CN108206204B - Spin filtering heterojunction device based on cobalt-molecular multiferroic material and preparation thereof - Google Patents

Abstract

The invention relates to a spin filtering heterojunction device based on a cobalt-molecular multiferroic material and a preparation method thereof, wherein the spin filtering heterojunction device comprises two cobalt electrodes which are respectively used as a source electrode and a drain electrode, and a molecular multiferroic material (NH) which forms a middle scattering region material thin layer between the two cobalt electrodes₄)₃Cr₂O₈And a linker molecule multiferroic material (NH)₄)₃Cr₂O₈And the two cobalt electrodes are thin layers formed by metal material cobalt. Compared with the prior art, the invention has the advantages that the molecular multiferroic material Cs (NH) is adopted₄)CrO₈Magnetic center chromium ion (Cr)⁵⁺) The 3d orbit is in spin interaction with the surface of the Co electrode to form a unique spin filtering effect, and in addition, the spin filtering effect has good and stable performance, is independent of the type of a semiconductor substrate of the device, is easy to realize, and can be generally applied to molecular semiconductor electronic devices.

Description

Spin-filtered heterojunction devices based on cobalt-molecular multiferroics and their fabrication

technical field

The invention relates to the technical field of spintronic devices, in particular to a spin filtering heterojunction device based on a cobalt-molecular multiferroic material system and its preparation.

Background technique

Molecular electronics studies electronics at the molecular level, and its goal is to replace solid electronic components such as silicon-based semiconductor transistors with single molecules, supramolecules or molecular clusters to assemble logic circuits, or even complete molecular computers. Its research content includes the synthesis of various molecular electronic devices, performance testing and how to assemble them together to achieve certain logical functions. Compared with traditional solid-state electronics, molecular electronics has strong advantages. The current microelectronics processing technology will be close to the limit of development in 10 years, and the continuous reduction of line width will make solid electronic devices no longer follow the traditional operating laws; at the same time, the reduction of line width will also increase the processing cost. Molecular electronics is expected to solve these problems. In a Pentium computer chip, 107 to 108 electronic components can be integrated in an area of 1 cm ² , while molecular electronics allows 1014 single-molecule electronic components to be integrated in the same size area, and the integration degree is improved. It will greatly improve the operation speed. At the same time, since molecular electronics adopts a bottom-up approach to assemble logic circuits, the components used are synthesized in large quantities through chemical reactions, so the production cost will be greatly reduced compared with the traditional photolithography method.

The combination of molecular electronics and spintronics produces a new discipline that will combine the advantages of molecular electronics and spintronics with greater advantages. This new interdisciplinary subject is called molecular spintronics (molecular spintronics). From a fundamental and technical point of view, the ability to manipulate the electron spin of organic molecular materials provides a new and convenient route for the study of spintronics. This is because organic molecules have the advantage of very weak spin-orbit coupling and hyperfine interactions, which can make spin coherence times and distances much longer than in conventional metals and semiconductors . The energy required to change the spin state of an electron is only one-thousandth of the energy required to propel the electron into motion. Obviously, the energy consumption of spintronic devices is negligible compared with that of traditional silicon-based semiconductor electronics. At present, in order to seize the commanding heights of future science and technology, many developed countries have formulated special plans for the development of nanoelectronics and molecular electronics, invested huge manpower and material resources, and achieved a series of breakthroughs. On December 21, 2001, the American "Science" magazine ranked the series of achievements in molecular electronics as the first of the top ten scientific and technological progress in 2001.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a spin-filtered heterojunction device based on cobalt-molecular multiferroic material and its preparation in order to overcome the above-mentioned defects of the prior art, which can realize the realization of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is connected to the cobalt electrode, due to the spin interaction between the 3d orbitals of the magnetic center chromium ion (Cr ⁵⁺ ) of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ , the surface of the oxygen atom of Cr 2 O 8 interacts with the 3d orbital. The coupling between the surface states of the cobalt electrode (Co) leads to the spin filtering effect of the molecular multiferroic heterojunction, and the molecular multiferroic tunnel junction is expected to be used for the preparation of high-performance molecular spintronic crystal devices.

The object of the present invention can be realized through the following technical solutions:

A spin-filtered heterojunction device based on cobalt-molecular multiferroic materials, comprising two cobalt electrodes as source and drain electrodes respectively, and molecular multiferroic molecular multiferroics constituting a thin layer of material in the intermediate scattering region between the two cobalt electrodes material (NH ₄ ) ₃ Cr ₂ O ₈ , and a gate connecting two sides of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ , the two cobalt electrodes are thin layers formed by the metal material cobalt.

