Temperature-dependent Magnetic Data Research Articles

Insights into crystal structure, temperature-dependent magnetization and dielectric relaxation mechanism in Bi substituted samarium iron garnet

The single phase Sm3−xBixFe5O12 with x=0.0,0.2,0.4 and 0.6 samples are synthesized by solid-state reaction method. We have systematically studied the structural, morphological, temperature dependent magnetic and electric properties. All the samples crystallize in simple cubic crystal structure and belongs to Ia3‾d space group. The lattice constant and bond angle between Fe(a)−O−Fe(d) network is enhanced with Bi substitution. As a result, the ferrimagnetic transition temperature is also enhanced from 565K to 569K. We have examined the applicability of Bloch's and Cojocaru's laws to our temperature-dependent magnetization data and find that Bloch's T3/2 law is not suitable due to geometrical asymmetry, long wavelength excitation, and local atomic disorders. However, Cojocaru's law which accounts for the geometrical aspects, provides a better fit to our magnetization data. Further, the impedance plots at room temperature exhibit no relaxation which is attributed to the limited mobility of charge carriers. On the other hand, the high temperature impedance data represents dielectric relaxation at a particular frequency. This frequency is used to determine the relaxation time which is fitted to Arrhenius law and activation energy is evaluated. The activation energy lies in between 0.24−0.32eV which corresponds to the singly ionized oxygen vacancies. The conduction mechanism is analyzed with Variable range hopping model and the average hopping length and hopping energies are also determined for these garnet samples.

Intertwined crystal structure, magnetic, and charge transport properties in mixed valent A-site ordered manganite NdBaMn2O6

In the present work, we report a correlation between crystal structure, magnetic, and electrical properties in an exotic magnetic compound, NdBaMn2O6 having ∼92% ordering between Nd and Ba, investigated using temperature-dependent synchrotron x-ray diffraction (XRD), dc magnetization and transport measurements. Temperature-dependent XRD data reveals that the compound undergoes various complex crystallographic phase transitions from high-temperature (T > 320 K) P4/mmm phase to intermediate (320 K – 280 K) Cmmm phase to a mixed (Cmmm and P21am) phase (280 K – 220 K) and to low-temperature P21am phase (T < 220 K). It is found that these crystallographic phase compositions play a crucial role in controlling its magnetic and transport properties. Temperature-dependent dc magnetization data show a sharp drop at the onset of mixed phase (Cmmm + P21am) at 280 K followed by a broad hump at ∼ 220 K where mixed phase to P21am transition occurs, thus indicating a correlation between the structural and magnetic properties. The dc magnetization in the mixed phase region is calculated by considering a superposition of the magnetic moments of Cmmm and P21am phases weighted by the fraction of each phase, which exactly follows the experimental magnetization data. Temperature variation of resistivity data shows a jump at ∼260 K, a temperature corresponding to 50–50% phase composition of Cmmm and P21am phases. The compound shows insulating behavior over a whole temperature range as confirmed from the resistivity data. Further, application of magnetic field causes a shift of magnetic and transport transition temperatures which may be due to the magnetic field induced structural transition in the system.

Observation of itinerant ferromagnetic behaviour in equiatomic medium entropy AlNiCo alloy

A medium entropy AlNiCo alloy has been synthesized by vacuum arc melting, which shows single phase B2 structure. Atom probe tomography analysis showed that the elements are homogeneously and randomly distributed in AlNiCo alloy without any clustering tendency. A detailed analysis of temperature and field dependent magnetization data suggests an itinerant ferromagnetic ground state of AlNiCo alloy. The magneto crystalline anisotropy is evaluated to be 1.03 × 105 ergs/cm3 at 5 K. A broad maximum at low temperature in zero-field-cooled (ZFC) magnetization, as well as irreversibility of ZFC and field-cooled (FC) curves indicated a glassy behavior. This observation is supported by the memory effects observed at low temperature. The observed anomalous magnetic behavior is discussed in terms of modifications in exchange interactions. To the best of our knowledge, this is the first report on microstructure and magnetic properties of an equiatomic ternary AlNiCo alloy.

