Scanning Raman spectroscopy for characterizing compositionally spread films

Composition-spread La1-xSrxMnO3 thin films were prepared by pulsed laser deposition technique from LaMnO3 and SrMnO3 targets. The films were epitaxial with a continuous variation of the out-of-plane lattice parameter along the direction of composition gradient. Scanning Raman spectroscopy has been employed as a nondestructive tool to characterize the composition-spread films. Raman spectra showed the variation of the structural, Jahn Teller distortions and the presence of coexisting phases at particular compositions that are in agreement with the previous observation on the single-crystal samples. Raman spectra on the continuous composition-spread film also reveal the effect of disorder and strain on the compositions

Large interfacial exchange fields in a thick superconducting film coupled to a spin-filter tunnel barrier

The differential conductance of NbN/GdN/TiN superconductor / ferromagnetic insulator / normal metal junctions, with a thick NbN layer shows a large zero-field voltage offset interpreted as a spin-filtered Zeeman splitting of the NbN density of states (DOS) by an effective exchange field (H₀) from the GdN. The splitting increases linearly with applied field (H_ext) enabling the relative sign of H₀ and H_ext to be determined. We show that the short NbN coherence length concentrates H₀ at the NbN/GdN interface and eliminates any averaging over the GdN domain structure leading to a large zero-field splitting.This work was supported by the ERC Advanced Investigator Grant (Superspin).This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevB.92.18051

Reversible control of spin-polarized supercurrents in ferromagnetic Josephson junctions.

Magnetic inhomogeneity at a superconductor (S)-ferromagnet (F) interface converts spin-singlet Cooper pairs into spin-one triplet pairs. These pairs are immune to the pair-breaking exchange field in F and support a long-range proximity effect. Although recent experiments have confirmed the existence of spin-polarized triplet supercurrents in S-F-S Josephson junctions, reversible control of the supercurrent has been impossible because of the robust preconfigured nature of the inhomogeneity. Here, we use a barrier comprising three F layers whose relative magnetic orientation, and hence the interfacial inhomogeneity, can be controlled by small magnetic fields; we show that this enables full control of the triplet supercurrent and, by using finite element micromagnetic simulations, we can directly relate the experimental data to the theoretical models which provide a general framework to understand the role played by magnetic states in long-range supercurrent modulation.This is the author's accepted manuscript. It will be under embargo until the 26th of February 2015. The final version is published by NPG in Nature Communications here: http://www.nature.com/ncomms/2014/140826/ncomms5771/full/ncomms5771.html

Triplet pair correlations and nonmonotonic supercurrent decay with Cr thickness in Nb/Cr/Fe/Nb Josephson devices

Supercurrents carry charge but not spin in the vast majority of superconductors. This is because the charge carriers are Cooper pairs which are formed of electrons with antiparallel spins (singlet pairs). It is now established that if singlet pairs propagate through a superconductor interface with an inhomogeneous ferromagnet, triplet pair correlations form so the supercurrents can acquire a net spin component. Although the spins at sputter-deposited Fe/Cr interfaces can be frustrated due to surface roughness and interdiffusion, an antiferromagnetic spin density wave (SDW) state can still form in Cr close to the interface. Here, we show evidence for triplet pair correlations in Josephson junctions with Cr/Fe and Cr/Fe/Cr barriers. Although the exact mechanism of pair conversion is unknown, we propose a simple model in which a SDW state in Cr and frustrated spins at the Cr/Fe interfaces serve as spin-mixers for generating triplet supercurrents and so provide a potential means to generating large superconducting spin current densities.The work was funded by the UK Royal Society and the European Research Council (AIG "Superspin").This is the accepted version of an article which originally appeared in Physical Review B. The full version is available at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.104505

Crossover from diffusive to tunneling regime in NbN/DyN/NbN ferromagnetic semiconductor tunnel junctions

