Toroidal moments as indicator for magneto-electric coupling: the case of BiFeO_3 versus FeTiO_3

In this paper we present an analysis of the magnetic toroidal moment and its relation to the various structural modes in R3c-distorted perovskites with magnetic cations on either the perovskite A or B site. We evaluate the toroidal moment in the limit of localized magnetic moments and show that the full magnetic symmetry can be taken into account by considering small induced magnetic moments on the oxygen sites. Our results give a transparent picture of the possible coupling between magnetization, electric polarization, and toroidal moment, thereby highlighting the different roles played by the various structural distortions in multiferroic BiFeO_3 and in the recently discussed isostructural material FeTiO_3, which has been predicted to exhibit electric field-induced magnetization switching.Comment: 6 pages, 2 figure

Interplay between breathing mode distortion and magnetic order in rare-earth nickelates RRNiO3_3 within DFT+UU

We present a systematic density functional theory (DFT) plus Hubbard $U$ study of structural trends and the stability of different magnetically ordered states across the rare-earth nickelate series, $R$NiO$_3$, with $R$ from Lu to La. In particular, we investigate how the magnetic order, the change of the rare-earth ion, and the Hubbard interaction $U$ are affecting the bond-length disproportionation between the nickel sites. Our results show that structural parameters can be obtained that are in very good agreement with present experimental data, and that DFT+$U$ is in principle able to capture the most important structural trends across the nickelate series. However, the amplitude of the bond-length disproportionation depends very strongly on the specific value used for the Hubbard $U$ parameter and also on the type of magnetic order imposed in the calculation. Regarding the relative stability of different magnetic orderings, a realistic antiferromagnetic order, consistent with the experimental observations, is favored for small $U$ values, and becomes more and more favorable compared to the ferromagnetic state towards the end of the series (i.e., towards $R$=Pr). Nevertheless, it seems that the stability of the ferromagnetic state is generally overestimated within the DFT+$U$ calculations. Our work provides a profound starting point for more detailed experimental investigations, and also for future studies using more advanced computational techniques such as, e.g., DFT combined with dynamical mean-field theory.Comment: 13 pages, 11 figures, 1 tabl

BaNiF4: an electric field-switchable weak antiferromagnet

We show that in the antiferromagnetic ferroelectric BaNiF4 the Dzyaloshinskii-Moriya interaction leads to a small canting of the magnetic moments, away from the easy axis, resulting in a noncollinear magnetic structure. The canting corresponds to an additional "weak" antiferromagnetic order parameter whose orientation is determined by the polar structural distortion and can be reversed by switching the ferroelectric polarization with an electric field. Our results point the way to a more general coupling mechanism between structural distortions and magnetic order parameters in magnetoelectric multiferroics which can be exploited in the design of electric field-switchable magnets.Comment: 4 pages, 4 figure

The impact of hysteresis on the electrocaloric effect at first-order phase transitions

We study the impact of thermal hysteresis at the first-order structural/ferroelectric phase transitions on the electrocaloric response in bulk BaTiO$_3$ by performing molecular dynamics simulations for a first-principles-based effective Hamiltonian. We demonstrate that the electrocaloric response can conceptually be separated in two contributions: a transitional part, stemming from the discontinuous jump in entropy at the first order phase transition, and a configurational part, due to the continuous change of polarization and entropy within each phase. This latter part increases with the strength of the applied field, but for small fields it is very small. In contrast, we find a large temperature change of $\sim 1$ K resulting from the transition entropy, which is essentially independent of the field strength. However, due to the coexistence region close to the first order phase transition, this large electrocaloric response depends on the thermal history of the sample and is generally not reversible. We show that this irreversibility can be overcome by using larger fields.Comment: 7 pages, 5 figure

First-Principles-Based Strain and Temperature Dependent Ferroic Phase Diagram of SrMnO3_3

Perovskite structure SrMnO$_3$ is a rare example of a multiferroic material where strain-tuning and/or cation substitution could lead to coinciding magnetic and ferroelectric ordering temperatures, which would then promise strong magnetoelectric coupling effects. Here, we establish the temperature and strain dependent ferroic phase diagram of SrMnO$_3$ using first-principles-based effective Hamiltonians. All parameters of these Hamiltonians are calculated using density functional theory, i.e., no fitting to experimental data is required. Temperature dependent properties are then obtained from Monte Carlo and molecular dynamics simulations. We observe a sequence of several magnetic transitions under increasing tensile strain, with a moderate variation of the corresponding critical temperatures. In contrast, the ferroelectric Curie temperature increases strongly after its onset around 2.5\,\% strain, and indeed crosses the magnetic transition temperature just above 3\,\% strain. Our results indicate pronounced magnetoelectric coupling, manifested in dramatic changes of the magnetic ordering temperatures and different magnetic ground states as function of the ferroelectric distortion. In addition, coexisting ferroelectric and ferromagnetic order is obtained for strains above 4\,\%. Our calculated phase diagram suggests the possibility to control the magnetic properties of SrMnO$_3$ through an applied electric field, significantly altering the magnetic transition temperatures, or even inducing transitions between different magnetic states.Comment: 13 pages, 8 figure

Labour markets in a Post-Keynesian growth model: the effects of endogenous productivity growth and working time reduction

We study endogenous employment and distribution dynamics in a Post-Keynesian model of Kalecki-Steindl tradition. Productivity adjustments stabilize employment and the labour share in the long run: technological change allows firms to replenish the reserve army of workers in struggle over income shares and thereby keep wage demands in check. We discuss stability conditions and the equilibrium dynamics. This allows us to study how legal working time and its reduction affect the equilibrium. We find that a demand shock is likely to lower the profit share and increase the employment rate. A supply shock in contrast tends to have detrimental effects on employment and income distribution. Labour market institutions and a working time reduction have no long-term effect on growth, distribution and inflation in the model. The effects on the level of capital stock and output however are positive in a wage-led demand regime. Furthermore, an erosion of labour market institutions dampens inflation temporarily. The model provides possible explanations as to the causes of several current economic phenomena such as secular stagnation, digitalisation, and the break-down of the Philips curve.Series: Ecological Economic Paper

Search Fatigue

Consumer search is not only costly but also tiring. We characterize the intertemporal effects that search fatigue has on oligopoly prices, product proliferation, and the provision of consumer assistance (i.e., advice). These effects vary based on whether search is all-or-nothing or sequential in nature, whether learning takes place, and whether consumers exhibit brand loyalty. We perform welfare analysis and highlight the novel empirical implications that our analysis generates.