Introduction

The advent of the 4th Industrial Revolution has highlighted the crucial role of Internet of Things (IoT) sensors in collecting extensive data and facilitating automated control across various domains, including public safety, industrial manufacturing, and environmental monitoring1,2,3,4,5,6,7. The deployment of the fifth-generation (5G) network is anticipated to support connectivity for up to 107 devices per km², and with the rapid expansion of IoT devices, the sixth-generation (6G) network is projected to sustain a connection density of 108 devices per km² by 20306,8,9,10. However, the widespread implementation of IoT systems, which encompasses environmental sensors, data loggers, management controllers, and wireless transceivers, faces substantial challenges related to ensuring a reliable power supply11,12,13,14. Utilizing capacity-limited batteries or installing external power cables for the expected billions of IoT sensors is impractical due to the high labor and financial costs associated with the installation and maintenance of these systems15,16,17. A promising solution to these challenges is the integration of energy harvesting technologies into individual IoT devices. This integration could render IoT sensing devices battery-free, sustainable, and autonomous, thereby significantly enhancing the feasibility and scalability of IoT deployments18,19,20.

Magneto-mechano-electric (MME) generators, which utilize ambient magnetic fields (typically below 10 Oe), exhibit significant potential for powering standalone IoT sensors21,22,23. These ambient magnetic fields at a specific alternating current (AC) frequency are ubiquitous, originating from power cables in electric transmission lines, infrastructures, factories, and residential settings. MME generators, developed with a variety of composite structures, incorporate energy conversion elements including piezoelectric, triboelectric, magnetostrictive, and magnetic materials24,25,26,27. These devices initially convert AC magnetic fields into vibrations through the magnetostriction effect or magnetic force, which are subsequently transformed into electricity via the piezoelectric or triboelectric effect1,4,28.

To enhance the performance of MME generators, extensive research has focused on optimizing component materials, improving structural designs, and exploring novel application-specific configurations23,29,30,31,32,33. Recently, a high-power MME generator was developed by hybridization of a piezoelectric single-crystalline Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 (PMN-PZT) produced through solid-state crystal growth (SSCG) and an electromagnetic induction (EMI) solenoid34. This generator achieved a notable total root mean square (RMS) power output of 51.7 mW (24.1 mW from the PMN-PZT component and 27.6 mW from the EMI component), corresponding to a power density of 4.43 mW/cm³, under a small AC magnetic field (7 Oe) at an optimal impedance matching condition34. Despite the high MME output reported in previous studies utilizing SSCG-based PMN-PZT crystals, these crystals are inherently prone to internal pore formation. This limitation arises from the nature of the SSCG process, which typically uses a sintered piezoelectric polycrystal as the source material. The SSCG process often leads to the formation of pores due to excess lead in the mother ceramic material, consequently resulting in reduced piezoelectric properties in the grown crystals35,36. These internal pores can act as defect centers that degrade domain alignment and electromechanical coupling efficiency, thereby limiting the energy conversion performance of the final harvesting device. Conversely, the modified Bridgman process for growing piezoelectric crystals, including second-generation Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT), involves the slow cooling of a molten mixture of raw materials from a high temperature37,38. The directional solidification technique enhances compositional uniformity and reduces internal porosity, resulting in single crystals with improved structural integrity, piezoelectric, the mechanical quality factor (Qm), and dielectric properties. This melt-based method enables the growth of single crystals with fewer internal pores and enhanced piezoelectric performance, making it a more effective approach for fabricating high-quality piezoelectric materials39.

Herein, we report an MME generator with an RMS power exceeding 0.1 W, utilizing Mn-doped piezoelectric PIN-PMN-PT crystals grown by the modified Bridgman method to demonstrate multiple battery-free IoT environment monitoring systems. The incorporation of acceptor Mn ions into the crystal structure aims to improve Qm and decrease the dielectric loss (tanδ), both of which are critical for efficient energy harvesting28,40. Although Mn doping for enhancing the Qm has been widely reported in Pb-based piezoelectric materials, our study demonstrates its application in a high-performance PIN-PMN-PT crystal-based MME generator that can continuously drive multiple IoT devices, achieving the RMS output under small magnetic field excitation41,42. Our approach includes finite element analysis (FEA) simulation to estimate the impact of acceptor doping on the harvesting performance of MME generators. The Mn-doped MME generator produced a significant improvement in RMS power output, achieving a maximum of 106.2 mW at a load resistance of 20 kΩ under AC magnetic field of 7 Oe, which is nearly 2.2 times higher than the output of the undoped generator. The calculated RMS power density is 35.8 mW/cm³, approximately 8 times higher than that of the recently reported hybrid-type MME generator (comparison data in Table S1, see the Supplementary Information) 34. Moreover, we investigated the practical application of the PIN-PMN-PT MME harvester in powering multiple multifunctional IoT devices, establishing a battery-free configuration. Real-time environmental monitoring data were displayed on smartphone applications, highlighting the potential of the Mn-doped MME generator for integration into sustainable and autonomous IoT applications. This integration of advanced crystal growth with device-level application distinguishes our work from previous SSCG-based studies and represents a critical advancement in both materials design and functional demonstration.

