Critical temperature for a system with Heisenberg-like nearest neighbour and Ising-like next nearest neighbour interactions

A ferromagnetic system with $\text{S =}\text{1}\text{2}$, where the interactions between nearest neighbours are isotropic (having Heisenberg character) and those between the next nearest neighbours anisotropic (Ising-like), is investigated. Using the effective hamiltonian approach we find the change in the critical temperature due to the inclusion of next nearest neighbour interactions of different character. The change is greatest in two dimensions where the critical temperature is shifted up from its zero value for a system with the Ising-like interactions switched off. We also calculate the T_c for a system with both nearest and next nearest neighbours interactions of the Ising-type. The results for the two models are compared.     

Structure magnetique en champ applique nul de l'acetate de manganese tetrahydrate Mn(CH3COO)2, 4H2O

The magnetic structure of manganous acetate Mn(CH₃COO)₂, 4H₂O has been solved by neutron diffraction. Manganous acetate crystallizes in the space group $\text{P2}{}_1\text{c}$ with Z = 6. Manganese atoms (in position 2a and 4e) are located in (100) planes. Below T_N = 3.18 K this compound is antiferromagnetic in a zero applied field with the k vector [$\text{1}\text{2}$ 00]. The plane (100) is ferromagnetic. The magnetic group is $\text{P}{}_{\mathrm{2a}}\text{2}{}_1\text{c}$.     

Constant coupling approximation for antiferromagnets with mixed ferro- and antiferromagnetic interactions

The constant-coupling approximation is extended to an antiferromagnetic spin-$\text{1}\text{2}$ system with two distinct anisotropic exchange interactions. The thermodynamic properties such as the transition temperature, magnetization, susceptibility and specific heat are discussed for three special cases: (i) ferro- and antiferromagnetic Ising interactions, (ii) isotropic ferro- and antiferromagnetic Heisenberg interactions; and (iii) isotropic ferromagnetic Heisenberg interactions and antiferromagnetic Ising interactions, allowing in each case for two different nearest-neighbour coupling constants. Numerical calculations have been performed for a layer structure with z = 6 intraplanar and z' = 6 interplanar nearest neighbours and the results are compared with those obtained in other approximations. Applying the theory to FeCl₂, the exchange constants are evaluated. It is shown that the calculated magnitudes of the interactions strongly depend upon the exchange-interaction model assumed.     

Further neighbour interactions in the quasi b.c.c. Heisenberg ferromagnets M2 Cu X4 · 2H2 O (M = NH4, K,Rb or Cs; X = Cl or Br)

The Curie-Weiss constant θ of the quasi b.c.c., $\text{S =}\text{1}\text{2}$, Heisenberg ferromagnets, (NH₄)₂ Cu Br₄ · 2H₂ O and Rb₂ Cu Br₄ · 2H₂ O, have been derived from measurements of the susceptibility in the paramagnetic region (4.2 < T < 100 K) and of the high-field magnetization curves at $\text{T = 4.2}\text{K}\text{(}\text{T}\text{T}{}_{\mathrm{c}}\text{? 2.3)}$. The $\text{T}{}_{\mathrm{c}}\text{θ}$ values obtained point to the presence of further neighbour interactions considerably stronger than previously assumed. The effective number of equivalently interacting magnetic neighbours is estimated to be at least seventeen.     

Collective excitations in the 1-D-ferromagnet CsFeCl3 with singlet ground state

By means of inalastic neutron scattering we have determined the dispersion relation of the magnetic excitations in CsFeCl₃ at different temperatures.The dispersion in c-direction, along the Fe-chains is typically ferromagnetic and in the hexagonal plane antiferromagnetic. Due to the lack of an applicable theory the data were parametrized by the simple heuristic formula:?ω = [2J[1 - cos πq_c] [A + 2J(1 - cos π)q_c)] + [C + J' (1.5 + γ(q))]²]^{$\text{1}\text{2}$}The gap was found to be C = 0.148 THz, the easy plane anisotropy A = 0.308 THz, the ferromagnetic interaction J = 0.148 THz and the antiferromagnetic interaction J' = -0.04 THz. At 1.25 K all excitations had a width smaller than the instrumental resolution ΔE = 0.025 THz. These results can be interpreted as follows: CsFeCl₃ is a singlet ground state system with strong ferromagnetic interaction J along the crystallographic c-axis and weak antiferromagnetic interaction J' in the plane perpendicular to c.In addition we have measured the influence of a magnetic field along the hexagonal c-axis. The splitting found agrees with the assumed level scheme yielding g = 2.5 for the first excited level.     

