What Is a Tunnel Junction?

A magnetic tunnel junction refers to an extremely thin insulating layer with a thickness of about 0.1 nm sandwiched between two ferromagnetic sheets, forming a so-called junction element. In ferromagnetic materials, 3d orbital local electron bands of ferromagnetic metals are cleaved due to quantum mechanical exchange, so that spin up and down electrons near the Fermi surface have different energy state densities. In MTJs, the mechanism of the TMR effect is the spin-dependent tunneling effect. The general structure of MTJs is a sandwich structure of ferromagnetic layer / non-magnetic insulation layer / ferromagnetic layer (FM / I / FM). In saturation magnetization, the magnetization directions of the two ferromagnetic layers are parallel to each other, and usually the coercive forces of the two ferromagnetic layers are different. Therefore, when the magnetization vector of the ferromagnetic layer with a small coercive force is reversed when the magnetization is reversed, the two ferromagnetic layers are first flipped, so that The magnetization direction becomes antiparallel. The tunneling probability of electrons from one magnetic layer to another is related to the magnetization directions of the two magnetic layers.

The quantum tunneling effect is one of the basic quantum phenomena, that is, when the total energy of a microscopic particle is less than the barrier height, the particle can still cross this barrier. According to classical theory, in order to escape from the energy barrier, particles must cross the top of the barrier. But due to the quantum uncertainty in quantum mechanics, time and energy are a set of conjugate quantities. In a short time (that is, the time is certain), the energy can be very uncertain, so that a particle looks like it has crossed a potential barrier from a "tunnel."

As early as 1975, Julliere was in Co / Ge / Fe Magnetic Tunnel Junctions (MTJs) (Note: The general structure of MTJs is ferromagnetic layer / non-magnetic insulation layer / ferromagnetic layer (FM / I / FM) TMR effect was observed in the sandwich structure). However, this discovery did not attract much attention at the time.

In 1988, when Brazilian scholar Baibich worked in a research group led by Professor Fert of the Department of Physics at the University of Paris, France, he first discovered the giant magnetoresistance (GMR) effect in Fe / Cr multilayer films. The discovery of the TMR effect and the GMR effect has led to the emergence of a new branch of condensed matter physics, magnetoelectronics.

(1) Under low junction voltage, the tunnel current changes linearly with the voltage; under high junction voltage, the tunnel current changes faster than the linear law.

(2) At room temperature, the magnetic resistance of the magnetic tunnel junction is between a few percent and 40%, and decreases with the increase of the junction voltage. When the temperature decreases from room temperature to 77 or 4.2K, the magnetic resistance value is significant Increase.

(3) Depending on the manufacturing technology, the junction resistance of the magnetic tunnel junction varies widely, and can be 10 ^ 2 · cm or less 10 ^ 7 · cm or more. * (4) Two irons in the magnetic tunnel junction The magnetic field between magnetic layers is very small, an order of magnitude lower than that of a ferromagnetic metal-nonmagnetic metal-ferromagnetic metal sandwich structure with the same ferromagnetic layer thickness.

Compared with magnetic multilayer films and granular films, the biggest advantages of magnetic tunnel junctions are their controllable high junction resistance (low power loss), saturation field, and relatively high field sensitivity.

An extremely thin insulating layer with a thickness of about 0.1 nm is sandwiched between two ferromagnetic sheets to form a so-called junction element. In ferromagnetic materials, 3d orbital local electron bands of ferromagnetic metals are cleaved due to quantum mechanical exchange, so that spin up and down electrons near the Fermi surface have different energy state densities. In MTJs, the mechanism of the TMR effect is the spin-dependent tunneling effect. The general structure of MTJs is a sandwich structure of ferromagnetic layer / non-magnetic insulation layer / ferromagnetic layer (FM / I / FM). In saturation magnetization, the magnetization directions of the two ferromagnetic layers are parallel to each other, and usually the coercive forces of the two ferromagnetic layers are different. Therefore, when the magnetization vector of the ferromagnetic layer with a small coercive force is reversed when the magnetization is reversed, the two ferromagnetic layers are first flipped, so that The magnetization direction becomes antiparallel. The tunneling probability of electrons from one magnetic layer to another is related to the magnetization directions of the two magnetic layers.

If the magnetization directions of the two layers are parallel to each other, in one magnetic layer, most of the electrons of the spin subband will enter the empty state of most of the spin subbands in the other magnetic layer, and the electrons of a few spin subbands will also enter the other. The empty state of a few spin sub-bands in the magnetic layer has a large total tunneling current. If the magnetization directions of the two magnetic layers are anti-parallel, the situation is exactly the opposite. The vacant states of a few spin subbands in another magnetic layer, and the electrons of a few spin subbands will enter the vacant states of most spin subbands in another magnetic layer. The tunneling current in this state is relatively small.

Therefore, the tunneling conductance changes with the change of the magnetization direction of the two ferromagnetic layers. The conductance when the magnetization vectors are parallel is higher than the conductance when anti-parallel.

Experimentally, the difficulties in preparing the tunnel junction are:

1. The oxide barrier is required to be uniform and dense, without defects such as pinholes, and thin enough (1.0 to 2.0 nm) to allow electrons to tunnel.

2. Avoid surface contamination and oxidation of the magnetic metal layer. The characteristics of the tunnel junction should reflect the intrinsic properties of the magnetic metal layer rather than the surface layer properties. These two problems can only be solved by strictly controlling the preparation conditions.

The most commonly used insulation barrier layer is

. Other materials such as MgO have also been reported, but no significant magnetoresistance effect was observed at room temperature. The insulating layer can be directly deposited on the magnetic metal layer, or a corresponding metal layer (such as Al) is deposited first, and then plasma in situ oxygen or natural oxidation in the air is performed. The resistivity of tunnel junctions made with different technologies varies widely.

In order to enable the magnetization of the two ferromagnetic layers to be arranged in parallel or antiparallel, the following methods can be used: (1) depositing a pinned layer; (2) depositing an antiferromagnetic coupling layer such as MnFe; (3) two ferromagnetic The layers are made of materials with different coercive forces, such as CoFe and NiFe; (4) The ferromagnetic layer has different coercive forces by controlling the preparation conditions.

To measure the transport characteristics of the tunnel junction, a current vertical film surface (CPP) method is required. Therefore, two special techniques are often used in the preparation of tunnel junctions, one is photolithography and the other is in-situ masking. The advantage of photolithography is that the number and size of the junctions are easier to control, but the preparation process is more complicated.

Since there is no or almost no interlayer coupling between the two ferromagnetic layers in MTJs, only a small external magnetic field is required to reverse the magnetization direction of one of the ferromagnetic layers, thereby achieving a huge change in tunneling resistance. MTJs have a much higher magnetic field sensitivity than metal multilayer films. At the same time, the structure of MTJs has high resistivity, low energy consumption, and stable performance. Therefore, MTJs are used as read heads, various sensors, or magnetic random sensors. Memory MRAM), has unparalleled advantages.

However, in the case of magnetic heads for computers, if the MTJs type TMR read head is superior to the current spin valve type GMR read head in terms of read rate and noise, the MTJs RA value should be lower than 4m. The RA value of the best PSV MTJs currently available is still 2 orders of magnitude higher than this value.

Research and development of TMR materials with high room temperature TMR value, good thermal stability, low RA value, and low cost will be the focus and key of future work in the field of magnetoresistive materials. The research and development of PSV type MTJs materials with low RA value is expected to become A breakthrough to achieve this goal ^[1] .