We report an experimental investigation of the magnetic microstructure of iron oxide multicore assemblies by means of polarized small-angle neutron scattering (SANS). Guided by a recently developed analytical theory for vortex-state magnetic nanoparticles, we provide a quantitative comparison between the measured and calculated cross sections, revealing signatures that are consistent with vortex-type magnetization configurations at low applied magnetic fields. In particular, the field evolution and the characteristic isotropic ring-type feature of the spin-flip scattering intensity at intermediate momentum transfers are in line with the formation of flux-closure states. The latter are stabilized by the interplay of exchange, Zeeman, and magnetostatic energies. The methodology allows for a statistically significant characterization of vortex states in densely packed nanoparticle systems, thereby complementing surface-sensitive techniques that are commonly limited to the observation of spin structures in individual particles.

It has long been known that paramagnetic materials are weakly attracted by external magnetic fields but this attraction has been little studied for porous paramagnetic materials whose composition depends on adsorption processes. In this work, how paramagnetic metal–organic frameworks (MOF) particles can be displaced and even kept stationary by strong external magnetic fields without falling down while immersed in a liquid medium is shown. The magnetic field required for this behavior is specific to each MOF but it also changes with each adsorption process in which the composition, and hence, the mass accumulation in the pores, varies. Therefore, the threshold at which this stasis phenomenon occurs can be correlated to the mass of adsorbate trapped by the porous paramagnetic MOFs. Based on this phenomenon, a simple device has been constructed that allows the determination of the amount of mass trapped, modulating the magnetic field with the variation of the distance between the sample holder containing the paramagnetic MOF particles and a permanent magnet.

Magnetotactic bacteria (MTB) are at the forefront of interest for biophysics applications, especially in cancer treatment. Magnetosomes biomineralized by these bacteria are high-quality magnetic nanoparticles that form chains inside the MTB through a highly reproducible, naturally driven process. In particular, Magnetovibrio blakemorei and Magnetospirillum gryphiswaldense MTB exhibit distinct magnetosome morphologies: truncated hexa-octahedral and cuboctahedral shapes, respectively. Despite having identical compositions (magnetite, Fe3O4) and dimensions within a similar size range, their effective uniaxial anisotropies significantly differ at room temperature, with M. blakemorei exhibiting ∼25 kJ/m3 and M. gryphiswaldense ∼ 11 kJ/m3. This prominent anisotropy variance provides a unique opportunity to explore the role of magnetic anisotropy contributions in the magnetic responses of these magnetite-based nanoparticles. This study systematically investigates these responses by examining static magnetization as a function of temperature (M vs T, 5 mT) and magnetic field (M vs μ0 H, up to 1 T). Above the Verwey transition temperature (∼110 K), the effective anisotropy is dominated by the shape anisotropy contribution, notably increasing the coercivity for M. blakemorei by up to twofold compared to M. gryphiswaldense. However, below this temperature, the effective uniaxial anisotropy rapidly increases in a nonmonotonic way, significantly changing the magnetic behavior. Computational simulations using a dynamic Stoner-Wohlfarth model provide insights into these phenomena, enabling careful interpretation of experimental data. According to our simulations, below the Verwey temperature, a uniaxial magnetocrystalline contribution progressively emerges, peaking around 22-24 kJ/m3 at 5 K. Our study reveals the complex evolution of magnetocrystalline contributions, which dominate the magnetic response of magnetosomes below the Verwey temperature. This demonstrates the profound impact of anisotropic properties on the magnetic behaviors and applications of magnetite-based nanoparticles and highlights the exceptional utility of magnetosomes as ideal model systems for studying the complex interplay of anisotropies in magnetite-based nanoparticles.

We experimentally report a hitherto unseen angular anisotropy in the polarized small-angle neutron scattering (SANS) cross section of a magnetically strongly inhomogeneous material. Based on an analytical prediction using micromagnetic theory, the difference between the spin-up and spin-down SANS cross sections is expected to show a spin-disorder-induced anisotropy. The effect is particularly pronounced in inhomogeneous magnetic materials, such as nanoporous ferromagnets, magnetic nanocomposites, or steels, which exhibit large nanoscale jumps in the saturation magnetization at internal pore-matrix or particle-matrix interfaces. Analysis of the experimental neutron data constitutes a method for determining the exchange-stiffness constant. Our results for the nuclear-magnetic interference terms contained in the polarized magnetic neutron scattering cross section might also be of relevance to other neutron techniques.