Phase transitions in classical vector models

The equation of state obtained earlier /6/ for 3-dimensional Ising ferromagnets in the first order in$\text{1}\text{z}$ (z is the number of the nearest neighbours) is generalized with regard to the classical D-component vector model. The results obtained are valid in a wide temperature range |τ|?[z²(D+2)]^?1 (τ=(T_c-T)/T_c) and become exact in the limit D → ∞.     

NMR study of magnetic properties in antiferromagnetic BaMnF4

¹⁹F NMR was used to study the magnetic structure of BaMnF₄. The spectrum consists of four clusters of lines, and the cluster positions are compatible with an antiferromagnetic coupling between nearest neighbours within each of the puckered layers in the crystal. The splitting of the lines indicate that the crystal symmetry at low temperatures is lower than the $\text{A}\text{2}{}_1\text{am}$ structure known for BaMnF₄ at room temperature. The angular dependence of the lines is explained by two sublattice uni-axial-antiferromagnet model. An NMR line which appears at about 10 kOe in a wide range of frequencies is attributed to the spin flop transition.     

Magnetic moments of composite fermions

There have been a considerable number of papers proposing composite models for leptons and quarks. Recently, Glück and Lipkin have stated that reproducing the observed magnetic moments of these fermions presents a serious difficulty for these composite models. We show for a renormalizable theory that, in contrast to Glück's and Lipkin's nonrelativistic arguments, a deeply bound system (with heavy constituent particle masses m_c) of (total) spin $\text{1}\text{2}$, charge e and mass m has the magnetic moment $\text{(e/2m) [1 +}\text{“usual” (QED + QCD + weak) corrections +O}\text{(m/m}{}_{\mathrm{<mtext>c</mtext>}}\text{)}\text{“new” bindng corrections]}$. Although there remains the considerable dynamical problem of obtaining “light” bound fermions from heavy constituents, there is no separate, additional magnetic moment difficulty.     

Duree de vie et facteur de lande du niveau (5s5p2)4P12 de L'ion etain Sn+

Oriented Sn⁺ ions in the (5s5p²)⁴P_{$\text{1}\text{2}$} level are produced by Penning collisions between Sn atoms in the ground state and optically oriented He (2³S₁) metastable atoms. If r.f. transitions are induced in the 2³S₁ He level, the circularly polarized light emitted by the ions in a direction perpendicular to the magnetic field is modulated at the r.f. frequency; the variation of the degree of modulation with the r.f. frequency allows a determination of the lifetime τ' of the ion level. The lifetime and the Landé g-factor of the Sn⁺ (⁴P_{$\text{1}\text{2}$}) level are also measured by direct magnetic resonance. The line shape is also computed when the resonances of the He atom and of the Sn⁺ ion overlap; the experimental signals are in good agreement with this computation.     

A comparison of direct photon, π0 in pp and pp interactions at the CERN ISR

We have measured the production of direct photons, π⁰'s, and η's in p$\text{p}$ and pp collisions at √s=53 GeV in the range 2≈p_T≈6 GeV/c for the central rapidity region |y| 0.4. No statistically significant difference between pp? and pp interactions is observed.     

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.     

