Observation of a spin wave exponent of 5/2 in the uniaxial antiferromagnet MnF2

Autor: Masayuki Kawakami, S. Abens, A. Hoser, Ulrich Köbler
Rok vydání: 2001
Předmět:
Zdroj: Physica B: Condensed Matter. 307:175-183
ISSN: 0921-4526
DOI: 10.1016/s0921-4526(01)00633-0
Popis: The temperature dependence of the sublattice magnetization (order parameter) of the uniaxial antiferromagnet MnF 2 ( S =5/2) has been measured using 19 F NMR, neutron scattering and magnetic X-ray scattering. All three methods reveal a T 5/2 power law for the deviation of the order parameter from its saturation value at absolute zero. This law holds perfectly up to 0.8 of the Neel temperature, T N =67.2 K. The unexpected spin wave exponent of 5/2 as well as the large validity range of the T 5/2 power law indicate that the spin dynamics deviates from that of classical spin wave theories. We propose that fourth-order exchange interactions, i.e., biquadratic, three- and four-spin interactions are responsible for this. Existence of these interactions is evidenced with measurements of the cubic susceptibility χ 3 . Surprisingly, χ 3 is negative for all temperatures (the magnetic isotherms increase faster than linearly with field) meaning that the fourth-order interactions are ferromagnetic. Since χ 3 is large but does not diverge at T N the existence of a long range ferromagnetically ordered component is questionable. The new quantum exponent of 5/2 is considered as characteristic for magnets with half-integral spin and axial interactions. This result completes the empirical systematics of the spin wave exponents in materials with half-integral spin. These exponents are 2 for isotropic interactions, 3/2 for anisotropic and/or two-dimensional interactions and 5/2 for axial interactions.
Databáze: OpenAIRE
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