Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2000-09-07
2003-06-17
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C365S158000, C360S324100, C360S324120
Reexamination Certificate
active
06580270
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a device comprising an elongated field-sensitive ferromagnetic layer of which the magnetization is sensitive to an applied magnetic field. Within the concept of the invention ‘elongated’ means that layer has an elongated shape i.e. the dimension of the layer in a first direction (the ‘length’) is greater than the dimension (the ‘width’) in a direction perpendicular to the first direction. The first direction is commonly called the ‘longitudinal direction’. The field-sensitive layer may be, for example, (part of) a flux guide or (part of) a magnetoresistive element. The field-sensitive layer may be the second layer of a device comprising a first and a second layer of ferromagnetic material separated by a spacer layer of non-ferromagnetic material, the first and second layer being formed such that the magnetization of the first ferromagnetic layer is maintained in a fixed or pinned direction in the presence of an applied magnetic field, while the magnetization direction of the second ferromagnetic layer is able to change in the presence of an applied magnetic field, the second layer comprising an elongated layer of magnetoresistive material, the device comprising means for directing a sensing current through the elongated layer in a current direction parallel to the longitudinal direction of the layer or perpendicular to the plane of the film. Such a device may be a magnetoresistive sensor or memory element, such as a TMR (Tunnel Magneto Resistance) or GMR (Giant Magneto Resistance) sensor or memory element. The layer of non-magnetic material, which separates the first and second layer of a ferromagnetic material, is often called “spacer layer” in the case of a GMR sensor or memory element, and “barrier layer” in the case of a TMR sensor or memory element. In the following text, the more general term “separation layer” will be used in order to indicate the spacer layer or the barrier layer of a GMR or a TMR element, respectively.
A device of the type described in the opening paragraph is known, for example, from U.S. Pat. No. 5,465,185, wherein a sensor is shown. The first layer is also called the ‘pinned layer’ and the second the ‘free layer’.
Within the concept of the invention, a ‘free layer’ is a layer whose magnetization direction is able to change at applied fields with a strength lower than the strength of the field required for changing the magnetization direction of the ‘pinned’ layer. Such changing may be a nearly free rotation so as to follow the applied magnetic field such as, for example, in sensors or a changing between two anti-parallel states such as, for example, in memory elements.
It is often preferred that the elongated field-sensitive layer, when used in a magnetoresistive element for GMR devices in which a sensing current is parallel to the longitudinal direction, has a relatively large ratio of the length (l) over the width (w) of the layer, and thus forming a stripe. Such a ratio leads to a large resistance of the stripe, and hence, for a given sense current I
sense
, a given relative resistance change &Dgr;R/R and a given sheet resistance R↑, to a large voltage difference between the two extreme states of the element:
&Dgr;
V/V
=(
l/w
)·&Dgr;
R/R·R↑·I
sense
(1)
However, such devices with a l/w ratio>1, and especially when having a large t(thickness)/w(width) ratio, prove to have a relatively large field that is required to change the magnetization direction of the free layer. In practice this hampers, for example, the use of GMR spin valves as MRAMs (Magnetic Random Access Memories), because large currents would be required to switch the element, leading to the necessity to use a transistor which is much larger than the magnetic element itself. Another example is the application of AMR and GMR sensors for measuring angles, using an elongated, preferably long and narrow stripe shaped MRE (magnetoresistive Element) (i.e. one having a l/w ratio>1) and more preferably having a large t(thickness)/w(width) ratio. A small and preferably zero net magnetic anisotropy is required, as the magnetization of such a sensor should be saturated for all directions of the (moderate) applied field to enable an accurate angle reading to be performed.
The above equation (1) refers to the situation with the current in the plane of the layers. Recently sensor or memory element structures in which the current is directed perpendicular to the plane of the layers have attracted much attention. Examples are spin tunneling structures (a sandwich structure of two ferromagnetic layers with an oxidic tunneling barrier in between). For these structures no large l/w ratio is required from the point; of view of output voltage. However, the need for a large l/w ratio may nevertheless arise in specific sensor (such as read heads) or magnetic element designs.
The problem discussed above becomes particularly serious upon the miniaturization of sensor and memory elements to a scale at which stripe widths enter the nanometer range (w<1 &mgr;m).
SUMMARY OF THE INVENTION
It is an object of the invention to provide a device with a reduced net magnetic anisotropy and/or reduced switching field.
To this end, a device in accordance with the invention is characterized in that the field-sensitive ferromagnetic layer comprises a sandwich structure comprising two magnetic layers, with parallel easy magnetization axes due to the magnetocrystalline anisotropy, and with opposite magnetization directions due to anti-ferromagnetic coupling by a non-magnetic layer
3
separating the two layers
1
and
2
, while the magnetization directions due to the magnetocrystalline anisotropy for the two magnetic layers are directed transversely to the longitudinal axis of the field-sensitive ferromagnetic layer.
Terms known to the person of skill in the art are used in the present specification. Some of these are summarized herebelow. The layer of non-magnetic material, which separates the first and second layer of a ferromagnetic material, is often called “spacer layer” in the case of a GMR sensor or memory element, and “barrier layer” in the case of a TMR sensor or memory element. In the following text, the more general term “separation layer” will be used in order to indicate the spacer layer or the barrier layer of a GMR or a TMR element, respectively. Magnetocrystalline anisotropy is the dependence of the (free) energy of a ferromagnetic material on the direction of magnetization of the material, resulting from the internal structure in the material. The total magnetic anisotropy of a ferromagnetic body such as a layer is the result of magnetocrystalline anisotropy, plus a contribution from the shape anisotropy of the body. The shape anisotropy is due to the dependence of the total energy in the magnetostatic field due to the magnetization of the body. The external shape of the body defines preferred directions of magnetization of the body. Magnetic anisotropy creates a situation in which the total energy of the body is lowest when the magnetization direction is parallel or antiparallel to one specific direction, or to several directions which are equivalent from the point of view of the symmetry of the body. Such a direction is called an easy axis direction. For layer structures the shape anisotropy is such that the preferred magnetization directions are confined to the plane of the layer or of a stock of layers (a film). This is a situation of application for a number of embodiments in this patent application (i.e.: there is no contribution from magnetocrystalline anisotropy which more than counteracts the shape anisotropy). In addition, the magnetization within the plane of the layer can have one or more easy directions. In a number of embodiments in this patent application the situation is considered in which the magnetocrystalline anisotropy of the magnetic free layer is uniaxial, i.e., in which there is only one preferred easy direction with the plane of the layer (film). This situation can be created, e.g., by growth o
Aurora Reena
Biren Steven R.
Koninklijke Philips Electronics , N.V.
Lefkowitz Edward
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