Chaleur spécifique et transformation martensitique dans le système Nb1−xSnx

Experimental data on the superconducting temperature, and low temperature specific heat, together with X-ray and neutron investigations are reported for two sets of Nb_xSn_1?x, samples with the A15 type structure. The first one, intended for a study of the effect of stoichiometry, consists of cast alloys with lattice constants ranging from 5.273 to 5.289 Å, γ's from 4.9 to $\text{9}\text{mJ}\text{K}{}^2$ g-at and T_c's from 6 to 17.9 K. The second one consists of nearly stoichiometric sintered alloys with lattice constants ranging from 5.288 to 5.291 Å, γ's from 5.6 to 13.5 $\text{mJ}\text{K}{}^2$ g-at and T_c's close to 18 K. The martensitic transformation ratio has been determined by neutron diffraction in the latter set and the values 32, 77 and 95% were obtained. The analysis of the specific heat jumps at T_c shows that the superconducting T_c of the tetragonal phase is 17.9 K and both T_c and γ are reduced in the cubic phase.Phonon information inferred from specific heat data are indicative of a structure with a typical energy of 9 meV in the density of states. The agreement with spectra obtained by inelastic neutron scattering is good.     

Influence des marches atomiques sur le transport de matière à la surface du nickel

Surface atomic transport on Ni was measured by mass transport technique on a sinusoidal profile. One of the studied surfaces was within 15' of a singular (001) orientation. Others were vicinal surfaces. Kinetic damping coefficients $\text{1}\text{τ}$ are shown to be dependent on the profile parameters, i.e. of the density n_tot, of all the monoatomic steps and also of the density σ_c of kinks of the monoatomic steps. If the parameter describes the random motion of adatoms on isolated terraces and if α and k are two coefficients linked to adsorption/emission processes from steps and kinks, a sinusoidal profile with 5 μm periodicity obeys to the relation: $\text{1}\text{τ}\text{= (}\text{1}\text{τ}\text{)}{}_{\mathrm{T}}\text{[1+αn}{}_{\mathrm{tot}}{}^2\text{(1+ kσ}{}^2{}_{\mathrm{c}}\text{)]}$. This result demonstrates the importance of surface atomic structure which has been neglected in Mullins' theory, which may be non-negligible in all processes where surface mass transport is involved.     

Critical dynamics in isotropic ferromagnets

Modern high resolution inelastic neutron scattering methods make very long wavelength fluctuations accessible to dynamical investigation. The new results on Fe and EuO show that: (a) the $\text{q}{}^{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ power law is well borne out at T = T_c; (b) strong zero-wavenumber relaxation due to spin non-conserving forces can make the predicted hydrodynamic spin diffusion at T >T_c unobservable; (c) this zero-wavenumber relaxation is the only appreciable deviation from the $\text{z =}\text{5}\text{2}$ exchange scaling behaviour; (d) there is no evidence for the predicted cross-over to dipolar dynamics a finite q's.     

Specific heat of the (La,Gd) Al2 system in the superconducting and normal state

Specific heat measurements between 0.5 and 4.2°K are reported for the system ($\text{La}$, Gd) Al₂ in both the superconducting and normal state. The observed specific heat jump at the superconducting transition temperature T_c is in excellent agreement with the Abrikosov-Gor'kov (AG) theory. This is in accordance with the previously reported close correspondence of the T_c vs. Gd concentration curve with the AG theory. Two very interesting features occur in the normal state specific heat. First, the Gd impurities cause a surprisingly strong enhancement of the electronic specific heat coefficient. Second, there is a large magnetic field dependent Schottky-like anomaly at low temperatures. This anomaly persists even in the superconducting state.     

