demagnetized state, the domain walls ofthe alloys induce their own local anisotropy. This local anisotropy tends to stabilize the position ofthe walls (wall pinning). Due to this wall pinning phenomenon there is inertia in the response of the magnetic material to an applied field until a certain "threshold" field Ht is reached. For H≥ Ht the walls move abruptly from their pinning state giving rise to a shaφ step in the magnetic flux . The presence of a step in the B-H loop, which characterizes the magnetization behavior ofthe marker, ensures a very unique detection signal rich in harmonic content. Markers of this type are described in US patent 4,980,670. Several other patents are directed to harmonic EAS markers (see, for example, US patent 4,298,862) and specifically to harmonic markers having a step change (see, for example, US patents 4,298,862; 4,823,113; 4,980,670; 4,938,267; 5,313,192; and 5,029,291). In these patents, substantial emphasis was placed on the detection system and the post processing (annealing) ofthe amoφhous alloy in order to achieve the desired property, namely the step change in the magnetization behavior. One ofthe problems with the annealing ofthese markers under a given set of conditions is the difficulty of consistently reproducing the targeted step change value ofthe threshold magnetic field . The inconsistency with which the step change is produced prevents accurate identification ofthe markers and reduces the yield rate of markers appointed for detection. There remains a need in the art for an improved harmonic EAS tag which is composed of a Co-Fe-B-Si alloy and which, owing to the casting conditions and chemistry requirements attending its manufacture, provides a reproducible step change in the magnetization behavior thereof upon post processing (annealing). Also needed is an improved method for annealing the marker to alter its material structure and thereby optimize the response thereof to the magnetic field applied within the interrogation zone in a reproducible way. SUMMARY OF THE INVENTION The present invention provides an EAS system marker and method for its manufacture. Generally stated, the marker comprises a strip of ferromagnetic metal that has amoφhous structure, and is composed of a Co-Fe-B-Si alloy which can be annealed to produce a step change in the magnetization flux (B). Upon being annealed, the ferromagnetic metal is especially suited for use as a harmonic marker in an antitheft detection system. More specifically, there is provided in accordance with the invention a unique correlation between the composition of a ferromagnetic metal within a near zero magnetostrictive Co-Fe-B-Si system and the annealing conditions required to achieve a step change in the magnetization behavior thereof at a threshold field Ht. The zero magnetostrictive metal has a composition consisting essentially ofthe formula: (Co Fe)*oo- x (Si B) *. where 20 ≤x≤23 and 7.5≤ B/Si≤ 9. The Co Fe ratio, which determines" the magnetostriction value is in the range of 15.4≤Co/Fe≤ 15.9 for the magnetostriction to be near zero. One example of a composition within the present invention is: Co 73.7 Fe .7 S12.5 B -9.1. The annealing time at a given temperature required to achieve a threshold field of a given value, depends upon the total Boron plus Silicon content. The invention further provides a unique correlation between the surface chemistry and structure ofthe annealed sample and the value ofthe threshold magnetic field. Annealing ofthe Co-Fe-B-Si alloy at temperatures in the range of 400-430 °C for 10 to 30 min. causes crystallization ofthe surface. This crystallization is driven by the diffusion ofthe B and Si into the surface where oxidation takes place. The immediate area underneath the surface oxides is depleted of B and Si and rich in Co and Fe. The remaining Co and Fe metals crystallize and form a layer of hard magnetic material ofthe order of 1 to 3 μm. This hard magnetic layer on top ofthe soft magnetic bulk alloy of Co-Fe- B-Si causes the domain wall pinning and the formation ofthe step in the magnetization B-H loop. The thickness ofthe crystalline Co-layer correlates with the value ofthe threshold magnetic field, H-. The present invention also requires a certain solidification rate ofthe alloy, which is necessary in order for the diffusion/oxidation and surface crystallization to take place. The ribbon exit temperature is a relative measure ofthe solidification rate ofthe alloy. The as cast surface composition