Magnetic Properties Of Diluted Magnetic Semiconductor Nanoribbons

With the progress of science and technology, the various new materials continually come up. In recent years, diluted magnetic semiconductor that has significant excellent properties compared with conventional semiconductor has been paid attention extensively. Diluted magnetic semiconductor has the advantages of fast speed, low energy consumption, small volume and non‐volatile. These characteristics could make the performance of future electronic devices break through the physical limitations of traditional electronic devices, and especially the low dimensional diluted magnetic semiconductor material can provide more operational characteristics.Using the s d exchange interaction model, we studied the magnetization curves in mean field approximation and the spin wave at low temperatures, and the effect of carrier density, magnetic impurity density and the width of nanoribbons on them. In mean field theory, the bands of carriers split into spin‐up sub bands and spin‐down subbands. The effective magnetic field that the spin splitting forms tends to make the local spin ordered alignment, while temperature tends to destruct this spin order. Both of the factors affect the magnetic properties of dilute magnetic semiconductor nanoribbons. The numerical result shows that the magnetization curve can be roughly divided into three stages. During the former two stages, carriers are always in the fully spin polarized state, the effective magnetic field system remain unchanged, and the increasing of temperature results in the decreaseing of magnetization of the system. In the third stage, with the increasing of temperature, carrier polarization begins to decline, and there are more carriers occupy spin‐up subbands, so the effective magnetic field starts to decrease, which causes the magnetization curve of the system drops down faster. By comparison, we found that there are difference effects that carrier density and impurity density on the properties of diluted magnetic semiconductor nanoribbon, especially on the critical temperature. Changing the carrier density affects the whole magnetization curve. Its effect on the second stage magnetization curve is very obvious. When we change the carrier density, the critical temperature varies, which is mainly caused by the fact whether the carriers are easy to occupy the spin‐up subbands on the second stage. Changing the impurity density only affects the part of magnetization curve when carriers are part polarization. The larger magnetic impurity density is, the higher temperature at which the transition of carriers from complete polarization state to part polarization state becomes, and the higher critical temperature of the system is. However, we cannot get stable numerical results when we change the width to observe its effect on the magnetization.Although the mean field theory has brought convenience to study, it has some limitations for its ignoring of the quantum fluctuations. We further studied the spin wave spectrum using spin wave approximation at low temperatures. Based on the comparative analysis of the numerical results, we found that carrier density, impurity density and width has different effects on the spin wave spectrum. Changing both carrier density and impurity density can affect the spin waves which are in the range of relatively large wave vector. In this range, the higher carrier density is, the higher spin wave energy is. On the contrary, the higher impurity density is, the lower spin wave energy is. When we changed the width of nanoribbon, we found that the larger width of nanoribbon is, the higher spin wave energy is.The result of our study shows that the magnetic properties of diluted magnetic semiconductor nanoribbon are strongly dependent on the carrier density, impurity density and nanoribbon width. The dependence on the width is caused by quantum size effect.