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The magnetic properties of the MnRuxRh1-xAs system with x = 0.05, 0.10 were studied under hydrostatic pressure up to 1.2 GPa in the 80–420 K temperature range. The (p,T) magnetic phase diagram was determined. The ferromagnetic component in an intermediate (AF2+f) domain was stabilized with pressure for both samples, and the critical field variation versus pressure was determined. In addition, dc magnetization measurements were performed in magnetic fields up to 10 T, and the magnetic field–induced phase transitions were pointed out. The reverse and conventional magnetocaloric effect for both compounds was analyzed. Electronic band structure computations for the MnRuAs and MnRhAs parent compounds were performed by using the Korringa–Kohn-Rostoker method. The results indicate that significant changes of electronic states in the vicinity of EF on the application of external pressure tend to favor a ferromagnetic-type ordering of the Mn magnetic moments.

The character of thermal diffusion of magnetic atoms has been investigated in Ni–Mn mechanical alloys around Ni3Mn composition by monitoring magnetic properties during the atomic ordering process. Observed susceptibility versus temperature curve after annealed at the ordering temperature of 693 K for 50 h shows at least four steps. Those multi steps came into two at the final stage of annealing for 1000 h. The step at the highest temperature corresponds to the Curie temperature TC of Ni3Mn. The observed variation of TC with annealed time was explained by solving a diffusion equation, and the range of the concentration fluctuation was determined to be 4 nm.

Both series of MnFe0.35−xCo0.65NixP and MnFe0.35Co0.65−xNixP orthorhombic compounds were analyzed for their structural characteristics versus x and versus temperature, as well for the changes induced to the magnetic behaviors (ordering temperature, saturation magnetization, magnetic entropy change). The operated Ni substitutions lead to decrease the Curie point closer to room temperature, especially when Ni substitutes to Co. However, in both systems, the saturation magnetization evolves unexpectedly. Even if remaining rather weak, the corresponding changes of magneto-caloric entropy at the transition are significantly increased, being more than twice and three times the initial value for Ni substituted to Fe and to Co respectively. Chemical and topological arguments are then proposed to discuss the noticeable difference of behaviors observed between the hexagonal (e.g. MnFeP1−xAsx) and the orthorhombic (e.g. MnFe1−xCoxP) series of polytype pnictides.

For the (Fe 1-x Ni x ) 2 P compounds with x = 0.015, 0.02, 0.025 and 0.25, both X-ray diffraction and X ac susceptibility experiments under pressure up to 1.5 GPa were performed from 80 to 420 K. For all the studied compounds, a clear cut metal atom distribution was found on the basis of the Fe 2 P structure-type (SG: P62m), the 3g sites being fully occupied by Fe, while both Fe and Ni are randomly distributed on the 3f sites. The variation of the ordering temperature under pressure ∂T C /∂p increases from negative values for all studied compounds up to zero for the Ni-rich one (Fe 0.75 Ni 0.25 ) 2 P. Additionally for the latter, neutron diffraction experiments were carried out in the 10-290 K range to characterise the transformation between the ferromagnetic (F) and paramagnetic (P) states. An electronic structure analysis of the (Fe 1-x Ni x ) 2 P compounds performed using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) shows that the total energy calculation fully supports the Ni preference for the 3f occupancy. Accounting to this trend, the calculated total magnetic moment was found to decrease almost linearly with x, in very fair agreement with the experimental data when measured for x < 0.25. However, the theoretical values of local magnetic moments computed in (Fe 0.75 Ni 0.25 ) 2 P remain in satisfying agreement with the results of neutron diffraction. Hence, it is suggested that the strong electron polarisation at E F (P ∼ 90%) as established theoretically for the (Fe 1-x Ni x ) 2 P compounds having the smallest Ni content, can be responsible for the marked magneto-elastic phase transition as observed, e.g. in (Fe 0.975 Ni 0.025 ) 2 P.

The magnetic properties and their pressure dependence in Fe 2 P-type quasi-ternary intermetallic compounds Mn(Rh 1− x Co x )As were investigated up to 8 GPa. The low temperature antiferromagnetic state AF(I) disappeared for x >0.2 and a new phase (Phase I) appeared with a small magnetic moment. The transition temperature T t2 from this phase to the canted state and the Curie temperature T c remained almost constant up to x =0.7. The pressure dependence of T t2 remained constant up to 3.5 GPa, and then Phase I disappeared above 4 GPa. On the other hand, T c increased with increasing pressure with a rate of 8 K/GPa up to 2–3 GPa and then with 22–25 K/GPa. This large increase in rate can be explained by considering an RKKY-type interaction between Mn–Mn atoms along the c -axis.

Measurements of detailed magnetization curves have been made in a hexagonal Fe 2 P-type ternary intermetallic compound, MnRhP around the Curie temperature by using a sample-subtracting type magnetometer. For the critical exponents β =0.43 and δ =2.67 were obtained. Though this material is called nearly two-dimensional ferromagnet, the temperature dependence of the spontaneous magnetization determined from Arrott plots has no two-dimensional character. This tendency is different from the case of Cu 2 Sb-type intermetallic compounds, MnZnSb and MnGaGe in which two-dimensional character was observed in β .

