Long Term Magnetic Stability of Alnico and Barium Ferrite MagnetsK. J. Kronenberg and M. A. Bohlmann Citation: Journal of Applied Physics 31, S82 (1960); doi: 10.1063/1.1984613 View online: http://dx.doi.org/10.1063/1.1984613 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/31/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Long-term magnetic field stability of Vega AIP Conf. Proc. 1429, 82 (2012); 10.1063/1.3701905 Long term stability of electrodynamic tethers AIP Conf. Proc. 608, 554 (2002); 10.1063/1.1449773 Evaluation of LongTerm Magnet Stability J. Appl. Phys. 37, 1101 (1966); 10.1063/1.1708352 Flexure Mounted Beam Balance for LongTerm Magnetic Stability Measurements Rev. Sci. Instrum. 32, 1051 (1961); 10.1063/1.1717611 Investigations on Barium Ferrite Magnets J. Appl. Phys. 27, 1051 (1956); 10.1063/1.1722540

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82S K. J. KRO.\ENBERG

to zero. Only these diffraction patterns, the dots of which originated over extended areas of the ~ample, represent the crystal lattice and were used for the struc­tural study.

RESULTS

Many earlier findings based on different experiments have been confirmed.

1. Samples of Alnico V, air cooled after casting, pro­duced exclusively the diffraction patterns of a body­centered cubic lattice, which was found by Heidenreich and Nesbitt with reflection diffraction.l [See Fig. 2 (a)]. It represents the crystal lattice of the matrix. Electron micrgraphs of transparent areas of the material show a milky cloudiness without structure.

2. After cooling in a field the material exhibits almost everywhere a completely new diffraction system in addi­tion to the one found without field cooling. Figure 2 (b) shows a ring pattern superimposed on the former spot pattern. The rings indicate a face-centered cubic lattice which has no correlation to the matrix lattice. The rings are granulated but otherwise exhibit a uniform intensity all around. They must originate from a great multitude of very small crysials which have little correlation to one another. A precipitation ha~ obviously started with very many nuclei. The dots of the matrix patterns are still intense. But they have grown less sharp and have changed their distance. While the precipitate grew, the matrix changed composition and lost a bit of its former degree of order. Electron micrographs look cloudy with dark granules of irregular arrangement.

3. After the final heat treatment, the diffraction

pattern of Alnico V has changed into a dot patt, representing a highly ordered crystal structure. ']~rn dots are the remainders of the rings found after fi l~ cooling. Almost everywhere in the sample the rna;':·

h . h d f' L'k fiX pattern as yams e to amt .tra~es. 1 e in Fig. 2(c1, the dots are often double or tnple If they originate from a transparent sample area which extends over about square micron. The dots have almost the same distan a as the rings had before the final h~at treatment. \{:! find areas of the sample where the rmgs are still intact but heavily emphasized at places where the final dotg would be expected. Such areas may have been delayed in precipitating and represent an intermediate situation of incomplete structure.

The growing concentration of the intensity of the rings at certain angles is the consequence of the in­creasing order of the precipitate. This proceeds in Alnico V to form very sharp dot patterns indicating perfect single crystal order. It has to be assumed that the precipitate particles grow together and recrystallize to produce a sort of single crystal network. The matrix becomes subdivided into separated particles producing only faint and diffuse diffractions.

4. The fact that an area of one square micron usually produces double or triple dots confirms an earlier a;. sumption concluded from electron micrographs of the Alnico V structure.4 The precipitated network of Alnico V is subdivided by dislocation lines into a mosaic type structure with 2 to 5 areas of slightly differing crystal orientations per square micron.

Diffraction patterns of alnico materials other than Alnico V differ mainly in the degree of order the precipi­tate is achieving.

JOGR?>JAL OF APPLIED PHYSICS SUPPLEMENT TO VOL. 31, :\'0.5 MAY. 1960

Long Term Magnetic Stability of Alnico and Barium Ferrite Magnets*

K. J. KRONENBERG AND M. A. BOHLMANN

The Indiana, Steel Products Company, Valparaiso, Tn-diana

Alnico and barium ferrite materials do not age at room temperature. The decreases in remanence, occurring with time, are adjustments by the magnet to its environm,ent, Remanence adjustment proceeds with the logarithm of time and amounts to 2% or less one year after magnetization. Remanences in magnets with Hei >2200 oe show no changes, Also, the smaller the irreversible susceptibility at the operating point, the more stable is remance. Remanences can be stabilized completely by preadjustment.

Alnico V magnets are more stable than similar Alnico III magnets, even when the Alnico V magnets operate at a higher susceptibility. NeeJ's theory suggests that the oriented material is more stable than the isotropic material because larger volumes are involved in each magnetization reversal. To explain relatively large domain-like regions of reversal, we have to assume that magnetic regions cooperate during the mag­netization reversal.

