Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers

Abstract

Abstract:We present an experimental study of spin transport in single layer graphene using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for spin injection. While h-BN is expected to be favorable for spin injection, previous experimental studies have been unable to achieve spin relaxation times in the nanosecond regime, suggesting potential problems originating from the contacts. Here, we investigate spin relaxation in graphene spin valves with h-BN barriers and observe room temperature spin lifetimes in excess of a nanosecond, which provides experimental confirmation that h-BN is indeed a good barrier material for spin injection into graphene. By carrying out measurements with different thicknesses of h-BN, we show that few layer h-BN is a better choice than monolayer for achieving high non-local spin signals and longer spin relaxation times in graphene.* Author to whom correspondence should be addressed: kawakami.15@osu.edu 2 Graphene is a promising spin channel material for next generation spintronic devices due to the experimental demonstration of long spin diffusion lengths at room temperature 1-3 and theoretical predictions of long spin relaxation times 4,5 arising from the weak spin-orbit and hyperfine couplings 5,6 .However, experimentally measured spin relaxation times [1][2][3]7,8 in graphene are orders of magnitude shorter than theoretically predicted 4,5 . In graphene spin valves, the tunnel barrier plays a crucial role for spin injection by circumventing the problem of impedance mismatch 9 between graphene and the ferromagnetic electrodes. As demonstrated by Han et. al. 8 , high quality tunnel barriers are critical for obtaining higher spin relaxation times (τ s ) in graphene because barriers with pinholes or rough surface morphology can cause additional contact-induced spin relaxation, which has received a great deal of interest recently. [10][11][12][13][14] As opposed to growing oxide tunnel barriers on graphene, a thin insulating twodimensional (2D) van der Waals material can also be used as a tunnel barrier. A particular material of interest is single (or few) layer h-BN because of its various suitable properties 15 : large energy band gap ~5.97 eV, high crystallinity, spin filtering 16 , absence of pinholes and dangling bonds, atomic lattice similar to graphene, and chemical stability at ambient conditions. In addition, atomically clean vertical heterostructures of h-BN/graphene can be mechanically assembled using polymer-based transfer techniques 17,18 . The first experimental report demonstrating spin injection into graphene using a monolayer h-BN tunnel barrier showed τ s less than 100 ps 19 . This was followed by the work of Kamalakar et. al. 20,21 and Fu et. al. 22 , which used chemically grown h-BN barriers, yielding τ s ~ 500 ps. Another recent study using an encapsulated geometry 23 with graphene sandwiched between a thick bottom layer of h-BN and a monolayer of h-BN on top showed τ s less than 200 ps. As evident from these studies, graphene spin valve devices ...

Spin transport in two-layer-CVD-hBN/graphene/hBN heterostructures

“…The growth of CVDhBN can suffer from the inhomogeneous surface coverage, and the copper etching steps could also damage the CVDhBN and leave some underetched residues, leading to uneven interfacial growth of ferromagnetic cobalt on top [15], which may cause spin dephasing in graphene via randomly oriented magnetic fringe fields near the contacts [39]. Moreover, during the wet transfer of CVD-hBN, some unwanted contamination may get trapped at the interface with graphene, and graphene itself comes in direct contact with DI water.…”

“…Furthermore, a fully hBN-encapsulated monolayer-graphene with exfoliated-hBN tunnel barriers showed differential spin polarizations of 1-2 % with monolayer-hBN contacts [13][14][15][16], up to 100% with bilayerhBN contacts [16], and up to 6% with trilayer-hBN contacts [17]. Thicknesses of more than three layers are not suitable for * Author to whom all correspondence should be addressed: m.gurram@rug.nl spin injection [15,17,18] due to very high tunneling interface resistance. However, for large-scale spintronics applications, it is important to incorporate large-area chemical vapor deposition (CVD) grown hBN tunnel barriers in spin valves [18][19][20][21] and magnetic tunnel junctions [22,23].…”

Strong Modulation of Spin Currents in Bilayer Graphene by Static and Fluctuating Proximity Exchange Fields

“…To prepare clean heterostructures of graphene/YIG we employ a dry transfer technique [13,25] To modulate the spin signal in graphene, we align the magnetization of electrode E2 and E3 in either parallel (P) or antiparallel (AP) configuration and apply a fixed magnitude of magnetic field, B ROT = 15 4 mT, in the plane of the graphene. Note that this magnetic field is smaller than what is required to change or switch the electrode but large enough to saturate the YIG magnetization [19].…”

“…To prepare clean heterostructures of graphene/YIG we employ a dry transfer technique [13,25] as discussed in supplementary material [26]. The optical image of the hexagonal boron nitride (h-BN)/graphene stack on YIG is shown in Figure 1(a), where thin h-BN is highlighted by black dotted lines.…”

“…The AFM topography of the h-BN/graphene/YIG surface is depicted in Figure 1(b) with the clean interface. In this structure, the h-BN serves as the tunnel barrier for spin injection into graphene [25]. shows R NL, (R NL =V NL /I), as a function of in-plane magnetic field.…”