Magnetic Tunnel Junction Elements Research Articles

A spintronics full adder for magnetic CPU

Spintronics devices are based on the up or down spin of the electrons rather than on electrons or holes as in the traditional semiconductor electronics devices. Magnetic processors using spintronics devices in principle are much faster and with the potential features of nonvolatile, lower power consumption and higher integration density compared with transistor-based microprocessor. Full adder is one of the most important basic units of the arithmetic/logic unit for any processors. The design of the full adder determines the speed and chip-density of a processor. In this paper, a novel spintronics full adder is proposed based on novel programmable spintronics logic devices. Only seven magnetic tunnel junction elements are needed for this full adder design.

Programmable spintronics logic device based on a magnetic tunnel junction element

A programmable spintronics logic device was designed and fabricated based on a single pinned magnetic tunnel junction (MTJ) element. In this work, a current input line C passing through the MTJ element itself was introduced. Two separated input current lines (A and B) could switch the magnetization of the pinned layer under the heat assistance from line C. Full logic functions (AND, OR, NAND, NOR, XOR, and XNOR) can be realized based on a normal pinned and a synthetic pinned MTJ element. A Wheatstone bridge was engineered to read this single MTJ element logic device. MTJ elements with 1μm2 and normal pinned structure: (Ta30Å∕NiFe40Å∕MnIr35Å∕CoFe30Å∕(Al7Å)+oxidation∕CoFe30Å∕NiFe40Å∕Ta200Å), have low resistance of 6.3Ω and high resistance of 7.2Ω, which gives the magnetoresistive (MR) ratio of 14%. Approximately a 3-mV output difference is obtained between logical 1 and 0.

Probing local spin modes in exchange-biased MTJ elements

We have studied the local magnetization dynamics in exchange-biased magnetic tunnel junction (MTJ) elements, by time-resolved magneto-optical Kerr effect measurements. We have observed a strong spatial dependence of the frequency and damping of the precessional motion in the free layer. A local edge mode has been detected, as well as increased damping at the edges. The experimental results are in good agreement with micromagnetic simulations, in which the stray field from the pinned layer of the MTJ is included.

A New Reference Signal Generation Method for MRAM Using a 90-Degree Rotated MTJ

A new reference signal generation method for high-density MRAM is reported. 0.4/spl times/0.8 /spl mu/m/sup 2/ magnetic tunnel junction (MTJ) elements were successfully integrated with 0.24-/spl mu/m CMOS technology. By using a 90-degree rotated MTJ as a new reference signal generator, the reference resistance could be always located in the exact midpoint between high-resistance state R/sub H/ and low-resistance state R/sub L/ regardless of applied voltage. When tested in 8/spl times/8 MTJ arrays, it is found to show good fidelity to our expectations. So it is supposed that this new method is more favorable for high-density MRAM.

A 1-Mbit MRAM based on 1T1MTJ bit cell integrated with copper interconnects

A low-power 1-Mb magnetoresistive random access memory (MRAM) based on a one-transistor and one-magnetic tunnel junction (1T1MTJ) bit cell is demonstrated. This is the largest MRAM memory demonstration to date. In this circuit, the magnetic tunnel junction (MTJ) elements are integrated with CMOS using copper interconnect technology. The copper interconnects are cladded with a high-permeability layer which is used to focus magnetic flux generated by current flowing through the lines toward the MTJ devices and reduce the power needed for programming. The 25-mm/sup 2/ 1-Mb MRAM circuit operates with address access times of less than 50 ns, consuming 24 mW at 3.0 V and 20 MHz. The 1-Mb MRAM circuit is fabricated in a 0.6-/spl mu/m CMOS process utilizing five layers of metal and two layers of poly.

Effects of two in-plane fields on the magnetization reversal mechanism in magnetic tunnel junction elements

One of the potential applications of the spin tunnel junction is as magnetic random access memory (MRAM) elements. For MRAM application purposes, two in-plane perpendicularly applied fields are required during magnetization reversal. In this article, Lorentz transmission electron microscopy has been used to study the magnetization reversal mechanism of tunnel junction elements under the influence of two in-plane perpendicular fields. Four hundred NiFe/MnFe/NiFe/Al2O3/NiFeCo elements with differing shapes and sizes have been observed. When an in-plane Y field was applied to the hard axis followed by sweeping the X field along the easy axis, it was found that the number of elements in which 360° domain walls are observed to form decreases as the strength of the Y field increases, and that the magnetization reversal mechanism in the tunnel junction elements gradually changes from a domain wall motion mechanism to a moment rotation mechanism as the Y field increases. As expected, the reversal field (X field) reduces as the Y field increases.

Lorentz microscopy study of magnetization reversal mechanism in magnetic tunnel junction elements

The magnetization reversal mechanism of four different shaped (rectangular, elliptical, trapezoidal, and hexagonal) tunnel junction elements with area ranging from 0.04 to 12 μm2 has been investigated using Lorentz transmission electron microscopy. It has been found that the reversal field is influenced by element size and aspect ratio but is not strongly dependent upon the element shape. 360° domain walls were often observed to form in the elements and were sustained to a high field when the relative magnetization configuration of the free and pinned layers changed from parallel to antiparallel. The formation of 360° domain walls depends strongly on the shape and aspect ratio of the element, with a lower probability of formation in elements with either an aspect ratio of 1:1 or a hexagonal shape.

The formation of 360° domain walls in magnetic tunnel junction elements

The formation of 360° domain walls has been observed in exchange-biased magnetic tunnel junction elements using Lorentz transmission electron microscopy. These domain walls occur under certain circumstances and remain stable up to a high external field (∼150 Oe) compared to the value observed to achieve the antiparallel state of the free- and pinned-layer magnetizations (∼20 Oe). They have been found to play an important role during reversal to the parallel magnetization state, inducing a much lower switching field and a very different reversal mechanism.

Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory (invited)

Exchange biased magnetic tunnel junction (MTJ) structures are shown to have useful properties for forming magnetic memory storage elements in a novel cross-point architecture. MTJ elements have been developed which exhibit very large magnetoresistive (MR) values exceeding 40% at room temperature, with specific resistance values ranging down to as little as ∼60 Ω(μm)2, and with MR values enhanced by moderate thermal treatments. Large MR values are observed in magnetic elements with areas as small as 0.17 (μm)2. The magnetic field dependent current–voltage characteristics of an MTJ element integrated with a silicon diode are analyzed to extract the MR properties of the MTJ element itself.