The longitudinal spin Seebeck effect (LSSE) is an efficient method for generating incoherent magnon excitations by applying a vertical temperature gradient across a magnetic material. To avoid the contamination of the thermally generated “pure” spin current by the electronic degrees of freedom, magnetic insulators are exclusively preferred for LSSE. Although the ferrimagnetic insulator Y3Fe5O12 (YIG) is known as the hallmark system for LSSE because of its ultra-low Gilbert damping,[1] other members of the insulating iron garnet family, including the compensated ferrimagnet Gd3Fe5O12 (GdIG), have recently attracted considerable interest due to their unique compensated spin configurations.[2] In particular, the fascinating observation of the change in sign of the LSSE signal in a GdIG/Pt bilayer around the magnetic compensation temperature due to reorientation of sublattice magnetizations has motived us to address to an emerging fundamental question: “Is there a general trend for the temperature dependence of LSSE in compensated ferrimagnets?” Therefore, we have performed a comprehensive investigation of LSSE in GGG/GdIG(t)/Pt(5nm) bilayers with 5 different thicknesses ranging between 31 and 272 nm. We find that all the GdIG films possess an in-plane magnetic easy axis, and the compensation temperature decreases from 270 to 220 K with decreasing film-thickness from 272 to 31 nm. Using a proposed rescaling method, we have demonstrated, for the first time, a “universal scaling” behavior for the temperature dependence of LSSE signal for our GdIG films with different compensation temperatures, which, we believe, holds true for other compensated ferrimagnetic systems as well. In addition, we have performed the LSSE study on a 31 nm GdIG film grown on lattice-mismatched GSGG substrate that exhibits an out-of-plane magnetic easy axis and observed a clear distinction in the magnetic field dependent LSSE signal, relative to the 31 nm GGG/GdIG film showing an in-plane magnetic easy axis. This highlights a strong correlation between the LSSE signal and the magnetic anisotropy in compensated ferrimagnets. Our findings will pave the way to explore LSSE in other novel compensated ferrimagnets.

Universal scaling of the temperature dependence of longitudinal spin Seebeck effect in compensated ferrimagnets

Controlled manipulation of spin texture in magnetic materials by external means, e.g., magnetic and electric fields, temperature etc., is a prerequisite for the cutting-edge spintronic applications. Magnetic materials with perpendicular magnetic anisotropy (PMA) are of special interest because of their wide-ranging functionalities starting from applications in magnetoresistive random access memory (MRAM) devices to modulation of topological spin textures. Recently, there has been a growing interest in rare earth (RE)-transition metal (TM) alloy thin films due to their tuneable spin configurations as a function of temperature and RE content. For example, GdFeCo shows in-plane spin configuration at room temperature, but PMA occurs near the compensation temperature (TCO).[1] Such spin reorientation is generally attributed to competition between PMA and shape anisotropy. In order to understand the behaviour of magnetic anisotropy around the spin reorientation temperature, several indirect methods were employed to determine the temperature dependence of anisotropy fields, e.g., longitudinal and polar MOKE, FMR etc.[1] Here, we examined the spin reorientation in single layer ferrimagnetic alloy thin films of FeGd by dc magnetometry and radio frequency transverse susceptibility measurement, which is a direct method to determine the effective anisotropy fields. The FeGd system belongs to the RE-TM family and is particularly interesting as TCO of this system can be tuned by varying the chemical composition. We prepared the FexGd100-x(90 nm)/Ta(8 nm) films onto a Si substrate coated with a 10 nm Ta layer using RF and DC sputtering; we focus below on a nominal composition of Fe75.7Gd24.3. From the temperature dependence of the in-plane magnetization M(T) curve, TCO for the Fe75.7Gd24.3 film was estimated to be ~ 70 K (main panel of Fig. 1(a)). Inset of Fig. 1(a) depicts the in-plane M(H) loops for this film measured at T = 10 and 300 K. While the in-plane saturation magnetization has drastically dropped between 300 and 10 K, the coercivity is enhanced significantly. In addition, the ratio of remanent magnetization to the saturation magnetization is an order of magnitude higher at 10 K than at 300 K. These observations indicate the occurrence of spin reorientation between 10 and 300 K. In order to probe the spin reorientation, we performed magnetic field dependent transverse susceptibility (TS) measurements for both in-plane and out-of-plane configurations at different temperatures between T = 300 and 10 K. Our TS measurement set up exploits a self-resonant ultra-sensitive tunnel diode oscillator circuit with a resonant frequency of 13 MHz and is a highly sensitive technique to precisely determine the effective anisotropy fields.[2] The inset of Fig. 1(b) shows the magnetic field dependence of the relative change in TS at 40 K for the Fe75.7Gd24.3 for in-plane configuration. As the dc magnetic field is swept from positive to negative saturation, the first (second) peak to positive (negative) anisotropy field (HK). As shown in the main panel of Fig. 1(b), HK remains the same for in-plane and out-of-plane configurations but begins bifurcating below 250 K. At lower temperatures, the out-of-plane HK shows a broad maximum between 200-150 K and then decreases smoothly. On the other hand, the in-plane HK continues to decrease down to 100 K below which it shows a small plateau around TCO and then decreases gradually. These observations indicate that the spin configuration in the system changes from a combination of in-plane and out-of-plane spins with equal contributions above 250 K, to a combination of in-plane and out-of-plane spins with a major contribution by the out-of-plane spins below 250 K. The origin of such temperature-driven spin reorientation might be related to phase-segregation. With the help of scanning transmission electron microscopy equipped with energy dispersive x-ray spectroscopy (STEM-EDS) and x-ray reflectivity (XRR) measurements, a recent study shows that modulation of chemical composition occurs in FeGd films with thickness more than 20 nm where Fe is partially diffused to the adjacent Ta layer.[3] Since the thickness of our film is ~ 90 nm, it is possible that there are different Fe-deficient (Gd-rich) regions which are separated by regions with nominal chemical composition. In such a hybrid phase-segregated spin system, different regions have distinct spin configurations and hence, different orientations of magnetic anisotropy. We have also performed out-of-plane M(H) measurements on the Fe75.7Gd24.3 film that showed a double-step reversal behaviour below 250 K and hence, confirms the chemical phase-segregation in our system. The spin reorientation in our system is thus determined by the competition between local magnetic anisotropy energies of adjacent phase-segregated regions. Our TS technique determines the average behaviour of the local anisotropy field and hence effectively probes the spin reorientations in these micron-scale phase-segregated magnetic systems. Results for other Fe/Gd compositions will also be discussed.

