Impurity-Pinned Solitons in the Two-Dimensional Antiferromagnet Detected by Electron Paramagnetic Resonance

Abstract

It is shown that the introduction of a very small amount of nonmagnetic impurities into the magnetic sites of a classical two-dimensional antiferromagnet creates a new type of static (impuritypinned) soliton that affects the Arrhenius, exp͑2E͞T͒, temperature-dependent electron paramagnetic resonance linewidth by drastically changing the parameter E. Data just above the transition temperature for ͑C 3 H 7 NH 3 ͒ 2 M x Mn 12x Cl 4 confirm the existence of these impurity-pinned solitons.[S0031-9007(98)05498-2] PACS numbers: 75.10. Hk, 75.40.Gb, 76.30.Fc Two-dimensional magnetic systems support interesting nonlinear excitations including solitons and vortices. For the two-dimensional (2D) isotropic ferromagnetic Belavin and Polyakov [1] obtained these solitonlike solutions (BP solitons) from topological considerations. The energy of this excitation is found to be independent of the soliton size resulting from scale invariance of the continuum Heisenberg Hamiltonian. The significance of these excitations was recognized early in connection with the critical properties of 2D magnets. For example, in [1] it was shown that the existence of large localized excitations will cause the correlation length to remain finite at any nonzero temperature as expected from the Mermin-Wagner theorem [2].Recently we have shown [3,4] that BP solitons dominate the thermodynamics in the fluctuation region immediately above the Néel temperature of a large class of nearly classical 2D antiferromagnets. Experimentally this is observed as an Arrhenius behavior of the temperature-dependent electron paramagnetic resonance (EPR) linewidth in layered manganese systems which was first predicted by Waldner [5,6]. In [3,4] the EPR linewidth was calculated from the dynamic spin correlation function with the time dependence from the solitonmagnon interaction; moveover, it was shown that the calculated linewidth matched the observed Arrhenius behavior.In this Letter we show that a new type of soliton pinned to a nonmagnetic impurity will form, and this pinned soliton has a lower energy than a large pinned soliton with a corresponding larger density in the lattice. This lowering of energy for the impurity solitons occurs simply because of elimination of exchange bonds at the impurity, which is a significant effect in the small and a negligible effect in the large impurity solitons. Because of this energy difference, the smaller pinned soliton will dominate the BP soliton in the fluctuation region. In order to relate these small impurity-pinned solitons to experimental data, we first obtain the temperature-dependent EPR linewidth resulting from these structures as a function of impurity concentration. This calculation shows that there will be large changes in the temperature dependence of the EPR linewidth as the impurity concentration is varied in a small (less than 1%) range. Finally, this effect is observed by EPR measurements on manganese compounds with nonmagnetic impurities where the calculated impurity dependence is indeed observed.We begin with...

Internal modes and magnon scattering on topological solitons in two-dimensional easy-axis ferromagnets

“…11) was used to explain the EPR line-width in easy-axis (EA) magnets. [12][13][14][15][16][17][18] The classical example of a localized soliton is BP-solution, which exists in isotropic magnets. The scattering problem for such solitons allows an exact analytical solution because of the scale-and conformal-invariance of the model.…”

Oscillating solitons pinned to a nonmagnetic impurity in layered antiferromagnets

“…Besides, these authors [6,7] have also shown that the introduction of a very small amount of nonmagnetic impurities into the magnetic sites of a classical 2D antiferromagnet creates a new type of static (impurity-pinned) soliton that affects the Arrhenius, e −Es/T , temperature-dependent EPR linewidth by drastically changing the parameter E s . Using an approach on a discrete lattice, they considered the spin vacancy located at the soliton center and obtained two different impurity-solitons ( the P-soliton and the I-soliton), which were detected in the layered antiferromagnet (C 3 H 7 NH 3 ) 2 M x Mn l−x Cl 4 through EPR measurements [6,7]. The P-soliton has the same structure of the BV-soliton but without a spin at its center, which results in a vortex singularity at the center.…”

Heisenberg spins on a cone: an interplay between geometry and magnetism

“…Such a result has leads to the appearance of a new type of soliton whose energy is lower than its counterpart in the absence of the spinless impurity [6,7,8]. This 'more fundamental soliton' has been theoretically [7,8,9,10,11] studied and observed in experiments [6].…”

“…[6], for details) while c = 2JSa/h = E j /M j , where M j is the soliton rest mass (for details, see Ref. [19]).…”