Magnetic materials

FIELD: magnetic materials whose axial symmetry is used for imparting magnetic properties to materials.

SUBSTANCE: memory element has nanomagnetic materials whose axial symmetry is chosen to obtain high residual magnetic induction and respective coercive force. This enlarges body of information stored on information media.

EFFECT: enhanced speed of nonvolatile memory integrated circuits for computers of low power requirement.

4 cl, 8 dwg

The technical field

The present invention relates to magnetic materials and, in particular, to the use of axial symmetry to set the magnetic properties of materials.

Prior art

The amount of information that can be stored on the hard drive of the computer has increased 10 times over the past 40 years and, apparently, will continue to increase exponentially over the coming decades. Traditionally used in modern magnetic materials will not be able to meet future requirements for performance products associated with storing data on magnetic media. One solution to this problem is to appeal to nanotechnology and quantum mechanics, with the prospect of producing magnetic particles of nanometer scale, called nanomagnetism. The latter, due to their very small size, have magnetic properties that differ from the original substrate material. Each nanomagnetic similar giant atom artificial element and allows the formation of new magnetic materials separate giant atoms. Rapidly developing area of nanomagnetism can provide advanced alternative media for hard disk drives and the new generation is ectrodactyly non-volatile memory chips for computers with low energy consumption.

The most important property of natural magnetic element or alloy is its anisotropy. It is characterized, in particular, the existence of preferred directions of magnetization in the material and, ultimately, influences the characteristics of magnetic material and defines the scope of its application. The traditionally used magnetic material anisotropy is a function of the shape and symmetry of the electronic Fermi surface and therefore is characteristic of a specific element or alloy and cannot be simply defined. In nanomagnetics, however, the anisotropy depends not only on the band structure of the source material, but also on the shape of nanomagnetic. One of the most attractive characteristics of artificial magnetic materials is that their magnetic properties can be specified in the select form components of nanomagnets.

Disclosure of inventions

In accordance with the first aspect of the present invention, the memory element contains nanomagnetic having axial symmetry, selected to obtain a high residual magnetic flux and an appropriate coercive force.

In accordance with the second aspect of the present invention, the sensor element contains nanomagnetic having axial symmetry, are chosen so that they are superb romagnani and show essentially zero hysteresis, so that the magnetization of nanomagnetic depends only on the current values of the applied field, and not from the background field.

In accordance with a third aspect of the present invention the magnetic logic element contains nanomagnetic having axial symmetry, are chosen so that they are superparamagnetic particles and are essentially zero hysteresis, so that the magnetization of nanomagnetic depends only on the current values of the applied field, and not from the background field.

Devices of the present invention are artificial magnetic materials formed on a substrate such as a silicon workpiece, using electron-beam lithography. The device may have dimensions in the range of 40-500 nm and to have a thickness in the range of 3-10 nm and can have a triangular or pentagonal geometry, which corresponds to the axial symmetry of the 3-rd and 5-th order. The procedure may, however, large. Source material may be supermalloy (Ni₈₀Fe₁₄Mo₅), which was selected for two reasons. First, this alloy is essentially isotropic and therefore any anisotropy in nanomagnetic must come from their form. Secondly, supermalloy and like him not containing molybdenum permalloy are soft-magnetic alloys, widely used in industry and in research and the research and as such are effectively new and changed properties, which can be given a simple material by nanometer structuring. As will be shown below, the artificial magnetic materials can be shaped magnetic properties in a very wide range by simply changing the symmetry of the constituent nanomagnetics.

