Perpendicular Hot Electron Spin-Valve Effect in a New Magnetic Field Sensor: The Spin-Valve Transistor

D. J. Monsma, J. C. Lodder, Th. J. A. Popma, B. Dieny
1995 Physical Review Letters  
A new magnetic field sensor is presented, based on perpendicular hot electron transport in a giant magnetoresistance (Co͞Cu) 4 multilayer, which serves as a base region of an n-silicon metal-base transistor structure. A 215% change in collector current is found in 500 Oe (77 K), with typical characteristics of the spin-valve effect. The in-plane magnetoresistance was only 3%. The transistor structure allows the investigation of energy resolved perpendicular transport properties, and in
more » ... es, and in particular spin-dependent scattering of hot electrons in transition-metal as well as rare-earth-based multilayers. PACS numbers: 72.15.Gd, 73.40.Vz, 75.50.Rr, 85.70.Kh The discovery of giant magnetoresistance in magnetic multilayers [1] (also called the spin-valve effect [2]) has led to a large number of studies on giant magnetoresistance systems. Usually, the resistance of the multilayer is measured with the current in plane (CIP). This is the easiest experimental approach of electrical transport in magnetic multilayers. Devices exhibiting CIP giant magnetoresistance are under development as magnetic field sensors, for instance, in read-back magnetic heads used in magnetic recording technology. However, from a fundamental point of view, the CIP configuration suffers from several drawbacks; the CIP magnetoresistance (MR) is diminished by shunting and channeling [2, 3] . In particular, uncoupled multilayers or sandwiches with thick spacer layers suffer from this problem, whereas the saturation field in such systems is usually small. Moreover, diffusive surface scattering reduces the MR for sandwiches [2] and thin multilayers [4] . Finally, fundamental parameters of the effect, such as the relative contributions of interface and bulk spin-dependent scatterings, are difficult to obtain using the CIP geometry [5] . Measuring with the current perpendicular to the planes (CPP) solves most of these problems, mainly because the electrons cross all magnetic layers, but a practical difficulty is encountered; the perpendicular resistance of the ultrathin multilayers is too small to be measured by ordinary techniques. The first CPP-MR experiments were reported on Co͞Ag multilayers [6], where the multilayer was sandwiched between superconducting Nb leads. In this way, CPP experiments could be performed, albeit only at liquid helium temperatures. The use of microfabrication techniques for CPP measurements from 4.2 to 300 K was first shown for Fe͞Cr multilayers [7] , where the multilayers were etched into micropillars to obtain a relatively large resistance (a few mV). Both types of measurements have confirmed the larger MR effect for the CPP configuration, but they suffered from the general complexity of realization and measurement techniques. Experiments using electrodeposited nanowires showed CPP-MR up to 15% at RT [8]. In this Letter, we present the design, prospects, and experimental results of a new magnetic field sensor and measurement tool based on perpendicular hot electron transport in a spin-valve multilayer: the spin-valve transistor. Here, a spin-valve multilayer serves as a base region of an n-silicon metal-base transistor structure. Metal-base transistors have been proposed for ultrahigh frequency operations [9] because of their negligible base transport time and low base resistance; however, low gain prospects have limited their advent. Its use for investigating transport properties has been shown for Au, Ag, Al, and Pd base films [10] . With the spin-valve transistor, we present the first evidence of a spin-valve effect for hot electrons ഠ1 eV above E F in (Co͞Cu) multilayers. We find a very large change (215% at 77 K) in collector current under application of a magnetic field of 500 Oe, with typical giant magnetoresistance characteristics, such as saturation field and hysteresis. The hot nature of the electrons and the possibility to vary the electron energy accurately in a range of about 0.2-3 eV raises exciting possibilities for fundamental research of the spin-valve effect and may lead to unforeseen effects and new spin valves. In contrast to usual conduction electrons which are sensitive to the density of states (DOS) at the Fermi energy (E F ), the hot electrons are sensitive to DOS above E F . Manipulation of electron energy may offer important insights into the relative importance of the band structure (DOS) and scattering potentials, which are understood to form the basis of the scattering asymmetry between majority and minority conduction electrons [11] . Spin polarization of hot electrons in magnetic films is known for energies larger than about 5 eV relative to the Fermi level [12] . The electron energy range of the spin-valve transistor is particularly attractive, because the asymmetry in DOS of spin-split 3d bands is most pronounced for energies up to 2 eV above the Fermi level in Co and Fe [13] . 5260 0031-9007͞95͞74(26)͞5260(4)$06.00
doi:10.1103/physrevlett.74.5260 pmid:10058723 fatcat:daxv5ngudbfqvnrjzdmiphgjqa