Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2001-03-08
2002-08-20
Le, Vu A. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S171000
Reexamination Certificate
active
06438026
ABSTRACT:
The invention relates to a magnetic field element provided with a stack of a first magnetic layer structure, a second magnetic layer structure having a substantially fixed direction of magnetization, and a spacer layer structure separating the first magnetic layer structure and the second magnetic layer structure from each other, the magnetic field element being further provided with a biasing means for applying a longitudinal bias field to the first magnetic layer structure.
U.S. Pat. No. 6,023,395 discloses a magnetic tunnel junction magnetoresistive sensor for sensing external magnetic fields. A magnetic tunnel junction device comprises a fixed ferromagnetic layer and a sensing ferromagnetic layer mutually separated by an insulating tunnel barrier layer and is based on the phenomenon of spin-polarized electron tunneling. The response of magnetic tunnel junction devices is determined by measuring the resistance of the magnetic tunnel junction when a sense current is passed perpendicularly through the magnetic tunnel junction from one ferromagnetic layer to the other ferromagnetic layer. Magnetic tunnel junction devices suffer from the problem of maintaining a single magnetic domain state. Shifting domain walls cause noise and reduce the signal-to-noise ratio. This may give rise to a non-reproducible response of the sensor, when a linear response is required. In view of this problem the known magnetic tunnel junction magnetoresistive sensor is provided with a biasing ferromagnetic layer in the magnetic tunnel junction stack of layers that is magnetostatically coupled with the sensing ferromagnetic layer. A non-magnetic electrically conductive spacer layer is present between the biasing layer and the other layers of the stack, such that a ferromagnetic coupling between the biasing layer and the sensing ferromagnetic layer is prevented. The demagnetizing field from the biasing ferromagnetic layer magnetostatically couples with the edges of the sensing ferromagnetic layer. In this way, wherein the spacer layer is of sufficient thickness, the magnetic moments of the sensing layer and the biasing layer are magnetically coupled to one another via an antiferromagnetic coupling field which results from the magnetostatic coupling of the edges of the sensing and biasing layers.
A disadvantage of the known sensor is that the degree of magnetic coupling of the sensing layer with the biasing layer depends on the geometry of the element, particularly the relevant layers thereof. Further, an antiferromagnetic magnetostatic coupling requires a thick spacer layer in order to suppress a possible ferromagnetic coupling, however, such a thick layer introduces undesired electrical shunting in case of a current-in-plane configuration. This effect makes an antiferromagnetic coupling mechanism practically unsuitable for application in GMR or AMR elements. In this context it is noted that the disclosed measure is only related to magnetic tunnel junction magnetoresistive sensors.
It is an object of the invention to improve the magnetic field element of the kind described in the opening paragraph, in such a way that the magnetic coupling induced by the biasing means is relatively independent of the geometry of the field element.
This object is achieved with the magnetic field element according to the invention which field element is provided with a stack of a first magnetic layer structure, a second magnetic layer structure having a substantially fixed direction of magnetization, and a spacer layer structure separating the first magnetic layer structure and the second magnetic layer structure from each other, the magnetic field element being further provided with a biasing means for applying a longitudinal bias field to the first magnetic layer structure, wherein the biasing means includes a biasing magnetic layer structure located opposite to the first magnetic layer structure, which biasing magnetic layer structure provides a magnetic coupling field component perpendicular to the direction of magnetization of the second magnetic layer structure and is separated from the first magnetic layer structure by a nonmagnetic layer structure, whereby the first magnetic layer structure is mainly ferromagnetically coupled to the biasing magnetic layer structure The longitudinal bias field is directed perpendicularly to the fixed direction of magnetization of the second magnetic layer structure. The applied measure causes a magnetic coupling between the biasing magnetic layer structure and the first magnetic layer structure through the non-magnetic layer structure. During manufacturing of the magnetic field element the biasing magnetic layer structure can be realized in a simple way using technology already existing. During forming of the biasing magnetic layer structure a magnetic field should be applied which makes an angle larger than 0° and smaller than 180° with the field applied during forming of the second magnetic layer structure.
The biasing means applied into the field element is applicable in sensors of different sensing principles. The magnetic field element according to the invention can be a spin tunnel junction element, in which case the spacer layer structure includes a tunnel barrier layer of an insulating material, such as Al
2
O
3
, or a spin-valve element of the giant magneto-resistive type. In both cases a very important magnetic characteristic is the magnetic hysteresis of the first magnetic layer structure. When the magnetic moment of this layer structure is aligned with a magnetic field of a magnetic source, e.g. a passing magnetic disk, an anti-parallel alignment with the magnetic moment of the second magnetic layer structure may be achieved. This effect gives rise to a change in resistance. In order to prevent a distortion in the output of the magnetic field element it is essential to prevent the introduction of domain walls moving erratically through the first magnetic layer structure during said magnetic alignment. Domain walls may be introduced if a magnetic hysteresis exists in the first magnetic layer structure. It has been proved that said hysteresis is reduced considerably and even completely eliminated in the sensor according to the invention.
In the magnetic field element according to the invention a magnetic coupling field, i.e. a longitudinal biasing field, between the biasing magnetic layer structure and the first magnetic layer structure exists. The resulting coupling is mainly ferromagnetic, i.e. that the ferromagnetic coupling is dominant over possibly present antiferromagnetic couplings. This dominance can be obtained by carefully choosing the thickness of the spacer layer structure as function of the ferromagnetic materials of the relevant layer structures. The ferromagnetic coupling is defined over a large area, i.e. in principle over the opposing faces of the biasing layer structure and the first layer structure, on a very local level, i.e. of the order of the size of the grains of the structures. More precise, the desired ferromagnetic coupling is obtained by exploiting the ferromagnetic coupling due to the waviness or roughness of magnetic layer structures. This coupling is also called orange-peel coupling or topological coupling. In this invention the correlated waviness of the biasing magnetic layer structure and the first magnetic layer structure, which structures are separated by the nonmagnetic spacer layer structure of sufficient thickness, causes the ferromagnetic coupling. In the case of parallel magnetizations the magnetic flux crosses the spacer structure from the magnetic structure to the others; this makes the situation with parallel magnetizations energetically favorable over an antiparallel configuration.
By making the spacer layer structure sufficiently thin, e.g. in the case of use of Ta as spacer material the thickness is typically less than about 3 nm, and choosing the saturation magnetization of a ferromagnetic layer in the biasing layer structure sufficiently large, it can be assured that the ferromagnetic coupling is dominant over magnetostatic
Gillies Murray Fulton
Kuiper Antonius Emilius Theodorus
Lenssen Kars-Michiel Hubert
Biren Steven R.
Koninklijke Philips Electronics , N.V.
Le Vu A.
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