# Presentation: 920799

## Cobalt/Copper Nanowire Synthesis & Spin-Transport Properties

**Peter Greene,* Randy K. Dumas and Kai Liu**
*University of California – Davis*
**University of Washington*
**Support: NSF-REU (PHY-0649297) and the Alfred P. Sloan Foundation**

## Overview

- Motivation
- Sample Fabrication
- Measurement Techniques
- Results

## Motivations and Goal

- 	 Motivations
- Multilayered nanowires as model system to study nanomagnetism & spintronics
- Magnetization reversal mechanisms of nanodiscs fascinating:
- 	Vortex state vs. single domain
- Spin-dependent electrical transport as manifestation of magnetic configurations
- Applications: Ultrahigh density magnetic storage & novel spintronic devices
- Goal
- Probe nanoscale magnetic configuration using giant magnetoresistance

## Nanowire Fabrication

## Electrodeposition

-  Electrolyte contains Co2+ and Cu2+ ions
-  Concentration of Co is 80 times that of Cu
-  Integrate current to estimate number of atoms deposited and obtain desired thickness

## Magnetoresistance (MR)

-  Modified VSM operating system to make resistance versus applied field measurements
-  Capability to make both AC and DC resistance measurements
-  Automation makes resistance FORCs feasible
-  Will R-H and M-H FORCs yield correlation between magnetic configuration & MR?

## Giant Magnetoresistance

-  Current flowing perpendicular to spaced magnetic planes can be decomposed into two oppositely spin polarized currents.
-  The current with electron spin parallel to the magnetization of a plane is preferred and thus is less likely to scatter.
-  Low fields allow the magnetic layers to point antiparallel or randomly causing a high resistance state as neither spin polarization is preferred.
- High fields align all of the magnetic layers causing one spin channel to be shunted throughout the wire.

**High Resistance**
**Low Field **

**Low Resistance**
**High Field**

**2007 Nobel Prize in Physics:**
- M. N. Baibich, J. M. Broto, A. Fert, F. N. V. Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas, Phys. Rev. Lett. **61**, 2472 (1988).
- G. Binasch, P. Grunberg, F. Saurenbach, and W. Zinn, Phys. Rev. B **39**, 4828 (1989).

## Single Domain vs. Vortex State

**Single Domain vs. Vortex State**

## Reversal Mechanisms

**Single Domain**

**Vortex State**

- M-H FORCs show information about the domain reversal process.
- Perhaps R-H FORCs will show similar information with smaller sample sizes.
- How does the two current model of GMR apply to vortex states?

- R. K. Dumas, *et al*, Phys. Rev. B, **75**, 134405 (2007); Appl. Phys. Lett. **91**, Nov. 12, 2007.

- K. Liu, *et al*, Appl. Phys. Lett., **81**, 4434 (2002).

## Results: Single Domain GMR

-  Fabricated 50- 200nm diameter nanowires with Cu and Co thickness 1- 60nm
- Obtained 7% room temperature CPP-GMR
- Strong evidence of single domain reversal

**50nm diameter [Co(5nm)/Cu(8nm)]****400**

## Results: Alternate GMR Signals

- Evidence of vortex state reversal if disks are isolated
- Low field: anisotropic MR
- Saturation resistances are not equal (?)
- Magnetostatic interactions within the wires are strong

**200nm diameter [Co(50nm)/Cu(10nm)]****150 ****nanowires**

## Summary

**Synthesized arrays of Co/Cu multilayered nanowires**
**Measured magnetization reversal signatures  and magneto-transport properties**
**50nm diameter [Co(5nm)/Cu(8nm)]****400**** nanowires: **
**Single-domain reversal**
**7% room temperature CPP-GMR**
**200nm diameter [Co(50nm)/Cu(10nm)]****150 ****nanowires:**
**Reversal via vortex state in Co nanodiscs with similar dimensions**
**Fine features in MR at low field – anisotropic MR due to vortex structures**