Corrosion Resistant Amorphous Magnetic Alloys

This invention relates to amorphous magnetic alloy compositions having improved corrosion resistance.

There has been extensive activity over the years in the field of corrosion prevention in magnetic materials. One area of particular interest is in the fabrication of magnetic transducers or heads for use in magnetic recording devices such as magnetic disk drives, magnetic tape devices and the like. Current versions of such transducers usually incorporate one or more layers and shapes of thin film material which are formed by deposition, etching or milling, or by a combination of these steps, in the fabrication of the finished devices. After fabrication, the devices are exposed to a variety of corrosive environments during use, and unless this corrosive action is prevented, or at least reduced, serious problems may arise over time with the performance of the transducers.

One possible approach to corrosion resistance or prevention is to coat the exposed surface of the device with a layer of protective material to effectively seal the device elements from the corrosive atmosphere. However, in the case of the magnetic transducers discussed herein, this approach is not practical because the magnetic reading and writing components of the transducers must be positioned as close as possible to the magnetic record medium from which the signals are to be reproduced and on which they are to be written or recorded in order to achieve maximum recording and reproduction density. Thus, the presence of a corrosion protection layer between the transducer components and the record medium would reduce the available recording density.

Other efforts at corrosion resistance or prevention have involved the addition of one or more different materials to the thin film material, or have involved the use of different steps during the manufacture of the transducer. However, none of these techniques have proved to be totally satisfactory to date.

U.S. Pat. No. 4,023,695 (Suits) describes NiFe magnetic thin film alloys having Rh added thereto for corrosion resistance. Suits does not teach the use of Co-based alloys.

U.S. Pat. 4,615,748 (Takino et al) describes amorphous soft magnetic thin films of alloys of Co and Hf and Pd with a saturation magnetic induction Bs approaching 15,000 gauss and a saturation magnetostriction constant .lambda.s less than 1.5.times.10.sup.-6.

U.S. Pat. 4,668,310 (Kudo et al) describes amorphous magnetic alloys including magnetic metals and metalloid alloys including Zr, Ti and Y.

A publication entitled Magnetic Properties And Corrosion Resistance Of (CoFeB) 100-x Cr x Thin Films, Journal of Applied Physics, 50 (11), November 1979, page 7603, Brunsch, describes tests to determine the effects of varying amounts of Cr in CoFeB alloys on the corrosion rate of the alloys. This paper concluded that a strong increase of the corrosion rate was observed in the alloys when the Cr amount ranged between 4 and 15 At %, and at lower Cr concentrations no significant reduction in corrosion rate of the base alloy was observed.

Another publication, Magnetic Corrosion and Surface Properties of NiFeCr Thin Films, Journal of Applied Physics, Vol 47, No. 3, March 1976, page 1158, Rice et al, describes the effect of Cr additions to Permalloy in the form of 82 atomic percent (At %) Ni and 18 At % Fe. The additions of Cr to Permalloy and its observed effect on the corrosion rate set out in this publication are summarized herein as FIG. 1 of the prior art.

A further paper entitled Properties of Thin Permalloy Films with Rhodium Addition For Corrosion Resistance, Journal of Applied Physics, Vol 50(a) September 1979, page 5899, Rice et al, describes the effects of Rh additions to Permalloy on the corrosion resistance of such alloys. The results of this paper are summarized and shown as prior art in FIG. 2 herein.

One of the principal discoveries involved in this invention is that both relative corrosion resistance and magnetostriction changes may be estimated for various elements added to a Co-based amorphous alloy system. The objective is to simultaneously achieve a magnetostriction value of .lambda.s <1.times.10.sup.-6 and a relative corrosion resistance which is equal to or greater than that of 80:20 NiFe (Permalloy).

For typical Co-based amorphous alloys, i.e., alloys that contain from 70-96 At % Co and up to approximately 6 At % Fe, with the remainder of the amorphous composition of the prior art being composed of an element selected from the group. Zr, Ta, Nb, Hf, Ti and B, the corrosion rate of such alloys is generally about 6 times greater than that of Permalloy, i.e. 81:19 At % NiFe, in typical corrosive environments.

For these general amorphous Co-based alloys such as Co from 70-96 At %, the base corrosion rate of such an alloy relative to that of Permalloy is identified as the base rate=(corrosion rate of amorphous Co alloy/corrosion rate of Permalloy). The base corrosion rate of an alloy of the present invention which is Co90 At %, Zr4.5 At % and Rh5.5 At %, if made without the addition of Rh, is approximately 6 times that of Permalloy. With the addition of Rh at 5.5 At % in accordance with this invention, the corrosion rate observed is 6.times.10.sup.-.times.At % Rh, where x is found to be 0.065, yielding a corrosion rate that is approximately 2.7 times that of Permalloy.

