Structural and magnetic properties of a two-dimensional spin- 5 2 triangular lattice antiferromagnet NH4Fe(PO3F)2 are explored via x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and 31P nuclear magnetic resonance experiments on a polycrystalline sample. The compound portrays distorted triangular units of the Fe3+ ions with anisotropic bond lengths. The magnetic susceptibility shows a broad maximum around T max chi 12 K, mimicking the short-range antiferromagnetic order of a lowdimensional spin system. It shows the onset of a magnetic ordering at TN 5.7 K, setting the frustration ratio of TN 5.7. Such a value of f reflects moderate magnetic frustration in the compound. The dM/dH versus H plots of the low-temperature magnetic isotherms exhibit a sharp peak at HSF 1.45 T, indicating a field-induced spin-flop transition and a small magnetic anisotropy. Despite a tiny anisotropy, the magnetic susceptibility and NMR shift could be modeled assuming the spin-5/2 isotropic triangular lattice model and the average value of the exchange coupling is estimated to be J/kB 1.7 K. This value of the exchange coupling is reproduced well from the saturation field of the pulse field data. The rectangular shape of the 31P NMR spectra below TN unfolds that the ordering is a commensurate antiferromagnetic type. Three distinct phase regimes are clearly discerned in the H-T phase diagram, redolent of a frustrated magnet with in-plane (XY-type) anisotropy.

The ytterbium-based delafossite NaYbSe2 is discussed as a prototype for a spin-orbit entangled, effective spin-1/2 triangular spin lattice with emerging antiferromagnetic correlations and a quantum spin liquid (QSL) ground state. We report on a comprehensive study of the static and dynamic anisotropic magnetism in singlecrystalline samples of NaYbSe2 using NMR spectroscopy as a local-probe technique. We performed 23Na and 77Se NMR measurements in magnetic fields up to 16 T, applied along the in-plane and out-of-plane crystallographic directions and at temperatures from 300 down to 0.3 K. We could determine the anisotropic hyperfine contributions from the angular dependence of the 23Na and 77Se NMR spectra. In the paramagnetic regime, we probed the temperature dependence of the 23Na and 77Se spectral shift and the hyperfine coupling constants for fields applied along the principal crystal axes. The spin-lattice relaxation-rate data indicate critical spin fluctuations and the absence of long-range magnetic order at low magnetic fields and temperatures down to 0.3 K, evidenced by a monotonic increase of 1/T1 and associated spectral broadening. This is clear proof of the evolution of a critical QSL ground state with residual fluctuations down to lowest temperatures. At elevated fields, we observe the emergence of long-range order as the temperature-dependent 1/T1 rate passes through a pronounced maximum at TN at a given field, followed by a decrease at lower temperatures. Further, we find an inhomogeneous broadening of the 23Na spectra below TN, probing the histogram of the local-field distribution in the presence of the field-induced order.

We present a 31 P NMR investigation of BaCdVO(PO 4 ) 2 focusing on the nearly saturated regime between mu 0 H c 1 = 4 . 05 T and mu 0 H c 2 = 6 . 5 T, which used to be considered a promising candidate for a spin-nematic phase. NMR spectra establish the absence of any dipolar order there, whereas the weak field dependence of the magnetization above H c 1 is accounted for by Dzyaloshinskii-Moriya interaction terms. The low -energy spin dynamics (fluctuations), measured by the nuclear spin -lattice relaxation rate T 1 - 1 , confirms the continuity of this phase and the absence of any low -temperature phase transition. Unexpectedly, the spin dynamics above H c 1 is largely dominated by two-magnon processes, which is expected above the saturation field of a spin-nematic phase, but not inside. This shows that BaCdVO(PO 4 ) 2 is indeed close to a spin-nematic instability; however, this phase is not stabilized. We thus confirm recent theoretical predictions that the spin-nematic phase can be stabilized, at most, in an extremely narrow field range close to saturation or is rather narrowly avoided [Jiang et al. , Phys. Rev. Lett. 130 , 116701 (2023)].

We probe the magnetic field -induced Tomonaga-Luttinger liquid (TLL) state in the bond -alternating spin1/2 antiferromagnetic (AFM) chain compound NaVOPO4 using thermodynamic as well as local mu SR and P-31 NMR probes down to mK temperatures in magnetic fields up to 14 T. The mu SR and NMR relaxation rates in the gapless TLL regime decay slowly following characteristic power -law behavior, enabling us to directly determine the interaction parameter K as a function of the magnetic field. These estimates are crosschecked using magnetization and specific heat data. The field -dependent K lies in the range of 0.4 < K < 1 and indicates the repulsive nature of interactions between the spinless fermions, in line with the theoretical predictions. This renders NaVOPO4 the first experimental realization of TLL with repulsive fermionic interactions in hitherto studied S = 1/2 bond -alternating AFM-AFM chain compounds.

