Spin–glass magnetism in RFeTi2O7 (R=Lu and Tb) compounds

20th International Conference on Magnetism.The compounds RFeTi2O7 (R=Lu and Tb) crystallize at room temperature in centrosymmetric orthorhombic space group Pcnb. There are five non-equivalent positions of the iron ions: the two positions, Fe’ and Fe”, in the octahedron consisting of the Fe’ tetrahedron and Fe” five-vertex polyhedron and the three positions, Fe1, Fe2 and Fe3 in the mixed Fe-Ti octahedra [1]. The populations of the mixed Fe-Ti sites are different. The crystal structure features lead to atomic disorder in the distribution of the magnetic ions in this compound. From low temperature heat capacity, magnetization and frequency dependent ac susceptibility we conclude that both compounds undergo a spin glass transition at TSG=4.5 and 6 K for R =Lu and Tb, respectively. Since Lu is not magnetic, in RFeTi2O7 the spin glass behavior is caused by the disordered distribution of the magnetic Fe3+ ions in the different crystallographic positions. The substitution of the magnetic and highly anisotropic Tb ion instead of Lu increases TSG because of the additional Tb-Fe exchange interaction, while the critical exponent of the frequency dependence on temperature hardly varies. The spin glass behavior in these crystalline compounds is caused by the presence of competitive interactions that lead to frustration.The financial support of the Spanish MINECO MAT2011-23791, MAT2014-53921-R and Aragonese DGA-IMANA E34 projects is acknowledged.Peer Reviewe

Magnetic anisotropy of maghemite nanoparticles probed by RF transverse susceptibility

We present radio frequency magnetic transverse susceptibility measurements on γ-Fe2O3 nanoparticles, which yield an estimation of their effective anisotropy constant, Keff as a function of nanoparticle size. The resulting values range from 4 to 8 × 104 erg/cm3, being on the order of the magnetocrystalline anisotropy in bulk maghemite. Keff values increase as the particle diameter increases. Evidences of anisotropy field distribution given by the size distribution in the samples, and interparticle interactions that increase as the particle size increases, are also observed in the TS measurements. The effects of such interparticle interaction overcome those of thermal fluctuations, in contrast with the behavior of other iron oxide particles.The financial support of the Spanish MINECO MAT2011-23791, MAT2014-53921-R, and MAT2014-54975-R and Aragonese DGA-IMANA E34 and M4 projects is acknowledged.Peer Reviewe

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

Magnetism of Cr10 wheels on Au(111) and Cu(111) surfaces

Trabajo presentado al X Meeting de GEFES (División de Física de la Materia Condensada de la Real Sociedad Española de Física), celebrado en Valencia del 24 al 26 de enero de 2018We report on the magnetic properties of {Cr10} wheels (Cr10(OMe)20(O2CCMe)10) UHVsublimated on Cu(111) and Au(111) single-crystals investigated by means of by XAS & XMCD, SQUID magnetometry and STM. We discuss the magnetic anisotropy and magnetic moment field-dependence of the grafted {Cr10} molecules, as a function of the metallic substrate and layer thickness, and compare it with bulk material. Our results demonstrate that evaporated molecules show negligible anisotropy and weak dependence with the substrate. A combination of XMCD and SQUID magnetometry show that Mono- and Multilayer {Cr10} samples exhibit a magnetic behavior stemming from a S = 9 ground state, ruling out some previous results. Montecarlo simulations show that the magnetic evolution can be explained by {Cr10} molecules magneticaly consisting of two semi-crowns containing 4 Cr ions interacting ferromagnetically, separated by 2 Cr ions with antiferromagnetic interactions, giving rise to the ground S = 9 state, in agreement with previous EPR data in literature. Evaporation of {Cr10} onto a substrate producing subtle structural changes induce strong changes in the Cr-Cr interactions and distinct magnetic behavior from the bulk.Peer Reviewe

