J. S. Kouvel

Anomalous Magnetic Moments and Transformations in the Ordered Alloy FeRh

Magnetization and electrical resistivity measurements on an iron‐rhodium alloy of approximate composition FeRh, having CsCl‐type structure, confirm recent x‐ray and neutron diffraction evidence for a first‐order antiferromagnetic‐ferromagnetic transition at about 350°K. From 350°K down to 77°K, an essentially constant, field‐independent susceptibility of about 1×10−4 emu/g is observed. Above 350°K, the alloy behaves like a normal ferromagnet with a Curie point of 675°K and a saturation magnetization (extrapolated to 0°K) of 130 emu/g. This σ0 value is equivalent to an average atomic moment of 1.85 μB, or to 3.85 μB per iron atom if the rhodium moment is assumed to be zero. Alternatively, if the iron moment is taken to be 2.2 μB, the rhodium moment has to be about 1.5 μB. Qualitative results are given for the effect of high magnetic fields and pressures on the first‐order transition temperature of this alloy.

Unusual Nature of the Abrupt Magnetic Transition in FeRh and Its Pseudobinary Variants

Previous measurements of the critical temperature (Tcrit) for the first‐order antiferromagnetic‐ferromagnetic transition in FeRh as a function of magnetic field have established that the total entropy change at the transition (ΔS) is much larger than the estimated change in lattice entropy (ΔS)lat. This study has now been extended to pseudobinary variants of FeRh where the Rh is partially replaced by Pd, Pt, or Ir, resulting in a large decrease or increase of Tcrit from its value (about 330°K) for FeRh. In each case, a value for ΔS was deduced from the measured field dependence of Tcrit. The difference between each ΔS and an estimated (ΔS)lat value, when plotted vs Tcrit, defines a smooth curve with a maximum at about 500°K, which is just below the Curie points of these alloys. It is therefore concluded that ΔS−(ΔS)lat represents an entropy change of magnetic origin. This anomalous change in magnetic entropy is attributed to thermal excitation of the Rh moments which in the ferromagnetic state are induced...

Magnetization of Iron‐Nickel Alloys Under Hydrostatic Pressure

Magnetization measurements were made on iron, nickel, and their alloys in hydrostatic pressures up to 3000 atm and in fields up to 12 000 oe. The results of these room‐temperature measurements, combined with Patricks results on the pressure dependence of the Curie temperatures, indicate that σ0, the saturation moment at 0°K, as well as θ, the Curie temperature, is rapidly decreased by compression in the fcc alloys of about 30% Ni. This is in agreement with the recent low‐temperature pressure measurements of Kondorsky and Sedov. For these alloys, both σ0 and θ are decreasing with increasing Fe concentration. When the 30% Ni alloy is partially transformed to bcc, its σ0 and θ rise to much higher values but its magnetization is much less pressure dependent. It is concluded that unlike the more simple ferromagnetic bcc alloys, the fcc Fe‐Ni alloys have a coexistence of ferromagnetic and antiferromagnetic order, similar to that recently proposed for the Ni‐Mn alloys.

Exchange Anisotropy in Cu-Mn and Ag-Mn Alloys

Cu-Mn and Ag-Mn alloy specimens of about 25 atomic % Mn were cooled to 4.2°K in a magnetic field, and it was found that their hysteresis loops measured parallel to this field were displaced from their symmetrical positions about the origin. The displacement of the loops decreases monotonically with increasing temperature, and its disappearance is accompanied by large hysteresis losses. These unusual properties are similar to those previously reported for disordered Ni-Mn alloys and are also attributed to exchange anisotropy interactions between small regions of ferromangetic and antiferromagnetic order. It is proposed that these regions arise from statistical composition fluctuations in the alloys and the existence of both ferromagnetic and antiferromagnetic interactions between Mn atoms. The essence of this microscopic exchange anisotropy model is illustrated for a square lattice representation of these alloys. The magnetic transformation temperatures are identified with anomalies in the 1/χ vs T curves;...

Mechanism for the First‐Order Magnetic Transition in the FeRh System

Measurements of the field dependence of the critical temperature (Tcrit) for the first‐order antiferromagnetic‐ferromagnetic transition in FeRh established that the total entropy change (ΔST) at Tcrit is much larger than the change in lattice entropy (ΔSL). Doping experiments (Pd, Pt, or Ir) showed that ΔST−ΔSL=ΔS varies almost linearly with Tcrit. Apparently the transition involves a change in the electronic density of states and thus in the electronic entropy, S=γTcrit, where γ is the electronic heat coefficient. To test this mechanism, low‐temperature specific‐heat measurements were made on several compositions in the Fe(Rh, Pd) system. The compositions and γ values are Fe53Rh47 (59 μJ/g·°K2), Fe51Rh49 (60 μJ/g·°K2), Fe48Rh46Pd6 (64 μJ/g·°K2), Fe49Rh51 (16 μJ/g·°K2), and Fe48Rh49Pd3 (31 μJ/g·°K2). The first three are ferromagnetic at T=0; the latter two are antiferromagnetic with Tcrit of 310° and 210°K, respectively. The insensitivity of γ to Fe concentration in the ferromagnetic samples suggests a br...

