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Method of annealing amorphous ribbons and marker for electronic article surveillance - Herzer, Giselher
Method of annealing amorphous ribbons and marker for electronic article surveillance
United States Patent Application 20060170554
Herzer, Giselher (Bruchkoebel, DE)
11/294914
148/113, 148/122, 148/540, 266/262, 428/832.1, 428/842.1, 148/108
C22C45/02; G08B13/14; C21D1/00; C21D1/04; C21D1/84; G08B13/24; G11B5/66; G11B5/708; H01F1/00; H01F1/04; H01F1/153; H01L41/12
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20090195373 INITIATOR SYSTEM AND METHOD FOR A TIRE PRESSURE MONITORING SYSTEM August, 2009 Lettieri et al.
20060255962 Dedicated learn mode November, 2006 Caren
1. A method of manufacturing a planar ferromagnetic element comprising the steps of: (a) providing a planar ferromagnetic ribbon having a thickness and a ribbon axis extending along a longest dimension of said ferromagnetic ribbon; and (b) annealing said ferromagnetic ribbon in a magnetic field having a substantial component normal to a plane containing said planar ferromagnetic ribbon and having, in addition to said substantial component normal to said plane containing said planar ferromagnetic ribbon, a component in said plane containing said ferromagnetic ribbon and transverse to said ribbon axis and a smallest component along said ribbon axis for producing a fine domain structure in said ferromagnetic ribbon regularly oriented transverse to said ribbon axis and having a maximum width of 1.5 times said thickness and oriented transverse to said ribbon axis, and an induced magnetic easy axis substantially perpendicular to said ribbon axis.
2. A method as claimed in claim 1 wherein step (b) comprises annealing said ferromagnetic ribbon A said magnetic field for giving ‘Said ferromagnetic ribbon a hysteresis loop which is linear up to a magnetic field substantially equal to a magnetic field which ferromagnetically saturates said ferromagnetic ribbon.
3. A method as claimed in claim 1, wherein step (a) comprises providing a ferromagnetic ribbon having a composition FeaCObNicSixByMz wherein a, b, c, x, y and z are in at %, wherein M is at least one glass formation promoting element and/or at least one transition metal and wherein 15<a<75 0<b<40 O≦c<50 15<x+y+z<25 0≦z<4 so that a+b+c+x+y+z=100.
4. A method as claimed in claim 3 comprising selecting the glass formation promoting element from the group consisting of C, P, Ge, Nb, Ta and Mo.
5. A method as claimed in claim 3 comprising selecting the transition metal from the group consisting of Cr and Mn.
6. A method as claimed in claim 3 wherein step (a) comprises providing a ferromagnetic ribbon having a composition Fe24Co30Ni26Si8.5B11.5.
7. A method as claimed in claim 3 wherein step (a) comprises providing a ferromagnetic ribbon having a composition Fe32Co10Ni40Si2B16.
8. A method as claimed in claim 3 wherein step (a) comprises providing a ferromagnetic ribbon having a composition Fe37Co5Ni40Si2B16.
9. A method as claimed in claim 3 wherein step (a) comprises providing a ferromagnetic ribbon having a composition Fe40CO2Ni40Si5B13.
10. A method as claimed in claim 3, wherein step (a) comprises providing a ferromagnetic ribbon having a composition FeaCObNicSixByMZ wherein a, b, c, x, y and z are in at %, wherein M is at least one glass formation promoting element and/or at least one transition metal and wherein 15<a<30 1<b<40 20<c<50 15<x+y+z<25 0<z<4 so that a+b+c+x+y+z=100.
11. A method as claimed in claim 10 wherein 15<a<27 10<b<20 30<c<50 15<x+y+z<20 0<x<6 10<y<20 0<z<3 so that a+b+c+x+y+z=100.
12. A method as claimed in claim 10 wherein step (a) comprises providing a ferromagnetic ribbon having a composition Fe24CO18Ni4OSi2B16.
13. A method as claimed in claim 10 wherein step ˜a) comprises providing a ferromagnetic ribbon having a composition Fe24CoI5Ni43Si2B16.
14. A method as claimed in claim 10 wherein step (a) comprises providing a ferromagnetic ribbon having a composition Fe22CO15Ni45Si2BI6.
15. A method as claimed in claim 10 wherein step (a) comprises providing a ferromagnetic ribbon having a composition Fe23CO15Ni45Si1B16.
16. A method as claimed in claim 1 comprising forming said ribbon into a transformer core.
The present application is a divisional application of Ser. No. 10/830,576, filed Apr. 23, 2004, which is a divisional of Ser. No. 10/358,950, filed Feb. 5, 2003 (abandoned), which is a divisional application of Ser. No. 09/703,913, filed Nov. 1, 2000, which issued as U.S. Pat. No. 6,551,416, which is a divisional application of Ser. No. 09/262,689, filed Mar. 4, 1999, which issued as U.S. Pat. No. 6,299,702, which is a divisional application of Ser. No. 08/968,653, filed Nov. 12, 1997, which issued as U.S. Pat. No. 6,011,475.
