A. Kovács

Enhanced exchange anisotropy in IrMn/CoFeB systems and its correlation with uncompensated interfacial spins

Bottom pinned exchange bias systems of IrMn/CoFe and IrMn/CoFeB on CoFe seed layers were studied. Enhanced exchange anisotropy has been observed for IrMn/CoFeB samples annealed at 350u2009°C. The ferromagnetic and antiferromagnetic layers of both samples are polycrystalline and textured {110} for the CoFe and CoFeB, and {111} for IrMn. Results demonstrated that the enhanced exchange anisotropy in the IrMn/CoFeB system is closely associated with the increased uncompensated interfacial spins as evidenced by the enhanced Mn x-ray magnetic circular dichroism (XMCD) signal strength. A quantitative correlation between the Mn XMCD signal and the exchange anisotropy constant Jk was observed.

Structural and electrical characterization of SiO2/MgO(001) barriers on Si for a magnetic transistor

We report a structural and electrical study of sputter-deposited SiO2/MgO barriers for developing magnetic Si-based transistors. We propose that SiO2/MgO tunneling barriers may utilize spin-filtering by achieving crystalline MgO (001) while reducing spin-scattering due to the Si/SiO2 interface. We find that MgO (<3u2002nm thick) crystallizes with (001) preferred orientation on thermally oxidized Si(<2u2002nm). Typical processing temperatures do not cause significant intermixing with SiO2 or ferromagnetic electrode. Conversely, MgO on Si is amorphous up to 2 nm thick. Capacitance-voltage characteristics of MgO capacitors are influenced significantly by the density of interface-states, as high as 5×1013u2002cm−2u2009eV−1 while Si/SiO2/MgO structures are electrically beneficial by reducing to 6×1012u2002cm−2u2009eV−1.

Evaluation of Schottky and MgO-based tunnelling diodes with different ferromagnets for spin injection in n-Si

In this work we present the electrical properties of sputter-deposited ferromagnetic (FM) Schottky diodes and MgO-based tunnelling diodes to n-doped (0u20090u20091) silicon. The effective Schottky barrier height (SBH) has been evaluated as a function of the FM electrode (Co70Fe30, Co40Fe40B20 and Ni80Fe20), the silicon doping density (1015 to 1018u2009cm−3), the MgO tunnelling barrier thickness (0, 1.5 and 2.5u2009nm) and post-deposition annealing up to 400u2009°C. The ideality factors of the Schottky diodes are close to unity, indicating transport by thermionic emission and the absence of an interfacial oxide layer, which is confirmed by transmission electron microscopy. The effective SBH is found to be approximately 0.65u2009eV, independent of the FM material and decreasing with increasing doping density. The changes induced by high temperature annealing at the current–voltage characteristic of the Schottky diodes depend strongly on the FM electrode. The effective SBH for the tunnelling diodes is as low as 0.3u2009eV, which suggests a high density of oxide and interface traps. It is again independent of the FM electrode, decreasing with increasing doping density and annealing temperature. The inclusion of MgO leads to higher thermal stability of the tunnelling diodes. The measured contact resistance values are discussed with respect to the conductivity mismatch for spin injection and detection.

Exchange-bias in amorphous ferromagnetic and polycrystalline antiferromagnetic bilayers: Structural study and micromagnetic modeling

We study the role of the structure of antiferromagnetic polycrystalline metallic films in determining the magnetic properties of an exchange-coupled amorphous ferromagnetic layer. The bilayers are sputter-deposited, highly textured {111} Ir22Mn78 and Co65.5Fe14.5B20 thin films. We focus on structural characterization of Ir22Mn78 as a function of layer thickness in the range having the strongest influence over the exchange-bias field and training effect. We have used transmission electron microscopy to characterize defects in the form of interface steps and roughness, interdiffusion, twin- and grain-boundaries. Such defects can result in uncompensated magnetic spins in the antiferromagnet, which then contribute to exchange-bias. These experimental results form the basis of a general model, which uses finite element micromagnetic simulations. The model incorporates the experimental structural parameters of the bilayer by implementing a surface integral technique that allows numerical calculations to solve t...

