Use of a Spin-Flop State for the Creation of Spin-Valve Elements for a Full Wheatstone Bridge
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TRICAL AND MAGNETIC PROPERTIES
Use of a Spin-Flop State for the Creation of Spin-Valve Elements for a Full Wheatstone Bridge M. A. Milyaeva, L. I. Naumovaa, *, R. S. Zavornitsyna, I. K. Maksimovaa, A. Yu. Pavlovaa, V. V. Proglyadoa, and V. V. Ustinova aMikheev
Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, Ekaterinburg, 620990 Russia *e-mail: [email protected] Received March 23, 2020; revised March 30, 2020; accepted April 6, 2020
Abstract—A method of universal thermomagnetic treatment forming a pairwise opposite exchange bias in spin valves, which are sensor elements of a Wheatstone bridge, has been developed. The method is based on the formation of two magnetic phases in a spin valve upon the transition of a synthetic antiferromagnet into a spin-flop state. Thermomagnetic treatment in a two-phase state leads to the formation of a pairwise mutually opposite exchange bias in different elements of a Wheatstone bridge. The direction of a formed exchange bias is governed by the uniaxial anisotropy of every element. Keywords: spin valve, magnetoresistance, synthetic antiferromagnet, spin-flop state, Wheatstone bridge DOI: 10.1134/S0031918X20080050
INTRODUCTION Multilayered nanostructures with a giant magnetoresistive effect, such as spin valves, are used in microelectronics as magnetosensitive materials. A spin valve is composed of two ferromagnetic layers separated by a copper layer. One of the ferromagnetic layers (pinned) is adjacent to an antiferromagnetic layer, and is connected with it by means of exchange interaction. This results in the appearance of unidirectional anisotropy characterized by the unidirectional anisotropy axis (UAA). The hysteresis loop of magnetization reversal in this layer is shifted towards higher fields. The second ferromagnetic layer is free and sustains magnetization reversal in low fields. To improve the thermal stability of the characteristics of a spin valve, the pinned layer is replaced by a synthetic antiferromagnet (SAF) [1–3]. SAF represents a trilayered nanostructure, in which two ferromagnetic layers F1 and F2 are separated by a ruthenium layer and connected by means of antiferromagnetic exchange interaction. Layer F1 is adjacent to an antiferromagnetic alloy layer and is pinned, and layer F2 is a reference layer. In magnetosensitive devices, four sensor elements are frequently combined into a Wheatstone bridge to reduce the temperature drift of an output signal. A full Wheatstone bridge, in which every element makes a contribution to an output signal, is used to increase the level of a signal [4–6]. To accomplish this, the characteristic dR/dH must be positive for two elements of this
bridge and negative for two others. In spin valves, such characteristics are obtained when their UAAs are pairwise mutually opposite to each other. There are different methods for the formation of such an arrangement of UAAs. The two-stage sputtering of structures with opposite UAAs onto different areas of a substrate is used [7] for spin valves with the upper location
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