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 thir 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 o 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. K 51 (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 Ku 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/m3 up to about 1 kJ/m3. Accordingly the anisotropy field which is given by HK=2 Ku/Js (d. Luborsky et al., “Magnetic Annealing of Amorphous Alloys”, IEEE Trans. on Magnetics MAG-11, p. 1644-1649 (1975); Js 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 HK=25 Oe.
FIGS. 1a and 1b represent a comparative example of the typical domain structure of an amorphous ribbon annealed according to the prior art in a saturating magnetic field across the ribbon width; FIG. 1a is a schematic sketch of this domain structure and FIG. 1b is an experimental example of this domain structure for an amorphous Fe24Co18Ni40Si2B16 alloy annealed for about 6 s at 350° C. in a transverse field of about 2 kOe.
FIG. 2a illustrates the typical domain structure of an amorphous ribbon annealed according to the prior art in a saturating magnetic field perpendicular to the ribbon plane, [FIG. 2a is a schematic sketch of this domain structure and] FIG. 2b is an experimental example of this domain structure for an amorphous Fe24Co18Ni40Si2B16 alloy annealed for about 6 s at 350° C. in a perpendicular field of about 10 kOe in accordance with the invention.
FIGS. 3a and 3b show the typical hysteresis loops as obtained after (a) transverse field annealing in a magnetic field of about 2 kOe and (b) after perpendicular field-annealing in a field of about 15 kOe, respectively; both loops were recorded on a 38 mm long, 6 mm wide and appr. 25 μm thick sample; the dashed lines in each case are the idealized, linear loops and serve to demonstrate the linearity and the definition of the anisotropy field Hk.; the particular sample shown in the figure is an amorphous Fe24Co18Ni40Si2B16 alloy annealed for about 6 s at 350° C. in each case.
FIGS. 6a and 6b illustrate the principles of the field annealing technique according to this invention; FIG. 6a is a schematic sketch of the ribbon's cross section (across the ribbon width) and illustrates the orientation of the magnetic field vector and the magnetization during annealing; FIG. 6b shows the theoretically estimated angle β of the magnetization vector during annealing as a function of the strength and orientation of the applied annealing field. The field strength H is normalized to the saturation magnetization Js(Ta) at the annealing-temperature.
FIGS. 8a and 8b show an example for the domain structure of an amorphous ribbon field-annealed according to this invention which yields a uniaxial anisotropy oriented perpendicular to the ribbon axis and oblique to the normal of the ribbon plane; FIG. 8a is a schematic sketch of this domain structure; FIG. 8b is an experimental example of such a domain structure for an amorphous Fe24Co18Ni40Si2B16 alloy annealed for about 6 s at 350° C. in a magnetic field of about 3 kOe strength and oriented at an angle of about 88° with respect to the ribbon plane and at the same time perpendicular to the ribbon axis.
FIGS. 9a and 9b show an inventive example for the (a) magnetic and (b) magnetoresonant properties of a magnetostrictive amorphous alloy when annealed according to the principles of this invention; FIG. 9a shows the hysteresis loop which is linear almost up to saturation at Hk; FIG. 9b shows the resonant frequency fr and the resonant amplitude A1 as a function of a static magnetic bias field H; the particular example shown here is to a 38 mm long, 6 mm wide and appr. 25 μm thick strip cut from an amorphous Fe24Co18Ni40Si2B16 alloy annealed for about 6 s at 360° C. in a magnetic field of about 2 kOe strength and oriented at an angle of about 85° with respect to the ribbon plane and simultaneously perpendicular to the ribbon axis.
FIGS. 11a, 11b and 11c demonstrate the effect of the strength of the magnetic field strength H applied during annealing on (a) the resonant signal amplitude, (b) the domain structure and (c) on the anisotropy field Hk; the annealing field was acting essentially normal to the ribbon plane i.e. at an angle between about 85° and 90° except for the data points given at H=0 where a 2 kOe field was applied across the ribbon width; FIG. 11a shows the maximum resonant signal amplitude and the resonant signal amplitude at the bias field where the resonant frequency fr exhibits its minimum; FIG. 11b shows the domain size and the estimated angle of the magnetic easy axis with respect to the ribbon plane; FIG. 11c shows the anisotropy field; region II represents one preferred embodiment of the invention; the particular results shown in this figure was obtained for an amorphous Fe24Co18Ni40Si2B16 alloy annealed for about 6 s at 350° C.
FIGS. 12a and 12b illustrate the role of the annealing field strength H on the linearity of the hysteresis loop for a field was acting essentially normal to the ribbon plane i.e. at an angle between about 85° and 90° except for the data points given at H=0 where a 2 kOe field was applied across the ribbon width; FIG. 12a shows the typical form of the hysteresis loop in its center part when annealed in a “perpendicular” field of a strength larger and smaller than the saturation magnetization at the annealing temperature, respectively; FIG. 12b shows the evaluation of the linearity of the hysteresis loop with the applied annealing field strength in terms of the coercivity Hc of the annealed ribbons; the results shown were obtained for an amorphous Fe24Co18Ni40Si2B16 alloy annealed for about 6 s at 350° C.
