DE102016004816A1 - Magnetoelectric memory for storing digital data - Google Patents

Abstract

The invention relates to a magnetoelectric storage device for storing digital data, comprising a plurality of memory cells (1), wherein each cell (1) comprises a with respect to a magnetic permeability anisotropic ferroelectric material (2), the ferroelectric material (2) in a circuit located or can be electrically loaded, z. B. between two electrodes (3a, 3b) is arranged, and between permanent magnets or electromagnets, the ferroelectric material (2) is arranged such that it is at least temporarily exposed to a magnetic field, the storage device at least one measuring device for measuring magnetic quantities , z. B. a permeability measuring device for measuring the magnetic permeability of the ferroelectric material (2), an electrical load circuit for the ferroelectric material (2) is present, wherein the electrical load circuit is such that upon electrical loading of the ferroelectric material (2) as a result the magnetic load effected Umklappvorgang the magnetic permeability takes place, so that a writing operation of the memory can take place by changing the magnetic permeability, and wherein a reading operation of the memory by measuring the magnetic magnitudes of the ferroelectric material (2), z. B. by measuring the magnetic permeability of the ferroelectric material (2) by the permeability measuring device, can be done.

Description

The invention relates to a magnetoelectric storage device for storing digital data.

So-called FeRAMs (Ferroelectric Random Access Memory) are known.

These use a nonvolatile electronic storage medium based on materials or crystals with ferroelectric properties, that is analogous properties to ferromagnetism. In this case, a folding over of a spontaneous polarization of the ferroelectric unit cell is used in order to store and read the binary data.

Frequently used as ferroelectric barium titanate (BaTiO ₃ ). The positively charged titanium ions align to one side of the crystal, while the negatively charged oxygen ions align to another side, creating a permanent polarization of the crystal.

These have the advantage that they do not require a composite material and have a high storage density. Compared to EEPROMs, data are retained longer, usually over ten years. The write time is higher than with EEPROMs, eg. 100 ns. While EEPROMs create about 10 ⁶ write and read cycles, with FeRAMs z. B. 10 ¹⁰ to 10 ¹⁵ cycles can be realized.

For storing and reading voltage pulses are used. Depending on whether the direction of polarization reverses or does not reverse, a different displacement current occurs. Since this read destroys the existing polarization, it is necessary to rewrite the memory cell with the original content.

However, this aspect is disadvantageous. The read operation or renewed overwriting of the memory cell causes damage, which leads to a significant reduction in the life of the storage medium. In addition, an unwanted depolarization of the unit cell occur, so that no correct detection of the current state of the memory cell after a certain period of operation is possible.

Further memories are so-called MeRAMs (Magnetoelectric Random Access Memory). Here, a nonvolatile electronic storage medium based on composites with ferroelectric and ferromagnetic properties is used. In this technology, a flip of a spontaneous polarization of the ferroelectric unit cell and a flip-over of the spontaneous magnetization of the ferromagnetic phase are used to store and read the binary data. It is written electrically and read magnetically.

MeRAM technology is still under development.

Compared to EEPROMs and partly also to FeRAMs, SRAMs, DRAMs and STT-RAMs, these memories (MeRAMs) have the advantages of low power consumption, a high write and read speed of sometimes less than 1 ns, non-volatile data protection, a high number of write and read accesses Read cycles and high data security. In addition, no standby power is required. Also, a refresh (refresher) is not required.

The biggest difficulty with MeRAMs is that composite materials have to be used. Chip design and chip production in very small dimensions are therefore difficult. Thus, the achievable storage density is lower than with FeRAMs. In addition, additional mechanical damage problems (eg delamination) can occur in the composites, shortening the life of the storage medium.

The US 3 909 809 A describes a combination of magnetostrictive and piezoelectric effect. Here, a layer of barium titanate (BaTiO ₃ ) is used.

From the EP 1318523 A1 a hybrid MeRAM with ferromagnetic layers is known, which comprises a piezoelectric layer and a magnetoresistance layer (or the tunneling electron effect (Tunneling Electrons) and a GMR effect (Giant Magnetoresistance) uses.

