Small Impurity-Pinned Solitons in Layered Antiferromagnets

Abstract

Layered manganese-halide compounds exhibit quasi-two-dimensional magnetic behavior in the temperature region immediately above the ordering temperature where soliton excitation can be experimentally detected as an Arrhenius, exp(E/T), temperature-dependent electron paramagnetic resonance linewidth, where E is the soliton energy. When a nonmagnetic impurity such as magnesium is introduced into the Mn lattice, experimental linewidth data indicate that the excitation energy is dramatically reduced and the temperature range over which Arrhenius behavior is observed is widened to include higher temperatures. These effects occur for very small (less than 1%) impurity concentrations.Quasi-two-dimensional (2D) magnetic systems can be physically realized as layered metal-halide compounds of the form (C n H 2n+1 NH 3 ) 2 MX 4 , where M is a transition metal ion and X is Cl or Br. For the case with M = Mn(II) and X = Cl these compounds are nearly-classical (the Mn ion has a spin of 5/2) spin systems with long-range antiferromagnetic order. However, as the ordering temperature is approached from above there is a range of a few Kelvins (referred to as the fluctuation region) where short-range order effects within the metal-halide layer become important. It is in this relatively narrow temperature range where the system exhibits 2D antiferromagnetic correlations and the corresponding 2D nonlinear excitations, or solitons, can be experimentally studied and compared with theoretical calculations of magnetic quantities. From the theoretical point of view it is known that the classical 2D magnet with an isotropic exchange interaction supports soliton excitations. The form and energy of these excitations was obtained by Belavin and Poyakov [1] from simple topological considerations indicating that the energy is E s ¼ 4pJS 2 , where J is the exchange constant and S is the value of the spin. Topology also dictates that these are vortex-like excitations, but to avoid a singularity at the excitation center, the magnetization rotates through an angle of 180 going radially out from the center. In the following we refer to these as Belavin-Polyakov (BP) excitations. It is expected that these structures should be present in layered magnetic compounds, and indeed, early EPR measurements [2, 3] and recent research [4,5] has shown that solitons rather than magnons are the dominant contribution to the temperature dependence of the electron paramagnetic resonance (EPR) linewidth in the fluctuation region. Experimentally this is noticed as an Arrhenius, exp E s =T ð Þ, temperature dependence where E s is the soliton excitation energy. In this work the effect of nonmagnetic impurities interacting with solitons on the EPR linewidth is investigated. There are various interesting effects that can be attribu-

Oscillating solitons pinned to a nonmagnetic impurity in layered antiferromagnets

“…Recently, Zaspel et al [4,5] have shown that the BelavinPolyakov solitons dominate the thermodynamics in the fluctuation region immediately above the Néel temperature of a large class of nearly classical 2D 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].…”

“…If the soliton center coincides with the impurity center, this field theory reproduces the two soliton solutions of Refs. [6,7] and still gives the sizes, energies and configurations of these solitons in an analytical way. For the P-soliton, almost entire space is mapped onto the plane, but part of the spin space sphere around the origin is not included in the mapping.…”

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

“…In magnetic materials, impurities may be considered for improving (magnetic impurity) or for vanishing (nonmagnetic, spinless) local magnetic interaction at the positions where they were placed. However, recent reported results have found that the antiferromagnetic correlations around a spin vacancy are not destroyed, rather they appear to be increased [5,6,7]. 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].…”

“…[8], the NLσM supplemented by a static nonmagnetic impurity potential was applied to study such a new soliton. In spite of the investigation be carried out in the low frequency limit, its results were shown to be in excellent agreement with numerical and discrete lattice ones [6,7]. Actually, as presented in Ref.…”

Heisenberg spins on a circular conical surface

“…Besides, the idea that topologically nontrivial excitations arise in real physical systems had a strong impact on modern physics. These structures became an object of intensive investigations in many condensed matter models and they are particularly believed to play an important role in magnetic systems [8][9][10][11][12]. Our main interest is to study the configuration and energy of such topological excitations on the surface of a cone.…”