In general, work on the magnetic effect of the conventional semiconductors, which has been the focus of intensive study in recent years, can be divided into three categories. In the first category, the generation/manipulation of spin-polarized current has been achieved via circularly polarized light excitation, electron tunneling, spin pumping, and Seebeck spin tunneling, and various effects such as the spin Hall effect and the spin torque transfer effect have been reported. The second category of work is aimed at the modification of the charge-transport process in semiconductors. It has been reported that unevenly distributed charge carriers in Si are susceptible to a magnetic field. There are also indications for positive magnetoresistance arising from the shrinkage of the electron wavefunction of the impurity states in a magnetic field. The last category of work is the p–n junction or Schottky junction. As reported, a magnetic field drives the turn-on voltage of diodes considerably upward, yielding magnetically tunable rectifying behaviors.
In most of the previous work, the magnetic effect was produced by direct tuning, i.e., applying a magnetic field directly to a semiconductor to modify the kinetic/dynamic behaviors of the charge carriers. In this case, usually a field on the magnitude of Tesla is required to get significant effects. Obviously, a low-field magnetic effect is no doubt more important for either fundamental research or practical application. Unfortunately, it is hardly achieved because of the insensitivity of the conventional semiconductors to magnetic field. Compared with semiconductors, soft ferromagnetic (FM) materials are much more susceptible to magnetic fields. Through the coupling of the FM material with semiconductor, an indirect approach for magnetic tuning could be developed, i.e., tuning the latter by changing the former. This is particularly important noting the fact that the search for suitable FM metals is relatively easier.
It is well established that Permalloy (Ni₈₀Fe₂₀) (Py) is an FM material with a very low coercive force field, usually a few oersteds, and a large anisotropic magnetoresistance, AMR ≈ 2% at room temperature. Ni₆₅Co₃₅ (NC) is another representative AMR material, with an AMR up to 5% at room temperature. The work functions of the Py and the NC are ca. 4.7 eV and ca.4.9 eV, respectively. Si is the mostly studied semiconductor. As tabulated, the electron affinity of Si is ca. 4.3 eV when electron-doped to ca. 10¹⁵ cm⁻³ and ca. 5.0 eV when hole-doped to 10¹⁵ cm⁻³. Due to the difference in work function (electron affinity), a strong coupling can be created between Si and an FM material when they form a Schottky junction (FM/Si). An important feature of the Schottky junction is the photovoltaic effect: illuminating the junction by photons with an energy higher than the interface barrier or bandgap, a photocurrent or photovoltage can be generated across the two poles of the diode. Here, we report a study on the lateral photovoltaic effect for FM/Si junctions, focusing on its magnetic tunability. Remarkably, the anisotropic photovoltage, driven by a field of several oersteds, appears not only in the FM layer but also in the Si, and its relative change is much larger in the latter than in the former. In addition to the lateral photovoltaic effect, a planar Hall photovoltage, a transverse photovoltaic effect, is also detected in the FM layer. Key factors affecting the magnetic effect are determined. The present work demonstrates an effective tuning of the electronic processes in Si by very low magnetic fields.