Magnetostrictive material

A magnetic material of at least two component parts arranged to have respective structures which mutually are not homogenous, the structure of one part cooperating with the structure of the other to assist the magnetostrictive behaviour of the material. The cooperation can be the structure of one part altering the atomic spacing of the other part. The alteration can be an increase from the usual atomic spacing. There can be contiguous component parts and the cooperation can be a pseudo free surface on at least one part or a reduced restriction of the local moment otherwise achieved by the contiguous component parts. A component can be at least partly an alloy and the alloy can have a ratio other than the usual one for the alloy. One of the components can be non-magnetic.

This application claims benefit from International No. PCT/GB92/02275 filed 
Dec. 9, 1992, which claims priority from British Patent Application 
9126207.1 filed Dec. 10, 1991. 
BACKGROUND OF THE INVENTION 
This invention relates to the production of materials with useful 
magnetostrictive properties. 
Magnetostriction is the property which relates magnetic characteristics of 
a body of material to a change of the shape of the material. The property 
is seen in the change in size of bodies of certain materials when the 
magnetic environment changes or the change in magnetic characteristic when 
a force is applied to such a body to change its shape. Magnetostriction is 
a dimensionless quantity represented by the magnetostriction constant 
.lambda..sub.S, relating magnetization and shape change and in the SI 
system of units useful values are some tens or hundreds of parts per 
million, particularly for use in sensors and transducers. For such uses a 
high magnetomechanical coupling factor is desirable. Also "soft" magnetic 
properties are generally preferred. 
While thin film amorphous alloys and magnetic multilayers individually 
provide some of the required properties there is still a strong need for a 
significant improvement in the properties available and for materials 
which exhibit a useful combination of such properties. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide materials with improved 
properties and methods of production of such materials. 
According to the invention there is provided a magnetic material of at 
least two component parts arranged to have respective structures which 
mutually are not homogenous, the structure of one part cooperating with 
the structure of the other to assist the magnetostrictive behaviour of the 
material. 
The cooperation may be a pseudo free surface on at least one of the two 
component parts. 
The cooperation may reduce restriction of the local moment of at least one 
of the two component parts. 
The cooperation may be the structure of one part altering the atomic 
spacing of the other. Preferably the alteration is an increase from the 
usual spacing. The structure may be the absence of a regular lattice. 
There may be an interface region in one component where the first and 
second components are contiguous. There may be an interface region where 
the layers are contiguous. The modification may be in the interface 
region. Where the interface region is in an elemental component the 
lattice structure may be one other than the usual one for the elemental 
component. Where the interface region is in a component which is at least 
partly an alloy the alloy may have a ratio other than the usual one for 
the alloy. 
A third component layer may be overlaid on the second and a lattice 
structure of the third layer may be modified in its turn. Further 
component layers may be overlaid with or without lattice structure 
modification. 
A layer may have a lattice structure modified by an overlaid layer, at 
least contiguous to the overlaid layer. 
Each of the component layers may have an inherent lattice structure and a 
modification may be that at least one inherent structure is modified by 
another contiguous inherent structure. 
The modification may be that the second lattice, at least where it is 
contiguous to the first component layer, adopts at least approximately the 
first lattice form. 
One of the first and second component layers may be a magnetic component 
and the other may be magnetic or a non-magnetic component. While each of 
the first and second layers will generally be different elements or alloys 
it is also possible that magnetic and non-magnetic forms of one component 
may be used. 
The magnetic component may be chosen from the group of ferromagnetic 
elements of at least iron, nickel, cobalt, gadolinium and terbium. 
The non-magnetic component may be chosen from aluminium, magnesium, gold 
and silver, silicon oxide. 
Preferably there are many first and second layers in the material forming a 
multilayer material. Generally the first and second layers are alternated. 
One first layer and the overlaid second layer may be identified as a 
bilayer. A first and second layer may have a third layer associated 
therewith as a trilayer and may further have a fourth layer associated 
therewith as a quadlayer. In general the material form can be expressed as 
having M multilayers each having N component layers. Typically N will be 
between two and five while M will have a value of a few tens, typically 20 
to 50. 
