Magnetic properties of mixed-valence iron phosphate glasses

Abstract

Magnetic properties of mixed-valence iron phosphate glasses, where there coexist Fe 2+ and Fe 3+ ions, have been investigated. The molar fraction of Fe 3+ with respect to the total iron ion, ͓Fe 3+ ͔ / ͓Fe total ͔, can be controlled by melting the glass at varied temperatures. Experiments of magnetic aging and memory effects as well as dynamic and static scaling analyses of relaxation time and nonlinear magnetic susceptibility have been performed to get insight into the nature of low-temperature magnetic phase of the glass system. The experimental results reveal that the iron phosphate glasses undergo paramagnet-spin-glass transitions at low temperatures. Temperature dependence of magnetic specific heat suggests that as the temperature is lowered, the magnetic moments start to be frozen at a temperature significantly higher than the spin-glass transition temperature accompanied by a deviation in magnetic susceptibility from Curie-Weiss law. The ratio of the absolute value of Weiss temperature to spin-glass transition temperature increases as the ratio ͓Fe 3+ ͔ / ͓Fe total ͔ becomes larger. This behavior is explainable in terms of the difference in single-ion anisotropy between Fe 3+ and Fe 2+ ions.

Magnetic properties of oxide glasses containing iron and rare-earth ions

“…A close look at the χ (T) curve reveals that χ FC (T) deviates from χ ZFC (T) below 4.5 K, which is higher than the peak temperature, i.e., 3.6 K, and increases monotonously at lower temperatures. This differs slightly from the behavior of canonical SGs such as CuMn 29 and oxide glasses such as iron tellurite and phosphate glasses, 11,17 where χ FC (T) deviates from χ ZFC (T) and becomes almost constant just below the peak temperature. This implies that some of the magnetic moments of Fe ions form magnetic clusters in the present glass and the larger clusters start to freeze at 4.5 K with lowering temperature.…”

“…The value of zν is comparable with those reported for conventional SGs, including amorphous oxide SGs. 11,17,30,33 The value of τ 0 is much larger than the typical values for atomic SGs, where τ 0 ∼ 10 −13 s, i.e., the spin-flip time of individual magnetic moments belonging to atoms or ions, but similar to those reported for strongly interacting nanoparticle systems and superspin glasses, for which τ 0 ∼ 10 −6 s. 34,35 This is consistent with the bifurcation of χ ZFC (T ) and χ FC (T ) above the peak temperature of χ ZFC (T ) discussed in Sec. III B, indicating that magnetic clusters consist of some of the Fe ions in the present glass.…”

“…1 Instead, short-range antiferromagnetic (AFM) superexchange interactions via 2p states of oxide ions are predominant in the magnetic oxide glasses, [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] with some exceptions such as Eu 2+ -containing glasses, where ferromagnetic (FM) interactions prevail. [19][20][21][22][23] In a system where magnetic moments are located at randomly distributed cations, the short-range AFM interactions inevitably bring about magnetic frustrations of geometrical origin.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Those 3d TM-containing glasses have negative Weiss temperatures, the amplitude of which is tens or hundreds of degrees Kelvin, and exhibit a paramagnetic (PM) to SG transition at several or tens of degrees Kelvin. On the other hand, most of the oxide glasses possessing 4f rare-earth (RE) ions seldom show any magnetic transition down to 2 K, 24,25 which can be easily accessed in magnetic measurements.…”

Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites

“…Normally, in amorphous magnetic oxides, random distribution of magnetic ions affects the antiferromagnetic (AFM) superexchange interactions, which usually dominate in these oxides and causes frustration in the arrangement of the spins891011. This induces a spin glass transition and a drastic suppression of the magnetic ordering temperature1213.…”