Origin of the unusual ground-state spin S = 9 in a Cr10 single-molecule magnet

The molecular wheel [Cr10(OMe)20(O2CCMe3)10], abbreviated {Cr10}, with an unusual intermediate total spin S = 9 and non-negligible cluster anisotropy, D/kB = −0.045(2) K, is a rare case among wheels based on an even number of 3d-metals, which usually present an antiferromagnetic (AF) ground state (S = 0). Herein, we unveil the origin of such a behavior. Angular magnetometry measurements performed on a single crystal confirmed the axial anisotropic behavior of {Cr10}. For powder samples, the temperature dependence of the susceptibility plotted as χT(T) showed an overall ferromagnetic (FM) behavior down to 1.8 K, whereas the magnetization curve M(H) did not saturate at the expected 30 μB/fu for 10 FM coupled 3/2 spin Cr3+ ions, but to a much lower value, corresponding to S = 9. In addition, the X-ray magnetic circular dichroism (XMCD) measured at high magnetic field (170 kOe) and 7.5 K showed the polarization of the cluster moment up to 23 μB/fu. The magnetic results can be rationalized within a model, including the cluster anisotropy, in which the {Cr10} wheel is formed by two semiwheels, each with four Cr3+ spins FM coupled (JFM/kB = 2.0 K), separated by two Cr3+ ions AF coupled asymmetrically (J23/kB = J78/kB = −2.0 K; J34/kB = J89/kB = −0.25 K). Inelastic neutron scattering and heat capacity allowed us to confirm this model leading to the S = 9 ground state and first excited S = 8. Single-molecule magnet behavior with an activation energy of U/kB = 4.0(5) K in the absence of applied field was observed through ac susceptibility measurements down to 0.1 K. The intriguing magnetic behavior of {Cr10} arises from the detailed asymmetry in the molecule interactions produced by small-angle distortions in the angles of the Cr–O–Cr alkoxy bridges coupling the Cr3+ ions, as demonstrated by ab initio and density functional theory calculations, while the cluster anisotropy can be correlated to the single-ion anisotropies calculated for each Cr3+ ion in the wheel.The authors acknowledge financial support from the Spanish Agencia Estatal de Investigación, through Projects MAT2017-83468-R (AEI/FEDER, UE) and PID2020-115159GB-I00/AEI/10.13039/501100011033, Aragonese Project RASMIA E12_20R (co-funded by Fondo Social Europeo) and of the European Union FEDER (ES). Also University of Padova Grants P-DISC#09BIRD2019-UNIPD SMOW.Peer reviewe

Temperature- and field-induced transformation of the magnetic state in Co2.5Ge0.5BO5

A tetravalent-substituted cobalt ludwigite Co2.5Ge0.5BO5 has been synthesized using the flux method. The compound undergoes two magnetic transitions: a long-range antiferromagnetic transition at TN1 = 84 K and a metamagnetic one at TN2 = 36 K. The sample-oriented magnetization measurements revealed a fully compensated magnetic moment along the a- and c-axes and an uncompensated one along the b-axis leading to high uniaxial anisotropy. A field-induced enhancement of the ferromagnetic correlations at TN2 is observed in specific heat measurements. The DFT+GGA calculation predicts the spin configuration of (↑↓↓↑) as a ground state with a magnetic moment of 1.37 μB/f.u. The strong hybridization of Ge(4s, 4p) with O (2p) orbitals resulting from the high electronegativity of Ge4+ is assumed to cause an increase in the interlayer interaction, contributing to the long-range magnetic order. The effect of two super–superexchange pathways Co2+-O-B-O-Co2+ and Co2+-O-M4-O-Co2+ on the magnetic state is discussed.This work has been financed by the Russian Foundation for Basic Research (project no. 20-02-00559). The authors acknowledge financial support from the Spanish Agencia Estatal de Investigación, through projects MAT2017-83468-R (AEI/FEDER, UE) and PID2020-115159GB-I00/AEI/10.13039/501100011033, the Aragonese project RASMIA E12_20R (co-funded by Fondo Social Europeo), and of the European Union FEDER (ES).Peer reviewe

