• What is TMR magnetic sensor ?

    A tunnel junction TMR magnetoresistive sensor formed on layers having nitrogen interspersed therein. The nitrogenation of the layers on which the sensor is deposited allows the sensor layers to have very smooth, uniform surfaces.

    The heart of a computer’s long-term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air-bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk and when the disk rotates, air adjacent to the surface of the disk moves along with the disk. The slider flies on this moving air at a very low elevation (fly height) over the surface of the disk. This fly height can be on the order of Angstroms. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.

    The write head includes a coil layer embedded in first, second and third insulation layers (insulation slack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air-bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to tire coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.

    In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. This sensor includes a nonmagnetic conductive layer, referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, and hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air-bearing surface (ABS) and the magnetic moment of the free layer is biased parallel to the ABS, but is free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.

    The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos Θ, where Θ is the angle between the magnetizations of the pinned and free layer’s. In a read mode, the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as read back signals.

    More recently, researchers have focused on the development of magnetic tunnel junction (MTJ) sensors, also referred to as tunneling magnetoresistance (TMR) sensors or tunnel valves. Tunnel valves or MTJ/TMR sensors offer the advantage of providing improved signal amplitude as compared with other GMR sensors. MTJ/TMR sensors operate based on the spin dependent tunneling of electrons through a thin, electrically insulating barrier layer. The structure of the barrier layer is critical to optimal MTJ/TMR sensor performance, and certain manufacturing difficulties such as target poisoning during barrier-layer deposition have limited the effectiveness of such MTJ/TMR sensors. Therefore, there is a strong felt need for a magnetic tunnel junction (MTJ) sensor that can provide optimal MTJ/TMR performance, and also, for a practical method of manufacturing such an optimized MTJ/TMR sensor.

    A Magnetic Tunnel Junction (MTJ) structure is essentially a metal-insulator-metal structure in which the metal electrodes are composed for ferromagnetic alloys.  This structure is called an MTJ because a potential applied between the ferromagnetic electrodes induces a tunnel current that is dependent on the relative orientation of the magnetization of the ferromagnetic electrodes on opposite sides of the insulating tunnel barrier.  Thus in effect, it shows a magnetoresistance effect, in which the value of the tunneling magnetoresistance (TMR) is dependent on the relative orientation of the magnetization between the ferromagnetic electrode layers.  In order to take advantage of the TMR effect for sensing purposes, the ferromagnetic electrodes on opposite of the insulating barrier are designed to respond differently to magnetic field so that their differing response produces a large change in resistance in response to an applied magnetic field.  Generally, one layer, called the Pinned Layer (PL) is designed so that its magnetization does not respond to a magnetic field, and the other layer, the Free Layer (FL) is designed so that its magnetization orientation changes readily in response to a magnetic field.   This type of structure using a PL and FL is often called a spin valve.  It can be sued for linear magnetic field sensing, detecting the angle of an applied magnetic field, and for magnetic switching and proximity sensing applications.