Zhenghong Qian

70% TMR at room temperature for SDT sandwich junctions with CoFeB as free and reference Layers

Spin dependent tunneling (SDT) wafers were deposited using dc magnetron sputtering. SDT junctions were patterned and connected with one layer of metal lines using photolithography techniques. These junctions have a typical stack structure of Si(100)-Si/sub 3/N/sub 4/-Ru-CoFeB-Al/sub 2/O/sub 3/-CoFeB-Ru-FeCo-CrMnPt with the antiferromagnet CrMnPt layers for pinning at the top. High-resolution transmission electron microscopy (HRTEM) reveals that the CoFeB has an amorphous structure and a smooth interface with the Al/sub 2/O/sub 3/ tunnel barrier. Although it is difficult to pin the amorphous CoFeB directly from the top, the use of a synthetic antiferromagnet (SAF) pinned layer structure allows sufficient rigidity of the reference CoFeB layer. The tunnel junctions were annealed at 250/spl deg/C for 1 h and tested for magneto-transport properties with tunnel magnetoresistive (TMR) values as high as 70.4% at room temperature, which is the highest value ever reported for such a sandwich structure. This TMR value translates to a spin polarization of 51% for CoFeB, which is likely to be higher at lower temperatures. These junctions also have a low coercivity (Hc) and a low parallel coupling field (Hcoupl). The combination of a high TMR, a low Hc, and a low Hcoupl is ideal for magnetic field sensor applications.

Magnetostriction effect of amorphous CoFeB thin films and application in spin-dependent tunnel junctions

CoFeB thin films and magnetic tunnel junctions using them are studied for magnetostriction effect. The single-layer films were sputter deposited with excellent soft magnetic properties including a high saturation magnetization of 1.5T, a near-zero hard axis coercivity, a low easy axis coercivity of 2.0Oe, and an induced magnetic anisotropy field of 32Oe. The saturation magnetostriction constant is measured to be 31ppm. Magnetic tunnel junctions (MTJs) were fabricated and tested for potential strain gauge applications. The gauge factor for the magnetostrictive MTJs, a measure of strain sensitivity, is many times of the best piezoresistive devices.

High frequency measurements of CoFeHfO thin films

High-frequency measurements of the transverse susceptibility and damping constant of CoFeHfO thin films have been made over a frequency range of 0.1 GHz to 6 GHz as a function of film resistivity, thickness, and temperature. The film resistivity varied from 250 /spl mu//spl Omega/cm to 2100 /spl mu//spl Omega/cm. The films show relatively low damping at high frequencies with the damping constant /spl alpha/ ranging from 0.01 to 0.06. The damping constant increases with film resistivity and, for the highest resistivity films, the damping constant decreases as the thickness increases. The damping constant, induced anisotropy, and film resistivity show weak temperature dependence over a temperature range from 4 K to 300 K. The low damping constant, in conjunction with the high anisotropy, large resistivity, and large spin-dependent-tunneling magnetoresistance, makes this material attractive for several high-frequency magnetic device applications.

Spin dependent tunneling junctions with reduced Neel coupling

A new structure of spin dependent tunneling (SDT) junctions has been demonstrated to have a much reduced Neel coupling field between the free and pinned ferromagnetic layers comparing with conventional SDT structures. The new structure consists of a modified synthetic-antiferromagnetic composite layer as the pinned layer with two Ru spacer layers and three ferromagnetic layers. The Neel coupling field is much reduced for both top- and bottom-pinned SDT structures using this new composite pinned layer. Furthermore, the net magnetic moment is kept at zero for the composite pinned layer to minimize the fringe field after patterning. The coupling reduction can be understood by considering the additive contribution from the first two interfaces with Ru in the composite pinned layer, which cancels that from the pinned layer interface with the barrier. By properly spacing these three most important interfaces, reducing the coupling to basically zero is realized. The coupling reduction allows the elimination of a...

