Search

The role of oxidation on the structural, magnetic and electronic properties of a Ni-NiO compound is reported. XRD results indicate that NiO is formed at the expense of metallic Ni and complete NiO phase formation was observed at 1000 °C. The average crystallite size of NiO is found to increase from 12 to 35 nm upon calcination. Using transmission electron microscopy (TEM) the average particle size for the samples calcined at 1000 °C was found to be approximately 126 nm. Hence, comparing it to the crystallite sizes estimated from the XRD, it is clear that the powder samples are polycrystalline in nature, consisting of smaller crystallites clustered together. Both the ferromagnetic and AFM contributions from Ni and NiO, respectively, were observed in the magnetization data. The saturation magnetization and coercivity were found to decrease with increasing calcination temperature with a shifting of hysteresis loop towards the negative axis. The optical band-gap of the powders was found to be unchanged upon calcination (3.3 eV or 3.4 eV). The density of electrons near the Fermi level was found to be higher in the non-calcined and dried sample, resulting in a higher magnetic moment compared to the other samples.

The effect of multiaxial mechanical stress on angle-dependent magnetization reversals is studied on a single 10-nm ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ film, a single 10-nm ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ film, and a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$(10 nm)/${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$(10 nm) bilayer. These films are grown on flexible $\mathrm{Al}$ ``kitchenlike'' foils. These flexible foils are bent on two convex optical lenses to apply multiaxial mechanical stresses of different magnitudes. In order to understand the mechanisms driving the physical properties of the flexible systems, the magnetic films are also grown on hard native-oxide-covered $\mathrm{Si}$(100) substrates. By probing the angular dependence of the coercive field and the remanent magnetization, we show that the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ single layer and the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/{\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ bilayer grown on $\mathrm{Si}$ present a uniaxial anisotropy. Along their easy axis, their domain structure at the coercive field exhibits large domains with sawtooth domain walls. However, the ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ layer grown on $\mathrm{Si}$ shows a cubic anisotropy and sharp domain walls with a right-angle geometry along its easy axis. All layers grown on $\mathrm{Al}$ foils show development or enhancement of a uniaxial anisotropy arising from the $\mathrm{Al}$ surface morphology. Multiaxial mechanical stress applied to the flexible $\mathrm{Al}$ foils results in very different nonreversible effects as a function of the composition of the layered system. For ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$, multiaxial mechanical stress has no effect, as expected for a nonmagnetostrictive system. For ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$, it results in a full in-plane randomization of its magnetization-reversal properties. For the bilayer, multiaxial mechanical stress does not fully randomize the in-plane magnetization-reversal properties: the bilayer retains a uniaxial character.

International audience; In this article, the magnetic properties of gadolinium chloride-filled double-walled carbon nanotubes (GdCl3@DWNTs) in the temperature range 2-300 K are explored. The temperature-dependent phonon frequencies of the G-band were studied from 80-300 K to investigate the effect of temperature on the magnetic ordering. Temperature-dependent susceptibility measurements show that the GdCl3@DWNTs sample has a pronounced superparamagnetic phase from 83 K. The temperature dependence of the G-band frequency for filled tubes exhibited a distinct difference compared to pristine nanotubes, where a sharp phonon hardening at low temperatures was observed. A correlation between the onset temperature of superparamagnetism and the abrupt G-band phonon hardening in the filled tubes was verified. GdCl3@DWNTs were characterized by a finite remnant magnetization at 300 K which decreased as the temperature was lowered because of the presence of the discontinuous magnetic nanoparticles, providing a superparamagnetic contribution characterized by an S-shaped non-saturating hysteresis loop at 2 K. Remarkably, the onset of superparamagnetism, marked by the bifurcation point, occurred at roughly the same temperature where the G-band phonon frequency showed a pronounced hardening at approximately 80 K, indicating a close correlation between phonon modes and spin clusters.

The compounds with the general chemical formula RCrTiO5 (R = rare-earth ions) possess an orthorhombic crystal structure having Pbam space group. However, there are limited works carried out on this group of materials, primarily in bulk to explore their structural and magnetic properties. The exploration of structural and magnetic properties of RCrTiO5 compounds in the nano dimension has not been reported to date. This report focuses on the synthesis of DyCrTiO5 nanoparticles using a simple, time saving and cost-effective sol-gel technique to investigate the role of size on structural as well as magnetic properties. The synthesized sample is calcined at 800 °C for three hours. The crystal structure is confirmed from the Le-Bail profile fitting of the x-ray diffraction (XRD) pattern. The DyCrTiO5 nanoparticles are crystallized in an orthorhombic structure with lattice parameters, a, b, c of 7.321 ± 0.001, 8.644 ± 0.002 and 5.840 ± 0.001 A, respectively. The average particle size is found to be 53 ± 3 nm from transmission electron microscopy (TEM). From the temperature-dependent magnetization measurement, the obtained Neel temperature ( T N ) is 147 ± 2 K. In contrast to the bulk form, DyCrTiO5 nanoparticles do not show spin reorientation (SR) which is observed below 37 K or the magnetic compensation because of the dominating contribution of Dy3+ moment. In addition, the irreversibility in magnetization is suppressed when measured as a function of temperature in the heating and cooling cycle of the measurement with increasing the probing magnetic field. This indicates a feature similar to kinetic arrest. In isothermal magnetization measurements, exchange bias effect is observed because of interaction between the uncompensated spins on the surface of the nanoparticles.

The temperature dependence of magnetic susceptibility, χ(T), of the (Cr84Re16)100−xVx alloy system is reported in this contribution. Anomalies are observed at the Neel temperature (TN), associated with the onset of the antiferromagnetic (AF) spin-density-wave (SDW) state, with these anomalies becoming more pronounced as x increases. The TN values determined from χ(T) measurements are in close agreement with that obtained from the measurement of temperature dependence of electrical resistivity, ρ(T). Prominent broad deep minima are observed in χ(T) upon cooling below 100 K which are followed by low temperature peaks in x = 10.4, 10.9 and 12.4 samples. A second anomaly, not associated with TN, is observed at a temperature TO TN.

