The investigation of the process of magnetization in FeNiBSi glasses by means of the Barkhausen jumps

The Barkhausen jumps occuring during the magnetization of the amorphous Fe-Ni-B-Si alloys have been studied by the measurements of the derivative of magnetization with respect to time (dM/dt). The peaks in dM/dt vs. applied field (H) are reproducible after averaging over some fifty cycles of magnetization. The investigation of the variations of dM/dt with the applied field (H) and with the instantaneous magnetization (M) accompanied with the same studies, but performed starting from different values of the initial remanence (M_ri), completes our earlier analysis of the process of magnetization in these alloys. It is shown that the model of Chikazumi used for the explanation of the variations of remanent magnetization, coercive field and loss with the maximum magnetization (M_m) is consistent with the observed Barkhausen jumps. The possibility of separation of contributions from particular domain walls to the process of magnetization in soft magnetic materials (which would enable the determination of the locations and strengths of the particular pinning centres) is briefly discussed.     

Coercivity of anisotropic γ-Fe2O3 particles at high temperatures

The coercivity of five different samples of anisotropic γ-Fe₂O₃ particles is studied in the temperature range 0°–600°C. It is found that their relative coercive force h _c = H _c (T)/H _c (0) is almost a linear function of the relative magnetization of the particles m _s = M _s (T)/M _s (0), where H _c (0) and M _s (0) are the values of H _c and M _s of the particles at 0°C. It is experimentally found that h _c = βm _s + α, where β = 1.103 ± 0.015 and α = ?0.114 ± 0.009. This character of the dependence of h _c on m _s suggests that, at high temperatures, H _c of anisotropic γ-Fe₂O₃ particles can depend on both their shape anisotropy and other factors. It is assumed that, as the temperature increases, anisotropic γ-Fe₂O₃ particles in a zero magnetic field are divided into small structurally and magnetically unstable nanoclusters with magnetization spontaneously changing its direction. As a result, H _c disappears near the Curie temperature, although the saturation magnetization of the particles in a field of 1 T is still retained at this temperature.     

Application of the Stoner-Wohlfarth model with interaction for the determination of the saturation magnetisation, anisotropy field, and mean field interaction in bulk amorphous ferromagnets

Magnetic hysteresis curves of bulk amorphous ferromagnet alloys of composition Nd₆₀Fe₃₀Al₁₀, Nd₆₀Fe₂₀Co₁₀Al₁₀ and Pr₅₈Fe₂₄Al₁₈ have been measured in applied magnetic fields up to 9 T at temperatures in the range 10-350 K. The behaviour of the demagnetisation curve in the first quadrant is interpreted using a mean field interaction model as proposed by Callen et al. [Phys. Rev. B 16 (1977) 263], which extends the Stoner-Wohlfarth model [Philos. Trans. Roy. Soc. A 240 (1948) 599] for a random distribution of non-interacting uniaxial grains. Application of the mean field interaction model enables the determination of the saturation magnetisation M_s, anisotropy field H_a, and interaction parameter d, and from these other magnetic parameters, such as the anisotropy constant, K, are deduced. For the three alloys, the temperature dependent behaviour of M_s, H_a, d and K over the range 20-350 K are found to be qualitatively similar, though there are quantitative differences. In all cases M_s increases with decreasing temperature, both H_a and K increase with decreasing temperature, reaching a peak in the range 75-120 K, and then decreasing, and d decreases approximately linearly as the temperature decreases. The physical mechanisms responsible for coercivity in these materials are discussed in the context of random anisotropy and a strong pinning model of domain walls.     

Phase composition and magnetic characteristics of Fe-filled multi-walled carbon nanotubes

The work addresses the correlation between the phase composition and the magnetic characteristics of aligned Fe-filled multi-walled carbon nanotubes (Fe-MWCNTs) grown by pyrolysis of ferrocene on oxidized Si substrates. In a combinatorial approach we exploited the extremely high gradients of the technological parameters temperature and ferrocene flow across the surface of a substrate positioned close to the reactor wall to obtain a large variation in the structural and magnetic properties of the Fe-MWCNTs. In this way, we established several clear correlations between the Fe-filling phase composition and the overall magnetic characteristics of the aligned Fe-MWCNTs. The α-Fe rich samples, which possess a more ordered graphitic sheet structure, a higher degree of preferred crystalline orientation of the metal filling and much larger metal crystallites in comparison with the carbide-rich samples, show a much stronger magnetic anisotropy with easy axis perpendicular to the substrate and unusually high values of the coercive field H_c and the saturation field H_s. The changes in the measured saturation magnetisation M_s and the H_c values correlate well with the variation of the α-Fe content and the filling crystallinity. A special annealing treatment of the samples causes a distinct increase of the α-Fe quantity and an increase of the measured average grain size. The respective magnetic characteristics show a significant increase of the overall magnetic moment and decrease of the coercive field. The correlation between the structural and the magnetic characteristics of the annealed samples matches quite well the respective correlations in the case of as-deposited samples.     

