Microstructure characterization and magnetic properties of nanomaterials

The microstructure and superparamagnetic properties of two systems of magnetic nano-particles are reviewed. A new type of magnetic core-shell Ni—Ce nanocomposite particle (15–50 nm) has been prepared. Typical HREM images and FFT patterns of HREM images showed that many planar defects (nanotwins and stacking faults) exist in the large Ni core zone (10–45 nm). The shell layers (3–5 nm) consist of an innermost Ni—Ce alloy and an outermost NiO oxide. FFT patterns from different regions of typical HREM images show well defined spots characteristic of core-shell nanocomposite materials. Magnetization measurements as a function of magnetic field and temperature showed that superparamagnetic behaviour is exhibited above the average block temperature (T_B = 170 K). This superpara-magnetic relaxation was found to be modified by interparticle interactions that depend on the applied field and size distribution. In addition, antiferromagnetic order occurred with a Neél temperature T_N of about 11 K. A spin-flip transition was observed below T_N at a certain applied field. Novel carbon encapsulated Ni nanoparticles assemblies have been synthesized by modified arc-discharge under a methane atmosphere. The presence of carbon encapsulation is confirmed by HR-TEM lattice imaging, and nanodiffraction. The intimate and contiguous carbon fringe around these Ni nanoparticles is good evidence for complete encapsulation by carbon shell layers. Superparamagnetic property studies show that the blocking temperature T_B is around 115K at 0.1 T applied field. Above T_B, the magnetization M(H,T) can be described by the classical Langevin function L using the relation M/M_s(T = 0) = coth (μH/kT) — kT/μH. The particle size can be inferred from the Langevin fit (particle moment μ), which is a little larger than the HR-TEM observation. It is suggested that these assemblies of carbon encapsulated Ni nanoparticles have typical single-domain, field-dependent superparamagnetic relaxation properties, and this typical superparamagnetic behaviour is consistent with the Stoner—Wohlfarth theory of single-domain particles.     

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.     

Magnetic properties of CdSb doped with Ni

Magnetic properties of the group II–V semiconductor CdSb single crystals doped with Ni (2 at%) are investigated. Deviation of the zero-field-cooled susceptibility, χ_ZFC, from the field-cooled susceptibility is observed below 300 K, along with a broad maximum of χ_ZFC (T) at T_b in fields below the anisotropy field B_K∼4 kG. T_b(B) obeys the law [T_b(B)/T_b(0)]^1/2=1–B/B_K with T_b(0)∼100 K. The magnetization exhibits saturation above ∼20–30 kG, a weak temperature dependence and anisotropy of the saturation value M_s. The coercive field is much smaller then B_K and displays anisotropy inverted with respect to that of M_s. Such magnetic behavior is expected for spheroidal Ni-rich Ni_1−xSb_x nanoparticles with high aspect ratio, broad distribution of the sizes and with orientations of the major axis distributed around a preferred direction.     

The Langevin formula for describing the magnetization curve of a magnetic liquid

It has been shown that, at the initial stage of the magnetization curve, the magnetic susceptibility of magnetic liquid determined as χ = Mμ₀/B (M is the magnetization, B is the magnetic induction in a sample) obeys the Curie law, and the magnetic susceptibility determined as χ = M/H (H is the magnetic field intensity in a sample) obeys the Curie–Weiss law. Since the Curie law is a particular case of the Langevin dependence, it is assumed that an experimental magnetization curve is described by the Langevin formula with a Langevin parameter ξ = PB/kT, where P is the magnetic moment of a particle and T is the temperature. Experimental verification has shown that, at parameter ξ, the mean relative deviation between the values of M measured and calculated by the Langevin formula is 5%. This deviation can be accounted for by the influence of dispersion of the magnetic moments of nanoparticles.     