We present new results on the crystal structure, magnetic and electronic properties of the Co x Fe 1− x MnP series of compounds. X-ray diffraction experiments were performed for x =0.525, x =0.65 and x =0.70. A jump both of the lattice parameters and the unit cell volume was evidenced at the metamagnetic phase transition between antiferromagnetic and ferromagnetic states. Moreover the ( P , T ) magnetic phase diagrams for x =0.45, x =0.50 and x =0.525 were determined. Mossbauer spectroscopy experiments were performed for several x contents allowing to determine the hyperfine field, the isomer shift and the quadrupole splitting of the iron sites. Spin-polarised electronic structure calculations were performed using the Korringa–Kohn–Rostoker (KKR) method both for CoMnP and FeMnP, in the ferromagnetic state. The calculated local magnetic moments are 0.25 μ B (Co), 2.90 μ B (Mn) and 0.83 μ B (Fe), 2.89 μ B (Mn) in CoMnP and FeMnP, respectively. The electronic structure of the Co x Fe 1− x MnP solid solutions ( x =0.525 and x =0.70) were calculated using the coherent potential approximation (CPA). The calculated values of the magnetic moments and hyperfine fields are compared with the experimental ones.

Pressure-induced structural transitions in MnRhP and MnRhAs were studied by in situ angle-dispersive X-ray diffraction using synchrotron radiation and a diamond anvil cell. A transition from a hexagonal Fe 2 P-type structure to an orthorhombic one was observed in MnRhP at a pressure of approximately 34 GPa. The first-nearest neighboring Mn–Mn interatomic distance at this transition pressure is close to the distance at which the sign of the effective exchange interaction coefficient between Mn atoms turns from positive to negative. A structural phase transition was also observed in MnRhAs at around 26 GPa, although the crystal system of the high-pressure phase was undetermined.

Measurements of detailed magnetization curves have been made in an orthorhombic TbFeSi2-type compound, LaMnSi2 around the Curie temperature by using a sample-subtracting type magnetometer. For the critical exponents, β = 0.33 and δ = 3.02 were obtained. Though this material is called nearly two-dimensional ferromagnet, the temperature dependence of the spontaneous magnetization determined from Arrott plots has no two-dimensional character. This tendency is different from the case of Cu2Sb-type intermetallic compounds, MnZnSb and MnGaGe in which two-dimensional character was observed in β.

The results of AC magnetic susceptibility measurements of zinc ferrite (Fe3−xZnxO4, x = 0, 0.0072, 0.0174, 0.049) single crystalline samples under pressure up to 1.2 GPa in the temperature range close to the Verwey transition temperature (TV) are reported. The isotropy point TIP increases with pressure for each x but the disappearance of primary anisotropy constant at TIP has no effect on Mossbauer spectrum at ambient pressure. Finally, Mossbauer effect data in the vicinity of the Verwey transition does not support the recent claim of the inverse to normal spinel crossover at the transition.

Summary The crystallographic and magnetic properties of Fe 2 P have been studied by neutron diffraction experiments, magnetization measurements and KKR method for band structure calculations. The measured atomic magnetic moments of tetrahedral Fe(3f) and pyramidal Fe(3g) sites are respectively (0.59±0.02)μ B and (2.22±0.01)μ B , resulting in a net moment of (2.82±0.03)μ B . This value is very close to that obtained from saturation magnetization measurements (H=150kOe, T=4K) which yield 2.87μ B per formula unit. The results obtained from the KKR method are 0.80, 2.33, −0.03(1), −0.02(8)μ B for Fe and P sites respectively. The values are comparable with the measured atomic magnetic moments. Thermal evolution of magnetic moments, magnetization at saturation and cell parameters presents a sharp drop at the Curie temperature (217±2)K indicating a first order ferro - paramagnetic transition.

The Ce 2 Fe 17 H x ternary hydrides with x = 0.8 and 2 have been studied under external pressures up to 1.2 GPa in order to check the stability of H-induced ferromagnetism. The resulting T vs P phase diagrams allow valuable comparisons with the x vs. T ones if hydrogen ( x ) is assumed to act as a negative pressure tool.

In both the series of ternary d-metal pnictides MnFeP1 − xAsx and MnRhP1 − xAsx the magnetic properties appear to be very sensitive to external parameters (temperature, pressure, magnetic field) and chemical composition. An extended analysis of previous experimental results is discussed in relation to the magneto-elastic phase transition in view of band structure calculations. The principal trends of these experiments are presented here.

The magnetization at high magnetic field and the a.c. magnetic susceptibility of RAgSn (RNd, Tb and Ho), TbPdSn and TbAgGa polycrystalline samples have been measured. The temperature dependence of the magnetic susceptibility indicates an antiferromagnetic order at low temperatures. The magnetization curves for all compounds show a multistep metamagnetic process in an ordered state.

Powder X-ray diffraction measurements on intermetallic compounds RERh2Si2 (RE = Gd, Tb) have been made in the temperature range of 78 to 300K. Below TN both samples show significant volume expansion with the coefficient δV V being as large as 5.54·10-4 and 6.2·10-4 for RE = Gd, Tb, respectively. The Neel temperature decreases with increasing applied pressure in GdRh2Si2, but it remains constant for TbRh2Si2.