Rakhi Acharyya

A way to measure electron spin-flipping at ferromagnetic/nonmagnetic interfaces and application to Co/Cu

We describe a technique, using the current-perpendicular-to-plane (CPP) geometry, to measure the parameter δF/N, characterizing flipping of electron spins at a ferromagnetic (F)/nonmagnetic (N) metallic interface. The technique involves measuring the CPP magnetoresistance of a sample containing a ferromagnetically coupled [F/N]n multilayer embedded within the 20 nm thick central Cu layer of a symmetric Py-based, double exchange-biased spin-valve. To focus on δF/N, the F- and N-layers are made thin compared to their spin-diffusion lengths. We test the technique using F/N=Co/Cu. Analyzing with no adjustable parameters, gives inconsistency with δCo/Cu=0, but consistency with our prior value of δCo/Cu=0.25±0.1. Taking δCo/Cu as adjustable gives δCo/Cu=0.33−0.08+0.03.

Specific resistance of Pd/Ir interfaces

From measurements of the current-perpendicular-to-plane (CPP) total specific resistance (AR = area times resistance) of sputtered Pd/Ir multilayers, we derive the interface specific resistance, 2AR(Pd/Ir) = 1.02 +/- 0.06 fOhmm^2, for this metal pair with closely similar lattice parameters. Assuming a single fcc crystal structure with the average lattice parameter, no-free-parameter calculations, including only spd orbitals, give for perfect interfaces, 2AR(Pd/Ir)(Perf) = 1.21 +/-0.1 fOhmm^2, and for interfaces composed of two monolayers of a random 50%-50% alloy, 2AR(Pd/Ir)(50/50) = 1.22 +/- 0.1 fOhmm^2. Within mutual uncertainties, these values fall just outside the range of the experimental value. Updating to add f-orbitals gives 2AR(Pd/Ir)(Perf) = 1.10 +/- 0.1 fOhmm^2 and 2AR(Pd/Ir)(50-50) = 1.13 +/- 0.1 fOhmm^2, values now compatible with the experimental one. We also update, with f-orbitals, calculations for other pairs

A study of spin–flipping in sputtered IrMn using Py-based exchange-biased spin-valves

To study spin–flipping within the antiferromagnet IrMn, we extended prior current-perpendicular-to-plane giant magnetoresistance studies of Permalloy (Py)-based exchange-biased-spin-valves containing IrMn inserts to thicker IrMn layers—5 nm ≤ tIrMn ≤ 30 nm. Unexpectedly, AΔR = A(RAP − RP)—the difference in specific resistance between the antiparallel (AP) and parallel (P) magnetic states of the two Py layers—did not decrease with increasing tIrMn, for tIrMn ≥ 5 nm, but rather became constant to within our measuring uncertainty. This constant looks to be due mostly to a new, small magnetoresistance in thin Py layers. The constant complicates isolating the spin-diffusion length, lsfIrMn, in bulk IrMn, but lsfIrMn is probably short, ≤1 nm. Similar results were found with FeMn.

Conduction electron spin–flipping at sputtered Co90Fe10/Cu interfaces

From measurements of the current-perpendicular-to-plane magnetoresistance of ferromagnetically coupled [Co90Fe10/Cu]n multilayers, within sputtered Permalloy-based double exchange biased spin-valves, we determine the parameter δCo(90)Fe(10)/Cu = 0.19 ± 0.04 that sets the probability P of spin–flipping at a Co90Fe10/Cu interface via the equation P = 1 – exp(−δ).From measurements of the current-perpendicular-to-plane (CPP) magnetoresistance of ferromagnetically coupled [Co(90)Fe(10)/Cu]xn multilayers, within sputtered Permalloy-based double exchange biased spin-valves, we determine the parameter delta[(Co(90)Fe(10))/Cu] = 0.19 +/- 0.04 that sets the probability P of spin-flipping at a Co(90)Fe(10)/Cu interface via the equation P = 1 - exp(-delta).

