Hafnium oxide-doped single-phase multiferroic material and preparation method thereof

Abstract

The invention relates to the technical field of functional materials, in particular to a hafnium oxide-doped single-phase multiferroic material and a preparation method thereof. The chemical expression of the single-phase multiferroic material is Hf1-xNbxO2 or Hf1-yTayO2, x is more than 0 and less than or equal to 0.5, and y is more than or equal to 0.25 and less than or equal to 0.5. According to the hafnium oxide-doped single-phase multiferroic material, Ta or Nb-doped hafnium oxide is used, the condition of ferromagnetic property is met by utilizing additional d orbital electrons provided by introduced transition metal Ta or Nb, meanwhile, introduced impurities do not change non-centrosymmetric arrangement of oxygen atoms, the ferroelectric property of hafnium oxide is reserved, and the ferroelectric property of the hafnium oxide is improved. Therefore, the hafnium oxide-doped single-phase multiferroic material is realized, and relatively strong magnetoelectric coupling is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of functional materials, and particularly to a doped hafnium dioxide single-phase multiferroic material and a preparation method thereof. Background Art

[0002] Multiferroic materials possess multiple ordered states of iron, such as ferroelectricity, ferromagnetism, ferroelasticity, and ferro-vorticity, and have extensive application potential, such as magnetoelectric coupling magnetoelectric random access memory devices, magnetic sensors, etc. Since the ferroelectricity of perovskite materials originates from the hybridization of the empty d orbitals of transition metal elements and the p orbitals of oxygen, which causes the arrangement of metal ions to change and generates spontaneous polarization of ferroelectricity. While ferromagnetism often originates from the hybridization of the partially occupied d orbitals of transition metal elements and the p orbitals of oxygen, that is, it requires the d orbitals of transition metals not to be empty. The conflict between these two principles results in the scarcity of perovskite multiferroic materials.

[0003] Hafnium dioxide is a material with a high dielectric constant and is compatible with silicon-based semiconductor processes, and has been widely studied and applied. Ferroelectric properties have been found in hafnium dioxide doped with various impurities such as Si, La, Zr, Y, Eu, Al, and Sc. The ferroelectricity of hafnium dioxide originates from the non-centrosymmetric arrangement of oxygen atoms, which does not conflict with the ferromagnetic principle of partially occupied d orbitals. Currently, there is only a multiferroic interface obtained by stacking hafnium dioxide ferroelectric thin films and other ferromagnetic materials, and there is no single-phase hafnium dioxide multiferroic material. The preparation of the stacked structure and large-scale integration are more difficult compared to single-phase materials. Single-phase materials are easier to prepare, easier to integrate on a large scale, more compatible with silicon-based CMOS processes, have stronger magnetoelectric coupling properties, etc., and have a wider application range. Therefore, it is very necessary to develop a single-phase hafnium dioxide multiferroic material. Summary of the Invention

[0004] In order to solve the above technical problems of the present invention, the present invention provides a doped hafnium dioxide single-phase multiferroic material and a preparation method thereof. Based on ferroelectric hafnium dioxide, the present invention realizes single-phase multiferroic hafnium dioxide by doping Ta or Nb, and in the single-phase multiferroic hafnium dioxide doped with Nb, a strong magnetoelectric coupling effect can also be realized.

[0005] According to the first aspect of the present invention, the present invention provides a doped hafnium dioxide single-phase multiferroic material, and its chemical formula is Hf 1-x Nb x O2 or Hf 1-y Ta y O2, where 0 ≤ x ≤ 0.5 and 0.25 ≤ y ≤ 0.5.

