J. Sánchez-Benítez

A mixed-valence Co-7 single-molecule magnet with C-3 symmetry

The synthesis, structure and magnetic properties of [Co(II)(4)Co(III)(3)(HL)(6)(NO(3))(3)(H(2)O)(3)](2+) [H(3)L = H(2)NC(CH(2)OH)(3)] are reported: the complex is an exchange-biased single molecule magnet.

Enhanced magnetoresistance in the complex perovskite LaCu3Mn4O12

Moderate-pressure techniques (P=2 GPa) have been used to prepare the complex LaCu3Mn4O12 perovskite. It has been characterized by neutron powder diffraction, magnetic, and magnetotransport measurements. This material is ferrimagnetic below TC=361 K. The magnetoresistance (MR) is enhanced with respect to that of CaCu3Mn4O12 due to the effective electronic injection that dramatically reduces the bulk resistivity, thus promoting the grain-boundary contribution to the electrical resistance. Values of low-field MR close to 3% at room temperature are achieved for magnetic fields of 1 T.

Synthesis and characterisation of a Ni(4) single-molecule magnet with S(4) symmetry

[Ni4Cl4(HL)4] () {H2L=HN(CH2CH2OH)2} has S4 symmetry and crystallises in the tetragonal space group I4(1)/a. Two exchange couplings are observed between the four Ni(II) centres, with J1=7.29 cm(-1) and J2=-2.08 cm(-1), leading to an S=4 ground state. The Ni4 complex shows the onset of frequency dependent signals in the out-of-phase ac susceptibility below 3 K. In single-crystal measurements carried out using a micro-SQUID, hysteresis loops are observed below 0.5 K, confirming that shows slow relaxation of magnetisation. The loops are temperature dependent but only weakly sweep rate dependent due to the presence of small intermolecular interactions, which hinder quantum tunnelling. This exchange bias between Ni4 molecules is also seen in high-frequency high-field EPR measurements, which give the parameters D=-0.75 cm(-1), B4 degrees=-6.7x10(-5) cm(-1) and gz=2.275.

[Mn6] under Pressure: A Combined Crystallographic and Magnetic Study†

Folding under pressure: Crystallographic studies on a Mn6 single-molecule magnet under high pressure conditions show the drastic structural changes of the magnetic core (see picture, Mn purple, O red, N blue), which impact on the magnetic properties of ferromagnetic exchange between the metal atoms will be in booster weaker, and under extremely high pressure, a transition to antiferromagnetic behavior.

Magnetic ground state of an experimental S=1/2 kagome antiferromagnet.

We present a detailed analysis of the heat capacity of a near-perfect S = 1/2 kagome antiferromagnet, zinc paratacamite Zn x Cu 4-x (OH) 6 Cl 2 , as a function of stoichiometry x → 1 and for fields of up to 9 T. We obtain the heat capacity intrinsic to the kagome layers by accounting for the weak Cu 2+ /Zn 2+ exchange between the Cu and the Zn sites, which was measured independently for x = I using neutron diffraction. The evolution of the heat capacity for x = 0.8... 1 is then related to the hysteresis in the magnetic susceptibility. We conclude that for x > 0.8 zinc paratacamite is a spin liquid without a spin gap, in which unpaired spins give rise to a macroscopically degenerate ground state manifold with increasingly glassy dynamics as x is lowered.

Pressure-induced Jahn–Teller switching in a Mn12 nanomagnet

Pressure-induced switching of a fast-relaxing single-molecule magnet to a slow-relaxing isomer is observed for the first time by using a combination of high pressure single-crystal X-ray diffraction and high pressure magnetic measurements.

Giant Magnetoresistance in the Half-Metallic Double-Perovskite Ferrimagnet Mn2FeReO6

The first transition-metal-only double perovskite compound, Mn(2+) 2 Fe(3+) Re(5+) O6 , with 17 unpaired d electrons displays ferrimagnetic ordering up to 520 K and a giant positive magnetoresistance of up to 220 % at 5 K and 8 T. These properties result from the ferrimagnetically coupled Fe and Re sublattice and are affected by a two-to-one magnetic-structure transition of the Mn sublattice when a magnetic field is applied. Theoretical calculations indicate that the half-metallic state can be mainly attributed to the spin polarization of the Fe and Re sites.

