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Introduction

The Variable Field Module (VFM3) is an ideal option for researchers who want to apply high magnetic fields to their atomic

force microscopy experiments. 

VFM3™ Variable Field Module for Magnetic AFM Applications

Asylum Research has developed a second generation Variable Field Module (Figure 1) for the MFP-3D™ Atomic Force Microscopes (AFM). This module is useful for magnetic force microscopy (MFM), conductive AFM (C-AFM), and other applications where the sample’s properties are magnetic field dependent. The VFM3 can apply static magnetic fields up to ±0.8 Tesla (~1 G resolution), parallel to the sample plane.

How it WorksThe VFM3 has a unique design incorporating a motor driven permanent magnet. Figure 2 shows how the rotation of a powerful rare earth magnet translates into a variable field at the sample. Once a field value is reached, the motor is turned off and the field is maintained without residual heat, thermal drift, or mechanical vibration.

FeaturesThe VFM3 attaches to the MFP-3D XY sample scanner by means of the included quick release plate. Adjustable pole pieces allow for optimal choice between maximum required field, sample placement and minimum field gradients. Field intensity is software controllable. An integrated Hall sensor provides a quantitative measure of the applied magnetic field and closed loop feedback control between motor and sensor allow for linear field ramps. A beryllium copper non-magnetic cantilever clip that minimizes field distortion is included.

Application ExamplesThe image sequence in Figure 3 shows the application of an increasing magnetic field to a piece of PMR (Perpendicular Magnetic Recording) hard disk. The written bits are progressively degaussed as the in-plane magnetic field is increased from zero to >0.7 Tesla.

Figure 4 presents MFM phase images of a Terfenol-D disk with and without fields applied perpendicular to the magnetization. Figure 4(a) and 4(b) show the MFM phase signal with an applied 2000G field and after the field is removed, respectively. Figures 4(c) and 4(d) similarly show phase images with and without a -2000 G field.

Optional High Voltage Tip BiasFor experiments where combined magnetic field and a high tip-sample voltage bias are required, such as for studies of ferroelectric and piezoelectric materials, the VFM3 HV Kit can be attached to the module with a few screws (Figure 5).

Figure 1: The VFM3 Variable Field Module.

Figure 2: The strength and sign of the magnetic field applied to the sample depends on the rotation angle of the rare earth magnet. At 0° or 180°, the magnetic flux is shunted away from the sample by the soft iron armature and pole pieces. As the magnet rotates, more and more flux is channeled instead through the sample. At 90,° the field is maximized.

AFMAsylum Research

Soft Iron Sample

H=0 G; θ=0˚

H=8000 G; θ=90˚

Flux LinesPermanentMagnet

The VFM3 HV Kit also requires an MFP-3D AFM upgraded with the High Voltage Option, which includes:

l LFM head

l High voltage sample holder

l High voltage cantilever holder

l HV220 high voltage amplifier

VFM3 Specificationsl Maximum field: ±0.8 Tesla

l Maximum field ramp: 7000 G/minute

l Sensor resolution: ~1 G

l Closed loop control minimum increment: ~1 G

l Open loop minimum step: ~0.2 G

l Other environmental conditions: Ambient

Figure 3: Five frames showing a piece of PMR hard disk degaussed with an in-plane ~0.7 Tesla magnetic field using the VFM3.

Figure 4: MFM phase images of the Terfenol-D disk under a series of in-plane fields perpendicular to the magnetization. (a) MFM phase images taken at 2000 G and (b) after field removal; (c) MFM phase images taken at -2000 G and (d) after field removal. The direction of the magnetic field is indicated by the arrow. 10 μm scan. Image courtesy of Shuhong Xie, Xiangtan University, China and JiangYu Li, University of Washington.

Figure 5: VFM3 with High Voltage Unit attached.

Visit www.AsylumResearch.com to learn more

The foregoing application note is copyrighted by Oxford Instruments Asylum Research, Inc. Oxford Instruments Asylum Research, Inc. does not intend the application note or any part thereof to form part of any order or contract or regarded as a representation relating to the products or service concerned, but it may, with acknowledgement to Oxford Instruments Asylum Research, Inc., be used, applied or reproduced for any purpose. Oxford Instruments Asylum Research, Inc. reserves the right to alter, without notice the specification, design or conditions of supply of any product or service. Data Sheet 39 – 6/2014.

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