R. C. Sousa

Thermally assisted switching in exchange-biased storage layer magnetic tunnel junctions

A thermally assisted writing procedure is proposed in a tunnel junction based magnetic random access memory cell. The magnetic layers of the tunnel junction are both exchange-biased with antiferromagnetic layers, the reference layer having a much higher blocking temperature than the storage layer. In the operating mode, a current pulse sent through the junction generates enough heat to raise the temperature of the storage layer above its blocking temperature, without affecting the pinning of the reference layer. The concept is demonstrated here for an isolated junction using an homogeneous external magnetic field.

Spin dependent tunnel junctions for memory and read-head applications

Spin dependent tunnel junctions with TMR exceeding 30-40% can now be prepared with AlO/sub x/ and AlN barriers, with junction resistance tuned from 30-40 /spl Omega//spl times//spl mu/m/sup 2/ to 10/sup 8/ /spl Omega//spl times//spl mu/m/sup 2/. Thermal stability is better than 3000% for thicker barriers (>11 /spl Aring/) but is degraded to 220/spl deg/C for 6 /spl Aring/ barriers. A 9 bit MRAM cell is demonstrated using tunnel junctions and vertically integrated a:Si diodes. Junction switching is achieved with on chip 20 ns field pulses. Requirements for tunnel junctions for 100 Gb/in/sup 2/ read head applications are discussed. First prototypes of tunnel junction read heads with 600 /spl Aring/ read gaps were fabricated, where the TJ is at the air bearing surface. TMR loss was observed during the final head lapping steps.

Enhancement of perpendicular magnetic anisotropy through reduction of Co-Pt interdiffusion in (Co/Pt) multilayers

We demonstrate that the effective magnetic anisotropy of sputtered (Co/Pt) multilayers can be doubled by limiting the interdiffusion occurring at Co/Pt interfaces. We present a way to decrease the interdiffusion by inserting an ultra-thin Cu layer at or near the Co/Pt interfaces. When such a material is sputtered on Co prior to the Pt deposition, the perpendicular magnetic anisotropy, as well as the thermal stability, is enhanced for Co layer thicknesses smaller than 1 nm. This is of great interest for out-of-plane magnetized spintronic devices which require high perpendicular magnetic anisotropy for down-size scalability reasons together with a free layer as thin as possible to reduce the writing energy when switched by spin transfer torque.

Co/Ni multilayers with perpendicular anisotropy for spintronic device applications

This letter presents a study of perpendicular anisotropy in Co/Ni multilayers, which could constitute a thick polarizer in spin torque oscillators or a magnetic electrode in magnetic tunnel junctions (MTJ) with perpendicular anisotropy. Perfectly square perpendicular loops are observed for as-deposited Co/Ni multilayers with various sublayer thicknesses and bilayer repetition numbers using a Pt buffer layer. An anisotropy energy of 1.0 × 106 erg·cm−3 is obtained for 9 nm thick Co/Ni multilayers. For Co/Ni multilayers deposited on MgO, no perpendicular magnetization component is observed in the as-deposited state, but it develops (even in 2.1 nm Co/Ni multilayers) after annealing at 250 °C.

100 ps precessional spin-transfer switching of a planar magnetic random access memory cell with perpendicular spin polarizer

Ultrafast spin-transfer precessional switching between two stable states of a magnetic random access memory device is demonstrated in structures comprising a perpendicularly magnetized polarizing layer (PL⊥), an in-plane magnetized free layer (FL), and an in-plane magnetized analyzing layer (AL) in a PL⊥/spacer/FL/spacer/AL stack. Back and forth switching can be achieved with sub-ns current pulses of the same polarity. The spin-torque influence from the analyzer leads to an asymmetric dependence of the switching properties as a function of the current sign and initial state.

Precessional spin-transfer switching in a magnetic tunnel junction with a synthetic antiferromagnetic perpendicular polarizer

This paper reports sub-nanosecond precessional spin-transfer switching in elliptical magnetic tunnel junction nanopillars. This result is obtained in samples integrating a synthetic antiferromagnetic perpendicular polarizer and a tunnel junction with in-plane magnetized electrodes. The out-of-plane precession of the free layer magnetization results in oscillations of the switching probability as a function of the pulse width. At 9.25 MA/cm2 current density, these oscillations have a period of 1 ns with a high degree of coherence.

Spin transfer torque switching assisted by thermally induced anisotropy reorientation in perpendicular magnetic tunnel junctions

A method to switch the magnetization of the free layer in magnetic tunnel junctions with perpendicular anisotropy is demonstrated. It consists in assisting the spin transfer switching of the magnetization by a thermally induced reorientation of the free layer magnetic anisotropy from out-of-plane to in-plane. The junction temperature increase is due to the Joule dissipation around the tunnel barrier produced by the same pulse of current which generates the spin transfer torque. This magnetic reorientation allows the spin transfer torque efficiency to be maximal since the spin polarization of the current is perpendicular to the magnetization of the free layer. Such a thermally assisted switching allows designing highly down-size scalable magnetoresistive random access memory cells with improved write efficiency.

Charge trapping-detrapping mechanism of barrier breakdown in MgO magnetic tunnel junctions

Endurance of MgO-based magnetic tunnel junctions has been studied using a time-dependent dielectric breakdown method. Series of successive electrical pulses were applied until electrical breakdown of the tunnel barrier. We show that two electrical breakdown regimes exist depending on the time interval Δt between pulses compared to a characteristic escape time of trapped electrons τ0 ∼ 100 ns. For Δt   τ0, breakdown is ascribed to large temporal modulation of trapped charges causing alternating stress in the barrier oxide. Between these two regimes, the tunnel junctions reach a very high endurance.

Spin-dependent phenomena and their implementation in spintronic devices

The general purpose of spinelectronis is to take advantage of the spin of electrons in addition to their charge to obtain new phenomena and conceive innovative electronic components. The first application of spinelectronics is in magnetoresistive heads for computer disk drives based on the giant magnetoresistance phenomenon. The discovery of tunnel magnetoresistance in magnetic tunnel junctions has allowed the emergence of a new kind of non-volatile memory called magnetic random access memory (MRAM). It potentially combines the advantages of all existing memories: non-volatility of FLASH, speed of SRAM, density of DRAM, hardness to ionizing radiations and endurance. Many research groups are nowadays investigating the use of these magnetic components for other logic applications. Spintronic phenomenon is the spin transfer effect allows controlling the magnetization of a magnetic nanostructure directly with a spin-polarized current. It attracts a considerable interest since it provides a new write scheme in MRAM and allows conceiving frequency tunable RF components.

Probing fast heating in magnetic tunnel junction structures with exchange bias

Heat diffusion in a magnetic tunnel junction (MTJ) having a ferromagnetic/antiferromagnetic free layer is investigated. The MTJ is heated by an electric current pulse of power PHP, flowing through the junction in current perpendicular to the plane (CPP) geometry, via Joule heat dissipation in the tunnel barrier. According to a proposed one-dimensional (1D) model of heat diffusion, when an electric voltage is applied to the MTJ, the free layer experiences a transient temperature regime, characterized by an exponential increase of its temperature TAF with a time constant ?TR, followed by a steady temperature regime characterized by TAF=TRT+?PHP, where TRT is the room temperature and ? is a constant. Magnetic transport measurements of exchange bias HEX acting on the free layer allow the determination of ? and ?TR. The experimental values of ? and ?TR are in agreement with those calculated using the 1D model and an estimation of the MTJ thermodynamic parameters based on the Dulong?Petit and Widemann?Franz laws.