Random fan-out state induced by site-random interlayer couplings

Abstract

We study the low-temperature properties of a classical Heisenberg model with site-random interlayer couplings on the cubic lattice. This model is introduced as a simplified effective model of Sr(Fe 1−x Mn x )O 2 , which was recently synthesized. In this material, when x = 0.3, (πππ) and (ππ0) mixed ordering is observed by neutron diffraction measurements. Using Monte Carlo simulations, we find an exotic bulk spin structure that explains the experimentally obtained results. We name this spin structure the "random fan-out state." The mean-field calculations provide an intuitive understanding of this phase being induced by the site-random interlayer couplings. Since Rietveld analysis assuming the random fan-out state agrees well with the neutron diffraction pattern of Sr(Fe 0.7 Mn 0.3 )O 2 , we conclude that the random fan-out state is reasonable for the spin-ordering pattern of Sr(Fe 0.7 Mn 0.3 )O 2 at the low-temperature phase. PACS number(s): 75.50. Lk, 75.40.Mg, 28.20.Cz Sr(Fe 0.7 Mn 0.3 )O 2 using the spin configuration obtained from Monte Carlo simulations at low temperature. A. Robustness of random fan-out stateIn this section, we discuss the relationship between decision rules of interactions and the spin-ordering pattern. In general, the spatial average of interlayer random interactions J and its fluctuation J corresponding to the random fields depend on the signs and absolute values of interactions:where J AA , J AB , and J BB denote the interactions between A-A, A-B, and B-B along the interlayer direction, respectively. Then, the spatial average of effective interaction between the NNLs, J nnl , is given by(36) Here, we assume that J AA is always AF and the absolute values of interactions are the same or zero. Then, the four rules in Table III are all possible decision rules of interactions where FM and AF interactions exist and J nnl < 0 regardless of x. Note that rule 1 is adopted in the previous sections. Under these rules and using Monte Carlo simulations and meanfield calculations, we find the appearance of the mixed phase, where the spin-ordering pattern is the random fan-out state. Furthermore, we also find that (πππ) and (ππ0) magnetic peaks develop at the same temperature and the angle between 214408-8 RANDOM FAN-OUT STATE INDUCED BY SITE-RANDOM . . .

Electronic structure and magnetic properties of SrFe1−Mn O2: A theoretical investigation

“…The results are in agreement with previous studies [7]. To examine the nature of the phase transition, we estimate values of critical exponents using a finitesize-scaling function:…”

“…We used a larger U in our first-principles calculation than theirs, and hence, smaller exchange parameters are obtained according to J∼t 2 /U. We note that if we rescale the transition temperature for x¼0 according to the experimental data, the transition temperatures for x ¼0.1, 0.2, and 0.3 would be 390, 320, and 270 K, respectively, in agreement with experiment results [6,7]. The specific heat curve peaks get more round with increasing x as shown in Fig.…”

“…While for other four atomic configurations (b), (c), (d) and (f), the magnetic structure changes to be the (ππ0) AF3 ordering. Experimentally, a mixed magnetic structure of (πππ) and (ππ0) ordering was found by neutron diffraction measurements in the compound SrFe 0.7 Mn 0.3 O 2 [7]. We also calculate the energy of the non-collinear magnetic structure for SrFe 0.75 Mn 0.25 O 2 and find that the non-collinear magnetic structure is less stable than AF1 and AF3 in all situations depicted in Fig.…”

“…For SrFe 1 À x Co x O 2 , the reduction of T N relative to SrFeO 2 is no more than 50 K. However, the Néel temperature of SrFe 1 À x Mn x O 2 decreases substantially with increasing x. Moreover, a simultaneous emergence of mixed magnetic (πππ) and (ππ0) ordering was detected by neutron diffraction measurements in SrFe 1 À x Mn x O 2 with x¼ 0.3 [7]. The mixed magnetic ordering means that the antiferromagnetic and ferromagnetic couplings coexist in the doped system.…”

“…Monte Carlo simulations based on this model show that there are three ordered phases in such systems, i.e., ferromagnetic ordered, antiferromagnetic ordered, and mixed phases [17]. To investigate magnetic properties of SrFe 1 À x Mn x O 2 , Tamura et al [7] performed an anisotropic siterandom Heisenberg model study on a simple cubic lattice using the Monte Carlo methods. Their simulations show that there are three ordered phases corresponding to conventional G-type antiferromagnetic (πππ) ordered, C-type antiferromagnetic (ππ0) ordered, and mixed phases.…”