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Scientists Make a Breakthrough in “Dead Layer” of AFM

Since the early 1930s when Louis Néel found the anomaly that some materials containing magnetic elements and showing zero remanence at all temperatures did not follow the paramagnetic Curie law, the constant low-temperature susceptibility behind this phenomenon which is called antiferromagnet(AFM)has drew tremendous attentions and found numerous applications, specially in the area of improved computer memory units.

Synthetic antiferromagnets (S-AFMs) are antiferromagnets built with FM layers periodically interleaved with metallic or insulating spacers, where the magnetization of adjacent FM layers alternates owing to the antiferromagnetic (AF) interlayer exchange coupling (IEC).Over the past few decades, the development of S-AFMs composed of transition metals and alloys has gained great achievements, however, the layer-resolved magnetic switching in S-AFMs with correlated oxide multilayers is rarely observed. Meanwhile, S-AFMs with correlated oxides has been challenging partially owing to the markedly degraded ferromagnetism of the magnetic layer at nanoscale thicknesses. And this “dead layer” effect can be one of the major obstacles to the development of all-oxide S-AFMs. 

Prof. WU Wenbin’s group recently has made a breakthrough to solve the “dead layer” effect problem. They constructed S-AFMs using LCMO as the magnetic layers, CaRu1/2Ti1/2O(CRTO) as the spacer layers, and (001)-oriented NdGaO3 (NGO) as the substrate. The layer-resolved magnetic switching leads to sharp steplike hysteresis loops with magnetization plateaus depending on the repetition number of the stacking bilayers. The magnetization configurations can be switched at moderate fields of hundreds of oersted.

Fig. 1. AF-IEC in LCMO/ CRTO multilayers. (A) Temperature (T) dependence of normalized magnetization (M) measured in LCMO/CRTO and LCMO/CRO SLs, [2.8/1.2]10. The paramagnetic background from the NGO(001) substrates is included for comparison. During these measurements, a cooling field of 250 Oe is applied along the in-plane easy-axis [010]. The inset shows the corresponding hysteresis loops measured at 50 K from each SL, with the paramagnetic background from the NGO substrate subtracted (figs. S3 and S4).(B) Hysteresis loops measured at 100 K from LCMO/CRTO SLs, [2.8/1.2]N, with N = 2, 3, and 4, respectively. For clarity, the steplike loop measured with the magnetic field (H) sweeping from positive (negative) to negative (positive) is denoted with solid (open) circles (fig. S6). (C) Two possible magnetic configurations of the intermediate state for the N = 4 SL, which has the magnetization plateau at 1/2MS. (D) PNR measured at 10 K from LCMO/CRTO SL, [2.8/1.2]10, with the field of 30 Oe (top) or 5000 Oe (bottom) applied along the in-plane easy-axis [010].

The main magnetic properties of the AF-IEC in LCMO/CRTO multilayers are showed in Fig.1. It displays in Fig. 1A a TC of 182 K, greatly enhanced compared with the plain LCMO film but lower than the corresponding comparison LCMO/CRO superlattice SL (TC ~ 265 K). Note that the LCMO/CRTO SL also shows a decrease of moments at 140 K, the authors believe such a drop compared to the LCMO/CRO SL in magnetization is recognized as a signature of the AF-IEC between FM LCMO layers across the CRTO spacers. To directly demonstrate the AF-IEC in these SLs, they also performed polarized neutron reflectivity(PNR) measurements on the LCMO/CRTO SL, [2.8/1.2]10, as shown in Fig. 1D. At a low field of 30 Oe, both the spin-up (R+) and spin-down (R-) polarized neutrons show reflections at QZ = 0.78 nm-1, leaving a periodicity of twice the bilayer thickness (8 nm). Such a magnetic periodicity without doubt means the AF-IEC in the SL. On the other hand, at a high field of 5000 Oe, these reflections are fully suppressed, and new ones appear at exactly the structural Bragg peak at Q= 1.57 nm-1, indicating the transition from antiparallel to parallel magnetic alignments of all the LCMO layers. 

In summary, the authors has demonstrated AF-IEC with layer-resolved magnetic switching for LCMO/CRTO SLs. And they also found that the AF-IEC is tunable via the Ti-doping level of the CRTO spacer and the growth orientations. The system they developed in the paper can also be extended for further possible applications, such as biotechnology applications and spintronic applications. 

This work was supported by the National Natural Science Foundation of China (gra, the National Basic Research Program of China, and the Hefei Science Center of the Chinese Academy of Sciences.

(CHEN Xiaofeng, USTC News Center, Hefei National Laboratory for Physical Sciences at the Microscale)


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