Search results for "Spontaneous magnetization"
showing 5 items of 15 documents
Phase transitions in nanosystems caused by interface motion: the Ising bipyramid with competing surface fields.
2005
The phase behavior of a large but finite Ising ferromagnet in the presence of competing surface magnetic fields +/- H_s is studied by Monte Carlo simulations and by phenomenological theory. Specifically, the geometry of a double pyramid of height 2L is considered, such that the surface field is positive on the four upper triangular surfaces of the bi-pyramid and negative on the lower ones. It is shown that the total spontaneous magnetization vanishes (for L -> infinity) at the temperature T_f(H), related to the "filling transition" of a semi-infinite pyramid, which can be well below the critical temperature of the bulk. The discontinuous vanishing of the magnetization is accompanied by a…
The McCoy-Wu model in the mean-field approximation
1998
We consider a system with randomly layered ferromagnetic bonds (McCoy-Wu model) and study its critical properties in the frame of mean-field theory. In the low-temperature phase there is an average spontaneous magnetization in the system, which vanishes as a power law at the critical point with the critical exponents $\beta \approx 3.6$ and $\beta_1 \approx 4.1$ in the bulk and at the surface of the system, respectively. The singularity of the specific heat is characterized by an exponent $\alpha \approx -3.1$. The samples reduced critical temperature $t_c=T_c^{av}-T_c$ has a power law distribution $P(t_c) \sim t_c^{\omega}$ and we show that the difference between the values of the critical…
Illustrating the processability of magnetic layered double hydroxides: layer-by-layer assembly of magnetic ultrathin films.
2013
We report the preparation of single-layer layered double hydroxide (LDH) two-dimensional (2D) nanosheets by exfoliation of highly crystalline NiAl-NO3 LDH. Next, these unilamellar moieties have been incorporated layer-by-layer (LbL) into a poly(sodium 4-styrenesulfonate)/LDH nanosheet multilayer ultrathin film (UTF). Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible light (UV-vis), and X-ray diffraction (XRD) profiles have been used to follow the uniform growth of the UTF. The use of a magnetic LDH as the cationic component of the multilayered architecture enables study of the resulting magnetic properties of the UTFs. Our magnetic data show the appearance of spontaneous …
Spontaneous Magnetization in Ni−Al and Ni−Fe Layered Double Hydroxides
2008
Layered double hydroxides containing paramagnetic Ni (II) and diamagnetic/paramagnetic Al (III)/Fe (III) ions have been prepared and characterized. Ni 2Al(OH) 6(NO 3). nH 2O ( 1), Ni 2Fe(OH) 6(NO 3). nH 2O ( 2), Ni 2Fe(OH) 6(C 6H 8O 4) 0.5. nH 2O ( 3), and Ni 2Fe(OH) 6(C 10H 16O 4) 0.5. nH 2O ( 4) were prepared by coprecipitation at controlled pH as polycrystalline materials with the typical brucite-like structure, with alternating layers of hydroxide and the corresponding anions, which determine the interlayer separation. Magnetic studies show the appearance of spontaneous magnetization between 2 and 15 K for these compounds. Interestingly, the onset temperature for spontaneous magnetizati…
Chiral Molecular Magnets: Synthesis, Structure, and Magnetic Behavior of the Series [M(L-tart)] (M = MnII, FeII, CoII, NiII;L-tart = (2R,3R)-(+)-tart…
2006
A new series of layered magnets with the formula [M(L-tartrate)] (M = Mn(II), Co(II), Fe(II), Ni(II); L-tartrate = (2R,3R)-(+)-tartrate) has been prepared. All of these compounds are isostructural and crystallize in the chiral orthorhombic space group I222, as found by X-ray structure analysis. Their structure consists of a three-dimensional polymeric network in which each metal shows distorted octahedral coordination bound to four L-tartrate ligands, two of which chelate through an alcohol and a carboxylate group and the other two bind terminally through a monodentate carboxylate group. The chirality of the ligand imposes a Delta conformation on all metal centers. Magnetically, the paramag…