Interparticle Interaction Effects in Nonmiscible CoAg Thin Films With High Co Concentration
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hown 5,6 that the magnetic percolation threshold in CoAg heterogeneous alloys is thickness dependent, quite probably because of the reduced dimensionality of very thin films. The comparison of the magnetic remanent state after different field cycles has been shown to be a very useful and powerful technique to study the magnetic interactions among particles in powder-like materials and thin films11 . The 5M curve is defined12 as the difference of two zerofield remanence curves: 56M= 2 Mr-1 - Md. Mr is the isothermal remanent magnetization (or IRM) and is obtained by starting from a demagnetized state and measuring the magnetization at zero field after applying fields of increasing amplitude. Md is known as the dc demagnetization curve (or DCD) and can be obtained by applying a large negative saturating field, then a positive field (that is increased after each measurement) and then measuring the remanent magnetization. Although several different ways to quantify the interparticle interactions have been proposed, the most widely accepted is the one proposed by Che and Bertram 12 in which two parameters, a and /3, can be derived from a phenomenological model considering magnetostatic and exchange interactions. An easy method to derive a and /3 from the experimental data was proposed by Harrell et al.13 , leading to
0
8=J8Mdh, o3
-
o~
-
(1)
3Mr-I
In the expression above h is the incremental applied field normalized to the remanent coercivity, Hrem is the reverse negative field that, after saturation in the forward direction, produces a zero magnetization at zero field, and Mr° is the remanent magnetization at the point where the 5M curve crosses zero. Positive values of the parameter a originate when the interparticle interactions favor a magnetized state and in thin film media are generally due to exchange coupling 14 . Negative values of a, on the other hand, are usually related to weakly interacting particles coupled via dipolar-type interactions. When the 6M curve does not cross zero, the 8 parameter is defined as zero. The time dependent effects are usually characterized through the magnetic viscosity S. If a magnetic sample is saturated in one direction and a magnetic field (close to H,) is applied in the opposite direction, M changes with time, even if H is kept constant. The change in magnetization with time is due to thermally activated magnetization transitions over anisotropy barriers. M(t) is generally 15 found to obey a logarithmic law 16 , M(t) = C + Sln(t / to),
(2)
where C and to are constants and t is the time elapsed since the field was applied. Sometimes the magnetization decay is expressed in percent decay/decade, 5,that is simply related to S by 8 = In(10) (S/Ms) x 100. The magnetic viscosity and the irreversible susceptibility, j,, (obtained from the derivative of the dc demagnetization remanence curve) are used to obtain the 17 fluctuation field, Hf, and the activation volume , Vac, S -By-VM kBT ___r (3) The fluctuation field is an imaginary field equal to the effect of thermal agitation
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