Onset Of Long-range Ferromagnetic Order Research Articles

Tuning the magnetic properties of transition metal MOFs by metal–oxygen condensation control: the relation between synthesis temperature, SBU nuclearity and carboxylate geometry

Five novel metal organic frameworks, belonging to three structural types, have been obtained hydrothermally. Three of them, with formula [M3(hfipbb)2(OH)2(H2O)] (M = Mn, Co, Ni; hfipbb = 4,4′-(hexafluoroisopropylidene)bis(benzoate) dianion) belong to the first structural type. The crystal structure of the Mn-based compound was obtained from single crystal synchrotron X-ray diffraction data, while the isostructural compounds of Co and Ni were studied by means of Rietveld analysis using powder X-ray diffraction data. The magnetic behavior of the compounds with M = Co and Ni is characterized by the onset of long-range ferromagnetic order at very low temperature with Curie temperatures near 8 and 2.5 K, respectively. Such spontaneous magnetization seems to be preceded by low-dimensional magnetic interactions, due to the nature of the secondary building units (SBUs) of the structure, which are triple chains built by M3O14 units. Structural and topological analyses of [Mn2(hfipbb)2(H2hfipbb)] and [Co2(hfipbb)2(H2hfipbb)] (2 and 3 structural types, respectively; H2hfipbb = 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)) demonstrate that the packing of the rod-shaped SBUs in 2 prevents the framework interpenetration giving rise to a bnn net whereas 3 is a 2-fold interpenetrated pcu net with isolated paddle-wheel clusters as SBUs pcu. Concerning the magnetic behavior of 2 and 3, antiferromagnetic interactions observed at very low temperature (magnetic susceptibility maximum at 2.8 and 18 K, respectively) are confined to the secondary building units.

Growth and magnetic anisotropy of Fe films deposited on Si(111) using an ultrathin iron silicide template

The growth and magnetic properties of thin Fe films deposited at room temperature on ultrathin iron silicide seed layer epitaxially grown on Si(111) single crystal were investigated as a function of Fe thickness ($0<{t}_{\mathrm{Fe}}<300$ monolayers). The growth mode and structure have been determined in situ by means of scanning tunneling microscopy, low energy electron diffraction, and x-ray photoelectron diffraction. The magnetic properties were characterized ex situ by conventional polar and longitudinal magneto-optical Kerr effect, transverse biased initial inverse susceptibility and torque (TBIIST) measurements, and superconducting quantum interference device magnetometry. Fe growth is of Volmer-Weber type (island growth) and the epitaxial film adopts the bcc $\ensuremath{\alpha}$-Fe structure. Onset of long-range ferromagnetic order occurs at 4.7 monolayers (ML), in the vicinity of the percolation threshold of the Fe islands. The Curie temperature increases continuously with Fe coverage, varying from 135 K for 4.7 ML to 260 K for 7.3 ML. Two different spin reorientation transitions have been observed versus Fe coverage. First, the magnetic easy axis rotates from normal to the film plane, for coverage below 6 ML, to in plane, for thickness above 7 ML. Then, in-plane magnetized films present tiny (anisotropy fields less than 4 Oe) uniaxial and sixfold magnetic anisotropies. From sixfold anisotropy the small higher order cubic anisotropy constant ${K}_{2}$ was measured precisely by TBIIST. It decreases monotonously with increasing coverage and changes its sign at approximately 20 ML, which in turn results in a switching of the sixfold anisotropy easy axis from ⟨1--10⟩ to ⟨1--21⟩ directions. It appears that TIBIIST magnetometry is a powerful method for a quantitative determination of the various contributions to in-plane magnetic anisotropies in ultrathin films.

Evidence for the pressure induced magnetic ordering in YbInCu4

Abstract We have carried out electrical resistivity ρ and AC-susceptibility χ AC measurements on YbInCu 4 under pressure. For pressures above 2.49 GPa, the first-order valence transition is completely suppressed and a clear peak appears in χ AC with a small kink in ρ at T M =2.4 K. The χ AC peak is easily diminished by applying low magnetic fields and disappears above ∼500 Oe. The characteristic behavior of χ AC at T M can generally be ascribed to the onset of long-range ferromagnetic ordering and, therefore, the ground state of the pressure-stabilized phase of YbInCu 4 is most probably a ferromagnetically ordered state.

