Al-Qayoudhi, Mohammed Salim
(2020)
The strain manipulation of nickel and permalloy spin valves.
PhD thesis, University of Nottingham.
Abstract
In spintronics devices, the manipulation of magnetisation in magnetic films and spin valves can play a crucial role in the development of novel technologies. The usage of strain-induced anisotropy utilised in spintronics devices can devise new methods for low-power storage and logic devices. One of these methods is using a multiferroic device to introduce strain throughout the magnetic materials. In this study, strain induced in ferromagnetic layers is accomplished by using a hybrid structure of piezoelectric transducer/ magnetic multi or single layers. The main aim of this thesis is to study the strain effect on the magnetisation and magnetic reversal of spin valves consisting of permalloy and nickel layers (with different magnetostriction values and sign) through the strain-induced magnetoelastic coupling.
Initially, this thesis examines the role of the strain on single layers of microdevices of sputtered permalloy (Ni80Fe20) and nickel layers using magnetotransport measurements. Varying strain in the permalloy sample rotates the magnetisation by 90^o when varying the strain between tensile and compressive. The induced magnetic anisotropy modulates the magnetic properties of the permalloy sample, particularly the anisotropic magnetoresistance (AMR), switching field, magnetic reversal, and remanent magnetisation. The coherent rotation model enables calculation of magnetic anisotropy as a function of applied voltage, showing positive magnetostriction of the permalloy layer, possibly enhanced by surface anisotropy, which shows dominance at low thickness. The stoichiometry divergence from 20:80 ratio might also explain the magnetostriction boost. The nickel layer under strain effect showed zero magnetostriction, which might be due to the critical thickness at which there is an inversion of the value of magnetostriction between positive and negative value.
Spin valves with incorporated IrMn layers, S548 (PZT/Cr/NiFe/Cr/Ni/IrMn/Cr), and S549 (PZT/Cr/Ni/Cr/NiFe/IrMn/Cr) were investigated under varying voltages through the transducer. The permalloy in these spin valves shows positive magnetostriction in S548, but no effect in S549, as the permalloy is coupled partially to the IrMn layer, while the nickel exhibited zero magnetostriction, similar to that in single layer sample. Despite the magnetisation reversal of both spin valves exhibiting two-step reversal, these multilayer systems’ magnetoresistance is dominated by AMR; applying strain does not enhance the giant magnetoresistance (GMR) effect in both spin valves. This might be attributed to the spacer suppressing of the flow of electron spins.
The core of this thesis is the investigation of the strain effect in sputtered pseudo spin valve with Cu spacer: IM008 (PZT/Ta/NiFe/Cu/Ni/Ta) and IM009 (PZT/Ta/Ni/Cu/NiFe/Ta), which is demonstrated using magnetotransport and ferromagnetic measurements. These spin valves show clear GMR curves responding to the strain. The GMR modulated with the tuning strain in the spin valves for both magnetic layers shows a dependence of strain. The nickel layer possesses negative magnetostriction in these spin valves opposite to the sign of the magnetostriction of the permalloy layer. The effect of strain on each layer manifested differently in the magnetic properties of the whole spin valves (i.e. coercivity, remnant magnetisation, and spin valve stiffness). The strain affects each magnetostrictive layer differently, which can be utilised for certain applications with low power cost. Finally, in addition to the transport measurements, the static and dynamic properties of pseudo spin valves were investigated using modulated FMR. The resonance field H_res of the permalloy varies with the strain in both spin valves. The anisotropy as a function of strain is extracted for the permalloy layer, showing positive magnetostriction in agreement with transport measurement. For IM008, the spectra show only one clear resonance, which might be related to the dynamic interaction between magnetic layers, while for IM009, the nickel spectra showed negative magnetostriction extracted for certain voltages. The linewidth ∆H of permalloy shows in-plane dependence, caused by extrinsic damping contributions. The strain control of FMR could be used as a basis for bringing faster, lighter, and energy-efficient prototype devices that could be used in communication and space technology.
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