Strong magnetic correlations to 900 K in single crystals of the trigonal antiferromagnetic insulators SrMn₂As₂ and CaMn₂As₂

The body-centered tetragonal $AM_{2}X_{2}$ ternary compounds ($A$ = rare or alkaline earth, $M$ = transition metal, $X$ = Si, Ge, P, As, Sb) with the ${\rm ThCr_{2}Si_{2}}$ structureJust1996 have generated tremendous interest in the scientific community due to their novel electronic and magnetic properties. Prominent among these is the iron-arsenide family of parent compounds $A{\rm Fe_{2}As_{2}}$ ($A$ = Ca, Sr, Ba, Eu).Johnston2010 ; Stewart2011 ; Scalapino2012 ; Dagotto2013 ; Fernandes2014 ; Hosono2015 ; Dai2015 ; Inosov2016 ; Si2016 These materials are metallic and show nearly contiguous antiferromagnetic (AFM) spin-density wave and structural transitions at temperatures $T$ up to $\sim 200$ K. The suppression of these transitions by external pressure or chemical doping leads to superconductivity with bulk superconducting transition temperatures $T_{\rm c}$ up to 56 K. It is believed that the FeAs layer as the conducting sheet in this structure plays a crucial role in the occurrence of superconductivity.Johnston2010 ; Stewart2011 ; Scalapino2012 ; Dagotto2013 ; Fernandes2014 ; Hosono2015 ; Dai2015 ; Inosov2016 ; Si2016 Hence, it is important to investigate other related materials with similar compositions and structures in the search for new superconductors and other novel phenomena. For example, SrNi₂As₂ ($T_{\rm c}=0.62$ K, Ref. Bauer2008, ) and BaNi₂As₂ ($T_{\rm c}=0.7$ K, Ref. Ronning2008, ) were both found to be superconductors.

On the other hand, SrCo₂As₂ (Refs. Pandey2013, ; Jayasekara2013, ; Wiecki2015, ) and BaCo₂As₂ (Refs. Sefat2009, ; Anand2014, ) are correlated metals with no structural, superconducting or long-range magnetic ordering transitions. From inelastic neutron scattering measurements, SrCo₂As₂ is found to exhibit strong AFM correlations at the same stripe wavevector as do the superconducting iron arsenides, which raises the interesting question of why SrCo₂As₂ is not a high-$T_{\rm c}$ superconductor.Jayasekara2013 The reason has been suggested from NMR measurements to be that SrCo₂As₂ exhibits strong ferromagnetic (FM) spin correlations/fluctuations in addition to the AFM correlations and these compete with the AFM correlations that are the presumptive glue for superconductivity in these systems. Subsequent NMR studies indicated that the large range of $T_{\rm c}$ observed within the FeAs-based systems may also arise from the competition between FM and AFM correlations.Wiecki2015b

Recently significant attention has focussed on Mn arsenides. Our studies of the properties of the parent and doped BaMn₂As₂ systems were originally motivated by their potential to be ThCr₂Si₂-type high-$T_{\rm c}$ superconductors analogous to the cuprates. The semiconductor BaMn₂As₂ shows G-type (checkerboard-type) local-moment collinear AFM order below its high Néel temperature $T_{\rm N}=625$ K with the ordered moments aligned along the tetragonal $c$ axis.Singh2009 ; Singh2009b ; Johnston2011 Thus magnetoelastic coupling does not cause a distortion of the crystal structure below $T_{\rm N}$, contrary to the orthorhombic distortion associated with AFM ordering in the $A{\rm Fe_{2}As_{2}}$ compounds due to the collinear Fe ordered moments aligned in the $ab$ plane. An optical gap of 48 meV was inferred for BaMn₂As₂ from the optical conductivity,Antel2012 consistent with results from the electrical resistivity $\rho$ versus temperature $T$ measurements in the $ab$ plane.Singh2009 Furthermore, this optical studyAntel2012 found that BaMn₂As₂ is much more two-dimensional in its electronic properties than are the $A{\rm Fe_{2}As_{2}}$ parent compounds.Singh2009 A neutron scattering study of isostructural ${\rm BaMn_{2}Bi_{2}}$ found the same G-type AFM structure as in ${\rm BaMn_{2}As_{2}}$ but with a lower $T_{\rm N}=387$ K.Calder2014

Only 1.6% K substitution for Ba transforms BaMn₂As₂ into a local-moment AFM metal.Pandey2012 ; Yeninas2013 Higher doping levels lead to the onset of FM at $\approx 16$% K-dopingBao2012 and half-metal FM behavior below the Curie temperature $T_{\rm C}\sim 100$ K at 40% K doping (Refs. Pandey2013b, ; Ueland2015, ) and at 60% Rb doping.Pandey2015 The FM is thus thought to be associated with FM ordering of the itinerant doped-hole spins and coexists with the G-type AFM order of the local Mn moments with $T_{\rm N}>300$ K.Ueland2015 ; Lamsal2013

