rus
Issue #1/2025
D.V.Vasilyev, A.N.Saurov, V.V.Amelichev
HIGH-SENSITIVE MAGNETIC FIELD TRANSDUCER BASED ON SPIN-TUNNEL MAGNETORESISTIVE NANOSTRUCTURES WITH SYNTHETIC ANTIFERROMAGNET
HIGH-SENSITIVE MAGNETIC FIELD TRANSDUCER BASED ON SPIN-TUNNEL MAGNETORESISTIVE NANOSTRUCTURES WITH SYNTHETIC ANTIFERROMAGNET
DOI: https://doi.org/10.22184/1993-8578.2025.18.1.60.69
The results of a study of mock-ups of magnetic field transducers (MFT) based on spin-tunnel magnetoresistive nanostructures (STMR) with a synthetic antiferromagnet (SAF) are presented. The absolute sensitivity to the magnetic field of the studied MFT-SAF mock-ups was 217 mV/Oe in the magnetic field range ±5 Oe (±0.5 mT) at a supply voltage of 5 V.
The results of a study of mock-ups of magnetic field transducers (MFT) based on spin-tunnel magnetoresistive nanostructures (STMR) with a synthetic antiferromagnet (SAF) are presented. The absolute sensitivity to the magnetic field of the studied MFT-SAF mock-ups was 217 mV/Oe in the magnetic field range ±5 Oe (±0.5 mT) at a supply voltage of 5 V.
Теги: magnetic field transducer magnetic flux concentrators spin-tunnel magnetoresistive nanostructures synthetic antiferromagnet wheatstone bridge scheme концентраторы магнитного поля мостовая схема преобразователь магнитного поля синтетический антиферромагнетик спин-туннельные магниторезистивные наноструктуры
INTRODUCTION
A number of modern scientific and technical tasks are difficult to solve without the use of highly sensitive magnetic field transducers (MFTs). One of the most promising design solutions for converting weak magnetic fields into an electrical signal are spin-tunnel magnetoresistive (STMR) nanostructures. In STMR nanostructures, the change in resistance results from electron tunnelling in parallel and antiparallel magnetic configuration of ferromagnetic layers, and the magnetoresistive effect can exceed 200% [1]. Magnetic tunnelling junctions (MTJ) usually consists of two ferromagnetic layers (Fe, Ni, Co alloys) with a barrier layer (MgO, Al2O3) between them, with one of the ferromagnetic layers connected to the antiferromagnetic layer (IrMn, PtMn, FeMn) by exchange coupling and is called the “fixed” layer. The magnetisation of the second ferromagnetic layer changes in small magnetic fields, and this layer is called the “free” layer. In STMR nanostructures based on amorphous Al2O3 film, electron tunnelling is incoherent, resulting in a magnetoresistive effect of less than 70% [2]. The CoFeB / MgO / CoFeB structure in MTJ provides coherent tunnelling of electrons through the coordinated structure of crystalline layers, thus increasing the magnetoresistive effect up to 600% [3].
The replacement of the fixed layer in MTJ by a synthetic antiferromagnetic agent (SAF) contributes to reduction of the magnetostatic interaction in STMR nanostructures between the fixed and free layers. SAF is usually represented by two ferromagnetic layers separated by a non-magnetic layer (Ru, Cu, Ag) and interconnected by an indirect exchange coupling, which has an oscillatory character. The SAF structure has a closed magnetic configuration and concentrates the magnetic field on a fixed layer, minimising its effect on the free layer.
At the same time, the SAF influence on the crystallisation mechanism of amorphous CoFeB films as a result of thermomagnetic treatment (TMT) is known [4]. In two types of structures, with a fixed CoFe (2.5 nm) / CoFeB (3 nm) layer and with a fixed layer with SAF CoFe (2.5 nm) / Ru (2.5 nm) / CoFeB (3 nm), it was found that the separating Ru layer in SAF provides crystallisation of amorphous CoFeB films from the side of the MgO barrier layer, forming a coordinated crystal structure of CoFeB / MgO / CoFeB layers and more efficient tunnelling of electrons, which ultimately has a positive effect on the main magnetic parameters and their stability.
