According to some recent studies, the magnetoresistance curves of ferromagnetic strip-shaped samples can significantly differ depending on whether the in-plane external applied magnetic field H is oriented in parallel to either the long or the short edge of the strip. To address this problem, in the present work magnetization curves M(H) were measured for similarly shaped samples with both magnetic field orientations used in the magnetoresistance measurements. It was found that the M(H) curves strongly depend on the saturation magnetization and shape of the samples as well as on the magnetic field orientations. For some samples with sufficiently large saturation magnetization, the effective demagnetizing factors could be deduced from the measured M(H) curves. By considering the investigated samples as a ferromagnetic slab, and approximating them with a general ellipsoid, the demagnetizing factors were calculated from known formulae and compared to the experimental values. A fairly good matching was observed, although the latter data were systematically slightly larger, certainly due to the not completely homogeneous magnetization within the rectangular slab as opposed to the case of a general ellipsoid. The differences in the M(H) curves for the two orientations of the magnetic field could be completely attributed to demagnetizing effects.

Evidence for the suppression of collective magnetic behavior of dipolarly interacting Fe nanoparticles is found in Fe-Ag granular multilayers. Interaction of Fe particles separated by an Ag layer is studied as a function of the nominal thickness of the Ag layer. The surprisingly increasing interaction with increasing Ag-layer thickness, verified by memory-effect measurements, is explained by the formation of pinholes in the Ag layer at small Ag thicknesses, allowing direct ferromagnetic coupling between the Fe particles. This coupling may hinder the frustration of superspins favored by dipolar interactions. At larger Ag thicknesses, the Ag layer is continuous without pinholes and frustration leads to the appearance of the superspin-glass state. The effect of increasing interactions correlates well with the growing deviation at low temperatures of the measured field-cooled (FC) magnetization from the interaction-free FC curve calculated by a model based on the relaxation of two-level systems. Similar phenomenon is reported in a recently published paper (Sánchez et al., Small 2022, 18, 2106762) where a dense nanoparticle system is studied. Here the collective magnetic behavior of the particles due to dipolar interactions is suppressed when the anisotropy energy of the individual particles exceeds a certain threshold.

In polar magnets, such as GaV 4 S 8 , GaV 4 Se 8 and VOSe 2 O 5 , modulated magnetic phases namely the cycloidal and the Néel-type skyrmion lattice states were identified over extended temperature ranges, even down to zero Kelvin. Our combined small-angle neutron scattering and magnetization study shows the robustness of the Néel-type magnetic modulations also against magnetic fields up to 2 T in the polar GaMo 4 S 8 . In addition to the large upper critical field, enhanced spin-orbit coupling stabilize cycloidal, Néel skyrmion lattice phases with sub-10 nm periodicity and a peculiar distribution of the magnetic modulation vectors. Moreover, we detected an additional single-q state not observed in any other polar magnets. Thus, our work demonstrates that non-centrosymmetric magnets with 4 d and 5 d electron systems may give rise to various highly compressed modulated states.

The longitudinal and transverse magnetoresistance curves MR( H ) and the magnetization isotherms M( H ) were measured at T = 3 K and 300 K up to high magnetic fields for a microcrystalline (µc) Ni foil with grain sizes above 1 µm (corresponding to bulk Ni) and for a nanocrystalline (nc) Ni foil with an average grain size of about 100 nm. At T = 3 K, the field-induced resistivity change was quite different for the two microstructural states of Ni and the evolution of resistivity with magnetic field was also different which could be explained as arising from their very different electron mean free paths. At T = 300 K, the MR( H ) curves of both the μc-Ni and nc-Ni samples were very similar to those known for bulk Ni. The MR( H ) data were analyzed at both temperatures with the help of Kohler plots from which the resistivity anisotropy splitting (Δ ρ AMR ) and the anisotropic magnetoresistance (AMR) ratio were derived, the latter values being very similar at both temperatures and for both microstructural states of Ni metal. The present high-precision MR( H ) data revealed that the available theoretical models do not accurately describe the suppression of thermally induced magnetic disorder at around room temperature in high magnetic fields.

