Magnetic nonuniformity and thermal hysteresis of magnetism in a manganite thin film

Interface morphology driven exchange interaction and magnetization reversal in a Gd/Co multilayer

Rare-earth (RE)/transition metal (TM) ferromagnetic heterostructures with competing interfacial coupling and Zeeman energy provide a rich ground to study different phase states as a function of magnetic field and temperature. The interface morphology as a knob in these RE/TM heterostructures provides an excellent opportunity to engineer the macroscopic magnetic response by tuning the interface dependent microscopic interactions between the layers. We have investigated the interface morphology driven structure and magnetic properties of a Gd/Co multilayer. The interface morphology of the multilayer was controlled by annealing the multilayer at a relatively low temperature of 573 K under vacuum conditions. Combining the different experimental techniques and a simple one-dimensional spin-based model calculation, we studied the detailed magnetic structure and magnetization reversal mechanism in this system across compensation temperature (T_comp), which suggested a strong interface dependent coupling in the system. We showed that changes in the interface morphology of the Gd/Co multilayer strongly influence the macroscopic magnetic properties of the system. The calculation also confirms the formation of a helical magnetic structure with a 2π domain wall in this system below T_comp. The experimental finding and the simulation of this technologically important system will help to understand the physics of all-optical switching and related applications.

Correlation of Magnetic and Superconducting Properties with the Strength of the Magnetic Proximity Effect in La0.67Sr0.33MnO3/SrTiO3/YBa2Cu3O7-δ Heterostructures

The effect of the stacking sequence on magnetic and superconducting properties in La_0.67Sr_0.33MnO₃ (LSMO)/YBa₂Cu₃O_7-δ (YBCO) and LSMO/SrTiO₃/YBCO heterostructures, which consequently affected the magnetic proximity effect (MPE), was investigated using spin-polarized neutron reflectivity experiments. The results established the intrinsic nature of MPE and its correlation with stacking sequence-dependent magnetic and superconducting properties in these oxide heterostructure systems. We found an increase in the superconducting transition temperature (T_sc) and magnetization for both of the heterostructures as compared to heterostructures with a reversed stacking order. The evolution of the magnetization of the interfacial ferromagnetic (FM) layer, studied as a function of temperature for both heterostructures, showed a decrease in the MPE-induced magnetic depleted layer thickness for heterostructures at a higher T_sc. A comparison of the results of different studies with the present results suggested that the average magnetization and transition temperatures of a FM and a superconductor (SC) were important parameters that dictate the strength of the proximity effect due to the complex interaction of SC and FM in these systems. Tuning the strength of MPE in FM/SC and FM/I/SC oxide heterostructures may provide a promising platform for the effective realization of devices.

Critical Slowing Down at the Abrupt Mott Transition: When the First-Order Phase Transition Becomes Zeroth Order and Looks Like Second Order

We report that the thermally induced Mott transition in vanadium sesquioxide shows critical slowing down and enhanced variance ("critical opalescence") of the order parameter fluctuations measured through low-frequency resistance-noise spectroscopy. Coupled with the observed increase of the phase-ordering time, these features suggest that the strong abrupt transition is controlled by a critical-like singularity in the hysteretic metastable phase. The singularity is identified with the spinodal point and is a likely consequence of the strain-induced long-range interaction.

Interface induced magnetic properties of Gd/Co heterostructures

Antiferromagnetic coupling between rare-earth and transition metal ferromagnetic layers gives rise to various magnetic ground states in heterostructures of these materials. Interface structure and morphology tend to play important roles in magnetic properties of such systems. Interface induced magnetization in Gd/Co heterostructures has been studied using a combination of structural and magnetic characterization techniques. The interface morphology of the Gd/Co system was varied by growing Gd/Co multilayers using magnetron sputtering under different argon partial pressures. Interfacial properties were further modified by annealing the multilayers under high vacuum. The macroscopic magnetization measurements have been correlated with depth dependent structure and magnetic properties of multilayers studied using X-ray and polarized neutron reflectometry techniques. Secondary ion mass spectrometry measurements from both as-deposited and annealed samples also confirmed modification at the interfaces. It has been shown that the interface structure, together with roughness, leads to a unique low-temperature magnetic phase characterized by twisting of Gd and Co moments.

High density nonmagnetic cobalt in thin films

Recently high density (HD) nonmagnetic cobalt has been discovered in a nanoscale cobalt thin film, grown on Si(111) single crystal. This form of cobalt is not only nonmagnetic but also superconducting. These promising results have encouraged further investigations of the growth of the nonmagnetic (NM) phase of cobalt. In the original investigation, the cobalt film had a natural cobalt oxide at the top. We have investigated whether the growth of HD NM cobalt layers in the thin film depends on (i) a capping layer on the cobalt film, (ii) the thickness of the cobalt film and (iii) the nature of the substrate on which the cobalt film is grown. The results of such investigations indicate that for cobalt films capped with a thin gold layer, and for various film thicknesses, HD NM cobalt layers are formed. However, instead of a Si substrate, when the cobalt films are grown on oxide substrates, such as silicon oxide or cobalt oxide, HD NM cobalt layers are not formed. The difference is attributed to the nature-crystalline or amorphous-of the substrate.

