Matthew T. Moneck

Co∕Pt multilayer based magnetic tunnel junctions using perpendicular magnetic anisotropy

Magnetic tunnel junctions that utilize perpendicular magnetic anisotropy have attracted growing attention due to their potential for higher storage densities in future high capacity magnetic memory applications. In this study, we present an experimental demonstration of magnetic tunnel junctions composed of perpendicularly magnetized Co∕Pt multilayer electrodes and an AlOx tunnel barrier. The emphasis has been on how to maximize the thickness of the Co layers adjacent to the tunnel barrier while still magnetized perpendicularly for possible spin torque utilization in future applications. It is found that the thickness ratio between the Co and Pt layers and the number of bilayers were significant parameters to customize the magnetic properties. The difference between the switching fields of the soft and the hard layers can be adjusted by the number of repeats of the Co∕Pt bilayers. The measured hysteresis shows virtually zero exchange coupling between the two layers through the tunnel barrier. Measured tun...

Optimization of Ta thickness for perpendicular magnetic tunnel junction applications in the MgO-FeCoB-Ta system

The impact of Ta thickness on magnetic anisotropy and interlayer magnetic coupling is evaluated for the Ta-FeCoB-MgO thin film system commonly used in magnetic tunnel junctions. It is shown that there exists a window of Ta thickness where strong magnetic coupling of FeCoB with another magnetic layer is achievable through Ta while still maintaining properties required for use in a perpendicular magnetic tunnel junction. We also expand on existing knowledge about the role of annealing temperature, film composition, and seedlayer sequence on magnetic anisotropy in Ta/FeCoB/MgO tri-layers of varying FeCoB thickness.

Annealing effects on structural and transport properties of rf-sputtered CoFeB∕MgO∕CoFeB magnetic tunnel junctions

Annealing effects on the structural and transport properties of sputtered CoFeB∕MgO∕CoFeB magnetic tunnel junctions deposited on SiO2∕Si were investigated. At the as-deposited state, the CoFeB was amorphous at the CoFeB∕MgO interface. High-resolution transmission electron microscope image clearly shows that after annealing at 270°C for 1h, crystallization of amorphous CoFeB (three to four monolayers) with lattice matching to MgO (100) occurred locally at the interface between MgO and CoFeB, producing a magnetoresistance (MR) around 35%–40%. After annealing at 360°C for 40min, the MR increased to 102%. The increase in the MR with annealing is attributed to the complete formation of (100) crystalline structure of CoFeB well lattice matched with the (100)-oriented MgO barrier. The bias voltage dependence of the MR shows a consistent correlation with each CoFeB∕MgO interface.

Return Field-Induced Partial Erasure in Perpendicular Recording Using Trailing-Edge Shielded Writers

Return field-induced partial erasure (RFPE) in trailing-edge shielded perpendicular writers has been studied, both by modeling and by experiments. For a given head-media combination, the return field underneath the trailing shield increases with increasing write current. Once exceeding a certain threshold, it will cause partial erasure of the bits that have just been written by the main pole. Recording performance, such as reverse overwrite, spectral signal-to-noise ratio, and bit-error rate are all found degraded at high write currents, due to RFPE. Design optimization of both head and media together is needed, in order to maximize the advantage of a trailing-edge shielded pole head and minimize the impact of RFPE

Fabrication and testing of deep submicron annular vertical magnetoresistive random access memory elements

Robust magnetic switching and nonvolatility make magnetic random access memory an attractive prospect for future memory designs. However, there has been some concern over whether or not standard fabrication processes can be used to produce annular shaped memory elements. In this paper we present the fabrication and test results of deep submicron annular memory elements defined by electron beam and optical lithographies. The annular memory cells consist of a current perpendicular to plane giant magnetoresistive (CPP GMR) stack containing two ferromagnetic layers with a nonmagnetic interlayer where thin Cu laminations were included in the ferromagnetic layers to enhance the CPP GMR effect.

