Prof. Rajeev J Ram

Magnetic Properties and Interactions of Single-Domain Nanomagnetic Arrays
Patterned arrays of single domain nanomagnets (SDNMs) have been proposed as candidates for ultra-high-density data storage and ultra-low-power data processing technology. For both applications of SDNMs, the magnetic properties and interaction between nanomagnetic elements must be understood.We made microscopic measurements on a square array of SDNM posts with the magnetic and spatial characteristics that would be desirable for the above applications.Special attention was paid to the interactions between the posts and fluctuations in the magnetic properties of the SDNMs, as these characteristics determine the signal-to-noise ratio in data storage applications and the power loss in magnetic cellular automata circuits.

The sample we studied was a 100 nm period nanomagnetic array.Each element in the array was a single crystal Ni post with an average height of 115 nm and diameter of 57 nm. The average switching field determined by bulk velocity sensitive magnetometry (VSM), was 710 Oe, with an easy axis parallel to the long axis of the post. A microscopic ‘hysteresis’ curve was measured by taking successive magnetic force microscope (MFM) scans of a fixed 20x20 grid of posts in a varying external magnetic field and recording the switching field f each post. The microscopic (MFM)and bulk (VSM) data compared well.

The unique information gained from MFM measurements is the microsopic map of the individual switching field. This map gives us the magnetic configuration of the neighboring states as the elements flip.Thus we were able to account for the interaction field of the neighbors in determining the switching field of each element.

Simultaneous atomic force microscope (AFM) measurements were made along with the MFM measurements in order to record the size dependence of the switching field. The measured mean switching field, standard deviation and size dependence were accurately reproduced by numerical simulations which assumed the magnetization switching mechanism to be curling.

In summary, we have investigated the switching field behavior of single domain nanomagnets in a dense array. By using magnetic force microscopy, we spatially resolve the hysteresis loops of individual nanomagnets.We showed that magnetostatic interactions play an important role and up to 24 nearest neighbors need to be taken into account to determine the switching field distribution. Also,the experimental features such as mean switching field,standard deviation,and size dependence are well reproduced when curling is assumed as the switching mechanism. The measurements made so far on the SDNMs were conducted using a room temperature open air scanning probe microscope (SPM). In order to study effects like superparamagnetism, temperature control and improved sensitivity of the MFM is essential. Currently we are in the process of installing a cryogenic high vacuum scanning probe microscope (SPM). The microscope is designed to operate in STM, AFM contact and AFM non-contact modes over a temperature range of 5K to 300K. The variable temperature range should allow us to study the superparamagnetic transition in SDNMs and also provide greater signal to noise ratio in MFM measurements made at low temperature due to the absence of thermal fluctuations of the cantilever. The performance of the cryo-SPM is further enhanced because the high vacuum environment in which the head is placed. The vacuum reduces the damping of the cantilever oscillation thereby substantially increasing the Q of the system and therefore the sensitivity of the measurement.