Intracluster interactions in butterfly {Fe3 LnO2} molecules with the non-Kramers ions Tb(III) and Ho(III)

The intracluster exchange interactions within the >butterfly> [Fe3Ln(μ3-O)2(CCl3COO)8(H2O)(THF)3] molecules, where Ln(III) represents a lanthanide cation, have been determined by a combination of x-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometry (VSM) along with an interaction model. We have studied the compounds with Ln=Tb and Ho, both non-Kramers lanthanides and with high uniaxial anisotropy, and Ln=Lu(III) and Y(III) as pseudolanthanides, which supply nonmagnetic Ln reference cases. At low temperature, the three Fe atoms can be considered as a self-unit with total spin SFe3=5/2. Using the element selectivity of the XMCD magnetometry, measured at the Ln L2,3 edges, together with the VSM measurements, the local magnetization of the Ln ion and the Fe3 subcluster, as a function of the field and low temperature (T≈2.5K), has been determined separately. These results are described quantitatively in the framework of a theoretical model based on an effective spin Hamiltonian, which considers the competing effects of intracluster interactions and the external applied magnetic field. The Ln-Fe3 exchange interaction within the {Fe3LnO2} cluster has been determined to be antiferromagnetic, in both Tb and Ho compounds, with JFeTb/kB=-0.13(1)K and JFeHo/kB=-0.18(1)K, respectively. In both cases, a field-induced reorientation of the Fe3 and Ln spins from antiparallel to parallel orientation takes place at a threshold field μ0H=1.1 and 2 T, for the {Fe3TbO2} and {Fe3HoO2} compounds, respectively. By comparison with other compounds of the series with uniaxial anisotropy, it is concluded that the polarizability of the Fe3 subcluster magnetic moment decreases in the trend {Fe3YO2}→{Fe3TbO2}→{Fe3HoO2}→{Fe3DyO2}, because of the increasing opposition of the exchange antiferromagnetic field caused by the Ln ion. In the Ln=Tb, Ho, and Dy, the magnetization of the whole molecule is dominated by the anisotropy of the Ln ion. The intracluster Fe3-Ln exchange interactions are very weak compared to the Ln ligand field and Fe-Fe exchange interactions.The projects MINECO (MAT2011/23791, MAT2011/27233-C02-02, and MAT2014/53921-R), DGA IMANA E34, and Alexander Von Humboldt Foundation (D.P.) are acknowledged for financial support.Peer Reviewe

Intracluster interactions in butterfly {Fe3 LnO2} molecules with the non-Kramers ions Tb(III) and Ho(III)

The intracluster exchange interactions within the "butterfly" Fe3Ln(µ3-O)2(CCl3COO)8(H2O)(THF)3] molecules, where Ln(III) represents a lanthanide cation, have been determined by a combination of x-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometry (VSM) along with an interaction model. We have studied the compounds with Ln=Tb and Ho, both non-Kramers lanthanides and with high uniaxial anisotropy, and Ln=Lu(III) and Y(III) as pseudolanthanides, which supply nonmagnetic Ln reference cases. At low temperature, the three Fe atoms can be considered as a self-unit with total spin SFe3=5/2. Using the element selectivity of the XMCD magnetometry, measured at the Ln L2,3 edges, together with the VSM measurements, the local magnetization of the Ln ion and the Fe3 subcluster, as a function of the field and low temperature (T˜2.5K), has been determined separately. These results are described quantitatively in the framework of a theoretical model based on an effective spin Hamiltonian, which considers the competing effects of intracluster interactions and the external applied magnetic field. The Ln-Fe3 exchange interaction within the {Fe3LnO2} cluster has been determined to be antiferromagnetic, in both Tb and Ho compounds, with JFeTb/kB=-0.13(1)K and JFeHo/kB=-0.18(1)K, respectively. In both cases, a field-induced reorientation of the Fe3 and Ln spins from antiparallel to parallel orientation takes place at a threshold field µ0H=1.1 and 2 T, for the {Fe3TbO2} and {Fe3HoO2} compounds, respectively. By comparison with other compounds of the series with uniaxial anisotropy, it is concluded that the polarizability of the Fe3 subcluster magnetic moment decreases in the trend {Fe3YO2}¿{Fe3TbO2}¿{Fe3HoO2}¿{Fe3DyO2}, because of the increasing opposition of the exchange antiferromagnetic field caused by the Ln ion. In the Ln=Tb, Ho, and Dy, the magnetization of the whole molecule is dominated by the anisotropy of the Ln ion. The intracluster Fe3-Ln exchange interactions are very weak compared to the Ln ligand field and Fe-Fe exchange interactions

Antiferromagnetic single-chain magnet slow relaxation in the {Tb(α-fur)3}n polymer with non-Kramers ions

