Nano Res

1

Tunnel Magnetoresistance with Atomically Thin

Two-Dimensional Hexagonal Boron Nitride Barriers

Andre Dankert1*, M. Venkata Kamalakar1, Abdul Wajid1,

R. S. Patel2#, Saroj P. Dash1†

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0627-4

http://www.thenanoresearch.com on November 7 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0627-4

1

Graphical Table of Contents -

Spin-polarized electron tunneling through a monolayer h-BN barrier in magnetic junction and

observation of tunnel magneto resistance at room temperature

2

Tunnel Magnetoresistance with Atomically Thin

Two-Dimensional Hexagonal Boron Nitride Barriers

Andre Dankert1*, M. Venkata Kamalakar1, Abdul Wajid1,

R. S. Patel2#, Saroj P. Dash1†

1Department of Microtechnology and Nanoscience, Chalmers University of Technology,

SE-41296, Göteborg, Sweden

2Department of Physics, Birla Institute of Technology and Science Pilani – K K Birla

Goa Campus, Zuarinagar– 403726, Goa, India

*andre.dankert@chalmers.se; #rsp@goa.bits-pilani.ac.in, †saroj.dash@chalmers.se

ABSTRACT

The two-dimensional atomically thin insulator hexagonal boron nitride (h-BN) constitutes a

new paradigm in tunnel based devices. A large band gap along with its atomically flat nature

without dangling bonds or interface trap states makes it an ideal candidate for tunnel spin

transport in spintronic devices. Here, we demonstrate the tunneling of spin-polarized

electrons through large area monolayer h-BN prepared by chemical vapor deposition in

magnetic tunnel junctions. In ferromagnet/h-BN/ferromagnet heterostructures fabricated over

a chip scale, we show tunnel magneto resistance at room temperature. Measurements at

different bias voltages and on multiple devices with different ferromagnetic electrodes

establish the spin polarized tunneling using h-BN barriers. These results open the way for

integration of 2D monolayer insulating barriers in active spintronic devices and circuits

operating at ambient temperature, and for further exploration of their properties and prospects.

Keywords: Hexagonal Boron Nitride, 2D layered materials, CVD, Spintronics, Magnetic

tunnel junction, Tunnel magneto resistance, Tunnel barrier.

1. Introduction

3

The quantum phenomenon of electron tunneling enables novel spintronic, electronic,

optoelectronic and superconducting nanodevices with enhanced efficiency and low power

consumption [1]. Such tunnel devices typically require growth of insulating materials of few

atomic layers thin, which is a major challenge in materials science. Mostly used conventional

tunnel barrier materials are made up of metal oxides, which have non-uniform thicknesses,

pinholes, defects and trapped charges. These issues compromise the performance and

reliability of the devices. Tunnel barriers are main building blocks of magnetic tunnel

junctions (MTJs), where two ferromagnetic (FM) contacts are separated by ultra-thin oxide

barriers [2-6]. Such MTJs are currently used in read-heads of hard drives and new emerging

technologies including magnetic random access memory and spin-transfer torque

devices [3-5]. For many of these applications, it is crucial to achieve tunnel

magnetoresistance (TMR) signals with optimal junction resistances [3], where the demand for

controlling thickness of metal oxide tunnel barriers with atomic level precession pose a

serious challenge.

The discovery of the two-dimensional (2D) atomic crystals has opened up the possibility of

exploring their fascinating properties [7, 8]. Recently, graphene has been explored as a barrier

for spin transport, where TMR and spin filtering effects have been observed [9-19], including

the bias dependence of the TMR [20]. However, the absence of a bandgap in semi-metallic

and low resistive graphene, leads to an increasing demand for an insulating 2D crystal [7-9].

Hexagonal boron nitride (h-BN) is an insulating isomorph of graphene with a large bandgap

of ~ 6 eV, making it an ideal candidate for dielectric substrates and tunnel barriers [8, 21-25].

The atomically thin and inert nature of h-BN can provide the ultimate control over the

morphology and can minimize defects related to interface states and interfacial alloy

formation. It has been theoretically proposed to use h-BN as tunnel barrier in MTJs to

4

achieve large magnetoresistance signals [26-28]. The spin filtering nature of h-

BN/ferromagnet interfaces and tailoring of TMR by uniaxial strain has been proposed [26-28].

