Perpendicular Magnetic Tunnel Junctions Research Articles

Proposal of a Novel Pole Type Structures in Perpendicular MRAM for High Gb/Chip

This paper proposes a novel MRAM using perpendicular magnetic tunnel junction device for high capacity. Conventional MRAM has weak points to realize high capacity in the design structure of the cell, one of which is that using simple current injection system can generate only weak switching field. As a solution, we propose a novel MRAM that has two additional poles in this paper. Proposed novel MRAM has a strong switching field owing to two poles added on both sides of the free layer, just like perpendicular magnetic recording heads. In this paper, analysis of the switching field and useful designs for high Gb/Chip are presented. This research was done using three dimensional FEM with injected current density of 8 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -6 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Pt/Co/oxide and oxide/Co/Pt electrodes for perpendicular magnetic tunnel junctions

This letter presents a study of perpendicular magnetic anisotropy in oxide/Co/Pt structures, which could constitute the upper magnetic electrode of magnetic tunnel junctions. The growth of cobalt layers on SiO2 substrates shows that all 0.6 nm thick Co films are superparamagnetic, whereas perpendicular magnetic anisotropy is obtained for 1.5 nm films after annealing. Co layers grown on various Al and Mg oxides prepared by sputtering also exhibit perpendicular magnetic anisotropy after annealing. Combined with inverse Pt/Co(CoFeB)/oxide stackings, these structures allow preparing tunnel junctions with thicker magnetic electrodes and much better thermal stability than those based on standard Pt/Co multilayers.

Switching characteristics and magnetoresistance of Co-based multilayered perpendicular magnetic tunnel junctions

AbstractPerpendicularly magnetized magnetic tunnel junctions (pMTJ) using Co based multilayers as free and reference layers have been optimized for perpendicular magnetic anisotropy (PMA) and high tunneling magnetoresistance (TMR). The effect of seed layers, Co thickness and Co/Ni thickness ratio on the anisotropy of these multilayer films has been studied. Intermixing of these multilayers was analyzed by local electrode atom probe (LEAP). The effect of Co thickness, Pd thickness and number of Co/Pd bilayers on the anisotropy and coercivity of the [Co/Pd]n multilayer films have been studied for both free and reference layers. The magnetic behavior of these PMA systems was studied by alternating gradient magnetometer. CoFeB/MgO/CoFeB trilayers sandwiched between the PMA multilayer material systems were studied. The transport properties of the patterned MTJ stacks were measured by PPMS from 10K to 400 K. A maximum TMR of 10% at 10K (5-10% at 300 K) was obtained for these perpendicular MTJ’s (pMTJ), regardless of whether they were magnetically annealed for MgO-CoFeB crystallization or not, indicating that the fcc-bcc-fcc transitions from the fcc multilayers to the bcc CoFeB/MgO/CoFeB do not promote the “giant MgO TMR effect” caused by symmetry filtering.

Spin transfer switching enhancement in perpendicular anisotropy magnetic tunnel junctions with a canted in-plane spin polarizer

We have studied spin transfer switching (STS) in a magnetic tunnel junction with perpendicular magnetic anisotropy for the reference and free layers using the Landau–Lifshitz–Gilbert formalism. We propose a multilayer structure in which the insertion of an additional spin polarizer with in-plane anisotropy can enhance the STS efficiency and switching speed of the device. It is revealed that a canted spin polarizer with an angle between 40° and 80° out of the film plane in the correct direction enhances the STS efficiency more than a fixed in-plane or perpendicular polarizer. Furthermore, we show that the spin transfer torque exerted on the in-plane polarizer layer by the free layer automatically tilts the in-plane polarizer in the direction that enhances STS for both magnetization states of the free layer.

Current-induced flip-flop of magnetization in magnetic tunnel junction with perpendicular magnetic layers and polarization-enhancement layers

We performed a micromagnetic investigation of current-induced magnetization switching in perpendicular magnetic tunnel junctions with polarization-enhancement layers. The pinned layer with a polarization-enhancement layer can be excited and eventually reverses at a current density lower than the value theoretically expected from that without a polarization-enhancement layer. The reversal results in continuous flip-flops of magnetizations as long as the current is applied. The flip-flop occurs at only one current polarity, caused by the precession amplification in polarization-enhancement layer. In order to prevent the unwanted flip-flop, the perpendicular anisotropy of the pinned layer must be severalfold larger than that of the free layer.

