Session X40: Proximity Effects and Spin Transport in Complex Oxides

Probing spin correlations using angle resolved photoemission in the coupled metallic/Mott insulator system PdCrO2

A nearly free electron metal and a Mott insulating state can be thought of as opposite ends of the range of possibilities for the motion of electrons in a solid. In the magnetic oxide metal PdCrO₂, these two coexist as alternating layers. Using angle resolved photoemission, we surprisingly find sharp band-like features in the one-electron removal spectral function of the correlated subsystem. We show that these arise because a hole created in the Mott layer moves to and propagates in the metallic layer while retaining memory of the Mott layer’s magnetism. This picture is quantitatively supported by a strong coupling analysis capturing the physics of PdCrO₂ in terms of a Kondo lattice Hamiltonian. Our findings open new routes to use the non-magnetic probe of photoemission to gain insights into the spin-susceptibility of correlated electron systems.   

Magnetic proximity effect in Pt/α-Fe2O3 bilayers

Recently antiferromagnetic insulators play an import role in spintronic research including the spin superfluid [1] and long distance spin transport [2]. In this work, we find the magnetic proximity effect (MPE) in non-magnet Pt on antiferromagnetic insulator α-Fe2O3 bilayers. Off-axis sputtering was used to grow the high-quality bilayers as confirmed by atomic force microscopy and X-ray diffraction. Magneto-transport reveals anomalous hall signals at different temperature is observed. To elucidate these features, angle-dependent magnetoresistance (ADMR) measurements at different magnetic fields and temperatures have been conducted to isolate the contributions of anisotropic magnetoresistance (AMR) and spin Hall magnetoresistance (SMR). We also find non-trivial MR signals from field sweeps that reflect novel surface magnetism from the uncompensated moments in α-Fe2O3.

[1] Yuan, Wei, et al. "Experimental signatures of spin superfluid ground state in canted antiferromagnet Cr2O3 via nonlocal spin transport." Science advances 4.4 (2018): eaat1098.
[2] Lebrun, R., et al. "Tunable long-distance spin transport in a crystalline antiferromagnetic iron oxide." Nature 561.7722 (2018): 222.   

First-Principles Studies of Influences of Capping Layers on Perpendicular Magnetic Anisotropy (Ki) in X/Co2FeAl/MgO (X = 4d/5d transition metals) Structures

The Co₂FeAl/MgO based magnetic tunnel junctions (MTJs) with interfacial perpendicular magnetic anisotropy (K_i) have attracted extensive interests because of their potential utilization in spin-transfer-torque magnetic random-access memory (STT-MRAM). In this study, we systematically investigated the capping layer impact on the K_i magnitude in Co₂FeAl/MgO system with 4d/5d transition metals (TMs) using density functional theory calculations. Our results show that TM capping layers, such as W, Pb, and Bi can effectively increase the K_i values as compared to the uncapped structures. The results are explained in terms of atomic resolved K_i contributions due to different orbital hybridizations. This work shed light on research and development of novel MTJs with high thermal stability during data storage in STT-MRAM in the future.   

Spin Seebeck Effect in Y3Fe5O12/NiO/Pt Thin Film Heterostructures

The spin Seebeck effect (SSE), where a thermal gradient in a ferromagnet generates spin current, has been in the focus of spintronics studies as it is one of the most efficient ways to generate spin currents. Also, antiferromagnets are gaining attention recently due to their properties like zero net magnetization, low magnetic susceptibility and very fast spin dynamics. In this work, we report SSE measured in Y3Fe5O12(YIG) (20 nm)/NiO(2 nm)/Pt (5 nm) structures as a function of temperature (5 K to 300 K) with the YIG magnetized in-plane, using a lock-in technique that probes the response at the first and second harmonic. By varying the current amplitude, magnetic field direction and temperature we characterize the amplitude of SSE in a comparative study with and without antiferromagnetic layer. Our results show that not only is spin transport possible through thin NiO layers but there is also an increase in the SSE effect magnitude at intermediate temperatures (~150 K). We correlate this increase in the SSE magnitude with an increase in the magnetic susceptibility of NiO shown by our XMLD/XMCD measurements. Our results suggest that the SSE can be used to characterize the response of thin antiferromagnetic materials and optimize their spin-transport characteristics.   

Spin transport and standing spin waves generation in heavy metal-magnetic insulator-ferromagnet hybrid structures

Magnons (or spin waves) are collective excitations of electrons’ spin angular momenta in magnetic or antiferromagnetic materials. The excitation and tunability of magnons in low-damping magnetic insulators, e.g., the yttrium iron garnet (YIG), is particularly interesting because it could offer much longer magnon propagation length and potential broad spintronics applications.
Here, we study a platinum/YIG/permalloy hybrid structure, where YIG is the magnetic spacer and permalloy (Py) is a low-damping ferromagnetic metal. Through external microwave excitation, the YIG layer and the Py layer can be excited to reach the ferromagnetic resonance modes individually, and we have detected the spin current signal from the Py spin pumping process transmitted through the YIG layer and converted to voltage signals in the platinum (Pt) layer by the inverse spin Hall effect. More importantly, we have detected additional resonance peak and spin pumping voltage signals which are corresponding to the perpendicular standing spin waves (PSSW) in the YIG layer. At specific frequency, this PSSW in YIG could be coupled with the magnon mode in Py, which could possibly facilitate the magnon spin transport from the Py layer to the bottom Pt layer.   

