Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1998-01-26
2001-08-14
Snow, Walter (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C338S03200R, C428S692100, C360S313000
Reexamination Certificate
active
06275033
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a magnetic field sensor comprising a layered structure E/F
o
/S/F
f
, in which:
E is an exchange-biasing layer, comprising nickel oxide;
F
o
is a ferromagnetic layer with a fixed magnetization, comprising cobalt;
S is a spacer layer;
F
f
is a ferromagnetic layer with a free magnetization. For the sake of clarity, a number of terms in this definition will here be further elucidated:
1. The terms “fixed” and “free” in relation to the magnetizations of the layers F
o
and F
f
are intended to have a relative meaning, viz. the magnetization M
f
of the layer F
f
is “free” with respect to the magnetization M
o
of the layer F
o
if, under the influence of an applied external magnetic field H, M
f
can be more easily rotated from its equilibrium position than M
o
;
2. The magnetization M
o
is “fixed” by exchange-biasing the layer F
o
to the layer E;
3. The term “nickel oxide” refers to any stoichiometric or non-stoichiometric compound of nickel and oxygen. Although the symbol “NiO” will frequently be used in this context, this symbol should be viewed as encompassing compounds of the form NiO
1±&dgr;
, in which &dgr; is a relatively small fraction.
Magnetic field sensors of this type may be employed inter alia:
as magnetic heads, which can be used to decrypt the magnetic flux emanating from a recording medium in the form of a magnetic tape, disc or card;
in compasses, for detecting the terrestrial magnetic field, e.g. in automotive, aviation, Maritime or personal navigation systems;
in apparatus for detecting position and/or angle, e.g. in automotive applications;
as field sensors in medical scanners, and as replacements for Hall probes in various other applications;
as memory cells in Magnetic Random-Access Memories (MRAMs);
as current detectors, whereby the magnetic field produced by such a current is detected.
A layered structure as described in the opening paragraph is known, for example, from an article by T. C. Anthony et al. in
IEEE Trans. Magn.
30 (1994) pp. 3819-3821, in which the system NiO/Co/Cu/Co is studied. The authors report advantageously large room-temperature magneto-resistance (MR) ratios of up to 17% in this system (as compared to typical values of about 4-6% in conventional multilayers, i.e. multilayers using an FeMn exchange-biasing layer). However, the coercivity of the free Co layer is disappointingly high, with values of the order of 3 kA/m being observed in inner loop measurements (as compared to values of the order of 0.2-0.4 kA/m in conventional multilayers). Such high coercivity values greatly hinder potential use of the NiO/Co/Cu/Co system in practical applications.
SUMMARY OF THE INVENTION
It is an object of the invention to alleviate this problem. More particularly, it is an object of the invention to provide a NiO-exchange-biased magnetic field sensor having a relatively large room-temperature MR ratio and yet having a relatively low free-layer coercivity.
These and other objects are achieved according to the invention in a magnetic field sensor as described in the opening paragraph, characterized in that the material of the layer F
f
has a magnetostriction constant of at most 1.5×10
−6
and a crystal anisotropy of at most 1.3 J/m
3
.
The invention exploits the realization that the coercivity of a ferromagnetic thin film is influenced by the size of spatial anisotropy fluctuations, since such fluctuations tend to lead to domain wall pinning. An important contributor to this effect is the magnetostriction anisotropy, which should therefore be as low as possible. However, the inventors have realized that there is a second contributor in the case of an NiO-based multilayer which simply does not play a role in the case of FeMn-based multilayers. This can be explained as follows.
Conventional multilayers are usually deposited on a Ta seed layer, which induces a (111) crystallographic texture in subsequently deposited layers. Metals with a face-centered-cubic (fcc) crystal structure do not display crystal anisotropy in the (111) plane, and, thus, any such metal (such as Co) deposited upon a Ta seed layer will be automatically free of crystal anisotropy. However, when such a metal is deposited upon NiO, the situation is radically different; it has been observed that layers grown on NiO display a weak or incomplete (111) texture, and, consequently, such layers will not in general be free of crystal anisotropy. It is therefore important in this case to choose a material which has an intrinsically low crystal anisotropy of itself. Now, in a conventional multilayer, the FeMn exchange-biasing layer is situated at or towards the top of the layered structure (as one progresses away from the substrate); this is permissible because FeMn is a relatively good electrical conductor, so that it is thus possible to make electrical contact with the structure via the top layer. However, in contrast, NiO is an electrical insulator, and thus cannot be located at the top of the layered structure if electrical contact is to be made with the structure via its top layer. The NiO exchange-biasing layer must thus be situated at the bottom of such a structure. Consequently, the NiO acts as a substrate for the subsequently deposited layers, which (according to the above arguments) must thus have an intrinsically low crystal anisotropy.
In the most ideal case, the crystal anisotropy of the free ferromagnetic layer F
f
will be exactly zero or at least substantially zero. In practice, this can be difficult to achieve; however, the inventors have determined that satisfactory results can still be obtained with non-zero crystal anisotropies less than 1.3 J/m
3
. In this context, it should be noted that the crystal anisotroy is in fact a series expansion of terms, in which the third and higher-order terms are, in general, substantially negligable with respect to the first and second-order terms. For practical purposes therefore, any reference to the crystal anisotropy in this text may be regarded as a reference to the sum of the first and second-order terms in the series expansion.
Free-layer (F
f
) materials with the inventive properties listed in claim
1
occur among the alloys Ni
x
Fe
y
Co
z
, in which:
x+y+z=100,
64≦x≦74 ,
15≦y≦22. Preferential examples of suitable such alloys include Ni
66
Fe
16
Co
18
and Ni
72
Fe
21
Co
7
. For example, the inventors have observed that a test multilayer with the structure:
Si(100)/50 nm NiO/3 nm Co
90
Fe
10/Cu/F
f
/Ta demonstrates an MR-ratio of 12.7% in combination with a free-layer coercivity of just 0.3 kA/m when the layer F
f
is comprised of 5 nm Ni
66
Fe
16
Co
18
.
A further embodiment of the sensor according to the invention is characterized in that a laminar portion of the layer F
f
along its interface with the layer S is predominantly comprised of cobalt. An example of a suitable material for use in such an “interface layer” is Co
90
Fe
10
; however, pure Co may also be used, for example. Specifically, in the case of the above-mentioned test multilayer, using the composite 0.8 nm Co
90
Fe
10/
4.2 nm Ni
66
Fe
16
Co
18
as the layer F
f
(with the Co
90
Fe
10
interface layer adjacent to the Cu spacer layer) produces a multilayer with an MR-ratio of 14.1% and a free-layer coercivity of 0.2 kA/m.
The quantity of any element is the above-mentioned alloys recited as having a specific composition may vary in the amount of ±2 at. %.
The spacer layer S may be comprised of various different materials, such as Cr, Mn, Ru, and Ag, for example. However, a preferential embodiment of the sensor according to the invention is characterized in that the spacer layer S is comprised of Cu or Au. These latter metals have resistivity values (of the order of about 2.2-2.5 &mgr;&OHgr;/cm) which are relatively low compared to those of the former metals (of the order of about 10-15 &mgr;&OHgr;/cm). Cu is particularly advantageous, because it is relatively inexpensive compared to Au.
The deposition of NiO on an underlying substrate can generally proceed
Snow Walter
Spain Norman N.
U.S. Philips Corporation
LandOfFree
Magnetic field sensor having nickel oxide and cobalt... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Magnetic field sensor having nickel oxide and cobalt..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Magnetic field sensor having nickel oxide and cobalt... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2538689