Previous electrical resistivity ( ρ ) and specific heat ( C P ) studies on the ternary (Cr100-xAlx)95Mo5 alloy system suggested that it harbours two quantum critical points (QCPs). This study reports comprehensive investigations of this alloy system through Seebeck coefficient ( S ) , thermal conductivity ( κ ) , magnetic susceptibility ( χ ) and neutron diffraction ( N D ) measurements in the concentration range 0 ≤ x ≤ 8.6. The results of S and χ show that spin-density-wave (SDW) antiferromagnetism is suppressed to temperatures below 2 K for concentrations in the range 1.4 ≤ x ≤ 4.4. Plots of d S / d T in the limit as T → 2 K depict two minima, i.e. just above x = 1.4 and 4.4. This parameter has been used as a key indicator of quantum critical behaviour (QCB) in Cr alloys. Analyses against the Nordheim-Gorter relationship demonstrates a positive slope for the incommensurate (I) SDW alloys and a negative slope for the commensurate (C) SDW alloys. Extrapolations of these two slopes intercept at a concentration of 3.2 at.% Al indicating the occurrence of band structure modifications when Al is added into the Cr95Mo5 base alloy. The Lorenz number ( L ) for the alloys with x = 0 and 0.5 shows interesting anomalous behaviour associated with band structure effects and SDW magnetic ordering. N D measurements as a function of temperature confirm that alloys with x 4.4 show commensurate (C) SDW order. Power law fits of the form T N ∝ 1.40 − x 0.35 ± 0.05 for the ISDW to P phase transition and T N ∝ x − 4.40 0.63 ± 0.03 for the P to CSDW phase transition rendered the critical exponents 0.35 ± 0.05 and 0.63 ± 0.03 respectively. Overall the results of S , κ , χ and N D corroborate the existence of two QCPs at x ≈ 1.4 and 4.4.

A series of spinel-type MnxNi1-xCo2O4 (x = 0, 0.3, 0.5, 0.7, 1) oxides were synthesized by using a citrate sol-gel technique. The structural, magnetic and electronic properties of the oxides with different Mn content were investigated using X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), vibrating sample magnetometer (VSM) and X-ray photoelectron spectroscopy (XPS). The XRD and FTIR studies revealed that MnxNi1-xCo2O4 formed in a cubic structure for all the compositions synthesized. Transmission electron microscopy (TEM) studies show cubic nano-plates for x = 0 and on increasing Mn concentration, agglomerated nanoparticles with sphere-like shape are observed with a particle size in the range of 11.7 nm–22.1 nm. This is in close agreement with the results of XRD analysis. The Mn 2p, Ni 2p, and Co 2p XPS spectra reveal the coexistence of Mn2+, Mn3+, Ni2+, Ni3+, Co2+, and Co3+ species on the surface of MnxNi1-xCo2O4 samples in differing proportions. The measurements of magnetic properties at room temperature show that the samples have some ferromagnetic behaviour. The saturation magnetization, remnant magnetization, and coercivity of the nanoparticles vary significantly with a concentration of Mn, which tends to be distributed in both tetrahedral and octahedral positions of the spinel lattice.

The study describes the preparation of graphene oxide based silica coated magnetic nanoparticles (Fe3O4@SiO2@GO) functionalized with 2-phenylethylamine (PEA) as an adsorbent for selected organophosphorus pesticides (OPPs) namely chlorpyrifos, parathion, and malathion from aqueous solution. The synthesised (Fe3O4@SiO2@GO-PEA) adsorbent was characterised using Fourier transform-infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), Zeta potential, Nitrogen adsorption/desorption at 77 K, and a Vibrating Sample Magnetometer (VSM). The maximum adsorption was found at 15 min, with 15 mg adsorbent dosage using 1 μg/mL concentration. There was no significant difference in the pH condition used highlighting the potential to use the material on a variety of samples. Using non-linear methods, the sorption isotherm data were fitted to Langmuir, Freundlich, Temkin, Dubinin–Radushkevich, Sips, and Redlich and Peterson models. All three pesticides were best fit with the Sips model. Pseudo-first and pseudo-second order kinetics models were fitted to the experimental data using non-linear methods, and pseudo-second-order kinetics gave the best fit to the data for all three pesticides. The absorbent was tested for its reusability, and there was little loss in the recovery of OPPs after 10 cycles. Finally, the adsorbent was tested using real world samples from the Vaal River and Dam (South Africa), and showed greater than 86.9% recovery. The results obtained showed that the synthesised Fe3O4@SiO2@GO-PEA is an efficient adsorbent for recovery and analysis of pesticides.

Mixed ternary spinel-type oxides MnxNi1-xCo2O4 (x = 0, 0.5, 1) were synthesized using a citrate sol-gel route. The morphologies and structures of the mixed oxide nanoparticles were characterized using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Fourier-Transform Infrared Spectroscopy (FTIR). Structural analysis from XRD and FTIR confirm the formation of single phase cubic spinels. TEM and SEM images show the formation of interconnected nanoparticles. The particle sizes obtained from TEM and XRD are consistent with each other. It was observed that the lattice constant increases with increasing Mn concentration from x=0 to x=0.5 and then decreases with further increase in Mn concentration to x=1. Magnetization measurements were carried out using Vibrating Sample Magnetometery (VSM), which showed that the oxides have weak ferromagnetic behavior with the presence of an antiferromagnetic component. The oxide with the highest Mn substitution was observed to be superparamagnetic at room temperature.