There may be a substrate on which the multilayer material is built up. The 
material may remain on the substrate for use or be released therefrom, for 
example by chemical means. The substrate may be glass or mica or KAPTON 
(RTM) or other suitable material. The magnetic material may be prepared in 
the place of eventual use, for example by deposition of successive 
component layers on a machine part or like object. There may be more than 
30 bilayers. 
Each layer of a bilayer may be at least a few nanometers thick, that is a 
few tens of atoms thick, and may be up to ten or more nanometers thick. It 
is believed that a well-defined boundary is useful where the second layer 
is overlaid on the first. This will produce a stress-free strain in the 
overlaid layer. 
One form of the material described above may be prepared by a method 
including overlaying the second component on the first, providing a 
well-defined boundary where the second component is contiguous with the 
first, and causing or producing a stress-free strain in the overlaid 
component, at least adjacent to the first component. 
Another form of the material described above may be prepared by a method 
including overlaying the second component on the first, and permitting or 
causing the second component structure to be influenced by the first to 
produce the cooperation. 
According to a particular aspect of the invention there is provided a 
magnetic material of a first component layer and a second component layer 
overlaid on the first, there being a lattice structure in at least one of 
the layers and in which magnetic material at least one lattice structure 
is modified where the first and second component layers are contiguous 
whereby the magnetostrictive property of the material is enhanced. 
There may be an interface region where the layers are contiguous. The 
modification may be in the interface region. Where the interface region is 
in an elemental layer the lattice structure in at least the interface 
region may be one other than the usual one for the layer element. 
According to the invention there is provided a method of producing a 
multilayer magnetic material including depositing a first component, 
causing or permitting a second component to be progressively overlaid on 
the first, controlling the deposition of the first component and the 
overlaying of the second component to produce respective different 
structures, and causing or permitting the structures to cooperate to 
assist the magnetostrictive behaviour of the material. 
The method may include providing a first component layer of a first lattice 
structure, causing or permitting a second component layer of a second 
lattice structure to be progessively overlaid on the first, and 
controlling the overlaying of the second component so that at least one 
component layer in a region contiguous to the interface of the layers has 
a lattice structure modified by the overlaying action. 
The method may include overlaying by sputtering. The method may include 
overlaying substantially atom-by-atom. The method may Include sputtering 
from a target which may be of one element or of two or more elements to 
enable the deposition of an alloy. 
The method may include causing or permitting the respective different 
structures to be distinct and form an interface which provides a pseudo 
free surface. 
A multilayer material as described above has an M/H loop characteristic 
which is altered by distorting the material, for example by bending. 
According to a particular aspect of the invention there is provided a 
multilayer magnetic material as described above in which the overlaid 
second layer is covered by an insulating layer, to enhance the frequency 
response. 
According to the invention there is provided a magnetic material 
arrangement including a magnetic material as described above and means to 
cooperate with the material to convey to an external connection a response 
of the material to an external action. 
The response may be a change of permeability or dimension and the external 
action may be the stressing of the material. The action may be flexure of 
a support for the material or an ambient pressure change, or an external 
magnetic field. 
The arrangement may form part of an accelerometer, a torque sensor or like 
device.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Various proposals for multilayer materials having magnetic properties have 
been made, for example de Wit, Witmer and Dirne (Advanced Materials, Vol. 
3 (1991) No. 7/8 pp 356 to 360) and Zeper, van Kesteren and Carcia 
(Advanced Materials, Vol. 3 (1991) No. 7/8 pp 397-399). In the first of 
these (de Wit, etc) a material with enhanced saturation magnetization and 
relative permeability but minimal magnetostriction, specifically for a 
video recorder read/write head, is described. To achieve this a very small 
grain size is sought for the magnetic material layer (iron, grain size 
below 10 nanometers) and to produce such a grain size the iron layers are 
around 10 nanometers thick. The layers are separated by thinner layers of 
another ferromagnetic material, specifically an iron/chromium/boron layer. 
