Optical and microwave-assisted digital detection of magnetic field with NV− centers in diamond

Magnetometry based on Optically Detected Magnetic Resonance (ODMR) of NV− colour centers in diamond is a well-established method for sensitive detection of magnetic field intensity and direction. However, ODMR spectra are inherently independent of magnetic field polarity which is a key parameter for applications such as reading of magnetic bits in Hard Disk Devices (HDD) and for the understanding of physical phenomena in nanomagnetism. Recently a method has been implemented for digital detection of magnetic field polarity by NV− centers in diamond simply by monitoring the changes of a magnetic field demodulated photoluminescence (PL) signal in condition of magnetic resonance (i.e. microwave assisted or ODMR). Unlike other methods, the demodulated PL signal shows a complete sign inversion rather than an intensity shift, thus making the detection of magnetic field polarity direct and unambiguous. Here, it is reported that the same method can be applied to off-resonance condition (i.e. optical), that is by detecting the photoluminescence signal locked to a demodulating magnetic field without microwave resonant excitation. This method simplifies the set-up and it avoids microwave heating effects. Furthermore, a sensitivity study has been conducted resulting in up-to-standard sensitivities of a few μT/√Hz for both microwave-assisted and only-optical detection in a low-cost cw-ODMR set-up. It is concluded that the simplified set-up required for the optical method comes at expense of reduction of sensitivity by 36 % compared to the microwave-assisted method. Overall, potentially these features demonstrate that the optical and microwave-assisted methods are suitable for real-time digital detection of magnetic field of magnetic bits in HDDs and for the study of magnetic phenomena at nanoscale.