Strong frustration due to competing ferromagnetic and antiferromagnetic interactions: Magnetic properties ofM(VO)2(PO4

Abstract

We present a detailed investigation of the magnetic properties of complex vanadium phosphates M(VO)2(PO4)2 (M = Ca and Sr) by means of magnetization, specific heat, 31 P NMR measurements, and band structure calculations. Experimental data evidence the presence of ferromagnetic and antiferromagnetic interactions in M(VO)2(PO4)2, resulting in a nearly vanishing Curie-Weiss temperature θCW ≤ 1 K that contrasts with the maximum of magnetic susceptibility at 3 K. Specific heat and NMR measurements also reveal weak exchange couplings with the thermodynamic energy scale Jc = 10 − 15 K. Additionally, the reduced maximum of the magnetic specific heat indicates strong frustration of the spin system. Band structure calculations show that the spin systems of the M(VO)2(PO4)2 compounds are essentially three-dimensional with the frustration caused by competing ferromagnetic and antiferromagnetic interactions. Both calcium and strontium compounds undergo antiferromagnetic long-range ordering at TN = 1.5 K and 1.9 K, respectively. The spin model reveals an unusual example of controllable frustration in three-dimensional magnetic systems.

Frustrated couplings between alternating spin-12chains in AgVOAsO4

“…In M(VO) 2 (PO 4 ) 2 , four comparable couplings in the frustrated unit lead to the strong frustration on a 3D spin lattice, and the enhancement of quantum fluctuations manifests itself as the suppression of the specific heat maximum. 27 In AgVOAsO 4 , J and J ′ dominate over J a and J c , hence the magnetic behavior is quasi-1D, with the spin gap imposed by the alternating spin chains (similar to non-frustrated systems). The effects of the frustration should be primarily expected in high magnetic fields that close the spin gap and induce the long-range order.…”

“…Turning to the structural aspects of the proposed spin model, we note that the frustration scenario in AgVOAsO 4 is very similar to one in M(VO) 2 (PO 4 ) 2 with M = Ca and Sr. 27 In both cases, most of the couplings are long-range and AFM, yet the presence of the short-range FM coupling (J c in AgVOAsO 4 ) between the cornersharing VO 6 octahedra makes the system frustrated. The magnitude of the frustration is controlled by the ratios of J's.…”

“…5) reveals an electronic structure, typical for V +4 compounds. 7,27,36 The lowlying valence bands are predominantly formed by O 2p states. The bands around −3 eV represent the filled 4d shell of Ag +1 (similar to Ref.…”

“…This arrangement of the magnetic orbital precludes the V-O-V superexchange along the structural chains, and makes the relationship between the crystal structure and the spin lattice rather complex. 11,13,27 In Sec. V, we apply extensive DFT calculations in order to find the correct microscopic model.…”

Unusual ferromagnetic superexchange in CdVO3: The role of Cd

“…The interchain couplings are long-range and likely AFM because of the underlying V-O-O-V superexchange pathways that typically lead to AFM couplings, while the FM superexchange is usually operative at short distances (see, e.g., Refs. [12][13][14]. Moreover, the next-nearest-neighbor intrachain coupling J 2 between the corner-sharing vanadium pyramids should also be AFM, as in CaV 2 O 5 and related compounds (see Sec.…”