Variable temperature study of the crystal and magnetic structures of the giant magnetoresistant materialsLMnAsO(L=La, Nd)

Abstract

A variable temperature neutron and synchrotron diffraction study has been performed on the giant magnetoresistant oxypnictides LMnAsO (L = La, Nd). The low-temperature magnetic structures have been studied, and results show a spin reorientation of the Mn 2+ spins below T N (Nd) for NdMnAsO. The Mn 2+ spins rotate from alignment along c to alignment into the basal plane, and the Mn 2+ and Nd 3+ moments refine to 3.54(4) μ B and 1.93(4) μ B , respectively, at 2 K. In contrast, there is no change in magnetic structure with temperature for LaMnAsO. There is no evidence of a structural transition down to 2 K; however, discontinuities in the cell volume and L-O and Mn-As bond lengths are detected at ∼150 K for both materials. This temperature coincides with the electronic transition previously reported and suggests a coupling between electronic and lattice degrees of freedom.

Large resistivity change and phase transition in the antiferromagnetic semiconductors LiMnAs and LaOMnAs

“…In contrast to these results, via neutron diffraction measurements performed at 290 K, the refined magnetic moment of Mn was 2.43μ B , which is more than one μ B lower compared to our theoretical result. In a later experiment, 21 however, it was shown that the moments at 2 K are 3.34 μ B , which is in good agreement with our calculations. In the case of LiMnAs, our neutron-diffraction data also show a similar trend as a function of temperature, where at 300 K the Mn moments are reduced from 3.72 to 2.62 μ B (see Table I).…”

“…Instead of canting, in agreement with Ref. 21 we explain the small remanent total moment of these compounds with the formation of MnAs as an impurity phase. This is because during synthesis, independent of the method used, tiny amounts of MnAs ( 1%) can easily be formed, which cannot be detected by techniques such as XRD or energy-dispersive x-ray spectroscopy (EDX).…”

“…By combining ab initio density functional theory (DFT) calculations with our experiments, we discuss the electronic structure and magnetic exchange coupling of the Mn atoms, further commenting on the non-Curie-Weiss-like behavior experimentally reported for related compounds. 20 We will also comment on the issue arising from a previous work 9 regarding the weak ferromagnetic behavior observed in LaOMnAs, which was first attributed to a small spin canting 9 and in a later paper 21 explained via the formation of small amounts ( 1%) of MnAs as impurities during synthesis.…”

Synthesis and characterization of Ca-doped LaMnAsO

“…Various studies of the parent LnMnP nO have shown that the MnP n planes are universally antiferromagnetic [3][4][5][6][7][8][9][10] with in-plane checkerboard magnetic structure similar to the in-plane magnetic ordering in the various Mn-122 compounds. For compounds with moment-bearing rare-earth ions (Ln), a second low temperature magnetic ordering of the rare-earth element (i.e., Ce, Nd, and Pr) accompanied by a Mn-spin reorientation transition have been observed [4][5][6][7][8][9]. Other Mn-1111 compounds with variations on the Ln or P n sites have also been shown to order with the same in-plane * yliu@ameslab.gov AFM structure, albeit with uniform or alternate stacking of the AFM planes [11,12].…”

Common effect of chemical and external pressures on the magnetic properties ofRCoPO (R=La,Pr,Nd,Sm).

“…Depending on the actual choice of the pnictide element X in combination with a transition metal M , different electronic properties can be achieved 1 ranging from high-T c bulk superconductivity (with T c values up to 55 K for M = Fe and X = As) [2][3][4][5][6] to ferromagnetism with rare Kondo features (typically for M = Ru and X = P) 7,8 or with large -even colossal -magnetoresistance (M = Co, Mn and X = As, P). [9][10][11][12][13][14][15] However, the complex multi-orbital nature of the fermiology, 16,17 together with the open issue about the importance of electronic correlations, 18,19 makes the overall understanding of these materials still not complete and currently highly debated.…”