In spite of the numerous works devoted to studying the magnetoresistance of Co metal, very diverging results have been reported in the literature on the magnitude of the anisotropic magnetoresistance (AMR) ratio of Co samples mostly without detailed structural characterization. Therefore, the main purpose of the present work was to establish if the crystal structure of Co has an effect on the AMR ratio. With the help of structural studies by X-ray diffraction (XRD) and transmission electron microscopy, fully hcp-Co and predominantly fcc-Co polycrystalline foils were produced by electrodeposition and their magnetoresistance curves MR(H) were measured at room temperature in magnetic fields up to H = 8 kOe. The MR(H) curves indicated much lower saturation fields for fcc-Co than for hcp-Co, in good agreement with the significantly larger magnetocrystalline anisotropy of the hcp phase. These findings were supported also by the measured magnetization isotherms. The coercive field and MR(H) peak position data indicated a magnetically softer behavior of the fcc-Co phase than that of the hcp-Co phase, in agreement with literature findings. Finally, it was established that the AMR ratio is about +1.2 % for hcp-Co whereas it is about +1.9 % for the predominantly fcc-Co samples. By having an estimated volume fraction of the hcp-Co phase in the latter samples from the XRD studies, we could assess an AMR ratio of about 2.2 % for pure fcc-Co. It is the first time that the AMR ratio has been determined separately for the two crystalline phases of Co and the finding that AMR(fcc-Co) approximate to 1.8 AMR(hcp-Co) is a hitherto unknown result. Finally, our AMR results for the two phases of Co metal are discussed in the light of recent progress on the microscopic mechanisms of the AMR effect.

Magnetic memory effect was measured in Fe-Ag granular multilayers by employing the so-called stop-and-wait protocol in order to reveal super-spin glass (SSG) behavior and explore the role of intra- and interlayer interactions which can influence the superparamagnetic (SPM) behavior of Fe particles separated by Ag layers. Calculations based on the relaxation of two-level systems are made to obtain the evolution of the magnetization of interaction-free SPM particles as a function of temperature and time for arbitrary annealing procedures, e.g., zero-field-cooled (ZFC) and field-cooled (FC) susceptibility (low-field magnetization) or temperature-cycling measurements. In samples with a single Fe layer of different nominal thicknesses, both the observed memory effect and the low-temperature deviation of the measured FC magnetization from the calculated interaction-free FC curve are attributed to the effect of dipolar interactions. Estimates on the anisotropy energy suggest the importance of the surface anisotropy of the Fe particles in the magnetic behavior.

The crystal structure, the magnetic properties, and the valence band density of states of an oxide dispersion strengthened (ODS) steel with nominal composition of Fe-18%Cr-13%Ni-2.5%Mo-3.2%Si-0.1%C+1 wt% Y2O3 was investigated applying volumetric and surface sensitive methods. The sample volume contained majority fcc and minority bcc phases according to XRD and volumetric magnetic measurements, while the top layer of the grains contained only the paramagnetic fcc phase, revealed by conversion electron Mossbauer spectrometry (CEMS) and by photoelectron measurements. The valence band photoelectron emission spectra reflected well the average distribution of the theoretically expected DOS, however, there were no signs of abrupt peaks in the region of 0-4 eV below the Fermi level that could have been attributed to 3d Fe states (bcc phase). It was demonstrated that the photoelectron emission can be utilized to distinguish the Fe atoms located in different crystallographic structures. The valence band data of the ODS steel was compared to that of originating from a bcc iron based magnetic alloy.

We report small-angle neutron scattering studies of the lacunar spinelGaV(4)S(8), which reveal the long-wavelength magnetic phases to be cycloidally modulated. Upon cooling, these modulated phases, including a recently proposed Neel-type skyrmion phase, transform into a simple ferromagnetic state. These results indicate the modulated phases in GaV4S8 gain their stability from thermal fluctuations, while at lower temperatures the ferromagnetic state emerges in accord with the strong easy-axis magnetic anisotropy. Crucially, our study provides microscopic evidence that the skyrmions in GaV4S8 indeed display a Neel-type helicity. More generally, our approach can be applied to evidence the helicity of any bulk skyrmion phase.

Magneto-optical effects have been investigated over the infrared{visible spectral range in ACr2O4 (A = Fe, Co) spinel oxides with non-collinear spin orders in their ground states. We found large magneto-optical Kerr rotation and ellipticity at the on-site d-d transitions of the A2+ ions located within the charge gap. The magneto-optical Kerr rotation of 12 deg observed in CoCr2O4 is unprecedentedly large among magnetic semiconductors and points towards the uniqueness of tetrahedrally coordinated Co2+ ions in generating a strong magneto-optical response. Criteria of strong magneto-optical effects emerging at on-site d-d transitions of transition metal ions are discussed.

