Theory of electric-field-controlled surface ferromagnetic transition in metals

Abstract

It is widely believed that in metals, unlike in the dilute magnetic semiconductors, the control of ferromagnetic ordering by external voltage is hardly achievable. We compare the two types of ferromagnets and show that there is no obvious reason why metals are less preferable for this phenomenon. A similar effect in metals, however, has a different physical picture and should be identified as a voltage-induced surface transition. We study its properties within the theory of the surface critical phenomena and discuss possible difficulties on the way to its experimental realization.The possibility to externally control ferromagnetic ordering will naturally render an entire additional degree of freedom to the functionality of the magnetoelectronics and spintronics devices. 1 Of primary importance is the capacitive manipulation of ferromagnetism. The voltage-controlled ferromagnetic ordering ͑VCFO͒ was demonstrated not long ago 2 in the dilute magnetic semiconductors ͑DMS͒. 3 The phenomenon was recognized as a controllable variation in the local ͑bulk͒ Curie temperature, T C b , in the surface layer, which the electric field penetrates varying the free-carrier concentration, , and consequently the free-carrier-mediated exchange coupling between the localized spins. For reasonable voltages, the available variation in and/or T C ͑Ref. 4͒ is minor, thus making it imperative that the operation temperature of the device, T 0 , be sufficiently close to the transition point,The recent challenge for the technological applications of the VCFO in DMS is that T C , and consequently T 0 , is much lower than the room temperature, T R .One of the possible ways to achieve the room-temperature VCFO is to switch to the high-T C metallic ferromagnets. The conventional wisdom, however, puts forward the two following widely accepted arguments against the possibility of the VCFO in metals: ͑i͒ due to the high the voltage-induced variation in T C is inconsiderable in metals; ͑ii͒ in metals, the injected carriers occupy only an atomic-size surface layer and so should the voltage-induced magnetization. In this Rapid Communication we show that both arguments are actually misconceptions and a similar effect must exist in metals as recently predicted in Ref. 5. Its physical picture, however, is in a sense opposite of the conventional interpretation. The phenomenon should be viewed not as a local bulk but as a surface transition, which can be isothermally and reversibly turned on and off by the external voltage. We find the conditions for the existence of the VCFO in metals, obtain the dependence of physical quantities on the applied voltage, and discuss complications, together with possible solutions, on the way to its experimental realization.The density of itinerant carriers in a ferromagnet defines the strength of the exchange coupling and consequently the magnitude of T C . The reserve statement is also qualitatively correct: that T C is high directly points out on high no matter what the ferromagnet is-a metal or a DMS. For example, in th...

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Magnetoelectric Devices for Spintronics

“…Spatially, the field effect operates over distances on the order of the Thomas-Fermi screening length (λ TF ), which can be a few nanometers in materials with very low carrier densities but much less than a nanometer in metals. However, in magnetic materials, changes in the magnetic properties may be perceived over distances set by the exchange length l ex (88), which is usually larger than λ TF and can approach 10 nm (89). Experiments by Chiba and colleagues (90, 91) on Co ultrathin (typically 0.4-nm-thick) films gated through HfO 2 or by ionic liquids showed strong electrical modulations of the Curie temperature near or across 300 K (Figure 5).…”