It is not clear, what are "residual magnetic properties" which you want to maintain. For what purpose and how long do you intend to maintain this property?

Of course, there are magnetic properties which depend on size, volume, prehistory of sample etc. Some magnetic properties can be changed if we replace objects surrounding the sample or if we measure magnetic properties applying external magnetic field.

If elemental, phase and mineral composition of the sample almost the same as had the original rock, you can measure specific properties of sample material, like susceptibility, permeability, saturation magnetization, coercive force, residual magnetization, taking into account effects of size, shape... Statistics of results of sufficient number of samples also could help to understand properties of the original rock. 

Thanks prof. Gregory. If I am not wrong, there may be a possibility for magnetic domains of rocks to get disturbed during the  sampling processing of rock samples from the origin site using metal tools. In that case what may be the sampling scheme?   

Structure of magnetic domains is forming inside of grains of magnetic phase as a result of competition between magnetic and exchange interactions of electrons. Domain structure is changing when the external field is applied during measurements. So we don't need and aren't able to maintain it. The correct goal is to maintain maximal number of magnetic phase grains in their original enviroment.

Processing affects usually the grains, which have been subjected to more or less near the treated surface. In order to reduce surface effects, sample has to be much bigger in all directions than the grain sizes. If the sample contains much larger number of grains of magnetic phase, which are inside of its volume, in comparing with number of grains near the treated surface, its magnetic properties sufficiently good correspond to properties of original rock.

You can apply different methods of sampling in order to compare processing effects. Different delicate methods are developed for cutting crystals, diamonds, metals (electrical discharge, slow sawing etc.) in order to diminish deepness of affected surface. If the rock is brittle, it is possible to break it and find pieces which are sufficiently small to be measured but they are big enough in comparing with grain size. Also there are methods of chemical etching, which can decrease stresses on the sample surface. Study of surface can help to determine shape and size of grains.

Yes and thanks Professor. As you said, the loss of grains may be negligible compared to the size of the sample and also thanks for various methods of sampling.

Professor, Is it possible to enhance the elastic properties of rock by doping or substituting minerals in to rock samples? If it is possible, how rapid, the doping would be achieved and what are the methods?  

You should not heat the sample, and keep in mind its original orientation since its remanent magnetization must be anisotropic (after cooling in the local terrestrial field).

Why not cool it before machining, since remaining below room temperature will preserve it's room temperature magnetization.

See:  http://en.wikipedia.org/wiki/Rock_magnetism

Old ref. :    L.Neel for model, E. Thellier for experiments

Obviously you must avoid any strong magnetic field perturbation (with magnetic tools), but moving the sample in the actual terrestrial field, at room temperature should not have much effect (since we are well below the melting tre of the rock or the blocking tre of grains inside).  I guess you will meausre the "remanent magnetization" and its anisotropy at room temperature before its thermal dependence until its disappearence, to get the characteristics of its magnetic history.

Then, (after heating) all the magnetization curves will give you the magnetic properties of the material, but no more its magnetic history

Hi,

When we collect rock specimens in the field we can collect them in a variety of different fashions. The most common, and with the most potential to affect the remanent magnetization (what you called "residual" magnetic properties) is by gasoline powered drill. It is essentially a modified chainsaw with diamond tipped drill bit (see http://www.ascscientific.com/drills.html).

Since things get very hot cutting rock the drill is also modified with a water valve that allows pressurized water (from a hand pump) to be used to cool the rock and the drill bit. Generally speaking, this works pretty well and temperatures will never go more than a few tens of degrees above ambient. There maybe some micro scale heating at the drill/rock interface, but this will still remain relatively low (I guess no more than 100C) and very localized. 

In terms of the effect on remanent magnetization, if the rock is very viscous it will pick up what we call a "drilling overprint" due to heating during drilling. In general, this is very easily removed in the lab to isolate the characteristic remanent magnetization.

If you have any questions specific to what you are trying to do, please ask.

Thanks prof. Jean and Greig. As said by prof. Jean, my intention is to know the magnetic signature of rock specimen. On my trials, I will come for further clarifications to you.