Showing papers by "Miguel A. Novak published in 2013"

Controlled synthesis of Mn3O4 nanoparticles in ionic liquids

Abstract: This work describes a simple one-step synthesis of Mn3O4 nanoparticles by thermal decomposition of [Mn(acac)2] (acac = acetylacetonate) using imidazolium ionic liquids (ILs) and a conventional solvent, oleylamine, for comparison. The Mn3O4 nanoparticles were characterized by XRD, ATR-FTIR, TEM, Raman, UV/VIS and magnetometry techniques. The addition of 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide IL (BMI·NTf2) yielded a smaller particle size (9.9 ± 1.8 nm) with better dispersion and more regular sizes than synthesis using oleylamine as the solvent (12.1 ± 3.0 nm). The complete conversion of the precursor to Mn3O4 nanoparticles occurred after 96 h at 180 °C for the reaction performed in BMI·NTf2. However, under these reaction conditions in oleylamine, no precursor was detected, but two different phases were observed: a major phase corresponding to Mn3O4 and a minor phase corresponding to MnO2. Magnetometry revealed that Mn3O4 nanoparticles synthesized in either oleylamine or BMI·NTf2 exhibited ferrimagnetic behavior at low temperatures, whereas they were paramagnetic at room temperature. As expected, the blocking temperature and the coercivity decreased with the size of nanoparticles. Our results demonstrate that reaction conditions such as time, and the nature of the ionic liquid play important roles in determining the size of Mn3O4 nanoparticles.

New Synthetic Route toward Heterometallic 3d–3d′ and 3d–4f Single-Molecule Magnets. The First CoII–MnIII Heterometallic Complex

Abstract: Four tetranuclear heterometallic complexes, [CoII2Mn2III(dpm)4(MeO)6] (1) and [LnIII2MnIII2(dpm)6(MeO)6(MeOH)n], where Ln = Gd (2, n = 2), Tb (3, n = 2), and Dy (4, n = 0), have been obtained follo...

Two co-crystalline M(hfac)2(IPhIN)2·M(hfac)2 (M = Mn, Co) compounds with a bis(iminoylnitroxide) biradical: structure and magnetism

Abstract: 1-Iodo-3,5-bis(4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1H-imidazole-1′-oxyl)benzene (IPhIN = L) forms co-crystalline [M(hfac)2(IPhIN)2]·[M(hfac)2] coordination complexes (M = Mn, Co) with [Mn(hfac)2] and [Co(hfac)2]. In each system only one of the iminoylnitroxide radical moieties is coordinated as part of the [M(hfac)2(IPhIN)2] structural units; the metal ions do not coordinate the radical nitrogen atoms, but rather the nitroxide oxygen to form a cis NO–M–ON grouping in a hexacoordinate ligand sphere. Solvate water molecules in the lattice provide an important driving force for assembly of the co-crystal structures in the cobalt(II) derivative; an additional factor is the formation of halogen bonds between hfac CF3 groups and the IPhIN iodine atom. The paramagnetic susceptibility (χM) versus temperature (T) measurements showed antiferromagnetic exchange downturns at lower temperature. Based on the crystal structures of the complexes, the χMT versus T data were modelled using a paramagnetic contribution from the [M(hfac)2] fragment, an exchange interaction J1 within the L–M–L fragment, plus an intramolecular radical–radical interaction J2 within the IPhIN biradical unit. For the Mn(II) complex, a good fit was obtained for gMn = 2.14(1), grad = 2.0 (fixed), J1 = −311(9) cm−1 and J2 = 11.1(7) cm−1. A reasonable fit for the cobalt compound was obtained for gCo = 2.72(1), grad = 2.0 (fixed), J1 = −400(5) cm−1, J2 = 28.3(3) cm−1 with a generalized zero field splitting parameter D = 64(2) cm−1.

Macroscopic quantum tunneling of magnetization explored by quantum-first-order reversal curves

Abstract: A method to study the fundamental problem of quantum double well potential systems that display magnetic hysteresis is proposed. The method, coined quantum-first-order reversal curve (QFORC), is inspired by the first-order reversal curve, based on the Preisach model for hysteresis. We successfully tested the QFORC method in the hysteresis of the Mn12Ac molecular magnet, which is governed by macroscopic quantum tunneling of magnetization. The QFORC reproduces well the experimental magnetization behavior. It is possible to separate the thermal activation and tunneling contributions from the magnetization variation, as well as associate the magnetization jumps with specific quantum transitions.

Macroscopic quantum tunneling of magnetization explored by quantum-first-order reversal curves (QFORC)

Abstract: A novel method to study the fundamental problem of quantum double well potential systems that display magnetic hysteresis is proposed. The method, coined quantum-first-order reversal curve (QFORC) analysis, is inspired by the conventional first-order reversal curve (FORC) protocol, based on the Preisach model for hysteretic phenomena. We successfully tested the QFORC method in the peculiar hysteresis of the Mn12Ac molecular magnet, which is governed by macroscopic quantum tunneling of magnetization. The QFORC approach allows one to quickly reproduce well the experimental magnetization behavior, and more importantly to acquire information that is difficult to infer from the usual methods based on matrix diagonalization. It is possible to separate the thermal activation and tunneling contributions from the magnetization variation, as well as understand each experimentally observed jump of the magnetization curve and associate them with specific quantum transitions.