Mn substituted MnxZn1−xCo2O4 (x = 0, 0.3, 0.5, 0.7, 1) oxides were synthesized by a facile co-precipitation method followed by calcination at 600 °C. The presence of manganese ions causes appreciable changes in the structural and magnetic properties of the Mn-substituted ZnCo2O4. The morphologies, structures, and electronic properties of Mn–Zn–Co oxide microspheres were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The X-ray diffraction and Fourier transform infrared spectroscopy results confirmed the formation of spinel MnxZn1−xCo2O4. It was shown that the Mn–Zn–Co oxide microspheres increase in size and become regular in shape with increasing Mn concentration with the crystal size lying in the range from 19.1 nm to 51.3 nm. Magnetization measurements were carried out using a vibrating sample magnetometer at room temperature and 10 K. The saturation magnetization is observed to increase with increasing Mn concentration from x = 0 to x = 1.

Spinel ferrimagnetic compounds of the form MCr2O4, where M is a divalent metal cation are of great interest due to the structural and magnetic phase transitions. In this class of materials, NiCr2O4 and CoCr2O4 are well studied in bulk or single crystals. NiCr2O4 demonstrates low temperature magnetic phase transitions together with structural phase transitions. However, CoCr2O4 displays only magnetic phase transitions at low temperature retaining stable cubic phase across the magnetic phase transition temperatures. Exchange interaction between the magnetic cations situated at the tetrahedral or octahedral site determines the net magnetic properties of the material. By site mixing with different cations, drastic changes in the magnetic exchange interaction leading to change in magnetic properties are observed. The present work focuses on the structural and magnetic properties of (Ni0.5Co0.5)(Cr0.75Fe0.25)2O4 nanoparticles. Co2+ and Fe3+ have been substituted at Ni and Cr sites, respectively, to modify the magnetic interactions. Rietveld refinement of the x-ra diffraction (XRD) data confirmed the cubic Fd3¯m space group of the samples. In addition to the site substitution the size of the particles was exclusively in the nano range. Transmission electron microscopy (TEM) analyses showed the average particle size DTEM = 82 ± 2 nm. MZFC and MFC demonstrate irreversibility at low temperature. Energy dispersive X-ray spectroscopy (EDS) confirmed the presence of Co, Ni, Cr, Fe and O without trace of any impurities in the samples. TC of the sample was found to be 138 ± 2 K by differentiating the MZFC data with respect to temperature measured with a probing magnetic field of 0.1 T. Hysteresis loops measured at 3 K demonstrated kinks around zero field range.

International audience; A temperature-dependent coherent magnetization reversal model is proposed for size-distributed assemblies of ferromagnetic nanoparticles (NPs). NPs are assumed to be of uniaxial and cubic anisotropies. The thermal dependence is included by considering thermal fluctuations, implemented through the Néel-Arrhenius theory. Theoretically calculated thermal and angular dependences of magnetization reversal loops are obtained. There are similar behaviors for a single NP and an assembly of NPs. In particular, it is shown that the fourfold anisotropy results in double slope loops along the hard axis in both cases. Also, the azimuthal dependence of coercive fields is similar in both cases and with or without the presence of a fourfold anisotropy. There are also important differences in the behaviors observed for asingle NP and that of an assembly of NPs. Firstly, the blocking temperature is barely enhanced by the presence of a fourfold anisotropy but is greatly enhanced in the assembly of NPs relative to a single NP. Secondly, along the easy axis, for a single size particle, the shape of the M-H loops is neither temperature dependent nor fourfold anisotropy dependent, as it is always rectangular. However, the shape of the M-H loops for an assembly of NPs is temperature dependent, but this shape is weakly dependent on the anisotropy ratio. Simulations of M-H loops using the model presented here would allow the quantitative determination of the anisotropy constants for either single sized NPs or an assembly with different sizes

The magnetic phase diagram of (Cr90Ir10)100-yVy alloy system has been constructed using the TN values obtained from the electrical resistivity, ρ, Seebeck coefficient, S, and magnetization, M, as function of temperature measurements. The present study furthermore proposes a V induced commensurate (C) spin-density-wave (SDW) to incommensurate (I) SDW phase transition on cooling, at a transition temperature TCI, in certain samples in the series investigated. M T measurements on the sample with y = 2.4 shows only one transition associated with the Neel transition at TN, with no additional transition that can be associated with TCI and is accepted to be in the CSDW phase at all temperatures. It is hypothesized that the addition of V destabilizes the CSDW phase and introduces an ISDW phase at lower temperatures in the samples with y = 3.4, y = 3.6 and y = 5.4, as the χ(T) curves of these samples show two transitions. It is suggested that all the samples with y ≤ 5.4 show magnetic phase transitions at TN on cooling from the paramagnetic (PM) to CSDW phase. On further addition of V the samples with y > 5.4 the obtained χ(T) curves show only one transition that suggest that these samples are in the ISDW phase at all temperatures below TN. Results suggest a quantum critical behavior at V concentration of yc = 9.033.

The spinel ferrimagnetic compound CoCr2O4 demonstrates a spin spiral (TS) ordering at 25 K, as well as an anomaly at 15 K termed as lock-in transition (TL). From crystallographic perspective CoCr2O4 retains the cubic phase down to 11 K. On the other hand, the normal spinel CuCr2O4 crystallizes into the cubic phase above 850 K, below which Jahn-Teller (J-T) activity of the Cu reduces the crystal symmetry by transforming it to a tetragonal phase. Contraction of CuO4 tetrahedra towards the formation of a square planar structure accounts for the tetragonal to orthorhombic structural transition at ∼ 130 K associated with the ferrimagnetic Curie temperature (TC). Considering the differences in crystal structure and magnetism of these two compounds, the current work investigates the modification in crystal structure and magnetic behaviour by mixing Co site with Cu in CoCr2O4. To achieve this, (Co1–xCux)Cr2O4 (x = 0.5, 0.25 and 0.75) nanoparticles were prepared by chemical routes. X-ray diffraction (XRD) revealed the retention of cubic structure for the samples calcined at a temperature of 600 °C for x = 0.25 and 0.5. On the other hand, J-T distortion becomes prominent for x = 0.75. Hence, only the compositions with x = 0.25 and 0.5 were studied in detail as unusual cubic phase retention is observed in these compounds. The temperature dependent magnetization studies revealed that the TC values of both the samples, 103 K for (Co0.5Cu0.5)Cr2O4 and 99 K for (Co0.75Cu0.25)Cr2O4, compare well with the value reported for CoCr2O4. However, the feature related to TS is quite prominent for x = 0.25, whereas it is suppressed for x = 0.5. The electronic properties of the cations associated with these compounds, probed using X-ray photoelectron spectroscopy (XPS), indicate that Cu and Co mostly has a 2+ oxidation state whereas that of Cr is 3+.

