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A high pressure neutron study of colossal magnetoresistant NdMnAsO0.95F0.05
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Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 27 (2015) 116001 (7pp)
doi:10.1088/0953-8984/27/11/116001
A high pressure neutron study of colossal magnetoresistant NdMnAsO0.95F0.05 E J Wildman1 , M G Tucker2 and A C Mclaughlin1 1 The Chemistry Department, University of Aberdeen, Meston Walk, Aberdeen, AB24 3UE, Scotland, UK 2 ISIS Facility, Rutherford Appleton Laboratory, Harwell, Didcot, OX11 0DE, UK
E-mail: [emailprotected] Received 10 October 2014, revised 16 December 2014 Accepted for publication 20 January 2015 Published 27 February 2015 Abstract
A high pressure neutron diffraction study of the oxypnictide NdMnAsO0.95 F0.05 has been performed at temperatures of 290–383 K and pressures up to 8.59 GPa. The results demonstrate that the antiferromagnetic order of the Mn spins is robust to pressures of up to 8.59 GPa. TN is enhanced from 360 to 383 K upon applying an external pressure of 4.97 GPa, a rate of 4.63 K GPa−1 . NdMnAsO0.95 F0.05 is shown to violate Bloch’s rule which would suggest that NdMnAsO0.95 F0.05 is on the verge of a localized to itinerant transition. There is no evidence of a structural transition but applied pressure tends to result in more regular As–Mn–As and Nd–O–Nd tetrahedra. The unit cell is significantly more compressible along the c-axis than the a-axis, as the interlayer coupling is weaker than the intrinsic bonds contained within NdO and MnAs slabs. Keywords: CMR, high pressure, neutron diffraction (Some figures may appear in colour only in the online journal)
ers, allowing the electronic properties to be easily manipulated upon applying external pressure. For LnFeAsO1−y (Ln = La, Nd, Pr, Sm) the value of Tc increases as the bond angles within the FeAs4 tetrahedron tend towards optimized values, and the maximum Tc is achieved when a regular tetrahedron is adopted with angles of 109.47◦ [6]. It is also possible to synthesise pnictides and oxypnictides so that other transition metals, such as Mn, Co or Ni replace Fe [7–9]. Manganese pnictides have received particular attention due to their interesting magnetic and electronic properties. BaMn2 As2 is a G-type antiferromagnetic insulator, within which the Mn spins order antiferromagnetically in a ‘checker-board’ fashion aligned along c [10]. Pressure induced metallization occurs at 36 K when ∼4.5 GPa is applied to the material and above 5.8 GPa the compound is metallic over the entire temperature range [11]. X-ray diffraction studies carried out as a function of pressure revealed an anomaly in the unitcell volume at ∼5 GPa which was not accompanied by a change in crystal structure, indicating an electronic transition that is consistent with the resistivity results. Colossal magnetoresistance (CMR) has recently been reported in the 1111-type manganese pnictide series for NdMnAsO1−x Fx , with a maximum CMR of −95% achieved
1. Introduction
Due to the recent discovery of high temperature superconductivity at 26 K in electron doped LaFeAsO [1] there has been much active research into oxypnictide materials. These layered 1111-type pnictides form with the primitive tetragonal ZrCuSiAs structure of space group P 4/nmm. Enhancing the superconducting transition temperature of this family of compounds has been achieved by using high pressure synthesis to create oxygen vacancies [2] and also by carrying out substitutions on the rare earth site, with the maximum Tc of 56.3 K currently held by Gd1−x Thx FeAsO [3]. Variable pressure x-ray diffraction results carried out at room temperature on the series LaFeAsO1−x Fx [4, 5] at pressures 10 GPa revealed no structural transition and tetragonal symmetry was preserved over the range studied. Anisotropic compression occurs as the axial compressibility of the c-axis (κc ) is almost twice that observed along a (κa ). The crystallographic direction normal to the FeAs layer is more effected by pressure than that parallel to the FeAs layer, and both structural parameters decrease monotonically so that the c/a ratio decreases linearly with pressure. It has been established that the anisotropic compression changes the charge distributions within the [FeAs]− and [LnO]+ lay0953-8984/15/116001+07$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
J. Phys.: Condens. Matter 27 (2015) 116001
E J Wildman et al
2. Methods