Interfacial Oxidation Enhanced Perpendicular Magnetic Anisotropy in Low Resistance Magnetic Tunnel Junctions Composed of Co/Pt Multilayer Electrodes

Low-resistance magnetic tunnel junctions utilizing perpendicular magnetic anisotropy of Co/Pt multilayer electrodes have been investigated. In these junctions, AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> tunnel barrier has been prepared by repeated natural oxidation processes. Each natural oxidation process has been optimized by varying the thickness of pre-oxidized Al layer prior to oxidation. The perpendicular magnetic anisotropy of Co/Pt multilayer has been found to be dependent on the degree of oxidation at the interface of tunnel barrier and Co adjacent layer. It has been observed that magnetization of Co/Pt multilayer is tilted when Co layer adjacent to tunnel barrier is relatively thick, which is attributed to the lack of interfacial anisotropy contribution near the interface of Co adjacent layer and AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> barrier. Interfacial oxidation treated on the monolayer thick Co adjacent layer has enhanced perpendicular magnetic anisotropy of the entire Co/Pt multilayer electrodes, enabling sharp transition between well-defined parallel and anti-parallel states at micro-fabricated perpendicular magnetic tunnel junctions of ~100 Omega ldr mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> resistance-area product.

Effect of Annealing and Barrier Thickness on MgO-Based Co/Pt and Co/Pd Multilayered Perpendicular Magnetic Tunnel Junctions

In this study, the effects of annealing conditions on the magnetic characteristics of multilayered perpendicular magnetic tunnel junctions (pMTJ) of the structures SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Pt/(Co/Pd) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> /MgO/(Co/Pt) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> /Pt with various MgO barrier thicknesses were explored. We found that both the fixed and free layers exhibit coercivity growth with increasing annealing temperature. Insertion of a 0.4 nm Mg layer under the MgO barrier layer increases the corecivities further. Magnetoresistance measured by the current-in-plane tunneling (CIPT) method reveals that the insertion of Mg layers on both side of the MgO layer can increase the MR ratio by up to 32%.

Effects of MgO Barrier Thickness on Magnetic Anisotropy Energy Constant in Perpendicular Magnetic Tunnel Junctions of (Co/Pd)$_{n}$/MgO/(Co/Pt)$_{n}$

In this study, we report experimental results on perpendicular magnetic tunnel junctions composed of the binary multilayer Co/Pt and Co/Pd with various MgO barrier thicknesses. The coercivities of both Co/Pt and Co/Pd layers varied substantially with the MgO thickness. Very interestingly, the perpendicular anisotropy constant Ku of the Co/Pt multilayer measured by the extraordinary Hall effect oscillated with the MgO thickness, and the period of this oscillation was about 0.3 nm. Furthermore, both the Ku value and the oscillation amplitude decreased after annealing at 150deg C and 200deg C, which suggest that the oscillation may result from the film morphology.

Fabrication of MgO barrier for a magnetic tunnel junction in as-deposited state using amorphous RE–TM alloy

MgO (1 0 0) textured films on Fe buffer layer with (1 0 0) preferential orientation were prepared by a reactive facing targets sputtering system at a substrate temperature of 100 °C during MgO deposition. This process can allow fabrication of MgO (1 0 0) tunneling barrier layer without high-temperature annealing process after the sputter-deposition. In addition, FeCo (1 0 0) preferred orientation films prepared on GdFeCo layers were improved with GdFeCo thickness. MgO films deposited on Fe (or FeCo) buffer layers revealed apparent (1 0 0) preferred orientation at the early stage of the film growth. 3 nm-thick MgO films deposited on GdFeCo [100 nm]/Fe [3 nm] exhibited (1 0 0) texture. Magnetic characteristic of perpendicular-magnetic tunnel junction (P-MTJ) element with the structure of GdFeCo [100 nm]/Fe [3 nm]/MgO [3 nm]/Fe [3 nm]/TbFeCo [100 nm] exhibited high squareness ratio of 0.8 and coercivity of free layer as low as 117 Oe by anomalous Hall effect, and (1 0 0) preferred orientation of 3 nm-thick MgO layer was observed by an X-ray diffractometer.

