| Ferromagnetic III-V Semiconductor, (Ga,Mn)As |
III-V ferromagnetic semiconductor (Ga,Mn)As exhibits hole induced ferromagnetism, which was established both from experimental and theoretical works. (Ga,Mn)As can be grown epitaxially on a GaAs substrate by molecular beam epitaxy (MBE). (Ga,Mn)As based heterostructures can be also grown by MBE. We demonstrated electrical spin-polarized carrier injection from (Ga,Mn)As into GaAs heterostructure. In the following, we introduce our results on the investigation of basic properties of (Ga,Mn)As.
| Fig. 1 |
The schematic of (Ga,Mn)As. In this material, doped
Mn(green) substitutes for Ga(blue) site in host semiconductor GaAs.
|
(Ga,Mn)As shows ferromagnetism at low temperatures, and the highest
Curie temperature TC reported so far is 185 K. According to the p-d Zener
model, which can explain many experimental observations semi-quantitatively,
TC can be expressed by the function of Mn composition and hole concentration.
The model shows that TC increases as Mn composition and/or hole concentration
increases (below figure). However, it was difficult to dope a large amount
of Mn due to its low solubility. We have succeeded to grow (Ga,Mn)As with
high Mn composition (~ 20%) without MnAs precipitates by reduction of growth
temperature and film thickness. The Curie temperature of (Ga,Mn)As with
high Mn composition is comparable with the highest reported value.
| Fig. 2 |
The hole concentration dependence of the Curie temperature predicted by the p-d Zener mode. |
| Fig. 3 |
The temperature dependence of sheet resistance Rsheet and spontaneous Hall resistance RHall.
Mn composition dependence of the Curie temperature.
|
Magnetic anisotropy, which determines magnetization direction in
ferromagnetic materials, is one of the most important parameters in order
to design magnetic devices. Recently, there are reports of electric-field
control of magnetic anisotropy in (Ga,Mn)As and ferromagnetic metal such
as Fe, CoFe, which is expected to open up a new scheme of bit writing (magnetization
direction switching) with much reduced power. In (Ga,Mn)As, perpendicular
magnetic anisotropy can be controlled by lattice strain. Figure 5 shows
the biaxial strain dependence of anisotropy field calculated by the p-d
Zener model. Magnetization easy axis turns toward perpendicular to plane
under tensile strain, whereas in-plane under compressive strain. In figure
5 we show the results of Hall measurements RHall on (Ga,Mn)As with compressive
(upper panel) and tensile strain (lower panel) under magnetic fields perpendicular
to the plane, where RHall is proportional to a perpendicular component
of magnetization. The results are consistent with the expectation from
the p-d Zener model. For (Ga,Mn)As with compressive strain, the magnetic
easy axes are in the plane, which are composed from two major axes, biaxial
easy axis (along [100] and [010] direction) and uniaxial easy axis (along
[110] or [-110] direction). Figure 6 shows the temperature dependence of
biaxial anisotropy and uniaxial anisotropy fields determined from ferromagnetic
resonance (FMR) and transport measurements.
| Fig. 4 |
Strain dependence of perpendicular anisotropy field calculated by the p-d
Zener model. |
| Fig. 5 |
Hall resistance of (Ga,Mn)As with compressive (upper) and tensile strain
(lower). |
| Fig. 6 |
The temperature dependence of biaxial anisotropy and uniaxial anisotropy fields determined from FMR and transport measurements. |
In ferromagnetic multilayer structures, the electrical resistance
depends on the magnetization alignment between ferromagnetic layers, which
can be applied to HDD reading head and magnetic random access memory (MRAM)
by utilizing the high-resistance state as output ¡È1¡É and low-resistance
state as ¡È0¡É. For (Ga,Mn)As/ GaAs/ (Ga,Mn)As multilayer devices, we have
observed tunnel magnetoresistance (TMR), whose ratio reach 290% at 0.4
K.
| Fig. 7 |
TMR observed in a (Ga,Mn)As/ GaAs/ (Ga,Mn)As structure. |
