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Title:
THERMALLY ACTIVE NANOMAGNETS FOR ENERGY HARVESTING AND SENSOR APPLICATIONS
Document Type and Number:
WIPO Patent Application WO/2019/038028
Kind Code:
A1
Abstract:
According to the present invention an innovative design is disclosed for performing energy harvesting from heat directly at the nanoscale. The advantage of the proposed device is that it has no moving parts, which means the lifetime of the device is theoretically infinite, similar to the lifetime of the photovoltaic solar cells. Because the magnetization dynamics of the nanomagnets can be tuned, the working temperature of the device can also be tuned. Such a power unit can harvest energy from heat sources and can be used for sensors installed at remote locations without reliable wind or solar power where changing the battery is too costly. The use of the proposed energy harvesting device will have direct benefits to the society by reducing the pollution caused by the low-power batteries around the world.

Inventors:
SENDETSKYI, Oles (Hertensteinstrasse 1a, Ennetbaden 5408, Ennetbaden, CH)
Application Number:
EP2018/070580
Publication Date:
February 28, 2019
Filing Date:
July 30, 2018
Export Citation:
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Assignee:
PAUL SCHERRER INSTITUT (5232 Villigen PSI, 5232, CH)
International Classes:
H01L37/04; H01F1/00
Domestic Patent References:
WO2014079505A12014-05-30
WO2009133047A22009-11-05
Foreign References:
US20140022036A12014-01-23
US20160141333A12016-05-19
Other References:
KUNTAL ROY: "Landauer limit of energy dissipation in a magnetostrictive particle", JOURNAL OF PHYSICS: CONDENSED MATTER, INSTITUTE OF PHYSICS PUBLISHING, BRISTOL, GB, vol. 26, no. 49, 7 November 2014 (2014-11-07), pages 492203, XP020274186, ISSN: 0953-8984, [retrieved on 20141107], DOI: 10.1088/0953-8984/26/49/492203
None
Attorney, Agent or Firm:
FISCHER, Michael (Siemens AG, Postfach 22 16 34, München, 80506, DE)
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Claims:
Patent claims 1. A method for current generation in a conductor using a thermally activated magnetization flip of a single domain nano-magnet associated with the conductor comprising the step of increasing the temperature of the single domain nano- magnet above the thermal blocking temperature, said thermal blocking temperature being the temperature below which the thermally activated magnetization flip of the single domain nano-magnet will not occur.

2. Indirect thermal energy to electric energy conversion using the magnetization flip of a thermally active single domain nano-magnet in order to induce an alternating current signal in a conductor according to the method defined in claim 1. 3. Energy harvesting device, comprising:

a) a single domain nano-magnet associated with a conductor wherein a thermally activated magnetization flip of a single domain nano-magnet induces an alternating current signal in said conductor; and

b) a semiconductor element being connected to said conductor.

4. Energy harvesting device according to claim 3, wherein the semiconductor element is a diode and/or a rectifier. 5. Energy harvesting device according to claim 3 or 4 wherein the magnetization flip of a single domain nano-magnet is induced by an external magnetic field.

6. Energy harvesting device according to any of the preceding claims 3 to 5, wherein an amplifier is provided to amplify the alternating current signal generated by the magnetization flip of the single domain nano-magnet; said magnetization flip being activated thermally and/or by an external magnetic field, being preferably used as temperature sensor magnetic field marker sensor.

Description:
Thermally active nanomagnets for energy harvesting and sensor applications

The present invention relates to thermally active nanomagnets for energy harvesting and/or sensor applications. The

invention further relates to a method for converting thermal energy into electric energy.

There is a huge demand for low power energy sources

worldwide. Potential market for the proposed here energy harvesting device includes, for example, watches, wearables, sensors, etc. Approximately 1.2 billion watches are sold worldwide annually and the industry is worth $40 billion a year with Switzerland having 54% of the market. Wearables are a very fast growing market currently worth $14 billion and are projected to be worth $34 billion in 2020. Sensors might not need power all the time, the sensor can wake up only once in a while, measure something, send the signal and then fall asleep again. Internet of things, the name for smart devices or sensors to collect and exchange data, needs an autonomous low-power energy source. Internets of Things (IoT) sensors are projected to be a $39 billion market by 2022.

