Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-07-13
2003-06-03
Ometz, David L. (Department: 2653)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C365S158000, C365S173000, C428S900000
Reexamination Certificate
active
06574079
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to the art of magnetic tunnel junction (MTJ) read head devices, which sense magnetic fields in a magnetic recording medium. More particularly, the present invention relates to a magnetic tunnel junction arrangement having a tunneling barrier made of particular materials that result in high tunneling performance. The invention finds particular application in conjunction with reading hard disk drives and will be described with particular reference thereto. However, it is to be appreciated that the invention will find application with other magnetic storage media. Further, it is to be appreciated that the invention will find application in other magnetic field detection devices as well as in other devices and environments.
2. Description of the Related Art
Magneto-resistive (MR) sensors based on anisotropic magneto-resistance (AMR) or a spin-valve (SV) effect are widely known and extensively used as read transducers to read magnetic recording media. Such MR sensors can probe the magnetic stray field coming out of transitions recorded on a recording medium by generating resistance changes in a reading portion formed of magnetic materials. AMR sensors have a low resistance change ratio or magneto-resistive ratio &Dgr;R/R, typically from 1 to 3%, whereas SV sensors have a &Dgr;R/R ranging from 2 to 7% for the same magnetic field excursion. SV heads showing such high sensitivity are able to achieve very high recording densities, that is, over several giga bits per square inch or Gbits/in
2
. Consequently, SV magnetic read heads are progressively supplanting AMR read heads.
In a basic SV sensor, two ferromagnetic layers are separated by a non-magnetic layer, an example of which is described in U.S. Pat. No. 5,159,513. An exchange or pinning layer of FeMn, for example, is further provided adjacent to one of the ferromagnetic layers. The exchange layer and the adjacent ferromagnetic layer are exchange-coupled so that the magnetization of the ferromagnetic layer is strongly pinned or fixed in one direction. The magnetization of the other ferromagnetic layer is free to rotate in response to a small external magnetic field. When the magnetizations of the ferromagnetic layers are changed from a parallel to an anti-parallel configuration, the sensor resistance increases yielding a relatively high MR ratio.
Recently, new MR sensors using tunneling magneto-resistance (TMR) have shown great promise for their application to ultra-high density recordings. These sensors, which are known as magnetic tunnel junction (MTJ) sensors or magneto-resistive tunnel junctions (MRTJ), came to the fore when large TMR was first observed at room temperature. See Moodera et al, “Large magneto resistance at room temperature in ferromagnetic thin film tunnel junctions,”
Phys. Rev. Lett.
v. 74, pp. 3273-3276 (1995). Like SV sensors, MTJ sensors basically consist of two ferromagnetic layers separated by a non-magnetic layer. One of the magnetic layers has its magnetic moment fixed along one direction, i.e., the fixed or pinned layer, while the other layer, i.e., free or sensing layer, is free to rotate in an external magnetic field. However, unlike SV sensors, this non-magnetic layer between the two ferromagnetic layers in MTJ sensors is a thin insulating barrier or tunnel barrier layer. The insulating layer is thin enough so that electrons can tunnel through the insulating layer. Further, unlike SV sensors, MTJ sensors operate in CPP (Current Perpendicular to the Plane) geometry, which means its sensing current flows in a thickness direction of a laminate film or orthogonal to the surfaces of the ferromagnetic layers.
The sense current flowing through the tunnel barrier layer is strongly dependent upon a spin-polarization state of the two ferromagnetic layers. When the sense current experiences the first ferromagnetic layer, the electrons are spin polarized. If the magnetizations of the two ferromagnetic layers are anti-parallel to each other, the probability of the electrons tunneling through the tunnel barrier is lowered, so that a high junction resistance R
ap
is obtained. On the other hand, if the magnetizations of the two ferromagnetic layers are parallel to each other, the probability of the electrons tunneling is increased and a high tunnel current and low junction resistance R
p
is obtained. In an intermediate state between the parallel and anti-parallel states, such as when the both ferromagnetic layers are perpendicular in magnetization to each other, a junction resistance R
m
between R
ap
and R
p
is obtained such that R
ap
>R
m
>R
p
. Using these symbols, the TMR ratio may be defined as &Dgr;R/R=(R
ap
−R
p
)/R
p
.
The relative magnetic direction orientation or angle of the two magnetic layers is affected by an external magnetic field such as the transitions in a magnetic recording medium. This affects the MTJ resistance and thus the voltage of the sensing current or output voltage. By detecting the change in resistance and thus voltage based on the change in relative magnetization angle, changes in an external magnetic field are detected. In this manner, MTJ sensors are able to read magnetic recording media.
Another problem is a trade-off between high TMR ratio and MTJ resistance. The TMR ratio is proportional to the spin polarization of the two ferromagnetic layers. A TMR ratio as high as 40% was achieved by choosing a preferable composition for the two ferromagnetic layers. See Parkin et al., “Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory,”
J. Appl. Phys.,
v. 85, pp. 5828-5833 (Apr. 15, 1999). However, despite this large TMR ratio, the application of such MTJs in read heads was, up to now, prohibitory due to the large resistance of the junctions, resulting in a large shot noise V
rms
and a poor signal to noise ratio S/N. Shot noise V
rms
=(2·e·I·&Dgr;f)
½
×R, where: e=1.6×10
−19
C; I=current; &Dgr;f=bandwith; and R=junction resistance.
It is possible to reduce the MTJ's resistance-area product R·A or RA using a natural, in situ oxidation method. RA is a characteristic of an insulating barrier and contributes to junction resistance R through the equation R=R·A/junction area. Using a 7 Å or less Al layer that is properly oxidized, an RA as low as 15 &OHgr;.&mgr;m
2
has been achieved. This remarkably low value together with the high TMR ratio make MTJs very attractive for application as read heads for very high recording densities.
However, yet another problem in MTJs is that the thin insulating barrier is very sensitive to one of the manufacturing processes called lapping. Lapping involves the definition of an air bearing surface (ABS) on the MTJ head. Because the insulating barrier is so thin, lapping can create electrical shorts between the two adjacent magnetic layers, rendering the sensor useless.
Tunneling magnetoresistance (TMR) was discussed by Julliere in “Tunneling Between Ferromagnetic Films”
Physics Letters,
54A 225 (1975). However, prior to 1995, the reported MTJ junctions only show very small TMR response at room temperature, at best being on the order of 1-2%.
An MTJ device with a large TMR over 10% at room temperature was reported by Moodera et al. in the aforementioned article “Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film tunnel Junctions”
Physics Review Letters,
74, 3273 (1995). It was hypothesized that increased TMR performance could be achieved by a decrease in surface roughness that results from the base electrode growth, by evaporation onto a cryogenically-cooled substrate, by the use of a seed layer, and by keeping the base electrode extremely thin. The tunnel barrier was formed by cryogenically depositing an Al layer and subsequently warming this layer and plasma oxidizing it to consume more of the Al. The resulting junction resistances were in the range of hundreds of Ohms to tens of k&OHgr; for junct
Araki Satoru
Sun Jijun
Morgan & Lewis & Bockius, LLP
Ometz David L.
TDK Corporation
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