Scanning SQUID Study of Vortex Manipulation by Local Contact

The overall goal of this procedure is to manipulate vortices in thin superconducting films by local vertical stress without applying current or magnetic field. The ability to control the position of individual vortices is necessary in order to study how vortices interact with each other, with the material, and with other magnetic objects. The main advantage of this technique is that it is a local method which allows a very effective manipulation of an individual vortex with no additional fabrication steps.

We first witnessed the effect of strain on submicro and ferro-magnetic objects when we used to SQUID to image and examine the way they change their orientation as a result of stress. Following this study, we wanted to investigate the effect of local stress on other nano-magnetic objects, and the first system we tested is the system of vortices in a superconductor. First, prepare a thin superconducting film sample.

Then, obtain a SQUID sensor fabricated on a silicon chip. Gently polish the chip with nonmagnetic polishing paper to form a symmetrical point containing the sensing area. The material must be polished all the way down to expose the sensing area.

Glue a flexible cantilever to a conducting plate with a dielectric layer. Then, glue the sensor chip to the cantilever. Mount the sample with varnish or silver paste.

Then, attach it to the Z-Axis Piezo element. Set up two telescopes on translation stages, and connect the telescopes to a computer via a camera. Connect the stick slip course motion system controller.

Direct the telescopes at the front and side of the chip to provide optical imaging. Using the Z-stick slip stage, move the sample to about one micrometer from the sensor so that the sensor is reflected in the sample. Do not allow the sample to contact the sensor.

Check the angles between the chip and its reflection on both sides of the tip when viewed from the front. Then, move the sample about 0.5 to one millimeter away from the sensor to prevent damage to the SQUID. Rotate the alignment screws so that the angles between the chip and its reflection are equal on both sides of the tip when viewed from the front.

Move the sample back to about one micrometer from the sensor, and check the angles again. Once the angles are equal, rotate the alignment screws until the angle between the chip and its reflection is at least four degrees when viewed from the side to ensure that the tip contacts the sample when moving the vortices. It is important to align the sensor at an angle so that only the sensor tip touches the sample.

This way, a distinct point of contact can be determined. Load the scanning head of the SQUID system into a four Calvin cooling system. To generate vortices, cool the sample to 4.2 Calvins in the presence of a magnetic field based on the sample size and desired number of vortices.

Turn off the magnetic field, and turn on the SQUID. Using the Z-stick slip, move the sample closer to the sensor chip. Apply voltage between the cantilever and the capacitance detection plate to bring the cantilever into the bridge circuit.

Sweep the Z Piezo element from zero to 200 volts while monitoring the capacitance between the cantilever and the plate. When the sample contacts the sensor chip, a large change in capacitance will occur. Once the sample has successfully contacted the chip, repeat this process at several locations on the sample to define the plane of the sample relative to the sensor.

To move the sample relative to the sensor, sweep the voltage on the X and Y Piezo elements. Once the plane is defined, use the Z Piezo element to move the sample so that the sensor scans at a constant height above the sample. Scan around one vortex to precisely determine the location of its center relative to the sensing area.

Turn off the SQUID, and then apply a voltage greater than the touch-down voltage to the Z Piezo element. Either tap the sample next to the vortex center, or slowly drag the sensor while in contact with the sample to the desired new vortex location. Turn the SQUID back on, and scan the sample at a constant height to determine the new vortex location.

Vortices were generated on a niobium thin film sample. Using this method, the vortices can be moved into defined patterns. A vortex may be moved multiple times.

Even after moving this vortex 820 micrometers, the vortex remained stable at its new location. The magnetic signal is mapped by recording the flux through the SQUID as function of position. The signal is recorded through the SQUID's pickup loop.

However, contact with the sample is made at the tip of the sensor chip. This distance must be compensated for when selecting a contact location for vortex manipulation. In this case, contact happens prior to recording the magnetic signal.

If the sensor is turned on during a manipulation scan, the displacement event appears as a faint step in the signal. The vortex, imaged at its new position, appears intact. This technique can be applied to study the effect of local mechanical stress on other nano-magnetic objects.

An effective and reproducible manipulation technique of nano-magnetic objects is essential to promote possible applications such as logic elements.