Single domain nanoscale magnets are typically used as

building blocks for artificial spin systems, where they are arranged on two-dimensional periodic lattices, the most famous examples are artificial square spin ice and artificial kagome spin ice, see Fig. 1. These systems have attracted a great interest as a fertile playground to test classical and unconventional models of frustrated magnetism, spin ice and spin liquid physics. The nanomagnets commonly used are elongated Permalloy (Ni80Fe20) nano-islands with the size sufficiently small that the atomic spins are

ferromagnetically aligned in a single domain. Due to the magnetic shape anisotropy in ellipsoidal nanomagnets, they are magnetized along the long axis of the ellipsoid. These tiny magnets are manufactured in the cleanroom using the state-of-the-art electron beam nanolithography . The present invention therefore has the objective to provide a method and a device for the conversion of thermal energy into electric energy in an efficient way without having moving parts.

These objectives are achieved according to the present invention by the features given in the independent claims 1, 2 and 3. The present invention beneficially uses the physical property that thermally activated single domain nano-magnets rapidly switch the direction of their magnetization above certain temperature (hereinafter also called blocking

temperature) . The associated rapid change in magnetic flux around the nanomagnet induces electromotive force in the nearby placed conductor and can be used for energy harvesting and/or sensor applications. The heat required to switch the magnetization of the nanomagnet is transformed indirectly into electric current and this is performed at the nanoscale with no moving parts. Combination of a thermally active single domain nanomagnet with a conductor and current

rectifier can be used for sensing applications or low-power energy harvesting from waste heat, solar energy or

potentially ambient heat sources.

According to the present invention an innovative design is disclosed for performing energy harvesting from heat directly at the nanoscale. Solutions for this task according to the present invention are listed in the independent claims 1, 2 and 3 as mentioned above. Advantageous embodiments of the present invention are given in the remaining dependent claims.

The advantage of the proposed device is that it has no moving parts, which means the lifetime of the device is

theoretically infinite, similar to the lifetime of the photovoltaic solar cells. Because the magnetization dynamics of the nanomagnets can be tuned in dependency from the material, the size of the nano-magnet, the orientation of its magnetic axis, the working temperature of the device can also be tuned. Such a power unit can harvest energy from heat sources and can be used for sensors installed at remote locations without reliable wind or solar power where changing the battery is too costly. The use of the proposed energy harvesting device will have direct benefits to the society by reducing the pollution caused by the low-power batteries around the world.

Embodiments of the present invention are hereinafter

described in more detail with reference to the attached drawings .

Figure 1 (a) shows a schematic drawing (left) and a magnetic force microscopy image (right) of artificial square spin ice. Figure 1 (b) depicts a schematic drawing (left) and a magnetic force microscopy image (right) of artificial kagome spin ice, black and white contrast correspond to the north and south poles of the nanomagnet respectively. Figure 1 (c) is a scanning electron microscopy image of artificial square ice consisting of Permalloy nanomagnets. The field of view is approximately 1.5 μιη.

An important development in this invention was the creation of thermally active nanomagnets, i.e. nanomagnets with spontaneously fluctuating magnetic moments at room

temperature. This property of the nanomagnets is a key element to the conversion of thermal energy to electric energy. The spontaneous fluctuation is induced by reducing the energy barrier between the two energetically equivalent stable moment orientations in a single nanomagnet.

As can be seen in the schematic drawing in Fig. 2(a), the magnetization of the nanomagnet can point to the left or to the right. The energy barrier is reduced by reducing the volume of the island and/or by increasing the temperature, until thermal energy k B T is of the same order as the barrier energy KV, where k B is the Boltzman constant, T is

temperature, K is the shape anisotropy constant and V is the volume of the nanomagnet. At this point thermal fluctuations induce spontaneous flipping of the magnetization and the temperature T b at which this happens is called the blocking temperature. T b can be controlled in a broad temperature range and can be estimated from zero field-cooled and field- cooled (ZFC/FC) magnetization measurements performed with a magnetometer, as shown in Fig. 2(b) .

A characteristic peak at T b represents a crossover from static to dynamic behaviour. This dynamics can be used for thermal energy harvesting. Figure 2(a) depicts a schematic drawing of a single nanomagnet with two possible

magnetization states and the energy barrier between them. Figure 2 (b) shows zero-field-cooled and field-cooled

magnetization.

(ZFC/FC) measurements of artificial kagome spin ice sample performed with a Superconducting Quantum Interface Device (SQUID) magnetometer. In this example the blocking

temperature Tb is equal to 160 K (indicated with an arrow) .