Nuclear magnetic resonance (NMR) techniques have been used to study the static and dynamic microscopic properties of the Weyl semimetal NbP. From a complete analysis of the angular dependence of the Nb-93-NMR spectra in a single crystal, the parameters for the electric quadrupole interactions and the magnetic hyperfine interactions were determined to be nu(Q) = 0.61 MHz, eta = 0.20, (K-XX, K-YY, K-ZZ)=(-0.06,0.11,-0.11)% at 4.5 K. The temperature and field dependence of the Nb-93 Knight shift revealed a characteristic feature of the shape of the density of states with nearly massless fermions. We clearly observed a quantum oscillation of the Knight shift associated with the band structure, whose frequency was in good agreement with the previous bulk measurements. The temperature dependence of the spin-lattice relaxation rate, 1/T1T, showed an almost constant behavior for 30 < T < 180 K, while a weak temperature dependence was observed below similar to 30 K. This contrasts with the behavior observed in TaP and TaAs, where the 1/T1T measured by the Ta-181 nuclear quadrupole resonance (NQR) shows 1/T1T proportional to T-2 and T-4 above approximately 30 K. In TaP, the temperature dependent orbital hyperfine interaction plays a significant role in nuclear relaxation, whereas this contribution is not observed in TaAs. Two-component spin echo oscillations were observed. The shorter-period oscillation is attributed to the origin of quadrupole coupling, while the longer-period oscillation indicates the presence of indirect nuclear spin-spin coupling, as discussed in other Weyl semimetal like TaP.

We present a ^31P nuclear magnetic resonance (NMR) investigation of BaCdVO(PO_4)_2 focusing on the nearly saturated regime between μ_0H_c1 = 4.05 T and μ_0H_c2 = 6.5 T, used to be considered as a promising candidate for a spin-nematic phase. NMR spectra establish the absence of any dipolar order there, whereas the weak field dependence of the magnetization above H_c1 is accounted for by Dzyaloshinskii-Moriya interaction terms. The low-energy spin dynamics (fluctuations), measured by nuclear spin-lattice relaxation rate (T_1^-1), confirms the continuity of this phase and the absence of any low-temperature phase transition. Unexpectedly, the spin dynamics above H_c1 is largely dominated by two-magnon processes, which is expected above the saturation field of a spin-nematic phase, but not inside. This shows that BaCdVO(PO_4)_2 is indeed close to a spin-nematic instability, however, this phase is not stabilized. We thus confirm recent theoretical predictions that the spin-nematic phase can be stabilized, at most, in an extremely narrow field range close to saturation or is rather narrowly avoided [Jiang et al., Phys. Rev. Lett. 130, 116701 (2023)].

Attaining milli-Kelvin temperatures is often a prerequisite for the study of novel quantum phenomena and the operation of quantum devices. Adiabatic demagnetization refrigeration (ADR) is an effective, easy and sustainable alternative to evaporation or dilution cooling with the rare and super-expensive ^3He. Paramagnetic salts, traditionally used for mK-ADR, suffer from chemical instability related to water of crystallization. We report synthesis, characterization as well as low-temperature magnetization and specific heat measurements of two new UHV compatible candidate materials NaYbP_2O_7 and KYbP_2O_7. Utilizing the PPMS at 2 K, the ADR of sintered pellets with Ag powder admixture starting at 5 T yields base temperatures (warm-up times) of 45 mK (55 min) and 37 mK (35 min) for NaYbP_2O_7 and KYbP_2O_7, respectively, slightly advantageous to KBaYb(BO_3)_2 (45 mK and 40 min) studied under similar conditions.

The mixed-anion compound with composition Sr2VO3Cl has been synthesized for the first time, using the conventional high-temperature solid-state synthesis technique in a closed silica ampule under inert conditions. This compound belongs to the known Sr2 TmO3Cl (Tm = Sc, Mn, Fe, Co, Ni) family, but with Tm = V. All homologues within this family can be described with the tetragonal space group P4/nmm (No. 129); from a Rietveld refinement of powder X-ray diffraction data on the Tm = V homologue, the unit cell parameters were determined to a = 3.95974(8) and c = 14.0660(4) Å, and the atomic parameters in the crystal structure could be estimated. The synthesized powder is black, implying that the compound is a semiconductor. The magnetic investigations suggest that Sr2VO3Cl is a paramagnet at high temperatures, exhibiting a μeff = 2.0 μB V-1 and antiferromagnetic (AFM) interactions between the magnetic vanadium spins (θCW = -50 K), in line with the V-O-V advantageous super-exchange paths in the V-O layers. Specific heat capacity studies indicate two small anomalies around 5 and 35 K, which however are not associated with long-range magnetic ordering. 35Cl ss-NMR investigations suggest a slow spin freezing below 4.2 K resulting in a glassy-like spin ground state.