[0006] In the above solution, the single-phase multiferroic material of hafnium dioxide doped by the present invention uses hafnium dioxide doped with Ta or Nb. By utilizing the additional d-orbital electrons provided by the introduced transition metals Ta or Nb, the conditions for ferromagnetic properties are satisfied. At the same time, the introduced impurities do not change the non-centrosymmetric arrangement of oxygen atoms, retaining the ferroelectric properties of hafnium dioxide, thus realizing a single-phase multiferroic material of doped hafnium dioxide. In the single-phase multiferroic material of hafnium dioxide doped by the present invention, before and after the switching of the ferroelectric polarization direction, the exchange interaction strength between doped atoms will be redistributed. Therefore, the reversal of ferroelectric polarization will cause the reallocation of the magnetic moment direction, forming a magnetoelectric coupling response. Especially in Hf 1-x Nb x O2 material, when the ferroelectric polarization direction is switched, the exchange interaction strength of the doped atoms therein will weaken. After the switching, the exchange interaction strength of the doped atoms during this period will be redistributed, and the change amplitude of the interaction strength can vary from 17.48 meV to 3.83 meV, up to a five-fold change. The change amplitude of these energies is stronger in Hf 1-x Nb x O2 material than in Hf 1-y Ta y O2 material. Compared with perovskite multiferroic materials, the single-phase multiferroic material of hafnium dioxide doped by the present invention has a higher spontaneous ferroelectric polarization intensity, and has a very high compatibility with the existing silicon-based CMOS process in terms of materials and processes, with great potential for practical applications.

[0007] Further, 0.25 ≤ x ≤ 0.5, 0.25 ≤ y ≤ 0.5.

[0008] Further, when the chemical formula of the single-phase multiferroic material is Hf 1-x Nb x O2, the Curie temperature of the single-phase multiferroic material is greater than or equal to 53.7 K, preferably 81.6 K.

[0009] Further, when the chemical formula of the single-phase multiferroic material is Hf 1-y Ta y O2, the Curie temperature of the single-phase multiferroic material is greater than or equal to 2.8 K, preferably 73.0 K.

[0010] Further, when the single-phase multiferroic material switches in the ferroelectric polarization direction, the exchange interaction strength of the doped atoms therein will weaken; after the switching, the exchange interaction strength of the doped atoms during this period will be redistributed, so that the reversal of ferroelectric polarization will cause the reallocation of the magnetic moment direction, forming a magnetoelectric coupling effect. Preferably, the single-phase multiferroic material is Hf 1- x Nb xWhen the O₂ content is present, when the ferroelectric polarization direction of the single-phase multiferroic material is switched, the exchange interaction strength of the doped atoms therein will weaken; after the switching, the exchange interaction strength of the doped atoms during this period will be redistributed, so that the reversal of the ferroelectric polarization will cause the re - distribution of the magnetic moment direction, forming a magnetoelectric coupling effect.

[0011] According to the second aspect of the present invention, the present invention also provides a preparation method of the above - mentioned single - phase multiferroic material, including the following steps:

[0012] Step 1: On a substrate, grow a layer of TiN electrode using atomic layer deposition technology;

[0013] Step 2: Use atomic layer deposition technology to alternately grow HfO₂ and TaO₂ or NbO₂ in a certain proportion, so as to obtain an HfO₂ thin film doped with Nb or Ta;

[0014] Step 3: Magnetron sputter TiN on the HfO₂ thin film doped with Nb or Ta, and then perform rapid annealing.

[0015] In the above - mentioned solution, the single - phase multiferroic material layer of the present invention is obtained by magnetron sputtering, and the preparation process is simple, with high reliability and is conducive to large - scale production.

[0016] Further, in step 1, the material of the substrate includes an Si substrate and SiO₂ formed on the Si substrate, and the thickness of SiO₂ is 1 - 2 nm.

[0017] Further, in step 1, the thickness of the TiN electrode is 50 - 200 nm, preferably 60 nm.

[0018] Further, in step 2, the etching rate of alternately growing HfO₂ and TaO₂ or NbO₂ using atomic layer deposition technology is 0.05 - 0.1 nm / cycle, preferably 0.08 nm / cycle.

[0019] Further, in step 3, the thickness of the magnetron - sputtered TiN is 50 - 200 nm, preferably 60 nm.

[0020] Further, in step 3, rapid annealing is to rapidly cool from 400 - 1000 °C to 100 °C in 20 - 60 s, preferably to rapidly cool from 500 - 600 °C to 100 °C in 60 s.

