Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2000-08-09
2003-04-01
Miller, Brian E. (Department: 2754)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06542343
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetic tunnel junction (MTJ) heads. More particularly, it relates to MTJ heads with flux guides defining the track widths of MTJ heads.
BACKGROUND ART
Magnetic tunnel junction (MTJ) devices based on the phenomenon of spin-polarized electron tunneling. A typical MTJ device includes two ferromagnetic layers separated by a thin insulating tunnel barrier layer. One of the ferromagnetic layers has a higher saturation field in one direction of an applied magnetic field, typically due to its higher coercivity, than the other ferromagnetic layer. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling of electrons can occur between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
MTJ devices have been proposed primarily as memory cells for solid state memory devices. The state of the MTJ memory cell is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of two ferromagnetic layers. The tunneling current is spin polarized, which means that the electrical current passing from one of the ferromagnetic layers, for example, a layer whose magnetic moment is fixed or prevented from rotation, is predominantly composed of electrons of one spin type (spin up or spin down, depending on the orientation of the magnetic moment of the ferromagnetic layer). The degree of spin polarization of the tunneling current is determined by the electronic band structure of the magnetic material comprising ferromagnetic layer at the interface of the ferromagnetic layer with the tunnel barrier layer. The first ferromagnetic layer thus acts as a spin filter. The probability of tunneling of the charge carriers depends on the availability of electronic states of the same spin polarization as the spin polarization of the electrical current in the second ferromagnetic layer. Usually, when the magnetic moments of the first and second ferromagnetic layers are parallel to each other, there are more available electronic states than when the magnetic moments of the two ferromagnetic layers are aligned antiparallel to each other. Thus, the tunneling probability of the charge carriers is highest when the magnetic moments of both layers are parallel, and is lowest when the magnetic moments are antiparallel. When the moments are arranged neither parallel nor antiparallel, the tunneling probability takes on an intermediate value. Thus, the electrical resistance of the MTJ memory cells depends on the spin polarization of the electrical current and the electronic states in both of the ferromagnetic layers.
MTJ heads have attracted more attention since a large tunneling magneto-resistance (TMR) was found at room temperature. These MTJ heads with TMR larger than 40% are potentially applicable as magnetoresistive read heads for high areal density recording. However, prior to head application, several design issues must be worked out. These issues include shield to shield distance, longitudinal biasing, and shorting across the insulating tunnel barrier during mechanical lapping processes. The shorting problem is particularly important since the head typically undergoes several lapping processes during the definition of the air bearing surface (ABS). More importantly, future higher areal density data storage requires that MTJ heads be able to operate at ever-decreasing track widths (TW). However, the cross-sectional area of an MTJ head decreases as the track width decreases. Generally, the resistance of an MTJ head depends inversely on the cross-sectional area of the MTJ valve. The resistance of an MTJ head can become very high for a sufficiently small cross-sectional area. A larger resistance generally means a larger noise level in the MTJ head, which leads to a poor signal to noise ratio (SNR).
U.S. Pat. No. 5,898,547 issued Apr. 27, 1999 and U.S. Pat No. 5,901,018 issued May 4, 1999 to Fontana, Jr. et al. disclose a magnetic tunnel junction (MTJ) magnetoresistive read head having an MTJ sensing or free ferromagnetic layer that functions as a rear flux guide to direct magnetic flux from a magnetic recording medium to the tunnel junction. The back edge of the free layer is located farther than the back edges of the tunnel barrier layer and the pinned layer from a sensing surface of the MTJ heads. Unfortunately, the track widths of the MTJ heads have not been reduced to optimize the efficiency of the MTJ heads. Furthermore, Fontana et al. does not teach the prevention of electrical shorts during the definition of air bearing surface (ABS).
U.S. Pat No. 5,930,087 issued Jul. 27, 1999 to Brug et al. discloses a robust recording head with a spin tunneling sensing element separated from an interface between the recording head and a recording media to prevent collisions and other ill effects at this interface. The spin tunneling sensing element includes a pair of magnetic elements, wherein one of the magnetic elements functions as a flux guide that conducts magnetic flux emanating from a recording medium away from the interface to an active area of the spin tunneling sensing element. However, Brug does not teach the use of a flux guide for reducing track width to optimize the efficiencies of the robust recording heads.
Japanese published patent application JP8-115511, published May, 7, 1996 discloses the use of a flux guide to improve the signal to noise ratio of a spin valve type GMR head by increasing a signal level from the spin valve as the track width is reduced. However, this publication does not address MTJ heads. Furthermore, the publication does not address reducing the noise level.
An article entitled “Evaluation of Front Flux Guide Type Magnetic Tunnel Junction Heads” published April 2000 in INTERMAG 2000 Conference Digest to Shimazawa et al. discloses magnetic tunnel junctions (MTJs) heads using the free layers as flux guide to prevent the electrical short during the definition of the air bearing surface (ABS). Unfortunately, Shimazawa et al. does not teach the use of a flux guide for reducing the track width so that the efficiencies of the MTJs heads are optimized.
Another article entitled “The Electrical and Magnetic Response of Yoke-Type Read Heads Based on A Magnetic Tunnel Junction” by Coehoorn et al. and published IEEE Transactions on Magnetics, vol. 35, No. 5, September 1999, discloses yoke-type read heads containing a tunnel junction magnetoresistive element (TMRE) that includes a front upper flux guide, a back upper flux guide, and a bottom flux guide. Coehoorn et al. does teach the use of the front flux guide with an anisotropic permeability to minimize side reading for a small track width, but does not teach the use of the flux guide for reducing the resistance to increase areal density of magnetic storage media.
There is a need, therefore, for an MTJ head including a flux guide that overcomes the above difficulties.
OBJECTS AND ADVANTAGES
Accordingly, it is a primary object of the present invention to provide MTJ heads with flux guides that prevent electrical shorting of the MTJ layers during the definition of air bearing surface.
It is a further object of the invention to provide MTJ heads with flux guides that reduce the track widths while leaving the cross-sectional areas unchanged. Therefore, the resistance of the tunnel junction valve is reduced, which reduces noise and improves the signal to noise ratio.
It is an additional object of the invention to provide MTJ heads with flux guides to prevent the leakage of the magnetic flux into the shields of the MTJ sensors.
It is another object of the invention to provide MTJ heads with flux guide enhancing the flux coupling to the MTJ sensor.
International Business Machines - Corporation
Lumen Intellectual Property Services Inc.
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