A 1 g polycrystalline sample of NdMnAsO0.95 F0.05 was synthesised via a two-step solid-state reaction. Initially, the NdAs precursor was obtained by the reaction of Nd pieces (Aldrich 99.9%) and As (Alfa Aesar 99.999%) at 900 ◦ C for 24 h in an evacuated, sealed quartz tube. The resulting precursor was then reacted with stoichiometric amounts of MnO2 , Mn and MnF2 (Aldrich 99.99%), all powders were ground in an inert atmosphere and pressed into pellets of 10 mm diameter. The pellets were placed into a Ta crucible and sintered at 1150 ◦ C for 48 h, again in a quartz tube sealed under vacuum. Powder x-ray diffraction patterns of NdMnAsO0.95 F0.05 were collected using a Bruker D8 Advance diffractometer with twin Gobel mirrors and Cu Kα radiation. Data were collected at room temperature over the range 10◦ < 2θ < 100◦ , with a step size of 0.02◦ , and could be indexed on a tetragonal unit cell of space group P 4/nmm, characteristic of the ZrCuSiAs structure type. X-ray diffraction patterns demonstrated that the material was of high purity. Powder neutron diffraction patterns were recorded on the high intensity diffractometer D20 at the ILL, Grenoble with a wavelength of 2.4188 Å. A 1 g sample of NdMnAsO0.95 F0.05 was inserted into an 8 mm vanadium can and data were recorded at selected temperatures between 350 and 400 K with a collection time of 10 min per temperature. High pressure time-of-flight neutron diffraction patterns were recorded using the instrument PEARL at the ISIS facility, UK. The sample was loaded into an encapsulated TiZr gasket [16] with 4 : 1 methanol–ethanol used as the pressure transmitting medium. A small pellet of lead was placed into the cell with the sample for use as a pressure calibrant [17]. The sample was then loaded into the Paris–Edinburgh cell [18]. The cell was used in transverse geometry giving access to scattering angles in the range 81.2◦ < 2θ < 98.8◦ . Data were recorded at room temperature using pressures up to ∼8.6 GPa with a collection time of ∼4 h per pressure. Data sets were also recorded at temperatures between 360 and 383 K using pressures of up to 4.97 GPa. The lead equation of state (EOS) [19] used to calculate the pressure was a Birch– Murnaghan equation of the form
Figure 1. Rietveld refinement fit to the 290 K, P = 3.65 GPa PEARL neutron powder diffraction pattern of NdMnAsO0.95 F0.05 . Tick marks represent reflection positions for NdMnAsO0.95 F0.05 (magnetic structure), ZrO2 (from the ceramic anvil), Al2 O3 (from the ceramic anvil), Pb and NdMnAsO0.95 F0.05 from top to bottom, respectively. The (1 0 1) and (1 0 0) magnetic peaks and (1 0 2) and (1 0 3) magnetic and nuclear peaks are highlighted. The inset shows the magnetic structure at 290 K, where the black spheres represent Mn and the grey spheres represent Nd.
at 3 K when x = 0.05 [12]. Magnetoresistance (MR) is defined as the change of electrical resistivity ρ in an applied magnetic field H , so that MR = (ρ(H ) − ρ(0)) /ρ (0), where ρ(H ) and ρ (0) are the resistivities in an applied field and zero field respectively. Magnetoresistant materials are important for magnetic memory device applications. Variable temperature neutron diffraction measurements on NdMnAsO0.95 F0.05 shows the same magnetic structure to that originally reported for the parent compound NdMnAsO (figure 1) [13, 14]. AFM ordering arises below 356(2) K (TN (Mn)), with Mn moments aligned parallel to the c-axis. At 23 K (TN (Nd)) the Nd3+ spins order with moments parallel to the basal plane, which results in a spin reorientation (TSR ) of Mn spins as they rotate from their previous alignment along c to along a. At the same time an Efros Schlovskii transition is observed which suggests that the reorientation of Mn spins into the basal plane at 20 K results in enhanced Coulomb correlations between localized electrons, resulting in a much higher resistivity below TSR [12]. A variable field neutron diffraction study has shown that upon applying a magnetic field there is a second order phase transition from antiferromagnetic to paramagnetic order of both the Nd and Mn spins [12]. The CMR then arises as a result of a second order phase transition from an insulating antiferromagnet to a semiconducting paramagnet upon applying a magnetic field, so that the electron correlations are diminished in field. It has been suggested that an antiferromagnetic instability is present in NdMnAsO0.95 F0.05 [12]. LaMnPO has very similar magnetic properties to NdMnAsO0.95 F0.05 (TN —375 K) [15]. A high pressure resistivity study on LaMnPO has revealed that an insulator-to-metal transition occurs at 20 GPa along with the complete suppression of longranged AFM order at higher pressures of 32 GPa. In this study we report the effect of pressure on the structure and antiferromagnetic ordering transition of NdMnAsO0.95 F0.05 .