Results

Fabrication process of MME generator and characterization of PIN-PMN-PT crystals

Even though single-crystalline PIN-PMN-PT exhibits a remarkable piezoelectric charge constant (d33) of up to approximately 2700 pC/N in bulk form, they typically demonstrate a relatively low Qm and high tanδ43. These characteristics can hinder the performance of high-efficiency piezoelectric MME generators due to increased energy losses under resonance frequency operation conditions. To enhance Qm and tanδ properties, restricting polarization switching and clamping the motion of the domain wall can be employed by the introduction of the hardening effect, leading to improved energy conversion efficiency28. These improvements could be achieved by the addition of acceptor ions into the B-site of perovskite ABO3-type piezoelectric single crystals. The incorporation of acceptor ions increases the oxygen vacancy concentration and enhances the electric internal bias, thereby enhancing the mechanical and dielectric properties of the piezoelectric material40,43,44. Among various acceptor dopants reported for Pb-based piezoelectric systems, such as Fe, Co, and Cu, Mn has shown particularly favorable effects45. Mn forms defect dipoles with oxygen vacancies, which effectively pin domain walls and suppress their mobility. This mechanism enhances the Qm without significantly degrading the piezoelectric coefficients, thereby maintaining both mechanical robustness and energy conversion performance. In addition to its functional advantages, the ionic radius of Mn²⁺ or Mn³⁺ is well matched to that of the B-site cations in the ABO3 perovskite structure40. This ionic compatibility reduces lattice strain and phase instability, facilitating efficient lattice incorporation45. In contrast, other dopants such as Cu or Fe may cause excessive distortion or redox-related degradation, potentially lowering long-term device reliability. Therefore, Mn-doped third-generation PIN-PMN-PT crystals with low tanδ and high Qm values represent a promising piezoelectric material for MME generators46,47. These enhanced properties could significantly improve energy harvesting efficiency under resonance conditions, making them suitable for high-performance MME applications in standalone IoT sensors and other related technologies28.

Figure 1a illustrates the schematic process for fabricating the acceptor Mn-doped perovskite structure of single-crystalline PIN-PMN-PT for application in MME generators. As shown in Fig. 1a (i), Mn ions are substituted at the B-site (In3+, Nb5+, Mg2+, and Ti4+) of the PIN-PMN-PT lattice to create mobile oxygen vacancies, thereby enhancing the hard-piezoelectric properties, including high Qm, low tanδ, and high coercive field (Ec)48. These enhancements can be introduced through two mechanisms: (1) The formation of oxygen vacancies increases the Young’s modulus of the piezoelectric crystals by a hardening effect, deriving in a higher Qm. (2) The introduced oxygen vacancies impede switching of spontaneous polarization and hinder the domain wall movement within the single crystal PIN-PMN-PT, resulting in an improved electric internal bias49. The undoped and Mn-doped (011) PIN-PMN-PT crystals were synthesized using the modified Bridgman method50,51. After the deposition of top and bottom Au electrodes, the PIN-PMN-PT crystal plate (thickness of 200 µm) was bonded to a Ti cantilever plate (300 µm in thickness) by epoxy adhesion, and a polydimethylsiloxane (PDMS) passivation layer was coated onto the cantilever structure, as shown in Fig. 1a (ii). When a longitudinal AC magnetic field was induced to the MME cantilever structure, the cantilever beam with fixed magnets oscillated due to the magnetic force. And then, this mechanical oscillation was transduced into electric energy by the PIN-PMN-PT crystal, as depicted in Fig. 1a (iii). Figure 1b presents an image of the setup to measure the output properties of the piezoelectric MME device. The inset of Fig. 1b presents one end of the MME harvester fixed by a clamping part, and NdFeB magnets serve as both the magnetic force generator and the proof mass at the other end of the cantilever. The assembled MME generator was positioned between Helmholtz coils, which generated a magnetic torque via the AC magnetic field.