Measurements of the temperature dependence of electrical resistance (R) and neutron powder diffraction (NPD) of a Cr84.7Re15.3 alloy is reported. The magnetism originates from the formation of a spin-density-wave (SDW) as a result of nesting between the electron and hole sheets at the Fermi surface (FS). NPD revealed the magnetic character of the SDW to be commensurate (C) with the lattice with a Neel temperature (TN) of 560 ± 5K that correlates to TN = 585 ± 5K determined from the saddle point in the R(T) results. A power law fit Iα(1−TTN)2β of the neutron data gave β = 0.36(1), indicating that the AF ordering follows the 3D Heisenberg model. An additional antiferromagnetic magnetic contribution, associated with Cr2O3, was observed to co-exist in the alloy. Quantification analyses revealed that the Cr2O3 content was less than 2 wt.%. A β value of 0.30(1) was determined for the magnetic phase transition of Cr2O3 in correspondence with the 3D Ising model.

The possible existence of the spin glass (SG) state in the (Cr84Re16)99.6Mn0.4 alloy was investigated using magnetisation measurements. The temperature dependence of magnetisation, M(T), indicated a splitting of the M(T) data in the zero-field-cooled (ZFC) and field-cooled (FC) curves at the pinning temperature, Tp which is an indication of magnetic anisotropy in the system. The (Cr84Re16)99.6Mn0.4 alloy showed time dependence of isothermal remanent magnetisation (IRM) measurements, which is a signature associated with canonical SG. Measurement of field dependence of magnetisation did not show saturation at the lowest temperature or highest magnetic field because of the lack of long-range magnetic ordering which is also a SG signature. Preliminary measurements indicate the possible existence of magnetic frustration leading to the SG state in the (Cr84Re16)99.6Mn0.4 alloy.

Rare-earth orthochromites (RCrO3) with orthorhombically distorted perovskite (ABO3) structure exhibit a wealth of magnetic phenomena including temperature-induced magnetization reversal (TMR), spin-reorientation (SR), spin-flipping (SF) and exchange-bias (EB). These occur as a result of magnetic interactions among the cations such as Cr3+–Cr3+, Cr3+–R3+ and R3+–R3+, where R is the rare earth element. In the present work, GdCrO4 samples were prepared using the sol-gel technique. The as-synthesized samples are amorphous in nature. Calcination of the samples at 600 °C for 1 h leads to the formation of the GdCrO4 phase whereas increasing the calcination temperature to 1000 °C for 1 h triggers the decomposition of GdCrO4 into GdCrO3. The role of thermal decomposition of GdCrO4 to GdCrO3 on crystal structure and magnetic transitions are discussed in this paper. Microstructure analyses show that the GdCrO4 powders have an intercalated porous structure that comprises a core-shell like construction with undistinguishable grain boundaries, whereas the morphology of the GdCrO3 demonstrates bulk nature with particles having micrometer size. Magnetization measurements as a function of temperature (M-T) with different probing magnetic fields show the ferromagnetic Curie temperature, (TC), of the GdCrO4 sample to be 24 K. GdCrO3 orders antiferromagnetically with a Neel temperature, TN (Cr), ascribed to the Cr magnetic moment at 171 K. A spin-flip transition (TSF) occurred at 22 K indicating the flipping of Cr3+ and Gd3+ spins and the spin-reorientation (TSR) transition is located at 5 K. The hysteresis loops measured across the transition temperatures validate the magnetic transitions as observed in the M-T curves.

In this report, x-ray photoelectron spectroscopy data for three types of Cu-doped nickel chromite nanoparticles are presented. CuCr2O4, Cu0.5Ni0.5Cr2O4, and NiCr2O4 were prepared using the citric acid-assisted sol-gel method. All samples were amorphous after the initial gel preparations were dried, but after calcination at temperatures higher than 700 °C, the samples crystallized in a spinel structure. X-ray diffraction patterns for the samples were well fitted with the I41/amd space group. Survey scans were conducted as well as detailed scans of Cu 2p, Ni 2p, Cr 2p, and O 1s levels. The O 1s core level was well-fitted with three components, including lattice oxygen (low binding energy) and two higher binding energy components, which corresponded to surface-absorbed hydroxyl groups originating from water and alcohol used in the synthesis of the particles. The line shape of the Cu 2p core level was compatible with mixed 2+ and 1+/0+ oxidation states for Cu ions in these nanoparticles. The strong intensity of the high binding energy satellite indicated that the majority of the Cu 2p ions were in 2+ oxidation state. Finally, Ni 2p showed a mixed 3+/2+ oxidation state for Ni ions in these nanoparticles.

