## 1 Introduction  
Spintronics, also known as spin-electronics, utilizing the spin degree of freedom in addition to the charge of electrons has attracted much attention in recent years from the viewpoint of the creation of new driving principles of electric and magnetic devices and energy conservation technology. While technical innovation in conventional semiconductor technology is approaching its limit, spintronics can open new paths to further development and will create new devices that exceed the limitations of conventional ones [1]. The important element in their development is how efficiently one can control the electron spin. Up until now, magnetic fields have been used as the most popular method for controlling the electron spin (or the magnetization of a magnetic material). In recent years, however, alternative methods such as a spin polarized current, an electric field, and a pure spin current, which are in principle not accompanied by Joule heating, are attracting much attention from the viewpoint of low power consumption spin control.  
On the other hand, the demand for an ever-increasing speed of manipulation in magnetic information storage has triggered a search for new methods for control of the spin functionality on the ultrafast timescale. Since the control speed of the spin by magnetic fields is already approaching its limit, how fast the spin functionality can be manipulated is one of the most important issues in modern technology. Under such circumstances, the recent development of ultrafast optical technology has opened the way to control the spin by light. In particular, the use of ultrashort optical pulses that are much shorter than the fundamental timescales such as spin-lattice relaxation or precession times allows us to excite magnetic media and to study the spin dynamics after ultrashort optical excitations. These new possibilities have recently led to exciting results demonstrating changes in the spin state on a sub-ps timescale [2]. For example, by the irradiation of \(\sim\)100 fs laser pulse with various pulse shapes, ultrafast demagnetization in ferromagnetic metals [3; 4; 5; 6], magnetization reversal in ferrimagnetic (ferromagnetic) alloys [7; 8], enhancement of magnetization in magnetic semiconductors [9; 10], generation of ferromagnetic order in strongly correlated electron systems [11], and generation and control of spin waves in magnetic insulators [12] have been realized. In the first place, in order to control the spin functionality on the ultrafast timescale, a strong magnetic interaction of the large energy scale (effective magnetic field acting on spins) should be utilized. Therefore, the use of exchange interaction or spin-orbit interaction might be desired for that purpose (Figure 1).  
In this review paper, I will show that ultrafast strengthening or quenching of the ferromagnetic order can be achieved via resonant photoexcitation in carrier-density-controlled ferromagnetic semiconductor EuO doped with Gd ions (Eu\({}_{1-x}\)Gd\({}_{x}\)O) [13]. The change of the magnetic order is established within 3 ps and detected by nonlinear magneto-optical effects. Such spin dynamics cannot be explained by a well-known three-temperature model. The associated spin dynamics is explained by the interplay of chemically and optically generated carriers and the resulting dynamic renormalization of the Ruderman-Kittel-Kasuya-Yoshida (RKKY)-like magnetic exchange coupling. The change from ultrafast enhancement to quenching of ferromagnetism is marked by the crossover from the predominantly semiconducting to the predominantly metallic doping range of Eu\({}_{1-x}\)Gd\({}_{x}\)O. Aside from the generalized insight into ultrafast spin as well as charge dynamics, a key requirement for spintronics technology, our investigation provides guiding information for the selection and design of materials for ultrafast optical control of magnetic interactions.