As a preference of the above solution, the two cobalt electrodes and the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ are arranged in the order of “cobalt electrode-molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ - Cobalt electrodes” are periodically arranged and constitute a three-dimensional periodic structure of spin-filtered heterojunction devices, wherein the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is connected with the two cobalt electrodes. vertical and in contact with the source and drain.

More preferably, the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ has a thickness along the z-axis direction of the order of nanometers.

As a preferred option of the above solution, the spin-filtering effect of the spin-filtering heterojunction device utilizes the magnetoelectric coupling between the magnetic center chromium ions of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ The interaction between the oxygen surface of the molecular multiferroic material and the surface state of the cobalt electrode forms a unique spin filtering effect.

The preparation method of spin-filtered heterojunction device based on cobalt-molecular multiferroic material includes the following steps:

(1) Utilize the cobalt surface as the source and drain of the spin-filtered heterojunction device;

(2) contacting the oxygen surface of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ , which constitutes the intermediate scattering region between the source electrode and the drain electrode, with the two cobalt electrodes to form an oxide layer;

(3) Press two cobalt electrodes and molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ to form a "sandwich structure""cobalt electrode-molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ - cobalt electrode" ” are arranged periodically to form a spin-polarized electrical transport system of a two-electrode system;

(4) Leave holes for making gates on both sides of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ of the spin-polarized electrical transport system, and make gates in the holes to obtain the cobalt-based - Spin-filtered heterojunction devices of molecular multiferroic materials.

As a preference of the above solution, the thickness formed by the two cobalt electrodes and the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ in the middle scattering region is in the order of nanometers.

The invention uses magnetic material cobalt atoms as left and right electrodes, and uses molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ as the tunneling region in the middle. The electrodes and the tunneling region are arranged in sequence along the z-axis direction to form a three-dimensional structure. In this two-electrode spin-polarized electrical transport system, the oxygen surface of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is in contact with the left and right cobalt electrodes. Due to the spin interaction between the 3d orbitals of the magnetic center chromium ion (Cr ⁵⁺ ) of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ , the oxygen atoms of the molecular multiferroic material and the electrode surface composed of cobalt atoms are in contact with each other. An interesting spin-filtering effect is formed. Among them, the length of the intermediate scattering region is kept in the nanometer scale. The left and right cobalt electrodes and the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ satisfy a three-dimensional periodic structure in the x-axis and y-axis directions.

Compared with the prior art, the present invention provides a spin filtering heterojunction device based on a cobalt-molecular multiferroic material, the working condition is at room temperature, and the preparation is easy. Since the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ contains paramagnetic center chromium ions (Cr ^{5 +} ), there is a magnetic exchange between the adjacent Cr ⁵⁺ molecules, through the oxygen surface of the molecular multiferroic material. Combined with cobalt electrodes, a two-electrode sandwich structure spin-polarized transport system can be formed. The length of the electrotransport system is in the nanometer range. Since the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is a three-dimensional periodic structure, it is located between the magnetic metal cobalt electrodes, and the molecular semiconductor heterojunction formed by the two is a non-equilibrium non-periodic system. The two-electrode spin-polarized electrical transport system can be solved by using the non-equilibrium Green's function method combined with the idea of density functional theory, and then the electrical transport problem of molecular multiferroic devices can be studied. Due to the unique magnetoelectric coupling effect and spin interaction of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ , a molecular electronic device with spin filtering effect was formed when in contact with the surface of the cobalt electrode.

Description of drawings

1 is a schematic structural diagram of a spin-filtered heterojunction device of the present invention;

2 is a current-voltage relationship diagram of the spin-filtered heterojunction device of the present invention when the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is in a ferromagnetic state;

3 is a current-voltage relationship diagram of the spin-filtered heterojunction device of the present invention when the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is in a ferrimagnetic state.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

Example 1

As shown in Fig. 1, a spin-filtered heterojunction device based on cobalt-molecular multiferroic material includes left and right electrodes composed of magnetic metal cobalt as source and drain, molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ serves as the intermediate tunneling region. The left and right electrode materials are both magnetic metal cobalt, and the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ with a length of nanometer scale and the double electrodes are arranged in sequence to form a sandwich structure. The two sides of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ are connected to the gate.