Unusual Magnetic Behavior of Disordered Ni3Mn

Measurements were made of the magnetization of disordered Ni3Mn for various fields between 945 and 7550 oe and at many temperatures between 1.5 and 300°K. The magnetization vs temperature curve for any given field is found to have a pronounced maximum at about 25°K and a long concave‐upward section extending from about 140°K to temperatures above 300°K. By means of a new method of analysis, the Curie point is deduced to be 132°K. An abnormally large induced magnetization, greatest at about 50°K, is also found. All the experimental results are consistent with the occurrence of a ferrimagnetic‐ferromagnetic transition at low temperatures.

Effects of Mechanical and Thermal Treatment on the Structure and Magnetic Transitions in FeRh

Well‐annealed bulk samples of the intermetallic compound FeRh exhibit a first‐order transition from antiferromagnetic to ferromagnetic order on heating near 330°K. Plastic deformation has been found to convert the normal CsCl‐type ordered structure of FeRh to a disordered fcc structure, which is only weakly magnetic and does not exhibit the first‐order transition. On annealing the disordered material at Ta=510°K, three stages of effects have been observed. In the first, the fcc phase converts to a highly ordered CsCl‐type structure, the magnetization at Ta increasing with a time constant of ∼10 min; the material is strongly ferromagnetic down to low temperatures. In the second stage, there is a much slower (τ≈400 min) increase in magnetization at Ta with a gradual return of the first‐order transition at lower temperatures. In the third stage, there is no further increase in magnetization at Ta, but the first‐order transition continues to increase in completeness and sharpness. The slow return of the first...

On the Determination of Magnetocrystalline Anisotropy Constants from Torque Measurements

According to simple theory, torque measurements on cubic single‐crystal disks should give values for the crystal anisotropy constants, K1 and K2, that are independent of field above some minimum saturating field. Our experiments on {100} and {110} disks of 3¼% silicon‐iron, and previous experiments of others, show that this theory is inadequate. In a more refined theory it is assumed that the edges of the disk are not saturated even in high fields. The observed increase in the peak values of the torque with increasing field can be attributed to a very small variation of the net magnetization I as the disk is rotated in the field. This variation must be such that I is largest in the hard directions of magnetization and smallest in the easy directions. Direct observation of the domain patterns on a {100} disk appears to confirm these assumptions. For both {100} and {110} disks, K1′ (the effective value of K1) varies as 1−c/√H over the range 1000 to 20 000 oersteds. The corresponding K2′ values for the {110}...

Magnetic Critical‐Point Behavior of CrO2

The magnetization σ of the ferromagnetic compound CrO2 was measured as a function of field H and temperature T near the Curie point Tc. From isotherms of σ2 vs H/σ, the initial susceptibility χ0 above Tc was obtained, which when tested against the relationship χ0−1 ∝ (T−Tc)γ gives a constant γ of 1.63±0.02 from just above Tc (386.5°K) up to about 1.15 Tc. This γ value contrasts with the values near43 recently computed for the Heisenberg model and later found experimentally in various ferromagnetic metals and compounds. At higher temperatures the effective γ decreases rapidly towards unity. Up to the highest field used (25 kOe), the critical isotherm obeys the relationship σ∝H1/δ with δ=5.75±0.05, which differs markedly from the theoretical δ values of 3 (molecular field) and 5.2 (3‐dimensional Ising) and from various experimental values. Gradual departure from this relationship below 1.5 kOe is attributed to the magnetocrystalline anisotropy that persists at Tc. Furthermore, we find that all the σ(H, T) d...

Ferromagnetic to Canted‐Ferrimagnetic Transition in Fe(Pd, Pt)3

Magnetization and neutron‐diffraction measurements were made on the pseudo‐binary Cu3Au‐type ordered alloys Fe(PdxPt1−x)3 down to 4.2°K. The magnetic transition reported earlier for compositions near x=0.6 is found to correspond to a rapid but continuous change, as the temperature is lowered below ∼140°K, from pure ferromagnetism (as in FePd3) to a state in which the Fe moments open out into a simple canted structure (with two sublattices arranged identically to those in the pure antiferromagnet FePt3). The Pd, Pt moments are always parallel to the net ferromagnetic Fe moment; these moments vanish for compositions x<0.5. The occurrence and nature of this transition can be readily explained within a molecular field model in which the Pd, Pt moments are induced by and are a rapidly saturating function of the net exchange field from the Fe moments.