Superparamagnetic Ni:SiO<Subscript>2</Subscript>–C nanocomposites films synthesized by a polymeric precursor method

In this work, we report on the magnetic properties of nickel nanoparticles (NP) in a SiO₂–C thin film matrix, prepared by a polymeric precursor method, with Ni content x in the 0–10 wt% range. Microstructural analyses of the films showed that the Ni NP are homogenously distributed in the SiO₂–C matrix and have spherical shape with average diameter of ~10 nm. The magnetic properties reveal features of superparamagnetism with blocking temperatures T _B ~ 10 K. The average diameter of the Ni NP, estimated from magnetization measurements, was found to be ~4 nm for the x = 3 wt% Ni sample, in excellent agreement with X-ray diffraction data. M versus H hysteresis loops indicated that the Ni NP are free from a surrounding oxide layer. We have also observed that coercivity (H _C) develops appreciably below T _B, and follows the H _C ∝ [1 – (T/T _B)^0.5] relationship, a feature expected for randomly oriented and non-interacting nanoparticles. The extrapolation of H _C to 0 K indicates that coercivity decreases with increasing x, suggesting that dipolar interactions may be relevant in films with x > 3 wt% Ni.     

Hydrothermal synthesis and magnetic studies of transition metal nocerites M3(BO3)F3 (M=Fe,Co, Ni)

Polycrystalline samples of M₃(BO₃)F₃ (M=Fe, Co, Ni), isostructural with nocerite Mg₃(BO₃)(OH,F)₃, have been prepared in supercritical hydrothermal conditions. These compounds represent with boracites, M₃B₇O₁₃F (M=Mg, Cr, Mn, Fe, Co, Zn), the only transition metal fluoride borates known to date. Co₃(BO₃)F₃ and Ni₃(BO₃)F₃ are antiferromagnetic with T_N=17(2) and 40(2) K, respectively. Spin-flop transitions at B_C1=4.0 T and B_C2=7.5 T occur at 1.6 K in Co₃(BO₃)F₃, while a parasitic ferromagnetism (0.02 μ_B/Ni²⁺ at 1.6 K) appears below T_N in Ni₃(BO₃)F₃. The magnetic structures consist of three spin sub-lattices of double rutile-type ferromagnetic chains.     

Improvement of the remanence properties and the weakening of interparticle interactions in BaFe12O19 particles by B2O3 addition

In the present study, the effects of B₂O₃ addition on the remanence properties of barium ferrite magnets are examined. The relationship between isothermal magnetization remanence M_R(H) and demagnetization remanence M_D(H) for non-interacting single domain particles, M_D(H)=M_R(H_max)−2M_R(H), was used in order to investigate the interactions between particles. We have found that remanence magnetization M_R increased by 40% in magnitude with B₂O₃ addition in addition to the weakened couplings between particles. The B₂O₃ addition seems to supply the required conditions for usage of these materials in the magnetic recording media.     

Three-dimensional ordering of impurity-doped TMMC in a magnetic field

The three-dimensional ordering temperatures of the quasi-one-dimensional antiferromagnet (CH₃)₄NMnCl₃ containing 0, 0.13, 0.25 and 0.48 at% Cu²⁺ ions were studied as a function of an external magnetic field up to 60 kOe by means of proton NMR. A strong increase of T_N(H) with H was found even for the impure system. The T_N versus H curve for the pure system cam be explained by introducing a simple form for the effect of the field on the correlation length, ξ(T, H) = ξ(T) × [1+(λJ/kT)H²], while that for the impure system is unlike the one for the pure system.     

Influence of disorder on superconductivity in non-magnetic rare-earth nickel borocarbides

The effect of substitutional disorder on the superconducting properties of YNi₂B₂C was studied by partially replacing yttrium and nickel by Lu and Pt, respectively. For the two series of (Y, Lu)Ni₂B₂C and Y(Ni, Pt)₂B₂C compounds, the upper critical field H _c2(T) and the specific heat c _p(T, H) in the superconducting mixed state have been investigated. Disorder is found to reduce several relevant quantities such as T _c, the upper critical field H _c2(0) at T=0 and a characteristic positive curvature of H _c2(T) observed for these compounds near T _c. The H _c2(T) data point to the clean limit for (Y, Lu) substitutions and to a transition to the quasi-dirty limit for (Ni, Pt) substitutions. The electronic specific heat contribution γ(H) exhibits significant deviations from the usual linear γ(H) law. These deviations reduce with growing substitutional disorder but remain even in the quasidirty limit which is reached in the Y(Ni_1−x , Pt _x )₂B₂C samples for x=0.1.     

