**Acknowledgement**

The authors thank Prof. John B. Wiley, Prof. Yoshitami Ajiro, Prof. Kazuyoshi Yoshimura, Dr. Atsushi Kitada, Prof. Kazuma Hirota, Dr. Masakazu Nishi, Prof. Keisuke Totsuka, Prof. Masayuki Hagiwara, Prof. Yasuo Narumi, Prof. Koichi Kindo, Prof. Hiroi Zenji, Dr. Makoto Yoshida, Prof. Masashi Takigawa, Dr. Masaki Ichihara, Dr. Cédric Tassel, Prof. Yasutomo J. Uemura, Prof. Graeme M. Luke, Dr. Tatsuo Goko, Prof. Seung-Hun Lee, Prof. Bella Lake, Prof. Mike H. Whangbo, Prof. Werner Paulus, Dr. Olivier J. Hernandez, Dr. Clemens Ritter, Dr. Kunihiro Nakano, Prof. Yutaka Ueda, Dr. Sk Mohammad Yusuf, Prof. John P. Attfield, Dr. Tsutomu Momoi, Prof. Nic Shannon, and Prof. Mikio Takano for their fruitful discussion and collaborations. This work was supported by the Japan Society for the Promotion of Science (JSPS) through its ''Funding Program for WorldLeading Innovative R&D on Science and Technology (FIRST) Program", Grants-in-Aid for Science Research in the Priority Areas ''Novel States of Matter Induced by Frustration" (No. 19052004) and Grant-in-Aid for Scientific Research (A) (No. 22245009) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and CREST.

#### **4. References**

28 Ion Exchange Technologies

**3. Conclusion** 

etc.) might be prepared.

**Author details** 

Yoshihiro Tsujimoto

Hiroshi Kageyama

**Acknowledgement** 

*National Institute for Materials Science, Japan* 

*Graduate School of Engineering, Kyoto University, Japan* 

We have demonstrated that the ion-exchange reaction using the DJ phase that involves the simultaneous co-exchange of metal cations and halide anions is effective approach to design a new class of two-dimensional quantum spin antiferromagnets, with tuned in-plane magnetic interactions and a variety of quantum phases. A series of (Cu*X*)*An*–1*Bn*O3*n*+1 with the *S* = 1/2 square lattice exhibits novel quantum spin phenomena such as spin-liquid state, quantized magnetization plateau, quantum phase separation, and field-induced BEC state. They are caused by the strong competition between FM and AFM interactions. Table 2 summarizes the structural and magnetic properties in (Cu*X*)*An*–1*Bn*O3*n*+1. Note that most studies on spin frustrated magnetism has been focused so far on the systems composed of only AFM interactions. It is only recently that attentions have been paid to frustrated systems including FM interactions from both experimentally and theoretically points of view. We hope that the (Cu*X*)*An*–1*Bn*O3*n*+1 system will continue to contribute to the development of new physics of the frustrated spin systems in the future. The ion-exchange reaction can be extended to other compounds with transition metals with different spin quantum number, for example, (*M*Cl)LaNb2O7 (*M* = Mn2+, Fe2+, etc.) [6265] and (NiCl)Sr2Ta3O10 [66]. Reflecting the classical nature of magnetic moment, (*M*Cl)LaNb2O7 (*M* = Co, Fe, Mn) exhibit antiferromagnetic order, but several anomalous behaviors have been still observed due to geometrical frustration. The nickelic chloride with *S* = 1 undergoes the successive magnetic phase transitions with an intermediate phase characterized by a partial magnetic order. The advantage of the ion-exchange reactions is that, once one finds an appropriate precursor or hosts, any desired spin lattices can be constructed in principle. Using this topochemical strategy, various magnetic lattices (triangle, kagomé, pyrochlore,

The authors thank Prof. John B. Wiley, Prof. Yoshitami Ajiro, Prof. Kazuyoshi Yoshimura, Dr. Atsushi Kitada, Prof. Kazuma Hirota, Dr. Masakazu Nishi, Prof. Keisuke Totsuka, Prof. Masayuki Hagiwara, Prof. Yasuo Narumi, Prof. Koichi Kindo, Prof. Hiroi Zenji, Dr. Makoto Yoshida, Prof. Masashi Takigawa, Dr. Masaki Ichihara, Dr. Cédric Tassel, Prof. Yasutomo J. Uemura, Prof. Graeme M. Luke, Dr. Tatsuo Goko, Prof. Seung-Hun Lee, Prof. Bella Lake, Prof. Mike H. Whangbo, Prof. Werner Paulus, Dr. Olivier J. Hernandez, Dr. Clemens Ritter, Dr. Kunihiro Nakano, Prof. Yutaka Ueda, Dr. Sk Mohammad Yusuf, Prof. John P. Attfield, Dr. Tsutomu Momoi, Prof. Nic Shannon, and


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