Linear Ac Magnetic Susceptibility Research Articles

Isotropic-Heisenberg to isotropic-dipolar crossover and finite-size scaling in Cr75−xFe25+x (x = 0, 5) thin films

‘Zero-field’ linear ac magnetic susceptibility, χ1(T), of the Cr75−xFe25+x (x=0, 5) thin films with thickness, t, ranging from 980 to 10nm has been measured at temperatures close to Tc, the temperature at which the ferromagnetic-paramagnetic phase transition occurs. An elaborate analysis of χ1(T⩾Tc) for the films with t⩾40nm yields the temperature dependence of the effective critical exponent for susceptibility, γeff(T), that is characteristic of the three-dimensional (3D) isotropic Heisenberg-to-3D isotropic dipolar crossover. In the asymptotic critical region (ACR), these thin-film samples behave as a 3D isotropic dipolar (ID) ferromagnet. As the film thickness reduces from t≃ 980nm to 40nm, ACR narrows down while the temperature, Tdip, at which a dip in γeff(T) occurs and the temperature, TIH∗, that marks the onset of the 3D isotropic Heisenberg (IH) behavior, shift to lower temperatures. For a given t, the width of ACR as well as the characteristic temperatures Tdip and TIH∗ increase with decreasing (increasing) Fe (Cr) concentration. Consistent with these observations, the ratios involving nonlinear ac magnetic susceptibilities obey the generalized magnetic equation of state with 3D ID critical exponents and the value of Tc same as that determined from χ1(T). A quantitative comparison between theory and experiment highlights certain limitations of the existing theories. The films with t≲20nm do not exhibit 3D IH-to-3D ID crossover. Instead, the critical behavior of Cr70Fe30 thin films with t=21nm and t=11nm is that of a 3D IH and 3D Ising ferromagnet, respectively. By contrast, a 3D Ising (spin glass) critical behavior is observed in the Cr75Fe25 thin film with t=19nm (t=12nm). Curie temperature, Tc, decreases with film thickness in accordance with the finite-size scaling.

Evidence of electronic phase arrest and glassy ferromagnetic behaviour in (Nd0.4Gd0.3)Sr0.3MnO3 manganite: comparative study between bulk and nanometric samples

The effect of the doping of rare earth Gd3+ ions replacing Nd3+ in Nd0.7Sr0.3MnO3 is investigated in detail. Measurements of resistivity, magnetoresistance, magnetization, and linear and nonlinear ac magnetic susceptibility on chemically synthesized (Nd0.7−xGdx)Sr0.3MnO3 show various interesting features with doping level x = 0.3. A comparative study has been carried out between a bulk and a nanometric sample (grain size ∼60 nm) synthesized from the same as a prepared powder to maintain an identical stoichiometry. The resistivity of the samples shows strong dependence on the magnetic field–temperature history. The magnetoresistance of the samples also shows strong irreversibility with respect to sweeping of the field between the highest positive and negative values. Moreover, the resistivity is found to increase with time after field cooling and then switching off the field. All these phenomena have been attributed to phase separation effects and the arrest of phases in the samples. Furthermore, the bulk sample displays a spin glass like behaviour as evident from the frequency dependence of linear ac magnetic susceptibility and critical divergence of the nonlinear ac magnetic susceptibility. The experimentally obtained characteristic time τ0 after dynamical scaling analysis of the frequency dependence of the ac susceptibility is found to be 10−17 s which implies that the system is different from a canonical spin glass. An unusual frequency dependence of the second harmonic of ac susceptibility around the magnetic transition temperature led us to designate the magnetic state of the sample to be glassy ferromagnetic. On reduction of the grain size low field magnetoresistance and phase arrest phenomena are found to enhance but the glassy state is observed to be destabilized in the nanometric sample.