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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. [ABSTRACT FROM AUTHOR]

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 (Ni 100-x Fe x )-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 FM state 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, x c 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. [ABSTRACT FROM AUTHOR]

Abstract: The magnetoresistance (MR) was measured at 200, 250 and 300K in magnetic fields up to B=12T for a nanocrystallized Fe63.5Cr10Nb3Cu1Si13.5B9 alloy. Both the longitudinal (LMR) and transverse (TMR) component of the magnetoresistance decreased from B=0 to about 0.1T. This could be ascribed to a giant MR (GMR) effect due to spin-dependent scattering of conduction electrons along their path between two Fe–Si nanograins via the non-magnetic matrix. Such a scattering may occur if the nanograin moments are not or only weakly coupled in the absence of a strong exchange coupling (due to the high Cr content in the matrix) and/or only weak dipole–dipole coupling is present (due to sufficiently large separations between the nanograins). For larger fields, the GMR saturated and a slightly nonlinear increase in MR with B was observed due to a contribution by the residual amorphous matrix. The anisotropic MR effect (AMR≡LMR−TMR) was negative for all fields and temperatures investigated. By measuring the MR of melt-quenched Fe100−x Si x solid solutions with x=15, 18, 20, 25 and 28, the observed AMR could be identified as originating from the Fe–Si nanograins having a D03 structure. [ABSTRACT FROM AUTHOR]

Nanocrystalline Fe90Zr7B2Cu1 with ferromagnetic BCC nanocrystals of about 10–20 nm size embedded in a residual amorphous matrix was produced from amorphous precursor by partial crystallization. A significant increase in the Curie temperature of the residual amorphous phase (TC) as compared to that of the amorphous precursor was found by combined bulk magnetic and Mössbauer measurements. The unusual increase of TC for alloys with different nanocrystalline fractions correlates with the quantity of the BCC phase. [Copyright &y& Elsevier]

Low-temperature spin freezing was observed below 25 K in the nanocrystalline (nc)-Fe92−xZr7BxCu1 (x=2–21 at%) alloys prepared from their amorphous precursors by various heat treatments. The freezing causes magnetic hardening with increasing B concentration and annealing temperature, shown by the temperature dependence of both the susceptibility and coercive field. The phenomenon is attributed to the gradual magnetic blocking of a fraction of nanocrystals decoupled from the ferromagnetic matrix. Similar effects are observed in a Cr-containing FINEMET-type nc alloy. [Copyright &y& Elsevier]

A correlation was found between the change of the saturation magnetization (Ms) and spin wave stiffness constant (Dsp) as a function of annealing temperature for FINEMET-type glassy precursors (Fe73.5Nb3Si13.5B9Cu1), annealed in the temperature range 400–540°C for times up to 1 h. During irreversible structural relaxation both Ms and Dsp increase. During the evolution of the nanocrystals Ms decreases while Dsp increases. [Copyright &y& Elsevier]

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 1 nm to 20 nm. 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 3 nm 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 ( H c ) on magnetic layer thickness ( d ) could be described for each of the layered structure types by the usual equation H c = H co + 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. [ABSTRACT FROM AUTHOR]

• Electrochemical preparation of fully hcp-Co and predominantly fcc-Co. • Structural study by combined X-ray diffraction and transmission electron microscopy. • First determination of the anisotropic magnetoresistance (AMR) ratio of hcp-Co. • First estimate of the AMR ratio of fcc-Co. • Hitherto unknown result: AMR(fcc–Co) ≈ 1.8 AMR(hcp-Co). 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) ≈ 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. [ABSTRACT FROM AUTHOR]

The magnetoresistance (MR) and magnetization were studied at room temperature and down to 4.2 K for Ni81Cu19(3 nm)/Cu(dCu) multilayers with 0.5 nm⩽dCu⩽3 nm and for a bulk Ni81Cu19 alloy, all produced by electrodeposition. For most of the multilayers, giant magnetoresistance (GMR) was observed with a maximum around 1–1.5 nm Cu-layer thicknesses. The room-temperature MR curves consisted of a rapidly varying low-field component up to about 1 kOe and a slowly varying high-field component persisting beyond 18 kOe. For Cu-layer thicknesses around the maximum GMR, the low-field component dominated whereas for dCu>1.5 nm, it became comparable to or smaller than the high-field component. For the bulk alloy, anisotropic magnetoresistance was observed. According to the room-temperature magnetization curves, both the bulk alloy and the multilayers exhibited ferromagnetic (FM) behaviour with technical saturation around 1–3 kOe. For the multilayers, the low-field MR curves with a small peak splitting were characteristic of uncoupled or weakly coupled magnetic layers. Magnetic measurements from 5 to 300 K indicated that besides an FM contribution, these multilayers also exhibit a superparamagnetic (SPM) component with blocking temperature around 220 K. From an analysis of the present results, the low-field MR component could be attributed to the FM domains with their magnetizations exhibiting a random in-plane distribution in the zero-field (demagnetized) state and the high-field MR component to a contribution due to SPM regions. All these considerations are supported by the complete saturation of the MR in a magnetic field of 4 kOe at T=4.2 K. [Copyright &y& Elsevier]

