Ferromagnetic insulators are extremely valuable components of novel devices based on spin 

(rather than charge) transport. The lack of conductivity means that spin waves can propagate 

more freely in ferromagnetic insulators, as those spin waves are undamped by eddy currents. 

However, there are a limited number of ferromagnetic insulators commonly found in nature. 

As such, a large fraction of research done on ferromagnetic insulator based devices involve one 

specific ferromagnetic insulator compound, yttrium iron garnet, Y3Fe5O12 (YIG). Despite being 

so commonly used, researchers continue to learn more about the fundamental properties of 

YIG, and how it interacts with other materials. An excellent example came from a recent study 

of the nature of the magnetic exchange coupling between YIG and the commonly used metallic 

ferromagnet permalloy, Ni80Fe20 (Py) [1]. In this work, the authors used polarized neutron 

reflectometry to characterize the magnetization of individual YIG and Py thin film layers in a 

multilayer stacks. They found very surprising results, and we'll reproduce their polarized 

neutron reflectometry (PNR) experiment for our NCNR summer school PBR experiment. It's an 

excellent paper, but SPOILER ALERT - don't read it ahead of time if you want to be surprised by 

the results!

The thin-film multilayer sample to be studied in our experiment was grown by magnetron 

sputtering in Luqiao Liu's lab at the Massachusetts Institute of Technology. The layer structure, 

as well as key results from initial characterization measurements are shown in Figure 1.

The atomic force microscopy (AFM) image in Fig. 1(b) shows a local surface roughness of about 

1 nm, suggesting that the sample is smooth enough for PNR measurements. Fig. 1(c) shows 

Figure 1. (a) Sample layer structure. (b) AFM image indicating low surface 

roughness. (c) VSM hysteresis loops for our sample (red) and control samples 

(black, blue, and green).
room temperature vibrating sample magnetometry (VSM) measurements of the applied field 

dependent magnetization of our YIG / Py experiment sample (red), as well as those for various 

control samples: no YIG (green), no Py (blue), and with a non-magnetic MgO spacer layer in 

between the YIG and Py layers. Notably, the hysteresis loop for our sample with adjacent YIG 

and Py layers looks qualitatively different than that of the other three. Instead of a tight, 

square, ferromagnetic loop, the net magnetization of the sample with a YIG/Py interface is 

qualitatively different. When the applied field drops below about 0.1 T, the magnetization 

starts to relax, slowly dropping before switching rapidly upon application of negative field. Can 

you guess what might be happening here?

The VSM measurements are only sensitive to the net magnetic moment associated with the 

samples. To understand this strange hysteresis loop, we'll need characterize the 

magnetizations of the individual layers using PNR. We will conduct room temperature specular 

PNR measurements at applied fields of 0.5 T (magnetic saturation), and 0.01 T (nearremanence). By model fitting this data, we will be able to deduce the depth profiles of the inplane magnetization, and the nuclear composition (which will allow us to reference how the 

depth-dependent magnetization correlates with the chemical composition). These depth 

profiles will reveal fascinating magnetic coupling between the ferromagnetic insulator YIG and 

the ferromagnetic metal Py.

[1] Yabin Fan, P. Quarterman, et. al, PHYSICAL REVIEW APPLIED 13, 061002 (2020).