Fig. 1: Fabrication process and dielectric/ferroelectric characterization of Mn-doped PIN-PMN-PT single crystals.
figure 1

a Schematic illustration of Mn element doping in a PIN-PMN-PT single crystal and the subsequent application of the MME generator. b Image of the setup to measure the output of the piezoelectric MME generator. The inset shows a top view of clamped PIN-PMN-PT generator. c EDS analysis of the top surface of the Mn-doped PIN-PMN-PT crystal, with the inset displaying an SEM image of the same surface. d Frequency variation of dielectric properties and e P-E hysteresis curves for undoped and Mn-doped PIN-PMN-PT crystals.

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The resonance (fr) and anti-resonance (fa) frequency values of PIN-PMN-PT crystals increased with Mn doping, as shown in the frequency-dependent impedance analyzer graph (refer to Supplementary Fig. S1). This alternation is attributed to the hardening effect by the doping of the Mn element into the piezoelectric material28. Specifically, the Qm increased to 515 for the Mn-doped sample, compared to 327 for the undoped sample. Figures 1c and S3 show the results of energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and elemental mapping performed on the top surface of the Mn-doped PIN-PMN-PT single crystal. The EDS spectrum confirms the presence of all necessary elements, with Pb, In, Nb, Mg, Ti, Mn, and O atoms uniformly distributed throughout the analyzed region, demonstrating the homogeneous composition of the piezoelectric crystal. Figure 1d illustrates the frequency-dependent dielectric constant (εr) and tanδ of undoped and acceptor-doped PIN-PMN-PT crystals, measured in the frequency range from 102 Hz to 104 Hz. The undoped PIN-PMN-PT exhibited an εr of 3309 and a tanδ of 0.00436 at 1 kHz, which decreased to 2288 and 0.00257, respectively, upon Mn doping. The dielectric constant as a function of temperature was measured for both undoped and Mn-doped PIN-PMN-PT single crystals to determine their phase transition temperatures, as shown in Figure S4 (see the Supplementary Information). For the undoped crystal, the rhombohedral-to-orthorhombic transition temperature (TRO), orthorhombic-to-tetragonal transition temperature (TOT), and Curie temperature (TC) were identified as 127 °C, 137 °C, and 185 °C, respectively. The Mn-doped crystal exhibited TRO, TOT, and TC at 143 °C, 156 °C, and 198 °C, respectively. Figure 1e presents the P-E hysteresis loops of undoped and Mn-doped PIN-PMN-PT single crystals, measured at a frequency of 10 Hz. The undoped sample showed a maximum polarization (Pm) of 41.37 µC/cm², a remnant polarization (Pr) of 38.16 µC/cm², and a coercive field (Ec) of 4.51 kV/cm. Upon Mn substitution, the Pr and Pm values decreased, while the Ec value increased. The observed reduction in dielectric and ferroelectric properties is ascribed to the domain wall pinning by the dopant.

The Mn ions substitute into the B-sites of the PIN-PMN-PT lattice owing to their similar ionic radii, creating higher oxygen vacancies to retain charge neutrality. The oxygen vacancy presence in the PIN-PMN-PT crystals was further investigated by X-ray photoelectron spectroscopy (XPS). Figure 2a shows the XPS graph of the O1s state for undoped and Mn-doped PIN-PMN-PT samples. The asymmetric O1s spectral band was deconvoluted into two peaks: the first peak (OI) near 529.5 eV corresponds to lattice oxygen, and the second peak (OII) near 531.5 eV is associated with oxygen vacancies52. The relative intensity of the OII peak increased with Mn doping, indicating a higher concentration of oxygen vacancies in the doped sample compared to the undoped sample53.

Fig. 2: Structural and chemical analysis revealing Mn-induced phase evolution in PIN-PMN-PT single crystals.
figure 2

a XPS spectra of the O1s state for undoped and Mn-doped PIN-PMN-PT single crystals. b XRD pattern of Mn-doped (011) PIN-PMN-PT crystal. The inset shows a magnified view of the diffraction peak located around 2θ = 65°. c Raman spectroscopy result of undoped and Mn-doped PIN-PMN-PT single crystals. d HRTEM image and illustration of the crystal structure of Mn-doped (001) PIN-PMN-PT crystal (i). Polarization map derived from atomic displacements in the HRTEM image, indicating the polarization direction (ii).

