Magnetic memory device and method for reading magnetic memory cell using spin hall effect

A magnetic memory device includes a substrate for reading and a magnetic memory cell. The substrate has a channel layer. The magnetic memory cell is formed on the substrate and has a magnetized magnetic material that transfers spin data to electrons passing the channel layer. Data stored in the magnetic memory cell are read by a voltage across both side ends of the channel layer that is generated when the electrons passing the channel layer deviate in the widthwise direction of the channel layer by a spin Hall effect.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No. 2008-75690 filed on Aug. 1, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to reading data stored in a magnetic memory device, and more particularly, to a magnetic memory device and a method for reading a magnetic memory cell using a spin Hall effect, which makes it possible to easily read stored data of a magnetic memory cell by using a spin Hall effect.

2. Description of the Related Art

Many laboratories and enterprises are conducting researches to implement the next-generation memory devices. The next-generation memory devices must have a multi-functional capability and a nonvolatile function of retaining data even when power supply is interrupted. Among the next-generation memory devices, a Magnetic Random Access Memory (MRAM) device is attracting much attention as a new conceptual memory device having the above functions.

A switching operation of the MRAM device is performed using a magnetic field generated by a current or using a spin torque transfer generated by directly injecting a current into a memory cell. A data read operation of the MRAM device is performed by reading a resistance value that changes depending on the arrangement states of a free magnetic layer and a pinned magnetic layer separated by magnetic tunneling junction. This data read operation requires many thin layers for magnetic tunneling. Also, the problem of a damage to a thin layer generated in an etching process during a memory fabrication process and the problems of the high resistance and the stability of a tunneling barrier are remaining to be solved.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a magnetic memory device that makes it possible to easily read stored data of a magnetic memory device by using a spin Hall effect.

Another aspect of the present invention provides a method for reading a magnetic memory cell, which does not necessarily need a magnetic tunneling junction when reading data stored in a magnetic memory cell.

According to an aspect of the present invention, there is provided a magnetic memory device including: a substrate having a channel layer formed therein; and a magnetic memory cell formed on the substrate and having a magnetized magnetic material that transfers spin data to electrons passing the channel layer, wherein data stored in the magnetic memory cell are read by a voltage across both side ends of the channel layer that is generated when the electrons passing the channel layer deviate in the widthwise direction of the channel layer by a spin Hall effect.

The magnetization direction of the magnetic material of the magnetic memory cell may be perpendicular to the top surface of the channel layer. In this case, the electrons with the spin data of the magnetic material is injected from the magnetic material into the channel layer in a read operation of the magnetic memory cell. The deviating direction of the electrons in the widthwise direction of the channel layer changes depending on the spin direction of the injected electrons.

The magnetization direction of the magnetic material of the magnetic memory cell may be in-plane direction of the top surface of the channel layer and may be parallel to the lengthwise direction of the channel layer. In this case, the spin direction of the electrons passing the channel layer is aligned by a stray magnetic field of the magnetic material perpendicular to the top surface of the channel layer in a read operation of the magnetic memory cell. The deviating direction of the electrons in the widthwise direction of the channel layer changes depending on the aligned electron spin direction.

In the read operation of the magnetic memory cell, the polarity of the voltage across the both side ends of the channel layer may change as the deviating direction of the electrons in the widthwise direction of the channel layer changes.

The magnetic material of the magnetic memory cell may be formed of one selected from the group consisting of CoFe, Co, Ni, NiFe, and a combination thereof. Also, the magnetic material of the magnetic memory cell may be formed of one selected from the group consisting of (Ga, Mn)As, (In, Mn)As, and a combination thereof. The magnetic memory cell may have a multilayer structure of ferromagnetic material/tunnel barrier/ferromagnetic material. In another embodiment, the magnetic memory cell may have a single pattern structure of ferromagnetic material.

The channel layer may be a two-dimensional electron gas layer. The two-dimensional electron gas layer may be formed of one selected from the group consisting of GaAs, InAs, InGaAs, InSb, and a combination thereof.