Perpendicular magnetic tunnel junction with tunneling magnetoresistance ratio of 64% using MgO (100) barrier layer prepared at room temperature

MgO (100) textured films can be prepared by reactive facing targets sputtering at room temperature without postdeposition annealing process when they were deposited on (100) oriented Fe buffer layers. This method allows fabrication of perpendicular magnetic tunnel junction (p-MTJ) with MgO (100) tunneling barrier layer and rare-earth transition metal (RE-TM) alloy thin films as perpendicularly magnetized free and pinned layers. The 3-nm-thick MgO tunneling barrier layer in p-MTJ multilayer prepared on glass substrate revealed (100) crystalline orientation. Extraordinary Hall effect measurement clarified that the perpendicular magnetic components of 3-nm-thick Fe buffer layers on the two ends of MgO tunneling barrier layer were increased by exchange coupling with RE-TM alloy layers. The RA of 35kΩμm2 and tunneling magnetoresistance ratio of 64% was observed in the multilayered p-MTJ element by current-in-plane-tunneling.

Effect of annealing and barrier thickness on MgO-based magnetic tunnel junctions with perpendicular anisotropy

The effect of annealing conditions on the magnetic characteristics of multilayered perpendicular magnetic tunnel junctions (pMTJs) of the structures SiNx∕Pt∕TbFeCo∕MgO∕(Co∕Pt)5 and SiNx∕Pt∕TbFeCo∕Mg∕MgO∕(Co∕Pt)5 with various MgO barrier thicknesses are explored. We found that both the hard and free layers have coercivity reduction with increasing annealing temperature. However, annealing at median temperature could improve the squareness of the hysteresis loop of the hard layer (TbFeCo). More interestingly, the annealing temperature at which the free layer (Co∕Pt) lost the perpendicular magnetic anisotropy (PMA) depends on the thickness of the MgO layer. Insertion of a thin Mg layer of thickness 0.4nm between the barrier and the hard layer can assist the junction in keeping the PMA at higher annealing temperature. The experimental results suggest that 150°C could be the suitable annealing temperature for these pMTJs.

垂直ＭＲＡＭ用スピン偏極増強用ＦｅＣｏ層およびＭｇＯバリア層の結晶性向上

Rare-earth-transition metal (RE-TM) alloy films, such as TbFeCo and GdFeCo layers were used as perpendicularly magnetized layers for pinned and free layers in a perpendicular magnetic tunneling junction (p-MTJ). (100) oriented MgO tunneling barrier layer can be deposited on (100) oriented Fe (and FeCo) buffer layers. The (100) texture of the Fe (and FeCo) layers were enhanced by depositing them on the RE-TM layers. X-ray diffraction study clarified that MgO layers deposited on the Fe (and FeCo) layers revealed apparent (100) orientation at the early stage of the film growth. 3nm-thick MgO layer deposited on GdFeCo(100nm)/Fe(3nm) exhibited (100) texture.

室温作製ＭｇＯ（１００）配向膜を用いた垂直ＭＴＪ構造の試作

MgO (100) textured films can be prepared by reactive facing targets sputtering at room temperature when they were deposited on (100) oriented Fe buffer layers. This method allows fabrication of perpendicular magnetic tunnel junction (p-MTJ) with MgO tunneling barrier layer and rare-earth transition metal amorphous alloy (RE-TM alloy) thin films as perpendicularly magnetized layers. The 3nm-thick MgO tunneling barrier layer in p-MTJ multilayer prepared on a glass substrate revealed (100) crystalline orientation. Anomalous Hall effect measurement clarified that the 3nm-thick Fe buffer layers attached to the two end of MgO tunneling barrier layer magnetized vertically to the film plane because of the strong exchange coupling with RE-TM alloy layers. TMR ratio of 31% was observed in the multilayered p-MTJ element.

MTJ Elements With MgO Barrier Using RE-TM Amorphous Layers for Perpendicular MRAM

Magnetic properties of an FeCo/TbFeCo bilayer were investigated by MFM and the anomalous Hall effect. In the case of FeCo(x nm)/TbFeCo(50 nm), FeCo as thin as 2 nm behave as perpendicular magnetization film while large thickness of RE-TM films, such as TbFeCo(100nm) causes the increase of the critical thickness up to 4 nm. The prototype of perpendicular MTJ, p-MTJ, consist of TbFeCo/Fe/MgO/Fe/GdFeCo exhibited high squareness ratio of 0.8 having low coercivity of 117 Oe while MgO(100) texture was confirmed in the Fe/MgO/Fe junction prepared on the amorphous GdFeCo layer even though the thickness of MgO is as thin as 3 nm