This layer needs to be at least two nanometers thick to prevent the grains 
in one iron layer from linking with those in another iron layer by 
columnar growth and specifically epitaxial growth is not desired. In the 
second (Zeper et al) a magneto optical recording material is described, 
for enhanced recording density, which has adequate Kerr rotation and low 
enough Curie temperature while having a preferred magnetization direction 
perpendicular to the material layer. This preferred direction only occurs 
with cobalt/platinum or cobalt/palladium multilayers when the cobalt 
layers are less than some 0.8 to 1.2 nanometers. The thickness of the 
non-magnetic but magnetically polarisable platinum or palladium layer is 
set by the required balance of Kerr effect and Curie temperature and for 
platinum is about 0.9 nanometers with a 0.4 nanometer cobalt layer. The 
cobalt layer is about two atom layers thick so that the required magnetic 
anisotropy is not reduced by "bulk" atoms between the surface layers. In 
Zuberek, Szymczak, Krishnan and Tessier (Journal de Physique, Vol. 12, No. 
9, Colloque C8, December 1988, pp 1761 to 1762) it is suggested that a 
"bilayer" of evaporated component materials depends for effectiveness on 
the thinness of a nickel layer. In Dirne, Tolboom, de Wit and Witmer (J. 
Magn. Magn. Mat., No. 83, (1990) pp 399 to 400) the possibility of 
interface mixing in Fe/Co multilayers is discussed and seen as a 
disadvantage. 
Techniques to avoid this disadvantage are suggested. 
Multilayers of the present invention do not follow the techniques of the 
above proposals. The present invention provides in one aspect that a layer 
of a first component overlaid on another layer, of a second component, is 
caused in a region contiguous with the other layer to adopt a structure 
other than that of the lattice which it would normally adopt. In this way, 
which can be described as lattice mismatch, a stress-free strain can be 
produced in at least the contiguous region of the overlaid layer. A result 
of this stress-free strain is an enhanced magnetostriction constant, 
.lambda..sub.S. 
Typically such layers of a first material overlaid on the second would be 
arranged as several bilayers on a substrate. Each layer of a bilayer would 
be some five to fifteen nanometers thick and can be between at least one 
nanometer thick and ten nanometers thick. The layers are deposited by 
sputtering so that atoms being deposited arrive at a rate slow enough to 
be able to be influenced in deposited position by the existing lattice of 
the second component layer or that of the first-deposited overlaid first 
component atoms. Care is needed to avoid building stress into the layers 
by the thermal effects of sputtering. Some ten to fifty bilayers can be 
deposited. The overlaid component applied adjacent to the second component 
lattice is conveniently caused to adopt a lattice spacing appropriate to 
that of the second component. This will cause a stress-free strain in at 
least the contiguous region of the overlaid component. The stress-free 
strain will be best exhibited if there is a sharp boundary between the 
layers. 
If required a pre-stress can be built in to the eventual material by, for 
example, keeping the substrate hot during deposition of components so that 
the substrate contracts on cooling. Also a magnetic bias can be built in 
by including a layer of a permanently magnetizable component for example 
as or immediately above the substrate. A buffer substrate can also be used 
but care must be taken that the buffer transmits any strain that is 
applied for action by or on the magnetostrictive material. An inert or 
insulating layer may be applied to protect the material. Films may be 
grown under the influence of an imposed magnetic field to enhance the 
desired effect. 
FIG. 1 represents in a schematic manner the lattice at and near the surface 
of the second component layer and the lattice in the contiguous region of 
the overlaid component layer. In the Figure overlaid component atoms are 
indicated by the smaller, crossed, circles and the second layer component 
atoms by the more widely spaced larger, open, circles. 
To investigate the magnetostrictive properties further examples have been 
prepared. Typically these are a number of alternate layers of magnesium 
and iron, each a few nanometers thick and having a few tens of bilayers. 
The layers are not necessarily of the same thickness. The layers are 
deposited by sputtering from alternately exposed sources using 
conventional techniques. The preparation is arranged to ensure that bulk 
effects are not significant. The effect of stress on the film on the shape 
of the magnetization/field (M/H) loop was also examined. A small amount of 
distortion, by bending the specimens, changed the loop shape by a 
detectable amount, increasing the value of M at lower values of H compared 
with the loop for the unstressed specimens. 