Experimentally, h-BN has been used as tunnel barrier for spin injection in lateral graphene

spin transport devices [29-31]. The chemical vapor deposited (CVD) h-BN is found to show

reproducible tunneling behavior circumventing the conductivity mismatch problem between

the ferromagnet and graphene for efficient spin injection [30]. In order to further establish the

potential of atomically thin h-BN tunnel barriers, it is important to demonstrate TMR in

technologically relevant MTJs, which has not been realized so far.

Here, we employ large area CVD grown monolayer h-BN as a tunnel barrier in MTJs having

ferromagnetic Ni80Fe20 and Co contacts in a vertical structure. We show that atomically thin

CVD h-BN exhibits quantum tunneling of spin polarized electrons for the demonstration of

TMR at room temperature. Measurements on multiple devices with different ferromagnetic

electrodes and bias dependence of the TMR signal establish the spin polarized tunneling

effects using h-BN barriers. Our results demonstrate the integration of atomically thin 2D

insulating tunnel barriers in magnetic tunnel devices and the observation of tunnel

magnetoresistance at ambient temperatures.

2. Experimental

Magnetic tunnel junctions incorporating a h-BN tunnel barrier between two FM metal

electrodes are schematically presented in Fig 1a. When the magnetizations of the FMs are in

a parallel orientation it is preferable that spin polarized electrons will tunnel through the h-

BN than if they are antiparallel, giving rise to a change in the resistance across the junction

[32]. We fabricated magnetic tunnel devices using a h-BN barrier and ferromagnetic Ni80Fe20

(Py) and Co electrodes (Fig. 1b). The main fabrication steps of our devices are presented in

5

Fig. 1c. The bottom Co or Py electrodes are prepared on Si/SiO2 substrate by photo

lithography, electron beam evaporation and lift off methods. Before deposition of FM

electrodes, we deposited a thin layer of Ti in order to increase adhesion of FM to the SiO2

substrate. Subsequently we transferred the CVD grown layer of h-BN on bottom FM

electrodes. We have been able to achieve a ripple free transfer of CVD h-BN over large areas

on our chips by using a frame assisted process. The CVD h-BN layer used in our experiment

was grown on copper substrate (purchased from Graphene supermarket). The h-BN surface is

first covered with PMMA, and then isolated from the Cu substrate by etching in a H2O2-HCl

solution. This Cu etchant produces very clean h-BN in comparison to other available

chemical etching procedures. The h-BN/PMMA layer is washed with deionized water, and

subsequently transferred onto the chip containing bottom FM electrodes in isopropyl alcohol

medium. We avoided the use of DI water on FM contacts to minimize the oxidation.

However, oxidation and contamination of bottom FM electrode due to air exposure could not

be avoided in the present process. After drying the chip in an ambient environment, we

annealed it at 150 C for 10min. It has been observed that this annealing step improved

adhesion of h-BN with bottom FM electrode. The chip was then washed with acetone to

remove the PMMA resulting in chip containing h-BN/FM heterostructures. The top

electrodes of ferromagnetic Co and capping Au layer were prepared on the h-BN/FM

heterostructures by photo lithography, e-beam evaporation and lift off techniques. The atomic

force microscopy was performed with a Bruker Dimension 3100 SPM using tapping mode.

The Raman spectrum was measured with a Horiba XploRA system using a 638 nm laser and

a grating of 1800 lines/mm. TMR measurements were performed at room temperature with

application of an in-plane magnetic field Bin. We sweep the Bin while applying a constant bias

current and measure the voltage change with a Keithley 2400 Sourcemeter. We fabricated

6

various devices consisting of Py/h-BN/Co and Co/h-BN/Co magnetic tunnel structures on

chip scale suing such CVD h-BN.

3. Results and Discussion

Figure 2a shows the optical micrograph of large area CVD h-BN transferred on the SiO2/Si

substrate. Atomic force microscope (AFM) measurements reveal an effective thickness of ~5

Å for the h-BN layer on the SiO2/Si substrate as shown in Fig 2 (b), which corresponds to

single layer h-BN [30]. The Raman shift for the CVD h-BN has been found to be at ~1369

cm-1 (Fig. 2(c)), which matches with the results of exfoliated h-BN [32]. We find the tunnel

resistances of the h-BN contacts are in the range of 0.5 – 1 kΩ.µm2 in our magnetic tunnel

structures. The detailed tunneling characteristics of CVD h-BN barriers have been presented

in our previous work, where an effective barrier height of φ ~ 1.49 eV was extracted [30].