Uniaxial magnetic anisotropy in Co2.25Fe0.75O2BO3 compared to Co3O2BO3 and Fe3O2BO ludwigites

12 páginas, 17 figuras, 1 tabla.-- PACS number(s): 75.50.Gg, 75.50.Vv, 75.30.Gw.-- et al.Magnetic and Mössbauer spectroscopy (MS) measurements have been performed on a single crystal of Co2.25Fe0.75O2BO3 with ludwigite structure. Two magnetic transitions (TN=115 K and TC=70 K) were traced from the ac susceptibility temperature dependence. The MS spectra as a function of temperature clearly show the onset of magnetic ordering below 115 K. Magnetization measurements on the parent Co3O2BO3 and Fe3O2BO3 compounds have been done for comparison. In Fe3O2BO3 the anisotropy of the different phases has been determined, showing that the anisotropy axis changes from the a to the b axis in the low-temperature antiferromagnetic phase. High magnetic uniaxial anisotropy has been detected for both Co3O2BO3 and Co2.25Fe0.75O2BO3. From the angle-dependent magnetization measurements it is found that in both compounds the easy axis of magnetization is the b [010] axis, where an antiferromagnetic component is superimposed on the main ferromagnetic component. In the c direction the behavior is purely antiferromagnetic. In Co2.25Fe0.75O2BO3 a strong reduction of the remanent magnetization and a very strong increase in coercive field along the b axis with respect to those found in Co3O2BO3 were observed from magnetic hysteresis cycles measured below TC. The increase of coercive field is caused by the increase of defects upon Co substitution by Fe.The financial support of Spanish MINCYT, Grant No. MAT08/1077. Also this study was supported by the Russian Foundation for Basic Research (Project No. 09-02-00171-a), the Federal Agency for Science and Innovation (Rosnauka) (Project No. MK-5632.2010.2), and the Physical Division of the Russian Academy of Science (Program “Strongly Correlated Electrons,” Project No. 2.3.1).Peer reviewe

Spin-glass behavior in single crystals of hetero-metallic magnetic warwickites MgFeBO4, Mg0.5Co0.5FeBO4, and CoFeBO4

et al.Magnetic properties of heterometallic warwickites MgFeBO4, Mg0.5Co0.5FeBO4, and CoFeBO4 are presented, highlighting the effect of Co substitution on the magnetic properties of these compounds. The analysis of magnetization and heat capacity data has shown that these compounds exhibit a spin-glass transition below TSG=10, 20 and 22 K, respectively. Using zero field ac susceptibility as entanglement witness we find that the low dimensional magnetic behavior above TSG show quantum entanglement behavior χ(Τ)∝T−α(Τ) up to TE≈130 K. The α parameters have been deduced as a function of temperature and Co content, indicating the existence of random singlet phase in this temperature region. Above TE the paramagnetism is interpreted in terms of non-entangled spins giving rise to Curie–Weiss paramagnetism. The different intra- and inter-ribbon exchange interaction pathways have been calculated within a simple indirect coupling model. It is determined that the triangular motifs in the warwickite structure, together with the competing interactions, induce frustration. The spin-glass character is explained in terms of the substitutional disorder of the Mg, Fe and Co atoms at the two available crystallographic sites, and the frustration induced by the competing interactions. The Co substitution induces uniaxial anisotropy, increases the absolute magnetization and increases the spin-glass freezing temperature. The entanglement behavior is supported in the intermediate phase irrespective of the introduction of anisotropy by the Co substitution.This work has been financed by the MINECO Project MAT11/23791, MAT2014-53921-R and DGA IMANA project E-34, Russian Foundation for Basic Research (project Nr. 13-02-00958, 13-02-00358 and 14-02-31051-mol-a), Council for Grants of the President of the Russian Federation (project Nr. NSh-2886.2014.2 and SP-938.2015.5). The work of one of coauthors (M.S.P.) was supported by the program of Foundation for Promotion of Small Enterprises in Science and Technology (“UMNIK” program).Peer reviewe