Explanation on Exotic Magnetic Field Dependence of Staircase Pattern in Magneto-resistance of CrO2 Films

2002 Fiscal Year Final Research Report Summary

• Project/Area Number: 13650034
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: 表面界面物性
• Research Institution: Miyagi National College of Technology
• Principal Investigator: SUZUKI Katsuhiko Miyagi National College of Technology, Dept. of Integrated Arts & Sciences, Professor, 総合科学系・理数科, 教授 (80187715)
• Co-Investigator(Kenkyū-buntansha): MATSUURA Makoto Miyagi National College of Technology, Dept. of Integrated Arts & Sciences, Professor, 総合科学系・理数科, 教授 (40042262)
• Project Period (FY): 2001 – 2002
• Keywords: CrO_2 / Half Metal / Magneto-resistance / MFM / Temperature Dependence of Resistance / Staircase Pattern / Thermal Activation Tunneling

Research Abstract

The exotic staircase patterns have been found in the magenetoresistance of CrO_2 films in low temperature ranges. The temperature dependence of resistance decreased with decreasing temperature and had the minimum near about 120K. The natural logarithmic values of resistance were proportional to the inverse of temperature in two temperature ranges (high range : 50K-90K ; low range : 30K-50K).
The value of the proportional coefficient was about 9.5K in the high temperature range and corresponds to 0.819meV. The value in the low temperature range was about 3.6-4.1K and does to 2.9-4.1meV. On the other hand, the value of step height in the staircase pattern was about 800μV in low magnetic field and corresponds to 0.8meV. The value in high magnetic field was about 360-390μV and does to 0.36-0.39meV. Therefore, the value of the proportional coefficient in high temperature range is close to one of step height in low field, and in low temperature, in high field.
There are probably the discontinu ous states in grains during the existence of the magnetic domains because The CrO_2 has the half metallic band structure and the MFM images showed the polymagnetic domain structure in a grain. The scanning probe microscopic measurements showed that the present samples were granular films, which means that there are the discontinuous states between grains. These discontinuous states give rises to the temperature dependence in the resistance of the type of the activative energy.
The activation energies in high and low temperature ranges must result from the discontinuous states in and between grains of films, respectively, by taking the magnitute of the coefficient and the mean free path of electrons into account. Although the value of coefficient in high temperature range depended on the magnetic field, one in low temperature range almost did not depend on the magnetic field, which supports the above model. Therefore, the exotic staircase patterns in the magnetoresitance are related to both the discontinuous states in the films and the half metallic properties.