Rajesh V. Chopdekar

Magnetoelectric coupling in epitaxial CoFe2O4 on BaTiO3

The authors have synthesized epitaxial CoFe2O4 films on piezoelectric BaTiO3 single crystal substrates as a model magnetoelectric system. The BaTiO3 substrate provides a surface lattice that can be dynamically changed in an attempt to alter the strain state and hence the magnetization of the CoFe2O4 film. Magnetization measurements indicate that the magnetic anisotropy of CoFe2O4 is dominated by compressive epitaxial strain effects and can be understood in terms of the symmetry of the substrate surface unit cell.

Control of the magnetic and magnetotransport properties of La0.67Sr0.33MnO3 thin films through epitaxial strain

The influence of epitaxial strain, in the form of tetragonal distortions, on the magnetic and magnetotransport properties of La{sub 0.67}Sr{sub 0.33}MnO{sub 3} thin films was studied. The tetragonal distortion (c/a ratio) was modulated through the choice of the substrate, ranging from c/a=1.007 on (001)-oriented (LaAlO{sub 3}){sub 0.3}(Sr{sub 2}AlTaO{sub 6}){sub 0.7} substrates to 0.952 on (110)-oriented GdScO{sub 3} substrates. In agreement with previous theoretical predictions, these large values of tensile strain cause the Curie temperature and the saturation magnetization to decrease, alter the temperature dependence of the resistivity and magnetoresistance, and increase the resistivity several orders of magnitude.

Thermal fluctuations in artificial spin ice

Artificial spin ice systems have been proposed as a playground for the study of monopole-like magnetic excitations, similar to those observed in pyrochlore spin ice materials. Currents of magnetic monopole excitations have been observed, demonstrating the possibility for the realization of magnetic-charge-based circuitry. Artificial spin ice systems that support thermal fluctuations can serve as an ideal setting for observing dynamical effects such as monopole propagation and as a potential medium for magnetricity investigations. Here, we report on the transition from a frozen to a dynamic state in artificial spin ice with a square lattice. Magnetic imaging is used to determine the magnetic state of the islands in thermal equilibrium. The temperature-induced onset of magnetic fluctuations and excitation populations are shown to depend on the lattice spacing and related interaction strength between islands. The excitations are described by Boltzmann distributions with their factors in the frozen state relating to the blocking temperatures of the array. Our results provide insight into the design of thermal artificial spin ice arrays where the magnetic charge density and response to external fields can be studied in thermal equilibrium.

Thermalized ground state of artificial kagome spin ice building blocks

We present a direct magnetic imaging study on the thermal macrospin ordering of artificial kagome spin ice building blocks. Using photoemission electron microscopy, employing x-ray magnetic circular dichroism, we are able to resolve the single domain magnetic nature of the macrospins and determine the states of the combined building block structures. The nano-patterning and material selection allows thermally activated magnetization reversal for the macrospins to occur. The ordering of the macrospins is dominated by the ground state, consistent with a thermal ground state ordering. This work paves the way for the realization of extended artificial spin ice structures exhibiting experimentally accessible thermal behavior.

Spin-polarized conduction in oxide magnetic tunnel junctions with magnetic and nonmagnetic insulating barrier layers

Epitaxial magnetic tunnel junctions with electrodes of spinel-structure Fe3O4 and perovskite La0.7Sr0.3MnO3 have been fabricated and characterized. Spinel barrier layers of magnetic FeGa2O4 and nominally nonmagnetic Mg2TiO4 were used to provide a good interface with the more sensitive Fe3O4 electrode interface. Junction magnetoresistances (JMRs) of up to −11% at 60K and −26% at 70K were observed for FeGa2O4 and Mg2TiO4 barrier junctions, respectively. The JMR bias dependence and junction resistance versus temperature data indicate that a majority of the junctions are dominated by elastic tunneling below about 50mV and by inelastic hopping through the barrier at higher bias.