FIGS. 13a and 13b demonstrate the influence of the strength and the orientation of the magnetic annealing field on the resonant signal amplitude; FIG. 13a shows the maximum resonant signal amplitude and FIG. 13b shows the resonant signal amplitude at the bias field where the resonant frequency fr exhibits its minimum; the particular results shown were obtained for an amorphous Fe24Co18Ni40Si2B16 alloy annealed in a continues mode for about 6 s at 350° C. in a magnetic field of orientation and strength as indicated in the figure.
FIGS. 15a and 15b show an example for the deterioration of the linearity of the hysteresis loop and the magnetoresonant properties if the induced anisotropy has component along the ribbon axis; FIG. 15a shows the hysteresis loop and the prevailing magnetization processes; FIG. 15b shows the resonant frequency f, and the resonant amplitude A1 as a function of a static magnetic bias field H; the particular example shown is a 38 mm long, 6 mm wide and appr. 25 μm thick strip cut from an amorphous Fe24Co18Ni40Si2B16 alloy annealed for about 6 s at 360° C. in a magnetic field of about 2 kOe strength and oriented “ideally” perpendicular to the ribbon plane such that no appreciable transverse field component was present.
FIGS. 16a and 16b respectively show cross sections through an annealing fixture in accordance with the inventive method which guides the ribbon through the oven; FIG. 16a demonstrates how the ribbon is oriented in the magnetic field if the opening is significantly wider than the ribbon thickness, FIG. 16b shows a configuration wherein the ribbon is oriented perfectly perpendicular to the applied annealing field in a strict geometrical sense.
FIGS. 22a and 22b compare the resonant signal amplitude of an amorphous Fe24Co18Ni40Si2B16 alloy after annealing in a magnetic field oriented transverse to the ribbon (prior art) or at angle of about 85° between the field direction and a line across the ribbon width (the invention); the field strength was 2 kOe in each case and the ribbons were annealed in a continuous mode for about 6 s at annealing temperatures between about 300° C. and 420° C.; FIG. 22a shows the maximum amplitude A1 and FIG. 22b shows the amplitude at the bias field where the resonant frequency has its minimum.
FIG. 23 is another comparison of the resonant signal amplitude of an amorphous Fe24Co18Ni40Si2B16 alloy after annealing in a magnetic field oriented transverse to the ribbon (prior art) or at angle of about 85° between the field direction and a line across the ribbon width (the invention); the maximum amplitude is plotted versus the slope |df/dH| at the bias where this maximum occurs; the field strength was 2 kOe in each case and the ribbons were annealed in a continuos mode for about 6 s-12 s at annealing temperatures between about 300° C. and 420° C.
properties. Js is the saturation magnetization, λs the saturation
magnetostriction constant and Tc is the Curie temperature. The Curie
temperature of alloys 8 and 9 is higher than crystallization temperature of
these samples (=440° C.) and, thus, could not be measured.
Alloy atomic constituents (at %) magnetic properties
Nr Fe Co Ni Si B Js (Tesla) λs (ppm) Tc (° C.)
1 24 30 26 8.5 11.5 0.99 13.0 470
2 24 18 40 2 16 0.95 11.7 415
3 24 16 43 1 16 0.93 11.1 410
4 22 15 45 2 16 0.87 10.1 400
5 32 10 40 2 16 1.02 16.7 420
6 37 5 40 2 16 1.07 18.7 425
7 40 2 40 5 13 1.03 18.9 400
8 37.5 15 30 1 16.5 1.23 22.1
9 34 48 — 2 16 1.52 27.3
Hk=2Ku·Js
The resonant frequency of the longitudinal mechanical vibration of an elongated strip is given by fr=12⁢L⁢EH/ρ
where L is the sample length, EH is Young's modulus at the bias field H and ρ is the mass density. For the 38 mm long samples the resonant frequency typically was between about 50 kHz and 60 kHz depending on the bias field strength.
FIGS. 1a and 1b show the typical slab domain structures obtained after transverse field-annealing which yields a uniaxial anisotropy across the ribbon width. FIGS. 2a and 2b show the stripe domain structure with closure domains after annealing the same sample in a perpendicular field of 12 kOe, which yields a uniaxial anisotropy perpendicular to the ribbon plane. FIG. 2a shows this structure schematically (as is known) and FIG. 2b snows this structure for an inventive resonator alloy.
The domain width for these examples can be reasonably well described by (cf. Landau et al., in Electrodynamics of Continuous Media, Pergamon, Oxford, England, ch 7. (1981)) w=2⁢γw⁢DKu(1)
where γw is the domain wall energy, Ku=HkJs/2 is the anisotropy constant and D is the dimension of the sample along which the magnetic easy axis is oriented. That is, D equals the ribbon width for an in-plane transverse anisotropy, while for a magnetic easy axis normal to the ribbon plane D corresponds to the ribbon thickness.