Other structures may be used to achieve the invention. Certain combinations 
of overlaid component layers will produce a degree of alloying and this in 
turn can produce "dislocations" in an atomic level structure which would 
otherwise be adopted. Provided one component layer is magnetic the other 
can be an insulator as the merging of the layers where they are contiguous 
can alter the spacing of the magnetic component layer lattice and thus 
usefully alter the magnetostrictive property. FIG. 2 shows in sketch form 
this action where atoms of a magnetic component, identified at m, are 
displaced by the presence of atoms of another component, which may be 
non-magnetic or even an insulator, identified at m. 
The overlaid component layers are formed of different elements, different 
alloys, a magnetic and a non-magnetic formed of the same element, or a 
magnetic and non-magnetic form of the same alloy. 
The present invention provides in another aspect an interface effect which 
can be a relatively sharp lattice mismatch so that the overlaid components 
cannot easily mix or in a yet further aspect a smoother mismatch which may 
be an intermetallic region as overlaid components merge. Component layers 
need not be deposited separately, the rate of a first layer may be reduced 
while the rate of a second layer is increased to take over. 
An insulating layer, for example silicon oxide, can be introduced to 
produce an improvement in performance, especially an improved high 
frequency performance. The insulating layer thickness will be dependent on 
the frequency at which the device is to operate but will also be dependent 
on the effect of the relative thickness of the insulating layer and the 
bilayer on the propagation velocities in the insulating and magnetic 
components. From these considerations it is possible to derive suitable 
thickness relationships for a particular application of the multilayer 
material. In general the insulating layer thickness will be several times 
that of the individual magnetic component layer. In this way 
magnetostrictive materials with several tri- or quad- layers can be built 
up, of the general form of L multilayers each having N component layers. 
At least one of the N component layers will be a magnetic, generally a 
ferromagnetic, substance such as iron, nickel, cobalt, gadolinium or 
terbium while another will be a non-magnetic substance and may be a metal 
or an insulator, examples including aluminium, magnesium, gold, silver and 
silicon oxide. If N is more than two one of the non-magnetic layers can be 
a metal and another an insulator. A further magnetic component layer may 
be included. A magneto-electric transducer can be formed by placing 
structures as exemplified above onto an electrostrictive structure such as 
PVDF, the dimensional changes in the magnetic structures imposing a strain 
on the electrostrictive structure, producing an electrical signal related 
to the action on the magneto strictive structure. 
Further examples of the invention will now be described. 
Multilayers are grown using an rf glow discharge sputtering system in a 
sputter-up configuration with three 15 cm water-cooled magnetron target 
electrodes on a carousel, and a water-cooled substrate platen electrode 
which allows the substrate to be pre-sputtered. The substrate platen also 
has a heater capable of temperatures up to 600.degree. C. The 
target-substrate distance was set at 60 mm, with an argon plasma pressure 
of 5 mtorr. The sputtering equipment is a NORDIKO machine. The deposition 
rates were calibrated using a "Talystep" stylus based film thickness 
measurement instrument. Structural characterization was also done using 
X-ray diffraction. 
The substrates were glass and Kapton (Registered Trade Mark) (polyimide). 
The substrate for SAMR measurements (see below) was 25 mm thick Kapton, 
which was thin enough and had a low enough Young's modulus to avoid overly 
constraining the films while still being able to support them. 
Fe/Ag multilayers were grown with 50 repeats (i.e. 50 bilayers). 
Fe--Co/Ag multilayers of 25 repeats (25 bilayers) were grown. The layers of 
Fe--Co alloy in the Fe--Co/Ag multilayers were produced by growing 
sandwiches of Fe/Co/Fe between the silver layers (FIG. 4). The multilayer 
was subsequently annealed to allow interdiffusion of the Fe and Co to form 
a layer of alloy of thickness t(Co)+2t(Fe) (the Ag is immiscible with 
both) (FIG. 5). The heat treatment was done in a non-inductively wound 
tube furnace, with the sample laid flat between two glass slides in a 
vacuum provided by a rotary pump and nitrogen trap to eliminate oxidation. 
The anneal time and temperature were 30 minutes and 290.degree. C. 
respectively. 
