Mapping AC Susceptibility with Quantum Diamond Microscope*

Measurement of AC susceptibility is critical and indispensable in investigating magnetic

relaxations and phase transitions in various magnetic materials.  

In AC susceptibility measurements, the magnetic system is externally perturbed by a small magnetic excitation for

frequencies ranging from few hertz to several mega hertz to

observe the dynamic changes in the magnetic system. Conventional AC susceptometry is performed

by placing the sample in a primary excitation  coil and measuring the response in a secondary detection

coil. This methods however, fails to give microscopic information of the magnetic system under study,

which can be of great importance to study local pinning and other effects due to defects.

To obtain microscopic information, we present a novel technique for determining the microscale AC susceptibility of

magnetic materials. We use magnetic field sensing properties of nitrogen-vacancy

(NV– ) centers in diamond to gather quantitative data about the magnetic state of

the magnetic material under investigation. In order to achieve the requisite speed in

imaging, a lock-in camera is used to perform pixel-by-pixel lock-in detection of NV–

photo-luminescence. In addition, a secondary sensor is employed to isolate the effect

of the excitation field from fields arising from magnetic structures on NV– centers.

We demonstrate our experimental technique by measuring the AC susceptibility of

soft permalloy micro-magnets at excitation frequencies of up to 20 Hz with a spatial

resolution of 1.2 μm and a field of view of 100 μm. Our work paves the way for micro-

scopic measurement of AC susceptibilities of magnetic materials relevant to physical,

biological, and material sciences.