Enhancement of the spin Seebeck voltage in a polycrystalline bulk-YIG by controlling microstructure

Abstract

Since the spin Seebeck effect (SSE) has been discovered [1], most of experiments has been performed on a single crystal Y3Fe5O12 (YIG) fabricated by a pulsed laser deposition (PLD) method [2, 3]. This is because a single crystal YIG has been believed to be able to generate sufficiently high SSE signal [4]. However, this method is known to be very difficult to use in mass-production. In contrast, the sol-gel synthesis can provide a low-cost and mass-production method. Here, we fabricated a bulk-YIG by adopting a sol-gel synthesis and a mechanical pressing process with heat-treatments [5]. To explore the role of the grain in the SSE, the bulk YIG samples with various gran size ranging from 4 to 7 μm were fabricated by manipulating the sintering time as displayed in Fig. 1(a). To measure the longitudinal-SSE (LSSE), a platinum (Pt) electrode was deposited on the bulk-YIG sample by rf sputtering on the surface of disk-shaped YIG with 1.8 mm-thickness and 14 ∅ mm-diameter. By applying 400 Oe magnetic field parallel to the disk plane (+y direction), magnetization in bulk YIG aligned along +y direction. In order to form temperature gradient along -z direction, bottom surface was heated, and top surface was air cooled as shown in Fig. 1(b). Figure 1(c) indicates thermoelectric voltage induced by LSSE versus temperature difference, 40 K between top and bottom surfaces of the bulk YIG. It is evident that the grain size can affect significantly to the thermoelectric voltage. These results can provide the method to enhance the SSE by controlling the microstructure of a polycrystalline bulk-YIG.