The physical mechanisms for this improvement can be derived from an earlier observation of the present inventor made for transverse field-annealed samples (Herzer G., “Magnetomechanical damping in amorphous ribbons with uniaxial anisotropy”, Materials Science and Engineering vol. A226-228, p. 631-635 (1997)). Accordingly the eddy current losses in an amorphous ribbon with transversely induced anisotropy do not follow the classical expression Peclass=(tpfB)26⁢ρel(2⁢a)
as commonly believed hitherto, but instead have to be described by Pc=Peclass1-(Jx/Js)2(2⁢b)
where t denotes the ribbon thickness, f is the frequency, B is the ac induction amplitude, ρel is the electrical resistivity, Jx is the component of the magnetization vector along the ribbon axis due to the static magnetic bias field, and Js is the saturation magnetization.
Since for non-zero bias fields (i.e. Jx>0) the denominator in eq. (2b) is smaller than one, the losses described by this equation are larger than the classical eddy current losses Peclass, in particular when the magnetization along the ribbon direction approaches saturation, i.e. Jx=Js. Only at zero static magnetic field, where loss measurements are usually being performed, both models yield the same result. The latter may be the reason why so far the disadvantageous excess eddy currents associated with the transverse anisotropy have not been appreciated.
Rejecting this assumption, the inventor has found that in the case of an arbitrary domain size a more correct description of the eddy current losses would be Pe=Peclass⁡[1-ɛ+ɛ1-(Jx/Js)2](3⁢a)
with ɛ=w2(w·cos⁢ ⁢β+t)2(3⁢b)
where Peclass are the classical eddy current losses defined in eq. (2a), w is the domain width, t is the ribbon thickness and β is the angle between the magnetic easy axis and the ribbon plane (i.e. β=0 for a transverse anisotropy and β=90° for a perpendicular anisotropy).
The orientation of the magnetization vector depends upon the strength and orientation of the applied field. It is mainly determined by the balance of the magnetostatic-energy gained if the magnetization aligns parallel to the applied field and the magnetostatic strayfield energy which is necessary to orient the magnetization out of the plane due to the large demagnetization factor normal to the plane. The total energy per unit volume can be expressed as φ=-HCDOTJs⁡(Ta)·(sin⁢ ⁢α⁢ ⁢sin⁢ ⁢β+cos⁢ ⁢α⁢ ⁢cos⁢ ⁢β)+Js⁡(Ta)22⁢ ⁢μ0⁢(Nzz⁢sin2⁢β+Nyy⁢cos2⁢β)(4)
where H is the strength and α is the out-of-plane angle of the magnetic field applies during annealing, Js(Ta) is the spontaneous magnetization at the annealing temperature Ta, β is the out-of-plane angle of the magnetization vector, μ0 is the vacuum permeability, Nzz is the demagnetizing factor normal to the ribbon plane and Nyy is the demagnetizing across the ribbon width. The angles α and β are measured with respect to a line across the ribbon width and a line parallel to the direction of the magnetic field and magnetization (or anisotropy direction), respectively. Numerical values given for α and β refer to the smallest angle between said directions. That is e.g. the following angles are equivalent 85°, 95°(=180°-85°) and/or 355°. Furthermore, the magnetic field and/or the magnetization shall nominally have no appreciable vector component along the ribbon axis. The ribbon or strip axis means the direction along which the properties are measured i.e. along which the bias-field or the exciting α-field is essentially acting. This is preferably the longer axis of the strip. Accordingly, across the ribbon width means a direction perpendicular to the ribbon axis. Principally, elongated strips can be also prepared by slitting or punching the strip out of a wider ribbon, where the long strip axis is at an arbitrary direction with respect to the axis defined by the original casting direction. In the latter case, “ribbon axis” refers to the long strip axis and not necessarily to the casting direction i.e. the axis of the wide ribbon. Although in the present examples the strip or ribbon axis is parallel to the casting direction, aforementioned or similar modifications will be clear to those skilled in the art.
The angle β at which the magnetization vector comes to lie can be obtained by minimizing this energy expression with respect to β. The result obtained by numerical thick amorphous ribbon. In case of the field being applied perpendicular the result can be analytically expressed as: β={arc⁢ ⁢sin⁢μ0⁢HJs⁡(Ta) μ0⁢H<Js⁡(Ta) for 90⁢° μ0⁢H≥Js⁡(Ta)(5)
recognizing that Nyy>>Nzz=1.