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Figures 2b and S5 (see the Supplementary Information) show the X-ray diffraction (XRD) results of both undoped and Mn-doped PIN-PMN-PT crystals. These XRD patterns reveal the presence of rhombohedral and tetragonal crystal structures, exhibiting nearly identical patterns for the (110) plane family, which is indicative of their single-crystal nature. The existence of rhombohedral and tetragonal phases in both undoped and Mn-doped PIN-PMN-PT samples is further confirmed by the splitting of the (220)R/(202)T/(220)T difraction peaks at a 2θ angle around 65°, as illustrated in the inset of Fig. 2b. Upon doping with Mn, there is a slight increase in the rhombohedral phase compared to the undoped sample, indicating the influence of Mn doping on the crystal structure.

Figure 2c presents the Raman spectra of PIN-PMN-PT crystals, measured over a range of 50–1000 cm−1. Both the undoped and Mn-doped samples exhibit nine distinct vibrational modes at 145, 204, 274, 336, 426, 512, 590, 764, and 807 cm−1, respectively. The A1g mode in the Raman spectra of 664–1000 cm−1 was observed into two distinct Raman modes of E(4LO) at 764 cm−1 and Rh at 807 cm−1, indicating the coexistence of tetragonal (P4mm) and rhombohedral (R3m) phases within the PIN-PMN-PT crystals. Furthermore, it was characterized that the amplitude of the A1g mode at 764 cm−1 decreased, while the amplitude at 807 cm−1 increased with Mn doping. This result indicates a reduction in the tetragonal element and an enhancement in the rhombohedral element contributions. The analysis findings from Raman spectroscopy are consistent with the XRD results, thereby providing comprehensive evidence of the structural modifications induced by Mn doping54.

In contrast to classical ferroelectrics, PbTiO3 (PT)-based relaxor ferroelectrics exhibit a unique feature known as polar nanoregions (PNRs), which play a significant role in enhancing the piezoelectric properties of these materials55. A high-resolution transmission electron microscopy (HRTEM) has enabled researchers to confirm the formation of PNRs in PT-based relaxor materials, providing detailed insights into their structural characteristics. In this study, we conducted similar atomic-level characterizations to further understand the role of PNRs in PIN-PMN-PT single crystals. The left side (i) of Fig. 2d illustrates the atomic positions of A and B-sites in Mn-doped (001) PIN-PMN-PT single crystals, with corresponding details for undoped samples available in Figure S6 (see the Supplementary Information). In these HRTEM images, the cations on A and B sites are marked by blue and green circles, respectively. However, the oxygen columns are not clearly visualized due to the weak scattering of electrons by oxygen atoms. The atomic displacements within a unit cell are represented by polar vectors (red arrows) that originate from the center of a B site cation (indicated by a green circle) and point towards the nearest surrounding A site cations (indicated by blue circles). As shown on the right side (ii) of Fig. 2d, polar vectors were observed along the rhombohedral <111> direction and the tetragonal <001> direction, indicating the presence of both structural phases. This HRTEM analysis confirms that Mn-doped PIN-PMN-PT crystals possess PNRs with a coexistence of rhombohedral and tetragonal nanoregions52. This coexistence is instrumental in contributing to the high piezoelectricity observed in these materials, as the presence of PNRs enhances the electromechanical coupling by facilitating the reorientation of dipoles under an induced electric field56,57. Doping Mn into PIN-PMN-PT single crystals has been observed to shift the equilibrium between the coexisting rhombohedral and tetragonal phases, leading to an increased proportion of the rhombohedral phase. This phase shift is accompanied by a reduction in tetragonality, as indicated by a decrease in the c/a ratio in Mn-doped samples compared to their undoped counterparts55. Specifically, as shown in Figure S7 (refer to Supplementary Information), the distribution of the c/a ratio derived from unit-cell scale mapping of HRTEM images (Figs. 2d and S6) reveals that the undoped sample exhibits a broader range of c/a ratios with a mean value of 1.034, whereas the Mn-doped sample shows a narrower distribution with a lower mean c/a ratio of 1.019. This reduction in tetragonality suggests enhanced stability of the rhombohedral phase, which typically exhibits less lattice distortion than the tetragonal phase58. The modification in phase composition due to Mn doping is expected to significantly influence the electromechanical coupling properties of the crystal, potentially improving its performance in specific piezoelectric applications.

