Magnetic Studies in Sputtered Ni/Ti Multilayers

Magnetic properties of Ni/Ti multilayers, prepared by the DC triode sputtering method, have been studied by magnetic measurements. Both metal layers are crystalline with a (111) fiber structure when they are thicker than 20 Å. The magnetization decreases with a decrease in Ni layer thickness t _Ni and the analysis of the results at 5 K indicates the presence of a dead Ni layer about 13 Å thick. The effective anisotropy K _eff of Ni/Ti multilayers is obtained using a torque magnetometer. Spin-wave theory has been used to explain the temperature dependence of the magnetization. Approximate values for bulk exchange interaction J _b, surface exchange interaction J _S and interlayer coupling strength J _I for various Ni layer thicknesses have been obtained.     

Ferromagnetic and spin wave resonances in thin layer of expanded austenite phase

Four samples of austenite coatings deposited by reactive magnetron sputtering on silicon substrate at four different temperatures and pressures were investigated by ferromagnetic resonance (FMR) method at room temperature. The expanded austenite phase S (γ _N ) layers with thickness in the 160–273 nm range and concentration of magnetic atoms: 72 % Fe, 18 % Cr and 10 % Ni, were obtained. The coatings with nanometric size grains were strongly textured and grown mostly in [100] direction, perpendicular to the sample surface. Intense FMR spectra were recorded at various angles between the static magnetic field direction and the sample surface. A strong magnetic anisotropy of the main uniform FMR mode was observed and the effective magnetization 4πM _eff determined. Spin wave resonance (SWR) modes were observed in all investigated samples in out-of-plane geometry of the magnetic field. The resonance fields of SWR modes in our samples varied linearly with the spin wave mode number. The value of the effective magnon stiffness constant was determined assuming a parabolic shape of the magnetization variation across the sample thickness.     

Dynamic Magnetic Hysteresis Behaviors in a Mixed Spin (3/2, 2) Bilayer System with Different Crystal-Field Interactions

Dynamic magnetic hysteresis (DMH) behaviors of the mixed spin-3/2 and spin-2 Ising bilayer system with different crystal-field interactions on a two-layer square lattice is studied by the use of dynamic mean field calculations based on the Glauber-type stochastic. The hysteresis loops are obtained for different reduced temperatures (T), magnetic field amplitudes (h), frequencies (w) and interlayer coupling constants (J ₃). Influences of the T, h, w and J ₃ on the DMH properties are investigated. We also study the temperature, frequency and interlayer coupling interaction dependence of the coercive field and remanent magnetization. We compare our results with some theoretical and experimental works and observe a quantitatively good agreement with some theoretical and experimental results.     

Methods of Extracting Current Density Dependence of the Effective Activation Energy <Emphasis Type="Italic">U</Emphasis><Subscript>eff</Subscript>(J) in High <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Superconductors

We have reviewed the methods of extracting current density dependence of the effective activation energy U_eff(J) from experimental data, including transport measurements and magnetic relaxations. Then we applied the method proposed by Maley etc. on our single-phase HgBaCaCuO-1223 sample to obtain the effective activation energy. The effective activation energy U_eff(J, H = 1~T) is extracted from the magnetization relaxation data. On the other hand, U_eff(J) can be theoretically estimated for the model of a sinusoidal washboard potential in superconductors. By comparing the two results we believe that the single curve obtained in the former way can be seen as real current density dependence of effective activation energy U_eff(J). In addition, we have analyzed the reasons why the magnetic decay data at various temperatures can be scaled onto a single curve. The pinning mechanism in the measured temperature range does not change, and the activation energy depends separately on the three variables: T, B, and J are thought as two important factors for this. In the temperature close to zero and near T_c, thermally assisted flux motion would no longer valid since other processes predominate.     