Synthesis, structural, impedance, optical, Mössbauer characterizations and magnetic studies of Pb-containing germanate compounds

The orthorhombic antiferromagnetic germanate minerals Pb2Fe2Ge2O9 and Pb2Mn2Ge2O9 have been prepared via solid state reaction procedure. An extensive study of structural, dielectric, optical and magnetic properties of both the germanates have been investigated by employing different measurements such as X-ray diffraction, FTIR, SEM, impedance, UV–Vis–NIR & Mössbauer spectroscopic and magnetometary. Rietveld refinement of room temperature XRD patterns revealed that both the compounds crystallize in the orthorhombic space group Pbcn. FTIR spectrum exhibits the presence of M−O & Ge–O bonds which confirms strong chemical bonding. Morphological analysis through SEM showed good crystallinity of both the samples. The variation of dielectric constant, loss tangent and impedance with frequency have been studied and it has been established that both the germanates are semiconducting at 300K (RT) which may be attributed to the stacking of ions at grain boundaries. The optical band gaps estimated by UV–Vis–NIR spectroscopic measurements further confirms the semiconducting nature of the prepared germanates. 57Fe Mössbauer spectroscopic measurement on synthesized Pb2Fe2Ge2O9 sample was carried out at RT. The obtained values of IS and QS showed that iron is in Fe3+ high spin state (t2g3eg2,s=5/2) and occupy two different sites, Fe1 and Fe2. The field dependent magnetization measurements show that both the compounds display dominant paramagnetic nature at 300K. The temperature dependent magnetization data reveal very sharp first order type AFM to PM transition at TN ∼50K and TN ∼8K for Pb2Fe2Ge2O9 and Pb2Mn2Ge2O9, respectively, and weak FM nature below Neel temperature. The field dependent properties of both the compounds have been analyzed by drawing the M versus H plots at low and room temperatures. The values of calculated effective magnetic moment 6.07μB and 3.95μB for Fe3+ and Mn3+ ions are in the high spin states (t2g3eg2,s=5/2 and )t2g3eg1,s=2.

Structural, Morphological, and Magnetocaloric Properties of Nanocrystalline Pr0.7La0.3MnO3

Abstract The structural, morphological, magnetic, and magnetocaloric properties of nanocrystalline Pr0.7La0.3MnO3 (PLMO), synthesized by using a wet chemical Sol-gel method, have been investigated. The single-phase nature of nanocrystalline PLMO with orthorhombic crystal structure along with the Pbnm space group was confirmed by Rietveld analysis of the X-ray diffraction pattern. The average crystallite size and lattice microstrain were determined using the Williamson-Hall technique, and they were found to be ~ 28 nm and ~ 0.00137, respectively. Transmission electron microscopy shows the particle size and structural characteristics of the sample. Temperature-dependent magnetization data show paramagnetic to ferromagnetic transition. According to Banerjee criterion, the positive slope of Arrott's plot for nanocrystalline PLMO indicates the second-order magnetic phase transition. In a 5 T external magnetic field, the maximum magnetic entropy change (∆SM) was found to be 2.47 J/kg K. The refrigerant capacity and relative cooling power were calculated and the values are 193.3 and 277.57 J/kg, respectively, under a 5 T applied external magnetic field.

Weak itinerant magnetic behaviour in Al substituted Ni 92 Cr 8 alloys

In the present work we report low temperature magnetic properties of Ni92−xAlxCr8 alloys. It is found that the Curie temperature and spontaneous magnetization (MSP) get suppressed around xC ≈ 7. From the analysis of the temperature and field dependent magnetization data we have obtained high value of Rhodes–Wohlfarth ratio, which indicates the weak itinerant ferromagnetic behavior for samples below xC. This is further supported by analysing the temperature variation of MSP in the framework of spin fluctuations, SCR theory. Furthermore, the Ginzburg-Landau formalism-based on spin fluctuation theory revealed that the contribution of spin fluctuations at low temperature increases as we approach xC.