is determined by the casting atmosphere and the ribbon exit temperature. The surface composition preferable for the formation of a distinct Co-layer by annealing consists of Boron and Silicon oxides. Each ofthese oxides is achieved for ribbon exit temperatures higher than 280 °C . For lower temperatures primarily Co-oxide or Fe-oxides are formed which prevent the formation ofthe crystalline Co-layer by postprocessing. BRIEF DESCRIPTION OF T F. IϊRAWTNGS The invention will be more fully understood and further advantages will become apparent when reference is had to the following detailed description and the accompanying drawings, in which: Figure 1 depicts a typical magnetization B-H loop, where B is the flux density and H is the applied magnetic field, of a soft Co-Fe-B-Si amoφhous alloy that has been annealed according to the teachings of US Patent 5,313,192 in order to cause domain wall pinning and generate the charaαeristic step change at the threshold magnetic field Ht; Figure 2 is a graph illustrating the effect ofthe total Boron plus Silicon content of a Co-Fe-B-Si alloy on the annealing time required to achieve coercivity He equal to 1 Oe at a temperature of 425 °C and frequency of 1 kHz; Figure 3 depicts the Auger depth profile ofthe top surface of a Co-Fe-B-Si sample annealed to achieve a threshold magnetic field of Ht ι equal to 0.85 Oe; Figure 4 depicts the factor analysis ofthe Co- signal from the ribbon top surface Auger spectrum and demonstrates the presence ofthe distinctive metallic Co-layer; Figure 5 depicts a transmission electron microscopy picture ofthe crystallized Co-metal layer on the top surface ofthe annealed Co-Fe-B-Si amoφhous alloy; Figure 6 depicts the Auger depth profile ofthe top surface of a Co-Fe-B-Si sample annealed to achieve a threshold magnetic field Hα equal to 0.7 Oe, which is less than the threshold field (Hi) ofthe sample shown in Figure 3; Figure 7 depicts the Auger depth profile ofthe bottom surface of a Co-Fe- B-Si sample annealed in accordance with the method described in US Patent 5,313,192; Figure 8a illustrates a schematic diagram ofthe crossection ofthe as cast Co-Fe-B-Si amoφhous alloy; Figure 8b illustrates a schematic diagram ofthe crossection ofthe annealed Co-Fe-B-Si amoφhous alloy, the top surface (40) ofthe ribbon consisting of an oxide layer followed by a Co crystalline layer on top ofthe amoφhous bulk alloy; Figure 9a is a schematic diagram ofthe as cast top ribbon surface where area 30 is the edge and area 32 is the middle ofthe top surface; Figure 9b is a schematic diagram ofthe annealed top ribbon surface showing that after the annealing at 410 ° C for 30 min in air, the edge (30) is light gray colored and the middle (32) is dark gray colored; Figure 9c depicts a crossection ofthe middle (32) ofthe annealed top surface showing the oxide layer T 3 followed by the Co crystalline layer T on top of the bulk amoφhous alloy (33); Figure 9d depicts a crossection ofthe edge (30) ofthe annealed top surface illustrating only the oxide layer T 3 followed by the amoφhous bulk alloy (33); Figure 9e depicts the X-ray photoemission (XPS) histograms ofthe edge (30) and the middle (32) ofthe as cast top surface, and the edge and the middle of the as cast bottom surface ofthe ribbon; Figure 9f depicts the XPS histograms ofthe edge (30) and the middle (32) ofthe annealed top surface, and the edge and the middle ofthe annealed bottom surface ofthe ribbon; and Figure 10 depicts a thermal profile ofthe top surface of Co-Fe-B-Si amoφhous alloy as it exits the chilling surface ofthe spinning casting wheel, demonstrating that the edges are colder than the middle part ofthe ribbon. DETAILED DESCRIPΗON OF THE INVENTION Referring to the drawings, in Figure 1 there is depicted a magnetization B- H loop for a Co-Fe-B-Si marker with the characteristic step change in the magnetic flux at the threshold field Ht. The marker consists of a piece of ribbon with the dimensions of 38. lmmX3.2mmX 20μm and is annealed according to the teaching of US Patent 5,313, 192. The near zero magnetostrictive material for the marker has a composition consisting essentially ofthe formula (Co Fe)*oo. x (Si B) x , where 20 ≤x≤23 and 7.5≤ B/Si≤ 9. The Co/Fe ratio, which determines the magnetostriction value is in the range of ISA≤Co/Fe≤ 15.9 for the magnetostriction to be near zero. Representative examples of compositions within the formula are: Co 73.7 Fe^ Sizs B -9.*; Co 7 .7 Fe 4 .tSizo B n. s ; Co 73.7 Fβ4.8 Si2.5 B 19.0; and Co 73.5 Fe .