A Mössbauer study of Pb4Fe3O8Cl,a three-layer magnet

The study of Mössbauer spectra from Pb₄Fe₃O₈Cl allows the observation of crystallographic sites associated with two kinds of magnetic layers present in the structure. Magnetic order appears simultaneously in the two sites below T_N at 602 K. The following features are observed (i) magnetic moments in a plane of easy magnetization perpendicular to the fourfold axis, (ii) antiferromagnetic behavior, as evidenced by the rotation of the magnetic moments under the effect of an external field, (iii) slightly different critical exponents β (0.311 ± 0.003 and 0.295 ± 0.003) for the two magnetic layers.     

Phase boundary between uniaxial and planar ferromagnets in layered perovskite manganite La2−2xSr1+2xMn2O7 (0.313⩽x⩽0.350)

We have examined magnetizations as a function of temperature and magnetic field in layered perovskite manganites La_2−2xSr_1+2xMn₂O₇ single crystals (x=0.313, 0.315, 0.318, 0.320 and 0.350) in order to determine the phase boundary between two ferromagnets (one is an uniaxial ferromagnet whose easy axis is parallel to the c-axis and the other is a planar ferromagnet whose easy axis is within the ab-plane) and following results are obtained: (i) all the present manganites exhibit magnetic transitions from a ferromagnet to a paramagnet at 76, 107, 116, 120 and 125 K for x=0.313, 0.315, 0.318, 0.320 and 0.350, respectively; (ii) for x=0.318, 0.320 and 0.350, the magnetic structure is a planar ferromagnet below Curie temperature; (iii) for x=0.313 and 0.315, the magnetic structure changes from an uniaxial to a planar ferromagnet at 66 and 85 K, respectively. From the results described above we have constructed the magnetic phase diagram of layered perovskite manganite La_2−2xSr_1+2xMn₂O₇ (0.313?x?0.350).     

Magnetic and magnetization properties of electrodeposited fcc CoPt nanowire arrays

Magnetic and magnetization properties of fcc Co_1−xPt_x (x?0.3) alloy nanowires fabricated by electrodeposition into self-synthesized anodic alumina templates are investigated. Magnetization curves, measured for varying wire geometries, show a crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis as a function of the diameter and length. The measured values of coercivity (H_c) and remanent squareness (SQ) of CoPt nanowire arrays, as a function of angle (θ) between the field and wire axis, support the crossover of easy axis of magnetization. The curling mode of the magnetization reversal process is observed for CoPt nanowire arrays. At low temperatures, the easy axis for magnetization of the nanowires is observed to deviate from the room-temperature orientation.     

Magnetostriction of a single cubic laves phase compound Tb0.27Dy0.73Fe2

The magnetostriction of a single crystal cubic Laves phase compound Tb_0.27Dy_0.73Fe₂ is measured with the applied field (0–25 kOe) in the [111] direction in the temperature range between 80 and 300 K. Evidence is seen of a possible transition to the[uv0] easy axis phase during the spin re-orientation form [100] easy axis direction to the [110] easy axis direction.     

On glancing scattering of light ions from tungsten single crystal surfaces

A computer calculation of light ion (200–800 keV protons and helium) scattering from a tungsten single crystal model with a (110)-type surface is presented. An axis (100) is considered. Thermal vibrations in 3 dimensions have been incorporated. The number, i.e. fraction, and spatial distribution of reflected ions for some angular scans are presented in an appendix. The importance of trajectory histories is demonstrated. 3 regimes of scattering behaviour as a function of the incidence angle ψ in to the string in the surface are found:

(i) for very small ψ_in surface-semi-proper-channelling of all ions;

(ii) for intermediate ψ_in mainly planar channelling and dechannelling resulting in a decreasing reflective index for increasing ψ _in;

(iii) axial channelling and transverse plane focussing are mainly of importance leading to a small reflective index, and surface semi-channelling like for (ii) occurs for very few ions.

The importance of the scattering potential is demonstrated and an expression is discussed.     

Experimental observation of negative group velocity in C60 PMMA

The authors experimentally observed the superluminal propagation at negative group velocity in C₆₀ polymethyl methacrylate (PMMA) firstly. The largest time advancement of 7.61 ms was obtained at 1 mg/cm³ of sample concentration, the corresponding group velocity was −0.657 m/s. Fast light in C₆₀ PMMA offers several advantages over liquids or vapors for a variety of possible applications: (i) easy for experimental measurement and practical applications (ii) easy for manufacture, (iii) sample concentration is another convenient parameter to vary the signal delay.     