V B P −1/B = 1+ V0 B0
where V0 is the unit-cell volume at zero pressure, V the unitcell volume at pressure P , B0 the zero pressure bulk modulus and B is the pressure derivative of the ambient bulk modulus. For Pb, the values were taken to be, B0 = 42 GPa, B = 5. A wavelength-dependent attenuation correction [20] was applied to account for the different sample environment materials before the data were analysed. The pressure dependency of the structure was obtained by Rietveld refinement [21] of the neutron data using the GSAS program [22]. 2
J. Phys.: Condens. Matter 27 (2015) 116001
E J Wildman et al
The cell parameters decrease upon increasing pressure and the variation in unit-cell volume (V /V0 ) with pressure is shown in figure 3. The axial compressibility of the lattice parameters, k, were calculated using linear fits of the normalized lattice constants, a/a0 and c/c0 (where for example ka = −1a (da/dP ) etc). The obtained values of ka = 3×10−3 GPa−1 and kc = 5.2 ×10−3 GPa−1 indicate that the unit cell is significantly more compressible along the c-axis than the a-axis, as the interlayer coupling is weaker than the intrinsic bonds contained within [NdO]+ and [MnAs]− slabs. The bulk modulus, B0 , of NdMnAsO0.95 F0.05 was calculated using a single parameter Birch–Murnaghan fit (shown as the red line in figure 3), with the volume of the unit cell at ambient pressure fixed at the refined value of 145.81 (1) Å3 . The resultant fits gave values of B0 = 74 (1) GPa and B0 = 3.4 (5), which are similar to the values extracted from pressure studies of other 1111-type pnictides (for example, B0 = 70 GPa for LaFeAsO0.95 F0.05 [4]). The Nd–O and Mn–As bond lengths also decrease upon applying pressure, P (table 1). The change in thickness of the respective layers is however interesting. The MnAs layers become more compressed as pressure is applied, shrinking by ∼3.5% upon increasing P from 0–8.59 GPa. In contrast the NdO layer expands by ∼2.2% (figure 4). The results demonstrate that in NdMnAsO0.95 F0.05 the interlayer spacing undergoes a reduction of ∼9.6% upon increasing P from 0–8.59 GPa (table 1). The atomic positions of Mn and (O, F) are constrained by symmetry (Wyckoff position 2b and 2a, respectively), whereas both Nd and As are located at 2c and hence their z atomic position can be affected by external pressure. Upon increasing the pressure from 0–8.59 GPa the z co-ordinate of Nd increases from 0.1295(3) to 0.1385(5). At the same time the z co-ordinate of As changes from 0.6734(5) to 0.6750(6) (table 1) and as a result of these combined effects the Nd–As bond length shrinks by 4.4%. The effect of external pressure therefore is to bring the [MnAs]− and [NdO]+ layers together. The variation of α1 and α2 Nd–O/F–Nd bond angles with applied pressure are shown in figure 5 and table 1. The Nd–O/F–Nd bond angles change more towards an ideal tetrahedron, as α1 decreases from 120.7(1)◦ to 118.3(1)◦ , while α2 increases from 104.16(6)◦ to 105.25(7)◦ upon increasing P from 0–8.59 GPa. The consequence of the change in the Nd z atomic position and changes in the α1 and α2 Nd–O/F– Nd bond angles is therefore that the NdO layer expands very slightly with pressure. Figure 6 shows the variation of α1 and α2 As–Mn–As angles with applied pressure. In contrast to the change in Nd–O/F–Nd bond angles described above, a much smaller variation of the As–Mn–As bond angles with P is observed. Over the pressure range studied for NdMnAsO0.95 F0.05 , the As–Mn–As bond angles tend very gradually towards an ideal tetrahedron, as α1 decreases from 111.55(8)◦ to 111.3(1)◦ , while α2 increases from 105.4(1)◦ to 105.9(1)◦ as P increases from 0 to 8.59 GPa. The change in As–Mn–As bond angle with pressure seems to be greater above P = 5.51 GPa and much higher pressures would be required in order to observe if the optimum tetrahedron can be obtained. Variable temperature, variable pressure measurements were also carried out using the Paris–Edinburgh cell. The
Figure 2. Neutron diffraction patterns of NdMnAsO0.95 F0.05
recorded at 290 K at pressures between 0–8.59 GPa as labelled. The Rietveld fit to the pattern is displayed at each pressure.
