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We prepared ultrafine MnCo 2 O 4 particles using a facile sol–gel autocombustion method. The structural and vibrational characterizations confirmed the formation of a pure cubic spinel phase in as-synthesized MnCo 2 O 4 particles. The surface morphology analyses revealed the porous structure with an average particle size of 5 nm and uniform distribution of manganese (Mn), cobalt (Co), and oxygen (O) in the particles. We observed the presence of multiple valence states of Mn and Co, occupying both tetrahedral and octahedral sites, enhancing our insight into the material's electronic properties and structural configuration. Our magnetization studies unveiled a Curie Temperature (T C) of 185 K with ferrimagnetic ordering below T C , resulting in substantial magnetization (9.8 emu/g) and coercivity (3.46 kOe) at 2 K and weak magnetic behavior above T C. This is attributed to the finite size and surface effects. Furthermore, the incorporation of MnCo 2 O 4 nanoparticles and graphene oxide into a polyurethane foam matrix resulted in an excellent sound absorption coefficient of 90 % at 2000 Hz. This outstanding acoustic performance underscores the versatile applications of MnCo 2 O 4 nanoparticles in diverse noise mitigation applications. [Display omitted] [ABSTRACT FROM AUTHOR]

Polycrystalline samples of DyCo 9 Si 4 with the tetragonal LaFe 9 Si 4 -type structure have been synthesized by arc melting and high-temperature annealing. Results of the magnetization M and specific heat C p measurements show the existence of two magnetic anomalies at T C = 40 K and T m = 20 K. The transition at T C = 40 K is attributed to the ferromagnetic transition of the Co sublattice. The magnetic moments of Dy3+ appear to align gradually below T C , and almost fixed below T m , at which a broad maximum in C p has been observed. Field-dependent M at T = 2 K up to H = 7 T suggested that the magnetic moment of Dy3+ is coupled antiferromagnetically with those of Co, forming a ferrimagnetic structure. Magnetization measurements up to 50 T at low temperatures using a pulse magnet demonstrated a spin-flip transition around H = 20 T, above which the magnetic moments of Co and Dy are in parallel. The spin-flip transition becomes broad with increasing temperature but still remains at T C = 40 K, indicating a strong antiparallel coupling between Co and Dy moments even in the paramagnetic state. The magnetocaloric effect was evaluated from C p and M , and also from the sample-temperature variation in the quasi-adiabatic process with the pulsed magnetic field. The maximum entropy change for the field change from 5 to 0 T was observed at around T = T m with the value Δ S = 5 J/kg K. The maximum temperature of Δ S shifts to higher temperatures in high magnetic fields, resulting in the large value of Δ S = 24 J/kg K at T ∼ T C by the field change from 50 to 0 T. The present results demonstrate that the pulsed-field measurement is a powerful method to evaluate the magnetocaloric effect of materials. ● Physical properties of DyCo 9 Si 4 are reported for the first time. ● Step-like increase of the magnetization curve is observed at low temperature. ● Antiparallel coupling of Dy/Co magnetic moments remains in the paramagnetic state. ● Pulsed field measurement enables a quick and reliable magnetocaloric evaluation. ● Large magnetocaloric effect of 24 J/kg K is observed at T = 40 K. [ABSTRACT FROM AUTHOR]

A detailed study has been carried out of the static and dynamic magnetic properties of ferrimagnetic spinel MgFe 1.2 Ga 0.8 O 4 nanoparticles (NPs), including the measurements of low-field dc -magnetization M (T) under zero-field-cooled (ZFC) and field-cooled (FC) conditions and ac -susceptibility χ ac (T) at different frequencies (f) and amplitudes of the ac -field (H ac). Below, a Curie temperature (T C), a clear magnetic irreversibility (M ZFC (T) ≠ M FC (T)), as well as a broad peak in both M ZFC (T)– M FC (T) magnetizations have been observed. At T ≪ T C , the two distinct, frequency-dependent peaks at T p1 ≈ 9 K and T p2 ≈ 14 K in the imaginary part of the ac -susceptibility χ ac '' (T) have been found. Their positions have been consistent with those of the anomalies detected on the temperature dependence of the magnetic contribution to the heat capacity C mag (T), and a sharp decrease of the M ZFC (T) magnetization near ≈ 10–20 K. It was revealed that the fitting of the frequency dependence of the peaks' positions (T p1 (f) and T p2 (f)) in the imaginary part of ac -susceptibility χ ac '' (T) using the Neel-Arrhenius law yields unphysical values of the relaxation time (τ 0) and, therefore, these results ruled out a possibility of an existence of superparamagnetic (SPM) clusters in the system. On the other hand, the T p1 (f) and T p2 (f) dependences followed both the Vogel-Fulcher law and the power law. These observations indicate that, a complex magnetic state in MgFe 1.2 Ga 0.8 O 4 , realized in the low temperature region, may likely be related to a progressive freezing of magnetic clusters. [ABSTRACT FROM AUTHOR]