Effect of Al Substitution in Structural and Magnetic Properties of MnCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

Single-phase samples of Mn(Cr_1?x Al _x )₂O₄ (x = 0 – 0.30) with cubic spinel structure were prepared and the lattice constant is found to decrease from a = 8.4396 Å for x = 0 to a = 8.3801 Å for x = 0.30. The substitution of Al at Cr site is confirmed from the blue shift of Raman modes. Magnetization measurements and analysis show all the prepared samples exhibit ferrimagnetic transition with transition temperature in the range of 46 K for x = 0 to 33 K for x = 0.30. The saturation magnetization (M _s ) and the estimated anisotropy constant (K) show an anomalous behavior up to x = 0.10 and beyond that they decrease monotonously. They are explained by considering different site preferences of Al ³⁺ ions as the doping concentration is increased. The theoretical and experimental effective magnetic moment of the samples is found to be comparable and it decreases with increase in Al concentration.     

Critical Behavior Near the Paramagnetic to Ferromagnetic Phase Transition Temperature in Sr<Subscript>1.5</Subscript>Nd<Subscript>0.5</Subscript>MnO<Subscript>4</Subscript> Compound

The magnetic properties and the critical behavior in Sr_1.5Nd_0.5MnO₄ have been investigated by magnetization measurements. The magnetic data indicate that the compound exhibits a second-order phase transition. The estimated critical exponents derived from the magnetic data using various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical magnetization isotherms M (T _C, H). The critical exponent values for this compound was found to match well with those predicted for the mean-field model (δ = 2.212 ± 0.124, γ = 0.975 ± 0.018, and β = 0.502 ± 0.012) at T _C = 228.59 ± 0.17. The critical exponent γ is slightly inferior than predicted from the mean-field model. Such a difference may be due, within the context of the quenched disorder and essentially the presence of the Griffiths phase. The temperature variation in the effective exponent (γ _eff) is similar to those for disordered ferromagnets.     

Ferromagnetism and Anti-ferromagnetism in Nano-films with Alternate Crystal Fields: Monte Carlo Study

Using Monte Carlo simulations, the magnetic properties of the Ising model based on a nano-film formed by alternating layers are studied. The effect of a reduced crystal field with the anti-ferromagnetic and ferromagnetic reduced exchange interactions is deduced for fixed system sizes: N= 9 and 10 layers. Indeed, the anti-ferromagnetic reduced exchange interaction increases when one increases of the number layers, for odd or even values of N. The total magnetization increases with increasing of the anti-ferromagnetic and the ferromagnetic reduced exchange interactions. It is found that the total magnetization remains almost constant for large values of layer numbers (N).     

Effect of the Mn/Fe Ratio on the Microstructure and Magnetic Properties in the Powder Form (Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>P System

The structure, morphology, and magnetic properties of the mechanically alloyed iron manganese phosphides (Fe_1?x Mn _x )₂P with 0.15 ≤ x ≤ 0.75 (Mn/Fe ratio = 0.17, 0.33, 0.66, and 3) have been studied by means of X-ray diffraction, scanning electron microscopy coupled with energy-dispersive X-ray spectrometry, and BS1 and BS2 magnetometry. The powder form (Fe_1?x Mn _x )₂P compounds exhibit multiphase structures that contain Fe(Mn)-type solid solution and Fe₂P-type, Mn₂P-type, Fe₃P-type, and MnP/FeP-type phosphides. The magnetization versus temperature reveals the existence of multiple magnetic phase transitions. The saturation magnetization, coercivity, and squarness M _r/M _s ratio values are discussed as a function of both the Mn content and the temperature. From the approach to saturation magnetization studies, several fundamental magnetic parameters were extracted. The local magnetic anisotropy constant K ₁ was determined.     

Structural,Morphological, Optical and Magnetic Properties of Al-Doped CoFe<Subscript>2</Subscript><Emphasis Type="Bold">O</Emphasis><Subscript>4</Subscript> Nanoparticles Prepared by Sol–Gel Auto-Combustion Method