The substrate may include a lower cladding layer and an upper cladding layer for sandwiching the channel layer forming the two-dimensional electron gas layer; the lower cladding layer includes a first lower cladding layer and a second lower cladding layer formed under the first lower cladding layer, the second lower cladding layer having a larger energy band gap than the first lower cladding layer; and the upper cladding layer includes a first upper cladding layer and a second upper cladding layer formed on the first upper cladding layer, the second upper cladding layer having a larger energy band gap than the first upper cladding layer.

The channel layer may be formed of InAs, the first lower cladding layer and the first upper cladding layer may be formed of undoped InGaAs, and the second lower cladding layer and the second upper cladding layer may be formed of undoped InAlAs.

The substrate may have a ridge structure obtained by removing both sides along the lengthwise direction of the channel layer, the channel width may be limited by the ridge structure, and an insulating layer for planarization may be formed at the removed both sides of the ridge structure. Specifically, the ridge structure may have a cross-shaped top portion in such a way that a lengthwise elongated portion and a widthwise elongated portion of the channel layer cross each other, and a voltage across both ends of the widthwise elongated portion may be read as stored data.

According to another aspect of the present invention, there is provided a method for reading data stored in a magnetic memory cell, including: disposing a magnetic memory cell with a magnetized magnetic material on a substrate with a channel layer in order to transfer spin data to electrons passing the channel layer; and reading a voltage across both side ends of the channel layer using a spin Hall effect by the electrons passing the channel layer in a read operation of the magnetic memory cell to read the data stored in the magnetic memory cell.

The magnetization direction of the magnetic material of the magnetic memory cell may be one of a spin-down direction and a spin-up direction perpendicular to the top surface of the channel layer, the electrons with the spin data of the magnetic material may be injected from the magnetic material into the channel layer in the read operation of the magnetic memory cell, and the deviating direction of the electrons injected into the channel layer may change depending on the spin direction of the electrons to read the voltage.

The magnetization direction of the magnetic material of the magnetic memory cell may be in-plane direction of the top surface of the channel layer and may be one of two opposite directions parallel to the lengthwise direction of the channel layer, and a stray magnetic field of the magnetic material perpendicular to the top surface of the channel layer may be used to change the deviating direction of the electrons passing the channel layer according to the spin direction of the electrons, to read the voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to embodiments of the present invention, a data read principle of a magnetic memory cell using a spin Hall effect is as follows. The spin Hall effect is a phenomenon that the deviating direction of an electric charge changes depending on the spin direction of an electron by spin-orbit coupling without an external magnetic field. A magnetic memory device according to an embodiment of the present invention may include a semiconductor substrate having a channel layer, a ferromagnetic material, and a metal layer. A two-dimensional electron gas structure in the semiconductor substrate may be implemented in various ways. For example, an InAs channel may be used for the two-dimensional electron gas structure.

FIGS. 1A and 1Billustrate a case where the magnetization direction of a magnetic substance is the z-axis direction. A write operation of a magnetic memory cell uses a spin transfer torque. The magnetization direction in a pinned layer101is fixed, and the magnetization state of free layers102aand102bis switched according to the direction of a current (one of the +z direction and the −z direction). A tunneling barrier layer103is sandwiched between the pinned layer and the free layer. The pinned layer101and the free layers102A and102B may be formed of a ferromagnetic material, and the tunneling barrier layer103may be formed of an oxide material.

First, when a memory cell is turned on by a transistor for a read operation of the memory cell and thus electrons110aand110bwith the spin information of a lower magnetic layer are injected into a channel, the electrons110aand110bare bent by spin-orbit coupling in the +y direction or the −y direction while passing a channel107according to the spin direction. For example, if the spin direction of the free layer102ais the +z direction as illustrated inFIG. 1A, the electron110ais bent in the −y direction104a. If the spin direction of the free layer102bis the −z direction as illustrated inFIG. 1B, the electron110bis bent in the +y direction104b. Accordingly, an asymmetric charge distribution is formed in the y-axis direction and a predetermined voltage (i.e., a spin Hall voltage) is across both ends L and R of channel width in a steady state.