Magnetic properties of Fe<Subscript>3</Subscript>C ferromagnetic nanoparticles encapsulated in carbon nanotubes

The low-temperature dependences of magnetic characteristics (namely, the coercive force H _c , the remanent magnetization M _r , local magnetic anisotropy fields H _a, and the saturation magnetization M _s ) determined from the irreversible and reversible parts of the magnetization curves for Fe₃C ferromagnetic nanoparticles encapsulated in carbon nanotubes are investigated experimentally. The behavior of the temperature dependences of the coercive force H _c (T) and the remanent magnetization M _r (T) indicates a single-domain structure of the particles under study and makes it possible to estimate their blocking temperature T _B = 420–450 K. It is found that the saturation magnetization M _s and the local magnetic anisotropy field H _a vary with temperature as ～T ^5/2.     

Size and anisotropy effects on magnetic properties of antiferromagnetic nanoparticles

Based on the Heisenberg model taking into account single-ion anisotropy and using a Green's function technique we have studied the influence of size and anisotropy effects on magnetization M, Neel temperature T_N, coercive field H_c and spin excitation energy of antiferromagnetic nanoparticles. The properties are compared with those of ferromagnetic nanoparticles. We have shown that the enhanced magnetization M and coercive field H_c of antiferromagnetic nanoparticles is a surface effect, which is due to uncompensated surface spins. Moreover, the shape of the coercive field curve can be significantly influenced by surface magnetic anisotropy.     

Size effects and magnetoelastic couplings: a link between Hall–Petch behaviour and coercive field in soft ferromagnetic metals

Size effects regarding Hall–Petch (HP) relation are studied in this work for cobalt, nickel and Fe–3wt.%Si (FeSi), from polycrystalline to multicrystalline states. The materials show a breakdown in HP plot for thickness (t) to grain size (d) ratio less than a critical value. This appears in the beginning of plasticity for cobalt and FeSi whereas a plastic strain threshold must be overcome for nickel. Measurements of the coercive field on strained samples are able to depict such modification for low t/d ratio. Values of the coercive field in the polycrystalline domain allow an estimation of the magnetocrystalline anisotropy energy, related to the grain volume fraction concerned by reversal mechanisms for magnetic domains. Multicrystalline samples of cobalt and FeSi becomes magnetically softer at the yield stress. This is linked to a delay of the maximum intergranular stress towards higher strains for FeSi. For cobalt, non-linear elasticity and strong basal texture modify the magnetoelastic effects in coarse grain samples. For nickel, size effect on the coercive field appears after a few per cent of plastic strain as for HP relationship. A mean internal stress can be captured by magnetic measurements on polycrystals, related to the intragranular part of the kinematic stress. The softening of the magnetic properties for strained nickel multicrystals is due to a competition between the apparition of dislocation cells, which increases the coercive field by mechanisms of magnetic domain wall pinning, and surface softening of multicrystals, which tends to decrease the value of H_c.     

Magnetic Properties of Composite Materials Based on a Solid Solution of Type (CuInSe<Subscript>2</Subscript>)<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript>(MeSe)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> (Me = Mn,Fe) and Polyethylene

Basic magnetic characteristics (coercive force H_c, residual magnetization M_r, magnetization M, and saturation magnetization M_s) of solid solutions of type (CuInSe₂)_1–x(MeSe)_x (Me = Mn, Fe) have been investigated in a wide temperature interval (100–300 K). The existence of a magnetic phase transition has been established for all studied solid solutions at low temperatures, and the Néel temperatures have been determined from the temperature dependences of the magnetization. It is shown that the temperature dependences of coercive force H_c and of magnetization M can be described using the thermal relaxation (fluctuation) theory.     

