I sputtered NiAl targets using a DC sputtering equipment onto glass substrates to produce 2 thin film samples.
Sample A : Particle size = 16 nm, Hc = 130 Oe
Sample B : Particle size = 28 nm, Hc = 102 Oe
The data shows that the increse in grain size lowers the Hc values.
However, this is againts previous research (Dunlop (1981), Herzer (1997)) that shows, for nanoparticles, increase in particle size leads to increase in Hc values.
Is there any explanation on why I am getting this result?
Coercivity vs. particle size starts at zero and goes through a maximum, before decreasing . The very smallest particles or grains have zero coercivity, they are superparamagnetic. This generally occurs below 10nm but it is strongly material dependant. The particle is so small that thermal energy is enough to randomize the moments as soon as the field is removed. As the particle size is increased the coercivity increases, its still a single domain, but there are more moments and thus more energy is required to remove their order. A maximum is reached for a single domain particle. As the particle increases further in size, more than one domain is developed, the moments within a domain may be aligned, but other domains are not lined up. Here the coercivity is lowered because all energy goes into moving domain walls. Domain walls require less coercive energy to move, giving again a lower coercivity.
Larger particles may be multidomain, while smaller particles are, as a rule, single-domain. Multidomain structure of large particles may lead to a decrease in coercivity force.
Coercivity is not an intrinsic property of magnetic materials. It strongly depends on parameters such as material defects, stress and surface smoothness [ "Study of the grain size, particle size and roughness of substrate in relation to the magnetic properties of electroplated permalloy", J.B. Yi a, X.P. Li a,b,∗, J. Ding c, H.L. Seet, Journal of Alloys and Compounds 428 (2007) 230–236]
I can add that Hc (according Herzer) depends not only on the grain size but also on the anisotropy constant and the magnitude of the exchange coupling between the crystallites. For example the sample with a smaller crystallite size is characterized by a greater anisotropy constant associated with magnetoelastic contribution from the particle surface and it results in greater Hc.
Coercivity vs. particle size starts at zero and goes through a maximum, before decreasing . The very smallest particles or grains have zero coercivity, they are superparamagnetic. This generally occurs below 10nm but it is strongly material dependant. The particle is so small that thermal energy is enough to randomize the moments as soon as the field is removed. As the particle size is increased the coercivity increases, its still a single domain, but there are more moments and thus more energy is required to remove their order. A maximum is reached for a single domain particle. As the particle increases further in size, more than one domain is developed, the moments within a domain may be aligned, but other domains are not lined up. Here the coercivity is lowered because all energy goes into moving domain walls. Domain walls require less coercive energy to move, giving again a lower coercivity.
Dr. Beliayev, Dr. Siddiqui, Dr. Komogortsev, Dr. Baker,
Thank you very much for the input. Dr. Siddiqui, thank you for providing the reference.
1. Is there any way I can determine whether a particle have dingle or multidomain, so that I can show proof that multidomain is formed as the particle size increases? Can the XRD data show this?
2. Dr. Komogortsev, this is the first time I came across the explanation regarding how the magnetoelastic contribution from the particle surface effects the Hc. Is there any reference/journal that I can refer to?
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