Variable magnetic field sample holder

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

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.
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.

The variable magnetic field sample holder (VMFSH) 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.




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.
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.

VMFSH

The VMFSH (Fig. 2) has a stack of permanent magnets in the base of the sample holder to generate the magnetic field. The magnets can be rotated using a precise stepper motor to generate a variable in-plane magnetic field of up to 720 mT (7200 G). When the magnetic field of the magnets is aligned with the magnetic poles, the field is at its maximum, and when the magnetic field of the magnets is normal to the poles, the field is zero (Fig. 3). The stray field is focused around the sample via ferromagnetic poles. The use of the permanent magnets in contrast to solenoid magnets ensures that the dissipated heat is constant and does not depend on the magnetic field strength, providing minimal thermal drift.

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.

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).

Applications

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.

Figure 4. MFM images 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.

Features

  • Unique design using permanent magnets eliminating thermal drift caused by solenoid magnets.
  • Integrated Hall sensor
  • Programmable stepper motor to vary field strength with a resolution of 0.1 mT
  • 720 mT maximum field
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
Table 1: Specifications of the VMFSH

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