Sensing the quantum behaviour of magnetic nanoparticles by electron magnetic resonance

We have investigated Magnetic Nanoparticles (MNPs) of spinel type iron oxide (of approximately 8 nm) mineralized in the internal cavity of the bioreactor ferritin nanocage. In particular, we have used Electron Magnetic Resonance, EMR, spectroscopy and taken advantage of the capacity of the protein shells to control the size of the MNPs. EMR measurements in perpendicular and parallel configurations have been recorded at various temperatures. A model based on the giant spin is used to interpret the experimental results. The analysis indicates that the observed quantum behaviour has to be ascribed to the whole MNP and that the thermal population of excited spin states has a strong influence in the EMR behaviour of MNPs.

Molecular and Crystal Structure, and EPR Studies of Tetraphenylarsonium [Bis-(1,3-dithia-2-one-4,5-dithiolato)-nickelate], (AsPh4)[Ni(dmid)2]

The crystal structure of (AsPh4)[Ni(dmid)2 (dmid2- = 1,3-dithia-2-one-4,5-dithiolato) has been determined by X-ray diffraction studies: crystals are monoclinic, space group P21/n a = 14.724(2), b = 14.305(5), c = 15.570(3) Å, β = 92.20(1)°, V = 3277 Å3 , ᵨx = 1.63 g cm-3 for Z = 4, final conventional agreement indices R = 0.038 and Rw = 0.049 for 2115 unique, observed reflections and 211 variable parameters. The asymmetric unit contains one (AsPh4)+ cation and one [Ni(dmid)2] anion, both in general positions. The anions are isolated from each other by the bulky cations. The [Ni(dmid)2]- ion deviates strongly from planarity: the dihedral angle between the two Ni(dmid) moieties is 10.7°; one of the dmid ligand is almost planar, while the second one is severely puckered. Polycristalline powder and single crystal EPR spectra have been recorded at room temperature. Only one signal was observed in all crystal orientations. The angular depend-ences of the linewidth and of the g values have been determined, as well as the principal g values and directions. These results have been compared to those previously obtained for the analogue (n-Bu4N)[Ni(dmit)2 ] (drnit2- = 1,3-dithia-2-thione-4,5-dithiolato), and discussed in terms of the localization of the unpaired electron on the ligands. leading to a formal (III) and actual (II) nickel oxidation state.

Single crystal EPR study at 95 GHz of a large Fe based molecular nanomagnet: toward the structuring of magnetic nanoparticle properties

A W-band single-crystal EPR study has been performed on a molecular cluster comprising 19 iron(III) ions bridged by oxo- hydroxide ions, Fe(19), in order to investigate magnetic nanosystems with a behavior in between the one of Magnetic NanoParticles (MNP) and that of Single Molecule Magnets (SMM). The Fe(19) has a disk-like shape: a planar Fe(7) core with a brucite (Mg(OH)(2)) structure enclosed in a "shell" of 12 Fe(III) ions. EPR and magnetic measurements revealed an S = 35/2 ground state with an S = 33/2 excited state lying ∼ 8 K above. The presence of other low-lying excited states was also envisaged. Rhombic Zero Field Splitting (ZFS) tensors were determined, the easy axes lying in the Fe(19) plane for both the multiplets. At particular temperatures and orientations, a partially resolved fine structure could be observed which could not be distinguished in powder spectra, due to orientation disorder. The similarities of the EPR behavior of Fe(19) and MNP, together with the accuracy of single crystal analysis, helped to shed light on spectral features observed in MNP spectra, that is a sharp line at g = 2 and a low intensity transition at g = 4. Moreover, a theoretical analysis has been used to estimate the contribution to the total magnetic anisotropy of core and surface; this latter is crucial in determining the easy axis-type anisotropy, alike that of MNP surface.

Molecular nanomagnets and magnetic nanoparticles: the EMR contribution to a common approach

The current status and future developments of the use of electron magnetic resonance (EMR) for the investigation of magnetic nano-systems is here reviewed. The aim is to stimulate efforts to provide a unified view of the properties of magnetic nanoparticles (MNP) comprising a few hundred magnetic centres, and molecular nanomagnets which contain up to ca. one hundred magnetic centres (MNM). The size of the systems is becoming the same but the approaches to the interpretation of their properties are still different, being bottom up for the latter and top down for the former. We make the point here of the need for a common viewpoint, highlighting the status of the two fields and giving some hints for the future developments. EMR has been a powerful tool for the investigation of magnetic nano-objects and it can provide a tool of fundamental importance for the development of a unified view.

