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Tuning anomalous Hall effect and spin polarized current in magnetic ultrathin films

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In this dissertation I studied the anomalous Hall effect in MgO/Permalloy/Nonmagnetic Metal(NM) based structure, spin polarized current in YIG/Pt based thin films and the origin of the perpendicular magnetic anisotropy(PMA)

In this dissertation I studied the anomalous Hall effect in MgO/Permalloy/Nonmagnetic Metal(NM) based structure, spin polarized current in YIG/Pt based thin films and the origin of the perpendicular magnetic anisotropy(PMA) in the Ru/Co/Ru based structures.

The anomalous Hall effect is the observation of a nonzero voltage difference across a magnetic material transverse to the current that flows through the material and the external magnetic field. Unlike the ordinary Hall effect which is observed in nonmagnetic metals, the anomalous Hall effect is only observed in magnetic materials and is orders of magnitude larger than the ordinary Hall effect. Unlike quantum anomalous Hall effect which only works in low temperature and extremely large magnetic field, anomalous Hall effect can be measured at room temperature under a relatively small magnetic field. This allows the anomalous Hall effect to have great potential applications in spintronics and be a good characterization tool for ferromagnetic materials especially materials that have perpendicular magnetic anisotropy(PMA).

In my research, it is observed that a polarity change of the Hall resistance in the MgO/Permalloy/NM structure can be obtained when certain nonmagnetic metal is used as the capping layer while no polarity change is observed when some other metal is used as the capping layer. This allows us to tune the polarity of the anomalous Hall effect by changing the thickness of a component of the structure. My conclusion is that an intrinsic mechanism from Berry curvature plays an important role in the sign of anomalous Hall resistivity in the MgO/Py/HM structures. Surface and interfacial scattering also make substantial contribution to the measured Hall resistivity.

Spin polarization(P) is one of the key concepts in spintronics and is defined as the difference in the spin up and spin down electron population near the Fermi level of a conductor. It has great applications in the spintronics field such as the creation of spin transfer torques, magnetic tunnel junction(MTJ), spintronic logic devices.

In my research, spin polarization is measured on platinum layers grown on a YIG layer. Platinum is a nonmagnetic metal with strong spin orbit coupling which intrinsically has zero spin polarization. Nontrivial spin polarization measured by ARS is observed in the Pt layer when it is grown on YIG ferromagnetic insulator. This result is contrary to the zero spin polarization in the Pt layer when it is grown directly on SiO2 substrate. Magnetic proximity effect and spin current pumping from YIG into Pt is proposed as the reason of the nontrivial spin polarization induced in Pt. An even higher spin polarization in the Pt layer is observed when an ultrathin NiO layer or Cu layer is inserted between Pt and YIG which blocks the proximity effect. The spin polarization in the NiO inserted sample shows temperature dependence. This demonstrates that the spin current transmission is further enhanced in ultrathin NiO layers through magnon and spin fluctuations.

Perpendicular Magnetic Anisotropy(PMA) has important applications in spintronics and magnetic storage. In the last chapter, I study the origin of PMA in one of the structures that shows PMA: Ru/Co/Ru. By measuring the ARS curve while changing the magnetic field orientation, the origin of the PMA in this structure is determined to be the strain induced by lattice mismatch.

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Date Created
  • 2018

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Andreev reflection spectroscopy: theory and experiment

Description

A theoretical study of a three-dimensional (3D) N/S interface with arbitrary spin

polarization and interface geometry is presented. The 3D model gives the same intrinsic

spin polarization and superconducting gap dependence as

A theoretical study of a three-dimensional (3D) N/S interface with arbitrary spin

polarization and interface geometry is presented. The 3D model gives the same intrinsic

spin polarization and superconducting gap dependence as the 1D model. This

demonstrates that the 1D model can be use to t 3D data.

Using this model, a Heusler alloy is investigated. Andreev reflection measurements

show that the spin polarization is 80% in samples sputtered on unheated MgO(100)

substrates and annealed at high temperatures. However, the spin polarization is

considerably smaller in samples deposited on heated substrates.