A series (S1) of five Fe/Ag multilayers were grown where 
t(Fe)=t(Ag)=1,2,3,4,5 nm. 
A series (S2) of Fe/Co/Fe/Ag (i.e. Fe--Co/Ag) multilayers were grown where 
the thickness of the Co layer was twice that of each of the Fe layers so 
that the total thickness was the same for both, resulting in a post-anneal 
alloy of a nominal composition of Fe.sub.50 Co.sub.50. The thickness of 
this layer was varied from 4 to 40 nm and t(Ag) was set at 5 nm. These 
were subsequently annealed in vacuum for 30 minutes at 290.degree. C. Two 
other Fe/Co/Fe/Ag (i.e. Fe--Co/Ag) series were grown with silver layer 
thicknesses of 2 (S4) and 5 (S3) nm. The anneal times were longer for the 
films with thicker Fe/Co layers because of the greater degree of 
interdiffusion required. The anneal times (at 300.degree. C.) were 10, 20, 
30 and 50 minutes for alloy layer thicknesses 4, 6, 8, 12 nm respectively. 
As the layer thicknesses are decreased the volume fraction of the interface 
regions is increased. This increases the proportion of the multilayer 
material which is formed by atoms at or near the surface of a layer. The 
local moment is increased in the vicinity of a free surface so the local 
moment effect is enhanced as the interface volume fraction is increased. 
The texture of a layer is also significant and its retention can be 
enhanced by depositing the alloy directly and avoiding or reducing the 
annealing step. This will also encourage growth of the alloy in the [110] 
direction normal to the layer. 
The coercive fields of Fe--Co/Ag multilayers are between 1 and 2 
kAm.sup.-1, reducing with layer thickness. Generally, the annealing 
rendered the multilayers magnetically softer. Also by annealing the 
multilayers were stress-relieved and the iron and cobalt layers were 
caused to mix more thoroughly. The interfaces with silver were probably 
sharpened, since silver is immiscible with iron and with cobalt. For the 
thinnest iron and cobalt layers (D=9nm, i.e. t.sub.Co =2nm) the region 
between silver layers would have been almost completely alloyed 
as-deposited [Dirne and Denissen, 1989]. Annealing this multilayer 
increased (approximately doubling) its coercive field in distinction to 
the other multilayers, and it became impossible to measure the 
magnetostriction. This is considered below. 
The length of an anneal required to achieve complete alloying at a given 
temperature will depend on the thickness of the component layers. Thus it 
is possible that the anneals did not completely alloy the layers, 
especially in the thicker multilayers, and that they were longer than 
required for the thinner multilayers. The optimum heat treatment may not 
have been used. 
Magnetostriction measurements were made using the Small Angle Magnetization 
method proposed by Norita et al. [1980] and widely used on amorphous 
ribbons. 
The basic principle of the method is as follows: the sample to be measured 
is saturated in the longitudinal direction (z) by a large dc field. A 
smaller ac field of frequency .omega. acts in the transverse direction 
(y). This results in a small degree of rotation of the macroscopic moment 
direction about the longitudinal direction. If a stress, .sigma., is also 
applied to the sample along this direction, the angle of moment rotation 
Is given by 
##EQU1## 
where H.sub.d is the demagnetizing field due to the sample shape, and 
H.sub..sigma. is the stress-induced anisotropy field. This is given by 
##EQU2## 
The sample is mounted within a search coil whose axis is in the 
longitudinal direction. Thus a rotation of the magnetization about this 
axis results in an induced ac voltage across the search coil with 
frequency 2.sub..omega., V.sub.2.omega.. 
A change in the stress on a magnetostrictive material effects a change in 
the stress induced anisotropy field. In the case of a positively 
magnetostrictive material, a tensile stress tends to make the stress axis 
magnetically easier, so as to aligning the moments towards this axis. This 
would cause the angle of moment rotation to decrease, so as to decrease 
the search coil output voltage. 