Evolution of microstructure and magnetic properties from amorphous Fe3O4/SiO2 nanocomposite

Amorphous Fe3O4/SiO2 composite particles have been synthesized by using a one-pot sol-gel route. The evolution of Fe3O4 nanoparticles from the amorphous matrix on annealing temperatures up to 600 °C is identified through structural and vibrational mode studies. Electron microscopy measurements confirm that particles of 2–4 nm are embedded in the SiO2 matrix. The detailed analysis of temperature and field dependent magnetization data shows that a weak magnetic ground state of the as-prepared sample develops into a single domain state on annealing at 600 °C. Annealed samples exhibit higher magnetization values of ∼27 emu/g and coercivity of ∼1927 Oe at temperature of 2.5 K yet negligible spontaneous magnetization, suggesting short range magnetic order. A suitable distribution model with single domain particle assemblies is employed to analyze the particle size and moment distribution. From the analysis of M-T and M-H data it is found that interacting and non-interacting SPM models explain the data in different tampretaure ranges. We show magnetic interactions and anisotropy can be effectively controlled by the size and interparticle interactions of Fe3O4 nanoparticles. These particles are useful for the design of thermal seeds for magnetic hyperthermia.

Effect of chemical disorder on Griffiths phase in weak itinerant ferromagnetic Ni92−xCuxCr8 alloy

We report magnetic and thermal properties of Cu substituted Ni92−xCuxCr8 polycrystalline alloys close to their critical concentration (xc~18%), where ferromagnetic order disappears. From the detailed analysis of field and temperature dependent magnetization data weak itinerant ferromagnetic character has been identified for the compositions below xc. Spin-fluctuations theory using the Ginzburg–Landau formalism further revealed the contributions of spin fluctuations, which increase near xc. Specific heat data shows unusual low-temperature variation with an enhanced Sommerfeld coefficient and non Fermi-liquid behavior due to strong magnetic fluctuations near xc. The magnetization data and the linear term of the specific heat follow the theoretical predictions of a ferromagnetic quantum critical point within the experimental uncertainties. The exponent of the power-law M(H) ∝Hλ fits to low-temperature magnetic isotherms and its variation with composition indeed suggests a possibility of quantum Griffiths phase in these alloy compositions. Finally, this study shows that the Cu substituted Ni-Cr system shows Griffiths phase at critical point even after the reduction of disorder.

Precise measurement of angles between two magnetic moments and their configurational stability in single-molecule magnets

A key parameter for the low-temperature magnetic coupling of in dinuclear lanthanide single-molecule magnets (SMMs) is the barrier $U_{FA}$ resulting from the exchange and dipole interactions between the two $4f$ moments. Here we extend the pseudospin model previously used to describe the ground state of dinuclear endofullerenes to account for variations in the orientation of the single-ion anisotropy axes and apply it to the two SMMs Dy$_2$ScN@C$_{80}$ and Dy$_2$TiC@C$_{80}$. While x-ray magnetic circular dichroism (XMCD) indicates the same $J_z=15/2$ Dy groundstate in both molecules, the Dy-Dy coupling strength and the stability of magnetization is distinct. We demonstrate that both the magnitude of the barrier $U_{FA}$ and the angle between the two $4f$ moments are determined directly from precise temperature-dependent magnetization data to an accuracy better than $1^{\circ}$. The experimentally found angles between the $4f$ moments are in excellent agreement with calculated angles between the quantisation axes of the two Dy ions. Theory indicates a larger deviation of the orientation of the Dy magnetic moments from the Dy bond axes to the central ion in Dy$_2$TiC@C$_{80}$. This may explain the lower stability of the magnetisation in Dy$_2$TiC@C$_{80}$, although it exhibits a $\sim 49\%$ stronger exchange coupling than in Dy$_2$ScN@C$_{80}$.