T HE permanent magnet materials, Alnico and barium ferrite, do not change in time at room

temperatures. Figure 1 compares the aging of a quench-

* Research was supported by the USAF under contract, moni­tored by tbe Aero. Res. Lab., WADe.

hardened steel to the stability of remanence Ed of the Alnicos. The decrease of remanence of the stable ma­terials is not a permanent one. Remagnetization to saturation restores the original remanence value exactly. Since the remanence behavior is not related to the age

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M .\ G :\ E TIC S T A. B I LIT Y 0 F .\ L :'-J 1 C 0 .\ ~ 0 BAR I C:-1 FER R I T E 83S

TABLE I. ~atural stability of remanence.

===- Relative remanence 10 000 hr after Measuring

Material Hei oe LID magnetization (percent) tolerance --- 4000 0.9 100.0 ±0.1 Indox I Indox V 2030 0.8 99.6 ±0.1 Alnico III 580 3.5 98.10 ±o.04

2.2 97.04 ±0.05 Alnico VII 12010 3.5 99.32 ±O.04

2.2 98.96 ±O.06 Alnico V 630 8.0 99.95 ±0.01

4.3 99.23 ±0.02 3.5 98.84 ±0.04 2.2 98.3 ±0.07

ofthese materials, one should not describe it as "aging." This word should be reserved for materials which do change with age.

The decrease of remanence with time in the non-aging materials could be described best as remanence self adjustment. The arrangement of magnetic domains or regions is disturbed constantly by randomly di~tributed influences from within and without. They provoke rearrangements to lower-energy configurations which result, generally, in a decrease of remanence. The amount depends on many parameters concerning the magnet as well as its environment. For quantitative details of our experiments, see references 1-3 and Table r. Here we can only qualitatively summarize the measured relations.

(1) Remanence adjustment proceeds with the loga­rithm of time after magnetization. In our measure­ments, we chose an hourly scale. Starting with 0.1 hr after magnetization, the first year contains 5 decades and 10 years contains 6 decades.

(2) The higher the coercive force, the more stable is remanence. Magnets with H ci > 2200 oe (isotropic barium ferrite) had unchanging remanences during the first year.

(3) The remanence adjustment in magnets of Hel <2200 oe (alnicos and oriented barium ferrites) amounts to values from fractions of a percent to a few percent during the first year. But, these remanences too can be stabilized completely by preadjusting the rema­ence. Partial demagnetizing after magnetization to satUration leads to a somewhat smaller, but stable, remanence. However, excessive or rather sudden de­magnetization results in erratic behavior. The proper knockdown for a magnet depends 011 individual circum­stances, but generally is about 5-15%. , (4) Incompletely magnetized magnets have a more

Stable remanence than fully magnetized magnets. How­f:ver, they are not as stable as those ",hich are fully ll1agnetized and properly stabilized. ~gneh of different shapes sho\\' different rates

:l( J. Kronenberg, ,\rchiv. Eisenhullenll'. 24, -l·B (19.13). ,R:.]. Kronenberg, Z .. -\nge\\'. Ph,·s. 5, 321 (1953).

J.s!. ]. Kronenberg and :\1. A. Bohlmarp1, WADe TR 58-535 dA Document No .. -\D 203387, O.T.S.-PB151716, (1958).

%

LID 80 08 35 35 35 59

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", , \ ,

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FIG. 1. .-\ging of steel compared to natural stability of alnicos and barium ferrite.

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of remanence adjustment. The smaller the irreversible susceptibility at the operating point, the more stable is the remanence. Thus, a longer magnet shape has the advantage over a shorter one.

(6) Effect of orientation. It might seem from (5) that highly oriented materials may have poor stability if one cannot avoid unfavorable shapes or operating condi­tions which may bring the operating point into the steep part of the demagnetization curve. This is not the case. Generally, oriented material has the advantage over isotropic material in spite of higher susceptibility. A glance at the theory may help to understand this.

Neel calculated quantitative relations of the problem, which we have verified experimentally.4 The basic idea of his theory is the assumption that, for each disturb­ance influencing the magnetic situation at a certain location in a magnet, one can assign a magnetic field. These fictitious local fields can represent mechanical shocks, effects of external fields, rotations in the earth field, influences of temperature, or thermal fluctuations inherent in every material above absolute zero. Under constant environmental conditions in storage or usage, the latter fictitious fields occur randomly with the pas­sing of time. Each occurrence provokes an adjustment­stabilizing against all ensuing occurrences of equal or smaller sizes. Only a larger one initiates a new adjust­ment. The effect of these fields on magnetization is directly proportional to field strength and susceptibility.

But, Neel's theory considers also the importance of the volume of material involved in Barkhausen jumps. The smaller the regions of simultaneous reversal are, the more likely are occurrences of disturbances exten­sive enough to initiate reversals, and ~'ice ~'el'sa. This offers an explanation of the surprisingly high stability of Alnico V magnets operating in the steep part of the demagnetization curve. The stability of such Alnico V magnets is many times better than one would expect from the susceptibility. To explain this, we assume that yolume'i illyolyed in reversals must be several hundred times larger in the oriented material. The stability

"L. Ned, 1- phys. radium 11, 49 1950; 12, 339 (1<)51).

