A. van der Bilt

Direct and Raman spin-lattice relaxations in Co2+ salts: I. Cobalt fluosilicate

Abstract Spin-lattice relaxation in concentrated and diluted CoSiF6·6H2O has been studied between 1 and 20 K. At liquid-helium temperatures the relaxation time is found to depend on the purity and the method of preparation of the sample, and on the size of the crystals (phono bottleneck). Moreover, the relaxation process cannot be characterized by a single relaxation time. A new interpolation technique enabled us to calculate direct-process spin-lattice relaxation times from the measurements. These times depend on temperature and magnetic field as theory predicts; they can be derived from the relation τ−1D = 0.12TH4 (H in kOe, T in K). Above 5 K the relaxation time has the T-9 temperature dependence of the Raman process.

Relaxation phenomena at the phase transitions in cobalt bromide dihydrate

Abstract The metamagnetic compound CoBr 2 .2H 2 O has been examined in the temperature range between 1.2 and 40 K. Longrange order of AFM sign sets in at 9.21 ±0.02 K. The complete H-T phase diagram was constructed from the field-dependent susceptibilities. The triple point, where the antiferromagnetic, ferrimagnetic and paramagnetic phases meet, occurs at 7.1±0.1 K and 17.0±0.1 kOe. Two new phase transitions are observed at liquid-helium temperature in addition to the two that are already known. An explanation for the occurence or absence of the anomalies in the X ( H ) curves is given on basis of the relaxation-time constants as measured at the phase transitions. The exchange interactions between the cobalt ions in different ferromagnetic chains are calculated, being J 1 / k =-2.77±0.02 K , J 2 / k =-1.20±0.05 K , J 3 / k =-0.02±0.05 K . Several other slight anomalies observed in the X ( H ) curves are explained considering irregular spin sites at domain boundaries.

Paramagnetic and antiferromagnetic properties of tris(ethylenediamine)nickel(II) nitrate

Abstract Crystals of uniaxial [Ni(NH 2 CH 2 CH 2 NH 2 ) 3 ](NO 3 ) 2 have been examined between 0.3 and 30 K by means of specific-heat and susceptibility measurements. Long-range antiferromagnetic order sets in at 1.25 K, with the preferred axis along the crystal c axis. The magnetic phase diagram has been determined from the field-dependent susceptibilities, the bicritical point occurring at 0.62 ± 0.01 K and 13.9 ± 0.1 kOe. Relaxation effects were observed in the frequency-dependent susceptibilities. The following quantities have also been determined: g = 2.21 ± 0.05, g = 2.24 ± 0.05, D / k = -1.00 ± 0.05 K , J / k = -0.19 K , H SF (0) = 12.8 ± 0.2 kOe , H c (0) = 24.0 ± 0.2 kOe , H cl (0) = 37.5 ± 0.2 kOe , H E = 15.4 ± 0.1 kOe , H A = 6.8 ± 0.1 kOe , and b = 9.45 JK / mol . An exchange path for this weakly anisotropic Heisenberg antiferromagnet is proposed.

Direct and Raman spin-lattice relaxation in Co2+ salts: II. Cobalt Tutton salts

Spin-lattice relaxation in some cobalt Tutton salts has been studied with the dispersion-absorption technique. Above 4 K a T9 Raman relaxation process was observed. At liquid-helium temperatures the direct-process spin-lattice relaxation time could be calculated from the observed relaxation time by taking into account the effects of phonon bottleneck and fast relaxation centres. The result for the spin-lattice relaxation time in strong external magnetic fields can be expressed as τ−1 = 0.13TH4 + 1.2 × 10−9 T9J8 (θD = 180 K) s−1 (H in kOe, T in K).

Spin-lattice relaxation at strong fields in the non-kramers system NiSiF6·6H2O

Spin-lattice relaxation times have been determined for the non-Kramers system NiSiF6·6H2O. At low temperature the direct relaxation process, hindered by a severe phonon bottleneck, is observed in strong fields (H < 100 kOe). Extending the experiments to magnetically diluted samples we obtain for the reciprocal direct relaxation time: τ-1d = (2Ak/gμB)H2T with A = (20 ± 6) × 10-9 s-1 Oe-3. At liquid-hydrogen temperatures two (different) Raman processes are recognized. At weak fields the data are satisfactorily described by the T-5 dependence of τ, characteristic for a Raman process of a multilevel system. At strong fields the time constants vary according to τ ∝ H-2 T-3.6. This behaviour of τ is ascribed to a new (field-dependent) Raman type of relaxation process, which yields a H-2 T-3 dependence of τ for a S = 1 system.

Magnetic ordering in [Ni en3] (NO3)2

Abstract The zero-field susceptibility of [Ni en3](NO3)2 indicates that this compound orders antiferromagnetically at 1.30°K. Part of the H2T phase diagram is also presented.

The observation of two additional phase transitions in the ferrimagnetic state of CoBr2⊙2H2O

Two new phase transitions are observed in CoBr2⊙2H2H at liquid helium temperatures in addition to the two that are already known. The exchange interactions are calculated and data about the phase diagram are given. An explanation for the occurrence of the extra anomalies in the χ(H) curves is given on basis of the time constants at the four phase transitions.