We report the synthesis, crystal structure and magnetic properties of a new molecular complex based on a Tb(iii) ion supported by 2-furancarboxylic molecules: {Tb(α-fur)(HO)} (α-fur = CHOCOO). Two slightly different Tb sites (A and B) exist depending on the position of one of the dangling ligands. Ab initio calculations predict that, for both sites, the magnetic ground state is highly anisotropic (g∗ = 17.8) and consists of a quasi-doublet with a small gap, well isolated from the next excited state. The α-fur ligand forms 1D polymeric chains of Tb ions of the same type (either A or B) running along the c-axis. The crystal structure is formed by the supramolecular stacking along the a-axis of 2D layers containing parallel chains of the same type. Static magnetization and heat capacity measurements show that, magnetically, the system can be modeled as an ensemble of Ising chains of non-Kramers Tb ions with effective spin S∗ = 1/2, antiferromagnetically (AF) coupled by a weak intrachain interaction (J∗/k = -0.135 K). At very low temperatures, the static susceptibility reflects the presence of a 2-4% concentration of defects in the chains. Ac susceptibility measurements at H = 0 performed down to mK temperatures have enabled us to observe the slow relaxation of magnetization through two different pathways. They are assigned to Single-Chain-Magnet (SCM) behavior in two different types of AF chains (A and B), triggered by the existence of defects breaking the chains into segments with short-range order. At temperatures below 0.1 K this mechanism is replaced by individual relaxation of the ions through direct processes. Under the application of a magnetic field the system slowly relaxes by two distinct direct processes, strongly affected by a phonon bottleneck effect.This work has been financed by MECOM Projects MAT11/23791 and MAT11/27233-C02-02, MAT2015-68204-R, MAT2014-53921-R, DGA IMANA E34 and MOLCHIP E98 Projects. Consolider Nanoselect (CSD2007-00041) and by a grant of the Ministry of National Education, CNCS – UEFISCDI, project number PN-II-ID-PCE-2012-4-0261. D. P. thanks the Alexander von Humboldt (AvH) Foundation for financial support.Peer Reviewe

Nanomagnetism of thin films and molecules: a spectroscopic study

La presente memoria comprende el estudio del comportamiento magnético de dos tipos de materiales de dimensión nanométrica, multicapas fe/si nanoestructuradas, por un lado, e imanes moleculares de tipo mariposa, por otro. En la primera parte de este trabajo determinamos las propiedades magnéticas de multicapas (fe/si)n en función del grosor nanométrico de las capas. Además, estudiamos la influencia de los procesos de difusión atómica fe-si en la modificación de las propiedades magnéticas, así como la estabilidad de las multicapas bajo calentamiento, la dinámica de dominios y los procesos de reversión magnética en función de temperatura y campo magnético aplicado. La segunda parte de esta memoria está dedicada a la investigación de compuestos moleculares (serie moléculas mariposa {fe3lno2}, ln=tierra rara) candidatos a comportarse como moléculas imán (smm de su nombre en inglés single molecule magnet). La combinación de técnicas macroscópicas y microscópicas nos han permitido investigar la naturaleza de las interacciones intramoleculares, la reorientación de espín inducida por campo magnético y las propiedades magnéticas estáticas y dinámicas a baja temperatura. En particular, para entender los mecanismos de relajación magnética y explorar la existencia de fenómenos de túnel cuántico, ha sido necesario realizar un análisis comparativo de los resultados de capacidad calorífica, susceptibilidad ac y dc y espectroscopía Mössbauer.Peer Reviewe

Iron silicide formation at different layers of (Fe/Si)3 multilayered structures determined by conversion electron Mössbauer spectroscopy

Under the terms of the Creative Commons Attribution 3.0 Unported License to their work.The morphology and the quantitative composition of the Fe-Si interface layer forming at each Fe layer of a (Fe/Si)3 multilayer have been determined by means of conversion electron Mössbauer spectroscopy (CEMS) and high-resolution transmission electron microscopy (HRTEM). For the CEMS measurements, each layer was selected by depositing the Mössbauer active 57Fe isotope with 95% enrichment. Samples with Fe layers of nominal thickness dFe  = 2.6 nm and Si spacers of dSi  = 1.5 nm were prepared by thermal evaporation onto a GaAs(001) substrate with an intermediate Ag(001) buffer layer. HRTEM images showed that Si layers grow amorphous and the epitaxial growth of the Fe is good only for the first deposited layer. The CEMS spectra show that at all Fe/Si and Si/Fe interfaces a paramagnetic c-Fe1− x Si phase is formed, which contains 16% of the nominal Fe deposited in the Fe layer. The bottom Fe layer, which is in contact with the Ag buffer, also contains α-Fe and an Fe1− x Si x alloy that cannot be attributed to a single phase. In contrast, the other two layers only comprise an Fe1− x Si x alloy with a Si concentration of ≃0.15, but no α-Fe.The financial support of the Spanish MINECO MAT2011- 23791, the Aragonese DGA-IMANA E34 (co-funded by Fondo Social Europeo) and that received from the European Union FEDER funds is acknowledged. L.B.-R. acknowledges the Spanish MINECO FPU 2010 grant.Peer Reviewe

CEMS Analysis of Phase Formation in Nanostructured Films (Fe/Si)3

Determination of stable phases formed at the Fe/Si interface in (Fe/Si)n structure, grown by thermal evaporation in an ultrahigh vacuum system was performed using conversion electron Mössbauer spectroscopy (CEMS).Peer Reviewe

