Enabling AFM Measurements with a Variable Magnetic Field Sample Holder (VMFSH)

The variable magnetic field sample holder (VMFSH) is an attachment for Nanosurf’s CoreAFM, DriveAFM and FlexAFM. The VMFSH accessory facilitates AFM measurements in a variable in-plane magnetic field.

Recently, magnetic materials have drawn a significant amount of interest across the scientific community. They can be found almost everywhere, from everyday electronics at home to cutting-edge diagnostic equipment in hospitals.

As continued efforts around miniaturization persist, new applications are consistently emerging in data storage, electronics, memory, robotics, spintronics and biomedical devices.¹ Nanoscale characterization techniques are critical to helping advance the field of magnetic (nano)materials.

Combining VMFSH with AFM or advanced AFM modes enables correlative measurements with a variable in-plane magnetic field for in-situ observations of magnetization, conductivity or topography changes at the nanoscale with the introduction of a magnetic field up to 720 mT.

Ferromagnets and ferrimagnets are the most frequently studied permanent magnets. They are renowned for their capacity to retain the memory of an applied field once it has been extracted. The magnetization properties also differ depending on whether there is an increase or decrease in magnetic field.

This behavior of magnetic hysteresis is exhibited in Figure 1. In a ferromagnet, the magnetic domains are aligned with the field via large magnetic, and when all the domains are fully aligned, the material achieves saturation magnetization.

Figure 1. Magnetic hysteresis loop of a ferromagnet. MR – residual magnetization, HC – coercive field. At high fields the magnetic domains are fully aligned. At intermediate fields, the domains align patterns reducing the magnetization strength. Image Credit: Nanosurf AG

When the field reverses, there is a loss of domain alignment, however not completely, and at zero field a residual magnetization remains.

To take the magnetization to zero, a coercive field (HC) should be applied, at which the net alignment of domains, and therefore the magnetization, would be equivalent to zero.

The domain behavior at fields close to the coercive field is of particular interest, as domains are seldomly chaotic in their alignment, and they produce structures and patterns that are contingent on the material parameters.

Additionally, the hysteresis parameters are not entirely intrinsic and typically depend on domain state, grain size, stress and temperature. These are all parameters that AFM has the capacity to probe.

VMFSH

The VMFSH (Fig. 2) incorporates a stack of permanent magnets in the base of the sample holder to produce the magnetic field. Rotation of magnets can be achieved using a precise stepper motor to create a variable in-plane magnetic field of up to 720 mT (7200 G).

Figure 2. The variable magnetic field sample holder. The base contains the permanent magnets, a precise stepper motor for their rotation, and an integrated calibrated Hall sensor. The sample mounting plate is positioned in the center of the top plate, between the magnetic poles. Image Credit: Nanosurf AG

When the alignment of the magnetic field of the magnets is allied with the magnetic poles, the field is at its maximum, and the field is zero when the magnetic field of the magnets is normal to the poles (Figure 3).

Figure 3. Working principle of the variable magnetic field sample holder. The permanent magnets in the base of the holder can rotate around the vertical axis. When the magnetic field of the magnets is aligned with the magnetic poles, the field is at its maximum (left image), and when the magnetic field of the magnets is normal to the poles, the field is zero (right image). Image Credit: Nanosurf AG

The stray field is concentrated on the sample via ferromagnetic poles. The use of permanent magnets compared with solenoid magnets ensures that the dissipated heat remains constant and is not contingent on the magnetic field strength, resulting in minimal thermal drift.

The strength of the field is contingent on the magnet rotation and gap width between the magnetic poles. The gap widths are set using spacers, on which the sample is placed. There are five spacers that allow 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 utilized for the field measurement.

The VMFSH automation software sets the necessary field setpoint, and using a user-defined field range with the required number of steps can make MFM scan series. The field resolution for the automated setpoint is 0.1 mT. With manual adjustment, a field resolution of 0.005 mT is possible.

Applications

The VMFSH sample holder can be used in addition to advanced modes, including conductive AFM or magnetic force microscopy (MFM). Figure 4 shows the effect of varying magnetic fields in a Shakti spin ice, a nanometer-scale configuration of magnets.

Figure 4. MFM images (5 x 5 µm²) of Shakti spin ice structure made at different magnetic fields imaged using the variable magnetic field sample holder using the CoreAFM. Image Credit: Nanosurf AG

Imaging of a Shakti lattice was conducted at its saturated state at -200 mT and in the magnetic behavior throughout the reversal process at -10 mT.

Features

• 720 mT maximum field
• Built-in Hall sensor
• Programmable stepper motor to vary field strength with a resolution of 0.1 mT
• Unparalleled design using permanent magnets, eradicating thermal drift caused by solenoid magnets.

Table 1. Specifications of the VMFSH. Source: Nanosurf AG

This information has been sourced, reviewed and adapted from materials provided by Nanosurf AG.

For more information on this source, please visit Nanosurf AG.