Tunnel magnetoresistance measurements were performed in a four probe cross bar geometry

(Fig. 3(a)), which enables the measurement of the tunnel junction resistance, while avoiding

other effects such as anisotropic magnetoresistance of the FMs. We applied a fixed bias

current between the top and bottom ferromagnetic electrodes of the junction while the voltage

drop across the junction was measured as a function of the external in-plane magnetic field

(B). A magnetization reversal of the FM electrodes occurs at fields corresponding to their

respective coercivities. The switching field is achieved by using different materials and also

through different geometries of the FM electrodes. Hence, their magnetizations can be

aligned either parallel or antiparallel and the system enables us to observe two well-defined

different resistance states due to difference between the threshold switching fields of the two

FM electrodes.

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The TMR measurement in Fig. 3b clearly shows that the tunnel resistance abruptly increases

upon switching the magnetization of the FM electrodes from a parallel to antiparallel

magnetization configuration. The sharp switching of the FM electrodes and the flat region for

the anti-parallel magnetization configuration is a strong indication of TMR [33-37]. The

TMR ~0.15 % was calculated by using the relation TMR = 𝑅𝐴𝑃−𝑅𝑃

𝑅𝑃 ×100 %, where 𝑅𝐴𝑃 and

𝑅𝑃 are the tunnel resistances measured at antiparallel and parallel magnetization

configuration of the two FM electrodes respectively [33]. As expected, the TMR signal also

changes sign with reversing bias polarity (shown in Fig. 3c) [33-37]. The observation of

TMR with h-BN barriers were reproduced on different devices and also in junction with

different FM contacts. Figure 4 shows a TMR signal of 0.5% for a Co/h-BN/Co MTJ at 300

K, where different widths of Co electrodes were chosen to achieve different switching fields.

These results suggest that one atomic layer of h-BN is sufficient enough to decouple the top

and bottom FM layers, which lead to observation of TMR signal at room temperature [33-37].

The observed TMR is a consequence of spin-dependent tunneling from a FM through the h-

BN barrier. The relationship between the observed TMR and tunnel spin polarizations (TSP)

of the FM/h-BN interface can be explained by the Julliere model [33]. The spin polarization

of the FM/h-BN contacts are estimated to be P1 ∼ P2 = P ∼ 0.05 – 0.25 % considering the

relation TMR = 2P1P2/(1 − P1P2), where P1 and P2 are the tunnel spin polarizations of the

Py/h-BN and Co/h-BN interfaces are assumed to be similar, respectively [33]. These values

are less than previously reported TSP of P ~ 14 % in Co/h-BN contacts for spin injection into

graphene in lateral spin transport devices, where FM electrodes are placed on the top of the h-

BN/graphene channel [30]. Such spin polarization obtained in graphene spin transport

devices are comparable to that obtained using Al2O3 and TiO2 tunnel barriers [43]. Much

higher values for spin polarization were predicted theoretically considering lattice matched

FM/h-BN/FM vertical MTJ structures [26 -28]. We attribute the lower value obtained for our

8

vertical MTJ devices to the possible contamination of the bottom Py or Co surface during the

air exposure and h-BN transfer process [38-41]. The oxidation or contamination of the

interface is known to produce strong spin-scattering that reduces the tunneling spin

polarization even with conventional metal oxide tunnel barriers such as Al2O3 and MgO [38-

41]. Higher spin polarization can be expected in structures grown in-situ without any air

exposure. Therefore, the direct growth of h-BN on FMs and in-situ preparation of vertical

MTJ layer stack is required to demonstrate the true spintronics potential.

Figure 5 shows the bias dependence of TMR for two devices with Py/h-BN/Co MTJs at room

temperature. The TMR data of the Dev1 at -10 mV and -50 mV are shown in Fig. 5a and the

complete bias dependence of TMR signal for both devices are plotted with bias voltage in Fig

5b. To be noted we restrict our measurements to low bias voltages < 50 mV, because of

atomically thin h-BN tunnel barriers. The low voltage study is also appropriate considering

the fact that tunneling dominates the conduction mechanism [35-37, 40]. The negative and

positive bias range corresponds to spins tunneling through h-BN barrier from Py → Co and

Co → Py respectively. We observe a decreasing trend of TMR at higher bias voltages with a

maximum around zero voltage, which is consistent with behavior generally observed in spin

polarized tunneling [35-41]. Our results reflect the typical behavior for MTJs, where the bias

dependence of TMR relies upon the quality and height of the h-BN barrier, electronic

properties and magnon excitations at the FM/h-BN interfaces [35, 40]. The observations of

such characteristics also exclude any artifacts such as anisotropic magnetoresistance. The

observed asymmetry in TMR with bias can be due to interfaces oxidation, different work

functions, and densities of states for the two different FM electrodes [35-42]. The difference

in the observed TMR values for two different devices can be due to slightly different

interface conditions, which is not known exactly. The CVD grown h-BN is having mostly