Unlike BaMn₂As₂ with the tetragonal ${\rm ThCr_{2}Si_{2}}$ structure, the compounds ${\rm SrMn_{2}As_{2}}$ and ${\rm CaMn_{2}As_{2}}$ both crystallize in the trigonal ${\rm CaAl_{2}Si_{2}}$-type structureMewis1978 ; Brechtel1978 containing a corrugated honeycomb Mn sublattice which can be viewed as a triangular lattice bilayer. The possibility of geometrically-frustrated triangular-lattice exchange connectivity exists and such compounds often show novel physical behaviors associated with the geometric frustration.Ramirez1994 ; Moessner2006 ; Balents2010 Single crystals of ${\rm SrMn_{2}As_{2}}$ were grown previously using Sn flux.Wang2011 These authors’ in-plane electrical resistivity $\rho(T)$ measurements indicated that the ground state is insulating with activation energies of 0.29–0.64 eV depending on the $T$ range, and their magnetic susceptibility $\chi(T)$ measurements indicated an AFM transition at $T_{\rm N}=125$ K.Wang2011

Two neutron powder diffraction studiesRatcliff2009 ; Bridges2009 of the related ${\rm CaAl_{2}Si_{2}}$-type ${\rm CaMn_{2}Sb_{2}}$ revealed AFM ordering below $T_{\rm N}=88$ K and 85 K, respectively, with an AFM propagation vector ${\bf k}=(0,0,0)$, i.e., the crystal and AFM unit cells are the same. In the former paper the AFM structure was deduced to be collinear, with the ordered moments aligned in the $ab$ plane with a low-$T$ ordered moment of 2.8(1) $\mu_{\rm B}$/Mn, where $\mu_{\rm B}$ is the Bohr magneton. In the latter paper, a model was favored with the ordered moments canted at $\pm 25^{\circ}$ with respect to the $ab$ plane with an ordered moment of 3.38(6) $\mu_{\rm B}$/Mn.

Herein, we report the growth, crystal structure, $\rho(T)$, magnetization as a function of magnetic field $M(H)$, $\chi(T)$ and heat capacity $C_{{\rm p}}(T)$ measurements of ${\rm CaMn_{2}As_{2}}$ and ${\rm SrMn_{2}As_{2}}$ single crystals. These studies were initiated because of the above-noted possibility that the Mn spin lattice might exhibit novel magnetic behaviors associated with the presence of geometric frustration within the triangular-lattice Mn layers. If the strongest AFM interactions are indeed within a triangular lattice layer, this should lead to a noncollinear AFM structure below $T_{\rm N}$. Instead, in a companion neutron diffraction study to the present work, the AFM structure of ${\rm SrMn_{2}As_{2}}$ was found to be collinear with the ordered Mn moments aligned in the $ab$ plane with magnitude 3.6 $\mu_{\rm B}$/Mn.Das2016 This magnetic structure is the same as one of the two AFM structures proposed for ${\rm CaMn_{2}Sb_{2}}$ (Ref. Ratcliff2009, ) discussed above.

We discovered that the $\chi(T)$ of ${\rm SrMn_{2}As_{2}}$ and ${\rm CaMn_{2}As_{2}}$ and the $C_{\rm p}(T)$ of ${\rm SrMn_{2}As_{2}}$ above their respective Néel temperatures $T_{\rm N}$ of 120 and 62 K exhibit behaviors characteristic of strong dynamic short-range AFM spin correlations up to at least 900 K, likely arising from quasi-two-dimensional connectivity of strong AFM Mn–Mn exchange interactions within the corrugated honeycomb Mn spin sublattice. This result is interesting because such strong AFM spin correlations up to high temperatures and the suppression of $T_{\rm N}$ to much lower temperatures than expected from molecular field theory, due to AFM fluctuations associated with the low dimensionality of the exchange interaction connectivity, may give rise to novel physical properties upon doping the compounds into the metallic state.

Single crystals of ${\rm SrMn_{2}As_{2}}$ and ${\rm CaMn_{2}As_{2}}$ were grown using Sn flux. High-purity elements Sr (99.95%) from Sigma Aldrich, and Ca (99.95%), Co (99.998%), As (99.9999%) and Sn (99.999%) from Alfa Aesar were taken in the ratio (Sr,Ca):Mn:As:Sn = 1:2:2:20 and placed in an alumina crucible that was subsequently placed in a silica tube that was evacuated, partially refilled with high-purity argon ($\approx$1/4 atm pressure) and then sealed. After preheating at 600^∘C for 5 h, the assembly was heated to 1150 ^∘C at the rate of 50 ^∘C/h and held at this temperature for 20 h for homogenization. Then the furnace was slowly cooled at the rate of 5 ^∘C/h to 700 ^∘C. At this temperature the molten Sn flux was decanted using a centrifuge. Shiny hexagonal-shape single crystals of maximum dimensions $4\times 3\times 1\ {\rm mm}^{3}$ were obtained.