Exchange bias interactions in polycrystalline/amorphous bilayers

For technologically relevant systems of polycrystalline antiferromagnetic layer coupled to an amorphous ferromagnetic layer, quantitative models and micromagnetic simulations are challenging due to inherent structural differences. We present a numerical study, performed using a surface integral technique with finite element micromagnetic simulations, that can incorporate structural and magnetic parameters such as grain crystallography, mixed spin-interface coupling and granular stability, arising from grain volume and anisotropy. We show that this model is in good agreement with experimental results for exchange bias and coercive fields as well as the training effect.

The formation mechanism of 360° domain walls in exchange-biased polycrystalline ferromagnetic films

The formation mechanism of 360° domain walls (360DW) created in an exchange-biased bilayer of Co65.5Fe14.5B20/Ir22Mn78 is described. The structural and magnetic properties are experimentally characterized and incorporated into a micromagnetic model of exchange-bias for granular anti-ferromagnetic films. This model is used to study and explain the formation mechanism of 360DWs in the ferromagnetic layer, which occur due to interface coupling to the antiferromagnetic layer. The validity of the resulting calculated magnetization maps are examined by comparing simulated and experimental Fresnel-contrast images of the bilayer. Energy barrier simulations are used to explain the dependence of the areal size and spatial frequency of the 360DW on the anisotropy energy of the anti-ferromagnetic layer. These calculations also show how such structures can form at room temperature at relatively low applied magnetic fields. Calculations based on this model are in agreement with imaging using Lorentz transmission electr...

Observation of single-spin transport in an island-shaped CoFeB double magnetic tunnel junction prepared by magnetron sputtering

Abstract The Co40Fe40B20(CFB)/MgO/CFB/MgO/CFB-based multilayer was prepared by conventional magnetron sputtering and utilised in the fabrication of double magnetic tunnel junctions (DMTJs) for which the middle CFB layers were island-shaped. By analysing the magnetic property of the CFB islands with Langevin’s equation, it was possible to identify their diameters of 7.6, 8.9 and 11.0 nm; accordingly submicron-scaled DMTJs were fabricated to investigate single-spin transport phenomena. The coulomb staircase and the oscillatory tunnel magnetoresistive (TMR) were able to be observed at 6 K, where the TMR ratio was enhanced up to 60%, which is the highest value ever achieved in this structure.

The role of structure on magneto-transport properties of Heusler Co2MnSi films deposited on MgO(001)

We present an experimental study identifying structural reasons that degrade spin-polarization of Co2MnSi thin films deposited on MgO(001) substrates. Through the fabrication of magnetic tunnel junctions, we measure a range of values for tunneling magneto-resistance (TMR) ratios following post-deposition annealing and epitaxial crystallization of the Heusler film. These TMR ratios reflect qualitatively the change in spin polarization of the Co2MnSi thin films. Low-temperature annealing results in low spin-polarization due to a high fraction of an amorphous phase. As annealing temperatures increase, the fraction of L21 and B2 chemically ordered phases increases, thus improving significantly the spin-polarization. However, for samples annealed at higher temperatures, significant degradation in the cubic magneto-crystalline anisotropy is observed, which we attribute to the detection of manganese diffusion into the MgO substrate. This Mn diffusion is manifested in a reduction of the value of the TMR ratio, na...

Influence of thermal energy on exchange-bias studied by finite-element simulations

In this article we describe the thermal relaxation in anti-ferromagnetic/ferromagnetic bilayers using a hybrid method that combines a kinetic Monte Carlo technique with magnetization dynamics following the Landau Lifshitz Gilbert equation. A granular anti-ferromagnetic layer is exchange coupled to an amorphous ferromagnetic layer and discretized using a finite element method. Calculations are made to help clarify how the underlying magnetic structure is related to the measured exchange bias fields as a function of temperature for the case of amorphous Co65.5Fe14.5B20/granular Ir22Mn78 bilayers. Our calculations are in excellent agreement with experimentally measured macro-magnetic properties of these bilayers.