Nanoscale magnetization in ferromagnetic thin films

Open Access
Author:
Balk, Andrew Lee
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 10, 2011
Committee Members:
  • Nitin Samarth, Dissertation Advisor
  • Nitin Samarth, Committee Chair
  • Peter E Schiffer, Committee Member
  • Jun Zhu, Committee Member
  • Srinivas A Tadigadapa, Committee Member
Keywords:
  • scanning probe
  • semiconductor
  • spintronics
  • Hall effect
  • ferromagnetism
  • Mn)As
  • (Ga
  • domain wall
  • spin ice
  • MOKE
  • cryogenics
Abstract:
We present data and analysis from two techniques for probing the nanoscale behavior of spins in ferromagnetic thin films. The first is an all electrical measurement we use to determine the position of magnetic domain walls in the ferromagnetic semiconductor (Ga,Mn)As at high precision and speed. We exploit these properties to create a scanning domain wall magnetometer, which uses the domain wall itself as a nanoprobe. Data from the magnetometer evinces a highly disordered, non-repeatable domain wall interaction potential, which we determine to be a result of an elastic stretching mode of domain wall motion. The stretching mode had been predicted theoretically1 but not directly measured. We then take higher resolution data of the stretching mode specifically, and use it to extract domain wall pinning site strength, energy, and density. We also observe much higher domain wall mobility in the stretching mode than has been previously measured at larger domain wall displacements, a property which may be useful for future spin based device applications. Our second measurement technique is based on the magneto-optic Kerr effect (MOKE), which we use to monitor the field dependent magnetization and susceptibility of interacting Pt/Co and permalloy nanomagnet arrays. We observe evidence for magnetostatically mediated collective behavior at small lattice spacings in both materials. Position resolved magnetization data taken on the Pt/Co arrays tentatively shows domain nucleation and growth.