[0021] The technical solution provided by the present invention has the following beneficial effects:

[0022] The present invention relates to a doped hafnium dioxide single-phase multiferroic material that uses hafnium dioxide doped with Ta or Nb. By utilizing the additional d-orbital electrons provided by the introduced transition metals Ta or Nb, the conditions for ferromagnetic properties are satisfied. At the same time, the introduced impurities do not change the non-centrosymmetric arrangement of oxygen atoms, retaining the ferroelectric properties of hafnium dioxide, thus realizing a doped hafnium dioxide single-phase multiferroic material. In the single-phase multiferroic material of the present invention, due to the switching of ferroelectric polarization, the strength of the exchange interaction between doped atoms changes, forming magnetoelectric coupling, especially in the Hf 1-x Nb x O2 material is more obvious. Compared with perovskite multiferroic materials, a doped hafnium dioxide single-phase multiferroic material of the present invention has a higher spontaneous ferroelectric polarization intensity and is highly compatible with the existing silicon-based CMOS process in terms of materials and processes, with great potential for practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0024] Figure 1 It is a structural diagram of a doped hafnium dioxide single-phase multiferroic material provided by Embodiment 1 and Embodiment 2 of the present invention at a doping concentration of 25%;

[0025] Figure 2 It is a spectrum diagram obtained by calculating the phonon spectrum when doping Nb or Ta at a concentration of 25% in Embodiment 1 and Embodiment 2 of the present invention;

[0026] Figure 3 It is a spectrum diagram obtained by using Berry Phase when calculating the polarization intensity in Embodiment 1 and Embodiment 2 of the present invention;

[0027] Figure 4 It is a diagram of the flipping potential barriers of five paths obtained by using the Neb method when calculating the ferroelectric polarization flipping in Embodiment 1 and Embodiment 2 of the present invention;

[0028] Figure 5 For Figure 3 It is a schematic diagram of the specific starting and ending structures among the five flipping paths in

[0029] Figure 6 It is a diagram of the Curie temperature calculated for 25% doping in Embodiment 1 and Embodiment 2 of the present invention;

[0030] Figure 7 It is a diagram of the change in the types of nearest neighbor sites during the switching in Embodiment 1 and Embodiment 2 of the invention;

[0031] Figure 8 Structural diagram, phonon spectrum, and Curie temperature diagram of 50% Nb-doped in Invention Example 1;

[0032] Figure 9 Structural diagram, phonon spectrum, and Curie temperature diagram of 50% Ta-doped in Invention Example 2. Detailed implementation manners

[0033] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without making creative efforts shall fall within the scope of protection of the present invention.

[0034] In the present invention, for those not specifying specific techniques or conditions in the embodiments, the techniques or conditions described in the literature in the field or the product specifications shall be followed. For those not specifying the manufacturer of the instruments used, they are all conventional products that can be obtained through regular channels. The raw materials used in the present invention can be conveniently purchased in the domestic market.

[0035] Example 1

[0036] This example provides a doped hafnium dioxide single-phase multiferroic material, whose chemical formula is Hf 0.75 Nb 0.25 O2, and its preparation method includes the following steps:

[0037] Step 1: On a substrate containing Si-based and SiO2 (with a thickness of 1 - 2 nm), grow a 60-nm-thick TiN electrode using atomic layer deposition technology.

[0038] Step 2: Use atomic layer deposition technology to alternately grow HfO2 and NbO2 at an etching rate of 0.08 nm / cycle in a certain proportion to obtain an Nb-doped HfO2 thin film.

[0039] Step 3: Magnetron sputter 60-nm-thick TiN on the Nb-doped HfO2 thin film, and then rapidly anneal it from 580 °C to 100 °C in 30 s.

