Young Sun

Magnetoelectric coupling in the paramagnetic state of a metal-organic framework

Although the magnetoelectric effects - the mutual control of electric polarization by magnetic fields and magnetism by electric fields, have been intensively studied in a large number of inorganic compounds and heterostructures, they have been rarely observed in organic materials. Here we demonstrate magnetoelectric coupling in a metal-organic framework [(CH3)2NH2]Mn(HCOO)3 which exhibits an order-disorder type of ferroelectricity below 185 K. The magnetic susceptibility starts to deviate from the Curie-Weiss law at the paraelectric-ferroelectric transition temperature, suggesting an enhancement of short-range magnetic correlation in the ferroelectric state. Electron spin resonance study further confirms that the magnetic state indeed changes following the ferroelectric phase transition. Inversely, the ferroelectric polarization can be improved by applying high magnetic fields. We interpret the magnetoelectric coupling in the paramagnetic state in the metal-organic framework as a consequence of the magnetoelastic effect that modifies both the superexchange interaction and the hydrogen bonding.

Cross coupling between electric and magnetic orders in a multiferroic metal-organic framework

The coexistence of both electric and magnetic orders in some metal-organic frameworks (MOFs) has yielded a new class of multiferroics beyond inorganic materials. However, the coupling between two orders in multiferroic MOFs has not been convincingly verified yet. Here we present clear experimental evidences of cross coupling between electric and magnetic orders in a multiferroic MOF [(CH3)2NH2]Fe(HCOO)3 with a perovskite structure. The dielelectric constant exhibit a hump just at the magnetic ordering temperature TN. Moreover, both the direct (magnetic field control of dielectric properties) and converse (electric field control of magnetization) magnetoelectric effects have been observed in the multiferroic state. This work opens up new insights on the origin of ferroelectricity in MOFs and highlights their promise as magnetoelectric multiferroics.

Large magnetic entropy change in the colossal magnetoresistance material La2/3Ca1/3MnO3

Abstract In this paper, we present a study of magnetocaloric effect in the colossal magnetoresistance material La 2/3 Ca 1/3 MnO 3 . From the measurements of temperature dependence of magnetization under various magnetic fields, we have discovered a large magnetic entropy change associated with the ferromagnetic–paramagnetic transition. This result suggests that perovskite manganites are suitable candidates as working substances in magnetic refrigeration technology.

Room temperature giant dielectric tunability effect in bulk LuFe2O4

We report the extreme sensitivity of dielectric permittivity to applied dc bias electric field in bulk LuFe2O4. A small bias field of 50V∕cm can greatly reduce the dielectric permittivity in the vicinity of room temperature, which is in strong contrast to conventional ferroelectric materials where a large electric field of the order of tens of kV/cm is required. This giant dielectric tunability effect within a broad temperature interval around room temperature is very promising for tunable device applications. The possible origins of this giant effect are discussed.

Observation of Resonant Quantum Magnetoelectric Effect in a Multiferroic Metal-Organic Framework.

A resonant quantum magnetoelectric coupling effect has been demonstrated in the multiferroic metal-organic framework of [(CH3)2NH2]Fe(HCOO)3. This material shows a coexistence of a spin-canted antiferromagnetic order and ferroelectricity as well as clear magnetoelectric coupling below TN ≈ 19 K. In addition, a component of single-ion quantum magnets develops below ∼ 8 K because of an intrinsic magnetic phase separation. The stair-shaped magnetic hysteresis loop at 2 K signals resonant quantum tunneling of magnetization. Meanwhile, the magnetic field dependence of dielectric permittivity exhibits sharp peaks just at the critical tunneling fields, evidencing the occurrence of resonant quantum magnetoelectric coupling effect. This resonant effect enables a simple electrical detection of quantum tunneling of magnetization.

Tuning colossal magnetoresistance response by Cr substitution in La0.67Sr0.33MnO3

We have studied the effects of Cr substitution in the perovskite La0.67Sr0.33MnO3 on the magnetic, electrical transport, and magnetoresistance properties. Cluster glass behaviors have been observed in the La0.67Sr0.33Mn1−xCrxO3 system. The most interesting feature is that extraordinary transport and colossal magnetoresistance (CMR) behaviors, characterized by double peaks, were observed with Cr substitution. When 0.1⩽x⩽0.2, the temperature range of CMR response is greatly broadened, from the lowest temperature to above room temperature. These results suggest that Cr substitution can be a potent way in tuning CMR response and also imply a ferromagnetic interaction similar to double exchange occurs between Mn and Cr.

Variable-range hopping of small polarons in mixed-valence manganites

Mixed-valence manganites Ln1-xAxMnO3 exhibit complicated transport behaviour resulting from complex interplays among charge, spin and lattice. In the paramagnetic phase, the carriers are trapped in localized states as small polarons due to the incorporation of three different localization features: (i) strong electron-phonon interaction, (ii) the variations in the Coulomb potential due to the presence of Ln3+ and A2+ ions in the lattice, (iii) the magnetic localization due to spin disorder on the interatomic scale. When the thermal energy is not enough for small polarons to hop between nearest-neighbour sites, the transport of small polarons could be accomplished by two steps: first, the small polaron is thermally activated into an intermediate state in which the carrier is weakly localized; then it feels the potential fluctuation due to an electrical and magnetic disorder and transports by variable-range hopping. We term this kind of transport process as variable-range hopping of small polarons, and derive the expression of resistivity as ρ = BTexp [Ea/kT + (T0/T)¼].

Cooling field dependence of exchange bias in phase-separated La0.88Sr0.12CoO3

We report the observation of exchange bias phenomena in the hole-doped perovskite cobaltite La0.88Sr0.12CoO3 in which a spontaneous phase separation occurs. When the sample is cooled in a static magnetic field through a freezing temperature, the magnetization hysteresis loops shift to the negative field. Moreover, the exchange bias strongly depends on the cooling field. These results highlight the important role of a glassy interface between the intrinsic inhomogeneous phases in a phase-separated system.

Low magnetic field reversal of electric polarization in a Y-type hexaferrite

We report on the magnetically tunable ferroelectricity and giant magnetoelectric sensitivity up to 250 K in a Y-type hexaferrite, BaSrCoZnFe11AlO22. Not only the magnitude but also the sign of electric polarization can be effectively controlled by applying low magnetic fields (a few hundreds of Oe) that modifies the spiral magnetic structures. The magnetically induced ferroelectricity is stabilized even in zero magnetic field. Decayless reproducible flipping of electric polarization by oscillating low magnetic fields is shown. The maximum linear magnetoelectric coefficient reaches a high value of ∼3.0 × 103 ps/m at 200 K.

Electric field induced phase transition in charge-ordered LuFe2O4

The measurements of resistance under various applied voltages as well as the highly nonlinear current-voltage characteristics reveal that a small electric field is able to induce an insulating to metallic phase transition in LuFe2O4. The threshold field at which the phase transition occurs decreases exponentially with the increasing temperature. We interpret this transition as a consequence of the breakdown of the charge-ordered state triggered by applied electric field. This electrically driven phase transition results in a colossal electroresistance effect around room temperature which makes LuFe2O4 a very promising material for many applications.