# Magnetic Properties and THz Emission from Co/CoO/Pt and Ni/NiO/Pt Trilayers

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## Abstract

**:**

## 1. Introduction

_{3}Fe

_{5}5O

_{12}(YIG)/NiO/Pt suggested that the NiO thin layers enhanced the spin current driven into the Pt layer due to AFM magnons or AFM fluctuations [39]. Furthermore, epitaxial NiO (001) layers were fabricated in MgO/Pt/NiO/FeNi/SiO

_{2}multilayers and spin-transfer ferromagnetic resonance (ST-FMR) experiments were performed which showed a highly efficient angular momentum transfer through the epitaxial NiO, a result that was attributed to the well-defined orientation of the antiferromagnetic moments and the spin quantization axis of the injected spin current from the Pt layer [40]. ST-FMR measurements were also used to quantify the magnon current in NiO in multilayers of Bi

_{2}Se

_{3}/NiO/Py, and it was shown that the magnon current in NiO is able to exert a magnon torque that is sufficient to control the magnetization of the magnetic Py layer [41]. Spin-pumping inverse spin Hall effect measurements have also been performed for the case of CoO in Y

_{3}Fe

_{5}5O

_{12}(YIG)/CoO/Pt [42], in which an enhancement of the inverse spin Hall voltage was recorded near the transition temperature from the AFM to the paramagnetic phase. The effect was attributed to spin fluctuations near the magnetic phase transition.

## 2. Materials and Methods

^{−1}(20 $\mathrm{W}$), respectively. The film thickness was determined using a quartz balance system (Inficon XTM/2, Kurt J. Lesker Company, Jefferson Hills, PA, USA). MgO (100) and Si (001) and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ polished on both sides were used as substrates. The ferromagnetic Co and Ni layer were directly deposited on each substrate, followed by the deposition of CoO and NiO oxides. After the deposition of the metallic layer, atmospheric air was allowed to flow into the system through a fine valve at a pressure of 2–3 $\times {10}^{-3}$ mbar. This led to the formation of a thin passive-oxide layer on the surface of the metal. More information about the growth process can be found in reference [44]. The thickness of the FM layers were kept at 10 $\mathrm{n}$$\mathrm{m}$, where the thickness for both oxides was $1.4$ $\mathrm{n}$$\mathrm{m}$. A Pt overlayer of 3 $\mathrm{n}$$\mathrm{m}$ thickness was finally deposited. In short, the grown structures had thicknesses of ${\mathrm{Co}}_{10\mathrm{n}\mathrm{m}}/{\mathrm{CoO}}_{1.4\mathrm{n}\mathrm{m}}/{\mathrm{Pt}}_{3\mathrm{n}\mathrm{m}}$ and ${\mathrm{Ni}}_{10\mathrm{n}\mathrm{m}}/{\mathrm{NiO}}_{1.4\mathrm{n}\mathrm{m}}/{\mathrm{Pt}}_{3\mathrm{n}\mathrm{m}}$.

## 3. Results

#### 3.1. Magnetization Reversal

_{B}[48]. The exchange bias further increases for lower temperatures. This is due to a stronger coupling between the FM and AFM layer. Furthermore, it is known that T

_{B}is generally lower than the Néel temperature [49]. However, even at T > T

_{B}the FM/AFM layers can be still coupled in cases in which the AFM order is only partially established due to the presence of different sizes of AFM grains [49]. Similarly, there is an increase in the EB at lower temperatures for the NiO case but the values are smaller compared to the CoO counterpart.

#### 3.2. Ferromagnetic Resonance Spectroscopy, FMR

^{−2}[55,56].

#### 3.3. THz Spectroscopy

## 4. Discussion

_{u2}[44,57], since we do not observe 100% remanent magnetization in the hysteresis loop. Temperature-dependent measurements show that the magnetization reversal of Ni-based trilayer develops an hysteretic behaviour and small exchange bias values below 150 $\mathrm{K}$.

## 5. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Abbreviations

FM | Ferromagnetic |

NM | Non-magnetic |

STEs | Spintronic THz emitters |

THz-TDS | Terahertz time-domain spectroscopy |

EB | Exchange Bias |

AFM | Antiferromagnetic |

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**Figure 1.**In-plane hysteresis curves obtained using Squid magnetometry. (

**a**) CoCoOPt curves measured at $T=300\mathrm{K}$ and (

**b**) at $T=4\mathrm{K}$. (

**c**) NiNiOPt hysteresis curve obtained at $T=300\mathrm{K}$ and (

**d**) at $T=10\mathrm{K}$.

**Figure 2.**Exchange bias values for Co/CoO/Pt. The values have been calculated as the sum of coercivity values for the positive and the negative field direction, ${\mathrm{H}}_{\mathrm{C}(+\mathrm{B})}$ + ${\mathrm{H}}_{\mathrm{C}(-\mathrm{B})}$. Below 150 $\mathrm{K}$, the antiferromagnetic phase of CoO drives the exchange bias to very large values. (Inset) The inset shows the exchange bias values for Ni/NiO/Pt that obtain significant smaller values.

**Figure 3.**Results of FMR Spectroscopy. (

**a**) Example of a differential FMR resonance curve recorded for the Co/CoO/Pt sample at constant frequency of 16 GHz. (

**b**) Calculation of the damping parameter $\alpha $ according to Equation (2). For the Co/CoO/Pt equal to $\alpha =1.93\times {10}^{-2}$ and for the Ni/NiO/Pt equal to $\alpha =6.78\times {10}^{-2}$.

**Figure 4.**(

**a**) THz-TDS system that was used in the measurements, (

**b**) typical THz signal from a reference ${\mathrm{Co}}_{10\mathrm{n}\mathrm{m}}/{\mathrm{Pt}}_{3\mathrm{n}\mathrm{m}}$ sample grown on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrate. (

**c**,

**d**) THz spectra from MgO/Co/CoO/Pt and MgO/Ni/NiO/Pt, respectively. The THz radiation is detected from the Pt-side.

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**MDPI and ACS Style**

Kanistras, N.; Scheuer, L.; Anyfantis, D.I.; Barnasas, A.; Torosyan, G.; Beigang, R.; Crisan, O.; Poulopoulos, P.; Papaioannou, E.T.
Magnetic Properties and THz Emission from Co/CoO/Pt and Ni/NiO/Pt Trilayers. *Nanomaterials* **2024**, *14*, 215.
https://doi.org/10.3390/nano14020215

**AMA Style**

Kanistras N, Scheuer L, Anyfantis DI, Barnasas A, Torosyan G, Beigang R, Crisan O, Poulopoulos P, Papaioannou ET.
Magnetic Properties and THz Emission from Co/CoO/Pt and Ni/NiO/Pt Trilayers. *Nanomaterials*. 2024; 14(2):215.
https://doi.org/10.3390/nano14020215

**Chicago/Turabian Style**

Kanistras, Nikolaos, Laura Scheuer, Dimitrios I. Anyfantis, Alexandros Barnasas, Garik Torosyan, René Beigang, Ovidiu Crisan, Panagiotis Poulopoulos, and Evangelos Th. Papaioannou.
2024. "Magnetic Properties and THz Emission from Co/CoO/Pt and Ni/NiO/Pt Trilayers" *Nanomaterials* 14, no. 2: 215.
https://doi.org/10.3390/nano14020215