Reversible Control of Interfacial Magnetism through Ionic-Liquid-Assisted Polarization Switching

The ability to control magnetism of materials via electric field enables a myriad of technological innovations in information storage, sensing, and computing. We use ionic-liquid-assisted ferroelectric switching to demonstrate reversible modulation of interfacial magnetism in a multiferroic heterostructure composed of ferromagnetic (FM) La_0.8Sr_0.2MnO₃ and ferroelectric (FE) PbZr_0.2Ti_0.8O₃. It is shown that ionic liquids can be used to persistently and reversibly switch a large area of a FE film. This is a prerequisite for polarized neutron reflectometry (PNR) studies that are conducted to directly probe magnetoelectric coupling of the FE polarization to the interfacial magnetization.

Scaling of thermal hysteretic behavior in a parametrically modulated cold atomic system

We observe the hysteresis of a spontaneous symmetry breaking (SSB) transition in a parametrically modulated magneto-optical trap by sweeping the total number of atoms and study thermal hysteretic behavior in the system by measuring the scaling exponent of hysteresis. It is shown that the relaxation time of the order parameter in the SSB transition becomes larger near the critical number. The scaling exponent of the hysteresis area with number sweeping rate is found to be 0.64±0.04, which is consistent with the value in the mean-field model.

Composition dependence of charge and magnetic length scales in mixed valence manganite thin films

Mixed-valence manganese oxides present striking properties like the colossal magnetoresistance, metal-insulator transition (MIT) that may result from coexistence of ferromagnetic, metallic and insulating phases. Percolation of such phase coexistence in the vicinity of MIT leads to first-order transition in these manganites. However the length scales over which the electronic and magnetic phases are separated across MIT which appears compelling for bulk systems has been elusive in (La_1−yPr_y)_1−xCa_xMnO₃ films. Here we show the in-plane length scale over which charge and magnetism are correlated in (La_0.4Pr_0.6)_1−xCa_xMnO₃ films with x = 0.33 and 0.375, across the MIT temperature. We combine electrical transport (resistance) measurements, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and specular/off-specular x-ray resonant magnetic scattering (XRMS) measurements as a function of temperature to elucidate relationships between electronic, magnetic and morphological structure of the thin films. Using off-specular XRMS we obtained the charge-charge and charge-magnetic correlation length of these LPCMO films across the MIT. We observed different charge-magnetic correlation length for two films which increases below the MIT. The different correlation length shown by two films may be responsible for different macroscopic (transport and magnetic) properties.

Superconductivity-induced magnetization depletion in a ferromagnet through an insulator in a ferromagnet-insulator-superconductor hybrid oxide heterostructure

Coupling between superconducting and ferromagnetic states in hybrid oxide heterostructures is presently a topic of intense research. Such a coupling is due to the leakage of the Cooper pairs into the ferromagnet. However, tunneling of the Cooper pairs though an insulator was never considered plausible. Using depth sensitive polarized neutron reflectivity we demonstrate the coupling between superconductor and magnetic layers in epitaxial La2/3Ca1/3MnO3 (LCMO)/SrTiO3/YBa2Cu3O7-δ (YBCO) hybrid heterostructures, with SrTiO3 as an intervening oxide insulator layer between the ferromagnet and the superconductor. Measurements above and below the superconducting transition temperature (TSC) of YBCO demonstrate a large modulation of magnetization in the ferromagnetic layer below the TSC of YBCO in these heterostructures. This work highlights a unique tunneling phenomenon between the epitaxial layers of an oxide superconductor (YBCO) and a magnetic layer (LCMO) through an insulating layer. Our work would inspire further investigations on the fundamental aspect of a long range order of the triplet spin-pairing in hybrid structures.

Antiferromagnetic coupling between surface and bulk magnetization and anomalous magnetic transport in electro-deposited cobalt film

Correlation of morphology and structure and magnetization depth profiles of Co films grown by two different techniques, e.g. electrodeposition (S1) and sputtering (S2).

Enhancement of the ferromagnetic metallic phase fraction by extrinsic disorder in phase separated La(5/8-y)Pr(y)Ca(3/8)MnO3 (y = 0.45) thin film

Our study shows that extrinsic disorder plays a decisive role in shaping inhomogeneities at large length scales in phase separated systems. Epitaxial La5/8-yPryCa3/8MnO3 (y = 0.45) thin films grown on SrTiO3, LaAlO3 and NdGaO3 substrates exhibited comparable biaxial strain while showing markedly dissimilar extrinsic disorder. Compressively strained film on LaAlO3 is found to be free from extrinsic disorder and has a robust insulating phase with small phase separation while film grown on SrTiO3 shows huge extrinsic disorder due to the strain relaxation process which invokes phase separation at a large length scale that is sufficient to cross the percolation threshold and cause a metal-insulator transition.