Magneto-optical studies of epitaxial EuS films on silicon

An improved simple method for magneto-optical display of ferromagnetic domains by means of laser light is reported. As an example, characteristic domain patterns are presented and discussed, which develop during the magnetization reversal of ferromagnetic EuS-films grown epitaxially on silicon substrates. The critical field strength for domain nucleation, H_K, the coercive field, H_c, the saturation field, H_s, and the direction and width of the domains have been studied as function of angle varied between the magnetical easy and hard axes of (110)- and (111)-EuS films, and as a function of temperature between 6 and 14.5 K. By extrapolating the linear decrease of H²_K,cs vs. T to zero the Curie temperature of the (110)- and (111)-EuS films has been determined to (15.3±0.2) K and (14.5±0.2) K, respectively.     

Magnetoresistance of single Co nanowires

Polycrystalline single Co nanowires are prepared by electron beam lithography on GaAs substrates at room temperature. The width of the Co nanowires is varied between 150 and 4000 nm. Magnetoresistance measurements are carried out in a temperature range between 1.5 and 45 K applying magnetic fields μ₀H up to 4.5 T parallel and perpendicular to the current direction. The in plane (longitudinal) magnetoresistance (MR) shows pronounced features at magnetic fields H_c (coercive fields) indicating the magnetization reversal process. From the MR-curves we determined H_c as a function of the angle α between current and field direction (from in plane to out of plane) and of the width w of the Co nanowires. The H_c=H_c(α,w) behavior allows to discuss the reversal process in more detail.     

The relationship between the transition voltage of the I–V curve of the ferroelectrics and the coercive field of the P–V hysteretic curve

The relationship between the transition voltage of the I–V curve of the ferroelectrics and the coercive field of the P–V hysteretic curve is calculated. The first mathematical analysis to explain the relation between the transition voltage V_t and the coercive voltage V_c is obtained. The origin of the interrelation between the transition voltage of the I–V curve and the coercive field is that the height of the boundary barrier is inversely proportional to the effective dielectric constant of the near-boundary region, which is dependent on a derivative of polarization on the electric field, ∂P/∂E. The term ξ(eV_t) plus the term (en_b²δ/dN_dP_s)(eV_c) equals a constant. V_t is the function of E_g, P_s, V_c, and E. There is a linear relation between V_c and V_t. This relationship will induce the matchable relations between the I–V curve and the E–P loop. As long as the V_c of the V–P loop exists, the correspondent V_t of I–V curve will certainly exist. It will be the foundation of a new ferroelectric memory, which operates by the I–V characteristics. These relations are the conditions that can enable nonvolatile memory and nondestructive readout.     

Temperature dependence of effective mobility edge and inelastic scattering time in silicon MOS inversion layers in strong magnetic fields

Temperature (T) dependence on Hall conductivity (σ_xy) in Si MOS inversion layers measured in 15 T at T=1.4?10 K is investigated by comparing dσ_xy/dN_s, N_s electron concentration, to the calculation based on an effective mobility edge (E_c) model. Temperature dependence of inelastic scattering time is discussed in connection to the T-dependence of E_c.     

Effect of a high magnetic field on the morphology and magnetic properties of the MnBi compound during the Mn1.08Bi-MnBi phase transformation process

Effect of a 10 T high magnetic field on the morphology and magnetic properties of the MnBi compounds during the Mn_1.08Bi-MnBi phase transformation has been investigated. Results indicate that the field has split the MnBi crystal along the (0 0 1)-crystal plane during the Mn_1.08Bi-MnBi phase transformation process and the split MnBi crystals align and aggregate along the magnetic field direction. Along with the change of the MnBi phase morphology, the magnetic property changes greatly. Indeed, with the alignment and aggregation of the spit MnBi phases, the saturation magnetization M_s and the magnetic susceptibility χ increase, and the coercive field H_c and the remnant magnetization M_r decrease. This implies that a high magnetic field may have caused the magnetic property of the MnBi phase to transform towards soft magnetism. Above results may be attributed to the enhancement of the magnetization and the Mn_1.08Bi-MnBi phase transformation in a high magnetic field.     

Effects of ion doping on magnetism of antiferromagnetic nanoparticles

Based on the Heisenberg model including single-ion anisotropy and using a Green's function technique we have studied the influence of doping effects on magnetization M, Neel temperature T_N and coercive field H_c of antiferromagnetic nanoparticles. We have shown that the experimentally obtained room temperature magnetization M is due to surface or/and doping effects in antiferromagnetic nanoparticles.     