Linear spin-valve bridge sensing devices

Spin-valves are applied in a variety of devices including magnetic sensors and isolators. The linear spin-valve sensing element is configured as a Wheatstone bridge consisting of four sensing resistors. The sensing devices are characterized by three parameters: the sensing field range, sensitivity and linearity. The devices using linear spin-valve sensing elements exhibit excellent performance. A Linear magnetic field sensor constructed of a bridge with 4 /spl mu/m wide line serpentine resistors shows a sensitivity of 1.6 mV/V-Oe in the operation range of -5 Oe to 5 Oe. An analog magnetic isolator constructed of a bridge with 4 /spl mu/m wide line serpentine resistors exhibits superior performance with a sensitivity of 1.27 mV/V-mA and a linearity error less than 0.05%.

Magnetic design and fabrication of linear spin-valve sensors

A spin-valve configuration with double free layers has been used in design and fabrication of a linear sensing resistor with excellent performance. In this novel structure, one free layer is the sensing layer which contributes to the GMR while another layer is the interacting layer which does not contribute to the GMR but improves the sensor performance. This approach not only can effectively reduce the hysteresis but also make it easy to optimize the bias point of the sensor.

3-Axis Magnetometers Using Spin Dependent Tunneling: Reduced Size and Power

A 3-axis magnetometer has been constructed using 3 Spin Dependent Tunneling (SDT) magnetic field sensors as transducers. This magnetometer has been designed for use in Unattended Ground Sensor (UGS) applications. As such, there has been an emphasis on low cost, size, and power. The present version is smaller than previous versions, and is ready for prototype sampling. This paper describes the basic properties of the SDT 3-axis magnetometer, including size, power, and noise floor.

Magnetoresistive signal isolators employing linear spin-valve sensing resistors

Feasibility of fabricating low power, high-speed, magnetoresistive signal isolators employing linear spin-valve resistors as sensing elements has been demonstrated. In the fabricated prototype digital isolators, linear spin-valve resistors are physically isolated from an on-chip coil by an 11 μm BCB isolation barrier, which provides the galvanic isolation with a breakdown voltage larger than 2000 V. The devices are high speed (>50 MHz), small size, and low power consumption. Only a 4.5 mA coil driving current is required for the device to be fully functional. The power consumption is estimated to be ∼1/6 of NVE’s present isolator products, which requires a 50 mA coil driving current. Besides digital signal isolators, linear spin-valve resistors can also be used in analog signal isolators. The response of the linear spin-valve resistors to the current passing through the coil has been demonstrated to have a very good linearity, with a linearity error less than 0.05%.

Prototype spin-dependent tunneling isolators integrated with integrated circuit electronics

Low power, fast speed, small size, wide temperature range, and common-mode-noise-rejection capability are some of the attributes of magnetoresistive devices for galvanic isolation over conventional devices such as optical and capacitive devices. We have fabricated prototype galvanic isolators using spin-dependent tunneling materials. The tunnel junctions have been deposited by rf diode sputtering and the Al2O3 barriers are formed by depositing a thin layer of Al, then oxidizing it with oxygen contained plasma. The junctions are then patterned using photolithography techniques to define the pinned and free layers separately. A series of tunnel junctions are connected in a Wheatstone bridge form and are fabricated directly on top of the integrated circuit (IC) electronics that are used to process the signals from the bridge. In its core magnetics design, this digital device simply employs a one-bit memory cell capable of operating at 5 V. The functions of the devices are tested using a function generator, a...

Fabrication and properties of spin dependent tunneling junctions with CoFeHfO as free layers

Spin-dependent tunneling (SDT) structures of Ta–Cu–Ta–CoFeHfO–Al2O3–FeCo–CrPtMn have been deposited by rf diode sputtering. The junctions have been fabricated using photolithographic techniques. A junction magnetoresistive ratio as high as 34% has been obtained after annealing the junctions at 250 °C for 1 h. The junctions have a typical bias voltage of 475 mV at half-maximum magnetoresistance values. The resistance–area–product is about 1 MΩ μm2, and the dc breakdown voltage is about 1.5 V. AlN has also been investigated as a barrier for the junctions. CoFeHfO layers have a high in-plane induced anisotropy field of 65 Oe and a high 4πMs value of 1.2 T, leading to a ferromagnetic resonance frequency higher than 2 GHz. This material has a high bulk resistivity of 1000 μΩ cm, resulting in a small eddy current effect. Therefore, a SDT device with CoFeHfO as the free layer is an attractive candidate for high-speed applications.