Quantization and confinement effects in superconducting nanostructures

The confinement of the flux lines by a lattice of submicron holes (‘antidots’) has been studied in nanostructured superconducting Pb/Ge multilayers. By introducing regular arrays of sufficiently large antidots, multi-quanta vortex lattices have been stabilized. Sharp cusp-like magnetization (M) anomalies, appearing at matching fieldsH_m=mφ₀/Sin superconducting films with the antidot lattices having a unit cell areaS, are successfully explained. These anomalies are, analogues of the well-knownM(H) cusp atH_c1, but for the onset of multi-quanta (m+1)φ₀-vortices penetration at each subsequent matching fieldH_m. It is shown that theM(H) curve between the matching fieldsH_m<H<H_m+1follows a simpleM∝ln(H−H_m) dependence. These experimental observations have revealed an unusual expansion of validity of the London limit in superconductors with lattices of relatively large antidots. The successful high quality fit of theM(H,T) curves convincingly demonstrates that a new type of the critical stateB=const (‘single-terrace critical state’) can be realized in superconductors with the antidot lattices.     

Spin-wave theory for finite classical magnets and superparamagnetic relation

Analytical calculations based on finite-size spin-wave theory and Monte Carlo (MC) simulations are performed to investigate the validity of the well-known relation m(H, T) = M(H, T)B _D[M(H, T) H/T] between the induced magnetization m of the magnetic particle and its intrinsic magnetization M for the Ising and isotropic classical models (B _D(x) is the Langevin function, D is the number of spin components, is the number of atoms in the particle). It follows from general arguments and from our analytical results for the Heisenberg model at T≪T _c that this relation is not exact for any finite D and nonzero temperature. Nevertheless, corrections to this formula remain very small practically in the whole range T < T _c if ≫ 1, as confirmed by our Monte Carlo calculations. At T T _c/4 there is a good agreement between the MC and finite-size spin-wave calculations for the field dependence of m and M for the Heisenberg model with free boundary conditions. Received 1st December 2000     

Components of the low-temperature heat capacity of rare-earth hexaborides

The temperature dependence of heat capacity C _p(T) was studied for nine rare-earth hexaborides MB₆(M=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) at temperatures of 5–300 K. Using the correspondence principle for lattice heat capacities of isostructural compounds, the lattice contribution C ₁(T) and the excess contribution ΔC(T) to the heat capacity of the hexaborides were determined. The lattice heat capacity C ₁(T) is represented as the sum of the Debye contributions of the metal and boron sublattices: C ₁(T)=C _M (T)+6C _B(T). The Debye temperatures π_M and π_B of the metal and boron sublattices were determined. The anomalies in the excess heat capacity ΔC(T)=C _p (T)?C ₁(T) are related to the magnetic ordering effects, the Schottky contribution, and the Jahn-Teller effect.     

Study of the field dependence of the magnetocaloric effect in Nd1.25Fe11Ti: A multiphase magnetic system

The field dependence of magnetic entropy change ΔS_M(T,H) has been studied in the crystalline sample Nd_1.25Fe₁₁Ti, a multiphase system constituted by three phases: Fe₁₇Nd₂, Fe₇Nd and Fe₁₁TiNd. The magnetic entropy change has been calculated from the numerical derivative of magnetization curves M(T,H) with respect to temperature and subsequent integration in field. To determine the field dependence of the experimental ΔS_M, a local exponent n(T,H) can be calculated from the logarithmic derivative of the magnetic entropy change vs. field. In contrast with the results for single phase materials, where n at the Curie temperature T_C is field independent, it is shown that for a multiphase system n evolves with field both at the Curie temperature of the system and the Curie temperatures of the constituent phases. This is in agreement with numerical simulations using the Arrott-Noakes equation of state.     

Equation of state and quantal crossover of the XY model in a longitudinal magnetic field

The equation of state of the XY model in a longitudinal (Γ) and transverse field (B) is investigated in the vicinity of a multicritical point (Γ_c, T =0) using the renormalization group method developed recently by the author and K. Walasek. For Γ = Γ_c a quantal crossover behaviour of the form H = M_{R³ψ (z)} is obtained, where H ～ B, M_R denotes the transverse magnetization, z ～ TM_R^?/βq, while ?,β_q are crossover and multicritical exponents, respectively.     