The EP 1901317 A1 describes a magnetic device and an assembly with a piezoelectric element, a magnetic element and two electrodes.

The WO 2009/090605 A also describes a memory cell for a RAM having two ferromagnetic layers. Here, the electrical resistance along a magnetic element is measured.

The WO 2010/039422 A describes a spin-torque-transfer MeRAM (STT-RAM).

The US 2010 003 273 8 A1 describes an arrangement with a magnetic tunnel and with two ferromagnetic layers.

The WO 2010/039422 A describes another non-volatile magnetoelectric memory (STT-RAM) using an anisotropic magnetic effect in a ferromagnetic material, taking advantage of the TMR magnetic effect.

The US 2012/0267735 A1 describes a magnetostrictive piezoelectric nanomagnet.

The WO 2013/020569 A1 describes another magnetorestrictive memory, in particular a further spin-transfer torque MeRAM (STT-MRAM).

From the publication of Artjom Avakiam et al. "Nonlinear modeling and finite element simulation of magnetoelectric coupling and residual stress in multiferroic composites", published on 14.04.2015 , by Springer Verlag Wien, is known to store data in multiferroic devices by controlling by means of an electric field. Here, the use of barium titanate (BaTiO ₃ ) and cobalt ferrite (CoFe ₂ O ₄ ) is discussed.

The invention has for its object to provide a memory that combines the advantages of FeRAMs with the advantages of MeRAMs.

This object is achieved by a memory having the features specified in claim 1.

The invention is based on the idea of combining the advantages of MeRAM and FeRAM in a single material, namely a ferroelectric. In this way, this storage medium enables an electrical writing and magnetic reading process, without having to resort to a composite material. Thus, the well-researched ferroelectric behavior in the storage media, which is already being used industrially in FeRAMs, is supplemented by the advantages of the MeRAMs. Thus, the advantages of MeRAM compared to other storage media are maintained, without causing additional disadvantages of this storage medium.

The invention is also based on the knowledge of the physical principle that the magnetic permeability, ie the magnetic conductivity, is anisotropic in a ferroelectric material. Depending on the spatial direction, the magnetic permeability is different. Thus, the magnetic permeability of the polarity direction differs from that of the other two spatial directions. The created memory is a Fe-M-RAM. This effect allows the reading process in Fe-M RAMs. Electrically controlled folding processes of electrical polarization (ferroelectric effect) change the local magnetic permeability in a defined direction. This change, as described above, follows from the change in local magnetic permeability. Consequently, the local magnetic quantities in ferroelectric materials are functions of the electrical but also mechanical (ferroelastic effect) load, because the mechanical stress of a ferroelectric can also change the polarization.

In a simple way, a polarized ferroelectric can be exposed to a constant externally applied magnetic field. There are z. B. two magnetic poles (N, S) in the ferroelectric, which can be realized by a permanent magnet. Alternatively, the field may be applied by an electromagnet. If the H-field is known, the B-field or vice versa is measured. The magnetic permeability results from μ = B / H. Thus, the magnetic permeability or its change due to the folding is known. Now, the polarization is caused by the electrical load of the ferroelectric. This takes place in a writing process.

The write operation also locally changes the magnetic permeability. This change in the magnetic magnitudes caused by the folding over can be measured. This measurement corresponds to a read operation.

All ferroelectrics which have the desired behavior with respect to spontaneous polarization and magnetic anisotropy can be used. Including z. B. BaTiO ₃ , Pb (ZrxTi1 - x) O3 (PZT) and other materials. It is relevant that each cell comprises an anisotropic ferroelectric material with respect to magnetic permeability.

The device according to the invention has all the advantages of the MeRAMs over conventional storage media, such as FeRAMs, EEPROMs, SRAMs, DRAMs, STT-RAMs, etc. It also has all the benefits of FeRAMs.