For the thin amorphous ribbon, the demagnetizing factor across the ribbon width is only about Nyy=0.004 (cf. Osborne, “Demagnetizing Factors of the General Ellipsoid”, Physical Review B 67 (1945) 351 (1945)). That is, the demagnetizing field across the ribbon width is only 0.004 times the saturation magnetization in Gauss when the ribbon is fully magnetized in this direction. Accordingly an alloy with a saturation magnetization of 1 Tesla (10 kG), for example, can be homogeneously magnetized across the ribbon width if the externally applied field exceeds about 40 Oe. The demagnetizing factor perpendicular to the ribbon, however, is close to unity, i.e. in a very good approximation can be put as Nzz=1. That is, when magnetized perpendicular to the ribbon plane the demagnetizing field in that direction virtually equals the saturation magnetization in Gauss. Accordingly a field of about 10 kOe is needed, for example, in order to orient the magnetization perpendicular to the ribbon plane if the saturation magnetization is 1 Tesla (10 kG).
FIG. 6b shows the calculated angle of the magnetization vector during annealing as a function of the strength and orientation of the applied annealing field. The field strength H is normalized to the saturation magnetization Js(Ta) at the annealing temperature. FIG. 7 shows, as an example, the temperature dependence of the saturation magnetization for the investigated Fe24Co18Ni4Si2B16 allay. Compared to its room temperature value of Js=0.95 T, the magnetization is reduced e.g. to about Js=0.6 T at an annealing temperature of about 350°. The latter value is ultimately relevant to the aforementioned demagnetizing fields during annealing.
The particular example shown in FIG. 8b is for an Fe24Co18Ni40Si2B16 alloy annealed for about 6 seconds at a temperature of 350° C. in a field of 3 kOe oriented at about α=88′ with respect to the ribbon plane. Very fine domains of about 12 μm in width are observed, i.e. considerably smaller than the slab domains of the transverse field annealed sample (cf. FIG. 1). The magneto-optical contrast seen in FIG. 6b corresponds to the closure domains A and B in FIG. 8a, respectively. In contrast to the “labyrinth” domain pattern observed for the sample annealed in a 10 kOe perpendicular field (cf. FIG. 2b) the domains are now regularly oriented across the ribbon width.
The applied field strength of 3 kOe is about half the magnetization in Gauss at the annealing temperature Ta (Js(360° C.) 0.6 Tesla 6 kG) i.e., μ0H/Js(Ta)=0.5. Accordingly (cf. FIG. 6b) the out-of-plane angle of the induced anisotropy can be estimated to be about 30°.
In order to verify the findings in more detail, a first set of experiments investigated the influence of the annealing field strength. The annealing field was oriented substantially perpendicular to the ribbon plane i.e. at an angle close to 90° (see also next section). The results are shown in FIGS. 11a, 11b and 11c, and 12a and 12b.
The reduction of magnetostatic stray field energy is counterbalanced by the energy needed to form domain walls and eventually to form the closure domains. By balancing these energy contributions (cf. Kittel C., “Physical Theory of Ferromagnetic Domains”, Rev. Mod. Phys. vol. 21, p. 541-583 (1949)) the domain wall width w of the inventive material can be estimated as w=2⁢γwtKu·(Nzz⁢sin2⁢β+Nxx⁢cos2⁢β)(6)
where γw is the domain wall energy, t is the ribbon thickness, Ku=HkJs/2 is the anisotropy constant, β is the out-of-plane angle of the magnetization vector, Nzz is the demagnetizing factor normal to the ribbon plane and Nyy is the demagnetizing across the ribbon width. The solid line in FIG. 11b was calculated with the help of this expression and reproduces well the experimental domain size determined by magneto-optical investigations (squares in FIG. 11b).
FIG. 11c shows the behavior of the anisotropy field Hk. Interestingly the anisotropy field of the perpendicularly annealed ribbons is about 10% smaller than the one of the transverse field annealed ribbons. This difference has been confirmed in many comparative experiments. The most likely origin of this effect is related to the closure domains being formed when the magnetic easy axis tends to point out of the ribbon plane. The closure domains reveal a magnetization component along the ribbon axis either parallel or antiparallel. When magnetizing the ribbon with a magnetic field along the ribbon axis, the domains oriented more parallel to that field will easily grow in size and the ones antiparallel to the field will shrink. Thus, the energy needed to turn the bulk domains out of their easy direction is diminished by the fraction of the magnetization component parallel to the ribbon compared to the magnetization component perpendicular to the ribbon axis. Accordingly a lower field strength Hk is needed to saturate the ribbon ferromagnetically. Quantitatively the effective anisotropy field thus can be expressed by Hk=2⁢KuJs·(1-w2⁢t⁢sin⁢ ⁢β)(7⁢a)
where Ku is the induced anisotropy constant, Js is the saturation magnetization, w is the domain width of the stripe domains, t is the ribbon thickness and β is the out-of-plane angle of the magnetic easy axis. Ku is experimentally obtainable by measuring the effective anisotropy field Hktrans of a transversely annealed sample where β=0 i.e. Ku=HktransJs/2. The ribbon thickness t can e.g. be determined by a gauge or other suitable methods and the domain width w is obtainable from magneto-optical investigations. Thus, given a ribbon with oblique anisotropy, the anisotropy angle β can be determined by measuring Hk of the ribbon and using the following formula β=arcsin⁡(2⁢tw⁢(1-HkHktrans.))(7⁢b)
where Hktrans is the anisotropy field of a sample annealed under the same thermal conditions in a transverse magnetic field across the ribbon width. The triangles in FIG. 11b represent the thus-determined anisotropy angle which coincides well with the expected anisotropy angle calculated with eq. (5), the latter result being represented by the dashed line in FIG. 11b.
