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Title: Helical magnetic structure and transport properties in epitaxial B20 Fe(1−x)Co(x)Ge films
Author: Spencer, Charles Steven Bolanos
ISNI:       0000 0004 7231 6573
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2018
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We have grown epitaxial single-phase B20 Fe(1−x)Co(x)Ge films on Si (111) substrates films by molecular beam epitaxy. This method is able to produce the whole range of Fe(1−x)Co(x)Ge which are of high quality due to homogeneous layer growth with low surface roughness of 1-2 nm. The films grown are racemic, showing an equal mix of left-handed and right-handed chiral grains, and are strained due the lattice mismatch between the film and substrate. Magnetic measurements showed the FeGe films grown (~ 70 nm thickness) have a saturation moment, Ms = 0.982(7) μB, and ordering temperature, Tc = 280(2) K, both close to bulk value. Ms and Tc were found to decrease monotonically with increasing x and all films were found to have an easy-plane anisotropy. A helical magnetic structure was observed using polarised neutron reflectometry and we found the helix wavelength to vary with composition. A divergence in the wavelength was found at a critical composition xc = 0.5 where a transition from helimagnet to collinear ferromagnet occurred. The temperature dependent resistivity, ρxx(T), was found to be metallic for all compositions and a broad peak with magnetic origin was found to arise for intermediate compositions 0.2 < x < 0.7. The magnetoresistance in Fe(1−x)Co(x)Ge with 0.1 < x < 1 was found to behave similarly to FeGe, however many differences were observed, such as a positive linear magnetoresistance for Fe(0.4)Co(0.6)Ge and a change in conical magnetoresistance scaling beyond Fe(0.5)Co(0.5)Ge. The Hall resistivity showed an increase as well as a sign change in the ordinary Hall effect coefficient, R0, from 0.0122 μΩ cm/T for FeGe to -0.6808 μΩ cm/T for Fe(0.5)Co(0.5)Ge, at 5 K, indicating a large reduction in the carrier concentration and change of carrier type. The anomalous Hall effect was found to increase dramatically on the introduction of Co, increasing by an order of magnitude from FeGe to Fe(0.9)Co(0.1)Ge at 5 K. We find evidence of potential skyrmion structures through the measurement of the topological Hall effect, with features as large as -0.39 μΩ cm for Fe(0.6)Co(0.4)Ge at 5 K. We conclude Fe(1−x)Co(x)Ge is a potential material for further skyrmion study, but direct observation of these topological structures are required to fully attribute the measured effect and progress further.
Supervisor: Marrows, Christopher ; Langridge, Sean Sponsor: Science and Technology Facilities Council ; Hitachi Cambridge ; University of Leeds
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available