A magnetically shielded instrument for magnetoresistance and noise characterizations of magnetic tunnel junction sensors

A magnetically shielded setup was developed for characterizing magnetoresistance (MR) and noise properties of magnetic tunneling junction (MTJ) sensors. A mu-metal shielding is installed to avoid the interference of external magnetic disturbance. Both MR curves and noise power spectra of MTJ sensors can be obtained for further data analysis. Moreover, a hard-axis magnetic field can be applied to eliminate the hysteresis and the linear field response of MTJ sensors can be measured. The preliminary measurement results on MTJ sensors are presented to illustrate the characterization capabilities of this setup. © 2010 IEEE.published_or_final_versionThe 2010 IEEE International Conference of Electronic Devices and Solid-State Circuits (EDSSC), Hong Kong, 15-17 December 2010. In Proceedings of EDSSC, 2010, p. 1-

Magnetic tunnel junction sensors with conetic alloy

Poster Session - F. Storage Applications and Others: PF-12This journal issue contain selected papers of APDSC'10Al 2O 3 magnetic tunneling junction (MTJ) sensors were fabricated with Conetic alloy Ni 77Fe 14Cu 5Mo 4 deposited as the free layer and pinned layer for its soft magnetic properties. It was observed that the Al 2O 3 MTJ sensors with Conetic exhibited relatively small easy-axis coercivity. Tunneling magnetoresistance (TMR) and noise measurements were carried out to characterize the sensors. TMR of 9.5% and Hooge parameter of 3.825 × 10 -7μm 2 were achieved without any hard-axis field. Hard-axis bias field was applied to eliminate the hysteresis and improve the linear field response of the MTJ sensor. The hysteresis was removed by applying an external magnetic field along the hard axis at 8 Oe and the sensor sensitivity was 0.4 %/Oe within a linear region at room temperature. The relationship between the Hooge parameter and hard-axis field was also investigated and the result demonstrated that the 1/f noise can be suppressed by an optimized hard-axis bias field. This work shows that it is feasible to use Conetic alloy as the soft magnetic layers in MTJ sensors for its small coercivity, and a hard-axis bias field can be used to linearize the sensor response and suppress the 1/f noise. © 2011 IEEE.published_or_final_versionThe Asia-Pacific Data Storage Conference (APDSC'10), Hualien, Taiwan, 27-29 October 2010. In IEEE Transactions on Magnetics, 2011, v. 47 n. 3, p. 714-71

Magnetic tunnel junction magnetic field sensor design tool

A spreadsheet-based magnetic tunnel junction (MTJ) sensor design tool is presented in this paper. The system is developed using Excel and Visual Basic Application. It allows users to optimize the various parameters of the sensor design with the goal of SQUID-like sensitivity. Users can input parameters of the design including magnetic properties, junction areas, and free layers thicknesses. The design tool will then calculate and display automatically various noise sources including Johnson noise, shot noise, 1/f noise, and thermal magnetic noise that must be considered when building MTJ magnetic field sensors. Graphs predicting the sensitivities, operating current and power of the finished sensors are shown and fine tuning of each design parameter is allowed using the scrollbars provided. Using this design tool, effects of changes made to any design parameter can be clearly observed and detailed noise analysis can be studied without manually repeating complex calculations. ©2010 IEEE.published_or_final_versionThe 3rd International Nanoelectronics Conference (INEC 2010), Hong Kong, China, 3-8 January 2010. In Proceedings of the 3rd INEC, 2010, p. 1149-115

Fabrication strategies for magnetic tunnel junctions with magnetoelectronic applications

In this paper, two lithographic fabrication processes for magnetic tunnel junctions (MTJs) with different mask designs and etching technologies are discussed. The advantages and disadvantages of both processes are compared. The crucial steps to protect the oxide insulating barriers and avoid side-wall redepositions (which may lead to short circuits) are developed, and important design considerations of the mask patterns and the device geometric structures are elaborated. We show that implementing the strategies developed greatly increases the successful manufacturing yield of MTJ magnetoelectronics devices.link_to_subscribed_fulltex

Fabrication of spintronic devices - Etching endpoint detection by resistance measurement for magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) have received tremendous interest since the discovery of substantial room temperature tunneling magnetoresistance (TMR) due to spin-dependent tunneling, and have been intensively investigated for applications in next-generation memory devices, hard disk drives, and magnetic sensors. In the fabrication of MTJs, etching is needed to remove the top cap layers, upper magnetic layers, and the middle oxide layer in order to form a tunneling junction. In view of this, we have devised an innovative, simple, low-cost endpoint detection method for fabricating MTJs. In this method, the endpoint is detected by measurement of the sheet resistance of the MTJ stack. Only a multimeter is needed in this method, hence it provides a simple low-cost alternative for spintronic device researchers to explore the research field of magnetic tunnel junctions. This technique is also of great use in other kinds of metallic stack etching experiments.link_to_subscribed_fulltex