Ni doped CoCr2O4, (Co1–xNix)Cr2O4 with x=0.5 and 0.25, nanoparticles were synthesized through sol-gel technique. The as synthesized samples were calcined at 600 °C for 1h. The phase identification of the samples was carried out using X-ray diffraction (XRD) techniques. The Davg for (Co0.5Ni0.5)Cr2O4 and (Co0.75Ni0.25)Cr2O4 were found to be 33 (1) nm and 22 (1) nm, respectively, from the transmission electron microscopy (TEM). Magnetic properties of the nanoparticles were measured down to 2 K using a vibrating sample magnetometer (VSM). Magnetization as a function of temperature measured under 0.1, 0.5 and 1 T applied magnetic field show a hump like feature at the peak temperature, Tp, similar to the superparamagnetic blocking feature observed in many nanoparticle systems. With increasing applied magnetic field the Tp shifts towards lower temperature. Magnetization as a function of applied magnetic field (μ0H= 0.1 T) showed an exchange bias like feature for x= 0.5, whereas no shifting of the loop was observed for x=0.25 sample. The transition related to spiral ordering at temperature TS was observed for both the samples, whereas no lock-in transition at temperature TL was noticed.

Cubic spinel CoCr2O4 has recently attained attention due to its multiferroic properties. However, the Co site substitution effect on the structural and magnetic properties has rarely been studied in thin film form. In this work, the structural and magnetic properties of Co1-x Ni x Cr2O4 (x= 0, 0.5) epitaxial thin films deposited on MgAl2O4 (100) and MgO (100) substrates to manipulate the nature of strain in the films using pulsed laser deposition (PLD) technique are presented. The epitaxial nature of the films was manifested through x-ray diffraction (XRD), reciprocal space mapping (RSM) and Rutherford backscattering spectrometry (RBS) measurements. Raman measurements revealed a disappearance of characteristic A 1g and F 2g modes of the CoCr2O4 with increase in the Ni content. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) studies show a modification of the surface morphology upon Ni substitution. Magnetic measurements disclose that the ferrimagnetic Curie temperature (T C) of the CoCr2O4 in thin film grown on MgAl2O4 (100) and MgO (100) substrates were found to be 100.6 ± 0.5 K and 93.8 ± 0.2 K, respectively. With Ni substitution the T C values were found to be enhanced to 104.5 ± 0.4 K for MgAl2O4 (100) and 108.5 ± 0.6 K for MgO (100) substrates. X-ray photoelectron spectroscopy (XPS) suggests Cr3+ oxidation states in the films, while Co ions are present in a mixed Co2+/Co3+ oxidation state. The substitution of Ni at Co site significantly modifies the line shape of the core level as well as the valence band. Ni ions are also found to be in a mixed 2+/3+ oxidation state. O 1s core level display asymmetry related to possible defects like oxygen vacancies in the films.

Three spinel compounds, namely CoCr2O4, (Co0.75Ni0.25)Cr2O4, and (Co0.5Ni0.5)Cr2O4 were investigated using x-ray photoelectron spectroscopy techniques. These compounds were synthesized using sol-gel techniques. The gel was dried and subsequently calcined at 600 °C to achieve crystallinity and the correct cubic phase. The survey scans of the analyzed samples with detailed spectra of the Co 2p, Ni 2p, Cr 2p, O 1s, and the valence band were collected. The core levels are found to be asymmetric and have multiple components. Ni and Co have 2+/3+ oxidation states, whereas Cr prefers 3+ oxidation state. The O 1s core level demonstrates three components. The low binding energy component refers to the lattice oxygen atoms, whereas the two high binding energy components correspond to the surface adsorbed hydroxyl groups resulting from water and the alcohol used for the synthesis of these nanoparticles. The data emanating from this study are reported.

NiCr2O4 and CoCr2O4 are two distinct members of the chromite family (ACr2O4) having unique structural and magnetic properties. This paper is a continuation of previous work [Mohanty et al., J. Magn. Magn. Mater. 451(2018)20] and is focused on the modification of ferrimagnetic properties in NiCr2O4 by substituting Co ions at Ni sites. In order to do so (Ni1−xCox)Cr2O4 (x = 0, 0.50, 0.75, 1.00) powder samples were synthesized using chemical co-precipitation techniques. Interest is primarily on (Ni0.5Co0.5)Cr2O4 and (Ni0.25Co0.75)Cr2O4 as characterization results on the end members have been reported. It was determined with in-situ temperature dependent X-ray diffraction measurements that these compositions become fully crystallized cubic spinels with space group Fd 3 - m above 700 °C. All samples were subsequently calcined at 900 °C to ensure phase purity and uniform crystallinity. Transmission electron microscopy showed non-uniform distribution of particle sizes with the majority of the particles having bi-pyramidal structures with flat tops for both samples. A comparative account of different ferrimagnetic Curie temperature, θ f or TC, estimations are presented as determined from measured temperature dependences of the following techniques: Fits of the magnetic susceptibility data indicated a minimum value for the (Ni0.5Co0.5)Cr2O4 sample at 56.1 ± 0.9 K, followed by an increase to 76.8 ± 0.4 K for the (Ni0.25Co0.75)Cr2O4 sample; Neutron diffraction measurements gave values of 80 ± 1 K and 87 ± 1 K; Their paramagnetic Curie points, θ p , were determined to be 73.3 and 84.9 K, respectively. The magnetic frustration index f = ΘCW/TC was found to decrease continuously over the series from 11.9 for NiCr2O4 to 7.1 and 6.5 for x = 0.5 and 0.75 respectively with (Ni0.5Co0.5)Cr2O4 being more frustrated than (Ni0.25Co0.75)Cr2O4. Magnetization as a function of applied magnetic field measured at 3 K demonstrates unusually high coercivity for (Ni0.5Co0.5)Cr2O4, supporting the notion for the higher degree of magnetic frustration in this sample. No exchange bias was observed for M (μ0H) in the (Ni0.25Co0.75)Cr2O4 sample measured at 3 K, but did however show a significant anomaly in its hysteresis loop at this temperature. The anomaly in hysteresis behaviour disappeared when measured at 1.7 K and also at higher temperatures. Frequency dependent ac-susceptibility measurements revealed no shift in the peak position with frequency refuting the existence of spin-glass like behavior in both these samples.

