Muon spin relaxation study of the magnetism in unilluminated Prussian Blue analogue photomagnets

Abstract

We present longitudinal field muon spin relaxation (µSR) measurements in the unilluminated state of the photo-sensitive molecular magnetic Co-Fe Prussian blue analogues M1−2xCo1+x[Fe(CN)6]·zH2O, where M=K and Rb with x = 0.4 and ≃ 0.17, respectively. These results are compared to those obtained in the x = 0.5 stoichiometric limit, Co1.5[Fe(CN)6]·6 H2O, which is not photo-sensitive. We find evidence for correlation between the range of magnetic ordering and the value of x in the unilluminated state which can be explained using a site percolation model. A. IntroductionPrussian Blue (PB) (Fe 4 [Fe(CN) 6 ] 3 ) is a long-known dye and prototypical transition metal co-ordination compound that exhibits ferro-magnetism [1,2,3] driven by superexchange coupling between iron spins. It is an important case of exchange coupling mediated through the CN − bridge [4]. Much of the recent interest in magnetic compounds related to PB (including a few that have been studied with µSR [5,6,7]) is motivated by potential novel behavior and related applications in molecular magnetism [8,9], including magnetism that is sensitive to exposure to light, i.e. photomagnetism.The compounds studied in this paper are the molecule based Co-Fe PB analogues (Co-Fe PBAs)1: (Color online) Crystal structure of M1−2xCo1+x[Fe(CN)6]·zH2O: (a) x = 0.5; (b) x = 0.5, with an Fe(CN)6 vacancy shown in gray coordinated by bound water molecules. Large, medium and small circles denote Fe, Co, and CN, respectively. The alkali ions which maintain charge neutrality occupy cubic interstitial sites (not shown).1 This chemical formula is a commonly used approximation though [8,9,12]. These compounds have the sodium chloride structure, with Co and Fe ions located on the vertices of a cubic lattice, each octahedrally coordinated by six cyano moieties (Fig. 1). The Co and Fe ions are connected via cyanide bridges with interstitial alkali metal ions and water molecules [9,13] (Fig. 1(b)). Co-Fe PBAs are in general non-stoichiometric and significant structural disorder (vacancies in the Fe(CN) 6 sites) is present. Depending on the stoichiometry and synthesis route, the materials are paramagnets or exhibit magnetic ordering at temperatures below ∼ 25 K, due to small superexchange coupling J between Fe III and Co II moments.Illumination of Co-Fe PBAs with broadband visible light in the range of ∼ 550 − 750 nm can cause dramatic changes in the magnetic properties, including an increase in magnetization and ordering temperature. The proposed origin of the photomagnetic effect is a light-induced charge transfer from the state: Fe II (t 6 2g , S = 0)-CNCo III (t 6 2g , S = 0) to the meta-stable self-trapped state: Fe III (t 5 2g , S = 1/2)-CN-Co II (t 5 2g e 2 g , S = 3/2) [9,14], effectively increasing the concentration of magnetic moments. The non-stoichiometry is believed essential for the photoinduced magnetization [14]. However, in spite of considerable experimental and theoretical effort, the microscopic mechanism of the photomagnetic effect and the nature of magnetic orde...

Size dependence of the photoinduced magnetism and long-range ordering in Prussian blue analogue nanoparticles of rubidium cobalt hexacyanoferrate

“…When irradiated, the low-spin regions, near the primordial high-spin clusters, are photoinduced to the high-spin magnetic state, resulting in a growth of the magnetic domain. This scenario is supported by our description of the anisotropic photoinduced magnetism observed in thin films [13,18] and by the local probe investigations of others [42,43]. All samples show a photoinduced increase in their magnetic signals, and the strength of the change is correlated with the size of the particles.…”

Optically induced dynamics of muonium centers in Si studied via their precession signatures

“…However, the rapid reactivity of hydrogen means that many of the techniques that are used to investigate hydrogen in semiconductors are not able to probe isolated H. Studies of muonium ͑Mu 0 = + e − ͒ using the muon spin rotation/relaxation/resonance ͑SR͒ techniques are now widely recognized to be the main experimental source of information on isolated H in many semiconductors. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Recall that the muon ͑ + ͒ is a radioactive particle with a lifetime of Ϸ2.2 s. It can be considered a pseudoisotope of hydrogen with Ϸ 1 9 th the mass of the proton, but is still much heavier than the electron ͑Ϸ200ϫ ͒. Hence, the electronic structures of Mu 0 and H 0 are very similar.…”

Probing the magnetic ground state of the molecular dysprosium triangle with muon spin relaxation

“…3(b)). These temperature dependencies are commonly seen in materials undergoing magnetic ordering at T c = T * [23], In such cases the increase of λ above T c is attributed to a critical slowing down of the fluctuations and ∆ is proportional to the order parameter (local magnetization) [23,24]. This behavior is quite unusual, and exhibits a sharp transition from a paramagnetic to an ordered/static regime, or the sudden appearance of static magnetic fields in the system.…”

“…The parameters λ and ∆ obtained from the fits are shown in Fig. 3 the order parameter (local magnetization) [23,24]. This behavior is quite unusual, and exhibits a sharp transition from a paramagnetic to an ordered/static regime, or the sudden appearance of static magnetic fields in the system.…”