Critical Current Oscillations of Josephson Junctions Containing PdFe Nanomagnets

Abstract

Josephson junctions with ferromagnetic layers are vital elements in a new class of cryogenic memory devices. One style of memory device contains a spin valve with one "hard" magnetic layer and one "soft" layer. To achieve low switching fields, it is advantageous for the soft layer to have low magnetization and low magnetocrystalline anisotropy. A candidate class of materials that fulfills these criteria is the Pd1−xFex alloy system with low Fe concentrations. We present studies of micron-scale elliptically-shaped Josephson junctions containing Pd97Fe3 layers of varying thickness. By applying an external magnetic field, the critical current of the junctions are found to follow characteristic Fraunhofer patterns. The maximum value of the critical current, extracted from the Fraunhofer patterns, oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across the junction between values of zero and π.

Spin-valve Josephson junctions for cryogenic memory

“…Those workers were the first to confirm the theoretical prediction 3,4 that the ground-state phase difference across an S/F/S Josephson junction could be either zero or π, depending on the thickness of the ferromagnetic F-layer inside the junction. Oscillations in the magnitude of the critical current as a function of F-layer thickness, signifying oscillations between 0 and π junctions, have now been demonstrated with a wide variety of ferromagnetic materials, including CuNi alloy, 1,[5][6][7] PdNi, 2,8 Ni, 9-14 Ni 3 Al, 15 Co, 11 Fe, 11 NiFe, 11,16,17 NiFeMo, 18 PdFe, 19 and NiFeCo.…”

Ferromagnetic Josephson junctions based on epitaxial NbN/Ni60Cu40/NbN trilayer

“…This shift along the applied field axis (H shift ) with respect to the ideal Fraunhofer diffraction pattern is attributed to the existence of the remanent magnetization (M r ) in the Ni 60 Cu 40 ferromagnetic barrier layer. The magnetic flux through a SFS junction is given by the equation 28 where H, L, and d F are the applied field, the junction width, and the thickness of the F layer, while µ 0 and λ L are the vacuum permeability and the London penetration depth of the NbN electrodes, respectively. The last term describes the contribution of the remanent magnetization M r of the F layer when external magnetic field is removed.…”

Ab initio Investigation of Impurity Ferromagnetism in the Pd1−xFex Alloys: Concentration and Position Dependences

“…Experimental findings and their understanding at the time of the 1970s were summarized in a review by Nieuwenhuys [3]. The current revival of interest is fueled by the potential use as a material of a choice for a weak and soft ferromagnet in superconducting Josephson magnetic random-access memory (MRAM) based on Josephson junctions [4][5][6][7][8][9][10][11][12][13]. The Pd-rich ferromagnetic Pd 1−x Fe x (0.01 < x < 0.1) alloy is used there in the form of a thin film fabricated utilizing magnetron sputtering [5][6][7][8][9][10][11][13][14][15], molecular-beam epitaxy (MBE) [15][16][17][18][19][20][21][22][23], and ion-beam implantation [24].…”

“…There is a scatter of opinions on the compositional and magnetic homogeneity of Pd 1−x Fe x films obtained by different deposition techniques: some papers report an indication of clustering [10,11,13,14] or formation of the nanograins of the Pd 3 Fe phase [15] in samples deposited with the magnetron sputtering technique; the others on MBE growth of thin Pd 1−x Fe x films [18][19][20][21][22][23] report on their high magnetic homogeneity; finally, the ionimplanted palladium films [24] show a significant influence of the Fe implant distribution on saturation magnetization (M s ) and Curie temperature (T C ) of the samples. It is not clear why and how the redistribution of iron solute in the Pd matrix influences M s and T C .…”