We report the synthesis, characterization, low-temperature magnetic, and thermodynamic measurements of the millikelvin adiabatic demagnetization refrigeration (mK-ADR) candidate material NaYbGeO4 which exhibits a distorted square lattice arrangement of YbO6 magnetic units. Magnetization and specific heat indicate weakly interacting effective spin-1/2 moments below 10 K, with a Curie-Weiss temperature of only 15 mK, that can be polarized by magnetic fields of order 1 T. For the ADR performance test, we start the demagnetization from 5 T at a temperature of -2 K and reach a minimum temperature of 150 mK at zero field. The warming curve indicates a sharp magnetic transition in the heat capacity at 210 mK, implying only weak magnetic frustration. The entropy density of SGS <^> 101 mJ K-1cm-3 and hold time below 2 K of 220 min are competitive while the minimal temperature is higher compared to frustrated Ytterbium-oxide ADR materials studied under similar conditions.

We report the synthesis, characterization, low-temperature magnetic, and thermodynamic measurements of the novel milli-Kelvin adiabatic demagnetization refrigeration (mK-ADR) candidate material NaYbGeO$_4$ which exhibits a distorted square lattice arrangement of YbO$_{6}$ magnetic units. Magnetization and specific heat indicate weakly interacting effective spin-1/2 moments below 10~K, with a Curie-Weiss temperature of only 15~mK, that can be polarized by magnetic fields of order 1~T. For the ADR performance test, we start the demagnetization from 5~T at a temperature of $\sim 2$~K and reach a minimum temperature of 150~mK at zero field. The warming curve indicates a sharp magnetic transition in the heat capacity at 210~mK, implying only weak magnetic frustration. The entropy density of $S_{\rm GS}\simeq 101$ mJ K$ ^{-1}$cm$^{-3}$ and hold time below 2~K of 220~min are competitive while the minimal temperature is higher compared to frustrated Ytterbium-oxide ADR materials studied under similar conditions.

Attaining milli-Kelvin temperatures is often a prerequisite for the study of novel quantum phenomena and the operation of quantum devices. Adiabatic demagnetization refrigeration (ADR) is an effective, easy and sustainable alternative to evaporation or dilution cooling with the rare and super-expensive $^3$He. Paramagnetic salts, traditionally used for mK-ADR, suffer from chemical instability related to water of crystallization. We report synthesis, characterization as well as low-temperature magnetization and specific heat measurements of two new UHV compatible candidate materials NaYbP$_2$O$_7$ and KYbP$_2$O$_7$. Utilizing the PPMS at 2 K, the ADR of sintered pellets with Ag powder admixture starting at 5 T yields base temperatures (warm-up times) of 45 mK (55 min) and 37 mK (35 min) for NaYbP$_2$O$_7$ and KYbP$_2$O$_7$, respectively, slightly advantageous to KBaYb(BO$_3$)$_2$ (45 mK and 40 min) studied under similar conditions.

We probe the magnetic field-induced Tomonaga-Luttinger liquid (TLL) state in the bond-alternating spin-$1/2$ antiferromagnetic (AFM) chain compound NaVOPO$_4$ using thermodynamic as well as local $\mu$SR and $^{31}$P NMR probes down to milli-K temperatures in magnetic fields up to 14~T. The $\mu$SR and NMR relaxation rates in the gapless TLL regime decay slowly following characteristic power-law behaviour, enabling us to directly determine the interaction parameter $K$ as a function of the magnetic field. These estimates are cross-checked using magnetization and specific heat data. The field-dependent $K$ lies in the range of $0.4 < K < 1$ and indicates repulsive nature of interactions between the spinless fermions, in line with the theoretical predictions. This renders NaVOPO$_4$ the first experimental realization of TLL with repulsive fermionic interactions in hitherto studied $S=1/2$ bond-alternating AFM-AFM chain compounds.

