M-type barium hexaferrite is known to be a hard magnetic material with a typical coercivity of few thousand Oe. Upon substitution of Fe ions by certain metallic ions and sintering at high enough temperature, XRD reveald the presence of two distinct phases: the M-type and the W-type, both of which are magnetic with different saturation magnetizations and coercivities. I would expect, therefore, to obtain a hysteresis loop for the synthesized sample which is characteristic of a superposition of that corresponding to the two phases, with some shoulders or kinks, possibly.The magnetic measurements, however, showd a hysteresis loop characteristic of a single ferromagnetic material with substantially lower coercivity in comparison with the pure phase. How can we then interpret the observed behavior?
If the material consists of two interacting magnetic phases and the phase size of the few nanometers, there is can be no kinks in magnetization curve.
Check these links please.
http://en.wikipedia.org/wiki/Exchange_spring_magnet
In exchange bias systems, e.g., you have also different hysteresis loops from different parts of the system which add up to one loop. When I'm performing simulations of these systems, I can see the different loops and the sum of them - which does normally not show any steps or other irregularities along the slope of the loop.
Maybe it's worth a try in your system, too, to subtract the one-phase loop from the double-face one, with a scaling factor for the one-phase loop according to the percentage of this phase in the mixture, and to see whether the difference looks similar to the loop expected for the second phase.
In fact I have done something similar, but the other way around. I have added the hyesteresis loops of the two phases with different proportions, and always obtain irregularities in the simulated loop. The problem seems to be due to the presence of the hard magnetic phase superimposed on the soft phase.
When both phases are mixed, they will additionally influence each other. As soon as one phase starts with the magnetization reversal, this will lead to field changes in the material which will trigger the other material's magnetization reversal. Maybe this could easily be simulated by adding two (or alternating) layers of two materials with properties such as those you would expect for the discrete phases - at least it would be worth a try.
If the material consists of two interacting magnetic phases and the phase size of the few nanometers, there is can be no kinks in magnetization curve.
Check these links please.
http://en.wikipedia.org/wiki/Exchange_spring_magnet
Thanks Sergey, this source is interesting. The effect we are observing, however, is just the opposite of the spring magnet effect, in the sense that the material is becoming soft rather than hard. However, the arguments in the suggested source could be intimately rlated to the observed properties.
Hysteresis loop being a linear combination of two other hysteresis loops only works correctly when both magnetic phasese are located in space far enough one from another, so they don't interact. This simplification may also work satisfactorily in case of two-layer sample, with layers thick enough to neglect interface interactions. This picture may, or may not, be applicable for interpenetrating phases, especially when at least one of them has the form of nanocrystals.
Thanks for this hint, Marek - my former exchange bias systems consisted of two layers, which is surely a difference from the samples examined here. However, even in my old samples, I found only a few hints for irregular hysteresis shapes, such as steps or "double hysteresis" loops. And even if I carry out simple (Ising) simulations of a layer-system with completely different parameters per layer, the final loop does not show any irregularities. Thus I would really assume that the interaction of both phases does not allow for finding irregularly-shaped loops.
As rightly pointed out above, in non-interacting systems, one more thing which matters is the volume fraction. Dominant behaviour in superimposed conditions
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