Ferrite desaturation in slow motion

Hi everyone,

The following appears to be more physics than electronics, but is very 
relevant to the latter and many of you have already amazed me with your 
knowledge. So here is the question.

A ferrite toroid is saturated by some current defined by the geometry of 
the core/winding and some material constants. The exact values of I and 
B(I) are not important, assume they are sufficiently high.

Now, as the current is decreased, B(I) eventually decreases to some B_r. 
This is a relatively accurate collective description of the underlying 
phenomena. But what are these phenomena? What causes the domains to lose 
their alignment? Thermal excitations? What is the time scale? What 
actually happens in the ferrite when observed at nanosecond resolution?
I know what the situation is going to look like after a microsecond, but 
what is the dynamics of the change?

Could you please suggest me some good reading on the transient phenomena 
in ferrite ceramic materials? I would like to understand that far better 
and beyond what the typical magnetics design books have to offer.

	Best regards, Piotr

On Saturday, September 26, 2020 at 7:40:42 AM UTC+10, Piotr Wyderski wrote:
> Hi everyone, 
> 
> The following appears to be more physics than electronics, but is very 
> relevant to the latter and many of you have already amazed me with your 
> knowledge. So here is the question. 
> 
> A ferrite toroid is saturated by some current defined by the geometry of 
> the core/winding and some material constants. The exact values of I and 
> B(I) are not important, assume they are sufficiently high. 
> 
> Now, as the current is decreased, B(I) eventually decreases to some B_r. 
> This is a relatively accurate collective description of the underlying 
> phenomena. But what are these phenomena? What causes the domains to lose 
> their alignment?

Back when I was a graduate student in inorganic chemistry - I bailed out after two years with a master's degree, and went on to do a Ph.D. in physical chemistry - the magnetic behaviour of transition metal nuclei was of interest. They were either paramagnetic (the nuclei tended to line up amplifying the field a little ) or diamagnetic (the magnetic axes of adjacent nuclei tended to point in opposite directions, attentuating the external field a little).

Ferromagnetism didn't come up. The very small energy differences involved meant that room temperature thermal excitation could the flip nuclei very easily.

Magnetic refrigeration exploits this down at liquid helium temperatures.

https://en.wikipedia.org/wiki/Magnetic_refrigeration

How fast it could happen is anybodies  guess. You are talking about the rotational inertia of an atomic nucleus which is very small indeed.

Nuclear magnetic resonance in a 20 Telsa field happens at frequencies from 60–1000 MHz, so it can be quite quick.

https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance

 >Thermal excitations? What is the time scale? What 
> actually happens in the ferrite when observed at nanosecond resolution? 
> I know what the situation is going to look like after a microsecond, but 
> what is the dynamics of the change? 
> 
> Could you please suggest me some good reading on the transient phenomena 
> in ferrite ceramic materials? I would like to understand that far better 
> and beyond what the typical magnetics design books have to offer. 

None of the books I've read about magnetic phenomena have been in the least helpful. Physicists may have access to better texts. Phil Hobbs or George Herold  come to mind.

-- 
Bill Sloman, Sydney

Bill Sloman wrote:

> How fast it could happen is anybodies  guess. You are talking about the rotational inertia of an atomic nucleus which is very small indeed.

People are using pretty typical ferrite toroids in pulse compression 
circuits, and the time scale is tens of nanoseconds or less. So the 
underlying physics is fast enough, but I am not sure what the physics is.

At the moment, I am interested in the transition phenomena only 
("edges", not "levels"). On a practical note, I would like to know how 
fast I can desaturate a piece of ferrite without making it explode, what 
ferrites would be the fastest and how to optimise the process. Most 
importantly, I want to understand what's going on under the hood, as I 
am not a big fan of voodoo science.

Thank you, Bill.

	Best regards, Piotr

"Piotr Wyderski" <peter.pan@neverland.mil> wrote in message 
news:rkmtnj$3u1fd$1@portraits.wsisiz.edu.pl...
>> How fast it could happen is anybodies  guess. You are talking about the 
>> rotational inertia of an atomic nucleus which is very small indeed.
>

I would propose a figure on the order of the electron paramagnetic 
resonance.  So, some GHz typically.  This isn't very noticeable in spinel 
ferrites (losses dominate), but garnets are useful in the GHz -- using the 
Faraday effect in circulators/isolators, and the EPR directly in YIG 
oscillators.

Losses are scale dependent, hence why large ferrite beads have a lower peak 
resonance than small ones, etc.

Higher loss materials, in relatively large shapes, mask the effect of 
underlying physics -- there's just so little material participating, there's 
practically no signal left at the ~GHz where interesting things might be 
observed.

So, MnZn (high loss, high mu) tends to be... "classical", in the sense that 
it can be described as a lossy bulk material with all the usual messy 
hysteresis and saturation properties.  NiZn, same thing but lower loss, mu 
and Bsat.  YIG lower still, but finally low enough that quantum effects are 
perceptible (like EPR).