βSi 2 .2 B 19.1 The metallic alloys ofthe present invention are produced generally by cooling a melt at a rate of at least about 10 s to 10 6° C/s. A variety of techniques are available for fabricating amoφhous metallic alloys within the scope ofthe invention such as, for example, spray depositing onto a chilled substrate, jet casting, planar fiow casting, etc. Typically, the particular composition is selected, powders or granules ofthe requisite elements (or of materials that decompose to form the elements, such as cobalt-boron, cobalt-silicon, etc.) in the desired proportions are then melted and homogenized, and the molten alloy is thereafter supplied to a chill surface, capable of quenching the alloys at a rate of at least about 10 s to 10 °C/s. The most preferred process for fabricating continuous metallic strip composed ofthe alloys ofthe invention is the process known as planar flow casting, set forth in U.S. Patent 4,142,571, to Narasimhan, assigned to AlliedSignal Inc., which is incoφorated herein by reference thereto. The planar flow casting process comprises the steps of (a) moving the surface of a chill body in a longitudinal direction at a predetermined velocity offrom about 100 to about 2000 meters per minute past the orifice of a nozzle defined by a pair of generally parallel lips delimiting a slotted opening located proximate to the surface ofthe chill body such that the gap between the lips and the surface changes from about 0.03 to about 1 mm, the orifice being arranged generally peφendicular to the direction of movement ofthe chill body, and (b) forcing a stream of molten alloy through the orifice ofthe nozzle into contact with the surface ofthe moving chill body to permit the alloy to solidify thereon to form a continuous strip. Preferably, the nozzle slot has a width offrom about 0.3 to 1 mm, the first lip has a width at least equal to the width ofthe slot and the second lip has a width offrom about 1.5 to 3 times the width ofthe slot. Metallic strip produced in accordance with the Narasimhan process can have widths ranging from 7 mm, or less, to 150 to 200 mm, or more. Amoφhous metallic strip composed of alloys ofthe present invention is generally about 0.020 mm thick, but the planar flow casting process described in U.S. Patent 4,142,571 is capable of producing amoφhous metallic strip ranging from less than 0.020 mm in thickness to about 0.14 mm or more, depending on the composition, melting point, solidification and crystallization characteristics of the alloy employed. The magnetic properties of alloys cast to a metastable state using the methods described hereinabove generally improve with increased volume percent of amoφhous phase. However, the alloys ofthe present invention are cast so as to be about 90 to 100% amoφhous (by volume), and preferably about 95 to 97% amoφhous. The volume percent of amoφhous phase in the alloy is conveniently determined by X-ray diffraction. A major problem encountered when annealing ofthe as cast alloy used in manufacture ofthe marker, is the difficulty of determining the appropriate time and temperature conditions required to produce a threshold step in the B-H loop at a given value. The problem is in large part due to insufficient knowledge concerning the parameters operative to produce the required step change in magnetization behavior. In accordance with the present invention, it has been discovered that the annealing time at a given temperature is a function ofthe alloy composition. Specifically, there has been observed a strong correlation between the annealing time and the total B plus Si content . Annealing ofthe amoφhous metallic material in the Co-Fe-B-Si series at temperatures in the range of 400 to 430 ° C causes crystallization ofthe surface followed by bulk crystallization at prolonged times. The coercivity (Hc)of the ferromagnetic metal increases with the increase in the crystallization according to Liebermann, IEEE Trans. Mag., Noi Mag-17, Νo.3, 1286, (1981); and Liebermann et al., Metallurgical Trans. A, Vol.20A, 63, (1989). Therefore the value of he coercivity ofthe ferromagnetic metal at a given temperature and frequency can be used as a measure ofthe crystallization amount ofthe material. Figure 2 depicts the correlation between the annealing time needed to achieve coercivity equal to lOe at the temperature of 425 °C in the Co-F-B-Si series as a function ofthe total B plus Si content. Annealing ofthe Co-Fe-B-Si material at these temperatures ( Ta < crystallization temperature) in an oxidizing atmosphere causes oxidation ofthe surface. Auger chemical surface analysis ofthe annealed surface confirms this observation. Removing material from the surface by sputtering and taking Auger spectra at each step leads to a depth profile ofthe chemistry ofthe marker. Figure 3 depicts such a depth profile ofthe top surface of a Co-Fe-B-Si marker with a threshold field of Hu equal to 0.85 Oe. The Y axis describes the intensity ofthe signal for each chemical element and the X-axis is the sputtering time. The sputtering rate remained constant during the profiling at 8θA min. By utilizing the sputtering rate, the time is translated into depth. The intensity ofthe oxygen peak decreases as sputter progresses into the material from the surface. The point at which the oxygen signal is diminished is used to estimate the oxide thickness. A similar trend (decreasing intensity) occurs in the B and Si signals, indicating that the oxides are primarily B-O and Si-O. XPS analysis and factor analysis ofthe Auger spectra indicate the presence of Co-O as well. Factor analysis ofthe Auger depth profile data is shown in Figure 4. The Co Auger spectrum was deconvoluted into the Co-oxide, Co-metal and the bulk Co-Fe-B-Si signal. As illustrated, the oxide layer is followed by a region depleted in B and Si and rich in metallic Co. TEM of this region confirms the presence of hexagonal bcc Co crystal, and is shown in Figure 5. An important aspect of this invention is the correlation ofthe thickness of the crystalline Co-layer and the value for the threshold magnetic field. The threshold magnetic field is proportional to the thickness ofthe crystalline Co-layer. Figure 6 describes the Auger depth profile ofthe top surface of a marker with H-2 equal to 0.7 Oe , which is less than the Hu of Figure 3. As illustrated, the thickness ofthe Co-layer is reduced as well. The physical mechanism for the formation of a step change in the magnetization of the annealed Co-Fe-B-Si alloy is the formation of a magnetically hard layer consisting of metallic Co and some Fe on the top surface (surface not in touch with the quenching substrate) ofthe ribbon. The threshold magnetic field where this step change occurs correlates with the thickness ofthe crystalline layer. The annealing time required to produce a step at a given field for a given temperature is proportional to specifically the total B plus Si content in the as cast alloy composition. Another important indicator is derived by observing the Auger depth profile (Figure 7) ofthe annealed markers bottom surface (surface in contact with the quenching substrate). The depth profile does not signify the formation of a distinct Co-crystalline layer. For that puφose, the active component ofthe marker is the top surface. Figure 8a is a schematic diagram of the crossection ofthe as cast ribbon. Ti is the thickness ofthe oxide (approximately 20A) on the top surface (40) ofthe ribbon and T 2 is the oxide thickness (approximately 30A) on the bottom surface (42) ofthe amoφhous ribbon. Figure 8b is a schematic diagram ofthe markers' structure. The top surface (40) ofthe ribbon consists of an oxide layer followed by a Co crystalline layer on top ofthe amoφhous bulk alloy. T 3 is the thickness ofthe oxide (approximately 300A) and T is the thickness ofthe Co crystalline layer (approximately 1000 A). The bottom surface ofthe ribbon consists of an oxide layer followed by a mixed crystalline and amoφhous transition layer. Ts is the oxide thickness (approximately 80A) and Tβ is the thickness ofthe mixed phase transition layer (approximately 40A). The fart that the bottom surface ofthe marker doesn't form a Co-crystalline layer in spite ofthe surface oxidation indicates that certain surface chemistry and structure is required fo.r this to occur. In order to prove this claim a piece of 2" (50.8 mm) wide Co-Fe-B-Si metal strips was annealed at 408 °C for 30 min in air. After annealing the middle part ofthe top surface ofthe 2" (50.8 mm) wide strip developed a dark gray color, whereas the edges ofthe top surface and the bottom surface remained light silver gray colored. Auger analysis ofthe dark gray area confirmed the presence ofthe suiface oxide and the distinctive crystalline Co-layer followed by the bulk amoφhous alloy. On the contrary, the light gray colored areas exhibited only the surface oxide followed by the bulk alloy with some Co- crystallites mixed in. Figures 9a, 9b, 9c, and 9d depict the top