CoFe_2?x Al _x O₄ (x = 0.0,0.5,1.0, and 1.5) ferrite nanoparticles have been synthesized by the sol–gel auto-combustion method. The effect of non-magnetic Al content on their structural, morphological, optical, and magnetic properties was also investigated. X-ray diffraction (XRD) diffraction analysis was applied and indicated that the synthesized nanopowders of samples with x<1.5 and calcined at 800 ^°C have single-phase spinel structure. It has shown also by increasing Al content, the particle size, lattice parameter, unit cell volume, coercivity, anisotropy constant, and magnetization decrease, while the energy band gap increases. The size of particles was measured by TEM being in the range of 65–75 nm (for x = 0.0) and 9–10 nm (for x = 1.0). For sample with x = 1.5, the minimum calcination temperature for obtaining a single-phase spinel structure was 1000 ^°C. By increasing the calcination temperature from 1000 to 1100 ^°C, the mean crystallite size and crystallinity increase, while the lattice parameter, coercivity, anisotropy constant, and magnetization decrease. The average grain size evaluated by SEM analysis was found to be \(\tilde 91\) and 166 nm for samples calcined at 1000 and 1100 ^°C, respectively.     

Effect of Annealing Temperature on the Microstructure and Magnetic Properties of MnGa Films

MnGa films are the promising magnetic recording materials and spintronic materials owing to their intrinsic properties, such as large magnetic anisotropy, high coercivity, moderate magnetization, and high spin polarization. In this paper, MnGa films with high coercivity and low surface roughness have been successfully fabricated onto MgO substrates by magnetron sputtering and post-annealing. Moreover, the effects of post-annealing temperature (T _a) on crystalline structure, surface morphology, and magnetic performances of MnGa films have also been investigated. It is found that the crystallization temperature for MnGa films is 400 ^°C. With increasing T _a, the crystallization degree enhances and an in-plane texture is formed. The grain size and surface roughness of MnGa films increase slowly when T _a is below 500 ^°C, but they exhibit a rapid rise when T _a is above 500 ^°C. As T _a increases, the coercivity (H _c) and remanence squareness ratio (S) for MnGa films improve monotonically, whereas saturation magnetization (M _s) increases firstly and then drops. The increases in H _c, S, and M _s with T _a are attributed to the grains’ growth and the improvement of crystallinity, and the decrease of M _s at higher T _a possibly is due to the partial oxidation of Mn.     

Ferromagnetism in Fe-doped BaTiO<Subscript>3</Subscript> Ceramics

Polycrystalline samples of BaTi_1?xFe_xO₃ (x = 0.00–0.30) are prepared by solid-state reaction method and their structural and magnetic properties are studied. Detailed investigation of XRD patterns reveal the coexistence of tetragonal (space group P4mm) and hexagonal phases (space group P6 ₃/mmc) for x ≥ 0.1. Magnetic measurements reveal room-temperature ferromagnetism in x = 0.15–0.3 samples, and their ferromagnetic transition temperature increases from 397 K for x = 0.15 to 464 K for x = 0.3. The initial magnetization curves for x = 0.15–0.3 are analyzed in terms of bound magnetic polaron (BMP) model. The analysis of susceptibility data in the paramagnetic region by Curie-Weiss law confirms the ferromagnetic transition for x ≥ 0.15 and the effective magnetic moment systematically increases with increase in Fe concentration.     

Magnetization reversal and tunable exchange bias behavior in Mn-substituted NiCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

Single phase samples of Ni(Cr_1?xMn _x )₂O₄ (x = 0–0.50) were synthesized by using sol–gel route. Investigation of structural, magnetic, exchange bias and magnetization reversal properties was carried out in the bulk samples of Ni(Cr_1?xMn _x )₂O₄. Rietveld refinement of the X-ray diffraction patterns recorded at room temperature reveals the tetragonal structure for x = 0 sample with I4₁/amd space group and cubic structure for x ≥ 0.05 samples with \( {\text{Fd}\bar{3}\text{m}} \) space group. Magnetization measurements show that all samples exhibit ferrimagnetic behavior, and the transition temperature (T_C) is found to increase from 73 K for x = 0 to 138 K for x = 0.50. Mn substitution induces magnetization reversal behavior especially for 30 at% of Mn in NiCr₂O₄ system with a magnetic compensation temperature of 45 K. This magnetization reversal is explained in terms of different site occupation of Mn ions and the different temperature dependence of the magnetic moments of different sublattices. Study of exchange bias behavior in x = 0.10 and 0.30 samples reveals that they exhibit negative and tunable positive and negative exchange bias behavior, respectively. The magnitudes of maximum exchange bias field of these samples are found to be 640 and 5306 Oe, respectively. Exchange bias in x = 0.10 sample originates from the anisotropic exchange interaction between the ferrimagnetic and the antiferromagnetic components of magnetic moment. The tunable exchange bias behavior in x = 0.30 sample is explained in terms of change in domination of one sublattice moment over the other as the temperature is varied.     