3. Results and discussion
Figure 1 shows the magnetic structure and the 290 K neutron diffraction pattern and Rietveld fit obtained for NdMnAsO0.95 F0.05 at a pressure of 3.65 GPa. There is no evidence of a phase transition up to 8.59 GPa, which is common for other 1111 oxypnictides of this structure type (figure 2) [23]. A tetragonal P 4/nmm unit cell was observed over the entire pressure range with fully occupied cation and anion sites. The Pr, Mn and As occupancies refined to within ±1% of the full occupancy and were fixed at 1.0. The O and F occupancies were fixed at 0.95 and 0.05 respectively. The corresponding fit parameters, refined lattice constants, bond lengths and angles at each pressure are given in table 1. The (1 0 1) and (1 0 0) magnetic peaks can be observed, alongside a magnetic contribution to the (1 0 2) and (1 0 3) structural peaks (figures 1 and 2), with no change in magnetic structure upon increasing the pressure. The Mn moment refines to 1.92 (7) µB and 1.93 (7) µB at P = 0 GPa and 8.59 GPa respectively (table 1). There is no evidence of a spin reorientation upon increasing the pressure so that the Mn spins remain aligned parallel to c for all P . 3
J. Phys.: Condens. Matter 27 (2015) 116001
E J Wildman et al
Table 1. Refined cell parameters, agreement factors, atomic parameters and selected bond lengths and angles for NdMnAsO0.95 F0.5 from Rietveld fits against neutron diffraction data at various pressures. Nd and As are at 2c (1/4, 1/4, z), Mn at 2b (3/4, 1/4, 1/2) and O,F at 2a (3/4, 1/4, 0). Pressure (GPa) Atom
Occupancy
Nd
1.00
Mn As
1.00 1.00
O/F
0.95/0.05
c
α1 α2
α2
a
b
α1
z Uiso (Å2 ) Uiso (Å2 ) z Uiso (Å2 ) Uiso (Å2 ) a (Å) c (Å) χ 2 (%) RWP (%) RP (%) Nd–O/F (Å) Mn–As (Å) Mn–Mn (Å) Nd–As (Å) α1 Nd–O/F–Nd (◦ ) α2 Nd–O/F–Nd (◦ ) α1 As–Mn–As (◦ ) α2 As–Mn–As (◦ ) MnAs Layer (Å) Nd(O/F) Layer (Å) Interlayer spacing (Å) Mn moment (µB )
0.46
2.03
3.65
5.51
7.55
8.59
0.1295(3) 0.007(1) 0.003(1) 0.6734(5) 0.011(1) 0.009(1) 4.04840(6) 8.8965(6) 0.539 4.14 5.25 2.329(1) 2.545(3) 2.86265(4) 3.357(3) 120.7(1) 104.16(6) 105.4(1) 111.55(8) 3.085(5) 2.304(5) 1.754(5) 1.92(7)
0.1304(2) 0.007(1) 0.009(1) 0.6734(3) 0.0114(9) 0.011(1) 4.04148(4) 8.8728(4) 0.499 3.09 3.84 2.329(1) 2.540(2) 2.85776(3) 3.346(2) 120.41(9) 104.29(4) 105.4(1) 111.52(6) 3.077(4) 2.314(4) 1.741(4) 1.87(6)
0.1319(2) 0.0016(9) 0.010(1) 0.6740(4) 0.010(1) 0.008(1) 4.01940(5) 8.7913(5) 0.453 3.25 3.96 2.320(1) 2.526(2) 2.84215(3) 3.315(2) 120.03(9) 104.46(4) 105.4(1) 111.52(6) 3.059(4) 2.319(5) 1.707(4) 1.89(5)
