Exchange Coupling in Synthetic Anion‐Engineered Chromia Heterostructures

Abstract

Control of magnetic states by external factors has garnered a mainstream status in spintronic research for designing low power consumption and fast‐response information storage and processing devices. Previously, magnetic‐cation substitution was the conventional approach to induce ferromagnetism in an intrinsic antiferromagnet. Theoretically, anion doping is proposed to be another means to change magnetic ground states. Here, the authors demonstrate the synthesis of high‐quality single‐phase chromium oxynitride thin films using in‐situ nitrogen doping. Unlike antiferromagnetic monoanionic chromium oxide and nitride phases, chromium oxynitride exhibits a robust ferromagnetic and insulating state, as demonstrated by the combination of multiple magnetization probes and theoretical calculations. With increasing the nitrogen content, the crystal structure of chromium oxynitride transits from trigonal (Rtrue3¯c) to tetragonal (4 mm) phase and its saturation magnetization reduces significantly. Furthermore, they achieve a large and controllable exchange bias field in the chromia heterostructures by synthetic anion engineering. This work reflects the anion engineering in functional oxides towards potential applications in giant magnetoresistance and tunnelling junctions of modern magnetic sensors and read heads.

Growth of 2D Cr2O3–CrN Mosaic Heterostructures with Tunable Room‐Temperature Ferromagnetism

“…In addition, the temperature-dependent M-H loops (Figure 3c; Figure S19, Supporting Information) of Cr 2 O 3 -CrN MH (N:Cr = 0.8) and magnetism-temperature (M-T) curves (Figure S14b,c, Supporting Information) of different MHs show that the ferromagnetism of all the MHs persists up to 400 K (T C > 400 K), which is one of the highest temperatures among previously reported heterostructures (Figure 3d; Table S2, Supporting Information). [9,15,16,22,[33][34][35] We also used MFM to probe the magnetization of individual Cr 2 O 3 -CrN MH flakes. In MFM, to ensure the phase shift stemming from the interaction between the out-of-plane component of the magnetic field originating from the sample and magnetized tip, the tip is lifted to a constant height above the surface [9,15,16,22,[33][34][35] to eliminate the influence of surface morphology as illustrated in Figure 4a,b.…”

“…[9,15,16,22,[33][34][35] We also used MFM to probe the magnetization of individual Cr 2 O 3 -CrN MH flakes. In MFM, to ensure the phase shift stemming from the interaction between the out-of-plane component of the magnetic field originating from the sample and magnetized tip, the tip is lifted to a constant height above the surface [9,15,16,22,[33][34][35] to eliminate the influence of surface morphology as illustrated in Figure 4a,b. [36] For nonmagnetic samples, no phase shift will occur, while ferromagnetic samples will generate a negative (positive) phase shift induced by the attractive (repulsive) force between the tip and the samples.…”

“…[16,21] Considering the significance of structure and magnetism modulation for specific applications, a facile and controllable approach to prepare 2D ferromagnetic heterostructures is highly desired. [4,[22][23][24] Herein, we report a one-pot chemical vapor deposition (CVD) method to grow air-stable 2D chromium oxide-nitride mosaic heterostructures (Cr 2 O 3 -CrN MHs) with controllable roomtemperature ferromagnetism. We found that the CrN domains were embedded in a 2D Cr 2 O 3 matrix in the form of the Cr 2 O 3 -CrN MH using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy.…”

“…a) SQUID characterization of Cr 2 O 3 , CrN, and Cr 2 O 3 -CrN MH at 300 K. b) N:Cr atomic ratio dependent H c and M s of the Cr 2 O 3 -CrN MHs with thicknesses of 20 nm on 3 mm × 3 mm sapphire substrates at 300 K. c) SQUID characterization of Cr 2 O 3 -CrN MH (N:Cr = 0.8) at different temperatures. d) Comparison of T C in previously reported heterostructures and the Cr 2 O 3 -CrN MHs [9,15,16,22,[33][34][35].…”

Unidirectional Magnetic Anisotropy in Molybdenum Dioxide–Hematite Mixed-Oxide Nanostructures

“…The exchange interactions between a ferromagnet (FM) and an antiferromagnet (AFM) across an interface, responsible for the unidirectional anisotropy, have attracted much attention because they give rise to both an exchange bias field as well as an enhanced coercive field, if compared with the case of a free (uncoupled) FM nanophase [ 10 , 11 , 12 , 13 , 14 ]. The so-called exchange bias systems have been exploited in spin valve devices such as magnetic read heads [ 15 , 16 ], nonvolatile memories [ 17 , 18 ], and various sensors [ 19 , 20 ]. Nanoparticulate systems involving unidirectional anisotropy are also intensively studied due to the possibility to increase the magnetic stabilization (magnetic blocked regime of fine nanoparticles) or the magnetic hyperthermia effects [ 21 , 22 ].…”