| Electric-field control of ferromagnetic properties |
| Electric-field control of ferromagnetism | |
| Properties of magnetic materials are often assumed not possible to alter once the material is prepared and put into use. For example, although magnetic materials used in the information technology store trillions of bits, the bits are recorded in the form of magnetization directions established by applying external magnetic fields, and the properties of the magnetic media itself remain unchanged upon magnetization reversal. Control of material properties by external means, as in semiconductors, where their conductivity can be varied by electric fields, will thus add a new and unexplored dimension to the usage of magnetic materials. It is, therefore, of great fundamental as well as technological interest, particularly in view of recent rapid developments of magnetoelectronics and spintronics, to demonstrate electric field manipulation of magnetism. In this page, we show electric field control of ferromagnetism in a semiconducting alloy (In,Mn)As thin film using an insulating gate field-effect transistor (FET) structure. By applying electric fields, we demonstrate that the transition temperature of hole-induced ferromagnetism in (In,Mn)As can be varied isothermally in a reversible way. |
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| Figure.1 | Field effect control of hole induced ferromagnetism in magnetic semiconductor (In,Mn)As field-effect transistors. Shown are the cross-sections of a metal-insulator-semiconductor structure under gate biases VG, which controls the hole concentration in the magnetic semiconductor channel (schematically shown in red circles). Negative VG increases hole concentration resulting in enhancement of the ferromagnetic interaction among magnetic Mn ions, whereas positive VG has an opposite effect. The InAs/(Al,Ga)Sb/AlSb structure under the (In,Mn)As layer serves as a buffer relaxing the lattice mismatch between the structure and the GaAs substrate to produce smooth surface on which the magnetic layer is grown. The thickness of channel layer and insulator are 5 nm and 0.8 µm, respectively. |
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| Figure.2 | RHall versus field curves under three different gate biases. Application of VG = 0, +125, and -125 V results in qualitatively different field dependence of RHall measured at 22.5 K. RHall is proportional to M due to anomalous Hall effect. When holes are partially depleted from the channel (VG = +125V), a paramagnetic response is observed (blue dash-dotted line), whereas a clear hysteresis at low fields (< 0.7 mT) appears as holes are accumulated in the channel (VG = -125 V, red dashed line). Two RHall curves measured at VG = 0 V before and after application of ±125 V (black solid line and green dotted line, respectively) are virtually identical. Inset, the same curves shown to higher magnetic fields. |
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| Figure.3 | Temperature dependence of spontaneous Hall resistance RHallS under three different gate biases. RHallS proportional to the spontaneous magnetization MS indicates ±1 K modulation of TC upon application of VG = ±125 V. TC is the temperature at which RHallS (hence MS) goes to zero. Data at VG = 0 V before and after application of ±125 V are shown by squares and down triangles, respectively. In order to minimize the effect of domain rotation and magnetic anisotropy, RHallS is determined by extrapolation of RHall from moderate fields (0.1 - 0.7 T) to 0 using Arrott plots (RHall2 versus B/RHall plots shown in inset). |
| Electrical Manipulation of Magnetization Reversal in a Ferromagnetic Semiconductor | |
| The coercive force HC at which magnetization reversal occurs can be manipulated by modifying the carrier density through application of electric fields in a gated structure. Electrically assisted magnetization reversal as well as electrical demagnetization has been demonstrated using the effect. This electrical manipulation offers a functionality not previously accessible in magnetic materials and may become useful for reversing magnetization of nanoscale bits for ultra-high density information storage. |
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| Figure.4 | Electric field dependence of the magnetic hysteresis curves measured by RHall at 40 K. Application of E = ±1.5 MV/cm results in a change of coercive force μ0HC by a factor of 5, from 1 mT at -1.5 MV/cm to 0.2 mT at +1.5 MV/cm, without affecting the square shape of the hysteresis. Magnetic field sweep rate is 0.37 mT/min. |
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| Figure.5 | Time evolution of RHall resulting from a sequence of applied electric fields in sample B at 32 K, showing an electrically assisted magnetization reversal (A). The initial positive RHall at t = 0 is prepared under E = -1.5 MV/cm by first applying a large enough positive magnetic field to saturate the channel magnetization and then reducing the field to μ0H0 = - 0.2 mT. This state corresponds to state A on the hysteresis curve under E = -1.5 MV/cm shown in the inset. The sign change of RHall, i.e. magnetization reversal, occurs in response to switch-off of the electric field (E = 0) at t = 25 sec, which makes the magnitude of HC smaller than H0. The state is then at state B on the hysteresis curve under E = 0. RHall (thus M) shows only a small variation upon switching E back and forth between 0 and -1.5 MV/cm, as the state goes back and forth between state C and state D of the inset. Electrical demagnetization of the channel layer by electric field (B). Here, a hysteresis loop is first measured under E = -1.5 MV/cm (closed triangles). Switching E to +1.5 MV/cm eliminates the hysteresis (closed circles) and then turning E back to -1.5 MV/cm results in an initial magnetization curve (a virgin curve, open triangles), demonstrating an electrical demagnetization. |
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