J. E. Millburn

Colossal magnetoresistance in ( x = 0.0, 0.1)

Magnetization and magnetotransport measurements have been used to study the composition dependence of the electronic properties of the Ruddlesden - Popper phases and . Although their behaviour differs in detail, both compounds show a colossal magnetoresistance (CMR) effect (> 10000% in 14 T) in the temperature range . However, neither material shows a transition to a ferromagnetic state above 4.2 K, and both materials have higher resistivities ( for ) than the metallic oxides previously found to show CMR. In view of the low conductivity and the absence of ferromagnetism, the CMR of these phases is not readily explained by a double-exchange mechanism.

Chemistry of naturally layered manganites (invited)

Experiments on three double-layer (n=2) Ruddlesden–Popper (RP) systems are reported. Doping Sr1.8La1.2Mn2O7 (Tc=126 K) with Nd to form Sr1.8La1.2−xNdxMn2O7 leads to a reduction in Curie temperature for low doping levels (x=0.2), and to behavior reminiscent of Sr1.8Nd1.2Mn2O7 for x⩾0.7. This suggests that it may be possible to control the temperature of maximum magnetoresistance chemically in these phases. The application of pressure (0<P/GPa⩽1.8) is shown to modify the magnetotransport properties of Sr2NdMn2O7 to resemble those of Sr1.9Nd1.1Mn2O7. The changes can be explained by considering the relative strength of ferromagnetic and antiferromagnetic interactions within the material. Finally, the need for careful phase analysis of n=2 RP materials is demonstrated by the misleading magnetization data recorded for a sample of Sr1.8Sm1.2Mn2O7 containing ∼2.8% of an n=∞ perovskite phase.

Control of electronic properties by lanthanide size and manganese oxidation state in the MnIII/MnIV Ruddlesden–Popper phases Ln2−xSr1+xMn2O7

The magnetic behaviour of then=2 Ruddlesden–Popper phases Sr 2 LnMn 2 O 7 is very sensitive to the Ln 3+ lanthanide cation. In samples with larger, more basic lanthanide cations (Ln=Nd, Pr) antiferromagnetic phases with ordering temperatures in the region of 130 K co-exist with phases showing a magnetic response suggestive of superparamagnetism or the development of small ferromagnetic clusters at high temperature. The magnetic transition temperature drops to 20 K in samples containing smaller, acidic cations (Ln=Gd–Er, Y). In the latter group of compounds, the transition is from a Curie–Weiss paramagnet to a spin-glass; there is no evidence for long-range magnetic order. This change in behaviour can be explained by considering the variation in the relative strength of superexchange and double exchange interactions as a function of the lanthanide cation. The influence of manganese oxidation state on magnetic response is investigated in the Sr 2-x Ln 1+x Mn 2 O 7 composition range (0.0≤x≤0.7) for Ln=Nd, Tb.

COUPLED METAL-INSULATOR AND MAGNETIC TRANSITIONS IN LNSR2MN2O7 (LN = LA, TB)

We report magnetisation, conductivity and structural data on the Ruddlesden-Popper MnIII/MnIV oxides LnSr2Mn2O7(Ln = La, Tb); while LnSr2Mn2O7 undergoes an insulator-metal transition and an increase in spontaneous magnetisation at 132 K, the isostructural terbium compound is semiconducting and paramagnetic throughout the range 4.2 < T/K < 300.

Extending the n = 2 Ruddlesden–Popper solid solution La2–2xSr1 + 2xMn2O7 beyond x= 0.5: synthesis of Mn4+–rich compounds

Reported herein are the synthesis and room temperature crystal structures of the heretofore unknown, metastable manganites La2–2xSr1 + 2xMn2O7 + δ (0.5 ⩽ x ⩽ 0.9) via high temperature (T = 1650 °C) quenching followed by low temperature (T = 400 °C) annealing to fill oxygen vacancies; this approach enables access to the electronic, magnetic, and structural properties of previously unexplored compositions in this important CMR system.