Perpendicular Magnetic Tunneling Junction With Double Barrier Layers for MRAM Application

A double-barrier-layer perpendicular magnetic tunneling junction (DpMTJ) structure consisting of Si substrate/Pt/GdFeCo/AlOx/GdFeCo/FeCo/AlOx/FeCo/TbFeCo/Pt/Ti-cap was prepared by a direct current (dc) and radio frequency (RF) magnetron sputtering method. An elliptical DpMTJ element with 3.5 mumtimes2.5 mum size was fabricated using a top-down technique. A conducting atomic force microscope (CAFM) was used to obtain I-V curves of DpMTJ structures. We obtained the magnetoresistance (MR) ratio value from measured I-V curves by applying two opposite magnetic fields value of plusmn200 Oe perpendicular to the plane of film. The MR ratio was reached as high as 74% at zero applied bias voltage. Furthermore, the MR ratio decreased as bias voltage increased. It could make the DpMTJ structure to be used in the high-density MRAM devices

Epitaxial growth of L1 0-FePt/MgO/L1 0-FePt (0 0 1) trilayer structures

Epitaxial growth of L1 0-FePt/MgO/L1 0-FePt (0 0 1) trilayer structures on an Au (0 0 1) monocrystalline buffer layer was investigated. By post-annealing at 500 °C, flat Au (0 0 1) buffer layers with roughness of Ra∼0.1 nm were obtained on a 10-nm-thick Fe (0 0 1) seed layer. Despite a large lattice mismatch between L1 0-FePt and MgO (∼10%), fully epitaxial growth of L1 0-FePt/MgO/L1 0-FePt (0 0 1) trilayers was achieved on the Au buffer layer, suggesting that the combination of L1 0-FePt and MgO is useful for fabrication of perpendicularly magnetized magnetic tunnel junctions.

Comparison of the interfacial structure between MgO and Al–O oxidation layers for perpendicular magnetic tunnel junction

We discuss the interfacial structure of MgO and Al–O barrier layers and influence on the magnetic properties of perpendicular magnetic tunnel junction (pMTJ) devices. The pMTJs layer structures analyzed were Si-wafer∕Pt∕Gd(FeCo)∕FeCo∕MgO (AlO)∕FeCo∕Tb(FeCo)∕Pt. The deposit of all pMTJs structures was carried out by rf and dc magnetron sputtering systems. Transmission electron microscopy (TEM) clearly showed that the interfacial structure of FeCo∕MgO or AlO∕FeCo in the pMTJs was very smooth and uniform. Hysteresis loops obtained by an alternating gradient magnetrometer (AGM) for the different oxide barrier layers of pMTJ structures showed that the Al–O layer performs better than the MgO layer. An additional discussion on the oxide layer thickness in the TEM and AGM measurements is also presented.

The magnetic characteristics of perpendicular magnetic tunnel junction with MgO and Al–O oxidation layers in various thickness

In this work we show the magnetic characteristics of perpendicular magnetic tunnel junction (pMTJ) with different oxidation layers. The pMTJs structures were made by RF and DC magnetron sputtering. Individual depositions of magnesium oxide layers and of aluminum oxide films were prepared by plasma oxidation. The experimental results showed that the initial switching field was decreased as the magnesium oxide thickness was increased. Further work of the aluminum oxide surface roughness and hysteresis loop influenced by different oxidation layers on pMTJs structures will be discussed as well.

Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory

We present here a magnetic tunnel junction device using perpendicular magnetization films designed for magnetic random access memory (MRAM). In order to achieve high-density MRAM, magnetic tunnel junction devices with a small area of low aspect ratio (length/width) is required. However, all MRAMs reported so far consist of in-plane magnetization films, which require an aspect ratio of 2 or more in order to reduce magnetization curling at the edge. Meanwhile, a perpendicular magnetic tunnel junction (pMTJ) can achieve an aspect ratio=1 because the low saturation magnetization does not cause magnetization curling. Magnetic-force microscope shows that stable and uniform magnetization states were observed in 0.3 μm×0.3 μm perpendicular magnetization film fabricated by focused-ion beam. In contrast, in-plane magnetization films clearly show the presence of magnetization vortices at 0.5 μm×0.5 μm, which show the impossibility of information storage. The PMTJ shows a magnetoresistive (MR) ratio larger than 50% with a squareness ratio of 1 and no degradation of MR ratio at 103 Ω μm2 ordered junction resistance.