Thus, the incorporation of SAF into the STMR nanostructure enables a number of advantages in the following main parameters:
increased magnetoresistive effect (more than 50%);
small displacement of the centre of the hysteresis loop relative to the zero value of the magnetic field;
stability of magnetic parameters after magnetic annealing.
The range of measured magnetic fields using STMR-based on MFTs nanostructures is from a few tens of picotesla (pT) to a few tens of millitesla (mT) [5–7]. This is one of the largest dynamic operating ranges of known MFTs that can be fabricated using IC technology.
MFTs-SAF find effective application in systems of control of movement and rotation of objects, control of electric current, detection of ferromagnetic objects and structural defects of ferromagnetic materials, counting of magnetic micro- and nanoparticles in biological material analysis, in information storage devices with limited energy resource.
Taking into account the advantages of STMR nanostructures with SAF, the new design and technological solutions development on their basis in the field of creating highly sensitive magnetic field transducers is an actual and promising direction of research.
EXPERIMENTS
The indirect exchange interaction between ferromagnets in SAF has an oscillatory character [8], with the first antiferromagnetic maximum having the highest energy, resulting in the remagnetisation of the fixed layer at high magnetic fields. The composition and thicknesses of the films comprising the STMR nanostructure Ta / CoFe / CoFeB / MgO / CoFeB / Ru (8 Å) / CoFe / IrMn / Ta were selected in such a way as to ensure antiferromagnetic interaction of ferromagnetic layers in SAF and the minimum coercivity of the free layer. The sketch of the structure and its remagnetisation loop are presented in Fig.1.
The free layer remagnetisation loop shown in Fig.1b was obtained using a MESA-200 magnetic measurement system (Shb Instruments, USA). The remagnetisation loop of the fixed layer is not shown in this figure because the MESA-200 measuring system has a limited measurement range (±1000 Oe), while for the remagnetisation of SAF with the thickness of the non-magnetic layer corresponding to the first antiferromagnetic maximum, it is necessary to set a magnetic field of 2 kOe or more [8].
In order to apply MFTs-SAF over a wide temperature range, it is necessary to minimise the effect of temperature variation on the output signal [9]. One of the tools to solve this problem is the use of the Wheatstone bridge circuit (Fig.2). There are several variants of bridge circuits: with one active arm, half-bridge and full bridge circuit. In terms of MFTs applications, the full Wheatstone bridge circuit, where all four arms contribute to the output signal, is of most interest. In contrast to a single active arm circuit, a full bridge circuit changes the impedance of the four arms almost equally with temperature and the output signal has minimal deviation from zero. In an external magnetic field, resistance of one diagonal pair of arms of the circuit increases and the other decreases, this imbalance leads to appearance of a signal at the output of the bridge circuit [9].
According to the data presented in [9], in order to reduce the detection threshold, each arm of the MFTs-SAF should consist of N MTJ. On the other hand, as the number of STMR elements in the arm increases, resistance of the bridge circuit increases, which limits the frequency range of the transducer operation. The necessity of using a chain of series-connected MTJs in each arm of the bridge circuit determines the design and technological peculiarities of the MFTs-SAF creation. The design variant of series-connected STMR elements in the form of a sketch is shown in Fig.3.
The dependence of the resistance of a group of series-connected MTJs on the external magnetic field is shown in Fig.4. The coercive force of the free layer of this structure is equal to 35 Oe, the hysteresis loop displacement field with respect to the zero value of the magnetic field is absent, the magnetoresistive effect is 45%.
The curve in Fig.4 has a rectangular shape due to the parallel arrangement of the easy axis (EA) and unidirectional anisotropy (UDA). The EA direction is set during the sputtering of the STMR nanostructure, while the UDA results from the exchange interaction between ferromagnetic and antiferromagnetic layers [10]. For correct operation of MFT it is necessary that dependence has a linear form in the area of weak magnetic fields. For this purpose, linearisation methods are used, which ensure the orthogonal arrangement of the magnetisation vectors of the free and reference layers and reduction of the width of the hysteresis loop of the free layer [11]. In the studied MTJs, the non-collinear configuration was achieved using two-stage TMT and shape anisotropy. During the first stage of TMT at 300 °C, crystallisation of amorphous CoFeB films occurs, resulting in an increase in the magnetoresistive effect up to 79% (Fig.5), with the external magnetic field direction coinciding with the EA direction. The second stage of TMT was carried out at a temperature of 250 °C, the external magnetic field direction was directed to form a perpendicular arrangement of unidirectional and uniaxial anisotropy axes. In the process of two stages of TMT, the external magnetic field was set using a system of permanent magnets and was of the order of 1 kOe.