FIGS. 12a and 12b summarize the effect of the annealing field parameters on the linearity of the hysteresis loop. FIG. 12a is an enlargement of the center part of the loop and shows the typical loop characteristics for a transverse, oblique and pure perpendicular anisotropy, respectively. FIG. 12b quantifies the linearity in terms of the coercivity of the sample. Almost “perfectly” linear behavior, in these examples, corresponds to coercivities less than about 80 mOe.
of 50 Hz. The examples refer to an amorphous Fe24Co18Ni40Si2B16
alloy annealed in a continuos mode at 350° C. for about 6 s in a
HAmax A1max Domain demag- as
Nr α β Hk (Oe) (mV) type netized annealed
1 0° 0° 11.4 6.5 72 I 120 150-200
2 30° 3° 11.0 6.8 76 I (II?) 30 125
3 60° 12° 10.6 6.8 88 II 16 20
4 88° 30° 10.0 6.3 90 II 12 14
FIGS. 13a and 13b demonstrate the effect of the field annealing angle α on the resonant signal amplitudes for various field annealing strengths. For field strengths above about 1.5 kOe the resonant susceptibility is significantly improved as the field annealing angle exceeds about 40° and approaches a maximum when the field is essentially perpendicular to the ribbon plane i.e. when α approaches 90°.
FIGS. 13a and 13b also demonstrate that there is virtually no significant effect of the annealing field strength on the magneto-resonant properties when a transverse (0°) field-anneal treatment according to the prior art is employed.
For moderate fields in the range between about 1.5 kOe up to the value or the saturation magnetization at the annealing temperature (i.e. about 6 kOe in these examples) the best signal amplitudes result if the field is oriented substantially perpendicular which means annealing angles above about 600 up to about 90°, which is a preferred embodiment of the invention.
The inventor became aware of this mechanism from an experiment at moderate annealing fields wherein special emphasis was put on orienting the ribbon plane “perfectly” perpendicular to the annealing field. The results are shown in FIGS. 15a and 15b and illustrate the non-linear hysteresis loop and the poor magneto-resonant response obtained in this experiment. The domain structure investigations showed that a substantial part of the ribbon revealed domains oriented along the ribbon axis being responsible for the non-linear hysteresis loop and the diminished resonant response.
Hy=H cos a (8)
This transverse field component Hy should be strong enough to overcome the demagnetizing field and the magnetoelastic anisotropy fields at the annealing temperature. That is the minimum field Hymin across the ribbon width should be at least
Hymin=NyyJs(Ta)/μ0−3λs(Ta)σ/Js(Ta) (9)
Accordingly, the angle of the annealing field should be α≤arccos⁢HyminH(10)
In eqs. (8) through (10) H is strength and α is the out-of-plane angle of the magnetic field applied during annealing, Js(Ta) is the spontaneous magnetization at the annealing temperature Ts, λs(Ts) is the magnetostriction constant at the annealing temperature Ts, μ0 is the vacuum permeability, Nyy is the demagnetizing across the ribbon width and σ is the tensile stress in the ribbon.
Typical parameters in the experiments are Ta=350° C., Nyy=0.004, Js(Ta) 0.6 T, λs(Ta)=5 ppm and σ=100 MPa. This yields a minimum field of about Hymin=55 Oe which is to be overcome in the transverse direction. Hence, for a total annealing field strength of 2 kOe this would mean that the annealing angle should be less than about 88.5°.
Even more, such small deviations from the 90° angle may naturally occur since the magnetic field tends to orient the ribbon plane into a position parallel to the field lines. FIGS. 16a and 16b give an illustrative example. FIGS. 16a and 16b show the cross section of a mechanical annealing fixture 5 which helps to orient the ribbon 4 in the oven. If the opening 5a of this fixture 5 is larger than the ribbon thickness, the ribbon 4 will automatically be tilted by the torque of the magnetic field although everything else is perfectly adjusted. The resulting angle α between the ribbon plane and the magnetic field is determined by the width h of the opening and the width b of the ribbon, i.e. α=arccos⁢hb(11)
Even for a relatively narrow opening width of about h=0.2 mm the resulting angle, for a 6 mm wide ribbon will be about α=88′. This deviation from 90° is enough to produce a sufficiently high transverse field to orient the in-plane component of the magnetization across the ribbon width. The width h of the opening 5a in the annealing fixture 5 should not exceed about half of the ribbon width. Preferably the opening should be not more than about one fifth of the ribbon width. In order to allow the ribbon to move freely through the opening the width h should be preferably at least about 1.5 times the average ribbon thickness.