As described above, in implementation of the magnetic memory device, the storage state of the magnetic memory cell can be easily read using the spin Hall effect. In the magnetic memory cell read operation, the stored data can be read by a positive or negative voltage and thus a reference voltage is unnecessary. Using this phenomenon, the magnetization direction of the magnetic memory cell can be electrically read without a complex magnetic tunneling junction, when considering only a data read operation. This phenomenon can be used when the electron passing the channel107spins in the direction perpendicular to a plane (i.e., the top surface of the channel). This vertical spin of the electron may be implemented using the vertical magnetization of the magnetic material as in the above embodiments (FIGS. 1A and 1B), or may be implemented using a vertical magnetic field leaking in horizontal magnetization. The vertical magnetization of the magnetic material may be implemented using a vertical magnetization material, or may be implemented using a multilayer structure formed by alternate lamination of ferromagnetic layers and nonmagnetic layers.

In the result, the deviating path of the electrons changes according to the arrangement direction of the spin and this can be easily measured using a voltage. The spin Hall effect is a phenomenon that occurs in a metal channel as well as in a semiconductor channel with a good electron mobility. The spin Hall effect can diversify materials and does not necessary require a magnetic tunneling junction in a data read operation, thus making it possible to simplify the fabrication process.

When measuring a voltage (i.e., a spin Hall voltage VM) across both ends in width direction of the channel107in the structures ofFIGS. 1A and 1B, the stored data can be read by a positive voltage value in the case ofFIG. 1Aand the stored data can be read by a negative voltage value in the case ofFIG. 1B. Herein, although the data storage or writing operation can use various magnetic memory storage methods,FIGS. 1A and 1Billustrates the case where the magnetization direction is controlled by the magnetization direction of a current by using a spin transfer torque. The illustration of a connection state of transistors and a metal line (bit line) on the top of the magnetic memory device is omitted for convenience in description.

The data read method using a spin Hall illustrated inFIGS. 1A and 1Bmay be similarly used when the magnetization direction of the free layers102aand102bis parallel to the x-axis direction. As illustrated inFIG. 2A, when the magnetization direction of the free layer102ais the +x direction, a stray magnetic field105agenerated in the magnetic material is generated in the +z direction in the channel107(the spin is aligned according the stray magnetic field), an electron110adeviates in the −y direction104a, and the spin Hall voltage VMhas a positive voltage value. On the contrary, as illustrated inFIG. 2B, when the magnetization direction of the free layer102bis the −x direction, a stray magnetic field105bis generated in the −z direction in the channel107, an electron110bdeviates in the +y direction104b, and a voltmeter can read a negative voltage across both ends L and R of channel width direction. At this point, in order to prevent the spin of the +x direction from being directly injected into the channel107, the current is applied only to the channel107without passing the junction between the magnetic free layer102aand the channel. In this case, the magnetization direction of the free layer102bmay be switched using a metal line130. It is preferable that the ferromagnetic free layer102aelongates sufficiently so that the spin of the electron passing the channel layer can be accurately aligned by the stray magnetic field.

FIG. 3Aillustrates a semiconductor substrate for a read operation for forming a spin channel used in a magnetic memory device according to an embodiment of the present invention. Referring toFIG. 3A, a semiconductor substrate120includes a semi-insulated InP substrate201, an InAlAs buffer layer202, an n31doped InAlAs charge supply layer204, an undoped InGaAs/InAlAs lower clad layer205, an InAs channel layer207, an undoped InAlAs/InGaAs upper clad layer205′, and an InAs capping layer206that are sequentially laminated on the semi-insulated InP substrate201. Herein, the InAs channel layer207corresponds to the channel107ofFIGS. 1A,1B,2A and2B.