The frequency dependence of permeability and power loss are investigated in FINEMET samples after isothermal heat treatments, leading to structural relaxation and/or nanocrystallization. The rapid decrease in power loss and increase in permeability at low frequencies are connected mainly with the structural relaxation and the first period of nanocrystallization. The time scale for the evolution of soft magnetic properties at high frequencies is similar to that for the density change. [Copyright &y& Elsevier]

Abstract: Anomalous (negative) irreversible and reversible Curie-point relaxation was observed in amorphous Fe76Cr8B15Cu1 alloy. This observation is in line with the results previously found in a FeNiCrSiB alloy series where the Cr-concentration dependence of the normal (positive) reversible Curie-point relaxation could only be explained by assuming two contributions of different signs to the relaxation. The normal contribution is related to the ordering of Fe and Ni atoms while the anomalous one is associated with the Cr atoms. The role of Cr on the resistivity relaxation is also presented. Existing literature results on anomalous structural relaxation of metallic glasses are critically discussed. [Copyright &y& Elsevier]

Electrodeposited Co–Cu/Cu multilayers were prepared by a galvanostatic/potentiostatic (G/P) pulse sequence in which the Co-rich magnetic Co–Cu layer was deposited by current control and the non-magnetic Cu layer by potential control. The Cu deposition potential has been chosen so as to exclude the occurrence of a Co dissolution during the Cu deposition cycle. This ensured that the actual amounts of metals deposited were equal to the nominal ones. The multilayers with a total thickness of about 1.7 μm were mechanically peeled off from their Ti substrate. Detailed room-temperature studies of the dependence of the magnetoresistance (MR) and the magnetic properties on Co and Cu layer thicknesses were performed with particular attention to analysing the field evolution of the MR. A giant magnetoresistance (GMR) effect was observed in most samples and a maximum GMR of 10% measured at 1 kOe could be achieved. No oscillatory GMR behaviour with increasing Cu layer thickness could be observed but rather a continuous evolution of the MR characteristics from anisotropic magnetoresistance (AMR) to GMR. This could be explained by the gradual increase of the Cu coverage on the Co-rich magnetic layer during each Cu pulse with increasing average Cu layer thickness. It was concluded that the Cu layer becomes continuous above about 2 nm thickness only. For 2.5 nm Cu layer thickness, the Co layer remained continuous down to 1.1 nm thickness. However, for 1.1 nm Cu layer thickness, the 1.1 nm thick Co layer was broken up into superparamagnetic islands. The evolution of the coercive force with layer thicknesses well corroborated the above picture. From these results, an asymmetry in the nucleation of Cu on Co and Co on Cu could be deduced in agreement with some previous reports on evaporated multilayers. [Copyright &y& Elsevier]

The influence of Cu content on the magnetic and magnetotransport properties of Fe60Co18Nb6B16−xCux (x=0, 1) alloys has been studied by means of hysteresis loops, permeability and magnetoimpedance measurements. The formation of Cu clusters enhances the nucleation of the α-FeCo phase, refining the microstructure. This can explain the smaller coercivity values obtained for the Cu-containing alloy. Magnetic permeability and magnetoimpedance results are in agreement with coercivity data. Larger permeability and magnetoimpedance ratio are found in samples of Cu-containing alloys annealed at the beginning of nanocrystallization. [Copyright &y& Elsevier]

Thermal evolution of magnetization and magnetic permeability of Fe39-xMnxCo39Nb6B15-yCuy (x=2,4; y=0,1) alloys is reported. Mn addition decreases the Curie temperature of the initial amorphous phase. For nanocrystalline samples an almost temperature independent value of the relative magnetic permeability is observed from room temperature up to ∼950 K. Permeability drops at the second crystallization stage due to the formation of boride phases. [Copyright &y& Elsevier]

Abstract: A combined magnetic, neutron diffraction and field-dependent 57Fe Mössbauer study of Sc-substituted Ba-hexaferrite in powder form is reported, and the findings are compared with the data for BaFe12O19. The five cation sublattice with collinear ferrimagnetic structure of uniaxial (Gorter) type for BaFe12O19, persists also for BaFe10.4Sc1.6O19 at room temperature and below down to 130–190K, depending on the applied field. Below room temperature, BaFe10.4Sc1.6O19 undergoes a magnetic transition as indicated by the magnetization and Mössbauer spectroscopic measurements. The neutron diffraction results are compatible with a gradual transformation below 190K into a complex canted structure with spins pointing out of the axial direction. The complex magnetic order at 10K can be described in terms of an incommensurate complex block-type conical structure. [Copyright &y& Elsevier]