Using 31 P nuclear magnetic resonance (NMR) we investigate the recently discovered presaturation phase in the highly frustrated two-dimensional spin system SrZnVO(PO 4 ) 2 [F. Landolt et al. Phys. Rev. B 104 , 224435 (2021)]. Our data provide two pieces of evidence against the presumed spin-nematic character of this phase: i) NMR spectra reveal that it hosts a dipolar spin order and ii) the T − 1 1 relaxation rate data recorded above the saturation field can be fitted by the sum of a single-magnon term, exponential in the gap, and a critical second-order term, exponential in the triple gap, leaving no space for a nematic spin dynamics, characterized by a double-gap exponential. We explain the unexpectedly broad validity of the simple fit and the related critical spin dynamics.

We have studied the microscopic magnetic properties, the nature of the 130-K phase transition, and the ground state in the recently synthesized compound Ce$_2$Rh$_2$Ga by use of $^{69,71}$Ga nuclear quadrupole resonance (NQR). The NQR spectra clearly show an unusual phase transition at $T_t$ $\sim$ 130 K yielding a splitting of the high-temperature single NQR line into two clearly resolved NQR lines, providing evidence for two crystallographically inequivalent Ga sites. The NQR frequencies are in good agreement with fully-relativistic calculations of the band structure. Our NQR results indicate the absence of magnetic or charge order down to 0.3 K. The temperature dependence of the spin-lattice relaxation rate, 1/$T_1$, shows three distinct regimes, with onset temperatures at $T_t$ and 2 K. The temperature-independent 1/$T_1$, observed between $T_t$ and 2 K, crosses over to a Korringa process, 1/$T_1$ $\propto$ $T$, below $\sim$ 2 K, which evidences a rare two-ion Kondo scenario: the system goes into a dense Kondo coherent state at 2.0 and 0.8 K for the two different Ga sites.

We have studied the microscopic magnetic properties, the nature of the 130 K phase transition, and the ground state in the recently synthesized compound Ce2Rh2Ga by use of Ga-69(,)71 nuclear quadrupole resonance (NQR). The NQR spectra clearly show an unusual phase transition at T-t similar to 130 K, yielding a splitting of the high-temperature single NQR line into two well-resolved NQR lines, providing evidence for two crystallographically inequivalent Ga sites. The NQR frequencies are in good agreement with fully relativistic calculations of the band structure. Our NQR results indicate the absence of magnetic or charge order down to 0.3 K. The temperature dependence of the spin-lattice relaxation rate 1/T-1 shows three distinct regimes, with onset temperatures at T-t and 2 K. The temperature-independent 1/T-1, observed between T-t and 2 K, crosses over to a Korringa process, 1/T-1 proportional to T, below similar to 2 K, which evidences a rare two-ion Kondo scenario: The system evolves into a dense Kondo coherent state below 2.0 and 0.8 K probed by the two different Ga sites.

While the Heisenberg model for magnetic Mott insulators on planar lattice structures is comparatively well understood in the case of transition metal ions, the intrinsic spin-orbit entanglement of 4f magnetic ions on such lattices shows fascinating new physics largely due to corresponding strong anisotropies both in their single-ion and their exchange properties. We show here that the Yb delafossites, containing perfect magnetic Yb$^{3+}$ triangular lattice planes with pseudospin $s=1/2$ at low temperatures, are an ideal platform to study these new phenomena. Competing frustrated interactions may lead to an absence of magnetic order associated to a gapless spin liquid ground state with a huge linear specific heat exceeding that of many heavy fermions, whereas the application of a magnetic field induces anisotropic magnetic order with successive transitions into different long ranged ordered structures. In this comparative study, we discuss our experimental findings in terms of a unified crystal-field and exchange model. We combine electron paramagnetic resonance (EPR) experiments and results from neutron scattering with measurements of the magnetic susceptibility, isothermal magnetization up to full polarization, and specific heat to determine the relevant model parameters. The impact of the crystal field is discussed as well as the symmetry-compatible form of the exchange tensor, and we give explicit expressions for the anisotropic g factor, the temperature dependence of the susceptibility, the exchange-narrowed EPR linewidth and the saturation field.