0.1338(3) 0.002(1) 0.005(1) 0.6744(4) 0.008(1) 0.005(1) 3.99869(6) 8.7117(7) 0.500 3.42 4.30 2.314(1) 2.511(2) 2.82750(4) 3.285(2) 119.5(1) 104.69(4) 105.6(1) 111.47(7) 3.039(6) 2.331(6) 1.671(5) 1.99(5)
0.1349(3) 0.001(1) 0.001(1) 0.6751(4) 0.006(1) 0.004(1) 3.97672(8) 8.6314(8) 0.484 3.51 4.27 2.304(1) 2.498(2) 2.81197(5) 3.255(2) 119.3(1) 104.79(5) 105.5(1) 111.48(7) 3.023(7) 2.329(7) 1.640(5) 1.98(5)
0.1371(3) 0.001(1) 0.005(1) 0.6757(4) 0.010(1) 0.008(1) 3.95391(7) 8.5301(7) 0.524 3.07 3.75 2.297(1) 2.481(2) 2.79584(5) 3.220(2) 118.8(1) 105.03(4) 105.7(1) 111.41(6) 2.997(6) 2.339(6) 1.597(6) 1.99(5)
0.1385(5) 0.004(1) 0.003(1) 0.6750(6) 0.009(1) 0.009(1) 3.9445(2) 8.5032(8) 0.467 4.49 5.34 2.297(1) 2.471(3) 2.7892(1) 3.208(3) 118.3(1) 105.25(7) 105.9(1) 111.3(1) 2.976(7) 2.355(7) 1.586(7) 1.93(7)
Figure 4. The variation of the MnAs and NdO layer thickness with applied pressure. Figure 3. Pressure dependence of the normalized unit-cell volume. The line represents the fit to the Birch–Murnaghan equation.
It is likely that the anisotropic compression of the NdMnAsO0.95 F0.05 structure upon application of external pressure results in changes in charge distributions within the [MnAs]− and [NdO]+ slabs [23]. However it has been shown that neither hole doping or electron doping results in an increase in TMn [12, 24], therefore it is highly unlikely that the enhanced TMn upon increasing external pressure is a result of a change in the charge distribution within the [MnAs]− layer. Instead it is more likely to be a structural effect. Table 2 shows the variation of the structural parameters with temperature and pressure for NdMnAsO0.95 F0.05 . It is clear that the Mn–As bond length decreases upon increasing both temperature and
sample was heated to temperatures above TN = 360 K where there was no apparent magnetic diffraction. The pressure was then increased at each temperature until the (1 0 1) and (1 0 0) magnetic diffraction peaks reappeared (figure 7, inset b). TN (at a specific temperature above TN = 360 K at P = 0) is defined as the pressure required in order to re-establish magnetic diffraction. Magnetic diffraction was re-established at 370 K (2.08 GPa), 380 K (4.23 GPa) and 383 K (4.97 GPa). The results show that it is possible to increase TN from 360 to 383 K upon applying an external pressure of 4.97 GPa, a rate of 4.63 K GPa−1 (figure 7). 4
J. Phys.: Condens. Matter 27 (2015) 116001
E J Wildman et al
Figure 5. The dependence of the α1 and α2 Nd–O–Nd bond angles
with pressure which show a clear change to a more regular tetrahedron upon applying pressure.