In 3 d − 4 f intermetallic compounds, a Co substitution for Fe usually strengthens the exchange interactions due to a shift of the Fermi energy to a region of the 3 d band with higher density of states. Here, we study the influence of Co on the magnetism of ferrimagnetic DyFe 5 Al 7 using magnetization measurements in static (up to 14 T) and pulsed (up to 58 T) magnetic fields. We find that the homogeneity range of DyFe 5− x Co x Al 7 is limited to x ≤ 2.5 . The Co substitution for Fe produces a strong detrimental effect on the 3 d − 3 d intrasublattice exchange interactions, which leads to a pronounced decrease of the Curie temperature and of the 3 d − 4 f intersublattice exchange field. A reduction of the density of states at the Fermi level might occur due to a broadening of the 3 d band. A decrease of the rare-earth contribution to the magnetic anisotropy energy is also observed with increasing Co content. This is attributed to a competition between the Fe and Co contributions to the crystal electric field at the Dy site. [ABSTRACT FROM AUTHOR]

This investigation presents the optical, structural, vibrational and magnetic studies of the cubic spinel Fe 2+ CrFeO 4 nano-ferrites, which transformed to maghemite structure by annealed at 250 °C for 3 h and to hexagonal hematite at temperatures T ≥ 500 °C in air. These nanomaterials were synthesized by the co-precipitation route and sintered at different temperatures. A split of the lattice constant a to a and c occurred for T ≥ 500 °C due to the transformation to hexagonal hematite, where c / a ≈ 2.7. The density and trend of crystallite size (R) values showed increase against T, whereas the calculated and experimental specific surface area, threshold energy, Debye temperature and elastic wave velocities showed decrease. The trend of dislocation density, inter-chain distance, distortion parameter, saturation magnetization (Ms), squareness, remnant magnetization, magnetic moment and anisotropy constant was decreased with T. The main IR absorption bands; ν 1 and ν 2 of the nanomaterials were observed in the IR spectra where their positions proved dependence on T. The porosity (P), strain (ɛ), force constants, elastic and stiffness constants and coercivity revealed dependence on T. Variation of all deduced parameters with T revealed the structure transformation from cubic to hexagonal hematite. The optical band gab energy (E g ) showed increase as T increases. E g , S and ɛ proved dependence on R, whereas Ms proved dependence on P. So, D x , Ms and Hc proved effect on the elastic wave velocities. [ABSTRACT FROM AUTHOR]

Near the paramagnetic to ferrimagnetic transition temperature, the dc magnetization data of two sulpho spinels, CoCr 2 S 4 and FeCr 2 S 4 , are analyzed by the modified Arrott plot, the Kouvel-Fisher method, log M vs log H, and the scaling analysis. Critical exponents β = 0.3946 ± 0.0064, 0.421 ± 0.04, γ = 1.101 ± 0.004, 1.041 ± 0.015, and δ = 3.77 ± 0.06, 3.512 ± 0.14 are obtained around the critical temperature ∼ 224.4 K and 166.5 K for CoCr 2 S 4 and FeCr 2 S 4 , respectively. Scaling analysis indicates that calculated critical exponents, as well as critical temperatures, are intrinsic to the systems. The values of critical exponents of CoCr 2 S 4 are between those predicted for a three dimensional Heisenberg model and those predicted by mean field theory. Probably, the observed large spin-phonon coupling and magnetocrystalline anisotropy caused by Co ion may play the key role in this critical behavior of CoCr 2 S 4 . The critical exponents of FeCr 2 S 4 are close to the mean field theory except for δ , which is higher than the theoretical value. The higher value of δ may arise due to the incomplete ferrimagnetic transition and presence of short range magnetic ordering above T C for FeCr 2 S 4 . [ABSTRACT FROM AUTHOR]

