arXiv subjects

J. F. Mitchell

Publications and source records attributed to J. F. Mitchell.

129 records · Page 8Linked to original sources

arXiv1999

Direct Imaging of the First Order Spin Flop Transition in the Layered Manganite La1.4Sr1.6Mn2O7

Magnetic field induced transitions in the antiferromagnetic layered manganite La1.4Sr1.6Mn2O7 were studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the first direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polarized domains at crystal defect sites and not by domain wall motion. A small magnetic hysteresis is caused by the nucleation and annihilation of domains in the mixed state. These results establish a direct link betweeen the magnetic structure on the atomic scale as seen in neutron scattering and the macroscopic properties of the sample as seen in magnetization and conductivity measurements.

cond-mat↗

arXiv1999

Evidence for a Purely Double-Exchange Mechanism for the Anisotropic Conductivities in Layered La-Sr-Mn-O Single Crystals Below TC

Experimental evidence supports the double-exchange (DE) mechanism for both in-plane and c-axis conductivity in the colossal-magnetoresistive (CMR) layered manganite La1.4Sr1.6Mn2O7. Below TC, the data determine both the DE and antiferromagnetic (AF) superexchange between bilayers. These agree with recent spin-wave data, such that the ratio of intra- to inter-bilayer DE constants is very close to the spin-independent conductance anisotropy of ~300. The conductivity is shown to be proportional to the square of the measured magnetization over a wide range of fields and for temperatures below the in-plane ferromagnetic ordering temperature, TC. This dependence is shown to be consistent with DE coupling and earlier zero-field studies of La1.2Sr1.8Mn2O7 near TC, which used neutron scattering for the local magnetization. A mixed AF and spin-flop state (similar to the intermediate state of type-I superconductors) is a rigorous prediction of the data and modeling.

cond-mat↗

arXiv1997

Neutron scattering investigation of magnetic bilayer correlations in $La_{1.2}Sr_{1.8}Mn_2O_7$

Neutron scattering investigations of the paramagnetic correlations in the layered manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$, which exhibits colossal magnetoresistance above the Curie transition at T$_C$ = 112 K, show that spins in neighboring layers within each bilayer are strongly canted at an average angle that is dependent on both the magnetic field and temperature, as predicted by de Gennes. The in-plane correlation length does not diverge at T$_C$, although the magnetic Bragg intensity obeys critical scaling below T$_C$, with the same temperature dependence as the zero-field electrical conductance.

cond-mat.str-el↗