Exchange-spring magnets based on L10-FePt ordered phase

Thin bi-layers constituted by a hard L1₀-FePt layer and a soft Fe layer were obtained using respectively an rf sputtering device and an UHV e-beam evaporation technique. Magneto-Optical Kerr Effect magnetometry, Atomic/Magnetic Force Microscopy and Conversion Electron Mössbauer Spectroscopy were used in order to correlate the magnetic properties of the bi-layers with the effects of the interdiffusion at the interfaces. It has been found that the evaporated Fe can easily diffuse into the hard film, giving raise to the formation of a region containing small particles of both Fe and Fe-rich FePt which show a superparamagnetic behaviour. The ferromagnetic Fe film can grow only on this region. The system shows (i) a preferred orientation of the easy magnetization axis along the direction normal to the film plane, and (ii) a single-phase magnetic behaviour due to the strong exchange coupling which established between the constituent phases.     

Magnetic properties of amorphous sputtered CoxSi1−x films

Magnetic properties have been measured by transverse magneto-optic Kerr effect (MOKE) in amorphous sputtered Co_xSi_1?x films which are ferromagnetic and amorphous at room temperature with 0.55<x<0.80. These as-sputtered films show an in-plane easy axis with the ratio of coercitive force in the hard axis H_c (HA) to easy axis H_c (EA) and the value of the blocking field H_B almost constant with 0.60<x<0.80. Transverse magnetic susceptibilities with bias magnetic field parallel (χ_t0) and perpendicular $\text{(χ}{}_{\mathrm{<mtext>t\pi </mtext><mtext>2</mtext>}}\text{)}$ to the easy axis has been measured. These measurements show a deviation of Hoffmann's micromagnetic law for the susceptibilities.     

The influence of easy direction of magnetisation on 59Co NMR spectrum in R2(Co1−xMnx)17 compounds

The spin echo NMR spectra of ⁵⁹Co in R₂(Co_1-xMn_x)₁₇, (R = Y, Gd) measured at 4.2 K are reported. The large shift of resonance lines is observed, that is explained as caused by reorientation of easy axis of magnetisation from easy plane to easy direction (c axis). It is suggested to explain quantitatively the spectra, that only two of four Co sites (9d and 18f) in R₂Co₁₇ structure play a dominant role in determining of anisotropy energy and the Co atoms at the 6c sites (“dumb-bell” atoms) give no direct contribution to the anisotropy energy of the compound. The corresponding changes of local anisotropy energy and the orbital part of cobalt magnetic moment characteristic for each of cobalt structural sites are calculated and discussed.     

Texture in NdFeB magnets analysed on the basis of the determination of Nd2Fe14B single crystals easy growth axis

We have shown by X-ray analysis and magnetic measurements, that the easy growth axis of Nd₂Fe₁₄B crystals corresponds to the “a” axis of the tetragonal structure while the easy magnetization axis is the “c” axis at temperatures above 135K. This correlation allows to understand some interesting features in NdFeB magnets:

• &#x02022;-anisotropic contraction during sintering in magnets obtained by classical powder metallurgy,
• &#x02022;-orientation mechanism during hot pressing (“die upset”) of magnets based on melt spun ribons.

     

Response of an isolated magnetic grain suspended in a liquid to a rotating field

We have studied the response of an isolated uniaxial magnetic grain suspended in a liquid to an applied fieldh rotating with frequencyω. In the presence of an applied static field (H?h), at low frequencies (i.e. for fast relaxation), the easy axis followsh, while at high frequencies the behavior is similar to that of a bulk sample. In zero static field, the response of a ferromagnetic grain is more complicated; there exists a critical frequencyω _e below which a steady state is reached, with the easy axis followingh. Forω>ω _e the mechanical behavior depends crucially on the initial conditions. Finally, a superparamagnetic grain has a (different) critical frequencyω _e, below which it reacts similarly to the ferromagnetic particle, while forω>ω _e it does not follow steadily the rotating field, but can only oscillate about its initial position.     

