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We report the observation of a Griffiths Phase in the geometrically frustrated antiferromagnet DyBaCo4O 7+δ . Its onset is determined using measurements of the thermoremanent magnetization, which is shown to be superior to conventional in-field measurement protocols for the identification of the Griffiths Phase. Within this phase, the temporal relaxation of magnetization exhibits a functional form which is expected for Heisenberg systems, reflecting the nature of spin interactions in this class of materials. Interestingly, the effective Co 2+ /Co 3+ ratio tailored by varying the oxygen non-stoichiometry δ is only seen to influence the antiferromagnetic ordering temperature (T N ), leaving the Griffiths Temperature (T G) invariant.Keywords: Keywords A Griffiths Phase (GP) pertains to the formation of arbitrarily large magnetically ordered regions within the global paramagnetic phase, at temperatures exceeding that of long range magnetic ordering. First postulated in the context of random site dilution in Ising ferromagnets [1], this phase is characterized by the magnetization remaining non-analytical at temperatures above the ferromagnetic Curie temperature (T C ), where the system is neither a pure paramagnet, nor does it exhibit long range order. Analyticity is restored only above the Griffiths temperature T G , which refers to the Curie Temperature of the pristine (undiluted) system. This model was further generalized to account for random magnetic spin systems, where T G now refers to the highest ordering temperature allowed by the bond probability distribution [2]. The physics of this regime between T C and T G is now known to be extremely rich, and has even been extended to Quantum Phase Transitions where these effects are thought to manifest themselves in the form of power laws in thermodynamical observables [3]. This area of research saw renewed interest, when it was suggested that the phenomenon of Colossal Magnetoresistance observed in mixed valent manganites could be understood in the context of a Griffiths Singularity [4]. A variety of factors like doping, Jahn Teller distortion [5], size variance of the A site ions in the ABO 3 structure [6], finite size effects [7] and magnetic site dilution [8] were seen to stabilize a GP in these materials. This phenomena appears to be ubiquitous to a number of strongly correlated electron systems, and materials as diverse as transition metal oxides [5][6][7][8][9], chemically substituted f -electron systems [10] and magnetocaloric intermetallics [11,12] are now reported to harbor such a state.The probability of a magnetically ordered rare region existing within a global paramagnetic phase would fall exponentially as the volume of this region [13] . Thus the experimental verification of the GP primarily relies on our ability to detect the contribution which this rare region makes to measurable thermodynamic quantities. The most popular route has been to evaluate the inverse of the measured zero field cooled (ZFC) dc magnetic susceptibility χ −1 = (T −T C ) 1−λ , w...

The geometrically frustrated antiferromagnetic Swedenborgite DyBaCo4O7 is investigated by a combination of xray diffraction, magnetization and dielectric measurements. Systematic doping in the series Dy1−xCaxBaCo4O7 causes a lifting of the geometrical frustration resulting in a structural transition from a Trigonal P31c to an orthorhombic Pbn21 symmetry at x = 0.4. This structural transition can also be accessed as a function of temperature, and all our orthorhombic specimens exhibit this transition at elevated temperatures. In line with previous reports, the temperature at which this structural transition occurs scales linearly with the mean ionic radii of the A site ion. However, CaBaCo4O7 which has an equal number of Co 2+ and Co 3+ ions clearly violates this quasilinear relationship, indicating that charge ordering could also play a critical role in stabilizing the orthorhombic distortion in this system. Using thermoremanent magnetization measurements to circumvent the problem of the large paramagnetic background arising from Dy 3+ ions, we chart out the phase diagram of the Dy1−xCaxBaCo4O7 series.

The quantum spin systems Cu 2 M BO 5 (M = Al, Ga) with the ludwigite crystal structure consist of a structurally ordered Cu 2+ sublattice in the form of three-leg ladders, interpenetrated by a structurally disordered sublattice with a statistically random site occupation by magnetic Cu 2+ and nonmagnetic Ga 3+ or Al 3+ ions. A microscopic analysis based on density-functional-theory calculations for Cu 2 GaBO 5 reveals a frustrated quasitwo-dimensional spin model featuring five inequivalent antiferromagnetic exchanges. A broad low-temperature 11 B nuclear magnetic resonance points to a considerable spin disorder in the system. In zero magnetic field, antiferromagnetic order sets in below T N ≈ 4.1 K and ∼2.4 K for the Ga and Al compounds, respectively. From neutron diffraction, we find that the magnetic propagation vector in Cu 2 GaBO 5 is commensurate and lies on the Brillouin-zone boundary in the (H 0L) plane, q m = (0.45, 0, −0.7), corresponding to a complex noncollinear long-range ordered structure with a large magnetic unit cell. Muon spin relaxation is monotonic, consisting of a fast static component typical for complex noncollinear spin systems and a slow dynamic component originating from the relaxation on low-energy spin fluctuations. Gapless spin dynamics in the form of a diffuse quasielastic peak is also evidenced by inelastic neutron scattering. Most remarkably, application of a magnetic field above 1 T destroys the static long-range order, which is manifested in the gradual broadening of the magnetic Bragg peaks. We argue that such a crossover from a magnetically long-range ordered state to a spin-glass regime may result from orphan spins on the structurally disordered magnetic sublattice, which are polarized in magnetic field and thus act as a tuning knob for field-controlled magnetic disorder.

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