Semiquantitative chemical analyses of the single crystals were performed using a JEOL scanning electron microscope (SEM) equipped with an EDX (energy-dispersive x-ray analysis) detector, where a counting time of 120 s was used. A room-temperature powder x-ray diffraction (XRD) pattern was recorded on crushed single crystals using a Rigaku Geigerflex powder diffractometer with Cu K$\alpha$ radiation at diffraction angles 2$\theta$ from 10^∘ to 110^∘ with a 0.02^∘ step width. The data were analysed by Rietveld refinement using FullProf software.fullprof

$M(T)$ measurements for ${\rm 1.8~K\leq}~T~{\rm\leq 300~K}$ and $M(H)$ measurements for $H\leq 5.5$ T were carried out using a Quantum Design, Inc., Magnetic Properties Measurement System (MPMS). The high-temperature $M(T)$ for ${\rm 300~K\leq T\leq 900~K}$ was measured using the vibrating sample magnetometer (VSM) option of a Quantum Design, Inc., Physical Properties Measurement System (PPMS). In this paper we exclusively use Gaussian cgs units for $M$, $\chi$ and $H$ (see Sec. 3.5.1 of Ref. Johnston2010, ). In this system of units, the Tesla (T) is a unit of convenience for $H$ defined as 1 T = $10^{4}$ Oe, where Oe is the conventional cgs unit for $H$.

$C{\rm{}_{p}}(T)$ data were obtained using a relaxation method with the heat capacity option of the PPMS. Four-probe $\rho(T)$ data were obtained with an ac current amplitude $I=1\,\rm{\mu A}$ at a frequency of 37.7 Hz using the ac transport option of the PPMS. Electrical contacts to a crystal were made by soldering 0.05 mm diameter Pt wire to a crystal using indium solder.

We have shown that ${\rm SrMn_{2}As_{2}}$ and ${\rm CaMn_{2}As_{2}}$ are AFM insulators with Néel temperatures $T_{\rm N}=120$ K and 62 K, respectively. The microscopic origin of this large difference in magnitude of the Néel temperatures together with the reason why the Sr compound has a higher $T_{\rm N}$ than the Ca one, in spite of the smaller unit cell of the latter, remain to be explained.

The $\chi(T)$ data at $T\leq T_{\rm N}$ indicate that the hexagonal $c$ axis is a hard axis, with the ordered Mn spin-5/2 moments lying within the $ab$ plane. Since a collinear AFM structure within the $ab$ plane is inferred for ${\rm SrMn_{2}As_{2}}$ from neutron diffraction measurements that were carried out in a companion study,Das2016 the nonzero limits of $\chi_{ab}(T<T_{\rm N})$ observed for this compound must arise from the three collinear AFM domains with their axes at $60^{\circ}$ to each other within the $ab$ plane. If the populations of the three domains are equal, within molecular field theory one obtains $\chi_{ab}(T\to 0)=\chi(T_{\rm N})/2$, in approximate agreement with the $\chi_{ab}(T)$ data in Fig. 4(a). The prediction of the easy axis arising from the Mn–Mn magnetic dipole interactions in a collinear magnetic structure of ${\rm SrMn_{2}As_{2}}$ obtained using the formalism of Ref. Johnston2016, and the experimental crystal structure is that the ordered moments should lie in the $ab$ plane as inferred here from the $\chi(T)$ data and also observedDas2016 in the neutron diffraction experiments.

Thus the potential geometric frustration for AFM ordering within the triangular-lattice bilayers parallel to the $ab$ plane that originally motivated this work is apparently not important in ${\rm SrMn_{2}As_{2}}$ and ${\rm CaMn_{2}As_{2}}$. In particular, if AFM Mn–Mn exchange interactions within a triangular sublattice layer were dominant, a noncollinear AFM structure would have resulted instead of the observedDas2016 collinear AFM structure.

Strong dynamic AFM short-range correlations up to at least 900 K as observed in our $\chi(T)$ measurements, consistent with our $S_{\rm mag}(T)$ data up to 200 K, are likely due to quasi-two-dimensional connectivity of strong AFM Mn–Mn exchange interactions within the corrugated honeycomb Mn layers. This in turn offers the possibility of novel electronic ground states arising upon doping these materials into the metallic state.