Each of the lower and upper cladding layers205and205′ is formed of a dual cladding structure including an undoped InGaAs layer and an InAlAs layer. That is, the lower cladding layer205includes a first lower cladding layer205aformed of InGaAs and a second lower cladding layer205bformed of InAlAs under the first lower cladding layer205a. Also, the upper cladding layer205′ includes a first upper cladding layer205a′ formed of InGaAs and a second upper cladding layer205b′. The second lower cladding layer205bhas a larger energy band gap than the first lower cladding layer205a, and the second upper cladding layer205b′ has a larger energy band gap than the first upper cladding layer205a′.

The channel layer207forms a quantum well by an energy barrier of the lower and upper cladding layers205and205′. Specifically, electrons are confined by the lower and upper cladding layers205and205′ of a dual cladding structure, and channel layer207forms a two-dimensional electron gas (2-DEG) layer. In the two-dimensional electron gas layer, the electron mobility is very high and also the spin-orbit coupling effect is very high. In the present embodiment, the channel layer207is formed of InAs, to which the present invention is not limited. For example, the channel layer207may be of GaAs, InGaAs or InSb.

The n−doped InAlAs charge supply layer204is formed under the channel layer207to supply electric charges to the channel layer207, and the InAlAs buffer layer204reduces a lattice mismatch between the InP substrate201and the lower cladding layer205. Also, the InAs capping layer206formed at the top of the substrate120prevents the oxidation and degradation of the semiconductor substrate120that may occur during the fabrication process.

FIG. 3Billustrates another example of a substrate for forming a spin channel used in a magnetic memory device according to an embodiment of the present invention. Referring toFIG. 3B, a semiconductor substrate121includes a semi-insulated GaAs substrate211, a GaAs buffer layer212, and a channel layer213that are formed on the semi-insulated GaAs substrate211. The channel layer213maybe used as the channel107ofFIGS. 1A,1B,2A and2B. Upper layers214and215on the channel layer213will be used to form a Schottky barrier and a ferromagnetic material of a memory cell. The channel layer213may be an undoped GaAs layer or an Al0.3Ga0.7As layer.

FIG. 3cillustrates a further another example of a substrate for forming a spin channel used in a magnetic memory device according to an embodiment of the present invention. Referring toFIG. 3C, a substrate122includes a single-layer spin channel223. The spin channel223may be used as the channel107ofFIGS. 1A,1B,2A and2B. The spin channel223may be formed using any metal, semiconductor or semi-metal that provides a spin Hall effect. For example, the metal channel may be formed using Au, Pt, Ag, Al or Cu. For example, the semi-metal channel may be formed using Sb. For example, the semiconductor channel may be formed using GaAs, InAs, InGaAs or InSb. As illustrated inFIG. 3C, an insulating layer222may be disposed between the channel223and a silicon layer221. The insulating layer222may be formed of an insulating material such as AlOx, MgO, or SiO2. The insulating layer22may be omitted.

FIGS. 4A to 4Eare cross-sectional views illustrating a method for fabricating a magnetic memory device with a read unit according to an embodiment of the present invention. The illustration ofFIGS. 4A to 4Efocuses on a method for fabricating a read unit for reading stored data, and a process for fabricating a storage unit or a magnetic memory cell may vary depending on the storage method. Thus, the read unit fabricated for the present embodiment can be used for any magnetic memory cell, andFIGS. 4A to 4Eillustrates a device with a two-dimensional electron gas layer structure as an example.

Referring toFIG. 4A, a multi-layer semiconductor substrate120as illustrated inFIG. 3Ais formed, and a lithograph process and an ion-milling process are used to removed both sides of the semiconductor substrate, thereby forming a ridge structure at the substrate. The ridge structure constrains a two-dimensional electron gas channel layer107. The channel layer107may be formed to a width of about 40 nm to about 100 nm. For easy measurement of a voltage (i.e., a spin Hall voltage) across both ends of channel width, the ridge structure may have a cross-shaped top portion in such a way that a lengthwise elongated portion and a widthwise elongated portion of the channel layer107cross each other (seeFIGS. 1A,1B,2A,2B and4A). A spin Hall voltage can be measured across both ends of the widthwise elongated portion of the channel layer107.

