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Institute
Since the discovery of spin torque in 1996, independently by Berger and Slonczewski, and given its potential impact on information storage and communication technologies, (e.g. through the possibility of switching the magnetic configuration of a bit by current instead of a magnetic field, or the realization of high frequency spin torque oscillators (STO), this effect has been an important field of spintronics research. One aspect of this research focuses on ferromagnets with low damping. The lower the damping in a ferromagnet, the lower the critical current that is needed to induce switching of a spin valve or induce precession of its magnetization. In this thesis ferromagnetic resonance (FMR) studies of NiMnSb layers are presented along with experimental studies on various spin-torque (ST) devices using NiMnSb. NiMnSb, when crystallized in the half-Heusler structure, is a half-metal which is predicted to have 100% spin polarization, a consideration which further increases its potential as a candidate for memory devices based on the giant magnetoresistance (GMR) effect. The FMR measurements show an outstandingly low damping factor for NiMnSb, in low 10-3 range. This is about a factor of two lower than permalloy and well comparable to lowest damping for iron grown by molecular beam epitaxy (MBE). According to theory the 100% spin polarization properties of the bulk disappear at interfaces where the break in translational symmetry causes the gap in the minority spin band to collapse but can remain in other crystal symmetries such as (111). Consequently NiMnSb layers on (111)(In,Ga)As buffer are characterized in respect of anisotropies and damping. The FMR measurements on these samples indicates a higher damping that for the 001 samples, and a thickness dependent uniaxial in-plane anisotropy. Investigations of the material for device use is pursued by considering sub-micrometer sized elements of NiMnSb on 001 substrates, which were fabricated by electron-beam lithography and measured by ferromagnetic resonance. The damping remains in the low 10-3 range as determined directly by extracting the Gilbert damping from the line width. Additionally magnetostatic modes are observed in arrays of elements, which is further evidence of high material quality of the samples. By sputtering various metals on top of the NiMnSb, spin pumping from the ferromagnet into the non-magnetic layer is investigated. After these material investigations, pseudo-spin-valves using NiMnSb as one of the ferromagnet, in combination with Permalloy were fabricating using a self-aligned lithography process. These samples show a GMR ratio of 3.4% at room temperature and almost double at low temperature, comparing favourably to the best single stack GMR structures reported to date. Moreover, current induced switching measurements show promisingly low current densities are necessary to change the magnetic orientation of the free layer. These current densities compete with state-of-the-art GMR devices for metal based structures and almost with tunnel junction devices. The true potential of these devices however comes to light when they are operated as spin torque oscillators to emit high frequency, tunable, narrow spectrum electromagnetic waves. These Heusler based STOs show an outstanding q-factor of 4180, even when operating in the absence of an external field, a value which bests the highest value in the literature by more than an order of magnitude. While these devices currently still suffer from the same limited output power as all STO reported to date, their sub-micron lateral dimensions make the fabrication of an on-chip array of coupled oscillators, which is a promising path forward towards industrially relevant output power.
Fabrication and characterization of CPP-GMR and spin-transfer torque induced magnetic switching
(2014)
Even though the unique magnetic behavior for ferromagnets has been known for thousands of years, explaining this interesting phenomenon only occurred in the 20th century. It was in 1920, with the discovery of electron spin, that a clear explanation of how ferromagnets achieve their unique magnetic properties came to light. The electron carries an intrinsic electric charge and intrinsic angular momentum. Use of this property in a device was achieved in 1998 when Fert and Gru¨nberg independently found that the resistance of FM/NM/FM trilayer depended on the angle between the magnetization of the two layers. This phenomena which is called giant magnetoresistance (GMR) brought spin transfer into mainstream. This new discovery created a brand new research fi called “spintronics” or “spin based electronics” which exploits the intrinsic spin of electron.
As expected spintronics delivered a new generation of magnetic devices which are currently used in magnetic disk drives and magnetic random access memories (MRAM). The potential advantages of spintronics devices are non-volatility, higher speed, increased data density and low power consumption. GMR devices are already used in industry as magnetic memories and read heads.
The quality of GMR devices can be increased by developing new magnetic materials and also by going down to nanoscale. The desired characteristic properties of these new materials are higher spin polarization, higher curie temperature and better spin filtering. Half-metals are a good candidate for these devices since they are expected to have high polarization. Some examples of half-metals are Half-Heusler alloy, full Heusler alloy and Perovskite or double Perovskite oxides. The devices discussed in this thesis have NiMnSb half-Heusler alloy and permalloy as the ferromagnetic layers separated by Cu as the nonmagnetic layer.
This dissertation includes mainly two parts, fabrication and characterization of nan- opillars. The layer stack used for the fabrication is Ru/Py/Cu/NiMnSb which is grown on an InP substrate with an (In,Ga)As buff by molecule beam epitaxy (MBE). A new method of fabrication using metal mask which has a higher yield of working samples over the previous method (using the resist mask) used in our group is discussed in detail. Also, the advantages of this new method and draw backs of the old method are explained thoroughly (in chapter 3).
The second part (chapters 4 and 5) is focused on electrical measurements and charac- terization of the nanopillar, specially with regard to GMR and spin-transfer torque (STT)
measurements. In chapter 4, the results of current perpendicular the plane giant mag- netoresistance (CPP-GMR) measurements at various temperatures and in-plane magnetic fi are presented. The dependence of CPP-GMR on bias current and shape anisotropy of the device are investigated. Results of these measurements show that the device has strong shape anisotropy.
The following chapter deals with spin-transfer torque induced magnetic switching measurements done on the device. Critical current densities are on the order of 106 A/cm2, which is one order of magnitude smaller than the current industry standards. Our results show that the two possible magnetic configurations of the nanopillar (parallel and anti-parallel) have a strong dependence on the applied in-plane magnetic fi Fi- nally, four magnetic fi regimes based on the stability of the magnetic configuration (P stable, AP stable, both P and AP stable, both P and AP unstable) are identified.