Long-range Ferrimagnetic Order Research Articles

Formation of long-range ferrimagnetic order in dilute spinels Li0.5Fe2.5−xGaxO4 near the multicritical point

The influence of spatial inhomogeneity of short-range exchange on the formation of long- range ferrimagnetic order in dilute spinels is investigated. The objects of investigation are two types of polycrystalline samples of Li0.5Fe1.1Ga1.4O4 differing in the extent of heterogeneity in the distribution of magnetic (Fe3+) and nonmagnetic (Ga3+) ions in the lattice. The heterogeneity was created by using two regimes of thermal treatment, i.e., slow cooling (S) and quenching (Q) of samples from 1550 to 300 K. The results of analysis of magnetization isotherms σT(H) at T=4.2 K, (77–300) K, H⩽16 kOe and temperature dependences of initial susceptibility χ0(T) show that the long-range ferrimagnetic order near the multicritical point (x0=1.5) of the x–T phase diagram is stable to variations of the degree of spatial heterogeneity of the exchange.

Long-Time Decay of Thermoremanent Magnetization through Stepwise Changes of the Reentrant Spin-Glass-Like State in the Ni2In-Type Mn7Sn4

The zero-field-cooled magnetization at the magnetic field H = 300 Oe shows anomolous kinks at T A and T e (> T A ) far below the reentrant spin-glass-like transition temperature T f (above which a long-range ferrimagnetic ordering is set up). The irreversibility between the zero-field-cooled and the field-cooled magnetizations, and the time decay of the thermoremanent magnetization (σTRM) are observed below the ferrimagnetic Curie temperature T C . The decay feature can be described by Ogielski's power law σTRM = At -a whose a exhibits peaks at T A , T B , T f and T C . The peak of α at Te can be fitted by the combination of Ising type and Ginzburg-Landau type expressions. Three stepwise changes of the spin-glass-like state below T f and the feature of the coexistence of short-range spin-glass and long-range magnetic ordering components above T f are discussed.

A magnetization study of the frustrated spinel compound Mg 1.55Fe 0.9Ti 0.55O 4

Former studies suggested a reentrant spin-glass-like behavior for the diluted spinel compound Mg 1.55Fe 0.9Ti 0.55O 4. Near equilibrium magnetization measurements in very low magnetic fields show, however, that there is no transition of any kind at any temperature. We suggest that the long-range ferrimagnetic order is destroyed by the dipolar interactions since a moderate magnetic field restores a ferrimagnetic critical behavior at 164 K. There is no evidence of a sharp spin glass or canting transition at low temperature. Short-range correlations are observed up to 300 K with a slow dynamics. High fields magnetizatization measurements confirm the reentrant character and give evidence of the frozen disorder or fluctuations which give rise to strong high field susceptibility.

X-ray, SEM and magnetic properties of Mg[sbnd]Cd ferrites

Magnetic properties of polycrystalline ferrites CdxMg1−xFe2O4(x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) are studied for their saturation magnetisation nB, AC susceptibility χ and initial permeability μi. Dependence of saturation magnetisation on Cd2+ content suggest that A—B and B—B superexchange interactions are comparable in strength. Néel's two sublattice model is applicable upto x ⩽ 0.4 while Y—K three sublattice model is predominent for x ⩽ 0.4. Variation of AC susceptibility and initial permeability with temperature revealed the long-range ferrimagnetic ordering in the compounds having x ⩽ 0.4. The sample with x ⩽ 0.2 exhibits, multidomain structure, while for x = 0.4 shows SD—SP transition and for x ⩾ 0.6 the samples show S—P domain structure. Low-frequency dispersion of initial permeability suggests domain wall displacement. Addition of Cd2+ results in the sharp decrease in crystallographic phase transition temperature Tp and Curite temperature Tc of the samples.

Magnetic behavior of nanocrystalline cobalt ferrite films

When prepared with a nanocrystalline structure, cobalt ferrite films often exhibit very low magnetization compared with that of the bulk material. We report here on low-temperature magnetization measurements which show that the long-range ferrimagnetic order is broken in the films and that a glassy behavior results. Even though we cannot ascertain the nature of the magnetic disorder, our data are consistent with the molecular field theory of Ising spin glasses with strong ferromagnetic interactions.

Ferrimagnetic long-range order of the Hubbard model.

In this paper, we show rigorously that there exists the ferrimagnetic long-range order in the ground state of the positive-U Hubbard model at half filling on some bipartite lattices. When ${\mathit{N}}_{\mathit{A}}$>${\mathit{N}}_{\mathit{B}}$(${\mathit{N}}_{\mathit{A}}$ and ${\mathit{N}}_{\mathit{B}}$ are the total site numbers of two sublattices A and B), except for the ferromagnetism which was found by Lieb [Phys. Rev. Lett. 62, 1201 (1989)], there also exists the antiferromagnetic long-range order in the ground state. This result only requires U>0 and is independent of the dimension of the lattices.

Magnetic state of intermetallic Ho 1− x Y x(Co 1.95Al 0.05) 2 compounds

In order to investigate the influence of Al atoms on the magnetic properties of the RCo 2 compounds, neutron diffraction, a.c. susceptibility, thermal expansion and electrical resistivity measurements have been performed on the Ho 1− x Y x (Co 0.95Al 0.05) 2 compounds. A comparison of this system with the Ho 1− x Y x Co 2 compounds with x > x c = 0.58 shows that the substitution of Co atoms by Al atoms decreases the critical field which produces the splitting of the d-band. This leads to a long-range ferrimagnetic order in Ho 1− x Y x (Co 0.95Al 0.5) 2 with 0.58< x<0.8, while in the Ho 1− x Y x Co 2 compounds the Ho subsystem shows a short-range order and the Co subsystem remains paramagnetic.