International audience; The temperature dependence of exchange bias properties are studied in polycrystalline BiFeO3/Ni81Fe19 bilayers, for different BiFeO3 thicknesses. Using a field cooling protocol, a non-monotonic behavior of the exchange bias field is shown in the exchange-biased bilayers. Another thermal protocol, the Soeya protocol, related to the BiFeO3 thermal activation energies was carried out and reveals a two-step evolution of the exchange bias field. The results of these two different protocols are similar to the ones obtained for measurements previously reported on epitaxial BiFeO3, indicating a driving mechanism independent of the long-range crystalline arrangement (i.e. epitaxial or polycrystalline). An intrinsic property of BiFeO3 is proposed as being the driving mechanism for the thermal dependent magnetization reversal: the canting of the BiFeO3 spins leading to a biquadratic contribution to the exchange coupling. The temperature dependence of the magnetization reversal angular behavior agrees with the presence of such a biquadratic contribution for exchange biased bilayers studied here

Coupled ferromagnetic and antiferromagnetic bilayers are an important class of materials that allow manipulating magnetic properties, including the interfacial exchange bias phenomenon. Bismuth ferrite, BiFeO 3, is the most studied single-phase magnetoelectric multiferroic due to its unique ferroelectric and antiferromagnetic orderings well above room temperature. We report on a systematic experimental study regarding the direct correlation between the Bi 2O 3 parasitic phase concentration in the BiFeO 3 and the magnetic properties of the polycrystalline heterostructure BiFeO 3/Ni 81Fe 19 deposited via magnetron sputtering. It was found that the macroscopic exchange field, that arises from exchange bias coupling, is zero for phase-pure BiFeO 3 and increases up to 18 Oe on increasing the concentration of Bi 2O 3. This trend is in agreement with the azimuthal behavior of the magnetization reversal. The structural characterization also indicates that phase-pure BiFeO 3 has a disordered mesoporous structure. The influence of the Bi 2O 3 parasitic phase and mesoporous state, that is known to exist and introduce defects in the polycrystalline bilayers of BiFeO 3/Ni 81Fe 19, on the magnetization reversal and exchange bias coupling is reported for the first time in this study.Coupled ferromagnetic and antiferromagnetic bilayers are an important class of materials that allow manipulating magnetic properties, including the interfacial exchange bias phenomenon. Bismuth ferrite, BiFeO 3, is the most studied single-phase magnetoelectric multiferroic due to its unique ferroelectric and antiferromagnetic orderings well above room temperature. We report on a systematic experimental study regarding the direct correlation between the Bi 2O 3 parasitic phase concentration in the BiFeO 3 and the magnetic properties of the polycrystalline heterostructure BiFeO 3/Ni 81Fe 19 deposited via magnetron sputtering. It was found that the macroscopic exchange field, that arises from exchange bias coupling, is zero for phase-pure BiFeO 3 and increases up to 18 Oe on increasing the concentration of Bi 2O 3. This trend is in agreement with the azimuthal behavior of the magnetization reversal. The structural characterization also indicates that phase-pure BiFeO 3 has a ...

Nanoparticles of (Co1–xNix)Cr2O4, with x = 0.5 and 0.25, were prepared utilizing the sol-gel technique, in order to investigate the effect of Ni substitution at the Co site. The crystal structure of the prepared samples was identified using X-ray diffraction. Transmission electron microscopy images indicate a non-uniform distribution in particle sizes. Temperature dependent magnetization measurements as a function of probing field demonstrate different magnetic transition temperatures to that of both the parent compounds. The magnetization as a function of applied magnetic field shows a wasp-waist like feature for (Co0.5Ni0.5)Cr2O4 nanoparticles measured at 10 K, which is absent in both NiCr2O4 and CoCr2O4. This feature diminished for other measurement temperatures below the Curie temperature and was also absent at all temperatures for the (Co0.75Ni0.25)Cr2O4 nanoparticles. X-ray photoemission spectroscopy results show that the Ni cations prefers the 3+ and Co the 2+ oxidation states, while that of Cr was found to be 3+. However, mixed oxidation states were observed for Ni and Co in both samples, which can influence the magnetic properties.

Dimensionality effects on epitaxial and polycrystalline Cr 1− x Ru x alloy thin films and in Cr/Cr–Ru heterostructures are reported. X-ray analysis on Cr 0.9965 Ru 0.0035 epitaxial films indicates an increase in the coherence length in growth directions (1 0 0) and (1 1 0) with increasing thickness ( d ), in the range 20≤ d ≤300 nm. Atomic force microscopy studies on these films shows pronounced vertical growth for d >50 nm, resulting in the formation of columnar structures. The Neel temperatures ( T N ) of the Cr 0.9965 Ru 0.0035 films show anomalous behaviour as a function of d at thickness d ≈50 nm. It is interesting to note that this thickness corresponds to that for which a change in film morphology occurs. Experiments on epitaxial Cr 1− x Ru x thin films, with 0≤ x ≤0.013 and d =50 nm, give T N – x curves that correspond well with that of bulk Cr 1− x Ru x alloys. Studies on Cr/Cr 0.9965 Ru 0.0035 superlattices prepared on MgO(1 0 0), with the Cr layer thickness varied between 10 and 50 nm, keeping the Cr 0.9965 Ru 0.0035 thickness constant at 10 nm, indicate a sharp decrease in T N as the Cr separation layers reaches a thickness of 30 nm; ascribed to spin density wave pinning in the Cr layers for d

