JAPAN NANONET BULLETIN - 87th Issue - January 11, 2007

Molecular magnets and shell nanomagnets
— Combining “controllability” of organic materials and “magnetism” of inorganic materials —

In a paramagnetic material, the magnetic moments of atoms caused by unpaired electron spins are randomly oriented. When magnetic moments of atoms spontaneously align parallel to each other, the material becomes ferromagnetic. Organic molecules with even numbers of electrons become diamagnetic because the electron spins’ magnetic moments are cancelled out. In organic molecules, although there are molecules with an isolated unpaired electron, such as a radical, it was extremely difficult to align magnetic moments parallel within or between molecules. However, in 1967, Prof. Koichi Ito (Professor Emeritus, Osaka City University) published his experimental results, which indicated that when a small number of unpaired electron spins align parallel to one another, an organic ferromagnet can be obtained. In 1991, Prof. Minoru Kinoshita (Professor Emeritus, The University of Tokyo) discovered the world’s first organic molecular ferromagnet, p-nitrophenyl nitronyl nitroxide (p-NPNN), in which electron spins between adjacent molecules in the molecular crystal align parallel to each other. Prof. Awaga, who was involved in the discovery with Prof. Kinoshita, says, “Adjacent unpaired electron spins tend to align antiparallel to each other. In other words, molecules are more stable when they are chemically bound. Obtaining a molecular ferromagnet was not about how to chemically bind the molecules but how to align parallel the spins of adjacent radicals.” He also says, “In order to avoid chemical bonding, spin polarization of the radicals must be increased, and the polarized spins must align with those of the adjacent molecules in antiphase.” p-NPNN has a NO group with a polarized unpaired electron spin and a substituent group with very little polarization. p-NPNN is crystallized by intermolecular electrostatic interaction between the NO group and the substituent. This contact between the NO ligand and the substituent group causes propagation of the antiphase spin polarization between the adjacent molecules.

The Curie temperature, below which p-NPNN shows ferromagnetism, is 0.65 K. Research on cyclic thiazyl radicals (SN radicals) to attain higher Curie temperatures has led to an increase in temperature of one order of magnitude. While researching the properties of SN radicals, Prof. Awaga discovered a unique property of TTTA (1,3,5-trithia-2,4,6-triazapentalenyl) derivatives. TTTA shows paramagnetism at high temperatures because the molecules arrange themselves in a regular array; however, it is diamagnetic at temperatures below 180 K due to dimerization of the molecules. This phase transition occurs with a thermal hysteresis loop over a wide temperature range including room temperature. Prof. Awaga says, “The magnetic bistability, in which paramagnetism and diamagnetism coexist, was discovered in TTTA. If we could control the transition between these two stable states, memory devices and sensors may be fabricated by using organic molecular crystals.” Since the crystal colors differ between high-temperature phases and low-temperature phases, photo-induced phase transitions are possible.

Prof. Awaga also conducted research on single-molecule magnets, which are magnets with the smallest possible size. At cryogenic temperatures, magnetization curves of some metal cluster complexes are hysteresis loops similar to those of ferromagnets, because their magnetic relaxation is slow below their blocking temperatures. Magnetization reversal also occurs due to a tunnel effect. Mn12 clusters are the most researched single-molecule magnets.

They contain four Mn⁴⁺ ions and eight Mn³⁺ ions and have huge magnetic moments as a molecule, and uniaxial magnetic anisotropy is induced by Jahn-Teller distortion caused by Mn³⁺. He discovered dipole-biased tunneling of magnetization and determined the origin of magnetic anisotropy through metal ion exchange and ligand substitution. He says, “It is quiet thrilling that physical phenomena, such as single-molecule magnetism and quantum tunneling of magnetization, could be clarified by molecular modification, which is a typical chemical method.”

Tremendous progress has been made in molecular magnets. However, they cannot be easily commercialized because their ferromagnetic transition temperature and their blocking temperature are low. Prof. Awaga says, “Molecular magnets exhibit changes in their magnetic properties by external stimulation, such as lights and chemical reactions. It is easy to control the magnetic properties in organic molecules but difficult in inorganic molecules. When the size of inorganic molecules becomes smaller, controlling the magnetic properties becomes easier. I felt that nanomagnets and molecular magnets are closely related because nanosized magnets are expected to exhibit quantum effects.” He began to research inorganic shell nanomagnets to develop nanomagnets for practical applications. He first prepared a hollow sphere of Co₃O₄ with a diameter of 500 nm and a wall thickness of 40 nm. When cobalt hydroxide is uniformly deposited on the surface of polystyrene beads and burned, hollow structures can be obtained. Although this Co₃O₄ spherical nanoshell shows antiferromagnetism at room temperature, spontaneous magnetization appears below the Neel temperature and exhibits spin glass-like characteristics. This could result from making Co₃O₄ a spherical nanoshell, because lattice defects in the wall of the spherical nanoshell are increased, and thus, spins, which are supposed to be cancelled out, remain. Prof. Awaga says, “In spherical nanoshells, remanent magnetization is larger than those of bulk materials by one or two orders of magnitude. However, remanent magnetization is smaller in nanomagnets with a diameter of 50 nm. I think nanomagnets with a diameter of 50 nm are too small to maintain spontaneous magnetization because magnetic relaxation occurs rapidly.” There may be unexplored mesoscopic properties in the magnets with a diameter of 500 nm.

Prof. Awaga also fabricated nanoshell magnets using cobalt, magnetite (Fe₃O₄), hematite (α-Fe₂O₃) and iron. He discovered properties similar to those of single molecule magnets, such as magnetization curves, which change remarkably with temperature below 300 K. He considered that, if the magnets could be properly surface-treated, they may become soluble magnets or may become catalysts extracted by magnets because of their large surfaces. He says, “Clockwise magnetization or counter-clockwise magnetization in the nanoshell wall may be generated by applying electric current to a nanosphere in one direction. Controlling magnetization with the current leads to spintronics, into which I want to expand the research on nanospheres.” It has been twenty years since he started to research on molecular magnets, and his research has expanded beyond molecular magnets to molecular spintronics.