Pressure-induced Magnetic Phase Transition Research Articles

Pressure-induced magnetic phase transition in the two-dimensional Heisenberg ferromagnet K 2CuF 4

A pressure-induced magnetic phase transition in K 2CuF 4 was investigated up to 13 GPa. We observed a sudden decrease in the Curie temperature starting from 7 GPa and a large decrease in the susceptibility above 10 GPa. High-pressure powder X-ray diffraction studies were also made at room temperature up to 14 GPa to investigate such a drastic change in magnetism from structural point of view. A structural phase transition was observed at around 9.5 GPa, above which the CuF 6 octahedra were found to be in the ferrodistortive (FD) order with the elongated axes in the basal plane. As magnetic and structural changes occur at nearly the same pressure, the observed change in the magnetism suggests a ferromagnetic-to-antiferromagnetic phase transition resulting from the distortive change in the basal plane if the critical pressure is assumed not to change so much in the temperature range investigated.

Raman study of pressure-induced magnetic phase transition in EuSe

Raman spectra of EuSe have been measured under hydrostatic pressure up to 2.1 GPa at 2 K by using a diamond anvil. With increasing pressure, distinct spectral changes occur twice, which can be interpreted in terms of successive magnetic phase transition: ferri → antiferro (AF-I) → ferro. The Raman peaks are attributed to phonons at special points in the Brillouin zone taking account of the folding effect on the phonon dispersion. This makes it possible to draw overall features of the dispersion curve of LO phonon along the [111] direction under high pressure.

Theory of pressure-induced magnetic and metal-insulator transitions.

A theory of pressure-induced simultaneous metal-insulator and magnetic phase transitions is presented. It is based on a model consisting of a periodic lattice with (1) an itinerant-electron band, (2) a narrow dispersionless band (localized states), (3) a hybridization term, (4) a very strong short-ranged repulsion between electrons of opposite spin in the localized states, and (5) a moderately strong short-ranged repulsion between electrons in the itinerant and localized bands. The problem is treated in the Hartree-Fock approximation, allowing spontaneous ferromagnetic broken symmetry. Pressure, which mainly changes the energy separation between the two bands, induces transitions from insulating, (ferro)magnetic states, to metallic states with no localized moments. The transition may be continuous or discontinuous, depending on the values of the interaction parameters. A richly structured phase diagram is obtained. For some values of the parameters the model reproduces in a reasonable fashion and for T\ensuremath{\rightarrow}0 the complex transition recently found in ${\mathrm{NiI}}_{2}$ at pressures of about 19 GPa.

A pressure-induced magnetic phase transition in Y 2Fe 17 intermetallic compound

The results of the investigation of the influence of pressure on the magnetic properties of Y 2Fe 17 intermetallic compound are reported. The Curie temperature is established as T c=320 K and its pressure derivative as d T c/d p=−47K/GPa. Th action of the hydrostatic pressure is found to produce a new magnetic phase. A Ginzburg-Landau functional is used to describe the origin of the non-collinear magnetic structure. The field-induced volume magnetostriction is calculated on the basis of the experimental data obtained.

Pressure-induced structural and magnetic phase transitions in CeZn.

Neutron-diffraction studies with pressures up to 12 kbar have been performed on a CeZn single crystal. At 12 kbar, a pressure-induced ferromagnetic phase with a Ce moment of 1.6{mu}{sub {ital B}}/atom exists below {Tc}=12 K and a cubic to rhombohedral transition occurs at {ital T}{sub {ital M}}=60 K. Below 8 kbar, a collinear (00{pi}) antiferromagnetic structure is stable below {ital T}{sub {ital N}}{approximately}30 K, where a cubic-to-tetragonal transition occurs. Moreover, in the antiferromagnetic region, an application of pressure induces a tetragonal I ({ital c}/{ital a}{gt}1)--to--tetragonal II ({ital c}/{ital a}{lt}1) phase transition. At the boundary between antiferromagnetism and ferromagnetism around {ital p}=10.6 kbar, a mixed phase or a ferrimagnetic phase appears below {ital T}{sub {ital c}} with a complex structure which is a coexistent with both the tetragonal and rhombohedral distortions.

Pressure-induced magnetic phase transition in strong itinerant ferromagnet Fe 3Pt

We have investigated the magnetic properties of strong itinerant ferromagnetic Fe 72Pt 28 (ordered and disordered phases) at pressures up to 8 GPa and at various temperatures (300 to 4.2 K) using the 57Fe high pressure Mössbauer effect technique. We observe (in both cases) for the first time a pressure-induced magnetic phase transition from the ferromagnetic (FM) high-moment state to a low-moment magnetic state. The result is consistent with recent theoretical calculations of the volume dependence of the Fe moment in FM pure fcc Fe. This observation clearly demonstrates a unique physical picture of the ferromagnetism in 3d ferromagnetic systems that exhibit large magnetovolume anomalies. The effect of atomic ordering on the stability of the FM ground state is also discussed.

Magnetism at high-pressure in fcc Fe-based alloys

We present new studies of the pressure effect on the stability of the ferromagnetic (FM) state in fcc Fe-based alloys up to 8 GPa and at different temperatures (300-2 K) using the 57Fe high pressure Mössbauer technique. The selected systems include weak (Fe68.3Ni31.5) and strong (Fe72Pt28) itinerant ferromagnets. We find in both weak and strong ferromagnets a pressure-induced magnetic phase transition from the FM, high-spin (HS) state to a low-spin (LS) state at a critical pressure. The existence of two energetically close lying magnetic states (HS and LS) reveals a unique physical picture of ferromagnetism in fcc Fe-based systems that exhibit large magnetovolume effects. The results are compared with spin polarised band structure calculation of fcc Fe. Differences between the behavior of weak and strong ferromagnets are also discussed.

Pressure-induced antiferromagnetism in fcc Fe-Ni Invar alloys

We report the experimental observation of a pressure-induced magnetic phase transition in the ferromagnetic fcc Fe 68.5Ni 31.5 Invar alloy. It is shown that above a critical pressure of 5.8 GPa the system displays antiferromagnetic order at low temperatures. The Néel temperature decreases on further increasing the pressure. The experimental results are compared with several related theoretical models.

Pressure-induced magnetic phase transition (spin glass → Kondo) in [formula omitted]Ce alloys

In most spin glass systems the freezing temperature T o lies well above the Kondo temperature T K. The relatively rapid increase of T K caused by pressure should lead to a spin glass to Kondo phase transition as predicted by Larsen. Evidence for such a transition has been found in high pressure resistivity measurement on La Ce alloys.