The magnetic behaviour of a Cr80−xFe20Mnx alloy system with x=2, 7, 10, 13 and 22 has been investigated in the temperature range 2–400 K through measurements of magnetization, electrical resistivity, magnetoresistivity, specific heat and thermal expansion. The temperature vs. Mn concentration magnetic phase diagram of the system is rich in magnetic behaviour with ferromagnetic (FM), antiferromagnetic (AFM) and paramagnetic phase regions and a spin-glass (SG) region at the lowest temperatures. Phase transition temperatures amongst these different magnetic phases could be identified from well-defined anomalies of magnetic origin that are displayed by graphs of the above-mentioned physical properties as a function of temperature. The time relaxation of the thermoremanent, isothermal remanent and field-cooled magnetizations below and above the SG freezing temperature show unusual aspects. These relaxations do not follow the usual superposition principle that is expected for typical SG materials. Negative giant magetoresistance (GMR) is observed in the alloys at 4 K. The GMR initially increases sharply on increasing the Mn content in the alloy system, followed by a tendency towards a saturation negative value for concentrations of more than about 10 at% Mn. Low-temperature plots of Cp/T vs. T2, where Cp is the specific heat, present anomalous behaviour for alloys with x=2, 10 and 22. For x=2 the plot shows an upturn at the lowest temperatures that changes over to a prominent downturn for x=10 and 22. This behaviour is attributed to Fe concentration fluctuations in the alloys, confirming the theoretical model of Matthews.

Multiferroics (MF) based nanostructures combining ferroelectricity (FE) and antiferromagnetism (AF) are attracting a lot of interest in the scientific community. Indeed, a controlled magnetoelectric coupling in a single nanostructure will bring a direct way to control the magnetic state of the MF material. Therefore, MF materials are good candidates for use in nanodevices, like magnetic memories or high frequency devices [1-3]. However, most of MF materials possess a Neel temperature well below 300 K, reducing the number of candidates available for room temperature applications. Still, a good candidate for exchange coupling at room temperature is BiFeO 3 (BFO). An interesting use of MF would be in MF/Ferromagnet exchange coupled bilayers as the net magnetization could then be controlled through the coupling by the use of an electric field. Such coupling named exchange bias was discovered by Meiklejohn and Bean [4]. It results in a characteristic horizontal shift of the hysteresis cycle (bias), and can be interpreted as unidirectional anisotropy.

Measurements of the electrical resistivity ( ρ ) of a Cr + 1.2 at.% Ga alloy doped with V or Mn, are reported in the temperature range 6–900 K. These measurements were complimented with thermal expansion ( α ) measurements for the undoped Cr + 1.2 at.% Ga alloy in the temperature range 77–450 K. The measurements show interesting behaviour with dopant concentration in that the residual resistivity ( ρ residual ) does not vary smoothly with dopant concentration as expected. On the contrary, ρ residual shows three peaks as a function of dopant concentration. This behaviour is ascribed to impurity resonant scattering effects which are included in a theoretical model used to explain the observed temperature dependence of the resistivity for these alloys. The resistivity and thermal expansion coefficient of the Cr + 1.2 at.% Ga alloy behaves anomalously close to the ISDW–CSDW phase transition temperature ( T IC ). The temperature derivative of the resistivity shows a minimum while the α – T curve shows a peak at T IC in contrast to what is expected. The magnetic phase diagram of the (Cr + 1.2 at.% Ga)(V, Mn) alloy system is constructed from the results of the measurements.

Electrical resistivity, thermoelectric power and specific heat measurements are reported for a Cr+2.2 at% Al alloy single crystal, having an Al content close to the triple point on the magnetic phase diagram of the Cr–Al alloy system. A weak resistivity component of spin-density-wave origin, not previously observed in alloys near the triple point concentration, is isolated at low temperatures. Its presence is confirmed by the thermoelectric measurements. The specific heat measurements suggest a maximum in the density of states at the Fermi energy for alloy concentrations close to the triple point, a point that appears to be a particular type of critical point for the Cr–Al alloy system.

Measurements are reported of the electrical resistivity of dilute Cr-Si and Cr-Ga alloy single crystals in the temperature range 4 to 1000 K. Analyses of the data show large magnetic contributions to the electrical resistivity of all of the alloys at temperatures , where is the Neel temperature. These magnetic contributions are partly attributed to spin-fluctuation and temperature-dependent resonant impurity scattering effects. A hysteretic resistivity anomaly, ascribed to the effects of mixed incommensurate (I) and commensurate (C) spin-density-wave (SDW) states, was observed near the I-C phase transition temperature of . The resistivity of shows a small and unusual anomaly near the spin-flip transition temperature.

This study provides additional information to complement the magnetic phase diagram of the Cr 1− x Ru x alloy system in the higher Ru content region where the spin-density-wave (SDW) magnetism becomes suppressed. Results from the neutron diffraction investigation on a Cr+15 at% Ru polycrystalline sample reveal that the SDW ordering is commensurate to the underlying crystallographic structure for all temperatures below the Neel temperature ( T N ). The Neel temperature determined from the temperature dependences of the (1 0 0) magnetic peak intensity and the longitudinal wave velocity give T N =228 K. This temperature is significantly lower than was concluded from magnetic susceptibility and electrical resistivity studies that gave T N =270 K.

A magneto-elastic anomaly at 243 K, previously ascribed to the incommensurate to commensurate phase transition, in a Cr+1.9 at% Fe alloy single crystal has now been identified as being related to the stabilization of the wave vector to a constant value over an extended temperature region within the incommensurate phase.

A temperature dependent coherent magnetization reversal model is proposed for size-distributed assemblies of ferromagnetic nanoparticles and ferromagnetic-antiferromagnetic core-shell nanoparticles. The nanoparticles are assumed to be of uniaxial anisotropy and all aligned along their easy axis. The thermal dependence is included by considering thermal fluctuations, implemented via the N\'eel-Arrhenius theory. Thermal and angular dependence of magnetization reversal loops, coercive field and exchange-bias field are obtained, showing that F-AF size-distributed exchange-coupled nanoparticles exhibit temperature-dependent asymmetric magnetization reversal. Also, non-monotonic evolutions of He and Hc with T are demonstrated. The angular dependence of Hc with T exhibits a complex behavior, with the presence of an apex, whose position and amplitude are strongly T dependent. The angular dependence of He with T exhibits complex behaviors, which depends on the AF anisotropy and exchange coupling. The resulting angular behavior demonstrates the key role of the size distribution and temperature in the magnetic response of nanoparticles., Comment: Revised arguments in Introduction and last section

Thermal expansion and neutron diffraction measurements are reported on a Cr+0.16 at. % Pt alloy single crystal. Anomalous hysteresis behavior is observed in the temperature dependence of the magnetovolume (Δω) and of the magnitude of the spin-density-wave (SDW) wave vector (Q) below the incommensurate-commensurate SDW phase transition temperature. The hysteresis effects in Δω and Q seem to be related, but are not linked in theoretical models.