Magnetic ordering in the pressure-stabilized high-temperature phase ofYbInCu4

To elucidate the ground state of the pressure-stabilized high-temperature (HT) phase of ${\mathrm{YbInCu}}_{4},$ we have carried out electrical resistivity $\ensuremath{\rho}$ and ac-susceptibility ${\ensuremath{\chi}}_{\mathrm{ac}}$ measurements at high pressures. For pressures above 2.49 GPa, the first-order valence transition is completely suppressed (below $\ensuremath{\sim}80\mathrm{mK}).$ Separately, above 2.39 GPa, a clear peak appears in ${\ensuremath{\chi}}_{\mathrm{ac}}$ with a small kink in $\ensuremath{\rho}$ at around ${T}_{M}=2.4\mathrm{K}.$ The ${\ensuremath{\chi}}_{\mathrm{ac}}$ peak is easily diminished by applying low magnetic fields and disappears above $\ensuremath{\sim}500\mathrm{Oe}.$ The characteristic behavior of ${\ensuremath{\chi}}_{\mathrm{ac}}$ at ${T}_{M}$ can generally be ascribed to the onset of long-range ferromagnetic ordering and, therefore, the ground state of the pressure-stabilized HT phase is most probably a ferromagnetically ordered state. This result is compatible with the occurrence of weak ferromagnetism recently reported for the Y-substituted compound ${\mathrm{Yb}}_{0.8}{\mathrm{Y}}_{0.2}{\mathrm{InCu}}_{4}$ under pressure of 1.2 GPa.

Magnetic properties of ZrNi 5− xIn x (0⩽ x⩽1) ternary system

Magnetisation was measured for the series of ZrNi 5− x In x ( x=0, 0.25. 0.50, 0.75 and 1.0) alloys over the temperature range 1.75–700 K in applied field up to 50 kOe. All materials studied crystallise in the AuGe 5-type crystal structure. Alloys with x=0, 0.25 and 0.5 were found to be ferromagnets with relatively high transition temperatures, T c, dependent on the indium concentration (from T c=368 K for x=0.5 up to 647 K for x=0). The measured saturation magnetic moments are fully connected with nickel atom; the Zr and In moments are negligible. An environment-dependent model for the formation of Ni moments is suggested. The critical concentration of Ni for the onset of long-range ferromagnetic order in ZrNi 5− x In x is estimated as 4.5 atoms/f.u. (75 at%), approximately. The long-range magnetic order is determined by the number of the nearest neighbours of Ni atoms occupying 16(e) positions. Alloys with x=0.75 and 1.0 exhibit Pauli paramagnetism.

Isotropic-Heisenberg to isotropic-dipolar crossover in amorphous ferromagnets with composition near the percolation threshold

ac (zero-field) susceptibility of the amorphous ferromagnetic alloys (Fe p Ni 1-p ) 80 B 16 Si 4 (0.0540 p' Ni 1-p' ) 80 B 16 Si 4 (0.1125 c or p c' for the onset of long-range ferromagnetic order has been measured to very high precision in the absence and presence of dc (static) magnetic field over a wide temperature range embracing the critical region near the ferromagnetic-paramagnetic phase transition. Elaborate data analyses permit the accurate determination of the asymptotic critical exponents β D , γ D , and δ D for spontaneous magnetization, susceptibility and the critical isotherm as well as the leading correction-to-scaling exponent for susceptibility (these exponents characterize the isotropic dipolar fixed point) and hence assert that the asymptotic critical behavior of amorphous ferromagnets with p c ≲p c '≲p' D , γ D , and δ D do satisfy the Widom scaling relation β+γ=βδ accurately. In addition, the temperature dependence of the effective critical exponent for susceptibility observed in the amorphous ferromagnetic alloys with p C as e=(T-T C )/T C ≃ 10 -5 in the alloys with p>0.1 or p'>0.2, which behave as normal ferromagnets down to 3.8 K. A sharp contrast in the critical behavior of ferromagnets that either do or do not exhibit reentrant behavior at low temperatures is shown to reflect the decisive role played by the isotropic dipolar long-range interactions in establishing long-range ferromagnetic order in dilute magnetic systems exhibiting reentrant behavior.

Magnetization reversal and spin reorientation in Fe/Cu(100) ultrathin films

The magnetization reversal in perpendicular applied magnetic fields in low-temperature grown Fe/Cu(100) ultrathin films was studied in situ by magneto-optical Kerr effect, Kerr-microscopy, and scanning tunneling microscopy. It was found that the onset of long-range ferromagnetic order at \ensuremath{\cong}0.9 monolayers (ML) is related to the coalescence of bilayer islands of Fe. Below 3.8 ML, the magnetization-reversal process takes place in a narrow field range and is characterized by domain-wall motion. While the domain boundaries are rather smooth at thin films, domains get increasingly irregularly shaped above 3 ML, which is assigned to a decrease of the domain-wall energy. At the spin-reorientation transition from out-of-plane to in-plane magnetization at \ensuremath{\cong}3.8 ML, two coexisting metastable spin configurations are found.

Onset of long-range ferromagnetic order in a system of ferromagnetic particles with giant magnetic moments

Magnetic, x-ray, and small-angle neutron scattering data obtained for the decomposed alloy Cu64Mn9Al27 are used to show that the onset of long-range ferromagnetic order in a system of small superparamagnetic grains dissolved in a nonmagnetic matrix is attributable to cooperative ordering of their magnetic moments.

Lyapunov spectral analysis of a nonequilibrium Ising-like transition.

