Superconducting phase domains for memory applications

Abstract

In this work we study theoretically the properties of S-F/N-sIS type Josephson junctions in the frame of the quasiclassical Usadel formalism. The structure consists of two superconducting electrodes (S), a tunnel barrier (I), a combined normal metal/ferromagnet (N/F) interlayer and a thin superconducting film (s). We demonstrate the breakdown of a spatial uniformity of the superconducting order in the s-film and its decomposition into domains with a phase shift π . The effect is sensitive to the thickness of the s layer and the widths of the F and N films in the direction along the sIS interface. We predict the existence of a regime where the structure has two energy minima and can be switched between them by an electric current injected laterally into the structure. The state of the system can be non-destructively read by an electric current flowing across the junction. PACS numbers: 74.45.+c, 74.50.+r, 74.78.Fk, 85.25.Cp Josephson junctions containing normal (N) and ferromagnetic (F) materials in a weak link region are currently the subject of intense research. An interest in such structures is due to the possibility of their use as control elements of superconductor memory compatible with Single Flux Quantum (SFQ) logic. A number of implementations of Josephson control elements were proposed recently, among which the structures containing F layers in the weak link region are of greatest interest 1-3 . Various types of superconducting spin-valve structures including two or more ferromagnetic layers have been proposed [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] . The mutual orientations, parallel or antiparallel, of magnetizations of the layers determine critical currents and critical temperatures of the structures. Recently, it was proposed to apply the phenomenon of triplet superconductivity in spin-valve devices with noncollinear magnetization of the layers [19][20][21][22][23][24][25][26][27] . The problem of small characteristic voltage I C R N in these structures was solved by using an additional tunnel barrier connected through a thin superconducting spacer [28][29][30][31] . However, to control an operation of these devices, the application of magnetic fields or strong spin-polarized currents is necessary in order to switch the structure to a different state. Such control requires the use of additional external circuits resulting in restriction of possible memory density. Moreover, characteristic operational times of such devices are limited by relatively slow processes of the remagnetization of the ferromagnetic layers.In this work we propose a S-N/F-s-I-S control unit for a superconducting memory cell (Fig.1) based on the principles completely different as the suggested earlier. The considered structure consists of the two superconductive electrodes (S) and of the two weak link regions: the tunnel barrier (I) and the metallic (N/F) interlayer. The (N/F) part is formed by the longitudinally oriented normal (N) and ferromagnetic (F) layers. The weak link areas are separated by a thin super...

Josephson junctions in a local inhomogeneous magnetic field

“…To conclude, we derived and analyzed numerically equations describing behavior of a Josephson junction in local inhomogeneous magnetic field. As discussed in the Introduction, such situation may have many different reasons and experimental realizations [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]27]. It was demonstrated that time-dependent local field provides an additional driving force, which may induce flux-flow type dynamics in long junctions.…”

“…However, in many experimental situations JJ's are subjected to a local, strongly inhomogeneous magnetic field. For example, it can originate from a self-induced flux in JJ's with a sign-reversal order parameter [7][8][9]; appear in JJ's containing ferromagnetic interlayers with spatially inhomogeneous thicknesses [10][11][12][13], nanoparticles or domain walls [14][15][16][17][18][19]; in JJ's with a local current injection [20,21]; can be induced by a nearby Abrikosov vortex [22][23][24][25], by a sharp tip of a Magnetic Force Microscope (MFM) [26,27], etc. Although such a situation has been considered previously, often this has been done without proper substantiation.…”

Two mechanisms of Josephson phase shift generation by an Abrikosov vortex

“…For example, they can operate as autonomous and persistent phase batteries -one of key elements of quantum [1,[5][6][7][8][9][10][11][12][13][14] and digital Josephson electronics [2,3,15,16]. Such junctions can be also used for development of novel types of cryogenic memory [17][18][19][20][21][22]. The π-phase shift is most commonly needed, e.g., for bringing the flux-qubit to the degeneracy point [1,[5][6][7], for realization of complementary digital Josephson electronics [2,3,15,16] and for maximum distinction between 0 and 1 state in a memory cell [17].…”

Фазовые Переходы В Гибридных SFS-структурах С Тонкими Сверхпроводящими Слоями

“…В гибридных SFS-структурах это достигается путем изменения толщины ферромагнитно-го слоя [13] и/или при использовании комбинированного барьера, состоящего из участков нормального (N) металла и ферромагнетика [14]. Подобные системы с искусственно созданным чередованием 0 и π контактов обладают необычной (отличной от фраунгоферовой) зависимостью критического тока I c от внешнего магнитного поля H 0 [15,16] и допускают спонтанную генерацию вихрей с магнитным потоком, равным доле кванта магнитного потока 0 = π c/|e| на границах между 0 и π участками перехода [17,18].…”

“…Джозефсоновская разность фаз в основном состоянии 0−π перехода может принимать произвольное значение 0 < ϕ < π [19,20], что позволяет использовать такой ϕ-контакт в качестве источника заданного фазового сдвига (фазовой батареи) в контурах сверхпроводящих квантовых интерферометров [21]. Предложенный в [14] дизайн гибридной структуры с комбинированным F/N-барьером может оказаться перспективным для разработки на его основе сверхпроводящей ячейки памяти. Заметим, что при отсутствии структурных неоднородностей на FS-границе продольная слоям фазовая модуляция сверхпроводящего параметра порядка возможна из-за формирования неоднородного состояния Ларкина−Овчинникова−Фульде−Феррелла (ЛОФФ) [22,23].…”