High bias voltage effect on spin-dependent conductivity and shot noise in carbon-doped Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions

Abstract

Low temperature (10K) high voltage bias dynamic conductivity (up to 2.7V) and shot noise (up to 1V) were studied in epitaxial Fe(100)/Fe-C/MgO(100)/Fe(100) magnetic tunnel junctions, as a function of the magnetic state. The junctions show large tunnel magnetoresistance (185% at 300K and 330% at 4K). Multiple sign inversion of the magnetoresistance is observed for bias polarity when the electrons scan the electronic structure of the bottom Fe-C interface. The shot-noise shows a Poissonian character. This demonstrates a pure spin dependent direct tunneling mechanism and validates the high structural quality of the MgO barrier. PACS numbers:Magnetic tunnel junctions (MTJs) [1,2] are nowadays one of the most active areas of material science and spintronics. Recent theoretical predictions [3,4] and experimental demonstration [5,6,7,8,9] of coherent spin-dependent tunneling in single crystal Fe(100)/MgO(100)/Fe(100) MTJs revolutionized this area providing new ways to create devices with room temperature Tunneling Magneto-Resistance (RT-TMR) exceeding 100%. The large TMR at low bias is mostly due to fully spin polarized ∆ 1 bulk electron states in Fe(001), reflected for antiparrallel ferromagnetic electrodes configuration (AP) or well transmitted for the parallel (P) state [3,4]. However, the tunneling mechanism gets more complex when taking into account the electronic structure of the interfaces[10] and when biasing the junction. Therefore, for finite bias polarities the antiparallel conductance may exceed the parallel one, resulting in TMR suppression [8] or its sign reversal [10]. By engineering the chemical and electronic structure of the Fe/MgO interface, the voltage variation of the TMR in amplitude and sign can be skilfully manipulated. It has been recently demonstrated that the carbon-doping of the bottom Fe/MgO interface leads to strongly asymmetric TMR vs bias, providing a root for creation of high-output voltage device applications [9].Our Letter presents a first study of dynamical conductance and TMR in a large bias window, up to 2.7 V, for Fe(100)/Fe-C/MgO(100)/Fe(100) MTJs. The shot noise analysis in different magnetization configuration is performed at voltages up to 1V. The experiments are done at room temperature (300K) and low temperature (4K-10K). The measured TMR ratio increases from 185% at 300K to 330% at 4K, mostly due to the strong temperature variation of the tunnel conductivity in the antiparallel (AP) state. Moreover, our tunneling spectroscopy experiments show a clear maximum in the AP conductivity for a finite bias and a multiple TMR sign inversion. These experiments demonstrate the role of the minority spin Fe interface resonance state (IRS) to the tunneling. Furthermore, in both parallel (P) and antiparallel magnetization configuration, the shot noise measurements demonstrate an uncorrelated direct tunneling mechanism across the MgO barrier. The shot noise analysis and the large breakdown voltage of the junctions (up to 3V) demonstrates the high quality of our MgO barriers (i.e. absence of...

Very low 1∕f noise at room temperature in fully epitaxial Fe∕MgO∕Fe magnetic tunnel junctions

“…This dependence, as is evidenced in Figures 1b and 1c, is rather asymmetric being the inversion of the noise asymmetry linked to the TMR inversion point (see vertical dotted line in Fig.2.) We note that in general for the A-MTJs, both conductance and 1/f noise level appeared to be much more stable to the applied external electric field above 10 6 V/cm, in agreement with our recent findings for carbon-doped epitaxial Fe/MgO/Fe with large MgO barriers [19].…”

Controlling Shot Noise in Double-Barrier Magnetic Tunnel Junctions

“…The efforts aimed at understanding spin coherency and SN, limited up to now to MTJs, revealed suppressed SN with Al 2 O 3 barriers (0:7 < F < 1) due to sequential tunneling [17] and also in serial MTJ arrays [18]. As for MTJs with MgO barriers, full SN (F ¼ 1) independent of the magnetic state was observed in epitaxial Fe=MgO=Fe [19]. Then, the noise was examined for ultrathin (less than 1 nm) MgO barriers, where F ' 0:92 was observed in the parallel state [20,21].…”

Shot noise in magnetic double-barrier tunnel junctions

“…14,15 It has also been shown experimentally that the Fano factor in a single-barrier magnetic tunnel junction depends on magnetic configuration of the junction, and can be remarkably enhanced for the antiparallel alignment of the electrodes' magnetizations. [16][17][18][19][20] Even more possibilities have been found in double-barrier tunnel junctions with two ferromagnetic electrodes and one ferromagnetic central layer, 21,22 where one can distinguish four magnetic configurations corresponding to different alignments of the magnetic moments of all three magnetic layers. 23 It has also been shown that a simple model based on two well-separated spin channels for electronic transport cannot properly describe the experimental observations without taking into account spin-flip transitions.…”