Studying Nanoferroics with Piezoresponse Force Microscopy

Nanoferroics are nanoscale materials with ferroic properties, meaning they are materials that undergo a reversible change to their physical properties in the presence of an external field. There can be a number of different mechanisms that cause a material to exhibit ferroic behavior. The interest in macroscale ferroic materials has arisen from applications such as information storage and the creation of optoelectronic and spintronic devices.¹

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Introduction to Nanoferroic Research

Nanoferroic research largely focuses on creating new materials for nanoferroic devices, as well as methods for device fabrication and more fundamental studies to understand how the material properties and device design affect the overall performance.²

In nanoferroic research, a number of different types of fields and forces can be used to induce a ferroic effect, including applying magnetic or electric fields as well as physical strain on the system. One tool that has been very successful in its applications in nanoferroic research since its development over 25 years ago is piezoresponse force microscopy (PFM).³

Role of Piezoresponse Force Microscopy (PFM) in Nanoferroic Studies

PFM is a type of atomic force microscopy (AFM) that utilizes the piezoelectric properties of materials to characterize and manipulate such materials. AFM has been a hugely successful technique in materials and nanoscience research due to its outstanding spatial resolution, making manipulating and imaging single atoms simultaneously possible.⁴

A standard AFM measurement will use an atomically sharp tip to scan over the material’s surface. However, there are different types of AFM scanning modes, as the amount of force exerted by the material on the tip changes, a change in current can be measured and quantified. As the 3D position of the tip is known very precisely, the variation in the measured current across the surface can be reconstructed to make the AFM image.

For nanoferroic research, PFM is a specific type of AFM that is used for mapping out the ferroic domains in nanoferroic materials. PFM makes use of the piezoelectric effect – where an applied electric field to a ferroelectric surface results in a surface vibration. The polarization state can be determined by measuring the magnitude and phase of the piezoelectric response.

Advantages of PFM in Investigating Nanoferroic Materials

The advantage of using PFM in nanoferroic research is that, with a variety of different scanning modes, PFM can be used to comprehensively characterize many material properties of interest for nanoferroics, such as the evolution of the domain structure as a function of time with response to voltage or make incredibly precise measurements of electromagnetic properties.^5,6

For nanoferroic research, investigation of the domain structure of materials is critical as the overall shape, size and behavior of domains influence the final material properties. Ferroic materials have localized regions where the electric dipoles, magnetic moments, or crystal lattices are arranged in a particular direction.

The effect of an external influence, such as mechanical stress or an external electromagnetic field, on the domains causes the ‘switching’ behavior that is exploited in nanoferroic research.

PFM has the spatial resolution to map domain sizes as well as look at boundaries and interfaces between neighboring domains. Now, with materials like BaTiO₃ being used to make multiferroic materials – which exhibit several orders of ferroic behavior – domain characterization has become increasingly complex as a single material may have several different domain structures and some of the properties will be due to the interfacial structures and interactions.⁷

Many new classes of material in nanoferroic research are now trying to exploit ultrathin layers and heterostructures, which PFM is well-suited to measuring as well as capturing the effects of local defects, which require the high spatial resolution to image directly.

Future Directions and Potential Developments in PFM for Nanoferroic Studies

There has been a significant amount of recent development in the creation of 2D nanoferroic materials in nanoferroic research.⁸ Some of the drive for this has arisen from the need for new materials that can be used for transistors that do not suffer the same thermal problems with miniaturization that more traditional silicon-based devices do.

Using multiferroic materials and exploiting their versatility of ferroic effects is highly appealing for data storage and information science.⁹ However, the complexity of understanding the different regions in a device or performing quality control on such nanoferroic materials will require even more extensive use of PFM.

Future developments of PFM may include greater attempts to use ‘stroboscopic’ type PFM measurements or improvements in the number of frames per second that can be recorded by making ultra-high resonant frequency cantilever tips.⁵

This would facilitate greater possibilities for real-time imaging of material behavior in response to external stimuli, such as heating or the application of electromagnetic fields. Being able to visualize domain-switching in real-time and how fast relaxation processes occur could help scientists working in nanoferroic research to design more efficient and effective materials with greater read/write speeds.

Automated AFM Solutions for the Semiconductor Industry

References and Further Reading

1. Wadhawan, V. K. (2002) Ferroic materials: A primer. Resonance, 7(7), pp. 15–24. doi.org/10.1007/bf02836749
2. Glinchuk, M. D., et al. (2013) Nanoferroics. Dordrecht: Springer, p. 378. doi.org/10.1007/978-94-007-5992-3
3. Gruverman, A., et al. (2019) Piezoresponse force microscopy and nanoferroic phenomena. Nature Communications, 10(1), pp. 1–9. doi.org/10.1038/s41467-019-09650-8
4. Newberry, D. (2020) Tools Used in Nanoscience. In Nanotechnology Past and Present, pp. 23–32. doi.org/10.1007/978-3-031-02084-1_3
5. Gruverman, A., et al. (2008) Piezoresponse force microscopy studies of switching behavior of ferroelectric capacitors on a 100-ns time scale. Physical Review Letters, 100(9), pp. 3–6. doi.org/10.1103/PhysRevLett.100.097601
6. Labuda, A. & Proksch, R. (2015) Quantitative measurements of electromechanical response with a combined optical beam and interferometric atomic force microscope. Applied Physics Letters, 106(25). doi.org/10.1063/1.4922210
7. Valencia, S., et al. (2011) Interface-induced room-temperature multiferroicity in BaTiO₃. Nature Materials, 10(10), pp. 753–758. doi.org/10.1038/nmat3098
8. Guan, Z., et al. (2020) Recent Progress in Two-Dimensional Ferroelectric Materials. Advanced Electronic Materials, 6(1), pp. 1–30. doi.org/10.1002/aelm.201900818
9. Scott, J. F. (2007) Multiferroic Memories. Nature Materials, 6, pp. 256–257. doi.org/10.1038/nmat1868

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