Tuning of Magnetic Activity in Spin-Filter Josephson Junctions Towards Spin-Triplet Transport

Abstract

The study of superconductor/ferromagnet interfaces has generated a great interest in the last decades, leading to the observation of equal spin spin triplet supercurrent and 0 − π transitions in Josephson junctions where two superconductors are separated by an itinerant ferromagnet. Recently, spin-filter Josephson junctions with ferromagnetic barriers have shown unique transport properties, when compared to standard metallic ferromagnetic junctions, due to the intrinsically non-dissipative nature of the tunneling process. Here we present the first extensive characterization of spin polarized Josephson junctions down to 0.3 K, and the first evidence of an incomplete 0 − π transition in highly spin polarized tunnel ferromagnetic junctions. Experimental data are consistent with a progressive enhancement of the magnetic activity with the increase of the barrier thickness, as neatly captured by the simplest theoretical approach including a non uniform exchange field. For very long junctions, unconventional magnetic activity of the barrier points to the presence of spin-triplet correlations.The interaction of superconductors with materials other than simple insulators or metals has made accessible a series of conceptually new challenges. Of particular interest to this work, Josephson junctions (JJs) with ferromagnetic materials separating two superconductors have been extensively characterized over the last decade. The simultaneous presence of the macroscopic phase coherence of superconductors and the exchange interaction of ferromagnetic materials is indeed of great value in the study of fundamental questions on possible pairing states in superconductors [1,2], demonstrating the presence of spin-polarized triplet supercurrents [3][4][5][6][7][8][9], and for potential applications in a wide range of cutting edge areas, such as spintronics [10,11], memory applications for high performance computing [12][13][14][15][16][17][18] and circuit components such as π shifters and phase qubits [19][20][21][22][23]. A playground where different forms of order can cooperate and interfere is of considerable value for inspiring other fields of physics [1,2].The existing literature focuses mostly on metallic superconductor/ferromagnet/superconductor (SFS) junctions, where the evidence of long-range spin triplet correlations is well established [3][4][5][6][7][8]: in the presence of equalspin Cooper pairs, the magnitude of the critical current I C decays much more slowly with magnetic barrier thickness than expected for standard singlet supercurrents [4,5]. In fact, spin-polarized Cooper pairs can survive at much longer length scales when compared to opposite spin Cooper pairs, and are practically immune to depairing induced by the presence of an exchange field [1,2]. Such junctions, together with superconducting spin valve devices, are likely to be the building blocks for future spintronic devices [11]. While metallic SFS junctions have been extensively characterized, the physics of ferromagnetic junctions with insulating barriers, l...

Electrodynamics of Highly Spin-Polarized Tunnel Josephson Junctions

“…As one can observe in Fig. 3, the critical current density J c (t), with cross section A = 49 µm 2 , and the R N A(t) curves at 300 mK obey to a typical tunnel behavior, thus confirming the insulating nature of the ballistic GdN barrier [32]. R N A(t) exhibits the characteristic exponential thickness dependence:…”

“…These values are higher than those usually achieved for SFS JJs and comparable to those of some SIFS heterostructures [1,3,7,11,15,16]. For barrier thicknesses lower than 2.5 nm, the characteristic voltage is as high as a few millivolts [28,31,32]. In Fig.…”

“…We can attribute this deviation to the unconventional magnetic activity discussed in Ref. [32], which is at a maximum in the case of most spinpolarized JJs, where the magnetic nature of the barriers manifests in a steep increase of I c (T ) below 2 K [ Fig. 6 (b)] [32].…”

“…[32], which is at a maximum in the case of most spinpolarized JJs, where the magnetic nature of the barriers manifests in a steep increase of I c (T ) below 2 K [ Fig. 6 (b)] [32]. The conventional TJM model does not take the magnetic activity in the I f barrier into account, nor the spin-dependent tunneling mechanism and the unconventional thermal behavior of I c , thus giving a systematic underestimation of R sg , as shown in Fig.…”

“…Some of their properties have been studied in Refs. [28][29][30][31][32]. The first evidence of macroscopic quantum tunneling (MQT) in ferromagnetic JJs is an indication that spin-filter JJs can be used as active quantum devices [31].…”