High Tunnel magnetoresistance in ultrathin MnGa-based perpendicular magnetic tunnel junctions utilizing ferromagnetic bcc-CoMn interlayers

Abstract

Perpendicular magnetic tunnel junctions (p-MTJs) utilizing tetragonal Mn-based alloy films attracts attention as key devices for magnetic random access memory and THz frequency devices 1. There were many past studies demonstrating those devices, whereas their TMR ratios were much smaller than those predicted by first-principles calculations2. Interlayers between the Mn-based alloy and the barrier effectively enhance those TMR ratio, and the TMR ratio of 60% was reported in MnGa/MgO MTJs with FeCo interlayer3. It is crucial to find new interlayer materials to further enhance the TMR ratio for realizing Mn-based p-MTJs for practical applications.Recently, we have reported in-plane type MTJs using bcc-Co75Mn25 electrodes. Bcc-Co75Mn25 is known for its metastable bcc-phase and for ferromagnetism having high Curie temperature4. Those MTJs showed a high TMR ratio of over 200%, which may be due to highly spin-polarized △1 band in bcc-Co75Mn25 5. Here we demonstrate high TMR ratio in ultrathin MnGa-based p-MTJs with the bcc-Co3Mn alloy interlayer. The result indicates that bcc-CoMn alloy is one of the promising interlayer materials for obtaining high TMR ratio in Mn-based p-MTJs6,7.The samples were prepared using a magnetron sputtering system. The stacking structure of the samples was MgO(001) substrate/Cr(40)/Co55Ga45(30)/ Mn61Ga39(3)/ CoxMn100-x(0.8) /Mg(0.4)/MgO(2)/Fe60Co20B20(1.2)/Ta(3)/Ru(5) (thickness is in nanometers) Here, the CoxMn100-x interlayer was deposited using a co-sputtering technique with Co and Mn targets. All the layers were deposited at room temperature, and the deposition process for the buffer and seed layers was adopted from a previous study6. Micro-fabrication of MTJs was performed by standard ultraviolet photolithography, and Ar ion milling etching. After microfabrication, the MTJs were annealed at 250 °C in a vacuum furnace. TMR measurements were performed by a physical property measurement system (PPMS, Quantum Design) using a standard four-probe method at a bias voltage of 1 mV.Figure 1 shows the TMR curves of the MTJs with/without Co75Mn25 interlayer measured at 300 K. The Co75Mn25 interlayer shows the TMR ratio up of approximately 80%, which is much enhanced as compared with that for MnGa-MTJs, ~ 10 %. Meanwhile, the shape of the TMR curve and the increase of coercivity indicate that Co75Mn25 interlayer strongly and antiferromagnetically couples with perpendicularly-magnetized MnGa layer. Figure 2 shows the composition x dependence of the TMR ratio for the MTJs with 0.8-nm-thick CoxMn100-x interlayer. The TMR ratio for the x=0 MTJ is 70% (145%) at 300 K (10K), which is larger than the value expected from Julliere’s relation with spin-polarization of fcc-Co and CoFe(B). Moreover, the TMR ratio increases by up to 85% (209%) at 300 K(10 K) for the case of x=0.8. Those high TMR ratio would originate from coherent tunneling between highly spin-polarized Δ1 bands in bcc-Co (Co-Mn) and CoFe(B) electrodes, even though the Co-Mn interlayer thickness is very small, 0.8 nm. This may be also consistent with the temperature dependence of resistance or conductance in these MTJs, which was similar to those observed in Fe/MgO/Fe MTJs.AcknowledgementWe would like to thank Y. Kondo for his technical assistance. This work was partially supported by JST CREST (No. JPMJCR17J5). **