arXiv subjects

J. -M. Liu

Publications and source records attributed to J. -M. Liu.

At least 37 records · Page 2Linked to original sources

arXiv2014

BaFe$_2$Se$_3$: a high $T_{\rm C}$ magnetic multiferroic with large ferrielectric polarization

The iron-selenides are important because of their superconducting properties. Here, an unexpected phenomenon is predicted to occur in an iron-selenide compound with a quasi-one-dimensional ladder geometry: BaFe$_2$Se$_3$ should be a magnetic ferrielectric system, driven by its magnetic block order via exchange striction. A robust performance (high $T_{\rm C}$ and large polarization) is expected. Different from most multiferroics, BaFe$_2$Se$_3$ is ferrielectric, with a polarization that mostly cancels between ladders. However, its strong magnetostriction still produces a net polarization that is large ($\sim$$0.1$ $μ$C/cm$^2$) as compared with most magnetic multiferroics. Its fully ferroelectric state, with energy only slightly higher than the ferrielectric, has a giant improper polarization $\sim$$2-3$ $μ$C/cm$^2$.

cond-mat.mtrl-sci↗

arXiv2014

Reversing ferroelectric polarization in multiferroic DyMn2O5 by nonmagnetic Al substitution of Mn

The multiferroic RMn2O5 family, where R is rare-earth ion or Y, exhibits rich physics of multiferroicity which has not yet well understood, noting that multiferroicity is receiving attentions for promising application potentials. DyMn2O5 is a representative member of this family. The ferroelectric polarization in DyMn2O5 is claimed to have two anti-parallel components: one (PDM) from the symmetric exchange striction between the Dy3+-Mn4+ interactions and the other (PMM) from the symmetric exchange striction between the Mn3+-Mn4+ interactions. We investigate the evolutions of the two components upon a partial substitution of Mn3+ by nonmagnetic Al3+ in order to tailor the Mn-Mn interactions and then to modulate component PMM in DyMn2-x/2Alx/2O5. It is revealed that the ferroelectric polarization can be successfully reversed by the Al-substitution via substantially suppressing the Mn3+-Mn4+ interactions and thus the PMM. The Dy3+-Mn4+ interactions and the polarization component PDM can sustain against the substitution until a level as high as x=0.2. In addition, the independent Dy spin ordering is shifted remarkably down to an extremely low temperature due to the Al3+ substitution. The present work not only confirms the existence of the two anti-parallel polarization components but also unveils the possibility of tailoring them independently.

cond-mat.str-el↗

arXiv2013

Experimental observation of ferrielectricity in multiferroic DyMn2O5: revisiting the electric polarization

The electric polarization and its magnetic origins in multiferroic RMn2O5, where R is rare-earth ion, are still issues under debate. In this work, the temperature-dependent electric polarization of DyMn2O5, the most attractive member of this RMn2O5 family, is investigated using the pyroelectric current method upon varying endpoint temperature of the electric cooling, plus the positive-up-negative-down (PUND) technique. It is revealed that DyMn2O5 at low temperature does exhibit the unusual ferrielectricity rather than ferroelectricity, characterized by two interactive and anti-parallel ferroelectric sublattices which show different temperature-dependences. The two ferroelectric sublattices are believed to be generated from the symmetric exchange-striction mechanisms associated with the Mn-Mn spin interactions and Dy-Mn spin interactions, respectively. The path-dependent electric polarization reflects the first-order magnetic transitions in the low temperature regime. The magnetoelectric effect is mainly attributed to the Dy spin order which is sensitive to magnetic field. The present experiments may be helpful for clarifying the puzzling issues on the multiferroicity in DyMn2O5 and probably other RMn2O5 multiferroics.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic and Orbital Order in ($R$MnO$_3$)$_n$/($A$MnO$_3$)$_{2n}$ Superlattices Studied via a Double-Exchange Model with Strain