[0040] The basic structure of the single-phase multiferroic material in this example is as Figure 1As shown, the stability of the structure was demonstrated by phonon spectrum calculation. The phonon spectrum calculation method was to use the VASP software and the DFPT algorithm, the DFT+U method and the PBE pseudopotential. After expanding the unit cell into a 2×2×2 supercell with 96 atoms, the phonon spectrum of the material was calculated at a k-point density of 3×3×3. The phonon spectrum diagram of the undoped HfO2 material is as shown in Figure 2 Figure (a) in Figure 2 Figure (b) in

[0041] Since there are no obvious imaginary frequencies on the phonon spectrum (that is, except for the points near the Γ axis on the horizontal axis, there are no obvious points with a vertical axis less than 0), the structure can be considered dynamically stable. 2 Using the Berry phase method for first-principles calculation of the single-phase multiferroic material in this example, a spontaneous ferroelectric polarization of 42 μC / cm Figure 3 Figure (a) in 3 is the spectrum obtained by using Berry Phase when calculating the spontaneous ferroelectric polarization intensity of the single-phase multiferroic material in this example. Calculate the translation distance along the y-axis between adjacent parallel lines (unit: electronic charge), and then divide by the calculated unit cell volume (unit: A

[0042] ) to obtain the spontaneous ferroelectric polarization intensity. Figure 4 Figure (a) in Figure 5 is the flipping barrier diagram of five paths obtained by using the Neb method when calculating the ferroelectric polarization flipping in this example. Figure 4 Figure (a) in Figure 4 is the schematic diagram of the specific starting and ending structures in the five flipping paths described in

[0043] It can be seen from Figure (a) in Figure 6 that the lower-energy paths 4 and 5 are more likely to occur, and the lowest is 21.4 meV / atom, indicating that the single-phase multiferroic material in this example has a polarization flipping barrier of 21.4 meV / atom, which is lower than the 30.7 meV / atom barrier in intrinsic HfO2.

[0044] Therefore, without considering the calculation of U, the doping method of this embodiment can obtain a single-phase doped hafnium dioxide multiferroic material.

[0045] In the 25% doping structure, since the considered structure has only one doped atom in a single unit cell, the nearest neighbor sites of metal atoms in intrinsic hafnium dioxide are not formed, and the interaction is not strong. The present invention further uses DFT+U calculation and considers the types of neighbors at the nearest neighbor sites using the VASP software, such as Figure 7 The figure shows the change in the types of nearest neighbor sites during the switching.

[0046] In the ferroelectric Pca21 phase, each metal atom has 12 nearest neighbor atoms. According to chirality and material symmetry, they can be divided into six types, which are marked by numbers on the atoms. These six types of neighbors will change in type before and after ferroelectric flipping, thus causing a change in the strength of magnetic exchange interaction. The types of neighbors and the interaction strength on the switching path are shown in Table 1 below. And in Nb-doped HfO2, in the intermediate state of ferroelectric flipping (the path is types 4 and 5 with the lowest potential barrier), there will be a state where the interactions of all 6 neighbors are very weak (i.e., the P42 / nmc phase in Table 1). Therefore, the magnetic moment on Nb will change again due to thermal noise and redistribute after the ferroelectric flipping is completed. Thus, in Nb-doped HfO2, the ferroelectric switching will cause the switching of the magnetic polarization direction, that is, the magnetoelectric coupling phenomenon.

[0047] Table 1

[0048] Neighbor types <![CDATA[Nb doping in Pca21 phase (meV)]]> <![CDATA[Nb-doping in P42 / nmc phase (meV)]]> 1 -17.48 -2.79 2 -3.83 -2.78 3 -16.69 -3.18 4 -3.31 -1.75 5 -9.14 -1.68 6 3.95 -1.74

[0049] Example 2

[0050] This embodiment provides a single-phase multiferroic material of doped hafnium dioxide, and its chemical formula is Hf 0.75 Ta 0.25 O2, and its preparation method includes the following steps:

[0051] Step 1: On a substrate containing Si and SiO2 (with a thickness of 1-2 nm), use atomic layer deposition technology to grow a TiN electrode with a thickness of 60 nm.

[0052] Step 2: Use atomic layer deposition technology to alternately grow HfO2 and TaO2 at an etching rate of 0.08 nm / cycle in a certain ratio to obtain a Ta-doped HfO2 thin film.

[0053] Step 3: Magnetron sputter TiN with a thickness of 60 nm on the Ta-doped HfO2 thin film, and then rapidly anneal at 580 °C for 30 s and cool down to 100 °C.