Griffiths-like phases (GP) are connected to disordered magnetic nanostructures. Here, we focus on the Giant Magnetocaloric compound Tb5-xLaxSi2Ge2 with x = 0.075 where a re-entrant cluster-glass state (CGS) emerges at a characteristic freezing temperature T-f similar to 140 K within the GP (110-180 K), i.e. above the Curie temperature (T-c). This unconventional magnetic state has been studied via temperature-dependent DC magnetization (5-300 K), time-dependent macroscopic AC susceptibility (80-200 K), including ageing and memory experiments, and magnetic small-angle neutron scattering (SANS), above T (110-250 K). This approach allows to reveal the microscopic structure of the GP at the nanoscale in this system. AC susceptibility and DC magnetization confirm the presence of interacting short-range (< 2 nm) ferromagnetic (FM) clusters in the GP. The Langevin-like field dependence of the isothermal magnetization provides a quantitative assessment of the temperature dependence of the cluster size. Memory effects and ageing phenomena within GP are indicative of magnetically-frustrated states. Our results reveal that the dynamics is affected by the progressive coupling among CGS and GP towards the FM state. SANS correlation lengths between 1-5 nm above T-f are determined from the calculated magnetic correlation function C(r), which is computed from the magnetic SANS intensity. A phenomenological model based on the formation of FM clusters with intercluster (FM) interactions within a PM matrix is proposed to explain the unusual re-entrant glassy behaviour in the PM state. These findings serve as another experimental reference for the global understanding of disordered magnetic compounds.

Magnetotactic bacteria are envisaged as potential theranostic agents. Their internal magnetic compass, chemical environment specificity and natural motility enable these microorganisms to behave as nanorobots, as they can be tracked and guided towards specific regions in the body and activated to generate a therapeutic response. Here we provide additional diagnostic functionalities to magnetotactic bacteria Magnetospirillum gryphiswaldense MSR-1 while retaining their intrinsic capabilities. These additional functionalities are achieved by incorporating Tb or Gd in the bacteria by culturing them in Tb/Gd supplemented media. The incorporation of Tb provides luminescence properties, enabling potential applications of bacteria as biomarkers. The incorporation of Gd turns bacteria into dual contrast agents for magnetic resonance imaging, since Gd adds T1 contrast to the existing T2 contrast of unmodified bacteria. Given their potential clinical applications, the diagnostic ability of the modified MSR-1 has been successfully tested in vitro in two cell models, confirming their suitability as fluorescent markers (Tb-MSR-1) and dual contrast agents for MRI (Gd-MSR-1).

Magnetotactic bacteria (MTB) are aquatic microorganisms that biomineralize chains of membrane-enclosed magnetic nanoparticles called magnetosomes. Taking advantage of their self-propulsion capability provided by their flagella and the guidance capabilities ensured by their magnetosome chain, MTB are envisaged as biorobots. These can be guided and manipulated by external magnetic fields and are naturally attracted toward hypoxic areas, such as the tumor regions, while retaining the therapeutic and imaging capacities of the magnetosomes. In addition, MTB exhibit a series of characteristics that also make them very attractive for studies on fundamental magnetic properties. In fact, the arrangement of the magnetosomes in a 1D assembly already presents a great opportunity to better understand self-assembled nanostructures, and the high quality of magnetosomes also enables them as ideal candidates to study the change of the magnetic properties at the nanoscale. In this work, we will review the latest progress in the use of the MTB biorobots for targeted therapies and model nanomagnetic systems.

Antiferromagnetic materials are receiving renewed interest on behalf of their potential for information technologies. Recent reports have also revealed how the physics governing such magnetic arrangements and their excitations become more complex compared to traditional ferromagnetic materials, especially at the nanoscale. Here, we address two main issues that are of prime interest to their technological transfer. First, using small-angle neutron scattering, we show the existence of a magnetic helix-like super-structure in a polycrystalline TbCu 2 alloy, preserved at both bulk and nanoparticle ensembles of 8 nm. Second, using inelastic neutron scattering, we elucidate the magnetic excitons and the crystalline electric field energy level schemes of TbCu 2 in bulk and nanoparticle ensembles. This allows to understad the effect of the surface broken symmetry on the quantum energy levels at the nanoscale, so as the key role of interfacial effects on the propagation of magnetic excitations. Our research provides insights for the realization of magnetic moment dynamics models based on complex nanometric super-structures, and for nanoparticles to be integrated in spintronics and information technology applications.