The effect of Mg substitution on structural, magnetic, and dielectric properties of Y-type hexaferrite Ba 0.5 Sr 1.5 Zn 2- x Mg x Fe 11 AlO 22 ( x = 0, 0.4, 0.8, 1.2, 1.6, 2.0) prepared by the solid state reaction method was investigated. The X-ray diffraction patterns are analyzed by Rietveld refinement method and the hexagonal structure with R 3 − m was confirmed. The magnetic transition temperatures associated with the longitudinal conical to proper screw phase transition ( T 1 ) and proper screw to ferrimagnetic phase transition ( T 2 ) can be modulated by varying Mg content. For x ≤ 1.2 samples, the transition temperature T 2 monotonically increases with increasing Mg substitution, suggesting the possibility that the helical spin order of Mg substituted Y-type hexaferrites is sustained up to higher temperatures than that of undoped samples. The saturation magnetization at room temperature increases to a maximum with Mg content x = 1.2 and then decreases with x . The variations of magnetic properties can be attributed to the difference in the preferential site for Zn and Mg and different distribution of Fe 3+ ions over tetrahedral and octahedral sites. The results of magnetic field dependence of dielectric constant, i.e., magnetodielectric effect, at various temperatures imply that the doped samples show magnetically induced ferroelectricity up to 250 K. The intrinsic magnetodielectric effect is observed in all the samples. It is worthy to note that the magnetodielectric effect measured below 200 K increases with increasing Mg content, which is probably attributed to the variation of magnetic structures. The Mg-doped hexaferrites Ba 0.5 Sr 1.5 Zn 2- x Mg x Fe 11 AlO 22 may be potential candidates for application in magnetoelectric devices. [ABSTRACT FROM AUTHOR]

Deep understanding and powerful control of the magnetization dynamics is of great importance for developing high speed spintronic devices. In this work, laser-induced ultrafast demagnetization process of Gd x (CoFe) 1- x alloy films has been extensively studied by using the fs-laser pump-probe technique. Interestingly, it is found that by adjusting the Gd content x and hence the intersublattice 3 d -4 f exchange coupling strength, the demagnetization dynamics varies from a fast one-stage decay to a continuous or a discrete two-stage decay, which is mainly resulted from the different energy transfer rates via the intersublattice exchange coupling and has been well simulated by considering the Elliott-Yafet type spin-flip scattering theory and the 4-Temperature model. Moreover, we observed that the second-stage decay time is strongly dependent on the external magnetic field, which is analyzed theoretically and suggests an origin of ultrafast giant magnetic cooling effect via the modulation of lattice-spin coupling. These studies are quite helpful for achieving the efficient manipulation of ultrafast magnetization behaviors. • A systematical modulation of the laser-induced ultrafast demagnetization processes of GdCoFe films has been achieved. • The energy transfer channels between 3 d and 4 f spins have been investigated in the ultrafast demagnetization evolution. • The field-dependent second-stage decays were observed, which attributed to the giant ultrafast magnetic cooling effect. [ABSTRACT FROM AUTHOR]

Polycrystalline sample of Bi 2 Cu 5 B 4 O 14 was synthesized and its structural and magnetic properties were investigated by means of temperature dependent x-ray diffraction, magnetization, and heat capacity measurements. Bi 2 Cu 5 B 4 O 14 is found to be a spin-1/2 layered compound with repeating Cu-pentamer units. DC magnetic susceptibility at high temperatures follows the Curie-Weiss law with a large and negative Curie-Weiss temperature θ CW ≃ −39 K, suggesting the dominant ferromagnetic interaction between Cu 2+ ions. A positive curvature in inverse susceptibility and a magnetization hysteresis with reduced ordered moment at T = 2.1 K pinpoint the ferrimagnetic nature of the compound with T C ≃ 25 K. The modified Arrott analysis shows that Bi 2 Cu 5 B 4 O 14 is a good example of Heisenberg ( β = 0.367 and γ = 1.388) ferrimagnet. Heat capacity data at low temperatures (i.e. in the ordered state) follows a power law ( γT α ) with a reduced exponent ( α ≃ 2.58). The change in magnetic entropy is calculated to be S mag ≃ 28 J/mol K at around 100 K which is close to the expected theoretical value of 28.81 J/mol K for Bi 2 Cu 5 B 4 O 14 . [ABSTRACT FROM AUTHOR]