Magnetic structure of magnetoelectric multiferroic HoFeWO6

The polar magnetic oxide, HoFeWO6, is synthesized, and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoFeWO6 forms the polar crystal structure (space group Pna21 (#33)) due to the cation ordering of W6+ and Fe3+. An antiferromagnetic transition at TN = 17 K is accompanied by a significant change in magnetic entropy with a value of ≈ 5 J kg−1 K−1 in a 70 kOe applied field. Temperature dependent neutron diffraction and magnetization data indicate that the Fe sublattice orders in a strongly non-collinear and non-coplanar arrangement below TN. The Fe ordering initially leads to induced ordering of the Ho spins such that the Ho spins also show behavior of long-range ordering that is evident from the neutron diffraction measurements. Below T ≈ 4 K, the Ho spins order independently and pull the Fe spins toward the direction of Ho spins. A comparison with the magnetic structures and corresponding ferroelectric properties of other members of RMWO6 (R = Y, Sm-Tm, M = Cr, Fe, V) family indicate that the spontaneous polarization is due to the magnetic structure specific to the Fe sublattice through magnetoelectric coupling whereas the polarization is independent of the Ho sublattice.

Effect of Bi-doping on structural, magneto-caloric and magneto-resistive properties of La0.67- xBixCa0.33MnO3 perovskites

We report the doping effect of Bi on structural, magnetic and magneto-resistive properties of nanocrystalline La0.67-xBixCa0.33MnO3 (x = 0, 0.05, 0.1) (LBCMO) samples. Rietveld refinement of the X-ray diffraction (XRD) patterns revealed that all the three samples crystallize into orthorhombic crystal structure having Pbnm space group. The average crystallite size was calculated using Debye Scherer's formula and the crystallite size varies from 25 to 30 nm. Scanning electron microscopy (SEM) images shows that the particles are spherical in nature and distributed uniformly. The temperature dependent magnetization data infers that all the three samples undergo paramagnetic to ferromagnetic (PM-FM) phase transition with transition temperature (Tc) ~245 K, 250 K and 230 K upon cooling. La0.67Ca0.33MnO3 (LBCMO1), La0.62Bi0.05Ca0.33MnO3 (LBCMO2) samples exhibit second order magnetic phase transition, whereas La0.57Bi0.1Ca0.33MnO3 (LBCMO3) sample exhibit first order like transition. The maximum magnetic entropy changes |ΔSMmax| were calculated for nanocrystalline LBCMO samples using thermodynamic relations and the values were 3.73 Jkg−1K−1, 4.72 Jkg−1K−1 and 4.47 Jkg−1K−1 respectively, where field change was μoH = 5 T. As a consequence of temperature dependence of electrical resistivity, data shows metallic behavior before metal-semiconductor transition temperature (TMS) and semiconducting behavior after TMS. The magnetic field dependent electrical resistivity data reveals a huge change in resistivity near TMS and the maximum magnetoresistance (MR) value around ~60% for an applied field of 5T around the TMS for LBCMO3 sample. The coexistence of magnetocaloric effect and moderately high value of magnetoresistance makes the studied samples promising for multifunctional device applications near its critical temperature.

Enhanced magnetic and magnetodielectric properties of Co-doped brownmillerite KBiFe2O5 at room temperature

We report significant enhancement in magnetic and magnetodielectric (MD) properties of cobalt (Co) doped polycrystalline KBiFe2O5 (KBFO). Rietveld refinement of X-ray diffraction data confirms monoclinic crystal structure with P2/c space group. SEM micrographs confirmed the polycrystalline nature of the samples with grain sizes varying from 1 µm to 5 µm. Raman spectra demonstrate the shifting of vibrational modes due to Co doping which is associated with FeO4 tetrahedral. The shift in vibrational modes due to Co doping is in line with the observed changes in FeO4 tetrahedral bond angles (O1-Fe-O1 & O1-Fe-O2) and bond lengths (Fe-O1 & Fe-O2) obtained from XRD data. Room temperature magnetization data reveal the maximum enhancement in Mr (~103 times) and Ms (~200 times) values for 5% Co-doped KBFO compared with KBFO. This enhancement in magnetization data is due to a spin-canted (Fe3+) antiferromagnetic spin structure and increased lattice strain with increased cobalt substitution. The temperature-dependent dielectric and magnetization data show an anomaly near 770 K (Tc) indicates the MD effect. Compared to KBFO, nearly ten times enhancement in MD and nearly 50 times reduction in magnetic loss (ML) is observed for KBFCO5 measured at 30 kHz frequency. Landau free energy approach confirms the biquadratic nature of MD coupling, and the strength is − 8 (emu/g)2 for 5% Co-doped KBFO and − 5(emu/g)2 for 10% Co-doped KBFO, respectively. The comparative study reveals that Co substitution in KBFO enhances maximum MD% and magnetization in 5% Co substitution. These results establish the coupling between electric and magnetic orders, which can find application in AC/DC magnetic field sensors and ME solar cells.

