Local Canting Research Articles

Frustration and Dzyaloshinsky-Moriya anisotropy in the kagome francisitesCu3Bi(SeO3)2O2X (X = Br, Cl)

We investigate the antiferromagnetic canting instability of the spin-1/2 kagome ferromagnet, as realized in the layered cuprates Cu$_3$Bi(SeO$_3)_2$O$_2$X (X=Br, Cl, and I). While the local canting can be explained in terms of competing exchange interactions, the direction of the ferrimagnetic order parameter fluctuates strongly even at short distances on account of frustration which gives rise to an infinite ground state degeneracy at the classical level. In analogy with the kagome antiferromagnet, the accidental degeneracy is fully lifted only by non-linear 1/S corrections, rendering the selected uniform canted phase very fragile even for spins-1/2, as shown explicitly by coupled-cluster calculations. To account for the observed ordering, we show that the minimal description of these systems must include the microscopic Dzyaloshinsky-Moriya interactions, which we obtain from density-functional band-structure calculations. The model explains all qualitative properties of the kagome francisites, including the detailed nature of the ground state and the anisotropic response under a magnetic field. The predicted magnon excitation spectrum and quantitative features of the magnetization process call for further experimental investigations of these compounds.

Microscopic description of twisted magnet Cu2OSeO3

Twisted structures of chiral cubic ferromagnets MnSi and Cu2OSeO3 can be described both in the frame of the phenomenological Ginzburg–Landau theory and using the microscopical Heisenberg formalism with a chirality arising from the Dzyaloshinskii–Moriya (DM) interaction. Recent progress in quantum first-principal methods allows us to calculate interatomic bond parameters of the Heisenberg model, namely isotropic exchange constants Jij and DM vectors Dij, which can be used for simulations of observed magnetic textures and comparison of their calculated characteristics, such as magnetic helix sense and pitch, with an experimental data. In the present work, it is found that unaveraged microscopical details of the spin structures (local canting) have a strong impact on the global twist and can significantly change the helix propagation number. Coefficients J and D of the phenomenological theory and helix propagation number k=D/2J are derived from interatomic parameters Jij and Dij of individual bonds for MnSi and Cu2OSeO3 crystals and similar cubic magnets with almost collinear spins.

Delta Doping of Ferromagnetism in Antiferromagnetic Manganite Superlattices

We demonstrate that delta doping can be used to create a dimensionally confined region of metallic ferromagnetism in an antiferromagnetic (AFM) manganite host, without introducing any explicit disorder due to dopants or frustration of spins. Theoretical consideration of these additional carriers shows that they cause a local enhancement of ferromagnetic double exchange with respect to AFM superexchange, resulting in local canting of the AFM spins. This leads to a highly modulated magnetization, as measured by polarized neutron reflectometry. The spatial modulation of the canting is related to the spreading of charge from the doped layer and establishes a fundamental length scale for charge transfer, transformation of orbital occupancy, and magnetic order in these manganites. Furthermore, we confirm the existence of the canted, AFM state as was predicted by de Gennes [Phys. Rev. 118, 141 (1960)] but had remained elusive.

Magnetic Behavior of the LiFeTiO4 Spinel

The LiFeTiO4 spinel obtained by the “liquid mix” technique has been studied by neutron powder diffraction, ac and dc magnetic susceptibility, and magnetization measurements. The cationic distribution in the tetrahedral (A) and octahedral (B) sites was determined to be (Li0.47Fe0.53)A(Li0.53Fe0.47Ti)BO4. The dc magnetic susceptibility measurements show two maxima at 29 and 282 K (at H = 1000 Oe) and a Curie−Weiss behavior is not observed in the temperature range 282−800 K. Neutron diffraction patterns shows that the B-site magnetic moments at 2 K temperature are lower than those expected for a collinear arrangement of spins. Careful analyses of the low-temperature neutron diffraction patterns of this material suggest a localized canting of spins (LCS) type of magnetic ordering where a long-range order (LRO) of longitudinal component of moments coexists along with a totally disordered transverse component of moments. The conclusion is also supported by the features seen in complementary magnetic measurements.

Localized canting of spins structure in the spinel oxide system: ZnzTizFe2−x−zCrx−zCoO4

The spinel oxide system ZnzTizFe2−x−zCrx−zCoO4; z=x2; x=0.60, 0.65, 0.70 and 0.80, was studied using neutron diffraction technique, low field DC magnetization measurements (ZFC–FC measurements), magnetic hysterisis, Mössbauer spectroscopy and low field AC susceptibility measurements. All the compositions show significantly less B-site magnetic moments at 10K temperature derived from neutron diffraction data than the free ions site moments deduced assuming collinear arrangement of spins. This combined with some other features seen in the low temperature neutron diffraction patterns suggest localized canting of spins (LCS) type of magnetic ordering in the present system where a long range order of longitudinal component of moments co-exists along with totally disordered transverse component of moments. The conclusion is also supported by the features seen in the other measurements. The magnetic moments derived from 10K neutron diffraction data are explained using the LCS approach for different exchange integrals ratios.

The magnetic state of diamagnetically diluted antiferromagnetic cobalt monoxide

Magnetic neutron diffraction measurements have been performed on (Co 1− x Mg x )O solid solutions with x = 0.1, 0.2, 0.3, 0.43, 0.53, 0.63 and 0.7 at T = 4.2 –300 K. It is shown that the concentration dependence of the average magnetic moment per atom does not obey the condition of simple dilution. The diamagnetic atom results in a local canting of the moments and an additional reduction of the average magnetic moment. One can consider this atom as a source of random magnetic field due to the appearance of a new superexchange interaction between magnetic atoms through the diamagnetic impurity. The sign of this interaction is opposite to the sign of the direct exchange interaction in the magnetic sublattice.

