Casey Israel

High-field magnetic force microscopy as susceptibility imaging

We describe an extension of variable-temperature magnetic force microscopy (MFM) that allows spatial discrimination between the different states that exist in magnetically phase-separated materials. Some manganites exhibit a micrometer-scale separation of phases that are either ferromagnetic, paramagnetic, or antiferromagnetic. In an applied field large enough to saturate the ferromagnetic phase, any MFM contrast arising from the variation of the magnetization (domain walls, domains of differing orientation) is eliminated, while the nonferromagnetic phases are magnetized according to their susceptibilities. The different phases can then be discerned by their respective contrast levels in the MFM images.

Compact variable-temperature magnetic force microscope with optical access and lateral cantilever positioning

We describe a compact design for a variable-temperature magnetic force microscope that incorporates a novel mechanical device for the lateral positioning of a piezoresistive cantilever under the guidance of an external optical microscope. The small size of the instrument makes it possible to perform low-temperature experiments by inserting the probe directly into a liquid-helium storage Dewar or into any open or closed liquid-nitrogen container. Besides convenience, this also means that the cycle time for exchanging tips and∕or samples can be as short as 4 h, including warm-up and cooldown. The probe is long enough to reach the middle of an 8 T superconducting magnet. We present the details of this design and show some results.

Sintering Effect of Annealed FePt Nanocrystal Films Observed by Magnetic Force Microscopy

Chemically synthesized FePt nanocrystals can exhibit room temperature ferromagnetism after being annealed at temperatures above 500degC. In thick films composed of FePt nanocrystals, the coercivity can be quite large. However, the coercivity of thin films has been found to decrease significantly with decreasing thickness, to the point that ferromagnetism at room temperature is lost. We studied 12 to 55 nm thick films by using magnetic force microscopy (MFM) under external applied fields. We made smooth films by spin casting 4-nm-diameter FePt nanocrystals and annealing them at 605degC-630degC. Thin FePt films showed lower coercivity than thick films. To help interpret the MFM images, we obtained complementary magnetic and structural data by superconducting quantum interference device (SQUID) magnetometry, transmission electron microscopy (TEM), and X-ray diffraction. We conclude that the magnetic properties of these films are strongly affected by nanocrystal aggregation that occurs during annealing