**References:**

[1] W. He, H. -L. Liu, H.-Y. Wu, J. -W. Cai, and Z. -H. Cheng, Appl. Phys. Lett. 106, 042401 (2015). [2] V. Kalappattil, R. Geng, R. Das, M. Pham, H. Luong, T. Nguyen, A. Popescu, L. M. Woods, M. Kläui, H. Srikanth and M. H. Phan, Mater. Horiz. 7, 1413-1420 (2020). [3] E. Kirk, C. Bull, S. Finizio , H. Sepehri-Amin, S. Wintz , A. K. Suszka, N. S. Bingham, P. Warnicke, K. Hono, P. W. Nutter, J. Raabe, G. Hrkac, T. Thomson and L. J. Heyderman, Phys. Rev. Mater. 4, 074403 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ad612b336fa97c19a15298613ec09cf9.png)
(a) Main panel: in-plane M(T) of Fe75.7Gd24.3 measured in 1 T field and inset: in-plane M(H) loops at 10 and 300 K, (b) main panel: in-plane and out-of-plane HK(T), inset: field dependent TS curve at 40 K for in-plane configuration.

 Competing Magnetic Anisotropies and Spin Reorientation in Phase-Segregated Single Layer Ferrimagnetic FeGd Films

Graphene (Gr) is a particularly interesting material for spintronic and technological applications due to its low intrinsic spin orbit coupling and hyperfine interaction, which lead to high spin lifetimes and coherence lengths [1]. Furthermore, it is common in spintronic applications to introduce a structural isomorph to Gr, hexagonal Boron Nitride (hBN) to improve lattice matching in heterostructure engineering[2]. Specifically for spintronic and spin-caloritronic applications, a ferrimagnetic (FM) layer is often used as a magnon source. Therefore, it is important to understand the interfacial properties of FM/Gr(hBN) interfaces. Here, we explore the change in magnetic anisotropy in the FM insulator Nickel Ferrite (NFO) due to the inclusion of monolayer Gr and hBN. This was done by performing radio frequency transverse susceptibility measurements utilizing a tunnel diode oscillator technique, which is a sensitive probe of the system’s effective anisotropy[3]. Specifically, in-plane (IP) and out-of-plane (OOP) measurements were taken for bare NFO, NFO/Gr, NFO/hBN samples. We find that the inclusion of the Gr layer reduces the overall anisotropy for both IP and OOP drastically, by a factor of 2, in the entire temperature of 20 – 300 K. On the other hand, the inclusion of the hBN layer slightly reduces the in-plane anisotropy at low temperature (T &lt; 180 K) but increases it at high temperature (T &gt; 180 K). Since NFO is an insulator and Gr is a semimetal there is a large conductivity difference at the interface which likely leads to charge transfer reducing the overall anisotropy[4]. This is further supported by the less drastic change with the inclusion of monolayer hBN, a semiconductor. Relative to NFO, magnetometry measurements show an enhancement of the saturation magnetization in NFO/Gr and NFO/hBN, highlighting an additional role played by the magnetic proximity effect.

Proximity and Charge-Transfer Mediated Magnetism at Nickel Ferrite Interfaces with Graphene and Hexagonal Boron Nitride Monolayers

**Session chair:** Chuanpu Liu, Colorado State University



GOI - Magnetization Control

technical paper

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

2022 Joint MMM-INTERMAG

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

*Session Chair: Aidan Hindmarch, University of Durham*

These on-demand presentations are available to view at your leisure. 

IG: Multilayered and Patterned Films, and Exchange Bias

INTERMAG 2021 Welcome Video



	
<figure class="video_container">
 <iframe src="https://screencast-o-matic.com/player/crflqFVnUpZ?v=6&ff=1&title=0&controls=1" width=640 height=360 frameborder="0" allowfullscreen="true"> </iframe>
</figure>

Want to buy a ticket? 

Register for INTERMAG 2021 here

[![](https://assets.underline.io/uploads/markdown_image/1/image/17ce76e7960db633258024eef8d6da7f.png)](https://intermag.org/register)

Got your ticket, but still can’t access the event?

Contact us and our team will look into it.

Noah Schulz

3

3

SHORT BIO

Presentations

Universal scaling of the temperature dependence of longitudinal spin Seebeck effect in compensated ferrimagnets

Competing Magnetic Anisotropies and Spin Reorientation in Phase-Segregated Single Layer Ferrimagnetic FeGd Films

Proximity and Charge-Transfer Mediated Magnetism at Nickel Ferrite Interfaces with Graphene and Hexagonal Boron Nitride Monolayers

Stay up to date with the latest Underline news!

PRESENTATIONS

CONFERENCES

COMPANY

RESOURCES