Quantum coherence in a ferromagnetic metal: Time-dependent conductance fluctuations

Quantum coherence of electrons in ferromagnetic metals is difficult to assess experimentally. We report measurements of time-dependent universal conductance fluctuations in ferromagnetic metal $\left({\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}\right)$ nanostructures as a function of temperature and magnetic field strength and orientation. We find that the Cooperon contribution to this quantum correction is suppressed, and that domain wall motion can be a source of coherence-enhanced conductance fluctuations. The fluctuations are more strongly temperature dependent than those in normal metals, hinting that an unusual dephasing mechanism may be at work.

Figure 1

(a) Noise measurement scheme. Only five consecutive leads are used for the noise measurement among seven leads. (b) MFM image of one segment of $27\text{−}\mathrm{nm}$-wide wire. The whole segment is a single domain. Left and right edges of image are $\mathrm{Ti}∕\mathrm{Au}$ leads covering the wire. (c) MFM image for $450\text{−}\mathrm{nm}$-wide wire. Arrows indicate some of the domain walls.

Figure 2

Anisotropic magnetoresistance data with a perpendicular magnetic field for the $100\text{−}\mathrm{nm}$-wide wire at $8\mathrm{K}$. Resistance of the sample was recorded while sweeping magnetic field between $\pm 2\mathrm{T}$. Solid line is for sweeping the field from $-2\mathrm{T}\text{to}+2\mathrm{T}$ and dashed line is for sweeping the other direction. Arrow indicates the discontinuity due to magnetization reorientation, and the switching field, $B_S$, for this sample was $\sim 0.59\mathrm{T}$. Left inset shows four-terminal scheme used for the AMR measurement. Right inset shows $R$ vs $T$ for this sample (slight rise below $10\mathrm{K}$ not apparent on this scale).

Figure 3

Noise power as a function of temperature for all four samples: (a) $w=27\mathrm{nm}$, (b) $50\mathrm{nm}$, (c) $100\mathrm{nm}$, and (d) $450\mathrm{nm}$. For all four sets of data, solid squares are $B=0\mathrm{T}$; open squares are $B_{\perp }=8\mathrm{T}$; open triangles are $B_{\Vert }=8\mathrm{T}$; and solid triangle is $B_{\perp }\sim B_{\mathrm{s}}$. Error bars are comparable in size to the symbols.

Figure 4

Comparison of noise power plot between $50\text{−}\mathrm{nm}$-wide Py wire (triangle points) and $35\text{−}\mathrm{nm}$-wide nonmagnetic samples (square points). Solid symbols are for $B=0\mathrm{T}$ and open symbols are for $B=0.512\mathrm{T}$ for nonmagnetic wire and $B=8\mathrm{T}$ for magnetic wire. Nonmagnetic wire data was shifted upward by a factor of 5 for clarity. Inset: Noise power as a function of sample volume at $2\mathrm{K}$. Solid squares are for Py wires and open square are for nonmagnetic AuPd wires (identical sample geometry, with thicknesses from $6.5\mathrm{nm}\text{to}9\mathrm{nm}$).