Enabling AFM Measurements with Variable Magnetic Field Sample Holder

The variable magnetic field sample holder (VMFSH) is an accessory that can be used in combination with Nanosurf’s CoreAFM, FlexAFM and DriveAFM. The VMFSH accessory facilitates AFM measurements in a variable in-plane magnetic field.

Magnetic materials have drawn a significant amount of interest within the scientific community and can be found almost everywhere, from everyday electronics to cutting-edge diagnostic equipment in hospitals.

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

Combining VMFSH with advanced AFM modes or AFM enables correlative measurements with a varying 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 analyzed 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 fields.

This behavior of magnetic hysteresis is exhibited in Fig. 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.

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.

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 persists. To take the magnetization to zero, a coercive field (HC) should be applied, at which the net alignment of domains, and 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 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, temperature and stress. These are all parameters that AFM has the capacity to probe.

VMFSH

The VMFSH incorporates a stack of permanent magnets in the base of the sample holder to produce the magnetic field, as shown in Figure 2. 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).

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.

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).

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).

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 can make MFM scan series within the user-defined field range with the 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 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 field in a Shakti spin ice, a nanometer-scale configuration of magnets.

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.

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

The 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 the thermal drift caused by solenoid magnets.

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

. .
Feature Variable in-plane magnetic field generator; Integrated hall sensor.
Platform compatibility CoreAFM, FlexAFM, DriveAFM
Requirements Accessory Interface, Isostage 300
Sample size 2 x 2 mm2 to 10 x 10 mm2
Maximum field 720 mT (7200 G), 2 mm gap
Field resolution 0.1 mT (1 G)
Magnetic field Permanent rare earth magnets
Field variation Programmable stepper motor
Dimensions 100 mm x 140 mm

 

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

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

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