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Magnetic vibes



Usually in solids the lattice structure and its vibrational excitations, the phonons, are only weakly influenced by magnetic fields. In the compound ZnCr2S4 this is different: Here the order of the magnetic spins strongly couples to the lattice degrees of freedom and employing a moderate external magnetic field one is able to affect the lattice expansion and the symmetry of the phonon modes. The reason for this particular behavior is connected to the "frustration" of both, the structure and the magnetism of this material. Frustration means that competing, contrary interactions weaken the order of the corresponding microscopic degree of freedom.

 thermal expansion coefficient

Thermal expansion coefficient of ZnCr2S4 near the magnetic phase transition at TN ≈ 8.5 K. The structural anomaly can be suppressed in an external magnetic field.

 

Concerning the magnetic structure it is the competition of antiferromagnetic and ferromagnetic bonds, which makes the spin system susceptible towards a distortion by an external magnetic field. At the same time the lattice structure at elevated temperatures is highly symmetric, namely cubic, and thus susceptible to external influences. Thus the usually moderate spin-lattice coupling is amplified and at the magnetic phase transitions at low temperatures additional phonon modes appear. The alteration of the magnetic structure (e.g. induced by an external magnetic field) in turn changes the lattice. The underlying microscopic mechanism which connects magnetic spin and the lattice is suggested to be local exchange-striction along the magnetic interaction pathes. The inter-atomic force constants which are responsible for the dynamic as well as static lattice properties are enhanced along the magnetic bonds which are realized in a particular magnetic structure.

 

splitting of the lowest phonon

Below the magnetic transition the phonon mode splits.

 

To learn more, see:

Spin-Driven Phonon Splitting in Bond-Frustrated ZnCr2S4
J. Hemberger, T. Rudolf, H.-A. Krug von Nidda, F. Mayr, A. Pimenov, V. Tsurkan, and A. Loidl
Phys. Rev. Lett. 97, 087204 (2006)