Band-structure-dependent nonlinear giant magnetoresistance in Ni${}_{1-x}$Fe${}_x$ dual spin valves

Conventional giant magnetoresistance (GMR) in spin valves is current-independent, so the resistance of a device depends only on the relative orientation of the magnetic layers. In dual spin valves consisting of three ferromagnetic (FM) layers separated by nonmagnetic (NM) spacers (i.e., a FM1/NM/FM2/NM/FM1), GMR can be current-dependent if spin can accumulate in FM2 when outer FM1 layers are aligned antiparallel. Currently the underlying physics is poorly understood, although spin accumulation in FM2 is likely to depend on the gradient in the density of states at the Fermi energy of the ferromagnet. To investigate this hypothesis, we have measured a series of dual spin valves with Ni${}_{1-x}$Fe${}_x$ as FM2 layers of varying composition. We show that both the magnitude and sign of the nonlinear GMR depend strongly on the Fe content and thus on the band structure of the ferromagnet FM2.

Figure 1

Minor loops as a function of applied bias current for DSVs with different FM2 layer compositions as indicated in each figure measured at room temperature. The numbers to the right of each graph show the magnitude of the dc bias current in mA. Positive current denotes electron flow from IrMn/CoFe to CoFe. The vertical bars in each figure denote the scale. The inset shows the layer sequence in the DSV stack.

Figure 2

Finite-element simulation of the Oersted field in a nanopillar device with dimensions of 250 $\times$ 250 nm${}^2$. (a) The nanopillar geometry with the current flow direction indicated by black arrows. The Oersted field distribution is calculated at the middle of the nanopillar through a cross section as shown. (b) The field distribution for a current of 1 mA with the color scale denoting the magnitude of the magnetic field generated by the current in units of mT; the maximum field ($\sim$1.5 mT) is at the middle of the edges of the pillar, and the bulk of the layer experiences a field of less than 1 mT.

Figure 3

The change in the resistance-area product with current and middle Ni${}_{1-x}$Fe${}_x$ layer composition. Each data point represents the average value of $A\Delta R$ from many devices and chips measured.

Figure 4

Normalized magnitude of nonlinear GMR ($\alpha$) defined by the slope of the linear region of resistance-area product ($A\Delta R$) vs current density ($J{}_{\mathrm{dc}}$) as a function of Ni$\%$. Each error bar represents one standard deviation measured from a set of $A\Delta R$ vs $J{}_{\mathrm{dc}}$ data collected from several devices on different substrates.

Figure 5

(a) Two possible locations of the Fermi energy with respect to the peak in density of states (DOS) without spin accumulation. (b) Spin accumulation on top of Fermi energy indicated by red bands. In the presence of a spin accumulation, electrons are transported through a different density of states (top of the red band).