Exchange Coupling in MnAlC/α-Fe Nanocomposite Magnets

Nanostructured composite materials consisting of exchange-coupled hard and soft magnetic phases are proposed as alternative for the development of high-energy product permanent magnets. In this work, we have examined the effects of soft magnetic α-Fe addition on the structure and magnetic properties of powders composed of hard magnetic Mn_54.3Al₄₄C_1.7 compound. The optimum melt-spun ribbon precursor (with τ-phase structure, with magnetization of 88 emu/g and coercive field of 1.6 kOe) was obtained after annealing the ribbons at 500 ^°C for 20 min. After the combination between the soft and the optimized hard phase, the intensity of τ-phase peaks measured by X-ray diffraction decreases. These changes can also be seen in the magnetic properties. The coercivity (～ 500 Oe) tends to decrease with the annealing temperature, while the magnetization tends to increase up to 141 emu/g. Evidence of good exchange coupling between particles of Mn_54.3Al₄₄C_1.7 and α-Fe, in the produced composite, was proved by the hysteresis loop and its corresponding Thamm-Hesse analysis.     

Electronic and Magnetic Investigations on Fe<Subscript>2</Subscript>NiTe Alloy with High Curie Temperature

Full-potential local-orbital minimum-basis and spin-polarized relativistic Korringa-Kohn-Rostoker along with Monte Carlo simulations are applied to study the electronic and magnetic properties of Fe₂NiTe with Hg₂CuTi structure. The analysis of orbital population shows the electrons of 4s, 3d, 4d, and 4p from transition metal and 5s, 5d, and 5p from Te atom participating in bonding. It is demonstrated by the density of states of d-d and p-d hybridizations. Calculations show the magnetic moment is carried mainly by Fe atoms. Accordingly, the exchanges of Fe constituents play a leading role in interactions. By using the calculated Heisenberg exchange coupling parameters, the Curie temperature is estimated to be 761.38 K within mean-field approximation. In order to obtain more accurate value of the Curie temperature, Monte Carlo method is adopted to model the normalized magnetization as functions of the temperature, the obtained 507.93 K value is noticeably higher than the room temperature, which is favorable in realistic spintronics application. Finally, the magnetic moments, exchange interactions, and Curie temperatures in the range of 5.4 to 6.5 Å are calculated; the results implies the Curie temperatures are still above room temperature between given lattice intervals.     

Spin State of Cobalt and Electrical Transport Mechanism in MgCo<Subscript>2</Subscript>O<Subscript>4</Subscript> System

MgCo₂O₄ samples were synthesized by inverse co-precipitation method. The formation of a single-phase spinel structure was confirmed by X-ray diffraction measurements and Fourier-transform infrared spectroscopy. The samples crystallized in a face-centered cubic structure with Fd-3m space group as revealed from the Rietveld refinement of X-ray diffraction data. Magnetic measurements carried out in a broad temperature range of 5–300 K showed antiferromagnetic to paramagnetic phase transition (Neel temperature) observed at 101 K. Magnetic susceptibility data fitted using the Curie Weiss law and effective Bohr magnetic moment (μ_eff) for Co atoms was determined. Calculated μ_eff comes out to be 3.05 μ_B. These results were correlated to the spin states of Co³⁺ atoms. A small hysteresis in the field-dependent magnetization MH loop taken at 5 K indicates the existence of weak ferromagnetism in this system. The electrical resistivity measurement in the temperature range 77–750 K displayed the semiconducting-like behavior for this system.     