We have investigated NbN-DyN-NbN junctions with 1 to 10 nm thick DyN barriers. A crossover from diffusive (hopping) to tunneling-type transport was found in these junctions as the DyN thickness is reduced below ∼4 nm. We have also made a detailed study of magnetic and electrical properties of thicker DyN thin films deposited under similar conditions; DyN films were found to be ferromagnetic with TCurie∼35±5 K. Electrical transport of the junctions with ∼10 nm DyN was understood in terms of Shklovskii-Efros (SE)-type variable range hopping (VRH) at low temperature between 90 and 35 K. We estimated localization length ξ=5.6 nm in this temperature range. Temperature dependence of resistance was found to deviate from SE-VRH below 35 K along with large suppression of resistance with magnetic field. This is correlated with onset of magnetism below 35 K. Large butterfly-shaped MR up to ∼40% was found for the ∼10 nm thick DyN junction at 2 K. In the tunneling regime, barrier height of the tunnel junction was estimated ∼50 meV from the Simmons model. Signatures of spin filtering was found in temperature dependence of resistance in tunnel junction with ∼3 nm thick DyN. Cooper pair tunneling in these junctions below TC (∼10.8 K) of NbN was understood according to S-I-S tunneling current model. We found coherent tunneling of Cooper pairs through a ∼1 nm thick DyN tunnel barrier with critical current IC∼12 μA. The critical current also showed modulation with magnetic field.This work was supported by the ERC Advanced Investigator Grant SUPERSPIN.This is the accepted version of the article. The final version is published in Physical Review B: http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.094414. ©American Physical Societ

The Potential of the Josephson Energy

AbstractThe concept of the Josephson energy emerges directly from the original predictions of Brian Josephson. Although it can be used to explain the behaviour of Josephson devices, the Josephson energy has no present applications. In this paper, we show that in the context of magnetic Josephson junctions, the Josephson energy offers a potential mechanism for controlling magnetic memory devices.</jats:p

Benefits of current percolation in superconducting coated conductors

The critical currents of MOD/RABiTS and PLD/IBAD coated conductors have been measured as a function of magnetic field orientation and compared to films grown on single crystal substrates. By varying the orientation of magnetic field applied in the plane of the film, we are able to determine the extent to which current flow in each type of conductor is percolative. Standard MOD/RABiTS conductors have also been compared to samples whose grain boundaries have been doped by diffusing Ca from an overlayer. We find that undoped MOD/RABiTS tapes have a less anisotropic in-plane field dependence than PLD/IBAD tapes and that the uniformity of critical current as a function of in-plane field angle is greater for MOD/RABiTS samples doped with Ca.EPSRC US Department of Energ

High curie temperatures in ferromagnetic Cr-doped AlN thin films

Al1-xCrxN thin films with 0.02less than or equal toxless than or equal to0.1 were deposited by reactive co-sputtering onto c-plane (001) sapphire. Room-temperature ferromagnetism with a coercive field of 85 Oe was observed in samples with chromium contents as low as x=0.027 (2.7%). With increasing Cr content the mean magnetic moment is strongly suppressed, with a maximum saturation moment of 0.62 and 0.71 mu(B) per Cr atom at 300 and 50 K, respectively. We show that the Curie temperature of Al1-xCrxN for x=0.027 is greater than 900 K. (C) 2004 American Institute of Physics

Large Superconducting Spin Valve Effect and Ultrasmall Exchange Splitting in Epitaxial Rare-Earth-Niobium Trilayers.

Epitaxial Ho/Nb/Ho and Dy/Nb/Dy superconducting spin valves show a reversible change in the zero-field critical temperature (ΔT(c0)) of ∼400  mK and an infinite magnetoresistance on changing the relative magnetization of the Ho or Dy layers. Unlike transition-metal superconducting spin valves, which show much smaller ΔT(c0) values, our results can be quantitatively modeled. However, the fits require an extraordinarily low induced exchange splitting which is dramatically lower than known values for rare-earth Fermi-level electrons, implying that new models for the magnetic proximity effect may be required.This work is partially supported by ERC AiG “Superspin”. Yuanzhou Gu acknowledges financial support from King’s College, Cambridge.This is the accepted manuscript of a paper published in Physical Review Letters (Phys. Rev. Lett. 115, 067201). The final version is available at http://dx.doi.org/10.1103/PhysRevLett.115.06720

Spin-transfer switching and low-field precession in exchange-biased spin valve nano-pillars

Using a three-dimensional focused-ion beam lithography process we have fabricated nanopillar devices which show spin transfer torque switching at zero external magnetic fields. Under a small in-plane external bias field, a field-dependent peak in the differential resistance versus current is observed similar to that reported in asymmetrical nanopillar devices. This is interpreted as evidence for the low-field excitation of spin waves which in our case is attributed to a spin-scattering asymmetry enhanced by the IrMn exchange bias layer coupled to a relatively thin CoFe fixed layer.Comment: 11 pages, 4 figures. To appear in APL, April 200