This change in output voltage can be compensated by adjustment of the dc 
field, H.sub.Z. The change in H.sub.Z is then equal to the change in 
H.sub..sigma.. Thus, the magnetostriction constant, .lambda..sub.S, is 
generally determined by plotting the change in H.sub.Z for a series of 
applied stresses, so that (where moment rotation is small) 
##EQU3## 
The multilayers are too thin to be self supporting and need to be measured 
while still on their substrates. Although these can have elastic moduli 
far lower than the multilayers, they are much thicker and so accommodate a 
significant proportion of the stress, so it is necessary to correct for 
their presence. 
This correction can be made by assuming that the multilayer is strongly 
adhered to the substrate so that they strain by the same amount and so 
that the Young's modulus, E, of the whole is equal to the volume average 
of the component layers and the substrate. This assumes that the reed as a 
whole (i.e. substrate +multilayer) is effectively homogeneous. In this 
case the magnetostriction is given by 
##EQU4## 
The same method of correction was also used by Wenda et al. [1987] on 
measurements of FeB and FeSiB single films sputtered onto polyester 
substrates. 
Measurements of multilayer samples were taken on two slightly different 
SAMR apparatus arrangements. The dc field was applied using a solenoid, 
while the ac field was applied with a pair of Helmholtz coils. The sample 
was placed within a search coil (4000 turns) coaxially with the solenoid, 
in the center of the Helmholtz coils. In the second configuration, a 
second, "bucking", coil was also present and was backed off against the 
search coil to cancel out any induced voltages common to both, i.e. due to 
sources other than the sample magnetization. 
The sample was clamped at both ends. The clamp at one end was fixed while 
that at the opposite end was connected to a cable which was loaded with 
known weights. The dc field was determined from the product of the current 
and the number of turns per unit length in the solenoid. 
The values of B.sub.S were obtained from literature, namely 2.0 T for iron 
and 2.3 T for Fe.sub.50 Co.sub.50 alloy. The value of B.sub.S for the 
whole multilayer was simply B.sub.S for the relevant magnetic layer 
multiplied by its volume fraction. 
The Young's modulus of the multilayers was measured using the vibrating 
reed method, which is fully described in Berry and Pritchet [19753]. It is 
based on the measurement of the resonant frequencies of various modes of 
cantilever vibration, f.sub.n, of reeds clamped at one end. 
Reeds were cut from the multilayer using a sharp blade and had free lengths 
and widths of approximately 13-20 mm and 1 mm respectively. 
The value of E for the multilayer can be obtained from the value for the 
reed from the expression (from Berry and Pritchet): 
##EQU5## 
where the subscripts f and s refer to the multilayer and substrate 
respectively, and t and d are the multilayer and reed thicknesses (t&lt;&lt;d). 
The value of .lambda..sub.S for the multilayer was calculated from equation 
4. In the case of the Fe--Co/Ag multilayers (t(Co)=2t(Fe)), two data sets 
are presented, corresponding to the two methods of calculating E.sub.f. 
For the Fe/Ag data, only the values of .lambda..sub.S derived from the 
assumption of a volume average E.sub.f are shown. 
The data for .lambda..sub.S is shown versus the reciprocal of the 
modulation period, D, in FIG. 6. At 1/D=0 the value of .lambda..sub.S 
should be that for bulk iron. The bulk value of .lambda..sub.S of 
polycrystalline iron is -7 ppm, although this becomes more positive as it 
becomes more textured, so that perfectly textured polycrystalline iron 
with the [110] direction normal to the film surface has a value of 
.lambda..sub.S of approximately 14 ppm. In practice, the measured value of 
.lambda..sub.S for a single iron film will generally lie between these two 
values. As a check a thick (0.8 .mu.m) iron film was deposited onto each 
side of a Kapton susbtrate (1.6 .mu.m total). .lambda..sub.S was measured 
to be -3.0 ppm. 
As D decreases so that the volume fraction of the Fe/Ag interface layers 
increases, it is found that .lambda..sub.S becomes more positive. Hence 
the presence of silver is seen to enhance the value of .lambda..sub.S in 
iron, as expected. 
A series of multilayers were grown with layers of Fe and Co of equal 
thickness (5 nm) with silver layers of varying thickness. Because there 
was approximately twice the amount of iron as cobalt in the alloy layer 
after annealing, it is probable that there was a composition gradient 
across the layer, with a corresponding variation in .lambda..sub.S. The 
.lambda..sub.S data from this series of multilayers did not show any 
systematic trend and there was a wide scatter in the data and is therefore 
not shown here. 