From the prior art findings which are illustrated in Table III herein and in FIGS. 1 and 2 herein and which show the effect of both Rh and Cr additives on Permalloy (a crystalline material), it would be expected that both Cr and Rh would be effective in reducing the corrosion rate of amorphous Co alloys as well. However, the use of Cr additives with amorphous Co alloys as shown in the prior art produces at best no change in the corrosion rate, and in fact indicates somewhat of an increase in the corrosion rate of such alloys. Thus, x for Cr in such systems is essentially equal to zero, i.e., no effect on corrosion rate. In contrast, the present invention shows that the addition of Rh to such amorphous Co-based alloys is unexpectedly effective in decreasing the corrosion rate of Co-based amorphous alloys.

In thin metal films, the amorphous state is produced largely as a function of atom size, the general rule being that the larger the atom diameter, the less the amount of that atom that is required to create an amorphous condition in an alloy. For example, the diameter of an atom of Ti is 2.88 .ANG. and it requires a minimum of 15 At % of Ti to render an alloy of CoTi amorphous; the diameter of an atom of Zr is 3.17 .ANG. and it requires a minimum of 3-4 At % of Zr to produce an amorphous condition in an alloy of CoZr.

The preferred elements to render Co amorphous are Zr, Ta, Y, Mg and Nb, or other rare earth metals, since these minimize the loss of moment due to the relatively low concentrations required to produce an amorphous condition in Co which has an atomic % in the range of 85.5% to 89.5%, i.e. approximately 2.5-4.5 At % for Zr and approximately 8-10 At % for Ta and Nb. In combinations of Zr Zr and/or Ta or Nb, the amorphous condition is achieved when the At % is at 7-9 At % of the combined elements. For alloys wherein the amorphous condition is achieved by the addition of either Zr alone or in combination with Ta or Nb, the magnetostriction can be controlled in accordance with this invention.

FIG. 1 is a graph of variations in corrosion resistance as a function of the atomic percent of Cr in Permalloy, an 80% Ni 20% Fe alloy, as shown in the prior art; and

FIG. 2 is a graph of variations in corrosion resistance as a function of the atomic percent of Rh in Permalloy, as shown in the prior art.

In alloys which are rendered amorphous by Zr, Ta, Nb, Ti, Y or rare earth elements, or combinations thereof, and which achieve a As of approximately 1.times.10.sup.-6, the following estimated changes in saturation magnetostriction .lambda. occur for the listed added elements. Thus, compensation of .lambda. s for complex alloys, e.g. quaternary and pentanary alloys, can be estimated. These are shown below in Table I.

Table II below lists the approximate dilutive effect on the magnetic moment of a Co alloy by the addition of different amounts of amorphous metals and combinations of metals. Table II is calculated using a saturation magnetic induction 4.pi.Ms for Co of 17.5 kilogauss, and subtracting therefrom the dilutive 4.pi.Ms for the atomic weight percent of the particular element or elements added to the alloy.

Below is a specific example of an alloy whose corrosion rate is 0.8 that of Permalloy and which has a 4.pi.Ms of approximately 14.3KG and a .lambda.s.ltoreq.1.times.10.sup.-6.

Co81.5 Fe3.5 (Pt,Pd)9 Zr 4 Rh2, .lambda.s.sup..+-.1.times.10-6

The range of atomic percentages for the elements of this alloy composition are 70-90% cobalt, 2-15% rhodium, 2-15% zirconium and 3-20% platinum or palladium wherein the selected atomic percentages of the elements of the alloy do not total more than 100%.

Referring to the values in Table II, the value of 4.pi.Ms for this alloy can be calculated as follows:

Examples of ternary, quaternary and pentanary alloy systems capable of excellent corrosion resistance and .lambda.s of approximately 1.times.10.sup.-6 or less are set out below. It is apparent that even more complex systems may be considered utilizing the teachings of this invention.

An alloy of Co90 Zr4.5 Rh5.5 was prepared and tested. The measured properties of this alloy were found to be

The corrosion resistance of this amorphous alloy is 0.44 that of 81:19 NiFe (Permalloy).

The Zr concentration can be varied by+1-1.5 At % greater than illustrated in Examples 1 and 2 and still maintain low magnetostriction and improved corrosion resistance. For a selected atomic % of cobalt in the range of 70-96%, a 2-20% atomic % of Rh and 2-15% atomic % of zircon can be added to the composition, provided that the total of the atomic percentages is 100%. In both Examples 1 and 2, 4.pi.Ms is greater than 12,000 Gauss.

As previously indicated, the Brunsch paper reports that the addition of Cr to an amorphous Co-based alloy of Co79 Fe6.5 B14.5, with Cr addition of up to .about.14At %, results in no decrease in corrosion rate. Thus, the relationship between Cr and Rh additions to Permalloy is unexpected for amorphous Co-based alloys, as shown in Table III below.

Thus, the effect of Rh on the corrosion rate in Co-based amorphous alloys is an unexpected and beneficial result.