Adsorption of gold on low index planes of rhenium

On atomically rough areas of a thermally cleaned rhenium field emitter, adsorbed gold behaves like it does on tungsten. The average work function $\text{}\text{?}$gf increases at low average gold coverage $\text{}\text{?}$gq due to formation of gold-rhenium dipoles, and at high coverage a structural transformation in the gold layer leads to a $\text{}\text{?}$gq-independent work function. Broadly similar behaviour is found for gold on the low-index planes of tungsten, but on low-index rhenium planes gold behaves rather differently. When thermally cleaned at > 2200 K and annealed below 800 K, the work function, φ(clean), of (101&#x0304;1&#x0304;) takes one of two values 5.25 ± 0.04 eV, and 5.36 ± 0.04 eV, which are tentatively attributed to the two possible structures of this plane. Similar behaviour is expected and observed for (101&#x0304;0),but the values taken by φ(clean) are not well defined. Both forms of (101&#x0304;1&#x0304;) are thought to undergo reconstruction above 800 K forming a single structure with φ(clean) = 5.55 ± 0.03 eV. (112&#x0304;0) and (112&#x0304;2&#x0304;) each have only one possible structure, and in keeping with this, φ(clean) has a single well-defined value for each plane. The flatness of (101&#x0304;1&#x0304;) and (101&#x0304;0) leads to field reduction at their centres which produces an increase in their measured work functions by up to 10%. The initial increase in φ produced by gold condensed at 78 K and spread at low equilibration temperatures T_s on (112&#x0304;2&#x0304;), (101&#x0304;1&#x0304;) and (112&#x0304;0) is attributed to gold-rhenium dipoles, which, on the latter two planes approximate to the Topping model, giving dipoles characterised by μ₀(1011) = 0.1 × 10^?30 C-m with α = 10 ų and μ₀(112&#x0304;0) = 0.32 × 10^?30 C-m with α = 22 ų, where μ₀ is the zero-coverage dipole moment and α its polarizability. Failure of the Topping model on (112&#x0304;2&#x0304;) is attributed to its atomically rough structure. No dipole effect is seen on (101&#x0304;0). Energy spectroscopy of electrons field emitted at (202&#x0304;1&#x0304;) and (101&#x0304;1&#x0304;) demonstrates the non-free character of electrons in rhenium, while the small effect of adsorbed gold strengthens the belief that gold is bound through a greatly broadened 6s level centred 5.6 eV below the Fermi level and the dipolar nature of the bond supports this model. At higher values of T_s and $\text{}\text{?}$gq gold appears to form states which are well-characterised by a coverage-independent work function. (101&#x0304;0), (101&#x0304;1&#x0304;) and (112&#x0304;0) each form two such states, one in the range 2 < $\text{}\text{?}$gq < 4 (state 1), and the second at $\text{}\text{?}$gq > 4 (state 2). The atomic radii of gold and rhenium are thought to be sufficiently similar to allow the possibility that state 1 is a replication of the Re plane structure by gold. The high work function and thermal stability of state 2, taken together with the observed temperature dependence of the transformation of state 1 to state 2, encourages the belief that state 2 results from atomic rearrangement of state 1 into a close-packed Au(111) structure. State 2 also forms on (112&#x0304;2&#x0304;) and the absence of state 1 on this plane suggests some surface alloying at coverages below 4 $\text{}\text{?}$gq.     

Quantum theory of longitudinal dielectric response properties of a two-dimensional plasma in a magnetic field

An analysis of dynamic and nonlocal longitudinal dielectric response properties of a two-dimensional Landau-quantized plasma is carried out, using a thermodynamic Green's function formulation of the RPA with a two-dimensional thermal Green's function for electron propagation in a magnetic field developed in closed form. The longitudinal-electrostatic plasmon dispersion relation is discussed in the low wavenumber regime with nonlocal corrections, and Bernstein mode structure is studied for arbitrary wavenumber. All regimes of magnetic field strength and statistics are investigated. The class of integrals treated here should have broad applicability in other two-dimensional and finite slab plasma studies.The two-dimensional static shielding law in a magnetic field is analyzed for low wavenumber, and for large distances we find $\text{V(}\text{r}\text{) ～}\text{Q}\text{k}{}_0{}^2\text{r}{}^3$. The inverse screening length $\text{k}{}_0\text{=}\text{2πe}{}^2\text{??}\text{?ξ}$ (? = density, ξ = chemical potential) is evaluated in all regimes of magnetic field strength and all statistical regimes. k₀ exhibits violent DHVA oscillatory behavior in the degenerate zero-temperature case at higher field strengths, and the shielding is complete when $\text{ξ = r′}\text{l}\text{z.shtsls;}\text{ω}$, but there is no shielding when $\text{ξ ≠ r′}\text{l}\text{z.shtsls;}\text{ω}{}_{\mathrm{c}}$. A careful analysis confirms that there is no shielding at large distances in the degenerate quantum strong field limit $\text{l}\text{z.shtsls;}\text{ω}{}_{\mathrm{c}}\text{> ξ}$. Since shielding does persist in the nondegenerate quantum strong field limit $\text{l}\text{z.shtsls;}\text{ω}{}_{\mathrm{c}}\text{> KT}$, there should be a pronounced change in physical properties that depend on shielding if the system is driven through a high field statistical transition. (It should be noted that the static shielding law of semiclassical and classical models has no dependence on magnetic field in two dimensions, as in three dimensions.) Finally, we find that the zero field two-dimensional Freidel-Kohn “wiggle” static shielding phenomenon is destroyed by the dispersal of the zero field continuum of electron states into the discrete set of Landau-quantized orbitals due to the imposition of the magnetic field.     

