This is certainly not a statement which is generally true. But anisotropy terms may (and usually do) exhibit temperature dependence, mostly such that the (effective) anisotropy constant is reduced at higher temperature. KV and Ki need not share the same temperature dependence. So, if KV decays faster than does Ki then it may in fact become "negligible" in comparison. But that will depend on the specifics of the system(s) considered.
Btw., where did you find the statement of your question?
In relation Keff = Kv+ (Ki/ t) ; if tCoFeB is higher film thickness in Ta (5 nm) /CoFeB ( 50 nm)/MgO (1 nm) , what will be the magneto crystalline anisotropy and is this (magneto crystalline anisotropy) negligible in Ta (5 nm) /CoFeB ( 50 nm)/MgO (1 nm) trilayer system . Could you please explain about this?
Isn't it the case that FeCoB is an amorphous layer? I would think so (addition of Boron tends to facilitate the formation of 'metallic glasses'). In this case, there will be very little (if any, I don't know that exactly) "magnetocrystalline" anisotropy, simply because there is no crystallinity. In this case, the perpendicular anisotropy derives (to a [very] large extent) from the interface.
Well, the simple-to-formulate yet not always simple-to-do advice is that if you produce these materials you have to analyze them. For the structure this could mean x-raying and/or doing SAD for example. X-ray reflectivity for thickness determination. If this is for PMA tunneling devices, then roughness is a huge issue -> maybe AFM. Btw., how do you deal with surface oxidation, does it matter?
This aside, DO check whether Boron is still in your material or not (e.g. XPS)
For magnetic characteristics you will need to do magnetic characterization, typically magnetometry or, maybe, MOKE.
In experimental science you don't get results unless you produce them and make sure they make sense. The "bad" part is that also you need to acquaint yourself sufficiently with the methods to gain justified confidence in what you do. Support by knowledgeable colleagues is often a must.
[I am not an expert by any means for the specific system you're dealing with.So I cannot give more specific advice out of the blue. I just can give you an example from my place: in my group we're now trying to grow thin film materials we never did before - it takes weeks and (maybe) months of labour to find out the best route to getting there. For the moment we don't even know whether we'll be successful ultimately.]
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