Reversible single-crystal to single-crystal transformation between triangular single-molecule toroics.

We report a method for synthesizing single-molecule magnets through a single-crystal to single-crystal transformation. This process yields two single-molecule magnets with similar triangular Dy3 cores but distinct solvents and space groups achieved via solvent exchange. Magnetic properties reveal that both Dy3 molecules exhibit similar toroidal moments but manifest diverse multiple magnetization dynamic behaviors owing to the spin-lattice coupling influence from different solvent molecules.

Magnetic-Field Tuning of Hydrogen Bond Order-Disorder Transition in Metal-Organic Frameworks.

The ordering of polar hydrogen bonds may break space inversion symmetry and induce ferroelectricity or antiferroelectricity. This process is usually immune to external magnetic fields so that magnetic control of hydrogen bonds is very challenging. Here we demonstrate that the ordering of hydrogen bonds in the metal-organic frameworks [(CH_{3})_{2}NH_{2}]M(HCOO)_{3} (M=Fe, Co) can be manipulated by applying magnetic fields. After cooling in a high magnetic field, the order-disorder transition of hydrogen bonds shifts to a lower or higher temperature, depending on antiferroelectricity or ferroelectricity induced by hydrogen bond ordering. Besides, the order-disorder transition leads to a giant thermal expansion, exceeding ∼3.5×10^{4} and ∼2×10^{4} ppm for M=Fe and Co, respectively, which is much higher than that of inorganic ferroelectrics. The influence of magnetic field on hydrogen bond ordering is discussed in terms of the magnetoelastic coupling.

Multiferroicity and magnetoelectric coupling in the paramagnetic state of the metal-organic framework [(CH3)2NH2]Ni(HCOO)3.

The metal-organic framework [(CH3)2NH2]Ni(HCOO)3 (DMA-Ni) has an ABX3 perovskite-like structure. At T C ~ 181 K, DMA-Ni displays a first-order ferroelectric transition, which is triggered by the disorder-order transition of hydrogen bonds. In addition, this compound exhibits a spin-canted antiferromagnetic order below T N ~ 37.6 K through the long-distance superexchange interaction, and a spin-reorientation transition appears near 15 K. The coexistence of magnetic and ferroelectric orders at low temperature testifies the multiferroic properties of DMA-Ni. Besides, the magnetoelectric (ME) coupling exists in the paramagnetic state, where the ferroelectric polarization can be modified by applying high magnetic fields. This behavior is attributed to the local magnetostriction effect.

Spin-induced multiferroicity in the binary perovskite manganite Mn2O3.

The ABO3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn2O3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn2O3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties.

A multilevel nonvolatile magnetoelectric memory.

The coexistence and coupling between magnetization and electric polarization in multiferroic materials provide extra degrees of freedom for creating next-generation memory devices. A variety of concepts of multiferroic or magnetoelectric memories have been proposed and explored in the past decade. Here we propose a new principle to realize a multilevel nonvolatile memory based on the multiple states of the magnetoelectric coefficient (α) of multiferroics. Because the states of α depends on the relative orientation between magnetization and polarization, one can reach different levels of α by controlling the ratio of up and down ferroelectric domains with external electric fields. Our experiments in a device made of the PMN-PT/Terfenol-D multiferroic heterostructure confirm that the states of α can be well controlled between positive and negative by applying selective electric fields. Consequently, two-level, four-level, and eight-level nonvolatile memory devices are demonstrated at room temperature. This kind of multilevel magnetoelectric memory retains all the advantages of ferroelectric random access memory but overcomes the drawback of destructive reading of polarization. In contrast, the reading of α is nondestructive and highly efficient in a parallel way, with an independent reading coil shared by all the memory cells.

Multiferroicity Broken by Commensurate Magnetic Ordering in Terbium Orthomanganite.

TbMnO3 is an important multiferroic material with strong coupling between magnetic and ferroelectric orderings. Incommensurate magnetic ordering is suggested to be vital for this coupling in TbMnO3 , which can be modified by doping at the site of Tb and/or Mn. Our study shows that a self-doped solid solution Tb1-x Mny MnO3 (y≤x) can be formed with Mn doped into the site of Tb of TbMnO3 . When y is small Tb1-x Mny MnO3 shows both ferroelectric and incommensurate magnetic orders at low temperature, which is similar to TbMnO3 . However, if y is large enough, a commensurate antiferromagnetic ordering appears along with the incommensurate magnetic ordering to prevent the appearance of multiferroicity in Tb1-x Mny MnO3 . That is to say, the magnetoeletric coupling can be broken by the co-existence of a commensurate antiferromagnetic ordering. This finding may be useful to the study of TbMnO3 .

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.

Quantum tunneling of magnetization in a metal-organic framework.

Resonant quantum tunneling of magnetization has been observed in a hybrid metal-organic framework where an intrinsic magnetic phase separation leads to the coexistence of long-range canted antiferromagnetic order and isolated single-ion quantum magnets. This unusual magnetic phenomenon is well interpreted based on a selective long-distance superexchange model in which the exchange interaction between transition metal ions through an organic linker depends on the position of hydrogen bonds. Our work not only extends the resonant quantum tunneling of magnetization to a new class of materials but also evokes the important role of hydrogen bonding in organic magnetism.