Magnetic defects in antiferromagnets

Neutron polarisation analysis scattering studies have been undertaken on a number of metallic antiferromagnetic alloys based on γ-Mn. Results obtained for the Mn-Cu alloy system will be reviewed. The common feature in this study and other γ-Mn alloys is a component transverse to the antiferromagnetic direction in the defect centred on the alloying element. The local canting of the moment appears to be due to the occurrence of unsatisfied antiferromagnetic interactions in the face centred cubic type-1 antiferromagnetic structure. A classical spin model of the defect structure will be presented. This local structure is derived by minimising the energy of the local spin configuration in the presence of a rigid long range antiferromagnetic order. The local configuration is then allowed to perturb the long range antiferromagnetic order through the wavevector dependent susceptibility estimated from the measured magnon dispersions in this material.

Local canting in a dilute triangular XY antiferromagnet

Computer simulations are used to show how diamagnetic impurities induce the formation of 'canted local states' in a triangular lattice of antiferromagnetically coupled XY spins, through relief of the magnetic frustration inherent in the lattice. The canted local states are shown to interact with each other through polarisation of the intervening medium. At low concentration of impurities the exchange energy of some of the remaining moments is found to increase, in contrast to the usual effect of impurities on the exchange energy of the moments in a non-frustrated lattice.

Localised canting models for substituted magnetic oxides

A free energy formulation of localised canting in substituted magnetic oxides is described. In contrast with earlier attempts at mathematical formulations of localised canting, quantities which may be replaced by average values and those which must be considered on a site-to-site statistical basis are clearly delineated. In addition, the use of local canting to determine average split-sublattice molecular-field directions is done in an iterative self-consistent manner. The theoretical curves of magnetisation versus level of non-magnetic substituent are somewhat rounded, compared with the linear dependences which result from the Yafet-Kittel model. For a given ratio of exchange parameters, canting commences at somewhat lower substituent levels than from the Yafet-Kittel model. Comparisons with experimental magnetisation data for YIG with Ge, Si, Zr, and Sc substitutions and for Li-Zn ferrite indicate several things: (a) no single exchange parameter can provide a good theoretical fit to the data for a given system over the full range of substitution; (b) the theoretical curves, when fitted to the data, indicate a decrease in the inter- to intrasublattice exchange integral ratio for large substitutions where canting effects become pronounced; (c) the fits, when based on the assumption of a randomly split sublattice and equal intra- and intersublattice interactions for the two parts of the split sublattice, are consistent with reported exchange constant ratios for the unsubstituted oxides.

High field magnetization studies in lithium-zinc ferritea)

The magnetization in Li0.5−z/2ZnzFe2.5−z/2O4 with z=0.0, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 has been measured at 30 K and at applied magnetic fields of up to 100 kOe. The magnetization for samples with z?0.2 saturated at less than 10 kOe. The magnetization for samples with z?0.3 did not saturate. Rather, for applied fields in excess of 30 kOe, these samples exhibited field-independent differential susceptibilities ranging from 1.68×10−5 emu/g-Oe for z=0.3 to 20.66×10−5 emu/g-Oe for z=0.7. The high-field susceptibilities are attributed to localized canting of octahedral site Fe3+ spins with fewer than three tetrahedral site Fe3+ nearest neighbors. Susceptibilities predicted by existing localized canting models are in qualitative agreement with the data, but predicted values for large z are too small.

On the magnetic arrangement in the diamagnetically substituted rare earths iron garnets

The magnetic arrangement in the rare earth iron garnet with diamagnetic substitution in octahedral sites, at 0 K, is calculated on the basis of the model of localized canting, when the exchange interaction between rare earth and iron ions is taken into account. The magnetization measurements on scandium substituted gadolinium iron garnet and neodymium iron garnet are reinterpreted.

Localized canting model for substituted ferrimagnets. II. Doubly substituted YIG systems

The localized canting model has been extended to the doubly substituted YIG systems. Canting criteria have been developed which indicate the possibility for localized canting in both sublattices for some of these systems. Application of this model to a doubly substituted YIG system with a known cation distribution has resulted in a determination for YIG of Jdd/Jad ~ 0.22 and Jaa/Jad ~ 0.07 in close agreement with the results obtained in Part I. The localized canting model has also been used to establish the cation distribution for various doubly substituted YIG systems. In particular, this model has been applied to the AI- and Ga-substituted YIG's providing interesting information about the magnetic structures of these systems.

Localized canting model for substituted ferrimagnets. I. Singly substituted YIG systems

A statistical model based on the concept of localized canting, originally proposed by Geller and coworkers, is developed to account for the magnetic properties of substituted ferrimagnets. This model is used to determine the exchange parameter ratios Jdd/Jdd and Jaa/Jad in YIG systems by evaluating the magnetic moments and Curie temperatures of two classes of singly substituted YIG over the entire substitution range. Good agreement with experimental data is obtained with the physically reasonable exchange parameter ratios of [Formula: see text] and Jaa/Jad *~ 0.07. It is also shown that both the Yafet-Kittel and the Nowik models may be regarded as particular limiting cases of the new model.