The two-orbital double-exchange model is employed for the study of the magnetic and orbital orders in ($R$MnO$_3$)$_n$/($A$MnO$_3$)$_{2n}$ ($R$: rare earths; $A$: alkaline earths) superlattices. The A-type antiferromagnetic order is observed in a broad region of parameter space for the case of SrTiO$_3$ as substrate, in agreement with recent experiments and first-principles calculations using these superlattices. In addition, also a C-type antiferromagnetic state is predicted to be stabilized when using substrates like LaAlO$_3$ with smaller lattice constants than SrTiO$_3$, again in agreement with first principles results. The physical mechanism for the stabilization of the A- and C- magnetic transitions is driven by the orbital splitting of the $x^2-y^2$ and $3z^2-r^2$ orbitals. This splitting is induced by the $Q_3$ mode of Jahn-Teller distortions created by the strain induced by the substrates. In addition to the special example of (LaMnO$_3$)$_n$/(SrMnO$_3$)$_{2n}$, our phase diagrams can be valuable for the case where the superlattices are prepared employing narrow bandwidth manganites. In particular, several non-homogenous magnetic profiles are predicted to occur in narrow bandwidth superlattices, highlighting the importance of carrying out investigations in this mostly unexplored area of research.

cond-mat.str-el↗

arXiv2011

Ab Initio Study of the Intrinsic Exchange Bias at the SrRuO$_3$/SrMnO$_3$ Interface

In a recent publication (S. Dong et al., Phys. Rev. Lett.103, 127201 (2009)), two (related) mechanisms were proposed to understand the intrinsic exchange bias present in oxides heterostructures involving G-type antiferromagnetic perovskites. The first mechanism is driven by the Dzyaloshinskii-Moriya interaction, which is a spin-orbit coupling effect. The second is induced by the ferroelectric polarization, and it is only active in heterostructures involving multiferroics. Using the SrRuO$_3$/SrMnO$_3$ superlattice as a model system, density-functional calculations are here performed to verify the two proposals. This proof-of-principle calculation provides convincing evidence that qualitatively supports both proposals.

cond-mat.mtrl-sci↗

arXiv2011

Neutron scattering study of magnetic phase separation in nanocrystalline La$_{5/8}$Ca$_{3/8}$MnO$_3$

We demonstrate that magnetic phase separation and competing spin order in the colossal magnetoresistive (CMR) manganites can be directly explored via tuning strain in bulk samples of nanocrystalline La$_{1-x}$Ca$_x$MnO$_3$. Our results show that strain can be reversibly frozen into the lattice in order to stabilize coexisting antiferromagnetic domains within the nominally ferromagnetic metallic state of La$_{5/8}$Ca$_{3/8}$MnO$_3$. The measurement of tunable phase separation via magnetic neutron powder diffraction presents a direct route of exploring the correlated spin properties of phase separated charge/magnetic order in highly strained CMR materials and opens a potential avenue for realizing intergrain spin tunnel junction networks with enhanced CMR behavior in a chemically homogeneous material.

cond-mat.str-el↗

arXiv2011

Multiferroic Properties of CaMn$_7$O$_{12}$

We report that CaMn$_7$O$_{12}$ is a new magnetic multiferroic material. The appearance of a ferroelectric polarization coinciding with the magnetic phase transition ($\sim90$ K) suggests the presence of ferroelectricity induced by magnetism, further confirmed by its strong magnetoelectric response. With respect to other known magnetic multiferroics, CaMn$_7$O$_{12}$ displays attractive multiferroic properties, such as a high ferroelectric critical temperature and large polarization. More importantly, these results open a new avenue to search for magnetic multiferroics in the catalogue of doped oxides.