Figure 7. (a) The variation of TN with applied pressure showing that TN increases at a rate of 4.63 K GPa−1 . The top inset shows the variation of log (TN ) against log(V ). The bottom inset shows the ambient pressure, 380 K neutron diffraction pattern recorded on diffractometer D20 at the ILL. The diffuse scattering observed above TN is indicated. (b) The 380 K, 4.23 GPa neutron diffraction pattern, recorded on the instrument PEARL at the ISIS facility, UK. Tick marks represent reflection positions for NdMnAsO0.95 F0.05 (magnetic structure), ZrO2 (from the ceramic anvil), Al2 O3 (from the ceramic anvil), Pb and NdMnAsO0.95 F0.05 from top to bottom, respectively. The (1 0 1) magnetic diffraction peak is indicated.
Figure 6. The pressure variation of the α1 and α2 As–Mn–As bond angles, which show a gradual change to a more regular MnAs4 tetrahedron upon increasing pressure.
pressure from 290 K, 0 GPa to 383 K, 4.68 GPa. At the same time the interlayer spacing also decreases from 1.754(5) to 1.622(9) Å. There is no real change in the α1 or α2 As–Mn– As bond angles. In localized antiferromagnets, the Heisenberg exchange interaction, JH , has been shown to increase with applied pressure as a result of better orbital overlap as the cation– anion bond length decreases. Numerous antiferromagnetic insulators have been shown to obey Bloch’s rule, where α = dlog (TN ) /dlog (V ) ∼ −3.3 where V is the cell volume [25]. A theoretical rationalisation of Bloch’s rule comes from calculations of the variation of the overlap integral with the cation–anion bond length. Neutron scattering studies of NdMnAsO0.95 F0.05 recorded on diffractometer D20 at the ILL demonstrate that above TMn there is magnetic diffuse scattering, characteristic of short range magnetic order up to 400 K (figure 7(a), bottom inset). Upon application of pressure, the reduction in both the Mn–As bond length and the interlayer spacing enhances the superexchange between Mn centres, both along Mn–As–Mn and between the planes,
which then results in the increase in TMn . The variation of log (TN ) against log(V ) is shown in the top inset of figure 7(a) and a small value of α = −0.95 (2) is obtained, violating Bloch’s law. The perturbation description for the superexchange spin–spin interaction should break down on the approach to a crossover from localized to itinerant electronic behaviour of a Mott–Hubbard insulator. The small value of α = −0.95 (2) would suggest that NdMnAsO0.95 F0.05 is on the verge of a localized to itinerant transition so that a purely localized Heisenberg description of the magnetic exchange is not applicable. This is further corroborated by variable field resistivity and neutron diffraction measurements which have shown that competing electronic phases are present in NdMnAsO0.95 F0.05 [12]. 5
J. Phys.: Condens. Matter 27 (2015) 116001
E J Wildman et al
Table 2. Refined cell parameters, agreement factors, atomic parameters and selected bond lengths and angles for NdMnAsO0.95 F0.5 from Rietveld fits against neutron diffraction data at various temperatures and pressures. Nd and As are at 2c (1/4, 1/4, z), Mn at 2b (3/4, 1/4, 1/2) and O,F at 2a (3/4, 1/4, 0). Temperature (K)/Pressure (GPa) Atom
Occupancy
Nd
1.00
Mn As
1.00 1.00
O/F
0.95/0.05
c
α1 α2
α2
a
b
α1
z Uiso (Å2 ) Uiso (Å2 ) z Uiso (Å2 ) Uiso (Å2 ) a (Å) c (Å) χ 2 (%) RWP (%) RP (%) Nd–O/F (Å) Mn–As (Å) Mn–Mn (Å) Nd–As (Å) α1 Nd–O/F–Nd (o ) α2 Nd–O/F–Nd (o ) α1 As–Mn–As (o ) α2 As–Mn–As (o ) MnAs Layer Nd(O/F) Layer Interlayer spacing (Å)
290/0
370/2.08
380/4.23
383/4.97
0.1295(3) 0.007(1) 0.003(1) 0.6734(5) 0.011(1) 0.009(1) 4.04840(6) 8.8965(6) 0.539 4.14 5.25 2.329(1) 2.545(3) 2.86265(4) 3.357(3) 120.7(1) 104.16(6) 111.55(8) 105.4(1) 3.085(5) 2.304(5) 1.754(5)
0.1328(3) 0.002(1) 0.015(1) 0.6749(4) 0.0104(9) 0.008(1) 4.015272(6) 8.7728(6) 1.037 2.67 2.84 2.321(1) 2.527(2) 2.83923(4) 3.302(2) 119.74(9) 104.59(4) 111.63(6) 105.2(1) 3.068(5) 2.330(5) 1.687(5)