Structural and magnetocaloric properties (0.75)La0.7Ca0.3MnO3/(0.25)La0.84Sr0.16MnO3 nanocomposite

We report the structural, magnetic, and magneto-caloric properties of as-synthesized (0.75)La0.7Ca0.3MnO3/(0.25)La0.84Sr0.16MnO3 nanocomposite material. The parent nanocrystalline samples synthesizes using pechini sol-gel route as well as sample's crystal structure was analyzed by Rietveld analysis of powder X-ray diffraction pattern. The nanocomposite sample showed two structural phases, which correspond to both the parent samples La0.7Ca0.3MnO3 (Orthorhombic structure, Pbnm space group) and La0.84Sr0.16MnO3 (Rhombohedral structure, R3‾c space group) respectively. The Williamson-Hall method was employed to calculate the crystallite size as well as strain in both parent samples. The values of crystallite size (D) and strain (ε) for La0.7Ca0.3MnO3 are 65 nm and 0.0009 respectively and for La0.84Sr0.16MnO3 are 37 nm and 0.0021. Homogeneity of the prepared compounds was confirmed with scanning electron microscopy (SEM) images. The temperature-dependent magnetization data displayed two successive transitions in the composite sample at 155 K and 255 K, which correspond to the parent samples. The respective maximum values of magnetic entropy change (−ΔSM) were observed to be 1.9 Jkg−1K−1 and 3.03 Jkg−1K−1 at 5 T magnetic field for La0.7Ca0.3MnO3 and La0.84Sr0.16MnO3. To assess the ordering of the samples, Landau's analysis was carried out that reveals a good concordance with our experimental results. The broadened temperature range was found in the composite sample, consequently, the isothermal entropy change is reduced. An enhancement of 20%–25% of relative cooling capacity is noticed in the composite sample as compared to the parent samples.

Synthesis, structure, magnetism, luminescence and DFT analysis of three metal-organic complexes based on 2,5-di(1H-1,2,4-triazol-1-yl)terephthalic acid

The three multi-dimensional coordination polymers, [Mn2(μ4-dttpa)(μ6-dttpa)(H2O)2]n(1), {[Co(μ4-dttpa)(H2O)2]·2H2O}n(2), and {[Zn3(μ4-dttpa)2(μ2-dttpa)(phen)2(H2O)2]·2H2O}n(3) (H2dttpa ​= ​2,5-di(1H-1,2,4-triazol-1-yl)terephthalic acid; phen ​= ​1,10- phenanthroline), are successfully synthesized and characterized by X-ray single crystal diffraction, powder X-ray diffraction (PXRD), FT-IR spectra, elemental analysis and thermogravimetric analysis. Structurally, complex 1 is a three-dimensional structure with the 3-nodal Schläfli point symbol of {4·63·82}2{42·62·82}{62·84}. Complex 2 displays a two-dimensional network with a 6-connected uninodal net. With the point symbol of {4·62}2·{43·63}2·{44·62}{6}, complex 3 shows the two-dimensional stacking pattern. Temperature-dependent magnetic data of complexes 1 and 2 indicate antiferromagnetic behaviors between adjacent metal ions from 2 ​K to 300 ​K. Further magnetic studies show that the best fitting parameters of comple×1 are obtained using the dinuclear Mn(II) model. From 50K to 300K, complex 2 displays antiferromagnetism over its mononuclear analogue with the zero-field splitting parameter (|D| ​= ​35.2 ​cm−1). The luminescent intensity of complex 3 is much stronger than that of free ligand (H2dttpa) and auxiliary ligand (phen). Moreover, the photoluminescence mechanism of complex 3 has been analyzed by theoretical calculation at ωB97XD/6–311++g(2d,p) level with DFT and TDDFT, which reveals that luminescence emission of 3 is attributed to the ligand-to-ligand charge transfer (π∗phen-πdttpa2-). Compared with the free ligand, the enhancement of the luminescent intensity of compound 3 might occur due to the construction of Zn(II) ion, dttpa2− and phen increases the efficiency of charge transfer between LUMO and HOMO in the solid state.