[0054] The basic structure of the single-phase multiferroic material of this embodiment is asFigure 1 As shown, the stability of the structure was demonstrated by phonon spectrum calculation. The phonon spectrum calculation method was to use the VASP software and the DFPT algorithm, the DFT+U method and the PBE pseudopotential. After expanding the unit cell into a 2×2×2 supercell of 96 atoms, the phonon spectrum of the material was calculated at a k-point density of 3×3×3. The phonon spectrum diagram of the single-phase multiferroic material in this example is as shown in Figure 2 Figure (c) in. There are no obvious imaginary frequencies in the phonon spectrum (that is, except for the points near the Γ axis on the horizontal axis, there are no obvious points with a vertical axis less than 0), so the structure can be considered dynamically stable.

[0055] The first-principles calculation of the single-phase multiferroic material in this example was carried out using the Berry phase method, and a spontaneous ferroelectric polarization intensity of 42 μC / cm 2 was obtained. As shown in Figure 3 Figure (b) in is the spectrum obtained by using the Berry Phase when calculating the spontaneous ferroelectric polarization intensity of the single-phase multiferroic material in this example. Calculate the translation distance along the y-axis between adjacent parallel lines (unit electronic charge), and then divide by the calculated unit cell volume (unit Å 3 ), and the spontaneous ferroelectric polarization intensity can be obtained.

[0056] The single-phase multiferroic material in this example was calculated using the NEB method. The NEB method used the VASP software and the CI-NEB algorithm, the DFT method and the PBE pseudopotential. The flipping barriers of the material under 5 flipping paths were calculated at a k-point density of 9×9×9. As shown in Figure 4 Figure (b) in is the flipping barrier diagram of the five paths obtained by using the Neb method when calculating the ferroelectric polarization flipping in this example. Figure 5 is the schematic diagram of the specific starting and ending structures in the five flipping paths described in Figure 4 . Figure 4 It can be seen from Figure (b) in that the lower energy is 30.5 meV / atom, indicating that the single-phase multiferroic material in this example has a polarization flipping barrier of 30.5 meV / atom, which is lower than the barrier of 30.7 meV / atom in intrinsic HfO2.

[0057] Without considering the electron Coulomb interaction term (U) and anisotropy calculations, the Heisenberg model Hamiltonian was used to describe the interaction between spin-polarized electrons in the single-phase multiferroic material in this example, and the Monte Carlo simulation (MC) was carried out using the Heisenberg model. As shown in Figure 6 Figure (b) in, the single-phase multiferroic material has a Curie temperature of 73.0 K and a ferromagnetic (rather than antiferromagnetic) ground state.

[0058] Therefore, without considering the calculation of U, the doping method in this example can obtain a single-phase doped hafnium dioxide multiferroic material.

[0059] In the 25% doping structure, since there is only one doping atom in a single unit cell of the considered structure, the nearest neighbor sites of metal atoms in the intrinsic hafnium dioxide are not formed, and the interaction is not strong. We further use DFT+U calculations and consider the types of neighbors at the nearest neighbor sites using the VASP software, such as Figure 7 The figure shows the change in the types of the nearest neighbor sites during the switching.

[0060] In the ferroelectric Pca21 phase, each metal atom has 12 nearest neighbor sites. According to chirality and material symmetry, they can be divided into six types, which are marked by numbers on the atoms. These six types of neighbors will change in type before and after the ferroelectric flip, thus causing a change in the strength of the magnetic exchange interaction. Therefore, in Ta-doped HfO2, the ferroelectric switching will be the switching of the magnetic polarization direction, that is, the magnetoelectric coupling phenomenon. However, according to the corresponding relationship between the types of neighbors and the interaction strength on the switching path in Table 2 below, this change amplitude is small, so the magnetoelectric coupling strength is lower than that of the material doped with Nb.

[0061] Table 2

[0062]

[0063]

[0064] Example 3

[0065] This example provides a doped hafnium dioxide single-phase multiferroic material with the chemical formula Hf 0.5 Nb 0.5 O2, and its preparation method is the same as that of Example 1.