Magnetic nanoparticles exhibit two different local symmetry environments, one ascribed to the core and one corresponding to the nanoparticle surface. This implies the existence of a dual spin dynamics, leading to the presence of two different magnetic arrangements governed by different correlation lengths. In this work, two ensembles of NdCu_2 nanoparticles with mean sizes of 18 nm and 13 nm have been produced to unravel the magnetic couplings established among the magnetic moments located within the core and at the nanoparticle surface. To this end, we have combined neutron diffraction measurements, appropriate to investigate magnetically-ordered spin arrangements, with time-dependent macroscopic AC susceptibility measurements to reveal memory and aging effects. The observation of the latter phenomena are indicative of magnetically-frustrated states. The obtained results indicate that, while the Nd^3+ magnetic moments located within the nanoparticle core keep the bulk antiferromagnetic commensurate structure in the whole magnetic state, the correlations among the surface spins give rise to a collective frustrated spin-glass phase. The interpretation of the magnetic structure of the nanoparticles is complemented by specific-heat measurements, which further support the lack of incommensurability in the nanoparticle state.

A study of the structural, Raman scattering and thermodynamic properties of a nanocrystalline ceramic rare earth oxide Yb2O3 and its bulk counterpart is reported. The nanosized sample was obtained by mechanical milling of bulk Yb2O3 (99.998%, high purity powder). The Rietveld analysis of the X-ray diffraction data indicates the presence of nanoparticles with a mean grain size of 12 +/- 1 nm after 75 h of milling time. The crystallographic structure within nanoparticles is cubic Ia-3 and the lattice parameter a = 10.455 +/- 0.002 angstrom. The nanocrystalline structure is confirmed by the evaluation of transmission electron microscopy images, showing a size distribution with a mean size D-TEM = 8 +/- 2 nm. Measurements of the specific heat (2 K-300 K) reveal an excess contribution respect to the unmilled (bulk) compound in the high temperature region above 70 K. At lower temperatures the results are consistent with a drastic change of the antiferromagnetic contribution (ordered below T-N = 2.2 K) as a result of the magnetic disorder arising from the size reduction process. The specific heat above T-N for the bulk and nanocrystalline samples are explained by the interplay among the phonon contribution, crystalline field and the presence of anharmonic effects. In the nanocrystalline state, broadening and shifts of the contribution of phonon modes to the Raman spectra, and a further reinforcement of the anharmonic contribution are found.

Correction for ‘Modifying the magnetic response of magnetotactic bacteria: incorporation of Gd and Tb ions into the magnetosome structure’ by E. M. Jefremovas et al., Nanoscale Adv., 2022, https://doi.org/10.1039/d2na00094f.

The spin dynamics in magnetically disordered GdCu 2 nanoparticles, varying the nanoparticle size in the range 53 to 7 nm, has been scrutinized. Dynamic χ A C susceptibility measurements have revealed the existence of dissipation at T g = 18 K, which is associated to the spin freezing transition, for all the ensembles. Besides, the superantiferromagnetic ensembles (〈 D 〉≥ 24 nm) also showcase a dissipation contribution close to the vicinity of the Néel transition, T N = 40.2 K. This dissipation, which takes the form of two humps located at T d 1 = 33.5(5) K and T d 2 = 40.0(5) K, is associated to uncompensated antiferromagnetic moments. Time-dependent phenomena (ageing and memory effects) are only evidenced below the spin freezing transition, evidencing that solely this low-temperature disordered phase is driven by the frustration of RKKY exchange interactions. Consequently, GdCu 2 nanoparticles display a dual disorder-driven magnetic dynamics, which are the one ascribed to the magnetically frustrated moments located at the nanoparticle surface; and that of uncompensated antiferromagnetic moments located within the nanoparticle core.

Results of the transport properties of the YbNi1−xCuxAl (x = 0, 0.2, 0.5, 0.8 and 1.0) series of alloys are reported. The previous analysis of X-ray diffraction patterns indicates that all compounds crystallize in the hexagonal ZrNiAl structure with a linear behavior of the unit cell volume as a function of the Cu concentration (x). This is not found in the unit cell parameters, showing a discontinuity between x = 0.5 and 0.8. Such discontinuities affect the behavior of the electrical resistivity, in which the position of the minimum temperature changes from 95 K to 175 K, and a rise in the low temperature slope in the magnetic contribution (with -lnT dependence) from 21 μΩcm to 212 μΩcm is observed. In addition, the electronic coefficient of the specific heat increases almost twofold from 125 mJ/mol·K2 (x = 0.5) to 246 mJ/mol·K2 (x = 0.8). These changes are attributed to the variation of the distance between Yb and transition metals (Ni and Cu) along the series and the different electronic properties of the transition metals (Ni and Cu).