Magnetism of the spin-$\frac12$ $\alpha$-KVOPO$_4$ is studied by thermodynamic measurements, $^{31}$P nuclear magnetic resonance (NMR), neutron diffraction, and density-functional band-structure calculations. Ferromagnetic Curie-Weiss temperature of $\theta_{\rm CW}\simeq 15.9$ K and the saturation field of $\mu_0H_s\simeq 11.3$ T suggest the predominant ferromagnetic coupling augmented by a weaker antiferromagnetic exchange that leads to a short-range order below 5 K and the long-range antiferromagnetic order below $T_{\rm N}\simeq 2.7$ K in zero field. Magnetic structure with the propagation vector $\mathbf k=(0,\frac12,0)$ and the ordered magnetic moment of 0.58 $\mu_B$ at 1.5 K exposes a non-trivial spin lattice where strong ferromagnetic dimers are coupled antiferromagnetically. The reduction in the ordered magnetic moment with respect to the classical value (1 $\mu_{\rm B}$) indicates sizable quantum fluctuations in this setting, despite the predominance of ferromagnetic exchange. We interpret this tendency toward ferromagnetism as arising from the effective orbital order in the folded chains of the VO$_6$ octahedra.

Single crystals of the hexagonal triangular lattice compound AgCrSe2 have been grown by chemical vapor transport. The crystals have been carefully characterized and studied by magnetic susceptibility, magnetization, specific heat, and thermal expansion. In addition, we used Cr-electron spin resonance and neutron diffraction to probe the Cr 3d(3) magnetism microscopically. To obtain the electronic density of states, we employed x-ray absorption and resonant photoemission spectroscopy in combination with density functional theory calculations. Our studies evidence an anisotropic magnetic order below T-N = 32 K. Susceptibility data in small fields of about 1 T reveal an antiferromagnetic (AFM) type of order for H perpendicular to c, whereas for H parallel to c the data are reminiscent of a field-induced ferromagnetic (FM) structure. At low temperatures and for H perpendicular to c, the field-dependent magnetization and AC susceptibility data evidence a metamagnetic transition at H+ = 5 T, which is absent for H parallel to c. We assign this to a transition from a planar cycloidal spin structure at low fields to a planar fanlike arrangement above H+. A fully ferromagnetically polarized state is obtained above the saturation field of H-perpendicular to S = 23.7 T at 2K with a magnetization of M-s = 2.8 mu(B)/Cr. For H parallel to c, M(H) monotonically increases and saturates at the same M-s value at H-parallel to S = 25.1 T at 4.2 K. Above T-N, the magnetic susceptibility and specific heat indicate signatures of two dimensional (2D) frustration related to the presence of planar ferromagnetic and antiferromagnetic exchange interactions. We found a pronounced nearly isotropic maximum in both properties at about T* = 45 K, which is a clear fingerprint of short range correlations and emergent spin fluctuations. Calculations based on a planar 2D Heisenberg model support our experimental findings and suggest a predominant FM exchange among nearest and AFM exchange among third-nearest neighbors. Only a minor contribution might be assigned to the antisymmetric Dzyaloshinskii-Moriya interaction possibly related to the noncentrosymmetric polar space group R3m. Due to these competing interactions, the magnetism in AgCrSe2, in contrast to the oxygen-based delafossites, can be tuned by relatively small, experimentally accessible magnetic fields, allowing us to establish the complete anisotropic magnetic H-T phase diagram in detail.

This study focuses on the control of exchange bias due to the modification of antiferromagnet and ferromagnet-antiferromagnet interface. Thin films of Ni-NiO were developed by thermal evaporation and subsequent annealing to realize a ferromagnet-antiferromagnet interface in which NiO is an antiferromagnet. The thickness and composition of individual magnetic layers were measured and optimized using Rutherford backscattering spectrometry. Ion implantation in these films is carried out using 100 keV and 200 keV Cu ions such that ions get deposited in the NiO layer and at Ni-NiO interface. This method offers a unique possibility to study the influence of different magnetic layers/interface on exchange bias. The experimental results show an initial enhancement followed by a decrease in exchange bias with ion fluence using 100 keV Cu ions. Interestingly, modifications in exchange bias are found to be more prominent in films where Cu ions are implanted in the NiO layer rather than in the Ni-NiO interface.

Here, we report the synthesis and magnetic properties of a Yb-based triangular-lattice compound LiYbS$_2$. At low temperatures, it features an effective spin-$\frac{1}{2}$ state due to the combined effect of crystal electric field and spin orbit coupling. Magnetic susceptibility measurements and $^7$Li nuclear magnetic resonance experiments reveal the absence of magnetic long range ordering down to 2~K, which suggests a possible quantum spin liquid ground state. A dominant antiferromagnetic nearest neighbour exchange interaction $J/k_{\rm B}\simeq$ 5.3~K could be extracted form the magnetic susceptibility. The NMR linewidth analysis yields the coupling constant between the Li nuclei and Yb$^{3+}$ ions which was found to be purely dipolar in nature.