Fe-Ag multilayers were prepared by vacuum evaporation in a wide range of Fe and Ag thicknesses (t(Fe), t(Ag)) and bilayer numbers (n), while the magnetic properties as a function of these parameters were measured. Samples with discontinuous Fe layers are superparamagnetic (SPM) and our paper shows that t(Ag) and n affect the superparamagnetic blocking temperature (T-B) not only by affecting the interactions between the Fe grains but also through influencing the growth process of Fe and thereby modifying the magnetic-grain-size distribution. A magnetic "phase diagram" of our Fe-Ag multilayers is constructed in the t(Fe), t(Ag), n parameter space, where the SPM and ferromagnetic behaviors are separated by a mixed zone showing the characteristics of both. The measured trend of the susceptibilities attained at the blocking temperature (T-B), increasing with increasing T-B, was explained by the volume distribution of the magnetic particles, as illustrated by calculations for an ensemble of interaction-free magnetic particles.

There have been extended studies on the appearance of ferromagnetism in transition-metal-metalloid (MD) glasses. In particular, the paramagnetic (PM) to ferromagnetic (FM) transition has been investigated on numerous (Ni100-xFex)-MD alloys upon the introduction of Fe where MD can represent a combination of various metalloid elements, while keeping the metal/metalloid ratio constant. It has been reported that adding a sufficient amount of Fe to a Pauli PM Ni-MD alloy matrix first induces a spin-glass (SG) state at low temperatures which goes over to a PM state at higher temperatures. Beyond a certain Fe content, x(c), the SG state transforms to a FMstate upon increasing the temperature. By plotting the characteristic transition temperatures as a function of the Fe content, a magnetic phase diagram can be constructed for each Ni-Fe-MD system which has a multicritical point (MCP) at x(c). By using the reported magnetic phase diagrams of various Ni-Fe-MD alloy systems, it is shown that the critical Fe content, xc scales inversely with the density of states at the Fermi level, N(E-F), of the parent Ni-MD matrix. This means that the higher the N(E-F), the lower the critical Fe content to induce ferromagnetism in the Ni-MD matrix. This is then discussed in terms of the Stoner enhancement factor, S, which characterizes the tendency of the matrix to become ferromagnetic. (C) 2017 Elsevier B.V. All rights reserved.

We use argon ion-bombardment during Fe layer deposition as a way to tailor the structural and magnetic properties of bilayer and multilayer systems containing Fe/FeV components. We present structural and magnetic results on Fe/FeV bilayer and Fe/Fe-oxide/Fe/FeV multilayer systems. Magnetization measurements were taken over a range of temperatures and show the expected ferromagnetic behavior for the Fe/FeV bilayer. The Fe/Fe-oxide/Fe/FeV multilayer demonstrates an enhanced coercivity and exchange bias at low temperatures, both due to the presence of the antiferromagnetic Fe-oxide layer. Polarized neutron reflectometry results (scattering length density depth profile and neutron spin asymmetry) were used to identify mixed interfacial layers resulting from ion-bombardment. These demonstrated a lower magnetic moment than bulk Fe layers and may undergo a reversal process that differs from non-mixed layers within the sample.

Magnetic nanoporous silica particles (MNS) with spherical morphology and 100 nm particle with advanced characteristics suitable for nanomedicine purposes were synthesized. The obtained nanoparticles were modified with SO3H groups in a two-step post synthesis procedure. An anticancer drug, mitoxantrone (MTX), and an anti-inflammatory drug, prednisolone (PRD), were loaded on the silica support. The mitoxantrone loaded MNS-SO3H nanoparticles were coated by chitosan and then prednisolone was infused in the chitosan layer. A second layer of alginate was then applied around the prednisolone and mitoxantrone containing formulation. All materials were characterized by XRD, N-2 physisorption, Mossbauer spectroscopy, magnetization measurements and transmission electron microscopy in order to demonstrate that by the applied preparation method around 11 nm sized maghemite crystals embedded in spherical mesoporous silica nanoparticles were obtained with high pore volume and surface area. Thermal gravimetric analysis, ATR FT-IR spectroscopy and in vitro release experiments proved that MTX and PRD were successfully loaded on the silica matrix. Alginate coating further improved the release properties by preventing the burst release of MTX and PRD. The cytotoxicity properties of the drugs loaded formulations and their ability to retain the intrinsic pharmacological properties of the encapsulated drugs were investigated on a panel of human tumor cell lines.