Study of the Magnetic Properties of Ni/Ag Superlattice

Thin multilayer films of alternating ultrathin Ni and Ag layers (L(Ni)=11,15,30 Å, bulk and \(L(\mathrm{Ag})=50~{\AA})\) have been prepared by evaporation in ultrahigh vacuum under controlled conditions and have been studied by the magnetic measurements. The critical temperature T _C is studied as a function of the surface exchange interaction (J _S). The dependence of T _C on the thickness L of the film has been investigated. A critical value of the surface exchange interaction in the film, above which the surface magnetism appears, is obtained. The shift of the critical temperature T _C(L) from the bulk value \([\frac{T_{\mathrm{C}}(\infty )}{T_{\mathrm{C}}(L)}-1]\) can be described by a power law L ^?λ , where \(\lambda =\frac{1}{\nu_{\mathrm{b}}}\) is the inverse of the correlation length’s exponent. The effective critical exponent associated with the magnetization M(β) is deduced for different thicknesses of Ni layers, and the thickness L(Ag) was being kept constant at 50 Å.     

Studying the mechanism of exchange coupling in ferro/ferrimagnet NiFe/DyCo film structures

Dependence of the magnetic and magnetooptical properties of an exchange-coupled NiFe/DyCo bilayer system on the thickness (t _DyCo) of a magnetically hard layer has been studied. It is established that the unidirectional anisotropy vanishes at t _DyCo ～ 400 Å, while the coercive field in the magnetically soft layer becomes comparable to the exchange-induced field shift. In this case, the DyCo layer magnetization is almost parallel to the film plane, whereas a reference DyCo film exhibits a perpendicular anisotropy. A model of the magnetic state of layers in the ferro/ferrimagnetic layer structure under consideration is proposed, which assumes that a 180° domain wall is formed at the interface upon magnetization reversal in the magnetically soft layer.     

First-Principles Studies of the Magnetic Anisotropy of Monolayer VS<Subscript>2</Subscript>

First-principles calculations are performed to study the magnetic anisotropy of monolayer VS₂. The magnetic anisotropy energy (MAE) for H-VS₂ is ?0.213 meV, and the magnetic preferential direction is in the monolayer plane, while the corresponding value for T-VS₂ is only 0.004 meV, which can be ignored for two-dimensional materials. According to the second-order perturbation of the spin–orbit coupling (SOC) interactions, the physical origin of magnetic anisotropy for H-VS₂ is derived from the occupied and unoccupied dxy/dx ²-y ² states with different spin channels in light of the electronic structure. In the reciprocal space, the negative contributions mainly stem from the corners of hexagonal Brillouin zone. Interestingly, there are non-equivalent K and K ^′ for the MAEs, which are observed for the first time for MAE in the reciprocal space. We predict that the lack of inversion symmetry results in the different signs of MAEs in the K and K ^′. Our studies open up broad prospects to trace the physical origin of magnetic anisotropy in the reciprocal space.     

Vortex Structure and Anisotropic Superconducting Gaps in Ba[Fe(Ni)]<Subscript>2</Subscript>As<Subscript>2</Subscript>

We studied nearly optimally Ni-substituted BaFe_2?x Ni _x As₂ (BFNA) single crystals with T _C ≈ 18.5 K. In irreversible magnetization measurements, we determined the field dependence of the critical current density and discuss the nature of observed strong bulk pinning. Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we directly determine two distinct superconducting gaps and resolve their moderate anisotropy in the momentum space. The BCS-ratio for the large gap 2Δ _L /k _B T _C > 4.1 evidences for a strong coupling in the Δ _L -bands.     

Effect of tensile stresses on the magnetoelasticity of Fe<Subscript>64</Subscript>Co<Subscript>21</Subscript>B<Subscript>15</Subscript> ferromagnetic ribbons

The effect of constant tensile elastic stresses on the field dependence of the magnetoelasticity (ΔE effect) of Fe₆₄Co₂₁B₁₅ amorphous ferromagnetic ribbons after thermomagnetic treatment in a temperature interval from 290 to 360°C has been studied. The maximum value of the negative ΔE effect increases upon application of a relatively small tensile stress and decreases under the action of large stresses. In addition, the application of tensile stresses decreases the magnetic field corresponding to a maximum negative ΔE effect. The results are explained based on notions about the influence of constant tensile stresses on the domain structure of ferromagnetic materials with positive magnetostriction and induced uniaxial anisotropy.