Referring toFIG. 4B, an insulating layer140is formed on both sides (removed portions) of the ridge structure in the resulting structure ofFIG. 4Afor planarization. For example, the insulating layer140may be formed of an oxide material such as TaOxor SiO2. The insulating layer140may provide the insulation from a neighboring (corresponding to the channel) channel.

Referring toFIG. 4C, an electron beam lithograph process and a sputtering process are used to deposit a memory cell (i.e., a storage unit of the magnetic memory device)150on the channel layer107. As illustrated inFIGS. 1A,1B,2A and2B, the memory cell150may be formed of a single pattern of a ferromagnetic material of a multi-layer structure of ferromagnetic material/tunnel barrier layer/ferromagnetic material. A variety of other layers may be formed to improve the performance of the memory device. Because the present invention focuses on a read unit for reading data stored in a magnetic memory cell150and a read method thereof, a method for fabricating the magnetic memory cell150is not described in detail but the present invention can be applied to read all magnetic memory cells including pre-existing magnetic memory cells. The ferromagnetic portions used in the magnetic memory cell150(e.g., the pinned layer101and the free layers102aand102bofFIGS. 1A,1B,2A and2B) may be formed of a ferromagnetic metal material selected from the group consisting of Fe, Co, Ni, CoFe, NiFe, and a combination thereof. Alternatively, the ferromagnetic portions used in the magnetic memory cell150may be formed using a magnetic semiconductor material such as (Ga, Mn)As or (In, Mn)As. The tunnel barrier layer (e.g., a reference numeral103ofFIGS. 1A and 1B) may be formed using Al2O3or MgO.

Referring toFIG. 4D, after the formation of the memory cell150, an outside of the memory cell150(i.e., a portion without a memory cell) is filled with an insulating layer140′ to formed a planarized structure. The insulating layer140′ may be formed using an oxide material such as TaOxor SiO2.

Referring toFIG. 4E, a metal line160serving as a bit line is formed on the resulting structure ofFIG. 4D. The metal line160is used as a simple bit line for spin transfer switching, but is also used as a current line for switching the memory cell150by generating a magnetic field in the magnetic memory device that performs a switching operation by a magnetic field generated from a current.

FIG. 5is a graph showing the relationship between a current and a voltage (i.e., a spin Hall voltage) depending on the magnetization direction of a magnetic memory cell, as a result of an experimental on a memory device structure as illustrated inFIGS. 1A and 1B. As shown in the graph ofFIG. 5, the slopes of the current-voltage graphs are opposite between the case where the spin direction of the free layer is the +z direction (i.e., spin-down) and the case where the spin direction of the free layer is the −z direction (i.e., spin-up). The reason for this is that the deviating direction of the electron varies depending on the magnetization direction of the magnetic memory cell as described above (seeFIGS. 1A and 1B).

FIG. 6shows the results ofFIG. 5in terms of a Hall resistance. As shown inFIG. 6, a positive or negative Hall resistance is measured throughout the total current region, depending on the magnetization direction of the magnetic memory cell, regardless of the current intensity, which can be easily used to accurately read the state of the memory.

As described above, the present invention makes it possible to easily read data of a magnetic memory cell in a relatively simple structure by using a spin Hall effect, thus providing a good cost competitiveness. Also, the present invention makes it possible to remove a tunneling layer (i.e., a magnetic tunneling junction) necessary for a read operation in a memory device not using a spin torque. Accordingly, the present invention can reduce the power consumption necessary for a switching operation by reducing the resistance value of the device, and also can conveniently implement a high-density memory device by reducing the size of a transistor for driving the memory device. Also, the present invention makes it possible to read stored data by positive and negative voltage values without the need of a reference resistance even in a magnetic memory device using a spin torque. When the output are generated in the form of positive and negative values, it can be used as an input in the next stage, thus making it possible to provide easy combination of logic circuits.