As a result of the second stage of TMT, the magnetoresistive effect and coercivity of the free layer are slightly reduced, the MTJs with the corrected characteristic can become the basis for the fabrication of highly sensitive MFTs-SAF.
One of the ways to increase sensitivity of magnetoresistive MFTs is introduction of magnetic flux concentrators (MFCs) into the design. As a rule, MFCs are made of magnetically soft ferromagnetic materials with high magnetic permeability, which makes it possible to amplify the useful signal several times and increase the signal-to-noise ratio [12]. As a rule, MFCs are extended rectangular parts, located at a small distance from each other, in the gap of which the sensitive elements of the transducer are located. Both the geometrical shape and the gap between them influence the gain of the MFCs [13].
Taking into account the above mentioned, the scientific team of the Scientific-Manufacturing Complex “Technological Centre” has developed and created a mock-up of the MFTs-SAF design, the photo of which is presented in Fig.6.
As a result of the study of the electrophysical characteristics of MFTs-SAF, it was found that the resistance of the bridge circuit can range from 2 to 20 MOhms, absolute sensitivity to the magnetic field, at a supply voltage of 5 V, reaches 217 mV/Oe in the range from minus 5 to 5 Oe (from minus 0.5 to 0.5 mT). The influence of the size of the gap between MFCs on sensitivity to the magnetic field of MFTs-SAF has been experimentally established. Figure 7 shows the volt-oersted characteristic of the MFTs-SAF mock-up with the gap between the MFCs 1.0 mm (blue curve), absolute sensitivity to the magnetic field (S) at a supply voltage of 5 V is 108 mV/Oe; the red curve of Fig.7 illustrates the output characteristic of the mock-up converter with the gap between the MFCs 0.35 mm, S = 217 mV/Oe. At the same time, the geometrical dimensions of MFCs of mock-up samples were the same.
In order to reduce the hysteresis of the volt-oersted characteristics of mock-up samples, linearisation methods are being developed that involve the introduction of additional magnetic components into the MFTs-SAF design.
Increasing the number of MTJs in each arm of the bridge circuit is necessary not only to reduce the detection threshold by magnetic field, but also to redistribute the supply voltage between the elements, which prevents the breakdown of the barrier layer and failure of the MFTs. In order to study the effect of supply voltage on the magnetic properties of MFTs-SAF, sensitivity to magnetic field of a mock-up sample was monitored at a fixed value of the external magnetic field and when the supply voltage was changed from 0 to 10 V with a step of 1 V (Fig.8).
At supply voltages higher than 5 V, the growth of absolute sensitivity slows down, and at a supply voltage of 8 V, the curve reaches saturation, which can be explained by the reduction of the magnetoresistive effect when the supply voltage to the MTJ is increased.
CONCLUSIONS
The studies of the created MFTs-SAF mock-up and the obtained results show the potentialities of the new technology of magnetosemiconductor microsystems, allowing to obtain devices with odd transfer characteristic and high magnetic field sensitivity. This opens up the possibility of creating highly sensitive MFT (217 mV/Oe and more) at low current consumption (less than 10 µA). The obtained experimental dependence of absolute sensitivity to magnetic field of MFT-SAF on the supply voltage shows high stability of MTJ with SAF at its change up to 10 V. The created mock-up samples of MFTs-SAF have no analogues of domestic production by the set of basic parameters and correspond to the products produced by one of the leaders of foreign manufacturers of magnetoresistive sensors based on STMR nanostructure technology [14].