11 nm diameter quasi-spherical and single phase CuO/Cu2O nanocomposites, with varying CuO:Cu2O ratio, were synthesized using solvothermal process. X-ray diffraction patterns refined with the Rietveld method show an evolution of the CuO:Cu2O ratio ( %) for the three samples (100:0, 66:34, and 9:91), along with an increased lattice deformation of the CuO unit cell as the amount of Cu2O increased: a = 4.653(2) angstrom, b = 3.411(1) angstrom, and c = 5.131(1) angstrom for the single phase CuO nanoparticles, similar to bulk, while a = 4.727(2) angstrom, b = 3.457(3) angstrom, and c = 5.247(2) angstrom for the 9/91 % CuO/Cu2O nanocomposites. Magnetic measurements as a function of the temperature (M vs T) and as a function of the magnetic field (M vs H) nanoparticles indicated the presence of a ferromagnetic phase in the whole range of temperatures for the single phase CuO nanoparticles, as revealed by the persistent hysteresis observed in the M vs H loops. In addition, an enhanced antiferromagnetic contribution, denoted by the increase in the antiferromagnetic susceptibility, chi(AF) similar to 4.8 10(-1) emu.g(-1).Oe,(-1) is also observed for these single phase CuO nanoparticles, while for bulk CuO, chi(AF) similar to 0.6 10(-6) emu.g(-1).Oe(-1). With increasing Cu2O content (>= 34 % Cu2O), the ferromagnetic phase is drastically suppressed for all temperatures, whereas the antiferromagnetic contribution at low temperatures (2-5 K) first increases (chi(AF) similar to 5.1 10(-6) emu.g(-1).Oe(-1) for 34 % Cu2O), but then, it gets reduced (chi(AF) similar to 1.46 10(-6) emu.g(-1).Oe(-1) for 91 % Cu2O). These magnetic changes showcase the relevance of the interface effects introduced by the Cu2O phase in CuO/Cu2O nanocomposites.

Magnetic Fluid Hyperthermia mediated by iron oxide nanoparticles is one of the most promising therapies for cancer treatment. Among the different candidates, magnetite and maghemite nanoparticles have revealed to be some of the most promising candidates due to both their performance and their biocompatibility. Nonetheless, up to date, the literature comparing the heating efficiency of magnetite and maghemite nanoparticles of similar size is scarce. To fill this gap, here we provide a comparison between commercial Synomag Nanoflowers (pure maghemite) and bacterial magnetosomes (pure magnetite) synthesized by the magnetotactic bacterium Magnetospirillum gryphiswaldense of $\langle D \rangle \approx ~40$ –45 nm. Both types of nanoparticles exhibit a high degree of crystallinity and an excellent degree of chemical purity and stability. The structural and magnetic properties in both nanoparticle ensembles have been studied by means of X–Ray Diffraction, Transmission Electron Microscopy, X–Ray Absorption Spectroscopy, and SQUID magnetometry. The heating efficiency has been analyzed in both systems using AC magnetometry at several field amplitudes (0–88 mT) and frequencies (130, 300, and 530 kHz).

An ensemble of superantiferromagnetic NdCu2 nanoparticles has been produced to perform a detailed analysis of magnetic excitations using inelastic neutron scattering. Neutron diffraction measurements indicate a mean nanoparticle size of < D > approximate to 13 nm, where the bulk commensurate antiferromagnetic structure is retained at the nanoparticle core. Magnetic measurements evidence the interaction among the magnetic moments located at the nanoparticle surface to be strong enough to establish a spin glass behavior. Specific heat analyses show a broad Schottky contribution, revealing the existence of a crystalline electric field. Inelastic neutron scattering analyses disclose that the splitting of the crystalline electric field levels associated with the Nd3+ ions, as well as the spin-wave excitations that emerged below the Neel transition (T-N approximate to 6 K) in polycrystalline NdCu2 are maintained in the nanoparticle state. We have been able to isolate the scattering contribution arising from the nanoparticle surface where both crystalline electric field splitting and the collective magnetic excitations are well-defined despite the symmetry breaking. Quantitative analyses of this surface scattering reveal that finite-size effects and microstrain lead to a partial inhibition of the transitions from the ground state to the first excited level, as well as a positive shift (similar to 15%) of the energy associated to collective magnon excitations.