Study of Lithographically Defined Data Track and Servo Patterns

We report on fabrication of discrete tracks on perpendicular magnetic recording (PMR) media with an e-beam lithographical process. We studied the recording performance of the e-beam media on a spinstand in parallel with conventional PMR media. Discrete track media show significant reduction in adjacent track erasure (ATE). We studied and quantitatively measured the source of the ATE improvement, and developed a triple track geometrical model to calculate achievable track density for both discrete track recording (DTR) and continuous media. From the model, we identify two factors of DTR that contribute to reaching a higher TPI. Using the same fabrication technique, we also studied servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts with servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better position error signal sensitivity and support higher TPI in the future.Discrete tracks are fabricated on conventional PMR media with an e-beam litho graphical process. The recording performance is studied on a spinstand in parallel with conventional PMR media. Discrete track media shows significant reduction in adjacent track erasure (ATE). The source of the ATE improvement is studied and quantitatively measured. A triple track geometrical model is developed to calculate achievable track density for both DTR and continuous media. From the model, we identify two factors of DTR, which contribute to reaching a higher TPI. Using the same fabrication technique, we also study servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts and servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better PES signal sensitivity and support higher TPI in the future.

Fabrication of Flyable Perpendicular Discrete Track Media

Discrete track media offers many potential recording advantages over conventional continuous media in hard disk drives. In this study, we present a novel fabrication process for discrete track perpendicular magnetic media via electron beam lithography, ion milling, and the use of a protective Al sacrificial layer. Physical characterization of the media confirms the process is able to produce patterned tracks with no damage to the media. Spin stand analysis verifies the disks are flyable and capable of recording sharp transitions without any degradation in the magnetic signal

Recording Performance Study of PMR Media With Patterned Tracks

Discrete track recording technology offers a potential advantage in reducing adjacent track erasure/interference. As nano-imprinting technology advances, fabrication of such media has been demonstrated. In recent years, discrete track recording has generally been viewed as one of the next promising technologies for areal density advancement. In this study, we evaluated the recording performance of PMR media with patterned tracks. To accurately assess the advantage of discrete track performance and compare with current continuous media, both recording performances were measured on one single track. The head flying height is monitored on the patterned and continuous media regions. At 100-nm data track width, patterned tracks show noticeably better signal-to-noise ratio and significantly lower adjacent track erasure compared with continuous media at the same track width. Such measured performance advantages are critical to increase track density beyond 300-400 ktpi

Increased Perpendicular TMR in FeCoB/MgO/FeCoB Magnetic Tunnel Junctions by Seedlayer Modifications

By modifying the seedlayer in perpendicular FeCoB/MgO/FeCoB magnetic tunnel junctions (MTJs), we observe an increase in maximum tunneling magnetoresistance (TMR) from 65% up to 138%. Its found that decreasing the Ta deposition rate in Ta/Ru/Ta underlayers allows for greater annealing temperatures (up to 350 ) while still maintaining a perpendicular easy axis. An improvement is also seen at a lower temperature where both seedlayers maintain a perpendicular FeCoB easy axis indicating that the increase in TMR is not solely related to annealing at a higher temperature.

Effects of the Crystalline Structure on the Switching Field and Its Distribution of Bit Patterned Co/Pt Multilayer Media

We studied the effects of medium microstructure on the switching field (SF) and its distribution (SFD). Specifically, quasi-single crystalline and polycrystalline Co/Pt based multilayers, chosen for its high perpendicular anisotropy and high exchange coupling, have been prepared via sputtering technique. The quasi-single crystalline was achieved by utilizing an Ag underlayer. Using electron-beam lithography, films were patterned into arrays of small islands, ranging from 1 mum to 0.050 mum (=50 nm) in size. The hysteresis loop for the both types of film, as unpatterned, showed similar anisotropy field and coercivity whereas, as patterned, the quasi-single crystalline samples consistently exhibited a higher SF than that of the polycrystalline samples. However, the interpretation on the SFD was rather complex. Additionally, a critical size where remanent magnetization changes a multi domain to a single domain state appeared to be notably smaller for the polycrystalline samples than that for the quasi-single crystal samples, suggesting a different nucleation volume in the films.