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Variation of the spontaneous magnetization m s (0), measured in units of g B , with the exchange interaction J 0 /W for a U/W0.125, b U/W0.25, and c U/W0.375.
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A study of the electrical transport and magnetic properties of a series of cluster compounds with the generic formula A0.5M2X4 suggests that the electrons at the Fermi surface are localized, and the ferromagnetism seen in these compounds arises from these electrons. The magnetism of these compounds shows some features characteristic of itinerant mo...
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... xUJ 0 m s (0)/2W. We plot the solutions of this equation for different values of U/W over a range of J 0 /W in Fig. 2. One notes that the value of m s (0) depends mainly on U, which as mentioned earlier controls the number of singly occupied states in the occupied band and is not much influenced by the choice of J 0 /W and it is always less than the high field saturation magnetization of 0.5g B ...
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Citations
... The n=½ corresponds to Efros-Sklovskii type variable range hopping in the presence of a soft gap at Fermi level due to the inter-site coulomb interaction [25]. Lamba et al devised a theoretical model to understand this type of T dependence [26]. They suggested that in the region of low conductivity i.e. at low T regions, electron hops in the background of frozen charge and experiences the coulomb gap leading to the T 1 /2 behavior. ...
GaV4S8 is a Mott insulator having structural transition at ∼ 44 K and ferromagnetic ordering below ∼12 K. In this article,we present the results of the dc resistivity, ac conductivity, dielectric constant and the magneto-dielectric effect (MDE) measurements on this compound. We observed a crossover from Mott type to Effros-Sklovskii type variable range hopping in going from higher to lower temperature. Frequency dependence of conductivity suggests the hopping transport of the carriers between the pair centers only. The Magneto-dielectric coupling (maximum value of 20%near 22 K) is maximum in the intermediate region of structural and magnetic transitions where exchange splitting of the Fermi surface develops the energetically favorable state for ferromagnetic ordering.
... Magnetic insulators are a class of materials realized in many various compounds. An example of them are materials known as transition metal cluster compounds with general formula AM 4 X 8 , where A-trivalence metal, M -transition metal, X-chalcogenide [1][2][3]. Phase separations can occur in magnetic insulators in various circumstances and their theoretical understanding is very current topic. Moreover, instabilities such as stripe formation (as well as charge order) can occur in high-T c superconductors (e.g. in cuprates) [4]. ...
In this work we focus on the study of phase separation in the zero-bandwidth
extended Hubbard with nearest-neighbors intersite Ising-like magnetic
interactions $J$ and on-site Coulomb interactions $U$. The system has been
analyzed by means of Monte Carlo simulations (in the grand canonical ensemble)
on two dimensional square lattice (with $N=L\times L =400$ sites) and the
results for $U/(4J)=2$ as a function of chemical potential and electron
concentration have been obtained. Depending on the values of interaction
parameters the system exhibits homogeneous (anti-)ferromagnetic (AF) or
non-ordered (NO) phase as well as phase separation PS:AF/NO state. Transitions
between homogeneous phases (i.e. AF-NO transitions) can be of first or second
order and the tricritical point is also present on the phase diagrams. The
electron compressibility $K$ is an indicator of the phase separation and that
quantity is of particular interest of this paper.
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The ternary transition metal compounds AlxVS2 (0.25 ≤ x ≤ 0.50) have structures derived from hexagonal CdI2–NiAs with an enlarged 2a0 × 2a0 × 2c0 × (2T) 2a0 × 2a0 × 3c0 × (3T) basic cell alongwith structural distortions. Transport properties are very sensitive to the temperature of preparation. The low temperature annealed (LT) phases show higher conductivity at room temperature than high temperature quenched (HT) phases. On cooling, these metal-like phases show two different transitions in conductivity, first one around 160 to 180 K giving insulator-like phases and the second around 30 to 40 K where the resistance shows a sharp decrease on cooling. The phases quenched above 1000 °C give widely different transport and thermoelectric properties depending upon Al concentration with metallic (x = 0.50), insulating (x = 0.33) or semiconducting (x = 0.25) behaviour. The magnetic susceptibility of all these phases shows strong temperature dependence above 80 K. These properties are discussed in terms of localization of carriers on V–V clusters and structural disorder.