Advantageously, compared to MeRAMs, no composite material for the cell according to the invention is necessary. Thus, there is no need to design and manufacture composites in very small dimensions. Thus, the storage density of the cell according to the invention corresponds to that of the FeRAMs. In addition, the additional mechanically damaging problems (eg delamination) of a composite material (MeRAM), which shorten the life of the storage medium, are avoided. This ultimately leads to a longer lifetime than that of MeRAMs.

Since the ferroelectric behavior in the storage media with ferroelectric material (FeRAM) is already well researched and FeRAMs are already used industrially, the invented memories can be used industrially very quickly. By contrast, MeRAMs are still in the development phase. The manufacturing costs of the memories according to the invention are lower than those of Me-RAMs.

Also compared to FeRAMs a significant increase in the lifetime is realized. Due to the magnetic reading process, which leads to a significantly lower load on the material compared to the electrical reading process, the life of the spoke medium according to the invention is significantly increased. There are also advantages in depolarizing the unit cell of the FeRAM. Because of the magnetic reading process, the current polarization level for reading is no longer needed. If folding processes take place, even if they are not clearly measurable electrically, the magnetic state of the cell is nevertheless changed. This can then be measured. For reading, the Fe-M-RAM according to the invention does not require any electric current.

For applying the magnetic field, for loading and for measuring, it is advantageous if the ferroelectric material is arranged between two electrodes and permanent magnets or electromagnets and is provided with a device having the permeability or the change or the magnetic resistance or the B or H field. The ferroelectric material according to the invention is at least temporarily exposed to a magnetic field. A B or H value or a magnetic resistance (Rm) can be measured, so that when one of these quantities is known, a measurement of the magnetic permeability of the ferroelectric material can be easily realized. With a device for measuring the magnetic permeability can then be measured directly. The electrical load takes place via the arranged electrodes.

The electrical load of the ferroelectric material according to the invention is such that, when the ferroelectric material is previously exposed to an externally applied magnetic field (electromagnet or permanent magnet) takes place as a result of the electrical load effected Umklappvorgang (anisotropy) of the magnetic permeability, so that a write operation of Memory takes place by changing the magnetic permeability. The reading of the memory takes place by measuring the magnetic permeability of the ferroelectric material (either by direct measurement or via B or H or Rm measurement).

Further advantageous embodiments of the invention are characterized in the subclaims.

In an advantageous embodiment of the device according to the invention it is provided that the permanent magnets or the electromagnets are mounted over the electrodes. In addition, the measuring device should be mounted in the plane of the electrodes. As a result, a simple cell structure and very small cell sizes can be realized. This is further improved if the electrodes are provided on the one hand for electrical loading and on the other hand for measuring the magnetic resistance or the permeability. In addition, the electrodes can be used to measure the H or B value and thus determine the magnetic permeability. As a measuring sensor z. B. a Hall element can be used.

In a further advantageous embodiment of the device according to the invention it is provided that the electrodes are arranged in a capacitor, so that the ferroelectric material acts as a dielectric.

In another preferred embodiment, the ferroelectric material consists of barium titanate (BaTiO ₃ ), ie a perovskite structure. Also possible is another crystal in perovskite structure but also other ferroelectric materials.

The memory according to the invention is preferably used in a non-volatile memory, which is provided as a RAM memory. This can be used as data storage or memory of a computer.

An embodiment will be explained in more detail with reference to the drawings, wherein further advantageous developments of the invention and advantages thereof are described. Show it:

1a a first perspective schematic diagram of a memory cell according to the invention;

1b a further perspective schematic representation of a second variant of the memory cell according to the invention;

1c a further perspective schematic representation of a third variant of the memory cell according to the invention;

1d a further perspective schematic representation of a third variant of the memory cell according to the invention;

2 a second perspective schematic diagram of the memory cell according to the invention;

3 a third perspective schematic representation of the memory cell according to the invention;

4 a fourth perspective schematic representation of the memory cell according to the invention;

5 a fifth perspective schematic representation of the memory cell according to the invention;

6 a perspective view of a crystal structure;

7a a first hysteresis curve;

7b a second hysteresis curve;

8th a known chip structure of a FeRAM, and

9 a chip structure according to the invention.