[0066] In the present invention, the Curie temperature of the material is calculated using the DFT+U method, the VASP software, and the Heisenberg model in Hf 0.5 Nb 0.5 O2, and the Curie temperature is characterized using the relative heat capacity. By selecting different doping sites, four 50% doped structures in a 2×2×2 supercell are constructed, such as Figure 8 The results show that the four considered structures have a Curie temperature higher than 53.7 K, the highest Curie temperature of one structure is 81.6 K, and the phonon spectra of these structures have no imaginary frequencies and are all stable structures.

[0067] Example 4

[0068] This example provides a doped hafnium dioxide single-phase multiferroic material with the chemical formula Hf 0.5 Nb 0.5 O2, and its preparation method is the same as that of Example 2.

[0069] In the present invention, in Hf0.5 Nb 0.5 Using the DFT+U method in NbO2, the Curie temperature of the material was calculated by the VASP software and the Heisenberg model, and the Curie temperature was characterized by the relative heat capacity. By selecting different doping sites, four structures with 50% doping in a 2×2×2 supercell were constructed, as Figure 9 The results show that the four structures considered have a Curie temperature higher than 2.8 K, the highest Curie temperature of one structure is 23.6 K, and the phonon spectra of these structures have no imaginary frequencies and are all stable structures.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A doped hafnium dioxide single-phase multiferroic material, characterized in that, Its chemical formula is Hf 1-x Nb x O2 or Hf 1- y Ta y O2, where 0 < x ≤ 0.5 and 0.25 ≤ y ≤ 0.

5.

2. The single-phase multiferroic material according to claim 1, wherein 0.25 ≤ x ≤ 0.5, 0.25 ≤ y ≤ 0.

5.

3. The single-phase multiferroic material according to claim 1, characterized in that, When the chemical formula of the single-phase multiferroic material is Hf 1-x Nb x O2, the Curie temperature of the single-phase multiferroic material is greater than or equal to 53.7 K, preferably 81.6 K.

4. The single-phase multiferroic material according to claim 1, characterized in that, When the chemical formula of the single-phase multiferroic material is Hf 1-y Ta y O2, the Curie temperature of the single-phase multiferroic material is greater than or equal to 2.8 K, preferably 73.0 K.

5. The single-phase multiferroic material according to claim 1, characterized in that, When the ferroelectric polarization direction of the single-phase multiferroic material is switched, the exchange interaction strength of the doped atoms therein will weaken; after the switching, the exchange interaction strength of the doped atoms during this period will be redistributed, so that the reversal of the ferroelectric polarization will cause the reorientation of the magnetic moment direction, forming a magnetoelectric coupling effect.

6. The preparation method of the single-phase multiferroic material according to any one of claims 1-5, characterized in that, It includes the following steps: Step 1: Grow a layer of TiN electrode on the substrate using atomic layer deposition technology; Step 2: Alternately grow HfO2 and TaO2 or NbO2 at a certain ratio using atomic layer deposition technology to obtain an HfO2 thin film doped with Nb or Ta; Step 3: Magnetron sputter TiN on the HfO2 thin film doped with Nb or Ta, and then perform rapid annealing.

7. The preparation method according to claim 6, characterized in that, In Step 1, the material of the substrate includes an Si substrate and SiO2 formed on the Si substrate, and the thickness of SiO2 is 1 - 2 nm; And / or, the thickness of the TiN electrode is 50 - 200 nm, preferably 60 nm.

8. The preparation method according to claim 6, characterized in that, In Step 2, the etching rate of alternately growing HfO2 and TaO2 or NbO2 using atomic layer deposition technology is 0.05 - 0.1 nm / cycle, preferably 0.08 nm / cycle.

9. The preparation method according to claim 6, characterized in that, In Step 3, the thickness of the magnetron sputtered TiN is 50 - 200 nm, preferably 60 nm.

10. The preparation method according to claim 6, characterized in that, In Step 3, the rapid annealing is to rapidly anneal by cooling from 400 - 1000 °C to 100 °C in 20 - 60 s, preferably to rapidly anneal by cooling from 500 - 600 °C to 100 °C in 60 s.

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