Variable magnetic field generator

The variable magnetic field generator is an accessory for Nanosurf’s CoreAFM, FlexAFM and DriveAFM. The VMFSH enables AFM measurements in a variable in-plane magnetic field.

Magnetic materials have attracted great interest across the scientific community and are found in everything from everyday electronics at home to state-of-the-art diagnostic equipment in hospitals. As miniaturization efforts continue, new applications are continually emerging in data storage, memory, spintronics, electronics, robotics, and biomedical devices.1 Nanoscale characterization techniques are crucial to advance the field of magnetic (nano)materials. The VMFSH combined with AFM or advanced AFM modes allows correlative measurements with a variable in-plane magnetic field to observe magnetization, conductivity, or topography changes at the nanoscale in-situ with the addition of a magnetic field up to 720 mT.

Ferrimagnets and ferromagnets are the most commonly studied permanent magnets. They are well known for their ability to retain memory of an applied field once it is removed. The magnetization properties also vary depending on whether the magnetic fields are increasing or decreasing. This behavior of magnetic hysteresis is shown in Fig. 1. In a ferromagnet, large magnetic fields align the magnetic domains with the field, and when all the domains are fully aligned, the material reaches the saturation magnetization. As the field reverses, the domains lose the alignment, however not entirely, and at zero field a residual magnetization remains. To bring the magnetization to zero, one needs to apply a coercive field (HC), at which the net alignment of domains, and therefore the magnetization, would be equal to zero. The behavior of the domains at fields close to the coercive field is of special interest, as domains are rarely chaotic in their alignment and they form structures and patterns that depend on the material parameters. Moreover, the hysteresis parameters are not fully intrisic and often depend on grain size, domain state, stress, and temperature. These are all parameters that can be probed using AFM.

The strength of the field depends on the magnet rotation and gap width between the magnetic poles. The gap widths are defined with spacers, on which the sample is placed. There are 5 spacers that provide 2, 4, 6, 8, or 10 mm spacing, with corresponding maximum fields of 720, 370, 240, 180 and 140 mT, respectively. An integrated Hall sensor is used for the field measurement.

The VMFSH has an automation software that sets the required field setpoint and can make MFM scan series within user defined field range with required number of steps. The field resolution for the automated setpoint is 0.1 mT. With manual adjustment, a field resolution of 0.005 mT can be acheived.

The VMFSH sample holder can be used in parallel with advanced modes such as conductive AFM or magnetic force microscopy (MFM). Fig. 4 demonstrates the effect of varying magnetic field in a Shakti spin ice, a nanometer-scale configuration of magnets. A Shakti lattice was imaged at its saturated state at -200 mT and in the magnetic behaviour during the reversal process was imaged at -10 mT.