Magnetic and Magneto-Transport Properties of Novel Nanostructured Networks


Kai Liu and C. L. Chien, The Johns Hopkins University, Baltimore, MD 21218



    I. Introduction

Systems with reduced dimensions often exhibit novel and enhanced magnetic and transport properties. Porous magnetic networks are a novel type of artificial structure with interesting properties such as enhanced coercivity. The networks are thin films deposited onto porous membranes, which replicate the nanometer-sized porous structure (Fig. 1). The magnetic properties of the networks depend on the layer thickness, as well as the pore size of the membranes. This type of novel magnetic network suggests a simple way of controlling the magnetic hardness of the material.

Fig. 1. Schematic of the nanoporous alumina membrane (left) and the replicated network on top of the membrane (right).  At small thickness of the network, the net width is the same as the wall width of the membrane between adjacent pores.

Furthermore, micromagnetic modeling by Zhu et al. [IEEE Trans. Magn. 34, 1609 (1998)] has predicted superior characteristics of networked magnetic recording media over the conventional thin film media. Because of the smaller number of neighboring grains in the networked media, the resultant reduction in the intergranular ferromagnetic exchange coupling leads to a suppression of the clustering magnetization reversal involving a large number of grains. Consequently, the networked media are expected to show lower switching noise at high recording density. They have also been predicted to be thermally more stable than conventional media. These studies further draw interest in magnetic networks as a new medium for exploring magnetism and other intricate physics.
 

   II. Fabrication and Characterization

Magnetron sputtering of permalloy (Py = Ni81Fe19) and Fe onto porous Anopore membrane.

Table 1. Specifications of the nanoporous alumina membrane used for the network fabrication.


Pore Diameter (nm)
Pore density (/cm2)
Porosity (%)
Wall thickness (nm)
20
1012
25-30
18
100
1010
40
58
200
109
50
82

Fig. 2. (Left) Atomic Force Microscopy image of the surface of a 100-nm pore-sized alumina membrane. (Right) Top view of Scanning Electron Microscope image of a 10 nm thick Py porous network grown on a 200-nm pore-sized alumina membrane.
 

  III. Magnetic Properties

    1. Coercivity enhancement

Fig. 3. Room temperature magnetic hysteresis loops of 20 nm thick Py grown on (a) Si and (b) 20-nm pore-sized membrane, and 30 nm thick Fe grown on (c) Si and (d) 20-nm pore-sized membrane.  Note the different scales for H.
 

    2. Layer thickness dependence of coercivity and squareness

Fig. 4. Layer thickness dependence of coercivity (a) Hc and (b) squareness at 300 K for Py networks grown on alumina membranes with various pore sizes.  Solid square, open square, and solid circle represent results with 20-nm, 100-nm, and 200-nm pore sizes, respectively.
 

  IV. Magnetoresistance


Fig. 5. Room temperature LMR and TMR of 20-nm (a) uniform Py film, and porous Py films grown on alumina membranes of pore sizes (b) 20 nm, (c) 100 nm, and (d) 200 nm.
 

Suppression of Anisotropy Magnetoresistance: Nanostructure of the networks


Fig. 6. Schematic of current distribution in (a) small and (b) big pore-sized porous membranes.

Both LMR and TMR in porous magnetic networks on the macroscopic scale are the superpositions of the usual LMR and TMR on the microscopic scale.  Since the usual LMR and TMR have opposite signs, they tend to cancel each other.  Consequently, the anisotropy of the MR in the networks is reduced, particularly so for larger pore sizes.
 
 

References

  1. Kai Liu and C. L. Chien, IEEE Trans. Magn. 34, 1021 (1998).
  2. Kai Liu, Ph.D. dissertation, The Johns Hopkins University, 1998.


Contact Us

Kai Liu:          kliu@pha.jhu.edu
C. L. Chien:    clc@pha.jhu.edu

Last Modified 2/7/99 by Kai Liu