Enhancement of the curie temperature along the perovskite series RCu3Mn4O12 driven by chemical pressure of R3+ cations (R = Rare Earths)

The compounds of the title series have been prepared from citrate precursors under moderate pressure conditions (P = 2 GPa) and 1000 degrees C in the presence of KClO(4) as oxidizing agent. The crystal structures are cubic, space group Im3 (No. 204); the unit cell parameters linearly vary from a = 7.3272(4) A (R = La) to a = 7.2409(1) A (R = Lu) at room temperature. A neutron or synchrotron X-ray diffraction study of all the members of the series reveals an interesting correlation between some structural parameters and the magnetic properties. The electron injection effect upon replacement of Ca(2+) with R(3+) cations in the parent CaCu(3)Mn(4)O(12) oxide leads to a substantial increment of the ferrimagnetic Curie temperature (T(C)). An essential ingredient is supplied by the internal pressure of the R(3+) cations upon a decrease in size along the rare-earth series, from La to Lu: the concomitant compression of the MnO(6) octahedral units for the small rare earths provides progressively shorter Mn-O distances and improves the overlapping between Mn and O orbitals, thereby promoting superexchange and enhancing T(C) by 50 K along the series. This interaction is also reinforced by a ferromagnetic component that depends on the local distortion of the MnO(6) octahedra, which also increases along the series, constituting an additional factor, via intersite virtual charge transfer t-e orbital hybridization, for the observed increment of T(C).

High-pressure dependence of Raman phonons of RMnO3 (R=Pr,Tb)

The insulating character of undoped manganites RMnO 3 is related to the Jahn-Teller (JT) distortion of the MnO 6 octahedra, which can be reduced by applying hydrostatic pressure. We analyze the dependence of the Raman phonons with pressure up to 10 GPa, for PrMnO 3 and TbMnO 3 . The variation of the stretching and bending modes indicates that the compressibility of Mn-O bonds is higher than R-O ones in PrMnO 3 , but smaller in TbMnO 3 . The variation of the tilt mode frequency, in TbMnO 3 , can be explained by an increase of the octahedra tilt angle, which is consistent with a larger compressibility of the Tb-O bonds. Therefore, while PrMnO 3 evolves towards the structure of the metallic-ferromagnetic doped perovskites, TbMnO 3 does not.

Magnetic and structural features of the NdNi1 − xMnxO3 perovskite series investigated by neutron diffraction

Selected members of the perovskite series NdNi(1 - x)Mn(x)O(3) (0 ≤ x ≤ 1) have been prepared by a soft chemistry technique, followed by thermal treatments either under high oxygen pressure (x ≤ 0.5) or in air (x > 0.5). The crystal and magnetic structures have been studied by means of neutron diffraction, complemented with magnetic susceptibility measurements. For x = 0.25, 0.75, the crystal structure of the perovskites can be defined in the orthorhombic Pbnm space group, with Ni and Mn distributed at random over the octahedral sites of the structure. In contrast, the x = 0.5 compound crystallizes in a monoclinic P 2(1)/n structure containing two different octahedral positions, occupied by Ni and Mn, respectively. This is a result of the charge disproportionation of Ni(3+) + Mn(3+) to give Ni(2+) + Mn(4+) cations. The Ni(2+)O(6) octahedra are considerably larger than the Mn(4+)O(6) octahedra. This compound can be considered as a double perovskite of composition Nd(2)NiMnO(6). Unlike NdNiO(3) and NdMnO(3), which exhibit an antiferromagnetic ordering at low temperatures, the intermediate samples for x = 0.25, 0.50, 0.75 exhibit a ferromagnetic arrangement of (Ni, Mn) spins, with the moments aligned along the z axis, as probed using neutron diffraction. A maximum T(C) of 200 K is observed for x = 0.5, whereas T(C) = 150 K and 130 K are observed for x = 0.25 and 0.75, respectively. While NdNiO(3) is metallic above 200 K, a semiconducting behavior is determined between 120-300 K for the intermediate compositions.