If a material has magnetic atoms and if they order below some temperature TC or TN then we can expect spin waves below that temperature. The magnetic ordering can be ferromagnetic (below TC) or antiferromagnetic (below TN) or even helimagnetic or some other exotic type of pordering. The spin waves exhibits dispersion as a function of the wave vector or momentum transfer. The nature of dispersion is often characteristic of the type of magnetic ordering. Now how to detect these spin waves? The most direct evidence of the spin waves can be obtained by inelastic neutron scattering. We can also determine the spin wave dispersion along different crystallographic directions. The method is very much similar to the determination of phonon dispersion by inelastic neutron scattering. Now the magnetic materials will show both magnon (spin wave) and phonon (lattice wave) dispersions at the same time. So one has to be able to distinguish the inelastic peaks which are due to phonons from those that are due to magnons. They can be usually done by looking at the temperature dependence and also Q (momentum transfer) dependence of the inelastic signals. The phonon intensity increase as a function of Q whereas the magnon intensity should decrease as a function of Q folowing the magnetic form factor. But the most reliable method is to use polarization analysis of the scattering. However polarization analysis needs more intense neutron beam and/or larger sample and therefore one cannot always afford this type of measurements. Also since magnons and phonons are elementary excitations that can interact with each other due magnon-phonon interactions. Sometimes we can have excitations that are neither purely magnetic nor purely phononic excitations. To determine their nature we have to use polarization analysis. But these are advanced topic but is extremely relevant for industrially important multiferroic materials. Investigation of spin waves in magnetic materials is extremely important because these investigations yield magnetic exchange interactions and also other microscopic parameters. Classically neutron triple axis spectroscopy (TAS) was used to investigate spin waves but now a days we use time of flight (TOF) speectroscopy as well for these investigations specially with Spallation Neutron Sources like SNS at Oak Ridge or ISIS at Didcot, UK or at J-PARK in Japan. At BARC you have triple axis spectrometer but unfortunately because of the limited neutron flux you need a vey lagre single crystal for the investigations. But if you have some good problem and reasonably large single crystal you can write an ILL research proposal and hope to get neutron beam time. But meanwhile you can read Chapter 6 (Magnetic excitations) of the book "Magnetic Neutron Scattering", ed. T. Chatterji, Elsevier 2006. If you do not understand something then you can contact me.

Thank you very much.

The problem I am having is for Ga0.7SC0.3FeO3 neutron diffraction data recorded at 5K one of the Fe sites has a very low moment (0.8muB). So can this happen if spin waves are excited at 5K which lower the moment.

I assume that you have done neutron powder diffraction measurements on Ga0.7SC0.3FeO3. The spin waves will take away some intensity from the magnetic Bragg peak but at T = 5 K this will be negligibly small especially when the ordering temperature of Fe atoms is very high compared to T = 5 K. So you have other problems that you must find out. If your manetic structure model is not right then you do not get the right moment. Also you may have made some mistake in the pcr file (in case you use Fullprof). The magnetic moment value is determined from the magnetic intensities put to absolute scale from the nuclear refinement. So both nuclear and magnetic refinements are involved for the moment determination. Any mistake anywhere will lead to wrong moment value.

You can safely ignore spin waves in your neutron powder diffraction data for insulating system at low temperature. So you do not have to read chapter 6 of Magnetic Neutron scattering, ed. T. Chatterji, Elsevier 2006. Read instead the Chapter 2 (Magnetic structures) of the same book.

OK Sir.Thank you once more.

I have redone the refinement but yet one of the Fe3+ ions is only giving a moment of 1muB. When I did a bond angle calculation for this F3+ ion I found O-Fe-O angles to be 176.1, 159.6 and 167.8 So why is the moment so small.

I would like to know at what energy transfer I will encounter the lowest energy magnon.  Is it possible to know it before actually performing a neutron inelastic scattering experiment. The material is Nd0.9Sr0.1CrO3.

In short I would like to know the position of my detector for the first magnetic inelastic peak. Will it be close to the first magnetic peak from a neutron diffraction experiment.

It wll be hidden as diffuse scattering in the background close to each magnetic Bragg peak. It will be stronger close to the low-Q position close to the first magnetic peak.You will not see this scattering from a powder sample and with a diffractometer where you cannot do the energy analysis. But you can see it with a time of flight spectromerter (even if you have powder sample) in a careful experiment by chosing proper incoming neutron energies. 

One more question Sir. How many branches will the dispersion curve have. Is it related to the different and  distinct exchange integrals?

In other words will the no. of branches exhibited by magnons relate to the different exchange integrals between various magnetic atoms in the unit cell?