Spin-Flipping Associated With the Antiferromagnet IrMn

We have used current-perpendicular-to-plane magnetoresistance measurements of Py-based exchange-biased spin-valves containing IrMn inserts of thickness <i>t</i> to estimate the spin-flipping probability of the antiferromagnet IrMn. From <i>t</i>=0 to <i>t</i>= 1 nm, we find a rapid decrease in <i>A¿R</i>=<i>A</i>(<i>RAP</i> - <i>RP</i>) , by about a factor of 50-here <i>A</i> is the area through which the CPP current flows, and <i>RAP</i> and <i>RP</i> are the resistances with the moments of the two Py layers oriented anti-parallel (AP) or parallel (P) to each other. We attribute this decrease to very strong spin-flipping in the IrMn/Cu interfacial region, with effective spin diffusion length <i>l</i> _sf ^IrMn/Cu= 0.24 nm, only about 1 monolayer (ML). But for <i>t</i> from 2 to 5 nm, the decrease of <i>A</i>¿<i>R</i> with increasing IrMn thickness is much slower. The reason for this slowing is not yet clear.

CPP Transport Properties of Ni/Ru and Interfaces

Using current-perpendicular-to-plane (CPP) magnetoresistance measurements, we derive values of twice the enhanced interface resistance, 2AR_F/N* and the interface scattering asymmetry, ¿_F/N, the ferromagnetic/non-magnetic (F/N) pairs Ni/Ru and Co₉₀Fe₁₀/Cu. For Ni/Ru, we find 2AR_NI/Ru* = 1.7_-0.3 ^+0.4 f¿m², similar to the value for Fe/Cr, but we estimate |¿_Ni/Ru| = 0.15 ± 0.03, much smaller than the value for Fe/Cr or the asymmetry found for Ni(Ru) alloys. For Co₉₀Fe₁₀ we find 2AR_CoFe/Cu* = 1.1 ± 0.2 and ¿_CoFe/Cu = 0.8 ± 0.1, both similar to the values for Co/Cu.

CPP Transport Properties of Ni/Ru and Co90Fe10/Cu Interfaces

Using current-perpendicular-to-plane (CPP) magnetoresistance measurements, we derive values of twice the enhanced interface resistance, 2AR_F/N* and the interface scattering asymmetry, ¿_F/N, the ferromagnetic/non-magnetic (F/N) pairs Ni/Ru and Co₉₀Fe₁₀/Cu. For Ni/Ru, we find 2AR_NI/Ru* = 1.7_-0.3 ^+0.4 f¿m², similar to the value for Fe/Cr, but we estimate |¿_Ni/Ru| = 0.15 ± 0.03, much smaller than the value for Fe/Cr or the asymmetry found for Ni(Ru) alloys. For Co₉₀Fe₁₀ we find 2AR_CoFe/Cu* = 1.1 ± 0.2 and ¿_CoFe/Cu = 0.8 ± 0.1, both similar to the values for Co/Cu.

CPP Transport Properties of Ni/Ru and

Using current-perpendicular-to-plane (CPP) magnetoresistance measurements, we derive values of twice the enhanced interface resistance, 2AR_F/N* and the interface scattering asymmetry, ¿_F/N, the ferromagnetic/non-magnetic (F/N) pairs Ni/Ru and Co₉₀Fe₁₀/Cu. For Ni/Ru, we find 2AR_NI/Ru* = 1.7_-0.3 ^+0.4 f¿m², similar to the value for Fe/Cr, but we estimate |¿_Ni/Ru| = 0.15 ± 0.03, much smaller than the value for Fe/Cr or the asymmetry found for Ni(Ru) alloys. For Co₉₀Fe₁₀ we find 2AR_CoFe/Cu* = 1.1 ± 0.2 and ¿_CoFe/Cu = 0.8 ± 0.1, both similar to the values for Co/Cu.