Tunnel magnetoresistance
Since this process is forbidden in classical physics, the tunnel magnetoresistance is a strictly quantum mechanical phenomenon, and lies in the study of spintronics.
The effect was originally discovered in 1975 by Michel Jullière (University of Rennes, France) in Fe/Ge-O/Co-junctions at 4.2 K. The relative change of resistance was around 14%, and did not attract much attention.
[1] In 1991 Terunobu Miyazaki (Tohoku University, Japan) found a change of 2.7% at room temperature.
Later, in 1994, Miyazaki found 18% in junctions of iron separated by an amorphous aluminum oxide insulator [2] and Jagadeesh Moodera found 11.8% in junctions with electrodes of CoFe and Co.[3] The highest effects observed at this time with aluminum oxide insulators was around 70% at room temperature.
In 2001 Butler and Mathon independently made the theoretical prediction that using iron as the ferromagnet and MgO as the insulator, the tunnel magnetoresistance can reach several thousand percent.
[4][5] The same year, Bowen et al. were the first to report experiments showing a significant TMR in a MgO based magnetic tunnel junction [Fe/MgO/FeCo(001)].
[7][8] In 2008, effects of up to 604% at room temperature and more than 1100% at 4.2 K were observed in junctions of CoFeB/MgO/CoFeB by S. Ikeda, H. Ohno group of Tohoku University in Japan.
[9] The read-heads of modern hard disk drives work on the basis of magnetic tunnel junctions.
TMR, or more specifically the magnetic tunnel junction, is also the basis of MRAM, a new type of non-volatile memory.
The 1st generation technologies relied on creating cross-point magnetic fields on each bit to write the data on it, although this approach has a scaling limit at around 90–130 nm.
[10] There are two 2nd generation techniques currently being developed: Thermal Assisted Switching (TAS)[10] and Spin-transfer torque.
If no voltage is applied to the junction, electrons tunnel in both directions with equal rates.
This property is theoretically predicted for a number of materials (e.g. CrO2, various Heusler alloys) but its experimental confirmation has been the subject of subtle debate.
Nevertheless, if one considers only those electrons that enter into transport, measurements by Bowen et al. of up to 99.6%[12] spin polarization at the interface between La0.7Sr0.3MnO3 and SrTiO3 pragmatically amount to experimental proof of this property.
Indeed, MgO filters the tunneling transmission of electrons with a particular symmetry that are fully spin-polarized within the current flowing across body-centered cubic Fe-based electrodes.
Thus, in the MTJ's parallel (P) state of electrode magnetization, electrons of this symmetry dominate the junction current.
In contrast, in the MTJ's antiparallel (AP) state, this channel is blocked, such that electrons with the next most favorable symmetry to transmit dominate the junction current.
Since those electrons tunnel with respect to a larger barrier height, this results in the sizeable TMR.
[14] The conceptually simpler experiment of inserting an appropriate metal spacer at the junction interface during sample growth was also later demonstrated[15][16] .
[7] This contradiction is lifted if one takes into account the localized states of oxygen vacancies in the MgO tunnel barrier.
Extensive solid-state tunnelling spectroscopy experiments across MgO MTJs revealed in 2014[13] that the electronic retention on the ground and excited states of an oxygen vacancy, which is temperature-dependent, determines the tunnelling barrier height for electrons of a given symmetry, and thus crafts the effective TMR ratio and its temperature dependence.
This low barrier height in turn enables the high current densities required for spin-transfer torque, discussed hereafter.
This may then be pinned to some selecting transistor in a magnetoresistive random-access memory device, or connected to a preamplifier in a hard disk drive application.
is the gauge-invariant nonequilibrium density matrix for the steady-state transport, in the zero-temperature limit, in the linear-response regime,[17] and the torque operator
The active region is attached to the left ferromagnetic electrode (modeled as semi-infinite tight-binding chain with non-zero Zeeman splitting) and the right N electrode (semi-infinite tight-binding chain without any Zeeman splitting), as encoded by the corresponding self-energy terms.
[21] The grain boundaries may act as short circuit conduction paths through the material, reducing the resistance of the device.
Recently, using new scanning transmission electron microscopy techniques, the grain boundaries within FeCoB/MgO/FeCoB MTJs have been atomically resolved.
This has allowed first principles density functional theory calculations to be performed on structural units that are present in real films.
[22] In addition to grain boundaries, point defects such as boron interstitial and oxygen vacancies could be significantly altering the tunnelling magneto-resistance.
Recent theoretical calculations have revealed that boron interstitials introduce defect states in the band gap potentially reducing the TMR further[23] These theoretical calculations have also been backed up by experimental evidence showing the nature of boron within the MgO layer between two different systems and how the TMR is different.
