Hi Stefan,

I do not know which magnetic field strength you will aim at and what temperature you will have.

Depending on the field strength you want to achieve, you may use a Helmholtz coil set up preferably  mounted in your furnace. The magnetic field is easily calculated via the current applied to. So there is no need for a magnetic field  senor permanently present in the furnace.  But indeed there is a draw back: depending on the envisaged temperature you have to use special designed coils.

Good luck.

https://en.wikipedia.org/wiki/Helmholtz_coil

hi Gerhard, thank you for answer, temperature is between 400-700 celsius, and magnetic strength 0.4 T, homogenous to be at least 2 inches long and wide.  Which one materials would you suggest me to use to prepare such coil?

Hi Stefan,

I did not suspected that the magnetic field is so  quite high and the temperature also.

The temperasture is not the real sticking point but the magnetic field strength. In the german wikipedia link an example is given. I have made a screen shot of the example and put it in the docx file which is attached. 

The authors of wikipedia use a radius of 0,15m, 130 windings per coil and a current of 1,7A. The result is a magnetic field of 1,33*10-3 T.

You may scale the radius up to at least 2 inches (0,05m)  for your case and the appropriate number of windings to achieve your magnetic field strength.

So form a short view on the formula (see the docx file) we see that you have to increase the (1,7A * 130windings)- product by a factor of 100 (while replacing R =0,15m by 0,05m). This is a quite large value when bearing in mind that due to the high temperature you cannot use a wire which is isolated by a special paint. Here you can use unpainted wire but the isolation has  be set up by lining up a lot of ceramic pearls onto the total length of wire. This however will significantly increase the volume of the coil. There may be another way of isolation.

Alternatively you may calculate the coil system for the case that the furnace is inside the coil system, but here a special design of furnace is needed which does not interfere with the magnetic field; here you have to increase the (1,7A * 130windings)-product by a factor at least 1000.

Sorry, but I think the Helmhotz coil  seem to be not the real choice for your magnetic field set up conditions. 

https://de.wikipedia.org/wiki/Helmholtz-Spule

Yes, I agree with you, but maybe I can use permanent magnet, and thermally isolate it with paint. 

So now is problem how to detect and determinate magnetic field in such condition and make sure that field is homogeneous.

Hi Stefan,

thermal isolation with paint does not work at all. You have to separate the magnet from excessive heat.

You have to design for example a furnace consisting of a sample chamber having two opposite lying windows, which is placed  in the gap of a for example circularly, quadratic or rectangular arranged magnet coil. Please see for example the sketch at the pdf file. Here you have the magnet system outside the furnace; the temperature does not affect the magnet system. The magnetic field is in the gap of the magnet coil system and passes the windows.

The windows may consist of a stack of parallel arranged ceramic sheets (to bring down the temperature), for example.

Dimensions are up to you.

So far my suggestions apart from the Helmholtz coil.

For designing the magnet with respect to  its shape, number of windings and the core material I am not the right one; sorry.

You may also use a permanent magnet system. But I think switching off the magnetic field and tuning its strength could be a very nice advantage of the system; you will have 0,4T and  that is very much.

Hi Stefan,

Taking into account the requirements for the field strength and homogeneity, one may think of a Halbach cylinder as a possible solution (see the link below). However, with the temperature range required, the magnetic system must be kept out of the furnace (as Gerhard Martens pointed out above).

The smaller you can make the furnace, the less bulky your magnetic system can be. You may estimate the outer radius of the required Halbach cylinder using the formula on the Wiki page (for k = 2). If the magnets can be sufficiently separated from heat you can, in principle, use NdFeB magnets that have Br (= mu0*Mr) ~ 1.2 T.  

You should also take care that the materials that the furnace itself is made of do not (strongly) modify the magnetic field inside.

https://en.wikipedia.org/wiki/Halbach_array#Halbach_cylinder

Dear Mikhail Kustov,

thank you for reply,

yes my furnace will be small, since I need homogeneity of filed in diameter of 2 inches, or minimum requirement for XRD, XPS, SQUID and SAXS, so I dont need big furnance.

Silicon wafers or  copper can they be good choice ?

The two proposed solutions (Helmholtz coils and Halbach array) differ in one detail, maybe essential: Helmholtz coils will produce field directed along the cylindrical furnace axis, while Halbach array's field will be perpendicular to the same axis.  Anyway, in both cases some kind of active thermal shield (non-magnetic) is a must.  I say 'active' as its temperature should be stabilized.

Dear Marek, do you think that my substrate in furnace should be positioned parallel or penderdicullary to field? 

Stefan, I'm not the correct person to answer your last question.  We have two things here: 1) the purpose of so grown sample, and 2) the physical possibility of arbitrary positioning a sample within furnace during growth.  Probably no problem with bulk material but a great difference for thin layer.

actually, I will prepare sample apriori putting in furnace, so annealing with  magnetic field will be applied on already prepared thin film ( amorphous or poly crystalline )  in order to influence magnetic structure, and phase

I guess your temp is *c ?   It depends on the magnetic  material of he permanent magnet . Iron has a Tc of > 1000 Co  so 1300 , not sure about  modern FeNb etc.  you are well below. There might be a bit of domain movement  but if you cycle thr u the temps used a few times that will stabilise.  In any event I doubt the field homogeneity would alter significantly,  field intensity might a bit 

yes, I need temperature in furnace to be between 400 and 700 degree celsius, 

"if you cycle thr u the temps used a few times that will stabilise" you are suggesting me to anneal in cycles?

To honest I doubt the field will change measurably  as long as the magnet(s)  stay below 300 c. Just if you worried, run the whole rig a coupl;e of times thru typical heating cycles . This will allow any domain movement to come to equilibrium  at your highest temp. After that the field really should not change to any significant degree. What variation in field either or time would actually worry you  could you notice ?

I agree with Chris.

the only variation is maybe the close contact with AC heater wire, 

emission of this electromagnetic field,

there are many papers, that simulated perfect conditions for fabrication of helbaich cylinder, but non of them take into account furnance application

Stefan,

If the straight heather wire is wound around the furnace cylinder then the field inside will oscillate around Halbach field, practically without spoiling its uniformity.  If the heating wire is helix-shaped, then this effect will be significantly reduced.  In addition, oscillating component will be of order ~100 Gs=0.01 T (assuming heather current around 10-20 A) at any location close to the wire itself and even smaller within the center of furnace, say at the level about 1% of fixed Halbach field.  I hope this is acceptable.