Thus “substantially” perpendicular means an orientation very close to 90°, but a few degrees away in order to produce a sufficiently high transverse field as explained above. This is also what is meant when sometimes the term “perpendicular” is used by itself in the context of describing the invention. This is in particular true for field strengths below about the saturation magnetization at the annealing temperature. Thus, the annealing arrangement as for example shown in FIG. 16b, where the applied field is perfectly perpendicular to the ribbon plane, is less suited.
FIG. 19a shows the cross section of such a magnet system 7,8 with an oven 6 in-between, in which the ribbon 4 is transported at the desired angle with respect to the field direction by the help of an annealing fixture 5. The outer shell of the oven 6 should be insulated thermally such that the exterior temperature does not exceed about 80° C.-100° C.
FIGS. 17a-d show a more detailed view of how the cross section of said annealing fixture may look. The annealing fixture preferably is formed by separate upper and lower parts (10 and 9 in FIG. 17a, and 12 and 11 in FIG. 17b) between which the ribbon can be placed after which these two parts are put together. The examples given in FIG. 17a and FIG. 17b are intended only to guide the ribbon through the furnace. As noted earlier, the annealing fixture additionally can be used to give the ribbon a curl across the ribbon width, as shown in FIG. 17c and FIG. 17d, respectively. The fixture shown in FIG. 17c has a lower part 13 and an upper part 14 which in combination define a curved opening. The fixture shown in 17d has a lower part 15 and, an upper part 16 which can be used to produce either a rectangular opening, by inserting respective strips into the uppermost rectangular channel in the upper part 16 and in the lowermost rectangular channel in the lower part 15 or, by leaving those uppermost and lowermost channels open and using a longitudinal supporting element 17, an opening suitable for producing curved ribbon can be obtained. These fixtures are equally suited for the annealing method according to this invention. In the latter type of annealing fixtures the ribbon has virtually no chance to be turned by the torque of the magnetic field. As a consequence, if such a curl annealing fixture is used it becomes important to properly orient the annealing field such that the normal of the ribbon plane is a few degrees away from the field direction which, as described before, is particularly important at moderate annealing field strengths.
For simplicity FIGS. 19a and 19b show only a single ribbon being transported through the oven 6. In a preferred embodiment, however, the annealing apparatus system should have at least a second lane with the corresponding supply and wind-up reels, in which a second ribbon is transported through the oven 6 independently but in the same manner as in the first lane. FIGS. 20a and 20b schematically show such a two lane system. Such two or multiple lane systems enhance the annealing capacity. Preferably, the individual lanes have to be arranged in such a way that there is enough space so that a ribbon can be “loaded” into the system while the other lane(s) are running. This again enhances capacity, particularly in the case of the ribbon in one lane breaks during annealing. This break can then be fixed while the other lanes keep on running.
In a first set of these experiments, an amorphous Fe24Co18Ni40Si2B16 alloy was investigated in detail as to the effect of the annealing temperature and the annealing time. The results are listed Table III and are illustrated in FIGS. 22a and 22b and FIG. 23. The resonant frequencies in all these examples were located at frequencies around about 57 kHz at Hmax and around about 55 kHz at Hfmin. In all examples of Table III the ribbon was ductile after the annealing treatment.
annealed in a continuous mode at the indicated annealing temperature Ta
at about the indicated time ta in a magnetic field of about 2 kOe
anisotropy field, Hmax is the bias field where the resonant amplitude A1
is maximum, Amax is said maximum signal, |df/dH| is the slope of the
results results 6.5-
at maximum A1 at fr, min >2 Oe
Exp. Ta ta Hk Hmax Amax |df/dH| Hfmin Afmin Δfr
Nr. (° C.) (s) (Oe) (Oe) (mV) (Hz/Oe) (Oe) (mV) (kHz)
1 300 6 10.2 6.5 81 582 8.8 50 2.2
2 320 6 11.1 7.3 81 559 9.5 55 1.9
3 340 6 11.3 7.5 82 608 10.0 52 1.8
4 360 6 10.8 7.0 88 662 9.5 52 2.1
5 370 6 10.6 7.1 93 730 9.3 46 2.2
6 380 6 10.4 6.6 93 723 9.3 48 2.3
7 400 6 9.7 6.3 95 827 8.8 44 2.7
8 420 6 9.8 6.1 95 850 8.3 49 2.9
9 300 12 11.3 7.5 79 506 9.8 53 1.8
10 320 12 11.9 7.8 78 507 10.3 55 1.6
11 340 12 11.9 7.8 83 546 10.3 57 1.7
12 360 12 11.4 7.5 85 587 10.0 56 1.8
13 370 12 11.1 7.4 90 677 9.8 55 2.0
14 380 12 10.7 7.1 91 701 9.5 55 2.2
15 380 12 10.7 6.9 90 673 9.5 53 2.2
16 420 12 9.4 5.5 96 887 8.0 44 31
T1 300 6 10.9 6.0 67 558 9.0 29 2.0
T2 320 6 11.9 6.9 68 552 10.3 20 1.6
T3 340 6 123 7.4 68 527 10.8 11 1.5
T4 360 6 12.0 7.1 70 575 10.5 9 1.7
T5 380 6 11.5 6.8 74 620 10.3 5 1.9
T6 400 6 10.8 6.0 75 660 9.5 3 2.3
T7 420 6 10.4 5.6 77 720 9.0 4 25
FIGS. 22a and 22b demonstrate that the inventive annealing technique results in a significantly higher magnetoresonant signal amplitude compared to the conventional transverse field-annealing at all annealing temperatures and times. As mentioned before, the inventive technique also results in more linear hysteresis loops, which is an advantage compared to [another] annealing techniques of the prior art where the induced anisotropy is perpendicular to the ribbon plane.