In the above-described spin-polarized transport heterojunction transistor, the source and drain electrodes are constituted by ferromagnets that play the roles of spin injection means and spin induction means, respectively. The electron spins injected into the drain are in the same direction as the transport. The electrons are transported ballistically through the middle channel, and when the electron reaches the drain, its spin can be detected. When the electron spins passing through the middle channel are parallel to the magnetization direction of the drain, the system presents an open (ON) state, and when it is antiparallel to the drain, the system presents a closed (OFF) state. The role of the gate (ie gate) is to generate an effective magnetic field, which is generated by the spin-orbit coupling of the substrate material, the structural confinement of the transport channel, or the static potential of the gate. This effective magnetic field can cause the electron spins to precess. By adjusting the bias voltage, the precession can be influenced to cause the electron spins through the intermediate channel to be parallel or anti-parallel to the drain, thereby effectively controlling the current.

When the magnetization directions of the two ferromagnetic layers are the same, the conductance is larger than the conductance when the magnetization directions of the two ferromagnetic layers are opposite. This is tunneling magnetoresistance (TMR), which can be defined as the following formula

产生，且使用了推迟格林函数。其中，非平衡情形的电子态从电极的深层能量开始并且被充满到左右电极的电化学势(μ_L，μ_R)，从左右电极的态构建密度矩阵。基于密度泛函理论方法来计算扩展分子(分子以及连接其两端的部分金属电极)的电子结构，在密度泛函水平上得到其表面格林函数。基于电子密度自洽迭代方法计算分子器件的电势分布以及电压降(voltage drop)，再利用非平衡态格林函数方法结合密度泛函理论方法求解其电流-电压曲线和输运谱等自旋极化电子输运性质。对于钴电极和分子多铁材料(NH₄)₃Cr₂O₈部分被充分弛豫，直到原子间的作用力小于

达到收敛状态。电流由如下公式得到The conductance and resistance are marked with the relative magnetization directions of the two ferromagnetic layers (applying a very small magnetic field of about 10 Gauss to the system will cause the relative magnetization directions of the two ferromagnetic layers to convert between ↑↓ and ↑↑) . Structure of spin-filtered devices based on molecular multiferroic heterojunctions with spin-polarized current flowing along the z-direction. The density matrix describing the electron distribution can be obtained from a series of Green's function matrices. The non-equilibrium state density matrix is the core part of the calculation of transport properties, and the relationship between the non-equilibrium state Green's function expression and the density matrix is established first from the scattering state. The scattered-state wavefunction _ψl starts from the left electrode and consists of the unperturbed incident state (labeled l) of the uncoupled semi-infinite electrode

is generated, and a delayed Green's function is used. Among them, the electronic states of the non-equilibrium case start from the deep energy of the electrodes and are charged to the electrochemical potentials ( _μL , _μR ) of the left and right electrodes, building a density matrix from the states of the left and right electrodes. Based on the density functional theory method, the electronic structure of the extended molecule (the molecule and part of the metal electrodes connecting its two ends) is calculated, and its surface Green's function is obtained at the density functional level. The potential distribution and voltage drop of molecular devices are calculated based on the electron density self-consistent iterative method, and the spin polarization such as the current-voltage curve and the transport spectrum are solved by the non-equilibrium Green's function method combined with the density functional theory method. Electron transport properties. For cobalt electrodes and molecular multiferroics (NH ₄ ) ₃ Cr ₂ O ₈ is partially relaxed until the interatomic forces are less than

reach a state of convergence. The current is obtained by the following formula

Where f _{L, R} (E-μ _L )=1/(1+e ^(E- μ _{L, R} )/κ _B T) represents the Fermi-Dirac (Fermi-Dirac) distribution, and μ _{L, R} represents the left and right chemical potential of the electrode. T(E, _Vb ) is the transmission coefficient at energy E and bias voltage _Vb .

FIG. 2 is a current-voltage relationship diagram of the spin filter device when the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is in a ferromagnetic state according to the present invention. The figure shows that the voltage varies in the range of -2 to 2V. First, in the forward bias voltage range of 0 to 1V, as the voltage increases, the spin-polarized upward current increases very slowly, and the value is close to 0, while the spin-polarized current increases very slowly. The downward current value is relatively large; in the range of 1-2V, the spin-polarized upward current increases at the beginning, and the spin-down current value is relatively large, with a certain oscillation. In the bias voltage range of 0~-2V, the value of spin-up current is close to 0, while the value of spin-down current is larger.

