**Preface to "Magnetoelectric Sensor Systems and Applications"**

In the field of magnetic sensing, a wide variety of different magnetometer and gradiometer sensor types, as well as corresponding read-out concepts, are available. Well-established sensor concepts such as Hall sensors and magnetoresistive sensors based on giant magnetoresistances (and many more) have been researched for decades. The development of these types of sensors has reached maturity in many aspects (e.g., performance metrics, reliability, and physical understanding), and these types of sensors are established in a large variety of industrial applications.

Magnetic sensors based on the magnetoelectric effect are a relatively new type of magnetic sensor. The potential of magnetoelectric sensors has not yet been fully investigated. Especially in biomedical applications, magnetoelectric sensors show several advantages compared to other concepts for their ability, for example, to operate in magnetically unshielded environments and the absence of required cooling or heating systems.

In recent years, research has focused on understanding the different aspects influencing the performance of magnetoelectric sensors. At Kiel University, Germany, the Collaborative Research Center 1261 "Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics", funded by the German Research Foundation, has dedicated its work to establishing a fundamental understanding of magnetoelectric sensors and their performance parameters, pushing the performance of magnetoelectric sensors to the limits and establishing full magnetoelectric sensor systems in biological and clinical practice. The research questions range from fundamental material modelling aiming to understand the underlying principles and physical limits, to the development of innovative sensor concepts and the establishment of thin-film processes technology, and to the usage of entire sensor systems in biomedical applications.

In many applications, magnetic sensors have several advantages if they are used either in addition or even instead of electric measurements. The advantages have been proven in science and research using magnetic sensors such as superconducting quantum interference devices (SQUIDs) or optically pumped magnetometers (OPMs). Application examples include spatially and temporally high-resolution medical analyses such as magnetocardiography (MCG) and combined electro- and magnetoencephalography (EEG/MEG). The drawbacks of these sensor technologies are mainly their high cost and their limited robustness against environmental influences. External magnetic fields, such as the magnetic field of the Earth or the fields created by power supplies, saturate SQUID and OPM sensors, which requires expensive and difficult-to-install magnetic shielding. Furthermore, SQUID sensor technology absolutely needs expensive liquid He cooling.

The magnetoelectric sensor principle—as a relatively new principle—has the potential to overcome these limitations at a very low cost. This would facilitate the transfer of medical research results into clinical practice. Recent advances, in terms of magnetic layer optimization, low-noise readout and dedicated signal processing for new read-out principles can potentially enhance the sensitivity of magnetoelectric sensor principles and bring them very close to that of OPMs or SQUIDs without robustness problems. Additional advantages are the large dynamic range—the requirement being insensitive to large external fields—and the very high bandwidth of certain magnetoelectric sensor approaches.

This book reports the latest research on magnetoelectric sensor systems and corresponding applications. The bandwidth of contributions ranges from biomedical application examples, specially tailored readout schemes for ME sensors, low-noise amplification circuits, and advances in the material science and improved understanding of the magnetic processes that are involved in magnetoelectric layers.

> **Gerhard Schmidt, Eckhard Quandt , Nian X. Sun, Andreas Bahr** *Editors*