0.1355(4) 0.010(1) 0.014(2) 0.6781(6) 0.014(1) 0.008(1) 3.98956(9) 8.654(1) 1.149 3.77 4.09 2.314(2) 2.521(3) 2.82105(7) 3.250(3) 119.1(2) 104.88(7) 112.0(1) 104.6(2) 3.082(9) 2.345(9) 1.614(9)
0.1353(4) 0.001(1) 0.011(2) 0.6767(6) 0.015(1) 0.006(1) 3.97916(9) 8.628(1) 0.916 3.50 3.84 2.307(2) 2.507(3) 2.81369(7) 3.247(3) 119.2(1) 104.84(6) 111.73(9) 105.1(2) 3.049(9) 2.335(9) 1.622(9)
CMR is observed in NdMnAsO0.95 F0.05 below the spin reorientation transition, TSR , of the Mn spins which is precipitated by the antiferromagnetic ordering of the Nd3+ spins at TNd [12]. It is possible that TNd will also increase with applied pressure which may result in CMR observed at higher temperatures. High pressure neutron diffraction and magnetoresistance measurements at low temperature are warranted to investigate this further. LaMnPO also has an AFM ordered state, with the same antiferromagnetic structure as NdMnAsO0.95 F0.05 and a comparable TN (Mn) of 375 K [15]. Hence it is worthwhile comparing the effects of pressure on TN (Mn). Pressure studies on this insulating material revealed a contrasting suppression of TN with increasing pressure (TN decreases from 375 to 290 K upon applying a pressure of 7.3 GPa). An induced crossover to a mixed state, in which insulating and metallic states coexist, is observed upon applying a pressure of 20 GPa followed by the collapse of long range AFM order at ∼32 GPa. The reduction in TN with pressure is therefore a result of increasing electron delocalization upon increasing pressure and by 32 GPa the localized/moment bearing electrons are fully delocalized. A high pressure x-ray diffraction study of the same compound shows that the tetragonal structure is stable up to 16.4 GPa [26]. At higher pressures an orthorhombic structure is observed followed by a collapsed orthorhombic state at 31 GPa. LaMnPO is reported to be less electronically stable than its substantial gap and ordered moment suggest. At ambient pressure it is close to an electron delocalization transition (EDT) that is driven by the nucleation of states with energies within the correlation gap [15, 26]. The electronic structure of NdMnAsO0.95 F0.05 appears to be more
robust to electronic delocalization upon applying external pressure, so that at modest pressures the shorter Mn–As bonds and interlayer spacing results in enhanced superexchange between Mn centres and higher TN upon increasing P . However the small value of α = −0.95 (2) obtained from the fit to the Bloch equation shows that Bloch’s rule is violated in NdMnAsO0.95 F0.05 and that NdMnAsO0.95 F0.05 is also on the verge of a localized to itinerant transition. Presumably, eventually in high enough pressures, an electron delocalization transition will occur and TN will decrease. Further neutron diffraction experiments with higher pressures will be necessary to see if there is an eventual collapse in the antiferromagnetic order and/or structural phase change with increasing pressure. Electronic structure calculations and variable pressure resistivity measurements are also warranted to explore this further. In summary we show that the antiferromagnetic order of the Mn oxyarsenide NdMnAsO0.95 F0.05 is robust to pressures of up to 8.59 GPa and that TN is enhanced with applied pressure at a rate of 4.63 K GPa−1 . This is in contrast to the oxyphosphide LaMnPO, where TN decreases in modest pressure. There is also no evidence of a crystallographic phase change or change in magnetic structure upon increasing the pressure up to 8.59 GPa. Acknowledgments
We acknowledge the UK EPSRC for financial support (Grant EP/L002493/1) and STFC-GB for provision of beamtime at ISIS and ILL. 6
J. Phys.: Condens. Matter 27 (2015) 116001
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