Coercivity of single-domain nickel wires

The coercivity H_c of compacts of thin (0.1?5 μ diameter) nickel wires embedded in a silver matrix has been measured as a function of the effective wire radius d_eff. H_c increases linearly with 1/d_eff², in agreement with Aharoni's theory of magnetization reversal by curling. From the slope, δH_{c/gd (deff^?2)}, upper and lower bounds for the exchange constant A of Nickel have been derived (77K): 1.5 × 10^?11 Wsec/m ? 3.0 × 10^?11 Wsec/m.     

Measurements of Hk and Ms in thin magnetic films by the angular dependence of the planar Hall effect

It is shown that the angular dependences of the planar Hall effect measured with infinite magnetic field and with magnetic field H⩾H_k have an intersection point and this fact is enough for measuring the anisotropy field H_k applying the method presented by Pastor, Ferreiro and Torres in J. Magn. Magn. Mat. 53 (1986) 349, 62 (1986) 101. The scaling of the Hall tension U proportional to M²_s in mV/Am^-1 gives a possibility for calculating the M_s-values of the films. These assumptions are verified for NiFe- and NiFeGe films with a uniaxial magnetic anisotropy.     

Pressure effect on magnetization for (CoTm)90Zr10 (Tm = Cr,Mo) amorphous alloys

Temperature dependences of the forced volume magnetostriction dω/dH and the saturation magnetization σ_s for (CoTm)₉₀Zr₁₀ (Tm = Cr, Mo) amorphous alloys have recently been measured by the 3-terminal capacitance method and a vibrating sample magnetometer and magnetic balance at temperatures from 77 K to the Curie temperature T_c or the crystallization temperature. The pressure coefficient of σ_s0 at 0 K, d ln σ_s0/dp, is estimated from (dω/dH)₀ extrapolated to 0 K using the thermodynamical relation. The values of d ln σ_s0/dp increase in negative value with increasing Tm concentration. The relation between d ln σ_s0/dp and the pressure coefficient of T_c, d ln T_c/dp, estimated indirectly from dω/dH is discussed.     

Production of muon pairs in the continuum region by 39.5 GeV/cπ±, K±, p and p beams incident on a copper target

General features of the inclusive hadroproduction of muon pairs of mass greater than 1.5 GeV/c² are shown. For incident π^?, the data extend up to M=5.7 GeV/c² (i.e.M²/s≈0.4). The scaling cross section M³dσ/dM for π^?N reactions is much flatter than that for pN reactions and exceeds the latter by two to three orders of magnitude at large M²/s.     

Magnetic properties and formation of Sr ferrite nanoparticle and Zn,Ti/Ir substituted phases

Strontium hexaferrite nanoparticles are prepared by the chemical sol–gel route. Specific saturation magnetization σ_s and coercive field strength H_c are determined depending on the heat treatment of the gel and iron/strontium ratio in the starting solution. These ultrafine powders with single-domain behavior have specific saturation magnetization σ_s=74 emu/g and coercive field strength H_c=6.4 kOe. Experimental results show that it is necessary to preheat the gel between 400 and 500°C for several hours . It can prevent the formation of intermediate γ-Fe₂O₃ and help to obtain ultrafine strontium ferrite single phase with narrow size distribution at a low annealing temperature. Additionally, the magnetic properties of sol–gel derived strontium ferrite with iron substituted by Zn²⁺, Ti⁴⁺ and Ir⁴⁺ are discussed. For an amount of substitution 0<x⩽0.6, the (Zn, Ti)_x substituted strontium ferrite shows higher values of both coercive field strength and saturation magnetization than the (Zn, Ir)_x substituted phase.     

Measurement of π−p → π0n at large momentum transfer

New results on a high statistics measurement of pion-nucleon charge exchange scattering at 40 GeV/c, extending in momentum transfer up to ?t = 1.8 (GeV/c)², are reported and compared with an optical impact parameter model, together with previous data for the reaction π^?p → ηn at the same energy. The imaginary part of the pole trajectory b₀(s) is determined from the slope of the tangent to the maxima of $\text{(?t)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{d}\text{σ}\text{d}\text{t}$. The linear increase of Im b₀(s) with log s, which has been observed at low energies, continues up to 40 GeV/c.