Demonstration of laser induced magnetization reversal in gdfeco nanostructures

Magnetization switching by a single femtosecond laser heat pulse is demonstrated for out-of-plane domains with sizes down to 200 nm in GdFeCo nanostructures. A complex magnetic domain configuration was revealed with a photoemission electron microscope employing x-ray magnetic circular dichroism at the Fe L3 edge and consisted of in-plane magnetized rims and out-of-plane domains, which results from the structuring process. No influence of this complex domain pattern on the switching efficiency of the structures was detected, constituting an important step towards the application of laser induced magnetization switching in storage devices.

Electrical transport and ferromagnetism in Ga1−xMnxAs synthesized by ion implantation and pulsed-laser melting

We present a detailed investigation of the magnetic and magnetotransport properties of thin films of ferromagnetic Ga1−xMnxAs synthesized using ion implantation and pulsed-laser melting (II-PLM). The field and temperature-dependent magnetization, magnetic anisotropy, temperature-dependent resistivity, magnetoresistance, and Hall effect of II-PLM Ga1−xMnxAs films have all of the characteristic signatures of the strong p‐d interaction of holes and Mn ions observed in the dilute hole-mediated ferromagnetic phase. The ferromagnetic and electrical transport properties of II-PLM films correspond to the peak substitutional Mn concentration meaning that the nonuniform Mn depth distribution is unimportant in determining the film properties. Good quantitative agreement is found with films grown by low temperature molecular beam epitaxy and having the similar substitutional MnGa composition. Additionally, we demonstrate that II-PLM Ga1−xMnxAs films are free from interstitial MnI because of the high-temperature proce...

Observation of inverse magnetoresistance in epitaxial magnetite/manganite junctions

We have fabricated and characterized epitaxial Fe3O4/CoCr2O4/La0.7Sr0.3MnO3 magnetic trilayer junctions (MTJs) grown on (110) and (100) SrTiO3 substrates. Large inverse junction magnetoresistance (JMR) as high as −25% has been observed at a field of 4 kOe in (110) oriented MTJs. The improvement of the JMR over that observed in previous epitaxial magnetite junctions is attributed to the choice of the spinel structure CoCr2O4 barrier, which minimizes structural disorder at the barrier/Fe3O4 interface. As a comparison, studies on (100) MTJs elucidate the effect of the magnetic domain state of the ferromagnetic electrodes on the JMR.

Probing the role of the barrier layer in magnetic tunnel junction transport

Author(s): Nelson-Cheeseman, BB; Chopdekar, RV; Alldredge, LMB; Bettinger, JS; Arenholz, E; Suzuki, Y | Abstract: Magnetic tunnel junctions with a ferrimagnetic barrier layer have been studied to understand the role of the barrier layer in the tunneling process-a factor that has been largely overlooked until recently. Epitaxial oxide junctions of highly spin polarized La0.7 Sr0.3 MnO3 and Fe3 O4 electrodes with magnetic NiMn2 O4 (NMO) insulating barrier layers provide a magnetic tunnel junction system in which we can probe the effect of the barrier by comparing junction behavior above and below the Curie temperature of the barrier layer. When the barrier is paramagnetic, the spin polarized transport is dominated by interface scattering and surface spin waves; however, when the barrier is ferrimagnetic, spin flip scattering due to spin waves within the NMO barrier dominates the transport.

Ferromagnetism in tetragonally distorted LaCoO3 thin films

Thin films of epitaxial LaCoO{sub 3} were synthesized on SrTiO{sub 3} and (La, Sr)(Al, Ta)O{sub 3} substrates varying the oxygen background pressure in order to evaluate the impact of epitaxial growth as well as oxygen vacancies on the long range magnetic order. The epitaxial constraints from the substrate impose a tetragonal distortion compared to the bulk form. X-ray absorption and x-ray magnetic circular dichroism measurements confirmed that the ferromagnetism arises from the Co ions and persists through the entire thickness of the film. It was found that for the thin films to show ferromagnetic order they have to be grown under the higher oxygen pressures, since a decrease in oxygen deposition pressure alters the film structure and suppresses ferromagnetism in the LaCoO{sub 3} films. A correlation of the structure and magnetism suggests that the tetragonal distortions induce the ferromagnetism.