Measurements are reported of the pressure dependences of the elastic constants of Cr+0.16 at. % Ga and Cr+0.88 at. % Ga alloy single crystals through the Neel temperature of both crystals and through the incommensurate–commensurate transition temperature of the latter crystal. The long-wavelength acoustic-mode Gruneisen parameters, γp, which quantify the lattice vibrational anharmonicity, are calculated from the measurements. Both alloys show, depending on the temperature, unusually large positive and negative values of γp which are ascribed to strong coupling of the spin-density wave to the acoustic phonons. The nature of these interactions differs for the two alloys.

We have measured the antiferromagnetic spin-density-wave (SDW) order in Cr/Cr{sub 97.5}Mn{sub 2.5}(001) superlattices. The Mn doping creates a high Neel temperature layer that confines the incommensurate SDW order within the Cr layers. With temperature cycling we observe a transition from commensurate to incommensurate SDW order and discrete changes in the SDW period. We find that these transitions show significant hysteresis (up to 75 K) when the number of SDW nodes within the Cr layer changes by an odd number, while there is no hysteresis for changes of an even number of nodes. This results from the competition between maintaining the spin structure at the interfaces and introducing a spin slip at the nodes of the Cr SDW.

Measurements of the temperature (T) dependence of the magnetic susceptibility (χ) and electrical resistance (R) on an antiferromagnetic (AFM) (Cr84Re16)89.6V10.4 alloy are reported in order to probe the existence of quantum critical behaviour (QCB) utilizing static magnetic fields (H) as a tuning parameter. The results indicate that an increase in H suppresses TN in such a way that it varies exponentially with increasing H. R(T) measurements show evidence of possible superconducting (SC) behaviour below 1 K at H = 0 T. These results therefore indicate the coexistence of the AFM and SC phases in the (Cr84Re16)89.6V10.4 alloy.

Quantum criticality (QC) in spin-density-wave antiferromagnetic Cr and Cr alloy systems is a topic of current interest. In the present study, V was used as a tuning parameter to drive the Neel transition temperature (TN) of the (Cr90Ir10)100−yVy alloy series with 0 ≤ y ≤ 14.3 to zero and search for effects of QC in the process. The magnetic properties and possible QC behaviour (QCB) in this alloy system were investigated through electrical resistivity (ρ), specific heat (Cp), and susceptibility (χ) measurements as a function of temperature (T), indicating that TN is suppressed to zero at a critical concentration yc ≈ 9. The Sommerfeld coefficient (γ) is considered a key indicator of QCB and a peak is observed in γ(y) at yc on decreasing y through this concentration, followed by a sharp decreasing trend. This behaviour is reminiscent of that observed for γ of the prototypical Cr100−xVx QC system and allows for the classification of yc in the (Cr90Ir10)100−yVy alloy system as a possible QC point.

Measurements of the electrical resistivity and specific heat of (Cr 86 Ru 14 ) 1− x V x alloys are reported. The Neel temperature of this alloy system is driven to 0 K on doping with a critical V concentration x c = 0.104. The results suggest the existence of a quantum critical point at x c .

Possible quantum critical behaviour in the (Cr84Re16)100−yVy alloy system, with 0 ≤ y

The Cr-Si system offers an ideal reference alloy as a case study to follow the evolution of the well-known first-order magnetic transition in dilute Cr-Si by doping with Mo, which is iso-electronic with Cr. This alloy system, in addition, then also offers the opportunity to search for effects of quantum criticality (QC) associated with the process of driving the Neel transition temperature (TN) to zero. Previous investigations on the ternary (Cr97.8Si2.2)100−yMoy alloy system (0 ≤ y ≤ 9) indicated possible quantum critical behaviour at a critical concentration yc ≈ 3.8. The present paper extends these studies to additional concentrations in the alloy series, as well as to explore key indicators of QC, such as Hall coefficient (RH) and magnetic susceptibility (χ) measurements. Recent results indicate the possible existence of two critical points in the (Cr97.8Si2.2)100−yMoy alloy system. The first critical point at yc1 is postulated to be related to the concentration where the first-order transition gives ...

Measurements of the elastic constants and nonlinear acoustic properties of a Cr +1.9 at % Fe alloy single crystal are reported. The results indicate anomalous magnetoelastic coupling of the spin-density-wave with the acoustic phonons below and around the Neel temperature.

The magnetic behavior and possibility of quantum criticality in a (Cr0.98Si0.02)1−yMoy alloy system (0 ≤ y ≤ 0.09) is investigated through electrical resistivity, specific heat, and thermoelectric power measurements in a temperature range 2–350 K. Alloys with 0 ≤ y ≤ 0.011 depict first-order Neel transitions that give way to continuous Neel transitions for 0.011

An analysis of experimental data on the electrical resistivity, Hall coefficient and electronic specific heat coefficient of a quantum critical (Cr86Ru14)1−xVx alloy system, is reported. The results give information on the role of spin-density-wave effects on the zero temperature relationships between these physical properties.

Electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (κ) measurements on Cr1-xSix alloy single crystals with x = 0.005, 0.012 and 0.016 reveal novel features not previously detected in polycrystalline alloys. Our results prove thermal transport to be an experimental probe that is especially well disposed towards exposing spin-density-wave ordering in an itinerant system such as Cr-Si.