Field-induced internal Fe and Ln spin reorientation in butterfly {Fe3LnO2} (Ln = Dy and Gd) single-molecule magnets

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).The intramolecular exchange interactions within the single-molecule magnet (SMM) "butterfly" molecule [Fe3Ln(μ3-O) 2(CCl3COO)8(H2O)(THF)3], where Ln(III) represents a lanthanide cation, are determined in a combined experimental [x-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometer (VSM)] and theoretical work. Compounds with Ln=Gd and Dy, which represent extreme cases where the rare earth presents single-ion isotropic and uniaxial anisotropy, on one hand, and with Ln=Lu and Y(III) as pseudolanthanide substitutions that supply a nonmagnetic Ln reference case, on the other hand, are studied. The Dy single-ion uniaxial anisotropy is estimated from ab initio calculations. Low-temperature (T 2.5 K) hard x-ray XMCD at the Ln L 2,3 edges and VSM measurements as a function of the field indicate that the Ln moment dominates the polarization of the molecule by the applied field. Within the {Fe3LnO2} cluster the Ln-Fe3 subcluster interaction is determined to be antiferromagnetic in both Dy and Gd compounds, with values J Dy-Fe3=-0.4 K and J Gd-Fe3=-0.25 K, by fitting to spin Hamiltonian simulations that consider the competing effects of intracluster interactions and the external applied magnetic field. In the uniaxial anisotropic {Fe3DyO2} case, a field-induced reorientation of the Fe3 and Dy spins from an antiparallel to a parallel orientation takes place at a threshold field (μ0H=4 T). In contrast, in isotropic {Fe3GdO2} this reorientation does not occur. © 2013 American Physical Society.The financial support of Spanish MINECO Grant No. MAT2011-23791, the Alexander Von Humboldt Foundation (D.P.), and Aragonese DGA-IMANA E34 (cofunded by Fondo Social Europeo) and that received from European Union FEDER funds are also acknowledged. L.B.R. acknowledges the Spanish MINECO FPU 2010 grant.Peer Reviewe

Negative magnetization in NdFexGa1-xO3 studied by XMCD

Resumen del trabajo presentado a la 20th International Conference on Solid Compounds of Transition Elements, celebrada del 11 al 15 de abril de 2016 en Zaragoza (España).Negative Magnetization (NM) in NdFe0.8Ga0.2O3 is reported, both by magnetization, ac-susceptibility and x-ray Magnetic Circular Dichroism measurements. The mechanism of NM in NdFe0.8Ga0.2O3 is the blocking of the weak-ferromagnetic domain walls and the polarization of the paramagnetic Nd sublattice by that of Fe at low temperatures.Peer Reviewe

Morphological and magnetic study at the interfaces of (Fe/Si)3 multilayers

Resumen del trabajo presentado al "V Euro-Asian Symposium Trends in MAGnetism: Nanomagnetism" celebrado en Vladivostok (Rusia) del 15 al 21 de septiembre de 2013.The field of research on nanostructures of ferromagnetic metal/semiconductormaterials is subject of interest due to their unique physical properties for applications in microelectronics, spintronics, etc. In this work we focus our research on the particular case of (Fe/Si)n multilayer magnetic structures with an odd number of Fe layers and their interfaces. The interlayer exchange coupling (IEC) between two ferromagnetic layers across a nonmagnetic spacer leads to applications in mass storage, nonvolatile memory and sensors. However, the presence of nonmagneticsilicides decreases the current spin polarization in the silicon spacerlayer and affects the mechanism of IEC. It is, therefore, of utmost importanceto control the interlayer composition. Samples are prepared by sequential deposition of Fe/Sibilayers in molecular beam epitaxy (MBE) set-ups in ultrahigh vacuum and at room temperature. Two different substrates have been used: a Si(100) wafer with a thin SiO2 buffer layer (type A) and an in situ prepared buffer layer of Ag(100) on Fe/GaAs(100) (type B). To characterize the morphology and chemical constitution of the films and their interfaces, high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS), angle-resolved x-ray photoelectron spectroscopy (ARXPS), high energy x-ray photoelectron spectroscopy (HAXPES), x-ray reflectivity (XRR), and conversion electron Mössbauer spectroscopy (CEMS) have been used. The magnetic properties were obtained from SQUID magnetometry and magneto-optic (MOKE) imaging of the magnetic domain structures. From the comprehensive analysis of all data we conclude that the as-deposited A and B samples show non-magnetic Fe silicide formation at the nominal Si spacers. As temperature is increased, at about 400 K, the A samples undergo diffusion of Si into the Fe films, reacting to form even more non-magnetic silicide. In contrast, in the B samples, the chemical reaction implies the transformation of non-magnetic silicides formed during deposition into ferromagnetic Fe3Si in a first step, and into non-magnetic silicides like ε-FeSi in a second step. The difference is attributed to the substrate, which in the case A is capable of supplying through the thin SiO2 buffer all Si needed to undergo the transition to ε-FeSi, while in the case B this cannot occur since the substrate contains no Si. The interface constitution, roughness, and asymmetry in the deposition are reviewed.Peer Reviewe