Below are examples relating to the present invention, with reference to the attached drawings figures, in which:

figure 1 depicts a micrograph obtained by scanning electron microscope, some artificial magnetic materials;

figure 2 is a hysteresis loop for nanomagnetics various sizes, thickness and geometric shapes;

figure 3 is a hysteresis loop of superparamagnetic particles triangular nanomagnetic;

4 is experimentally measured coercive force depending on the size of nanomagnetic;

figure 5 - experimentally measured anisotropy field inside various nanomagnetics;

6 is a main member of the anisotropy figure 5, expressed as a field of anisotropy (a) and as energy anisotropy of the individual nanomagnetics (B) for different nanomagnetics;

Fig.7 - magnetic susceptibility depending on the size and symmetry of nanomagnetics; and

Fig - schematic device superparamagnetic particles of nanomagnetic when used as a magnetic field sensor or logic is lament.

The best option of carrying out the invention

Standard inverse process of electron-beam lithography may be used to form devices in accordance with the present invention. The substrate sample was selected oriented monocrystalline silicon. The distance between nanomagnetism was always at least equal to the diameter of nanomagnetics, and for the smallest structures up to 3 diameters. The surface roughness of nanomagnetic was less than 0.5 nm and microstructure contained a 5-nm irregularly oriented grain. The upper surface of each nanomagnetic was covered with a layer of gold with a thickness of 5 nm to prevent oxidation. It was found that the integrity of the geometric shape was preserved in structures with sizes up to 50 nm. Figure 1 shows pictures of some of the structures obtained by scanning electron microscope.

In order to determine the magnetic properties of these various artificial materials were measured by their hysteresis loops (loops M-N), using well-known high-sensitivity magneto-optical method. The silicon surface can be viewed through the optical microscope, while the laser spot (size ≈5 μm) is moved over the surface until, until you focus on the upper part of one of the of Chertkov artificial material. The reflected laser beam is subjected to polarization analysis to achieve the meridional Kerr effect, which serves as an indicator for detecting the component of the magnetization lying in the optical plane of incidence. This magnetization in the future is recorded, while the alternating magnetic field frequency of 27 Hz with intensity up to 1000 e is applied in the sample plane. All measurements were performed at room temperature.

Figure 2 and 3 shows some hysteresis loops measured at nanomagnetic with different sizes, thickness and geometry. It seems obvious that the loops are very different from each other and from those obtained from conventional unstructured material. Noteworthy is the variety of technological applications covered by the samples shown on this drawing. Known small rectangles shown in Figv, can find application for replacement of artificial materials media for hard disk drives: when used in this scenario nanomagnetics in principle, it is possible to achieve densities of data storage of more than 100 Gbit/square inch, that is 10 times greater than that traditionally used media known from the prior art. Known large rectangles on Fig.2D well suited for magnetic memory about solinym access - close alternative to semiconductor memories: memory chip using magnetic elements such size, may have a capacity of 1 GB non-volatile high-speed memory. It was found that a wide range of magnetic properties of the detected y triangular and pentagonal nanomagnetics. Triangular nanomagnetic on File, which have a high residual magnetic induction and low coercive force, can be used as a memory element, while the pentagonal nanomagnetics on fig.2F and superparamagnetic particles triangular elements figure 3 can be made of high-quality magnetic field sensors with high sensitivity or read heads for hard disk drives, having an effective relative magnetic permeability of 3000 and zero hysteresis. These diverse applications are a direct result of changes in the size, thickness, and, most importantly, the symmetry of nanomagnetic that make up the artificial material.

In order to quantify this important effect was measured coercivity in the hysteresis loops as a function of size, thickness and order of symmetry of nanomagnetics. Coercivity is a measure of the applied field required to reduce the magnetization to zero (that is, the loop is of hysteresis has a Central width, equal to twice the coercive force), and is essentially a measure of how easily an external field can reverse the magnetization of nanomagnetic. The coercive force is a key parameter in assessing the suitability of this magnetic material for industrial use. Figure 4 presents the results, which, in order to compare different geometry, size nanomagnetics was expressed through the square root of their area. Was confirmed by the repetition of some of these experimental results on the second set of samples.

