Magnon Valve Effect between Two Magnetic Insulators

The key physics of the spin valve involves spin-polarized conduction electrons propagating between two magnetic layers such that the device conductance is controlled by the relative magnetization orientation of two magnetic layers. Here, we report the effect of a magnon valve which is made of two ferromagnetic insulators (YIG) separated by a nonmagnetic spacer layer (Au). When a thermal gradient is applied perpendicular to the layers, the inverse spin Hall voltage output detected by a Pt bar placed on top of the magnon valve depends on the relative orientation of the magnetization of two YIG layers, indicating the magnon current induced by the spin Seebeck effect at one layer affects the magnon current in the other layer separated by Au. We interpret the magnon valve effect by the angular momentum conversion and propagation between magnons in two YIG layers and conduction electrons in the Au layer. The temperature dependence of the magnon valve ratio shows approximately a power law, supporting the above magnon-electron spin conversion mechanism. This work opens a new class of valve structures beyond the conventional spin valves.

Figure 1

(a) Illustration of the magnon valve effect: When a temperature gradient is applied, the magnon current in the top YIG comes from two sources. One is generated by the presence of the local temperature gradient, and the other is the magnon current injected from the bottom YIG layer. If the magnetization directions of the YIG layers are parallel (antiparallel), these two magnon currents are additive (subtractive). Since the ISHE voltage measured by the Pt layer is proportional to the total magnon current through the top layer, a magnon valve effect is observed. (b) The cross-sectional scanning transmission electron microscopy and selected area electron diffraction patterns of the $\mathrm{GGG}/\mathrm{YIG}$ interface. (c) The cross-sectional high-resolution transmission electron microscopy of the $\mathrm{YIG}/\mathrm{Au}/\mathrm{YIG}$ region. The magnon valve structure measured in (b) and (c) is $\mathrm{GGG}/\mathrm{YIG}(40)/\mathrm{Au}(15)/\mathrm{YIG}(20)/\mathrm{Pt}(10\mathrm{nm})$.

Figure 2

The magnetic and magnon transport properties of the magnon valve. (a) Magnetization of the magnon valve structure $\mathrm{GGG}/\mathrm{YIG}(40)/\mathrm{Au}(15)/\mathrm{YIG}(20)/\mathrm{Pt}(10\mathrm{nm})$ as a function of the magnetic field applied in the plane of the layers. The arrows indicate the magnetization directions of the two YIG layers. (b) The ISHE voltage in Pt as a function of the magnetic field for the same magnon valve structure in the presence of the temperature gradient created by a 20 mA electric current applied at the heating electrode.

Figure 3

The ISHE voltage of different samples for controlled study. (a)–(d) Sample structures are marked in the figures, and the ISHE voltage is normalized by the resistance of the Pt detector to eliminate the sample-to-sample variation.

Figure 4

Thickness, temperature, magnetization direction, and heating current dependences of the magnon valve effect. (a)–(d) were measured in the $\mathrm{GGG}/\mathrm{YIG}(40)/\mathrm{Au}(15)/\mathrm{YIG}(20)/\mathrm{Pt}(10\mathrm{nm})$ sample. (a) The interlayer Au thickness dependence of magnon valve ratio MVR. The dashed line shows the exponential decay fitting curve. (b) The temperature dependence of MVR, and the dashed line shows the $T^{5/2}$ fitting curve. (c) The ISHE voltage as a function of the angle between the directions of the voltage probe and the in-plane magnetic field. The magnetic field (5 kOe) is large enough such that both YIG layers are in parallel with the magnetic field. The dashed line shows the sine fitting curve. (d) The heating current dependence of $V_{\uparrow \uparrow }-V_{\downarrow \downarrow }$. The dashed line shows the parabolic fitting curve.