Manipulating magnets with light: photoinduced magnetism of cobalt± iron Prussian blue analogs Dusan A. Pejakovic a, * , Jamie L. Manson b,1 , Joel S. Miller b , Arthur J. Epstein a,c a Department of Physics, The Ohio State University, Columbus, OH 43210-1106, USA b Department of Chemistry, University of Utah, Salt Lake City, UT 84112-0850, USA c Department of Chemistry, The Ohio State University, Columbus, OH 43210-1106, USA Received 28 August 2000; accepted 8 September 2000 Abstract An overview of the magnetic properties and photoinduced magnetism of cobalt±iron Prussian blue analogs is given. The pre- sented results for K 12x Co 1x FeCN 6 y H 2 O 0:2 6 x 6 0:4; y 5 indicate that the materials are in a cluster glass state. Upon illumination with visible light, the materials exhibit substantial changes in magnetization, linear and nonlinear ac susceptibilities, characteristic temperatures and magnetic hysteresis. The results are consistent with our model of photoinduced spin-doping of a cluster glass. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 75.50.Lk; 75.50.Xx; 75.90.+w; 78.90.+t Keywords: Photoinduced magnetization; Cluster glass; ac susceptibility; Nonlinear susceptibility; Prussian blue 1. Introduction The design of `smart' magnetic materials, with mag- netic properties tunable by external stimuli, is of great interest for possible applications. Particularly attractive is the possibility of optical control of magnetic order. The recent discoveries of photoinduced magnetization (PIM) in diluted magnetic semiconductors [1] and mol- ecule-based magnets [2] gave a new impetus to this area of research. Among molecule-based magnets, striking eects of light irradiation on the magnetic state were reported for cobalt±iron Prussian blue analogs M x Co y FeCN 6 zH 2 O (M alkali metal) [3±5]. The materials have a face-centered cubic lattice, in which Co and Fe ions are connected via cyanide bridges, and al- kali metal ions and water molecules are interstitial (Fig. 1(a)) [3,4]. The compounds are in general nonstoichio- metric, and signi®cant structural disorder due to the vacant FeCN 6 sites is present. Depending on the sto- ichiometry and synthesis route, the materials are para- magnets or exhibit magnetic ordering at temperatures <25 K. The negative Weiss temperature and expected predominance of the kinetic superexchange in the Co II ±Fe III system suggest that the predominant inter- action between the magnetic ions in the system is an- tiferromagnetic nearest-neighbor superexchange [4]. Upon illumination with light of wavelengths in the re- gion 550±750 nm, the materials exhibit dramatic changes in magnetization, linear and nonlinear dynamic susceptibilities, and magnetic hysteresis (Fig. 1(b)) [3±8]. The photoexcited state persists for several days at low temperatures. The photoinduced eects are completely reversible, and materials are returned to the ground state (state before illumination) by heating above 150 K. Partial inversion of the eect can also be obtained by blue or near-infrared light illumination (Fig. 1(c)). The proposed origin of the eect is the light-induced charge transfer from the state Fe II t 6 2g ; S 0±CN±Co III t 6 2g ; S 0 to form the state Fe III t 5 2g ; S 1=2±CN± Co II t 5 2g e 2 g ; S 3=2 [3,4]. Thus, the observed magnetic transition is caused by a change in the ions' electronic states, which leads to an increase in the spin concen- tration (n s ). This mechanism is dierent from the one leading to the PIM in diluted magnetic semiconductors, in which photogenerated charge carriers eect the Current Applied Physics 1 (2001) 15±20 www.elsevier.com/locate/cap * Corresponding author. E-mail addresses: pejakovi@mps.ohio-state.edu (D.A. Pejakovic), epstein.2@osu.edu (A.J. Epstein). 1 Present address: Chemistry and Materials Science Divisions, Argonne National Laboratory, Argonne, IL 60439. 1567-1739/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 7 - 1 7 3 9 ( 0 0 ) 0 0 0 0 4 - 3