From Figure 4 it follows that double and fourfold symmetry reproduce the characteristics of one class, while three-fold and five-fold symmetry reproduce the characteristics of another class: nanomagnetite with double/quadruple symmetry have a high coercive force, which first increases with decreasing values along the axis; nanomagnetite with three/five-fold symmetry have a low coercive force, which decreases to zero with decreasing values along the axis. The second conclusion that can be made regarding Figure 4, is that in all cases, the increase in thickness leads to a significant increase in the coercive force, which is not in unstructured magnetic films.

Characteristics is nanomagnetics with twofold symmetry predictable and are a consequence of the well-known phenomenon, called shape anisotropy. Magnetization preferably “ordered” on the longest axis of nanomagnet, with the aim of minimizing the square surfaces of the poles. Field that creates this effect, is the demagnetizing field passing between the surfaces of the poles inside the magnet. Demagnetizing field is scaled approximately with the ratio t/a, where t is the thickness of nanomagnetic and dimensions, and Figa it is seen that the coercive force increases with decreasing size.

In contrast, more difficult to understand the effects of three -, four-and five-fold symmetry. This is because the demagnetizing field of any structure describes a Cartesian tensor of second order and therefore can be only uniaxial (double) symmetry. Therefore, there is no shape anisotropy, at least in the usual sense, in the plane of these structures with the symmetry of a higher order. However, square nanomagnet have a sufficiently strong opposition to changing the direction of magnetization and therefore should be some anisotropy. The fact that this anisotropy is weaker in the triangular magnets, proves that, although it is not a classic shape anisotropy, it is still linked with the form of nanomagnetic.

The square on which magneticum inherent in a newly opened property, named configuration anisotropy. It is associated with very small deviations from the uniform magnetization, which takes place in almost all nanomagnetic. Up to the present time was not yet clear to what extent this new anisotropy important in determining the magnetic properties of nanomagnetic. To test the hypothesis, according to which variable characteristics, are shown in Fig.2-4, due to the configuration anisotropy was performed direct measurement of anisotropy in nanomagnetic using a method called magnetooptical anisometry with modulated field. A strong static magnetic field H (=350 OE) is applied in the plane of nanomagnetics and more weak oscillating field H_t(with an amplitude of 14 e) is applied in the plane of nanomagnetic perpendicular to N. The amplitude of the resulting oscillations of the magnetization recorded the same magneto-optical method used to obtain the hysteresis loops in figure 2 and 3, and is directly related to the amplitude and symmetry of any anisotropy in nanomagnets. Experimentally, we measured the anisotropy of the triangular, square and pentagonal nanomagnetics with sizes in the range of 50-500 nm at a thickness of 5 nm and the results are presented in figure 5. On figure 5 presents the polar diagrams of the angle determines the square is ckastne direction φ in nanomagnetic, the radius defines the radius of nanomagnetic in this direction and tone determines the experimentally measured value

for nanomagnetic with the same size, where M_Sthe saturation magnetization (800 unit tow CGS EMU cm^-3) and E(φ) - the average density of the magnetic energy of nanomagnetic when the magnetization direction φ. Although H_andis not the generally accepted definition of the field of anisotropy, it is possible to show that any fluctuations in N_andhave the same symmetry as the underlying anisotropy and amplitude equal to the magnitude of the anisotropy field. Figure 5 shows the experimental data for 22 different samples of the artificial magnetic material (8 sizes of triangles, 8 sizes of squares and 6 sizes pentagons), and each was measured either 19 or 37 different directions φ (0-180° - steps 10° for triangles and squares, 0-180° - steps 5° for pentagons), and the number of measurements amounted to 526.

From Figure 5 it should be the presence of strong anisotropy fields in all the investigated nanomagnetic. Triangular nanomagnetic exhibit anisotropy with sixfold symmetry, square nanomagnet exhibit anisotropy with fourfold symmetry, and pentagonal nanomagnet have anisotropy with des the fold symmetry. Frequency doubling occurs in the triangular and pentagonal structures, because the energy is always determined by the square of the magnetization, and therefore may not be supported odd orders of symmetry.

