qPlus Magnetic Force Microscopy in Frequency-Modulation Mode with milli-Hertz Resolution

TL;DR

This paper demonstrates a novel qPlus sensor capable of simultaneously imaging atomic structures and magnetic domain features by bridging the force gradient sensitivity gap, enabling combined STM, AFM, and MFM measurements.

Contribution

It introduces a high-stiffness qPlus sensor that detects milli-Hertz magnetic frequency shifts, unifying atomic and magnetic imaging with a single probe.

Findings

Successfully measured milli-Hertz magnetic frequency contrast.

Bridged the force gradient sensitivity gap between atomic and magnetic interactions.

Enabled combined STM, AFM, and MFM imaging with one sensor.

Abstract

Magnetic force microscopy (MFM) allows one to image the domain structure of ferromagnetic samples by probing the dipole forces between a magnetic probe tip and a magnetic sample. The magnetic domain structure of the sample depends on the atomic arrangement of individual electron spins. It is desirable to be able to image both individual atoms and domain structures with a single probe. However, the force gradients of the interactions responsible for atomic contrast and those causing domain contrast are orders of magnitude apart - ranging from up to 100N/m for atomic interactions down to 0.0001N/m for magnetic dipole interactions. Here, we show that this gap can be bridged with a qPlus sensor, with a stiffness of 1800N/m (optimized for atomic interaction), that is sensitive enough to measure milli-Hertz frequency contrast caused by magnetic dipole-dipole interactions. Thus we have succeeded to establish a sensing technique that performs Scanning Tunneling Microscopy, Atomic Force Microscopy and MFM with a single probe.