The .lambda..sub.S data for the second series of multilayers is shown for 
these films versus the reciprocal of the modulation period in FIG. 7. Also 
shown is the effective .lambda..sub.S of the magnetostrictive material 
alone, derived via the data assuming a volume average Young's modulus. 
There are pairs of data points obtained via the Berry and Pritchet E.sub.f 
correction method for most samples, corresponding to two values of E.sub.f 
obtained from the third and fourth modes of reed vibration. There are two 
samples measured with t(alloy)=30 nm since two sections were taken from 
the same substrate and were annealed independently. However, E.sub.reed 
for this sample was only measured using a reed from one of the anneals, so 
the pair of points from this sample with the higher .lambda..sub.S values 
may be subject to large error. This figure illustrates the difference in 
the data resulting from the two methods of correction of the Young's 
modulus. 
Before anneal, most samples were found to have small negative 
magnetostriction (generally smaller than -5 ppm), and large positive 
.lambda..sub.S after anneal confirming that alloying of the component TM 
layers had alloyed. The sample with t(alloy)=4 nm gave a large positive 
.lambda..sub.S before annealing, indicating that the iron and cobalt 
layers had intermixed to a high degree. It is suggested in Dirne and 
Denissen [1989] that Fe--Co alloys form at interfaces during deposition, 
with thicknesses of approximately 1 nm This means that approximately 50% 
of the Fe--Co in this sample was intermixed before annealing. 
Unfortunately, no SAMR response could be obtained from the sample after 
anneal. As stated earlier, this was accompanied by a remarkable increase 
in coercive field. It may be that since the layers were so thin, the 
anneal caused disruption of the coherency of the multilayer. 
From the volume average data, it is seen that .lambda..sub.S of the 
multilayer increases to approximately 20 ppm as the alloy thickness is 
reduced to 8 nm, equivalent to a value of .lambda..sub.S of about 36 ppm 
for the alloy layers. 
The magnetostriction of all three series (S2, S3, S4) of the Fe--Co/Ag 
multilayers were measured and are shown in FIG. 8. These values were 
calculated using the volume average Young's moduli. The corresponding 
values of .lambda..sub.S for the alloy alone are shown In FIG. 9. A clear 
trend is seen for the series of multilayers (S4) with t.sub.Ag =2 nm, 
where .lambda..sub.S decreases with decreasing alloy layer thickness. The 
data for the series of multilayers with t.sub.Ag =5 nm also show some 
reduction of .lambda..sub.S with t.sub.Fe --Co, but in FIG. 8, the values 
of .lambda..sub.5 for the alloys are approximately constant (within 
error), indicating that the reduction in .lambda..sub.S for the multilayer 
is due to loading by the silver layers. The reduction in .lambda..sub.S 
for the t.sub.Ag data set cannot be accounted for in this way. 
The decrease of .lambda..sub.s of the thicker multilayers from the series 
(S2) is thought to be due to the iron and cobalt layers not fully 
intermixing during the half-hour anneal. It should be noted that the 
values of .lambda..sub.S for the two multilayers comprising layers of 2 nm 
Fe, 4 nm Co and 5 nm Ag, annealed at approximately 300.degree. C. for 30 
minutes were measured to be the same within error. This adds confidence to 
the repeatability and accuracy of the data. If intermixing is poor, 
composition gradients can arise. 
The value of .lambda..sub.S of the alloy layer was measured to be 
approximately 35.+-.3 ppm. This compares with approximately 80 ppm for the 
bulk. A multilayer of Fe/Co was deposited without silver layers, and was 
annealed nominally to form a single layer (0.5 .mu.m total thickness) of 
Fe--Co alloy, T.sub.a =300.degree. C., t.sub.a =30 minutes. The value of 
.lambda..sub.S of the film was measured to be 43 ppm. A further heat 
treatment of 30 minutes at 300.degree. C. was found to cause an increase 
in .lambda..sub.S to approximately 65 ppm. 
The magnetostriction constant obtained from SAMR values depends on the 
Young's modulus of the reed and also on that of the film only. 