The totality of the obtained results allows us to conclude about a wide range of application of MTJ with SAF: from an information storage element in a non-volatile memory cell to an array of magnetic introscope matrix elements. An important advantage of MTJ with SAF is its application versatility; it can be used both for magnetic field changes detection, for example, in biosensor devices, and to detect the magnetic induction vector direction as part of a highly sensitive MFTs-SAF. Due to their high sensitivity, MFTs-SAF can be integrated into magnetic introscopy devices, ferromagnetic object detection systems, low magnitude non-contact DC and AC current measurement transducers and other control and diagnostic devices.
ACKNOWLEDGEMENTS
This work was financially supported by the Ministry of Education and Science of Russia under the Scientific research “Theoretical and experimental studies of spin-tunnel magnetoresistive nanostructures with synthetic antiferromagnet for creation of highly sensitive magnetic field transducers and elements of non-volatile magnetoresistive memory cells”, agreement FNRM-2022-0010.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
A number of modern scientific and technical tasks are difficult to solve without the use of highly sensitive magnetic field transducers (MFTs). One of the most promising design solutions for converting weak magnetic fields into an electrical signal are spin-tunnel magnetoresistive (STMR) nanostructures. In STMR nanostructures, the change in resistance results from electron tunnelling in parallel and antiparallel magnetic configuration of ferromagnetic layers, and the magnetoresistive effect can exceed 200% [1]. Magnetic tunnelling junctions (MTJ) usually consists of two ferromagnetic layers (Fe, Ni, Co alloys) with a barrier layer (MgO, Al2O3) between them, with one of the ferromagnetic layers connected to the antiferromagnetic layer (IrMn, PtMn, FeMn) by exchange coupling and is called the “fixed” layer. The magnetisation of the second ferromagnetic layer changes in small magnetic fields, and this layer is called the “free” layer. In STMR nanostructures based on amorphous Al2O3 film, electron tunnelling is incoherent, resulting in a magnetoresistive effect of less than 70% [2]. The CoFeB / MgO / CoFeB structure in MTJ provides coherent tunnelling of electrons through the coordinated structure of crystalline layers, thus increasing the magnetoresistive effect up to 600% [3].
The replacement of the fixed layer in MTJ by a synthetic antiferromagnetic agent (SAF) contributes to reduction of the magnetostatic interaction in STMR nanostructures between the fixed and free layers. SAF is usually represented by two ferromagnetic layers separated by a non-magnetic layer (Ru, Cu, Ag) and interconnected by an indirect exchange coupling, which has an oscillatory character. The SAF structure has a closed magnetic configuration and concentrates the magnetic field on a fixed layer, minimising its effect on the free layer.
At the same time, the SAF influence on the crystallisation mechanism of amorphous CoFeB films as a result of thermomagnetic treatment (TMT) is known [4]. In two types of structures, with a fixed CoFe (2.5 nm) / CoFeB (3 nm) layer and with a fixed layer with SAF CoFe (2.5 nm) / Ru (2.5 nm) / CoFeB (3 nm), it was found that the separating Ru layer in SAF provides crystallisation of amorphous CoFeB films from the side of the MgO barrier layer, forming a coordinated crystal structure of CoFeB / MgO / CoFeB layers and more efficient tunnelling of electrons, which ultimately has a positive effect on the main magnetic parameters and their stability.
Thus, the incorporation of SAF into the STMR nanostructure enables a number of advantages in the following main parameters:
increased magnetoresistive effect (more than 50%);
small displacement of the centre of the hysteresis loop relative to the zero value of the magnetic field;
stability of magnetic parameters after magnetic annealing.
The range of measured magnetic fields using STMR-based on MFTs nanostructures is from a few tens of picotesla (pT) to a few tens of millitesla (mT) [5–7]. This is one of the largest dynamic operating ranges of known MFTs that can be fabricated using IC technology.
MFTs-SAF find effective application in systems of control of movement and rotation of objects, control of electric current, detection of ferromagnetic objects and structural defects of ferromagnetic materials, counting of magnetic micro- and nanoparticles in biological material analysis, in information storage devices with limited energy resource.