9

monolayer thickness, with few patches of bi-layer and thicker layers [30, 31]. Using these

different thicknesses in lateral graphene spin transport devices, an enhancement in tunnel spin

polarization for higher contact resistances corresponding to the thickness of h-BN barrier has

been observed previously [30]. It would be interesting to investigate such thickness

dependence of TMR in h-BN based MTJ structures and its proposed spin filtering attributes

[26 -28].

4. Conclusions

In conclusion, we have demonstrated the feasibility of spin-dependent tunneling employing

single-layer insulating h-BN barriers in MTJs. We measured a TMR effect of 0.3 – 0.5 % at

room temperature corresponding to tunnel spin polarization P of 0.05 – 0.25 % for FM/h-BN

junctions. The lower values of TMR and P are attributed to oxidation and contamination of

the bottom FM electrode during h-BN transfer process. It is expected that much higher TMR

ratios can be obtained through improvements in fabrication process with cleaner interfaces,

incorporating multi-layer h-BN barriers [26-28], and use of h-BN/graphene heterostructures

to achieve spin filtering [26]. These results demonstrate that uses of atomically thin large area

CVD h-BN tunnel barrier are generic for a large class of devices requiring tunneling of spin

polarized carriers and particularly interesting for technologically important magnetic tunnel

junctions. Such h-BN tunnel barriers can also be employed for efficient spin injection into

silicon and other semiconductor materials [44, 45]. Opportunities are growing with

improvements in the growth process of large area CVD h-BN and h-BN/graphene van der

Waals heterostructures on ferromagnets, which can be used in future spintronic devices to

achieve desired contact resistances and large magnetoresistance signals [46-49].

Acknowledgement

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The authors acknowledge the support from colleagues of Quantum Device Physics

Laboratory and Nanofabrication Laboratory at Chalmers University of Technology. The

authors would like to thank Jie Sun and Niclas Lindvall for sharing the recipe for 2D layer

transfer process. This project is financially supported by the Nano Area of the Advance

program at Chalmers University of Technology, EU FP7 Marie Curie Career Integration

grant and the Swedish Research Council (VR) Young Researchers Grant. RSP acknowledges

the financial support from the Department of Science and Technology, Government of India

through nanomisson project (grant No. SR/NM/NS-1002/2010).

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Figure 1| Hexagonal boron nitride magnetic tunnel junction. (a) Schematics of spin

polarized tunneling in a FM/h-BN/FM tunnel junction. (b) Optical micrograph of a fabricated

magnetic tunnel junction with Py/h-BN/Co contact. (c) Fabrication steps of magnetic tunnel

junction with CVD h-BN and FM electrodes.

14

Figure 2| CVD Hexagonal boron nitride: (a) Optical micrograph of CVD h-BN on a

SiO2/Si substrate. (b) Atomic force microscopy image of a CVD h-BN layer on a SiO2/Si

substrate. The red line is the step scan showing an effective h-BN height of ~5 Å. (c) The

Raman peak of CVD h-BN on a SiO2/Si substrate.

Figure 3| TMR with Py/h-BN/Co MTJ at room temperature. (a) Schematics of a Py/h-

BN/Co MTJ in a four probe cross bar geometry. (b, c) TMR measurements with in-plane

magnetic field B in Py/h-BN/Co MTJ for applied bias voltages of +/- 5 mV at 300 K. The

arrows indicate the up and down B field sweep directions.

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Figure 4| TMR with Co/h-BN/Co MTJ at room temperature. (a) Schematics of a Co/h-

BN/Co MTJ in a four probe cross bar geometry. (b) TMR measurements with in-plane

magnetic field in Co/h-BN/Co MTJ for applied bias voltages of 125µV at 300 K. The arrows

indicate the up and down B field sweep directions.

Figure 5| Bias dependence of TMR with h-BN barrier at room temperature. (a) TMR

plots for Dev1 at bias voltage of -10 and -50 mV at 300 K. The arrows indicate the up and

down B field sweep directions. (b) Full bias dependence of TMR signal for two different

Py/h-BN/Co devices measured at 300 K (bule circles - Dev 1, and red circles - Dev2). The

lines are guide to the eye.