Tuning Exchange Coupling in NiO-Based Bimagnetic Heterostructured Nanocrystals.

A series of bimagnetic heterostructured nanocrystals having an antiferromagnetic NiO core and a ferrimagnetic MnxNi1-xO and/or FiM Mn3O4 island nanophase overgrowth has been synthesized under varying aqueous solution pH conditions. The two-step self-assembly process employs a thermal decomposition method to synthesize NiO nanoparticles, followed by growth of the MnxNi1-xO and/or Mn3O4 nanophase over the NiO core using hydrothermal synthesis at pH values ranging from 2.4-7.0. The environmentally benign hydrothermal process involves pH control of the protonation vs hydroxylation reactions occurring at the nanoparticle surface. TEM analysis and Rietveld refinement of XRD data show that three distinct types of heterostructured nanocrystals occur: NiO/MnxNi1-xO core-shell-like heterostructures at the pH of 2.4, mixed NiO/MnxNi1-xO and/or/Mn3O4 core-overgrowth structures for 2.4 < pH < 4.5, and predominantly NiO/Mn3O4 core-island structures for pH > 4.5. The magnetic coercivity and exchange bias of the heterostructured nanocrystals vary systematically with the pH of the aqueous solution used to synthesize the samples. The temperature-dependent magnetization and hysteresis loop data are consistent with the nature of overlayer coverage of the NiO core. Our DFT based calculations show that the MnxNi1-xO phase has ferrimagnetic properties with a stable spin orientation along the ⟨111⟩ orientation. Furthermore, the calculations show that the magnetic anisotropy constant (K1) of the Mn3O4 phase is considerably larger than that of the MnxNi1-xO phase, which is confirmed by our experimental results. The coercivity and exchange bias field are the largest for the NiO/Mn3O4 core-island nanocrystals, synthesized at a pH value of 5.0, with robust values of nearly 6 kOe and 3 kOe, respectively. This work demonstrates the tunability of hydrothermal deposition, and concomitant magnetic coercivity and exchange bias properties, of MnxNi1-xO and/or Mn3O4 nanophase overgrowth over a NiO core with pH, that makes these heterostructured nanocrystals potentially useful for magnetic device, biomedical, and other applications.

Effect of Ru substitution in tellurium bridged copper spin [formula omitted] system Ba2CuTeO6: Structure, magnetism and optical properties

In this paper we have explored and analyzed the crystal structure, magnetic and optical properties of Ru doped double perovskite compound Ba2CuTe1-xRuxO6 (x = 0, 0.05 and 0.1). Detailed analysis of powder x-ray diffraction data at room temperature confirms the formation of single phase with monoclinic crystal structure (space group C2/m). A careful deconvolution of Ru and Cu x-ray photoelectron spectra indicates the formation of mixed oxidation states: Ru (4+ and 5+) and Cu (2+ and 3+). Electron paramagnetic resonance (EPR) spectra also support the presence of EPR silent Cu3+ (S = 0) ions. A broad overturn around 66 K in temperature dependent magnetization data of pure compound gets gradually suppressed in Ru doped samples. Magnetic frustration decreases with increase of Ru doping concentration due to different magnetic competitive interactions. Optical properties (change of color) are influenced due to the formation of 4d (Ru) energy levels in the vicinity of Fermi level.