The onset of a long-range magnetic order in the spin glasses Y(Co 1− xMn x) 2 induced by Gd substitution

The influence of the f-d exchange interaction (induced by substitutions of Gd for Y) on the magnetic behaviour of the itinerant spin glasses Y(Co 1− x Mn x ) 2 is studied for x = 0.07 and 0.25. The substitutions give rise to a long-range ferrimagnetic order in Y 1− t Gd t (Co 1− x Mn x ) 2 systems at t ≥ 0.05 ( x = 0.25) and t ≥ 0.2 ( x = 0.07), with both f- and d-subsystems being magnetically ordered. At lower Gd concentrations the spin glass state is kept in these systems. The mean field approximation is used for analyzing the magnetization processes in compounds with homogeneously polarized d-band. The magnetization curve in effective fields up to ⋍ 1200 kOe is drawn for Y(Co 0.75Mn 0.25) 2.

MICROWAVE MAGNETIC RESONANCE IN REENTRANT SPIN GLASS SPINEL CoxZn1-x(FeyCr1-y)2O4

A microwave magnetic resonance experiment has been performed on the spinel system CoxZn1-x(FeyCr1-y)2O4 in the reentrant spin glass region (0.4≤x≤0.8, y = 0.5, 0.8). The temperatue dependence of the line shape, linewidth, resonant field, g-factor, and the relationship of the linewidth to the concentration of magnetic ions have been studied. The mechanism of the experimental phenomena has been discussed using the spin clustering effect and the local random field model. The microwave magnetic resonance properties in the system can be attributed to the formation of spin clusters, the establishment of the long-range ferrimagnetic order, the collapse of the ferrimagnetic order and random freezing of spin clusters.

Magnetostructural correlations in some one-dimensional exotic systems (invited) (abstract)

The thermodynamics of one-dimensional (1D) magnetic systems is now well established for uniform ferro-or antiferromagnetic chains. In the last few years, several exotic arrays of metal ions exhibiting unusual magnetic behaviors have been isolated both in molecular and solid-state chemistry. Among these, the engineering of structurally ordered bimetallic chains and the breakthrough in the understanding of magnetostructural correlations have allowed the development of new classes of magnetic materials. This paper will focus on several types of low-dimensional materials characterized by exotic stackings of the metal ions. (i) The bimetallic complexes MnM(edta) 6H2O (M=Ni, Cu). The low-temperature magnetic behavior will be discussed in terms of alternating Heisenberg ferrimagnetic chains. (ii) The orbitally degenerate chain CoCu(pbaOH)(H2O)3⋅4H2O. The similarity with a branch chain, due to the spin-orbit-coupling effect for Co (II) ions, will be used for the magnetic analysis. (iii) The Cu(II) phosphates A3Cu3(PO4)4, where A=Ca, Sr. In this case, 1D ferrimagnetism will be related for the first time to a pure topology effect. The bahavior of Cu3(PO4)2 in which Cu(II) trimers interact through outer ions will be examined. (iv) The vanadyl phosphate VO(HPO4)⋅4H2O exhibiting frustrated chains. For all these low-dimensional systems, the behavior will be closely related to the stacking of magnetic ions and their connections through bridging anions. Specific models of exchange coupling will be developed. On the other hand, the conditions for the occurrence of long-range ferrimagnetic ordering will be investigated.

On the magnetic ordering in the insulating spinel Ga 0.8Fe 0.2NiCrO 4 with the A sublattice diluted to below the percolation threshold

The results of a.c. susceptibility, d.c. magnetization and Mössbauer measurements on the insulating spinel system Ga 0.8Fe 0.2NiCrO 4 are reported. This system has the A sublattice diluted to below the percolation threshold and hence cannot retain any long-range ferrimagnetic order. The a.c. susceptibility shows a peak at T f ≈ 50K. For T#62; 1.5T f the magnetization is highly non-linear and tend to saturate above H #62; 2kOe. The Mössbauer spectra show broad doublets at 77 and 300 K. These observations lead to the picture of gradually evolving spin clusters ferrimagnetically coupled within, as the temperature is lowered, till the intercluster interaction brings about a cluster spin-glass ordering with randomly frozen cluster moments at T f. The absence of the Zeeman splitting in the Fe 57 Mössbauer spectra could be understood in terms of the dynamical effects arising from the thermal excitation of spin clusters.

On the Role of Intrasublattice Interactions in YIG with Diamagnetically Substituted a-Sublattice

Neutron diffraction and computer simulation on the yttrium–iron garnet (YIG) with diamagnetically substituted octahedral a-sublattice show that the intrasublattice interactions shift the concentration at which the long-range ferrimagnetic order appears from the percolation threshold xp = 0.205(3) up to xc = 0.40(2). In the vicinity of xc they cause the random spin canting reducing at low temperature the sublattice magnetizations as well as their mean values in the frozen magnetic fluctuations, especially for the tetrahedral d-sublattice. [Russian Text Ignored.]