Spintronic Properties and Spin-Orbit-Torque Switching Characteristics of Pt/REIG Heterostructures

Ferrimagnetic insulator (FMI) thin films with perpendicular magnetic anisotropy (PMA) have recently attracted attention for spin-orbit torque (SOT) switching applications. We report the properties of epitaxial terbium iron garnet (TbIG) and europium iron garnet (EuIG) thin films with PMA.¹ EuIG can be grown with PMA on (100) and (111) GGG substrates, making it ideal for orientation-dependent studies. For instance, Pt/EuIG had similar (001) and (111) Im[G] in contrast to similar studies on the Pt/CFO system. The TbIG films had a low M_s at room temperature due to their magnetic compensation point of 330K, and AHE measurements of Pt/TbIG showed a sign change at the compensation point. In addition, the (111) Im[G] of Pt/TbIG was similar to the Pt/EuIG system. Finally, we report the SOT switching characteristics of Pt/TbIG going through its compensation point, as well as the orientation dependence of the SOT switching characteristics of the Pt/EuIG system.

1. Rosenberg, Ethan R., et al. "Magnetism and spin transport in rare-earth-rich epitaxial terbium and europium iron garnet films." Physical Review Materials 2.9 (2018): 094405.   

Probing boundary magnetization with spin Hall magnetoresistance in high TN boron-doped magnetoelectric Cr2O3

Voltage controlled magnetization is a promising route for next-generation low-energy magnetic recording and logic devices. Utilizing magnetoelectric Cr₂O₃ based heterostructures, electric control of ferromagnetic exchange bias has been achieved up to the bulk Néel temperature. Recently spin Hall magnetoresistance has been shown to be present in Cr₂O₃ Pt bilayers, potentially providing a readout mechanism of the antiferromagnetic order parameter which avoids the need for a ferromagnet. In this study we combine this novel readout mechanism with the technique of boron-doping the Cr₂O₃ which increases the Néel temperature up to 400 K. Using a process of magnetoelectric annealing, spin Hall magnetoresistance in boron-doped Cr₂O₃ films is examined for the first time, with significant implications for future spintronic devices.
  

Interfacial electronic states and correlated magnetic order across BiFeO3/La0.7Sr0.3MnO3 complex oxide heterointerfaces

Atomic-scale electronic states across termination engineered BiFeO₃/La_0.7Sr_0.3MnO₃ (BFO/LSMO) complex oxide heterointerfaces have been revealed using cross-sectional scanning tunneling microscopy and spectroscopy. The evolution of the Mn, O, and Fe states across MnO₂-BiO and La_0.7Sr_0.3O-FeO₂ interfaces are demonstrated. Analysis of spectroscopy provides the electronic results of interfacial band gaps and interface bound charges at two kinds of terminations in the BFO/LSMO system. The result exhibits the MnO₂-BiO terminated interface with wide band gaps and dominant Mn⁴⁺ oxidation state, which can refer to an antiferromagnetic state at the interface. In contrast, the metallic behavior of the La_0.7Sr_0.3O-FeO₂ terminated interface can be correlated to the ferromagnetism at the interface.
Reference:
B.-C. Huang, P. Yu, Y. H. Chu, C.-S. Chang, R. Ramesh, R. E. Dunin-Borkowski, P. Ebert, and Y.-P. Chiu, ACS Nano 12, 1089 (2018).   

Unusual Magnetotransport Properties of Epitaxial NiCo2O4 Thin Films at Ultralow Temperatures

The inverse spinel NiCo₂O₄ (NCO) is a room temperature ferrimagnetic material that is promising for developing nanoscale energy and spintronic applications. In this work, we report the unusual thickness-dependent magnetotransport properties of high quality epitaxial NCO thin films. We have characterized the longitudinal magnetoresistance (MR) and Hall effect in NCO films with thickness varying from 5 unit cell (~4.1 nm) to 30 uc (24.6 nm) at ultralow temperatures down to 100 mK. The 10 and 30 uc films exhibit an intrinsic linear negative magnetoresistance, which persists up to 17 T without showing any saturation or upturn, while the 5 uc sample exhibits a positive MR that is possibly due to 2D weak antilocalization (WAL). We also observe robust anomalous Hall effect in the 10 and 30 uc samples, while the switching hystereses are in opposite directions. We discuss the effects of the Berry curvature, disorder scattering, and dimensionality crossover on the observed unusual thickness effect.   

Simulation of Hall sign change in ferromagnet SrRuO3

We employed a modified two-band model to theoretically investigate the mechanism of Hall sign change in SrRuO3, in which an mpurity band couple with conduction and valence bands. Apparently, our theory can interpret the experimental results, which implies the proposed impurity band arising from Ru vacancies is an important factor in Hall sign change. We found that the electron density in the impurity band is getting increase as the impurity band energy approaches the chemical potential resulting in a decrease of Curie temperature of SrRuO3. Besides, the Hall sign change occurs at the Curie temperature only when the band oupling is weak. Further more, the increase of the Ru vacancies makes the impurity band become solid but suppress the Curie temperature. Moreover, from the theoretical prediction a more relatively intrinsic p-type SrRuO3 has higher Curie temperature and opposite for intrinsic n-type SrRuO3.