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84S K. ]. KRONENBERG AND M. r\. BOHLMANN

measurements in the light of Neel's theory suggest a grouping of reversing portions of the magnet into larger cooperating regions. This agrees with earlier findings obtained by observing colloid patterns. 5 To explain

5 R. K. Tenzer, Proceedings of Conference on Jifagnelism and Jllaterials, AlEE Spec. Publ. T-91, pp. 203-211.

relatively large domain-like regions of reversal w h_~ h .. , e '''llII

to assume t at magnetic regIOns cooperated durin be magnetization reversal. The stability behavi g t oriented barium ferrite suggests similar condition:

r tf

tends to confirm our assumptions of domain groups ~ versing cooperatively. r-.

J 0 U R :\ .\ L 0 F .\ P P LIE D PH Y SIC S SCPPLEMENT TO VOL. 31. :\0. 5 MAY. 19611

Hot Working of Alnico 5 Alloys

C. L. KOLBE AND D. L. MARTIN

General Electric Research Laboratory, Schenectady, New York

Alnico permanent magnet alloys have been successfully shaped into rod, wire and strip by hot working at elevated temperature. It has been found that by careful processing, certain Alnico alloys can be fabricated by extrusion, swaging, and rolling. The wrought Alnico after heat treatment was found to have comparable permanent magnet properties and improved mechanical properties compared to cast specimens of the same alloy.

INTRODUCTION

AL~ICO magnets are made commercially by casting molten metals or by pressing and sintering of

powders. The brittleness of these alloys, which makes them unworkable at room temperature, and moderately high temperatures, is associated with a two-phase microstructure.

At a high temperature the Alnico alloys usually transform into a ductile body-centered cubic phase. This transformation is the basis of the successful hot fabrication of Alnico. In practice, the solution to the hot shaping problem is more involved because of the effect of alloying elements on the phase fields, and the effect of impurities, segregation, etc. on the workability.

Alnico-type alloys have been successfully hot de­formed before. Bieberl added 2% titanium to a 25 Ni, 9 AI, 64 Fe alloy and hot forged to !-in. fiats. He also added titanium to Fe-Ni-AI-Co alloys2 and hot rolled to i-in. strip. Finch and WhiteS hot forged Fe-Ni-AI­Co-Cu alloys containing 4.8% vanadium. Hansen4 de­scribes successful 180° hot bend tests on alloys to which titanium and zirconium had been added. The alloys covered in these three investigations were of the Alnico 1-4 types with energy product values under 2 million gauss-oersteds in contrast to values of 4-5 million gauss­oersteds reported in the present investigation.

Earlier studies by R. Ahles, W. Reich and R. lVIcKechnie5 in General Electric had established the

1 C. G. Bieber, lPermanent Magnet, U. S. Patent 2,285,406 (1942).

2 C. G. Bieber, Alloy for Permanent Magnet, U. S. Paten,t 2,384,450 (1945).

3 O. J. Finch and J. H. White, Permanent Magnet Material, U. S. Patents 2,347" 817 and 2,349, 857 (1944).

4]. R. Hansen, Permanent Magnet and Magnets Therefor, U. S. Patent 2,499,862 (1950).

5 R. Ahles, W. Reich, and R. McKechnie (private communica­tions).

workability of Alnico 5 alloys. This present study is an extension of their work and includes several of the alloys melted by R. McKechnie.

EXPERIMENTAL

~lore than forty compositions were prepared and evaluated for their hot fabrication characteristics. Iu this paper only a general summary of results is given. The twelve alloys listed in Table I to illustrate our ob­servations are divided into four groups: The first, Nos. 1-3, second, Nos. 4--8, and fourth, Nos. 11-12, groups are vacuum melted. Groups two and three, Nos. 4--10, contain zirconium additions, and the fourth group has high cobalt-titanium Alnico 10 modifications (Ticonal X).

Extrusion

In the early experiments it was found that the alloys containing zirconium successfully extruded at low tem­peratures in the range of 1050-1100°C. Sounder bars with fewer surface cracks were obtained than with the straight Alnico 5 composition.

An improved surface condition of the extruded bars was obtained by encasing the billet in a steel jacket. In one case (Alloy 9) the surface cracked badly when the alloy was extruded bare, 'but had a crackfree surface when re-extruded within a steel jacket. A heavy steet jacket is not necessary. A wall thickness of i in. was found satisfactory.

Extrusion Was a method of preconditioning prior to further working by swaging or rolling. Extrusion, how­ever, is not a prerequisite for satisfactory swaging and rolling results. Cast bars of several alloys were swaged without prior extrusion (Alloys 5 and 11). .

While acceptable extrusions were obtained with. mr­melted as well as with vacuum-melted alloys, prOVIded

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