Temperature dependence of electron magnetic resonance and magnetization in NiO nanorods

For NiO nanorods of 5 nm diameter prepared by sol-gel technique, variations of the magnetization M with temperature T (5-370 K) and magnetic field H up to 55 kOe are reported. Also, temperature variations of the EMR (electron magnetic resonance) parameters (intensity I₀, linewidth ΔH and resonance field H_r) of an observed line due to uncompensated spins are followed for The M vs. H and T variations yield a blocking above which the data fits modified Langevin function with magnetic moment μ_p?1240 μ_B/particle. For the EMR line, I₀ decreases rapidly for T<T_B, and the line broadens and shifts to lower H with lowering T, following the lineshift δH_r=(ΔH)ⁿ with n?2.8. This is close to the value of n=3 expected for randomly oriented particles.     

FMR and EPR in Ni@C nanocomposites: Size and concentration effects

One-domain Ni@C nanoparticles encapsulated in carbon coating have been investigated depending on the size and concentration of Ni in carbon. The nanoparticles of nickel were prepared with the average diameters changing in a broad range of 4–45 nm, and the concentration of Ni in C varies in 2–12 wt%. To prepare the Ni@C nanocomposites the solid solutions of nickel phthalocyanine–metal-free phthalocyanine (NiPc) _x (H₂Pc)_1–x , 0 ≤ x ≤ 1 were synthesized and the solidphase pyrolysis of these compounds was performed. In the case of ultradispersive Ni nanoparticles (the interval of quantum dots is 1–10 nm), a considerable shift of the resonance field and broadening of resonance absorption field were revealed in the spectra of FMR at room temperature. The data were interpreted taking into account the essential contribution of the surface magnetic anisotropy, the magnetic field of which far exceeds the magnetic field of volume anisotropy.     

Effect of annealing on the magnetic and superconducting properties of single-crystalline UCoGe

Single-crystals of the new ferromagnetic superconductor UCoGe have been grown. The quality of as-grown samples can be significantly improved by a heat-treatment procedure, which increases the residual resistance ratio (RRR) from ∼5 to ∼30. Magnetization M(T) and resistivity ρ(T) measurements show the annealed samples have a sharp ferromagnetic transition with a Curie temperature T_C is 2.8 K. The ordered moment of 0.06 μ_B is directed along the orthorhombic c-axis. Superconductivity is found below a resistive transition temperature T_s=0.65 K.     

Probing the field-induced variation of the chemical potential in Bi2Sr2CaCu2Oy via magneto-thermopower measurements

Approximating the shape of the magneto-thermoelectric power (TEP) ΔS(T,H) measured in Bi₂Sr₂CaCu₂O_y by an asymmetric linear triangle of the form ΔS(T,H)≃S _p (H)±B ^±(H)(T _c −T) with positive B ⁻(H) and B ⁺(H) defined below and above T _c , we observe that B ⁺(H) ≃2B ⁻(H). To account for this asymmetry, we explicitly introduce the field-dependent chemical potential μ(H) of holes into the Ginzburg-Landau theory and calculate both an average ΔS _av(T,H) and fluctuation contribution ΔS _fl(T,H) to the total magneto-TEP ΔS(T,H). As a result, we find a rather simple relationship between the field-induced variation of the chemical potential in this material and the above-mentioned magneto-TEP data around T _c , viz. Δ μ(H)∝S _p (H). Zh. éksp. Teor. Fiz. 116, 257–262 (July 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Scaling behavior of the Hall coefficient of Si:P at the metal-insulator transition

Measurements of the Hall coefficient R _H (T, B) of Si:P with P concentration N between 3.54 and 7.0·10¹⁸ cm^?3 are reported for the temperature range 0.04 K ≤ T ≤ 4K and in magnetic fields up to 7 T. Even far above the metal-insulator transition (MIT), a sign change of the temperature coefficient similar to the behavior of the conductivity σ(T) in moderate fields is not observed in R _H (T). Field and temperature dependence of R _H both increase as the MIT (at the critical concentration N _c = 3.52 · 10¹⁸ cm^?3) is approached. A careful extrapolation to T → 0 and B → 0 indicates that R _H ^?1 scales to zero as R _H ^?1 ～| N ? N _c ^μH with μ _H = (0.44 ± 0.04) in agreement with previous results.