1a) shows a schematic diagram of a memory cell 1 a magnetoelectric storage device for storing digital data. Construction methods 1b . 1c and 1d are also possible.

The cell 1 consists of a magnetic permeability anisotropic ferroelectric material 2 , in particular a crystal, of z. B. barium titanate.

The ferroelectric material 2 is between two electrodes 3a . 3b arranged.

The ferroelectric material 2 is arranged so that it is at least temporarily exposed to a magnetic field, which by the poles N and S in 1 is illustrated. The magnetic field is created by a magnetic field generating element.

The magnetic permeability is over one of some possible devices attached to the cell 1 can be attached, measured. For example, the measurement of the magnetic resistance Rm over in the 1a and 1c shown connections 6 and 7 respectively. 8th and 9 or the contacts 5a . 5b respectively. Alternatively or additionally, for. B. the measurement of the B field via a Hall sensor 10 or 11 be performed, like the 1b and 1d illustrate. In addition, the cell 1 over the electrodes 3a . 3b electrically loaded.

The electrical load causes a Umklappvorgang the magnetic permeability of the material 2 such that a write operation of the memory takes place by changing the magnetic permeability.

There is a read operation of the memory by measuring the magnetic permeability of the ferroelectric material 2 through the appropriate measuring mechanism.

The 2 to 4 illustrate the individual process steps: In the first step, the in 2 is shown, the poled ferroelectric material or the material 2 exposed to an externally applied magnetic field, so that magnetic poles (N, S) arise. In the second step, the in 3 is shown, an H-field and / or a B-field is measured.

In the third step, the in 4 is shown, the ferroelectric material 2 electrically loaded or not loaded, depending on which data information or low level or high level or logic 0 or logic 1 is to be achieved, whereby a change or no change in the magnetic permeability of the ferroelectric material takes place, resulting in a write operation by a voltage source V corresponds to an electrical load.

In the fourth step, the in 5 is shown, a change in the magnetic permeability is measured, which corresponds to a read operation.

As 4 shows is the electrical load for the ferroelectric material 2 connected to the electrodes of the ferroelectric material.

As 2 shows, the poles N and S are provided for applying the magnetic field. In this illustration, a permanent magnet has been indicated, thus creating an H-field and causing a B-field. Using a solenoid, a B-field is created, which instead of the in 2 indicated B-field can measure an H-field.

As 3 or 5 show, the terminals of R _m and 6.7 are provided for measuring the magnetic permeability. The magnetic permeability can also be measured using the Hall sensor 10 respectively. 11 be determined how the 1b and 1d demonstrate.

As the 1 to 5 show, the electrodes are arranged in a capacitor, so that the ferroelectric material acts as a dielectric. Other electrode arrangements are equally feasible.

6 shows the structure of one of the possible materials to be used, for. As a barium titanate crystal. Because of the ferroelectric, dielectric and magnetic properties, barium titanate and related perovskites such as Pb (ZrxTi1-x) O3 are well suited as material for the memory according to the invention. Particularly suitable are ferroelectrics with tetragonal crystal structures, but in principle also all other ferroelectric crystal types. With p, the spontaneous polarization in the 1a to 1d characterized.

In 7 Two typical hysteresis curves of the ferroelectric material are shown. Due to the pronounced hysteresis loop and the high coercive field strength, this material ensures a high degree of data security.

In 2 and 4 is the sequence of magnetic anisotropy illustrated. It can be seen that as a result of the electrical load on the ferroelectric material, the magnetic variables, in particular the B field, also change. In this case, the ferroelectric in the first step, 2 , constantly magnetically loaded. At the second step, 4 , the ferroelectric is electrically stressed while maintaining the constant magnetic load. The electrical stress causes the folding of the polarization within the unit cell, which also change the magnetic sizes. Thus, it can be seen that the magnetic induction, B-field vectors, differ before and after the second step.

In 8th A known, typical chip structure of a FeRAM is shown. A cell consists of N- and P-layers, a ferroelectric material 2 , Bit lines 11 and word lines 12 ,

9 shows an embodiment of the cell 1 a RAM chip according to the invention.