Detailed investigation of Ca2CoSi2O7 was performed in its low-temperature magnetoelectric state combining neutron diffraction with magnetization measurements on single crystals. The crystal and magnetic structures well below the antiferromagnetic transition temperature of T-N approximate to 5.7 K were determined using neutron diffraction. Neutron diffraction data imply no structural phase transition from 10 K down to 2.5 K and are well described within the orthorhombic space group P2(1)2(1)2 with a 3 x 3 x 1 supercell compared with the high-temperature unmodulated state (tetragonal space group P (4) over bar2(1)m). We found that in zero magnetic field the magnetic space group is P2(1)2'(1)2' with antiferromagnetic order along the [100] or [010] axes for two types of 90(omicron) twin domains, while neighboring spins along the [001] axis are ordered ferromagnetically. A noncollinear spin arrangement due to small canting within the ab plane is allowed by symmetry and leads to the existence of the tiny spontaneous magnetization below T-N. The ordered moment with a magnitude of about 2.8 mu B/Co2+ at 2.5 K lies in the ab plane. Distinct differences between the magnetic structure of Ca2CoSi2O7 as compared to those of Ba2CoGe2O7 and Sr2CoSi2O7 are discussed.

We report on the slow magnetization dynamics observed upon the magnetic phase transitions of GaV4S8, a multiferroic compound featuring a long-ranged cycloidal magnetic order and a Neel-type skyrmion lattice in a relatively broad temperature range below its Curie temperature. The fundamental difference between GaV4S8 and the chiral helimagnets, the prototypical skyrmion host compounds, lies within the polar symmetry of GaV4S8, promoting a cycloidal instead of a helical magnetic order and rendering the magnetic phase diagram essentially different from that in the cubic helimagnets. Our ac magnetic susceptibility study reveals slow relaxation dynamics at the field-driven phase transitions between the cycloidal, skyrmion lattice and field-polarized states. At each phase boundary, the characteristic relaxation times were found to exhibit a strong temperature dependence, starting from the minute range at low temperatures, decreasing to the micro-to millisecond range at higher temperatures.

The magnetic properties and the magnetoresistance behavior were investigated for electrodeposited nanoscale Co films, Co/Cu/Co sandwiches and Co/Cu multilayers with individual Co layer thicknesses ranging from 1nm to 20nm. The measured saturation magnetization values confirmed that the nominal and actual layer thicknesses are in fairly good agreement. All three types of layered structure exhibited anisotropic magnetoresistance for thick magnetic layers whereas the Co/Cu/Co sandwiches and Co/Cu multilayers with thinner magnetic layers exhibited giant magnetoresistance (GMR), the GMR magnitude being the largest for the thinnest Co layers. The decreasing values of the relative remanence and the coercive field when reducing the Co layer thickness down to below about 3nm indicated the presence of superparamagnetic (SPM) regions in the magnetic layers which could be more firmly evidenced for these samples by a decomposition of the magnetoresistance vs. field curves into a ferromagnetic and an SPM contribution. For thicker magnetic layers, the dependence of the coercivity (Hc) on magnetic layer thickness (d) could be described for each of the layered structure types by the usual equation Hc=Hco+a/d n with an exponent around n=1. The common value of n suggests a similar mechanism for the magnetization reversal by domain wall motion in all three structure types and hints also at the absence of coupling between magnetic layers in the Co/Cu/Co sandwiches and Co/Cu multilayers.

The 1/4-filled organic compound, delta-(EDT-TTF-CONMe2)(2)AsF6 is a frustrated two-dimensional triangular magnetic system as shown by high-frequency (111.2 and 222.4 GHz) electron spin resonance (ESR) and structural data in the literature. The material gradually orders antiferromagnetically below 40 K, but some magnetically disordered domains persist down to 4 K. We propose that in defect free regions frustration prevents true magnetic order down to at least 4 K in spite of the large first- and second-neighbor exchange interactions along chains and between chains, respectively. The antiferromagnetic (AFM) order gradually developing below 40 K nucleates around structural defects that locally cancel frustration. Two antiferromagnetic resonance modes mapped in the principal planes at 4 K are assigned to the very weakly interacting one-dimensional molecular chains in antiferromagnetic regions.

Multilayers of 0.4nm Fe and 5nm Ag with repetition number, n=1, 2, 5, 10 and 20 were prepared by vacuum evaporation onto Si wafer. The blocking temperature was determined by measuring the field cooled and zero field cooled magnetization curves with a SQUID magnetometer and it was found to increase by almost an order of magnitude from around 20K for the single Fe layer sample up to around 160K for n=20. Significant increase of the average size of the superparamagnetic Fe grains by increasing the number of the Fe layers was excluded by conversion electron Mössbauer spectroscopy measurements of the paramagnetic state. The role of the dipole–dipole interactions and their interplay with the out-of-plain magnetic anisotropy in the variation of the blocking temperature has been investigated by Monte-Carlo simulations.

