Antiferromagnetic phase transition and spin correlations in NiO

Abstract

We have investigated the antiferromagnetic ͑AF͒ phase transition and spin correlations in NiO by hightemperature neutron diffraction below and above T N . We show that AF phase transition is a continuous second-order transition within our experimental resolution. The spin correlations manifested by the strong diffuse magnetic scattering persist well above T N Ϸ 530 K and could still be observed at T = 800 K which is about 1.5T N . We argue that the strong spin correlations above T N are due to the topological frustration of the spins on a fcc lattice. The Néel temperature is substantially reduced by this process. We determined the critical exponents ␤ = 0.328Ϯ 0.002 and = 0.64Ϯ 0.03 and the Néel temperature T N = 530Ϯ 1 K. These critical exponents suggest that NiO should be regarded as a 3dXY system. DOI: 10.1103/PhysRevB.79.172403 PACS number͑s͒: 75.25.ϩz Transition-metal oxides have been the subject of renewed interest ever since high-temperature superconductivity was discovered in cuprate materials. The transition-metal oxides with narrow d bands form strongly correlated electron systems or a Mott-Hubbard system in which Coulomb interactions between electrons lead to a breakdown of the conventional band theory. In such a system, hole or electron doping may transform an insulating antiferromagnet into a highly correlated metal. 1,2 In selected copper oxides, the metal can superconduct, while in others it is a strongly renormalized Fermi liquid. In manganites for example the doping transforms the insulating antiferromagnet into a ferromagnetic metal with colossal magnetoresistive properties. 3 Among the transition-metal oxides the antiferromagnetic NiO plays a key role in the studies of the electronic and magnetic properties of correlated 3d-electron system. NiO, along with other transition-metal oxides MnO, FeO, and CoO, crystallizes with the face-centered-cubic ͑space group Fm3m͒ NaCl-type structure. The magnetic properties of pure NiO are well known. Ni 2+ ions in NiO are in an S = 1 state and order at T N = 530 K in the type-II antiferromagnetic structure with the propagation vector k = ͑ 1 2 , 1 2 , 1 2 ͒. Ferromagnetic ͑111͒ layers are antiferromagnetically stacked along the ͓111͔ direction. The magnetic moments of Ni lie in the ͑111͒ plane and there is now considerable evidence that they lie in the ͓1,1,−2͔ direction. 4 There is a rhombohedral distortion at T N which increases as the temperature is lowered. The electronic structure of NiO has been extensively investigated. [5][6][7] There are strong indications that NiO is a charge-transfer insulator 5 in which the top of the valence band is primarily formed by oxygen 2p states, while the bottom of the conduction band is Ni-3d like. The insulating state is characterized by a band gap of about 4 eV. Magnetic exchange interactions in NiO have been calculated 7-9 and have also been determined by measuring the spin-wave dispersion by inelastic neutron scattering. 10 The spin-wave dispersion in NiO could be described by a nearest-neighbor ͑NN͒ ferromag...

Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator

“…This indicates the prominent roles of spin fluctuation and short range spin correlation in AF on spin transport [17,18,28]. Note that short range spin correlation in AF still exists at temperatures much higher than T N , as revealed by neutron scattering [29]. Above the T N , the magnons whose wavelength is shorter than the spin correlation length remain.…”

“…Above the T N , the magnons whose wavelength is shorter than the spin correlation length remain. The spin correlation length of NiO is ξ ¼ l½ðT=T N Þ − 1 −0.64 , where l ¼ 0.5 nm [29]. The number of magnons participating the spin transport decreases due to the loss of the magnons whose wavelength is longer than the spin correlation length.…”

Spin transport in antiferromagnetic insulators mediated by magnetic correlations

“…Despite the difference in the dispersion relations of YIG and the AFs, our result clearly demonstrates highly efficient spin transport across the AFs. Considering that strong AF spin correlations have been observed well above T N for NiO [27], we believe the excitations responsible for spin transport in AFs must be magnons in ordered…”

Y3Fe5O12 spin pumping for quantitative understanding of pure spin transport and spin Hall effect in a broad range of materials (invited)

“…of NiO as the underlying mechanism for the observed robust spin transport in NiO [35]. Since the range of tNiO covers NiO layers with ordering temperatures [36] both above (large tNiO) and below (small tNiO) room temperature, this indicates that both AF ordered and AF fluctuating [37] spins in 8 NiO can be excited by exchange coupling to the precessing YIG magnetization and efficiently transporting spin current over a long distance.…”