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Register a new userMicroscopic theory of the Coulomb based exchange coupling in magnetic tunnel junctions
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We study interlayer exchange coupling (IEC) based on the many-body Coulomb interaction between conduction electrons in magnetic tunnel junction (MTJ). This mechanism complements the known IEC based on virtual electron hopping (or spin currents). We find that these two mechanisms have different behavior on system parameters. The Coulomb based IEC may exceed the hopping based exchange coupling. We show that the Coulomb based exchange coupling, in contrast to the hopping based coupling, depends strongly on the dielectric constant of the insulating layer. The dependence of the IEC on the dielectric properties of the insulating layer in MTJ is similar to magneto-electric (ME) effect where electric and magnetic degrees of freedom are coupled. We calculate the IEC as a function of temperature and electric field for MTJ with ferroelectric (FE) layer and show that IEC has a sharp decrease in the vicinity of the FE phase transition and varies strongly with external electric field.
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Heterostructures composed of ferromagnetic layers that are mutually interacting through a nonmagnetic spacer are at the core of magnetic sensor and memory devices. In the present study, layer-resolved ferromagnetic resonance was used to investigate the coupling between the magnetic layers of a Co/MgO/Permalloy magnetic tunnel junction. Two magnetic resonance peaks were observed for both magnetic layers, as probed at the Co and Ni L3 x-ray absorption edges, showing a strong interlayer interaction through the insulating MgO barrier. A theoretical model based on the Landau-Lifshitz-Gilbert-Slonczewski equation was developed, including exchange coupling and spin pumping between the magnetic layers. Fits to the experimental data were carried out, both with and without a spin pumping term, and the goodness of the fit was compared using a likelihood ratio test. This rigorous statistical approach provides an unambiguous proof of the existence of interlayer coupling mediated by spin pumping.
We study the combined effects of spin transfer torque, voltage modulation of interlayer exchange coupling and magnetic anisotropy on the switching behavior of perpendicular magnetic tunnel junctions (p-MTJs). In asymmetric p-MTJs, a linear-in-voltage dependence of interlayer exchange coupling enables the effective perpendicular anisotropy barrier to be lowered for both voltage polarities. This mechanism is shown to reduce the critical switching current and effective activation energy. Finally, we analyze the possibility of having switching via interlayer exchange coupling only.
Magnetic tunnel junctions (MTJs) are basic building blocks for devices such as magnetic random access memories (MRAMs). The relevance for modern computation of non-volatile high-frequency memories makes ac-transport measurements of MTJs crucial for exploring this regime. Here we demonstrate a frequency-mediated effect in which the tunnel magnetoimpedance reverses its sign in a classical Co/Al{_2}O{_3}/NiFe MTJ, whereas we only observe a gradual decrease of tunnel magnetophase. Such effects are explained by the capacitive coupling of a parallel resistor and capacitor in the equivalent circuit model of the MTJ. Furthermore, we report a positive tunnel magnetocapacitance effect, suggesting the presence of a spin-capacitance at the two ferromagnet/tunnel-barrier interfaces. Our results are important for understanding spin transport phenomena at the high frequency regime, in which the spin-polarized charge accumulation at the two interfaces plays a crucial role.
We study interlayer exchange interaction in magnetic tunnel junctions with ferroelectric barrier. We focus on the influence of image forces on the voltage dependence of the interlayer magnetic interaction (magneto-electric effect). The influence of the image forces is twofold: 1) they significantly enforce magneto-electric effect occurring due to the surface charges at the interface between ferroelectric and ferromagnets; 2) in combination with voltage dependent dielectric constant of the ferroelectric barrier image forces cause an additional contribution to the magneto-electric effect in magnetic tunnel junctions. This contribution can exceed the one coming from surface charges. We compare the interlayer exchange coupling voltage variation with spin transfer torque effect and show that for half-metallic electrodes the interlayer exchange coupling variation is dominant and defines the magnetic state and dynamics of magnetization in the tunnel junction.
We demonstrate a voltage-controlled exchange bias effect in CoFeB/MgO/CoFeB magnetic tunnel junctions that is related to the interfacial Fe(Co)Ox formed between the CoFeB electrodes and the MgO barrier. The unique combination of interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp switching of the perpendicularly-magnetized CoFeB allows sensitive detection of the exchange bias. It is found that the exchange bias field can be isothermally controlled by magnetic fields at low temperatures. More importantly, the exchange bias can also be effectively manipulated by the electric field applied to the MgO barrier due to the voltage-controlled antiferromagnetic anisotropy in this system.
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