In this study, the magnetic ground state of the hollandite type material K2Cr8O16 was tuned by externally applied pressure and investigated using µ+SR method in Zero-field (ZF) and weak-transversefield (wTF) configurations. As a result, the obtained magnetic transition temperature for the measuredpressures differs notably from magnetization measurements. Moreover, both wTF and ZF data reveala transition between two different magnetically ordered states at low temperatures for higher pressures. Further theoretical and experimental studies are currently being planned in order to elucidatethe detailed nature of the magnetically ordered phase. 

The K 2 Cr 8 O 16 compound belongs to a series of quasi-1D compounds with intriguing magnetic properties that are stabilized through a high-pressure synthesis technique. In this study, a muon spin rotation, relaxation and resonance (μ + SR) technique is used to investigate the pressure dependent magnetic properties up to 25 kbar. μ + SR allows for measurements in true zero applied field and hereby access the true intrinsic material properties. As a result, a refined temperature/pressure phase diagram is presented revealing a novel low temperature/high pressure (p C1 = 21 kbar) transition from a ferromagnetic insulating to a high-pressure antiferromagnetic insulator. Finally, the current study also indicates the possible presence of a quantum critical point at p C2 ~ 33 kbar where the magnetic order in K 2 Cr 8 O 16 is expected to be fully suppressed even at T = 0 K.

The magnetic nature of the quasi-one-dimensional BaVS3 has been studied as a function of temperature down to 0.25 K and pressure up to 1.97 GPa on a powder sample using the positive muon spin rotation and relaxation (mu(+) SR) technique. At ambient pressure, BaVS3 enters an incommensurate antiferromagnetic ordered state below the Neel temperature (T-N)31 K. T-N is almost constant as the pressure (p) increases from ambient pressure to 1.4 GPa, then T-N decreases rapidly for p > 1.4 GPa, and finally disappears at p similar to 1.8 GPa, above which a metallic phase is stabilized. Hence, T-N is found to be equivalent to the pressure-induced metal-insulator transition temperature (T-MI) at p > 1.4 GPa.

The magnetic nature of a quasi-one-dimensional compound, BaVSe3, has been investigated with positive muon spin rotation and relaxation (mu+SR) measurements at ambient and high pressures. At ambient pressure, the mu+SR spectrum recorded under zero external magnetic field exhibited a clear oscillation below the Curie temperature (T-C similar to 41 K) due to the formation of quasistatic ferromagnetic order. The oscillation consisted of two different muon spin precession signals, indicating the presence of two magnetically different muon sites in the lattice. However, the two precession frequencies, which correspond to the internal magnetic fields at the two muon sites, could not be adequately explained with relatively simple ferromagnetic structures using the muon sites predicted by density functional theory calculations. The detailed analysis of the internal magnetic field suggested that the V moments align ferromagnetically along the c axis but slightly canted toward the a axis by 28 degrees that is coupled antiferromagnetically. The ordered V moment (M-v) is estimated as (0.59, 0, 1.11) mu(B). As pressure increased from ambient pressure, T-C was found to decrease slightly up to about 1.5 GPa, at which point T-C started to increase rapidly with the further increase of the pressure. Based on a strong ferromagnetic interaction along the c axis, the high-pressure mu+SR result revealed that there are two magnetic interactions in the ab plane; one is an antiferromagnetic interaction that is enhanced with pressure, mainly at pressures below 1.5 GPa, while the other is a ferromagnetic interaction that becomes predominant at pressures above 1.5 GPa.

We report a muon spin rotation and relaxation (μ+SR) study on Na0.7CoO2 powder samples, where the sodium (Na) has been intercalated via an electrochemical reaction inside a Na-ion battery. The zero field μ+SR measurement at T = 2 K shows a paramagnetic state for the as-grown sample whereas an antiferromagnetic (AF) ordered state is seen for the electrochemically cycled one. Furthermore, the temperature dependence of the muon-spin precession frequencies reveals a Néel transition temperature of TN = 22 K. The results demonstrate the importance of having high-quality homogenous samples, and put the existing NaxCoO2 magnetic phase diagram under debate.

The ferromagnetic (FM) nature of the metallic LaCo2P2 was investigated with the positive muon spin rotation, relaxation and resonance (μ+SR) technique. Transverse and zero field μ+ SR measurements revealed that the compound enters a long range FM ground state at   K, consistent with previous studies. Based on the reported FM structure, the internal magnetic field was computed at the muon sites, which were predicted with first principles calculations. The computed result agree well with the experimental data. Moreover, although LaCo2P2 is a paramagnet at higher temperatures T > 160 K, it enters a short range ordered (SRO) magnetic phase for   K. Measurements below the vicinity of   revealed that the SRO phase co-exists with the long range FM order at temperatures 124 K  . Such co-existence is an intrinsic property and may be explained by an interplay between spin and lattice degree of freedoms.

An A-type antiferromagnet, NaNiO2, was examined by means of positive muon spin rotation and relaxation (mu+SR) measurements and first-principles calculations based on a density functional theory (DFT). Below T-N = 20 K, a clear muon spin precession signal was observed in the mu+SR time spectrum recorded under zero field, due to the formation of a static internal magnetic field. The microscopic origin of such an internal field was computed as a sum of dipolar and hyperfine contact fields at the site (0.624, 0, 0.854), where both the muon site and the local spin density at such a site were predicted with DFT calculations. While the computed values were consistent with experimentally obtained ones, in both the antiferromagnetic and the paramagnetic states, the contribution of the hyperfine contact field was shown to be insignificant even below T-N. Finally, measurements at higher temperatures signified thermally activated Na-ion diffusion with E-a = 50(20) meV and D-Na(300K) = 8.8 x 10(-11) cm(2)/s, commonly observed in layered-type compounds.

The magnetic ground states in highly ordered double perovskites LaSr1-xCaxNiReO6 (x = 0.0, 0.5, 1.0) are studied in view of the Goodenough-Kanamori rules of superexchange interactions in this paper. In LaSrNiReO6, Ni and Re sublattices are found to exhibit curious magnetic states separately, but no long range magnetic ordering is achieved. The magnetic transition at similar to 255 K is identified with the independent Re sublattice magnetic ordering. Interestingly, the sublattice interactions are tuned by modifying the Ni-O-Re bond angles through Ca doping. Upon Ca doping, the Ni and Re sublattices start to display a ferrimagnetically ordered state at low temperature. The neutron powder diffraction data reveals long range ferrimagnetic ordering of the Ni and Re magnetic sublattices along the crystallographic b-axis. The transition temperature of the ferrimagnetic phase increases monotonically with increasing Ca concentration.

We report a muon spin rotation (mu+SR) study of the magnetic properties of the double perovskite compound LaSrNiReO6. Using the unique length and time scales of the mu+SR technique, we successfully clarify the magnetic ground state of LaSrNiReO6, which was previously deemed as a spin glass state. Instead, our mu+SR results point toward a long-range dynamically ordered ground state below T-C = 23 K, for which a static limit is foreseen at T = 0. Furthermore, between 23 K < T <= 300 K, three different magnetic phases are identified: a dense (23 K < T < 75 K), a dilute (75 K <= T <= 250 K), and a paramagnetic (T > 250 K) state. Our results reveal how two separate yet intertwined magnetic lattices interact within the unique double perovskite structure and the importance of using complementary experimental techniques to obtain a complete understanding of the microscopic magnetic properties of complex materials.