The structural dependences of the galvanomagnetic properties of Co-Al and Fe-Al alloy films were investigated in this study. Ordered and disordered alloy films with thicknesses of 150 nm were prepared by using the flash evaporation technique on the heated and cooled substrates, respectively. The temperature dependence of resistance was measured in the range of 2∼300 K range with and without a magnetic field of 0.5 T. The influence of the order-disorder structural transition on the temperature dependence of the resistance is discussed in connection with the results for the magnetic properties and is analyzed in the framework of the partial localization of the electronic states and variable-range hopping conductivity.
A simple effective model for a description of magnetically ordered narrow-band insulators is studied. The hamiltonian considered consists of the effective on-site interaction ( ) and intersite magnetic exchange interactions ( , ) between nearest-neighbours. The phase diagrams and properties of this model for arbitrary chemical potential and arbitrary electron density have been determined within several approaches: (i) the variational method (which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation) for any , (ii) the Monte Carlo simulations on a square lattice with periodic boundary conditions for , and (iii) other approximate methods (inter alia: random phase approximation and spin-wave approximation) as well as (iv) rigorous treatment to obtain results concerning the ground state phase diagrams (the two last also for ). The investigations of the general case show that, depending on the values of interaction parameters and electron concentration , the system can exhibit not only homogeneous phases: (anti-)ferromagnetic ( , ) and nonordered (NO), but also phase separated states ( : /NO). For a fixed one finds the following phase transitions (both continuous and discontinuous ones) and their sequences, which can occur with increasing temperature: NO, NO, NO, NO. The system analyzed exhibits also tricritical behaviour.
The influence of the order-disorder structural transition on the transport properties of Fe-rich B2-phase Fe0.52Al0.48 alloy films has been investigated. The disordered alloy film was obtained by vapor-quenching deposition onto glass substrates cooled by liquid nitrogen. The electric resistance of the ordered and disordered Fe0.52Al0.48 alloy films have been measured in the 5 300 K temperature range in the presence and absence of a magnetic field of 0.5 T. The influence of the order-disorder structural transition on the location of the resistance minimum at low temperature was analyzed within the framework of the ``variable-range-hopping conductivity'' approach.
The influence of the order-disorder structural transition on the magnetic and transport properties of Co-rich β-phase CoxAl1-x (x=0.54 and 0.62) alloy films has been investigated. The disordered state in these alloy films was obtained by vapor-quenching deposition onto glass substrates cooled in liquid nitrogen. The transport properties of the ordered and disordered CoxAl1-x alloy films have been measured between 2 and 300 K with and without a magnetic field of 0.5 T. The influence of the order-disorder structural transition on the temperature dependence of the resistance was discussed in the context of magnetic (magnetization, magnetic susceptibility, and ferromagnetic resonance) measurements, and analyzed in terms of the partial localization of the electronic states and variable-range hopping conductivity.
We present the results of the magnetic and specific heat measurements on V4 tetrahedral-cluster compound GaV4S8 between 2 and 300 K. We find two transitions related to a structural change at 42 K followed by ferromagnetic order at 12 K on cooling. Remarkably similar properties were previously reported for the cluster compounds of Mo4. These compounds show an extremely high density of low energy excitations in their electronic properties. We explain this behavior in a cluster compound as due to the reduction of coulomb repulsion among electrons that occupy highly degenerate orbits of different clusters.
Magnetization measurements on Ga <sub> 0.6 </sub> Mo <sub> 2 </sub> S <sub> 4 </sub> cluster compound of modified spinel were performed at temperatures from 2 to 30 K. Temperature irreversibility of the magnetization develops just below the ferromagnetic transition temperature. Relaxation isotherms of the thermoremanent magnetizations can be described by a superposition of a stretched exponential and a constant term, and shows distinctive aging effects. This implies the presence of the orientational spin freezing in the ferromagnetic regime of the Ga <sub> 0.6 </sub> Mo <sub> 2 </sub> S <sub> 4 </sub>. © 1998 American Institute of Physics.
In this report we have analyzed a simple effective model for a description of
magnetically ordered insulators. The Hamiltonian considered consists of the
effective on-site interaction (U) and the intersite Ising-like magnetic
exchange interaction (J) between nearest neighbors. For the first time the
phase diagrams of this model have been determined within Monte Carlo simulation
on 2D-square lattice. They have been compared with results obtained within
variational approach, which treats the on-site term exactly and the intersite
interactions within mean-field approximation. We show within both approaches
that, depending on the values of interaction parameters and the electron
concentration, the system can exhibit not only homogeneous phases:
(anti-)ferromagnetic (F) and nonordered (NO), but also phase separated states
(PS: F-NO).
