1 Perpendicular magnetic anisotropy tunneling magnetoresistance and spin- transfer torque effect in magnetic tunnel junctions with Nb layers

2025-04-30 0 0 1.14MB 11 页 10玖币

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Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque

effect in magnetic tunnel junctions with Nb layers

Bowei Zhou,1 Pravin Khanal,1 Onri Jay Benally,2 Deyuan Lyu,2 Daniel B. Gopman,3 Arthur Enriquez,1 Ali

Habiboglu,1 Kennedy Warrilow, 1 Jian-Ping Wang,2 and Wei-Gang Wang1*

1. Department of Physics, University of Arizona, Tucson, AZ 85721, USA

2. Department of Electrical & Computer Engineering, University of Minnesota, Minneapolis, MN

55455, USA

3. Materials Science & Engineering Division, National Institute of Standards and Technology,

Gaithersburg, MD 20899, USA

Nb and its compounds are widely used in quantum computing due to their high

superconducting transition temperatures and high critical fields. Devices that combine

superconducting performance and spintronic non-volatility could deliver unique

functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy

metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m2

was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was

evaluated in junctions with different thickness combinations and different annealing

conditions. An optimized magnetoresistance of 120% was obtained at room temperature,

with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition,

spin-transfer torque switching has also been successfully observed in these junctions with a

quasistatic switching current density of 7.3 ×105 A/cm2.

*wgwang@arizona.edu

Magnetic tunnel junctions (MTJs) 1–7 are critical components in magnetic random access

memory (MRAM), 8,9 spin-logic, 10,11 spin-torque oscillator, 12–14 and neuromorphic 15

applications. Particularly, MTJs with perpendicular magnetic easy-axis (pMTJs) are promising

due to their potential in ultrafast and ultralow energy operations,16–19 and deep scaling

capability down to a lateral size of 2-3 nm. 20 The perpendicular magnetic anisotropy (PMA) in

many of these pMTJs has an interfacial origin, where the thickness of the ferromagnetic (FM)

layer has to be maintained around 1 nm. 21–23 Typically the MgO barrier is sandwiched by two

such thin FM layers, forming the core structure of the pMTJ. The other side of the thin FM layer

is usually interfaced with a heavy metal (HM) layer. The perpendicular easy axis of the system is

established due to the proper hybridization of the 3d wavefunction of the FM layer and the 2p

wavefunction of the oxygen from the MgO barrier, which can be further influenced by the 3d-4

or d 3d-5d hybridization at the FM/HM interfaces. 21–23 Clearly in this case the magnetic and

transport properties of pMTJs are sensitively dependent on the HM layers. A variety of HM

layers such as Pt,24 Ta,16 Mo,25,26 and W22,27 have been explored to investigate the performance

of pMTJs.

Niobium (Nb) and its compounds are one of the most important components in quantum

computation due to their high superconductivity transition temperatures and high critical

fields.28 In addition to quantum computing, it is also interesting to study the behavior of pMTJs

with Nb for cryogenic memory applications. Recently, it has been demonstrated that the

magnetic anisotropy of Fe can be modified by the superconductivity of Vanadium in the

MgO/Fe MTJs at low temperatures.29 On the other hand, electrically modifying the magnetic

properties of 3d ferromagnets is important in many applications at room temperature (RT).30–32

For example, through the voltage-controlled magnetic anisotropy (VCMA) effect, 31,33 a

switching energy as low as a few femtojoules has been achieved in pMTJs. 16–19 Therefore it

would be interesting to investigate the potential interaction between superconductivity and

PMA, as well as the VCMA effect in Nb-based pMTJs. In the past, Nb has been successfully

employed to obtain PMA in Nb/CoFeB/MgO heterostructures and the interfacial PMA energy

density of 2.2 mJ/m2 was obtained.34–36 However, transport properties have not been evaluated

in MTJs with Nb layers.

In this study, we report the RT performance of pMTJs with Nb as the HM layers. Magnetic,

transport, and spin-dynamic properties of the junctions were investigated in blanket MTJ films

as well as patterned junctions. The PMA and the thermal robustness of the pMTJs were studied

by varying the thickness of the Nb layers and annealing conditions. A reasonably large TMR of

120 % was obtained at RT. Spin-transfer torque (STT) 37,38 switching has also been successfully

observed in these junctions with a quasistatic switching current density of 7.3 ×105A/cm2.