Two-step magnetic ordering in quasi-one-dimensional helimagnets: possible experimental validation of villain's conjecture about a chiral spin liquid phase

Low-temperature specific heat, magnetic susceptibility, and zero-field muon spin resonance (microSR) measurements have been performed in the quasi-one-dimensional molecular helimagnetic compound Gd(hfac)3NITEt. The specific heat presents two anomalies at T(0)=2.19+/-0.02 K and T(N)=1.88+/-0.02 K, which both disappear upon the application of a weak magnetic field. Conversely, magnetic susceptibility and muSR data show the divergence of two-spin correlation functions only at T(N)=1.88+/-0.02 K. These results suggest an experimental validation of Villain's conjecture of a two-step magnetic ordering in quasi-one-dimensional XY helimagnets; i.e., the paramagnetic phase and the helical spin solid phase are separated by a chiral spin liquid phase, where translational invariance is broken without violation of rotational invariance.

Combined charge and spin density experimental study of the yttrium(III) semiquinonato complex Y(HBPz3)2(DTBSQ) and DFT calculations

High-resolution X-ray diffraction and polarized neutron diffraction experiments have been performed on the Y-semiquinonate complex, Y(HBPz3)2(DTBSQ), in order to determine the charge and spin densities in the paramagnetic ground state, S = (1/2). The aim of these combined studies is to bring new insights to the antiferromagnetic coupling mechanism between the semiquinonate radical and the rare earth ion in the isomorphous Gd(HBPz3)2(DTBSQ) complex. The experimental charge density at 106 K yields detailed information about the bonding between the Y3+ ion and the semiquinonate ligand; the topological charge of the yttrium atom indicates a transfer of about 1.5 electrons from the radical toward the Y3+ ion in the complex, in agreement with DFT calculations. The electron density deformation map reveals well-resolved oxygen lone pairs with one lobe polarized toward the yttrium atom. The determination of the induced spin density at 1.9 K under an applied magnetic field of 9.5 T permits the visualization of the delocalized magnetic orbital of the radical throughout the entire molecule. The spin is mainly distributed on the oxygen atoms [O1 (0.12(1) mu B), O2(0.11(1) mu B)] and the carbon atoms [C21 (0.24(1) mu B), C22(0.20(1) mu B), C24(0.16(1) mu B), C25(0.12(1) mu B)] of the carbonyl ring. A significant spin delocalization on the yttrium site of 0.08(2) mu B is observed, proving that a direct overlap with the radical magnetic orbital can occur at the rare earth site and lead to antiferromagnetic coupling. The DFT calculations are in good quantitative agreement with the experimental charge density results, but they underestimate the spin delocalization of the oxygen toward the yttrium and the carbon atoms of the carbonyl ring.

High-field/high-frequency EPR studies of spin clusters with integer spin: the multi-frequency approach

In this paper a rapid overview of the main results obtained from the study with multi-frequency HF-EPR of molecular spin clusters possessing integer spin values is presented. In the first part, two antiferromagnetic rings with zero ground spin state are reported. It is illustrated how the HF-EPR study of the first excited states allows obtaining important information on this kind of spin clusters. In the second part, selected examples of single-molecule magnets (SMM) are treated, starting with complexes involving only a few magnetic ions and going on to more complex systems. Indeed, because of their large zero-field energy gaps, EPR studies of SMM deserve the use of high frequencies and high fields. The approach presented here is twofold. First the interest of studying a series of 'simple' SMM in order to understand the subtle mechanisms underlying their properties is stressed. Then a summary of our HF-EPR studies of the most investigated SMM, Mn12ac and Fe8 is presented.

Finite-size effects in single chain magnets: an experimental and theoretical study

The problem of finite-size effects in s=1/2 Ising systems showing slow dynamics of the magnetization is investigated introducing diamagnetic impurities in a Co2+-radical chain. The static magnetic properties have been measured and analyzed considering the peculiarities induced by the ferrimagnetic character of the compound. The dynamic susceptibility shows that an Arrhenius law is observed with the same energy barrier for the pure and the doped compounds while the prefactor decreases, as theoretically predicted. Multiple spin reversal has also been investigated.

Origin of second-order transverse magnetic anisotropy in Mn12-acetate

The symmetry breaking effects for quantum tunneling of the magnetization in Mn12-acetate, a molecular nanomagnet, represent an open problem. We present structural evidence that the disorder of the acetic acid of crystallization induces sizable distortion of the Mn(III) sites, giving rise to six different isomers. Four isomers have symmetry lower than tetragonal and a nonzero second-order transverse magnetic anisotropy, which has been evaluated using a ligand field approach. The result of the calculation leads to an improved simulation of electron paramagnetic resonance spectra and justifies the tunnel splitting distribution derived from the field sweep rate dependence of the hysteresis loops.