By simulating a nonequilibrium coupled map lattice that undergoes an Ising-like phase transition, we show that the Lyapunov spectrum and related dynamical quantities such as the dimension correlation length~$\xi_\delta$ are insensitive to the onset of long-range ferromagnetic order. As a function of lattice coupling constant~$g$ and for certain lattice maps, the Lyapunov dimension density and other dynamical order parameters go through a minimum. The occurrence of this minimum as a function of~$g$ depends on the number of nearest neighbors of a lattice point but not on the lattice symmetry, on the lattice dimensionality or on the position of the Ising-like transition. In one-space dimension, the spatial correlation length associated with magnitude fluctuations and the length~$\xi_\delta$ are approximately equal, with both varying linearly with the radius of the lattice coupling.

Growth and morphology of ultrathin Fe films on Cu(001).

The relationship between magnetism and morphology of pseudomorphic ultrathin films of fcc Fe on Cu(001) is reexamined in view of morphological data obtained with scanning tunneling microscopy. Two types of films showing different magnetic properties were investigated. Films prepared at 300 K are found to grow in a good layer-by-layer mode. Their magnetism appears to be influenced by structural instabilities connected with a martensitic fcc\ensuremath{\rightarrow}bcc transformation. Films deposited at 130 K and later annealed at 300 K grow in a Stranski-Krastanov-like mode, resulting in a drastically different surface morphology. They are found to be structurally stable in the region where the spin reorientation takes place. An attempt is made to correlate the enhanced roughness of these films with their magnetic properties. Finally, the connection between the morphological percolation and the onset of long-range ferromagnetic ordering is discussed. The percolation of the first layer is found to be insufficient to establish long-range ferromagnetic ordering in these films.

Magnetic ordering in the three-dimensional frustrated Heisenberg model.

Monte Carlo simulations of moderately frustrated three-dimensional Heisenberg spin systems show evidence for two distinct ordering events. The upper one, at ${\mathit{T}}_{\mathit{c}}$, marks the onset of long-range ferromagnetic order and exhibits fluctuations which scale as the system size. The lower event, at ${\mathit{T}}_{\mathit{x}\mathit{y}}$, marks the freezing of transverse degrees of freedom and does not affect the long-range order in the system. Critical levels of frustration for the onset of ferromagnetism have been determined on several lattices and a phase diagram is presented.

Evidence against the existence of unstable magnetic moments in the system Ce(Fe1-xCox)2

57Fe Mossbauer studies in the system Ce(Fe1-xCox)2 for x=0.15 and 0.25 have shown the sudden loss in magnetisation in this system, well bellow the onset of long-range ferromagnetic order, arises due to the reorientation and locking of Fe moments in such a way as to produce almost zero net magnetisation. The thermal variation of the average hyperfine field, however, does not have any signature of the reorientation transition temperature.

Elastic diffuse neutron scattering from weakly ferromagnetic Pt alloys

Unpolarised neutron diffraction measurements on weakly ferromagnetic PtCo and PtFe alloys give elastic diffuse magnetic cross sections which exhibit marked forward peaks confirming the inhomogeneous distribution of magnetisation in these alloys. The local moments on the solute atoms are 2.00+or-0.06 mu B and 3.16+or-0.04 mu B for Co and Fe respectively. These are approximately independent of composition, but it is not possible to obtain a characteristic polarisation range for the Pt matrix algorithm indications are that this should be of the nearest-neighbour distance only. The forward cross sections are much larger than would reasonably be expected from the bulk magnetisation values of (d mu sat/dc), where mu sat is the saturation magnetisation. It is therefore argued that the Marshall model is inapplicable to unpolarised neutron data for weakly ferromagnetic alloys. Consequently an alternative model has been invoked wherein the cross sections have been interpreted as arising from the critical scattering of neutrons by concentration-driven critical fluctuations of the spontaneous magnetisation which occur near the percolation concentration for the onset of long-range ferromagnetic order.

Magnetization processes caused by iron substitution in cubic laves phases

The magnetic properties of cubic Laves phases RE(Fe, M) 2, M = Al, Co, Rh are discussed. Pauli-paramagnetism is obtained for all three YM 2 compounds. The appearance of a percolation limit for the onset of long-range ferromagnetic order which depends strongly on the matrix susceptibility is characteristic for all investigated series Y (Fe, M) 2. Mictomagnetic behaviour can be deduced from magnetic, Mössbauer- and neutron depolarization measurements below the percolation limit. A Co moment of approximately 1 μ B is induced in RECo 2 compounds, RE = Gd, Dy, Ho. On the contrary Rh has no magnetic moment in these compounds. Within the RE(Fe, Co) 2 series a Co moment is induced by both the RE and Fe atoms, whereas within RE(Fe, Rh) 2 the Rh atoms will only be influenced by the substituted Fe. In all compounds the spin of the heavy RE atoms couples antiparallel to the spin of the transition metal atoms. Local environment effects are important for all Fe containing series whereas the properties of RECo 2 compounds evidence itinerant electron metamagnetism.