Possible magnetic-field-induced voltage and thermopower in diluted magnetic semiconductors

In diluted magnetic semiconductors, the carrier concentration and the magnetization of local moments are strongly coupled, since the magnetic interaction is mediated by the carriers. It is predicted that this coupling leads to an electric polarization due to an applied magnetic-field gradient and to the appearance of a magnetic-field-dependent voltage. An expression for this voltage is derived within Landau theory and its magnitude is estimated for (Ga,Mn)As. Furthermore, a large contribution to the thermopower based on the same mechanism is predicted. The role of fluctuations is also discussed. These predictions hold both if the magnetization is uniform and if it shows stripe-like modulations, which are possible at lower temperatures.Comment: 6 pages revtex, 5 figure

Comment on "Strong dependence of the interlayer coupling on the hole mobility in antiferromagnetic La2−x_{2-x}Srx_xCuO4_4 (x<0.02x<0.02)"

Using the experimental data given in Phys. Rev. B70, 220507 (2004), we show that -- unlike the effective coupling discussed in this paper -- the net average antiferromagnetic interlayer coupling in doped lanthanum cuprates depends only weakly on the doping or on the temperature. We argue that the effective coupling is proportional to the square of the staggered magnetization, and does not supply new information about the origin of the suppression of the magnetic order in doped samples. Our analysis is based on a modified version of the equation describing the spin-flip transition, which takes into account the decrease of the staggered moment with temperature and doping.Comment: Phys. Rev. B (in press

Competing frustration and dilution effects on the antiferrromagnetism in La(2-x)Sr(x)Cu(1-z)Zn(z)O(4)

The combined effects of hole doping and magnetic dilution on a lamellar Heisenberg antiferromagnet are studied in the framework of the frustration model. Magnetic vacancies are argued to remove some of the frustrating bonds generated by the holes, thus explaining the increase in the temperature and concentration ranges exhibiting three dimensional long range order. The dependence of the Neel temperature on both hole and vacancy concentrations is derived quantitatively from earlier renormalization group calculations for the non--dilute case, and the results reproduce experimental data with no new adjustable parameters.Comment: 4 pages, 2 figure

Suppression of antiferromagnetic correlations by quenched dipole--type impurities

The effect of quenched random ferromagnetic bonds on the antiferromagnetic correlation length of a two--dimensional Heisneberg model is studied, applying the renormalization group method to the classical non--linear sigma model with quenched random dipole moments. It is found that the antiferromagnetic long range order is destroyed for any non--zero concentration, of the dipolar defects, even at zero temperature. Below a line T ~ concentration, the correlation length is independent of T, and decreases exponentially with concentration. At higher temperatures, itdecays exponentially with an effective stiffness constant which decreases with concentration/T. The results are used to estimate the three--dimensional N\'{e}el temperature, which decays linearly with $x$ at small concentrations, and drops precipitously at a critical concentration. These predictions are compared with experiments on doped copper oxides, and are shown to reproduce successfully some of the prominent features of the data.Comment: 34 pages, LateX, 4 figures Rport-no: TAU

Magnetization driven metal - insulator transition in strongly disordered Ge:Mn magnetic semiconductors

We report on the temperature and field driven metal-insulator transition in disordered Ge:Mn magnetic semiconductors accompanied by magnetic ordering, magnetoresistance reaching thousands of percents and suppression of the extraordinary Hall effect by a magnetic field. Magnetoresistance isotherms are shown to obey a universal scaling law with a single scaling parameter depending on temperature and fabrication. We argue that the strong magnetic disorder leads to localization of charge carriers and is the origin of the unusual properties of Ge:Mn alloys.Comment: 10 pages, 5 figure

Quantum Fluctuations in the Frustrated Antiferromagnet Sr_2Cu_3O_4Cl_2

Sr_2Cu_3O_4Cl_2 is an antiferromagnet consisting of weakly coupled CuO planes which comprise two weakly interacting antiferromagnetic subsystems, I and II, which order at respective temperatures T_I \approx 390K and T_{II} \approx 40K. Except asymptotically near the ordering temperature, these systems are good representations of the two-dimensional quantum spin 1/2 Heisenberg model. For T< T_{II} there are four low-energy modes at zero wave vector, three of whose energies are dominated by quantum fluctuations. For T_{II} < T < T_I there are two low energy modes. The mode with lower energy is dominated by quantum fluctuations. Our calculations of the energies of these modes (including dispersion for wave vectors perpendicular to the CuO planes) agree extremely well with the experimental results of inelastic neutron scattering (in the accompanying paper) and for modes in the sub meV range observed by electron spin resonance. The parameters needed to describe quantum fluctuations are either calculated here or are taken from the literature. These results show that we have a reasonable qualitative understanding of the band structure of the lamellar cuprates needed to calculate the anisotropic exchange constants used here.Comment: 84 pages, 7 figure

Quantum critical point in the spin glass-antiferromagnetism competition in Kondo-lattice systems

A theory is proposed to describe the competition among antiferromagnetism (AF), spin glass (SG) and Kondo effect. The model describes two Kondo sublattices with an intrasite Kondo interaction strength $J_{K}$ and an interlattice quantum Ising interaction in the presence of a transverse field $\Gamma$. The interlattice coupling is a random Gaussian distributed variable (with average $-2J_0/N$ and variance $32 J^{2}/N$) while the $\Gamma$ field is introduced as a quantum mechanism to produce spin flipping. The path integral formalism is used to study this fermionic problem where the spin operators are represented by bilinear combinations of Grassmann fields. The disorder is treated within the framework of the replica trick. The free energy and the order parameters of the problem are obtained by using the static ansatz and by choosing both $J_0/J$ and $\Gamma/J \approx (J_k/J)^2$ to allow, as previously, a better comparison with the experimental findings. The results indicate the presence of a SG solution at low $J_K/J$ and for temperature $T<T_{f}$ ($T_{f}$ is the freezing temperature). When $J_K/J$ is increased, a mixed phase AF+SG appears, then an AF solution and finally a Kondo state is obtained for high values of $J_{K}/J$. Moreover, the behaviors of the freezing and Neel temperatures are also affected by the relationship between $J_{K}$ and the transverse field $\Gamma$. The first one presents a slight decrease while the second one decreases towards a Quantum Critical Point (QCP). The obtained phase diagram has the same sequence as the experimental one for $Ce_{2}Au_{1-x}Co_{x}Si_{3}$, if $J_{K}$ is assumed to increase with $x$, and in addition, it also shows a qualitative agreement concerning the behavior of the freezing and the Neel temperatures.Comment: 11 pages, 3 figures, accepted for publication in J. Phys.

Polaron percolation in diluted magnetic semiconductors

We theoretically study the development of spontaneous magnetization in diluted magnetic semiconductors as arising from a percolation of bound magnetic polarons. Within the framework of a generalized percolation theory we derive analytic expressions for the Curie temperature and the magnetization, obtaining excellent quantitative agreement with Monte Carlo simulation results and good qualitative agreement with experimental results.Comment: 5 page