Theoretical simulations, optimization, and output performance of the MME generator

Finite element analysis (FEA) software (COMSOL Multiphysics) was employed to estimate the piezoelectric potential distribution and normalized power output from the PIN-PMN-PT material59. This approach allowed for a comparative analysis of the performance enhancement of the MME generator using undoped and Mn-doped PIN-PMN-PT samples. To improve the piezoelectric MME output, the second harmonic bending mode was utilized on the cantilever structure. In cantilever-based MME generators with identical geometry, the resonant frequency of the second bending mode is typically an order of magnitude higher than that of the first bending mode34. Although the tip displacement is generally greater in the first bending mode, the second mode involves significantly more oscillation cycles per second, resulting in higher deformation velocity and acceleration across the cantilever. This increased dynamic deformation leads to larger strain rates in the piezoelectric layer, thereby enhancing energy conversion efficiency and maximizing power output34,60. The eigenfrequency mode of the harvesting device was investigated to estimate the deformation shape of the MME cantilever at the second resonance mode. In the software setup, one end of the MME device was fixed, while magnets were located at the other end in a simplified two-dimensional model. This simulation model comprised the PIN-PMN-PT, adhesion layer, Ti plate, PDMS passivation, and mass magnets. The metal electrodes (with a thickness of approximately 200 nm) coated on the piezoelectric single crystals were not included in the geometric model, as their thickness is significantly smaller than that of the piezoelectric layer (200 μm) and their mechanical influence is negligible61. Instead, the electrodes were represented using electrical boundary conditions (electric potential and ground) applied to the surfaces of the piezoelectric domain, following a widely accepted approach in finite element modeling of piezoelectric devices. In this simulation, sinusoidal displacement was applied to the location of the magnet to emulate vibration induced by the external AC magnetic field, and the excitation frequency was set to match the second bending resonance frequency obtained from the eigenfrequency analysis, which were 125.7 Hz for the undoped PIN-PMN-PT device and 125.2 Hz for the Mn-doped device. As depicted in the left side (i) of Fig. 3a, the mid-region of the cantilever has the largest displacement of mechanical vibration at the second harmonic bending mode. The upper section depicts the potential distribution inside the piezoelectric PIN-PMN-PT when subjected to the induced vibration on the magnets with the second resonance mode. The key material constants used for the COMSOL simulations of the undoped and Mn-doped PIN-PMN-PT samples, including elastic compliance and piezoelectric charge constant are summarized in Table S2 and S3 of the Supplementary Information62,63,64. Given the relationship between the Qm and the damping ratio (ζ), expressed as equation (1) Qm = 1/2ζ, the ζ values were incorporated into the simulation to reflect the Qm of 53.7 for undoped and 92.0 for Mn-doped PIN-PMN-PT samples, as extracted from the fully assembled MME devices (Figure S2 in the Supplementary Information)65. The right side (ii) of Fig. 3a presents the normalized power outputs for both undoped and Mn-doped PIN-PMN-PT MME generators as a function of external load resistance, calculated using the piezoelectric d32 mode in COMSOL software. The Mn-doped sample exhibited a maximum power output approximately 2.8 times greater than that of the undoped sample, further confirming the substantial enhancement in energy harvesting performance enabled by Mn doping. While this simulation does not perfectly reproduce the experimental results, it effectively captures the trend in power enhancement due to Mn doping. The simplified 2D geometry and modeling assumptions inevitably introduce certain limitations in quantitative accuracy, which are acknowledged in this study.

Fig. 3: Simulation and experimental comparison of enhanced output performance in Mn-doped MME generators.
figure 3

a Simulated analysis showing the relative piezopotential distribution on the PIN-PMN-PT layer and relative displacement of the MME generator at its second resonance bending mode (i). Normalized output power derived from simulations for both undoped and Mn-doped PIN-PMN-PT MME generators (ii). Experimental results displaying b open-circuit voltage, c short-circuit current, d RMS output voltage (i), and RMS power output (ii) for the undoped and Mn-doped PIN-PMN-PT MME generators.

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To characterize the effect of acceptor doping on MME energy harvesting, both undoped and Mn-doped d32 mode PIN-PMN-PT crystals were incorporated into an MME generator operating at an AC magnetic field of 7 Oe in the second resonance bending mode. Despite the same operation conditions, the undoped piezoelectric MME device shows a slightly lower resonance frequency (~118 Hz) compared to the Mn-doped PIN-PMN-PT crystal-based MME generator (~124 Hz) (Figure S8 in the Supplementary Information). This increase in resonance frequency is attributed to the hardening behavior induced by acceptor Mn doping, which enhances the Young’s modulus of the piezoelectric single crystal. Figures 3b and 3c present the open-circuit voltage and short-circuit current generated from the PIN-PMN-PT MME harvesters, respectively. The MME device with Mn-doped PIN-PMN-PT produced a maximum open-circuit output voltage of ~180 V (shown in Movie S1) and a maximum short-circuit output current of 8.2 mA, while the undoped PIN-PMN-PT-based sample generated only ~96 V and 4.1 mA, respectively. To assess the long-term operational reliability of the harvester, a durability test was conducted using the Mn-doped PIN-PMN-PT-based MME generator under continuous vibration for 107 cycles, as shown in Fig. S9 (see the Supplementary Information). During the test, the open-circuit voltage was continuously monitored, and only an 8.9% reduction in the maximum output voltage was observed at the end of the test. This result confirms the stable energy harvesting performance of device over prolonged operation.

