Inducing supercurrents in thin films of ferromagnetic CrO₂

Abstract

It is becoming gradually clear that it is possible to induce superconducting correlations in ferromagnets by generating so-called odd-frequency triplet Cooper pairs, which have two equal (rather than opposite) spins. Half-metallic ferromagnets (HMF) such as CrO 2 are of special interest here since the presence of only a single spin band and no possibility for spin flip scattering can lead to very long lengths over which a supercurrent is induced. Early experiments with superconducting (S) contacts on CrO 2 films found supercurrents over distances of up to a micrometer. The generation of triplet correlations is assumed to involve inhomogeneous magnetization or disordered magnetic moments in the ferromagnet or at the interface, and a major question still is how this is brought about in the S/CrO 2 case. Here we report on experiments in which CrO 2 thin films are grown on two different substrates, TiO 2 and sapphire (single crystalline Al 2 O 3 ). Using amorphous Mo 70 Ge 30 superconducting contacts, we find supercurrents can flow in films grown on sapphire, but we have not been able yet to induce supercurrents in films grown on TiO 2 . We discuss this in terms of the differences in film morphology and the magnetic anisotropy for both film systems. We also show that the characteristic behavior of the clean S/HMF interface is a jump at the superconducting transition temperature of the resistance of the device to larger values.

Anomalous transport in half-metallic ferromagnetic CrO2

“…Note that the values for H c , around 15 mT, which for a 100-nm-thick film is as expected from earlier magnetization measurements. 24 The MR behavior was already studied by König et al, 15 with results similar to these. They interpreted their results assuming the c axis as easy axis; regardless of the angle between H and I , the magnetization switches sharply for H c. For H b domains start to form well above H c , which leads to a dip or a plateau in the variation of R. For their sample, a magnetization measurement confirmed that the c axis is indeed the easy axis.…”

“…The growth on an a-axis oriented substrate is in the form of rectangular grains with the long axis aligned along the film c axis and the short axis along the b axis. It has been reported that pretreatment of the TiO 2 substrates with hydrofluoric acid (HF) can enhance the strain in the films [23][24][25][26] and it affects both magnetic and electronic properties. Growth on sapphire is more complicated because of its hexagonal structure (a = 0.4754 nm), which is close to Cr 2 O 3 .…”

“…We earlier reported in some detail on the film growth, the morphology of the films, and also on the magnetization of the films. 24 In particular, we discussed the magnetic anisotropy resulting from the strain as function of thickness of the films. Such data are of relevance to the MR behavior, and similar reports on the magnetization can be found in Refs.…”

The critical current reaction on hydrostatic pressure of a superconductor–semimetal composite

“…[1][2][3][4][5][6][7][8][9][10][11][12] However, despite a vast number of works on the proximity effect in superconductor-ferromagnet junctions, the critical current has been determined only in a few cases. [13][14][15][16][17] The studies have been mostly limited to the measurements of the temperature dependences in heterostructures, which is largely due to the difficulty of measuring the current-voltage characteristics (CVC) in a direction normal to the thin layers. [18][19][20][21][22] In the present study we measured the temperature dependences of the resistance and the CVCs of a bulk composite made of ferromagnetic lanthanum manganite (La 0.7 Ca 0.3 MnO 3 ) and superconductor MgB 2 with different volume content of the magnetic material under high hydrostatic pressure.…”

Andreev spectroscopy of CrO 2 thin films on TiO 2 and Al 2 O 3

“…However, data taken at a lower temperature or the employment of superconductor with a higher critical temperature would facilitate the comparison between models. As it stands, the SMM model assumes that the films have 100% polarisation, which is not unreasonable given the fact they strongly support LRSTPE [22], but this is not definitive proof. Spin-polarised photoemission although 67005-p3 restricted to the top few nanometers of the surface would be a useful additional characterisation tool.…”