Defect properties of CuInSe2 and CuGaSe2

Using first-principles electronic structure theory, we have calculated defect formation energies and defect transition levels in CuInSe₂ and CuGaSe₂. We show that (i) it is easy to form Cu vacancies in CuInSe₂, and (ii) it is also relatively easy to form cation antisite defects (e.g. In_Cu) for this ternary compound. Consequently, defect pairs such as (2V_Cu+In_Cu) have a remarkably low formation enthalpy. As a result, the formation of a series of Cu-poor compounds (CPCs) such as CuIn₅Se₈ and CuIn₃Se₅, is explained as a repeat of (2V_Cu+In_Cu) pairs in CuInSe₂. The very efficient p-type self-doping ability of CuInSe₂ is explained by the easy formation of the shallow Cu vacancies. The electrically benign character of the natural defect in CuInSe₂ is explained in terms of an electronic passivation of the by . For CuGaSe₂, we find that (i) the native acceptor formation energies and transition energy levels are similar to that in CuInSe₂, but the donor formation energy is larger in CuGaSe₂. (ii) The Ga_Cu donor level in CuGaSe₂ is deeper than In_Cu donor level in CuInSe₂, therefore, Ga_Cu behaves as an electron trap in CuGaSe₂, even when it is passivated by V_Cu. We have also calculated the band alignment between the CPCs and CuInSe₂, showing that it could have significant effect on the solar cell performance.     

57Fe Mössbauer and magnetic studies of DyFe12−x Ta x compounds

Refinements of the X-ray diffraction patterns show that DyFe_12?x Ta _x compounds with x = 0.5–0.7 crystallise in the ThMn₁₂-type structure and that the Ta atoms occupy the 8i sites. Spin reorientations have been detected by ac magnetic susceptibility for all compounds below room temperature. First the moments shift direction from easy axis to easy cone at T _sr1, then to easy plane at T _sr2. Both T _sr1 and T _sr2 increase with increasing Ta content up to x = 0.65 before decreasing with further increase in Ta content. Analyses of the Mössbauer spectra indicate that the individual Fe site hyperfine fields derived at 4.5 K for DyFe_11.35Ta_0.65 are B _hf = 37.4 T, 32.2 T and 27.6 T for the 8i, 8j and 8f sites, respectively.     

Phase front motion in uniaxial ferromagnet

It is well known that in the uniaxial ferromagnet in the presence of an external magnetic field perpendicular to the easy axis (h_x) a continuous phase transition occurs for a critical value of this field. There are metastable and stable states if one includes a small field parallel to the easy axis (h_z). The motion of the relaxation front of the metastable state is investigated. It is found that an “interphase wall of Neel-type” exists, its velocity is proportional to h_z and increases when the critical point is approached.     

Mössbauer studie of Co2B by doping 1 at %57Fe

In order to clarify the direction of the easy axis of Co₂B, we examined Co₂B doped with 1 at %⁵⁷Fe using Mössbauer spectroscopy. In the temperature range 4.2 K≤T≤r.t., two well resolved six-line patterns having equal intensity were observed. This shows that the easy axis lies in the c plane. The displacement of magnetically split absorption lines by quadrupole interaction also supports this conclusion. In the temperature region between 4.2 K and 1.6 K, the direction of the easy axis seems to begin to deviate from the c plane.     

Fe100－xPdx合金纳米线阵列的制备及其磁性研究

利用直流电沉积法在多孔阳极氧化铝模板中制备出了一系列Fe_100-xPd_x磁性纳米线阵列. Pd的增加使纳米线的总体磁性降低，各向异性和矫顽力也发生了较大的变化. 当Pd含量高达x=30时，纳米线仍有相当高的矫顽力(7.48 kA/m)和较明显的各向异性，但当Pd的含量增加到50%时，纳米线的易磁化方向由平行线的方向反转到垂直线的方向. 实验证明，这是由于在Fe₈₀Pd₂₀和Fe₇₀Pd₃₀中连续的磁性相在Fe₅₀Pd₅₀纳米线中变成了与非磁性相相互间隔的非连续片状结构. 片状磁性相的形状各向异性使易磁化方向转变到垂直纳米线轴的方向. 从生长动力学的角度对Fe₅₀Pd₅₀纳米线中这种片状的形成进行了解释. 关键词： 纳米线 电化学沉积 磁性     

