Voltage Control of a van der Waals Spin-Filter Magnetic Tunnel Junction

Abstract

Atomically thin chromium triiodide (CrI 3 ) has recently been identified as a layered antiferromagnetic insulator, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled 1,2 . This unusual magnetic structure naturally comprises a series of anti-aligned spin filters which can be utilized to make spin-filter magnetic tunnel junctions with very large tunneling magnetoresistance (TMR) 3-6 . Here we report voltage control of TMR formed by four-layer CrI 3 sandwiched by monolayer graphene contacts in a dual-gated structure. By varying the gate voltages at fixed magnetic field, the device can be switched reversibly between bistable magnetic states with the same net magnetization but drastically different resistance (by a factor of ten or more). In addition, without switching the state, the TMR can be continuously modulated between 17,000% and 57,000%, due to the combination of spin-dependent tunnel barrier with changing carrier distributions in the graphene contacts. Our work demonstrates new kinds of magnetically moderated transistor action and opens up possibilities for voltage-controlled van der Waals spintronic devices. Main Text:Electrical manipulation of magnetism is central to spintronics 7-13 . Voltage-controlled switching between bistable magnetic states can be employed in energy efficient magnetic memory and logic technologies. In this regard, the recently discovered two-dimensional (2D) magnetic insulator chromium triiodide (CrI3) has several assets as a building block for van der Waals (vdW) spintronics 1,2,14 . First, the extreme thinness of few-layer CrI3 enhances the probability that the magnetism will be amenable to electrostatic control [15][16][17][18][19][20] . Second, the layered antiferromagnetic structure at zero field naturally forms a series of interlayer spin filters, and their relative alignment can be changed by a moderate magnetic field via spin-flip transitions. This unusual property underpins the recent demonstration of multiple-spin-filter magnetic tunnel junctions (sf-MTJs) that exhibit giant tunneling magnetoresistance (TMR) [3][4][5][6]21,22 .

Van der Waals magnets: Wonder building blocks for two‐dimensional spintronics?

“…However, when the magnetic field is enough to drive the bilayer into fully spin‐polarized states, the suppression in the tunneling is lifted. For the case of four‐layer CrI 3 , a large tunneling magnetoresistance ratio exceeding 25 000% has been reported at zero gate voltage (Figure C) . An obvious shift of the thresholds in these curves toward a positive gate bias may be caused by changes of Fermi level or magnetic proximity effects induced by CrI 3 in graphene contacts …”

“…39 C, Tunneling magnetoresistance ratio at different gate voltages for four layers CrI 3 MTJ. 108 Figure 6D). This in turn gives rise to an observed 160% magnetoresistance, corresponding to a spin polarization of 66% at the Fermi level.…”

“…B, Tunneling current verses out‐of‐plane magnetic field at bias voltage (−290 mV) (upper panel) and the corresponding RMCD signal of device in A (lower panel) . C, Tunneling magnetoresistance ratio at different gate voltages for four layers CrI 3 MTJ . D, Tunneling resistance of a Fe 3 GeTe 2 /h‐BN/Fe 3 GeTe 2 vdW heterostructure measured at 4.2 K with out of plane magnetic field (upper panel).…”

“…For the case of four-layer CrI 3 , a large tunneling magnetoresistance ratio exceeding 25 000% has been reported at zero gate voltage ( Figure 6C). 108 An obvious shift of the thresholds in these curves toward a positive gate bias may be caused by changes of Fermi level or magnetic proximity effects induced by CrI 3 in graphene contacts. 111 A more conventional MTJ architecture is composed of thin insulators sandwiched by two magnetic electrodes, such as Fe 3 GeTe 2 /h-BN/Fe 3 GeTe 2 .…”

Ultrathin Scattering Spin Filter and Magnetic Tunnel Junction Implemented by Ferromagnetic 2D van der Waals Material

“…According to the Mermin–Wagner theorem,18 2D long‐range FM order cannot exist in an isotropic magnetic system. Recently, people found that 2D layered CrI 3 ,19–27 Cr 2 Ge 2 Te 6 ,28 and CrSiTe 3 29 possesses intrinsic magnetocrystalline anisotropy against thermal fluctuations. However, the Curie temperatures of CrI 3 , Cr 2 Ge 2 Te 6 , and CrSiTe 3 are only dozens of Kelvins.…”