Development of ultra-low magnetic field sensors with magnetic tunnel junctions

The discovery of tunneling magnetoresistance (TMR) has enhanced the magnetoresistance (MR) ratio from the giant magnetoresistance (GMR) regime of around 10% to over 400% at room temperature. A combination of magnetic tunnel junctions with high magnetoresistance ratio and soft magnetic layers enables the development of ultra-low magnetic field sensor with sensitivity down to the scale of picoTesla. A magnetic field sensor with such high sensitivity would have important applications in biomedicine, information storage, and remote sensing such as higher resolution images for cardiograph and magnetic resonance imaging and thus earlier detection of abnormal health condition; higher hard-disk density; and remote sensing of metallic objects. We have constructed an automated four-probe electrical measurement system for measuring TMR of magnetic tunnel junctions with high throughput, enabling us to optimize the properties of the devices. Magnetron sputtering is used to deposit thin films with thickness ranged from angstroms to nanometers. Photolithography and ion plasma etching are applied to pattern the devices. The devices have a range of size from 10 μm × 10 μm to 80 μm × 80 μm. The device is composed of the bottom electrode, free soft magnetic layer, insulating oxide layer, pinned layer, pinning layer, and top electrode. The magnetization of the free layer can be rotated by the external magnetic field which in turn changes the resistance of the device and provide the sensing capability. The system structure, design consideration, fabrication process, and preliminary experimental results are discussed and presented in this paper.link_to_subscribed_fulltex

A novel low-cost high-throughput probe card scanner analyzer for characterization of magnetic tunnel junctions

The advancement of the technology of magnetic tunnel junctions (MTJs) greatly hinges on the optimization of the magnetic materials, fabrication process, and annealing conditions which involve characterization of a large number of samples. As such, it is of paramount importance to have a rapid-turnaround characterization method since the characterization process can take even longer time than the fabrication. Conventionally, micropositioners and probe tips are manually operated to perform 4-point electrical measurement on each individual device which is a time-consuming, low-throughput process. A commercial automatic probe card analyzer can provide high turnaround; however, it is expensive and involves much cost and labor to install and maintain the equipment. In view of this, we have developed a novel low-cost, home-made, high-throughput probe card analyzer system for characterization of MTJs. It can perform fast 4-probe electrical measurements including current vs voltage, magnetoresistance, and bias dependence measurements with a high turnaround of about 500 devices per hour. The design and construction of the system is discussed in detail in this paper.link_to_subscribed_fulltex

Preliminary design and noise considerations for an ultrasensitive magnetic field sensor

In this paper, we discuss the structural design including the materials, junction areas, and magnetic layers thicknesses, and various noise sources including Johnson noise, shot noise, 1/f noise, and thermal magnetic noise, that must be considered when building a magnetic tunnel junction (MTJ) magnetic field sensor with the goal of SQUID-like sensitivity. Analytical derivations of the sensor sensitivity and different noise sources are provided. A highly-portable software design tool is developed to optimize the various parameters of the sensor design, while also predicting the expected sensitivity, operating frequency range, operating current and power of the finished sensor. The functions and operations of this design tool are described. The relationships between the sensor detectivity and some critical design parameters are studied with this design tool. A possible design for the construction of an ultrasensitive magnetic field sensor is proposed.link_to_subscribed_fulltex

Detection of pinholes in magnetic tunnel junctions by magnetic coupling

Pinholes in tunnel barriers are detrimental to the performance of magnetic tunnel junctions (MTJs) since they create direct magnetic exchange coupling between the free and pinned magnetic films and may act as current short circuits. A simple and straightforward technique which enables observation of pinholes and distinguishes pinhole coupling from orange-peel coupling would aid greatly in optimizing the performance of MTJs. However, the existing methods for this determination are quite complex and destructive and do not work on complete structures. We have developed a simpler, nondestructive method that works on full MTJ structures which is able to identify whether an observed coupling arises primarily from magnetic exchange coupling through pinholes or from orange-peel coupling. The method is based on the shift in the free layer hysteresis loop at low temperatures. It is well known that the shift in the pinned layer loop at low temperatures is due to the sharp increase of the IrMn pinning strength. If pinholes exist, the free layer loop will also exhibit a shift due to direct exchange coupling. If there are no pinholes, no shift will be observed since orange-peel coupling is magnetostatic and cobalt has essentially no increase in magnetization below 300 K. In this way, a quick diagnosis can be made of whether or not pinholes exist in the MTJ. © 2008 American Institute of Physics.link_to_subscribed_fulltex