LIST OF REFERENCE NUMBERS

1

Cell, memory cell

2

material

3a

electrode

3b

electrode

4a

magnet

4b

magnet

5a

Contact

5b

Contact

6

connection

7

connection

8th

connection

9

connection

10

Hall sensor

11

Hall sensor

12

Bit series (bit line)

13

Word Series (Word Line)

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 3,909,809 A [0012]
• EP 1318523 A1 [0013]
• EP 1901317 A1 [0014]
• WO 2009/090605 A [0015]
• WO 2010/039422 A [0016, 0018]
• US 20100032738 A1 [0017]
• US 2012/0267735 A1 [0019]
• WO 2013/020569 A1 [0020]

Cited non-patent literature

• Artjom Avakiam et al. "Nonlinear modeling and finite element simulation of magnetoelectric coupling and residual stress in multiferroic composites", published on 14.04.2015 [0021]

Claims (11)

Magnetoelectric memory device for storing digital data, comprising a plurality of memory cells ( 1 ), each cell ( 1 ) a with respect to a magnetic permeability anisotropic ferroelectric material ( 2 ), the ferroelectric material ( 2 ) is in a circuit or can be electrically loaded, z. B. between two electrodes ( 3a . 3b ), the ferroelectric material ( 2 ) is arranged such that it is at least temporarily exposed to a magnetic field, the storage device at least one measuring device for measuring magnetic quantities, for. B. a permeability measuring device for measuring the magnetic permeability of the ferroelectric material ( 2 ), an electrical loading circuit for the ferroelectric material ( 2 ), wherein the electrical load circuit is such that when electrical loading of the ferroelectric material ( 2 ) takes place as a result of the electrical load effected Umklappvorgang the magnetic permeability, so that a writing operation of the memory can take place by changing the magnetic permeability, and wherein a reading operation of the memory by measuring the magnetic magnitudes of the ferroelectric material ( 2 ), z. B. by measuring the magnetic permeability of the ferroelectric material ( 2 ) by the permeability measuring device. Magnetoelectric storage device according to claim 1, characterized in that the electrical loading circuit for the ferroelectric material ( 2 ) z. B. on the electrodes ( 3a . 3b ) of the ferroelectric material ( 2 ) connected. Magnetoelectric memory device according to claim 2, characterized in that the electrodes ( 3a . 3b ) are provided for electrical loading by means of an electric field. Magnetoelectric memory device according to claim 3, characterized in that the permanent magnets or electromagnets ( 4a . 4b ) are provided for applying the magnetic field. Magnetoelectric memory device according to claim 4, characterized in that contacts ( 5a . 5b ) or connections ( 6 . 7 . 8th . 9 ) are provided for measuring the magnetic permeability. Magnetoelectric memory device according to one of the preceding claims, characterized in that the electrodes ( 3a . 3b ) are arranged in a capacitor, so that the ferroelectric material ( 2 ) acts as a dielectric. Magnetoelectric memory device according to one of the preceding claims, characterized in that the ferroelectric material ( 2 ) consists of barium titanate (BaTiO ₃ ) or another ferroelectric material. Use of a magnetoelectric memory device according to one of the preceding claims as a nonvolatile memory and / or as a non-volatile random access memory or as an Fe-M RAM. Use of a magnetoelectric memory device according to one of the preceding claims as a data memory of a computer. Use of a magnetoelectric memory device according to any one of claims 1 to 8 as a working memory of a computer. Method for operating a magnetoelectric storage device according to one of the preceding claims, characterized by the following method steps: - the ferroelectric is exposed to an externally applied magnetic field, so that z. B. magnetic poles (N, S) arise, - an H-field and / or a B-field, magnetic resistance or magnetic permeability is measured directly, - the ferroelectric material ( 2 ) is electrically loaded or not loaded, depending on which data information or low level or high level or logical 0 or logical 1 is to be achieved, whereby a change or no change in the magnetic permeability of the ferroelectric material ( 2 ), which corresponds to a write operation, and a change in magnetic permeability is measured, which corresponds to a read operation.

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