The RMS voltage (Vrms) output values of the MME devices were recorded by changing the external resistance (R) from 500 Ω to 100 MΩ to calculate the RMS output power (Prms) of the undoped and Mn-doped samples, as shown in the righe side (i) of Fig. 3d. The RMS power generated from the piezoelectric harvester is estimated utilizing the equation (2) Prms = (Vrms)2/R, by input of measured Vrms at various load resistances. The left side (ii) of Fig. 3d shows that the doped MME harvester generated a maximum Prms of 106.2 mW at a resistance of 20 kΩ, which is nearly 2.2 times higher compared to the undoped PIN-PMN-PT MME device (Prms of 48.1 mW at a resistance of 20 kΩ). This exceptionally high RMS output is attributed to the adoption of Mn-doped PIN-PMN-PT crystal grown by the modified Bridgman method with improved electromechanical coupling efficiency at resonance operating condition via hardening effect. The RMS output power above 0.1 W is favorable for demonstrating practical battery-free IoT sensors near electric cables. The RMS output (106.2 mW) of this work is approximately 2.1 times higher than the previous hybrid-type MME generator (51.5 mW under an AC magnetic field of 7 Oe at an impedance matching condition) 34. The power density of the MME harvesting device is calculated by dividing the Prms by the volume (VMME) of the MME device. This metric, expressed as equation (3) Prms/VMME, provides a comprehensive measure of the efficiency of the energy harvesting device by considering both the electrical output and the physical device size. The total volume of the PIN-PMN-PT MME harvester is 2.97 cm3, including the volume of the single-crystalline PIN-PMN-PT (0.0864 cm3), adhesion layer (0.00864 cm3), Ti cantilever (0.408 cm³), PDMS passivation (0.068 cm3), and magnet proof mass (2.4 cm3). Given the RMS power of our piezoelectric MME harvesting device is 106.2 mW, the power density is calculated to be 35.8 mW/cm3, which is about 8 times higher than the recently reported hybrid-type MME harvester with an RMS power density of 4.44 mW/cm334.

Electronic applications using PIN-PMN-PT MME generator

To effectively utilize the electrical energy generated by the MME harvester, it is crucial to convert the AC output into direct current (DC) power66. This conversion is achieved using a commercial power management circuit (LTC 3588, Linear Technology, USA), which is designed to regulate the DC output to a stable 3.7 V. The LTC3588 is highly regarded for its low-power consumption and efficient energy conversion capabilities, which include a low-loss rectifier and DC converter. However, this integrated circuit is not specifically tailored for our MME generator, which can produce relatively high output voltages up to 180 V. Consequently, this voltage mismatch can lead to significant energy losses during the conversion process. To address this problem, it is essential to develop advanced energy conversion circuits specifically designed to manage high-voltage inputs66. This development would minimize energy loss and maximize efficiency, thereby enhancing the overall performance of energy harvesting systems. Figure 4a shows the circuit diagram used for practical electronic applications, integrating the piezoelectric MME generator, supercapacitor, and various electronic devices. The performance of the MME harvester was further assessed by not only charging various supercapacitors with different capacities (0.11, 0.22, and 0.47 F) but also subsequently powering an array of white light-emitting diodes (LEDs) and a dot laser diode. The charging times for the supercapacitors up to 3.7 V using the Mn-doped piezoelectric crystal were 40, 88, and 200 seconds, respectively, at an AC magnetic field of 7 Oe, as presented in the left side (i) of Fig. 4b. In contrast, the undoped MME generator required significantly longer charging times of 82, 178, and 400 seconds to charge the same supercapacitors, as shown in the right side (ii) of Fig. 4b. The charging power by generator into the 0.11 F supercapacitor was calculated to be 18.8 mW, with applying the equation (4) (E = 1/2·C·V2, where C is the capacitance of supercapacitor and V is the charged voltage) for stored energy in a capacitor. This corresponds to charging energy of 18.8 mJ per second, derived by dividing the stored energy of 0.75 J in the supercapacitor by the charging time of 41 s. Figure 4c shows continuous illumination of 20 white LEDs powered by the MME generator, providing sufficient light to read a textbook in a dark environment. Furthermore, the power generated by the MME generator was used to operate a red dot laser diode, which has a threshold voltage of 3 V and an operating current of 25 mA, as depicted in Fig. 4d. These real-world demonstrations highlight the significant potential of the piezoelectric MME harvester with Mn-doped PIN-PMN-PT crystal for powering small electronic devices that require relatively high-power consumption. The results underscore its promise for practical, battery-free IoT applications, particularly in scenarios near electrical power cables where reliable and efficient energy harvesting is essential.