cond-mat.mtrl-sci↗

arXiv2011

Microscopic model for the ferroelectric field effect in oxide heterostructures

A microscopic model Hamiltonian for the ferroelectric field effect is introduced for the study of oxide heterostructures with ferroelectric components. The long-range Coulomb interaction is incorporated as an electrostatic potential, solved self-consistently together with the charge distribution. A generic double-exchange system is used as the conducting channel, epitaxially attached to the ferroelectric gate. The observed ferroelectric screening effect, namely the charge accumulation/depletion near the interface, is shown to drive interfacial phase transitions that give rise to robust magnetoelectric responses and bipolar resistive switching, in qualitative agreement with previous density functional theory calculations. The model can be easily adapted to other materials by modifying the Hamiltonian of the conducting channel, and it is useful in simulating ferroelectric field effect devices particularly those involving strongly correlated electronic components where ab-initio techniques are difficult to apply.

cond-mat.mtrl-sci↗

arXiv2010

Highly anisotropic resistivities in the double-exchange model for strained manganites

The highly anisotropic resistivities in strained manganites are theoretically studied using the two-orbital double-exchange model. At the nanoscale, the anisotropic double-exchange and Jahn-Teller distortions are found to be responsible for the robust anisotropic resistivities observed here via Monte Carlo simulations. An unbalanced in the population of orbitals caused by strain is responsible for these effects. In contrast, the anisotropic superexchange is found to be irrelevant to explain our results. Our model study suggests that highly anisotropic resistivities could be present in a wide range of strained manganites, even without (sub)micrometer-scale phase separation. In addition, our calculations also confirm the formation of anisotropic clusters in phase-separated manganites, which magnifies the anisotropic resistivities.

cond-mat.str-el↗

arXiv2010

Enhancement of ferroelectricity in Cr-doped Ho_2Ti_2O_7

A series of polycrystalline pyrochlore rare-earth titanate Ho_{2-x}Cr_xTi_2O_7 are synthesized in order to enhance the ferroelectricity of pyrochlore Ho2Ti2O7. For the sample close to the doping level x=0.4, a giant enhancement of polarization P up to 660\muC/m2 from 0.54\muC/m2 at x=0 is obtained, accompanied with an increment of ferroelectric transition point Tc up to ~140K from ~60K. A magnetic anomaly at T~140K together with the polarization response to magnetic field, is identified, implying the multiferroic effect in Ho2-xCrxTi2O7.

cond-mat.mtrl-sci↗

arXiv2009

Enhanced polarization, magnetoelectric response and possible bi-multiferroic phase separation in Tb_{1-x}Ho_xMnO_3

A series of manganites Tb1-xHoxMnO3 (0<=x<=0.6) with orthorhombic structure are synthesized and detailed investigations on their multiferroicity are performed. Successive magnetic transitions upon temperature variation are evidenced for all samples, and both the Mn3+ spiral spin ordering and rare-earth spin ordering are suppressed with increasing x. Significant enhancement of both the polarization and magnetoelectric response within 0.2<x<0.4 is observed, which may be ascribed to the competition possibly existing between spiral and E-type spin orders. Theoretical calculation is given based on two eg-orbital double-exchange model, and the result supports the scenario of the multiferroic phase separation.

cond-mat.str-el↗

arXiv2009

Flux state and anomalous quantum Hall effect in square Kondo lattice

The anomalous Hall effect (AHE) around the flux state in square Kondo lattice is investigated. By introducing the lattice distortion and local chirality, the square Kondo lattice can break the parity symmetry and time reversal symmetry spontaneously, and thus generate a topological nontriviality in the band structure associated with the AHE. Moreover, a possible realization of this AHE in multiferroic TbMnO$_3$ is discussed.

cond-mat.str-el↗

arXiv2009

Exchange bias driven by the Dzyaloshinskii-Moriya interaction and ferroelectric polarization at G-type antiferromagnetic perovskite interfaces

Exchange bias is usually rationalized invoking spin pinning effects caused by uncompensated antiferromagnetic interfaces. However, for compensated antiferromagnets other extrinsic factors, such as interface roughness or spin canting, have to be considered to produce a small uncompensation. As an alternative, here we propose two (related) possible mechanisms, driven by the intrinsic Dzyaloshinskii-Moriya interaction and ferroelectric polarization, for the explanation of exchange bias effects in perovskites with compensated G-type antiferromagnetism. One of the mechanisms is only active when a multiferroic material is involved and it is controllable by electric fields.