Was applied Fourier analysis to the graphs in Figure 5, to receive the margins of the anisotropy as a function of the size of nanomagnetics and symmetry and show the results in two different forms figure 6. On Figa represented directly by the graph of field anisotropy, while Figv depicts a graph of the dependence of anisotropy energies one nanomagnetic (in units of kt, where K is the Boltzmann constant, and T is equal to 298K), using theoretical dependence of U_a=2M_SH_aV/n². In this equation, U_a- energy anisotropy one nanomagnetic (in ergs), N_and- anisotropy field (in oersteds), n is the order of symmetry of anisotropy (4 - squares, 6 - triangles, 10 - pentagons) and V is the volume (in cm³) nanomagnetic. An important element for understanding the impact of symmetry order on the magnetic properties is a member of n²in this equation. This means that, although all forms of geometry show a similar field anisotropy on Figa, they demonstrate a very distinguish energy anisotropy on Figv.

Energy anisotropy is of particular interest from a-Z. the phenomena, called superparamagnetism, which is a process by which you can overcome energy barriers anisotropy by fluctuations of thermal energy kt in nanometer-scale magnets. As a rough guide the barrier can be overcome in the time scale of the considered measurements, if its height is less CT. This means that if energy anisotropy less CT, it can be expected that the coercive force of will quickly drop to zero. According Figv this occurs when the element size is less than approximately 150 nm, and the square of the elements of this decrease to zero as the size reduction is weaker. Conversely, if the energy anisotropy more CT, the coercive force is about to repeat field anisotropy. This explains the difference in the characteristics that can be observed in figure 4 the square elements on one side and triangular and pentagonal elements on the other. Energy anisotropy square elements (Figa) has a maximum with decreasing element size and the maximum direct effect on the data o the coercive power of the square elements (Figs). Field anisotropy pentagonal elements not observed maximum, and this is also reflected directly on the values of the coercive force (Fig.4D), while falling to zero occurs when several is of like larger size data of the coercive force, than the anisotropy field due to thermal excitation. Finally, field anisotropy of the triangular element really has a maximum exactly the same as that of the square, but because the energy of anisotropy is less than the triangular element, thermal excitation occurs at a larger size and prevents the maximum data coercive force (Pigv). Thus it is possible to explain the experimentally defined data coercive force as resulting from the combination of the configuration of anisotropy and thermal excitations.

Materials with a high enough magnetic induction with limited coercive force (and therefore memory function), are not only technologically important magnetic materials. Equally important are the materials with zero residual magnetic induction and zero coercive force, which are used in magnetic sensors and logic elements, and in this case, it is the magnetic susceptibility χ is a key parameter. χ defined aswhere M is the magnetization of nanomagnetics, and H is the applied magnetic field. χ is directly proportional to the slope of the hysteresis loop zero field, such as shown in Fig.2F. χ was measured using visualizingeconomics.com experiment (at a frequency of 27 Hz) as a function of the size of nanomagnet and symmetry at a constant thickness of 3.7±0.5 nm, and the results are shown in Fig.7. χ makes sense only when the coercive force is equal to zero, and the experiments were limited to this case. For comparison, the drawing also shows theoretical values of the magnetic susceptibility, derived from the Langevin function, which is a statistical thermodynamic representation applied to one giant back in free space. When considering Fig.7 we can come to three conclusions. First, all experimentally defined values of the magnetic susceptibility of the smallest magnets close to the Langevin model in free space, even when not using the approximation parameters, which proves that good control of the experimental system. Second, deviations from the model Langevin maximum for nanomagnetics with square symmetry. This is consistent with the fact that they have the largest energy configuration anisotropy, on the assumption that the giant spin, available square nanomagnetics, in the least degree like a giant back in free space. It configurational anisotropy ultimately causes the deviation from the model Langevin for all kinds of symmetries with increasing size. Thirdly, the measured value of the magnetic susceptibility on d is an order exceed the values which would have been obtained from the magnetic particles with the same shape and aspect ratio, but is made in a larger scale of a nanometer (i.e., micrometers and above who use the most conventional magnetic field sensors). In this latter case, the magnetic susceptibility is derived from the motion of domain walls against internal demagnetizing field, which can be very strong. Thus visible to the unique role of structuring in the nanometer scale.