Despite the intensive studies for decades, it is still not well understood how qualitatively different magnetic behaviors can occur in a narrow composition range for the Fe-rich Fe-transition metal (TM) amorphous alloys. In this study of amorphous $\mathrm{F}{\mathrm{e}}_{100\ensuremath{-}x}\mathrm{Z}{\mathrm{r}}_{x}$ ($x=7$, 9, 12) metallic glasses, normal ferromagnetism (FM) is found at 12 % Zr where only the FM-paramagnetic (PM) transition is observed at the Curie temperature, ${T}_{C}$. In contrast, spin-glass (SG)-PM transition at a temperature, ${T}_{g}$, called SG temperature, is only observed at 7 % Zr, while in the transient re-entrant composition range $(x=8\ensuremath{-}11)$, an SG-FM transition at a temperature, ${T}_{f}$, called spin-freezing temperature, is also observed at low temperature besides the normal FM-PM transition at ${T}_{C}$. In order to understand this unusual behavior, a detailed characterization of pressure (atomic volume), composition, and temperature dependence of the magnetic properties is coupled with high pressure synchrotron x-ray diffraction determination of the pressure dependence of the atomic volume. The results on $\mathrm{F}{\mathrm{e}}_{100\ensuremath{-}x}\mathrm{Z}{\mathrm{r}}_{x}$ ($x=7$, 9, 12) are compared to those obtained for the FM $\mathrm{C}{\mathrm{o}}_{91}\mathrm{Z}{\mathrm{r}}_{9}$ metallic glass not showing any kind of anomalous magnetic properties. It is confirmed that the unusual behavior is caused by a granularlike magnetic structure where weakly coupled magnetic clusters are embedded into a FM bulk matrix. Since the mechanism of the magnetization reversal was found to be of the curling type rather than homogeneous rotation, the energy barrier determining the blocking temperature of the clusters is calculated as AR, where A is the exchange constant and R is the cluster size, in contrast to the usual characterization of the energy barrier by KV where K is the anisotropy energy and V is the cluster volume. The volume fraction of the FM part is a fast changing function of the bulk composition: Almost 100% FM fraction is found at 12 % of Zr while no trace of real FM is observed at 7 at % Zr. The driving force of this surprising magnetic character is the atomic volume: The lower the Zr content, the higher is the fraction of Fe atoms with compressed atomic volume having low magnetic moment. The percolation of their network separates the clusters from the FM bulk. The complex magnetic behavior of the Fe-rich Fe-Zr amorphous system at low temperatures can thus be interpreted with the only assumption of a cluster-size distribution and a weak coupling of the clusters to the FM matrix. The introduction of this coupling is able to explain the opposite pressure dependence of ${T}_{g}$ and ${T}_{f}$. The threshold atomic volume in the low magnetic moment regions is found to be comparable to the atomic volume characteristic to the low-spin limit of the face-centered-cubic Fe alloys. The extensive literature results on the anomalous magnetism for various Fe-rich Fe-TM amorphous alloys and especially for the Fe-rich Fe-Zr glassy system are also found to be in agreement with this granular magnetic behavior.

A new method is developed to synthesize magnetic carbon microspheres decorated with carbon nanofibers and iron nanoparticles (nanofurry microspheres) for separation techniques in chemistry and biology. Microspheres are synthesized by carbonizing polystyrene–divinylbenzene-based, iron-loaded ion exchange resins. The phase composition, magnetic properties, and surface area and morphology of these materials are characterized by various techniques. It is detected that superparamagnetic (SPM) magnetite is present in microspheres exclusively upon carbonization at 400–500 °C, elemental iron, both α- and γ-Fe, is the major component at 600 °C, and cementite dominates between 700 and 1000 °C. Nanofiber formation is observed to be pronounced at high temperatures. The synthesized carbon microspheres have high surface area (100–300 m2 g−1) and can be separated easily by a magnet or by filtration. Saturation magnetization of selected samples is obtained between 5 and 28 emu g−1, depending on the phase composition. The novel microcomposites are expected to be effective adsorbents or support materials in various chemical processes, for example in water and air cleaning, catalysis, and biotechnological separations. Preliminary experimental studies for Cr(VI) removal from water and for platinum deposition are provided.