The present inventions propose to use thermal magnetization flip of the nanomagnet and associated change in magnetic flux around the nanomagnet to induce small bursts of current that can be extracted using metallic conductors or a metallic strip combined with a diode or a diode bridge or a rectifier.

A schematic drawing of the flipping process is shown in Fig.

3 for both in-plane (Permalloy Ni80Fe20) and out-of-plane nanomagnets (Co/Pt multilayers) . Metallic strip is a thin layer of metal deposited on top of a dielectric substrate.

The magnetization of the nanomagnet is confined to point along a single axis, either using the shape anisotropy or interface anisotropy. The magnetisation can point in two directions along this axis, thus creating two stable

configurations with an energy barrier between them, see Fig.

2. To confine magnetization, either the shape anisotropy is used for in-plane nanomagnets or the interface anisotropy is used for out-of-plane nanomagnets. Both geometries can be used for thermal energy harvesting. The method for manufacturing thermally active out-of-plane nanomagnets using Co/Pt

multilayers was developed only recently and is described in the inventor's PhD thesis.

The current generation is based on the phenomenon of

electromagnetic induction. During the magnetization flipping of the nanomagnet, magnetic flux generated by the nanomagnet rapidly changes and this induces electromotive force in the conductive metallic strip where Φ is the magnetic flux penetrating the metallic strip, t is time and E is the electromotive force induced in the conductor or strip. This electromotive force will result in a short burst of current in the strip. Employing proper strip geometry this current can be extracted and used as a power source. Devices based on the phenomenon of

electromagnetic induction are known to have very high

efficiency of energy conversion, therefore with proper conductor geometry high efficiency of thermal to electric energy conversion is achieved.

In Fig. 3, the magnetisation flip from up to down direction is described; however, the nanomagnet' s magnetisation will also flip from down to up direction. The generated current will switch polarity, or, in other words, an alternating current will be generated. This alternating current can be converted to the direct current using either a single diode (nano-diode) or a rectifier (nano-rectifier) comprising diodes (nano-diodes ) , such as Schottky or metal-oxide-metal nano-diodes, for example Ni-NiO-Cr/Au . These diodes have low forward voltage drop and a very fast switching action. One of the main challenges is to combine a low-power nano- rectifier with the nano-magnet and design a circuit for efficient extraction of electric current. A nano-diode/nano- rectifier probably will have to be kept at a lower

temperature compared to the temperature of nano-magnets. This is required in order to keep the thermal voltage of a diode lower than the induced voltage in the metallic strip.

Therefore, the device requires a temperature difference between the nano-magnets and the diodes. Using numerical simulations, it is estimated that one flip of a typical single domain nanomagnet generates about 3-10 -12 W of electric power in the strip at around 50 °C. With the nanomagnet density of 1 nanomagnet per 800 x 800 nm 2 and taking into account the losses in the rectifier, one potentially gets about 0.15 μΐ/ί continuous power per cm 2 . This is already enough to power an ultra-low-power quartz resonator used in electronic watches. By stacking the layers of the nanomagnets one could potentially reach higher output power suitable for applications like sensors or wearable devices. Heat can be supplied to the nano-magnets either through the heat exchange or from the solar radiation. The magnetization dynamics of the nanomagnets can be tuned to change the working

temperature of the device. This is achieved by changing the volume of the nanomagnets, for example increasing the

thickness increases the blocking temperature and therefore slows down magnetization dynamics.

Nanoscale temperature sensor can be manufactured based on the principle described above. Instead of using a diode or a rectifier, the alternating current generated by a thermally active nanomagnet can be amplified and the frequency of the alternating current can be used to measure temperature. The frequency will increase with temperature following Neel- Arrhenius equation: where o ~ 10 s is the attempt time or the inverse attempt frequency .

Nanoscale magnetic field marker sensor can be manufactured based on the principle described above. Instead of using a thermally active nanomagnet combined with a diode or a rectifier, a nanomagnet with higher volume is used with a blocking temperature higher than the temperature of the environment. When the external magnetic field reaches a specific magnetic field - coercive field of the nanomagnet, the moment reorientation will generate a current pulse that can be amplified and detected. Thus, a magnetic field marker sensor can be built. Figure 3 shows a schematic drawing of the current generation during a flip of (a) an in-plane magnetized nanomagnet (top view) and (b) an out-of-plane magnetized nanomagnet (side view) . Schematic drawings from top to bottom show temporal snapshots of the flipping process with the time axis pointing down. The diode allows the extraction of electric current.