Ferromagnetic FexSi􀀀x alloys have been proposed as potential spin injectors into

silicon with a substantial spin polarization. Andreev Reflection Spectroscopy (ARS) is

utilized to determine the spin polarization of both amorphous and crystalline Fe65Si35

alloys. The amorphous phase has a significantly higher spin polarization than that of

the crystalline phase.

In this thesis, (1111) Fe SmO0:82F0:18FeAs and Pb superconductors are used to

measure the spin polarization of a highly spin-polarized material, La0:67Sr0:33MnO3.

Both materials yield the same intrinsic spin polarization, therefore, Fe-superconductors

can be used in ARS. Based on the behavior of the differential conductance for highly

spin polarized LSMO and small polarization of Au, it can be concluded that the Fe-Sc

is not a triplet superconductor.

Zero bias anomaly (ZBA), in point contact Andreev reflection (PCAR), has been

utilized as a characteristic feature to reveal many novel physics. Complexities at a

normal metal/superconducting interface often cause nonessential ZBA-like features,

which may be mistaken as ZBA. In this work, it is shown that an extrinsic ZBA,

which is due to the contact resistance, cannot be suppressed by a highly spin-polarized

current while a nonessential ZBA cannot be affected the contact resistance.

Finally, Cu/Cu multilayer GMR structures were fabricated and the GMR% measured

at 300 K and 4.5 K gave responses of 63% and 115% respectively. Not only

do the GMR structures have a large enhancement of resistance, but by applying an

external magnetic eld it is shown that, unlike most materials, the spin polarization

can be tuned to values of 0.386 to 0.415 from H = 0 kOe to H = 15 kOe.

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Created

Date Created
  • 2015

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Spin polarized currents in magnetic and superconducting structures

Description

An electrical current with high spin polarization is desirable for the performance of novel spintronics devices, such as magnetic tunnel junction and giant magnetoresistance devices. The generation of spin polarized

An electrical current with high spin polarization is desirable for the performance of novel spintronics devices, such as magnetic tunnel junction and giant magnetoresistance devices. The generation of spin polarized current can be from ferromagnetic materials or triplet superconductors.

Anomalous Hall effect (AHE) is an effective way to study the properties of magnetic structures. The scattering of electrons by the magnetic moments affects the change of resistance, which can be used to detect the magnetization. In this dissertation, AHE is used to study the perpendicular magnetic anisotropy (PMA) structures, including Co/Pt and Ta/CoFeB/MgO.

Domain walls exist in all ferromagnetic materials. This dissertation studies the domain wall movement in the Ta/CoFeB/MgO structure. A single domain is observed by measuring the anomalous Hall effect. On the other hand, a zero Hall step is successfully observed in a single layer of magnetic material for the first time, which can be used to fabricate advanced domain wall spintronics devices.

Besides the normal ferromagnetic material, the generation of spin polarized current in superconductor is also important for Spintronics. The electrons in superconductors form Cooper pairs. In this dissertation, Andreev Reflection Spectroscopy (ARS) is used to study the spin configuration in Cooper pairs.

Generally, ferromagnetism and superconductivity can not co-exist. In this dissertation, the Bi/Ni bilayer structure has been studied with ARS, and the measurement results show a triplet superconductivity below 4K. The appearance of superconductivity is believed to be attributed to the Bi-Ni interface, and the triplet Cooper pair makes it a promising candidate in superconducting spintronics.

Besides, a Bi3Ni single crystal is also studied with ARS. The measurements show a singlet superconductivity in this material, which further proves the importance of the Bi/Ni interface to achieve triplet superconductivity.

Finally, ARS is also used to study NbSe2 monolayer, a 2D superconductor. The monolayer is verified by the measurements of critical temperature and critical field, which are different from the values of multilayer or bulk. Andreev reflection results show that NbSe2 monolayer is a singlet superconductor and there is no node exist in the superconducting gap for a in plane magnetic field up to 58 kOe.

Contributors

Agent

Created

Date Created
  • 2018