Spin-Resolved Tunneling Studies of the Exchange Field inEuS/AlBilayers

Abstract

We use spin-resolved electron tunneling to study the exchange field in the Al component of EuS/Al bilayers, in both the superconducting and normal-state phases of the Al. Contrary to expectation, we show that the exchange field, Hex, is a non-linear function of applied field, even in applied fields that are well beyond the EuS coercive field. Furthermore the magnitude Hex is unaffected by the superconducting phase. In addition, Hex decreases significantly with increasing temperature in the temperature range of 0.1 -1 K. We discuss these results in the context of recent theories of generalized spin-dependent boundary conditions at a superconductor/ferromagnet interface. PACS numbers: 74.50.+r, 74.45.+c, 75.70.Ak, Owing to their different symmetries, itinerant ferromagnetic (FM) order and spin-singlet superconducting (SC) order are generally mutually exclusive. With rare exception, nature does not allow ferromagnetic order to coexist with BCS superconductivity [1]. This immiscibility can, however, lead to interesting effects in the vicinity of a FM/SC interface, as electrons moving from one region to the other try to accommodate the differing order parameters. Over the last decade significant progress has been made in understanding the nature of the SC order parameter in the proximity of a FM/SC interface [2][3][4][5]. In fact, much of the research on FM/SC structures has focused on the evanescent SC condensate residing on the FM side of the interface in properly prepared bilayers [4,[7][8][9]. Remarkably, not only can SC Cooper pairs exist in the FM layer, but the exchange field in the FM induces a triplet component to the SC wavefunction [4]. This results in oscillations in the SC order parameter [5,7,8] that are reminiscent of the order parameter modulations predicted by Fulde and Ferrel [10], and Larkin and Ovchinnikov [11] for a BCS superconductor in a critical Zeeman field. In the present Letter, we present the results of a detailed study of the exchange field induced in the SC side of a FM/SC bilayer. We show this proximityinduced exchange field is not static, but has unexpected temperature and applied-field dependencies that are not attributable to the temperature and/or field dependence of the FM magnetization.Since we are primarily interested in the behavior of the exchange field induced in the SC layer, we have chosen an insulating material for the FM layer, EuS [12]. This greatly simplifies the interpretation of the data since electrons from the SC only enter the FM via evanescent wavefunctions. For the superconductor we chose Al since it has a very low spin-orbit scattering rate and its spin-paramagnetic phase diagram is well understood [13]. The FM/SC bilayers were fabricated by first depositing a 5 nm-thick EuS film via e-beam evaporation onto fire-polished glass at 84 K. Then a 2.4 nm thick Al film was deposited on top of the EuS film. The depositions were made at a rate of ∼ 1 nm/s and ∼ 0.1 nm/s, respectively, in a typical vacuum of < 3×10 −7 Torr. The samples were then exposed to air to form a n...

Manifestation of a spin-splitting field in a thermally biased Josephson junction

“…Such exchange correlations penetrate into the bulk of S over distances of the order of the coherence length [10]. This results in a splitting of the density of states (DOS) of the superconductor, as observed in a number of experiments [14][15][16][17]. Yet, the spin-split DOS of a superconductor may lead to interesting effects such as, for instance, the absolute spinvalve effect [18][19][20], the magnetothermal Josephson valve [21,22], and the large enhancement of the Josephson coupling observed in F-S-I-S-F junctions (I stands for a conventional insulator) when the magnetic configuration of the F layers is arranged in the antiparallel state [23][24][25][26].…”

Origin of Excess Low-Energy States in a Disordered Superconductor in a Zeeman Field

“…067003-2 our films as d is decreased from 3 to 2 nm, D decreases by an order of magnitude, but Gð0Þ hardly changes. Furthermore, recent tunneling measurements of Al-EuS bilayers have shown that a comparable Gð0Þ is produced by an interface-induced exchange field, which is a pure Zeeman field with no orbital depairing effects [24].…”

Stimulation of a Singlet Superconductivity in SFS Weak Links by Spin–Exchange Scattering of Cooper Pairs

“…Here we demonstrate that a tip of the magnetic exchange force microscope (MExFM)34, which induces localized in space magnetic exchange fields can play the role of such a local probe for a spin state of superconducting Cooper pairs. Existing experimental evidences of the externally induced exchange fields h in metals of the order of few56 or even few tens7 millielectronvolts place the electronic coupling to such field in a range of energies comparable with (or even exceeding) superconducting energy scale ~1 meV. We will show that the effect of exchange induced gating of Cooper pairs leads to a new phenomenon - stimulation of long- range singlet superconductivity in SFS (superconductor-ferromagnet-superconductor) weak links.…”