Fig. 4: Energy conversion and practical powering of LEDs and laser diodes using Mn-doped MME harvesters.
figure 4

a Schematic diagram of the energy conversion circuit used by the MME harvester for various electronic applications. b Charging curves of supercapacitors with capacities of 0.11, 0.22, and 0.47 F using the undoped (i) and Mn-doped (ii) MME generators. c Continuous operation of 20 white LEDs powered by the harvested energy. d Operation of a red dot laser diode using the harvested power. The insets in Figs. 4c and 4d show the LEDs and laser diode before illumination.

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Demonstration of multiple battery-free IoT monitoring systems

To utilize the high output of single-crystal PIN-PMN-PT MME generator as a power source for an environmental IoT monitoring system, multiple IoT sensors were operated by the self-power source, as depicted in Fig. 5a. The electric energy produced by the MME harvester is capable of powering both Bluetooth sensors of SimpleLink (Texas Instruments, USA) and Thunderboard Sense 2 (Silicon Labs, USA) at the same time. These IoT devices include multifunctional environmental sensing terms such as infrared temperature, ambient temperature, humidity, air pressure, ultraviolet index, ambient light, magnetic field, sound level, and physical motion. The collected environmental data can be transmitted to smartphones via Bluetooth communication. Figure 5b illustrates the experimental setup utilized to demonstrate the battery-free IoT systems. The setup includes the PIN-PMN-PT MME device, an energy conversion circuit, a storage supercapacitor, the SimpleLink module, the Thunderboard Sense 2 module, two smartphones, and an oscilloscope. The power output from the MME harvester, subjected to a 7 Oe magnetic field, was first rectified and then used to charge a 0.47 F supercapacitor, which subsequently powered the multiple IoT modules. Prior to connecting the IoT environmental sensors, it was essential to ensure that the energy harvested could sufficiently power the modules. Given this substantial power demand, maintaining continuous operation of both the SimpleLink and Thunderboard Sense 2 modules presented a challenge when using previously reported MME generators. Figures 5c and S10 (see the Supplementary Information) compare the charging and discharging behavior of a 0.47 F supercapacitor when charged by Mn-doped and undoped PIN-PMN-PT MME generators. Once the charging voltage of supercapacitor reached up to 3.6 V, the wireless environment monitoring systems activated and instantly transmitted all data to the smartphones (see Movie S2, Supplementary Information). The undoped MME generator required 400 s to charge the supercapacitor up to 3.6 V, whereas the Mn-doped MME generator accomplished this in 204 s. Upon connecting the charged capacitor to the sensor, the voltage from the undoped MME generator (Figure S10 in the Supplementary Information) initially dropped sharply and then gradually declined to approximately 2.0 V within 200 s, causing the IoT modules to cease operation due to the charging rate being lower compared to the consumption rate. In contrast, the supercapacitor charged by the Mn-doped MME generator (Fig. 5c) initially dropped to around 3.2 V but quickly recovered to the original 3.6 V, ensuring sustained operation of the IoT sensors over an extended period. This cycle of sensing and data transmission occurred every 1 second for the SimpleLink module and 10 seconds for the Thunderboard Sense 2 module, with real-time monitoring displayed on the smartphone screens, as shown in Fig. 5d. These results indicate that the acceptor-doped PIN-PMN-PT MME generator can produce exceptional electrical output under small magnetic fields, thereby enabling the operation of multiple wireless sensors for battery-free IoT environmental monitoring.

Fig. 5: Demonstration of multiple battery-free IoT monitoring systems powered by Mn-doped PIN-PMN-PT generators.
figure 5

a Schematic representation of multiple battery-free IoT monitoring systems. b Image of the experimental setup to operate multiple IoT sensors. c Voltage graph of the supercapacitor during the charging phase and operation of multiple IoT sensors powered by the MME harvester. d Screenshots of environmental monitoring displayed on smartphone applications for SimpleLink (i) and Thunderboard Sense 2 (ii) before and after sensing.