The variation of the amplitude with the annealing temperature and annealing time is correlated with a corresponding variation of the resonant frequency versus bias field curve in FIGS. 22a and 22b. The latter is best characterized by the susceptibility of the resonant frequency fr to a change in the bias field H, i.e. by the slope |dfr/dH|. Table III list this slope at Hmax where the resonant amplitude has its maximum. At Hfmin, where the resonant frequency has its minimum, this slope is virtually zero i.e. |dfr/dH|=0.
at maximum A1 at fr, min 6.5-
Alloy Hk Ta Hmax Amax |df/dH| Hfmin Afmin Δfr
Nr. (Oe) (° C.) (Oe) (mV) (Hz/Oe) (Oe) (mV) (kHz)
1 370 10.7 6.3 89 652 9.3 59 2.3
2 360 10.8 7.0 88 662 9.5 52 2.1
3 340 9.8 6.5 83 654 8.5 55 2.4
4 360 8.0 4.9 91 797 6.8 64 3.0
5 360 9.8 5.0 97 1064 8.3 40 4.2
6 360 9.0 4.0 97 1388 7.3 42 6.0
7 340 7.1 2.5 80 1704 5.8 35 4.5
8 360 14.8 8.3 82 725 12.5 49 2.2
9 360 14.1 6.0 75 829 11.5 21 3.1
1 370 11.9 6.8 76 614 10.3 17 1.9
2 380 11.5 6.8 74 620 10.3 5 1.9
3 340 11.0 6.3 68 624 9.3 15 2.2
4 360 8.8 5.0 70 769 7.5 17 2.9
5 360 10.7 5.0 86 1024 9.0 8 3.9
6 360 9.8 4.3 93 1371 8.0 10 5.7
7 340 7.8 2.5 46 1519 6.25 12 4.8
8 360 16.4 8.8 80 702 14.3 11 1.8
9 360 15.3 6.3 77 729 12.8 10 26
Alloy Nr. Amax Afmin
1 1.17 3.5
2 1.19 10
3 1.22 3.7
4 1.30 3.8
5 1.13 5
6 1.04 4.2
7 1.74 2.9
8 1.03 4.5
9 0.97 21
According to Livingston, “Magnetomechanical Properties of Amorphous Metals”, phys. stat. sol. (a) vol 70, pp 591-596 (1982) the resonant frequency for a transverse-annealed amorphous ribbon for H<Hk can reasonably well be described as a function of the bias field by fr⁡(H)=fr⁡(H=0)1-9⁢λs2⁢EsJs⁢HK3⁢H2(12)
where λs is the saturation magnetostriction constant, Js is the saturation magnetization, Es is Young's modulus in the ferromagnetically saturated state, HK is the anisotropy field and H is the applied bias field.
This relation also applies to the annealing technique according the principles of the present invention. The signal amplitude behaves as shown in FIG. 24, which shows the resonant frequency f, and the amplitude as a function of the bias field normalized to the anisotropy field Hk. The signal amplitude is significantly enhanced by domain refinement which is achieved with the annealing techniques described herein. This enhancement becomes particularly efficient when the sample is pre-magnetized with a field H larger than about 0.4 times the anisotropy field. As demonstrated in FIG. 24, this yields a significantly higher amplitude in a significantly wider bias field range than is obtainable when annealing in a transverse field according to the prior art.
The requirement for a certain level of the resonant frequency is easily adjusted by choosing an appropriate length of the resonator. Another application requirement is a well-defined susceptibility of the resonant frequency to the magnetic bias field. The latter corresponds to the slope |dfr/dH|, which from eq. (12) can be derived as ⅆfrⅆH=fr⁢H⁢9⁢λs2⁢EsJs⁢HK3⁢(1+9⁢λs2⁢EsJs⁢HK3⁢H2)-Fr⁢H⁢9⁢λs2⁢EsJs⁢HK3.(13)
Other applications such as electronic identification systems or magnetic field sensors rather require 8 high sensitivity of the resonant frequency to the bias field i.e. in such case a high value of |df/dH|>1000 Hz/Oe is required. Accordingly, it is advantageous to choose an alloy with a magnetostriction larger than about 15 ppm as exemplified by alloys Nos. 5 through 7 of Table I. At the same time the alloy should have a sufficiently low anisotropy field, which is also necessary for a high susceptibility of f, to the bias field.
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