Studies of interface phonons

The vibrational modes localized at the interface between two distinct crystals have been studied for a simple crystal model obeying all of the invariance conditions required for models used in studies of dynamical properties of crystal surfaces, and giving rise to Rayleigh surface waves. The two crystals are assumed to be semi-infinite simple cubic and to have the same lattice parameter a. They differ by their mass (M and M_A) and the central force interactions between first (K and K_a) and second nearest neighbors $\text{1}\text{2}\text{K}$ and $\text{1}\text{2}\text{K}{}_{\mathrm{A}}$. The interface is obtained by coupling the (001) free surfaces of these distinct crystals by central force intractions (K^'). We find that the variation of the interaction conditions (K^') at the interface and of the $\text{(}\text{K}\text{M}\text{)}\text{(}\text{K}{}_{\mathrm{A}}\text{M}{}_{\mathrm{A}}\text{)}$ parameter has the following qualitative effects on the properties of surface and bulk phonons. When (K^') increases from zero to a finite value, the frequencies of the surface phonons increase and are splitted in the case of two identical crystals. One can say that the surface phonons are transformed into interface modes. For some values of $\text{K}{}^{\mathrm{\text{'}}}\text{K}$ and $\text{(}\text{K}\text{M}\text{)}\text{(}\text{K}{}_{\mathrm{<mtext>A</mtext>}}\text{M}{}_{\mathrm{<mtext>A</mtext>}}\text{)}$ parameters these interface phonons may be admixed with bulk phonons and thus become virtual interface states.     

Crystal growth,structure, electron paramagnetic resonance and magnetic properties of (NH4)2V3O8

A method to grow single crystals of ammonium vanadate (IV, V) (NH₄)₂V₃O₈ has been devised. The crystal structure is tetragonal P4bm; residual factor is R = 0.030. Cell parameters are $\text{a = 8.891 ± 0.004}\text{A}\text{?}$ and $\text{c = 5.582 + 0.002}\text{A}\text{?}$. The V⁵⁺ atom lies at the center of a triangular pyramide (VO₄ tetrahedron) while the V⁴⁺ atom is on A 4-fold rotation axis at the center of a square-based pyramide VO₅ whose symmetry point group is almost C_4v with the short V = O bond lying along the 4-fold axis parallel to the c edge of the tetragonal cell. Crystals are thin platlets with (001) cleavage planes. The platlets have very often a square or rectangular shape limited by {100} or {110} planes. Each single crystal was not large enough to record a good e.p.r. spectrum, but by sticking on the same quartz plate a score of them it was possible to gather enough crystals so to record correct spectra and by orienting the plate to obtain resonance lines separately for g_∥ = 1.9263 et g_τ = 1.9755. Measurements at 283 K on powder samples gave times for spin-spin relaxation T₂ = 0.4 × 10^?7s and for spin-lattice relaxation T₁ = 1.6 × 10^?7s. The magnetic structure is characterized by an exchange narrowing ω_e = 3 × 10¹⁰rad/s which corresponds to a transition temperature of about 0.5 K. Static susceptibility measurements at high magnetic field show a paramagnetic behaviour with an antiferromagnetic interaction which is interpreted in the magnetic space group P_2c4bm as the interaction between V⁴⁺ ions from consecutive planes parallel to (001).