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Discussion

In summary, we have demonstrated Mn-doped PIN-PMN-PT crystal-based MME generators to operate multiple IoT monitoring systems. Through careful material design and optimization using the modified Bridgman method, we have achieved substantial improvements in the Qm and tanδ of the piezoelectric single crystals, resulting in enhanced energy harvesting performance. The experimental results show that the Mn-doped MME generator produces an open-circuit output voltage of ~180 V, a short-circuit current of 8.2 mA, and a maximum RMS output power of 106.2 mW, corresponding to an RMS power density of 35.8 mW/cm3. This performance underscores the effectiveness of Mn doping in enhancing the piezoelectric energy conversion efficiency of PIN-PMN-PT crystals at a resonance condition. The practical application of the Mn-doped MME generator was demonstrated by successfully powering multiple IoT sensors, including SimpleLink and Thunderboard Sense 2 modules. The capability of battery-free and real-time environmental monitoring highlights the potential for widespread adoption of MME devices in IoT applications, particularly in areas near electrical power cables where reliable energy harvesting is essential. Future research will focus on further integrating this technology into more complex and diverse IoT systems, with the aim of developing fully autonomous and battery-free solutions for a wide range of electronic applications67,68,69,70,71.

Methods

Fabrication of the PIN-PMN-PT MME generator

The undoped and Mn-doped PIN-PMN-PT crystal ingots (iBULe photonics Co.) were grown by the modified Bridgman process. The modified Bridgman method involves melting the raw material powders to form a homogeneous liquid in a platinum crucible. The melt is then slowly cooled from the bottom under a controlled temperature gradient. As the temperature is gradually lowered, a seed crystal or an oriented solidification front initiates crystal growth. The slow downward movement of the crucible through the furnace ensures directional solidification. This process enables the growth of PIN-PMN-PT single crystals with controlled composition and crystallographic orientation. The piezoelectric single crystal ingots were cut in a thick plate with crystallographic orientation of [011] (dimension of 36 mm × 12 mm x 0.2 mm) and [001] in the thickness direction, and Cr/Au electrode layers (10/200 nm in thickness, respectively) were deposited on top and bottom surfaces. The poling process was performed under a DC electric field of 4 kV/cm for 1 hour at room temperature for the undoped PIN-PMN-PT crystal, and under 5 kV/cm for 1 hour at room temperature for the Mn-doped PIN-PMN-PT crystal. Although this electric field is much lower than the conventional poling field for such single crystals, it was deliberately chosen as an optimized condition to suppress mechanical damage induced by the inverse piezoelectric stress during poling. The [011] PIN-PMN-PT crystal plate was bonded on a grade 5 Ti alloy plate (dimension of 20 mm × 68 mm x 0.3 mm) by an epoxy adhesive. Subsequently, the PDMS encapsulation layer (dimension of 20 mm × 68 mm x 0.5 mm) was coated on the cantilever structure. Commercially available neodymium block magnets (rectangular shape, 20 mm × 10 mm × 5 mm, total weight: 18 g, grade N35) were used and firmly bonded to the tip of the cantilever using epoxy adhesive to ensure mechanical stability and prevent shifts in the resonance frequency.

Simulation

For the COMSOL simulation, the mechanical properties, including Young’s modulus, density, and Poisson’s ratio, for materials such as Ti, adhesive epoxy, PDMS, and the NdFeB magnet are provided in Table 1.

Table 1 Mechanical properties of materials for COMSOL simulation
Full size table

Characterization

The microstructural features of the PIN-PMN-PT single crystals were investigated using HRTEM (JEM-2100F, JEOL). For HRTEM analysis, focused ion beam (FIB) milling was performed to prepare site-specific thin lamellae. EDS was conducted using a SEM (Quattro S, Thermo Fisher Scientific) to analyze the elemental composition of the samples. The surface oxygen concentration and crystallinity of the piezoelectric single crystals were characterized by XPS (AXIS SUPRA, KRATOS Analytical Ltd.) and XRD (X’Pert3, PANalytical), respectively. The dielectric and Qm properties of the crystalline PIN-PMN-PT were characterized using an impedance analyzer (HP4294A, Agilent Technologies). The P-E hysteresis curve was recorded using a ferroelectric measurement system (TF Analyzer 2000, aixACCT Systems GmbH) on the same sample dimensions. An AC magnetic field was generated by a Helmholtz coil linked to a function signal generator (WF1948, NF Corporation) and a bipolar amplifier (HSA 4051, NF Corporation). The output voltage and current signals were measured by a digital oscilloscope (MSO4034B, Tektronix) and a source meter (2611, Keithley).