Likewise for metals, bulk forms are lossy in a classical skin-effect manner 
(laminated iron, amorphous/nanocrystalline strip).  Powder is lossy in a 
similar way, given a range of particle size and some bulk conductivity 
(depending on binder fraction and pressing).

I'm not sure offhand if there's a metal powder composition that has 
particles fine enough, or of the right alloy, such that quantum effects are 
measurable at high frequencies.


Mind, this is all very hand-wavey, partly because I know very little about 
the physics itself, and partly because physics itself knows very little 
about ferromagnetism.  There isn't much explanatory value in theoretical 
studies of such a complex material; empirical studies tend to be more 
useful.

Also just my rough understanding; I haven't played with a lump of YIG for 
example.


> People are using pretty typical ferrite toroids in pulse compression 
> circuits, and the time scale is tens of nanoseconds or less. So the 
> underlying physics is fast enough, but I am not sure what the physics is.
>
> At the moment, I am interested in the transition phenomena only ("edges", 
> not "levels"). On a practical note, I would like to know how fast I can 
> desaturate a piece of ferrite without making it explode, what ferrites 
> would be the fastest and how to optimise the process. Most importantly, I 
> want to understand what's going on under the hood, as I am not a big fan 
> of voodoo science.

Under the above assumptions, I think you'll find that, even if you set 
external fields to zero, the bulk will take some time to "relax" to whatever 
level it does (remenance), and that time will be determined in essence by 
the L/R time constant of the bulk material.  Which again, depends on size 
(it's a stretch to call it a "bulk time constant", it's scale dependent).

This can also be understood in terms of wave propagation: the speed of light 
inside the material is quite low (high mu, modest e_r, modest rho), so the 
external field change is transmitted through the bulk at a corresponding 
rate.  And because the material is lossy, the field doesn't reach the center 
intact, it's attenuated and dispersed.  (The variable mu and loss with 
frequency causes velocity to change as well.)  So you get some standing 
waves, but they're largely damped, and there's a tail as the internal field 
eventually settles out.

I have seen a few articles where magnetic compressors or shock lines were 
built with stacks of alternating washers, of ferrite and dielectric.  Same 
idea as laminated iron, done at proportionally higher bandwidth, and with 
proportionally higher frequency material. :-)

Neat fact: standing waves are measurable on ferrite beads.  Compare long and 
thin to short and fat shapes.  Most parts have a more-or-less simple 
resonance (that's reasonably well fit by a single RLC unit, plus some 
diffusion RL on the LF side, plus DCR), others have an inflection point or 
even a double peaked response.

Somewhat less useful fact: standing waves occur in metals, too.  This is why 
round wires have skin effect given by Bessel functions.  The limit, as 
delta/R --> 0, does indeed equal the exponential solution found in the 
infinite-plane case.  That is to say, as the curvature of the wire, relative 
to skin depth, goes to zero, the geometry and solution are equivalent to the 
plane case.  (Less useful, because the difference in AC resistance isn't 
much, in the end.)

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/ 

Tim Williams wrote:

> Losses are scale dependent, hence why large ferrite beads have a lower 
> peak resonance than small ones, etc.

Thank you very much for your very practical input.

It led me to a follow-up question: how do ferrite losses depend on 
saturation? If the frequency is sufficiently high, say ~1GHz, and the 
wire passes through the core, can I turn on/off the losses by saturating 
the ferrite? Say the attenuation range of interest is 2:1 or more.

I am thinking about a magamp-like structure, but the controlled 
parameter would be attenuation, not inductance. A HF magnetoresistor, in 
fact.

	Best regards, Piotr

On Sun, 27 Sep 2020 20:43:09 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

>Tim Williams wrote:
>
>> Losses are scale dependent, hence why large ferrite beads have a lower 
>> peak resonance than small ones, etc.
>
>Thank you very much for your very practical input.
>
>It led me to a follow-up question: how do ferrite losses depend on 
>saturation? If the frequency is sufficiently high, say ~1GHz, and the 
>wire passes through the core, can I turn on/off the losses by saturating 
>the ferrite? Say the attenuation range of interest is 2:1 or more.
>
>I am thinking about a magamp-like structure, but the controlled 
>parameter would be attenuation, not inductance. A HF magnetoresistor, in 
>fact.
>
>	Best regards, Piotr

Attenuation/impedance at non-gigahertz frequencies is illustrated 
in the Fair-Rite catalog for selected commercial part types at 
varying levels of DC bias. Magnetic fields imposed from external 
sources on unbiased parts should be expected to have similar 
characteristics.