Figure 4 and Figure 5 illustrate the manner in which nanometer structuring truly emulates the creation of new magnetic materials. Supermalloy grown with a face-centered cubic crystallography, which usually leads to anisotropy with a twofold or fourfold symmetry. However, Figa shows a sixfold symmetry, which usually belongs to the materials with face-centered cubic crystallography. In this case, the symmetry of the shape nanomagnet (triangular) can be used to simulate changes in crystallographic phase. Similarly, a relatively high coercive force square and rectangular nanomagnetics (Figa and Figs) generally found in magnetic materials with high anisotropy and weak microstructure. This is the case careful choice of symmetry of nanomagnetic emulates the change in the element and changes in the microstructure. Finally, elementary crystallography determines that the lattice cannot have a ten-fold symmetry, and therefore it is impossible to expect the discovery of natural crystalline element or alloy magnetic anisotropy with tenfold symmetry. On Figs shown, however, that was a successful attempt to create a single item, artificially using nanostructuring. In this case it was used nanometer molding to give a crystalline material properties, which is usually found in quasicrystals.

On Fig schematically depicts the device using the superparamagnetic particles nanomagnetic as part of the sensor or logic element. Three-layer rotary valve 12 is connected on both sides with connecting lines 14. The valve 12 includes a lower magnetic layer 16, the nonmagnetic spacer layer 18, and one or more nanomagnetics 20 in super-paramagnetic state as the top layer. Arrow 22 indicates the current passing through the valve 12, and the arrow 24 - magnetization in a magnetic bottom layer 16.

Finally, it was determined the effect of symmetry forms nanomagnetics on their magnetic properties, which has a practical application. It was found that symmetry plays an important role, allowing you to control the magnetic properties in a very wide the m range. It was shown that a key effect of linking the symmetry of the magnetic properties is the configurational anisotropy. This allows you to create new artificial materials whose magnetic properties can be set for specific applications with a very high degree of precision.

The first new idea is the use of the configuration anisotropy through the symmetry of the element to set the magnetic properties. To date, the experts in this field of technology considered only rectangular, square or circular elements. It was determined that the configurational anisotropy induced by other forms, such as triangles, pentagons and hexagons can be used to control the magnetic properties of the element. The second new idea is to use superparamagnetism in nanostructures to eliminate hysteresis. In ordinary materials superparamagnetism leads to very strong fields saturation and as such is not used in magnetic sensors. In the present application is shown, however, that in nanostructures superparamagnetism can lead to very weak fields saturation (several OE - see Fig.2F). By itself, this fact can be very useful for sensor and logic element. Another important aspect of the invention is what is superparamagnetism ensures almost zero hysteresis, that is a necessary requirement for a good sensor (see figure 3). The biggest problem that is faced with this using nanostructures as sensors, is the fact that usually the hysteresis becomes larger with decreasing transverse dimensions of the device.

These two ideas can be combined fixation configuration anisotropy at a low value by choosing an appropriate shape (triangular, pentagonal or circular), which gives the nanostructure super-paramagnetic properties and, therefore, allows it to perform the function of a good sensor or logic element.

1. Memory element containing nanomagnetic with geometric axial symmetry of rotation is selected to obtain a high residual magnetic induction and appropriate coercive force, characterized in that it has an axial symmetry 3 - or 5-th order.

2. The element according to claim 1, characterized in that it is made of artificial magnetic material formed on the substrate surface.

3. The element according to claim 1 or 2, characterized in that it is made of superalloy (Ni₈₀Fe₁₄Mo₅).

4. Element according to one of claims 1 to 3, characterized in that the width is in the range of 40-500 nm, and a thickness in the range of 3-10 nm.