Experimental study of interaction field between the nanostructured Ni columns

Abstract

Nanoshuctu~td m y of magnetic pillars has been proposed for a high density data storage medium.[l] At a high recording density, the pillars are likely to be interacting with each other to the extent that it can be fatal to the data storage application. In this work, for the first time, we will provide an experimental way of measuring the interaction field between the pillars at a recording density higher than 16 Gbit". Sample Preparation The experiments were performed on an array of small size nickel pillars with a period, ranging from 150 nm to 300 nm. In the may, the cylindrical shaped pillars of a diameter down to 90 nm had the aspect ratio of 2. In the fabrication of test media, the electron beam lithography was used to expose a pattern on the resist, PMMA. The PMMA is used as a template for the electroplating of Nickel and later cleaned off with acetone for the magnetic measurements on the resulting structure. lnteraction Field Measurement The pillars were found out to he the single domain and have the easy axis along the out of plane direction. The net interaction field applied to a pillar from the adjacent pillars can be calculated from &,=(H*-H-)n, where H' and K are the switching fields of the pillar in two opposite directions along the vertical axis.[2] To measure Wand H-, the magnetic force microscope with an in situ electromagnet was used. A known field is applied to the target pillar and it is increased by step of 50 Oe till the pillar reverses the magnetization stale. The pillar's magnetic state was 0bsew.d by the MFM pbase detection imaging. In Fig. I , the MFM images show that the pillar in a dotted square reverses its magnetization at a certain applied field. A key factor in the measurement is to keep the neighbor pillars in the same magnetic state while the target pillar's switching field is measured. Point magnetic recording( PMR ) method is used for such purpose.[3] In PMR, the MFM tip comes in contact with the sample at the top center of the pillar and the field is applied from the electromagnet. The field, concentrated near the tip, was Jungmok Bae Subject Category 14 Daewoo Electonics Co., LTD. TMA Module Team 60-8 Kasan-dong, Kumchun-gu Seoul, Kored 153-023 Preference Oral Tel) 02-818-9870 Fax) 02-862-0386 Email) jungmok@tma.dwe.co.kr 0-7803-5555-5/99/$10.00 0 1 9991EEE. HC-07 controlled by the external field from the electromagnet. We were able to flip the pillar at the period as small as 200 nm. The exact field applied to the pillars were found out experimentally and verified by the simulation using commercial Maxwell software. Using the scheme, the interaction field for the period of 200 nm, 25n nm, and 300 nm in various neighboring pillars' states were measured.