The MTJ films in this work were fabricated in a 12-source UHV sputtering system (AJA

International) with a base pressure of 10-7 Pa (10-9 Torr). The stack structure of the films is

Si/SiO2/Ta(8)/Ru(7)/Ta(9)/Nb (d1)/Co20Fe60B20(1)/MgO(0.9-3.5)/ Co20Fe60B20(0.9-1.4)/Nb (d2)

/Ta(5)/Ru(15), where numbers in parentheses are thicknesses in nm and hyphenated numbers

indicate a linearly varying thickness in certain layers across the specimen. pMTJs with different

Nb thicknesses d1 and d2 have been fabricated for different purposes as detailed below. Circular

junctions with diameters ranging from 100 nm to 100 µm were patterned and subsequently

annealed under varying conditions. Detailed information on sample fabrication and

characterization can be found in our previous publications.39–41

FIG. 1. (a) and (b) Hysteresis loops of the samples with CoFeB thicknesses of 0.98 and 2.08 nm, respectively. The

samples were annealed at 300 ℃ for 10 min. (c) The plot of  ⋅ as a function of the CoFeB thickness t in a

series of samples.

First, the magnetic properties of the films were studied by vibrating sample magnetometry

(VSM) as shown in Figure 1. To isolate the role of the Nb buffer layer on the magnetic

properties of CoFeB, an additional stack structure was produced, consisting of

Ta(3)/Ru(5)/Ta(3)/Nb(10)/CoFeB(0.8-2.2)/MgO (2)/Ta(10)/Ru(20), where numbers in

parentheses are thicknesses in nm. Figure 1(a) shows the easy axis along the out-of-plane

direction for the sample with 0.98 nm CoFeB. The saturation magnetization () and the

anisotropy field (0) of this sample was determined to be 1162 kA/m (1162 emu/cm3) and

0.67 T, respectively. The easy axis changes to the in-plane direction when the top CoFeB layer

thicker than 1.85 nm and the hysteresis loops of the sample with a 2.08 nm top CoFeB is shown

in Figure 1(b). For each sample, the effective perpendicular magnetic anisotropy energy density

() can be obtained as  = 1

2 ⋅ . The interfacial PMA energy density () can be

obtained by =  - 2

2 + 

, where  and 2

2 are bulk anisotropy energy density and

shape anisotropy density, respectively, and  is the thickness of CoFeB on top of MgO. In figure

1(c),  ⋅ is plotted as a function of , and the y-intercept gives , which is found to be 1.85

mJ/m2 (1.85 erg/cm2) in this study for the samples annealed at 300 ℃ for 10 min.

Unfortunately, PMA could not be maintained when the films were annealed at 400 ℃. More

research is needed as stability at 400 ℃ is required by the back-end-of-line integration of MTJ

with CMOS. It was widely recognized that the formation energy between the HM layer and Fe

(60% Fe in the CoFeB alloy used here) plays an important role in determining the thermal

stability of pMTJs during annealing. 23,26 Generally, larger formation energy indicates the HM

layer and Fe are less likely to diffuse into each other to form compounds. The formation energy

of Nb-Fe is about -23 kJ/mol, which is substantially smaller than that of Mo-Fe (-3 kJ/mol), but it

is similar to Ta-Fe (-22 kJ/mol).42 From this point of view the thermal stability of Nb-pMTJs can

be potentially increased to a degree that is similar to Ta-pMTJs through additional materials

and process engineering.

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1Perpendicularmagneticanisotropy,tunnelingmagnetoresistanceandspin-transfertorqueeffectinmagnetictunneljunctionswithNblayersBoweiZhou,1PravinKhanal,1OnriJayBenally,2DeyuanLyu,2DanielB.Gopman,3ArthurEnriquez,1AliHabiboglu,1KennedyWarrilow,1Jian-PingWang,2andWei-GangWang1*1.DepartmentofPhysics,Univers...

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