To conclude, MnSb(0001) with ac lattice parameter of 5.795(2) ˚A have been success- fully grown on GaAs(111)B substrates. These films were grown using a new two stage growth procedure as this limited the amount of surface displaying the “mesa” morphol- ogy. A third antimony capping stage has been added to the growth procedure. It has been shown that this cap is easily removed through careful IBA cycles, resulting in a (2×2) surface reconstruction. Uncapped thin films can be prepared using a simple HCl and IBA method, after which the surfaces are seen to be free of contamination, well ordered and reconstructed, which are necessary requirements before undertaking quan- titative surface structure determination work. However, annealing above ∼375–400 ◦C
the surface morphology of MnSb(0001) is particularly sensitive to the local beam flux ratio. Variations in J of a few percent result in large scale morphological changes in the epilayer, with a previously unseen surface morphology having been discovered during this work.
An XRD investigation of MnSb thin films show some reflections which cannot be indexed onto any obvious material or compound. However, of much greater significance is clear, unequivocal evidence for c-MnSb crystallites within four of the six samples investigated. Each of these c-MnSb crystallites appear to be in different strain states. The mechanism which drives the formation of c-MnSb within these films is unclear, but the result is unique, with no similar results documented within the literature for MnSb or any other BP.
Magnetic properties of epitaxial MnSb(0001)
Bulk MnSb has a magnetic moment of 3.3 µB per manganese atom with a reported
Curie temperature (Tc) of between 550 K and 590 K [95, 101, 102]. To characterise
the magnetic properties of the MnSb thin films vibrating sample magnetometry (VSM) and the superconducting quantum interference device (SQUID) were used to collect hysteresis loops and magnetisation versus temperature (MvT) curves respectively. Cir- cularly polarised soft x-ray spectroscopy experiments were performed on thin films of MnSb(0001) in order to characterise the surface magnetic properties.
5.1 Bulk magnetometry
VSM and SQUID experiments were conducted in-house on MnSb(0001) samples of approximately 2×2 mm. Samples were mounted on polyetherether ketone (PEEK) sample holders using polytetrafluoroethylene (PTFE) tape. The combined PEEK holder and PTFE tape had a small diamagnetic response which was measured separately and subtracted from the MnSb(0001) hysteresis data. The GaAs substrates also have a small diamagnetic response which was not taken into consideration.
Hysteresis loops acquired with the applied field parallel and perpendicular to the sample surface are shown in Fig. 5.1. The negative gradient apparent in both data sets at large field are a diamagnetic contribution to the overall magnetisation from the GaAs substrate. From these data MnSb thin films show an “easy plane” and “hard axis” type magnetisation response which is in agreement with first principles calculations and experimental results of the MnSb magnetocrystalline anisotropy energy [101, 103]. The inset in Fig. 5.1 is a close up around the origin of the in-plane magnetisation. The
-3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 M a g n e t i s a t i o n 1 0 - 3 ( e m u ) H 10 4 (Oe) T = 300 K H perpendicular to c H // c -0.02 -0.01 0.00 0.01 0.02 -2 0 2
Figure 5.1: Hysteresis loops of MnSb(0001) with the applied field in-plane and out-
of-plane. The sample has an easy plane and hard axis parallel to <0001>. The inset shows a close up of the in-plane magnetisation data. The coercive field is found to be (14.7 ±0.5) Oe with a remnant magnetisation of (0.79 ±0.05)Msat.
coercive field,Hcis found to be (14.7±0.5) Oe in-plane with a remnant magnetisation
of (0.79 ± 0.05)Msat. The value for the coercive field is considerably smaller than
the previously published values of 120–560 Oe [104, 105, 106]. It must be noted that the quality of the thin films used in those experiments was relatively low, being either polycrystalline or highly textured and granular, which is in contrast to the high quality and well oriented samples grown in this study. Finally on the basis of a crude estimate of the epilayer thickness the saturation magnetisation is found to be (3.1±0.5)µB per
manganese atom. This is in broad agreement with published values.
Magnetisation versus temperature data was acquired using the SQUID on a sample of thickness 250 nm and is shown in Fig. 5.2. The data was acquired in the presence of a 4 kOe external field applied parallel to the sample surface (H0 ⊥c) with
300 400 500 600 700 0 2 4 6 8 300 350 400 450 500 550 600 650 700 750 800 0. 0 0. 5 1. 0 1. 5 2. 0 2. 5 3. 0 3. 5 T (K ) warming cooling M 1 0 - 4 ( e m u ) M a g n e t i s a t i on 10 - 3 ( e m u ) T ( K)
Figure 5.2: Magnetisation versus temperature for a 250 nm thick MnSb(0001) film in
a 4 kOe external field applied parallel to the sample surface. The TC for this sample is found to be 590 K. The inset shows equivalent data for a sample with a thickness of 50 nm. The sample shows the correct behaviour during heating but shows no ferro- magnetism upon cooling, demonstrating the sample has decomposed during the heating cycle.
to be 590 K which is in very good agreement with published values. The inset in Fig. 5.2 shows equivalent data for a sample with an estimated thickness of 50 nm. Upon heating the sample shows the correct behaviour and has aTc similar to that of the thicker film.
However there is no ferromagnetic moment apparent as the film is cooled. It should be noted that the atmosphere within which the sample sits is a poor vacuum with helium being the dominant residual gas. As such, and with specific reference to the the XPS and SEM results in Sec. 4.2, it is believed the loss of ferromagnetic ordering is a result of thermal decomposition during the prolonged heating cycle and not extreme oxidation from heating the thin epilayer in an atmosphere containing a large partial pressure of oxygen.