surfece ofthe as cast alloy and the annealed alloy as well as the crossections ofthe dark gray middle area and the light gray edge areas ofthe top surface ofthe annealed strip, correspondingly. The areas in the as cast surface, which correspond to the light gray and dark gray colored annealed areas were analyzed by X-ray photoemission spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM). Figures 9e and 9f summarize the XPS results. The as cast surface chemistry ofthe top surface, which after annealing forms the distinctive crystalline layer (middle dark gray area consists of B-O, Si-O and Co- O The as cast bottom surfece chemistry as well as the top surfece areas, which do not form the crystalline layer after annealing (light silver gray edges ofthe top surface and, bottom surface), consist primarily ofthe same oxides, however the amount of B-oxide and Si-oxide is reduced compared to the middle area. The crystallite size in the light and dark gray areas ofthe annealed sample was determined by TEM analysis. Table 1 summarizes the results latau. Area c alli ? nig* an tvpe bottom surfece middle (light gray) 30-50 nm Co-hexagonal twinned, C0 3 O.1 top surfece middle(dark gray) 50-100 nm Co- hexagonal twinned top surfece edge (light gray) 25-50 nm Co- hexagonal twinned Since the annealing rate was the same for all areas, the differences in the crystallite size are attributed to solidification rate differences. Theπnai profiles ofthe ribbon exiting the casting substrate confirm this observation. The areas, which have high cooling rates, such as the surfece in touch with the substrate (bottom surfece) and the edges ofthe top surfece, have lower exit temperature and smaller crystallite size. Figure 10 is a temperature profile ofthe top surfece ofthe exiting ribbon and h demonstrates that the edges ofthe ribbon are colder than the middle. Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all felling within the scope ofthe present invention as defined by the subjoined claims.

Claims: What i s claimed is : 1. For use in a magnetic theft detection system, a glassy metal alloy strip having a value of magnetostriction near zero, said strip having been annealed to produce a step change in the magnetization versus applied field behavior (B-H loop) thereof and having a composition consisting essentially ofthe formula. (Co Fe)ιoo-x (Si B) x where 20 <x<23 and ϊSA≤Co/Te≤ 15.9 and 7.5< B/Si< 9. 2. An alloy as recited by claim 1 , having a composition selected from the group consisting of Co 73.7 Fe 4 .7 Si 2 . 5 B 19.1, Co 7 B ι 8 . 5 , Co 73.7 Fe .8 Si2.5 B 19.0 and Co 73.5 Fe .βSi 2 .2 B -9.8- 3. An alloy as recited by claim 1, wherein said annealing is carried out at a temperature ranging from about 395 to 425 °C and an annealing time ranging from about 2 to 34 min., and said step change in the magnetization flux is produced at apphed magnetic fields ranging from about 0.4 to 1.5 Oe. 4. An alloy as recited by claim 1, wherein said annealing step produces a step change in the magnetization flux at a threshold magnetic field Ht, the annealing time and temperature conditions varying in direct proportion to the total B plus Si content. 5. An alloy as recited by claim 1, wherein said annealing is carried out in an oxidizing atmosphere, to thereby form on a surface of said strip a surface oxide followed immediately by a crystalline metallic layer. 6. An alloy as recited by claim 5, where the crystalline layer consists essentially of the magnetically hard Co with some Fe. 7. An alloy as recited by claim 6, wherein said metal strip has a top surface and the crystalline Co-layer is formed only on the top surface. 8. An alloy as recited by claim 6, wherein the presence ofthe magnetically hard crystalline Co-layer causes the formation ofthe step change in the magnetization flux ofthe metal strip at a threshold applied magnetic field. 9. An alloy as recited by claim 6, wherein the crystalline Co-layer thickness determines the value ofthe threshold applied magnetic field at which the step change in the magnetization flux occurs. 10. An alloy as recited by claim 6 wherein a surface ofthe alloy as cast consists essentially of B and Si oxides, and said as cast surface has a composition that promotes the formation ofthe crystalline Co-layer.

Inventors: AMBASZ VICTOR, COLLINS ALIKI, MARTIS RONALD, AMBASZ, VICTOR, COLLINS, ALIKI, MARTIS, RONALD
Applicants: ALLIED SIGNAL INC, ALLIEDSIGNAL INC.
Application year: 1996
Main classification: G08B13
IPC codes: G08B13, H01F1