The magnetic and magnetocaloric properties of the Th 6 Mn 23 -type structure Gd 6 (Mn 1−x Co x ) 23 (x ≤ 0.3) compounds are investigated from DC magnetization measurements. The Curie temperature of these ferrimagnetic materials evolves non-continuously with the Co content from T C = 489 K for x = 0 to T C = 118 K for x = 0.3. In relation with the lower magnetic moment of Co atoms compared with that of Mn atoms, the Co to Mn substitution leads to a decrease of the maximal magnetization, recorded at 5 K in an applied magnetic field of 9 T, from 54.7 μ B /f.u. for x = 0 to 36.3 μ B /f.u. for x = 0.3. These materials present an anomalous magnetocaloric effect, characterized by two broad –ΔS M maxima of moderate magnitude at T C and around 100 K, yielding to wide magnetocaloric response from T C down to the lowest temperature, and consequently, a relatively large cooling capacity q 10–350K (μ 0 ΔH = 5 T) reaching ∼ 3.7 J cm −3 for Gd 6 (Mn 0.875 Co 0.125 ) 23 . The results are analyzed and discussed in connection with previously published data. [ABSTRACT FROM AUTHOR]

An analysis of magnetic properties of powder of Mg-ferrite processed with a centrifugal mill was carried out. The starting ferrite powder was prepared by solid-state reaction at 1400 °C for 48 h. The powder:balls mass ratio was 1:14, and the container and balls were made of agate. The crystalline size and internal strain were evaluated by XRD patterns using Williamson–Hall and Scherrer methods. The powders get a nanometric scale for low milling time. The X-ray study showed that after 17 h of milling peaks corresponding to α-Fe2O3 appear. The TC were also evaluated by thermogravimetric analysis and correlate to crystalline size. Hysteresis curves are typical of soft magnetic materials with higher values of saturation magnetisation and lower coercivity. The starting sample has a saturation magnetisation of 39.2 emu/g and 0.4 Oe. At longer milling times, saturation magnetisation keeps nearly constant indicating the lack of inversion degree, while that coercivity increase up 576.7 Oe as a consequence of high density of internal stresses due to the mechanical grinding. [Copyright &y& Elsevier]

Tunability of structural and magnetic properties of Mn 3- x Co x Ga films is presented by Co substitution, where critical behavior emerges 0.37 ≤ x ≤ 0.56 exhibiting a transition from tetragonal hard ferrimagnetic with perpendicular magnetic anisotropy (PMA) to a cubic soft ferrimagnetic phase with in-plane magnetic anisotropy (IMA). In the critical regime, coexisting state of tetragonal and cubic phases possesses significantly low coercive field (H C = 1.9 kOe) with relatively low saturation magnetization (M S = 100~150 emu/cc) while maintaining the PMA. From first-principles calculations, moments of two Mn sites do not change upon Co substitution. However, moments of substituted Co almost vanish in tetragonal while giving around 1 μ B in cubic phase, which agrees with overall M S behaviors of the experimental results. These findings not only uncover the hidden magnetic state in Mn 3- x Co x Ga by altering the parallel and antiparallel coupling between Mn and Co, but also provide an additional degree of freedom for spintronic applications using Mn-based Heusler compounds. • Critical regime during the transition from hard ferrimagnetic tetragonal to soft ferrimagnetic cubic in Co-substituted Mn3Ga. • Dramatic reduction of the coercive field on remaining perpendicular magnetic anisotropy in the critical regime. • Compensation point near this critical regime due to a large reduction of the Co moments when substituted in Mn 3 Ga. [ABSTRACT FROM AUTHOR]