Time Dependent Magnetic Switching in Spin Valve Structures
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J. B. RESTORFF, M. WUN-FOGLE, S. F. CHENG, AND K. B. HATHAWAY Naval Surface Warfare Center, 10901 New Hampshire Ave., Silver Spring, MD 20903 ABSTRACT We have observed time dependent magnetic switching in spin-valve sandwich structures of Cu/Co/Cu/Fe films grown on silicon and Kapton substrates and Permalloy/Co/Cu/Co films grown on NiO or NiO/CoO coated Si substrates. The giant magnetoresistance (MR) values ranged from 1 to 3 percent at room temperature. The films were grown by DC magnetron sputter deposition. Measurements were made on the time required for the MR to stabilize to about 1 part in 104 after the applied field was incremented. This time depends almost linearly on the amplitude of the timedependent MR change with a slope (time / AMR) of 20 000 to 30 000 s. Some samples took as long as 70 s to stabilize. The time dependent effects may be important for devices operating in these regions of the magnetoresistance curve. In addition, measurements were made on the time history of the MR value for a period of 75 s following a step change in the field from saturation. We observed that the time dependent behavior of the MR values of both experiments produced an excellent fit to a function of the form AMR(t) = cc + f0ln(t) where ax and f0 are constants. This time dependence was consistent with the behavior of the magnetic aftereffect. INTRODUCTION Layered structures consisting of alternating ferromagnetic and nonmagnetic materials are capable of exhibiting giant magnetoresistancel. A brief review in given in Ref. 2. Because the resistance of these devices depends on the relative orientation between the spins in the ferromagnetic layers, they are referred to as spin-valves. Consider a simple example. Fig. 1 shows a layered device (Sample C of this paper). The copper serves as a "magnetic insulator" to reduce the coupling between the ferromagnetic layers. When a large magnetic field is applied, the spins in both the Co and Fe layers line up in the same direction. When the magnetic field is reversed, the Fe spins will change direction first since Fe has a lower coercivity, - 4.5 kA/m (56 Oe). As the field intensity is increased further, the spins in the Co layers [coercivity - 15.9 kA/m (200 Oe)] will eventually change direction. This can be seen in Fig. 2(a), which shows the magnetization vs field loop of sample C. The resistance of the spin valve is given by 3 Co (30 A) A)(1 5) 1 (1) R = RP + (RAP -Rp)(1-cos0) 2 Fe (30oA) where R is the resistance, 0 is the angle between the spins, RAP is the resistance when the spins are antiparallel (0 = 7t), and Rp is the resistance when the spins are parallel (0 = 0). We measure the magnetoresistance (MR), which is defined as
Si substrate Fig. 1. Example of a spin-valve structure.
385 Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
MR=ARIR=(R-Rp)IRp.
(2)
MR = RAP- Rp (I _cos0) 2Rp
(3)
With equation (1), this gives
The measured magnetoresistance of Sample C is shown in Fig. 2(b). Field Oe-70 150 00 -150el(Oe) . (a)
-700 -350 (b)
300 ,
0(350
700
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