Fabrication and magnetic characterization of Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pt<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> nanowire arrays

Co _x Pt_1−x (x≥0.7) alloy nanowires are grown into self-synthesized anodic alumina templates by electrodeposition. Magnetic and magnetization properties of Co _x Pt_1−x alloy nanowires are measured as functions of wire length, temperature, and field orientation. X-ray diffraction shows that as-prepared CoPt nanowires are of fcc polycrystalline structure. A crossover of easy axis of magnetization is observed from parallel to perpendicular of the nanowire axis as a function of length. The coercivity (H _c) and remanent squareness (SQ) of Co _x Pt_1−x nanowire arrays are derived from hysteresis loops measured at various angles (θ) between the field and wire axis. H _c(θ) and SQ(θ) curves show bell-shaped or otherwise bell-shaped behavior corresponding to the easy axis of their magnetization.     

Sn Mössbauer spectroscopy of Gd0.8L0.2Mn6Sn6 and Tb0.8L0.2Mn6Sn6 compounds (L=Sc,Y, Lu)

The HfFe₆Ge₆-type compounds Gd_0.8L_0.2Mn₆Sn₆ and Tb_0.8L_0.2Mn₆Sn₆ (L = Sc, Y, Lu) have been studied by ¹¹⁹Sn Mössbauer spectroscopy. The values of the apparent quadrupolar splitting clearly evidence the easy plane magnetization of the gadolinium compounds and the easy axis one in the terbium compounds. The three tin sites behave differently with the nature and size of the substituting L element. For a given series, the hyperfine field of the Sn_2d site is almost unchanged whatever the size of the L element. The hyperfine field of the Sn_2e site strongly varies with the L size in relation with atomic displacements. The hyperfine field of the Sn_2c site exhibits a more complicated behavior. The field difference in the easy plane and easy axis compounds confirms the angle-dependent anisotropic contribution of the Mn moment to the hyperfine field. The analysis of the results also suggests the play of angle-dependent contributions arising from the rare earth moment.     

Cholesteric pitch-transitions induced by a magnetic field in a sample containing incomplete number of pitches

We investigate the pitch transitions induced by an external bulk field in a Cholesteric Liquid Crystal slab of finite thickness ℓ that contains an incomplete number of π-twists. The analysis is performed for a magnetic field that is (i) perpendicular to the helical axis, and (ii) tilted with respect to one of the easy directions imposed by planar and rigid boundary conditions. For finite ℓ we obtain a cascade of transitions, where the bulk expels a half-pitch at a time with increasing field to avoid divergences in the elastic energy. The dependence of the threshold magnetic field inducing the expulsion on the easy axes twist angle δ is investigated for all the cascade of pitch transitions and in particular for the final one, corresponding to the Cholesteric-Nematic transition. In the ℓ → ∞ limit this dependence disappears and we reobtain the results of de Gennes for an infinite sample.     

Microstructure and magnetic properties of tubular cobalt–silica nanocomposites

Co-based (Co and Co₃O₄) nanoparticles were self-integrated into SiO₂ nanotubes with a methodology based on the use of a Co salt as a template structure for the formation of SiO₂ nanotubes. Within the confinement of tubular matrix of SiO₂, the nanofibres of cobalt precursor, i.e., [Co(NH₃)₆](HCO₃)(CO₃)·2H₂O, were treated in a H₂ atmosphere with different parameters. With a sufficient reduction on the cobalt precursor, sphere-like Co-based nanoparticles are obtained, being well aligned in the interior space of the SiO₂ nanotubes. With an insufficient reduction, platelet-like Co-based nanoparticles are formed, being arranged in a random manner inside the SiO₂ nanotubes. The sufficiently reduced Co–SiO₂ nanocomposite exhibits an open hysteresis loop in the low field region (<3 kOe) and a paramagnetic response in high field (>3 kOe) at 300 K. An observed wide separation between the zero-field-cooling (ZFC) and field-cooling (FC) curves over the whole temperature region has demonstrated a characteristic feature of ferromagnetism with a magnetically anisotropic barrier diverting the easy axis from the axis of the applied field. The predominant factor leading to this anisotropic potential barrier is attributed to the shape anisotropy native to the one-dimensional arrangement of Co-based nanoparticles within the tubular matrix, i.e. SiO₂ nanotubes.