Weak itinerant ferromagnetism and non-Fermi liquid behavior in Ni-TM (TM = Cr, Nb) alloys near critical concentration

We report a comprehensive study on the magnetic, electrical and thermal properties of Ni100−x TM x (TM = V, Cr, Nb,) alloys around their critical concentration. Analysis of field and temperature dependence magnetization data suggests a weak itinerant ferromagnetic behavior in x = 8 and 10 compositions and the ferromagnetic ordering suppresses in the concentration range 10 < x < 12. Further, the temperature dependence of specific heat shows an unusual low temperature variation with an enhanced Sommerfeld coefficient, γ, with a signature of non-Fermi-liquid (NFL) behavior close to critical concentration. Further, the enhancement in Kadowaki–Woods ratio suggests it to be a strongly correlated electron system near critical concentration. Present analysis of experimental data consistently revealed that the NFL behavior is caused by spin fluctuations near critical concentration. The temperature dependencies of the electrical resistivity, the magnetization and linear term of the electronic specific heat appear to follow the theoretical predictions of a quantum phase transition and it is tempting to suggest that the presently studies Ni-rich alloys can be candidates for the observation of Griffith phase.

Aggregation vs Surface Segregation: Antagonism over the Magnetic Behavior of NiCr Nanoparticles.

Annealing is a valuable method for fine-tuning the ultrasmall magnetic properties of alloy nanoparticles (NPs) by controlling their sizes, modifying their surfaces, and affecting their magnetic interactions. Herein, we study the effect of moderate annealing (450 °C) on strongly interacting NiCr nanoparticle assemblies (0 ≤ atom % Cr ≤ 15) immediately after deposition. Concurrent temperature-dependent electron microscopy and magnetization data demonstrate the interplay of two competing factors, namely, enhanced particle aggregation and element-specific surface segregation, on the magnetic properties, with the former boosting and the latter suppressing them. Strong interparticle interactions can lead to a magnetic response different from that of superparamagnetic particles, namely, from canonical spin-glass (0 atom % Cr) to correlated spin-glass (5–15 atom % Cr) behavior below higher spin-glass transition temperatures Tg (20–350 K). The observation of “high-field susceptibility” below cryogenic temperatures (≤20 K) is ascribed to the presence of inhomogeneity/defects caused by Cr segregation. This work emphasizes the necessity of taking into account the delicate balance of such competing factors to understand the magnetic properties of nanoparticulate samples.

Critical behavior and magnetocaloric effect in Co-doped nanocrystalline La0.7Te0.3Mn1-xCoxO3

We have explored the impact of cobalt doping on magnetic, magnetocaloric and electrical transport properties of the nanocrystalline La0.7Te0.3Mn1-xCoxO3 (0.1 ≤ x ≤ 0.5) samples synthesized using sol-gel method. The Rietveld refinement of the X-ray diffraction (XRD) patterns confirmed the rhombohedral crystal structure for x ≤ 0.2 samples and orthorhombic crystal structure for x ≥ 0.3 samples. The temperature dependence magnetization data proposed that each sample experienced a transition in phase from paramagnetic to ferromagnetic. The maximum values of magnetic entropy change |ΔSMmax| were calculated to be 2.93 Jkg−1K−1, 2.62 Jkg−1K−1, 2.04 Jkg−1K−1, and 1.71 Jkg−1K−1 for ΔH = 5 T respectively for x = 0.1, 0.2, 0.4, and 0.5 samples. Detailed critical exponent analysis was done for each sample and the nature of magnetic phase transition for x = 0.1 follows the 3D Heisenberg model, while for x = 0.2 and 0.3 pursue the mean field model. For x = 0.4 and 0.5, none of the models describe completely the magnetic phase transition. The temperature-dependent electrical resistivity ρ(T) data exhibit semi-conducting like behavior within the studied temperature range for x = 0.1 and 0.2 compositions. The magnetoresistance (MR %) values for x = 0.1 and x = 0.2 respectively were 44%, and 35% at a field change of 5 T around their respective transition temperatures. Relatively high values of |ΔSMmax| and MR% made the studied samples suitable for multifunctional device applications.

Magnetocaloric Effect for NaFeO2 Nanoparticles

The magnetocaloric effect (MCE) of NaFeO2 nanoparticles has been investigated by phenomenological model (PM). The simulated temperature-dependent magnetization data are in good agreement with the experimental one. Based on PM, the MCE parameters obtained as results of simulation for temperature-dependent magnetization data. Furthermore, the simulation shows that NaFeO2 nanoparticles have a prospective importance in magnetic refrigerants over a wide temperature range, including room temperature. This gives NaFeO2 nanoparticles potentially practical for MR.