Discrepancies have arisen between the values of E.sub.film corrected from 
E.sub.reed from two different methods of obtaining this correction. The 
method assuming a volume average of the moduli is thought to be the more 
reliable. 
The values of .lambda..sub.S of the Fe/Co/Fe/Ag multilayers were small and 
negative, as would be expected from an average of Fe and Co. After 
annealing, the .lambda..sub.S values were large and positive, indicating 
that the Fe and Co had alloyed to a large extent. Annealing also had the 
effect of making the films softer, predominantly due to relief of Internal 
stresses. Values of H.sub.C of the post-anneal samples were typically of 
the order of 1 kA/m. 
FIG. 3 shows the effect of stress on the M/H loop for one of the 
multilayers from series S4. The specific one used is that where n=3. The 
multilayer had 25 bilayers, each deposited as 3 nm Fe/6 nm Co/3 nm Fe/2 nm 
Ag, supported on a 25 micrometer Kapton layer. Annealing was 50 minutes at 
300.degree. C. under vacuum. The dotted line is the M/H characteristic at 
stress .sigma.=0. When a stress of 110 MPa is applied the shape and size 
of the M/H loop changes very significantly, as shown by the solid line. M 
is in arbitrary units. 
In the case of the Fe/Ag multilayers, reduction in the thickness of the 
layers (i.e. an increase in the volume fraction of the interface regions) 
has the effect of making the magnetostriction more positive. The measured 
values of .lambda..sub.S in this study increased from approximately -0.3 
ppm for 5 nm Fe/5 nm Ag to approximately +3 ppm for 1 nm Fe/1 nm Ag. This 
is consistent with an increase in the local magnetic moment of iron at the 
Fe--Ag interfaces. 
In the case of the Fe--Co multilayers, .lambda..sub.S was seen to decrease 
with decreasing modulation period, where this was shown to be due to 
mechanical loading by the silver layers, where t.sub.Ag =5 nm. When this 
correction was made, the value of .lambda..sub.Fe--Co were found to be 
approximately 35 ppm, which is considerably lower than those of bulk 
Fe--Co alloys. Non-optimal heat treatments are thought to contribute to 
the reduction in the values, as indicated by the decrease in 
.lambda..sub.S for the thicker multilayers annealed for 30 minutes. A 
definite decrease in .lambda..sub.S with decreasing alloy layer thickness 
was seen for the multilayers where t.sub.Ag =2 nm, which cannot be 
accounted for by the loading by the silver layers. The reason for this is 
not clear. It is possible that the silver layers are so thin that the 
alloy layers have percolated through, especially where these are 
appreciably thicker than the silver, so that the parameters of the 
multilayer approach those of a bulk material. It may be that inclusion of 
silver in the multilayer causes a reduction in .lambda..sub.Fe--Co. 
It is seen from the above that thickness and crystallographic texturing of 
the alloy layer and silver layer loading all can affect the value of 
.lambda..sub.S. Those skilled in the art will be able to proceed on the 
basis of the techniques set out above to produce multilayers with 
significant values of .lambda..sub.S using appropriate component layers 
and dimensions. 
Some of the above examples show that the technique of causing a magnetic 
component layer to adopt, in a surface region at least, a lattice 
structure other than the one it would normally adopt produces a magnetic 
material which exhibits a non-zero magnetostrictive coefficient, albeit at 
a coercivity more appropriate to a "hard" magnetic material than a "soft" 
one, but noting that this parameter was not controlled. 
Others of the above examples show that the technique of causing a sharp 
lattice mismatch or smoother mismatch intermetallic region in overlaid 
component layers produces a magnetic material which exhibits a non-zero 
magnetostrictive coefficient. In particular a sharp lattice mismatch, such 
as occurs with immiscible component layers, could produce a pseudo free 
surface. The term pseudo free surface is used to describe the arrangement 
where the surface of a thin layer can emulate a free surface although 
another layer is contiguous. 
The magnetic material can be used in sensing devices, such as pressure 
gauges, accelerometers and torque sensors, by attaching the material to a 
structure or by depositing the material in the place where it is to be 
used.