Taking into account the advantages of STMR nanostructures with SAF, the new design and technological solutions development on their basis in the field of creating highly sensitive magnetic field transducers is an actual and promising direction of research.
EXPERIMENTS
The indirect exchange interaction between ferromagnets in SAF has an oscillatory character [8], with the first antiferromagnetic maximum having the highest energy, resulting in the remagnetisation of the fixed layer at high magnetic fields. The composition and thicknesses of the films comprising the STMR nanostructure Ta / CoFe / CoFeB / MgO / CoFeB / Ru (8 Å) / CoFe / IrMn / Ta were selected in such a way as to ensure antiferromagnetic interaction of ferromagnetic layers in SAF and the minimum coercivity of the free layer. The sketch of the structure and its remagnetisation loop are presented in Fig.1.
The free layer remagnetisation loop shown in Fig.1b was obtained using a MESA-200 magnetic measurement system (Shb Instruments, USA). The remagnetisation loop of the fixed layer is not shown in this figure because the MESA-200 measuring system has a limited measurement range (±1000 Oe), while for the remagnetisation of SAF with the thickness of the non-magnetic layer corresponding to the first antiferromagnetic maximum, it is necessary to set a magnetic field of 2 kOe or more [8].
In order to apply MFTs-SAF over a wide temperature range, it is necessary to minimise the effect of temperature variation on the output signal [9]. One of the tools to solve this problem is the use of the Wheatstone bridge circuit (Fig.2). There are several variants of bridge circuits: with one active arm, half-bridge and full bridge circuit. In terms of MFTs applications, the full Wheatstone bridge circuit, where all four arms contribute to the output signal, is of most interest. In contrast to a single active arm circuit, a full bridge circuit changes the impedance of the four arms almost equally with temperature and the output signal has minimal deviation from zero. In an external magnetic field, resistance of one diagonal pair of arms of the circuit increases and the other decreases, this imbalance leads to appearance of a signal at the output of the bridge circuit [9].
According to the data presented in [9], in order to reduce the detection threshold, each arm of the MFTs-SAF should consist of N MTJ. On the other hand, as the number of STMR elements in the arm increases, resistance of the bridge circuit increases, which limits the frequency range of the transducer operation. The necessity of using a chain of series-connected MTJs in each arm of the bridge circuit determines the design and technological peculiarities of the MFTs-SAF creation. The design variant of series-connected STMR elements in the form of a sketch is shown in Fig.3.
The dependence of the resistance of a group of series-connected MTJs on the external magnetic field is shown in Fig.4. The coercive force of the free layer of this structure is equal to 35 Oe, the hysteresis loop displacement field with respect to the zero value of the magnetic field is absent, the magnetoresistive effect is 45%.
The curve in Fig.4 has a rectangular shape due to the parallel arrangement of the easy axis (EA) and unidirectional anisotropy (UDA). The EA direction is set during the sputtering of the STMR nanostructure, while the UDA results from the exchange interaction between ferromagnetic and antiferromagnetic layers [10]. For correct operation of MFT it is necessary that dependence has a linear form in the area of weak magnetic fields. For this purpose, linearisation methods are used, which ensure the orthogonal arrangement of the magnetisation vectors of the free and reference layers and reduction of the width of the hysteresis loop of the free layer [11]. In the studied MTJs, the non-collinear configuration was achieved using two-stage TMT and shape anisotropy. During the first stage of TMT at 300 °C, crystallisation of amorphous CoFeB films occurs, resulting in an increase in the magnetoresistive effect up to 79% (Fig.5), with the external magnetic field direction coinciding with the EA direction. The second stage of TMT was carried out at a temperature of 250 °C, the external magnetic field direction was directed to form a perpendicular arrangement of unidirectional and uniaxial anisotropy axes. In the process of two stages of TMT, the external magnetic field was set using a system of permanent magnets and was of the order of 1 kOe.
As a result of the second stage of TMT, the magnetoresistive effect and coercivity of the free layer are slightly reduced, the MTJs with the corrected characteristic can become the basis for the fabrication of highly sensitive MFTs-SAF.
