Size and voltage dependence of effective anisotropy in sub-100-nm perpendicular magnetic tunnel junctions

Abstract

Magnetic tunnel junctions with perpendicular magnetic anisotropy are investigated using a conductive atomic force microscope. The 1.23 nm Co 40 Fe 40 B 20 recording layer coercivity exhibits a size dependence which suggests single domain behavior for diameters ≤ 100 nm. Focusing on devices with diameters smaller than 100 nm, we determine the effect of voltage and size on the effective device anisotropy K eff using two different techniques. K eff is extracted both from distributions of the switching fields of the recording and reference layers, and from measurement of thermal fluctuations of the recording layer magnetization when a field close to the switching field is applied. The results from both sets of measurements reveal that K eff increases monotonically with decreasing junction diameter, consistent with the size dependence of the demagnetization energy density. We demonstrate that K eff can be controlled with a voltage down to the smallest size measured, 64 nm. PACS numbers: 85.75.-d,73.40.Gk,75.78.-n,75.70.-i 1 I. INTRODUCTION Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) are an attractive building block for non-volatile memories. PMA MTJs (p-MTJs) show promisein terms of the key requirements for implementation into products competitive with current data storage and memory technologies: large tunnel magnetoresistance (TMR), low writing energy cost, non-volatility over ∼ 10 years, and scalability of these properties toward ∼ 1 Tbit/inch 2 densities. Room temperature TMR ratios greater than 100% have long existed in in-plane MTJs 1,2 . In state of the art in-plane MTJs, TMR in excess of 600% is achieved by controlling the diffusion of Ta in the film stack through the addition of boron to the magnetic electrodes 3 . Despite these achievements, in-plane MTJs suffer from scalability issues due to their dependence on shape anisotropy for thermal stability and the high energy cost of switching the magnetization by the spin transfer torque (STT) effect 4,5 . For in-plane MTJs, switching energies E sw = I 2 c Rt, where I c is the critical switching current, R is the resistance, and t is the length of the pulse, of approximately 10 µJ/bit were achieved for current pulses on the order of 10 ms 6 . This value was drastically reduced using nanosecond pulses, yielding E sw on the order of single pJ/bit in purely in-plane MTJs 7 . In high TMR p-MTJs the large out-of-plane demagnetization energy does not contribute to E sw 8 . Recently, TMR ratios up to 162% were obtained in p-MTJs by further controlling interlayer diffusion through the substitution of Ta with Mo in the film stack 9 . PMA is achieved when the CoFeB thickness is less than about 1.5 nm, so that the effective anisotropy K eff is dominated by the interfacial anisotropy between Fe in the CoFeB and oxygen in the MgO 10 . In such p-MTJs, switching energies of hundreds of fJ/bit were achieved in 60 nm × 170 nm ellipses 11 . One of the most promising aspects of p-MTJs is that the interface anisotropy can be controlled by applying a...

Spin-Orbit-Torque Switching in 20-nm Perpendicular Magnetic Tunnel Junctions

“…Because major loops could not be measured, the effective anisotropy Keff,bottom was estimated indirectly. Keff,top was found from switching field distributions of minor loops measured multiple times [21], and related to the interface anisotropy Ki, using a method that has been described previously [24]. The interface anisotropy Ki was determined from , where t is the thickness.…”

“…The instrument was an RHK UHV350 with R9 controller operating in contact mode, in air at 300 K. Si-doped AFM probe tips (Arrow-FM nanoworld) were made conducting by sputtering 200 nm (nominal thickness) of Pt on a Ta adhesion layer. The details of these point contact measurements have been reported previously [22], [23], [24]. All C-AFM measurements were made in air and at room temperature.…”

Magnetoresistance Dynamics in Superparamagnetic Co−Fe−B Nanodots

“…CAFM measurements used a commercial silicon-doped atomic force microscope tip (NanoAndmore USA) coated with a 5-nm Ta layer for adhesion followed by 200 nm Pt. Further details about these measurements have been reported previously [9,19,20].…”

Role of CoFeB thickness in electric field controlled sub-100 nm sized magnetic tunnel junctions

“…The switching fields differ across several runs due to the stochastic nature of the magnetization reversal process. 34 Fig. 2(b) shows the switching probability derived from Fig.…”

“…In order to account for both AP (antiparallel) → P (parallel) and P→AP transitions, the modulus inside the function has been included. S. K. 34 We can, therefore, assume that our MTJs are also in the single domain regime.…”