Strain-induced high ferromagnetic transition temperature of MnAs epilayer grown on GaAs (110)
© Xu et al; licensee Springer. 2011
Received: 12 August 2010
Accepted: 9 February 2011
Published: 9 February 2011
MnAs films are grown on GaAs surfaces by molecular beam epitaxy. Specular and grazing incidence X-ray diffractions are used to study the influence of different strain states of MnAs/GaAs (110) and MnAs/GaAs (001) on the first-order magnetostructural phase transition. It comes out that the first-order magnetostructural phase transition temperature T t, at which the remnant magnetization becomes zero, is strongly affected by the strain constraint from different oriented GaAs substrates. Our results show an elevated T t of 350 K for MnAs films grown on GaAs (110) surface, which is attributed to the effect of strain constraint from different directions.
PACS: 68.35.Rh, 61.50.Ks, 81.15.Hi, 07.85.Qe
Today, there is growing interest for realization of new technologies utilizing spin degree of freedom of electrons in semiconductor devices . The technology of manipulating spin in semiconductors promises devices with enhanced functionality and higher speed. A prerequisite for realization of such kind of devices is development of solid-state spin injectors at room temperature. Diluted magnetic semiconductors (DMSs) and ferromagnet/semiconductor hybrids are two important components for efficient spin injection. The exploitation of DMSs, however, is severely hindered by their low Curie temperature due to the low solubility of transition metals in semiconductors [2, 3]. With room-temperature ferromagnetism and high crystal quality, MnAs has been epitaxied on (001)-, (110)-, (111)-, and (113)-oriented GaAs substrates [4–9]. Moreover, MnAs/GaAs having sharper interface than that of Fe/GaAs has been presented [10, 11]; the sharp interface is considered to be crucial for obtaining higher transmission efficiencies. Recently, spin injection from MnAs into GaAs has been demonstrated , and the spin-dependent tunneling experiments show that the spin polarization at MnAs/GaAs interfaces is high [13, 14]. Therefore, MnAs/GaAs hybrid is attracting more and more attention for its potential applications in spin injection, magnetic tunneling junctions, and magnetically logic devices.
The first-order magnetostructural phase transition is a long-standing topic in magnetism [15–20]. Bulk MnAs shows a coupled first-order magnetostructural phase transition from the ferromagnetic hexagonal α-phase (P6(3)/mmc) to the paramagnetic orthorhombic β-phase (Pnma) which is contracted in volume by 2% at about 318 K. For epitaxial MnAs films on GaAs substrate, the transition proceeds continuously over a broad temperature range with coexistence of the two phases. The phase coexistence results in a considerable fraction of MnAs epitaxial films which are usually in paramagnetic phase at ~30°C, a strong limitation for room temperature spintronic devices. The first-order magnetostructural phase transition temperature T t, at which the remnant magnetization becomes zero, can be enhanced either by applying an external magnetic field or by growing MnAs films on different oriented GaAs substrates . For example, T t for MnAs films grown on GaAs (111)B is higher than that grown on GaAs (001) [18, 22, 23].
Epitaxial MnAs films on GaAs (001) and GaAs (111)B have been thoroughly investigated [4–6], while little attention has been paid to MnAs films grown on GaAs (110) . The spin relaxation time is considered crucial important for practical application of spin memory devices or spin quantum computers. In GaAs (110) quantum wells, the spin relaxation time is in nanosecond range, much longer than that in GaAs (001) where the spin relaxation time is in picoseconds range . More work is expected for investigation of MnAs films grown on GaAs (110) surfaces. In this work, we will present that epitaxial MnAs films grown on GaAs (110) are with a different strain states from MnAs films grown on GaAs (001), and α-phase can coexist with β-phase to a higher temperature (remnant magnetization becomes zero when the temperature reaches 350 K).
Growth parameters and the thickness for samples A-D
Growth temperature (°C)
As4/Mn BEP ratio
Results and discussion
Lattice parameters, primitive cell volume, and transition temperature of samples A-D and MnAs bulk 
T t (Å)
In summary, we have shown that the ferromagnetic order in MnAs can be extended to higher temperature by growing MnAs on GaAs (110). Ferromagnetic α-phase can coexist with paramagnetic β-phase to 350 K. By XRD measurements, it is found that T t is not a simple function of primitive cell volume, and stretching of lattice parameters in the basal plane or compressing of lattice parameter in the perpendicular direction results in a higher T t. The result described here attests to a strong link between anisotropic strain and epilayer properties. Understanding and mastering these characterizations may open a possibility to control magnetic properties via selection of substrate orientation and provide new possibilities for using MnAs epilayer in spintronic devices.
This work was supported in part by the National Natural Science Foundation of China under Grant No. 60836002 and the special funds for the Major State Basic Research Contract No. 2007CB924903 of China and the Knowledge Innovation Program Project of Chinese Academy of Sciences No. KJCX2.YW.W09-1.
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