cond-mat.mtrl-sci↗

arXiv2009

Striped Multiferroic Phase in Double-Exchange Model for Quarter-Doped Manganites

The phase diagram of quarter-hole-doped perovskite manganites is investigated using the doubleexchange model. An exotic striped type-II multiferroic phase, where 25% of the nearest-neighbor spin couplings are orthogonal to each other, is found in the narrow-bandwidth region. Comparing with the spiral-spin ordering phase of undoped manganites, the multiferroic Curie temperature of the new phase is estimated to be ~4 times higher, while the ferroelectric polarization is similar in magnitude. Our study provides a path for noncollinear spin multiferroics based on electronic self-organization, different from the traditional approach based on superexchange frustration.

cond-mat.str-el↗

arXiv2009

Multiferroicity, The coupling between magnetic and polarization

Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review article, we outline some of the progressive milestones in this stimulating field, specially for those single phase multiferroics where magnetism and ferroelectricity coexist. Firstly, we will highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we will summarize various strategies used to combine the two types of orders. Special attentions to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry, (2) the ferroelectricity originating from the charge ordered states, and (3) the ferrotoroidic system, will be paid. Then, we will address the elementary excitations such as electromagnons, and application potentials of multiferroics. Finally, open questions and opportunities will be prospected.

cond-mat.mtrl-sci↗

arXiv2009

Double-exchange model study of multiferroic $R$MnO$_3$ perovskites

In this proceeding, recent theoretical investigations by the authors on the multiferroic $R$MnO$_3$ perovskites are briefly reviewed at first. Using the double-exchange model, the realistic spiral spin order in undoped manganites such as TbMnO$_3$ and DyMnO$_3$ is well reproduced by incorporating a weak next-nearest neighbor superexchange ($\sim10%$ of nearest neighbor superexchange) and moderate Jahn-Teller distortion. The phase transitions from the A-type antiferromagnet (as in LaMnO$_3$), to the spiral phase (as in TbMnO$_3$), and finally to the E-type antiferromagnet (as in HoMnO$_3$), with decreasing size of the $R$ ions, were also explained. Moreover, new results of phase diagram of the three-dimensional lattice are also included. The ferromagnetic tendency recently discovered in the LaMnO$_3$ and TbMnO$_3$ thin films is explained by considering the substrate stress. Finally, the relationship between our double-exchange model and a previously used $J_1$-$J_2$-$J_3$ model is further discussed from the perspective of spin wave excitations.

cond-mat.str-el↗

arXiv2009

Non-magnetic B-site Impurities Induce Ferromagnetic Tendencies in CE Manganites

Using a two-orbital model and Monte Carlo simulations, we investigate the effect of nonmagnetic B-site substitution on half-doped CE-type manganites. The lattice defects induced by this substitution destabilize the CE phase, which transforms into (1) the ferromagnetic (FM) metallic competing state, or (2) a regime with short-range FM clusters, or (3) a spin-glass state, depending on couplings and on the valence of the B-site substitution. While a C-type antiferromagnetic state is usually associated with an average $e_{\rm g}$ charge density less than 0.5, the nonmagnetic B-site substitution that lowers the $e_{\rm g}$ charge density is still found to enhance the FM tendency in our simulations. The present calculations are in qualitative agreement with experiments and provide a rationalization for the complex role of nonmagnetic B-site substitution in modulating the phase transitions in manganites.

cond-mat.str-el↗

arXiv2008

Metal-insulator transition in $(LaMnO_3)_{2n}/(SrMnO_3)_n$ superlattices

The modulation of charge density and spin order in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ ($n$=1-4) superlattices is studied via Monte Carlo simulations of the double-exchange model. G-type antiferromagnetic barriers in the SrMnO$_{3}$ regions with low charge density are found to separate ferromagnetic LaMnO$_{3}$ layers with high charge density. The recently experimentally observed metal-insulator transition with increasing $n$ is reproduced in our studies, and $n=3$ is found to be the critical value.

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