Heavily saturated parts show impedance as low, but still increasing 
with frequency, at 1GHz, for some structures.

https://static6.arrow.com/aropdfconversion/f054de036df0d70bc5a88a0415bdb3ea4c7af4d4/fr_catalog-14thed_rev3.pdf

RL

On Friday, September 25, 2020 at 5:40:42 PM UTC-4, Piotr Wyderski wrote:
> Hi everyone,
> 
> The following appears to be more physics than electronics, but is very 
> relevant to the latter and many of you have already amazed me with your 
> knowledge. So here is the question.
> 
> A ferrite toroid is saturated by some current defined by the geometry of 
> the core/winding and some material constants. The exact values of I and 
> B(I) are not important, assume they are sufficiently high.
> 
> Now, as the current is decreased, B(I) eventually decreases to some B_r. 
> This is a relatively accurate collective description of the underlying 
> phenomena. But what are these phenomena? What causes the domains to lose 
> their alignment? Thermal excitations? What is the time scale? What 
> actually happens in the ferrite when observed at nanosecond resolution?
> I know what the situation is going to look like after a microsecond, but 
> what is the dynamics of the change?
> 
> Could you please suggest me some good reading on the transient phenomena 
> in ferrite ceramic materials? I would like to understand that far better 
> and beyond what the typical magnetics design books have to offer.
> 
> 	Best regards, Piotr

I know almost nothing of magnetics.  But we made this ~flux gate 
magnetometer out of an inductor* (kinda over driven) and observing 
it come in and out of saturation was very interesting to me.  

My only thoughts,
George h. 

*in some external B-field (over-wrapped coil)

On Sunday, September 27, 2020 at 6:07:42 PM UTC-4, George Herold wrote:
> On Friday, September 25, 2020 at 5:40:42 PM UTC-4, Piotr Wyderski wrote:
> > Hi everyone,
> > 
> > The following appears to be more physics than electronics, but is very 
> > relevant to the latter and many of you have already amazed me with your 
> > knowledge. So here is the question.
> > 
> > A ferrite toroid is saturated by some current defined by the geometry of 
> > the core/winding and some material constants. The exact values of I and 
> > B(I) are not important, assume they are sufficiently high.
> > 
> > Now, as the current is decreased, B(I) eventually decreases to some B_r. 
> > This is a relatively accurate collective description of the underlying 
> > phenomena. But what are these phenomena? What causes the domains to lose 
> > their alignment? Thermal excitations? What is the time scale? What 
> > actually happens in the ferrite when observed at nanosecond resolution?
> > I know what the situation is going to look like after a microsecond, but 
> > what is the dynamics of the change?
> > 
> > Could you please suggest me some good reading on the transient phenomena 
> > in ferrite ceramic materials? I would like to understand that far better 
> > and beyond what the typical magnetics design books have to offer.
> > 
> > 	Best regards, Piotr
> 
> I know almost nothing of magnetics.  But we made this ~flux gate 
> magnetometer out of an inductor* (kinda over driven) and observing 
> it come in and out of saturation was very interesting to me.  
> 
> My only thoughts,
> George h. 
> 
> *in some external B-field (over-wrapped coil)

One purpose of the over wrapped coil was to cancel the 
Earth's B-field... so that gives you some estimate of the 
fields involved.  
GH

"Piotr Wyderski" <peter.pan@neverland.mil> wrote in message 
news:rkqmfv$1gad0$1@portraits.wsisiz.edu.pl...
> It led me to a follow-up question: how do ferrite losses depend on 
> saturation? If the frequency is sufficiently high, say ~1GHz, and the wire 
> passes through the core, can I turn on/off the losses by saturating the 
> ferrite? Say the attenuation range of interest is 2:1 or more.
>
> I am thinking about a magamp-like structure, but the controlled parameter 
> would be attenuation, not inductance. A HF magnetoresistor, in fact.

Yes; mu falls and, probably losses remain a constant fraction of that, but 
because the magnetic path is effectively getting more air gap (which is 
lossless), the Q rises.

legg linked the Fair-Rite catalog that shows some plots with DC bias; and 
Laird's catalog is even more expansive (if blurry).

Here's a part with curve fitting besides:
https://www.seventransistorlabs.com/Modeling/Images/HI0603P600R_Overlay.jpg
and the model:
https://www.seventransistorlabs.com/Modeling/SPICE/HI0603P600R_NL.ckt
(Saturation isn't quite right, but it's probably close enough to do a crude 
nonlinear circuit.)

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/ 

George Herold wrote:

> One purpose of the over wrapped coil was to cancel the
> Earth's B-field... so that gives you some estimate of the
> fields involved.

Yes, fluxgates can be sensitive and accurate, but they are pretty slow.
The BW of those I know of is <1MHz. Here I have quite the opposite 
problem: accuracy can be low, and no linearity is required (a magnetic 
window comparator is what I need), but the time scale of H change is on 
the order of 10ns. I am trying to figure out what the B change would 
then be.

	Best regards, Piotr