One of the ways to increase sensitivity of magnetoresistive MFTs is introduction of magnetic flux concentrators (MFCs) into the design. As a rule, MFCs are made of magnetically soft ferromagnetic materials with high magnetic permeability, which makes it possible to amplify the useful signal several times and increase the signal-to-noise ratio [12]. As a rule, MFCs are extended rectangular parts, located at a small distance from each other, in the gap of which the sensitive elements of the transducer are located. Both the geometrical shape and the gap between them influence the gain of the MFCs [13].
Taking into account the above mentioned, the scientific team of the Scientific-Manufacturing Complex “Technological Centre” has developed and created a mock-up of the MFTs-SAF design, the photo of which is presented in Fig.6.
As a result of the study of the electrophysical characteristics of MFTs-SAF, it was found that the resistance of the bridge circuit can range from 2 to 20 MOhms, absolute sensitivity to the magnetic field, at a supply voltage of 5 V, reaches 217 mV/Oe in the range from minus 5 to 5 Oe (from minus 0.5 to 0.5 mT). The influence of the size of the gap between MFCs on sensitivity to the magnetic field of MFTs-SAF has been experimentally established. Figure 7 shows the volt-oersted characteristic of the MFTs-SAF mock-up with the gap between the MFCs 1.0 mm (blue curve), absolute sensitivity to the magnetic field (S) at a supply voltage of 5 V is 108 mV/Oe; the red curve of Fig.7 illustrates the output characteristic of the mock-up converter with the gap between the MFCs 0.35 mm, S = 217 mV/Oe. At the same time, the geometrical dimensions of MFCs of mock-up samples were the same.
In order to reduce the hysteresis of the volt-oersted characteristics of mock-up samples, linearisation methods are being developed that involve the introduction of additional magnetic components into the MFTs-SAF design.
Increasing the number of MTJs in each arm of the bridge circuit is necessary not only to reduce the detection threshold by magnetic field, but also to redistribute the supply voltage between the elements, which prevents the breakdown of the barrier layer and failure of the MFTs. In order to study the effect of supply voltage on the magnetic properties of MFTs-SAF, sensitivity to magnetic field of a mock-up sample was monitored at a fixed value of the external magnetic field and when the supply voltage was changed from 0 to 10 V with a step of 1 V (Fig.8).
At supply voltages higher than 5 V, the growth of absolute sensitivity slows down, and at a supply voltage of 8 V, the curve reaches saturation, which can be explained by the reduction of the magnetoresistive effect when the supply voltage to the MTJ is increased.
CONCLUSIONS
The studies of the created MFTs-SAF mock-up and the obtained results show the potentialities of the new technology of magnetosemiconductor microsystems, allowing to obtain devices with odd transfer characteristic and high magnetic field sensitivity. This opens up the possibility of creating highly sensitive MFT (217 mV/Oe and more) at low current consumption (less than 10 µA). The obtained experimental dependence of absolute sensitivity to magnetic field of MFT-SAF on the supply voltage shows high stability of MTJ with SAF at its change up to 10 V. The created mock-up samples of MFTs-SAF have no analogues of domestic production by the set of basic parameters and correspond to the products produced by one of the leaders of foreign manufacturers of magnetoresistive sensors based on STMR nanostructure technology [14].
The totality of the obtained results allows us to conclude about a wide range of application of MTJ with SAF: from an information storage element in a non-volatile memory cell to an array of magnetic introscope matrix elements. An important advantage of MTJ with SAF is its application versatility; it can be used both for magnetic field changes detection, for example, in biosensor devices, and to detect the magnetic induction vector direction as part of a highly sensitive MFTs-SAF. Due to their high sensitivity, MFTs-SAF can be integrated into magnetic introscopy devices, ferromagnetic object detection systems, low magnitude non-contact DC and AC current measurement transducers and other control and diagnostic devices.
ACKNOWLEDGEMENTS
This work was financially supported by the Ministry of Education and Science of Russia under the Scientific research “Theoretical and experimental studies of spin-tunnel magnetoresistive nanostructures with synthetic antiferromagnet for creation of highly sensitive magnetic field transducers and elements of non-volatile magnetoresistive memory cells”, agreement FNRM-2022-0010.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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