FIG. 3 is a current-voltage relationship diagram when the spin filter device of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ according to the present invention is in a ferrimagnetic state. The voltage variation range shown in Figure 3 is also -2 to 2V. First of all, for the spin-up current: in the forward bias voltage range of 0-2V and 0--2V, with the increase of the absolute value of the voltage, the current increases extremely slowly, and the value is close to 0; for the spin-up direction For the lower electrons: in the range of 0 ~ 1V, the value increases steadily, in the range of 1 ~ 2V, there is a certain oscillation in the value, the voltage range is 1.25 ~ 1.5V, the current decreases with the increase of the voltage, there is an interesting In the range of 0~-1V, the absolute value of the spin-up current continues to increase, and in the range of -1~-2V bias voltage, the value of the current has a certain oscillation.

Combining Fig. 2 and Fig. 3, it can be seen that no matter in the ferromagnetic state or the ferrimagnetic state, in the spin filter device based on the cobalt-molecular multiferroic (NH ₄ ) ₃ Cr ₂ O ₈ heterojunction, the spin The upward current and the spin-down current produce obvious splitting, and in the bias voltage range of 0-1V and 0--1V, the spin filtering effect with good performance is produced. The spin filtering effect of the molecular multiferroic material The device can be widely used in molecular spintronics devices.

The foregoing description of the embodiments is provided to facilitate understanding and use of the invention by those of ordinary skill in the art. It will be apparent to those skilled in the art that various modifications to these embodiments can be readily made, and the generic principles described herein can be applied to other embodiments without inventive step. Therefore, the present invention is not limited to the above-mentioned embodiments, and improvements and modifications made by those skilled in the art according to the disclosure of the present invention without departing from the scope of the present invention should all fall within the protection scope of the present invention.

Claims (6)

1. a spin-filtered heterojunction device based on cobalt-molecular multiferroic material, is characterized in that, comprises two cobalt electrodes as source and drain respectively, constitute the intermediate scattering region material between two cobalt electrodes A thin layer of molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ , and a gate connecting both sides of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ , the two cobalt electrodes are formed of metallic material cobalt thin layer. 2 . The spin-filtered heterojunction device based on cobalt-molecular multiferroic material according to claim 1 , wherein the two cobalt electrodes and the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is periodically arranged along the z-axis in the manner of "cobalt electrode-molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ -cobalt electrode", and constitutes a three-dimensional periodic structure of spin-filtered heterojunction devices, in which , the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is perpendicular to the connecting line between the two cobalt electrodes, and is in contact with the source and drain electrodes. 3 . The spin-filtered heterojunction device based on cobalt-molecular multiferroic material according to claim 1 , wherein the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ is along the z-axis. 4 . The thickness of the directions is on the order of nanometers. 4. a kind of spin filtering heterojunction device based on cobalt-molecular multiferroic material according to claim 1, is characterized in that, the spin filtering effect of described spin filtering heterojunction device The magnetoelectric coupling interaction between the magnetic center chromium ions of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ makes the hybridization between the oxygen surface of the molecular multiferroic material and the surface state of the cobalt electrode to form a unique self- Spin filter effect. 5. The preparation method of a cobalt-molecular multiferroic material-based spin-filtered heterojunction device according to any one of claims 1 to 4, wherein the method comprises the following steps: (1) Utilize the cobalt surface as the source and drain of the spin-filtered heterojunction device; (2) contacting the oxygen surface of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ , which constitutes the intermediate scattering region between the source electrode and the drain electrode, with the two cobalt electrodes to form an oxide layer; (3) Press two cobalt electrodes and molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ to form a "sandwich structure""cobalt electrode-molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ - cobalt electrode" ” are arranged periodically to form a spin-polarized electrical transport system of a two-electrode system; (4) Leave holes for making gates on both sides of the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ of the spin-polarized electrical transport system, and make gates in the holes to obtain the cobalt-based - Spin-filtered heterojunction devices of molecular multiferroic materials. 6. The method for preparing a spin-filtered heterojunction device based on cobalt-molecular multiferroic material according to claim 5, wherein the two cobalt electrodes and the molecular multiferroic material (NH ₄ ) ₃ Cr ₂ O ₈ forms the thickness in nanometer scale.

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